A Universal Descriptor in Determining H2 Evolution Activity for Dilute Aqueous Zn Batteries

Abstract Large‐scale energy storage with aqueous Zn batteries (AZBs) have bright future, but their practical application is impeded by H 2 evolution reaction (HER), which results in poor stability of Zn–metal anodes. Here, using linear sweep voltammetry in dilute salt aqueous electrolytes, it is discovered that as the salt concentration decreases, HER is gradually suppressed, which is contrary to prior beliefs. Combining calculations and experiments, it is demonstrated that HER derives predominantly from the sum of Zn 2+ ‐solvated water rather than the average amount of water in the Zn 2+ ‐solvation structural unit or free water without interaction with Zn 2+ , which answers the puzzle from above. This result, which differs fundamentally from the previous understandings, sheds new light on the mysterious role of water chemistry in controlling HER and contributes to a more rational design of advanced electrolytes for AZBs.