Competitive Tradeoff between Zn Deposition and Hydrogen Evolution Reaction on Zn-Metal Anode

In aqueous zinc batteries, the potential of the hydrogen evolution reaction (HER) is higher than that of Zn deposition, making HER unavoidable in actual charge/discharge cycles. Generally, concentrated electrolytes can reconfigure the solvation structures of electrolytes and suppress the HER. However, by analyzing various thermodynamic characteristics, concentrated electrolytes show a thermodynamic HER advantage, which seems "contradictory" to the dynamical HER disadvantage. Herein, based on ZnCl2 electrolytes, we quantitatively assess the consumption of Zn2+ using the variation in hydrogen bonds by correlating the dynamic evolution of interfacial hydrogen bonds and find the above contradiction lies in the ratio of the sum of Zn2+-H2O to Zn2+-Cl– coordination structures. Under the same Zn-deposition potential, a Zn2+-Cl–- rich and Zn2+-H2O-poor layer was formed at the electrode/electrolyte interface in concentrated electrolytes, contributing to Zn deposition rather than the HER. This work will deepen the understanding of how concentrated electrolytes regulate the competitive tradeoff between HER and Zn deposition.