Platinum-Alloy Cathode Catalyst Degradation in Proton Exchange Membrane Fuel Cells: Nanometer-Scale Compositional and Morphological Changes

Electrochemical measurements showed an approximate to 75% Pt surface area loss and an approximate to 40% specific activity loss for a membrane electrode assembly (MEA) cathode with acid-treated "Pt3Co" catalyst particles in a H-2/N-2 proton exchange membrane fuel cell after 24 h voltage cycling between 0.65 and 1.05 V vs reversible hydrogen electrode. Transmission electron microscopy, scanning transmission electron microscopy, associated X-ray energy dispersive spectroscopy, and high angle annular dark-field techniques were used to probe the microstructural changes of the MEA cathode and the compositional changes along the MEA cathode thickness and within individual PtxCo nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated PtxCo particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell PtxCo particles was largely responsible for the reduction in the specific activity of PtxCo nanoparticle after potential cycling. The Pt weight loss associated with the formation of Pt crystallites near the cathode/membrane interface largely contributed to the measured electrochemical surface area loss, while particle growth of the PtxCo particles via Ostwald ripening played a lesser role. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3258275] All rights reserved.