A non-noble metal doped cobaltosic oxide-based catalyst, a preparation method and application thereof

Abstract

The present application belongs to the technical field of oxygen evolution catalysts, and particularly relates to a non-noble metal doped cobalt trioxide-based catalyst, a preparation method and application. A soluble cobalt salt and a non-noble metal salt are used as raw materials, dissolved in a solvent to form a mixed salt solution; a conductive substrate is covered on the mixed salt solution, and calcination oxidation treatment is performed to obtain a conductive substrate loaded non-noble metal doped cobalt trioxide-based catalyst. The present application is based on a solution blending and pyrolysis calcination synergistic strategy, the microstructure of the catalyst is synergistically regulated through the electronic effect and geometric effect of the doped elements, the chemical composition and charge characteristics of the catalyst-electrolyte contact phase are changed, the interface water molecule arrangement and proton transport path are optimized, thereby the OER reaction microenvironment is reconstructed on the atomic scale, the uniform doping of non-noble metal ions in the cobalt trioxide lattice is realized, and the strong interface combination between the carbon paper and the active component and the three-dimensional porous conductive network are constructed, thereby the electronic transmission capacity and the electrochemical active area of the material are effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of oxygen evolution catalyst technology, specifically relating to a non-precious metal-doped cobalt tetroxide-based catalyst, its preparation method, and its application. Background Technology

[0002] Hydrogen energy, as a clean, renewable, and high-energy-density carrier, has seen its green production technology become a research focus. Among these technologies, proton exchange membrane (PEM) water electrolysis, driven by renewable energy, is considered one of the most promising hydrogen production routes due to its advantages such as high current density, fast response speed, and high hydrogen purity.

[0003] However, the PEM water electrolyzer, operating under highly acidic and high oxidation potential conditions, places extremely stringent demands on its core component—the oxygen evolution reaction (OER) catalyst. OER is a kinetically slow four-electron transfer process and the rate-determining step that limits the overall efficiency of water electrolysis. Currently, the only commercially available catalyst that can maintain both high activity and stability under acidic OER conditions is iridium-based oxides (such as IrO2). However, iridium is one of the rarest elements in the Earth's crust, and its extremely high cost and limited reserves severely hinder the large-scale commercial application of PEM technology.

[0004] Currently, non-precious metal elements (such as Fe, Ni, and Mn) are used as active components for precise doping to control the electronic and structural properties of spinel-structured Co3O4, aiming to achieve a focus on abundant and low-cost transition metal oxide systems. However, in acidic media, the stability of the electric double layer structure on the catalyst surface directly determines the proton transport efficiency and intermediate adsorption behavior at the reaction interface. Traditional non-precious metal catalysts often face challenges such as double layer reconstruction instability and disordered interfacial water molecule structure at high potentials, leading to passivation of active sites and structural collapse. Summary of the Invention

[0005] The purpose of this invention is to provide a non-precious metal doped cobalt tetroxide-based catalyst, its preparation method, and its application. The core innovation lies in the synergistic strategy of solution blending and pyrolysis-calcination: by synergistically regulating the microstructure of the catalyst through the electronic and geometric effects of the doping elements, the chemical composition and charge characteristics of the catalyst-electrolyte contact phase (i.e., the double layer, a microstructure with charge separation characteristics formed at the interface) are altered, optimizing the interfacial water molecule arrangement and proton transport pathway. This reconstructs the OER reaction microenvironment at the atomic scale, achieving uniform doping of non-precious metal ions in the cobalt tetroxide lattice. Simultaneously, a strong interfacial bond and a three-dimensional porous conductive network are constructed between the carbon paper and the active components, effectively enhancing the material's electron transport capability and electrochemical active area.

[0006] To achieve the above objectives, the specific technical solution provided by the present invention is as follows: The first objective of this invention is to provide a method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst, comprising the following steps: S1. Using soluble cobalt salt and soluble non-precious metal salt as raw materials, they are dissolved in a solvent and stirred to form a mixed salt solution.

[0007] S2. The conductive substrate is coated on a mixed salt solution and calcined and oxidized at 250℃~350℃ in an air atmosphere. The non-precious metal-induced electronic structure redistribution causes cobalt ions to be oxidized and reconstructed into Co. 2+ [Co 3+ The 2O4 spinel structure was selectively replaced with cobalt sites in the lattice to form an electric double layer structure, resulting in a non-noble metal-doped cobalt tetroxide-based catalyst supported on a conductive substrate.

[0008] Furthermore, the molar ratio of cobalt ions in soluble cobalt salts to non-precious metal ions in non-precious metal salts is 4:1 to 2.

[0009] Furthermore, in the mixed salt solution, the concentration of cobalt metal ions is 0.1 mM to 0.4 mM, and the concentration of non-precious metal ions is 0.1 mM to 0.4 mM.

[0010] Furthermore, the non-precious metal ions in the non-precious metal salt are molybdenum, lanthanum, manganese, chromium, iron, or copper.

[0011] Furthermore, during calcination, the heating rate is 1℃ / min to 3℃ / min, and the calcination time is 12h to 24h.

[0012] Furthermore, the loading of the non-precious metal-doped cobalt tetroxide-based catalyst on the conductive substrate was 1 mg / cm³. 2 ~4mg / cm 2 The conductive substrate is carbon paper, carbon cloth, or nickel foam.

