In this report, we show that inexpensive and environmentally friendly manganese oxides can achieve high ion removal capacities in a hybrid capacitive deionization (HCDI) configuration and highlight the importance of understanding the relationship between material crystal structure and ion size in electrochemical water desalination. As population and pollution levels continue to rise and threaten fresh water sources, the scarce availability of potable water has become a critical issue for society. Therefore, it is imperative to develop efficient water desalination techniques, and capacitive deionization (CDI) has emerged as a low cost and low pressure deionization technique [1]. In a typical CDI process, brackish water is flown through a channel between two carbon electrodes, and a potential is applied across the electrodes. Ions from the water are then attracted to the electric double layer on the surface of the carbon materials and removed from solution, resulting in a desalinated water stream exiting the channel. When the electrodes are saturated, the potential can be reversed, releasing ions from the surface of the carbon electrodes and regenerating them for subsequent desalination.

The ion removal capacity of carbon electrodes in CDI is limited by the surface area of the carbon since the mechanism of ion removal is based on the electric double layer formed on the carbon surfaces. Recently, an approach has emerged (HCDI) that utilizes one carbon electrode and a redox active material as the opposite electrode [2]. This approach has the potential for higher ion removal capacities since the redox active material can remove and release ions from solution via oxidation and reduction reactions with bulk of the material and is therefore not limited by the available surface area. A tunnel manganese oxide (TuMO), Na₄Mn₉O₁₈, demonstrated the effectiveness of this approach by achieving a high capacity in NaCl solution while in an HCDI configuration [2].

Motivated by this past work, we study the behavior of other TuMOs in an HCDI configuration. Manganese oxides are of great interest for electrochemical water desalination due to their inherently low cost, environmentally friendliness, high electrochemically activity, and stability in aqueous environments. More specifically, TuMOs consist of MnO₆ octahedra arranged in various tunnel configurations around stabilizing cations. The large, open tunnel structures provide ample volume to store ions removed from solution. Further, by varying synthesis conditions, the size, shape, and ionic content of the structural tunnels can be modified, thus making TuMOs an attractive materials system to investigate for the relationship between crystal structure and ion removal capacity. TuMOs can all be synthesized with a flexible nanowire morphology as well, which allows for excellent access of water and ions to the surface of the materials.

In this work, we investigate the ion removal behavior of four TuMOs in an HCDI configuration in NaCl, KCl, and MgCl₂ solutions. Two of the phases studied are well-known tunnel structures, α-MnO₂ with 2x2 octahedra square tunnels and manganese oxide with todorokite crystal structure (Tod-MnO₂) containing tunnels of 3x3 octahedra dimensions. The other two phases are novel TuMOs with ordered (2xn-MnO₂) and disordered (Hybrid-MnO₂) combinations of structural tunnels with multiple different dimensions. The ion removal capacities of the four TuMOs are shown in Figure 1, and it was found that each material demonstrated high ion removal capacities above 20 mg g^-1 in all solutions tested. α-MnO₂, with the smallest tunnels, demonstrated lower capacities in the solution containing the largest hydrated cation, MgCl₂. The other three phases, which contain larger structural tunnels, were found to demonstrate superior removal of larger hydrated ions from solution.

Extended ion removal experiments showed the TuMOs to be stable for over 20 ion removal/ion release cycles, indicating the efficacy of using TuMOs for repeated water desalination via HCDI. Moreover, ex-situ XRD and EDS show that the ion removal mechanism is a result of chemical reaction between the ions in solution and the TuMO electrode. In summary, this work demonstrates not only the efficacy and high ion removal capacities of TuMOs in an HDCI configuration, but also shows the importance of understanding the relationship between material crystal structure and the size of ions in solution for maximizing performance in electrochemical water desalination.

References:

[1] M.E. Suss et al., Energy Environ. Sci., 2015, 8, 2296

[2] J. Lee et al., Energy Environ. Sci., 2014, 7, 3683