Hydrophilic Anthraquinone-Substituted Polymer: Its Environmentally Friendly Preparation and Efficient Charge/Proton-Storage Capability for Polymer-Air Secondary Batteries

Organic redox-active materials have been extensively studied as electrode-active materials to enable innovative battery designs with low environmental burdens. Facile condensation of 4,5-dihydroxyanthraquinone-2-carboxylic acid and poly(allylamine) in water produces hydrophilic 1,8-dihydroxyanthraquinone (DHA)-substituted poly(allylamine) (PDHA). Its high hydrophilicity originates from the poly(allylamine) main chain, and the distorted structure of the DHA inhibits intermolecular stacking of the polymer side chain. This made it possible to achieve a high electron and proton diffusion coefficient of 10-11 cm2/s, which is an order of magnitude higher than that of conventional redox polymers that have one-step, two-electron redox capability. The electrode, composed of appropriately synthesized PDHA, showed a full capacity of 140 mAh g-1 with excellent cyclability (>97% capacity maintenance after 500 cycles) and high rate capabilities (e.g., 120 C) in an acidic aqueous electrolyte under inert gas. These electrochemical properties were maintained even in air, making PDHA a promising candidate for a robust, electrode-active material. A polymer-air secondary battery was fabricated with PDHA, Pt/C, and a 0.5 M H2SO4 aqueous solution as the anode material, cathode material, and electrolyte, respectively, without any separator, inert gas, or strict sealing. This open-air battery displayed a discharge voltage of 1.05 V, with high cyclability greater than 500 cycles and high rate capabilities (e.g., 60 C), demonstrating a new battery concept and potential for large-scale applications.