Charge enhanced capacitive deionization electrodes for deionized water production

The deionized (DI) water of high purity standards is used in several industrial processes to manufacture products and technologies for high end applications. Deionized water is used in industrial processes such as to manufacture semiconductor and pharmaceuticals among others. Therefore, it is of significant importance to develop the cost effective technologies to provide DI water given the tremendous growth of industrial and manufacturing processes. This study investigated production of deionized water by capacitive deionization (CDI) using nitric acid treated activated carbon (NTAC) electrodes. Activated carbon (AC) was etched in nitric acid solution to introduce oxygen functional groups on its surface. The electrochemical measurements and N2 adsorption-desorption isotherms were used to study the electrochemical performances and textural properties of the materials respectively. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to confirm the surface oxygen containing groups presented by nitric acid treatment. Then AC, symmetric NTAC, and asymmetric CDI electrode cells were constructed to compare their performance to produce DI water. It was shown that the treatment of the AC with nitric acid increased the percentage of oxygen containing functional groups from 12.6 to 17.8 that enhanced its salt adsorption capacity. As a result the asymmetric electrode cell produced DI water with 1.6 μS/cm (grade III deionized water, according to International Standard Organization) starting with water solution of 12.0 μS/cm. Electrochemical impedance spectroscopy tests revealed good capacitive behavior due to improved conductivity with NTAC having a series resistance of 0.51 Ω much lower than AC achieved 2.0 Ω. Also NTAC exhibits highest specific capacitance of 381.7 F/g compared to 106.6 F/g of untreated AC electrode. Based on the experimental results NTAC is proposed as a potential electrode material for CDI applications. Keywords: Capacitive deionization; CDI; deionized water; activated carbon; enhanced electrosorption; oxygen functional groups.