Proton Storage Chemistry in Aqueous Zinc-Inorganic Batteries with Moderate Electrolytes

The proton (H⁺) has been proved to be another important energy storage ion besides Zn²⁺ in aqueous zinc-inorganic batteries with moderate electrolytes. H⁺ storage usually possesses better thermodynamics and reaction kinetics than Zn²⁺, and is found to be an important addition for Zn²⁺ storage. Thus, understanding, characterizing, and modulating H⁺ storage in inorganic cathode materials is particularly important. In this review, recent advances regarding the proton storage chemistry in aqueous zinc-inorganic batteries with moderate electrolytes are systematically reviewed. First, the four proton storage reaction patterns of H⁺ insertion, H⁺/Zn²⁺ co-insertion, H⁺-dependent conversion, and H⁺-dependent dissolution/deposition reaction are explicitly presented. Meanwhile, the proton storage processes of multi-sites and multi-steps, and the Hopping and Grotthuss proton transport mechanisms are carefully introduced. Second, the characterization techniques of proton storage are systematically classified into four types of electrochemical characterization techniques of batteries, structural characterization techniques of inorganic cathodes, pH characterization technique of electrolyte, and quantitative analysis technologies of H⁺ storage contribution. Third, the structural engineering of proton storage modulation is preliminarily refined to be interlayer engineering, doping engineering, defect engineering, composite engineering, and other engineering. Finally, the emerging challenges and perspectives about future directions of proton storage chemistry are proposed.