Efficient Sodium Ion Intercalation into the Free-Standing Prussian blue

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Efficient Sodium Ion Intercalation into the Free-Standing Prussian blue/ Graphene Aerogel Anode in Hybrid Capacitive Deionization System Sareh Vafakhah, Lu Guo, Deepa Sriramulu, Shaozhuan Huang, Mohsen Saeedikhani, and Hui Ying Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18746 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Materials & Interfaces

Efficient Sodium Ion Intercalation into the FreeStanding Prussian blue/Graphene Aerogel Anode in Hybrid Capacitive Deionization System Sareh Vafakhah, † Lu Guo, † Deepa Sriramulu, † Shaozhuan Huang, † Mohsen Saeedikhani, ‡ and Hui Ying Yang*, † †

Pillar of Engineering Product Development, Singapore University of Technology and Design, Singapore 487372

‡

Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576

*Corresponding

Author

Email: [email protected]

KEYWORDS Hybrid Capacitive Deionization, 3D graphene network, Prussian Blue, Aqueous In-situ XRD, Intercalation mechanism, desalination. energy consumption.

ABSTRACT

In this study, we introduced an efficient Hybrid Capacitive Deionization (HCDI) system for removal of NaCl from brackish water, in which Prussian blue (PB) nanocubes embedded in a 1 ACS Paragon Plus Environment

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highly conductive reduced graphene oxide aerogel (rGA) has been used as a binder-free intercalation anode to remove Na+ ions. The combination of redox-active nanocubes and the 3D porous graphene network yielded a high salt removal capacity of 130 mg g-1 at the current density of 100 mA g-1. Moreover, energy recovery and energy consumption upon different desorption voltages of the HCDI system were investigated and the result showed a notably low energy consumption of 0.23 Wh g-1 and a high energy recovery of 39%. Furthermore, the realtime intercalation process was verified by In-situ XRD measurements, which confirmed the intercalation and deintercalation processes during charging and discharging, respectively. Eventually, the perfect stability of the desalination unit was confirmed through the steady performance of 100 cycles. The improved efficiency as well as the ease of fabrication open a shiny horizon for our HCDI system towards commercialization of such technology for brackish water desalination.

INTRODUCTION Due to the growth in demand for high efficiency and low energy consumption technology for brackish water desalination, many research efforts have been devoted to explore new methods to overcome the water scarcity 1,3. Reverse osmosis (RO) is one of the conventional techniques used to revitalize freshwater supplies. However, high energy consumption and high cost put restrictions on its application in many areas

4,5.

Therefore, the development of

alternative techniques with low energy consumption is strongly encouraged. Electrochemical deionization-based techniques such as capacitive deionization (CDI), electrochemical oxidation, and electrodialysis have emerged as future remedies for water desalination

6,7.

Among all, CDI is an emerging technology which is suitable to remove the salt from brackish water. CDI is operating at low pressure and room temperature, which has low energy requirement and ease of operation 8. However, one of the drawbacks of CDI is the co-ions 2 ACS Paragon Plus Environment

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adsorption which reduces the charge efficiency significantly. A significant improvement in the CDI system was obtained by introducing membrane capacitive deionization (MCDI), which prevents the passage of co-ions into the electrodes by using ion exchange membranes

9,10.

Nevertheless, MCDI is yet to reach the desired salt removal capacity, whereas both CDI and MDCI are based on the electrostatically holding of salt ions in the Electrical double layer (EDL). In terms of salt removal, EDL faces the capacity limitation originated from lack of suitable electrode materials and insufficient electrical conductivity 5. To overcome these obstacles, an alternative concept was introduced to boost salt removal capacity by chemistry reaction which is called Hybrid CDI (HCDI) 6, 11-13. In this system, the battery-like electrode materials are chosen upon their redox reaction with either sodium or chloride ions in an aqueous solution 14,15. These materials are required to retain their stability during the adsorption/desorption stages in the aqueous environment and accommodate intercalated ions during the operation 15. Despite the successful application of several materials specifically as cation-selective battery materials, the necessity of using binder is a limitation that imposes an extra step in the assembling process 16. Herein, we introduced a binder-free anode consisting of Prussian Blue (PB) nanocubes embedded in a highly conductive reduced graphene oxide aerogel (rGA) network to remove Na+ ions using the intercalation mechanism. PB is chosen as a non-toxic and low-cost intercalation/deintercalation material with excellent redox activity 17-19. The three-dimensional (3D) rGA network is used as a suitable matrix owing to its high conductivity and mechanical stability as well as the uniform structure and high surface area 20-22. Nevertheless, there are some fundamental challenges to obtain PB embedded rGA structures due to the non-uniform particle distribution and precipitation as well as a weak synergetic interaction between the components. In a typical method a quick direct precipitation reaction takes place in a neutral aqueous solution. Therefore, control over the morphology and size of the final particles is not easy to achieve. Various solutions to tackle these challenges 3 ACS Paragon Plus Environment


