Preparation And Performance Of Electrolyzed Water Oxygen Evolution Catalyst Based On Cobalt-based ZIFs

The development of highly efficient and low cost electrolytic water hydrogen production technology is vital to achieving“carbon peaking and carbon neutrality goals”in China.However,the slow kinetics of oxygen evolution reaction（OER）and the dependence on scarce noble metal ruthenium,iridium and their oxide catalysts limit the commercialization of water electrolysis technology.Therefore,the design and development of low-cost and non-noble metal OER catalysts with high performance is of great significance to the development of water electrolytic water technology.Zeolitic imidazolate frameworks（ZIFs）,as one of the metal-organic frameworks（MOFs）,have significant advantages in the OER catalytic reaction of electrolytic water due to their periodic metal-nitrogen coordination structure,high specific surface area and adjustable pores.In this paper,a series of cobalt-based ZIFs electrocatalytic materials were prepared by utilizing cobalt-based metal organic framework material（ZIF-67）through approaches such as high-temperature sulfurization,electric field activation and electrodeposition of nanosheet loading.By studying the influence of catalyst composition and structure on OER catalytic performance,the structure-activity relationship between active site composition and performance is revealed.The main results are as follows:（1）Cobalt sulfide anchored on Co,S and N heteroatoms doped carbon（Co₉S₈/Co NSC）catalysts were prepared by solvothermal method using ZIF-67 as precursor and sublimed sulfur as sulfur source.The uniform distribution of Co metal ions in ZIF-67 ensures the good dispersion of Co₉S₈nanoparticles anchored on the multi-heteroatom doped carbon.The average particle size is about 10 nm,the specific surface area of the catalyst is as high as 411.24 m²g^-1,and the mesoporous percentage is 81.19%.Electrochemical tests show that the OER overpotential is 336 m V at a current density of 10 m A cm^-2in 1 M KOH electrolyte,which is comparable to commercial Ru O₂（306 m V）.The highly dispersed Co₉S₈nanoparticles and Co-N_xcoordination structure in the catalyst can synergistically catalyze OER and promote OER performance.（2）In terms of the problems of limited exposed active sites and low intrinsic activity caused by structural collapse during the pyrolysis of ZIFs,a non-carbonized nano-spherical cobalt-based metal hydroxyl compound catalyst was prepared by electric field-assisted activation of cobalt-based ZIFs.X-ray near-edge absorption spectroscopy（XANES）and ultraviolet-visible absorption spectroscopy（UV-vis）have shown that,OH^-ions in the electrolyte attack Co ions in the ZIFs under an electric field,resulting in cleavage of Co-N bond and formation of Co-O bonds,forming metal hydroxyl oxide species（Co（OH）₂/Co OOH）.The activation of Co-Mn bimetallic ZIFs shows that the addition of the second metal Mn not only regulates the electronic structure of Co,but also enhances the oxidation state of Co,and promoting the phase transition process of Co-Mn bimetallic ZIFs under electric field.Electrochemical tests show that under the voltage range of 0.947-1.747 V vs.RHE and the scan rate of 100 m V s^-1,cobalt-based metal hydroxyl compound has an overpotential of 332 m V at 10 m A cm^-2after 400 potential cycles,which is significantly higher than that of cobalt-manganese bimetallic ZIFs material（390 m V）.（3）In terms of the problems of poor intrinsic conductivity and inferior dispersion of ZIFs,cobalt hydroxide（Co（OH）₂）nanosheets were prepared by electrodeposition using nickel foam as the substrate.Using Co in Co（OH）₂to coordinate with N in dimethylimidazole,ZIF-67 was in-situ grown to obtain Co（OH）₂nanosheets loaded with ZIF-67 composite catalyst.The ZIF-67 loaded in the catalyst exhibits a polyhedral morphology with an average particle size of 320 nm and is highly dispersed on Co（OH）₂nanosheets with a thickness of about 30 nm.The results of X-ray photoelectron spectroscopy（XPS）show that the content of Co³⁺was increase in the sample of Co（OH）₂/ZIF-67 compared with ZIF-67 sample,which is beneficial to promote the electrocatalytic activity of OER.The electrocatalytic performance test showed that the overpotential of Co（OH）₂/ZIF-67 catalyst for OER was 266 m V at a current density of 20m A cm^-2,which was significantly better than that of Co（OH）₂（313 m V）and ZIF-67（463 m V）.The stability test showed that performance of Co（OH）₂/ZIF-67 composite catalyst has no significant decrease after 30 h test,showing excellent electrochemical stability.