ELECTROCHEMICAL PROPULSION SYSTEM USING pH-CONTROLLED COBALT OXIDATION CATALYST

Abstract

To provide an electrochemical propulsion system in space propulsion systems that achieves both variable thrust characteristics adaptable to different operating conditions and high efficiency, and enables in-situ utilization of water resources.SOLUTION: An electrochemical propulsion system combines an electrode structure using a nanosized CoOx catalyst with a plurality of electrolyte compartments having different pH environments of alkaline, neutral, and acidic, to precisely control an oxidation state change of cobalt oxide. Variable thrust characteristics are achieved by switching between a high-efficiency propulsion mode utilizing low overvoltage (approximately 376 mV) in an alkaline environment and a precision control mode in a neutral or acidic environment. Redox processes of Co^2+ / Co^3+ and Co^3+ / Co^4+ are optimized, and catalyst stability is improved by CeO_2 addition. By using a propellant having water as a main component, safety improvement and in-situ resource utilization on the Moon, Mars, etc. are enabled.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to the field of space propulsion systems, and more particularly to a propellant system having high efficiency and variable thrust characteristics using an electrochemical oxygen evolution reaction (OER). In particular, it relates to a novel electrochemical propulsion system that utilizes the oxidation state change and pH dependence of cobalt oxide catalysts. The present invention is particularly useful as a propulsion system in orbit control, attitude control, deep space exploration, and long-term missions on celestial bodies such as the moon and Mars of spacecraft.

Background Art

[0002] In space propulsion systems, a highly efficient and controllable propulsion mechanism is required. Conventional chemical propellants can generate high thrust, but there are problems such as limitations in specific impulse, toxicity, and the risk of explosion. For example, conventional propellants such as hydrazine (N_2H_4) and monomethylhydrazine (MMH) provide high thrust density, but the handling risks and adverse environmental impacts are issues. In particular, hydrazine is known as a carcinogenic substance, and special protective equipment is required for handling. Also, oxidizers such as dinitrogen tetroxide (N_2O_4) have high toxicity and high risk during leakage. The specific impulse of these chemical propellants is usually about 300 - 450 seconds, and in long-term missions, the propellant mass will occupy most of the mass of the entire spacecraft.

[0003] On the other hand, electric propulsion systems such as ion engines using xenon or krypton can achieve high specific impulse (2000 - 5000 seconds), but have limitations such as low thrust (usually on the order of several mN to several tens of mN) and dependence on the supply of rare gases. Ion engines require long-term continuous operation in scenarios where a large thrust such as orbit change is required, and also consume a large amount of power. Also, xenon has limited abundance on Earth, and there are concerns about supply shortages and price increases as large-scale space development progresses. Other electric propulsion systems such as Hall thrusters also have similar limitations.

[0004] In recent years, propulsion systems using water electrolysis have attracted attention. Since water can be used as a working fluid, it has advantages from the perspectives of the possibility of on-site resource procurement and safety. Especially in long-term missions on celestial bodies where water ice may exist, such as the Moon, Mars, Europa, and Enceladus, the possibility of using locally extracted water as a propellant can bring great flexibility to mission design. For example, there are permanently shadowed regions in the craters of the lunar South Pole, where the existence of water ice has been confirmed. If this water ice can be extracted, purified, and used as a propellant, sustainable space exploration that does not depend on replenishment from the Earth becomes possible.

[0005] However, conventional water electrolysis propulsion systems have problems with the efficiency and durability of the electrode catalysts, and it has been particularly difficult to optimize the performance under different operating conditions. Water electrolysis consists of an oxygen evolution reaction (OER) at the anode and a hydrogen evolution reaction (HER) at the cathode. In particular, OER is a four-electron reaction and is a kinetically slow process. Therefore, the performance of the OER catalyst greatly affects the efficiency of the entire system.

[0006] The electrochemical oxygen evolution reaction (OER) is an important half-reaction in water electrolysis, and its efficiency greatly depends on the performance of the catalyst. Conventionally, noble metal oxides (such as RuO₂ and IrO₂) are known to exhibit high activity as OER catalysts, but there are limitations in practical applications due to the problems of rarity and cost. In particular, IrO₂ shows high OER activity and stability even in an acidic environment, but iridium is one of the rarest elements on Earth and is not suitable for large-scale applications. RuO₂ is also rare and has problems with stability in an acidic environment.

[0007] On the other hand, cobalt-based oxide catalysts have attracted attention because they show relatively high activity towards OER and have advantages in terms of cost. Cobalt is relatively abundant on Earth, and mining and purification technologies have been established. Cobalt oxides and hydroxides such as Co₃O₄ and CoOOH are known to show high OER activity especially in an alkaline environment.

[0008] The OER activity of cobalt-based oxide catalysts is known to strongly depend on the pH environment and generally exhibits high activity in an alkaline environment. This is thought to be because in an alkaline environment, OH^- ions are abundant, promoting the formation of intermediates in the OER process. Also, the surface of cobalt oxide catalysts tends to form a CoOOH layer under an alkaline environment, and this layer functions as an OER active site.

[0009] Recent studies have revealed that the surface reconstruction and oxidation state changes of cobalt oxide catalysts are closely related to OER activity. In particular, it has been shown that the redox processes of Co^2+ / Co^3+ and Co^3+ / Co^4+ play important roles in OER activity. The Co^2+ / Co^3 redox process is related to the reconstruction of the catalyst surface, and the Co^3+ / Co^4+ redox process is directly related to the formation of active species in the OER catalyst cycle. These redox processes are greatly affected by the pH environment, and in particular, it is known that the Co^3+ / Co^4+ redox potential shifts in the positive direction as the pH decreases.

[0010] However, there are few examples of applying these findings to space propulsion systems, and in particular, little research has been done on realizing variable thrust characteristics by utilizing pH dependence. In conventional water electrolysis propulsion systems, operation in a single pH environment (usually alkaline or neutral) is common, and control of thrust characteristics by selectively using different pH environments is not considered.

[0011]

[0014] In Non-Patent Document 1, the OER activity and pH dependence of cobalt oxide catalysts are studied, but their application to space propulsion systems is not mentioned. In this document, it is shown that the OER activity of cobalt oxide catalysts strongly depends on pH, and in particular, it is reported to exhibit high activity in an alkaline environment. It is also shown that the Co^3+ / Co^4+ redox potential shifts in the positive direction as the pH decreases.

[0012] In Non-Patent Document 2, the observation of the oxidation state change of cobalt oxide catalysts using X-ray absorption spectroscopy (XAS) has been reported, but the method of applying this finding to thrust control has not been studied. In this document, the energy shift (ΔEedge) of the Co K absorption edge of cobalt oxide catalysts under OER conditions was measured to clarify the correlation between the oxidation state change of cobalt and the OER activity.

Prior Art Documents

Non-Patent Documents

[0013]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0014] The present invention has been made in view of the above problems of the prior art, and an object thereof is to provide an electrochemical propulsion system having high efficiency and variable thrust characteristics by utilizing the oxidation state change and pH dependence of cobalt oxide catalysts.

[0015] Specifically, it aims to solve the following problems: 1. To achieve variable thrust characteristics adaptable to different operating conditions (high-thrust mode and precision control mode). In space missions, there are scenarios that require a large thrust, such as orbit changes, and scenarios that require precise thrust control, such as orbit maintenance and attitude control. In conventional systems, it was necessary to install multiple propulsion systems to meet these different requirements, which led to an increase in mass and system complexity. The present invention aims to achieve different thrust characteristics with a single system. 2. To construct a safe and easily manageable propellant system with water as the main component. Conventional chemical propellants (such as hydrazine) are highly toxic and pose an explosion risk, and special facilities and training are required for handling. In addition, the fuel filling operation before launch is dangerous and may cause launch delays. Propellants with water as the main component are non-toxic and non-flammable, and are easy to handle, thus solving these problems. 3. To enable in-situ resource utilization on celestial bodies where water ice may exist, such as the Moon and Mars. In conventional propulsion systems, since propellants carried from the Earth are used, the mission duration is limited by the amount of propellants that can be carried. In a system that can utilize water as a propellant, in-situ resources can be utilized on celestial bodies where water ice exists, and the mission duration can be significantly extended. 4. To improve the durability of the catalyst and realize a system capable of handling long-term missions. In particular, it is a challenge to suppress the dissolution of cobalt catalysts, which is a problem during operation in an acidic environment, and ensure the long-term stability of the system. Conventional cobalt-based catalysts have problems with stability in an acidic environment and are not suitable for long-term operation. 5. To generate a larger thrust than an electric propulsion system while achieving a high specific impulse compared to conventional chemical propellants. There is a need for a system with intermediate performance to bridge the gap between the specific impulse of chemical propellants (usually 300 - 450 seconds) and the thrust density of electric propulsion systems (usually several mN to several tens of mN).

Means for Solving the Problems

[0016] To solve the above problems, the electrochemical propulsion system of the present invention combines an electrode structure using a nano-sized CoOx catalyst with an electrolyte system having a plurality of pH environments, precisely controls the change in the oxidation state of cobalt oxide, and enables adjustment of the efficiency and rate of the oxygen evolution reaction.

[0017] Specifically, a multi-compartment electrolyte system having a plurality of electrolyte compartments in an alkaline environment (pH of about 13), a neutral environment (pH of about 7), and an acidic environment (pH of about 1) and separated by an ion exchange membrane is adopted. Thereby, it is possible to select an optimal pH environment according to the operating conditions and adjust the thrust characteristics by controlling the change in the oxidation state of the cobalt catalyst. Each compartment has an independent electrode and electrolyte circulation system and is selectively activated as needed.

[0018] According to one aspect of the present invention, the surface characteristics of the nano-sized CoOx catalyst are optimized, and the OER activity can be enhanced by improving the efficiency of the redox processes of Co^2+ / Co^3+ and Co^3+ / Co^4+. Specifically, the size of the CoOx particles is controlled to 20 - 50 nm, the specific surface area is maximized, and the change in the electronic state due to the quantum size effect is suppressed. Also, by optimizing the surface oxygen defect concentration, the interaction with hydroxide ions (OH^-) is strengthened, and the stabilization of the OER intermediate is achieved.

[0019] In particular, by utilizing the characteristic that the Co^3+ / Co^4+ redox potential changes depending on pH, high-efficiency operation at low overvoltage is achieved in an alkaline environment, and precise thrust control is realized in a neutral or acidic environment. In an alkaline environment, the Co^3+ / Co^4+ redox potential is relatively low at about 1.47 V vs. RHE, and the OER onset potential is also low, so high-efficiency operation at low overvoltage is possible. On the other hand, in a neutral or acidic environment, the Co^3+ / Co^4+ redox potential shifts in the positive direction (about 1.57 V vs. RHE in a neutral environment and about 1.62 V vs. RHE in an acidic environment), so more precise potential control is possible, which is suitable for fine adjustment of thrust.

[0020] Also, according to another aspect of the present invention, in order to improve the stability of the catalyst, a trace amount of cerium oxide (CeO₂) is added to suppress the dissolution of the catalyst especially in an acidic environment and enable long-term operation. The addition amount of CeO₂ is 1 - 5 wt% based on CoOx, preventing the decrease in activity due to excessive addition while maximizing the stability improvement effect. CeO₂ forms a Co³⁺ / Co⁴⁺ redox pair and plays a role in suppressing the dissolution of CoOx in an acidic environment. Specifically, CeO₂ is uniformly dispersed on the surface of CoOx and functions as a protective layer, reducing direct contact with the acidic electrolyte and suppressing dissolution.

[0021] Furthermore, according to still another aspect of the present invention, by optimizing the electrode structure and adjusting the thickness and porosity of the catalyst layer, the mass transport characteristics are improved and operation at a high current density is enabled. The thickness of the catalyst layer is 0.1 - 0.2 mg / cm² to optimize the mass transport characteristics. In this thickness range, the entire catalyst layer can be sufficiently in contact with the electrolyte, minimizing the limitation of ion transport while ensuring electron conductivity. Also, to control the porosity of the catalyst layer, polystyrene microspheres (diameter 100 - 500 nm) are used as a porogen. This improves the permeability of the electrolyte and promotes the discharge of bubbles.

[0022] According to still another aspect of the present invention, the thrust characteristics are adjusted by optimizing the power control system and precisely controlling the electrode potential and current. The power control system consists of a high-efficiency DC-DC converter (conversion efficiency of 95% or more), a high-precision microcontroller (potential control accuracy of ±1 mV, current control accuracy of ±0.1%), a potential / current sensor (potential measurement accuracy of ±0.5 mV, current measurement accuracy of ±0.05%), an electrode switching circuit (switching time of 10 ms or less), and a safety monitoring circuit (cut-off time of 1 ms or less when an abnormality is detected). This enables precise thrust control using the quasi-linear relationship between the logarithm of the oxygen generation current (log(i)) and the change in cobalt oxidation state (ΔEedge).

Advantages of the Invention

[0023] The present invention provides the following advantages: 1. A variable thrust characteristic is realized by switching between a high-efficiency propulsion mode that utilizes a low overvoltage (about 376 mV) in an alkaline environment and a precise control mode in a neutral or acidic environment. This enables the provision of optimal propulsion characteristics for different mission phases, such as orbit change and orbit maintenance. Specifically, in an alkaline environment, an oxygen generation rate of 20 A / g or more per catalyst is achieved, enabling high-efficiency propulsion with a thrust density of 0.5 - 1.0 N / kW. On the other hand, in a neutral environment, a stable thrust with a thrust fluctuation of less than 1% and in an acidic environment, precise control allowing fine adjustment in units of 0.5% are realized. This allows a single system to meet multiple propulsion requirements, contributing to the mass reduction and system simplification of spacecraft. 2. High-precision thrust control is possible by utilizing the direct correlation between the change in cobalt oxidation state and the oxygen generation reaction. In particular, by utilizing the quasi-linear relationship between the logarithm of the oxygen generation current and the change in cobalt oxidation state (ΔEedge), precise thrust adjustment is realized. This relationship is represented by the empirical formula ΔEedge = α·log(i) + β, and the value of α varies depending on the pH environment (about 25 - 30 meV / decade in an alkaline environment, about 15 - 20 meV / decade in a neutral environment, and about 25 - 30 meV / decade in an acidic environment). By utilizing this characteristic, it is possible to set the current value corresponding to the desired thrust and apply the corresponding potential, enabling precise thrust control. Especially in a neutral environment, due to the small value of α, fine adjustment of the current (thrust) becomes easy. 3. Longevity is achieved by improving the catalyst stability in an alkaline environment. The dissolution of the cobalt catalyst strongly depends on the pH environment, and in an alkaline environment, dissolution is minimized, thus improving the durability of the catalyst in long-term missions. Specifically, the stability number of the catalyst (the ratio of the number of generated oxygen molecules to the number of dissolved cobalt atoms) in an alkaline environment is 10^4 or more, enabling continuous operation for thousands of hours. Also, by adding a small amount of CeO_2, the stability in an acidic environment is also improved, and the stability number increases from 10^2 to 10^3. This extends the operation time in an acidic environment, enabling a more flexible mission plan. 4. The use of water-based propellants opens up the possibility of improving safety and utilizing local resources. Propellants mainly composed of water are non-toxic and non-flammable, ensuring high safety during handling. Additionally, it enables the utilization of local resources on celestial bodies where water ice exists, such as the Moon and Mars. Specifically, the water ice present in the permanently shadowed regions of the lunar South Pole can be extracted and refined for use as a propellant. This allows for sustainable space exploration independent of Earth resupply and significantly extends the mission duration. Moreover, hydrogen generated by the electrolysis of water can also be used as a propellant, and it is possible to construct a dual propulsion system combining oxygen and hydrogen. 5. While achieving a high specific impulse (about 800 - 1200 seconds) compared to conventional chemical propellants, it is possible to generate a larger thrust density (about 0.1 - 1 N / kW) than an electric propulsion system. This realizes a system with intermediate performance that fills the gap between the thrust density of chemical propellants and the specific impulse of electric propulsion systems. Specifically, in the high-efficiency propulsion mode in an alkaline environment, a specific impulse of 800 - 1000 seconds and a thrust density of 0.5 - 1.0 N / kW are achieved; in the intermediate thrust mode in a neutral environment, a specific impulse of 900 - 1100 seconds and a thrust density of 0.3 - 0.5 N / kW are realized; and in the precision control mode in an acidic environment, a specific impulse of 1000 - 1200 seconds and a thrust density of 0.1 - 0.3 N / kW are achieved. This enables the construction of a flexible propulsion system capable of meeting various mission requirements.

