Theoretical Design Of Dual Atom Catalysts And Study On The Catalytic Mechanism Of Oxygen Reduction

Proton exchange membrane fuel cell（PEMFC）has attracted extensive attention because of its high energy conversion rate and environmental friendliness,which is of great significance to solve the problems of environmental pollution and energy shortage faced by mankind.The fuel cell cathode oxygen reduction reaction（ORR）has slow kinetics and requires Pt as a catalyst.However,the low content and high cost of Pt in the earth’s crust seriously hinder its commercial application.Therefore,the development of ORR catalysts with high catalytic activity,high stability and low cost will greatly promote the wide application of fuel cells.In this paper,the effects of the coordination environment and metal-metal interactions on the catalytic performance of ORR were systematically investigated by density functional theory（DFT）.（1）The ORR activity and reaction mechanism of single and dual Pd atom catalysts were investigated.The electronic structure of Pd was regulated by different electronegative coordination atoms（C,B and N）and homonuclear metal-metal interaction,so as to obtain highly active ORR catalysts.The results show that B with low electronegativity transfers electrons to Pd,which makes Pd with high electron density state,reduces its work function,and enhances the ability to transfer electrons to O₂.However,the catalysts coordinated with C and N make Pd show low electron density state.In addition,the metal-metal interaction of dual-atom catalyst（DAC）is beneficial to weaken the O-O bond,and further reduces the work function of the catalyst to improve its catalytic performance compared with the single-atom catalyst（SAC）.Calculating the Gibbs free energy change and oxygen reduction overpotential of each step of the elementary reaction,it is found that Pd₂B₆/C has high ORR activity and 4e^-selectivity.（2）The ORR catalytic activity and mechanism of transition metal（Fe,Co,Ni）as active center for homonuclear and heteronuclear dual-atom catalyst（DAC）were studied.The O₂adsorption strength of homonuclear DAC showed the order of Ni₂B₆/C<Co₂B₆/C<Fe₂B₆/C.However,the adsorption strength of heteronuclear DAC for O₂ is between the strongest（Fe₂B₆/C）and the weakest（Ni₂B₆/C）,achieving a more accurate adjustment.The results of d-band center and work function explain the law of O₂ adsorption energy on homonuclear and heteronuclear catalysts.In addition,the volcano diagram and crystal orbital Hamiltonian layout（COHP）analysis showed that Ni₂B₆/C has moderate adsorption strength for O₂ and O-containing intermediates,thus exhibiting high 4e^-selectivity and ORR catalytic activity.（3）For the dual-atom（Ni）catalyst with the best activity in the above studies,its electronic structure was further adjusted by B,C or N co-coordination,and its ORR catalytic activity and catalytic mechanism were studied.It is found that the co-coordination of B,C or N regulates the d-band center and work function of catalyst,resulting in the difference of ORR activity.The coupling of co-coordination effect and metal-metal interaction significantly weakens the linear relationship betweenΔG_*O andΔG_*OH,which can relatively independently adjust the adsorption strength of O-containing intermediates and improve the catalytic activity of ORR.