X‐ray Photoelectron Investigation of Phosphotungstic Acid as a Protonconducting Medium in Solid Polymer Electrolytes

An X-ray photoelectron spectroscopic study was performed on phosphotungstic acid, H3PW12O40, a cluster compound based on interlinked WO3 units. It was observed that the W4f transition could span a range of 2.4 eV, depending on the type of cation substitution and the number of waters of hydration (0-13). The W4f(7/2) transition for alkali metal-substituted PTA averaged 35.7 eV, while PTA with low or no waters of hydration was 36.2 eV. Fully hydrated PTA (13 waters and higher) gave binding energies averaging 37.4 eV. Other W6+ compounds were studied as well, revealing another binding energy trend in going from octahedral to tetrahedral geometry. The chemical shift behavior in PTA was attributed to waters of hydration forming hydrogen bonds with the terminal oxygens of the Keggin ion, pulling valence electron density away from the W nucleus and increasing its net positive charge, thus increasing W4f binding energy.