Study On Synthesis And Electrocatalytic Performance Of Cobalt Phosphide-based Nanocatalysts

The increasing demand for global energy bring us more and more environmental pollution problems.Hydrogen,as a new type of energy carrier,has attracted huge attentions.Producing hydrogen by water electrolysis is one of the most promising technology,but this reaction is inert and need electrocatalyst.Pt-based noble metal materials are the most effective electrocatalysts,but the high price and resource scarcity make it is difficult to achieve mass industrialization.So it is significative to develop non-noble metal catalysts with low cost and high abundance.In this paper,with cobalt phosphides based catalyst as study object,controllable synthesis strategy and electrocatalytic performance for water electrolysis of the single and binary metal phosphides were studied.By the design and composition of cobalt phosphide nanorods with heteroatoms doped carbons,the positive effect of doping effect and synergistic effect on electrocatalytic performance for hydrogen evolution reaction?HER?is revealed.Basing on this,the controllable synthesis law of bimetal phosphides derived from different metal precursor was developed and the composition,structure and morphology state of the active center in different nanostructured bimetal phosphides were also studied.Moreover,by associating the influence of the electronic structure with electrocatalytic performance,the electrocatalytic mechanism was explored.Our work produced theories basis for the design of high efficiency electrocatalyst for water electrolysis.The electronic structure and properties of carbon material can be modified by the doping of the heteroatom,which equip the inert carbon material with electrocatalytic activity.Graphite oxide?GO?was first synthesized with the Hummers method,then,N,S atoms were co-doped in graphite oxide?NSGO?.Lastly,cobalt phosphides nanorods were in situ growth on NSGO by solution phase reaction.The influence of the doping effect on HER was studied.The results suggested the synergistic effect of different component and the doping effect undertake the good electrocatlytic performance of CoP/NSGO.The introduction of the N and S atoms promoted the formation of the pyridinic N,pyrrolic N and thiophene S species,which also can be worked as the active sites.The CoP/NSGO displayed long time durability and could keep the activity for at least of 60000 s.Owing to the diversity of carbon based materials,carbon materials with different defect structure could be achieved by deriving from different carbon source,then produced different catalytic effect.Basing on this,using low cost biomass sodium alginate as the C source,ammonium hypophosphite as N,P sources,by the direct high temperature pyrolysis under a flow of argon,the N,P atoms could be co-doped in the carbon skeleton of the porous carbon flakes?NPCFs?.Lastly,the CoP decorated NPCFs was constructed.The studies suggested the defect structure riched biomass derived NPCFs showed synergistic effect with CoP,so the electrocatalytic performance for HER was improved greatly.Binary metal phosphides showed synergistic effect and the improved proton discharge process,which could promote the electrocatalytic performance greatly.Basing on this,we design binary metal?Co,Mo?phosphides.Using CoMoO₄ nanorods as precursor,by the gas-solid phase reaction with PH₃?generated by the Na₂HPO₂?in different temperature,a series of Co-Mo-P catalysts with different composition and morphology were produced.The results suggested phosphatization temperature is vital in determining the composition and morphology of the products.When the phosphatization temperature is 500?,the phosphatized products were single metal phosphides,and a rough rod-like morphology structure was formed.When the temperature was increased to 600?,besides the single metal phosphides,binary metal phosphides CoMoP phase was appeared,and the hollow tube-like structure was formed.When the temperature sequentially increased to 700?and 800?,the phosphidation process was not efficient enough which results in phosphates or pyrophosphates were included in the products.Due to the larger surface area and the more favorable component composition in CoMoP-600,it showed the best HER electrolytic performance among these as-synthesized phosphides catalysts.The rational comparison of single metal phosphides and bimetal phosphides for their catalytic activities is highly desired for understanding the origin of activity and catalytic mechanism.Single phase bimetal NiCoP nanocrystals?NCs?were prepared by a self-templating synthesis strategy using single metal phosphides?CoP and Ni₅P₄?as precursor.The structure,morphology and composition were analyzed by a variety of characterization methods.The HER and oxygen evolution reaction?OER?catalytic performance of the bimetal NiCoP were measured,and the full water electrolysis was also performed.The results suggested bimetal NiCoP showed better catalytic activity than the single metal phosphides?CoP and Ni₂P?on water electrolysis.The electron structure was studied with X-ray photoelectron spectroscopy and synchrotron-radiation-based X-ray absorption spectroscopy.The results suggested NiCoP showed different electronic structure with the single metal phosphides?Ni₂P and CoP?.The valence of Ni in NiCoP is higher than that of Ni₂P and lower than that of NiO?+2?.Similarly,the valence of Co in NiCoP is lower than that of Co₃O₄?+3,+2?,CoO?+2?and CoP,the valence of P in NiCoP is lower than that in single metal phosphides,which suggested electron transformation was not only exist between metal atoms and P atom,but also exist between the metal atoms?Ni,Co?,which make Ni in NiCoP has stronger electron collect ability and P has stronger protons capture ability,both of which could promote the water electrolysis process.What's more,the density functional theory suggested NiCoP showed more optimal free energy of hydrogen adsorption,significantly increased charge density around Fermi level and the reduced filling of the antibonding states,proving the superiority of the NiCoP catalyst in the electrolytic process.