Continuous cycling of carbon-based capacitive deionization systems: An evaluation of the electrode performance and stability

Abstract

• Electrode pore size distribution and apparent morphology are immune to oxidation. • Electrode oxidation will increase oxygen-containing functional groups on the surface. • Slight oxidation helps to improve the desalination performance of the electrode. • The degree of oxidation of the carbon electrode corresponds to the number of cycles. • Electrode oxidation evaluation system is of great significance to CDI technology. Activated carbon is widely used as the electrode in capacitive deionization technology (CDI), because of its simple manufacturing and low price. However, the evaluation system of carbon electrode oxidation degree and running time is lacking in existing studies, and the reasons for the changes caused by carbon electrode oxidation are not in-depth enough and limits its application. In this study, polyvinylidene fluoride/activated carbon (PVDF/ACP) composite electrode was prepared through coating. Electrode characterization and electrochemical testing were used to characterize PVDF/ACP anode after long-term circulation in NaCl solution. It was found that the pore size distribution and apparent morphology of the electrode were immune to the influence of carbon electrode oxidation, but the acidic functional groups on the electrode surface increased significantly and the electrochemical properties changed. Combined with the CDI desalination experiment, the relationship between oxidation degree and cycle number was revealed. The results show that the electrode with 0 ∼ 20 cycles is relatively stable and its performance is slightly improved. A small amount of oxygen-containing functional groups contributes to the electrode performance. The performance of the electrode with 20 ∼ 40 cycles to deteriorate suddenly, and a large number of oxygen-containings functional groups will affect the equilibrium potential and ion transports of electrode. The electrode performance degrades slowly after 40 ∼ 80 cycles. After 80 cycles, the electrode performance remained unchanged and the carbon electrode reached the oxidation threshold.