Surface/Interface Design Of Inorganic Solid Materials In Alkaline Electrocatalytic Water Splitting

The conversion and storage of renewable energy is important for sustainable development of human society.Electrochemical water splitting is one of the most promising options among existed technologies to achieve renewable energy conversion efficiently.Overall water splitting reaction can divide into two separate half reactions,namely,hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).Economic and efficient H2 production through water splitting relies heavily on high-performance and cost-effective HER and OER electrocatalysts.At present,electrocatalytic water splitting is mainly divided into two systems:acid proton membrane water electrolysis and alkaline water electrolysis.Among them,alkaline water electrolyzed system materials are widely available and do not depend on precious metals,and the life of catalytic devices is relatively long,so it is expected to achieve scale application in the future.Therefore,exploiting efficient electroctalysts with cheap price that used in alkaline water splitting is crutial.However,the development of electrocatalysts used in alkaline media faces with some challenges.For example,the surface transformation of materials during oxygen evolution makes the establishment of structure-activity relationship complicated and difficult;the generation of adsorbed hydrogen in alkaline hydrogen evolution involves the process of water molecule dissociation,which makes the overall catalytic kinetics slow.In view of these challenges,this thesis takes the cheap transition metal non-oxide as the material system,studies the behavior of surface structure transformation of the material for oxygen evolution reaction,and optimizes key factors such as electronic transmission to improve the overall catalytic activity of the material.Additionally,this thesis also proposes strategies for rational design of interface to boost slow hydrogen evolution reaction in alkaline solution.This thesis includes the following contents:1.Surface/interface structure and alkaline oxygen evolution reaction.Transition metal non-oxide materials are unstable under the condition of oxygen evolution reaction,and electrochemical surface oxidation will occur to produce actual catalytic activity sites.Such surface oxidation behavior makes the establishment of the structure-activity relationship between the original material and the activity of oxygen evolution reaction complicated.In this chapter,based on the research on the surface structure of materials before and after oxygen evolution reaction,the structure-activity relationship between material activity and structure is summarized through the exploration of the interface electrical behavior regulation of materials and the relationship between crystal structure and surface oxidation degree.(I).The rapid transfer of electrons from the reaction interface to the electrode directly determines the catalytic activity of the material after surface oxidation.In this work,we propose an interfacial electrical regulation strategy to enhance the OER catalytic activity of materials.Specifically,metallic Ni3C grows on conductive carbon to form Ni3C/C.Ni3C undergoes surface oxidation and generate NiOx surface layer during OER.However,the preserved metallic Ni3C core enable rapid electrons transfer in bulk part of material.Moreover,conductive carbon benefit to electrons transfer from oxide surface to current collector.Together,facile electrons transfer during catalysis render Ni3C/C enhanced OER performance.(2).The electrochemical surface oxidation of catalytic materials is closely related to its crystal structure.The surface oxidation of transition metal non-oxides during OER creating huge obstacle for establish corresponding structure-activity relationship.In this work,a series of nickel-based materials(Ni,Ni3Se2,NiSe,and NiO)with similar geometric morphology were prepared as our model of OER catalytic materials.Systematic characterization suggest all those nickel non-oxides experienced surface oxidation during OER forming a core-shell structure with bulk structure maintained.In combination of electrochemical analysis,HRTEM and EXAFS characterization,we found Ni3Se2 that with smaller coordination numbers,longer Ni-Ni bond length and metallic electrical behavior has optimum OER catalytic activity,which indicates that the material that is prone to surface oxidation and has good electrical conductivity has a high catalytic activity.2.Surface/interface structure and alkaline hydrogen evolution reaction.In the process of hydrogen evolution under acidic conditions,the adsorbed hydrogen atoms directly come from the protons in the solution,while the generation of hydrogen adsorption under alkaline conditions involves the dissociation of water molecules,which makes the whole catalytic process relatively slow.In this chapter,the synergistic optimization of water molecule adsorption and dissociation and free energy of hydrogen adsorption was achieved through surface interface regulation strategies such as heteroatomic modification and composite interface construction of the material,and the overall alkaline hydrogen evolution reactivity of the material was finally improved.(1).Heteroatomic modification can regulate the electronic structure of materials and further influence the catalytic behavior of surface active sites.To tackle the slow kinetics of HER in alkaline solution,we use Co2P as our model catalyst,and promote its alkaline HER by decorating with high electronegative oxygen atoms.The introduction of oxygen atom can induce generation of high valence state Co(δ+),which is beneficial to water adsorption and dissociation.Beside,oxygen incorporation can also lead to the optimization of hydrogen adsorption free energy.These advantages render oxygen incorporated Co2P as a better alkaline HER catalysts.However,a large amount of oxygen incorporation will decrease conductivity of materials drastically,leading to inferior HER activity.In this sense,a moderate oxygen incorporation can have optimum alkaline activity.(2).The presence of composite interface always facilitate acceleration of complex reaction.However,the creation of interface always accompanies many structural defects that would increase the amount of active sites,creating challenges for in-depth understanding of catalytic mechanism.In this work,our theoretical analysis predicts the synergistic effect in water dissociation and hydrogen adsorption free energy of creating NiSe/MoSe2 interface in alkaline solution.Subsequently,we design two materials(SC-NiSe/MoSe2 and WC-NiSe/MoSe2)with different interfacial interactions.A series of characterization reveal their microstructure difference.Moreover,electrochemical measurements suggest kinetic optimization contribute mainly to the enhancement of HER activity in NiSe/MoSe2 hybrids with abundant interface,whereas increase of electrochemical active area is a major contribution to the enhancement of activity in NiSe/MoSe2 hybrids with relatively weak interfacial interaction.This result demonstrates the importance for delicate design of interface for promoting alkaline HER.