Developing graphene like electrodes for capacitive deionisation

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. Porous carbon materials are the most important component in the CDI processes, as they are used as the electrodes that play a significant role in the efficiency of the desalting process. The ideal electrode materials for CDI should be both highly conductive and of high surface area and suitable pore structures. The currently available carbon electrodes limit the desalination efficiency of CDI due to their low conductivity and non-ideal pore structure and pattern. Previous research has demonstrated that the efficiency of CDI strongly depends upon the surface properties of the carbon electrodes, such as surface area and pore microstructure. Many kinds of carbon materials have been investigated as CDI electrodes such as carbon aerogel, carbon cloth, carbon nanotubes and mesoporous carbons. Recently, graphene-like nano-flakes (GNFs) with relatively high specific surface area have been prepared and used as electrodes for capacitive deionization. The GNFs were synthesized by a modified Hummers' method using hydrazine for reduction. The GNFs prepared by this process had the specific surface area as high as 222.01 m²/g. It was found that the ratio of nitric acid and sulfuric acid plays a vital role in determining the specific surface area of GNFs. They were characterized by atomic force microscopy, N₂ adsorption at 77 K and electrochemical workstation. Its electrosorption performance was much better than commercial activated carbon (AC), suggesting a great potential in capacitive deionisation application. Further, the electrosorptive performance of GNFs electrodes at various operational conditions, include different electrical potentials, flow rates and ionic strengths were measured and the electrosorption isotherm and kinetics were investigated. It is found that the specific electrosorptive capacity of the GNFs was 23.18 μmol/g for sodium ions (Na⁺) when the initial concentration was at 25 mg/l, which was higher than that of previously reported data using graphene and AC under the same experimental condition. In addition, the equilibrium electrosorption capacity was determined as 73.47 μmol/g at 2.0 V by fitting data through the Langmuir isotherm, and the rate constant was found to be 1.01 min^-1 by fitting data through pseudo-first-order adsorption. The results suggested that GNFs synthesized by a modified chemical oxidation and reduction method can be used as effective electrode materials in CDI process for brackish water desalination. It was found that highly charged multivalent ions with smaller hydrated radius would be strongly attracted to the electrodes. For cations with same charge, the one with smaller hydrated radius would be more effectively removed.