Pulse joul heat preparation of binder-free electrode and quinoxaline-based electro-hydrogen coupling flow battery system for long-time energy storage

Abstract

The application discloses a kind of long-time energy storage-oriented pulse joule heat preparation binderless electrode and quinoline base electric-hydrogen coupling flow battery system, belong to long-time energy storage, organic hydrogen carrier and electrochemical catalytic material technical field.The application is pre-constructed on the surface of foamed nickel coordination precursor containing cobalt, molybdenum, nitrogen and phosphorus, then through the multiple pulse joule heat with cooling interval promotes rapid carbonization and phase transition, obtains binderless Co-Mo2C / NF self-supporting electrode that can be used simultaneously for cathode hydrogenation and anode dehydrogenation.The application realizes the matching between molybdenum carbide phase formation, cobalt-containing active phase dispersion and composite interface quantity, the active phase distribution of electrode is uniform, with abundant effective interface, high stability of catalytic layer adhesion, with excellent bidirectional electrocatalytic performance, can be used as cathode and anode respectively, flexible realization energy storage mode and release mode switch, and constitute electric-hydrogen coupling flow battery system, with wide application prospect.

Description

Technical Field

[0001] This invention relates to the fields of long-term energy storage, organic hydrogen storage carriers and electrochemical catalytic materials, and particularly to a pulsed Joule heating method for preparing binder-free electrodes and a quinoxaline-based electro-hydrogen coupled flow battery system for long-term energy storage. Background Technology

[0002] Renewable energy power generation, such as wind and solar power, is affected by weather and time conditions, exhibiting intermittency and fluctuations. Converting electrical energy into chemical energy that can be stored long-term, transported, and released on demand is an important technological approach to improving the absorption capacity of renewable energy. Hydrogen energy is clean in its end-use process, but gaseous hydrogen has a low volumetric energy density, and its compression, liquefaction, storage, and transportation processes place high demands on equipment strength, insulation conditions, and safety management.

[0003] Liquid organic hydrogen supports achieve the chemical storage and release of hydrogen through reversible hydrogenation and dehydrogenation reactions between hydrogen-poor and hydrogen-rich compounds. They offer advantages such as convenient liquid storage and transportation, recyclability, and good compatibility with existing liquid storage and transportation facilities. Traditional organic hydrogen storage supports typically employ thermocatalytic hydrogenation and dehydrogenation. Hydrogenation often requires the application of high-pressure hydrogen gas, while dehydrogenation typically requires high temperatures and may rely on precious metal catalysts such as platinum (Pt), palladium (Pd), or ruthenium (Ru), resulting in complex equipment, high energy consumption, and high catalyst costs.

[0004] Electrochemical hydrogenation utilizes water or proton-type species in the electrolyte as a hydrogen source, forming a hydrogen-containing intermediate in situ on the cathode surface, allowing the organic substrate to undergo hydrogenation without the application of high-pressure molecular hydrogen. Electrochemical dehydrogenation, on the other hand, causes hydrogen-rich organic molecules to lose hydrogen and electrons at the anode, regenerating hydrogen-poor organic molecules. Therefore, by altering the electrode operating conditions to achieve hydrogenation and dehydrogenation of organic hydrogen storage carriers, it is hoped that an energy storage system capable of interconverting electrical energy and chemical hydrogen storage can be established.

[0005] Flow batteries store energy-storing active materials in an external tank and transport them to an electrochemical reactor via a circulating pump. The system's energy storage capacity and power can be adjusted by changing the liquid volume and reactor area. Introducing the reversible conversion of quinoxaline / 1,2,3,4-tetrahydroquinoxaline into the flow battery system allows electrical energy to be stored in the circulating electrolyte as organic molecular hydrogen. However, water molecule activation, surface hydrogen-containing intermediate formation, organic substrate adsorption, and hydrogen-containing intermediate transfer place different demands on the catalytic site. When non-noble metal catalysts are used for cathode hydrogenation, problems such as insufficient water activation and competition from hydrogen evolution side reactions easily arise; when used for anode dehydrogenation, the catalyst must be able to activate carbon-hydrogen bonds (CH) and form a suitable oxidized working surface. Single-component materials typically cannot simultaneously meet these requirements. Molybdenum carbide (Mo2C) possesses good electrical conductivity and water activation ability, and cobalt-containing (Co) species can participate in the adsorption and electrochemical conversion of nitrogen-containing heterocyclic compounds. Constructing a cobalt-containing active phase and a molybdenum carbide phase into a tightly packed composite structure is beneficial for the synergistic effects of water activation, substrate adsorption, and interfacial hydrogen transfer. However, while conventional high-temperature treatment in a tubular furnace can promote molybdenum species carbonization, it can easily lead to particle growth, migration of active components, cracking or detachment of the catalyst layer, and a reduction in the effective interface. Joule heat treatment has the advantages of rapid heating rate, short treatment time, and adjustable temperature program, but direct Joule heat treatment of simple mixtures may still result in problems such as uneven component dispersion and insufficient bonding between the active phase and the substrate.

