Electrochemical study in acid aqueous solution and ex-situ X-ray photoelectron spectroscopy analysis of metallic rhenium surface

Abstract

• Exchange current density approximately at 10 –6 Acm −2 and Tafel slope at 0.06 Vdec −1 . • Volmer–Heyrovsky is the mechanism of hydrogen evolution reaction on metallic rhenium. • Double layer capacitance is controlled by adsorption hydroxide and sulfate anions. • There is a formation of surface complex between Re(I) and sulfate. The electrochemical behavior of metallic rhenium in acid aqueous solution was investigated in this study. Ex-situ X-ray photoelectron spectroscopy (XPS) spectra were recorded to the rhenium electrode, after controlled cathodic potential electrolysis, which decrease the amount of oxide present on the surface. This electrolysis implies the lowest quantity of surface oxide reported, to the best of our knowledge, in literature. The electrochemical behavior of Re(0) in 0.5 M H 2 SO 4 displays a separation of the overall potential interval into hydrogen region (potential inferior than −0.11 V vs RHE), double layer region (-0.11 V to 0.43 V vs RHE) and oxygen region (potential superior than 0.43 V vs RHE). The hydrogen region displayed an exchange current density in the order of magnitude of 10 −6 Acm −2 , and the Tafel slope was approximately at 0.06 Vdec −1 , which suggest that the Volmer–Heyrovsky reaction is the mechanism of hydrogen evolution reaction. The capacitance (≈500 μFcm −2 ) in the double layer region is controlled by adsorption of sulfate and hydroxide anions. The formation of the surface complex ( R e 2 1 + S O 4 2 - ), was suggested by ex-situ XPS analysis, which is to the best of our knowledge reported for the first time. In the oxygen region, the Tafel slope of the anodic branch is 130 mVdec −1 and the current corrosion is in the order of magnitude of 10 −7 Acm −2 . Ex-situ XPS provides further evidence indicating that the f levels of Re (0) are Re4 f 7/2 at 40.0 eV and Re4 f 5/2 at 42.4 eV. The oxidation state of Re(I) displayed the Re4 f 7/2 level at 41.3 eV and the Re4 f 5/2 level at 43.6 eV.