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have been reported, such as in-situ growth of PB on the graphene sheets as well co-reduction of graphene oxide and PB precursors or the single-source precursor approach

15, 21, 23-25.

However, the application of such free-standing electrodes with detailed mechanism analysis in HCDI is not reported yet. In this study, we investigate the desalination performance of the PB at rGA conjointly with rGA to perform as the binder-free anode and cathode in an HCDI system. The Constant current operation mode is preferred due to the better stability of concentration over the cycles, and ease of operation 11. Moreover, we present a detailed mechanism analysis based on the experimental illustration of In-situ XRD, which broaden the fundamental insights into the real-time situation during the charging and discharging process. These results provide direct evidence to prove the sodium ions removal mechanism. The developed HCDI unit cell delivered remarkable removal capacity of 130 mg g-1, and the excellent long-term stability of the electrode materials was confirmed through the cycling desalination experiments. Furthermore, the effects of different voltage ranges and flow rates on the consumed energy were fully studied, and the optimized experiment parameters were investigated.

EXPERIMENTAL SECTION PREPARATION OF PB/rGA The PB nanocubes were embedded into the graphene sheets via direct nucleation and growth method 24,25. 2.5 ml of graphene oxide solution (4 mg/ml) was added to 16 ml of HCl (0.1 M), and the obtained suspension was sonicated for 2 hours. The well-dispersed solution was subsequently kept at 80 C oil bath for a few minutes. The synthesizing process was followed by adding 52 mg of Potassium hexacyanoferrate, K3Fe(CN)6, dropwise. The stirred mixture was kept in an oil bath for 80 min under vigorous stirring. The color of the dispersion gradually turned to dark cyan indicating the successful formation of Fe4[Fe(CN)6]3.

X

H2O nanocubes. 4

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After that, the solution was centrifuged and washed three times with water. The obtained product was redispersed in 3 ml of Deionized water, followed by adding of 2 ml of HCL (0.1M) under sonication for 5 min. Finally, it was subjected to hydrothermal reaction in a Teflon- lined autoclave at 150 C for 10 hours. Eventually, the PB incorporated at rGA was obtained by freeze-drying of as-prepared hydrogel for 3 days, and it was labelled as PB/rGA. Accordingly, the cathode electrode was prepared as follows

26.

10 ml of GO solution (4

mg/ml) was mixed and sonicated for 10 min with 1ml of Ammonia hydroxide (NH4OH). The dispersed solution was kept in a Teflon- lined autoclave for 10 hours at 150 C. The obtained hydrogel was washed several times to remove the residual ammonia until the pH was almost neutral. The as-prepared hydrogel was dried via freeze drying to achieve the robust aerogel which was labelled as rGA. All chemicals including Potassium hexacyanoferrate, K3Fe(CN)6, Hydrochloric acid (HCl), and ammonia hydroxide (NH4OH) were purchased from Sigma-Aldrich. The commercial graphene oxide (GO) solution with 4 mg/ml concentration was purchased from graphene supermarket. All the chemicals were used without further treatment.

MATERIAL CHARACTERIZATION The morphologies of PB/rGA were investigated by field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7600F) equipped with an energy dispersive X-ray spectrometer (EDS). The field-emission scanning electron microscopy (TEM) was performed using a JEOL JEM-2100F microscope running at 200 kV acceleration voltage. The Lattice structures were recorded by X-ray powder diffraction (XRD, Bruker D8) using Cu Kα radiation (λ=1.5406 Å, 40 kV, and 40 mA). Raman spectra were obtained using a WITEC CRM200 Raman system equipped with a 532 nm laser source (WITec Instruments Corp Germany). XPS analysis was carried out on a PHI-5400, Physical Electronics, US with Al Ka source. 5 ACS Paragon Plus Environment


