Aging Studies of Voltammetrically Dealloyed Pt-Cu Nanoparticle ORR Electrocatalysts

Abstract

Voltammetrically dealloyed Pt-Cu alloy nanoparticles show significantly improved Pt-mass and specific activities for the oxygen reduction reaction in acidic media in both RDE and MEA configurations. The active phase of the catalyst is prepared from very Cu-rich precursor materials which are formulated into catalyst inks and electrode layers. The high Cu content of the precursor materials raises fundamental and practical questions as to the molecular stability / oxidation state of the Cu rich catalyst surface and the shelf-life of precursor powders, inks and electrode layers. Here we present aging studies of a carbon-supported Pt25Cu75 nanoparticle catalyst precursor. We monitored how the surface area (ECSA) and the oxygen electroreduction (ORR) reactivity of dealloyed precursors (active form of catalyst) change as a function of the exposure time to air of the Pt25Cu75 precursor at room temperature. Similarly, we aimed to investigate how ECSA and ORR of the active catalyst changed with the time the liquid catalyst ink was exposed to air. We find little detrimental effect of air aging of the precursor on ORR activity over the time scales investigated. The effect of air aging of the catalyst inks is more complex, however. Initially, aged inks show increased ORR reactivity, that is, aged inks result in more active catalyst layers. After about 4-5 days of aging of the inks in air, however, we observed a drop in ORR activity of the resulting catalyst layers. This complex aging behavior of the inks seems independent of prior air aging of the dry powder precursor.