Current-mediated suppression of hydrogen evolution reaction in determination of Zn-metal Coulombic efficiency

Coulombic efficiency (CE) is a crucial metric in battery research, particularly for aqueous Zinc (Zn)-metal batteries. Nonetheless, the accurate determination of Zn CE is complicated due to a lack of awareness about charge loss triggered by the hydrogen evolution reaction (HER) and non-standardized testing conditions. This study reveals the governing factors affecting the Zn CE measurement under different testing conditions, such as applied current density, Zn-plating capacity, and half-cell platforms. Through literature and experimental studies, it is evident that the Zn CE inherently increases with higher current densities and capacities. When decoupling the actual potentials of HER and Zn deposition, HER-triggered parasitic reactions can be self-suppressed owing to greater overpotential for HER than for Zn-plating at higher current densities. A consistent trend was observed when using different Zn salts and current collectors. This awareness can help standardize CE measuring protocols for validating novel concepts and materials.