Capacitive deionisation (CDI) is a promising alternative technology in desalination (Welgemoed and Schutte 2005; Farmer et al. 1997; Oren, 2008) which is particularly suitable for small-scale inland brackish water desalination due to its lower energy demand and low maintenance requirements.
CDI targets the removal of the salt ions, which are only a small percentage of the feed solution, unlike most other desalination technologies that aim to shift water, which accounts for 90% of the feed solution (Fig 1). As a result, CDI operates on low energy and the electrodes are easily regenerated.
Porous carbon electrodes play a critical role in determining CDI performance. The ideal electrode materials for CDI should be highly conductive, with a high surface area and suitable pore size distribution.
Figure 1. Schematic diagram of capacitive deionization process
Desalination Performance of Various Carbon Electrodes
Previous research has shown that the efficiency of CDI strongly depends on the surface properties of the carbon electrodes, including their surface area and pore microstructure (Zou et al., 2008a, 2008b Li et al., 2009;).
Many kinds of carbon materials have been investigated as CDI electrodes, including carbon aerogel, activated carbon, carbon cloth, carbon nanotubes and mesoporous carbons (Ryoo and Seo, 2003; Zhang et al., 2006a, 2006b; Dai et al., 2006; Zou et al., 2008; Xu et al., 2008; Li et al., 2008). Custom design of the specific characteristics of the porous carbon materials will lead to improved electrosorption and desorption capacity in desalination applications.
The electrosorptive capacities of various carbon electrodes were compared in the batch-mode experiments that were conducted in a continuously recycling system including an electrosorptive unit cell and conductivity monitor as shown in Fig. 2 (a, b). Fig. 3 shows the TEM images of ordered mesoporous carbon (OMCs) (Li et al., 2009), the single and double wall carbon nanotubes and the random irregular pore arrangement in activated carbons, as well as their desalting capacity.
Figure 2. (a,b) Schematic diagram of CDI unit.
Figure 3. Comparison of porous carbon materials
Graphenes Prepared by Prof Zou’s Research Group
Graphene is considered ‘the thinnest material in our universe’ (Giem and MacDonald, 2007). Its exceptional electrical conductivity and two-dimensional flat structure have stimulated much research to optimise its properties and open pathways for various applications.
Professor Zou’s group has successfully synthesised graphene nano-flakes and was the first to report of using them as electrodes in the CDI process.
Graphenes’ thin flat structure has been confirmed by TEM and AFM images as shown in Fig. 4 (a–c). In our recent research, graphene nano-flakes (GNFs) have been chemically synthesised by a modified Hummers method (Li and Zou et al., 2010a).
Through optimisation of the experimental conditions, it was found that the ratio of nitric acid and sulphuric acid plays a significant role in determining the specific surface area of GNFs. The results showed that GNFs prepared by this process had a specific surface area of 222.0 m2/g, which is much higher than the previous 14.2 m2/g.
Figure 4. (a) TEM image of graphenes. (b, c) AFM images
Comparison of Graphene Nano Flakes (GNFs) and Activated Carbon (ACs) electrodes
The GNFs and ACs electrodes have been used in a bench scale CDI cell for desalting experiments under the same conditions. Their electrosorptive capacities as well as BET surface area are shown in Figure 5(a).
Although having the larger surface area (989.54 m2/g) than GNFs (222.0 m2/g), ACs have an electrosorptive capacity of only 13.73 µmol/g, which is much lower than that of GNFs (whose electrosorptive capacity is 23.18 µmol/g). This can be attributed to the fact that GNFs have a thin and interlayered structure that is more accessible for ions, while ACs have a large fraction of inaccessible small micropores. As a result, the effective surface area of GNFs is higher than that of ACs.
Figure 5(b) illustrates the ion electrosoprtion mechanism onto ACs and GNFs electrodes, respectively. The TEM image (Fig 5 (c-e)) of GNFs indicates that GNFs are aggregated together, showing a semi-transparent flower shape interlayer pattern. It also shows that GNFs are homogenous flakes with micro-size that are beneficial to ions accessing and are adsorbed on the surface of the flakes.
In contrast, the structure of ACs on the TEM image shows that it presents a beehive-type pore structure so that the ions cannot gain access to the inner pores and therefore a high electrosorptive capacity is difficult to achieve.
Considering both effective specific surface area and electrical conductivity, it is believed that the GNFs with high specific surface area have the potential as an excellent candidate electrode material for the CDI.
Figure 5. (a) Comparison of electrosorptive performance by employing GNFs and AC at the same experimental condition, the pictures at top-left and top-right depict the pore size distribution of AC and GNFs below 10 nm, respectively. (b) Mechanism of CDI employing AC and GNFs electrode. TEM observation images of AC (c) and GNFs in low (d) and high (e) magnification, respectively.
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