High-porosity activated
carbon (AC) was prepared from low-cost
coconut shells (CS) using KOH as an activating agent with different
KOH/char mass ratios. To cut down the amount of KOH used for AC preparation,
freezing pretreatment for a certain number of times was carried out
on CS before carbonization, which resulted in the maximum increase
in the specific surface area (
S
BET
) and
total pore volume of 92.8 and 44.4%, respectively, in the resultant
biochar. For the sake of description, AC from CS undergoing
x
times of freezing pretreatment and with a KOH/char ratio
of
y
is denoted as AC-
xy
. The
S
BET
values of AC-13 and AC-24 are 193 and 166
m
2
g
–1
larger than that of AC-05 (2217
m
2
g
–1
), respectively. At a current density
of 0.25 A g
–1
, the specific gravimetric capacitance
(
C
g
) values of AC-05, AC-13, and AC-24
are 386, 403, and 425 F g
–1
. Moreover, a symmetric
supercapacitor based on AC-24 exhibits a high energy density of 14.7
Wh kg
–1
at a power density of 120 W kg
–1
. The energy density retention rate of AC-24 is 71.1% with the power
density increased by about 110 times, indicating excellent rate capability.
Additionally, a capacitance retention rate of about 95% after 3000
cycles implies an outstanding cycle lifetime of an AC-24-based capacitor.
The freezing strategy developed here provides a novel route for low-cost
and eco-friendly production of AC from biomass wastes for high-performance
supercapacitors.
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