Voltammetric quantification of H:Pd ratio complicated by α↔β phase transition in PdHx: Electrodes with low Pd loadings

Abstract

The Pd hydride (PdHx) is a typical system to study the fundamentals of solute intercalation and phase transformations or to determine the effect of strain on the rate of electrocatalytic reactions. A crucial methodological aspect, however, involves quantitatively determining the hydrogen-to-palladium atomic ratio (H:Pd) under experimental conditions where various incidental contributions to the total electrode charge are comparable to the charge spent for H adsorption/absorption and desorption. This is the case for electrodes with relatively low (dozens of μg/cm2) loadings of Pd on various porous supports, typically used to study the oxidation of small organic molecules or the reduction of oxygen in acidic or alkaline fuel cells. Ultra-low Pd loadings (often in the range of a few μg/cm2) are also typical for Pd nanoparticles on smooth supports, which are commonly considered as model systems for in situ structural studies of hydrogen sorption. To determine accurately the H content in Pd particles, we propose a technique based on analysis of charge disbalance at anodic and cathodic scans of a cyclic voltammogram, and also address the contribution of adsorbed H. To illustrate the possibilities and limitations of this technique, we present the voltametric study of thin electrodeposited Pd layers on glassy carbon and of a carbon-supported Pd catalyst.