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The specific surface area (SSA) of PB/rGA and rGA were measured using nitrogen adsorption/desorption results by multipoint Brunauer–Emmett–Teller (BET) method using an Autosorb-iQ-MP-XR system. The pore size distribution of the material was determined with a Density Functional Theory (DFT) method. Thermogravimetric analysis (TGA) was measured by a TA Q50 with a heating rate of 20 °C min−1 under air environment in the temperature range of between 20 to 1000 °C. CV measurement has been conducted using a three electrodes setup in a 1M NaCl solution via Bio-logic VMP3, France electrochemical workstation. The IR spectra were recorded by the attenuated total reflectance (ATR) technique using the VERTEX 70v ATR-FTIR Spectrometer (BRUKER, Germany). A Keithley 6220 precision current source and Keithley 2000 multimeter system were used to measure electrical conductivity (four-probe method) of the samples. The inductively coupled plasma atomic emission spectroscopy (ICPAES, ICPE-9820, Shimadzu) was used to detect the existence of PB nanocubes into the effluent. In-situ XRD measurement was conducted in a coin type cell with a few drops of 1M NaCl, which is connected to a battery analyzer (Neware, Shenzhen, China), and the XRD patterns were recorded by X-ray powder diffraction (XRD, Bruker D8) with Cu Kα (λ=1.5406 Å) radiation.

PREPARATION OF ANODE AND CATHODE ELECTRODES The whole mass loading is kept ~ 10 mg for each electrode. The free-standing aerogels are pressed manually on the center zone of graphite sheets with 25 * 25 mm2 area and thickness of 0.67 mm without using any binder. The as-prepared electrodes are used directly for further desalination experiments.

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ELECTROCHEMICAL DEIONIZATION TEST Desalination process has been conducted through the electrochemical desalination set up which is called HCDI 15. The desalination cell is composed of the graphite paper as a collector with gently pressed self-standing PB/rGA, cation exchange membrane (CEM), spacer, anion exchange membrane (AEM), and self-standing rGA which is pressed at the second graphite current collector. The whole cell works under the constant current mode and in a flow-by system. The output solution (effluent) is repumped back to the system as a feed solution using the peristaltic pump with the same flow rate. The stack is fed from a 50 mL NaCl solution with the fixed concentration of 2500 ppm. The conductivity of the outlet solution is monitored and measured simultaneously by an inline conductivity meter (DDSJ-308F, Leici) at 298K, and the current and voltage are applied using a battery analyzer (Neware, Shenzhen, China). When the electrodes are being charged using the external DC energy supplier, the ions from the influent will be adsorbed and intercalated to the systems. Moreover, the adsorbed ions will be released back to the effluent during the discharging step. The Salt Removal Capacity of the electrodes was obtained from equation (1) 10, 15: Salt Removal Capacity = (Ci – Cf) V / m

(1)

Where, Ci and Cf are initial and final concentrations of NaCl solution in mg L-1, V is the volume of NaCl solution (L) and M is the total mass of the electrodes (g). RESULT AND DISCUSSION CHARACTERIZATION OF PB/rGA XRD measurements were used to examine the composition of the PB/rGA, and the rGA as well. Additionally, the XRD patterns for pure PB, rGO and pristine GO were considered as control data. Figure 1(a) shows the XRD pattern for GO, rGO, rGA, PB nanocubes, and PB/rGA. As illustrated in Figure 1(a), the formation of face-centered-cubic phase of PB 7 ACS Paragon Plus Environment


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nanocubes on graphene sheets was confirmed due to the constituent indexed peaks at 2 = 17.5, 24.8, 35.3, 39.6, 43.6, 50.7, 54.0, 57.2, which correspond to (200), (220), (400), (420), (422), (440), (600), (620) reflections of Prussian blue nanocubes, respectively 27,28. The result is consistent with the pure PB pattern which shows the nanocubes remained intact after the hydrothermal reaction. Moreover, there is no obvious peak ascribed to rGA, which denotes the anchored PB restricted restacking of the GA 24, 28. Therefore, the d-spacing of the graphene sheets increased and consequently the ordering degree was decreased

23, 28, 29.

