WORCESTER BOSCH SET OF ELECTRODES 87186643010

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P. Srimuk, J. Lee, A. Tolosa, C. Kim, M. Aslan and V. Presser, Chem. Mater., 2017, 29, 9964–9973 CrossRef CAS. On a system level, dependence of cell selectivity on operation parameters such as applied current, cell voltage, ion concentration, and pH among others will have to be systematically studied to find optimum conditions that enhance selectivity for the CDI cell. Intercalation electrodes such as TiS 2 show switchable preference depending on the potential of the electrode, as shown by Srimuk et al. 43 Such insights will be useful in realizing the full potential of existing (and the search for new) electrode materials. A heavy metal (Pb 2+) and salt (Na +) recovery method from wastewater using 3D graphene-based electrodes was proposed by Liu et al. 63 They used 3D graphene electrodes modified with ethylenediamine triacetic acid (EDTA) and 3-aminopropyltriethoxysilane (APTES) as the cathode and the anode, respectively. Two different mechanisms were presented for Pb 2+ and Na + removal. Pb 2+ is adsorbed via a chelation reaction with EDTA ( Fig. 6E), whereas Na + is adsorbed via electrosorption in the pores. Based on these mechanisms, the separation of ions was achieved during the desorption stage. First, Na + was desorbed by applying an inverse potential, followed by a short circuit potential. Afterwards Pb 2+ was desorbed in a separate step using nitric acid as an eluent. S. Porada, R. Zhao, A. van der Wal, V. Presser and P. M. Biesheuvel, Prog. Mater. Sci., 2013, 58, 1388–1442 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.

The removal of an unconventional ion, uranium( VI), using phosphate-functionalized graphene hydrogel electrodes was studied by Liao et al. 66 The electrodes were tested in equimolar solutions (0.3 mM) containing uranium( VI) and a series of interfering metals ions (Cs +, Co 2+, Ni 2+, Sr 2+, and Eu 3+). The authors reported that the electrodes preferred uranium( VI) over all the other metals that were tested. Furthermore, they observed that the uranium( VI) is more selective against monovalent metal ions compared to that of divalent or trivalent ions. This phenomenon was attributed to the stronger electrostatic interaction between trivalent ions and the electrode surface, thus adsorbing more trivalent ions resulting in reduced selectivity of uranium( VI). Apart from ion valence, the selectivity of the electrode is also attributed to the formation of strong acid–base complexes with the phosphate groups attached to the electrode ( Fig. 6E). 2.2 Anion selectivity The mechanisms used to achieve cation selectivity may be extrapolated, and used to achieve anion selectivity in CDI. One of the pioneering studies in CDI anion selectivity is the one of Farmer et al., reported in 1996, which showed a difference in electrosorption capacity of different anions. 67 In their work, the authors employed 192 pairs of carbon aerogel electrodes to investigate the desalination of NaCl and NaNO 3 in single-salt experiments. Although this work was not intended to investigate ion selectivity, the difference in electrosorption observed by the authors was the first clue that CDI could be a valuable technology for selective anion adsorption. 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 Recently, Zhang et al. used activated carbon in flow CDI to selectively remove Cu 2+ from a solution which also contained Na +. 65 A higher affinity towards Cu 2+ was obtained in the system. This was attributed to the preferential adsorption of Cu 2+ on the carbon particles and was also reduced to Cu. The preference of carbon towards divalent over monovalent cations, as shown in Fig. 6A was also reported here. The Na + removed from the feed remained in the electrolyte of the flow electrode. J. Jiang, D. I. Kim, P. Dorji, S. Phuntsho, S. Hong and H. K. Shon, Process Saf. Environ. Prot., 2019, 126, 44–52 CrossRef CAS. R. Zhao, S. Porada, P. M. Biesheuvel and A. van der Wal, Desalination, 2013, 330, 35–41 CrossRef CAS.Years later, Eliad et al. investigated the sieving effect of carbon electrodes based on the pore size distribution of different carbon electrodes. 42 It was shown that SO 4 2− ions were not able to penetrate the carbon pores with an average pore size of 0.36 nm due its large hydrated radius ( Fig. 6B). The electrosorption capacity for this carbon electrode was NO 3 −> Cl − ≫ ClO 4 − ⋙ SO 4 2−. The higher electrosorption capacity of NO 3 − compared to Cl − was attributed to a combination of the slit-shaped pores of the carbon electrode and the planar shape of hydrated NO 3 −, facilitating its storage in the micropores. The use of a larger average carbon pore size (0.58 nm) resulted in the electrosorption of all anions and no sieving effect was observed. A. Hassanvand, G. Q. Chen, P. A. Webley and S. E. Kentish, Water Res., 2018, 131, 100–109 CrossRef CAS. Y. Gao, L. Pan, H. B. Li, Y. Zhang, Z. Zhang, Y. Chen and Z. Sun, Thin Solid Films, 2009, 517, 1616–1619 CrossRef CAS.

S. P. Hong, H. Yoon, J. Lee, C. Kim, S. Kim, J. Lee, C. Lee and J. Yoon, J. Colloid Interface Sci., 2020, 564, 1–7 CrossRef CAS. To further improve the performance of graphene electrodes, several groups prepared three-dimensional graphene structures by using sponge 61 or polysterene 39 templates, increasing the accessible surface area. In the former, the specific surface area reached 305 m 2 g −1 leading to greater ion adsorption capacity of 4.95 mg g −1 for a 0.5 M NaCl solution. The total electrosorption capacity of graphene-based electrodes was pushed beyond that of activated carbon and carbon aerogels by increasing the frequency of defects in the graphene sheets, which effectively increases the density of micropores and dramatically increases the ion adsorption capacity (see Fig. 8a). 62,63

References

Y. Liu, W. Ma, Z. Cheng, J. Xu, R. Wang and X. Gang, Desalination, 2013, 326, 109–114 CrossRef CAS. J. Pan, Y. Zheng, J. Ding, C. Gao, B. van der Bruggen and J. Shen, Ind. Eng. Chem. Res., 2018, 57, 7048–7053 CrossRef CAS.

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. 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).

T. Rijnaarts, D. M. Reurink, F. Radmanesh, W. M. de Vos and K. Nijmeijer, J. Membr. Sci., 2019, 570–571, 513–521 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. For an ionic mixture with ions of all possible valencies z, typically ranging between −2 and +2, an overall micropore charge balance is