Deciphering the capacitive behavior of heteroatom–doped carbon materials with small mesopores

The charge storage processes of heteroatom–doped carbon materials with small mesopores (2 ∼ 4 nm) occur by both double–layer capacitance and pseudocapacitance. Yet, owing to the ambiguous understanding on how electrolyte ions affect their capacitive behavior, it is hard to form a unified design principle to guide the selection of electrolytes, with the purpose of boosting supercapacitors. Herein, an all–in–one carbon electrode with abundant oxygen–containing groups and an average pore size of 2.5 nm was used as a model electrode to investigate the capacitive behavior of this class of carbon materials. It is found that in ammonium acetate aqueous electrolyte the model electrode shows a significantly increased specific capacitance, and an ion exchange mechanism dominates charge storage. Acetate anions with a larger ion radius can better match small mesopores in size and thus increase double–layer capacitance due to the suppression of the “over–screening” effect in the first adsorbed layer on carbon surfaces. Besides, ammonium cations forms hydrogen bonds with oxygen–containing groups maximizing the Gibbs free energy change related to faradic reactions, which enhances pseudocapacitance. Our work elaborated the effect of electrolyte ions on the capacitance of heteroatom–doped carbon materials with small mesopores and provided new understanding on designing high–performance aqueous supercapacitors.