Reversible and high-density energy storage with polymers populated with bistable redox sites

Redox-active polymers with charging/discharging reversibility are employed to develop electrode-active materials in organic batteries, which are characterized by high power rates, flexibility/bendability, and environmentally benign properties. Reversible charge storage with polymers is achieved by redox "bistability" and exchange reactions. Redox bistability is a feature of electrochemical reversibility, which refers to the properties of redox pairs in which both the reduced and oxidized states are chemically robust and do not fade during substantial storage periods. The electron self-exchange reactions of the redox-active sites populated in the polymer layer give rise to charge propagation in support of exhaustive charging and discharging. The concept of charge storage reversibility is extended to hydrogen storage reversibility based on the bistability of the hydrogenation/dehydrogenation pair and the electron/proton exchange reaction, creating hydrogen carrier polymers as a new class of energy-related functional polymers. In this review, we show that reversibility of charge storage occurs in polymers with bistable redox-active groups populated in the repeat units of a nonconjugated backbone, especially when an electron self-exchange reaction spreads throughout the polymer. We will also show that extending the idea of electron exchange to electron/proton exchange leads to reversible hydrogen storage based on the bistability of hydrogenated and dehydrogenated states and the equilibrium for hydrogenation.