Electroreduction of [formula omitted] anions on chemically etched and electrochemically polished Bi(1 1 1) electrode

Abstract

Electroreduction kinetics of S 2 O 8 2 - to SO 4 2 - anions at chemically etched (CHE) and electrochemically polished (EP) Bi(1 1 1) electrodes has been studied using rotating disc electrode method. The surface nanostructure of CHE Bi(1 1 1) and EP Bi(1 1 1) electrodes has been studied by in situ STM and the very different values of root mean squared roughness (Rms) have been obtained (∼1000 times higher for CHE Bi(1 1 1) (Rms ⩾ 143 nm) than for EP Bi(1 1 1) (Rms ⩾ 0.145 nm)). The influence of the nanoroughness of CHE Bi(1 1 1) on the current density, heterogeneous reaction rate constant and corrected Tafel plots (cTp) has been demonstrated. For CHE Bi(1 1 1) the more pronounced inhibition of electroreduction reaction at moderate negative surface charge density has been observed in comparison with EP Bi(1 1 1), caused by the differences in surface charge density and also in diffuse layer ψ 0 potential drop values at crystallographically different homogeneous regions (planes) exposed at the surface of the macroheterogeneous polycrystalline CHE Bi(1 1 1) surface. The very low apparent transfer coefficient α app obtained indicates the nearly activationless charge transfer mechanism for S 2 O 8 2 - electroreduction at the CHE Bi(1 1 1) electrode similarly to EP Bi(1 1 1). However, α app only very weakly depends on Rms for the Bi electrodes at high negative surface charge densities where the values of ψ 0 potential are nearly equal for different planes at fixed electrode potential. At very high negative surface charge densities the cationic catalysis through the adsorbed ion pairs Na + S 2 O 8 2 - ) is possible.