WORCESTER BOSCH SET OF ELECTRODES 87186643010

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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. L. Eliad, G. Salitra, A. Soffer and D. Aurbach, J. Phys. Chem. B, 2001, 105, 6880–6887 CrossRef CAS.

In an electrode, the Donnan potential can be modulated by changing the cell voltage between two electrodes, or by changing the bulk electrolyte composition. By contrast, for a given IEM, the Donnan potential depends solely on electrolyte composition. 157 Both for electrodes and membranes, the charge density in the confined pore geometry is of importance, and in the Donnan approach this is defined per volume of micropores, thus has unit C m −3 or mol m −3 = mM. We will denote micropore charge with the symbol σ 0 with unit mol m −3. It can be multiplied by Faraday's number, F, and the microporosity to obtain the charge per volume of total electrode. This electronic charge σ 0 can be changed from negative to positive in carbon micropores, to adsorb either cations or anions, respectively. Meanwhile, in some other materials, such as PBA, an intercalation material, the charge is very negative and so, this material only absorbs cations. 78 On this count it resembles a subset of IEMs containing negatively charged groups, such as sulfonic groups, known as CEMs. Unlike in CEMs, in PBA the negative charge can be modulated up or down via injection or removal of electronic charge. Formation of an electrical double-layer (EDL) is a fundamental feature of many topics in physics and chemistry, and is also exploited in CDI. The first EDL model, the Helmholtz model, was proposed by Hermann Helmholtz in 1879. This model was later revised by Louis Gouy and David Chapman in 1910 and in 1913, respectively. The Helmholtz model and the Gouy–Chapman model were combined into the widely utilized Gouy–Chapman–Stern (GCS) model by Otto Stern in 1924. 35 W. Shi, X. Liu, C. Ye, X. Cao, C. Gao and J. Shen, Sep. Purif. Technol., 2019, 210, 885–890 CrossRef CAS. At 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 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.Q. Dong, X. Guo, X. Huang, L. Liu, R. Tallon, B. Taylor and J. Chen, Chem. Eng. J., 2019, 361, 1535–1542 CrossRef CAS. We introduce the volumetric partitioning function Φ exc, i = exp( μ exc, i,∞ − μ exc, i), and a similar term for affinity-based effects, Φ aff, i = exp( μ aff, i,∞ − μ aff, i), which lumps together all effects acting on the ion that are not ideal (entropy), volumetric, or charge-related. These factors Φ exc, i and Φ aff, i will be between 0 and 1 when such effects act to repel the ion from the micropore environment but will be >1 when they act to adsorb the ion into the micropore. We use Φ i = Φ exc, i· Φ aff, i. We obtain from eqn (3) a modified Boltzmann relation M. Asai, A. Takahashi, K. Tajima, H. Tanaka, M. Ishizaki, M. Kurihara and T. Kawamoto, RSC Adv., 2018, 8, 37356–37364 RSC. In addition to the properties of the electrode and the adsorbing ion, the operational parameters in CDI can affect the ion selectivity. Zhao et al. proposed and validated a theory of selectivity for a solution with 5 : 1 Na + and Ca 2+ feed ratio. 24 The authors reported a time-dependent selectivity as Na + was electrosorbed 5 times more than Ca 2+ at the early stage of desalination cycle. The higher electrosorption of sodium ions is explained by the higher concentration, causing higher diffusion to the pores of the electrode ( Fig. 6C). However, with time, the preference switches to Ca 2+ due to the stronger interaction between the divalent ion and the electrode surface, causing a ion-swapping effect, shown in Fig. 6A. Hou and Huang also studied the effect of feed concentration on ion selectivity. 51 By varying the concentrations of K +, Na +, Ca 2+, and Mg 2+, the authors observed that an increase in Na + concentration over other cations yielded preferential electrosorption of Na +, which was attributed to the higher availability of sodium ions. Apart from varying the feed concentration, they also studied the effect of applied potential on the electrosorption capacities of different ions, and concluded that increasing the voltage increased the preferential removal of K + over Na + and Na + over Ca 2+.

K. Singh, H. J. M. Bouwmeester, L. C. P. M. De Smet, M. Z. Bazant and P. M. Biesheuvel, Phys. Rev. Appl., 2018, 9, 064036 CrossRef CAS. C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins and Y. Cui, J. Electrochem. Soc., 2011, 159, A98–A103 CrossRef.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. Another recent approach that has provided viable results for selectivity between mono/divalent ions is the use of monovalent ion-selective membranes. Pan et al. investigated the use of such membranes to separate fluoride and nitrite from sulfate. 137 Using an equimolar solution, the authors observed a selectivity ( ρ) of ≈1.4 for fluoride ions over sulfate ions. Furthermore, it was found that the pH of the feed solution was an important parameter to control and improve the ion selectivity. Higher pH values increased the selectivity towards fluoride, while for acidic solutions the selectivity was lost due to an interaction between protons and the surface of the membrane. The effect of the feed concentration was also explored, keeping the concentration ratio between the two anions constant. An increasing fluoride selectivity was observed upon increasing the concentration of both F − and SO 4 2−. When the cell voltage was increased, the selectivity was reduced towards F − demonstrating that high cell voltages cannot attain high selectivity. This result is in line with other works that show lower selectivity at higher cell voltages. 41,77 K. Zuo, J. Kim, A. Jain, T. Wang, R. Verduzco, M. Long and Q. Li, Environ. Sci. Technol., 2018, 52, 9486–9494 CrossRef CAS. S. J. Seo, H. Jeon, J. K. Lee, G. Y. Kim, D. Park, H. Nojima, J. Lee and S. H. Moon, Water Res., 2010, 44, 2267–2275 CrossRef CAS. N. Pugazhenthiran, S. Sen Gupta, A. Prabhath, M. Manikandan, J. R. Swathy, V. K. Raman and T. Pradeep, ACS Appl. Mater. Interfaces, 2015, 7, 20156–20163 CrossRef CAS.

