Frozen-State Electrochemistry Enables Proton-Selective Reaction Environments for Deterministic Atomic Construction.

Electrochemical reactions are conventionally confined to liquid electrolytes, where charge transport and mass migration are intrinsically coupled, defining both the reaction kinetics and structural evolution. Here, we show that frozen acidic electrolytes can serve as proton-conductive, mass-transport-restricted media for electrochemical single-atom construction. Within a defined subzero temperature window, frozen aqueous electrolytes support proton-selective charge transport through hydrogen-bonded ice lattices while suppressing the long-range migration of heavier ionic species. This transport asymmetry enables the charge-programmable, aggregation-suppressed construction of isolated metal sites. The resulting atomic configurations remained stable under electrochemical operation. This frozen-state, proton-selective reaction environment is general across diverse metals and supports, providing a low-temperature electrochemical strategy for single-atom-catalyst construction.