Electrochemical performance of nitrogen-enriched carbons in aqueous and non-aqueous supercapacitors

Carbon materials with significant nitrogen contents were investigated as the electrode materials of supercapacitors. The preparation procedure involved the polymerization of melamine in the interlayer space of template fluorine mica and carbonization at 750, 850, and 1000 degrees C. Some samples were also stabilized prior to carbonization. We have shown previously that these carbons possess very interesting capacitive behavior in an acidic medium despite small surface areas. High capacitance values in H2SO4 were attributed to the pseudocapacitive interactions between the protons and nitrogen atoms. This paper further discusses the results obtained in a base and an aprotic electrolyte, KOH and TEABF(4)/PC, respectively. Electrochemical properties were evaluated with cycling voltammetry, a galvanostatic charge/discharge technique, and electrochemical impedance spectroscopy. High capacitance values were obtained in proton-free KOH, and the presence of pseudocapacitive interactions between the ions of the electrolyte and the nitrogen atoms of the carbon matrix is proposed. Compared to those in sulfuric acid, greater capacitances of nonstabilized samples were obtained in KOH, i.e., for the sample carbonized at 1000 degrees C, the capacitance was 84.61 F/g in KOH vs 47.92 F/g in H2SO4. On the other hand, less porous but more nitrogen-rich stabilized samples gave better performances in H2SO4, i.e., 62.24 F/g in H2SO4 compared to 49.86 F/g in KOH for the sample stabilized and carbonized at 1000 degrees C. The sample heat-treated at 750 degrees C with a surface area of ca. 400 m(2)/g performs similarly in both electrolytes, i.e., similar to 200 F/g. Significantly lower gravimetric capacitances were obtained in TEABF(4)/PC from the samples carbonized at 750 degrees C. On the other hand, the almost nonporous sample subjected to stabilization prior to carbonization at 1000 degrees C gave a capacitance of similar to 20 F/g. Hence, we suggest that the faradaic interactions between the carbon electrode material and the electrolyte, although much less significant than those in H2SO4 and KOH, play an important role in the nonaqueous electrolyte as well. Narrow micropores were detected by CO2 adsorption/desorption, and their importance to the interpretation of capacitive behavior is also discussed.