Influence of Electrolyte Composition and pH on Platinum Electrochemical and/or Chemical Dissolution in Aqueous Acidic Media

Comprehension of the impact of electrolyte nature and concentration on Pt degradation is essential for the improvement of durability of catalyst layers (CLs), which are the heart of polymer electrolyte membrane fuel cells (PEMFCs). Electrochemical and chemical dissolution of polycrystalline Pt in aqueous CF3SO3H, H2SO4, and HClO4 solutions of different concentrations (c = 0.1 and 0.5 M) upon potential switching and holding in the 0.60–1.20 V versus RHE range is analyzed using inductively coupled plasma mass spectroscopy. This potential range mimics the conditions encountered in operating PEMFCs. Trifluoromethane­sulfonic acid (CF3SO3H) is employed because it is the smallest fluorinated sulfonic acid and can serve as a model molecule. Degradation of Pt in H2SO4 and HClO4 solutions is examined for comparative analysis. The results reveal that the electrolyte concentration has a significant impact on Pt electrochemical and chemical dissolution. The amount of dissolved Pt in 0.1 M solutions of CF3SO3H, H2SO4, and HClO4 is practically the same and lower than that in analogous 0.5 M solutions. However, the amount of dissolved Pt in 0.5 M H2SO4 solution is greater than that in 0.5 M solutions of CF3SO3H or HClO4. The influence of anion nature and pH on Pt dissolution is examined in 0.1 and 0.5 M HClO4 solutions without and with 1.0 × 10–2 M H2SO4 addition. The results show that under these conditions the anion nature has no or negligible impact on Pt dissolution, but pH significantly affects the process. An analysis of potential versus pH diagrams (Pourbaix diagrams) for acid solutions of different pH values suggests that Pt degradation (with the formation of Pt2+(aq) and Pt4+(aq)) might proceed through both electrochemical and chemical pathways.