[0006] In existing technologies, cobalt-containing imidazole coordination materials are typically prepared first as independent powders, then modified with phosphorus-containing reagents and subjected to prolonged furnace heat treatment. Other technologies employ flash Joule heating to subject conductive carbon powder or mixed precursors to instantaneous high-temperature treatment. These methods do not address the simultaneous coordination and nucleation of cobalt species, molybdenum species, 2-methylimidazole, and phytic acid on the surface of nickel foam, nor do they reveal the impact of multiple pulsed Joule heating with cooling intervals on the gradual carbonization of Co, Mo, N, and P-supported precursors, Mo2C formation, and interface retention. Therefore, how to co-design the precursor co-assembly process with intermittent pulsed Joule heating to simultaneously obtain the Mo2C phase, the Co-containing active phase, a stable composite interface, and a complete binder-free catalytic layer remains an unsolved problem in existing technologies. Summary of the Invention

[0007] In view of the above-mentioned deficiencies of the prior art, the purpose of this invention is to provide a method and product for preparing a binder-free Co-Mo2C / NF self-supporting electrode by pulsed Joule heating, thereby solving the problems of active phase agglomeration, reduced effective interface, and insufficient adhesion stability of the catalyst layer caused by conventional high-temperature treatment. In this invention, cobalt source, molybdenum source, 2-methylimidazole, and phytic acid participate in coordination, nucleation, and crosslinking in the same precursor reaction solution. The molybdenum species and phytic acid are introduced during the formation of the cobalt-containing coordination structure, rather than being externally modified after the formation of the independent cobalt-based imidazole framework material. Therefore, the resulting precursor is a multi-component coordinated crosslinked precursor containing Co, Mo, N, and P, which can be crystalline, low-crystallinity, amorphous, or a combination of the aforementioned states. Whether it has a complete ZIF-67 crystal structure is not a necessary limitation of this invention. Another objective of this invention is to establish the structure-activity relationship between Joule heating parameters, molybdenum carbide phase formation, cobalt-containing phase dispersion, interface structure, and bidirectional electrocatalytic performance by controlling the peak temperature, duration, number of treatments, and cooling interval of pulsed Joule heating. The multiple pulsed Joule heating is not merely used to replace conventional tubular furnace heating, but rather to regulate the staged transformation of precursors. The preceding pulse promotes desolvation, ligand pyrolysis, and initial carbonization, while the subsequent pulse further promotes the conversion of molybdenum-containing species to Mo2C and the formation of a Co-Mo2C composite interface. The cooling interval between adjacent pulses is used to release thermal stress, limit particle migration, and reduce catalyst layer shrinkage and cracking. Therefore, the pulse peak temperature, single-pulse duration, number of treatments, and cooling interval collectively determine the degree of Mo2C formation, the dispersion state of Co-containing species, the retention of nitrogen- and phosphorus-containing components, and the number of composite interfaces. This invention also provides an electro-hydrogen coupled flow battery system using quinoxaline / 1,2,3,4-tetrahydroquinoxaline as the hydrogen storage active material and Co-Mo2C / NF as the bifunctional electrode.

[0008] To achieve the above objectives, the technical solution provided by the present invention is as follows: In a first aspect of the present invention, a method for preparing a binderless Co-Mo2C / NF self-supporting electrode for long-term energy storage using pulsed Joule heating is provided, comprising the following steps: (1) Surface cleaning of nickel foam is performed to obtain pretreated nickel foam; (2) Add cobalt source, molybdenum source and 2-methylimidazole to a mixed solvent of methanol and water, then add phytic acid and mix to obtain a precursor reaction solution containing cobalt precursor, molybdenum precursor, 2-methylimidazole and phytic acid; (3) The pretreated nickel foam is placed in the precursor reaction solution and subjected to a solvothermal reaction under closed conditions, so that molybdenum species and phytic acid participate in the construction of the precursor during the nucleation and growth of the cobalt-containing coordination structure, and a supported coordination crosslinking precursor containing Co, Mo, N and P is formed in situ on the surface of the nickel foam to obtain a supported precursor electrode. (4) The loaded precursor electrode is washed and dried; (5) The supported precursor electrode is subjected to 2 to 5 pulse Joule heat treatments in an inert atmosphere, with a cooling interval between two adjacent pulse Joule heat treatments, so that the precursor undergoes ligand pyrolysis, carbonization and carbothermic conversion of molybdenum species to form a composite active layer containing Co phase, Mo2C phase and nitrogen and phosphorus carbonaceous components, thus obtaining a binder-free Co-Mo2C / NF self-supporting electrode.

[0009] Preferably, in step (2), the water-soluble organic solvent is selected from at least one of monohydric alcohols having 1 to 4 carbon atoms; the cobalt source is selected from at least one of cobalt nitrate, cobalt chloride, cobalt acetate, cobalt sulfate and their hydrates; and the molybdenum source is selected from at least one of ammonium molybdate, ammonium heptamolybdate, sodium molybdate, phosphomolybdic acid and their hydrates.

[0010] Preferably, in step (2), the ratio of the cobalt source and the molybdenum source is 25:1 to 1:1, based on the molar ratio of cobalt to molybdenum; more preferably, it is 20:1 to 10:1.

[0011] Preferably, in step (2), the molar ratio of 2-methylimidazole to the total amount of cobalt and molybdenum in the cobalt and molybdenum sources is 5:1 to 30:1, and the molar ratio of phytic acid to the total amount of cobalt and molybdenum in the cobalt and molybdenum sources is 0.01:1 to 1:1.

[0012] Preferably, in step (2), the volume ratio of water-soluble organic solvent to water is 1:10 to 10:1, and more preferably 1:2 to 5:1.

[0013] Specifically, those skilled in the art can select a suitable water-soluble organic solvent according to actual conditions, such as at least one of methanol, ethanol, n-propanol, and isopropanol. The volume ratio of the water-soluble organic solvent to water is 1:10 to 10:1, more preferably 1:2 to 5:1. Under the most preferred conditions of this invention, methanol is used as the water-soluble organic solvent, and the volume ratio of methanol to water is 3:1.

[0014] Preferably, in step (3), the solvothermal reaction temperature is 80~160 ℃ and the reaction time is 2~24 h.

[0015] Preferably, in step (5), the peak temperature of the pulse Joule heat treatment is 800~1100 ℃, the single treatment time is 0.5~10 s, and the interval between two adjacent pulse Joule heat cooling is 5~60 s.

[0016] In this step, those skilled in the art can select a suitable inert gas to create an inert atmosphere based on actual conditions, such as at least one of nitrogen, argon, or helium.

