Desalination stability of capacitive deionization using ordered mesoporous carbon: Effect of oxygen-containing surface groups and pore properties

• The desalination showed remarkable degradation after 50 charge/discharge cycles.
• The surface properties had little effect on the desalination stability.
• The pore characteristics affected the desalination stability.
• The drop in surface area and increase in oxygen group led to degraded desalination.

Ordered mesoporous carbons (OMCs) with various pore properties and oxygen-containing functional groups were obtained by changing preparation conditions and thermal or chemical modification after preparation. The modification hardly changed the pore structure of OMCs but did tailor surface properties. Desalination performance of capacitive deionization (CDI) by OMCs was investigated during 50 continuous cyclic adsorption/desorption experiments using a 500 mg/L NaCl solution and an applied potential of 1.2 V. The results showed pronounced degradation in the desalination performance for all materials as the cycle number increased, and an inversion in conductivity appeared at the beginning of the adsorption or desorption stage after several cycles. The surface properties are important for initial desalination, with the material with moderate oxygen functional groups displaying the largest desalination capacity, rate and charge efficiency. However, these groups had a negligible effect on the long-term desalination stability. The material which had a very small surface area and pore size underwent the most remarkable performance degradation, and large surface area and pore diameter were more favorable towards long-term stability. The deterioration in CDI performance was attributed partially to the decrease in surface area and mostly to an increase in oxygen functional groups on the electrodes, especially the anodes.