Enhancing the Capacitance of Poly(Heptazine Imide) Electrodes in Aqueous Electrolytes via Hybrid Material Design

Potassium poly(heptazine imide) (K‐PHI), an ionic carbon nitride, has gathered interest as functional material in energy conversion, in particular as photocatalyst. As a semiconductor, it lacks of electric conductivity. Paired with its low surface area, its potential is limited for electric and adsorption‐based applications. This work reports the design of a PHI/carbon hybrid using micro‐ and mesoporous carbon as support in order to increase electric conductivity and the electrochemically‐active surface area of K‐PHI. By that, the specific surface area is increased from 9 m 2 g −1 in bulk K‐PHI up to 312 m 2 g −1 when designing the mesoporous hybrid. Furthermore, transmission electron microscopy (TEM) images indicate a growth of K‐PHI within the mesopores of the carbon, whereas the microporous hybrid exhibits K‐PHI at the external surface. Both hybrids show enhanced capacitance up to 23 F g −1 compared to 2 F g −1 for bulk K‐PHI. The mesoporous hybrid also shows enhanced capacitance retention at higher specific currents as the open mesopores enable faster ion diffusion combined with increased electric conductivity and shorter dielectric relaxation times. Overall, the design of K‐PHI@carbon hybrids enables the study of electrochemical processes at the PHI surface without being limited by its low electric conductivity and low surface area.