[0017] In a second aspect of the invention, a binder-free Co-Mo2C / NF self-supporting electrode is provided, manufactured using the method of the first aspect of the invention, comprising: Nickel foam substrate and composite active layer in situ loaded on the surface of nickel foam substrate; The composite active layer includes a cobalt-containing active phase, a molybdenum carbide phase, and a nitrogen- and phosphorus-containing carbonaceous component formed by the thermal conversion of 2-methylimidazole and phytic acid. The cobalt-containing active phase and the molybdenum carbide phase are dispersed in or in contact with the nitrogen- and phosphorus-containing carbonaceous components. The composite active layer is loaded on the surface of the nickel foam substrate and does not contain polymer binders.

[0018] In a third aspect of the invention, the application of the binderless Co-Mo2C / NF self-supporting electrode of the second aspect of the invention is provided, including: for electrochemical hydrogenation of quinoxaline, electrochemical dehydrogenation of 1,2,3,4-tetrahydroquinoxaline, and reversible electrochemical hydrogen storage and release of quinoxaline-based organic hydrogen storage carriers.

[0019] Preferably, the application method of the quinoxaline electrochemical hydrogenation includes the following steps: The binder-free Co-Mo2C / NF self-supporting electrode is used as the working electrode and is brought into contact with an electrolyte containing quinoxaline, water and alkaline supporting electrolyte. A reduction potential or reduction current is applied to the working electrode to convert quinoxaline into 1,2,3,4-tetrahydroquinoxaline.

[0020] More preferably, the alkaline supporting electrolyte is potassium hydroxide or sodium hydroxide, the concentration of the alkaline electrolyte is 0.1~3 M, the concentration of quinoxaline is 0.1~20 mM, the reaction temperature is 5~60 ℃, and the working electrode potential is -0.4~0 V.

[0021] Preferably, the application method of the electrochemical dehydrogenation of 1,2,3,4-tetrahydroquinoxaline includes the following steps: The binder-free Co-Mo2C / NF self-supporting electrode is used as the working electrode and is brought into contact with an electrolyte containing 1,2,3,4-tetrahydroquinoxaline and an alkaline supporting electrolyte. An oxidation potential or oxidation current is applied to the working electrode to convert 1,2,3,4-tetrahydroquinoxaline into quinoxaline.

[0022] More preferably, the alkaline supporting electrolyte is potassium hydroxide or sodium hydroxide, the concentration of the alkaline supporting electrolyte is 0.1~3 M, the concentration of 1,2,3,4-tetrahydroquinoxaline is 0.1~20 mM, the reaction temperature is 5~60 ℃, and the working electrode potential is 1.0~1.9 V.

[0023] Preferably, the application method of the quinoxaline-based organic hydrogen storage carrier in reversible electrochemical hydrogen storage and release includes the following steps: Using the binder-free Co-Mo2C / NF self-supporting electrode as the cathode, quinoxaline undergoes electrochemical hydrogenation to obtain 1,2,3,4-tetrahydroquinoxaline. Using the binder-free Co-Mo2C / NF self-supporting electrode as the anode, 1,2,3,4-tetrahydroquinoxaline undergoes electrochemical dehydrogenation to regenerate quinoxaline; The electrochemical hydrogenation and electrochemical dehydrogenation are performed sequentially using the same binderless Co-Mo2C / NF self-supporting electrode, or separately using two binderless Co-Mo2C / NF self-supporting electrodes with the same composition.

[0024] In a fourth aspect of the invention, a quinoxaline-based electro-hydrogen coupled flow battery system employing a binderless Co-Mo2C / NF self-supporting electrode according to the second aspect of the invention is provided, comprising: The first electrolyte storage tank is used to store a first electrolyte containing quinoxaline, 1,2,3,4-tetrahydroquinoxaline, or a mixture of both. The first circulation pump and the first circulation pipeline are used to drive the first electrolyte to circulate between the first electrolyte storage tank and the flow battery reactor. The flow battery reactor includes a first electrode chamber, a second electrode chamber, and an ion-conducting membrane disposed between the two. A binderless Co-Mo2C / NF self-supporting electrode is disposed within the first electrode cavity; A binderless Co-Mo2C / NF self-supporting electrode is disposed in the second electrode cavity; The second electrolyte storage tank, the second circulation pump, and the second circulation pipeline are used to circulate the second electrolyte containing the reversible redox couple between the second electrolyte storage tank and the second electrode cavity. And a charge / discharge control unit electrically connected to the binderless Co-Mo2C / NF self-supporting electrode and the counter electrode.

[0025] Based on the above system structure, those skilled in the art can select an ion-conducting membrane according to requirements or actual conditions, such as anion exchange membrane, cation exchange membrane, bipolar membrane or porous ion-conducting membrane; similarly, the reversible redox couple in the second electrolyte can be selected from at least one of ferrocyanide / ferricyanide redox couple, ferric ion redox couple, quinone / hydroquinone redox couple, soluble polyoxometalate redox couple or solid redox energy storage electrode.

[0026] Preferably, the quinoxaline-based electro-hydrogen coupled flow battery system has an energy storage operation mode and an energy release operation mode: In the energy storage operation mode, the binder-free Co-Mo2C / NF self-supporting electrode serves as the cathode, causing the quinoxaline in the first electrolyte to be electrochemically hydrogenated to 1,2,3,4-tetrahydroquinoxaline. In the energy release operation mode, the binder-free Co-Mo2C / NF self-supporting electrode serves as the anode, causing the 1,2,3,4-tetrahydroquinoxaline in the first electrolyte to be electrochemically dehydrogenated to quinoxaline.

[0027] Preferably, at least one of the following is provided between the first electrolyte storage tank and the flow battery reactor: a temperature regulation unit, a flow regulation unit, an inert gas protection unit, and a sampling and detection unit.