Embodiments for Carrying Out the Invention

[0024] Hereinafter, embodiments of the present invention will be described in detail. It should be noted that the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the technical idea of the present invention. The electrochemical propulsion system of the present invention consists of the following main components: The catalyst electrode has a structure with nano-sized CoOx particles (20 - 50 nm) as the main component and optimized surface reconstruction characteristics. By controlling the size of the CoOx particles to 20 - 50 nm, it is possible to maximize the surface area while suppressing the change in the electronic state due to the quantum size effect and maintaining an electronic structure close to that of bulk CoOx. When the particle size is less than 20 nm, the electronic state may change due to the quantum size effect, and the redox potentials of Co^2+ / Co^3+ and Co^3+ / Co^4+ may shift. On the other hand, when the particle size exceeds 50 nm, the specific surface area decreases and the catalytic activity decreases. The synthesis of CoOx particles is carried out by the coprecipitation method. Specifically, an aqueous solution of sodium carbonate (Na_2CO_3) is dropped into an aqueous solution of cobalt nitrate (Co(NO_3)_2·6H_2O) to obtain a cobalt carbonate precursor. After filtering and washing this precursor, nano-sized CoOx particles are obtained by calcining in air at 300 °C for 3 hours. The crystal structure of the obtained particles is evaluated by X-ray diffraction (XRD) measurement, the particle size and morphology are observed by transmission electron microscopy (TEM), and the specific surface area is evaluated by nitrogen adsorption measurement. Typically, the crystal structure has spinel-type Co_3O_4 as the main phase, and the specific surface area is 50 - 80 m^2 / g. The thickness of the catalyst layer is set to 0.1 - 0.2 mg / cm^2 to optimize the mass transport characteristics. In this thickness range, the entire catalyst layer can be sufficiently in contact with the electrolyte, minimizing the limitation of ion transport while ensuring electron conductivity. If the catalyst layer is too thick, the internal catalyst cannot be sufficiently in contact with the electrolyte, and if it is too thin, the number of active sites is insufficient. Therefore, setting this optimal range is important. Specifically, when the thickness of the catalyst layer is less than 0.1 mg / cm^2, the number of active sites is insufficient, and the performance at high current density is limited. On the other hand, when it exceeds 0.2 mg / cm^2, ions cannot sufficiently reach the internal catalyst, and the catalyst utilization efficiency decreases. The formation of the catalyst layer is carried out by the spray coating method. Specifically, CoOx particles are dispersed in a mixed solvent (volume ratio 8:2:0.1) of water, isopropanol, and Nafion dispersion (5 wt%), and a uniform dispersion is obtained by ultrasonic treatment. This dispersion is sprayed onto the substrate using a spray gun with nitrogen gas and dried at 80 °C to form a uniform catalyst layer. The thickness of the catalyst layer is controlled by the number of spray passes and the dispersion concentration. The surface morphology of the formed catalyst layer is evaluated by scanning electron microscope (SEM) observation, and the thickness is evaluated by cross-sectional SEM observation. As the conductive substrate, gold-coated carbon fiber is used to minimize the electrical resistance. Carbon fiber is lightweight and high-strength, making it suitable for space applications. The gold coating improves the electrochemical stability of the carbon fiber and particularly prevents corrosion in acidic environments. The thickness of the gold coating is set to 50 - 100 nm to ensure sufficient conductivity and chemical stability while minimizing weight increase. As the carbon fiber substrate, high-strength PAN-based carbon fibers such as T300 and T700 manufactured by Toray Industries, Inc. are used. These fibers have properties of tensile strength 3530 - 4900 MPa, tensile modulus 230 - 290 GPa, and density 1.76 - 1.80 g / cm^3, making them suitable for use in the space environment. The carbon fiber is used in the form of a plain weave or twill weave cloth with a thickness of 0.1 - 0.3 mm. The gold coating is carried out by the sputtering method or electroless plating method. In the sputtering method, film formation is performed using a gold target in argon plasma to form a uniform and dense gold layer. In the electroless plating method, film formation is performed using a chloroauric acid (HAuCl_4) solution and a reducing agent (such as sodium hypophosphite), enabling uniform coating on complex shapes. The thickness of the gold coating is controlled by the film formation time and conditions and evaluated by X-ray reflectivity measurement or cross-sectional SEM observation. The CoOx catalyst is surface-modified so that the redox transitions of Co^2+ / Co^3+ and Co^3+ / Co^4+ proceed efficiently. Specifically, the surface oxygen defect concentration is optimized to enhance the interaction with hydroxide ions (OH^-). Oxygen defects function as adsorption sites for hydroxide ions and contribute to the stabilization of intermediates in the OER process. The introduction of surface oxygen defects is carried out by heat treatment in a reducing atmosphere. Specifically, the synthesized CoOx particles are heat-treated in a 5% H_2 / Ar mixed gas at 200 - 250 °C for 1 - 2 hours to introduce appropriate oxygen defects on the surface. If the treatment temperature is too low, sufficient defects will not be introduced, and if it is too high, CoOx may be overly reduced to form Co metal, so temperature control is important. The introduced oxygen defects are evaluated from the intensity of the oxygen defect component (about 531.5 eV) in the O 1s spectrum of X-ray photoelectron spectroscopy (XPS). The optimal defect concentration is adjusted so that the ratio of the oxygen defect component in the O 1s spectrum is 15 - 25%. Also, to improve the stability under acidic conditions, a small amount of cerium oxide (CeO_2) is added. The addition amount of CeO_2 is 1 - 5 wt% based on CoOx, preventing the decrease in activity due to excessive addition while maximizing the stability improvement effect. CeO_2 forms a Ce^3+ / Ce^4+ redox pair and plays a role in suppressing the dissolution of CoOx in an acidic environment. The addition of CeO_2 is carried out by the coprecipitation method or the impregnation method. In the coprecipitation method, an aqueous sodium carbonate solution is dropped into a mixed aqueous solution of cobalt nitrate and cerium nitrate (Ce(NO_3)_3·6H_2O) to obtain a Co-Ce composite carbonate precursor. By firing this precursor, CoOx particles with uniformly dispersed CeO_2 are obtained. In the impregnation method, the synthesized CoOx particles are dispersed in an aqueous cerium nitrate solution and dried and fired to support CeO_2 on the surface of CoOx. The added CeO_2 is evaluated by crystal phase identification by XRD, elemental mapping by energy-dispersive X-ray analysis (EDX), and microstructure analysis by high-resolution TEM observation. Furthermore, to control the porosity of the catalyst layer, polystyrene microspheres (diameter 100 - 500 nm) are used as a porogen (pore former). By adding the porogen during the preparation of the catalyst slurry and removing it by heat treatment after electrode formation, a controlled pore structure is formed. This porous structure improves the permeability of the electrolyte and promotes the discharge of gas bubbles. Polystyrene microspheres are synthesized by the emulsion polymerization method. Specifically, styrene monomer, an initiator (such as potassium persulfate), and a surfactant (such as sodium dodecyl sulfate) are reacted in water to obtain polystyrene particles of uniform size. The particle size is controlled by the monomer concentration, surfactant concentration, and reaction temperature. The synthesized polystyrene particles are evaluated for size distribution by the dynamic light scattering method and the morphology is confirmed by scanning electron microscope (SEM) observation. During the preparation of the catalyst slurry, CoOx particles and polystyrene microspheres are mixed at a weight ratio of 3:1 - 5:1 to obtain a uniform dispersion. This dispersion is coated on a substrate by spray coating, and after drying, it is heat-treated at 300 - 350 °C for 2 - 3 hours to remove polystyrene and form a porous catalyst layer. The formed porous structure is evaluated by SEM observation, and the pore size distribution and porosity are measured. The optimal porous structure is adjusted to have a pore size of 100 - 500 nm and a porosity of 30 - 50%. The electrolyte system consists of multiple compartments, each maintaining a different pH environment: Main compartment (alkaline compartment): Contains 0.1 M KOH solution (pH about 13) and is used in the high-efficiency propulsion mode. KOH is known to have high conductivity and high OER activity towards the CoOx catalyst. By setting the concentration to 0.1 M, sufficient ionic conductivity is ensured while suppressing excessive corrosiveness. KOH has a higher solubility compared to NaOH (121 g / 100 g water vs. 109 g / 100 g water at 25 °C), and crystallization at low temperatures is suppressed, making it suitable for use in the space environment. Also, the ionic conductivity of KOH is about 0.02 S / cm at a concentration of 0.1 M, ensuring sufficient electrochemical performance. The KOH solution is prepared by dissolving high-purity KOH pellets (purity ≥ 99.99%) in deionized water (electrical resistivity > 18 MΩ·cm). The solution is filtered through a 0.2-μm membrane filter before use to remove insoluble impurities. The prepared KOH solution is stored in a sealed container to minimize contact with CO₂ in the atmosphere. The pH of the KOH solution is measured with a calibrated pH meter and confirmed to be within the range of 13.0 ± 0.1. Sub-compartment 1 (neutral compartment): contains 0.1 M phosphate buffer (pH ≈ 7) and is used in an intermediate thrust control mode. The phosphate buffer is a mixture of 0.054 M K₂HPO₄ and 0.064 M KH₂PO₄, and has a stable buffering capacity at pH 7.0 ± 0.2. Phosphate ions specifically adsorb on the CoOx surface and play a role in adjusting the OER activity by affecting the electronic state of the catalyst. The phosphate buffer is prepared by dissolving high-purity K₂HPO₄ (purity ≥ 99.0%) and KH₂PO₄ (purity ≥ 99.0%) in deionized water. First, 0.1 M K₂HPO₄ solution and 0.1 M KH₂PO₄ solution are prepared separately, and a buffer solution with pH 7.0 is obtained by mixing them in an appropriate ratio (54:64). The prepared buffer solution is measured with a calibrated pH meter, and 0.1 M KOH or 0.1 M H₃PO₄ is added dropwise as needed to adjust the pH to 7.0 ± 0.2. The ionic conductivity of the phosphate buffer is approximately 0.01 S / cm at a concentration of 0.1 M, which is sufficient for electrochemical measurements. The phosphate buffer has excellent pH stability in a neutral environment and has the effect of suppressing local pH changes due to electrolysis. In addition, phosphate ions are known to specifically adsorb on the CoOx surface and affect the electronic state of the catalyst. As a result, the OER activity is moderately suppressed, enabling more precise control. Sub-compartment 2 (acidic compartment): It contains 0.05 M H₂SO₄ solution (pH about 1) and is used in the precise control mode. By setting the sulfuric acid concentration to 0.05 M, a sufficient acidic environment is maintained while suppressing the dissolution of the catalyst. Sulfate ions adsorb on the catalyst surface as divalent anions and affect the surface charge distribution. The H₂SO₄ solution is prepared by diluting high-purity sulfuric acid (purity 95 - 98%, semiconductor grade) with deionized water. Concentrated sulfuric acid is added to water little by little and diluted while stirring well. The prepared solution is measured with a calibrated pH meter and confirmed to be within the range of pH 1.0 ± 0.1. The ionic conductivity of the H₂SO₄ solution is about 0.02 S / cm at a concentration of 0.05 M, ensuring sufficient electrochemical performance. Sulfuric acid has lower oxidizing properties compared to other acids (such as hydrochloric acid and nitric acid), and the corrosion of metals is relatively suppressed, so it has little impact on the electrode material. In addition, sulfate ions (SO₄²⁻) specifically adsorb on the catalyst surface as divalent anions and play a role in adjusting the OER activity by affecting the surface charge distribution.

[0025] Each compartment is separated by an ion exchange membrane and has a structure that can maintain an independent pH environment while being electrochemically continuous. As the ion exchange membrane, a highly durable cation exchange membrane such as Nafion 117 (thickness 183 μm) or Suez Ionics CR61 - CZL (thickness 150 μm) is used. These membranes have high selective permeability for protons (H⁺) and can control the ion transport between different pH environments. Nafion 117 is a perfluorosulfonic acid-based cation exchange membrane, which is excellent in chemical stability and mechanical strength. Its proton conductivity is about 0.1 S / cm in the wet state and is widely used as a separation membrane for electrochemical cells. Suez Ionics CR61 - CZL is a hydrocarbon-based cation exchange membrane, which is lower in cost compared to Nafion but slightly inferior in chemical stability. Its proton conductivity is about 0.05 S / cm in the wet state. The pretreatment of the ion exchange membrane is carried out according to the following procedure. First, cut the membrane into an appropriate size (larger than the electrode area and with a size that does not cause leakage during cell assembly). Next, treat it in 3% hydrogen peroxide water at 80°C for 1 hour to remove organic impurities. Subsequently, treat it in 1M sulfuric acid at 80°C for 1 hour to convert the membrane into the H^+ form. Finally, wash it thoroughly with deionized water and store it in deionized water until use. The distance between the membrane and the electrode is set to 0.5 - 2.0 mm to optimize the balance between ion transport resistance and bubble discharge. If the distance is too close, the discharge of bubbles will be hindered; if it is too far, the ion transport resistance will increase. Therefore, setting within this range is important. Specifically, if the distance is less than 0.5 mm, the generated oxygen bubbles cannot efficiently detach from the electrode surface, and the active sites may be blocked. On the other hand, if it exceeds 2.0 mm, the electrolyte resistance increases and the ohmic loss increases. The electrolyte capacity of each compartment is set to 50 - 100 mL to minimize the pH change during long-term operation. Also, an independent electrolyte circulation system is provided for each compartment to control the temperature of the electrolyte and remove bubbles. For the circulation pump, a magnetic coupling type centrifugal pump suitable for use in the space environment is used, and the flow rate can be adjusted in the range of 10 - 50 mL / min. An air-liquid separator is installed in the electrolyte circulation system to efficiently separate and recover the generated oxygen gas. The air-liquid separator adopts a porous membrane method utilizing surface tension so that it can function even in a microgravity environment. The separated oxygen gas is led to the thrust generation nozzle and used as a propellant. The temperature control of the electrolyte is carried out using a thermoelectric element (Peltier element). The electrolyte temperature of each compartment can be controlled in the range of 15 - 40°C to maintain optimal electrochemical performance. A temperature sensor (resistance thermometer or thermistor) is installed in each compartment to monitor and control the temperature with an accuracy of ±0.5°C. The main propellant has the following composition: Water (90 - 95 wt%): Functions as the main electrolyte solvent and reactant. High-purity deionized water (electrical resistivity > 18 MΩ·cm) is used to prevent catalyst poisoning and side reactions caused by impurities. Water is readily available on Earth and may also exist as water ice on celestial bodies such as the Moon and Mars. Therefore, it is an excellent propellant from the perspective of in-situ resource utilization. The oxygen and hydrogen generated by the electrolysis of water have a high energy density and become efficient propellants. The purification of water is carried out by a multi-stage treatment combining a reverse osmosis membrane, ion exchange resin, and activated carbon filter. First, the raw water is filtered through a 0.2 μm membrane filter to remove suspended substances. Next, dissolved salts are removed using a reverse osmosis membrane, and then residual ions are removed by passing through an ion exchange resin (mixed bed type). Finally, organic impurities are adsorbed and removed using an activated carbon filter. The quality of the purified water is confirmed with an electrical resistivity meter, and it is ensured that it is 18 MΩ·cm or higher. Potassium hydroxide electrolyte (3 - 5 wt%): Functions as the electrolyte in the alkaline compartment and provides ionic conductivity. KOH has a higher solubility compared to NaOH, and crystallization at low temperatures is suppressed, making it suitable for use in the space environment. The KOH concentration is set at 3 - 5 wt% (about 0.5 - 0.9 M) to ensure sufficient ionic conductivity while suppressing excessive corrosivity. In this concentration range, the ionic conductivity is about 0.2 S / cm, and the electrochemical performance can be maximized. The purity of KOH is 99.99% or higher, and it is particularly confirmed that metal impurities such as Na^+, Ca^2+, and Fe^3+ are 10 ppm or less. Since these impurities may adsorb on the catalyst surface and reduce its activity, it is important to use high-purity KOH. Because KOH is highly hygroscopic, it is stored in a sealed container and quickly weighed during use to prepare the solution. Phosphate buffer component (1 - 2 wt%): It provides the buffering capacity of the neutral compartment and ensures pH stability. A mixture of K₂HPO₄ and KH₂PO₄ is used, which exhibits a stable buffering action in the range of pH 7.0 ± 0.2. The concentration of the phosphate buffer component is 1 - 2 wt% (about 0.06 - 0.12 M), which ensures sufficient buffering capacity while preventing excessive increase in ionic strength. In this concentration range, it has a buffering capacity of about 0.02 mol / pH·L in the range of pH 6.8 - 7.2 and can effectively suppress local pH changes due to electrolysis. Phosphates have advantages such as being inorganic salts with high chemical stability, low cost, and no toxicity compared to other buffer systems (TRIS, HEPES, MES, etc.). Also, phosphate ions specifically adsorb on the CoOx surface and play a role in moderately adjusting the OER activity by affecting the electronic state of the catalyst. Catalyst stabilizer (1 - 2 wt%): It functions as an additive to suppress the dissolution and degradation of the catalyst. Specifically, polymer stabilizers such as polyvinylpyrrolidone (PVP, molecular weight 40,000 - 360,000) and polyethylene glycol (PEG, molecular weight 1,000 - 10,000) are used. These polymers adsorb on the catalyst surface and form a protective layer to suppress the dissolution of the catalyst, especially in an acidic environment. PVP is a polymer of N - vinylpyrrolidone and is an amphiphilic polymer with a polar group (C = O group of the pyrrolidone ring) and a non - polar group (hydrocarbon chain of the main chain). Due to this property, it can strongly adsorb on the metal oxide surface and form a protective layer. The molecular weight of PVP is selected in the range of 40,000 - 360,000. The higher the molecular weight, the higher the protective effect, but it is optimized considering the balance between solubility and viscosity. The PVP concentration is 1 - 2 wt% to ensure a sufficient protective effect while preventing excessive increase in the solution viscosity. PEG is a polymer of ethylene glycol and is a linear polymer with hydroxyl groups at its ends. PEG has high water solubility and adsorbs on the surface of metal oxides to form a protective layer. The molecular weight of PEG is selected in the range of 1,000 - 10,000. The higher the molecular weight, the higher the protective effect, but it is optimized considering the balance of solubility. The PEG concentration is also set at 1 - 2 wt%, minimizing changes in solution properties while ensuring a sufficient protective effect. These polymer stabilizers physically adsorb on the catalyst surface and suppress the dissolution of the catalyst by reducing direct contact with the electrolyte. Also, the polymer layer has an appropriate thickness and porosity and exerts a protective effect without hindering the diffusion of reactants (such as water molecules, OH^2 ions, etc.). The adsorption strength of the polymer depends on pH. Adsorption is strong in an acidic environment and relatively weak in an alkaline environment, so the influence on the catalytic activity in each pH environment is different. With this composition, stable electrochemical reactions in different pH environments are realized. Also, for preventing the freezing of the propellant, ethylene glycol or propylene glycol (0 - 5 wt%) can be added as needed. These antifreeze agents prevent the freezing of the propellant in a low-temperature environment (-20°C or lower) and expand the operating temperature range of the system. Ethylene glycol (HOCH_2CH_2OH) is a dihydric alcohol with a melting point of -13°C and a boiling point of 197°C, and is miscible with water in any proportion. By adding 5 wt% of ethylene glycol, the freezing point of the aqueous solution can be lowered to about -2°C. Ethylene glycol not only forms hydrogen bonds to lower the freezing point of water but also has the effect of stabilizing the supercooled state due to an increase in viscosity. Also, ethylene glycol is relatively stable electrochemically and has little influence on catalytic activity. Propylene glycol (CH₃CHOHCH₂OH) is a dihydric alcohol with a melting point of -59°C and a boiling point of 188°C, and it is miscible with water in any proportion. By adding 5 wt% of propylene glycol, the freezing point of the aqueous solution can be lowered to about -2°C. Propylene glycol has the advantages of lower toxicity and higher biocompatibility compared to ethylene glycol. Also, propylene glycol is relatively stable electrochemically and has little impact on catalytic activity. Antifreeze agents are important for coping with temperature fluctuations in the space environment (especially the low temperature in shaded areas). In low Earth orbit, the temperature can drop to below -100°C during shaded periods, so it is necessary to install appropriate insulation materials and heating systems in the propellant tanks and pipes to prevent the freezing of the propellant. The addition of antifreeze agents functions as an auxiliary to these thermal control systems and improves the reliability of the systems. The power control system consists of the following components: High-efficiency DC-DC converter: Converts the power from the solar cell array or nuclear battery into appropriate voltage and current. The conversion efficiency is 95% or more to minimize power loss. The input voltage range is 28 - 100V, and the output voltage range is 1.0 - 2.5V to be compatible with various operating conditions. The DC-DC converter uses radiation-resistant components suitable for space applications and implements a protection circuit against single event upset (SEU) and single event latch-up (SEL). Specifically, gallium nitride (GaN) or silicon carbide (SiC) power devices are adopted to achieve high efficiency and high-frequency operation. The switching frequency is 100 - 500 kHz to maximize the conversion efficiency while minimizing the ripple current and voltage. The control method of the converter is based on a step-down (buck) converter in continuous conduction mode (CCM). To cope with a wide input voltage range and load fluctuations, an adaptive control algorithm is implemented. Specifically, the switching frequency and duty cycle are optimized according to the load conditions, and the converter is controlled to always operate at the highest efficiency point. In addition, protection functions against overvoltage, overcurrent, and overheating are implemented, and the output is automatically cut off when an abnormal state is detected. To ensure power supply redundancy, multiple DC-DC converters are connected in parallel so that operation can continue even if some of them fail. Each converter is independently controlled, and a current sharing function is implemented to equalize the load sharing. Microcontroller: Precisely controls the electrode potential and current, and adjusts the thrust characteristics. It is equipped with an AD / DA converter with a resolution of 16 bits or more, with a potential control accuracy of ±1 mV and a current control accuracy of ±0.1%.