[0013] Furthermore, the solvent is water, and the stirring speed is 600 rpm to 900 rpm.

[0014] The second objective of this invention is to provide a non-precious metal-doped cobalt tetroxide-based catalyst, which is prepared using the above-described preparation method.

[0015] Furthermore, the non-precious metal-doped cobalt tetroxide-based catalyst uses a conductive substrate as a support to load a non-precious metal-doped cobalt tetroxide matrix, with the amount of non-precious metal doping ranging from 5 wt.% to 33 wt.% of the mass of the cobalt tetroxide matrix.

[0016] The third objective of this invention is to provide the application of the above-mentioned non-precious metal-doped cobalt tetroxide-based catalyst in the electrocatalytic splitting of water.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention utilizes a triple mechanism of precursor atomic-level mixing, precise control of oxidation pathways, and optimization of lattice integration kinetics to synergistically construct a non-noble metal-doped Co3O4 catalyst with uniform doping, complete crystallization, and controllable surface defects. Its optimized electronic structure (such as d-band center modulation) and abundant active oxygen species provide a foundation for high activity and structural stability in the catalytic oxidation reaction. Through a synergistic strategy of solution blending and pyrolysis-calcination, the electronic and geometric effects of the dopant elements are used to synergistically control the electric double layer structure on the catalyst surface, optimizing the interfacial water molecule arrangement and proton transport pathways. This reconstructs the OER reaction microenvironment at the atomic scale. The non-noble metal doping utilizes doping-induced lattice strain and charge redistribution effects: on the one hand, it optimizes the d-band electronic structure of the cobalt active center, modulates the electric field distribution and water molecule orientation within the electric double layer, and accelerates the oxygen intermediate conversion kinetics; on the other hand, by stabilizing the interfacial water structure and electric double layer configuration, it enhances the structural robustness of the material in strongly acidic media, effectively suppressing cobalt ion dissolution and lattice oxygen loss, and inhibiting the performance degradation caused by the instability of the electric double layer in acidic OER environments of traditional cobalt-based oxides.

[0018] The non-precious metal-doped cobalt tetroxide-based catalyst provided by this invention reconstructs cobalt tetroxide through non-precious metal doping, altering the microenvironment of the cobalt tetroxide interface, inducing local lattice distortion and surface charge redistribution, and promoting the formation of an electric double layer structure with different ion adsorption / desorption activities at the interface. By modulating the surface electronic properties and local chemical environment, it reshapes the orientation arrangement of water molecules and the hydrogen bond network structure within the electric double layer, affecting the adsorption configuration of water at the interface and its potential-dependent dynamic behavior, optimizing the arrangement of water molecules at the interface and the proton transport path, thereby reconstructing the microenvironment of the OER reaction at the atomic scale. Attached Figure Description

[0019] Figure 1 The XRD patterns are those of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention. Figure 1 In the examples, a represents Comparative Example 1, b represents Example 1, c represents Example 2, d represents Example 3, and e represents Example 4.

[0020] Figure 2 The polarization curves are those of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention.

[0021] Figure 3The figures show the cyclic voltammetry diagrams and the fitted double-layer capacitance diagrams of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention. a and a1 are the cyclic voltammetry diagram and double-layer capacitance diagram of Comparative Example 1, respectively; b and b1 are the cyclic voltammetry diagram and double-layer capacitance diagram of Example 1, respectively; c and c1 are the cyclic voltammetry diagram and double-layer capacitance diagram of Example 2, respectively; d and d1 are the cyclic voltammetry diagram and double-layer capacitance diagram of Example 3, respectively; and e and e1 are the cyclic voltammetry diagram and double-layer capacitance diagram of Example 4, respectively.

[0022] Figure 4 The polarization curves of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention are normalized based on the electrochemical active area (ECSA).

[0023] Figure 5 This is a graph showing the change in apparent activation energy Ea / pre-exponential factor A as a function of potential for the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention. Figure 5 In the examples, a represents Comparative Example 1, b represents Example 1, c represents Example 2, d represents Example 3, and e represents Example 4.

[0024] Figure 6 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Comparative Example 1 of this invention as a function of potential.

[0025] Figure 7 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 1 of the present invention as a function of potential.

[0026] Figure 8 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 2 of the present invention as a function of potential.

[0027] Figure 9 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 3 of the present invention as a function of potential.

[0028] Figure 10 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 4 of the present invention as a function of potential.

[0029] Figure 11 This is a graph showing the quantitative results of the interfacial water structure of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 as a function of potential.

[0030] Figure 12 The graph shows the characteristic peak distribution of the catalysts prepared in Example 2 and Comparative Examples 1-5 of this invention after deconvolution processing of the distributed relaxation time in an electrochemical impedance spectroscopy test at 1.66 V vs. RHE. The vertical axis represents the double-layer capacitance after normalization. Figure 12 In the diagram, a is the initial plot, and b and c are plots with the ordinate magnified at different scales.

[0031] Figure 13 The graph shows the rate of change of potential increase in the catalysts prepared in Example 2 and Comparative Examples 1 to 5 of this invention as a function of the acidic OER durability test.

[0032] Figure 14 This invention provides a quantitative representation of Figure 13 The evolution of the intensity of each relaxation peak is shown in the figure. Figure 14 In the diagram, a represents the ion diffusion impedance of the double layer, b to d represent the electron transfer impedances of different types, e represents the diffusion impedance of bubbles and water, and f represents the mass transfer and redox impedances. Detailed Implementation

[0033] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention.