[0028] Based on the above technical solutions, the design concept and principle of this invention are as follows: This invention designs a coordination precursor containing cobalt, molybdenum, nitrogen and phosphorus to be pre-constructed on the surface of nickel foam, and then promotes rapid carbonization and phase transformation through multi-pulse Joule heating with cooling intervals to obtain a binder-free Co-Mo2C / NF electrode that can be used for both cathode hydrogenation and anode dehydrogenation. Furthermore, it constructs a quinoxaline-based electro-hydrogen coupled flow battery system.

[0029] Specifically, this invention employs 2-methylimidazole and phytic acid to construct a cobalt-molybdenum (Co-Mo) coordination precursor. 2-methylimidazole participates in the coordination of metal species and provides carbon- and nitrogen-containing components during subsequent heat treatment; phytic acid participates in the coordination, cross-linking, and confinement of cobalt and molybdenum species through multiple phosphorus-containing functional groups. The coordination precursor is grown in situ on the surface of nickel foam, followed by multiple pulsed Joule heat treatments, allowing the sample to sequentially undergo rapid heating, short-term high-temperature holding, and cooling. The cooling process between adjacent pulses reduces particle migration caused by the accumulation of sustained high temperatures, while subsequent pulses continue to promote the carbonization of incompletely converted molybdenum species.

[0030] Under the above-mentioned optimized parameter design, it is helpful to further optimize the process and product properties while achieving the basic objectives of this invention. When the number of pulses is small or the peak temperature is low, the molybdenum-containing precursor may not be fully carbonized, the degree of molybdenum carbide phase formation is insufficient, and the electrode conductivity and water activation ability are limited. As the number of pulses increases and the peak temperature rises, the molybdenum carbide phase gradually forms, the cobalt-molybdenum carbide (Co-Mo2C) contact interface increases, and the charge transfer resistance decreases. When the peak temperature is too high, the single treatment time is too long, or the number of pulses is too many, it may lead to the growth of cobalt-containing components and molybdenum carbide particles, loss of nitrogen-containing or phosphorus-containing components, and local cracking of the catalyst layer, thereby reducing the number of effective interfaces and the Faraday efficiency of organic matter conversion. Therefore, the Joule heating parameters need to achieve a balance between phase transformation and interface retention. The preferred pulse Joule heating conditions are a peak temperature of 800~1100 ℃, a single treatment time of 0.5~10 s, and a cooling interval of 5~60 s. Under the preferred conditions, the formation of the molybdenum carbide phase, the dispersion of cobalt-containing components, and the integrity of the catalyst layer structure can be further considered. The resulting binder-free Co-Mo2C / NF can be used for quinoxaline cathode hydrogenation, 1,2,3,4-tetrahydroquinoxaline anode dehydrogenation, and electro-hydrogen coupled liquid flow energy storage processes consisting of the two reactions.

[0031] In application, the same Co-Mo2C / NF electrode can be used for both cathode hydrogenation and anodic dehydrogenation, reducing the number of electrode types required by the system; the first electrolyte is stored in an external tank and circulated through the reactor, which can expand the system's chemical energy storage capacity by increasing the amount of electrolyte stored; as presented in one or more embodiments of the present invention, under preferred conditions, the quinoxaline hydrogenation conversion rate and tetrahydroquinoxaline selectivity both reach 100%, and the Faraday efficiency reaches 89%; the tetrahydroquinoxaline dehydrogenation conversion rate reaches 99%, the quinoxaline selectivity reaches 100%, and the Faraday efficiency reaches 80%.

[0032] Compared with the prior art, the present invention has the following advantages and beneficial effects: This invention provides a method for preparing binder-free Co-Mo2C / NF self-supporting electrodes using pulsed Joule heating for long-term energy storage. This method uses non-precious metal components such as cobalt and molybdenum, reducing dependence on precious metals. The active precursor grows directly on the surface of nickel foam without polymer binders, reducing the impact of binders on the obscuring of active sites and the increase in interfacial resistance. This invention enables spatial coupling of Co species, Mo species, 2-methylimidazole, and phytic acid during the precursor nucleation stage, which is beneficial for shortening the diffusion distance of substances in subsequent carbothermic conversion and promoting the in-situ interface construction of the Mo2C phase and the Co-containing phase on the surface of nickel foam. Furthermore, the multi-pulse Joule heating used in this process does not simply shorten the heat treatment time, but rather uses a "short-time heating-intermittent cooling-reheating" method to gradually complete the ligand pyrolysis, carbonization, and Mo2C phase formation of the precursor, while maintaining the integrity of the Co-Mo2C composite interface and catalyst layer while restricting particle sintering.

[0033] This invention provides a binder-free Co-Mo2C / NF self-supporting electrode, its application, and a corresponding quinoxaline-based electro-hydrogen coupled flow battery system. The electrode of this invention features a uniform active phase distribution, abundant effective interfaces, high catalyst layer adhesion stability, and excellent bidirectional electrocatalytic performance. It can flexibly switch between energy storage and energy release modes, and has broad application prospects. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the synthesis process of the binder-free Co-Mo2C / NF self-supporting electrode in Example 1; Figure 2 Linear sweep voltammetry (LSV) curves for hydrogenation in blank potassium hydroxide (KOH) and potassium hydroxide with quinoxaline in Example 5; Figure 3 The linear sweep voltammetry curves for dehydrogenation in blank potassium hydroxide (KOH) and potassium hydroxide in quinoxaline are shown in Example 6. Figure 4 The selectivity and Faraday efficiency (left figure) and conversion (right figure) of Example 5 at different hydrogenation potentials; Figure 5 The selectivity and Faraday efficiency (left figure) and conversion (right figure) at different dehydrogenation potentials in Example 6. Figure 6 The results are the hydrotreating-dehydrogenation cycle performance test results of Example 7; Figure 7 This is a schematic diagram of the quinoxaline-based electro-hydrogen coupled flow battery system in Example 8. Detailed Implementation