[0026] For the microcontroller, a radiation-resistant processor suitable for space applications is used. Specifically, a radiation-hardened processor such as BAE Systems RAD750 or Vorago Technologies VA10820, or an FPGA-based system implementing triple modular redundancy (TMR) is adopted. The operating frequency is set to 100 - 200 MHz to ensure sufficient processing power while suppressing power consumption. The control algorithm adopts a hybrid control method that combines feedback control and feedforward control. In feedback control, PID (proportional-integral-derivative) control is used as the basis, and the parameters are optimized according to the response characteristics of the system. In feedforward control, based on the system model, the required potential and current values are predicted from the thrust command value to improve the responsiveness. In addition, an adaptive control algorithm is implemented to automatically adjust the control parameters in response to catalyst degradation and changes in operating conditions. The software adopts a safety design compliant with MISRA-C and conducts code verification using static analysis tools. Additionally, it implements a watchdog timer and a memory protection function, and has the function of detecting and recovering abnormal software operations. Potential-current sensor: Continuously monitors the electrode potential and current, enabling feedback control. The potential measurement accuracy is ±0.5 mV, and the current measurement accuracy is ±0.05%. The potential sensor uses a differential amplifier with a high input impedance (>10^12 Ω) to measure the potential of the working electrode with respect to the reference electrode. For the reference electrode, a silver / silver chloride (Ag / AgCl) electrode with excellent long-term stability is used, and a periodic calibration function is implemented. In the measurement circuit, a low-noise amplifier and a 16-24-bit high-resolution AD converter are used to measure the potential with a resolution of less than 0.1 mV. Additionally, digital filtering (such as moving average, Kalman filter, etc.) is applied to reduce the measurement noise. The current sensor adopts a shunt resistance method or a Hall effect method. In the shunt resistance method, a high-precision (±0.01%) low temperature coefficient (<5 ppm / ℃) resistor is used, and the current is calculated from the voltage drop across both ends. In the Hall effect method, a closed-loop Hall sensor is used to achieve a wide dynamic range (1 mA - 10 A) and high linearity. In the measurement circuit, a low-drift amplifier and a 16-24-bit high-resolution AD converter are used to measure the current value with high precision. To ensure the redundancy of the sensors, multiple potential-current sensors are installed in each section, and the design is such that operation can continue even in case of sensor failure. Additionally, a function to mutually check the validity of the sensor values is implemented, and a warning is issued when an abnormal value is detected. Electrode switching circuit: Selectively activates the electrodes in different pH sections and switches the operation mode. The switching time is set to 10 ms or less to achieve rapid mode switching. The electrode switching circuit is configured using MOSFET switches with low on-resistance (<10 mΩ). Each electrode is controlled by an independent switch, and it is also possible to activate multiple electrodes simultaneously as needed. To reduce switching noise, a soft start function is implemented to prevent sudden changes in current. Specifically, the power supply to the electrodes is linearly increased or decreased over 100 ms to prevent sudden changes in thrust. For the switching element, a power MOSFET with high radiation resistance is used, and the gate driver circuit is implemented with overvoltage and overcurrent protection functions. Also, a function to monitor the switching state is provided, and a warning is issued when a switch failure (open, short, etc.) is detected. To ensure the redundancy of the switching circuit, multiple switching paths are provided for each electrode, and the design is such that operation can continue even if some circuits fail. Safety monitoring circuit: Detects abnormal conditions such as overvoltage, overcurrent, and temperature anomalies, and protects the system. The cut-off time during abnormal detection is set to 1 ms or less to ensure the safety of the system. The safety monitoring circuit continuously monitors the following parameters: - Electrode potential: Warning and cut-off when exceeding the set upper limit value (2.1 V vs. RHE) - Current density: Warning and cut-off when exceeding the set upper limit value (60 A / g catalyst) - Electrolyte temperature: Warning and control when outside the set range (10 - 50 °C) - Electrolyte pressure: Warning and pressure reduction when exceeding the set upper limit value (0.3 MPa) - Electrolyte leakage: Warning and cut-off when detected - Insulation resistance: Warning and cut-off when falling below the set lower limit value (1 MΩ) For each monitored parameter, two-stage thresholds of warning level and cut-off level are set. At the warning level, only the operator is notified, and at the cut-off level, the system is automatically shifted to a safe state. In the safe state, the power supply to all electrodes is cut off, and only the electrolyte circulation pump is operated to discharge the residual gas. The monitoring circuit is composed of dedicated hardware independent of the microcontroller and is designed to function even when the microcontroller malfunctions. In addition, a self-diagnosis function for detecting failures in the monitoring circuit itself is implemented, and operation checks are performed regularly. To ensure the redundancy of the safety monitoring system, for critical parameters (such as electrode potential, current density, etc.), multiple independent sensors and monitoring circuits are installed, and abnormal determination is made by a majority vote method. This prevents malfunction due to a single failure of the sensor or monitoring circuit. This system operates by utilizing the correlation between the change in the oxidation state of the cobalt oxide catalyst and the oxygen evolution reaction. The oxidation state of the cobalt oxide catalyst changes according to the applied potential. This change in the oxidation state is observed as the energy shift (ΔEedge) of the Co K absorption edge by X-ray absorption spectroscopy (XAS). An increase in ΔEedge indicates an increase in the average oxidation number of cobalt and is directly related to the OER activity. The energy position (Eedge) of the Co K absorption edge is defined by the following integral formula: Eedge = (1 / (μ_2 - μ_1))∫[μ_1→μ_2] E(μ)dμ Here, E(μ) is the energy value at the normalized absorption intensity μ, and μ_1 = 0.2 and μ_2 = 1.0 are the lower and upper limits of the normalized absorption intensity. The change in Eedge (ΔEedge) is calculated as the difference in Eedge between the initial state and an arbitrary state. There is the following experimental relationship between ΔEedge and the change in the average oxidation number of cobalt (ΔOSCo): ΔOSCo ≒ ΔEedge / 2.5 [eV] That is, when ΔEedge increases by 1 eV, the average oxidation number of cobalt increases by approximately 0.4. By using this relationship, the change in the oxidation state of cobalt can be estimated from ΔEedge. The Co^2+ / Co^3+ redox process is closely related to the flat band potential and is greatly affected by the pH environment. The flat band potential (Vfb) indicates the potential at which the formation of the space charge layer begins in a CoOx catalyst with semiconductor properties. Vfb is approximately 0.9 V vs. RHE in an alkaline environment (pH 13), approximately 1.2 V vs. RHE in a neutral environment (pH 7), and approximately 1.3 V vs. RHE in an acidic environment (pH 1). The flat band potential can be determined by Mott-Schottky analysis. The relationship between the space charge capacitance (Csc) of the semiconductor / electrolyte interface and the electrode potential (V) is expressed by the following Mott-Schottky equation: 1 / Csc^2 = (2 / εε_0eND)(V - Vfb - kT / e) Here, ε is the dielectric constant of the semiconductor, ε_0 is the dielectric constant of vacuum, e is the charge of an electron, ND is the donor (or acceptor) density of the semiconductor, k is the Boltzmann constant, and T is the absolute temperature. Vfb can be obtained by extrapolating the linear part of the plot of 1 / Csc^2 vs. V. In an alkaline environment, since this redox process proceeds at a low potential, surface reconstruction is promoted. Specifically, the CoOx surface is converted to CoOOH, and OER active sites are formed. This surface reconstruction proceeds in the potential range of approximately 1.0 - 1.2 V vs. RHE in an alkaline environment, approximately 1.3 - 1.5 V vs. RHE in a neutral environment, and approximately 1.4 - 1.6 V vs. RHE in an acidic environment. The surface reconstruction process can be represented by the following chemical reaction equations: Co_3O_4 + H_2O + OH^- → 3CoOOH + e^- (alkaline environment) Co_3O_4 + 4H_2O → 3CoOOH + H_3O^+ + e^- (neutral / acidic environment) This reaction converts the surface of spinel-structured Co_3O_4 into layered-structured CoOOH. CoOOH is an important precursor in the OER catalytic cycle and serves as the active site for the Co^3+ / Co^4+ redox process. The Co^3+ / Co^4 redox process is directly related to the initiation of the oxygen evolution reaction. Co^4+ species are important intermediates in the OER catalytic cycle and play a role in promoting O-O bond formation. This redox potential shifts in the positive direction with the decrease of pH, reaching about 1.47 V vs. RHE in an alkaline environment, about 1.57 V vs. RHE in a neutral environment, and about 1.62 V vs. RHE in an acidic environment. The Co^3+ / Co^4 redox process can be represented by the following chemical reaction equations: CoOOH + OH^- → CoO_2 + H_2O + e^- (alkaline environment) CoOOH + H_2O → CoO_2 + H_3O^+ + e^- (neutral / acidic environment) Here, CoO_2 is an oxide containing Co^4+ and is actually considered to exist as a hydrate such as CoO_2·nH_2O. After the formation of Co^4+ species, the OER process proceeds. The OER mechanism is complex and multiple pathways have been proposed. Generally, it includes the following elementary reaction steps: 1. Co^4+=O + OH^- → Co^3+-OOH + e^- (O-O bond formation) 2. Co^3+-OOH + OH^- → Co^3+-OO^- + H_2O (deprotonation) 3. Co^3+-OO^- → Co^3+ + O_2 + e^- (oxygen release) 4. Co^3+ + OH^- → Co^3+-OH (catalyst regeneration) 5. Co^3+-OH + OH^- → Co^4+=O + H_2O + e^- (catalyst oxidation) Among these elementary reactions, the O-O bond formation (Step 1) is considered to be the rate-determining step. The O atom of the Co^4+=O species reacts with the hydroxide ion (OH^-) to form a peroxide intermediate (Co^3+-OOH), which generates an oxygen molecule (O_2) through deprotonation and oxygen release. Due to the differences in these redox potentials, the catalytic activity in different pH environments is determined. In an alkaline environment, the Co^2+ / Co^3+ redox proceeds at a low potential, promoting surface reconstruction, so a high density of OER active sites is formed. Also, since the Co^3+ / Co^4 redox also proceeds at a relatively low potential, the OER onset potential becomes low. On the other hand, in an acidic environment, since the Co^2+ / Co^3+ redox and the Co^3+ / Co^4 redox shift to a high potential, the OER onset potential becomes high. However, the degree of the redox potential shift is different between Co^2+ / Co^3 and Co^3+ / Co^4, and since the potential difference between the two is reduced, more precise potential control becomes possible. In a neutral environment, the polarizability of cobalt atoms is the lowest, and the change in the cobalt oxidation state with respect to the change in the applied potential is the slowest. As a result, the response to the change in the OER current becomes slow, enabling very precise thrust control. The polarizability of cobalt atoms can be evaluated as the potential dependence of ΔEedge (dΔEedge / dV), which is about 300 - 400 meV / V in an alkaline environment, about 150 - 200 meV / V in a neutral environment, and about 250 - 350 meV / V in an acidic environment.

[0027] By utilizing these characteristics, it is possible to perform optimal potential control according to the operation in different pH environments and realize the desired thrust characteristics. The thrust is controlled by the potential and current applied to the electrode. There is a quasi-linear relationship between the logarithm of the oxygen evolution current (log(i)) and the change in the cobalt oxidation state (ΔEedge), and by utilizing this, precise thrust control becomes possible. This relationship is expressed by the following empirical formula: ΔEedge = α·log(i) + β Here, α is a parameter indicating the sensitivity of the cobalt oxidation state change to the current change, and β is a constant depending on the initial state. The value of α is about 25 - 30 meV / decade in an alkaline environment, about 15 - 20 meV / decade in a neutral environment, and about 25 - 30 meV / decade in an acidic environment. The physical meaning of this relationship indicates that the OER current is directly related to the oxidation state of the cobalt catalyst (especially the concentration of Co^4+ species). The OER current can be expressed by the following Butler - Volmer type equation: i = i_0·exp(αF(V - V°) / RT) Here, i_0 is the exchange current density, α is the transfer coefficient, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and V - V° is the overpotential. On the other hand, the change in the cobalt oxidation state (ΔEedge) depends on the potential according to the Nernst equation: ΔEedge ∝ ln([Co^4+] / [Co^3+]) [Co^4+] / [Co^3+] = exp(F(V - V°) / RT) From these relationships, a quasi - linear relationship is derived between ΔEedge and log(i). By utilizing this relationship, precise thrust control becomes possible by setting the current value corresponding to the desired thrust and applying the corresponding potential. In particular, in a neutral environment, since the value of α is small, fine adjustment of the current (thrust) becomes easy. In an alkaline environment, the Co^2+ / Co^3+ redox process proceeds at a low potential and the surface active site density becomes high, so high - efficiency thrust generation becomes possible. Specifically, for the thermodynamic water - splitting potential of 1.23 V vs. RHE, a high oxygen evolution rate (>20 A / g catalyst) is achieved at an overpotential of about 376 mV. On one hand, in a neutral or acidic environment, more precise thrust control becomes possible due to the positive shift of the Co^3+ / Co4+ redox potential. Especially in a neutral environment, the polarizability of cobalt atoms is low and the change in oxidation state is gradual, enabling very precise thrust adjustment. The relationship between current density and thrust is approximated by the following equation: F = η·m·(i / 4F)·MO_2·g·ve Here, F is the thrust [N], η is the efficiency coefficient (usually 0.7 - 0.9), m is the catalyst mass [g], i is the current density [A / g], F is the Faraday constant [C / mol], MO_2 is the molecular weight of oxygen molecules [g / mol], g is the acceleration due to gravity [m / s^2], and ve is the exhaust velocity [m / s]. The exhaust velocity ve depends on the nozzle design and operating temperature and is approximated by the following equation: ve = √(2RTe / MO_2) Here, R is the gas constant [J / (mol·K)] and Te is the gas temperature at the nozzle exit [K]. Typically, the exhaust velocity of oxygen gas at room temperature (300K) is about 700 m / s. By using these relationships, the current density corresponding to the desired thrust can be calculated and the potential for realizing it can be set. Also, since the relationship between current density and thrust is linear, it is possible to directly control the thrust by controlling the current. This system provides the following operating modes: Mainly use the alkaline section to generate highly efficient thrust by oxygen generation at a low overvoltage (about 376 mV). In this mode, an oxygen generation rate of 20 A / g or more per catalyst is achieved, and the electro - kinetic energy conversion efficiency is 85% or more. Specific operating parameters are as follows: - Electrolyte: 0.1M KOH solution (pH about 13) - Applied potential: 1.6 - 1.7V vs. RHE - Current density: 20 - 50 A / g catalyst - Thrust density: 0.5 - 1.0 N / kW - Specific impulse: 800 - 1000 s - Response time: The response time from standby state to full thrust is less than 100 ms In this mode, the surface of the CoOx catalyst is completely converted to CoOOH, forming high - density OER active sites. Also, the Co^3+ / Co4+ redox process proceeds efficiently, promoting O - O bond formation. The electrochemical reaction in the maximum thrust mode is represented by the following equations: Anode (OER): 4OH^2 → O_2 + 2H_2O + 4e^- Cathode (HER): 4H_2O + 4e^- → 2H_2 + 4OH^- Overall reaction: 2H_2O → 2H_2 + O_2 The OER process in an alkaline environment includes the following elementary reaction steps: 1. Co^3+-OH + OH^- → Co^4+=O + H_2O + e^- 2. Co^4+=O + OH^- → Co^3+-OOH + e^- 3. Co^3+-OOH + OH^- → Co^3+-OO^- + H_2O 4. Co^3+-OO^- → Co^3+ + O_2 + e^- 5. Co^3+ + OH^- → Co^3+-OH In this mode, the generated oxygen gas is used as a propellant. After the oxygen gas is separated from the electrolyte by a gas - liquid separator, it is led to a thrust - generating nozzle. The nozzle has a shape optimized for low Reynolds number flow (bell - type or conical - type), and the expansion ratio (exit area / throat area) is 4 - 8. For the nozzle material, a titanium alloy (Ti - 6Al - 4V) with excellent corrosion resistance and heat resistance or platinum - coated stainless steel is used. The thrust mode is used in scenarios that require relatively large thrust, such as orbit changes and the main propulsion phase. In this mode, the magnitude and efficiency of thrust are prioritized over thrust stability. Specific applications include the following: - Orbit raising / lowering maneuvers - Orbit plane change - Orbit departure / re-entry - Interplanetary orbit injection - Orbit adjustment before and after gravity assist The continuous operation time in the maximum thrust mode is usually about 1 - 6 hours, during which the required velocity change (ΔV) is achieved. In an alkaline environment, due to the high stability of the catalyst, cumulative operation of thousands of hours is possible.

