Water‐in‐Acid Strategy for Corrosion‐Free Proton Storage: Phosphoric Acid Electrolyte Engineering Toward Sustainable Aqueous Batteries

Abstract Aqueous proton batteries, leveraging the intrinsic advantages of protons such as minimal hydrated radius, natural abundance, and rapid transport kinetics, have emerged as promising candidates for next‐generation energy storage. However, conventional strong acid electrolytes like H 2 SO 4 suffer from critical limitations including electrode dissolution and incompatibility with battery components. To circumvent these challenges, weak acids (e.g., HCOOH and H 3 PO 4 ) have been strategically selected as alternative electrolytes due to their non‐corrosive characteristics. Particularly, the implementation of high‐concentration “water‐in‐acid” (WIA) effectively suppresses undesirable interactions between electrode materials and free water molecules. Through electrolyte engineering, we developed a 9.5 M H 3 PO 4 WIA system that synergizes with a molybdenum trioxide electrode, achieving remarkable electrochemical performance: a high reversible capacity of 229.8 mAh g −1 at 3 A g −1 and exceptional cycling stability with 83.86% capacity retention after 1000 cycles at 5 A g −1 , surpassing conventional H₂SO₄‐based systems by both capacity and cyclability. This innovative approach establishes a new paradigm for developing high‐performance aqueous energy storage systems through acid‐dominated electrolyte design.