Rational Design Of High-efficiency And Durable Platinum-based Cathodic Catalysts Utilizing Surface Decorating Or Core Ordering Strategies

Proton exchange membrane fuel cells（PEMFC）have shown great application prospects in solving various environmental and energy problems.The sluggish kinetics of oxygen reduction reaction（ORR）at cathode and the poor electrocatalyst durability are the main obstacles,and it is thus urgent to develop high-performance electrocatalysts,such as Platinum（Pt）-based nanostructures.However,it is challenging to simultaneously achieve weak adsorption and strong cohesion on one surface,and thus high activity and durability for Pt-based electrocatalysts.In this thesis,utilizing density functional theory（DFT）calculations,the stabilization mechanism of two strategies,the introduction of chemically ordered phase into the substrate or inert Au onto Pt surface,were explored for core-shell nanocatalysts.The main contents and findings are as follows:（1）For the core protection strategy,using Pt（111）over-layers stressed and modified by Pt-M（M=Fe,Co,Ni,V,Cu,Ag,and Pd）intermetallic（L1₀,L1₁,L1₂,and L1₃ ordered phases）as examples,the effects of substrate ordering and skin thickness on catalytic and cohesive properties were systematically investigated.Among varied substrate-skin combinations,the strain and ligand effects differently influence the surface catalytic and cohesive properties.On the one hand,high durable slabs require a stable ordered substrate and a small lattice mismatch between substrate and skin.More specifically,slabs with single-layer Pt skin usually exhibits the highest dissolution potential thresholds but too weak adsorption energy,except for several Pt₃M/Pt systems.On the other hand,high active slabs generally feature two to four mono-layer of Pt skin but lower thresholds of dissolution potential,except for L1₂-Pt₃M/Pt systems.When benchmarked against the extended Pt（111）,10 promising catalysts were theoretically identified,for which slight weaker adsorption and stronger cohesion properties were simultaneously achieved by synergistically tailoring the skin thickness and substrate chemical ordering.More specifically,all candidates exhibit 10-fold enhanced activity of Pt（111）,half of which show improved durability,such as mono-layer skin on L1₂-Pt₃Co or Pt₃Fe,double-layer Pt on L1₃-Pt₃Ni or Pt₃Cu,and triple-layer skin on L1₁-Pt Cu,while double-or triple-layer skin on L1₀-Pt Co or Pt Ni and double-layer skin on L1₂-Pt Fe₃show slightly poor durability.Although L1₀ and L1₂ based nanocrystals have been demonstrated extensively as outstanding catalysts,L1₁ and L1₃ ones hold great application potential.The coexistence of high activity and durability on the same surface is because the different responses of surface adsorption and cohesion properties on the strain effects and ligand effects.When constructing intermetallic-core@Pt-shell nanocrystals from this slab model,the necessity of eliminating low-coordinated Pt and the possibility of maximizing Pt（111）facets and core ordering by morphology engineering was highlighted.（2）For the surface passivation strategy,using stepped surface and faceted nanocrystal as examples,several dissolution-resistant candidates were theoretically identified,by utilizing the preferential segregation and chemical inertness of Au element,and simultaneously avoiding too much Au as the neighbor of Pt atoms.More specifically,when benchmarked against clean Pt samples,an increase in dissolution potential threshold,0.12-0.25 V for stepped surfaces and 0.02-0.12 V for faceted nanocrystals,can be archived for decorated configurations featuring fully or partly naked Pt（111）facets/terrace.The existence of these novel configurations is because Au atoms not only prefer to occupy undercoordinated sites in Pt,but also exhibit higher dissolution potentials than Pt atoms with identical coordination number,making selective passivation feasible.Moreover,the predicted area specific activities of these novel configurations are promoted several-fold comparing with clean Pt ones.These detailed understanding of selective defect passivation and synergy between skin thickness and substrate ordering provide guidance on the rational design of promising Pt-based ORR catalysts.