Hybrid capacitive deionization (HCDI) is a derivative of capacitive deionization (CDI) method for water desalination, in which one carbon electrode is replaced with a redox-active intercalation electrode, resulting in substantial improvements in ion removal capacity over traditional CDI. The search for high-performing intercalation host compounds is ongoing. In this study, two-layered manganese oxides (LMOs), with sodium (Na-birnessite) and magnesium (Mg-buserite) ions stabilizing the interlayer region, were for the first time evaluated as HCDI electrodes for the removal of ions from NaCl and MgCl2 solutions to understand structural/compositional dynamics and electrochemical stability of LMO electrodes over extended cycling. Both materials demonstrated excellent initial ion removal performance with the highest capacities of 37.2 mg g-1 (637 μmol g-1) exhibited by Mg-buserite in NaCl solution and 50.2 mg g-1 (527 μmol g-1) exhibited by Na-birnessite in MgCl2 solution. The performance decay observed over the course of 200 ion adsorption/ion release cycles was attributed to two major phenomena: oxidation of carbon electrode and evolution of the structure/composition of LMO electrodes. The latter involves disorder in stacking of Mn-O layers and changes in the interlayer spacing/interlayer ions reflecting the composition of the solution being desalinated. This work highlights the importance of understanding the interactions between the HCDI electrodes and solutions containing different ions and the structural analysis of redox-active material in intercalation electrodes over the course of operation for gaining insight into the fundamental processes governing desalination performance and developing next-generation HCDI systems with long-term electrochemical stability.
Keywords: Mg-buserite MnO2; Na-birnessite MnO2; electrochemically induced phase transformation; hybrid capacitive deionization (HCDI); layered manganese oxides; water desalination.