Deciphering the cooperative roles of NH 4 + and H + in aqueous ammonium-ion batteries

Both ammonium and proton ions may serve as the charge carriers in aqueous ammonium-ion batteries (AAIBs); yet, their respective roles remain elusive. Here, we elucidate the contributions of NH₄⁺ and H⁺ to the redox behaviors of the copper hexacyanoferrate (CuHCF) electrode using operando pH monitoring, isotopic substitution, and acidity-controlled electrochemical analyses. We identify NH₄⁺ (de)insertion as the bulk redox process, while H⁺-coupled reactions act as auxiliary interfacial pathways that modulate charge-transfer kinetics. This work highlights interfacial proton management as a key design principle for achieving both high-rate performance and long-term electrochemical stability in aqueous energy-storage systems.