2.3. Electrochemical capacitors
2.3.2. Carbon nanostructured electrodes
2.3.2.2. Graphene
The name graphene (G) is derived from graphite and alkene [2.100] and G is a form of graphite with less than ten layers of six-membered carbon species in a honeycombed network [2.101-
50
2.103]. Applications of G range from flexible electronics to functional nano-devices and G is so far the best carbon nanostructured material-based electrode for ECs [2.85,2.104,2.105].
Types of G include monolayer, plane sheet, mesh and ribbons [2.101,2.106,2.107] and there are two generations of G, i.e. first (2D) and second (3D) [2.32]. Focus on G began in earnest in 2004 with publications on G increasing exponentially since 2005 [2.80]. G nanosheets are now regarded as the next generation electrode, this is principally due to their theoretical superior conductivity and surface area [2.8]. The high surface area of G nanosheets comes from interconnected open channels between layers, not pores [2.72, 2.108]. Electrons move unimpeded at higher speeds throughout the lattice in G than in ordinary metals [2.100]. The conductivity, specific surface area, thermal conductivity and intrinsic mobility of G are approximately 2000 S cm-1, 3100 m2 g-1, 3000 W m-1 K-1 and 200 000 cm2 V-1 s-1, respectively [2.32,2.102,2.109-2.111].
Traditionally, G is regarded as the mother of conjugated carbon nanomaterials and is the thinnest known material [2.30,2.92,2.105,2.112]. It resembles polycyclic aromatic hydrocarbons and benzene. G is made up of sp2 carbon atoms and its π-electrons are delocalised throughout the 2D network [2.103,2.105,2.109]. However, the role of monatomic thick G and the 2D honeycomb network of sp2-hybridised carbons is still not fully understood [2.113,2.114]. G is chemically inert, and this is a setback to its manipulation. Its chemistry can be viewed to involve breaking and formation of conjugated sp2 C-C bonds. Oxidation of G enhances the hydrophilic character and enhances agglomeration and precipitation [2.115].
G easily agglomerates due to van der Waals forces arising from π β π stacking interactions between sheets. This makes fabrication of a porous electrode a difficult task [2.30], especially in the G first generation, due to limited accessibility of internal pores by electrolyte ions [2.32].
The stacking order and coupling between layers determines the electronic properties. The actual capacitive performance of G-based nanocomposites is often lower than anticipated due to aggregation of G sheets [2.116]. Theoretical studies of G postulate high Cs valuesof 550 F g-1 if restacking problems are eliminated [2.32, 2.60].
Since G has a zero band gap, the properties of G such as band gap opening and conductivity can be enhanced through covalent modification [2.106]. In principle, G modification is possible at the edges and surface planes, altering the energetic characteristics in the process.
The zigzag edges are more reactive than arm chair but in practice G nanosheets have a mixture of configurations [2.105]. This makes altering chemical functionalities a difficult task.
51
Controlling the configuration length of a delocalised carbon lattice via covalent functionalization influences band gap tunability [2.101,2.106]. Band gap and type of charge carriers are important in the semiconductor industry. Substitution of carbon with heteroatoms is a possible way of altering charge carrier types.
G has found numerous applications in EDLCs mainly as a substrate for nanomaterials, such as carbon, and this gives them better functionalities. For instance, Haiyang et al. [2.74] recently reported G nanosheets synthesised at low temperatures of 100 and 400 ΒΊC, with the G nanosheets at the latter temperatures achieving the best Cs of 212 F g-1. The work on G nanosheets by Nasbi et al. [2.72] achieved low current responses due to low mobility of OH- ions and the large K+ ionic radius in the electrolyte. Their low surface G nanosheets achieved a Cs of 3.62 F g-1 in 3 mol L-1 NaCl electrolyte at a scan speed of 5 mV s-1.
Recently, a cauliflower-fungus graphene (CFG) with a mesoporous 3D structure, surface area of 462 m2 g-1 and rectangular cyclic voltammetry (CV) at 50 mV s-1, achieved a Cs of 103 F g-
1 [2.32]. This value was greater than 92.4 F g-1, the theoretical value, and also their energy density was 15.6 W h kg-1. They attributed these findings to the creation of fast 3D channels and hierarchical mesoporous structure for electrolyte ion mobility synthesized from CO2 via a one-step exothermic reaction.