[0035] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0036] Example 1 This embodiment provides a method for preparing binder-free Co-Mo2C / NF self-supporting electrodes using pulsed Joule heating for long-term energy storage. A schematic diagram of the synthesis is shown below. Figure 1 As shown, the steps are as follows: (1) Cut the nickel foam into 1 cm × 1 cm sheet materials, and place them in anhydrous ethanol, 1~3 mol / L hydrochloric acid, deionized water and anhydrous ethanol in sequence for ultrasonic cleaning for 10~30 min each time to remove surface oil and oxide layer. Then dry them at 50~80℃ for later use to complete surface purification and obtain pretreated nickel foam. (2) Weigh 1 mmol cobalt nitrate hexahydrate, 0.06 mmol ammonium molybdate tetrahydrate and 20 mmol 2-methylimidazole, add them to a mixed solvent consisting of 30 mL methanol and 10 mL deionized water, and stir magnetically for 20-60 min; then add 1 mL 1.4 mol / L phytic acid solution and continue stirring for 20-60 min to obtain a homogeneous precursor reaction solution; (3) The pretreated nickel foam is immersed in the precursor reaction solution and transferred together to a polytetrafluoroethylene-lined reactor for a solvothermal reaction at 120 °C for 6 h. After the reaction is completed, it is naturally cooled to room temperature, and the nickel foam loaded with the precursor is taken out, which is the supported precursor electrode. (4) The sample was washed three times with methanol and deionized water respectively, and dried at 60 °C for 12 h to obtain the Co-Mo precursor / NF; (5) Place the Co-Mo precursor / NF in a Joule heating device and perform the first Joule heat treatment under nitrogen or argon protection to rapidly raise the sample temperature to 900 °C and hold it for 3 s; stop the power supply and cool for 10 s before performing the second treatment, and then cool for another 10 s before performing the third treatment; after the treatment is completed, cool to room temperature in a protective atmosphere to obtain the Co-Mo2C / NF bifunctional electrocatalyst, i.e., the binder-free Co-Mo2C / NF self-supporting electrode.

[0037] Example 2 This embodiment provides a method for preparing binderless Co-Mo2C / NF self-supporting electrodes for long-term energy storage using pulsed Joule heating. Except for adjusting the molar ratio of cobalt to molybdenum in step (2) to 20:1, 15:1 and 10:1 respectively, the other conditions are the same as in Example 1. The resulting binderless Co-Mo2C / NF self-supporting electrode samples are denoted as CoMo-20, CoMo-15 and CoMo-10 respectively.

[0038] Example 3 This embodiment provides a method for preparing binder-free Co-Mo2C / NF self-supporting electrodes using pulsed Joule heating for long-term energy storage. Except for the solvothermal temperatures in step (3) being set to 100 ℃, 120 ℃, and 140 ℃ respectively, the other conditions are the same as in Example 1. The obtained precursors are all subjected to pulsed Joule heating at 900 ℃ for 3 s and three times.

[0039] Example 4 This embodiment provides a method for preparing binder-free Co-Mo2C / NF self-supporting electrodes using pulsed Joule heating for long-term energy storage. Referring to the steps of Embodiment 1, the same batch of Co-Mo precursor / NF is subjected to one, two, and three pulsed Joule heating treatments, respectively. Each treatment has a peak temperature of 900 °C, a treatment time of 3 s, and a cooling interval of 10 s. The same precursor is also subjected to three Joule heating treatments at 800 °C, 900 °C, and 1000 °C, respectively, with each treatment lasting 3 s.

[0040] In this embodiment, it was observed that when the number of treatments increased from one to three, the degree of carbonization of the molybdenum-containing precursor improved, the formation of the molybdenum carbide phase was more complete, and the charge transfer resistance decreased. The sample treated with three pulses achieved a better balance between phase formation and catalyst layer integrity. When the peak temperature was lower than the preferred temperature, the conversion of molybdenum-containing species was insufficient; when the peak temperature was too high, particle agglomeration and loss of nitrogen- and phosphorus-containing components were aggravated. 900 ℃, 3 s, three pulses, and a 10 s cooling interval are the preferred parameters in this embodiment.

[0041] Example 5 This embodiment provides an electrochemical hydrogenation method for quinoxaline, employing a three-electrode system. The binder-free Co-Mo2C / NF self-supporting electrode obtained in Example 1 is used as the working electrode, a Pt sheet as the counter electrode, and mercury / mercury oxide (Hg / HgO) as the reference electrode. The working area is 1 cm². 2The electrolyte was 10 mL of a solution containing 1 mol / L KOH and 0.002 mol / L quinoxaline. The LSV curves of the blank electrolyte and the quinoxaline-containing electrolyte were tested at 25 °C and 10 mV / s. Electrolysis was performed at a constant potential of -0.2 V for 6 h. No molecular hydrogen was introduced into the reaction chamber containing the working electrode during the reaction. Quantification was performed by High Performance Liquid Chromatography (HPLC), and qualitative analysis was performed by Gas Chromatography-Mass Spectrometry (GC-MS) or Nuclear Magnetic Resonance (NMR). The quinoxaline conversion reached 100%, the tetrahydroquinoxaline selectivity reached 100%, and the Faraday efficiency reached 89%.

[0042] Example 6 This embodiment provides an electrochemical dehydrogenation method for 1,2,3,4-tetrahydroquinoxaline, using the binder-free Co-Mo2C / NF self-supporting electrode obtained in Example 1 as the working electrode, a Pt sheet as the counter electrode, and Hg / HgO as the reference electrode. The electrolyte contains 1 M KOH and 0.002 M tetrahydroquinoxaline. The LSV curves of the blank electrolyte and the electrolyte containing tetrahydroquinoxaline were tested at 25 °C, and electrolysis was performed at a constant potential of 1.45 V for 3 h. The tetrahydroquinoxaline conversion reached 99%, the quinoxaline selectivity reached 100%, and the Faraday efficiency reached 80%.

