Chemical Routes to Graphene-Based Flexible Electrodes for Electrochemical Energy Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 423

are significantly compromised in a bulk form of graphene such as thin film or
paper. Effective prevention of intersheet restacking or creating fast ion/electrolyte
transportation ways are essential to allow the individual sheets in multilayered
graphene structures to behave as graphene in electrochemical energy storage
applications. Up-to-date, several strategies have been applied to enhance the
performance of flexible graphene electrode.
2.1 Exfoliating Stacked Graphene Sheets by Rapid
Expanding
Rapid expanding is an effective technique for exfoliating stacked graphene sheets
[ 42 ]. High temperature treatment can reduce GO and spontaneously generate a
tremendous amount of gaseous species. Kaner et al. present a strategy for the
production of flexible graphene electrode for supercapacitor by a simple laser
scribing approach that avoids the restacking of graphene sheets [ 58 ]. The process is
schematically illustrated in Fig. 5 . Initially, GO thin film was drop-cast onto the
DVD disk. Then, irradiation of the film with the infrared laser in a commercially
available LightScribe CD/DVD optical drive can reduce the GO to laser-scribed
graphene (LSG). In this rapid reduction process, the closely stacked GO sheets can
be well-exfoliated to a porous LSG sheets. The as-formed LSG film possessed
excellent conductivity of 1,738 S m -1 and excellent mechanical flexibility with
only *1 % electrical resistance change after 1,000 bending cycles. When two LSG
films were directly assembled in a supercapacitor without any binders or conductive
additives, the areal capacitance can be obtained as 3.67 mF cm -2 in 1.0 M H 3 PO 4
aqueous electrolyte (4.04 mF cm -2 in 1.0 M H 2 SO 4 aqueous electrolyte) with the
current density of 1 A g -1 , a capacitance of 1.84 mF cm -2 can still be achieved
even when operated at an high charge/discharge rate of 1,000 A g -1 , indicating the
high rate capability of such electrode. Additionally, the LSG electrode retained
96.5 % of its initial capacitance after 10,000 cycles. The liquid electrolyte can
further be replaced with poly(vinyl alcohol) (PVA)-H 3 PO 4 polymer gelled elec-
trolyte, which also acts as the separator. In this condition, the performance of
electrode was comparable with those obtained with an aqueous electrolyte. The high
performance of LSG electrode can be attributed to the porous structure of the
graphene film, which can provide an electrolyte reservoir to facilitate ion transport
and minimize the diffusion distance to the interior surfaces.
2.2 Separating Graphene Sheets with ''Nanospacers''
Another approach to prevent the restacking of graphene nanosheets is blending
graphene sheets with other nanoparticles as ''spacers'' to form composites with
high porosity and surface area. For example, Lian et al. demonstrated that flexible