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 2.5nm depth, yet vastly different ORR reactivities. Based on 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 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 alkaline electrolyte, acid or alkaline, even non-pretreated Pt alloy catalyst may be employed

A unique oxygen ligand environment facilitates water oxidation in hole doped IrNiOxcore shell electrocatalysts

The electro-oxidation of water to oxygen is expected to play a major role in the development of future electrochemical energy conversion and storage technologies. However, the slow rate of the oxygen evolution reaction remains a key challenge that requires fundamental understanding to facilitate the design of more active and stable electrocatalysts. Here, we probe the local geometric ligand environment and electronic metal states of oxygen-coordinated iridium centres in nickel-leached IrNi@IrOx metal oxide core–shell nanoparticles under catalytic oxygen evolution conditions using operando X-ray absorption spectroscopy, resonant high-energy X-ray diffraction and differential atomic pair correlation analysis. Nickel leaching during catalyst activation generates lattice vacancies, which in turn produce uniquely shortened Ir–O metal ligand bonds and an unusually large number of d-band holes in the iridium oxide shell. Density functional theory calculations show that this increase in the formal iridium oxidation state drives the formation of holes on the oxygen ligands in direct proximity to lattice vacancies. We argue that their electrophilic character renders these oxygen ligands susceptible to nucleophilic acid–base-type O–O bond formation at reduced kinetic barriers, resulting in strongly enhanced reactivities