Molten salt strategy: a universal approach for constructing high-performance bifunctional carbon-based electrocatalysts for zinc–air batteries

The molten salt strategy exhibits significant advantages and potential in the construction of carbon-based electrocatalysts, including environmental protection, recyclability, the promotion of graphite formation, efficient doping functionalization, and highly controllable product structures. In recent years, most explorations have focused on optimizing the oxygen reduction reaction (ORR) performance of carbon-based materials prepared by the molten salt strategy while neglecting improved oxygen evolution reaction (OER) activity. As a result, such materials with a poor balance between the ORR and the OER cannot achieve a high performance in zinc-air batteries (ZABs). In this review, we first discuss the methodology for selecting the appropriate molten salt to enhance the catalyst's ORR or OER performance. Combined with the requirements of ZABs, the standards required for the electrocatalysts' cathodes are elaborated in detail. Additionally, the review emphasizes the research progress in molten salt systems for preparing versatile carbon-based electrocatalysts for high performance ZABs, including metal-free carbon materials, single-atom catalysts, metal clusters/carbon composites, and metal nanoparticles/carbon composite materials. For various catalysts, the mechanism by which the molten salt method enhances the catalytic performance through structural modification is systematically elaborated, providing a theoretical foundation and design guidance for the selection of appropriate molten salts in different catalytic systems. Finally, we proposed some limitations and future development directions for the molten salt method and provided a feasible pathway for designing high-efficiency bifunctional catalysts.