[0043] Example 7 This embodiment provides a reversible hydrogenation-dehydrogenation cycle method. After hydrogenation of quinoxaline according to Example 5, the binder-free Co-Mo2C / NF self-supporting electrode is cleaned, and then dehydrogenation of tetrahydroquinoxaline is performed according to Example 6. One hydrogenation and one dehydrogenation are defined as one cycle, which is performed three times. After the third cycle, the hydrogenation conversion rate is 95% of that of the first cycle, with a selectivity of 94%; the dehydrogenation conversion rate is 82% of that of the first cycle, with a selectivity of 94%.

[0044] Example 8 This embodiment provides a quinoxaline-based electro-hydrogen coupled flow battery system. The flow battery system includes a first electrolyte storage tank, a first circulation pump, a first circulation pipeline, a flow battery reactor, an ion conduction membrane, a second electrolyte storage tank, a second circulation pump, a second circulation pipeline, and a charge / discharge control unit.

[0045] The first electrode chamber of the flow battery reactor is equipped with a self-supporting Co-Mo2C / NF electrode without binder prepared in Example 1, and the second electrode chamber is equipped with a counter electrode. The two electrode chambers are separated by an ion-conducting membrane. A first electrolyte containing 1 M KOH and a quinoxaline / tetrahydroquinoxaline hydrogen storage pair is added to a first electrolyte tank. The first electrolyte is circulated between the first tank and the first electrode chamber by a circulation pump. A second electrolyte containing a reversible redox couple is added to a second electrolyte tank and circulated between the second tank and the second electrode chamber by a second circulation pump. In energy storage mode, Co-Mo2C / NF is set as the cathode, and quinoxaline is converted to tetrahydroquinoxaline by an external power source, so that the input electrical energy is stored in the first tank in the form of organic chemical hydrogen. In energy release mode, the self-supporting Co-Mo2C / NF electrode without binder is set as the anode, so that tetrahydroquinoxaline is converted back to quinoxaline. Electrons are transferred to the counter electrode side through an external circuit, and the corresponding reverse conversion is completed by the reversible redox couple in the second electrolyte. The first electrolyte circulation flow rate is 5 mL / min, and the effective electrode area is 1 cm². 2 The initial electrolyte volume was 100 mL. Record the system's charging capacity, discharging capacity, coulombic efficiency, energy efficiency, and cycle stability.

[0046] This embodiment demonstrates that the binder-free Co-Mo2C / NF self-supporting electrode can not only catalyze the bidirectional conversion of quinoxaline in a static three-electrode system, but also serve as a bifunctional electrode in a flowing electrolyte reactor, providing a foundation for constructing a quinoxaline-based electro-hydrogen coupled liquid flow energy storage system.

[0047] Testing and inspection: The binder-free Co-Mo2C / NF self-supporting electrode prepared in Example 1 was subjected to electrochemical hydrogenation and dehydrogenation performance tests on organic hydrogen storage carrier.

[0048] Linear sweep voltammetry was performed using a three-electrode system, with the sample as the working electrode, the electrode as the counter electrode, and a Pt sheet as the reference electrode. The electrolyte for the quinoxaline hydrogenation assay was a mixture of 1 M KOH and 0.002 M quinoxaline, with a test potential range of -0.1 V to -0.4 V and a scan rate of 10 mV / s relative to the reversible hydrogen electrode. The blank group used a KOH solution of the same concentration without quinoxaline.

[0049] The electrolyte for the 1,2,3,4-tetrahydroquinoxaline dehydrogenation test was a mixed solution of 1 M KOH and 0.002 M 1,2,3,4-tetrahydroquinoxaline, with a test potential range of 1 V to 1.9 V relative to the reversible hydrogen electrode; the blank group used a KOH solution of the same concentration without 1,2,3,4-tetrahydroquinoxaline.

[0050] Following the method described in Example 5, constant potential electrolysis was performed at different cathode potentials, and the conversion rate, selectivity, and faradaic efficiency of quinoxaline were determined. Following the method described in Example 6, constant potential electrolysis was performed at different anode potentials, and the conversion rate, selectivity, and faradaic efficiency of 1,2,3,4-tetrahydroquinoxaline were determined.

[0051] The reaction products were quantified by high-performance liquid chromatography (HPLC) and qualitatively analyzed by gas chromatography-mass spectrometry (GC-MS) or nuclear magnetic resonance (NMR). Standard solutions of quinoxaline and 1,2,3,4-tetrahydroquinoxaline were prepared separately, and standard curves were established between concentration and chromatographic peak area.

[0052] The conversion rate of quinoxaline is calculated according to the following formula: Quinoxaline conversion rate = (initial amount of quinoxaline - amount of quinoxaline after reaction) / initial amount of quinoxaline × 100%.

[0053] The selectivity of 1,2,3,4-tetrahydroquinoxaline is calculated according to the following formula: Selectivity = Amount of 1,2,3,4-tetrahydroquinoxaline produced / Amount of quinoxaline converted × 100%.

[0054] The Faraday efficiency for the hydrogenation of quinoxaline to 1,2,3,4-tetrahydroquinoxaline is calculated according to the following formula: FE = 4Fn / Q × 100%.

[0055] Where F is the Faraday constant, n is the amount of 1,2,3,4-tetrahydroquinoxaline produced, and Q is the total amount of electricity passing through the working electrode during electrolysis.

[0056] The dehydrogenation conversion, selectivity, and dehydrogenation Faraday efficiency of 1,2,3,4-tetrahydroquinoxaline were calculated using the corresponding material balance and electron transfer number.

