Na3V2(PO4)3@C as Faradaic Electrodes in Capacitive Deionization

Jan 18, 2019 - Byles, B. W.; Cullen, D. A.; More, K. L.; Pomerantseva, E. Nano Energy 2018, 44, 476– 488, DOI: 10.1016/j.nanoen.2017.12.015. [Crossr...
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Na3V2(PO4)3@C as Faradaic Electrodes in Capacitive Deionization for High Performance Desalination Jianglin Cao, Ying Wang, Lei Wang, Fei Yu, and Jie Ma Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04006 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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Na3V2(PO4)3@C as Faradaic Electrodes in Capacitive Deionization for High Performance Desalination Jianglin Cao1,3, Ying Wang1, Lei Wang1, Fei Yu2*, Jie Ma1,3* 1 State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China. Tel: 86-21-6598 1831; E-mail: [email protected] 2 College of Marine Ecology and Environment, Shanghai Ocean University, Shanghai 201306, P.R. China, E-mail: [email protected] 3 Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, P.R. China Abstract Among various desalination technologies, capacitive deionization (CDI) has rapidly developed due to its low energy consumption and environmental compatibility, among other factors. Traditional CDI stores ions within the electric double layers (EDLs) in the nanopores of the carbon electrode, but carbon anode oxidation, the coion expulsion effect, and a low salt adsorption capacity (SAC) block its further application. Herein, the faradaic-based electrode is proposed to overcome the above limitations, offering an ultrahigh adsorption capacity and a rapid removal rate. In this paper, the open framework structure Na3V2(PO4)3@C is applied for the first time as a novel faradaic electrode in the hybrid capacitive deionization (HCDI) system. During the adsorption and desorption process, sodium ions are intercalated/de-intercalated through the crystal structure of Na3V2(PO4)3@C while chloride ions are physically trapped or released by the AC electrode. Different concentrations of feed water are investigated, and a high SAC of 137. 20 mg NaCl g-1 NVP@C and low energy consumption of 2.157 kg NaCl kWh-1 are observed at a constant voltage of 1.0 V, a concentration of 100 mM and a flow rate of 15 mL·min-1. The outstanding performance of the Na3V2(PO4)3@C faradaic electrode demonstrates that it is a promising material for desalination and that HCDI offers great future potential. Keywords: Faradaic electrode; Desalination; Na3V2(PO4)3@C; Hybrid Capacitive deionization; Electric double layer;

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1. Introduction Potable water demand has become a severe problem due to population growth and industry development. Many desalination technologies have been proposed1, and among them, capacitive deionization (CDI) has attracted great attention due to its low energy-consumption, lack of secondary pollution and other factors. Traditional CDI traps ions by applying a constant voltage and forming an electric double layer (EDL) on the carbon material surface. If an opposite voltage is applied, the stored ions are released from the electrodes and facilitate the recycling use of the electrodes. However, several limitations have emerged with the development of the traditional CDI. First, continuous oxidation occurs at the carbon anode during prolonged adsorptiondesorption processes, leading to a decrease in specific capacitance, high energy consumption and limiting of the cycle life2, 3. Also, excessive co-ion expulsion results in low charge efficiency and blocks the use of CDI in the higher-salinity feed water. Moreover, the specific surface area of the EDL-based electrode is a major restrictive factor because the pore surface area is available for ion adsorption while the bulk structure is rarely used, which creates a low salt adsorption capacity (SAC)4. In contrast, the proposed faradaic electrode is a novel method that can overcome above limitations5. Compared with the traditional EDL-based electrode, the main storage mechanism of the faradaic electrode is the intercalation or conversion effect between applied material and ions rather than the EDL effect, which shows a higher energy density and a higher salt desorption5. Additionally, no significant co-ion expulsion occurs in the faradaic electrode6. The faradaic electrode has rapidly developed in recent years. “A Desalination Battery” was first published by Pasta et al7, and this device is composed of two faradaic electrodes with a Na2 ‑ xMn5O10 nanorod as a positive electrode and Ag/AgCl as a negative electrode. However, the silver electrode is too expensive for applying in large-scale production, and the desorption rate is lower than that of the traditional CDI. Lee. et al. proposed using Na4Mn9O18 as a faradaic electrode and porous carbon as an EDL electrode, known as hybrid capacitive deionization (HCDI). The results demonstrate that this configuration produces a higher SAC (31.2 mg g-1 at 1.2 V with 580 mg L-1 feed water) and a rapid ion removal rate8. The faradaic electrode has subsequently undergone rapid development, and various electrode materials have been applied. For oxides in faradaic electrodes, the Na4Mn9O18 (NMO) crystal is orthorhombic

