Developing Nano-Structured Carbon Electrodes for Capacitive Brackish Water Desalination

Abstract

Capacitive deionisation (CDI) is a promising alternative technology in desalination. It targets to remove the salt ions which are only a small percentage of the feed solution, as compared to most other technologies that aim to shift water which accounts of 90% of the feed solution. As a result, CDI requires less energy to operate and the electrodes are easily regenerated. The basic concept of CDI is electrical potential induced surface adsorption of ions on to electrodes. An electrical field forces charged sodium (Na+) and chloride (Cl-) ions in the brackish water to move towards oppositely charged electrodes, by forming electrical double layers, and remove them from the water. It needs low voltage to operate, and it does not require harsh chemical cleaning process. The most critical component of CDI is the carbon electrode materials, as reported that the electrosorptive capacity strongly depends on the physical properties such as surface area and conductivity of the electrode. This chapter reports the current research efforts and progress in the development of porous carbon electrodes conducted in our research group, the commercial activated carbons (ACs), carbon nanotubes (CNTs) including single walled CNTs (SWCNTs) and double walled CNTs (DWCNTs), ordered mesoporous carbons (OMCs), and more recently, graphene nano-flakes were prepared and used as the electrodes of capacitive deionization device. Their salt removal performances were investigated. Their morphology and specific surface area were characterized. The electrosorption experiment results showed that their electrosorptive capacities are in the order of OMCs >SWCNTs>DWCNTs>ACs, ie. 0.93, 0.81, 0.80, 0.62 mg/g respectively. This is attributed to their differences in pore size distribution, pore pattern arrangement and specific surface area. In addition, the ion sorption onto these mentioned materials follows a Langmuir isotherm, indicating monolayer adsorption. Finally, we have investigated the regeneration property of both CNTs and AC through charge-discharge experiment and found that their regeneration was effective. Chemically modified graphene has been studied in the context of many applications due to its excellent electrical, mechanical and thermal properties. Chemical modification of graphene oxide, which is generated from graphite powder as start material, has been a promising route to achieve mass production of graphene platelets. Hummers and Offeman (1958) developed a powerful oxidation method involving reacting graphite with a mixture of potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4) to