Improved capacitive deionization performance of mixed hydrophobic/hydrophilic activated carbon electrodes

Capacitive deionization (CDI) is a promising salt removal technology with high energy efficiency when applied to low molar concentration aqueous electrolytes. As an interfacial process, ion electrosorption during CDI operation is sensitive to the pore structure and the total pore volume of carbon electrodes limits the maximum salt adsorption capacity (SAC). Thus, activation of carbons as a widely used method to enhance the porosity of a material should also be highly attractive for improving SAC values. In our study, we use easy-to-scale and facile-to-apply CO2-activation at temperatures between 950 °C and 1020 °C to increase the porosity of commercially available activated carbon. While the pore volume and surface area can be significantly increased up to 1.51 cm3g−1and 2113 m2g−1, this comes at the expense of making the carbon more hydrophobic. We present a novel strategy to capitalize on the improved pore structure by admixing as received (more hydrophilic) carbon with CO2-treated (more hydrophobic) carbon for CDI electrodes without using membranes. This translates into an enhanced charge storage ability in high and low molar concentrations (1 M and 5 mM NaCl) and significantly improved CDI performance (at 5 mM NaCl). In particular, we obtain stable CDI performance at 0.86 charge efficiency with 13.1 mg g−1SAC for an optimized 2:1 mixture (by mass).