Facile preparation of porous carbons from rice husk by microwave heating for supercapacitors

Porous carbons (PCs) including microporous and mesoporous carbons for supercapacitors were made from rice husk through zinc chloride (ZnCl
₂
) activation in one-step by microwave heating with microwave power at 600 W at 20 min heating time. PCs were characterized by field emission scanning electron microscope, transmission electron microscope and x-ray diffraction. The pore structure parameters and surface chemistry of PCs were obtained by nitrogen adsorption, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. The results show that the average pore size of PCs increases from 2.02 nm to 5.99 nm, and the total pore volume of PCs increases from 0.75 cm
³
/g to 2.07 cm
³
/g with the mass ratio of ZnCl
₂
/rice husk. The specific surface area of PCs passes through a maximum with the ZnCl
₂
/rice husk mass ratio, and reaches 1727 m
²
/g at 3/1 of ZnCl
₂
/rice husk mass ratio. At 4/1 of ZnCl
₂
/rice husk mass ratio, the specific capacitance of PC (PC
_4/1
) only decreases from 182 F/g to 171 F/g with the increase of discharge current density from 0.05 A/g to 2 A/g, possessing a capacitance retention up to 94.0% due to its low densities of surface functional groups, good conductivity and mesopore structure. The energy density of PC
_4/1
capacitor retains at 6.23 Wh/kg after 1000 cycles, showing perfect cycle stability. The results evidenced that microwave heating ZnCl
₂
activating rice husk at 20 min heating time in one-step is an efficient approach to the rapid preparation of low cost yet high performance PCs for supercapacitors.