Humic acids-based hierarchical porous carbons as high-rate performance electrodes for symmetric supercapacitors

Two kinds of hierarchical porous carbons (HPCs) with specific surface areas of 2000 m2 g−1 were synthesized using leonardite humic acids (LHA) or biotechnology humic acids (BHA) precursors via a KOH activation process. Humic acids have a high content of oxygen-containing groups which enabled them to dissolve in aqueous KOH and facilitated the homogeneous KOH activation. The LHA-based HPC is made up of abundant micro-, meso-, and macropores and in 6 M KOH it has a specific capacitance of 178 F g−1 at 100 A g−1 and its capacitance retention on going from 0.05 to 100 A g−1 is 64%. In contrast, the BHA-based HPC exhibits a lower capacitance retention of 54% and a specific capacitance of 157 F g−1 at 100 A g−1 which is due to the excessive micropores in the BHA-HPC. Moreover, LHA-HPC is produced in a higher yield than BHA-HPC (51 vs. 17 wt%).