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with S-shaped Na tunnels, which facilitates sodium ion diffusion. Therefore, NMO/activated carbon systems have been selected8. Wu et al. used a cation-selective MnO2 anode as a faradaic electrode and reported a desorption capacity of SAC of 14.9 mg g-1 in 500 mg L-1 NaCl9. Prussian blue analogues of faradaic electrodes were also chosen because their open framework can accommodate large ions without structure distortion. Guo et al. prepared FeFe(CN)6 (PB) for HCDI and obtained a high SAC of 120.0 mg g-1 at 1 C and a peak removal rate of 0.5430 mg g−1 s−1 at 40 C10. However, the relatively low desalination capacity of NMO and poor theoretical capacity resulting from structural defects in the aqueous electrolytes of Prussian blue analogues limit their application and performance11,

12.

Compared with the above

materials, the sodium (Na) super-ionic conductor (NASICON) is a stable structure because of its open framework in which ions can freely intercalate/de-intercalate. Therefore, the NASICON structure can facilitate ionic mobility with a small volume change during the charge and discharge (adsorption and desorption) processes. Additionally, the high ionic conductivity of NASICON makes it an attractive choice13. Among all NASICON compounds, Na3V2(PO4)3@C (NVP@C) exhibits a high theoretical reversible capacity (117.6 mAh g−1), good thermal stability (450 ℃ ). Moreover, NVP@C contains two intercalation/de-intercalation plateaus at 3.4 and 1.6 V vs. Na+/Na theoretically which makes it a good candidate for use in a rechargeable sodium battery (RSB). However, it should take into account issues regarding the working voltage and O2/H2 evolution potential range14 15. Additionally, NVP@C holds a relatively low energy consumption for Na intercalation in NVP, and it implies NVP@C is an energy saving material as energy consumption plays a crucial factor in desalination filed. Above all, NVP@C possesses a good crystal structure, excellent electrochemical properties and shows great prospects for ARSB, but no research exists on the use of NVP@C as a faradaic electrode for HCDI. Besides, relatively low energy consumption of NVP@C is obtained compared with other desalination methods. In this work, we prepared NVP@C using the sol-gel method and chose NVP@C as the faradaic electrode and active commercial carbon (AC) as the EDL electrode in the HCDI system. The electrochemical behaviors and deionization performances of HCDI are discussed, and the result shows an ultrahigh SAC (137.30 mgNaCl gNVP@C-1), a adsorption rate (0.076 mgNaCl gNVP@C-1 s-1) and a low energy consumption (2.16 kgNaCl kWh-1)

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compared with the traditional CDI system. Therefore, Na3V2(PO4)3@C shows a promise as a novel faradaic electrode for HCDI. 2. Experimental section 2.1 Preparation of NVP@C All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. NH4VO3, NH4H2PO4, Na2CO3 and citric acid were used as vanadium, phosphorus, sodium and carbon sources, respectively, and were mixed at a molar ratio of 2:3:1.5:3. Citric acid was added to the NH4VO3 solution and formed a transparent solution with gas emission. NH4H2PO4 and Na2CO3 were subsequently added, and a homogeneous solution was obtained. A few minutes later, the solution was heated to 80℃ under stirring to create a gel from the sol. After 12hour drying, the precursor was pre-heated at 350℃ for 4 h and then calcined at 800℃ for 7 h under an argon atmosphere. Finally, NVP@C was prepared. 2.2 Electrochemical measurement The NVP@C and AC electrode were prepared by mixing active material, acetylene black and polyvinylidene difluoride (PVDF) with a ratio of 8:1:1. The N-methyl-2pyrrolidone (NMP) was added in the mixture and then was stirred for 12 h. The resulting slurry was coated on a graphite sheet (mass ratio). Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) were measured on an electrochemical workstation (CHI660D, Chenhua Instruments Co., China) in a three-electrode system with Pt, Ag/AgCl, and 1 M Na2SO4 as the counter electrode, reference electrode and electrolyte, respectively. The scan rates of CV measurement were 0.1, 0.5 and 10 mV·s1

and the current densities in GCD tests were 5, 10, 15, and 20 mA·cm-2.

