Elucidating the Complex Oxidation Behavior of Aqueous H₃PO₃ on Pt Electrodes via In Situ Tender X-ray Absorption...

In situ tender X-ray absorption near-edge structure (XANES) spectroscopy at the P K-edge was utilized to investigate the oxidation mechanism of aqueous H₃PO₃ on Pt electrodes under various conditions relevant to high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) applications. XANES and electrochemical analysis were conducted under different tender X-ray irradiation doses, revealing that intense radiation induces the oxidation of aqueous H₃PO₃ via H₂O yielding H₃PO₄ and H₂. A broadly applicable experimental procedure was successfully developed to suppress these undesirable radiation-induced effects, enabling a more accurate determination of the aqueous H₃PO₃ oxidation mechanism. In situ XANES studies of aqueous 5 mol dm^-3 H₃PO₃ on electrodes with varying Pt availability and surface roughness reveal that Pt catalyzes the oxidation of aqueous H₃PO₃ to H₃PO₄. This oxidation is enhanced upon applying a positive potential to the Pt electrode or raising the electrolyte temperature, the latter being corroborated by complementary ion-exchange chromatography measurements. Notably, all of these oxidation processes involve reactions with H₂O, as further supported by XANES measurements of aqueous H₃PO₃ of different concentrations, showing a more pronounced oxidation in electrolytes with a higher H₂O content. The significant role of water in the oxidation of H₃PO₃ to H₃PO₄ supports the reaction mechanisms proposed for various chemical processes observed in this work and provides valuable insights into potential strategies to mitigate Pt catalyst poisoning by H₃PO₃ during HT-PEMFC operation.