Operando and ex situ comparison of Pt and Pt3Co catalyst degradation under ORR in PEMFC

Platinum bimetallic alloys represent a promising class of catalysts for the oxygen reduction reaction in proton exchange membrane fuel cells. Among them, Pt3Co is characterised by higher performances than bare Pt, but also by different stability, as Co leaching is known to take place during fuel cell operation. To underline differences of catalyst behaviour in real operating conditions, the evolution of bare Pt and Pt3Co catalyst nanoparticles are here compared from pristine conditions, up to catalyst activation and aging via specific break-in and Accelerated Stress Tests (ASTs) procedures, respectively. Changes in catalyst chemistry were monitored via x-ray absorption and photoelectron spectroscopies, and via SEM-EDX. Results were combined with morphology analysis carried out via small-angle x-ray scattering. Results from both operando and ex situ measurements show as for bare Pt catalyst, both particle morphology and the ratio among metal-to-oxidised Pt do not change remarkably after the break-in, and that the Electrochemically Active Surface Area (ECSA) strongly reduces due to average particle size growth from 2.28 to 6.21 nm within the first 3000 AST cycles. Conversely, in Pt3Co catalyst, Co leaching strongly affects the break-in stage, by reducing particle size and decreasing the fraction of metallic Co. During ASTs, leaching continues also after the formation of the Pt-rich skin, which formation contributes to slow down ECSA reduction, with particle size growing from 2.59 to 6.14 nm in between 3000 and 6000 cycles only.