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1. Tunnel structured manganese oxide nanowires as redox active electrodes for hybrid capacitive deionization

   https://doi.org/https://doi.org/10.1016/j.nanoen.2017.12.015  

   Byles, Bryan W. ; Cullen, David A. ; More, Karren L. ; Pomerantseva, Ekaterina ( January 2018 , Nano energy)

   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 »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.« less
2. Comparing energy demands and longevities of membrane-based capacitive deionization architectures

   https://doi.org/10.1039/D2EW00188H  

   Pothanamkandathil, Vineeth ; Gorski, Christopher A. ( June 2022 , Environmental Science: Water Research & Technology)

   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.
3. Toward sustainable desalination using food waste: capacitive desalination with bread-derived electrodes

   https://doi.org/10.1039/D0RA10763H  

   Wood, Adam R. ; Garg, Raghav ; Cohen-Karni, Tzahi ; Russell, Alan J. ; LeDuc, Philip ( March 2021 , RSC Advances)

   Each year approximately 1.3 billion tons of food is either wasted or lost. One of the most wasted foods in the world is bread. The ability to reuse wasted food in another area of need, such as water scarcity, would provide a tremendous sustainable outcome. To address water scarcity, many areas of the world are now implementing desalination. One desalination technology that could benefit from food waste reuse is capacitive deionization (CDI). CDI has emerged as a powerful desalination technology that essentially only requires a pair of electrodes and a low-voltage power supply. Developing freestanding carbon electrodes from food waste could lower the overall cost of CDI systems and the environmental and economic impact from food waste. We created freestanding CDI electrodes from bread. The electrodes possessed a hierarchical pore structure that enabled both high salt adsorption capacity and one of the highest reported values for hydraulic permeability to date in a flow-through CDI system. We also developed a sustainable technique for electrode fabrication that does not require the use of common laboratory equipment and could be deployed in decentralized locations and developing countries with low-financial resources.
4. Structurally and chemically engineered graphene for capacitive deionization

   https://doi.org/10.1039/d0ta10087k  

   Chang, Liang ; Fei, Yuhuan ; Hu, Yun Hang ( January 2021 , Journal of Materials Chemistry A)

   Highly efficient capacitive deionization (CDI) relies on unimpeded transport of salt ions to the electrode surface. Graphene is an ideal candidate to provide superb conditions for ion adsorption as it possesses high theoretical surface area and electrical conductivity. When ions are stored solely within the electric double layers (EDLs), a hydrophilic graphene surface with hierarchical pores can maximize the accessible surface area and promote the ion transport. In the case of synergistic ion storage via electrostatic adsorption and faradaic redox reaction, graphene can act as both the electron highway and the reciprocal spacer to provide surface-confined effects. Substantially, structural and chemical engineering towards graphene can enhance the ion removal capacity and rate, and improve the charge efficiency and ion selectivity. In this review, we keep pace with the in-depth studies of CDI technologies and recent progress on graphene-based materials for CDI. Major challenges in the rational assembly of the desired material functionalities in terms of surface area, pore structure, and hydrophilicity are addressed. As electrode materials develop, the ultimate goal is to achieve highly efficient, energy-saving, and environment-friendly CDI.
5. Emerging investigator series: capacitive deionization for selective removal of nitrate and perchlorate: impacts of ion selectivity and operating constraints on treatment costs

   https://doi.org/10.1039/c9ew01105f  

   Hand, Steven ; Cusick, Roland D. ( April 2020 , Environmental Science: Water Research & Technology)

   Treating toxic monovalent anions such as NO 3 − or ClO 4 − in drinking water remains challenging due to the high capital and environmental costs associated with common technologies such as reverse osmosis or ion exchange. Capacitive deionization (CDI) is a promising technology for selective ion removal due to high reported ion selectivity for these two contaminants. However, the impacts of ion selectivity and influent water characteristics on CDI life cycle cost have not been considered. In this study we investigate the impact of ion selectivity on CDI system cost with a parameterized process model and technoeconomic analysis framework. Simulations indicate millimolar concentration contaminants such as nitrate can be removed at costs in the range of $0.01–0.30 per m 3 at reported selectivity coefficient ranges ( S = 6–10). Since perchlorate removal involves micromolar scale concentration changes, higher selectivity values than reported in literature ( S > 10 vs. S = 4–6.5) are required for comparable treatment costs. To contextualize simulated results for CDI treatment of NO 3 − , CDI unit operations were sized and costed for three case studies based on existing treatment facilities in Israel, Spain, and the United States, showing that achieving a nitrate selectivitymore »of 10 could reduce life cycle treatment costs below $0.2 per m 3 .« less