Comparative Study of the Electrocatalytically Active Surface Areas (ECSAs) of Pt Alloy Nanoparticles Evaluated by Hupd and CO-stripping voltammetry

This study intends to provide some insight in the up-to-date elusive assessment of a correct choice of method for estimating the active surface area of Pt alloy nanoparticle catalysts. Taking PtNi3 nanoparticles as an example, we have compared three types of electrochemically active surface area (ECSA) data, CO-ECSA, Hupd-ECSA, and Hupd/CO-ECSA, which were evaluated from CO stripping and underpotentially deposited hydrogen stripping steps applied at different times along a reference catalyst activity test protocol. Considering a total of six different detailed voltammetric test protocols, we address Pt alloy particle size effects, analyze the effect of the time of application of CO and hydrogen stripping, and study their effect on the Pt mass and Pt surface-specific activities for the oxygen reduction reaction (ORR). In a discussion of the ratio of CO charge to hydrogen charge, it is shown that this quantity is more complex than previously thought and not associated with a specific surface structure. The Hupd/CO-ECSA data are found to be a reasonable balance for the estimate of surface area normalized, so-called specific catalytic ORR activities.