WORCESTER BOSCH SET OF ELECTRODES 87186643010

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P. Srimuk, J. Lee, Ö. Budak, J. Choi, M. Chen, G. Feng, C. Prehal and V. Presser, Langmuir, 2018, 34, 13132–13143 CrossRef CAS. Activated carbon. Activated carbon, defined by its high surface area to volume ratio, was used in the first CDI system 16 developed in the 1960's; in recent years this material has been modified to achieve even higher surface areas and hierarchical pore geometries with fast charge transfer and ion diffusion kinetics. In general, activated carbon, comprised of aggregates of microporous particles, is fabricated through pyrolysis of a carbon precursor, such as wood, then is activated ( i.e. micropores are created) via chemical etching or gasification of the product. 45 Although the typical performance of activated carbon electrodes does not match those of 1D and 2D materials (see Fig. 8a for a comparison), the low cost of activated carbon makes it an appealing electrode material for commercial applications. 47,48 S. Sahin, J. E. Dykstra, H. Zuilhof, R. L. Zornitta and L. C. P. M. de Smet, ACS Appl. Mater. Interfaces, 2020, 12, 34746–34754 CrossRef CAS. P. M. Biesheuvel, H. V. M. Hamelers and M. E. Suss, Colloid Interface Sci. Commun., 2015, 9, 1–5 CrossRef CAS.

Peng Liang is a professor in the School of Environment at Tsinghua University. His research interests include energy and resource recovery from wastewater through microbial electrochemical technologies, capacitive deionization, and autotrophic denitrification. He is now serving as the chair of the international working group for capacitive deionization and electrosorption (CDI&E), founded in 2014 and aims to establish interdisciplinary collaborations towards better understanding and application of CDI&E-related technologies. where η′ is a modified volume fraction of ions in the pore, which is the real volume fraction η, to which is added an empirical term γα′ which relates to the ion size to pore size ratio. The volume fraction η is given by a summation over all ions in the pore of their concentration in the micropores times the molar volume, i.e., the volume (per mole of ions), which can include the water molecules that are tightly bound to the ion (ion plus hydration shell). For larger ions, the γα′ term is larger, and thus for this ion, Φ exc, i will be lower and it will be excluded from the pores relative to the smaller ion. Though this function is derived from a Carnahan–Starling equation of state, which considers mixtures of ions of the same size, 160 we utilize this simplified expression here to describe a size-based selectivity in mixtures of ions of different sizes. X. Su, H. J. Kulik, T. F. Jamison and T. A. Hatton, Adv. Funct. Mater., 2016, 26, 3394–3404 CrossRef CAS.D. M. Mohilner, in Electroanalytical Chemistry, ed. A. J. Bard, Marcel Dekker, New York, 1966, pp. 241–409 Search PubMed.

E. Avraham, B. Yaniv, A. Soffer and D. Aurbach, J. Phys. Chem. C, 2008, 112, 7385–7389 CrossRef CAS. M. A. Lilga, R. J. Orth, J. P. H. Sukamto, S. M. Haight and D. T. Schwartz, Sep. Purif. Technol., 1997, 11, 147–158 CrossRef CAS. M. C. Zafra, P. Lavela, C. Macías, G. Rasines and J. L. Tirado, J. Electroanal. Chem., 2013, 708, 80–86 CrossRef CAS. J. Chang, Y. Li, F. Duan, C. Su, Y. Li and H. Cao, Sep. Purif. Technol., 2020, 240, 116600 CrossRef CAS. In summary, the use of various electrode material, operational conditions, and surface modifications for selective ion separation was reviewed in this section. Thus, it is evident that electrodes can act as selective elements in CDI processes. In the following section, we will review the use of membranes for selective ion separation in CDI. 3. Membranes for ion selectivity In the previous section, ion selectivity in terms of electrodes was discussed. The use of membranes also plays a vital role in CDI. This section is dedicated for exploring the studies which rely on membranes for achieving ion selectivity. 3.1 Cation selectivity Several different studies have demonstrated the advantages of using IEMs to prevent co-ion repulsion, reduce anode oxidation, and to boost the salt removal by employing gradient of solutions in multi-chamber cells. 7,114 An IEM can also be used as a barrier for specific ions, and therefore, improve the ion selectivity.S. Porada, A. Shrivastava, P. Bukowska, P. M. Biesheuvel and K. C. Smith, Electrochim. Acta, 2017, 255, 369–378 CrossRef CAS. The most accurate approach in describing the ion transport in combination with adsorption has been the porous electrode theory, put forward in 2010. 140 It was further developed by Biesheuvel and co-workers when they used this framework in a model that combined faradaic reactions and capacitive electrode charging for a mixture of a monovalent anion, a monovalent cation, and divalent cations, making use of the mD model to describe ion adsorption ( μ att = 0). 141 The same porous electrode theory was also used by Zhao et al. for a purely capacitive electrode, and extended by Dykstra et al. 48 for a solution with two types of monovalent cations and a monovalent anion. Here for the first time, a full cell with two electrodes is considered. Furthermore, the simple mD model with μ att = 0 is replaced by the improved mD model which considers a salt-concentration dependent ion adsorption energy. In Dykstra et al., the only mechanism causing a difference in adsorption between different monovalent cations was the diffusion coefficient of the ions leading to a selectivity for K + over Na + of up to S ≈ 1.4, in close agreement with detailed experiments. Theoretical calculations predict this selectivity to be at a maximum at intermediate cycle times, a result that was not fully corroborated by the experiments. Recently, Guyes et al. presented a theory which predicted an enhancement of size-based selectivity towards K + over Li + and Na +, with increasing chemical charges in the micropore added by surface modification. 142 R. Zhao, S. Porada, P. M. Biesheuvel and A. van der Wal, Desalination, 2013, 330, 35–41 CrossRef CAS. Amid the limited supply of freshwater, the mounting pressures of a rising population, fast paced economic growth and technological development are pushing demand for crucial resources such as lithium (battery applications), uranium (nuclear energy production) and rare-earth elements (advanced consumer electronics and renewable energy technology). Extracting such sought after resources from seawater, brines and wastewater can mitigate the negative environmental impacts of traditional mining methods. Therefore, we are not only faced with the significant task of efficiently sourcing, treating and distributing water to high-stress regions of the world but also of securing critical elements from dilute sources with minimal impact to environment. S. Buczek, M. L. Barsoum, S. Uzun, N. Kurra, R. Andris, E. Pomerantseva, K. A. Mahmoud and Y. Gogotsi, Energy Environ. Mater., 2020, 3, 398–404 CrossRef CAS.

