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−1more »
This content will become publicly available on June 29, 2023
Comparing energy demands and longevities of membrane-based capacitive deionization architectures
Several capacitive deionization (CDI) cell architectures employ ion-exchange membranes to control the chemistry of the electrolyte contacting the electrodes. Here, we experimentally examined how exposing carbon electrodes to either a saline electrolyte or an electrolyte containing a soluble redox-active compound influenced deionization energy demands and long-term stability over ∼50 hours. We specifically compared the energy demands (W h L −1 ) required to deionize 20 mM NaCl to 15 mM with a 50% water recovery as a function of productivity (L m −2 h −1 ). Relative to a conventional membrane capacitive deionization (MCDI) cell, flowing saline electrolyte over the electrodes did not affect energy demands but increased electrode salt adsorption capacities and capacity retention over repeated cycles. Exposing the electrodes to an electrolyte containing a redox-active compound, which made the cell behave similarly to an electrodialysis system, dramatically reduced energy demands and showed remarkable stability over 50 hours of operation. These experimental results indicate that using a recirculated soluble redox-active compound in the electrolyte contacting the electrodes to balance charge leads to far more energy efficient brackish water deionization than when charge is balanced by the electrodes undergoing capacitive charging/discharging reactions.
- Award ID(s):
- 1749207
- Publication Date:
- NSF-PAR ID:
- 10340965
- Journal Name:
- Environmental Science: Water Research & Technology
- Volume:
- 8
- Issue:
- 7
- Page Range or eLocation-ID:
- 1489 to 1496
- ISSN:
- 2053-1400
- Sponsoring Org:
- National Science Foundation
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