Electrocatalytic Performance Regulation Strategies For Oxygen Reduction And Oxygen Evolution Reactions

With the current energy crisis and serious environmental pollution,research on electrochemical energy storage and conversion technologies,such as fuel cells,metal-air batteries,water splitting devices,and carbon dioxide reduction technologies,is crucial.However,oxygen reduction reaction（ORR）as the cathode reaction of fuel cells,the slow reaction kinetics leads to the low Coulombic efficiency and low output current,which is a huge obstacle to its large-scale application.It is necessary to find efficient catalysts to improve its reaction kinetics.Studies have shown that composites of metals（especially transition metals）and nitrogen-doped carbon materials can effectively catalyze ORR due to the easy tuning of their structures and properties.Similarly,oxygen evolution reaction（OER）is the bottleneck of the water splitting reaction,although nickel hydroxide has good catalytic effect and is widely used,its performance still needs to be further improved.Therefore,it is of great significance to design and construct ORR and OER electrocatalysts with high catalytic performance,good stability and low price.The specific research of this paper is mainly divided into the following two parts:1.A high performance ORR catalyst with multiple active centers（MZn-NC）is prepared by high temperature carbonization,using the transition metal ion M²⁺（M=Fe,Co,Ni,etc.）to partially replace Zn²⁺in the framework of ZIF-8 to form bimetallic ZIFs.Among them,in Fe Zn-NC prepared from ferrous sulfate heptahydrate as iron source,in which ORR catalytic active centers Fe N_x and Fe₃C are derivated from the reaction of Fe with N and C in organic ligands,respectively,and Fe₂O₃ active substances are also generated,while the framework of ZIFs materials generates porous graphitized nitrogen-doped carbon substrate which can promote ORR during carbonization.The as-prepared Fe Zn-NC catalyzes ORR along the ideal 4e^-pathway and exhibits excellent catalytic performance with an onset potential of 0.947 V,a limiting diffusion current density of 6.07 m A cm^-2,and a Tafel slope of 57.3 m V dec^-1.After 10,000 cyclic voltammetry tests,the half-wave potential is only moved forward by 16 m V,and the limiting current density is only lost by 5.8%.The catalytic performance is better than that of commercial 20 wt%Pt/C.2.Using chiral amino acid molecules as chiral structure-directing agent and modifiers,polyvinylpyrrolidone（PVP）as a dispersant and stabilizer,ultra-small two-dimensional chiral nickel hydroxide nanosheets are prepared by hydrothermal method.The synthesized nickel hydroxide nanosheets are about 30 nm in size and 3 nm in thickness.Firstly,the two-dimensional flake structure can increase its specific surface areas and fully expose its active sites,which is beneficial to the mass transfer of reactants and products,thereby enhancing its catalytic activity.Secondly,the intrinsic chirality of the materials induced by chiral molecules makes the materials have spin selectivity for electrons,which is conducive to the propagation of ground-state oxygen and suppression of by-product hydrogen peroxide.Thirdly,the residual chiral amino acid molecules on the catalyst surface change the charge distribution of the materials,which also affects the catalytic performance.Compared with the achiral molecule-induced catalysts,the chiral molecule-induced catalysts exhibit superior OER catalytic performance,lower overpotential and higher current density,the current density can be increased by up to 160%.