Activated carbon (AC) based supercapacitors exhibit intrinsic advantages in energy storage. Traditional two-step synthesis (carbonization and activation) of AC faces difficulties in precisely regulating its pore-size distribution and thoroughly removing residual impurities like silicon oxide. This paper reports a novel coupled ultrasonication-milling (CUM) process for the preparation of hierarchically porous carbon (HPC) using corn cobs as the carbon resource. The as-obtained HPC is of a large surface area (2288 m(2) g(-1)) with a high mesopore ratio of similar to 44.6%. When tested in a three-electrode system, the HPC exhibits a high specific capacitance of 465 F g(-1) at 0.5 Ag-1, 2.7 times higher than that (170 F g(-1)) of the commercial AC (YP-50F). In the two-electrode test system, the HPC device exhibits a specific capacitance of 135 F g(-1) at 1 A g(-1), twice higher than that (68 F g(-1)) of YP-50F. The above excellent energy storage properties are resulted from the CUM process which efficiently removes the impurities and modulates the mesopore/micropore structures of the AC samples derived from the agricultural resides of corn cobs. The CUM process is an efficient method to prepare high-performance biomass-derived AC materials. (C) 2018 Elsevier Inc. All rights reserved.