[0057] The following combination Figures 2-6 The above embodiments will be described in detail based on the provided experimental results: like Figure 2 As shown, compared with the blank KOH electrolyte, the binder-free Co-Mo2C / NF self-supporting electrode prepared in Example 1 showed a significantly increased reduction current in the cathode potential range after the addition of quinoxaline, indicating that the catalyst can promote the electrochemical hydrogenation of quinoxaline.

[0058] like Figure 3 As shown, after the addition of 1,2,3,4-tetrahydroquinoxaline, the oxidation current of the binder-free Co-Mo2C / NF self-supporting electrode in the anodic potential range was significantly higher than that of the blank KOH electrolyte, indicating that the catalyst can promote the electrochemical dehydrogenation of 1,2,3,4-tetrahydroquinoxaline.

[0059] like Figure 4 As shown, when quinoxaline was hydrogenated at a constant potential under the conditions of Example 5, the conversion rate of quinoxaline gradually increased as the cathode potential shifted negatively; however, when the potential was too negative, the hydrogen evolution side reaction was enhanced, leading to a decrease in the Faraday efficiency. At the preferred potential of -0.2 V relative to the reversible hydrogen electrode, the conversion rate of quinoxaline reached 100%, the selectivity of 1,2,3,4-tetrahydroquinoxaline reached 100%, and the Faraday efficiency reached 89%.

[0060] like Figure 5 As shown, during the electrochemical dehydrogenation of 1,2,3,4-tetrahydroquinoxaline under the conditions of Example 6, the substrate conversion and quinoxaline formation gradually increased with increasing anode potential; however, as the potential increased further, competition for oxygen evolution intensified, and the Faraday efficiency decreased. At the preferred potential of 1.45 V relative to the reversible hydrogen electrode, the conversion of 1,2,3,4-tetrahydroquinoxaline was 99%, the quinoxaline selectivity was 100%, and the Faraday efficiency was 80%.

[0061] A hydrogenation-dehydrogenation cycle test was conducted according to Example 7, such as... Figure 6 As shown, after three cycles, the hydrogenation conversion rate of the catalyst remained at 95% of the initial value, with a selectivity of 94%, while the dehydrogenation conversion rate remained at 82% of the initial value, with a selectivity of 94%. This indicates that Co-Mo2C / NF can switch between cathode hydrogenation and anodic dehydrogenation conditions and maintain a certain degree of bifunctional catalytic stability.

[0062] like Figure 7 As shown in Example 8, the electrode can be configured in the flow battery reactor, so that the quinoxaline first electrolyte circulates between the external storage tank and the electrode chamber, and can complete the switching between energy storage mode and energy release mode.

[0063] The above results demonstrate that the process of this invention solves the problems of active phase agglomeration, reduced effective interface, and insufficient catalyst layer adhesion stability caused by conventional high-temperature treatment. The binder-free Co-Mo2C / NF self-supporting electrode prepared by this invention can be used for the electrochemical hydrogenation of quinoxaline and the electrochemical dehydrogenation of 1,2,3,4-tetrahydroquinoxaline, and has the potential for application in reversible organic hydrogen storage electro-hydrogen coupled flow battery systems.

[0064] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for preparing binder-free Co-Mo2C / NF self-supporting electrodes using pulsed Joule heating for long-term energy storage, characterized in that, Includes the following steps: (1) Surface cleaning of nickel foam is performed to obtain pretreated nickel foam; (2) Add cobalt source, molybdenum source and 2-methylimidazole to a mixed solvent consisting of water and at least one water-soluble organic solvent, then add phytic acid and mix, so that cobalt species, molybdenum species, 2-methylimidazole and phytic acid coordinate and crosslink in the same reaction system to obtain precursor reaction solution; (3) The pretreated nickel foam is placed in the precursor reaction solution and subjected to a solvothermal reaction under closed conditions, so that molybdenum species and phytic acid participate in the construction of the precursor during the nucleation and growth of the cobalt-containing coordination structure, and a supported coordination crosslinking precursor containing Co, Mo, N and P is formed in situ on the surface of the nickel foam to obtain a supported precursor electrode. (4) The loaded precursor electrode is washed and dried; (5) The supported precursor electrode is subjected to 2 to 5 pulse Joule heat treatments in an inert atmosphere, with a cooling interval between two adjacent pulse Joule heat treatments, so that the precursor undergoes ligand pyrolysis, carbonization and carbothermic conversion of molybdenum species to form a composite active layer containing Co phase, Mo2C phase and nitrogen and phosphorus carbonaceous components, thus obtaining a binder-free Co-Mo2C / NF self-supporting electrode.

2. The method for preparing binder-free Co-Mo2C / NF self-supporting electrodes for long-term energy storage using pulsed Joule heating according to claim 1, characterized in that: In step (2), the water-soluble organic solvent is selected from at least one monohydric alcohol having 1 to 4 carbon atoms; the cobalt source is selected from at least one of cobalt nitrate, cobalt chloride, cobalt acetate, cobalt sulfate and their hydrates; the molybdenum source is selected from at least one of ammonium molybdate, ammonium heptamolybdate, sodium molybdate, phosphomolybdic acid and their hydrates; the ratio of the cobalt source and the molybdenum source is 25:1 to 1:1, based on the molar ratio of cobalt to molybdenum.

3. The method for preparing binder-free Co-Mo2C / NF self-supporting electrodes for long-term energy storage using pulsed Joule heating according to claim 1, characterized in that: The molar ratio of 2-methylimidazole to the total amount of cobalt and molybdenum in the cobalt and molybdenum sources is 5:1 to 30:1; the molar ratio of phytic acid to the total amount of cobalt and molybdenum in the cobalt and molybdenum sources is 0.01:1 to 1:1; and the volume ratio of the water-soluble organic solvent to water is 1:10 to 10:

1.

