Revealing the mechanism of solid-state electrochemical conversion reactions in strong alkaline solutions

Abstract

Solid-state conversion reactions are ubiquitous in various electrochemical systems such as metal-ion batteries, lead-acid batteries, electrochemical reduction of solid oxides, etc. However, a deep understanding of the reaction mechanism in the solid electrode remains challenging. In this paper, we systematically study the deoxidation of porous CuO in strong alkaline solution using three types of electrode configurations. The reduction rate of CuO depends on the cell voltage, temperature, reduction depth, and solubility of CuO, and the reduction follows a geometrical contraction model. The diffusion coefficients (cm2 s−1) of O2− increase from 7.38 × 10−11 at 80 °C to 1.19 × 10−9 at 120 °C. In addition, the diffusion coefficients of O2− during the reduction of solid Bi2O3 and Sb2O3 are 3.00 × 10−10 and 1.22 × 10−9, respectively. Besides, a dissolution-deposition mechanism is proven to play a key role in controlling the reduction rate and morphologies of the electroactive species. Overall, this paper sheds light on deeply understanding various solid-state electrochemical reactions in terms of ion diffusion and the interplay of electrode materials and electrolytes.