Hydroquinone-substituted polyallylamine: redox capability for aqueous polymer–air secondary batteries and recyclability

Abstract

Organic redox polymers, composed of earth-abundant elements, offer rapid charge storage and are promising electrode-active materials for aqueous batteries, potentially replacing metals and overcoming their resource and performance limitations. However, the hydrophobicity of many organic redox molecules hinders their application using aqueous electrolytes. This necessitates molecular designs that impart hydrophilicity while immobilizing redox-active moieties onto electrodes. Polyallylamine, with its high density of hydrophilic amino groups, serves as an effective backbone. In this work, functionalizing it with hydroquinone enabled the use of the polymer in aqueous batteries. While hydroquinone provides high theoretical capacity, irreversible quinhydrone formation limits its reversible and thorough charge storage. We addressed this by covalently attaching hydroquinone to polyallylamine via condensation. The resulting polymer exhibited reversible and thorough charge storage, which was attributed to electrostatic repulsion between amino groups that suppressed quinhydrone formation. A polymer–air secondary battery was fabricated with the polymer, Pt/C, and a 0.5 M H2SO4 aqueous solution as the anode, cathode, and electrolyte, respectively, without any separator. This polymer–air secondary battery displayed a constant discharge voltage with high cyclability (>99% capacity retention after 100 cycles) and high-rate capability. Moreover, the polymer demonstrated recyclability, as raw materials were generated simply by acid treatment. This work demonstrates a polymer design strategy for integrating typically hydrophobic organic redox molecules into recyclable aqueous batteries.