**4. Conclusions**

Nitrogen self-doped 3D honeycomb-like porous carbon was successfully prepared via one-step KOH activation (a-CSHs). When the carbonization temperature was adjusted, the obtained carbon materials showed high specific surface areas (1257.8–2063.0 m2/g), abundant nitrogen contents (1.51–2.56 atom %), and hierarchical pore structures. The a-CSH-700 based electrode delivered a high specific capacitance of 238 F/g and 200 F/g at current densities of 0.5 A/g and 20 A/g, respectively, demonstrating a good capacitance retention of 84%. Moreover, the assembled a-CSH-700 based symmetric supercapacitor also displayed a high specific capacitance of 52 F/g at 0.5 A/g, with an energy density of 10.4 Wh/kg at 300 W/kg, and 91% capacitance retention after 5000 cycles at 10 A/g. This facile and cost-effective method for the preparation of 3D honeycomb-like porous carbon material from waste cottonseed husk was also renewable and easy for industrial production.

**Author Contributions:** F.Y. designed and administered the experiments. H.C. performed the experiments; H.C., G.W., L.C., and B.D. characterized the samples and collected the data; and F.Y., G.W., and B.D. gave technical support and conceptual advice. All of the authors contributed to the analysis and discussion of the data and to writing the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant No. U1303291), the Program for Changjiang Scholars and Innovative Research Team in University (Grant No. IRT\_15R46), and the Program of Science and Technology Innovation Team in Bingtuan (No. 2015BD003).

**Conflicts of Interest:** The authors declare no conflict of interest.
