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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.
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Green growth of mixed valence manganese oxides on quasi-freestanding bilayer epitaxial graphene-silicon carbide substrates
Nanostructured manganese oxides (MnOx) have shown incredible promise in constructing next-generation energy storage and catalytic systems. However, it has proven challenging to integrate with other low-dimensional materials due to harsh deposition conditions and poor structural stability. Here, we report the deposition of layered manganese dioxide (δ-MnO2) on bilayer epitaxial graphene (QEG) using a simple three-step electrochemical process involving no harsh chemicals. Using this process we can synthesize a 50 nm thick H–MnO2 film in 1.25s. This synthetic birnessite is inherently water-stabilized, the first reported in the literature. We also confirm that this process does not cause structural damage to the QEG, as evidenced by the lack of D peak formation. This QEG heterostructure enhanced MnO2's redox active gas sensing, enabling room temperature detection of NH3 and NO2. We also report on transforming this δ-MnO2 to other MnOx compounds, Mn2O3 and Mn3O4, via mild annealing. This is confirmed by Raman spectroscopy of the films, which also confirms limited damage to the QEG substrate. To our knowledge, this is the first synthesis of Mn2O3 and Mn3O4 on pristine graphene substrates. Both methods demonstrate the potential of depositing and transforming multifunctional oxides on single-crystal graphene using QEG substrates, allowing for the formation of nanostructured heterostructures previously unseen. Additionally, the electrochemical nature of the deposition presents the ability to scale the process to the QEG wafer and adjust the solution to produce other powerful multifunctional oxides.
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- Award ID(s):
- 2145549
- PAR ID:
- 10488492
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Materials Today Advances
- Volume:
- 21
- Issue:
- C
- ISSN:
- 2590-0498
- Page Range / eLocation ID:
- 100467
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.mtadv.2024.100467