In addition,

compared with the pure PB, there is a small shift of 0.7° in (220) peak of PB/rGA which might be attributed to the decreasing of the interplanar spacing between the atoms in the crystal. The difference in the interplanar spacing might be due to the distortion in PB lattice originated from intrinsic water molecules 29. Accordingly, rGA and rGO reveal a broad peak at 24, exhibiting the disordered stacking of graphene sheets along their stacking direction in comparison to pristine GO (10.77), revealing the removal of high amount of oxygen-based functional group as a consequence of reduction process 30.


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Figure 1. (a) XRD pattern of GO, rGO, rGA, PB/rGA, and pure PB, and (b) Raman spectra of GO, rGA, PB/rGA, and pure PB, (c) Nitrogen adsorption-desorption isotherms, (d) pore size distribution of PB/rGA obtained from DFT method (inset is the pore size distribution of rGA).

In addition, the existence of PB nanocubes can be thoroughly investigated using the IR spectra. Figure S1(supporting Information) shows the spectra of both rGA and PB/rGA. The two spectra show bands at about 1200 cm-1 and 1573 cm-1, which are ascribed to aromatic C=C and C-N stretching vibration

20, 26.

Besides, there are three new peaks in the IR spectra of

PB/rGA at 495 cm-1, 601 cm-1, and 2077 cm-1 which match to the stretching absorption band of the CN group in the Fe2+-CN-Fe3+, Fe-O, and Fe-CN, respectively, confirming the successful formation of PB on graphene sheets 27, 29. Representative Raman spectra of pristine GO, as well as rGA, pure PB, and PB/rGA, are illustrated in Figure 1 (b). All graphene-based materials including GO, rGA, and PB/rGA show 9 ACS Paragon Plus Environment


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two strong peaks located at around 1360 cm-1 and 1600 cm-1 corresponding to D band and G band, respectively

20, 31, 32.

The final value of ID/IG has been calculated to obtain the

graphitization degree on the graphene sheets. The intensity ratio for pristine GO is 0.98 while it is increasing for rGA and PB/rGA to 1.03, and 1.06, respectively. The ratio enhancement demonstrates an increase in the number of sp2 domains which confirms the efficient reduction of GO into rGA and PB/rGA

33.

In addition, the characteristic peak observed at 2187 cm-1

which is in accord with the Raman spectra for pure PB confirms the presence of PB in PB/rGA structure 28, 34. Moreover, a small G band shift (15 cm-1) can be observed in PB/rGA spectrum compared to rGA which may be correlated to the chemical interaction between PB nanocubes and the rGA nanosheets 23. From the aforementioned results of XRD, IR, and Raman it can be concluded that the formation of the graphene-based structure with PB nanocubes has been successfully done. Figure 1 (c) and 1 (d) show the adsorption-desorption measurements and pore structure of the aerogels which were investigated to obtain the surface areas and pore distributions in the anode and cathode materials. The PB/rGA isotherm exhibits relatively lower nitrogen uptake compared to rGA and thus the Brunauer-Emmett-Teller surface areas are calculated 88.4 m2g1

versus 253.68 m2g-1, respectively. As shown in Figure 1 (d) the pore size distribution obtained

by the DFT method, located mostly at 2.73 nm for the PB/rGA and at 1.73 nm and 2.79 nm for rGA sample. The low percentage of porosity within the micropore range for PB/rGA might be a result of the deposition of Fe species during the PB formation process 35. The pore size is a determinant factor in the resultant rate performance of the desalination system therefore the suitable pore size of PB/rGA and rGA facilitate the diffusion of sodium ions and chloride into the structure, respectively 36. Both the adsorption-desorption curves as indicated in Figure 1(c) have a conformity with Type IV of adsorption isotherm according to IUPAC classification, which implies the mesoporous structure of the aerogels 20. 10 ACS Paragon Plus Environment

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The chemical attributes of the PB/rGA and rGA are characterized using X-ray photoelectron spectroscopy (XPS) as presented in Figure S2 (a) and S2 (b). The three peaks at 285.35 eV, 398.83 eV, and 531.6 eV are attributed to C1s, N1s, and O1s for both samples, respectively 21, 37.

