Organic Redox Polymers as Electrode-Active Materials for Aqueous Rechargeable Air and Manganese Batteries

Using renewable energy to reliably supply electricity without relying on conventional fossil fuels and to achieve carbon neutrality is an ambitious undertaking. Looking ahead, environmentally friendly batteries that can store electricity reliably are required. Aqueous batteries offer advantages such as environmental compatibility, cost efficiency, and long-term stability. Traditionally, inorganic materials have been used as electrode-active materials in aqueous batteries, such as fuel cells, metal-air batteries, and lead-acid batteries. However, inorganic materials face the problems of scarce metal availability, dendrite formation, and/or toxicity. To address these problems, organic redox polymers have attracted considerable attention. Organic redox polymers are composed of Earth-abundant elements (H, C, N, O, S···) and feature a high density of organic redox molecules capable of storing charge. The application of organic redox polymers as electrode-active materials in aqueous batteries requires fixing these polymers on the electrode while maintaining sufficient hydrophilicity to allow redox reactions by accounting for the hydrophobic nature of the organic redox molecules, which is the core of the polymers. To understand the molecular design strategy of organic redox polymers for aqueous batteries, this review describes organic redox polymers as electrode-active materials, particularly for aqueous rechargeable air and manganese batteries as representative examples, and discusses their molecular design, methods for electrode fixation, and redox capability in aqueous environments. Subsequently, this review introduces aqueous rechargeable air and manganese batteries using organic redox polymers, and summarizes future perspectives.