Applications Of Density Functional Theory Calculations To The Study Of Nickel-based Hydrogen Evolution Electrodes

Because its only by-product is water hydrogen is considered to be the ideal clean fuel to replace traditional fossil resources.The development of highly active and low-cost Ni based hydrogen evolution electrodes has been a hot research topic.At present,nickel-based noble metal oxide electrodes such as Ni/RuO2 electrodes have been main commercial electrodes in water electrolysis.With the increase of price for RuO2,it is necessary to exploit hydrogen evolution electrodes with excellent catalytic performance and low price.Although there exist numerous studies about nickel-based alloy electrodes,the relationship between electrode structure and hydrogen evolution behavior is not clear so far.In order to exploit excellent hydrogen evolution electrodes,it is necessary to build the relationship between electrode structure and hydrogen evolution behavior.Besides,the preparation of Ni-Mo alloy electrodes usually takes place in the reactive environment with various adsorbates,and in practical applications the alloy catalysts can not avoid interacting with reactive adsorbates.A reactive environment may induce an enrichment of the surface Mo composition and thus change electro-catalytic performance of catalyst.Therefore,it is necessary to investigate adsorb ate-induced segregation in nickel-based hydrogen evolution electrodes.The interfacial cohesive strength of Ni/RuO2 electrode is usually found to be poor,which causes the active layer of RuO2 falling off from the Ni plate and thus reduce the electro-catalytic performance of catalysts.Based on the issues mentioned above,we focus our studies on these existing problems and difficulties in the process of the design,preparation and application of nickel-based hydrogen evolution electrodes.The main results are summarized as follows:(1)We had proposed a multiple-descriptor strategy for rational design of efficient and durable nickel-based bimetallic alloy electrode.We argued that excellent alloy catalysts for hydrogen evolution reaction(HER)should simultaneously have negative alloy formation energy,negative surface segregation energy of nickel and lower hydrogen binding strength than on pure Ni.By performing detailed DFT calculation on the thermodynamics,surface chemistry and electronic properties of Ni-M alloys,addition of a small amount of transition metal(M),such as Mo,Cr,Fe,V,or Co,to a Ni catalyst were predicted to have significantly improved the HER electro-catalytic activity of the nickel-based bimetallic alloy electrode.The relationships between the structures of the bimetallic alloy electrodes and their hydrogen evolution behaviors were studied.It was concluded that surface structure plays an important role in determinating the electro-catalytic activity,while the lower surface layers do not have an important influence on the electro-catalytic activity.Interestingly,Ni-M(M=Mo,Co)bimetallic alloy electrodes showed better catalytic performance when the ratio of M content between top atomic layer and second atomic layer was less than 1,while the Ni-M bimetallic alloy electrodes showed poorer catalytic performance when the ratio of M content between top atomic layer and second atomic layer was greater than 1.Besides,the relationships between the structures of the Ni3Mo-based trimetallic alloy electrodes and their hydrogen evolution behaviors were studied.It was concluded that the addition of third transition metal(M),such as Ti,V,Cr,Mn,Fe,Co,Cu,Hf or W could significantly improve the HER electro-catalytic activity and stability of the Ni3Mo-based trimetallic alloy electrode.Our results also indicated that the Ni surfece segregation behavior in the Ni3Mo/M(111)surface was governed by the synergistic influence of the surface energy effect,the strain effect and the solution heat effect.(2)The surface segregation in nickel-based bimetallic alloy electrodes with chemisorbed atomic oxygen had been investigated by first-principles calculations.The theoretical calculation results showed that the Mo surface enrichment could be due to the presence of chemisorbed atomic oxygen during the decarbonation of the Ni-Mo-Co electrode.The segregation trend of an alloy system under adsorbate environments could be governed by the effects of the segregation energy under vacuum conditions and the adsorption energies for the segregated and non-segregated alloy systems.The oxygen coverage could observably affect Mo or Co segregation behavior in NiM(111)surfaces.The Mo atom would segregate to the top-most layer when the oxygen coverage was thicker than 1/9 ML,while the Co atom would segregate to the top-most layer when the oxygen coverage was thicker than 2/9 ML.The structural evolution of the Ni3Mo(111)surface under oxidizing conditions was studied by ab initio atomistic thermodynamics.The thermodynamic phase diagram with the oxygen coverage up to one monolayer was constructed from numerous possible surface structures.We also discussed how to solve Mo surface enrichment in the Ni-Mo-Co electrode.(3)The surface stabilities of low-index surfaces of Ni and RuO2 were investigated by first-principles calculations.It was concluded that(111)was stable for Ni,both(110)and(101)were stable for RuO2.Based on the first-principles calculations and surface thermodynamic analysis the surface stabilities of RuO2 was evaluated as functions of oxygen partial pressure and temperature and the "surface phase digram" of RuO2 was constructed.The interfacial binding strength of Ni(111)/RuO2(101)were assessed by the work of adhesion(Wad).The Ni(111)/RuO2(101)interface energy was presented as a function of oxygen partial pressure and temperature.With the combination of first-principles calculations and surface thermodynamic analysis,a "interface phase diagram" of Ni(111)/RuO2(101)was constructed.We also discussed how to improve the interface stability of Ni/RuO2 electrodes.