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

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journal contribution
posted on 05.05.2021, 11:33 by Kouki Oka, Saki Murao, Miho Kataoka, Hiroyuki Nishide, Kenichi Oyaizu
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.