Chemical Routes to Graphene-Based Flexible Electrodes for Electrochemical Energy Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 425

138 F g -1 in 6 M KOH aqueous electrolyte and 83.2 F g -1 in 1 M LiPF 6 organic
electrolyte at a scan rate of 10 mV s -1 , and 80 F g -1 can be reached at a high scan
rate of 500 mV s -1 in aqueous electrolyte. Meanwhile, after 2,000 cycles, the
degradations of its specific capacitance in aqueous and organic electrolytes are
only 3.85 and 4.35 %, respectively. The ability to maintain high conductivity and a
high surface area provides an effective route to improve the performance of
graphene paper for energy-storage electrodes.
Shi et al. reported that flexible mesoporous graphene films with high conduc-
tivities can be prepared by graphitizing the composite films of GO and nanodia-
mond (ND) [ 60 ]. After graphitization, ND was changed into onion-like carbon
(OC) and GO was reduced to conductive graphene. In this kind of graphene films,
OC nanoparticles were sandwiched between graphene sheets, which not only
prevented the aggregation of graphene sheets, but also formed mesopores in the
range of 2-11 nm. The mesopores film possesses high specific surface area of
about 420 m 2 g -1 and high conductivities in the range of 7,400-20,300 S m -1 .
Such films are flexible and can be directly used as the electrodes of supercapac-
itors. In a three electrode configuration with 1.0 M H 2 SO 4 aqueous electrolyte, the
supercapacitor showed specific capacitances of 143 and 78 F g -1

at the current

density of 0.2 and 10 A g -1 , respectively.
A three-component, flexible electrode has also been developed for superca-
pacitors utilizing CNT decorated with c-MnO 2 nanoflowers (c-MnO 2 /CNT) as
spacers for graphene nanosheets [ 61 ]. Such electrode can deliver a high specific
capacitance of 308 F g -1 for two-electrode symmetric supercapacitors with
30 wt% KOH aqueous electrolyte, at a scan rate of 20 mV s -1 . A maximum
energy density of 43 Wh kg -1 can be obtained for this kind of supercapacitors at a
constant current density of 2.5 A g -1 . The fabricated supercapacitor device also
exhibits excellent stability by retaining &90 % of the initial specific capacitance
after 5,000 cycles.
Instead of the above-mentioned ''hard'' spacers, Li et al. developed a ''soft''
approach to prevent the restacking of graphene sheet in the paper form by delicately
employing the solvent molecule as spacers [ 62 ]. They discovered that the assembly
of graphene can be manipulated using the principles of colloidal chemistry and
water can serve as an effective ''soft spacer'' to prevent the restacking of graphene
sheets. This kind of hydrated graphene paper can be obtained when the filtration is
just completed and the resultant paper is still wet. Without any drying, graphene
paper is found to contain &92 wt% water. The hydrated graphene paper containing
0.045 mg cm -2 of graphene gives a sheet resistivity of 1,860 X square -1 . Hydrated
graphene sheets can remain significantly separated as assembled in the bulk paper
form. The high specific surface area of graphene sheets can be effectively reserved.
When testing the supercapacitance properties of hydrated graphene paper in a two-
electrode configuration, a specific capacitance of up to 215.0 F g -1 in 1.0 M H 2 SO 4
aqueous electrolyte can be obtained at the current density of 1.08 A g -1 , and more
importantly, a capacitance of 156.5 F g -1 can still be retained and even discharged
at an ultrahigh current density of 1,080 A g -1 . And the hydrated graphene paper can
retain over 97 % of its initial capacitance after cycling in a high operation current of