Preparation And Modification Of Iron-Family Compounds On Conductive Substrate For Water Splitting

At the primary stage of the development of water electrolysis technology,researchers designed and synthesized many powder catalysts fixed on glassy carbon electrode with polymer binders such as Nafion to catalyze water splitting for hydrogen and oxygen evolution.The stability of the catalyst fixed by the binder is poor,the catalyst easily to fall off at high current density.In addition,the introduction of the binder prevents the catalyst from contacting with water,and as a result,the efficiency of charge transfer and the catalytic performance of the catalyst are reduced.The conductive substrates with unique porous structure such as nickel foam and carbon cloth provide a pathway for charge transfer and mass transfer during catalytic reactions,accelerating the electrolyte penetration and gas diffusion and promoting the progress of catalytic reactions.In recent years,iron-family catalysts,especially iron-family（hydroxide）oxide with good structural stability,have attracted tremendous attention in the field of water electrolysis due to their excellent electrocatalytic oxygen evolution reaction（OER）activity.Therefore,loading iron-family element（hydroxide）oxide on surface of a suitable conductive substrate and exploring their catalytic mechanism are of great significance for water electrolysis.With nickel foam（NF）as the conductive substrate,the Fe OOH/Ni（OH）₂/NF composite catalysts were synthesized by electrodeposition-electrophoretic deposition technology.The electrochemical synthesis realizes the controllable preparation of nickel-iron-based composite materials at room temperature.By adjusting the parameters during the electrochemical synthesis,the composition of samples in the composite catalysts can be precisely modulated to achieve an optimal OER performance.Among them,the composite material obtained by cathodic electrodeposition for 550 s and electrophoretic deposition for 300 s displays the best catalytic performance.The catalyst requires an overpotential of only 245 m V to achieve the current density of 100 m A/cm² in 1 mol/L KOH solution.Density functional theory（DFT）calculation results show that the composite catalyst optimizes the thermodynamic process of the reaction with the Gibbs free energy of rate-determining step becoming the smallest.Therefore,the composite catalyst processes the best oxygen evolution reaction activity compared with the monomer catalysts.The experimental results are consistent well with the DFT results.Electrodeposition was used to prepare Ru-doped NiFe layered double hydroxide（NiFe LDH）precursor on surface of nickel foam,and Ru-doped NiFe₂O₄/NF was obtained through sintering process at high temperature.The introduction of Ruaccelerates the water adsorption and dissociation on surface of catalysts.Furthermore,Rudoping reduces hydrogen adsorption energy and changes the hydrogen evolution reaction process of nickel ferrite so that the material undergoes the Volmer-Tafel process under alkaline conditions to achieve hydrogen evolution.It requires an overpotential of only 18 m V to reach a current density of-10 m A/cm².Considering that nickel ferrite itself is a good OER catalyst,we employed Ru-doped NiFe₂O₄/NF as both the cathode and the anode catalysts to construct an overall water splitting cell,which requires the voltage of 1.52 V to drive overall water splitting and achieve a current density of 10 m A/cm².Furthermore,the catalysts can be used to catalyze water splitting for up to 50hours stably and efficiently.RuO₂/（Co,Mn）₃O₄ with low precious metal loading was prepared on carbon cloth（CC）substrate through the process of hydrothermal derivatization-ion exchange-high temperature calcination.The catalyst is superior to commercial RuO₂/CC in terms of OER performance and stability.An overpotential of 270 m V is required to reach the current density of 10 m A/cm² in 0.5 mol/L sulfuric acid solution for OER.During the long-term water splitting process for oxygen evolution,the overpotential only increases by 131 m V.Spectroscopic characterizations and theoretical calculations confirmed that the introduction of manganese enriches the electron to Ruin the composite catalyst,and improves the intrinsic catalytic activity of the active sites.