4. The method for preparing binder-free Co-Mo2C / NF self-supporting electrodes for long-term energy storage using pulsed Joule heating according to claim 1, characterized in that: In step (3), the solvothermal reaction temperature is 80~160 ℃ and the reaction time is 2~24h; in step (5), the peak temperature of the pulse Joule heat treatment is 800~1100 ℃, the single treatment time is 0.5~10 s, and the interval between two adjacent pulse Joule heat cooling is 5~60 s.

5. A binder-free Co-Mo2C / NF self-supporting electrode, characterized in that, Made by the method described in any one of claims 1 to 4, comprising: Nickel foam substrate and composite active layer in situ loaded on the surface of nickel foam substrate; The composite active layer includes a cobalt-containing active phase, a molybdenum carbide phase, and a nitrogen- and phosphorus-containing carbonaceous component formed by the thermal conversion of 2-methylimidazole and phytic acid. The cobalt-containing active phase and the molybdenum carbide phase are dispersed in or in contact with the nitrogen- and phosphorus-containing carbonaceous components. The composite active layer is loaded on the surface of the nickel foam substrate and does not contain polymer binders.

6. An application of the binder-free Co-Mo2C / NF self-supporting electrode as described in claim 5, characterized in that, include: Applications include electrochemical hydrogenation of quinoxaline, electrochemical dehydrogenation of 1,2,3,4-tetrahydroquinoxaline, and reversible electrochemical hydrogen storage and release of quinoxaline-based organic hydrogen storage carriers.

7. The application of the binder-free Co-Mo2C / NF self-supporting electrode according to claim 6, characterized in that, The application method of the electrochemical hydrogenation of quinoxaline includes the following steps: The binder-free Co-Mo2C / NF self-supporting electrode is used as the working electrode and is brought into contact with an electrolyte containing quinoxaline, water and alkaline supporting electrolyte. A reduction potential or reduction current is applied to the working electrode to convert quinoxaline into 1,2,3,4-tetrahydroquinoxaline; The application method of the electrochemical dehydrogenation of 1,2,3,4-tetrahydroquinoxaline includes the following steps: The binder-free Co-Mo2C / NF self-supporting electrode is used as the working electrode and is brought into contact with an electrolyte containing 1,2,3,4-tetrahydroquinoxaline and an alkaline supporting electrolyte. An oxidation potential or oxidation current is applied to the working electrode to convert 1,2,3,4-tetrahydroquinoxaline into quinoxaline. The application method of the quinoxaline-based organic hydrogen storage carrier in reversible electrochemical hydrogen storage and release includes the following steps: Using the binder-free Co-Mo2C / NF self-supporting electrode as the cathode, quinoxaline undergoes electrochemical hydrogenation to obtain 1,2,3,4-tetrahydroquinoxaline. Using the binder-free Co-Mo2C / NF self-supporting electrode as the anode, 1,2,3,4-tetrahydroquinoxaline undergoes electrochemical dehydrogenation to regenerate quinoxaline; The electrochemical hydrogenation and electrochemical dehydrogenation are performed sequentially using the same binderless Co-Mo2C / NF self-supporting electrode, or separately using two binderless Co-Mo2C / NF self-supporting electrodes with the same composition.

8. The application of the binder-free Co-Mo2C / NF self-supporting electrode according to claim 7, characterized in that: In the electrochemical hydrogenation method of quinoxaline, the alkaline supporting electrolyte is potassium hydroxide or sodium hydroxide, the concentration of the alkaline electrolyte is 0.1~3 M, the concentration of quinoxaline is 0.1~20 mM, the reaction temperature is 5~60 ℃, and the working electrode potential is -0.4~0 V; in the electrochemical dehydrogenation method of 1,2,3,4-tetrahydroquinoxaline, the alkaline supporting electrolyte is potassium hydroxide or sodium hydroxide, the concentration of the alkaline supporting electrolyte is 0.1~3 M, the concentration of 1,2,3,4-tetrahydroquinoxaline is 0.1~20 mM, the reaction temperature is 5~60 ℃, and the working electrode potential is 1.0~1.9 V.

9. A quinoxaline-based electro-hydrogen coupled flow battery system, characterized in that, The binderless Co-Mo2C / NF self-supporting electrode as described in claim 5 comprises: The first electrolyte storage tank is used to store a first electrolyte containing quinoxaline, 1,2,3,4-tetrahydroquinoxaline, or a mixture of the two. The first circulation pump and the first circulation pipeline are used to drive the first electrolyte to circulate between the first electrolyte storage tank and the flow battery reactor. The flow battery reactor includes a first electrode chamber, a second electrode chamber, and an ion-conducting membrane disposed between the two. A binderless Co-Mo2C / NF self-supporting electrode is disposed within the first electrode cavity; A binderless Co-Mo2C / NF self-supporting electrode is disposed in the second electrode cavity; The second electrolyte storage tank, the second circulation pump, and the second circulation pipeline are used to circulate the second electrolyte containing the reversible redox couple between the second electrolyte storage tank and the second electrode cavity. And a charge / discharge control unit electrically connected to the binderless Co-Mo2C / NF self-supporting electrode and the counter electrode.

10. The quinoxaline-based electro-hydrogen coupled flow battery system according to claim 9, characterized in that: The quinoxaline-based electro-hydrogen coupled flow battery system has an energy storage operation mode and an energy release operation mode: In the energy storage operation mode, the binder-free Co-Mo2C / NF self-supporting electrode serves as the cathode, causing the quinoxaline in the first electrolyte to be electrochemically hydrogenated to 1,2,3,4-tetrahydroquinoxaline. In the energy release operation mode, the binder-free Co-Mo2C / NF self-supporting electrode serves as the anode, causing the 1,2,3,4-tetrahydroquinoxaline in the first electrolyte to be electrochemically dehydrogenated to quinoxaline; The first electrolyte storage tank and the flow battery reactor are further provided with at least one of the following: a temperature regulation unit, a flow regulation unit, an inert gas protection unit, and a sampling and detection unit.

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