Study On The Application Of Cobalt-Based Catalysts In High-Efficiency Electrolysis Of Water For Oxygen Evolution

The consumption of fossil fuels and the rapidly growing energy demand force people to develop clean and sustainable energy.As a clean secondary energy,hydrogen energy has great potential for wide application in the future.Electrocatalytic splitting water into hydrogen is a highly efficient technology for hydrogen production,but its efficiency is significantly affected by the oxygen evolution reaction（OER）.The complicated four-electron transfer step leads to slow OER kinetics,which severely restricts its practical application.Although noble metal-based catalysts such as Ir O₂and Ru O₂have high OER activity,high price and scarcity of resources hinder their further development.Therefore,developing efficient and stable OER catalysts has become an important way to solve this problem.Cobalt-based metal catalysts are the research objects in this thesis.By adjusting microscopic morphology,electronic structure and conductive substrate of the catalyst,a series of cobalt-based metal catalysts was designed and synthesized with excellent performance.The specific research contents are as follows:（1）A series of X-Co₂O₄containing different metal elements was synthesized by hydrothermal method.The electrochemical test results showed that Ni Co₂O₄had best OER performance.Further,Mn-doped Ni Co₂O₄urchin-like catalysts were prepared.The physicochemical properties of catalysts were characterized by XRD,SEM,XPS,BET and so on.The XPS results showed that the introduction of Mn enlarges the Ni Co₂O₄lattice,forming a strong interaction between Mn-Ni-Co.At the same time,Mn⁴⁺occupied octahedral sites,enabling partial electron transfer from Co and Ni to Mn during the CV cycle.Moreover,Mn⁴⁺/Mn³⁺can be interconverted,which rationally regulated the electronic structure of catalyst and was beneficial to the formation of defect sites on the surface of catalyst.DFT calculation showed that Mn occupied the octahedral position of Ni Co₂O₄,which reduced the reaction energy barrier of the catalyst.Therefore,Mn-Ni Co₂O₄-3 achieved higher OER activity,with the smallest overpotential of 489.3 m V at a current density of 50 m A·cm^-2and Tafel slope of 182.5 m V·dec^-1.（2）The polymerized porphyrins supported on C₃N₄were prepared by solvothermal method.The Co₃O₄/NC catalyst was prepared by Co modification and heat treatment of BDA-PY/C₃N₄.Then Co₃O₄/NC-600 was treated with acid to explore the active center.The physicochemical properties of the catalyst were characterized by XRD,SEM,TEM,XPS,FT-IR and so on.The results indicated that Co₃O₄/NC-600 possessed a super-small nano-Co₃O₄structure and high nitrogen content.The strong chemical bond between pyridine N of nitrogen-doped carbon and Co was formed,resulting in a synergistic effect,which maked that showed good catalytic performance in OER reaction.DFT calculations found that Co₃O₄and（HO）₂-Co/NC coexisted as catalytic active centers in Co₃O₄/NC-600,which acted together in the OER reaction.In Co₃O₄/NC-600,Tafel slope was only 66.4m V·dec^-1and the minimum overpotential was 343.3 m V when current density reached 10m A·cm^-2.（3）The edge-rich Co-Fe-Cr nanosheet composites were prepared on NF by electrodeposition technology.And the effects of ion deposition sequence and deposition time on the performance of catalysts were explored.The physicochemical properties of catalyst were characterized by XRD,SEM,XPS and other methods.The results showed that the activity was better when the element deposited in the first stage was Co.Among them,Co-FCC/NF-4 had the best OER catalytic performance with the fastest electron transport efficiency and the most active sites.The overpotential at 10 m A·cm^-2was 252.3m V,Tafel slope was 44.2 m V dec^-1and it exhibited excellent stability.And DFT calculation proved that prepared Co-Fe-Cr nanosheets had the lowest reaction energy barrier,which was more conducive to the occurrence of OER reaction.The main reason was that Co-Fe-Cr nanosheets had a large specific surface area,exposing abundant active sites.Also,due to the close combination of Co-Fe-Cr nanosheets and the conductive substrate,electron transport resistance was greatly reduced,and its activity was effectively improved.