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. 43,44 For an ionic mixture with ions of all possible valencies z, typically ranging between −2 and +2, an overall micropore charge balance isAt each x-coordinate, the relationship between electrode potential ϕ e, solution potential ϕ mA and occupancy of a cation in the IHC, ϑ i, is implemented. This is given by the extended Frumkin equation eqn (12) for binary mixtures, 78 J. W. Blair and G. W. Murphy, Saline Water Conversion, Washington, DC, 1960, pp. 206–223 Search PubMed. A. Hemmatifar, J. W. Palko, M. Stadermann and J. G. Santiago, Water Res., 2016, 104, 303–311 CrossRef CAS. c School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151-742, Republic of Korea

C. Erinmwingbovo, M. S. Palagonia, D. Brogioli and F. La Mantia, ChemPhysChem, 2017, 18, 917–925 CrossRef CAS. Apart from more commonly targeted alkali and alkaline-earth metals, selective removal of heavy metals has also been of interest in CDI. In 2010, Li et al. utilized electrodes made of graphene nanoflakes to remove Fe 3+ and compared the electrosorption capacity with Mg 2+, Ca 2+, and Na + in single-salt experiments. 61 The Fe 3+ were preferred over the others, which was attributed to its higher valence ( Fig. 6A). Between Ca 2+ and Mg 2+, Ca 2+ were preferred due to their smaller hydrated radii ( Fig. 6B), as described before, whereas Na + exhibited the lowest electrosorption among all. In another study, Huang et al. employed activated carbon electrodes to remove Cu 2+ from aqueous solutions. 62 They also compared the Cu 2+ electrosorption in the presence of NaCl, natural organic matter (NOM), and dissolved reactive silica in binary salt solutions, and reported that Cu 2+ removal decreases with an increasing amount of the competitive species. However, no significant decrease in Cu 2+ electrosorption was observed in the presence of dissolved reactive silica. In addition to the pore size and morphological characteristics of the electrodes, the valence of the adsorbing ion has an influence on its selectivity. Studies have reported that ions with a higher valence are more effectively adsorbed in the EDL due to their stronger interactions with the electrodes. 25,52,54,55 In a mixture of mono- and divalent ions, at equilibrium the divalent ions were preferably electrosorbed as a result of the higher electrostatic attraction ( Fig. 6A). 56 Gao et al. obtained a higher divalent ion selectivity using carbon nanotube and carbon nanofiber electrodes due to charge-exclusion effect as depicted in ( Fig. 6B). 50 They also stated that ions with smaller hydrated radii were preferred if they have the same valence. Ions with identical valence are electrosorbed according to their hydration energy ( Fig. 6D). Thus, ions with lower hydration energy are preferred as their hydration shell can be readily rearranged inside the pores. 57 J. E. Dykstra, K. J. Keesman, P. M. Biesheuvel and A. van der Wal, Water Res., 2017, 119, 178–186 CrossRef CAS.R. K. Kalluri, M. M. Biener, M. E. Suss, M. D. Merrill, M. Stadermann, J. G. Santiago, T. F. Baumann, J. Biener and A. Striolo, Phys. Chem. Chem. Phys., 2013, 15, 2320 RSC. The works of Eliad et al, Gabelich et al., and later of Huang et al., provided evidence on electrosorption behavior of different anions on porous carbon electrodes. They demonstrated that CDI could be used to selectively remove different species of ions from aqueous solutions. However, at this early stage of ion selectivity with CDI, some questions regarding the parameters involved and the accurate mechanisms behind the selectivity, still remained unanswered. 68 P. M. Biesheuvel, S. Porada, M. Levi and M. Z. Bazant, J. Solid State Electrochem., 2014, 18, 1365–1376 CrossRef CAS.