Hydrogel-constrained interfacial kinetics enable reversible aluminum electrochemistry in aqueous rechargeable aluminum-air batteries

Aqueous aluminum-air batteries (AABs) are attractive for sustainable energy storage; however, their rechargeability remains fundamentally limited because Al/Al³⁺ redox reaction is thermodynamically less favorable than hydrogen evolution. Here, we address this challenge using a dual solid cellulose hydrogel electrolyte that spatially regulates interfacial reactions at the aluminum anode and air cathode. Cellulose hydrogels derived from coir fiber via an alkali-urea regeneration route and borax cross-linking form mechanically robust (∼0.8 mm), ionically conductive, and hydrated electrolyte matrices. Electrochemical evaluation across different hydrogel anolytes reveals a clear transition from passivation-controlled to kinetically accessible interfacial regimes, with acidic chloride hydrogel (HGE–HCl) exhibiting the most favorable aluminum redox behavior. Hydrogel confinement mitigates parasitic hydrogen evolution, lowers interfacial resistance, and redistributes cathodic reaction pathways to sustain charge transfer under aqueous conditions. The resulting dual-hydrogel Al-air cell employing a NiFe₂O₄ air cathode demonstrates stable charge–discharge cycling for approximately 800 h without external humidification. Morphological analysis further reveals that hydrogel-confined operation promotes a continuous and mechanically adaptive interphase that preserves interfacial accessibility during prolonged cycling. These findings establish hydrogel-mediated interfacial regulation as an effective strategy for sustaining reversible aluminum electrochemistry in aqueous media and provide a promising pathway toward rechargeable aluminum-air batteries.