Moreover, the peak of 709.91 eV in Figure S2 (a) is matched to Fe for PB/rGA which

originates from PB. To further investigate the composition, the high-resolution XPS spectra were examined (Figure S2 (c), S2 (d), S2 (e), and S2 (f)). Figure S2 (e) shows the Fe2p pattern, in which peaks of Fe2p3/2 at 712.8eV, and Fe2p1/2 at 721.7 eV are assigned to FeIII in Fe4[Fe(CN)6]3, and a peak at 708.7 eV reveals the existence of Fe2p3/2 in [FeII(CN)6]4- 38,39. The investigation of the morphology of PB/rGA was carried out using SEM and TEM. As indicated in SEM images (Figure 2 (a) before the hydrothermal reaction, and 2 (b) after freeze drying), PB nanocubes were uniformly distributed in the rGA network with the average size of 500 to 600 nm. The SEM image of 3D porous interconnected aerogel shows there is no isolated PB nanocubes in the structure indicating the strong interaction between rGA matrix and nanocubes which prevents future leakage of PB nanocubes through desalination experiments 28-29.

Besides, the ICP result indicates there is no significant change of the Fe ions (originating

from PB nanocubes) concentration in mg L-1 before and after 100 cycles of desalination, which confirms the strong interaction of PB nanocubes and graphene sheets. Besides, high loading of nanocubes clearly pictured in the images which is further investigated using TGA experiment. In comparison with the SEM image of the sample before the hydrothermal reaction (Figure 2 (a)), PB/rGA after the freeze-drying (Figure 2 (b)) demonstrates that not only the size and shape of nanocubes have been remained intact but also the porous structure has been achieved. As shown in TEM image (Figure 2 (c)), the PB nanocubes were observed surrounding with a few layers of graphene sheets implying encapsulation of nanocubes within the matrix 34. Moreover, Energy dispersive X-ray detector (EDS) mapping of the composite was conducted to further



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confirm the presence of PB nanocubes on graphene sheets (Figure 2 (d)). It indicates the presence of Fe and N (from PB), and carbon (from PB and rGA) in the composite.

Figure 2. SEM image of PB/rGA (a) before hydrothermal reaction, (b) after freeze drying, and (c) TEM Image of PB/rGA, and (d) corresponding EDS mapping of the PB/rGA.

The electrochemical analyses of the electrodes materials were evaluated using threeelectrode cell configuration in 1M NaCl solution. The cyclic voltammograms (CVs) for rGA and PB/rGA at the scan rate of 20 mV/s and in a voltage range of -0.7 V to 0.7 V are as illustrated in Figure S3. As it can be seen for rGA there is no obvious redox peak in the CV curve, but the quasi-rectangular shape representing the electrochemical double-layer capacitance behavior. Importantly, for anode material the intrinsic electrochemical activity (redox behavior) of PB is preserved after nanocubes anchored to rGA structure. It could be observed that successful formation of PB in rGA structure resulted in a distinct and symmetric 12 ACS Paragon Plus Environment

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pair of redox peak -0.05/ +0.25, presenting the sodium ions insertion and extraction happening reversibly and it is in agreement with the following reaction (Eq. 2) 40,41. FeIII4[FeII(CN)6]3 + 4Na+ + 4e- Na4FeII4[FeII(CN)6]3

(2)

Besides, the CV curve for PB/rGA shows the curve close to an expanded rectangular shape which implies the effective conjunction of properties of active nanocubes and graphene materials 29. Figure S4 illustrates the result of thermogravimetric analysis (TGA) of PB/rGA in the air environment and up to 1000 °C. The result shows 3-steps weight loss. The first step which occurs below 211 C is attributed to the residual moisture loss as well as the crystal water molecules (~13%). The second step at a temperature around 274 C (between 211 C to 300 C) corresponds to ferrocyanide structure decomposition

42.

The indicated weight loss is

attributed to the transformation of Fe4[Fe(CN)6]3 to Fe2O3 43. And finally, the TGA curve depicts a significant loss of 28% at approximately 405 C, which is related to ignition of graphene 31. Therefore, the content of PB in PB/rGA is estimated to be 58%, and the contained water molecules can be estimated as 10 (Fe4[Fe(CN)6]3 . 10 H2O).