A. Hassanvand, G. Q. Chen, P. A. Webley and S. E. Kentish, Water Res., 2018, 131, 100–109 CrossRef CAS.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. The mD model was further improved by allowing the chemical potential term to vary with the micropore salt concentration, thereby eliminating the prediction of unrealistically large adsorption capacities at high salt concentrations. 14 It has also been extended from the one-dimensional case to cell-level, two-dimensional systems, 92 and has been corroborated by molecular dynamic simulations. 93 The mD model was applied to describe a series of CDI developments such as “inverted CDI” by fixing charge in the micropores to emulate chemical treatment, 15 inclusion of surface transport 94 and an explanation of the benefits of pulsed-flow CDI over continuous flow systems. 95 Fig. 2 A graphical timeline depicting the evolution of ion selectivity in CDI and MCDI. The works employing membranes are denoted in italics. Electrochemical 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.

Introduction Fresh water scarcity and rapidly increasing global demand for clean water have stimulated scientists to seek out innovative methods of securing potable water supplies. Even though water desalination is deeply rooted within the human history, spanning across centuries, 1 it was not until the latter half of 20th century that desalination techniques became commercialized. 2 Conventional desalination methods, such as reverse osmosis (RO), electrodialysis (ED), multi-stage-flash (MSF), and multi-effect desalination (MED), are commonly used, but in some cases require significant energy input to produce fresh water. Furthermore, the majority of these systems often desalinate ‘to completion’, or do not preferentially remove the ions that are desired to be removed or even harvested. Ion selectivity is of key importance because it is often not necessary, and perhaps even detrimental, to remove the vast majority or entirety of ions from water. There are ample examples where one specific ion is to be removed because of its toxicity (arsenic, boron, heavy metals, ions leading to fouling, and sodium in irrigation water) or value (lithium, gold). In this review we focus on the ion selectivity ( i.e. preferential removal of a particular ion of interest within a mixture of ions) aspect of water desalination via capacitive deionization (CDI).Fig. 4 (A) GCS model – EDL formation on a charged surface, and (B) mD model – EDL formation inside a charged carbon pore. Akin to the work of Hawks et al., Mubita et al. investigated the selectivity of nitrate over chloride for carbon electrodes, analyzing pure carbon adsorption, ion concentration, and cell voltage. 77 In addition, a model was proposed for ion electrosorption which was validated by the experimental results. Compared to the work of Hawks et al., the activated carbon used by Mubita et al. has larger pore sizes than the radii of hydrated nitrate and chloride. Therefore, no sieving effect was considered. The authors observed that by increasing the cell voltage from 0 V (short-circuit) to 1.2 V, the selectivity ( ρ) towards nitrate reduced from ≈10 to ≈6. It is also shown in this work that nitrate ions have stronger affinity towards the carbon electrode surface, since chloride ions are replaced by nitrate ions similarly to the time-dependent effect described by Zhao et al for a mixture of mono/divalent ions, and well aligned with the work of Lin et al. 24,56 In this case, time-dependent selectivity is observed due to higher diffusion of chloride ions in the early stage of electrosorption ( Fig. 6C), later replaced by nitrate ions during the electrosorption process due to the better affinity of nitrate with the carbon surface ( Fig. 6A). 2.3 Intercalation materials Application of intercalation materials in desalination via CDI has been reported with an increasing interest in the past years. 10 High SACs have been reported for CDI cells with electrodes fabricated from various intercalation materials including Prussian blue (PB) and its analogues (PBAs), 34,78,79 NaMnO 2 (NMO), 80–82 NaFe 2P 2O 7, 83 and NaTi 2(PO 4) 3 84 among others. 85,86 The mechanism of charge storage in these materials involves intercalation of cations (of multiple valences 87) in a lattice or between layers. As a result, they do not require high surface areas to achieve high storage capacity. In some materials like the PBAs, 88 this insertion is accompanied by a redox change in the lattice. Interestingly, this mechanism results in the absence of co-ion repulsion, 89 enhancing the charge efficiency of electrosorption of intercalation materials without the use of membranes, as reported in literature. 78,80,90 T. Wu, G. Wang, S. Wang, F. Zhan, Y. Fu, H. Qiao and J. Qiu, Environ. Sci. Technol. Lett., 2018, 5, 98–102 CrossRef CAS. Fig. 8 Generalized selectivity mechanisms in MCDI based on (A) selective resins, (B) charge repulsion, and (C) ion diffusion in membranes. D. I. Oyarzun, A. Hemmatifar, J. W. Palko, M. Stadermann and J. G. Santiago, Water Res.: X, 2018, 1, 100008 Search PubMed.