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0035] In acidic media, the stability of the electric double layer structure on the catalyst surface directly determines the proton transport efficiency and intermediate adsorption behavior at the reaction interface. Currently, traditional non-precious metal catalysts often face challenges such as double layer reconstruction instability and disordered water molecule structure at high potentials, leading to passivation of active sites and structural collapse. In the spring of 2018, the development of non-iridium-based acidic OER catalysts with both high activity and high stability was proposed. The key lies in achieving precise control of the electric double layer structure at the catalyst-electrolyte interface.

[0036] Based on this, this invention employs a synergistic strategy of solution blending and pyrolysis-calcination, cleverly utilizing doping-induced lattice strain and charge redistribution effects through non-noble metal doping. On one hand, it optimizes the d-band electronic structure of the cobalt active center, modulates the electric field distribution and water molecule orientation within the electric double layer, and accelerates the oxygen intermediate conversion kinetics. On the other hand, by stabilizing the interfacial water structure and electric double layer configuration, it enhances the structural robustness of the material in strongly acidic media, effectively suppressing cobalt ion dissolution and lattice oxygen loss, and inhibiting the performance degradation caused by electric double layer instability in acidic OER environments of traditional cobalt-based oxides. Specifically:

[0037] A method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst includes the following steps: S1. Using soluble cobalt salt and soluble non-precious metal salt as raw materials, they are dissolved in a solvent and stirred to form a mixed salt solution.

[0038] In this invention, the molar ratio of cobalt ions in the soluble cobalt salt to non-noble metal ions in the soluble non-noble metal salt is 4:1 to 2. The non-noble metal ions in the non-noble metal salt are molybdenum, lanthanum, manganese, chromium, iron, or copper, etc. It should be noted that this invention utilizes the electronic structure redistribution induced by the doping of non-noble metal ions. However, excessively high doping concentrations can lead to lattice mismatch, causing local lattice collapse, phase separation, and active interface reconstruction. Higher doping concentrations result in more severe lattice distortion and more significant thermodynamic instability, ultimately manifesting as rapid performance degradation in the early stages of durability testing.

[0039] In this invention, soluble cobalt salt and non-precious metal salt are stirred in a solvent at 600 rpm to 900 rpm for 30 min. The solvent, such as water with a high dielectric constant, promotes the complete dissociation of the salt into free hydrated ions, achieving uniform mixing of metal cations at the molecular level at the nanoscale. This lays a thermodynamically homogeneous foundation for subsequent doping. In the mixed salt solution, the concentration of cobalt metal ions is 0.1 mM to 0.4 mM, and the concentration of non-precious metal ions is 0.1 mM to 0.4 mM.

[0040] S2. The conductive substrate is coated on the mixed salt solution, and calcined and oxidized at 250℃ to 350℃ for 12h to 24h under air atmosphere at a heating rate of 1℃ / min to 3℃ / min. At high temperature, cobalt ions are oxidized and reconstructed into Co. 2+ [Co 3+ The 2O4 spinel structure, with non-noble metal ions selectively replacing cobalt sites in the lattice, causes lattice distortion and charge redistribution, forming doped spinel structures with different electronic properties, thus obtaining a non-noble metal-doped cobalt tetroxide (Co3O4) based catalyst supported on a conductive substrate.

[0041] It should be noted that the scientific principle of the synthesis method of this invention is rooted in the synergistic design of "molecular precursor homogenization" and "controlled oxidation crystallization kinetics". It aims to achieve atomic-level uniform doping and high crystallinity construction of non-noble metal ions in Co3O4 spinel lattice. First, soluble cobalt salt and non-noble metal salt are stirred in deionized water. The high dielectric constant of water promotes the complete dissociation of salt into free hydrated ions. The concentration gradient is eliminated by fluid shear force, realizing the uniform mixing of metal cations at the molecular level at the nanoscale, laying a thermodynamic homogeneous foundation for subsequent doping. This process avoids precipitation or agglomeration caused by local supersaturation, ensuring the precise controllability of the precursor chemical composition. Secondly, the calcination stage is carried out in an air-atmosphere tube furnace with programmed temperature control: in the initial stage of heating (<150℃), the solution undergoes gradient dehydration and thermal decomposition of nitrates (e.g., 2Co(NO3)2→Co2O3+4NO2↑+O2↑) to form an amorphous mixed oxide precursor; subsequently, the temperature is increased to 250℃ at an extremely slow rate of 1℃ / min and held for 12 hours. This condition has multiple subtle effects—thermodynamically, 250°C is precisely within the kinetic window for the formation of Co3O4 (avoiding incomplete low-temperature phase transformation or high-temperature sintering), and oxygen in the air drives the directional oxidation and reconstruction of cobalt ions into Co. 2+ [Co 3+ ]2O4 spinel structure (Fd-3m space group), in which Co 2+ Occupying the 8a position of the tetrahedron, Co 3+ Occupying the 16d site of the octahedron; kinetically, slow heating suppresses gaseous byproducts (NO). x The explosive release of H2O reduces lattice stress and void defects, while the 12-hour heat treatment provides sufficient bulk diffusion time for non-noble metal ions, enabling them to selectively replace lattice cobalt sites through thermally activated migration (preferentially occupying octahedral sites to reduce lattice distortion energy), and achieve lattice relaxation and stress release through charge compensation mechanisms (such as high-valence doping-induced oxygen vacancy formation or adjustment of adjacent cobalt valence states), ensuring a high degree of uniformity in the spatial distribution of doped elements. Crystallographically, the low-temperature long-term treatment promotes the ordered epitaxial growth of crystal nuclei and the annihilation of point defects (such as dislocation climb and vacancy recombination), effectively suppressing grain coarsening caused by Ostwald ripening. At the same time, continuous gas flow removes decomposition byproducts and maintains a stable oxidation potential, ensuring phase purity and stoichiometry. Ultimately, this process synergistically constructs a non-noble metal-doped Co3O4 catalyst with uniform doping, complete crystallization, and controllable surface defects through a triple mechanism of precursor atomic-level mixing, precise control of oxidation pathways, and optimization of lattice integration kinetics. Its optimized electronic structure (such as d-band center modulation) and abundant reactive oxygen species provide a foundation for high activity and structural stability in the catalytic oxidation reaction. The core innovation lies in the synergistic regulation of the catalyst surface electric double-layer structure through the electronic and geometric effects of the doping elements, optimizing the interfacial water molecule arrangement and proton transport pathways, thereby reconstructing the OER reaction microenvironment at the atomic scale.

