Modification of carbon xerogel electrodes for more efficient asymmetric capacitive deionization

Since a capacitive deionization (CDI) cell is a type of electrochemical device, its performance will be limited by the electrode with a weaker ion-adsorption. A cyclic voltmmogram for a freshly prepared carbon xerogel (CX) electrode showed Na⁺ adsorption at the cathodic branch is not as favorable as Cl^- adsorption at the anodic branch in a ∼4 mM (∼233 ppm) NaCl solution. To balance this performance, the CX electrode was specifically modified by coating with SiO₂ particles and by the formation of surface -COOH groups from oxidation. It was observed that these modifications cannot only increase the cathodic current density but also adjust the location of the potential of zero charge (PZC) for the CX electrode, resulting in the enhancment of Na⁺ adsorption. In CDI tests, electrosorption performance showed that cells with modified negative CX electrodes had much higher electrosorption capacities, e.g., at 1.2 V, electrosorption capacity for the modified electrodes increased by 36-50% in comparison to that for the unmodified CX negative electrode. This improvement of electrosorption performance is attributed to both an increase in total charge passed through the cell and the adjustment of the PZC leading to diminished co-ion repulsion.