DESALINATION PERFORMANCE The prepared electrode material and the experimental set up of the HCDI device are shown in Figure S5. In order to achieve the optimized desalination performance, three different electrodes weight ratios, different cell voltages (the applied voltage between the two electrodes), different current densities (100 mA g-1 to 500 mA g-1 with respect to total mass of PB/rGA), and different flow rates of 50 mL min-1 to 280 mL min-1 are investigated. The adsorption cell voltage of 1.4 is kept without any alteration, whereas the desorption voltage is varied between -1.4 to 0 with 0.2 V intervals. Three different weight ratios of PB/ rGA as an 13 ACS Paragon Plus Environment


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anode to rGA as a cathode of 2:1, 1:1, and 1:2 are studied. Besides, the desalination performance of both sides rGA as a control sample is considered. As shown in Figure 3 (a), the salt removal capacities of two weight ratios of 2:1 and 1:1 are almost equal, and it reaches to 130 mg g-1 which is calculated based on the whole mass of the aerogels. The obtained removal capacity can be considered as one of the highest result reported yet in CDI systems 13-15, 44, 45. Although the corresponding removal capacity for 2:1 is slightly better in the higher current density of 200 mA g-1, and 300 mA g-1, but as it is obviously shown in Figure 3 (b), the time consumption is higher and consequently the rate performances of the weight ratio of 2:1 is much lower. This can be explained due to the lower conductivity of PB comparing to rGA, which leads to longer charging and discharging duration 18. The individual conductivity of the rGA and PB/rGA were measured by Four Points Probe method in room temperature which shows the conductivity of 17 Sm-1 and 5 Sm-1, respectively. Furthermore, the lower removal capacity and the rate performance for the weight ratio of 1:2 were confirmed by the experiments. Hence the obtained results lead to choose 1:1 as an optimal weight ratio for the remained experiments. Moreover, the removal capacity for the control samples confirm the impact of PB nanocubes as electrochemical active sites 15. Importantly, it is worth mentioning that the combination of active materials with the conductive graphene network in a 3D structure amazingly enhance the desalination properties by introducing the efficient electron transport pathway to the redox active sites, and it can be proved by comparing the high removal capacity of 130 mg g-1 obtained based on the total mass of the electrodes, whereas in previous studies done on PB composites the salt removal capacity was only 120 mg g-1 while the mass of additives were neglected and the results were reported based on the active materials masses.


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Figure 3. (a) The corresponding salt removal capacity, and (b) the rate performance for different weight ratios of the anode to cathode, 2:1 and 1:1, control sample, and 1:2 (c) The removal capacity with the weight ratio of 1:1, and (d) The adsorption and desorption capacity with the weight ratio of 1:1 and for different current densities of 100mA g-1, 200 mA g-1, 300mA g-1, and 500mA g-1. Different current densities of 100 mA g-1, 200 mA g-1, 300 mA g-1, and 500 mA g-1 with respect to the mass of anode aerogel have been chosen to study the current density influence on salt removal capacity of HCDI system. As shown in Figure 3 (c) and 3 (d), the deionization capacity of 130 mg g-1 is decreasing to 85 mg g-1, 55 mg g-1, and 25 mg g-1 while the current density is increased from 100 mA g-1 to 200 mA g-1, 300 mA g-1, and 500 mA g-1, respectively, and this can be explained by the concept of incomplete redox reaction for higher current densities 10, 14, 15, 46. However, the time consumption is reducing with the higher current density 15 ACS Paragon Plus Environment


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as well. And eventually, the removal capacity returned back to the initial value once the low current density of 100 mA g-1 has been applied once more, and it indicates the durability of the aerogels. The energy consumption of an entire cycle can be analyzed by integrating the voltage over time multiply by the applied current 14, 47-49. The desalination experiments were carried out for different desorption voltage of -1.4 to 0, and to satisfy the same condition for all the experiments, the current density was kept at 100 mA g-1 with the flow rate of 50 mL min-1, and the initial concentration of 2500 ppm. As illustrated in Figure 4 (a), the area is separated into charging and discharging sections. The blue and red areas are discharging and charging sections, respectively. The whole process is separated into four steps. The individual salt removal capacity of all applied voltage ranges is presented in Figure 4 (b). In step 1 showing in Figure 4 (a), the voltage is approaching zero. This step is considered as the self-discharging process 15, and the corresponding salt intercalation amount is relatively low compared to the step 2 (exhibiting in Figure 4 (b)). During the second step while the voltage is increased from zero to 1.4, the PB/rGA is negatively charged, and the sodium ions are intercalated into the crystal structure of PB, which boosts the salt removal capacity significantly since the capacity is no longer limited by the surface adsorption of carbon-based materials 45. In this step, the PB reduction is happening, and the salt removal capacity of 130 mg g-1 is achieved with the current density of 100 mA g-1, and the flow rate of 50 mL min-1. In the third step, the Na ions is deintercalated from the PB/rGA due to the PB oxidation in response to the discharging process. Finally, in the 4th step, the rest of the ions are released back from the electrodes to the effluent while the absolute value of voltage is increased from zero to -1.4. As it is illustrated in Figure 4 (b) the removal capacity of voltage range of 1.4 V to -1.2 V is almost equal to 1.4 V to -1.4 V (< 3 mg g-1), the energy consumption is 0.03 W h g-1 lower for -1.2 V though, which makes it a good candidate for optimizable operation of the desalination system. As shown in Figure 4 16 ACS Paragon Plus Environment

