Study On The Interface Structure Control And Water Splitting Performance Of Transition Metal Materials

The increasingly exhausted fossil fuel resources and environment pollution have largely limited the world-widely sustainable development.As a result,the developing of low-carbon,highly efficient and clean energy has been long for all over the world.As a high calorific fuel,hydrogen is easily gained from varieties of sources and product nothing but water.Consequently,hydrogen is regarded as the best substitute of fossil fuel.And it is an ideal way to generate hydrogen by using the electric power coming from world-widely renewable energy,like wind power,water power,solar power and so on to electrolyze water.However,the high price and scarcity of noble-metal electrocatalysts have greatly limited the large-scale production of hydrogen.Moreover,using traditional electrocatalysts to drive water splitting need larger overpotential,and thus is unecononmical on energy conversation.As a result,designing efficient and low-cost electrocatalysts is very significant for the developing of hydrogen energy and the reformation of traditional energy structure.Comparing with other potential electrocatalytic materials,transiton metal materials(TMMs)are rich reserves,wide varieties,and the complicated crystal and electric structures make it possible to obtain an excellent performance of electrocatalysis by material design.Benefited from the above advantages,TMMs are the highest potential materials as efficient and cheap electrocatalysts for hydrogen production from water splitting.In this thesis,promoting the formation of intermediate states during water splitting process to improve the performance of TMMs is taken as the starting point.Combining with the synchrotron radiation XAFS and other characterization methods,we investigate and discuss the effect of composite structure on the local atomic coordination configuration of TMMs’ active sites,electronic structure and intermediate states,aiming to provide valuable guidance and reference for designing new TMMs with lower cost and better performance.The main contents in this dissertation are as following:1.Study of CoOOH-graphene nanosheets with strong surface hydrophilicity for water oxidationAs a part of the water electrolysis,oxygen evolution reaction(OER)involves multiple proton-coupled electron transfer processes,and thus greatly limits the overall efficiency of water splitting because of a high overpotential.In the work of ChapterⅢ,the composite structure of CoOOH with graphene was elaborately designed to regulate the surface electronic structure of CoOOH nanosheets so as to realize a high-efficient electrochemical catatlysis of water oxidation.XAFS results showed that the d orbit electrons of Co active site in CoOOH were rearranged to(t2g)5(eg)Ⅰ after combining with graphene.The decrease of surface water contact angle from 70° to 23 ° indicated the improved surface hydroxyl species absorption and reaction kinetics.Moreover first-principles calculations demonstrated that the strong interface electron coupling between CoOOH and graphene effectively improved the absorption energy of OH-from-0.25 eV to-1.18 eV,and so as to decrease the Gibbs free energy of intermediates during reaction.Benefited from above mentioned,the overpotential at 10 mA/cm2 of CoOOH-graphene in alkaline solution(pH=13.6)was reduced to 248 mV.2.In-situ XAFS study of FeP@NC nanoparticles with potential-driven surface active structure rearrangementUnderstanding the variation of active sites or structures on TMMs is of great importance for seeking the optimized design of efficient electrocatalysts.In the work of Chapter IV,based on the in-situ XAFS technology sensitive to local structure,we investigated the relationship between surface structure and electrochemical performance by in-situ observing the structure change during hydrogen evolution reaction.FeP nanoparticles with～15 nm diameter and N-dpoed carbon layers’ coat(FeP@NC)were well designed and showed an excellent electrochemical hydrogen evolution performance with lower overpotential---135 mV@10 nmA/cm2.In-situ XAFS and XPS results indicated that surface of FeP nanoparticles rearranged to more ordered coordination of[FeP5(OH)].The octahedral structure on surface was advantage to adjacent P atoms as favorable proton acceptor,and increased the absorption energy from-0.12 eV to-0.22 eV.This finding may open up a new avenue to optimize the design of earth-abundant and efficient HER electrocatalysts.3.Study of CoOOH/BiVO4 photoanode with surface trapping states for photoelectrochemical water oxidationPhotoelectrochemcial water oxidation can greatly reduce the bias potential of overall water splitting with the help of photo-generate carriers in photoanode.Consequently,it is important to improve the utilization of photo-generate carriers for a higher energy conversion efficiency.In the work of Chapter V,we coated BiVO4 with a thin layer of CoOOH nanosheet to reduce the trapping states by an order of magnitude.With the help of CoOOH nanosheet,the average consumption of photo-generate carriers decreased from 0.183 mC/cm2 to 0.019 mC/cm2.XPS and theory calculation indicated that there were Co-O-V oxo-bridge bonds forming in the interface after combining BiVO4 with CoOOH,which helped to eliminate the trapping states density in forbidden band of surface.Benefited from the coupling effect of composite structure,CoOOH/BiVO4 photoanode showed an excellent photoelectrochemical water oxidation performance with a lower onset potential of 0.20 VRHE and larger photo-current density of 4.0 mA/cm2 under 1 sun illumination in a neutral solution.