Proton storage and transfer in aqueous batteries

Aqueous batteries are promising energy-storage devices due to their high safety, large capacity, and low cost. Recent studies have revealed significant proton involvement in aqueous batteries, even in non-acidic environments, attributed to the unique proton-transfer mode via hydrogen bonds in water. This review summarizes proton storage in solid electrodes and generalizes the impact of proton transfer on aqueous batteries from conceptual insights to practical examples. The specialized storage mode for protons as charge carriers is introduced, and the principles for electrode-material selection are proposed. Then, two distinct proton-transfer mechanisms are discussed, and the strategies to enhance aqueous-battery performance are analyzed. These strategies include reinforcing proton transfer in electrode materials for higher capacity and faster rates and impeding proton transfer in electrolytes and interfaces to reduce side reactions and expand the electrochemical stability window. Contradictions in proton-tuning strategies across different components are illustrated through detailed cases. This review addresses the general phenomena and challenges related to proton storage and transfer in rocking-chair-type aqueous batteries, aiming to inform the future design and utilization of protons in energy-storage systems.