Highly redox-active polymer with extensive electron delocalization and optimized molecular orbitals for extraordinary proton storage

Due to the unique "Grotthus mechanism" of the proton as the charge carrier, aqueous proton batteries (APBs) have the potential to become a type of feasible energy storage device with low cost and high security. Organic polymers with tunable molecule structures are promising for APB electrodes, whereas unsatisfactory redox capacity and low electron affinity hinder their real applications. Herein, a highly redox-active polymer, poly(diquinoxalino-phenazine) (PDQPZ), has been designed and synthesized with extended imine-conjugated polymeric backbones. Beneficial from the reasonable molecular configuration, the PDQPZ polymer shows extensive electron delocalization and optimized molecular orbitals with an extremely small LUMO value of −3.26 eV, leading to excellent electron affinity and high redox activity for fast, ultra-stable and extraordinary proton storage, which are further demonstrated by localized orbital locator-π (LOL-π) and iso-chemical shielding surface (ICSS) techniques. As such, the PDQPZ polymer as electrode material possesses a superior proton-storage capacity of 205.2 mAh g−1 and excellent cycling stability after 10,000 cycles with as low as 0.0008% decline per cycle. Multiple in-operando monitoring techniques further confirm highly reversible proton uptake/removal and the protonation pathways are proposed in detail. Finally, a high-performance and long-life APB device has been fabricated and operated over a wide temperature range.