Electrochemical impedance spectroscopy (EIS) was carried out with an amplitude of 5 mV in the frequency range from 105 Hz to 0.1 Hz (CHI760E, Chenhua Instruments Co., China). Galvanostatic Intermittent Titration Technique (GITT) was carried out on the battery testing system (LAND, China); detailed information is shown in Supporting Information (SI). The area of NVP@C electrode used in the electrochemical tests mentioned above was 1×1 cm2. 2.3 Deionization setup The batch model of two electrodes system was used in this paper. During the desalination process, Na+ intercalated into to the lattice of NVP and Cl- was removed by EDL effect offered by AC electrode (schematic diagram shown in Figure 1(a)). The

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deionization cell composed of an NVP@C faradaic electrode, an anion exchange membrane (AEM) that is close to an AC EDL electrode, a channel with a volume of 4×4×1 cm3 and several rubber gaskets. Before the desalination test, NVP electrode was pre-charged for 20 minutes with a current density of 50 mA·g-1, leading to Na defect in NVP electrode. A constant voltage of 1.0 V and -1.0 V was applied during the desalination and recovery process, respectively. The flow rate was 15 mL min-1, and the feed water contained 10, 50 and 100 mmol·L-1 of NaCl. The mass of NVP electrodes were 10-20 mg. The desalination capacity (Γ ), salt removal rates (v) and energy consumption (W) were calculated by equations (1) - (3), respectively. Γ=

( C - C0 ) V M Γ

(2)

v=𝑡 W =

(1)

1000 × 3600 × (C - C0) × V v × ∫i dt

(3)

Where C (mg·L-1) and C0 (mg·L-1) are the final and initial concentrations, respectively; V (L) represents the solution volume; and M (g) is the mass of the NVP@C faradaic electrode; t is the time (s), v stands for the voltage (V), and i is the current (A); W is the energy consumption (mg·kW·h-1). 2.4 Material characterization The crystalline structure was analyzed by X-ray diffraction characterization (D8 Advance X) with Cu Kα radiation (λ = 0.1542 nm). Scanning electron microscopy (SEM, FEI Nova Nano SEM 450) and transmission electron microscopy (TEM, JEOL2010F, Japan) were used to analyze the morphology and microstructure. 3. Results and discussion 3.1 Material characterization It is widely accepted that two types of sodium sites are present in NVP. During sodium ion adsorption and desorption, only Na2 can be intercalated and de-intercalated in the lattice (as shown in Figure 1(a)), which achieves the transformation between V4+and V3+ 16. From the SEM images (Figure 1(b)), we observe that NVP@C particles are stacked and that a single particle is square with a scale on the micro level. The TEM images illustrate that synthesized NVP particles are wrapped by amorphous carbon layers, as shown in Figure 1(c). The carbon layer enhances the electric conductivity and the cycling stability17. The XRD pattern of NVP@C is shown in Figure 1(d), which is consistent with the standard card JCPDS 53-0018. The peak sharpness and peak

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positions show that the prepared NVP@C exists in a crystalline phase without obvious impurity. The element analysis is displayed in Figure 1(e), and the carbon content is 12.91 % by weight.

Figure 1 Schematic diagram(a) of HCDI with the faradaic electrode and EDL electrode and the process of sodium ion intercalation and de-intercalation in NVP@C; SEM images (b) at different magnification and TEM images (c) of NVP@C; XRD patterns (d) and EDS analysis (e) of the NVP@C particles; 3.2 Electrochemical analysis A three-electrode electrochemical workstation is used to test the electrochemical properties of NVP@C. Figure 2(a) shows the cyclic voltammetry (CV) curves with a potential window of 0-0.8 V vs. Ag/AgCl at different scan rates. A pair of redox peaks can be observed at each scan rate, which indicates sodium ion intercalation/deintercalation behaviors18. As the scan rates increase, the peak currents are higher because the current is related to the concentration gradient of the active material at the diffusion layer. Normally, a higher diffusion gradient exists at a higher scan rate. Therefore, as the scan

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rate increases, the peak current increases. Also, a slight variation of the redox peaks occurs as the scan rate increases, resulting from concentration polarization at high scan rates. At low rates, the internal pores and the active surface can be used to store ions, but at high rates, only the outer active surface area can be used due to the time limitation of ion motion, which leads to the variation.