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(c), the calculated energy consumption of HCDI cell is presented in Wh g-1 unit, and the amount of energy consumed by the unit system is decreasing for narrower voltage ranges 46.

Figure 4. (a) The demonstration of charge and discharge segments for energy consumption calculation; blue sections represent the energy recovery and red sections represent the energy consumption, (b) The corresponding illustration of removal capacity vs time for different desorption voltages, (c) The energy consumption value and (d) percentage of energy recovery for different applied desorption voltage of -1.4, -1.2, -1.0, -0.8, -0.6, -0.4, -0.2, and 0 V.

Importantly, the obtained energy consumption for the last voltage range of 0 to 1.4V is higher than 0.2 to 1.4 V, although the time duration is obviously lower. It is necessary to mention that by increasing the desorption voltage from -0.2V to 0V, the extraction process might not be fully completed. Therefore, the lower salt removal capacity affects the final energy 17 ACS Paragon Plus Environment


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consumption value. As a result, the desorption voltage of -0.2 V has the best performance regarding the energy consumption. Furthermore, the energy recovery is calculated upon the different desorption voltage, as illustrated in Figure 4 (d). it should be mentioned that the energy recovery quantity is measured exclusive of the salt removal values. The best energy recovery of HCDI system for a single cycle was achieved for 0 V, which is 39%. In addition, the influence of flow rate on removal capacity and energy consumption has been evaluated with different flow rates of 50, 100, 150, 200, and 280 mL min-1, and the result is shown in Figure S6 (a) and (b). The higher removal capacity and the better rate performance of the HCDI system with increasing the flow rate (up to 150 mL min-1) is due to the reduction of overpotential as a result of the better ions mobility

14, 15.

Accordingly, the energy

consumption (Wh g-1) is improved for the higher removal capacity.


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Figure 5. (a) and (b) cycling performances for voltage ranges of 1.4 to -1.4V and 1.4 V to -0.2 V, respectively (Insets are the TDS pattern of each voltage). (c) The SEM image of PB/rGA after 100 cycles with voltage range of -1.4 V to 1.4 V. Figure 5 (a) and 5 (b) illustrate the cycling performance of the HCDI system for the desorption voltage of -1.4 V for 100 cycles and -0.2 V for 50 cycles, respectively. And the SEM image after 100 cycles with voltage range of -1.4 V to 1.4 V was shown in Figure 5 (c). To be consistent with previous desalination experiments, the current density is kept at 100 mA g-1 regarding the anode mass and the flow rate is 50 mL min-1. The inset images are the typical pattern for the TDS changes for the mentioned voltage ranges. It can be noticed from Figure 5 (a) that the excellent performance could be maintained for 100 cycles, and besides, after a few first cycles of slight dispersion (due to initial time requirement for system stabilization) the intercalation and deintercalation capacities are comparable which indicates the satisfactory reversibility of the materials. The excellent stability of the performance can be contributed to the sufficient interaction between the PB nanocubes and rGA so that it stops the depletion of the active nanocubes from the structure which is verified with the SEM image after the cycling (Figure 5 (c)) 50. The cycling performance for the voltage range of -0.2 V to 1.4 V as the best energy consumption performance candidate is also investigated. The average removal capacities are 80 mg g-1 for intercalation and 78 mg g-1 for deintercalation processes, which specifies the slightly higher capacity during the intercalation compared to deintercalation stage. This observation is confirmed by the TDS pattern (inset of Figure 5 (b)) which has a mild downward slope, indicating the incomplete release of ions to the effluent 11, 14.



