Seawater electrolyte-based metal-air batteries: from strategies to applications

Aqueous metal-air batteries are promising next-generation energy storage and supply technologies due to their advantages of high energy density and intrinsic safety. As an abundant natural resource, applying seawater-based electrolytes is proposed to have considerable economic and environmental benefits, and will significantly broaden the applications of metal-air batteries. However, the existence of complex components in seawater, in particular chloride ions, inevitably has a complex influence on air electrode processes, including the oxygen reduction and evolution reactions (ORR and OER), requiring the development of efficient chloride-resistant electrocatalysts. Meanwhile, a few seawater-based metal-air battery prototypes have shown great application potential but are still at an early stage of development. In this review, we first propose the concept of seawater-based metal-air batteries and comprehensively analyze the essential air electrode reactions in terms of thermodynamics and kinetics. Subsequently, rational design strategies for ORR and OER electrocatalysts suitable for use in chloride-containing and seawater-based electrolytes are comprehensively discussed. Moreover, the development history and potential applications of seawater-based metal-air batteries are demonstrated. Finally, a summary and outlook are provided for future innovations. For applying metal-air batteries using seawater-based electrolytes, the influence of chloride anions on the ORR/OER electrocatalytic mechanisms and rational design strategies for chloride-resistant electrocatalysts are summarized.