Boosting the Electrical Double-Layer Capacitance of Graphene by Self-Doped Defects through Ball-Milling

Improving the capacitance of carbon materials for supercapacitors without sacrificing their rate performance, especially volumetric capacitance at high mass loadings, is a big challenge because of the limited assessable surface area and sluggish electrochemical kinetics of the pseudocapacitive reactions. Here, it is demonstrated that "self-doping" defects in carbon materials can contribute to additional capacitance with an electrical double-layer behavior, thus promoting a significant increase in the specific capacitance. As an exemplification, a novel defect-enriched graphene block with a low specific surface area of 29.7 m(2) g(-1) and high packing density of 0.917 g cm(-3) performs high gravimetric, volumetric, and areal capacitances of 235 F g(-1), 215 F cm(-3), and 3.95 F cm(-2) (mass loading of 22 mg cm(-2)) at 1 A g(-1), respectively, as well as outstanding rate performance. The resulting specific areal capacitance reaches an ultrahigh value of 7.91 F m(-2) including a "self-doping" defect contribution of 4.81 F m(-2), which is dramatically higher than the theoretical capacitance of graphene (0.21 F m(-2)) and most of the reported carbon-based materials. Therefore, the defect engineering route broadens the avenue to further improve the capacitive performance of carbon materials, especially for compact energy storage under limited surface areas.