WORCESTER BOSCH SET OF ELECTRODES 87186643010

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Recently, Hu et al. proposed a new electrode based on layered metal oxide with Pd to remove nitrate using an approach similar to CDI. 113 However, the main difference was the reduction of NO 3 − to N 2 in the cathode of the cell by faradaic reactions. Although the authors did not provide a selectivity value, the electrodes are expected to exhibit high selectivity towards NO 3 − since its concentration in the electrode did not reach saturation. c i,inf, c i,eff, c j,inf, c j,eff are concentrations ( c) of influent (inf) and effluent (eff) of two competing ions, i and j, respectively.

The promising results of this seminal study have motivated further research of intercalation materials for desalination batteries, focused mainly on development of high capacity Na-insertion electrodes and alternatives for the Ag electrode. A potential alternative to Ag electrodes for Cl − storage, bismuth electrodes exploit the conversion of Bi to BiOCl. 123 The primary motivation for this alternative is its lower cost. 134,135 Unfortunately, Bi electrodes suffer from both H + production during oxidation of Bi to BiOCl, which lowers the solution pH, and slow kinetics of the reduction reaction in non-acidic conditions. The Na + removal was performed with a NASICON-type NaTi 2(PO 4) 3 electrode due to its high theoretical capacity. This electrode paring with Bi removes 3 Na + for every Cl −, which decreases the pH. Due to the imbalance of the ion removal and the decreasing pH, the NaTi 2(PO 4S. Porada, L. Borchardt, M. Oschatz, M. Bryjak, J. S. Atchison, K. J. Keesman, S. Kaskel, P. M. Biesheuvel and V. Presser, Energy Environ. Sci., 2013, 6, 3700–3712 RSC.

Modification of the electrode surface by adding functional surface groups is another approach to enhance the anion selectivity, as similarly observed in terms of cations ( Fig. 6E). Oyarzun et al. modified the carbon electrode surface with cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzenesulfonate (SDBS) to obtain a higher selectivity towards nitrate via inverse CDI (i-CDI). 76 The process of i-CDI can occur when the surface of the electrode is covered with functional surface groups. Therefore, during charging at high voltages, there is discharge of ions, while at lower voltages, there is adsorption of ions. The surface modification preferentially adsorbed nitrate by a factor of ≈7.7 over chloride. However, when using the i-CDI process, the selectivity was reduced to 6.5 at low cell voltages, 16% lower than the value observed for adsorption. Interestingly, the authors did not observe strong differences in the selectivity by varying the chloride ion concentration while keeping the nitrate ion concentration constant. P. M. Biesheuvel, S. Porada, M. Levi and M. Z. Bazant, J. Solid State Electrochem., 2014, 18, 1365–1376 CrossRef CAS.

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J. E. Dykstra, J. Dijkstra, A. Van der Wal, H. V. M. Hamelers and S. Porada, Desalination, 2016, 390, 47–52 CrossRef CAS.

X. Su, K. J. Tan, J. Elbert, C. Rüttiger, M. Gallei, T. F. Jamison and T. A. Hatton, Energy Environ. Sci., 2017, 10, 1272–1283 RSC.

K. Singh, S. Porada, H. D. de Gier, P. M. Biesheuvel and L. C. P. M. de Smet, Desalination, 2019, 455, 115–134 CrossRef CAS. E. N. Guyes, A. N. Shocron, A. Simanovski, P. M. Biesheuvel and M. E. Suss, Desalination, 2017, 415, 8–13 CAS.

c i,in and c i,f are initial and final concentrations of the target ion. c j,in and c j,f are initial and final concentrations of the competing ion. S. Choi, B. Chang, S. Kim, J. Lee, J. Yoon and J. W. Choi, Adv. Funct. Mater., 2018, 28, 1–9 Search PubMed. Moving forward, research into new electrode materials and chemistries, modification and optimization of existing materials, investigation of parameters in selectivity operation, modeling of selectivity at the system and molecular level, and finally, techno-economic analysis into the viability of selective ion separation via CDI will be crucial for fully realizing the potential of ion-selectivity via CDI. For CDI, the capacity to store ions is of paramount importance, and is important to study by electrosorption experiments at different values of the charging and discharging voltages that define a CDI cycle. In addition, we can use methods to measure the charge stored in the EDLs in the CDI electrodes, using the GITT method (galvanostatic intermittent titration technique). The charge that can be stored is often formulated as a capacity in C per gram electrode material which is typically defined by total mass of both electrodes 38 (also reported as mA h g −1 in some literature) while the change of capacity with voltage is the capacitance, expressed in F g −1. Additional information can possibly be inferred from electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Data for charge (capacity) provide valuable information for electrodes used for desalination since it can indicate whether an electrode is feasible as a CDI electrode. Although the storage capacity cannot be directly translated into desalination capacity, good correlations between capacitance and salt adsorption capacity have been reported. 39 In terms of selectivity, the storage capacity values in different single-salt solutions is a simple and fast way to compare whether an electrode has preference for a target ion or not. Comparisons between capacitance values were used by different research groups to explore the preference of one ion over another. 40–42 In case of intercalation materials, CV can provide information about the preference of the active materials towards different ions. Higher cathodic peak potential associated with intercalation of an ion indicate a higher preference for intercalation of the electrode towards that ion. This technique of determination has been used in CDI literature for selective separation from cationic mixtures. 43,44Electrochemical deionization processes have found many applications in selective electrosorption/electrodeposition of ions, such as remediation of toxic ions from contaminated freshwater and resource mining from seawater. Tailored electrode coatings have been especially instrumental in advancing the field of selective electrosorption; researchers have used electroactive polymers, chelating polymers and redox-active polymers (conjugated and pendant-bearing) to coat electrodes for high capacity, highly selective separations. 32,33 Some prominent examples of the latter include uranium extraction from seawater 34 and chromium/arsenic oxyanion removal 35 from wastewater. Such electrode modifications coupled with modulated electric field techniques have resulted in further enhanced selectivity. The mechanisms of this process, while capacitive/pseudocapacitive in nature ( Fig. 1a), are covered in more detail in section 5 of this review.