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

Abstract

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[subscript 3]Co ” catalyst particles in a H[subscript 2]/N[subscript 2] proton exchange membrane fuel cell after 24h voltage cycling between 0.65 and 1.05V 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[subscript x]Co nanoparticles before and after voltage cycling. Further Co dissolution from acid-treated Pt[subscript x]Co particles that leads to an increased thickness of a Pt-enriched surface layer and the development of core/shell Pt[subscript x]Co particles was largely responsible for the reduction in the specific activity of Pt[subscript 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 electrochemical surface area loss, while particle growth of the Pt[subscript x]Co particles via Ostwald ripening played a lesser role.