Nanoengineered PtVFe/C Cathode Electrocatalysts in PEM Fuel Cells: Catalyst Activity and Stability

The understanding of factors controlling electrocatalytic activity and stability of carbon-supported multimetallic catalysts is essential for advancing the design of fuel-cell electrocatalysts for oxygen reduction. The structural and compositional changes of trimetallic PtVFe/C catalysts have been investigated by several techniques, including XRD, X-ray absorption fine structure (XAFS), and inductively coupled plasma (ICP) analyses, in addition to electrochemical and fuel-cell performance tests. The investigation aims at understanding whether the base metals incorporated into the multimetallic catalysts are stable and how the base metals in the Pt-alloy are stabilized mechanistically. Changes are detected for the lattice properties and the composition of the base metals in the PtVFe nanoparticles after long-term exposure to air, after thermal treatment at different temperatures, and after use in proton-exchange membrane (PEM) fuel cells. These changes show certain correlations with the structure, activity, and stability of the catalysts. The fcc alloy phase is found to be predominant for catalysts treated at low temperature, but changes to a tetragonal-type alloy phase when the catalysts are treated at a higher temperature. In the low-temperature-treated catalyst, Fe has more bonds with O atoms, whereas in the high-temperature-treated catalyst, Fe has fewer O neighbors and Fe-Fe bonds are detected. Analysis of the catalysts after testing in fuel cells indicates that the percentage of iron oxide is reduced, reflecting the propensity of dissolution of Fe oxide in the electrolytes. The smaller lattice parameter for the higher-temperature treated catalyst is considered to be an important factor in determining the catalyst stability under conditions related to fuel-cell operation.