Relationship between capacitance of activated carbon composite electrodes measured at a low electrolyte concentration and their desalination performance in capacitive deionization

Capacitive deionization (CDI) has attracted attention as a novel desalination process, which would be one of solutions to global water scarcity. In this process, a porous electrode is the most important component, providing high capacitance and subsequent large desalination capacity. Activated carbon satisfies this requirement and is an economically competitive material over other types of carbon, so that lots of studies have been carried using various activated carbons. However, the relationship between capacitance and desalination performance remains unclear, especially at low electrolyte concentrations. In this study, we investigated the relationship between capacitance and desalination performance using various activated carbon composite electrodes. Capacitance was specifically measured at a low electrolyte concentration (0.01 M NaCl), in which condition the CDI works effectively, and was transformed to charge capacity, assuming that 1.2 V was applied to the system. The results showed that deionization capacity had a good linear relationship to the charge capacity evaluated from capacitance. In particular, we found out that 60–80% of the charge capacity (0.01 M) could be utilized in deionization. Also, the ratio became 50–60% with the charge capacity evaluated by capacitance measured at a high electrolyte concentration (1 M NaCl). This approach allows simple estimation of desalination performance from capacitance of electrodes, which would eventually make the systematic design and characterization of CDI electrodes convenient.