Localized Water Restriction in Ternary Eutectic Electrolytes for Ultra‐Low Temperature Hydrogen Batteries

Proton batteries are promising candidates for next-generation large-scale energy storage in extreme conditions due to the small ionic radius and efficient transport of protons. Hydrogen gas, with its low working potentials, fast kinetics, and stability, further enhances the performance of proton batteries but necessitates the development of novel electrolytes with low freezing points and reduced corrosion. This work introduces a localized water restriction strategy by incorporating a tertiary component with a high donor number, which forms strong bonds with water molecules. This approach restricts free water molecules and reduces the average hydrogen bond ratio and strength. As-prepared ternary eutectic electrolytes lowered the freezing point to -103 °C, significantly lower than the traditional binary electrolyte (9.5 m H₃PO₄, -93 °C). This electrolyte is highly compatible with the Cu_0.79Co_0.21[Fe(CN)₆]_0.64 ⋅ 4H₂O (CoCuHCF) cathode, reducing material dissolution and current collector corrosion. The H₂||CoCuHCF battery using this electrolyte demonstrated a high-power density of 23664.3 W kg^-1, excellent performance at -80 °C, and stable cyclability over 1000 cycles (>30 days) at -50 °C. These findings provide a framework for proton electrolytes, highlighting the potential of hydrogen batteries in challenging environments.