Beyond the Artifact: In Situ Quantification of True HER Kinetics During Zn Electrodeposition in Aqueous Zinc Metal Batteries

Aqueous zinc metal batteries (AZMBs) present a safe, low-cost, and sustainable solution for stationary, grid-scale energy storage; however, their long-term stability is compromised by the parasitic hydrogen evolution reaction (HER) during zinc electrodeposition. While zinc electrodeposition kinetics (i_{0, Zn}²⁺_/Zn⁰) have been extensively studied, direct quantification of HER kinetics (i_0,HER) during zinc electrodeposition remains elusive. Common approaches in literature decouple HER from zinc electrodeposition, measuring i_0,HER in inert electrolytes without zinc ions. This fails to capture the true electrochemical environment and can lead to misleading conclusions regarding HER kinetics during zinc electrodeposition. Here, we introduce a novel method that combines electrochemical mass spectrometry (EC-MS) with electrochemical measurements to quantify HER kinetics directly and simultaneously during zinc electrodeposition by real-time monitoring of evolved H₂ gas. This approach captures the true i_0,HER under coupled reaction dynamics and reveals the limitations of conventional decoupling strategies. The platform enables rapid screening of current collectors and electrolyte additives, offering unprecedented insight into the interplay between the HER and zinc electrodeposition. Combined, this strategy provides a powerful framework for the rational design of materials and chemistries that enhance the stability and efficiency of AZMBs.