Boosting the Supercapacitance of Nitrogen-Doped Carbon by Tuning Surface Functionalities

The specific capacitance of a highly porous, nitrogen-doped carbon is nearly tripled by orthogonal optimization of the microstructure and surface chemistry. First, the carbons’ hierarchical pore structure and specific surface area were tweaked by controlling the temperature and sequence of the thermal treatments. The best process (pyrolysis at 900 °C, washing, and subsequent annealing at 1000 °C) yielded a carbon with a specific capacitance of 117 F g−1—nearly double that of a carbon made by a typical single-step synthesis at 700 °C. Following the structural optimization, the surface chemistry of the carbons was enriched by applying an oxidation routine based on a mixture of nitric and sulfuric acid in a 1:4 ratio at two different treatment temperatures (0 and 20 °C) and different treatment times. The optimal treatment times were 4 h at 0 °C and only 1 h at 20 °C. Overall, the specific capacitance nearly tripled relative to the original carbon, reaching 168 F g−1. The inherent nitrogen doping of the carbon comes into interplay with the acid-induced surface functionalization, creating a mixture of oxygen- and nitrogen-oxygen functionalities. The evolution of the surface chemistry was carefully followed by X-ray photoelectron spectroscopy and by N2 sorption porosimetry, revealing stepwise surface functionalization and simultaneous carbon etching. Overall, these processes are responsible for the peak-shaped capacitance trends in the carbons