Mechanistic study of the reduction of copper oxides in alkaline solutions by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy was used to study the mechanism by which copper oxides are reduced in alkaline solutions. For the reductions of CuO and Cu₂O, a capacitive loop and also an inductive loop under certain conditions were observed in the complex plane. The electrochemical impedance for CuO reduction was not greatly dependent on the solution alkalinity and the kind of alkali hydroxide. However, the electrochemical impedance for Cu₂O reduction was considerably affected by the kind and concentration of alkali hydroxide. The diameter of the capacitive loop, i.e., the charge-transfer resistance (R_ct), was increased with increase in solution alkalinity. It should also be noted that R_ct was increased in the order of KOH < NaOH < LiOH. These dependences were consistent with the good separation between the reduction potentials of CuO and Cu₂O in chronopotentiometric and voltammetric measurements with strongly alkaline electrolytes containing Li⁺. The inductive loop observed for the Cu₂O reduction at higher concentrations of KOH (>6 M) and LiOH (>0.2 M) suggested the existence of an intermediate species (probably CuOH). The specific inhibitory effect of Li⁺ ions on the reduction of Cu₂O might be explained by a possible stabilization of CuOH by Li⁺ ions.