Capacitive Deionization of NaCl Solutions with Modified Activated Carbon Electrodes

A pyrolysis product derived from a coal-tar pitch was chemically activated using KOH in a KOH/carbon proportion of 5:1. The activated carbon was used as electrode-active material for capacitive deionization (CDI). Electrochemical parameters, such as current values, charge, specific charge and charge-discharge efficiencies were investigated using a unit cell and solutions of NaCl in different concentrations. The parent activated carbon shows an excellent behavior as electrode-active material in CDI, removing more salt than other carbons previously described in the literature. The activated carbon electrode presents an efficiency higher than 99% after 20 cycles. The parent activated carbon was modified by thermal treatment under nitrogen at different temperatures and by treatment with hydrogen and carbon dioxide. The modified activated carbons were also evaluated as electrode-active material to study the influence of the texture and surface chemistry on the CDI process. The results show the importance of both the texture and surface chemistry of the active material on the CDI process. The best behavior as electrode-active material was obtained for the materials with a high apparent specific surface area and a large quantity of-oxygenated functional groups, i.e., the parent activated carbon and the sample modified by hydrogen treatment.