Environmental Engineering Reference
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100 A g
-1
for 10,000 times. Thus, this strategy has been proven to be very effective
to make graphene nanosheets remain largely separated in a solvated state, providing
a simple strategy for addressing the key challenge that has limited the large-scale
application of graphene.
2.3 Creating Fast Ion/Electrolyte Transportation Ways
in Graphene Paper
Besides preventing graphene nanosheets from restacking, generating fast ion/
electrolyte transporting ways is another approach to improve the electrode per-
formance of graphene paper. As an illustration, Kung et al. reported an approach
by introducing in-plane carbon vacancy defects (pores) into graphene sheets using
a facile solution method to enhance the electrochemical energy storage perfor-
mance of graphene paper electrodes [
63
]. In-plane defects was firstly introduced
into the basal planes of GO by treating it in hot HNO
3
with the assistance of
ultrasonic vibration. As illustrated in Fig.
7
a. Under such conditions, sections of
GO can be transformed into soluble polyaromatic hydrocarbons, left a holey GO
(HGO) sheet. Thermal reduction of a free-standing HGO paper led to an electri-
cally conducting graphene paper constructed by holey graphene sheets. The in-
plane porosity can provide a high density of cross-plane ion diffusion channels that
facilitate ion transport and storage at high rates. The electrochemical performance
of holey graphene papers with a thickness of *5 lm was examined as LIB anode;
reversible capacities of 454 and 178 mAh g
-1
can be obtained with optimized
reduction conditions at the current densities of 50 and 2,000 mA g
-1
, respectively.
Recently, Ruoff et al. reported that KOH activation can generate holes or
defects on the surface of graphene [
64
]. Based on the similar method, activated
graphene films can also be fabricated (Fig.
7
b) [
65
]. First, an ''ink paste'' was
prepared by adding KOH into GO colloidal suspension and then concentrated by
heating. Films composed of stacking GO nanosheets decorated with KOH were
obtained through vacuum filtration method. The activation step was carried out
under flowing argon at 800 C for 1 h. The final activated graphene film was
obtained after washing and drying, which is flexible free-standing porous carbon
film with high specific surface areas of up to 2,400 m
2
g
-1
and a very high in-plane
electrical conductivity of 5,880 S m
-1
. Such electrode showed a capacitance
normalized to BET surface area of 14 lFcm
-2
at the scan rates of 50 mV s
-1
and
can be retained at 11 lFcm
-2
at 400 mV s
-1
. The high rate performances of the
above two graphene film electrodes is closely related to their highly interconnected
3D structure and short diffusion pathway, which favor fast ion transportation.
Up to now, the mentioned fabrications of graphene thin film or paper are all
based on the vacuum filtration method, as an alternative strategy, Xue et al.
developed a novel graphene aerogel pressing approach to fabricate graphene paper
with folded structured graphene sheets, the process is illustrated in Fig.
8
[
66
]. The
graphene aerogel was prefabricated by freeze-drying GO aqueous dispersion and
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