Abstract
Hybrid capacitive deionization (HCDI), which combines a capacitive carbon electrode and a redox active electrode in a single device, has emerged as a promising method for water desalination, enabling higher ion removal capacity than devices containing two carbon electrodes. However, to date, the desalination performance of few redox active materials has been reported. For the first time, we present the electrochemical behavior of manganese oxide nanowires with four different tunnel crystal structures as faradaic electrodes in HCDI cells. Two of these phases are square tunnel structured manganese oxides, α-MnO2 and todorokite-MnO2. The other two phases have novel structures that cross-sectional scanning transmission electron microscopy analysis revealed to have ordered and disordered combinations of structural tunnels with different dimensions. The ion removal performance of the nanowires was evaluated not only in NaCl solution, which is traditionally used in laboratory experiments, but also in KCl and MgCl2 solutions, providing better understanding of the behavior of these materials for desalination of brackish water that contains multiple cation species. High ion removal capacities (as large as 27.8 mg g−1, 44.4 mg g−1, and 43.1 mg g−1 in NaCl, KCl, and MgCl2 solutions, respectively) and high ion removal rates (as large as 0.112 mg g−1 s−1, 0.165 mg g−1 s−1, and 0.164 mg g−1 s−1 in NaCl, KCl, and MgCl2 solutions, respectively) were achieved. By comparing ion removal capacity to structural tunnel size, it was found that smaller tunnels do not favor the removal of cations with larger hydrated radii, and more efficient removal of larger hydrated cations can be achieved by utilizing manganese oxides with larger structural tunnels. Extended HCDI cycling and ex situ X-ray diffraction analysis revealed the excellent stability of the manganese oxide electrodes in repeated ion removal/ion release cycles, and compositional analysis of the electrodes indicated that ion removal is achieved through both surface redox reactions and intercalation of ions into the structural tunnels. This work contributes to the understanding of the behavior of faradaic materials in electrochemical water desalination and elucidates the relationship between the electrode material crystal structure and the ion removal capacity/ion removal rate in various salt solutions.
| Original language | English |
|---|---|
| Pages (from-to) | 476-488 |
| Number of pages | 13 |
| Journal | Nano Energy |
| Volume | 44 |
| DOIs | |
| State | Published - Feb 2018 |
Funding
E. Pomerantseva acknowledges funding from the National Science Foundation used to develop materials synthesis methods (Grant CBET-1604483 ) and to support water desalination experiments (Grant CMMI-1635233 ). The authors would also like to acknowledge M. Clites for her assistance with SEM and EDX characterization, as well as L. Gomes and Y. Gogotsi for use of the Malvern Instruments Zetasizer Nano ZS for conductivity measurements. The authors also thank the Electrochemical Energy Systems Laboratory in the Mechanical Engineering Department at Drexel University for the many helpful discussions on HCDI cell design and performance. Aberration-corrected STEM imaging was conducted at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences, which is a U.S. DOE Office of Science User Facility.
Keywords
- Electrochemical water desalination
- Hybrid capacitive deionization
- Manganese oxides
- Tunnel crystal structures