Fluorination effect of activated carbons on performance of asymmetric capacitive deionization

Abstract

Activated carbons (ACs) were fluorinated and fabricated into electrodes to investigate the effect of fluorination on asymmetric capacitive deionization (CDI). Fluorine functional groups were introduced on the AC surfaces via fluorination. The specific capacitance of the fluorinated AC (FAC) electrode increased drastically from 261 to 337F/g compared with the untreated AC (RAC) electrode at a scan rate of 5mV/s, despite a decrease in the specific surface area and total pore volume after fluorination. The desalination behavior of asymmetric CDI cells assembled with an RAC electrode as the counter electrode and an FAC electrode as the cathode (R || F-) or anode (R || F+) was studied. For R || F-, the salt adsorption capacity and charge efficiency increased from 10.6mg/g and 0.58–12.4mg/g and 0.75, respectively, compared with the CDI cell assembled with identical RAC electrodes at 1V. This CDI cell exhibited consistently better salt adsorption capacity and charge efficiency at different applied voltages because FAC electrodes have a cation attractive effect originating from the partially negatively charged fluorine functional groups on the AC surface. Therefore, co-ion expulsion in the FAC electrode as the cathode is effectively diminished, leading to enhanced CDI performance.