Na0.71CoO2 promoted sodium uptake via faradaic reaction for highly efficient capacitive deionization

Capacitive deionization is a promising method to produce portable water. However, its salt removal capacity is unsatisfied up to now. In this work, we proposed novel asymmetrical dual-ion capacitive deionization (A-CDI) device where the negative and positive electrodes are Na0.71CoO2 (NCO) and Ag@reduced graphene oxide (rGO), respectively. NCO exhibited laminated nanostructure, providing abundant vacancies for inserting sodium ions during the charge. Instantaneously, Ag@rGO is responsible for reacting with chloride ions, forming AgCl. As a result, Ag@rGO parallel to NCO A-CDI device demonstrates high desalination capacity, i.e. 34.8 mg/g with charge efficiency of 97% can be achieved in NaCl solution with an initial concentration of 500 mg/L at cell voltage of 1.2 V, which is much higher than that of most carbon electrodes. Moreover, the salt removal capacity of Ag@ rGO parallel to NCO A-CDI device in the first cycle was up to similar to 29 mg/g and decreased to similar to 19 mg/g after 50 cycles, demonstrating 66% remaining. Significantly, it is found that the (0 0 4) phase of NCO after A-CDI shifted to left, which implies the d-spacing increased due to the intercalation of Na+. Benefit from the novel structure and active Faradaic reaction, NCO should be a promising candidate as electrode for high-efficient CDI.