Structure And Performance Investigation Of Cobalt Molybdenum Oxide Derived Electrocatalyst For Hydrogen Evolution Reaction From Electrolytic Water Splitting

Electrolysis of water is a simple way for hydrogen production.However,the energy barrier of direct water splitting is relatively high,which needs to provide a high external voltage.Therefore,it is necessary to develop efficient electrocatalysts to reduce the energy consumption of the hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?,thus to improve energy conversion efficiency and lower the cost.It is well known that noble metals?Pt,Rd?and noble metal oxides?IrO₂,RuO₂?are regarded as the best electrocatalysts for the efficient HER and OER,respectively.However,the rare reserves and high cost directly limit their large-scale application.Thus,developing earth-abundant,low-cost electrocatalysts with high catalytic activity for the electrochemical water splitting is critical.Cobalt-molybdenum binary metal oxide is cost-effective and environmentally benign material,and is widely used in catalyzing OER under alkaline conditions.However,the performance is poor when used as catalysis for the HER due to the limitations of intrinsic activity and conductivity.In order to improve its catalytic activity for the HER and broaden its application in electrolysis of water,the dissertation takes this binary transition metal oxide as the research object and conducts experiments and theory calculations to establish the relationship between surface structure and catalytic activity.The specific results of research are as follows:Firstly,we prepared a Co-Mo binary oxide with crystal water?CoMoO₄?nH₂O?by solvothermal method,and then subjected to heat treatment under reduction condition?Ar/H₂?at different temperatures.The electrochemical performance tests showed that the catalyst possessed the best catalytic activity when the heat treatment temperature was 450°C,and the overpotential at the current density of 10 mA?cm^-2 was only 25 mV.According to the related characterization of the material structure and the corresponding theoretical calculations,we found that there were three reasons for the high activity of the catalyst:Firstly,calcination treatment of this precursor material under a reductive atmosphere resulted in the formation of Co nanoparticles on the Co₂Mo₃O₈ surface,which worked in concert to act as active sites for the HER to increase intrinsic activity;Secondly,the oxide?Co₂Mo₃O₈?contained certain structural defects during the precipitation of metallic cobalt,which contributed to increasing the conductivity;Thirdly,nanostructure and dehydration during the heat treatment resulting in porous structure,which allowed the catalyst to possess a large specific surface area,so that more active sites could be exposed.In our previous work,we found that the activity of the catalyst first became better and then decreased as the heat treatment temperature increased.Through the combination of experimental and theoretical calculations,we further studied the reasons for the activity variation of CoMoO₄?nH₂O after heat treatment at different temperatures.After the comprehensive analysis of electrochemical performance tests,structural characterization and theoretical calculation,we concluded that:?1?metallic cobalt?Co?,cobalt-molybdenum alloy?Co₃Mo?and metallic molybdenum?Mo?precipitated on the surface of the TMO and worked as active sites with the resulting TMO as the heat treatment temperature increased;?2?precipitation of Co was beneficial to the intrinsic activity due to its appropriate H adsorption energy,while the precipitation of Co₃Mo and Mo did the opposite owing to their stronger H adsorption energy,and that was also the main reason for the variation of catalyst activity.The results of this study will give meaningful guidance for the modification of other binary or multicomponent TMOs as catalysts for the HER so as to expand their application in electrocatalytic water splitting.