Theoretical Study On The Hydrogen Oxidation Activity Of Graphene Supported Non-noble Metal Catalysts

Alkaline fuel cell（AFC）has received extensive attention due to its outstanding advantages of cleanness and high-efficiency in recent years.Especially,with the development of the basic anion exchange membrane technology,the problem of carbonation of the electrolyte can be effectively avoided,which has led to the rapid development of AFC.At present,the hydrogen oxidation reaction（HOR）catalysts used in the anode of AFC mainly base on platinum group noble metals,which are expensive and scarce.The stability and activity of non-precious metal HOR catalysts are affected by the composition and structure,and the microscopic mechanism is complex and unclear,which greatly limits the development of such catalysts.This paper intends to use density functional theory（DFT）to systematically study the HOR mechanism of graphene-supported non-noble metal catalysts at the atomic level,and investigates the effect of catalyst types,sizes,substrates,doping on the oxidation resistance and HOR activity of the materials.The work carried out as follows:1.We constructe nickel nanoclusters(Ni₁₃)and graphene,nitrogen-doped graphene,and boron-doped graphene supporting nickel nanoclusters(Ni₁₃/G,Ni₁₃/BG and Ni₁₃/NG)models.DFT calculations demonstrate that the alkaline HOR proceeds via the Tafel-Volmer mechanism with the rate determining step of Volmer reaction on Ni₁₃,Ni₁₃/G,Ni₁₃/BG and Ni₁₃/NG.Graphene supporting and B/N-doping can effectively lower the d-band center of nickel nanoclusters,weaken the adsorption of H^*,and thus promote the Volmer reaction.Therefore,the HOR activity of the catalysts gradually increases in the order of Ni₁₃<Ni₁₃/G<Ni₁₃/BG<Ni₁₃/NG.In the Tafel-Volmer mechanism of HOR,only the adsorption of H^*is involved,without OH^*,so the free energy of hydrogen adsorption can be used as a descriptor of the HOR activity of the catalyst.However,pure Ni₁₃ clusters are strongly adsorbed by OH^*and thus are easily passivated and deactivated at low potentials.The phase diagram analysis show that after graphene loading and B/N-doping,the onset potential of the OH^*phase of Ni₁₃/G,Ni₁₃/BG and Ni₁₃/NG move up significantly,so the oxidation resistance is enhanced.In addition,after graphene loading and B/N-doping,the activation barrier of the H^*and OH^*recombination on Ni₁₃/G,Ni₁₃/BG and Ni₁₃/NG has also been effectively reduced,which favors the elimination of OH^*and thus reduces the passivation caused by the OH^*accumulation.Therefore,graphene loading and B/N-doping cannot only improve the HOR activity of nickel nanoclusters,but also enhance their oxidation resistance.2.A series of graphene-supported metal single-atom catalysts M/G（M=V,Cr,Mn,Fe,Co,Ni,Pt）are constructed.The strong binding energy between metal atoms M（M=V,Cr,Mn,Fe,Co,Ni,Pt）and graphene shows the high stability of the M/G structure.The electronic structure analysis shows that the catalytically active site is formed between the metal atom M and the adjacent carbon atoms in M/G,due to the unique microscopic coordination chemical environment.The HOR on M/G occurs via the Heyrovsky-Volmer mechanism with the rate determining step of the Heyrovsky reaction under alkaline conditions.The analyses of criteria such as reaction energy barrier,free energy of hydrogen adsorption,and reaction rate show that M/G（M=V,Cr,Mn,Fe,Co,Ni,Pt）catalysts prove suitable free energy of hydrogen adsorption,low reaction energy barrier,and high reaction rate.The sequence of HOR activity is Pt/G>Mn/G>Ni/G>Co/G>Fe/G>Cr/G>V/G.However,V/G,Cr/G,Mn/G,Co/G,and Fe/G have higher formation energy and lower oxidation potential,so they are prone to oxidative deactivation in an alkaline electrochemical environment.Therefore,the noble metal Pt/G and the non-noble metal Ni/G,which have both high HOR activity and high oxidation resistance,are very suitable candidates for AFC anode electrocatalysts.