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Figure 6. In-situ XRD measurements during charging and discharging processes in the voltage range of -1.4 to 1.4 and current density of 30 mA g-1. (a) the evolution illustration of peaks in different scan numbers within the (15- 41) 2 range domain with the corresponding charging/ discharging curve in the right side. (b) the selection of corresponding XRD patterns for the indicated oval. (c) higher magnification for the indicated oval in section (a). (d) schematic crystal structure of Prussian Blue before Na+ intercalation during discharging process which corresponds to I in (c). (e) schematic crystal structure of Prussian blue after Na+ intercalation during charging process which corresponds to II in (c). To further investigate the salt removal mechanism, the In-situ XRD measurements have been performed, and the XRD patterns were recorded during charge/discharge cycles. The In-situ XRD set up was constructed as shown in the Figure S7. The voltage domain range of -1.4 V to 1.4 V was selected to achieve a complete insertion/deinsertion of ions into the system, and the low current density of 30 mA g-1 regarding the anode mass was chosen to provide enough time 20 ACS Paragon Plus Environment

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duration to fulfill the scanning time. The In-situ XRD patterns of the anode electrode at different

charging/discharging

stages

are

illustrated

in

Figure

6.

The

intercalation/deintercalation mechanisms are easily perceived due to the shift of the constituent peaks of (200), (220), (400), (420), which were selected from the full XRD pattern. As it can be found in Figure 6 (a) the position of the peaks started shifting to the lower angles during the charging process indicating the sodium ions intercalation into the Prussian Blue structures and increase of the lattice parameters, while they are reversibly shifted back to the origin position during the Na+ extraction process referring to the highly reversibility and stability of the electrode during the desalination cycles

17, 51-53.

The slight difference in time duration of the

cycles may be contributed to the initial required activation time of the electrode. The XRD 2D diffraction patterns for a selected range of the scanned data are shown in Figure 6 (b), and as it can be seen the three-middle pattern are ascribed to the intercalation process during the charging in which the peaks are shifted to the lower angles. Further magnified evolution illustration of XRD measurements is depicted in Figure 6 (c). Figure 6 (d) and (e) represent the PB crystal structure after deintercalation and intercalation which correspond to the indicated area of I and II in section (c), respectively.

CONCLUSION To summarize, an ultrahigh salt removal performance has been achieved through a freestanding and binder-free hybrid capacitive deionization system, and it demonstrates the successful combination of intercalation/deintercalation mechanism with physically electrical double layer mechanism beneficial to a superior removal of salt ions from brackish water environment. The In-situ XRD measurements were conducted for the first time in an aqueous HCDI systems as a strong tool to prove the intercalation mechanism directly. The simple hydrothermal method was utilized to synthesize the PB/rGA as well as rGA as a binder-free 21 ACS Paragon Plus Environment


Page 22 of 31

anode and cathode materials for further desalination experiments which carried out under the constant current mode. The outstanding removal capacity of 130 mg g-1 (calculated based on the total mass of the electrodes) was obtained at a current density of 100 mA g-1 and the voltage range of -1.4 V to 1.4 V. The calculation of energy consumption is conducted for different discharge voltage of -1.4 to 0 V with the interval of 0.2 V, and the lowest energy consumption of 0.23 Wh g-1 was achieved at voltage range of -0.2 V to 1.4 V, while the highest energy recovery is found for discharge voltage of 0 V. Moreover, the integrity of the HCDI electrodes’ materials and the binder-free design system has been confirmed through the excellent cycling stability in an aqueous sodium chloride solution. Thus, it can be concluded that the reported free-standing structure augmented with redox-active nanocubes is a good candidate for brackish water desalination with high removal performance and low energy consumption.

Supporting Information, The IR spectra, XPS survey spectra, and CV curves for PB/rGA and rGA as an anode and cathode materials; TGA curve revealing the percentage of PB nanocubes in the anode; Photography images of the electrodes and HCDI set up; the removal capacity and energy consumption study upon different flow rate; and the In-Situ XRD set up configuration. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID ID: Hui Ying Yang: 0000-0002-2244-8231 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 22 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research work is supported by the National Research Foundation of Singapore, Prime Minister’s Office under its Environment & Water Research Programme with Grant No. 1301IRIS-17 and administered by the Environment & Water Industry Programme Office (EWI) of the PUB, Singapore’s national agency.



Page 24 of 31

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Efficient Sodium Ion Intercalation into the Free-Standing Prussian blue

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