Chemical Routes to Graphene-Based Flexible Electrodes for Electrochemical Energy Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 430

regular graphene paper obtained by vacuum filtration (84 mAh g -1 ). Such
graphene paper also possesses high rate capability and stability; at the current
densities of 200, 500, 1000, and 1500 mA g -1 , the corresponding reversible
specific capacities are 557, 268, 169 and 141 mAh g -1 , respectively. And
568 mAh g -1 can still be reached after 100 cycles at 100 mA g -1 (Fig. 10 b).
The performance of folded structured graphene paper as supercapacitor elec-
trode has also been tested in a two-electrode configuration with 1 M H 2 SO 4
aqueous electrolyte. Figure 10 c shows the CV curves at the scan rates of
50-1,000 mV s -1 over the voltage range from 0 to 1 V, which all display
quasirectangular shape, indicating the excellent capacitance characteristics of the
folded structured graphene paper. From the galvanostatic charge/discharge test, a
specific capacitance up to 172 F g -1 can be obtained at the current density of
1Ag -1 , which can still be retained at 110 F g -1 even when the supercapacitor is
operated at a fast rate of 100 A g -1 (Fig. 10 d). Additionally, after 5,000 times
cycling at a current density of 20 A g -1 , the graphene paper can retain over 99 %
of its initial capacitance, indicating its excellent stability.
The high performance of such graphene paper is originated from its folded
structured graphene sheets, and when used as LIB anode, such folds can provide
slightly increased intersheet spacing and nucleation sites, which can facilitate Li-ion
diffusion and SEI formation, leading to higher reversible capacity. And in the
application as supercapacitor electrode, the folded structure is helpful for graphene
paper to contact the electrolyte and remarkably improve the capacitance properties.
3 Graphene as Conductive Matrix for Flexible Electrode
To further increase the specific energy density of graphene paper as flexible LIB
and supercapacitor electrodes, incorporation of materials such as metal oxides or
conducting polymers into the graphene matrix has been proved to be an effective
approach [ 67 ]. From another point of view, due to the large specific surface area
and the conductive robust structure, graphene can be a promising matrix for dis-
persing functional materials in which the charge transfer, redox reaction, as well as
the mechanical stability will be enhanced. Moreover, the aggregation of graphene
sheets can be partly prevented by sandwiching with other nanomaterials [ 67 ].
Therefore, anchoring redox active materials on graphene will yield highly porous
composites attractive for fabricating high performance LIBs and supercapacitors.
3.1 Fabricating Graphene Composite Papers by In Situ
Reaction
Strategies for preparing graphene-based composites can be generally categorized
into two types: in situ reaction and blending. Many graphene-based composites