[0042] In some embodiments, the loading of the non-noble metal-doped cobalt tetroxide-based catalyst on the conductive substrate is 1 mg / cm³. 2 ~4mg / cm 2 The conductive substrate is carbon paper, carbon cloth, or nickel foam.

[0043] It should be noted that the core function of the conductive substrate (carbon paper, carbon cloth, or nickel foam) is to construct a low-impedance electron transport channel: given the intrinsic semiconductor properties of Co3O4, a highly conductive substrate (conductivity 10) is required. 2 S / cm~10 5 S / cm) can effectively eliminate interparticle contact resistance, suppress ohmic polarization and local potential distortion under high current conditions, ensure the instantaneous conduction of electrons from active sites to external circuits, and its three-dimensional porous framework provides mechanical anchoring points for the catalyst and maintains the integrity of the electrode structure; while 1 mg / cm 2 ~4mg / cm 2 The loading capacity is the key window for mass transfer-activity synergistic optimization—below 1 mg / cm³. 2 Insufficient active site density limits current output; exceeding 4 mg / cm³ 2 This results in an excessively thick catalyst layer, leading to an extended ion diffusion path, O2 bubble retention, and "electrochemical shielding" of the inner active sites, significantly increasing concentration polarization and reducing mass activity.

[0044] This invention also provides a non-precious metal-doped cobalt tetroxide-based catalyst, prepared using the above-described method. The non-precious metal-doped cobalt tetroxide-based catalyst uses a conductive substrate as a support to load a non-precious metal-doped cobalt tetroxide matrix, with the non-precious metal doping amount ranging from 5 wt.% to 33 wt.%.

[0045] The non-precious metal-doped cobalt tetroxide-based catalyst provided by this invention reconstructs cobalt tetroxide through non-precious metal doping, altering the microenvironment of the cobalt tetroxide interface, inducing local lattice distortion and surface charge redistribution, and promoting the formation of an electric double layer structure with different ion adsorption / desorption activities at the interface. In a preferred embodiment, the amount of non-precious metal doping is 5% of the mass of the cobalt tetroxide matrix. By modulating the surface electronic properties and local chemical environment, the orientation and hydrogen bond network structure of water molecules within the electric double layer are reshaped, affecting the adsorption configuration and potential-dependent dynamic behavior of interfacial water, optimizing the arrangement of interfacial water molecules and proton transport pathways, thereby reconstructing the OER reaction microenvironment at the atomic scale. This not only provides a feasible path to break free from dependence on the precious metal iridium, but also lays a key material foundation for promoting the large-scale application of low-cost, high-efficiency PEM water electrolysis technology. This is the core starting point and innovative value of this invention.

[0046] The following specific examples will provide further explanation.

[0047] Example 1 A method for preparing a molybdenum-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 132.7 mg of cobalt nitrate hexahydrate and 29.7 mg of ammonium molybdate tetrahydrate in 2 mL of deionized water and stir at 600 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0048] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0049] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniformly mixed salt solution is oxidized to obtain a molybdenum metal-doped Co3O4 catalyst, wherein the molybdenum doping amount is 5% of the mass of the cobalt tetroxide matrix, and it is named 5%-Mo-Co3O4.

[0050] Example 2 A method for preparing a lanthanum-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 132.7 mg of cobalt nitrate hexahydrate and 7.8 mg of lanthanum nitrate hydrate in 2 mL of deionized water and stir at 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0051] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0052] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniformly mixed salt solution is oxidized to obtain lanthanum metal-doped Co3O4 catalyst, wherein the lanthanum doping amount is 5% of the mass of the cobalt tetroxide matrix, and it is named 5%-La-Co3O4.

[0053] Example 3 A method for preparing a manganese-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 132.7 mg of cobalt nitrate hexahydrate and 6 mg of manganese nitrate tetrahydrate in 2 mL of deionized water and stir at a speed of 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, and then transfer it to a small magnetic boat.

[0054] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0055] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniformly mixed salt solution is oxidized to obtain a manganese metal-doped Co3O4 catalyst, wherein the manganese doping amount is 5% of the mass of the cobalt tetroxide matrix, and it is named 5%-Mn-Co3O4.

