WORCESTER BOSCH SET OF ELECTRODES 87186643010

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Fig. 2 A graphical timeline depicting the evolution of ion selectivity in CDI and MCDI. The works employing membranes are denoted in italics. M. C. Zafra, P. Lavela, C. Macías, G. Rasines and J. L. Tirado, J. Electroanal. Chem., 2013, 708, 80–86 CrossRef CAS.

X. Gao, A. Omosebi, N. Holubowitch, A. Liu, K. Ruh, J. Landon and K. Liu, Desalination, 2016, 399, 16–20 CrossRef CAS. Fig. 4 (A) GCS model – EDL formation on a charged surface, and (B) mD model – EDL formation inside a charged carbon pore. Fig. 8 Generalized selectivity mechanisms in MCDI based on (A) selective resins, (B) charge repulsion, and (C) ion diffusion in membranes. P. M. Biesheuvel, R. Zhao, S. Porada and A. van der Wal, J. Colloid Interface Sci., 2011, 360, 239–248 CrossRef CAS. I. Cohen, B. Shapira, E. Avraham, A. Soffer and D. Aurbach, Environ. Sci. Technol., 2018, 52, 6275–6281 CrossRef CAS.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 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.

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 Energy Environ. Sci., 2021, 14, 1095-1120 Recent advances in ion selectivity with capacitive deionization In 2013, Zafra et al. evaluated the electrosorption capacity of high surface area electrodes using single-salt solutions consisting of nutrients (Cl −, NO 3 −, and H 2PO 4 −/HPO 4 2−). 40 The authors found a lower phosphate electrosorption compared to nitrate or chloride. It was suggested that this reduced capacity was caused by the sieving effect of the prepared activated carbon (average pore size of 0.855 nm) towards to the smaller ions (Cl − and NO 3 −) compared to the large phosphate species (H 2PO 4 −/HPO 4 2−). This investigation agrees well with the report about the sieving effect of the porous carbon described by former authors ( Fig. 6B). In the same line of nutrient recovery, Ge et al. investigated the competition between physical adsorption and electrosorption of phosphate anions. 69 The authors suggested that electrosorption could only overcome the effect of physical adsorption at very high cell voltages. Therefore, to improve phosphate electrosorption the authors applied a cell voltage as high as 3.0 V, which also cause faradaic reactions. Although the authors suggest that some species formed during the faradaic reactions could also promote a disinfection of the treated water, there is an expressive reduction of the charge efficiency. Nevertheless, this work is important in understanding the lower electrosorption capacity of phosphate at neutral pH compared to other ions. 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. 78,80,90To 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 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 Y. Liu, W. Ma, Z. Cheng, J. Xu, R. Wang and X. Gang, Desalination, 2013, 326, 109–114 CrossRef CAS.

Z. Sun, L. Chai, M. Liu, Y. Shu, Q. Li, Y. Wang and D. Qiu, Chemosphere, 2018, 195, 282–290 CrossRef CAS. K. Singh, L. Zhang, H. Zuilhof and L. C. P. M. de Smet, Desalination, 2020, 496, 114647 CrossRef CAS. 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+. 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.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.

M. Tedesco, H. V. M. Hamelers and P. M. Biesheuvel, J. Membr. Sci., 2018, 565, 480–487 CrossRef CAS. M. Tedesco, H. V. M. Hamelers and P. M. Biesheuvel, J. Membr. Sci., 2016, 510, 370–381 CrossRef CAS.

Author Contributions

P. M. Biesheuvel, S. Porada, M. Levi and M. Z. Bazant, J. Solid State Electrochem., 2014, 18, 1365–1376 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 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.