Promoting Electrocatalytic Oxygen Reduction Reaction At Step/edge Sites:A DFT Study

Proton exchange membrane fuel cell（PEMFC）is a state-of-the-art technology that can directly convert hydrogen into electrical energy with high efficiency and emission free.Efficient approaches for reducing and/or eliminating the employment of platinum are thus actively pursued by scientific community employing catalyst-by-design strategy to achieve the economic benefit but meanwhile not to compromise and even to enhance the ORR activity.Adding gold as the third element forming ternary Pt-based core-shell nanoparticles（NPs）is an efficient strategy for enhancing the durability of cathode during oxygen reduction reaction（ORR）.Herein,using high-index（221）facets as a model catalyst,we discovered from density functional theory calculations that the inert Au atoms localized at the surface edge sites of Pt-based NPs can effectively promote elementary steps of ORR.The main results were as follow:（1）To mimic the step/edge structure of NP,we built a five-layer p（2×1）Pt（221）slab,and the Au-terminated edge Pt surface with Ni at subsurface models were established.Our results show that at working potential（U=0.8V）,ORR at step/edges sites on Ni@Au Pt core-shell NPs is kinetically much facile than those on pure Pt NPs,and the rate-determining step is likely to be O2 protonation forming*OOH against*O protonation forming*OH on Pt NPs.We re-confirm that the surface activity of Pt atoms of core-shell NPs can be well explained by either d-band model or coordination-number model,which have the same origin of chemical nature.Our study provides fundamental understanding of functionality of step/edge sites of Pt-based core-shell NPs for ORR that can be used for engineering of practical cathode catalyst with atomic efficiency and precision by maximizing occupancy of gold atoms at edge sites.（2）Moreover,exploring the comprehensive effect of an electric field and solvation is vital to understanding the mechanistic aspects of the oxygen reduction reaction（ORR）under PEMFC working conditions.In this work,we investigated the ORR on Pt（111）,Pt（221）,and Ni/Au₁Pt₃（221）surfaces using periodic density functional theory（DFT）calculations.Overall,our calculations provide a detailed description of the oxygen reduction reaction on several different Pt-based surfaces,and this should provide important benchmarks for the chemical and structural design of future electrocatalysts.