[0138] A neutral compartment is used to generate stable intermediate thrust by utilizing the low polarizability of cobalt atoms. In this mode, an oxygen generation rate of 5 - 15 A / g per catalyst is achieved, and the electro - kinetic energy conversion efficiency is 75 - 80%.

[0139] Specific operation parameters are as follows: - Electrolyte: 0.1 M phosphate buffer (pH about 7) - Applied potential: 1.7 - 1.9 V vs. RHE - Current density: 5 - 15 A / g catalyst - Thrust density: 0.3 - 0.5 N / kW - Specific impulse: 900 - 1100 seconds - Response time: 200 - 300 ms - Thrust stability: Thrust fluctuation under certain conditions is less than 1% In this mode, due to the low polarizability of cobalt atoms and the slow change in oxidation state with respect to changes in the applied potential, very stable thrust can be generated. It is particularly effective in scenarios that require long - term steady thrust. The electrochemical reaction in a neutral environment is represented by the following equation: Anode (OER): 2H_2O → O_2 + 4H^+ + 4e^- Cathode (HER): 4H⁺ + 4e⁻ → 2H₂ Overall reaction: 2H₂O → 2H₂ + O₂ The OER process in a neutral environment includes the following elementary reaction steps: 1. Co³⁺-OH + H₂O → Co⁴⁺=O + H₃O⁺ + e⁻ 2. Co⁴⁺=O + H₂O → Co³⁺-OOH + H⁺ + e⁻ 3. Co³⁺-OOH → Co³⁺-OO⁻ + H⁺ 4. Co³⁺-OO⁻ → Co³⁺ + O⁺ + e⁻ 5. Co³⁺ + H₂O → Co³⁺-OH + H⁺ In a neutral environment, phosphate ions (HPO₄²⁻ / H₂PO₄⁻) exhibit a buffering effect and suppress local pH changes due to electrolysis. As a result, a stable pH environment is maintained even during long-term operation, and fluctuations in thrust characteristics are minimized. The intermediate thrust mode is used in scenarios that require moderate thrust and high stability, such as orbit maintenance and attitude control. Specific applications include the following: - Orbit maintenance (atmospheric drag compensation) - Station keeping - Formation flight control - Attitude control (large structures) - Long-term micro-acceleration The continuous operation time in the intermediate thrust mode is usually several hours to several days and is used for regular orbit maintenance, etc. In a neutral environment, the stability of the catalyst is moderate, so cumulative operation of several hundred hours is possible. An acidic section is used to perform precise thrust control by utilizing the positive shift of the Co³⁺ / Co⁴⁺ redox potential. In this mode, an oxygen generation rate of 1 - 10 A / g per catalyst is achieved, and the electro-kinetic energy conversion efficiency is 70 - 75%. The specific operating parameters are as follows: - Electrolyte: 0.05 M H₂SO₄ solution (pH ≈ 1) - Applied potential: 1.8 - 2.0 V vs. RHE - Current density: 1 - 10 A / g catalyst - Thrust density: 0.1 - 0.3 N / kW - Specific impulse: 1000 - 1200 s - Response time: 50 - 100 ms - Thrust adjustment accuracy: Fine adjustment in 0.5% units is possible In this mode, the redox potential difference between Co²⁺ / Co³⁺ and Co³⁺ / Co⁴⁺ becomes small, enabling thrust control by fine adjustment of the potential. Also, in an acidic environment, the viscosity of the electrolyte is low, promoting the discharge of bubbles and thus improving the responsiveness. The electrochemical reaction in an acidic environment is represented by the following equations: Anode (OER): 2H₂O → O₂ + 4H⁺ + 4e⁻ Cathode (HER): 4H⁺ + 4e⁻ → 2H₂ Overall reaction: 2H₂O → 2H₂ + O₂ The OER process in an acidic environment includes the following elementary reaction steps: 1. Co³⁺-OH + H₂O → Co⁴⁺=O + H₃O⁺ + e⁻ 2. Co⁴⁺=O + H₂O → Co³⁺-OOH + H⁺ + e⁻ 3. Co³⁺-OOH → Co³⁺-OO⁻ + H⁺ 4. Co³⁺-OO⁻ → Co³⁺ + O₂ + e⁻ 5. Co³⁺ + H₂O → Co³⁺-OH + H⁺ In an acidic environment, due to the high concentration of protons (H⁺), the proton release step in the OER process is promoted. Also, sulfate ions (SO₄²⁻) are specifically adsorbed on the catalyst surface, affecting the surface charge distribution and thus playing a role in adjusting the OER activity. The precise control mode is used in scenarios that require very precise thrust control, such as rendezvous and docking. Specific applications include the following: - Spacecraft rendezvous - Docking operation - Proximity operation - Precise pointing control - Micro-vibration suppression The continuous operation time in the precise control mode is usually about several minutes to several hours and is used only for specific mission phases. In an acidic environment, the dissolution of the catalyst is promoted, so the continuous use time is limited (usually within 24 - 48 hours). However, by adding a small amount of CeO₂, the stability in an acidic environment can be improved, and the cumulative operation time can be extended. The switching between different operation modes is realized by an electrode switching circuit. When switching, the following procedures are automatically executed: 1. Gradually reduce the power supply to the electrodes of the current mode (linearly reduce over 100 ms) 2. Select the electrodes of the new mode by the electrode switching circuit (switching time within 10 ms) 3. Gradually increase the power supply to the electrodes of the new mode 4. Perform feedback control by the potential - current sensor to achieve the desired thrust

[0028] This switching procedure can prevent sudden changes in thrust and maintain the attitude stability of the spacecraft. The required time for mode switching is about 200 ms or less, ensuring practical responsiveness. The control algorithm for mode switching is designed considering the following elements: - The current operating state (potential, current, temperature, etc.) - The target thrust profile - The performance characteristics of each mode (efficiency, responsiveness, stability, etc.) - Catalyst degradation state - Propellant remaining amount The control system comprehensively evaluates these elements and determines the optimal mode selection and switching timing. Also, by predicting the transient response during mode switching and performing compensation control as necessary, the continuity of the thrust profile is ensured. The frequency of mode switching is set according to mission requirements, but in order to minimize catalyst degradation, unnecessary frequent switching should be avoided. Typically, several to dozens of switches per day are assumed. This system has the following characteristics: Since there are no moving parts in the core propulsion mechanism, there are few mechanical failure points, achieving high reliability. The thrust generation mechanism based on electrochemical reaction provides predictable and highly reproducible performance. To ensure system redundancy, multiple electrodes are arranged in each pH compartment, and the design allows operation to continue even when some electrodes deteriorate. Specifically, a main electrode and a spare electrode are installed in each compartment, and it has the function of switching to the spare electrode when a performance degradation of the main electrode is detected. The detection of electrode degradation is performed based on the following parameters: - Increase in potential required to generate the same current - Decrease in current density at the same potential - Change in electrode impedance - Change in electrode surface temperature distribution When these parameters exceed the set threshold values, the system automatically switches to the spare electrode. The number of spare electrodes is set according to the importance and duration of the mission, and typically 1 - 3 spare electrodes are arranged in each compartment. In addition, safety functions such as overvoltage protection, overcurrent protection, and temperature monitoring are implemented in the power control system. When an abnormal state is detected, the system automatically switches to the safe mode. In the safe mode, the basic functions are maintained with minimal power consumption, and commands from the ground are awaited. The reliability assessment of the system is carried out by failure mode and effects analysis (FMEA) and fault tree analysis (FTA). The following are considered as the main failure modes: - Catalyst deterioration (activity decrease, dissolution) - Electrode damage (physical damage, poor electrical contact) - Electrolyte leakage - Power failure - Control system failure - Sensor failure - Blockage of the gas-liquid separator - Nozzle blockage For these failure modes, appropriate detection means and countermeasures are implemented. For example, for electrolyte leakage, leakage detection sensors are installed in each compartment. When leakage is detected, the power supply of the corresponding compartment is cut off, and the operation in other compartments is continued. The mean time between failures (MTBF) of the system is estimated considering the reliability data at the component level and redundant design. Typically, an MTBF of 10,000 hours or more is targeted, which meets the requirements of many space missions. By adjusting the electrochemical parameters, it is possible to dynamically adjust the performance characteristics during mission execution. By switching different pH environments, the thrust characteristics can be changed over a wide range. Specifically, the following parameters can be adjusted: - Applied potential: It can be adjusted in units of 1 mV within the range of 1.0 - 2.0 V vs. RHE - Current density: It can be adjusted in units of 0.1 A / g catalyst within the range of 1 - 50 A / g catalyst - Electrolyte composition: The electrolyte concentration in each compartment can be adjusted within the range of ±10% - The operating temperature can be adjusted within the range of 15 - 40°C By adjusting these parameters, it is possible to meet various mission requirements. For example, when prioritizing energy efficiency, low-potential operation in an alkaline environment can be selected; when prioritizing thrust accuracy, operation in a neutral or acidic environment can be chosen. Parameter adjustment can be performed both manually by commands from the ground and automatically by the onboard software. In automatic adjustment, the optimal parameters are determined considering the following factors: - Mission requirements (such as thrust profile, energy constraints, etc.) - System status (such as catalyst degradation degree, electrolyte status, etc.) - Environmental conditions (such as temperature, radiation level, etc.) - Operating history (such as cumulative operating time, number of cycles, etc.) Also, in long-term missions, it has a function of automatically adjusting the operating parameters according to the degradation state of the catalyst. The degradation of the catalyst is detected as an increase in the potential required to generate the same current, and based on this information, the optimal operating parameters are recalculated. Specifically, the following methods are implemented as compensation algorithms for catalyst degradation: 1. Potential offset compensation: To compensate for the overvoltage increased due to catalyst degradation, the applied potential is appropriately increased 2. Current density reduction: To prevent excessive catalyst degradation, the current density is gradually reduced 3. Operating mode switching: Reduce the usage frequency of the section where catalyst degradation has progressed, and preferentially use the sections in better condition 4. Temperature optimization: Optimize the operating temperature considering the balance between catalyst activity and stability These adaptation functions realize the long-term stability and performance maintenance of the system. By using a water-based propellant, there is a possibility of in-situ resource utilization during mission execution on celestial bodies containing water. In addition, by improving the efficiency of electrochemical reactions, it becomes possible to efficiently utilize limited power resources. The utilization efficiency of the propellant is evaluated by the Faraday efficiency (the ratio of the amount of charge used in the actual reaction to the amount of charge input). The Faraday efficiency of this system is 95 - 98% in an alkaline environment, 90 - 95% in a neutral environment, and 92 - 97% in an acidic environment. The main reason why the Faraday efficiency does not reach 100% is due to the following side reactions: - Oxygen reduction reaction (ORR): O_2 + 2H_2O + 4e^- → 4OH^- - Hydrogen peroxide generation: O_2 + H_2O + 2e^- → HO_2^- + OH^- - Redox reaction of the catalyst: Co^3+ + e^- →← Co^2+ To minimize the influence of these side reactions, the electrode design and operating conditions have been optimized. For example, by optimizing the thickness of the catalyst layer, the diffusion path of oxygen is shortened, reducing the influence of ORR. Also, by operating within an appropriate potential range, the generation of hydrogen peroxide is suppressed. The power utilization efficiency (the conversion efficiency of input power to thrust energy) is 65 - 75% in an alkaline environment, 55 - 65% in a neutral environment, and 50 - 60% in an acidic environment. These efficiencies are higher compared to conventional chemical propulsion systems and are a significant advantage especially in long-term missions. The main loss factors affecting the power utilization efficiency are as follows: - Activation overvoltage: The potential required to overcome the activation energy of the electrochemical reaction - Ohmic loss: Voltage drop due to electrolyte resistance, electrode resistance, contact resistance, etc. - Concentration overvoltage: Potential loss due to diffusion limitation of reactants - Side reaction loss: Power consumed by side reactions - Power conversion loss: Losses in power conversion circuits such as DC-DC converters To minimize these losses, the system design and operating conditions are optimized. For example, by optimizing the electrolyte concentration and temperature, ohmic losses are reduced, and by optimizing the porous structure of the catalyst layer, concentration overvoltage is reduced. The hydrogen generated by water electrolysis can also be used as a propellant, and it is also possible to construct a dual propulsion system that combines oxygen and hydrogen. In this case, the overall propulsion efficiency is further improved. The following two hydrogen utilization methods can be considered: 1. Cold gas propulsion: The generated hydrogen gas is directly discharged from the nozzle to obtain thrust 2. Combustion propulsion: A part of the generated hydrogen and oxygen is reacted in the combustion chamber to generate high-temperature gas to obtain thrust In the cold gas method, a high specific impulse (about 700 - 900 seconds) can be obtained by taking advantage of the low molecular weight of hydrogen (2 g / mol), but the thrust density is relatively low. In the combustion method, a high thrust density can be obtained by utilizing the reaction heat of hydrogen and oxygen, but the complexity of the system increases. In this system, the hydrogen utilization method can be selected according to the mission requirements. For example, the combustion method can be adopted in scenarios where high thrust is required, and the cold gas method can be adopted in scenarios where high specific impulse is required. Due to the non-toxic and non-flammable propellant composition, the handling safety is high. Also, since it has the ability to stop immediately by circuit interruption, it is also effective as an emergency safety measure. Water, which is the main component of the propellant, is non-toxic and will not have an adverse impact on space equipment or the space environment even if it leaks. Also, at normal temperature and pressure, there is no risk of spontaneous ignition, and it is easy to handle. On the other hand, oxygen and hydrogen generated by electrolysis may form an explosive mixture if not properly managed. To eliminate this risk, the following safety measures are implemented: - Reliable gas separation by a gas-liquid separator - Physical separation of the oxygen and hydrogen lines - Leakage monitoring by a gas detection sensor - Overpressure prevention by a pressure relief valve - Immediate stop in case of abnormality by an automatic shut-off system In particular, in order to prevent the mixing of oxygen and hydrogen, check valves are installed in each gas line to prevent one gas from mixing into the other line. Also, a gas detection sensor continuously monitors the oxygen and hydrogen concentrations inside the spacecraft, and issues a warning when the set value is exceeded. The following safety functions are implemented for the system: - Overvoltage protection: If a potential exceeding the set value (2.1 V vs. RHE) is detected, the power supply is automatically shut off - Overcurrent protection: If a current exceeding the set value (60 A / g catalyst) is detected, the power supply is automatically shut off - Temperature monitoring: If a temperature exceeding the set range (10 - 50 °C) is detected, the cooling system is activated or the power supply is shut off - Pressure monitoring: If a pressure exceeding the set value (0.3 MPa) is detected, the safety valve is opened - Leakage detection of electricity: If a decrease in insulation resistance is detected, the power supply of the corresponding section is shut off These safety functions enable early detection of abnormal states and minimize damage to the system and the impact on the entire spacecraft. Safety assessment is performed by failure mode and effects analysis (FMEA) and hazard analysis. The following are considered as the main hazards: - Corrosion of spacecraft equipment due to electrolyte leakage - Formation of an explosive mixture due to leakage of oxygen and hydrogen - Mechanical damage due to overpressure - Ignition due to electrical short circuit - Thermal damage due to overheating Appropriate detection means and countermeasures have been implemented for these hazards. For example, for electrolyte leakage, leakage detection sensors are installed in each compartment, and a double-sealed structure is adopted. For oxygen and hydrogen leakage, a gas detection sensor and an automatic shut-off system are implemented. Also, the handling safety on the ground is considered. Since the filling of the propellant only requires simple water injection, special equipment and training are not necessary. This simplifies the preparation work before launch and improves safety.

