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

Electrochemical measurements showed an ≈75% Pt surface area loss and an ≈40% specific activity loss for a membrane electrode assembly (MEA) cathode with acid-treated "Pt 3 Co" 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 Pt x Co nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated Pt x Co particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell Pt x Co particles was largely responsible for the reduction in the specific activity of Pt x Co 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 electro- chemical surface area loss, while particle growth of the Pt x Co particles via Ostwald ripening played a lesser role.

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