Ionic Potential-Driven Interfacial pH Control Enables High Anode Utilization for Neutral Aluminum-Air Batteries

Abstract

Aqueous aluminum-air batteries (AABs) hold great promise as a sustainable energy conversion technology. However, they encounter significant challenges, including self-corrosion of the anode in alkaline electrolytes and passivation in neutral NaCl-based systems. Herein, we propose an ionic potential (φ)-guided design of a NH₄Cl/MnCl₂ neutral electrolyte that maintains interfacial pH between 4.1 and 4.4 via regulated hydrolysis equilibrium. Selected NH₄⁺ and Mn²⁺ (φ = 29.85) synergistically inhibit Al(OH)₃ passivation layers while suppressing hydrogen evolution, as evidenced by in situ pH monitoring and comprehensive characterization analysis. The optimized electrolyte exhibits a high output voltage of 0.97 V (20% improvement over pure NaCl systems) and an anode utilization efficiency exceeding 84.5%, corresponding to a specific capacity of 2516.5 mAh g^–1. Electrochemical impedance spectroscopy (EIS) results indicate that the charge-transfer resistance is reduced by an order of magnitude when compared with conventional electrolytes. In Swagelok-type cells with a restricted electrolyte-to-Al anode ratio (E/Al = 267 mg mL^–1), a high discharge capacity over 30 mAh cm^–2 is achieved (anode utilization >24%). Moreover, molded batteries featuring structural engineering innovations can deliver 130 mAh at 1 mA cm^–2 (anode utilization >94%). This research establishes a universal φ-based design principle for metal-air battery electrolytes, balancing corrosion inhibition and activation kinetics through thermodynamic regulation.