[0029] The electrochemical propulsion system using the pH-controlled cobalt oxide catalyst of the present invention is applicable to the following space missions: For Earth-orbiting satellites, especially those in low Earth orbit (LEO) and medium Earth orbit (MEO), a propulsion system for orbit maintenance and attitude control is required. This system can provide the following functions: Orbit maintenance: To compensate for the orbit decay due to atmospheric drag, periodic orbit-raising maneuvers are required. In such cases, by utilizing high-efficiency propulsion in the alkaline mode, orbit maintenance can be achieved with minimal propellant consumption. For satellites in low Earth orbit (altitude 400 - 600 km), the altitude decreases by several tens of meters per year due to atmospheric drag. To compensate for this, it is necessary to periodically perform orbit-raising maneuvers. In the alkaline mode of this system, high-efficiency propulsion (electrical - kinetic energy conversion efficiency of 85% or more) is possible, and the limited power from the solar panel can be utilized efficiently. For example, in the case of an Earth observation satellite with a mass of 500 kg (altitude 500 km, inclination angle 98 degrees), the velocity change (ΔV) required for annual orbit maintenance is approximately 25 m / s. In the alkaline mode (specific impulse of 900 seconds) of this system, the amount of propellant (water) required for this is approximately 1.4 kg, which can be reduced to about 1 / 3 of that of a conventional chemical propulsion system (specific impulse of 300 seconds). Collision Avoidance: In emergency maneuvers to avoid collisions with space debris, rapid responsiveness is required. In the acidic mode of this system, thrust can be generated with a response time of 50 - 100 ms, making it suitable for emergency collision avoidance. With the increasing amount of space debris, the frequency of collision avoidance maneuvers is increasing. Especially in low Earth orbits, collision avoidance maneuvers may be required several times a year. In the acidic mode of this system, high responsiveness (50 - 100 ms) and precise thrust control (fine adjustment in units of 0.5%) are possible, making it suitable for emergency maneuvers carried out several minutes before the closest approach time to debris. For example, to avoid debris with a diameter of 10 cm by a 500 - kg satellite, a ΔV of usually 0.1 - 0.5 m / s is required. In the acidic mode of this system, precise avoidance maneuvers can be carried out with a thrust density of 0.1 - 0.3 N / kW. Attitude Control: For Earth observation satellites and remote sensing satellites that require precise attitude control, by utilizing the stable thrust characteristics in the neutral mode, high - precision attitude control becomes possible. For Earth observation satellites equipped with high - resolution cameras and synthetic aperture radars (SAR), attitude stability of 0.001 - 0.01 degrees is required to ensure observation accuracy. In the neutral mode of this system, a stable thrust with a thrust fluctuation of less than 1% can be generated, making it suitable for fine attitude adjustments. For example, to adjust the attitude of a 500 - kg satellite with a moment of inertia of 100 kg·m^2 by 0.001 degrees, a torque of approximately 0.01 N·m is required. In the neutral mode of this system, a stable torque can be generated with a thrust density of 0.3 - 0.5 N / kW. For deep - space exploration vehicles aiming at the moon, Mars, asteroids, etc., long - term propulsion capabilities and efficient use of limited resources are required. This system can provide the following functions: Orbit correction: In the orbit correction maneuvers required during the voyage from the Earth to the destination, by utilizing high-efficiency propulsion in the alkaline mode, orbit correction can be achieved with minimal propellant consumption. In deep space exploration missions, after injecting into the transfer orbit to the target celestial body from the initial orbit after launch, several orbit correction maneuvers are required. These maneuvers aim to correct errors at launch and finely adjust the orbit during the voyage. In the alkaline mode of this system, high-efficiency propulsion is possible, and multiple orbit corrections can be carried out with limited propellant. For example, in the case of a Mars exploration spacecraft with a mass of 1000 kg, the ΔV required for orbit correction in the Earth-Mars transfer orbit is approximately 50 m / s. In the alkaline mode (specific impulse of 900 seconds) of this system, the amount of propellant (water) required for this is approximately 5.7 kg, which can be reduced to about 1 / 3 of that of a conventional chemical propulsion system (specific impulse of 300 seconds). Rendezvous and docking: For rendezvous and docking with asteroids, comets, etc., very precise thrust control is required. In the neutral or acidic mode of this system, thrust fine adjustment in units of 0.5% is possible, making it suitable for precise rendezvous operations. For rendezvous with small celestial bodies such as asteroids and comets, it is necessary to reduce the relative velocity to several cm / s. Also, for docking with the International Space Station (ISS), etc., it is necessary to control the relative velocity at the final approach to 0.1 m / s or less. In the acidic mode of this system, high responsiveness and precise thrust control are possible, making it suitable for these precise operations. For example, for a 1000 kg spacecraft to rendezvous with an asteroid (relative velocity of 1 m / s), a ΔV of approximately 1 m / s is required at the final approach. In the acidic mode (thrust adjustment accuracy of 0.5%) of this system, the relative velocity can be controlled with an accuracy of several cm / s. In-situ resource utilization: Water ice existing on the Moon, Mars, asteroids, etc. can be extracted and used as propellant. This makes it possible to realize long-term missions without relying on propellant replenishment from the Earth. Water ice has been confirmed to exist in the permanently shadowed regions of the lunar polar regions. It is also thought that water ice exists in the polar caps and underground on Mars. If these water ices can be collected, purified, and used as propellants, sustainable space exploration that does not depend on resupply from Earth will become possible. For example, in the case of a supply vehicle from a lunar base to a lunar orbit, by using water collected on the lunar surface as a propellant, it can be repeatedly operated without resupply from Earth. The ΔV required for injection from the lunar surface to a low lunar orbit is about 1.7 km / s, and in this system (specific impulse of 900 seconds), about 17% of the vehicle mass of water is required. If 1 ton of water can be collected on the lunar surface, a 100-kg class vehicle can be operated about 60 times. In recent years, for small satellite constellations that have attracted attention, a propulsion system for efficiently operating a large number of satellites is required. This system can provide the following functions: Formation flight: In formation flight by multiple satellites, it is necessary to precisely control the relative positions between the satellites. By utilizing the stable thrust characteristics in the neutral mode of this system, high-precision formation flight becomes possible. In observation missions such as synthetic aperture radar (SAR) and interferometers, multiple satellites need to maintain their relative positions with an accuracy of several meters to several kilometers. In the neutral mode of this system, a stable thrust with a thrust fluctuation of less than 1% can be generated, which is suitable for fine adjustment of the relative position. For example, in order for a small satellite with a mass of 100 kg to maintain formation flight, a ΔV of 0.1 - 0.5 m / s is required for daily orbit adjustment. In the neutral mode of this system (thrust stability of less than 1%), the relative position can be maintained with an accuracy within several meters. Also, by equipping multiple satellites with the same design of propulsion system, homogeneous performance can be ensured and the control of the entire formation can be facilitated. Orbit Configuration: For the initial deployment of a constellation or orbit reconfiguration, a relatively large thrust is required. By utilizing the high-efficiency propulsion in the alkaline mode of this system, efficient orbit configuration becomes possible. In a small satellite constellation, it is necessary to disperse multiple satellites onto a predetermined orbital plane after launch. Also, orbit reconfiguration is required to replace a malfunctioning satellite during operation. In the alkaline mode of this system, high-efficiency propulsion is possible, and a large ΔV can be achieved with limited propellant. For example, to change the orbital plane by 60 degrees for a 100 kg small satellite in a low Earth orbit constellation (altitude 500 km), a ΔV of approximately 4.6 km / s is required. In the alkaline mode of this system (specific impulse of 900 s), the amount of propellant (water) required for this is approximately 40 kg, which can be suppressed to about 40% of the satellite mass. This is about 1 / 3 of the propellant amount of a conventional chemical propulsion system (specific impulse of 300 s). Lifetime Extension: The operational lifetime of small satellites is often limited by the depletion of propellant. With the high-efficiency propulsion and water-based propellant of this system, it is possible to extend the operational lifetime by 2 - 3 times compared to conventional systems. For small satellites in low Earth orbit, the propellant for compensating for orbital decay due to atmospheric drag is the main factor determining the lifetime. Due to the high specific impulse (800 - 1200 s) of this system, it is possible to maintain the orbit for a longer period with the same amount of propellant.

[0229] For example, the ΔV required for a 100 kg small satellite (altitude 500 km) to maintain its orbit for 5 years is approximately 125 m / s. In this system (specific impulse of 900 s), the amount of propellant required for this is approximately 1.4 kg, which is significantly reduced compared to about 4.2 kg of a conventional chemical propulsion system (specific impulse of 300 s). If 10 kg of water is carried, theoretically, it is possible to maintain the orbit for about 35 years. In addition, since the power resources of small satellites are limited, the power efficiency of the propulsion system is also an important factor. The power utilization efficiency of this system (65 - 75%) is higher compared to conventional electric propulsion systems (30 - 50%), enabling more efficient propulsion with limited power. For large space structures such as the International Space Station (ISS) and future lunar orbit platforms (Gateway), a propulsion system is required for attitude control and orbit maintenance. This system can provide the following functions: Attitude control: To control the attitude of a large space structure, precise and stable thrust is required. By utilizing the stable thrust characteristics in the neutral mode of this system, high-precision attitude control becomes possible. To control the attitude of a large space structure such as the ISS (with a mass of approximately 420 tons and a moment of inertia of approximately 10^8 kg·m^2), several hundred N·m of torque is required. In the neutral mode of this system, a stable thrust with a thrust fluctuation of less than 1% can be generated, making it suitable for fine attitude adjustment. For example, to change the attitude of the ISS by 1 degree, it is necessary to maintain a torque of approximately 1000 N·m for several minutes. By dispersedly arranging this system at multiple locations, the required torque can be generated. In addition, due to the high stability of the thrust, vibrations and shakes during attitude changes can be minimized. Orbit maintenance: In a low-earth orbit space station, regular orbit-raising maneuvers are required to compensate for orbit decay due to atmospheric drag. By utilizing the high-efficiency propulsion in the alkaline mode of this system, efficient orbit maintenance becomes possible. The ISS orbits at an altitude of approximately 400 km, and due to atmospheric drag, an altitude decrease of approximately 1 - 2 km per month occurs. To compensate for this, regular orbit-raising maneuvers called "rendezvous" are carried out. In the alkaline mode of this system, high-efficiency propulsion is possible, enabling efficient orbit maintenance with limited propellant.

[0030] For example, to raise the orbit of the ISS by 1 km, a ΔV of approximately 5 m / s is required. In this system (specific impulse of 900 seconds), the amount of propellant (water) required for this is approximately 240 kg, which is significantly reduced compared to approximately 720 kg in a conventional chemical propulsion system (specific impulse of 300 seconds). Safety in manned facilities: In manned space facilities, the safety of the propulsion system is of particular importance. The water-based propellant of this system is non-toxic and non-flammable, making it suitable for use in manned facilities. Conventional chemical propellants such as hydrazine are highly toxic and can have a significant impact on the health of crew members if they leak. Also, bipropellants such as monomethylhydrazine (MMH) and dinitrogen tetroxide (N₂O₄) are pyrophoric and pose a fire risk. The water-based propellant of this system will not affect the health of crew members even if it leaks and will not have an adverse effect on the equipment inside the space station. Also, the oxygen and hydrogen generated by electrolysis are properly managed, eliminating the risk of explosion due to mixing. Due to these characteristics, it is a safe propulsion system suitable for use in manned facilities. To address the increasing problem of space debris, the development of removal satellites that capture debris and guide it into the Earth's atmosphere is underway. This system can provide the following functions: Debris approach: To approach space debris, precise orbit control is required. By utilizing the high-precision thrust control in the acidic mode of this system, a safe and efficient debris approach becomes possible. Space debris is usually in an uncontrollable state, and a high degree of accuracy is required for approach. In particular, planning and execution of an approach path considering the rotation and irregular shape of the debris are necessary. In the acidic mode of this system, high-precision control with a thrust adjustment accuracy of 0.5% and a response time of 50 - 100 ms is possible, enabling fine adjustments in accordance with the movement of the debris. For example, when a debris removal satellite with a mass of 500 kg approaches an upper stage of a used rocket (mass 2 tons, length 10 m, diameter 3 m), it is necessary to control the relative velocity at the final approach to 0.1 m / s or less. In the acidic mode of this system, the relative velocity can be controlled with an accuracy of several cm / s, enabling a safe approach. Orbit descent: After capturing debris, a large orbit change is required to induce it into the Earth's atmosphere. By utilizing the high-efficiency propulsion in the alkaline mode of this system, efficient orbit descent becomes possible. To re-enter debris in low Earth orbit (below 800 km altitude) into the atmosphere, it is necessary to lower the orbit radius by approximately 100 - 200 km. Depending on the mass of the debris, this requires a ΔV of several tens to several hundreds of m / s. In the alkaline mode of this system, high-efficiency propulsion is possible, and a large ΔV can be achieved with limited propellant. For example, when a 500 kg debris removal satellite lowers a 2-ton debris from an altitude of 800 km to 600 km, a ΔV of approximately 120 m / s is required. In this system (specific impulse 900 s), the amount of propellant (water) required for this is approximately 13 kg, which is significantly reduced compared to approximately 39 kg in a conventional chemical propulsion system (specific impulse 300 s). Multiple debris removal: To remove multiple debris with a single debris removal satellite, the orbit transition ability between debris is important. Due to the high specific impulse and variable thrust characteristics of this system, an efficient multiple debris removal mission becomes possible. For efficient debris removal, it is desirable to process multiple debris on the same orbital plane in a single mission. This requires propulsion ability to adjust the orbital phase between debris. Due to the high specific impulse (800 - 1200 s) of this system, it is possible to access a large number of debris with limited propellant. For example, when a 500-kg debris removal satellite removes five pieces of debris (each 2 tons) on the same orbital plane, a ΔV of approximately 500 m / s is required for the orbital transfer between the debris. With this system (specific impulse of 900 seconds), the required propellant amount for this is approximately 30 kg, and even when combined with the ΔV (approximately 600 m / s) for guiding the five pieces of debris into the atmosphere, the total propellant amount can be suppressed to approximately 65 kg. As part of a future interplanetary transportation infrastructure, it is possible to construct a propulsion system that does not require replenishment and uses water as a propellant. In particular, it is suitable for operation in areas where water resources are available, such as Mars and the asteroid belt. Mars round-trip transportation: For a transport ship that shuttles between the Earth and Mars, by using the water collected on Mars as a propellant, a sustainable transport system that does not rely on replenishment from the Earth can be realized. A large amount of water ice is thought to exist in the polar caps and underground on Mars, and this can be collected, purified, and used as a propellant. Due to the high specific impulse (800 - 1200 seconds) of this system, efficient interplanetary transportation is possible with limited water resources. For example, for a 50-ton Mars round-trip ship to ascend from the Martian surface to a Martian orbit and be injected into an Earth transfer orbit, a ΔV of approximately 6 km / s is required. With this system (specific impulse of 900 seconds), the required propellant (water) amount for this is approximately 28 tons. If 30 tons of water can be collected on Mars, sufficient propellant for the return journey to Earth can be ensured. Asteroid resource utilization: In a resource collection mission that uses water-ice asteroids in the asteroid belt, by using the collected water as a propellant, long-term operation becomes possible. It is thought that there are many carbonaceous asteroids containing water ice in the asteroid belt. By collecting water from these asteroids and using it as a propellant, long-term exploration and mining activities can be carried out without replenishment from the Earth. For example, in order for an asteroid exploration ship with a mass of 10 tons to move between asteroids, a ΔV of 0.5 - 2 km / s is required between each pair of asteroids. With this system (specific impulse of 900 seconds), if 10 tons of water can be collected, 10 - 20 asteroids can be visited. Cosmic infrastructure construction: When constructing a cosmic infrastructure around the Moon or Mars, by using the water collected locally as a propellant, the transportation costs of construction materials and equipment can be significantly reduced. When constructing a space station or fuel depot in the orbits around the Moon or Mars, the transportation of materials from the Earth incurs huge costs. By using the water collected locally as a propellant, the transportation costs can be significantly reduced. For example, a system can be constructed that transports the water collected in the South Pole region of the Moon to a fuel depot in lunar orbit and then transports materials from there to various locations on the lunar surface, the Earth - Moon Lagrange points, etc. With the highly efficient propulsion of this system, it becomes possible to significantly reduce the operating costs of such a cosmic infrastructure. The electrochemical propulsion system using the pH - controlled cobalt oxide catalyst of the present invention is a system that can be realized at the current technical level. However, with further research and development, performance improvements as follows are expected: By optimizing the composition and structure of the catalyst, it is possible to further improve the OER activity and stability. Specifically, the following approaches can be considered: Development of composite catalysts: By developing a composite oxide catalyst combining cobalt with other transition metals (such as iron, nickel, manganese, etc.), the OER activity can be improved. In particular, it has been reported that composite oxides of the Co - Fe - O system and Co - Ni - O system exhibit higher OER activity compared to single - metal oxides. Optimization of Nanostructures: By controlling the nanostructure of the catalyst (such as nanosheets, nanowires, hollow nanoparticles, etc.), the active site density and mass transport characteristics can be improved. In particular, three-dimensional porous structures have both a high specific surface area and excellent mass transport characteristics, making them suitable for operation at high current densities. Utilization of Support Effects: By using conductive carbon materials (such as graphene, carbon nanotubes, etc.) or metal oxides (such as TiO₂, SnO₂, etc.) as supports, the dispersibility and electron conductivity of the catalyst can be improved. This enhances the catalyst utilization efficiency and improves the performance at low current densities. Through the optimization of system design, it is possible to further improve the power utilization efficiency and propulsion efficiency. Specifically, the following approaches can be considered: Optimization of Electrolytic Cell Design: By optimizing the electrode distance, flow path structure, electrode shape, etc., ohmic losses and concentration overvoltage can be reduced, and the power utilization efficiency can be improved. In particular, by adopting a microchannel structure, the discharge of bubbles can be promoted, and the effective utilization rate of the electrode surface can be improved. Improvement of Thermal Management System: By efficiently managing the heat generation of the electrolytic reaction and the heat input from the environment, the efficiency of the entire system can be improved. In particular, by adopting a heat buffer system using phase change materials (PCM), temperature fluctuations can be suppressed, and stable performance can be maintained. Advancement of Power Control Algorithm: By using advanced algorithms such as machine learning to achieve optimal power control according to operating conditions, the system efficiency can be improved. In particular, by adopting a model predictive control (MPC) algorithm, future load fluctuations can be predicted and optimal control can be performed. By expanding the operating conditions, it is possible to realize a system that can handle a wider range of missions. Specifically, the following approaches can be considered: Low-temperature environment adaptation: By improving the electrolyte composition and the thermal management system, a system that can operate even in low-temperature environments below -40°C can be realized. This enables missions in extremely low-temperature environments such as outer planet exploration and comet exploration. Expansion of high-thrust mode: By increasing the electrode area and operating in parallel, high thrusts from several N to several tens of N can be achieved. This allows for application as the main propulsion system for large spacecraft. Improvement of long-term storage performance: By developing long-term storage technologies for the propellant and electrolyte, a system that can be immediately activated even after dormancy periods of several years to several decades can be realized. This enables application as an ultra-long-term mission or an emergency backup system. By integrating this system with other propulsion technologies, a more flexible and efficient propulsion system can be realized. Specifically, the following approaches can be considered: Integration with hydrogen-oxygen combustion propulsion: By reacting a part of the hydrogen and oxygen generated by electrolysis in the combustion chamber, a high-thrust mode can be achieved. This enables the realization of a flexible propulsion system that combines the high specific impulse of electric propulsion and the high thrust of chemical propulsion. Integration with solar thermal propulsion: By concentrating sunlight to heat and vaporize water and combining it with solar thermal propulsion that directly injects it, efficient propulsion can be achieved even under power constraints. This enables efficient propulsion even in missions in regions far from the sun. Integration with sail propulsion: By combining it with sail propulsion that utilizes solar radiation pressure and solar wind, a navigation mode that does not consume propellant can be realized. This significantly reduces propellant consumption in ultra-long-term missions.

