Beyond vanadium: inorganic redox chemistries for next-generation aqueous flow batteries

Rapid renewable electricity integration into power grids demands safe and stable long-duration energy storage, positioning aqueous redox flow batteries (RFBs) as promising candidates. All-vanadium RFBs, the most mature technology, have achieved megawatt-scale deployment but remain constrained by costly vanadium, motivating exploration for alternatives. Organic molecule-based RFBs have been widely investigated because of earth-abundant constituent elements and extensive molecular tunability; yet challenges with chemical stability, practical concentration, and oxygen sensitivity of organic molecules hinder their applicability. Inorganic species beyond vanadium, by contrast, offer superior chemical robustness and practical operability but lack a comprehensive and focused assessment. This review critically evaluates inorganic aqueous RFB chemistries, including zinc, chromium, iron, manganese, sulfur, iodine, bromine, and polyoxometalates. Emphasis is placed on their distinctive electrochemical properties, merits, key limitations, and recent advances in both laboratory research and industrial deployment. Rather than simply cataloguing recent progress, disparate strategies are systematically organized into coherent categories with representative studies underscored. By presenting a timely and integrated picture of diverse inorganic RFBs, this review enables direct comparison of performance trade-offs and technological maturity across systems, identifies interconnected barriers to commercialization, and highlights key directions for future research.