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

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 (k_o) and the formal redox potential (E_o) 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 pK_a value of 10.20 upon the completion of the bicarbonate reduction reactions, confirmed the conversion of bicarbonate to carbonate ions.