Unlocking pH-universal charge storage: electrochemically engineered N-doped graphene beyond nanoporosity

Despite excellent charge-discharge performance in carbon-based supercapacitors, their mechanisms across pH solutions remain unclear due to difficulties decoupling capacitive effects from porous structures and intricate micro-nano molecular/ionic channels. To investigate these mechanisms, we electrochemically synthesize structure-defined, nanohole-free N-doped graphene (ENG) as a model system. Attenuated total reflection in situ infrared spectroscopy (ATR-FTIR) reveals splendid electrochemical activity for pyridinic N and carboxyl functional groups in acidic medium. In contrast, hydroxyl groups dominate the charge-discharge behavior of ENG in alkaline medium. This behavior differs from the neutral medium scenario, where only CO bonds produce detectable ATR-FTIR peaks. Theoretical calculations identified strong adsorption affinity of pyridinic N and carboxyl functional groups for H⁺ over OH^-, which directly enhances the electrochemical activity of ENG in acidic medium. This work pioneers a targeted synthesis strategy for ENG, establishing it as a robust model to decode charge-discharge mechanisms of carbon-based materials across the full pH range.