[0056] Example 4 A method for preparing a chromium-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 132.7 mg of cobalt nitrate hexahydrate and 9.6 mg of chromium nitrate nonahydrate in 2 mL of deionized water and stir at 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0057] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0058] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniformly mixed salt solution is oxidized to obtain a chromium-doped Co3O4 catalyst, wherein the chromium doping amount is 5% of the mass of the cobalt tetroxide matrix, and it is named 5%-Cr-Co3O4.

[0059] Comparative Example 1 A method for preparing a cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 139.7 mg of cobalt nitrate hexahydrate in 2 mL of deionized water and stir at 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0060] S2, 2×1cm 2The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0061] S3. In an air-filled tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The homogeneous mixed salt solution is oxidized to obtain a molybdenum metal-doped Co3O4 catalyst, which is named Co3O4.

[0062] Comparative Example 2 A method for preparing a lanthanum-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 125.7 mg of cobalt nitrate hexahydrate and 15.6 mg of lanthanum nitrate hydrate in 2 mL of deionized water and stir at 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0063] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0064] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniform mixed salt solution is oxidized to obtain lanthanum metal-doped Co3O4 catalyst, wherein the lanthanum doping amount is 10% of the mass of the cobalt tetroxide matrix, and it is named 10%-La-Co3O4.

[0065] Comparative Example 3 A method for preparing a lanthanum-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 118.8 mg of cobalt nitrate hexahydrate and 23.4 mg of lanthanum nitrate hydrate in 2 mL of deionized water and stir at 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0066] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0067] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniform mixed salt solution is oxidized to obtain lanthanum metal-doped Co3O4 catalyst, wherein the lanthanum doping amount is 15% of the mass of the cobalt tetroxide matrix, and it is named 15%-La-Co3O4.

[0068] Comparative Example 4 A method for preparing a lanthanum-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 118.8 mg of cobalt nitrate hexahydrate and 31.2 mg of lanthanum nitrate hydrate in 2 mL of deionized water and stir at 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0069] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0070] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniformly mixed salt solution is oxidized to obtain lanthanum metal-doped Co3O4 catalyst, wherein the lanthanum doping amount is 20% of the mass of the cobalt tetroxide matrix, and it is named 20%-La-Co3O4.

[0071] Comparative Example 5 A method for preparing a lanthanum-doped cobalt tetroxide-based catalyst includes the following steps: S1. Dissolve 93.6 mg of cobalt nitrate hexahydrate and 51.5 mg of lanthanum nitrate hydrate in 2 mL of deionized water and stir at 600 rpm to 900 rpm for 30 min to obtain a homogeneous mixed salt solution, which is then transferred to a small magnetic boat.

[0072] S2, 2×1cm 2 The hydrophilic carbon paper was placed in anhydrous ethanol solution and sonicated for 20 minutes to remove surface impurities. It was then transferred to a vacuum drying oven and dried at 50°C for 6 hours. Subsequently, the dried hydrophilic carbon paper was placed on a molybdenum-cobalt mixed salt solution.

[0073] S3. In an air-purified tube furnace, the calcination temperature is set to 250℃, the heating rate is 1℃ / min, and the calcination time is 12h. The uniform mixed salt solution is oxidized to obtain lanthanum metal-doped Co3O4 catalyst, wherein the lanthanum doping amount is 33% of the mass of the cobalt tetroxide matrix, and it is named 33%-La-Co3O4.

[0074] The structure and performance of the non-noble metal-doped Co3O4-based catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 5 were tested, and the results are as follows.

[0075] Figure 1 The XRD patterns are those of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention. Figure 1 In the examples, a represents Comparative Example 1, b represents Example 1, c represents Example 2, d represents Example 3, and e represents Example 4. Figure 1 As shown, their crystal structure characteristics are reflected. The main diffraction peaks of all samples are highly consistent with the spinel-type Co3O4 crystal phase in JCPDS #42-1467 standard card, indicating that the main crystal structure is maintained during the doping process. However, the introduction of different metal ions produced significantly different modulation effects on the crystal structure: for Mn, Cr, and La-doped samples, the positions and intensities of the diffraction peaks remained basically unchanged, indicating that these metal ions successfully entered the Co3O4 lattice, achieving atomic-level doping without causing obvious lattice distortion or phase separation. Notably, the Mo-doped sample showed additional diffraction peaks, which, upon searching, perfectly matched the orthorhombic α-MoO3 phase in JCPDS #21-0569. This indicates that under the experimental conditions, some molybdenum species failed to be effectively incorporated into the cobalt oxide lattice, but instead formed an independent molybdenum oxide crystal phase on the material surface. This surface coating structure may be due to a large lattice mismatch between molybdenum ions and cobalt oxide, or a strong tendency for the migration and aggregation of molybdenum species during the preparation process, thus leading to the formation of heterostructures.

[0076] Figure 2 The figures show the polarization curves of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention. Figure 2 As shown, doping with various metal ions alters the OER current density of the Co3O4 catalyst to varying degrees. Mo ion doping significantly reduces the current density, while Mn, Cr, and La ion doping hardly changes the current density at low potentials. However, the current density at high potentials (greater than 1.75 V vs. RHE) shows significant differences, following the order Mn > undoped = Cr > La.

