Chlorinated hexaazatrinaphthylene as an efficient solid-state redox mediator for membrane-free acidic decoupled water...

Membrane-free decoupled water electrolysis offers a promising route for hydrogen production under intermittent renewable electricity, yet its operation in strong acids remains challenging due to mediator instability and slow proton-coupled electron transfer (PCET). Herein, we report a molecularly engineered hexaazatrinaphthylene derivative, HATN-6Cl, achieved through chlorination, which serves as a durable solid-state redox mediator for acidic membrane-free decoupled electrolysis. Chlorination preserves the CN-rich conjugated scaffold while redistributing electron density, leading to enhanced PCET kinetics and improved charge transport. As a result, HATN-6Cl exhibits exceptional high-rate capacity (153.6 mAh g^-1 at 50 A g^-1) and cycling stability (66.3% retention after 10,000 cycles at 25 A g^-1). In a two-step membrane-free electrolyzer, HATN-6Cl enables temporally separated H₂ and O₂ evolution with high Faradaic efficiency, stable step voltages, and rapid response to voltage perturbations. Step-specific photovoltaic voltage matching further demonstrates its compatibility with solar inputs. This work highlights substituent-level electronic tuning as an effective strategy for designing durable mediators for acidic decoupled water electrolysis.