Study On Modified Hydrogen Evolution Performance Of Molybdenum Carbide-based Electrocatalysts

In recent years,the excessive use of traditional fossil fuels has caused a series of resource and environmental problems,and exploring new types of environmentally friendly renewable energy has become the focus of worldwide attention.Hydrogen energy is considered to be a new type of renewable energy that is expected to replace traditional fossil fuels in the future because of its high energy density and environmental protection of emissions products.The products of electrochemical water splitting are only hydrogen and oxygen with environmentally friendly reaction process,which is considered to be the most promising large-scale hydrogen producing method.Hydrogen evolution reaction（HER）is a half-reaction in electrolyzed water reaction,and its reaction rate directly determines the hydrogen production efficiency.Electrocatalysts play an important role in the hydrogen evolution reaction and significantly affect the rate of electrochemical reactions.Among the hydrogen evolution catalysts,the precious metal catalyst platinum（Pt）has good catalytic activity,but its abundance in the earth’s crust is very low and the price is expensive,which limits its large-scale industrial application.Therefore,researchers began to study nonprecious metal catalysts to solve the problems faced by precious metal catalysts.However,at present,the use of non-noble metal catalysts to catalyze hydrogen evolution reaction still has a series of problems such as high overpotential,poor stability,and narrow pH environment,which cannot meet the needs of large-scale industrial applications.Transition metal carbides（TMCs）,especiallyβ-phase molybdenum carbide（β-Mo₂C）,have the advantages of low cost,high conductivity and good stability.In particular,it has a platinum-like d band electronic structure and catalytic behavior,which has attracted more and more attention.It is considered to be a nonprecious metal catalyst for hydrogen evolution reaction that can replace precious metal catalysts in the entire pH range.The use of heteroatom doping to adjust the electronic structure to improve the intrinsic catalytic activity ofβ-Mo₂C,and to construct a suitable morphological structure to increase the number of surface active sites,is widely recognized as an effective strategy to increase the catalytic activity of non-noble metal catalysts for hydrogen evolution reaction.This thesis is mainly aimed at the performance and mechanism of reduced graphene oxide（rGO）loaded heteroatom-dopedβ-Mo₂C(Mo₂E_xC_1-x/rGO,E=F,N,B or S)and cobalt foam supported binary metal carbide Co_xMo_xC（x=3 or 6）nanoparticles.The main content of this thesis is as follows:（1）Reduced graphene oxide supported heteroatom-dopedβ-Mo₂C nanoparticles(Mo₂E_xC_1-x/rGO,E=F,N,B or S)with nano size（average particle size of about 7.0 nm）and a clean surface were designed and synthesized by a simple hydrothermal-annealing process followed by treatment under CH₄（10%）/H₂ atmosphere.Their performance as a non-noble metal catalyst for hydrogen evolution reaction in a full pH range was systematically investigated.The doping of different heteroatoms has different promotion effects on the catalytic activity of Mo₂C hydrogen evolution.Among them,the F-doped electrocatalyst(Mo₂F_xC_1-x/rGO)showed the best activity in acidic,basic and neutral media,with initial overpotentials of 37,45 and 105 mV,and only 95 mV over-potential can reach the current density of 10 mA cm^-2 in acidic solution.In particular,the Mo₂F_xC_1-x/rGO electrocatalyst can reach a current density of 400 mA cm^-2 with an overpotential of only 232 mV and 397 mV in acidic and alkaline solutions.Under a neutral condition,only a 480 mV overpotential is needed to achieve a current density of 200 mA cm^-2.In addition,the Mo₂F_xC_1-x/rGO electrocatalyst showed excellent stability in catalyzing the HER reaction over the entire pH range.（2）Cobalt Foam（CF）supported binary metal carbides Co_xMo_xC（x=3 or 6）nanoparticles were used as hydrogen evolution non-noble metal catalysts to catalyze the HER reaction under full pH environment.Through the hydrothermal synthesis method,the ratio of reagents is adjusted,and the cobalt foam is used as the substrate to support the binary metal carbides of two phases of Co₃Mo₃C and Co₆Mo₆C nanoparticles（Co₃Mo₃C NP/CF and Co₆Mo₆C NP/CF）,respectively.Both Co₃Mo₃C NP/CF and Co₆Mo₆C NP/CF electrocatalysts exhibited an initial overpotential close to the commercial Pt/C in the HER reaction under full pH environment,that is,almost zero.Among them,Co₆Mo₆C NP/CF exhibits a more excellent HER performance,which the current density of 10 mA cm^-2 can be achieved only by an over-potential of 120 mV,128 mV,and 174 mV in acidic,alkaline,and neutral solutions.And the Tafel slope are 63 mV dec^-1,78 mV dec^-1,and 109 mV dec^-1,respectively.At the same time,Co₆Mo₆C NP/CF electrocatalysts showed good stability in catalyzing HER over the full pH range.