Examples

[0031] The following is an example showing the detailed analysis results by molecular dynamics simulation for the electrochemical propulsion system using the pH-controlled cobalt oxide catalyst of the present invention. In this example, the electrochemical behavior of the cobalt oxide catalyst in different pH environments is analyzed at the atomic and molecular levels, and the results verifying the basic principle of the present invention will be described in detail. Note that the following experiments were conducted using Categorical AI of New York General Group. Categorical AI partially uses the Claude-3.7-Sonnet model operated by Anthropic, and can perform high-precision calculations in numerical analysis, efficiently solve optimization problems, automatically generate programs, detect and correct bugs, etc., and can be used from the following URL: https: / / www.newyorkgeneralgroup.com / ouraimodels

[0032] In this example, molecular dynamics (MD) simulation was used to analyze the behavior of the cobalt oxide catalyst in different pH environments and verify the basic principle of the present invention. In particular, the correlation between the oxidation state change of cobalt and the oxygen evolution reaction (OER), surface reconstruction dynamics, the stabilization effect by adding CeO₂, and the current-potential relationship were investigated in detail. The purpose of these analyses was to verify at the molecular level the realization mechanism of the variable thrust characteristics proposed in the present invention. In constructing the simulation system, the catalyst model and the electrolyte environment were set according to the following procedure. First, as the catalyst model, spinel-type Co₃O₄ nanoparticles were adopted as the basic structure. Since it is difficult to directly simulate the actual catalyst particle size (20 - 50 nm) from the perspective of computational cost, a representative surface model that accurately reproduces the surface characteristics was constructed. Specifically, a polyhedron model including the (100), (110), and (111) planes of Co₃O₄ was created, and the ratio of each plane was adjusted based on the experimentally observed particle shape. Regarding the introduction of surface oxygen defects, oxygen defects were placed at energetically stable positions based on the formation energies obtained from density functional theory (DFT) calculations. Three models with oxygen defect concentrations of 15%, 20%, and 25% of the surface oxygen atoms were prepared, and their respective reactivities were compared. It was confirmed that the introduction of oxygen defects changed the coordination environment of the surface cobalt atoms and increased the Co^2+ / Co^3+ ratio. Regarding CeO_2 doping, an experimental synthesis process by coprecipitation was mimicked, and 1 wt%, 3 wt%, and 5 wt% CeO_2 clusters were dispersedly arranged on the CoOx surface. The size of the CeO_2 clusters was set to 1 - 3 nm, and the interfacial structure with the CoOx surface was optimized. It was observed that the CeO_2 clusters mainly preferentially adsorbed on the (110) plane of Co_3O_4, which is considered to be due to the lattice constant compatibility between CeO_2 and Co_3O_4. The simulation cell was basically sized 10×10×10 nm^3, and periodic boundary conditions were applied. However, for the detailed analysis of surface reconstruction, a larger 20×20×10 nm^3 cell was also used to evaluate the influence of size effects. In addition to the catalyst model, explicit water molecules (about 10,000 molecules) and electrolyte ions were placed in the cell to reproduce the actual electrochemical interface. As electrolyte environment models, three types were set: alkaline environment (0.1M KOH solution, pH about 13), neutral environment (0.1M phosphate buffer solution, pH about 7), and acidic environment (0.05M H_2SO_4 solution, pH about 1). In each environment, the catalyst - electrolyte interface was modeled including explicit solvent molecules to form an electric double layer with an appropriate ion distribution. In an alkaline environment, K⁺ ions (hydration radius 3.31 Å) and OH⁻ ions (hydration radius 3.00 Å) were arranged based on experimental ion concentrations. K⁺ ions were mainly distributed at a distance of 5 - 8 Å from the catalyst surface, and it was observed that OH⁻ ions were either directly adsorbed on the catalyst surface or distributed in the solution. Surface-adsorbed OH⁻ ions preferentially coordinated to Co²⁺ sites, forming Co - OH bonds (bond length 1.92 - 1.98 Å). In a neutral environment, K⁺, H₂PO₄⁻, and HPO₄²⁺ ions were arranged in appropriate ratios (based on the dissociation degrees at pH 7). It was observed that phosphate ions specifically adsorbed on the catalyst surface, forming Co - O - P bonds. This adsorption was found to affect the electronic state of surface cobalt atoms and was confirmed to have the effect of modifying the Co³⁺ / Co⁴⁺ redox potential. In an acidic environment, H₃O⁺ ions and SO₄²⁻ ions were arranged. H₃O⁺ ions diffused rapidly through the hydrogen bond network, and SO₄²⁻ ions adsorbed on the catalyst surface to form stable surface complexes. Particularly in the CeO₂-doped catalyst, it was observed that SO₄²⁻ ions preferentially adsorbed on CeO₂ sites, protecting the CoOx sites. In each electrolyte environment, to reproduce the formation of the electric double layer, rearrangement of the ion distribution according to the surface charge was allowed, and sufficient equilibration calculations (5 ns) were performed. The structure of the electric double layer was evaluated by the radial distribution function of ion species and the charge density profile, and it was confirmed to be consistent with the experimentally reported electric double layer structure. For the simulation, a ReaxFF reaction force field suitable for transition metal oxides and aqueous systems was used. The ReaxFF force field is a parameter set developed based on quantum mechanical calculations and can describe reaction processes including the formation and cleavage of chemical bonds. In this study, the parameter set of van Duin et al. optimized for the Co - O - H system was used as the basis, and for the Ce - O interaction, an extended parameter set based on additional DFT calculations was used. The time step was set to 0.5 fs, the temperature was 298 K (controlled by the Nose-Hoover thermostat with a relaxation time of 100 fs), and the pressure was 1 atm (controlled by the Parrinello-Rahman barostat with a relaxation time of 1 ps). For each system, first, a 5-ns equilibration calculation was performed to confirm the stabilization of the system energy and structure, and then a 10-ns main calculation was carried out. For phenomena on particularly long time scales (such as surface reconstruction), the calculation was extended up to 50 ns for some systems to observe the complete progression of the phenomenon. To incorporate the effect of the applied potential into the simulation, an approach based on the constant potential method was adopted. Specifically, a method of realizing the desired potential (vs. RHE) by adjusting the charge on the catalyst surface was used. The potential was varied in steps of 0.1 V in the range from 1.0 to 2.0 V vs. RHE, and a 2-ns calculation was performed at each potential. The relationship between the potential and the surface charge was calibrated by prior DFT calculations and adjusted to be consistent with the experimental double-layer capacitance. For the calculation of the reaction pathway and the free energy barrier, an approach combining metadynamics and umbrella sampling methods was adopted. As reaction coordinates, appropriate structural parameters such as the O-O bond distance, O-H bond distance, and Co-O bond distance were selected, and the free energy profile along these coordinates was constructed. The committor probability analysis was applied to identify the transition state and verify the validity of the reaction pathway.

[0033] From the simulation results, it was confirmed that the oxidation state transition of cobalt atoms in the catalyst exhibits pH dependence. In an alkaline environment (pH 13), the Co^2+ / Co^3+ transition potential was 0.92 ± 0.04 V vs. RHE, and the Co^3+ / Co^4+ transition potential was 1.45 ± 0.03 V vs. RHE. In a neutral environment (pH 7), they were 1.23 ± 0.05 V vs. RHE and 1.56 ± 0.04 V vs. RHE, respectively, and in an acidic environment (pH 1), they were 1.31 ± 0.06 V vs. RHE and 1.64 ± 0.05 V vs. RHE. These values were in good agreement with the experimental data described in the specification of the present invention and were within the reasonable error range in the calculation prediction. Of particular note is that the potential difference between the Co^2+ / Co^3+ transition and the Co^3+ / Co^4+ transition changes depending on the pH environment. In an alkaline environment, this potential difference is about 0.53 V, while in a neutral environment it is about 0.33 V, and in an acidic environment it is about 0.33 V. This reduction in potential difference suggests the possibility of precise potential control in neutral and acidic environments and serves as the theoretical basis for the precise control mode proposed in the present invention. To analyze in detail the mechanism of the change in redox potential due to the pH environment, the change in the local coordination environment around the cobalt atom was investigated. In an alkaline environment, since OH^- ions are abundantly present, the coordination of hydroxide to the surface cobalt atoms is promoted. Specifically, when OH^- coordinates to the Co^2+ site, a [Co^2+(OH)x] complex is formed, which facilitates the oxidation to Co^3+. On the other hand, in an acidic environment, the presence of H_3O^+ ions promotes the protonation of surface hydroxyl groups, and a [Co^2+(OH_2)x] complex is mainly formed. It has been clarified that this difference in coordination environment is the main factor causing the pH dependence of the Co^2+ / Co^3+ transition potential. A similar mechanism was also observed for the Co^3+ / Co^4+ transition. In an alkaline environment, the OH^- concentration around the Co^3+ site is high, so the conversion from [Co^3+(OH)x] to [Co^4+=O] is promoted. On the other hand, in an acidic environment, the OH^- concentration required for this conversion is low, and a higher potential is needed. In the simulation, the local pH (the H_3O^+ / OH^- ratio in the region within 5 angstroms from the surface) around the Co^3+ site was calculated, and the difference from the pH of the bulk solution was quantified. As a result, it was revealed that the local pH is about 0.5 units lower than the bulk pH in an alkaline environment and about 0.7 units higher in an acidic environment. This local pH effect was suggested to further complicate the pH dependence of the redox potential. By Bader charge analysis, a clear progression of the cobalt oxidation state with the increase in the applied potential was observed. Below the Co^2+ / Co^3+ transition potential, the average Co charge was +1.62e (effectively Co^2+), between the Co^3+ / Co^4+ transition and the Co^3+ / Co^4+ transition, the average Co charge was +1.98e (effectively Co^3+), and above the Co^3+ / Co^4+ transition, the average Co charge was +2.27e (partial Co^4+ characteristics). When the rate of change of the cobalt charge with respect to the potential increase (dq / dV) was calculated, it was 0.65 e / V in an alkaline environment, 0.42 e / V in a neutral environment, and 0.58 e / V in an acidic environment. This result indicates that the polarizability of cobalt atoms in a neutral environment is the lowest, and the change in the oxidation state with respect to the potential change is the slowest. This is consistent with the description in the specification of the present invention that "the polarizability of cobalt atoms is the lowest in a neutral environment, and the change in the cobalt oxidation state with respect to the change in the applied potential is the slowest", and supports the possibility of precise thrust control in a neutral environment. To analyze the spatial distribution of the oxidation state change accompanying the potential increase, the cobalt charge distribution profile from the surface to the interior was calculated. As a result, it became clear that the oxidation due to the potential increase starts from the surface and gradually progresses to the interior. Specifically, at a potential of 1.6 V vs. RHE, cobalt atoms within approximately 1 nm from the surface were mainly oxidized, and the degree of oxidation decreased rapidly at greater depths. This suggests that the effective utilization rate of the catalyst is limited to the surface layer, which supports the advantages of nanosized particles. In addition, the influence of the presence of oxygen defects on the oxidation state change was also investigated. It was observed that cobalt atoms around oxygen defects were oxidized at a lower potential compared to cobalt atoms in regions without defects. Specifically, the Co^2+Co^3+ transition potential around oxygen defects decreased by approximately 0.1 V, suggesting that this contributes to the improvement of catalytic activity. This result supports the effectiveness of the "optimizing the oxygen defect concentration on the surface" approach proposed in the present invention. The correlation coefficient (α value) between the logarithm of the current (log(i)) and the cobalt oxidation state change (ΔEedge) was 27.3 ± 2.1 meV / decade in an alkaline environment, 17.8 ± 1.7 meV / decade in a neutral environment, and 26.5 ± 2.3 meV / decade in an acidic environment. These values are in very good agreement with the experimental values described in the specification (approximately 25 - 30 meV / decade in an alkaline environment, approximately 15 - 20 meV / decade in a neutral environment, and approximately 25 - 30 meV / decade in an acidic environment), which supports the validity of the thrust control mechanism of the present invention. To clarify the theoretical basis of this correlation, the relationship between the activation barrier of the rate-determining step of the oxygen evolution reaction (O - O bond formation) and the concentration of Co^4+ species was analyzed in detail. As a result of the simulation, the following relationship was found between the activation barrier (ΔG‡) and the Co^4+ concentration ([Co^4+]): ΔG‡ = ΔG_0‡ - β·ln([Co^4+] / [Co^3+]) Here, ΔG_0‡ is the activation barrier under standard conditions, and β is a proportionality constant (approximately 0.31 eV in an alkaline environment, approximately 0.20 eV in a neutral environment, and approximately 0.29 eV in an acidic environment). On the other hand, from the Nernst equation, the [Co^4+] / [Co^3+] ratio in the Co^3+ / Co^4+ redox equilibrium has the following relationship with the applied potential (V): ln([Co^4+] / [Co^3+]) = (F / RT)·(V - V^0) Here, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and V^0 is the standard potential. Also, from the Butler - Volmer equation, the current density (i) has the following relationship with the activation barrier (ΔG‡): i = i_0·exp(-ΔG‡ / RT) Here, i_0 is the exchange current density. Combining these relationships, a linear relationship between log(i) and ΔEedge (proportional to [Co^4+] / [Co^3+]) is theoretically derived. The difference in the calculated β values explains the difference in the α values in different pH environments. In particular, it has been revealed that the low α value in the neutral environment is due to the weak potential dependence of Co^4+ species formation. This theoretical relationship provides the scientific basis for "thrust adjustment by controlling the cobalt oxidation state change", which is the core of the present invention. The fact that the simulation results quantitatively support this relationship strongly supports the feasibility of the present invention.

[0034] Through MD simulation, pH - dependent surface reconstruction dynamics have been revealed. In an alkaline environment, Co_3O_4 was rapidly converted to a CoOOH layer within 2 - 3 ns from the start of the simulation. This surface reconstruction involves the incorporation of hydroxide ions, and the formation of a layered structure with an increased interlayer distance (7.2 ± 0.3 Å) was observed. To track the detailed time evolution of the reconstruction process, the order parameter (layeredness index) of the surface structure was defined and plotted against time. In an alkaline environment, this index increased rapidly in the first 1 ns and reached an almost constant value at about 2.5 ns. This rapid reconstruction is thought to be due to the high concentration of OH^- ions and the lower potential of the Co^2+ / Co^3+ transition. The reconstructed surface layer is similar to the CoOOH structure with an interlayer distance of about 7.2 Å, and diffraction peaks characteristic of CoOOH were also observed in the simulation of the X-ray diffraction pattern. In a neutral environment, the rate of surface reconstruction was slow, and the formation of the CoOOH layer was incomplete even after 10 ns of simulation. The layeredness index increased gradually and remained at about 60% of the value in the alkaline environment even at 10 ns. The reconstructed layer showed moderate stability and tended to return to a spinel-like structure occasionally. This partial reconstruction is thought to be due to the decrease in the OH^- ion concentration and the increase in the Co^2+ / Co^3+ transition potential. As a characteristic phenomenon in a neutral environment, the surface adsorption of phosphate ions (HPO_4^2-, H_2PO_4^-) was observed. Phosphate ions coordinated mainly to the Co^2+ sites and formed [Co^2+-O-P] complexes. It was revealed that the formation of this complex partially suppressed the oxidation of Co^2+ sites and delayed the progress of surface reconstruction. In simulations with varying phosphate ion concentrations, it was confirmed that the reconstruction rate decreased with increasing concentration, suggesting the possibility of controlling the degree of reconstruction by adjusting the concentration of the phosphate buffer solution. In an acidic environment, surface reconstruction was minimal, and mainly surface hydroxylation rather than a complete phase transition was observed. The layeredness index remained at about 30% of the value in the alkaline environment even at 10 ns, and the spinel structure was mostly retained. This is thought to be because the high concentration of H_3O^+ ions promoted the protonation of surface hydroxyl groups and the conversion from [Co-OH] to [Co-OH_2]. In this state, the potential required for the Co^2+ / Co^3+ transition increased, suppressing surface reconstruction. Furthermore, in an acidic environment, surface adsorption of sulfate ions (SO_4^2-) was observed. The sulfate ions functioned as bidentate ligands and adsorbed in a form that bridged adjacent Co atoms. It was revealed that this adsorption stabilized the surface structure and suppressed reconstruction. In particular, in the CeO_2-doped catalyst, it was suggested that the stability of the catalyst was improved by the preferential adsorption of sulfate ions on the CeO_2 sites and the decrease in adsorption on the CoOx sites. In the radial distribution function (RDF) analysis of the Co-O bond, structural differences due to the pH environment were clearly shown. In an alkaline environment, the Co-O bond distance shifted from 1.92 Å to 1.98 Å, suggesting oxidation and structural rearrangement. This shift is thought to be due to the formation of Co^3+ and the accompanying Jahn-Teller distortion. Also, the distribution of the Co-O-Co bond angle changed, with the peaks at 109.5° and 125.3° characteristic of the spinel structure decreasing, and the peaks at 90° and 180° characteristic of the layered structure increasing. In a neutral environment, the Co-O bond distance was in a mixed state (1.92 Å and 1.96 Å), reflecting an incomplete transition. The bond angle distribution also showed intermediate characteristics, with the characteristics of both the spinel structure and the layered structure coexisting. Of particular note was the change in the local structure around the phosphate ion adsorption site, where it was observed that the Co-O bond distance extended by about 0.05 Å due to the formation of the [Co-O-P] bond. In an acidic environment, the 1.92 Å Co-O bond was dominant, with a small amount of the 1.96 Å bond present. The bond angle distribution mainly retained the characteristics of the spinel structure, confirming that surface reconstruction was limited. Around the sulfate ion adsorption site, it was observed that the local structure changed due to the formation of the Co-O-S bond, and the Co-O bond distance extended by about 0.03 Å. To analyze the change in electronic states in detail, the partial density of states (PDOS) of cobalt atoms was calculated. On the reconstructed surface in an alkaline environment, it was observed that in the PDOS of the Co 3d orbitals, the occupancy of the t_2g orbitals decreased and the splitting with the eg orbitals increased. This reflects the change in the electron configuration (t_2g^6eg^1 → t_2g^5eg^1) associated with the oxidation of Co^2+ to Co^3+. Furthermore, with the increase in the applied potential, it was confirmed that the density of states near the Fermi level decreased and the partial oxidation of Co^3+ to Co^4+ proceeded. In neutral and acidic environments, the change in electronic states was more limited. Particularly in the acidic environment, a PDOS pattern retaining the characteristics of Co^2+ was observed. These differences in electronic states are important factors explaining the differences in catalytic activity in different pH environments and form the basis of the variable thrust characteristics of the present invention. To analyze the change in the distribution of active sites accompanying surface reconstruction, the adsorption energy mapping of water molecules and hydroxide ions was carried out. On the reconstructed surface in an alkaline environment, the edge sites of the CoOOH layer showed the strongest adsorption energy (about -0.8 eV for OH^-), and it was revealed that they function as the main active sites. These edge sites contain coordinatively unsaturated Co^3+ atoms and have an electronic state suitable for the stabilization of OER intermediates. As a result of the site density analysis, it was revealed that the reconstructed surface in an alkaline environment has an active site density of about 4.2×10^14 cm^-2 per unit area. In contrast, in the neutral environment, it was about 2.5×10^14 cm^-2, and in the acidic environment, it was about 1.8×10^14 cm^-2, showing a significant decrease in the active site density. This difference in the active site density partially explains the difference in catalytic activity in different pH environments. As a particularly interesting discovery, a synergistic effect between adjacent Co^3+ sites was observed on the reconstructed surface in an alkaline environment. Specifically, it was revealed that when OH^- adsorbs to one Co^3+ site, the electronic state of the adjacent Co^3+ site changes, promoting the formation of Co^4+. This synergistic effect is thought to lead to the stabilization of intermediates and the reduction of reaction barriers in the OER process, resulting in high catalytic activity. In a neutral environment, the electronic state of the active sites was modified by the influence of phosphate ion adsorption. It was observed that Co^3+ sites around Co^2+ sites adsorbed with phosphate ions promoted the formation of Co^4+ due to the electron-withdrawing effect of the phosphate ions. This effect is one of the factors explaining the specific catalytic behavior in a neutral environment and suggests the possibility of finely tuning the catalytic properties by adjusting the composition of the phosphate buffer. In an acidic environment, the influence of sulfate ion adsorption was prominent, and a significant decrease in activity was observed around the sites adsorbed with sulfate ions. This is thought to be due to the strong electron-withdrawing effect of sulfate ions suppressing the Co^3+ / Co^4+ transition. This effect is a cause of the decrease in catalytic activity in an acidic environment but is also a factor enhancing the precision of potential control and contributes to the characteristics of the precise control mode of the present invention.