[0077] Figure 3 The figures show the cyclic voltammograms and fitted double-layer capacitance diagrams of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention. a and a1 are the cyclic voltammogram and double-layer capacitance diagram of Comparative Example 1, respectively; b and b1 are the cyclic voltammogram and double-layer capacitance diagram of Example 1, respectively; c and c1 are the cyclic voltammogram and double-layer capacitance diagram of Example 2, respectively; d and d1 are the cyclic voltammogram and double-layer capacitance diagram of Example 3, respectively; and e and e1 are the cyclic voltammogram and double-layer capacitance diagram of Example 4, respectively. Figure 3As shown, this study reveals the regulatory mechanism of metal ion doping on the electrochemically active surface area of ​​Co3O4 catalysts. The electric double-layer capacitance, as a key parameter characterizing the number of actual active sites on the catalyst, directly reflects the effective surface area available for electrochemical reactions. Experimental results show that pure-phase Co3O4 exhibits the highest electric double-layer capacitance (72.36 mF·cm⁻¹). -2 This suggests that it possesses a relatively abundant number of surface active sites. However, doping with different metal ions significantly alters this characteristic, exhibiting a decreasing trend of undoped > La > Cr > Mn > Mo. 3+ The capacitance value after doping is 71.01 mF·cm. -2 The fact that the ion radius of the chromium is close to that of the pure phase indicates that doping with its large ionic radius did not significantly change the surface structure and the degree of exposure of active sites; while Cr 3+ (31.57mF·cm) -2 ) and Mn 2+ / Mn 3+ (24.79mF·cm) -2 The introduction of Mo leads to a significant decrease in capacitance, which may be due to doping-induced lattice or surface reconstruction, reducing the exposure of effective active sites; this is particularly significant for Mo. 6+ Doping caused the double-layer capacitance to drop sharply to 4.93 mF·cm. -2 The decrease of more than 93% strongly suggests that the introduction of high oxidation state Mo ions may have triggered severe surface passivation or formed a low conductivity surface phase, which greatly limits the electrochemically active surface area.

[0078] Figure 4 The graphs show the polarization curves of the catalysts prepared in Examples 1-4 and Comparative Example 1 of this invention after normalization based on the electrochemical active area (ECSA). Figure 4 As shown, this standardization method effectively eliminates the interference of catalyst surface area differences on activity evaluation, thus accurately revealing the intrinsic catalytic activity (characterized by current density per unit active site) of different doped systems. The results show that the undoped Co3O4 catalyst exhibits relatively low intrinsic OER activity after normalization, which is closely related to the electronic configuration of cobalt sites in its spinel structure and the adsorption barrier of oxygen intermediates. It is worth noting that La... 3+Doping hardly altered the intrinsic activity of the catalyst, suggesting that lanthanum ions may primarily act as structural stabilizers rather than active site modifiers. Their large ionic radius and stable +3 oxidation state make them difficult to participate in redox cycles, limiting their impact on the electronic structure of cobalt sites. In contrast, metal ion doping exhibited significantly different effects on intrinsic activity, following the order Mo > Mn > Cr > undoped > La. This anomaly—especially the highest intrinsic activity exhibited by Mo doping—contrasts sharply with the apparent activity trend before normalization, profoundly revealing the dual mechanism of doping effects on catalyst performance: on the one hand, Mo... 6+ The high oxidation state may optimize the d-electron distribution of cobalt sites through strong electronic interactions, thereby reducing the thermodynamic barrier of the OER reaction; on the other hand, its introduction may lead to surface reconstruction, forming interface phases or defect sites with higher intrinsic activity.

[0079] Figure 5 This is a graph showing the change in apparent activation energy Ea / pre-exponential factor A as a function of potential for the catalysts prepared in Examples 1 to 4 and Comparative Example 1 of this invention. Figure 5 In the examples, a represents Comparative Example 1, b represents Example 1, c represents Example 2, d represents Example 3, and e represents Example 4. Figure 5 As shown, it is clear that different metal doping significantly modulates the evolution of the apparent activation energy (Ea) of the Co3O4 catalyst with respect to potential during the OER process. Each catalyst exhibits a characteristic Ea peak, and the corresponding potential (i.e., the potential at which the activation energy is at its highest) can be regarded as the "bottleneck potential" with the greatest resistance to reaction kinetics, i.e., the rate-determining potential. The rate-determining potentials of pure Co3O4 and Mn-doped samples are located at approximately 1.60 V vs. RHE, with the highest being Mo-doped sample (1.66 V), Cr-doped sample at 1.64 V, and La-doped sample at the lowest (1.58 V). This indicates that the doping elements systematically shift the energy barrier peak position of the key OER step by modulating the electronic structure and surface properties. This potential shift directly reflects the difference in the thermodynamic / kinetic threshold of the rate-determining step under different doping systems.

