Charge- and Proton-Storage Capability of Naphthoquinone-Substituted Poly(allylamine) as Electrode-Active Material for Polymer–Air Secondary Batteries

1,4-Naphthoquinone (NQ) undergoes a two electron and two proton redox process in one step at a potential of −0.05 V (vs Ag/AgCl) in 0.05 M H2SO4 aqueous solution. Its heterogeneous electron transfer rate constant is comparable to those of electrochemically reversible redox-active molecules. Therefore, NQ is a potential electrode-active material candidate in acidic aqueous electrolyte. Simple nucleophilic substitution of 2-bromo-1,4-naphthoquinone and poly(allylamine) yields NQ-substituted poly(allylamine) (PNQ), enabling us to adjust the hydrophilicity of the redox polymer according to the NQ introduction rate. A polymer–air secondary battery composed of the PNQ anode, the Pt/C cathode, and 0.05 M H2SO4 aqueous electrolyte displayed high cyclability (>100 cycles) and C-rate capability (∼15 C) with a moderate voltage of 0.8 V during discharging.