Electronic and Steric Engineering of a Methyl-Functionalized Hexaazatrinaphthalene for Proton-Coupled Energy Storage

The development of high-capacity organic electrodes for aqueous proton batteries is hindered by incomplete redox activity and structural instability. Here, we report a methyl-functionalized phenazine derivative, 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinaphthalene (HMHATN), engineered to synergize electronic delocalization and steric control. Methyl groups at peripheral positions electronically stabilize the intermediate redox states via charge redistribution, lowering the redox potential to 0.23 V vs SHE and enabling a six-electron transfer process. Sterically, methylation induces a herringbone molecular packing, shortening nitrogen-to-nitrogen distances by 9% and creating dual proton transport pathways: a Grotthuss-type hopping network via optimized hydrogen bonds and a vehicle-type highway for protons, as validated by AIMD simulations. These structural and electronic modifications suppress aggregation-induced passivation with a limited electron transfer number observed in nonmethylated analogues, unlocking full redox reversibility. HMHATN delivers a near-theoretical capacity (298 mAh/g at 1 A/g), exceptional rate capability (226 mAh/g at 50 A/g), and ultralong cyclability (85.9% retention over 10,000 cycles). Full-cell configurations with a CuHCF cathode further demonstrate practical viability, achieving 276 mAh/g at 2 A/g with 92.7% capacity retention over 3000 cycles. This work is anticipated to establish a universal way of designing organic electrodes, where targeted functionalization harmonizes electronic and steric effects to overcome limitations in proton-coupled energy storage.