Electrochemical reduction of bicarbonate on carbon nanotube-supported silver oxide: An electrochemical impedance spectroscopy study

Abstract

Bicarbonate reduction on a silver-oxide (Ag2O)-based electrode was studied via cyclic voltammetry and electrochemical impedance spectroscopy techniques. The effects of electrode composition, electrolyte concentration and the scan rate (10–250 mV/s) were investigated at a temperature of 27 °C. An optimum mass ratio of 70/30 (Ag2O/CNT) led to a maximum current density of 83 mA cm−2 at −0.43 V (VS Ag/AgCl). At scan rates between 10 and 250 mV/s, a negative shift with a displacement of around 1.032 V was observed – indicating the presence of irreversible reduction reactions. The observed irreversibility suggested that the reaction mechanism can be described by both diffusion and adsorption phenomena. The standard heterogeneous rate constant (ko) and the formal redox potential (Eo) were found to be 1.51 × 10−4 cm/s and 1.218 V, respectively. The EIS results confirmed the formation of the inductive loops at reduction potentials – a consequence of the adsorption of the generated species. A reduction in charge transfer resistance and a continual drop in the potential from −0.1 down to −1.4 V was also observed. This was accompanied by H2 evolution and bicarbonate production. The calculated pKa value of 10.20 upon the completion of the bicarbonate reduction reactions, confirmed the conversion of bicarbonate to carbonate ions.