Electrochemical deposition and characterization of mixed-valent rhenium oxide films prepared from a perrhenate solution

Cathodic electrodeposition of mixed-valent rhenium oxides at indium tin oxide, gold, rhenium, and glassy carbon electrodes from acidic perrhenate solutions (pH =1.5 +/- 0.1) prepared from hydrogen peroxide and zerovalent rhenium metal is described. Cyclic voltammetry, variable angle spectroscopic ellipsometry (VASE), X-ray photoelectron spectroscopy (XPS), UV-vis spectroclectrochemistry, and electrochemical quartz crystal microbalance (EQCM) data indicate that the chemical nature of the electrodeposited rhenium species depends mainly upon the potential and supporting electrolyte. The presence of SO42- as a supporting electrolyte inhibits the adsorption of perrhenate, ReO4-, at non-hydrogen adsorbing electrode materials. However, in acidic perrhenate solutions containing only protons and ReO4- anions, strong adsorption of ReO4- at potentials preceding hydrogen evolution occurs. This leads to the formation of an unstable (R2O3)-O-III intermediate which catalytically disproportionates to form mixed-valent rhenium films consisting of 72% (ReO2)-O-IV and 28% Re-0. During the hydrogen evolution reaction (HER), hydrogen polarization causes the principle deposit to be more reduced, consisting of. roughly 64% (ReO2)-O-IV and 36% Re-0. Conclusively, metallic rhenium can be deposited at potentials preceding the HER at non-hydrogen adsorbing electrode materials, especially in the absence of SO42- anions.