pH-Induced versus Oxygen-Induced Surface Enrichment and Segregation Effects in Pt-Ni Alloy Nanoparticle Fuel Cell Catalysts

We present a voltammetric, spectroscopic, and atomic-scale microscopic study of how initial interfacial contact with high- and low-pH electrolytes affects the surface voltammetry, near-surface composition, CO binding, and electrocatalytic oxygen reduction reaction (ORR) of dealloyed Pt-Ni alloy nanoparticles deployed in fuel cells. The first contact of the catalyst with the electrolyte is critical for the evolution of the catalytically active surface structure, yet still insufficiently understood. Counter to chemical intuition, we find that voltammetric activation protocols in both pH 1 and pH 13 electrolytes result in similarly Ni-depleted surfaces with similar near-surface Ni/Pt ratios to a 2.5 nm depth, yet vastly different ORR reactivities. On the basis of our combined voltammetric, scanning transmission electron microscopy with the spectroscopic mapping by energy dispersive X-ray (STEM-EDX) microscopic and X-ray photoelectron spectroscopy (XPS) analysis, we conclude that oxygen-saturated alkaline electrolytes causes a strong surface segregation of the more oxophilic Ni component toward the particles surface, however in distinctly different ways depending on the pretreatment pH. Data suggest a controlling role of the initial thickness of the Ni-depleted Pt shell for the catalysis-driven segregation process. We analyze and discuss how such subtle differences in initial surface composition can unfold such dramatic subsequent variations in ORR activity as a function of pH. Our findings have practical bearing for the design of active Pt bimetallic ORR catalysts for alkaline exchange membrane fuel cells. If the non-noble oxophilic Pt alloy component is insoluble in the alkaline electrolyte, our results call for an imperative acid pretreatment to avoid surface blocking by oxygen-induced segregation. If the non-noble oxophilic Pt alloy component is soluble in an alkaline electrolyte, acid or alkaline, even nonpretreated Pt alloy catalyst may be employed.