Figure 2 CV curves (a) at different scan rates; corresponding relationship (b) between the square root of the scan rates and the peak currents; (c) Nyquist curve and related resistances; (d) cycle performance at a constant current of 1 mA·cm-2 and related specific capacities; profile of charge-discharge curves (e) at different current densities; capacities (f) at different current densities; The relationship between the peak current and scan rate can be described by the classical Randles-Sevcik equation, which implies a diffusion-controlled process, as

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shown in SI. The relationship between the anodic/cathodic peak currents and the square root of the scan rates is shown in Figure 2(b). The calculated sodium ion diffusion coefficients of the anodic/cathodic electrodes are 1.228×10-10, 8.337×10-11, respectively. Also, the sodium ion diffusion coefficients calculated by GITT method are 1.485×10-13, 9.507×10-11 (shown in Figure S1 and Table S1)for charge and discharge processes. The above results present a rapid ion diffusion intercalation/de-intercalation process through NVP@C in the aqueous electrolyte, which is contributed to salt removal rate19. The Nyquist plot of NVP@C and the equivalent circuit are displayed in Figure 2(c). The simulated results show that the value of the charge-transfer resistance (Rct) is 148.4 Ω (which reflects the resistance between the electrode and electrolyte), Re is the uncompensated Ohmic resistance of NVP@C, and the simulated result is 29.45 Ω. The CPE and Warburg (Wo) values describe the surface layer capacity and the double layer capacitance and ion diffusion motion. Galvanostatic charge-discharge results are shown in Figure 2(d). The upper portion of Figure 2(d) shows an obvious charge-discharge platform and presents the redox reaction of sodium intercalation and de-intercalation in every cycle, in agreement with the CV profile. As the equation shown in SI, the calculated capacities are also presented in Figure 2(d). After the first 5 cycles, the capacity retention remains at 74.99 %, indicating that NVP@C is stable in the aqueous electrolyte. The performances and capacities of different current densities are illustrated in Figure 2(e) and 2(f). Attenuation is noted as the current densities increase, which is similar to the result in Song’s research20. The possible reasons for this observation include water oxidation, slight dissolution, and distortion of the electrode during the charge and discharge process. Moreover, the voltage profile of AC electrode that adsorbs Cl- is shown in Figure S2 and S3. 3.3 Deionization The deionization performances were analyzed at various concentrations of feed water with constant voltage operation (1.0 V). Figure 3(a) shows that the adsorption capacities and removal rates vary with the concentrations, which are calculated using equations (1) and (2).

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Figure 3 Salt adsorption capacities and salt removal rates (a) and energy consumptions (b) at various feed concentrations (mM); It is clear that as the concentrations increase, the capacities increase as well. Normally, a higher concentration can reduce the ionic resistance in the feed water by facilitating ion transport21. Moreover, the open framework of NVP@C favors the intercalation and de-intercalation reactions of sodium ions. Hence, the above factors lead to the higher capacity and the low energy consumption of the feed water. The SACs and salt removal rates of various feed concentrations are shown in Figure 3(a). As the concentration of feed water increases, the SAC increases as well. A high SAC of 137.20 mgNaCl gNVP-1 and a removal rate of 0.076 mgNaCl gNVP-1 s-1 can be obtained when feed water concentration is 100 mM, which is much higher than those of the traditional CDI22. Besides, energy consumptions by equation (3) are shown in Figure 3(b), 2.16 kgNaCl kWh-1 of 100mM shows a relatively promising energy consumption for desalination, compared with MCDI (1.54 kgNaCl kWh-1), CDI with activated carbon (0.90 kgNaCl kWh-1)23, 24. The table of energy consumptions from different desalination methods and materials are shown in Table S2. It can be seen that the energy consumption of this work is higher than traditional CDI and RO. 4. Conclusion The faradaic electrode is used in deionization because it can overcome the limitations of the traditional CDI, which include low SAC and excessive co-ion expulsion, among others. In this paper, an open framework structure NVP@C is successfully applied for the first time as the faradaic electrode in HCDI, where AC acts as a capacitive electrode. In the HCDI system, sodium ions are intercalated/deintercalated into NVP@C by faradaic capacitance, and EDL captures chorine ions. The

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results demonstrate that an excellent SAC and a high removal rate are achieved. Moreover, as the feed water concentrations (10-100 mM) increase, the adsorption capacities and removal rates increase as well. When the feed water concentration is 100 mM at a constant voltage of 1.0 V with a flow rate of 15 mL·min-1, the SAC is 137.20 mgNaCl gNVP-1 and a removal rate of 0.076 mgNaCl gNVP-1 s-1. Furthermore, relatively low energy consumption of 2.16 kgNaCl kWh-1 is achieved, and these values are higher than those produced by previous carbon electrodes. In summary, the NVP@C faradaic electrode offers great promise for removal of salt ions from highconcentration feed water. Acknowledgment This research was supported by the National Natural Science Foundation of China (grant nos. 21577099 and 21777118). We are also thankful to anonymous reviewers for their valuable comments to improve this manuscript.

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Na3V2(PO4)3@C with a low energy consumption of 2.157 kgNaCl kW·h-1 and a high salt adsorption capacity of 137. 20 mg NaClg-1 in HCDI system.

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