By Professor Linda
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.
Fig 1 Schematic diagram of capacitive deionization
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.,
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.
Fig 2 (a,b) Schematic diagram of CDI unit.
Fig 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
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.
Fig. 4 (a) TEM image of graphenes. (b, c) AFM images
Comparison of Graphene Nano Flakes (GNFs) and Activated Carbon (ACs)
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.
Fig.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,
- Dai, K., Shi, L., Zhang, D. and Fang, J. (2006). NaCl adsorption in
multi-walled carbon nanotube/active carbon combination electrode. Chemical
Engineering Science, 61, 428–433.
- Farmer, J.C., Tran, T.D., Richardson, J.H., Fix, D.V., May, S.C. and
Thomson, S.L. (1997a). The application of carbon aerogel electrodes to
desalination and wast treatment. Annual Meeting of the American Institute of
Chemical Engineers, Los Angeles, CA, November 16–21.
- Geim, A.K. and MacDonald, A.H. (2007) Graphene: Exploring the carbon
flatland. Physics Today, 60, 35–41.
- Li, L., Zou, L. Song, H. and Morris, G. (2009). Ordered mesoporous carbons
synthesized by a modified sol-gel process for electrosorptive removal of sodium
chloride. Carbon, 47, 775–781.
- Li., H., Gao, Y., Pan, L., Zhang, Y., Chen, Y. and Sun, Z. (2008).
Electrosorptive desalination by carbon nanotubes and nanofibres electrodes and
ion-exchange membranes. Water Research, 42, 4923–4928.
- Li, H., Zou, L., Pan, L. and Sun, Z. (2010a). Novel graphene-like electrodes
for capacitive deionization. Environmental Science & Technology,
- Li, H., Zou, L., Pan, L. and Sun, Z. (2010b). Using graphene nano-flakes as
electrodes to remove ferric ions by capacitive deionisation. Separation and
Purification Technology, 75, 8–14.
- Oren, Y. (2008). Capacitive deionization (CDI) for desalination and water
treatment—past, present and future (a review). Desalination, 228,
- Ryoo, M., Kim, J. and Seo, G. (2003). Role of titania incorporated on
activated carbon cloth for capacitive deionization of NaCl solution. Journal
of Colloid Interface Science, 264, 414–419.
- Xu, P., Drewes, J.E., Heil, D. and Wang, G. (2008) Treatment of brackish
produced water using carbon aerogel-based capacitive deionization technology.
Water Research, 42, 2605–2617.
- Welgemoed, T.J. and Schutte, C.F. (2005). Capacitive Deionization
TechnologyTM: An alternative desalination solution. Desalination,
- Zhang, D., Shi, L., Fang, J., Dai, K. and Li, X. (2006a). Preparation and
desalination performance of mutiwall carbon nanotubes. Materials Chemistry
and Physics, 97, 415–419.
- Zhang, D., Shi, L., Fang, J., Dai, K. and Liu, J. (2006b). Influence of
carbonization of hot-pressed carbon nanotubes on removal of NaCl from saltwater
solution. Materials Chemistry and Physics, 96, 140–144.
- Zou, L., Morris, G. and Qi, D. (2008b). Using activated carbon electrode in
electrosorptive deionisation of brackish water. Desalination, 225,
- Zou, L., Li, L., Song, H. and Morris, G. (2008a). Using mesoporous carbon
electrodes for brackish water desalination. Water Research, 42,