Decoupling proton reactivity from Zn²⁺ interfacial electrochemistry through solvent isotope substitution

The parasitic hydrogen evolution reaction (HER) and corrosion fundamentally limit the reversibility of aqueous zinc batteries, yet their mechanistic origin remains unresolved. Here, we establish an experimental framework that probes bulk/diffuse-layer electrolyte properties via isotopic substitution (H₂O → D₂O) in hydrated zinc electrolytes. This approach largely preserves Zn²⁺-centered interfacial electrochemical behavior, as evidenced by indistinguishable nucleation overpotentials and steady-state fast scan voltammetric responses at ultramicroelectrodes (UMEs), while selectively modulating bulk proton transport properties and HER kinetics. Despite largely preserved interfacial energetics, electrochemical mass spectrometry (ECMS) reveals a pronounced suppression of gaseous side products in D₂O-based electrolytes, directly implicating bulk and diffuse-layer H/D solvent dynamics as an important control over side reaction kinetics. Notably, this framework reveals that gas evolution emerges at comparable overpotentials across isotopic systems, indicating that the onset of parasitic activity is not governed by independent heterogeneous reaction pathways. Instead, these observations are consistent with a Zn⁰-associated activation process that includes corrosion-mediated HER during zinc electrodeposition, direct cathodic water reduction, or a combination of both pathways. Moreover, these findings reconcile and resolve long-standing observations in the zinc battery field in terms of HER mechanism and the additive literature, where trace components disproportionately enhance stability, by highlighting the critical roles of interfacial adsorption and double-layer structure alongside bulk solvation effects.