A long-range planar polymer with efficient π-electron delocalization for superior proton storage

Due to the unique "Grotthus mechanism", aqueous proton batteries (APBs) are promising energy devices with intrinsic safety and sustainability. Although polymers with tunable molecular structures are expected as ideal electrode materials, their unsatisfactory proton-storage redox behaviors hinder the practical application in APB devices. Herein, a novel planar phenazine (PPHZ) polymer with a robust and extended imine-rich skeleton based on phenazine units is synthesized and used for APB application for the first time. The long-range planar configuration achieves ordered molecular stacking and reduced conformational disorder, while the high conjugation with strong π-electron delocalization optimizes energy bandgap and electronic properties, enabling to the polymer with ultra-low proton diffusion barriers (≤ 0.12 eV), high redox activity and superior electron affinity. As such, the PPHZ polymer as an electrode material exhibits fast, stable and unrivaled proton-storage redox behaviors with a record capacity of 254.5 mAh g-1 in aqueous acidic electrolyte, which is the highest value among all proton-inserted organic electrodes. Dynamic in-situ monitoring techniques confirm the high redox reversibility upon proton uptake/removal, and the corresponding protonation pathways are elucidated by multiple theoretical calculations. Moreover, a soft-packaged APB cell using PPHZ electrode exhibits an ultralong lifespan over 30,000 cycles, further verifying its promising application prospect.