(Nanowerk Spotlight) The potential impact areas for nanotechnology in water applications are divided into three categories – treatment and remediation, sensing and detection, and pollution prevention – (read more: "Nanotechnology and water treatment") and the improvement of desalination technologies is one key area thereof. Nanotechnology-based water purification devices have the potential to transform the field of desalination, for instance by using the ion concentration polarization phenomenon (see: "Nanotechnology makes portable seawater desalination device possible").
Another, relatively new method of purifying brackish water is capacitive deionization (CDI) technology. The advantages of CDI are that it has no secondary pollution, is cost-effective and energy efficient. The basic concept of CDI, as well as electrosorption, is to force charged ions toward oppositely polarized electrodes through imposing a direct electric field: brackish water flows between pairs of high surface area carbon electrodes that are held at a potential difference of about 1-2 volts. The ions and other charged particles, such as microorganisms, are attracted to and held on the electrode of opposite charge. The negative electrode attracts positively charged ions (cations) such as calcium, magnesium and sodium, while the positively charged electrode attracts negative ions (anions) such as chloride, nitrate, and silica.
CDI is energy efficient because it aims to remove only the salt ions, which are a small percentage of the feed solution, as compared to most other technologies that aim to separate water, which accounts for 90% of the feed solution.
Suitable carbon materials are the most important component in CDI devices, as they are used as the electrodes that play a significant role in electrosorptive process.
A research team from the SA Water Centre for Water Management and Reuse at the University of South Australia and the Engineering Research Center for Nanophotonics & Advanced Instrument at East China Normal University in Shanghai, has now developed a CDI application that uses graphene-like nanoflakes as electrodes for capacitive deionization. They found that the graphene electrodes resulted in a better CDI performance than the conventionally used activated carbon materials.
Mechanism of CDI
employing activated carbon (AC) and graphene nanoflakes (GNFs) electrode. (Reprinted with permission from American Chemical Society)
The researchers conducted experiments in which they compared their graphene nanoflake-based electrodes with conventional activated carbon electrodes. They note that "although having the larger surface area (989.54 m2/g) than graphene nanoflakes (222.01 m2/g), activated carbon has an electrosorptive capacity of only 13.73 µmol/g, which is much lower than that of graphene nanoflakes (whose electrosorptive capacity is 23.18 µmol/g)."
They attribute this to the fact that the nanoflakes have an interlayer structure that is more accessible for ions, while activated carbon has a large fraction of inaccessible small micropores. As a result, the effective surface area of graphene nanoflakes is higher than that of activated carbon.
TEM and SEM images of activated carbon and graphene nanoflakes are used to confirm their hypothesis. According to the researchers, "the TEM image of graphene nanoflakes indicates that graphene nanoflakes are aggregated together, showing a semitransparent flower shape interlayer pattern. It also shows that graphene nanoflakes are homogeneous flakes with microsize that are beneficial to ions accessing and are adsorbed on the surface of the flakes.
"In contrast, the structure of activated carbon 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."
They point out that another possible reason for the high conductivity for graphene nanoflakes is the presence of conductive graphitized chunks in the graphene nanoflakes, which was caused by the incomplete grapheme preparation from the graphite precursor.
Bottom line though, considering both effective specific surface area and electrical conductivity, the research team believes that graphene nanoflakes with high specific surface area have the potential as an excellent candidate electrode material for the CDI.