Modification of platinum surfaces with cerium species for promoting oxidative desorption of adsorbed sulfur

Abstract

Adsorption of sulfur (S) significantly reduces the electrochemically active surface area of platinum (Pt) electrocatalysts in polymer electrolyte membrane fuel cells (PEMFCs), namely, S poisoning. Mitigation techniques against S poisoning are strongly desired for highly durable PEMFCs. A Pt single-crystal surface was demonstrated to be modified with cerium (Ce) species by being immersed in a Ce-containing aqueous solution with hydrogen (H₂) gas bubbling or potential holding at −0.2 V vs. Ag/AgCl. For a Ce-free Pt electrode, electrochemical responses characteristic of the adsorption/desorption of hydrogen and hydroxyl species at the bare Pt surface disappeared due to the adsorbed elemental sulfur, S_ad, while the oxidative desorption of S_ad from the Pt electrode occurred at around 0.80 V vs. Ag/AgCl. In contrast, for the Ce-modified Pt electrode, the oxidative desorption of S_ad occurred at a potential around 0.3 V which is less positive (more negative) than that of Ce-free Pt electrode, showing the enhanced oxidative desorption capability due to the presence of Ce species on the surface. While the Ce species was desorbed from the Pt electrode simultaneously with the oxidative desorption of S_ad, the Pt surface can be re-modified with the Ce species by H₂ gas bubbling or potential holding at −0.2 V vs. Ag/AgCl, which is a similar condition to that of anode of PEMFC under operations. Thus, the Ce-modification of Pt surfaces potentially acts as a practical mitigation measure against the S poisoning.