High-density charge
(energy) storage under supercapacitive mode
requires an electrode that would deliver larger space for charge accumulation
and offer a larger electrochemical potential difference at an electrode–electrolyte
interface. Porous carbon has been a preferred electrode for commercial
supercapacitors; however, its charge storability is much lower than
that of state-of-the-art charge-storage devices such as lithium-ion
batteries. We show that one of the primary limiting factors is the
voids in porous carbon, which do not contribute to the capacitance
because their sizes are much larger than the size of the solvated/unsolvated
ions in the electrolyte. We partially activate these voids by filling
them with a flower-shaped 3D hierarchical pseudocapacitive material
(MnCo2O4) by assuming that flower-shaped fillers
would provide an additional easily accessible surface for charge adsorption.
Less than 10 wt % MnCo2O4 in the porous carbon
from palm kernel shells through simple wet impregnation results in
a five-fold increase in the charge storability. Laboratory prototypes
of symmetric supercapacitors are fabricated using the MnCo2O4-filled carbon electrodes, which show five times higher
specific energy than pure carbon and are cycled over 5000 times with
>95% capacitance retention. The present strategy of activating
the
voids by hierarchical 3D nanostructures could be applied to build
high-performance energy-storage devices.
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