[0080] Figure 6 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Comparative Example 1 of this invention as a function of potential. Figure 7 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 1 of the present invention as a function of potential. Figure 8 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 2 of the present invention as a function of potential. Figure 9 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 3 of the present invention as a function of potential. Figure 10 This is an in-situ Raman diagram of the interfacial water structure of the catalyst prepared in Example 4 of the present invention as a function of potential. Figure 11This is a graph showing the quantitative results of the interfacial water structure change with potential for the catalysts prepared in Examples 1-4 and Comparative Example 1 of this invention. Figures 6 to 11 As shown, the initial (low potential region) free water content order is: Mo>La>Cr>Mn>undoped Co3O4, indicating that metal doping generally enhances the weak adsorption or non-hydrogen bonding of water molecules on the catalyst surface, i.e., increases the proportion of interfacial free water. The free water content of pure Co3O4 changes gradually throughout the potential scan range, indicating that its interfacial water structure has a weak potential response. The free water content of the Mn-doped sample is basically stable below 1.55 V vs. RHE, but increases sharply above 1.55 V, possibly reflecting that the surface oxidation state transition at high potential induces water molecule desorption or local double layer reconstruction, leading to more water molecules leaving the strongly adsorbed state. The free water content of the Mo-doped sample decreases significantly in the 1.40 V to 1.62 V range. Combined with its high OER activity, it is speculated that water molecules are more inclined to participate in interfacial reactions (such as OH-) during this stage. - (Adsorption or OH bond activation) leads to directional dissociation or transformation into reaction intermediates. The Cr-doped sample shows a slight decrease at 1.44 V, followed by a sharp increase to 1.66 V, and then a rapid decrease. This non-monotonic behavior may correspond to a multi-step surface reconstruction process: initially, a small amount of water molecules participate in the formation of oxygen vacancies or high-valence metal oxygen species; in the middle stage, some bound water is released due to enhanced surface hydroxylation; and in the later stage, accelerated O2 precipitation leads to interfacial dehydration. The La-doped sample is stable before 1.55 V, with a sharp increase in free water in the 1.55 V–1.62 V range, followed by a decrease and stabilization after 1.62 V, suggesting a reversible hydration / dehydration transition on its surface at moderate overpotentials, possibly related to La… 3+ This is related to the enhanced surface proton transfer capability induced by the dopant. Overall, different dopants significantly affect the adsorption configuration and potential-dependent dynamic behavior of interfacial water by modulating surface electronic properties and local chemical environment, thereby influencing the OER reaction pathway and kinetic characteristics.

[0081] Figure 12 The graph shows the characteristic peak distribution of the catalysts prepared in Example 2 and Comparative Examples 1 to 5 of this invention after deconvolution processing using the distribution of relaxation times (DRT) in electrochemical impedance spectroscopy at 1.66 V vs. RHE. The vertical axis represents the double-layer capacitance after normalization. Figure 12 In the diagram, a is the initial image, and b and c are images with the ordinate magnified at different scales, respectively. For example... Figure 12As shown, the pure Co3O4 catalyst exhibits six characteristic relaxation peaks: peak 1 (log(τ)≈−4.5) is attributed to the ion migration process at the CoOOH double-layer interface; peak 2 (log(τ)≈−3.5) is attributed to the ion migration process at the Co(OH)2 double-layer interface; peaks 2 and 3 (log(τ)≈−2.8 to −1.8) correspond to charge transfer kinetics; peak 4 (log(τ)≈−1) reflects water diffusion behavior; and peak 5 (log(τ)≈0.8) is related to bulk mass transport and surface oxidation reconstruction processes. Regarding the ion transfer impedance of the CoOOH and Co(OH)2 active phases of Co3O4, Cr and La doping promote ion transfer, while Mn and Mo doping inhibit it. Regarding the electron transfer impedance of Co3O4, Cr and La doping promote electron transfer, while Mn and Mo doping inhibit it. Peak 5 of Co3O4 is actually produced by the oxidation of Co3O4 to a higher valence state. This reconstruction process is accompanied by mass transport and electron transfer. Only Mo inhibits this reconstruction process, while Mn, Cr, and La promote it. These results indicate that the introduction of various metal ions alters the microenvironment of the Co3O4 electrode / electrolyte interface: the incorporation of metal ions with different ionic radii into the spinel lattice induces local lattice distortion and surface charge redistribution, promoting the formation of a double-layer structure with different ion adsorption / desorption activities at the interface.

[0082] Figure 13 This is a graph showing the rate of change of potential increase in the catalysts prepared in Example 2 and Comparative Examples 1 to 5 of the present invention as a function of acidic OER durability testing. Figure 13 As shown, in the galvanostatic durability test, the overpotential of the pure Co3O4 catalyst exhibited a continuous decreasing trend (negative potential shift), indicating that its surface underwent gradual electrochemical reconstruction, gradually transforming into the highly active CoOOH phase (passing through the Co(OH)2 intermediate state). This activation process tended to stabilize after 21 hours, and the system reached a catalytic-structural dynamic equilibrium. After introducing La doping, the electrode potential-time curve showed significant non-monotonic fluctuations, especially in the 10% and 20% highly doped samples, where the overpotential increased rapidly in the initial stage (positive potential shift), revealing a severe structural instability behavior. This phenomenon originates from La... 3+ (Ionic radius 1.032 Å) and Co 3+ Significant lattice mismatch between (0.545 Å) leads to high-density strain in the spinel framework; under electrochemical polarization, La 3+ Selective dissolution occurs, triggering local lattice collapse, phase separation, and active interface reconstruction. The higher the doping concentration, the more severe the lattice distortion and the more significant the thermodynamic instability, ultimately manifesting as rapid performance degradation in the early stages of durability testing. This highlights the crucial influence of dopant size matching on the robustness of the catalyst structure.

