Research On The Design And Performance Of Metal/Oxides Based Water Splitting Electrocatalysts

As a green and clean energy source,hydrogen plays an important role in many electrochemical energy conversion and storage devices.E lectrolyzing water splitting is an efficient and clean way for hydrogen generation,which has been widely concerned by researchers.However,both HER and OER in water splitting reaction have slow kinetics and large overpotential,which seriously lowers the energy conversion efficiency of electrocatalytic reaction.It is necessary to use an efficient catalyst to reduce the overpotential of the reaction to improve the energy efficiency.Transition metals and their oxides are commonly used electrocatalysts for water splitting.Metals have excellent conductivity and stability;oxides have various structures and can be easily synthesized and modified.Current commercial water splitting electrocatalysts are mainly based on precious metals and their oxides,however,their high costs and scarcity seriously hindered the large-scale applications.In order to reduce the costs and achieve commercialization,the design strategies for water splitting electrocatalysts mainly include reducing the content of precious metals in the catalyst and using non-precious metal catalysts replacing precious metal-based materials.Benefiting from the advantages of metals and oxides,the metal/oxides interactions,interfacial synergetic effects and structural evolution,metal/oxides-based catalyst is a kind of promising water splitting electrocatalyst with low-cost,high activity,and stability.In this paper,we mainly synthesized metal/oxide water splitting electrocatalysts by designing the element composition,crystal structure and surfa ce electronic structure of nanomaterials and investigated their electrocatalytic performance.Firstly,molybdenum oxides modified nickel-iron alloy was synthesized with assisted by interlayer limiting effect.Its'high specific surface area,porosity and high oxidation state metal sites on the surface were conducive to improving OER catalytic activity.Secondly,cobalt borate nanosheets were in-situ grown on the surface of cobalt metal.The rich defective structures and interfaces on the surface of nanoshee ts maintained the excellent conductivity,exposed more active sites,and improved the intrinsic activity of catalyst.Furthermore,a WO₃ supported low-platinum catalyst was synthesized by a photochemical method and the in-situ characterization revealed the promoting effect of supporter in electron transport and intermediates adsorption for the HER catalytic activity of Pt.Finally,the alloy catalyst supported by low valence molybdenum oxides was synthesized by co-reduction method,which integrated the good conductivity of the oxide supporter and the optimization of catalytic adsorption process,realized the high performance of HER in alkaline solution.The specific research contents are as follows:(1)Utilizing the interlayer confined effect of molybdenum intercalated layered hydroxide,molybdenum oxide-modified nickel-iron alloy nanosheets have been synthesized as OER electrocatalysts.Molybdenum oxides limited the aggregation of nickel-iron alloys at high temperature,the nanosheets formed have a higher specific surface area and good porosity,which improves the oxidation state of the metal on the alloy surface and promotes the formation of metal/oxyhydroxide core-shell structures during the OER process.This structure ensured the high conductivity and high OER intrinsic activity of catalysts.The electrochemical test results indicate that the catalyst has excellent OER activity in alkaline solution with an overpotential of 276 m V at th e current density of 10 m A·cm^-2 and the slope of Tafel of 55 m V·dec^-1.In addition,this confined synthesis strategy can also be extended to synthesize other similar metal/oxide catalysts.(2)A core-shell catalyst composed of metal core and nanosheet shel l was synthesized by in situ growth of cobalt borate nanosheets on metallic cobalt surface.and.There are many defect structures including grain boundaries,dislocations,vacancies and metal/oxide interfaces on the material surface,which not only exposes more active sites,but also enhances the intrinsic activity of the surface metal sites;moreover,the core-shell structure also ensured fast electronic transfer.The catalyst has excellent OER activity with an overpotential of 373 m V at the current densit y of 100m A·cm^-2 and a large working current density of 500 m A·cm^-2 under an overpotential of 400 m V.In addition,the characterization after electrochemical testing found the structure evolution of catalyst and the formation of oxyhydroxide on the surface as the real active location of catalysis.(3)A series of WO₃ supported low platinum HER catalysts with Pt nanoclusters anchored on the surface of WO₃ were prepared by photochemical methods.The HER activity and reaction kinetics of these low platinum catalysts in acidic solution are close to commercial Pt/C.When the loading of Pt is as low as 0.365 wt.%,the mass specific activity is 33 times than that of Pt/C catalyst.Electrochemical impedance spectroscopy and in-situ Raman spectroscopy were used to study the electronic characteristics of catalysts and the structure evolution during the electrochemical process.It was found that the WO₃ undergone a reversible phase transition during electrochemical process to form H_xWO₃,the H_xWO₃ phase with fast proton and electron transfer ability not only promoting the electron transfer process but also provide a hydrogen adsorption pathway on the Pt/H_xWO₃ interface.Therefore,this study reveals the important role of the in-situ phase transition of WO₃ in accelerating electron-proton transfer and hydrogen intermediates adsorption process,thereby promoting the HER activity of Pt.(4)A series catalyst with MoO_x nanosheets supported PdRu alloy were prepared by high temperature co-reduction method.The material has a porous nanosheet structure,which can expose more active sites and promote the diffusion of reactive species.Characterization results demonstrated the charge transfer effect between Pd and Ru in PdRu alloy catalysts,which changed the electronic structure and optimized the hydrogen adsorption energy of metal atoms.Furthermore,this bimetallic site catal yst possesses excellent water cleavage ability and appropriate hydrogen adsorption energy so that the HER reaction kinetics have been largely improved.The Pd₂Ru₂/MoO_x alloy catalyst has a higher HER activity than Pt/C in alkaline solution with an overpotential of 50 m V at current density of 10 m A·cm^-2 and Tafel slope of 27.5 m V·dec^-1.Electrochemical impedance spectroscopy results confirmed tha t the excellent HER activity is not only due to good electron transfer ability,but also because of the fast water cleavage and hydrogen adsorption process at the catalytic interface.