Products of SO ₂ adsorption on fuel cell electrocatalysts by combination of sulfur K-edge XANES and electrochemistry

Electrochemical adsorption of SO ₂ on platinum is complicated by the change in sulfur oxidation state with potential. Here, we attempt to identify SO ₂ adsorption products on catalyst coated membranes (CCMs) at different electrode potentials using a combination of in situ sulfur K-edge XANES (X-ray absorption near-edge structure) spectroscopy and electrochemical techniques. CCMs employed platinum nanoparticles supported on Vulcan carbon (Pt/VC). SO ₂ was adsorbed from a SO ₂/N ₂ gas mixture while holding the Pt/VC-electrode potential at 0.1, 0.5, 0.7, and 0.9 V vs a reversible hydrogen electrode (RHE). Sulfur adatoms (S ⁰) are identified as the SO ₂ adsorption products at 0.1 V, while mixtures of S ⁰, SO ₂, and sulfate/bisulfate ((bi)sulfate) ions are suggested as SO ₂ adsorption products at 0.5 and 0.7 V. At 0.9 V, SO ₂ is completely oxidized to (bi)sulfate ions. The identity of adsorbed SO ₂ species on Pt/VC catalysts at different electrode potentials is confirmed by modeling of XANES spectra using FEFF8 and a linear combination of experimental spectra from sulfur standards. Results on SO ₂ speciation gained from XANES are used to compare platinum-sulfur electronic interactions for Pt ₃Co/VC versus Pt/VC catalysts in order to understand the difference between the two catalysts in terms of SO ₂ contamination.