Protonated C₃N₄ Nanosheets for Enhanced Energy Storage in Symmetric Supercapacitors through Hydrochloric Acid Treatment

Next-generation electrochemical energy storage materials are essential in delivering high power for long periods of time. Double-layer carbonaceous materials provide high power density with low energy density due to surface-controlled adsorption. This limitation can be overcome by developing a low-cost, more abundant material that delivers high energy and power density. Herein, we develop layered C₃N₄ as a sustainable charge storage material for supercapacitor applications. It was thermally polymerized using urea and then protonated with various acids to enhance its charge storage contribution by activating more reaction sites through the exfoliation of the C–N framework. The increased electron-rich nitrogen moieties in the C–N framework material lead to better electrolytic ion impregnation into the electrode, resulting in a 7-fold increase in charge storage compared to the pristine material and other acids. It was found that C₃N₄ treated with hydrochloric acid showed a very high capacitance of 761 F g^–1 at a current density of 20 A g^–1 and maintained 100% cyclic retention over 10,000 cycles in a three-electrode configuration, outperforming both the pristine material and other acids. A symmetric device was fabricated using a KOH/LiI gel-based electrolyte, exhibiting a maximum specific capacitance of 175 F g^–1 at a current density of 1 A g^–1. Additionally, the device showed remarkable power and energy density, reaching 600 W kg^–1 and 35 Wh kg^–1, with an exceptional cyclic stability of 60% even after 5000 cycles. This study provides an archetype to understand the underlying mechanism of acid protonation and paves the way to a metal–carbon-free environment.