Products of SO2 Adsorption on Fuel Cell Electrocatalysts by Combination of Sulfur K-Edge XANES and Electrochemistry

Electrochemical adsorption of SO2 on platinum is complicated by the change in sulfur oxidation state with potential. Here, we attempt to identify SO2 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). SO2 was adsorbed from a SO2/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 SO2 adsorption products at 0.1 V, while mixtures of S-0, SO2, and sulfate/bisulfate ((bi)sulfate) ions are suggested as SO2 adsorption products at 0.5 and 0.7 V. At 0.9 V, SO2 is completely oxidized to (bOsulfate ions. The identity of adsorbed SO2 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 SO2 speciation gained from XANES are used to compare platinum sulfur electronic interactions for Pt3Co/VC versus Pt/VC catalysts in order to understand the difference between the two catalysts in terms of SO2 contamination.