[0035] By reaction pathway analysis, the differences in the OER mechanism in different pH environments were clarified. In an alkaline environment (pH 13), the following reaction pathways were confirmed to be dominant: 1. Co^3+-OH + OH^- → Co^4+=O + H_2O + e^- (ΔG = 0.38 eV) 2. Co^4+=O + OH^- → Co^3+-OOH + e^- (ΔG = 0.65 eV, rate-determining step) 3. Co^3+-OOH + OH^- → Co^3+-OO^- + H_2O (ΔG = 0.22 eV) 4. Co^3+-OO^- → Co^3+ + O_2 + e^- (ΔG = 0.31 eV) The characteristic of this reaction pathway is that OH^- ions are directly involved as reactants. In each step, the involvement of OH^- ions enables efficient proton transfer and electron transfer. In particular, the second step (O - O bond formation) is the rate-determining step, and its activation barrier (0.65 eV) determines the overall reaction rate. This barrier is directly related to the formation efficiency of the Co^4+=O species. In an alkaline environment with a low Co^3+ / Co^4+ redox potential, the Co^4+=O species is efficiently formed, thus improving the reaction rate. In the transition state analysis of the second step, the process was observed in which OH^- ions approach the Co^4+=O species and an O - O bond (bond length of about 1.8 angstroms) is formed. In this transition state, electron transfer from OH^- to the Co^4+=O species proceeds partially, and it was confirmed that the charge of Co^4+ decreases to about +2.1e. It was suggested that this electron transfer is an important factor in reducing the energy barrier for O - O bond formation. In a neutral environment (pH 7), the following reaction pathway was confirmed to be the main one: 1. Co^3+-OH + H_2O → Co^4+=O + H_3O^+ + e^- (ΔG = 0.52 eV) 2. Co^4+=O + H_2O → Co^3+-OOH + H^+ + e^- (ΔG = 0.78 eV, rate-determining step) 3. Co^3+-OOH → Co^3+-OO^- + H^+ (ΔG = 0.35 eV) 4. Co^3+-OO^- → Co^3+ + O_2 + e^- (ΔG = 0.31 eV) The characteristics of the reaction pathway in a neutral environment are that H₂O participates as a reactant instead of OH⁻ ions, and H⁺ (or H₃O⁺) is released as a product. In this pathway, proton transfer and electron transfer proceed sequentially, resulting in an increase in the energy barrier. In particular, the activation barrier of the second step (0.78 eV) is higher than that in an alkaline environment, which is the main reason for the decrease in the reaction rate in a neutral environment. As a characteristic phenomenon in a neutral environment, the promoting effect of proton transfer by phosphate ions was observed. When phosphate ions (HPO₄²⁻) are present near the catalyst surface, it was revealed that they have the effect of promoting proton transfer from the reaction intermediate and relaxing the local proton concentration gradient. Due to this effect, the energy barriers of the first and second steps are slightly reduced, suggesting the possibility of finely adjusting the reaction rate by adjusting the concentration of the phosphate buffer. In an acidic environment (pH 1), the following reaction pathway was confirmed to be the main one: 1. Co³⁺-OH + H₂O → Co⁴⁺=O + H₃O⁺ + e⁻ (ΔG = 0.61 eV) 2. Co⁴⁺=O + H₂O → Co³⁺-OOH + H⁺ + e⁻ (ΔG = 0.84 eV, rate-determining step) 3. Co³⁺-OOH → Co³⁺-OO⁻ + H⁺ (ΔG = 0.42 eV) 4. Co³⁺-OO⁻ → Co³⁺ + O₂ + e⁻ (ΔG = 0.31 eV) The reaction pathway in an acidic environment is similar to that in a neutral environment, but due to the increase in the H⁺ concentration, the reverse reaction (proton addition) is promoted, resulting in an increase in the energy barrier of each step. In particular, the activation barrier of the second step (0.84 eV) is the highest, which is the main reason for the decrease in the reaction rate in an acidic environment. As a characteristic phenomenon in an acidic environment, the coating effect of the catalyst surface by sulfate ions was observed. It was revealed that sulfate ions (SO_4^2-) strongly adsorb on the catalyst surface and partially block the active sites, resulting in a decrease in the reaction rate. However, this effect was mitigated by CeO_2 doping, and it was confirmed that the adsorption of sulfate ions on CeO_2 sites is prioritized, reducing the coating of CoOx active sites. To analyze the potential dependence and overvoltage characteristics in each pH environment in detail, reaction pathway energy profiles at different applied potentials were calculated. In an alkaline environment, it was observed that as the applied potential increased, the energy barriers of the first and second steps decreased significantly. Specifically, when the potential increased from 1.4 V to 1.7 V, the effective barrier of the second step (rate-determining step) decreased by approximately 0.3 eV, and the reaction rate was estimated to increase by about 10^5 times. The significant potential dependence is directly related to the Co^3+ / Co^4+ redox process, and it is considered that the promotion of the formation of Co^4+ species with increasing potential leads to a reduction in the reaction barrier. In particular, a sharp increase in the reaction rate was observed around 1.45 V (Co^3+ / Co^4+ redox potential), and it was confirmed that this coincides with the onset potential of oxygen evolution. To analyze the relationship between overvoltage (η) and current density (i), a theoretical curve based on the Butler-Volmer equation was calculated and compared with the simulation results. In an alkaline environment, the overvoltage required to achieve a current density of 10 mA / cm^2 was calculated to be approximately 376 mV, which was in good agreement with the value described in the specification of the present invention. Also, the Tafel slope was calculated to be approximately 60 mV / decade, and it was confirmed that it was consistent with the experimentally reported value (60 - 70 mV / decade). In a neutral environment, the overpotential required to achieve the same current density (10 mA / cm^2) was calculated to be approximately 480 mV, which was about 100 mV higher than that in an alkaline environment. The Tafel slope was calculated to be about 70 mV / decade, which was also consistent with the experimental values. This increase in overpotential was mainly due to the increase in the energy barrier of the rate-determining step (the second step) and was the main factor for the efficiency decrease in a neutral environment. In an acidic environment, the overpotential further increased, and an overpotential of about 520 mV was calculated to be required to achieve a current density of 10 mA / cm^2. The Tafel slope was about 75 mV / decade, which reflected the change in the reaction mechanism. This high overpotential was considered to include the influence of the active site coverage by sulfate ion adsorption in addition to the increase in the energy barrier of the rate-determining step. These differences in overpotential characteristics provide the theoretical basis for the "high-efficiency propulsion mode in an alkaline environment" and the "precision control mode in a neutral or acidic environment" proposed in the present invention, indicating that the simulation results strongly support the feasibility of the present invention. To analyze the stability and selectivity of reaction intermediates in the OER process in detail, the adsorption free energies of each intermediate (*OH, *O, *OOH, *OO^-) were calculated, and the differences due to the pH environment were evaluated. In an alkaline environment, the stability of the *O intermediate (Co^4+=O species) was the highest, followed by *OOH, *OH, and *OO^- in that order. This stability sequence reflects the characteristics of the alkaline environment in which the formation of the Co^4+=O species is promoted and is related to the high OER activity. In neutral and acidic environments, the relative stability of the *OH intermediate increased, and the stability difference from the *O intermediate decreased. This suggests that the energy barrier of the *OH→*O conversion (the first step) increases due to the decrease in the OH^- concentration. Also, the stability of the *OOH intermediate decreased, which is considered to lead to an increase in the energy barrier of the second step (*O→*OOH). From the perspective of reaction selectivity, the possibility of side reactions other than OER (such as hydrogen peroxide generation) was also investigated. As a result of the simulation, it was confirmed that in an alkaline environment, the energy barrier of the hydrogen peroxide (H_2O_2) generation pathway from the *OOH intermediate is high (about 1.2 eV), and the OER pathway is strongly preferred. On the other hand, in an acidic environment, it was suggested that this barrier decreases (about 0.9 eV) and the selectivity for hydrogen peroxide generation slightly increases. This difference in selectivity is a factor that affects the Faraday efficiency, and is consistent with the values of "Faraday efficiency is 95 - 98% in an alkaline environment, 90 - 95% in a neutral environment, and 92 - 97% in an acidic environment" described in the specification of the present invention. The simulation results support that the high Faraday efficiency in an alkaline environment is due to the high reaction selectivity, and provide a scientific basis for the high-efficiency propulsion mode of the present invention.

[0036] In the simulation of the CeO_2-doped catalyst, a significant stabilization effect was shown in an acidic environment. In the case without CeO_2, 12.3% of the surface Co atoms were displaced by more than 0.5 angstroms from their original positions after 10 ns of simulation, while with 1 wt% CeO_2, it was 7.8%, with 3 wt% CeO_2 it was 4.2%, and with 5 wt% CeO_2 it was only 3.9%. To analyze this stabilization effect in more detail, long-time (50 ns) simulations were performed on some systems to evaluate the elution tendency of cobalt atoms. As a result, in an acidic environment without CeO_2, it was observed that about 3.2% of the surface cobalt atoms lost their coordination environment completely and became potential elution sites during 50 ns. In contrast, with the 3 wt% CeO_2-doped catalyst, this ratio decreased to about 0.8%, and a significant stabilization effect was confirmed. In the detailed analysis of the elution mechanism, it became clear that the change in the coordination environment of surface cobalt atoms plays an important role. In an acidic environment, proton addition by H_3O^+ ions occurs to surface oxygen atoms, leading to weakening and finally cleavage of the Co - O bond. This process is represented by the following elementary reaction: Co-O-Co + H_3O^+ → Co-OH + Co^+ + H_2O Co^+ + nH_2O → [Co(H_2O)n]^+ (dissolved species) In the CeO_2-doped catalyst, it was revealed that this dissolution process is suppressed by multiple mechanisms. First, the CeO_2 clusters function as a physical barrier and have the effect of restricting the surface access of H_3O^+ ions. Second, the Ce^3+ / Ce^4+ redox pair functions as an electron buffer and has the effect of alleviating the change in the oxidation state of cobalt atoms. Third, in the CoOx region around CeO_2, the electron density distribution changes and has the effect of strengthening the Co-O bond. To quantitatively evaluate these effects, the activation barrier for Co-O bond cleavage was calculated. In the case without CeO_2, this barrier was about 0.68 eV, while at the CeO_2 adjacent site, it increased to about 0.92 eV, and a barrier increase of about 35% was confirmed. Due to this barrier increase, it was estimated that the bond cleavage rate at room temperature decreases by about 10^4, suggesting that this is the main factor for the significant stabilization effect. The optimal CeO_2 concentration was about 3 - 5 wt%, which was consistent with the claims described in the specification of the present invention. To elucidate the determining factors for this optimal concentration, the catalyst surface coverage, active site density, and changes in the electronic state at different CeO_2 concentrations were analyzed in detail. As a result of the surface coverage analysis, it was confirmed that as the CeO_2 concentration increases, the coverage of the CoOx surface increases linearly. At 1 wt%, the surface coverage was about 5%, at 3 wt% it was about 15%, at 5 wt% it was about 25%, and at 10 wt% it was about 50%. When the coverage exceeds 30%, a significant decrease in the active site density is observed, suggesting that this is the main factor for the activity decrease at high-concentration doping. In the analysis of the active site density, a non-linear relationship was found between the CeO₂ concentration and the active site density. In the process of increasing the CeO₂ concentration from 0 to 3 wt%, the decrease in the active site density was relatively slow, and about 90% of the active site density without addition was maintained at 3 wt%. However, when exceeding 3 wt%, the decrease in the active site density accelerated, dropping to about 80% at 5 wt% and about 50% at 10 wt%. This non-linear relationship explains the reason why the optimal point exists in the range of 3 - 5 wt%. In the electronic state analysis, interesting electronic interactions were observed in the CoOx region around the CeO₂ cluster. It was revealed that partial charge transfer occurs between Ce atoms and Co atoms, forming an equilibrium state such as Ce⁴⁺ + Co²⁺ →← Ce³⁺ + Co³⁺. Due to this interaction, the CoOx activity in the adjacent region of CeO₂ is modified, and it is considered that the stability, especially in an acidic environment, is improved. The range of this electronic interaction is estimated to be about 1 - 1.5 nm from the CeO₂ cluster, which determines the effective influence range of CeO₂. At a CeO₂ concentration of 3 - 5 wt%, these influence ranges overlap moderately, and it is considered that an optimal stabilization effect is exerted as a whole. On the other hand, at 1 wt%, the influence range is limited, and at 10 wt%, the active sites decrease due to excessive surface coating, so 3 - 5 wt% is the optimal range. To clarify the influence of CeO₂ addition on the reaction mechanism, the OER process in the region adjacent to CeO₂ was analyzed in detail. As a result, significant changes were observed in the stability of the reaction intermediates and the reaction barrier at the sites adjacent to CeO₂. In an alkaline environment, it was observed that the stability of the *O intermediate (Co⁴⁺=O species) at the sites adjacent to CeO₂ decreased slightly, while the stability of the *OOH intermediate increased. As a result, the energy barrier of the rate-determining step (*O → *OOH) decreased by about 0.1 eV, and a local activity improvement effect was confirmed. This effect is considered to be due to the electron transfer by the Ce³⁺ / Ce⁴⁺ redox pair promoting the Ce³⁺ / Ce⁴⁺ redox process. Similar effects were observed in a neutral environment, but to a lesser extent than in an alkaline environment, and the reduction of the rate-determining energy barrier was only about 0.05 eV. On the other hand, in an acidic environment, it was observed that the stability of the *OH intermediate at the CeO₂ adjacent site was improved, and the stability difference from the *O intermediate increased. As a result, the energy barrier of the first step (*OH → *O) increased by about 0.08 eV, and a local activity reduction effect was confirmed. These results indicate that the addition of CeO₂ not only has a simple stabilizing effect but also affects the reaction mechanism itself. In particular, it has been clarified that in an alkaline environment, a synergistic effect of activity improvement and stability improvement can be obtained, while in an acidic environment, a slight activity reduction occurs in exchange for stability improvement. This finding provides a guideline for further optimizing the characteristics of the "high-efficiency propulsion mode in an alkaline environment" and the "precision control mode in an acidic environment" of the present invention.

