Effect of Particle Size on the Dissolution of Pt3Co/C and Pt/C PEMFC Electrocatalysts

Abstract

Potentiostatic and potentiodynamic Pt and Co dissolution were investigated for three Pt3Co/C catalysts with particle sizes of 4.9, 8.1, and 14.8 nm in aqueous electrolyte at potentials encountered by the PEMFC cathode. For all three Pt3Co/C catalysts under prolonged potentiostatic dissolution, the dissolved Pt steady state concentration increases from 0.85 V to reach a maximum at 1.1−1.15 V and decreases at higher potentials. The dependence of the dissolved Pt steady state concentration on particle size reveals that catalyst stability decreases with decreasing mean particle size, whereas the stability under potential cycling conditions is non-monotonic with particle size. Preferential dissolution of Co from all three Pt3Co catalysts was observed at all potentials, which increases at >1.1 V, the region over which Pt dissolution decreases, reflecting the opposing effects of Pt oxide formation on Pt and Co dissolution. Comparison of Pt3Co/C and Pt/C with similar mean particle sizes (4.9 vs 5.0 nm) and particle size distributions reveals that the Pt3Co/C has a higher potentiostatic dissolution rate (4–5 times), a higher steady state concentration of dissolved Pt (∼2 times), and a larger change in electrochemically-active surface area (ECA) (18 times) after prolonged cycling, indicating that Pt3Co has inferior stability. The higher rates of Pt dissolution for Pt3Co vs Pt have been attributed to higher extents of surface Pt oxidation, as determined using voltammetric oxide reduction charges and the white line intensities in Pt L3 X-ray absorption spectra.