Rapid Inversion of Surface Charges in Heteroatom-Doped Porous Carbon: A Route to Robust Electrochemical Desalination

Abstract

Given that a considerably large population suffers from shortage of water, there are numerous on-going efforts to turn seawater into freshwater, and electrochemical desalination processes—particularly capacitive deionization (CDI)—have gained significant attention due to their high energy efficiency and reliable performance. Meanwhile, carbonaceous electrode materials, which are most commonly used in CDI systems, have poor long-term stability due to unfavorable interactions with oxygen in saline water. Herein, rapid and vigorous inversion of surface charges in heteroatom-doped carbon electrodes, which leads to a robust operation of CDI with high desalination capacity, is reported for the first time. By carbonization of coffee wastes, nitrogen- and sulfur-codoped activated carbon with hierarchical micro/mesopores are prepared in an environmentally-friendly manner, and this carbon results in a significantly higher inverted capacity than that of various activated carbon counterparts in long-term CDI operations, without any sign of drop in performance. Investigations on the changes in physicochemical properties of the electrodes during the inversion disclose the favorable roles of nitrogen and sulfur dopants, which can be summarized as enlarging the difference between the surface charges of the two electrodes by chemical interactions with oxygen in the anode and carbon in the cathode.