Chemical Routes to Graphene-Based Flexible Electrodes for Electrochemical Energy Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 421

2 Graphene as the Sole or Dominant Part
of Flexible Electrode
The charging/discharging process of batteries and supercapacitors is generally
dominated by the electron and counter-ion transport at the surface of the electrodes
[ 5 , 50 ]. Since the invention of LIB in 1990s, there has been an intensive research
on the insertion of Li-ions into the lattice of graphite and, recently, in graphene-
based materials [ 2 , 51 ]. Graphitic carbon can form LiC 6 structures with lithium,
which leads to a relatively low theoretical capacity of graphite, 372 mAh g -1 .In
the condition of individual graphene sheets, LiC 3 structures can be formed in
which lithium is stored on both sides of the graphene sheet, which can increase the
theoretical capacity to 744 mAh g -1 .
The lithium storage in graphene papers was firstly investigated by Wallace and
coworkers (Fig. 3 c, d) [ 49 ]. The initial discharge capacity of a graphene paper
obtained by vacuum filtration of graphene dispersion was measured to be
680 mAh g -1 at a current density of 50 mA g -1 , but rapidly decreased to only
84 mAh g -1 in the second cycle. Nguyen et al. investigated in detail the LIB
anode performance of graphene paper prepared via hydrazine reduction of GO
paper compared with graphene powder-based electrodes fabricated by conven-
tional method with polyvinylidene fluoride (PVDF) binder [ 52 ]. Under a current
density of 50 mA g -1 , graphene paper and PVDF-graphene powder anodes
exhibited initial reversible capacities of 84 and 288 mAh g -1 , respectively
(Fig. 4 a, b). The larger reversible capacity for the graphene powder anode may be
attributed to the more disordered packing of the graphene sheets (Fig. 4 d), which
is conducive to anisotropic Li diffusion. Significant increases in the reversible
capacity of graphene paper (from 84 to 214 mAh g -1 ) were observed as the
current density decreased from 50 to 10 mA g -1 (Fig. 4 c), suggesting the well-
ordered structure of the graphene nanosheets in the bulk paper form would create a
kinetic barrier for the diffusion of Li-ions during electrochemical process.
Graphene with its maximal surface area of 2,630 m 2 g -1 is an ideal medium for
supercapacitors as the EDLC is directly proportional to the surface area. Their
application as supercapacitor electrodes was first explored by Ruoff and cowork-
ers, who found that chemically derived graphene powder exhibits specific
capacitances of 135 and 99 F g -1 in aqueous and organic electrolytes, respectively
[ 53 ], and then different forms of graphene with improved performances have been
reported [ 54 - 56 ]. The different values obtained with different forms of graphene
mainly because the graphene nanosheets used tends to restack thus reducing the
surface areas as only the surfaces can contact with the electrolyte contribute to the
specific capacitance.
The supercapacitor application of graphene-based flexible electrodes was first
studied by Chen et al. [ 57 ]. The ultrathin graphene films were also prepared using
the vacuum filtration method. The thickness of graphene films can be tuned by
choosing different volume of graphene solution and films of 25, 50, 75, and
100 nm thick can be obtained. The supercapacitor tests were performed using a