Mitigating Hydrogen Evolution Reaction with Polyzwitterionic Hydrogel Electrolyte in Aqueous Aluminum‐ion Batteries

ABSTRACT Aqueous aluminum‐ion batteries are promising for grid‐scale energy storage due to their safety, low‐cost, and high theoretical specific capacity. However, the development is hindered by the hydrogen evolution reaction from water splitting and corrosion, causing poor reversibility in aluminum plating/stripping. Utilizing strong anti‐polyelectrolyte effect from aluminum cations and perchlorate anions, and the outstanding hydration strength from 2‐methacryloyloxyethyl phosphorylcholine (MPC), this study introduces a novel polyzwitterionic hydrogel electrolyte (PZHE) for AAIBs with MPC monomer and aluminum perchlorate electrolyte. PZHE binds water molecules under lean‐water conditions, greatly reducing free water activity and expanding the electrochemical stability window into 2.5 V. Despite limited water activity, ion migration channels created by zwitterionic phosphorylcholine groups enable high ionic conductivity of 4.22 mS cm −1 at 25°C. Consequently, the PZHE symmetrical cell achieves 600 h of reversible aluminum plating/stripping at a low overpotential of less than 0.2 V. With a potassium nickel hexacyanoferrate (KNHCF) cathode, the coin cell exhibits an initial discharge capacity of 66 mAh g −1 with a 1.2 V voltage plateau and retains 71% capacity after 400 cycles. Additionally, it demonstrates excellent capacity stability in the rest‐cycling test (6 months) and pouch cell setup (200 cycles), highlighting its potential for grid‐scale energy storage.