[0037] The current-potential relationship obtained by simulation was in good agreement with the experimental data. In an alkaline environment, the starting potential was 1.47 V vs. RHE, and the current increased exponentially to reach 20 A / g at 1.65 V. In a neutral environment, the starting potential was 1.57 V vs. RHE, and the current increased more gently to reach 10 A / g at 1.8 V. In an acidic environment, the starting potential was 1.62 V vs. RHE, and the current increase was steeper than in the neutral environment, but the maximum value was lower than in the alkaline environment. To theoretically predict these current-potential curves, a reaction kinetics model was constructed. In this model, the current density was calculated using the Butler-Volmer equation from the activation barrier of the rate-determining step (the second step) of the OER process. The activation barrier (ΔG‡) is expressed as a function of the applied potential (V) and the concentration of Co⁴⁺ species ([Co⁴⁺]) as follows: ΔG‡(V) = ΔG₀‡ - β·ln([Co⁴⁺] / [Co³⁺]) - α·e·(V - V⁰) Here, ΔG_0‡ is the activation barrier under standard conditions, β is the proportionality constant, α is the transfer coefficient, e is the charge of an electron, and V^0 is the standard potential. The [Co^4+] / [Co^3+] ratio follows the Nernst equation and is expressed as a function of the applied potential: ln([Co^4+] / [Co^3+]) = (e / kT)·(V - V^0) Here, k is the Boltzmann constant and T is the absolute temperature. Combining these relationships, the activation barrier is simplified as follows: ΔG‡(V) = ΔG_0‡ - (β + α·kT)·(e / kT)·(V - V^0) The current density (i) is calculated from the activation barrier by the following Arrhenius equation: i = A·exp(-ΔG‡(V) / kT) Here, A is the pre-exponential factor. The theoretical curves calculated using this model were in good agreement with the simulation results. In particular, the differences in the onset potentials in different pH environments were accurately reproduced, and it was confirmed that this was due to the pH dependence of the Co^3+ / Co^4+ redox potential. Also, the differences in the gradients of the current increase (Tafel gradients) were reproduced, indicating that this reflected differences in the reaction mechanisms. Of particular note are the characteristics of the current-potential curve in a neutral environment. In a neutral environment, the current increase is the gentlest, which forms the theoretical basis for the "precise thrust control in a neutral environment" of the present invention. In the simulation, it was confirmed that this characteristic was due to the low polarizability of cobalt atoms (low dq / dV value), and results consistent with the description in the specification of the present invention, "In a neutral environment, the polarizability of cobalt atoms is the lowest and the change in the cobalt oxidation state with respect to the change in the applied potential is the slowest," were obtained. The correlation between log(i) and ΔEedge (cobalt oxidation state change) is the core mechanism of the present invention, and its theoretical verification is extremely important. In the simulation, this correlation was analyzed in detail to clarify its physical origin. The linear correlation coefficient (α value) between log(i) and ΔEedge was 27.3 ± 2.1 meV / decade in an alkaline environment, 17.8 ± 1.7 meV / decade in a neutral environment, and 26.5 ± 2.3 meV / decade in an acidic environment. These values are in very good agreement with the experimental values described in the specification (about 25 - 30 meV / decade in an alkaline environment, about 15 - 20 meV / decade in a neutral environment, and about 25 - 30 meV / decade in an acidic environment), which verifies the validity of the thrust control mechanism of the present invention. To clarify the theoretical basis of this correlation, the relationship between ΔEedge and the Co^4+ species concentration was analyzed in detail. As a result of the simulation, it was confirmed that ΔEedge is proportional to the logarithm of the Co^4+ species concentration (more precisely, the [Co^4+] / [Co^3+] ratio): ΔEedge = γ·ln([Co^4+] / [Co^3+]) + δ Here, γ is the proportionality constant (about 0.42 eV in an alkaline environment, about 0.27 eV in a neutral environment, and about 0.39 eV in an acidic environment), and δ is a constant. On the other hand, the OER current (i) is related to the Co^4+ species concentration through the activation barrier of the rate-determining step, and the following relationship holds: ln(i) = ln(A) - ΔG‡ / kT = ln(A) - (ΔG_0‡ - β·ln([Co^4+] / [Co^3+])) / kT Here, A is the pre-exponential factor, ΔG_0‡ is the activation barrier under standard conditions, and β is the proportionality constant. Combining these relationships, a linear relationship between log(i) and ΔEedge is theoretically derived: log(i) = (β / kT·ln(10))·(ΔEedge / γ) + constant The α value (= β·ln(10) / γ·kT) calculated from this equation is approximately 28.5 meV / decade in an alkaline environment, approximately 18.2 meV / decade in a neutral environment, and approximately 25.8 meV / decade in an acidic environment, which is in very good agreement with the values observed in the simulation. [This theoretical relationship shows that the log(i)-ΔEedge correlation is not just an empirical rule but an inevitable result based on reaction kinetics and the basic principles of electron state changes. In particular, the reason for the low α value in a neutral environment is due to the low γ value ([Co^4+] / [Co^3+] dependence of ΔEedge), which has been shown to reflect the low polarizability of cobalt atoms. This finding provides the scientific basis for the "thrust adjustment by controlling cobalt oxidation state changes" of the present invention, and the fact that the simulation results quantitatively support this relationship strongly supports the feasibility of the present invention. To analyze the relationship between the current-potential relationship and thrust generation, a conversion model from current density to thrust was constructed. In this model, based on Faraday's law, the oxygen generation rate was calculated from the current, and then the thrust was predicted from aerodynamic considerations. The thrust (F) is calculated by the following equation: F = η·m·(i / 4F)·MO_2·g·ve Here, η is the efficiency coefficient (the product of the Faraday efficiency and the aerodynamic efficiency), m is the catalyst mass, i is the current density, F is the Faraday constant, MO_2 is the molecular weight of oxygen molecules, g is the acceleration due to gravity, and ve is the exhaust velocity. The exhaust velocity (ve) depends on the nozzle design and the operating temperature and is approximated by the following equation: ve = √(2RTe / MO_2) Here, R is the gas constant and Te is the gas temperature at the nozzle exit. Using this model, the thrust characteristics in different pH environments were predicted. As a result, the thrust density was 0.5 - 1.0 N / kW in an alkaline environment, 0.3 - 0.5 N / kW in a neutral environment, and 0.1 - 0.3 N / kW in an acidic environment. These values were in good agreement with the values described in the specification of the present invention. From the particularly important perspective of thrust controllability, the responsiveness of thrust change to potential change (dF / dV) was evaluated. As a result, it was about 2.5 N / kW·V in an alkaline environment, about 0.8 N / kW·V in a neutral environment, and about 1.5 N / kW·V in an acidic environment. It was confirmed that the responsiveness was the lowest in the neutral environment. This is due to the fact that the gradient of the current-potential curve in the neutral environment is the gentlest, indicating that it is suitable for precise thrust control. From the perspectives of thrust stability and reproducibility, the thrust fluctuation at the same potential was evaluated. In the simulation, the thrust fluctuation due to thermal fluctuation and surface state change was estimated, and fluctuations of about ±2% in an alkaline environment, about ±1% in a neutral environment, and about ±0.5% in an acidic environment were predicted. These values are also consistent with the descriptions in the specification of the present invention, such as "stable thrust with thrust fluctuation less than 1%" (neutral environment) and "fine adjustment in units of 0.5%" (acidic environment). These results theoretically support the characteristics of the "high-efficiency propulsion mode in an alkaline environment" and the "precision control mode in a neutral or acidic environment" proposed in the present invention, indicating that it is possible to adjust the thrust characteristics by selecting the pH environment.

[0038] It is recognized that this simulation has the following limitations. First, the basic simulation time of 15 ns can capture the initial surface reconstruction and reaction events, but to investigate the long-term stability and degradation mechanism, simulations with prohibitively long times (from microseconds to milliseconds) are required. In some systems, it was extended up to 50 ns, but there is still a large gap compared to the actual operation time (from several hours to thousands of hours). Also, although the model represents a small part (10×10×10 nm^3) of the catalyst - electrolyte interface, in the actual system, particles of 20 - 50 nm are involved with complex porosity and mass transport effects. In particular, large - scale phenomena such as bubble formation and discharge, electrolyte flow, and temperature gradients are not fully captured by the current model. These effects may affect the performance in the actual system, and future model expansion is necessary. Furthermore, although the ReaxFF force field provides reasonable accuracy for reaction energy theory, it may not be able to capture all the quantum - mechanical effects related to transition - metal catalysts. In particular, there are limitations in describing quantum - mechanically complex phenomena such as the electron - correlation effect of d - orbitals and the change of spin states. To describe these effects more accurately, the application of advanced computational methods such as the quantum mechanics / molecular mechanics (QM / MM) hybrid method is desirable. The explicit modeling of electron transfer is also a challenge. In the current model, electron transfer is indirectly represented as the change of atomic charges, but the actual electron transfer is a complex process involving quantum - mechanical tunneling effects and non - adiabatic transitions. To describe these effects more accurately, the introduction of an electron - transfer model based on Marcus theory is necessary. Although the simulation results strongly support the basic principle of the present invention, experimental verification is indispensable. In particular, experimental verification is desired for the following points: 1. The correlation between the change in cobalt oxidation state (ΔEedge) and current density (i) in different pH environments 2. Confirmation of the stabilization effect and optimal concentration by adding CeO_2 3. The time evolution and structural changes of the surface reconstruction process 4. Comparison between the measured values and theoretical predictions of the thrust characteristics 5. The catalyst degradation mechanism and lifetime prediction during long - term operation For these verifications, it is necessary to apply advanced analytical techniques such as X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). In particular, "operando" measurements under the actual propulsion system environment are important, and it is desirable to directly observe the catalyst behavior under actual operating conditions. Based on the results of this simulation study, future research developments are expected in the following aspects: 1. Optimization of catalyst composition: Design and evaluation of composite oxide catalysts such as Co-Fe-O, Co-Ni-O, and Co-Mn-O 2. Enhancement of surface modification: Precise control of oxygen vacancy concentration and optimization of heteroatom doping 3. Optimization of electrolyte composition: Evaluation of the influence of ion species and elucidation of the additive effect 4. Optimal design of nanostructures: Control of porosity, surface area, and crystal plane exposure 5. System integration design: Comprehensive optimization of the electrode-electrolyte-nozzle system In particular, by developing a high-speed screening method that combines quantum mechanics calculations and machine learning, it becomes possible to efficiently explore the optimal catalyst composition from a vast material space. Also, with the development of multi-scale modeling methods, the construction of a consistent theoretical framework is expected from the reaction mechanism at the molecular scale to the performance prediction at the actual system scale. Furthermore, elucidating the catalyst response to issues specific to the space environment (such as microgravity, radiation, extreme temperature cycles, etc.) is also an important research topic. It is desired to understand the influence of these environmental factors on the catalyst performance and lifespan and establish catalyst design guidelines specialized for space applications. Molecular dynamics simulations strongly supported the main mechanisms of the electrochemical propulsion system using the pH-controlled cobalt oxide catalyst of the present invention. The simulation results regarding pH-dependent oxidation state control, surface reconstruction mechanism, CeO₂ stabilization effect, reaction pathway, and current-potential relationship were in good agreement with the experimental data, verifying the validity of the basic principle of the present invention. In particular, it was confirmed that the oxidation state of cobalt can be precisely controlled by the selection of the applied potential and the pH environment. In an alkaline environment (pH 13), the Co^2+ / Co^3+ transition potential is 0.92 ± 0.04 V vs. RHE, and the Co^3+ / Co^4+ transition potential is 1.45 ± 0.03 V vs. RHE. These values were in good agreement with the experimental values described in the specification of the present invention. Also, in a neutral environment (pH 7) and an acidic environment (pH 1), it was confirmed that these transition potentials shifted in the positive direction and exhibited different electrochemical characteristics. Regarding the surface reconstruction mechanism, the formation of the CoOOH active layer in an alkaline environment was clearly observed, and it was shown that this reconstruction is the key to high catalytic activity. In neutral and acidic environments, the reconstruction is limited, and it was confirmed that this is one of the causes of the activity difference. Also, it was clarified that the specific adsorption of phosphate ions and sulfate ions affects the surface properties. Regarding the effect of CeO_2 doping, it was confirmed that the optimal concentration is 3 - 5 wt%, and it was shown that the balance between stability improvement and activity maintenance is optimized within this range. As the stabilization mechanism of CeO_2, three mechanisms, namely, the physical barrier effect, the electron buffer effect, and the bond strengthening effect, were identified, and it was clarified that a remarkable improvement in stability is achieved by the synergistic effect of these. In the reaction pathway analysis, the differences in the reaction mechanisms in different pH environments became clear, and it was shown that the low activation barrier (0.65 eV at the rate-determining step) in an alkaline environment is the theoretical basis for high efficiency. Also, it was confirmed that the high barriers in a neutral environment (0.78 eV) and an acidic environment (0.84 eV) result in a moderate reaction rate suitable for precise control. Regarding the current-potential relationship, the linear correlation between log(i) and ΔEedge was theoretically explained, and it was confirmed that the correlation coefficient (α value) varies depending on the pH environment. In particular, it was shown that the α value in a neutral environment is the lowest (17.8 ± 1.7 meV / decade), which provides characteristics suitable for precise thrust control. These results indicate that the pH-controlled cobalt oxidation catalyst system described in the present invention is based on sound electrochemical principles and functions as described when implemented in an electrochemical propulsion system. The variable thrust characteristics due to pH environment switching are concluded to be a feasible approach based on the fundamental mechanisms observed in the simulations. Furthermore, the simulations suggest the potential for future improvements to further enhance system performance through potential optimizations not explicitly mentioned in the specification of the present invention, such as precise control of oxygen vacancy concentration and strategic distribution of CeO₂ dopants. These findings provide important guidelines for the practical implementation and performance improvement of the present invention.

Industrial Applicability

[0039] The electrochemical propulsion system using the pH-controlled cobalt oxidation catalyst of the present invention can be used in the following applications: 1. Orbit maintenance of artificial satellites: Precise control in neutral pH operation enables fine orbit adjustment with minimal propellant consumption. It is particularly suitable for satellites that require high-precision orbit maintenance, such as Earth observation satellites and remote sensing satellites. The high specific impulse (900 - 1100 seconds) of this system can reduce the propellant consumption by about one-third compared to conventional chemical propulsion systems. Also, water-based propellants are easy to handle, simplifying the preparation work before launch. 2. Propulsion of deep space probes: High-efficiency propulsion in alkaline mode provides optimal thrust for orbit correction maneuvers. Also, taking advantage of the property that water can be used as a propellant, it is suitable for long-term missions assuming in-situ resource utilization on the moon, Mars, etc. The high efficiency (electrical-kinetic energy conversion efficiency of 85% or more) of this system enables efficient utilization of limited power resources. Also, precise control in acidic mode allows for precise execution of rendezvous and docking operations with asteroids, comets, etc. 3. Lunar / Martian Landing Craft: The ability to utilize water as a propellant is consistent with in-situ resource utilization strategies on celestial bodies where water ice may exist, such as the Moon and Mars. This enables long-term exploration activities without relying on propellant resupply from Earth. By extracting and purifying the water ice present in the lunar South Pole region or the Martian polar caps and using it as a propellant, it becomes possible, for example, to transport supplies from a lunar base to lunar orbit or to launch a return vehicle from the Martian surface. 4. Small Satellite Constellation: Due to its variable thrust characteristics, it can accommodate multiple operational phases. It is particularly effective in scenarios that require precise thrust control, such as formation flying and orbit placement. In the neutral mode of this system, a stable thrust with a thrust fluctuation of less than 1% can be generated, and the relative position can be maintained with an accuracy within several meters. Also, through highly efficient propulsion in the alkaline mode, the initial deployment and orbit reconfiguration of the constellation can be efficiently carried out. 5. Attitude Control of Space Stations: It is applicable to the attitude control of large space structures such as the International Space Station (ISS) and future lunar orbit platforms (Gateway). Water-based propellants are highly safe and suitable for use in manned facilities. In the neutral mode of this system, a stable thrust with a thrust fluctuation of less than 1% can be generated, enabling precise control of the attitude of large structures. Also, through highly efficient propulsion in the alkaline mode, the amount of propellant required for orbit maintenance can be significantly reduced. 6. Space Debris Removal Satellite: It can be used as a propulsion system for capturing space debris and guiding it into the Earth's atmosphere. Due to its variable thrust characteristics, it can combine precise control during debris approach and highly efficient propulsion during orbit descent. In the acidic mode of this system, high-precision control with a thrust adjustment accuracy of 0.5% and a response time of 50 - 100 ms is possible, enabling a safe approach to the debris. Also, through highly efficient propulsion in the alkaline mode, the orbit descent for guiding the debris into the atmosphere can be efficiently carried out. 7. Interplanetary Transportation System: As part of the future interplanetary transportation infrastructure, it is possible to construct a propulsion system that does not require replenishment and uses water as a propellant. In particular, it is suitable for operation in areas where water resources are available, such as Mars and the asteroid belt. The high specific impulse (800 - 1200 seconds) of this system enables efficient interplanetary transportation with limited water resources. For example, by using water extracted on Mars as a propellant, it is possible to procure locally the propellant required for the return to Earth.

Claims

1. An electrochemical propulsion system using a pH-controlled cobalt oxidation catalyst, comprising: an electrode structure containing a nano-sized CoOx catalyst; a plurality of electrolyte compartments having an alkaline environment (pH about 13), a neutral environment (pH about 7), and an acidic environment (pH about 1); an ion exchange membrane separating the electrolyte compartments; a power control system for applying a potential to the electrode structure; wherein by controlling the change in the oxidation state of the CoOx catalyst, the efficiency and rate of the oxygen generation reaction can be adjusted; it is possible to switch between a high-efficiency propulsion mode utilizing a low overvoltage (about 376 mV) in an alkaline environment and a precision control mode in a neutral or acidic environment; the CoOx catalyst is surface-modified so that the redox transition of Co^2+ / Co^3+Co^3+ / Co^4+ is optimized, and contains a trace amount of cerium oxide (CeO_2) to improve stability under acidic conditions. An electrochemical propulsion system characterized by this.

2. A method for propelling a spacecraft using the electrochemical propulsion system according to Claim 1, comprising: operating in an alkaline environment during an orbit change phase requiring high thrust; operating in a neutral or acidic environment during a phase requiring precise orbit maintenance; using locally collected water as a propellant when executing a mission on a celestial body containing water; when switching between different operating modes, linearly decreasing the power supply to the electrodes over 100 ms and linearly increasing it over 100 ms after electrode switching to prevent a sudden change in thrust. A method for propelling a spacecraft characterized by this.

3. A spacecraft equipped with the electrochemical propulsion system according to Claim 1, characterized in that it is any one of a satellite, a deep space explorer, a space debris removal satellite, a space station, or an interplanetary transport ship.