Du et al. [2.53] also reported good charge propagation on oxidised G nanosheets (GO) with narrow mesopores of 4 nm and redox peaks from oxygen functionalities. They also reported that disordered G nanosheets with many edges in the electrode store more energy with increase in scan rate. Also, 3D GO, G functionalised with oxygen moieties, achieved a Cs of 352 F g-1 at a sweep rate of 5 mV s-1 [2.117]. A composite of G/CeO2/carbon black in electrodes with a narrow deep morphology was reported to increase charge storage but reduced current response [2.95]. This in turn increased the specific surface area, Faradaic reactions and charge storage capabilities, but reduced the current response and power delivery capabilities. The same researchers were able to achieve a Cs of 11.84 F g-1 at 10 mV s-1 in their recent report based on G nanosheets modified with zirconium dioxide[2.27].
In another study by Battumur et al. [2.62], an optimum mass of 5% of multiwalled carbon nanotubes (MWCNTs) in 1% G nanosheets and Co3O4 nanocomposites provided additional nano-channels for flow of charge and electrolyte penetration. This therefore improved charge storage in their typical G nanosheet materials by reducing diffusion and the migration length
52
of the electrolyte ions. Higher loadings of G nanosheets were reported to hinder formation of a 3D network for an effective conductive system. G can also add conductivity to nanoparticles [2.42,2.80,2.85]. Additionally, electron injection from G into oxides increases the concentration of holes in G and may improve the conductivity of the entire hybrid material.
Corrugations in G nanosheets can achieve a 10-fold enhancement of electron transfer rates relative to the basal graphitic plane [2.82] but G has limited capability to store charge in aqueous electrolytes [2.85].
Cobalt-based composites, such as Co3O4/GO nanosheet, and Co(OH)2/GO nanosheet are also reported to have high capacitance values and a capacitance retention of 85-95% after 1000 cycles with 6 mol L-1 KOH [2.85]. Such observations were attributed to both the Co2+/Co3+
system in CoO4/CoOOH and Co3+/Co4+ in the CoOOH/CoO2 phase change. The Co3O4 form is preferred in capacitors because of the low costs involved in using it, environmental friendliness, and favourable microstructure and morphology [2.62]. Ramakrishnan et al. [2.85]
are of the opinion that a uniform distribution of metal oxide (MO) in nano-carbon based capacitor architectures is still a common challenge to most reported work.
Anosori et al. [2.80] designed a hybrid structure consisting of Nb2O5 nanoparticles with sizes between 10 and 20 nm deposited on a 3D graphene aerogel. Their design reduced the diffusion limitation of electrolyte ions moving through the electrode and increased electrode conductivity. They attributed their findings to the orthorhombic structure of niobium oxide which offers a 2D transport pathway for fast Li+ (cation from electrolyte) intercalation. Their stance was that thin films and microelectrodes perform better in supercapacitors.
In the report by Deng et al. [2.42], a G/V2O2 composite was prepared by a single-step hydrothermal technique. A nickel foam current collector, Pt counter electrode and SCE reference electrode were used with the scan speed applied at 5 mV s-1, whilst 3 mg was the mass load. They observed a deviation from the classic rectangular CV curve with a high profile and no redox peaks, this inferred high Cs values. In explaining their findings, they referred to the ability of the star fruit-like V2O2 structure to shorten the path length and this increased the contact area between V2O2 and G. Hence, ion accessibility in pores was improved and according to their interpretation, typical structures that shorten electrolyte diffusion length during charge/discharge improve electrochemical utilization of nanoparticles. After 1000 cycles their capacitance retention for G, VO2 and G/VO2 was 99%, 47% and 65%, respectively.
They proposed that the star fruit-like G/VO2 conversion to hollow hemispheres during
53
galvanostatic charge/discharge as a manifestation of poor cycle stability. They ascribed it to oxidation of V4+ to V5+. In their work, they also pointed out the Cs lowering effect of the electrode high charge transfer resistance.