Accelerated degradation of Pt3Co/C and Pt/C electrocatalysts studied by identical-location transmission electron microscopy in polymer electrolyte environment

Abstract

Identical-location transmission electron microscopy (ILTEM) coupled with X-ray energy dispersive spectroscopy (X-EDS) analyses were used to characterize the changes in the morphology and composition of Pt and Pt3Co nanoparticles deposited on high surface area carbon (Vulcan XC72) before and after electrochemical ageing tests performed in polymer electrolyte environment, using a “dry cell”. The Pt/C and Pt3Co/C electrocatalysts are modified upon electrochemical ageing, following changes in particle size, geometry, and composition; these changes are however milder to what happens upon aging in H2SO4 electrolyte, because of the lack of liquid water, a reactant in both carbon corrosion and Pt (Pt3Co) corrosion/dissolution reactions. The negative vertex potential of the ageing procedure also matters: Pt redeposition occurs at 0.1V vs. RHE and not at 0.6V vs. RHE, while carbon corrosion is emphasized after incursions at the lower vertex potential, in agreement to what demonstrated in liquid electrolyte. Besides, the presence of Co in Pt3Co alloys enables to somewhat slow-down the Pt corrosion from Pt3Co/C electrocatalysts, since cobalt acts as a sacrificial anode, which also lowers carbon corrosion.These morphology and composition changes were further used to explain the changes in ORR intrinsic activity of the electrocatalysts upon electrochemical aging; the ORR activity and the accelerated stress tests (AST) were measured/performed in a similar setup (at the interface with a polymer electrolyte) using an ultramicroelectrode with cavity. The ORR activity only improved for Pt/C nanoparticles when the AST contained the lower vertex potential (0.1V vs. RHE), thanks to the favorable increase of the particle sizes, favored because the Ptz+ ions released by the corrosion of the Pt/C nanoparticles at 0.9V vs. RHE remains trapped in the Nafion®, thereby easing its redeposition in the subsequent step at 0.1V vs. RHE. In all the other cases, the ORR activity decreased upon the AST. On Pt3Co/C the positive effect of Pt redeposition in the 0.1–0.9V vs. RHE ageing procedure is counterbalanced by a detrimental (and large) effect of Co dissolution, which adversely affects the nanoparticles composition for the ORR and pollutes the polymer electrolyte membrane (the Coy+ cations hinder O2 and H+ transport in the electrolyte membrane). After the 0.6–0.9V vs. RHE ageing procedure, the ORR activity always decreases, because the redeposition of Pt is not likely, therefore suppressing the positive effect of particle size increase monitored in the 0.1–0.9V vs. RHE ageing procedure, and because Co dissolution and adverse effect is maintained.