[0083] Figure 14 This invention provides a quantitative representation of Figure 13 The evolution of the intensity of each relaxation peak is shown in the figure. Figure 14 In the diagram, a represents the ion diffusion impedance of the electric double layer, b to d represent the electron transfer impedances of different types, e represents the diffusion impedance of bubbles and water, and f represents the mass transfer and redox impedance. For example... Figure 14 As shown in Figure a, the intensity of the relaxation peak (log(τ) ≈ −4.5) induced by La doping increases with increasing doping concentration, and is particularly strong at 10 mA cm⁻¹. -2 The continuous increase in constant current during the durability test indicates a significant increase in the ion migration resistance of this interfacial process, which further intensifies with electrochemical aging. Figure 14 As shown in Figure b, the intensity of the original electric double layer peak (peak 1) increases sharply when the La doping concentration exceeds 20%, reflecting that high-concentration doping leads to impeded ion transport at the basic interface, and that the impedance continues to accumulate during long-term operation. Figure 14 As shown in Figures c and d, corresponding to the charge transfer process, peaks 2 and 3 indicate that when the La doping concentration is below 20%, the charge transfer impedance decreases significantly, confirming that appropriate doping optimizes the electronic conduction efficiency of active sites and promotes interfacial charge exchange kinetics. Figure 14 The mid-e (water molecule diffusion impedance, peak 4) indicates that low doping levels (<20%) have a weak effect on reactant diffusion, while the diffusion impedance is significantly increased in the 33% highly doped sample, suggesting that lattice distortion caused by excessive La hinders electrolyte permeation and reactant transport. Figure 14 The mass transfer impedance (peak 5) in the middle f shows that La doping generally weakens the mass transfer resistance associated with the oxidation reconstruction of Co3O4 surface (such as the Co3O4→CoOOH phase transition), especially under moderate doping conditions. This indicates that the introduction of La effectively reduces the kinetic barrier to the formation of the active phase, providing favorable conditions for the rapid activation and structural stability of the catalytic interface.

[0084] This invention establishes a comprehensive in-situ characterization system for electric double-layer structures. In-situ Raman spectroscopy and in-situ electrochemical impedance spectroscopy are used to characterize the changes in the water structure at the electric double-layer interface with voltage, and the changes in the electron / ion transfer impedance of the electric double layer with durability testing. In-situ Raman spectroscopy is used to track the dynamic behavior of the water molecule structure at the interface with potential changes in real time, and the potential dependence of the ratio of free water to bound water is quantitatively analyzed. Electrochemical impedance spectroscopy is combined to analyze the potential response characteristics of the electric double-layer capacitance and charge transfer resistance. This method organically combines the regulation of the electric double-layer structure with in-situ characterization techniques, enabling visualization of the dynamic evolution process of the electric double-layer structure. It provides direct evidence for understanding the mechanism of electrocatalytic reactions and is applicable not only to the oxygen evolution reaction but also to the interface engineering research of other electrocatalytic systems.

[0085] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the invention.

[0086] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst, characterized in that, Includes the following steps: Soluble cobalt salt and soluble non-precious metal salt are used as raw materials, dissolved in a solvent, and stirred to form a mixed salt solution; A conductive substrate is coated onto a mixed salt solution and then calcined and oxidized at 250°C–350°C in air to oxidize and reconstruct cobalt ions into Co. 2+ [Co 3+ The 2O4 spinel structure, with non-noble metal ions selectively replacing cobalt sites in the lattice, causes lattice distortion and charge redistribution, resulting in a non-noble metal-doped cobalt tetroxide-based catalyst supported on a conductive substrate.

2. The method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst according to claim 1, characterized in that, The molar ratio of cobalt ions in soluble cobalt salts to non-precious metal ions in non-precious metal salts is 4:1 to 2.

3. The method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst according to claim 1, characterized in that, In the mixed salt solution, the concentration of cobalt metal ions is 0.1 mM to 0.4 mM, and the concentration of non-precious metal ions is 0.1 mM to 0.4 mM.

4. The method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst according to claim 1, characterized in that, The non-precious metal ions in non-precious metal salts are molybdenum, lanthanum, manganese, chromium, iron, or copper.

5. The method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst according to claim 1, characterized in that, During calcination, the heating rate is 1℃ / min to 3℃ / min, and the calcination time is 12h to 24h.

6. The method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst according to claim 1, characterized in that, The loading of non-precious metal-doped cobalt tetroxide-based catalysts on conductive substrates is 1 mg / cm³. 2 ~4mg / cm 2 The conductive substrate is carbon paper, carbon cloth, or nickel foam.

7. The method for preparing a non-noble metal-doped cobalt tetroxide-based catalyst according to claim 1, characterized in that, The solvent is water, and the stirring speed is 600 rpm to 900 rpm.

8. A non-precious metal-doped cobalt tetroxide-based catalyst, characterized in that, It is prepared using any one of the preparation methods of claims 1 to 7.

9. The non-noble metal-doped cobalt tetroxide-based catalyst according to claim 8, characterized in that, The non-precious metal-doped cobalt tetroxide-based catalyst uses a conductive substrate as a support to load a non-precious metal-doped cobalt tetroxide matrix. The amount of non-precious metal doping is 5 wt.% to 33 wt.% of the mass of the cobalt tetroxide matrix.

10. The application of a non-precious metal-doped cobalt tetroxide-based catalyst as described in claim 8 or 9 in the electrocatalytic splitting of water.

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