Graphene Hybrids for

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Calcined MgAl-layered double hydroxide/ graphene hybrids for capacitive deionization Qidi Ren, Gang Wang, Tingting Wu, Xin He, Jianren Wang, Juan Yang, Chang Yu, and Jieshan Qiu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04983 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Industrial & Engineering Chemistry Research

Calcined MgAl-layered double hydroxide/graphene hybrids for capacitive deionization Qidi Ren†, Gang Wang*, †, Tingting Wu†, Xin He‡, Jianren Wang†, Juan Yang†, Chang Yu† and Jieshan Qiu*, † †

State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical

Engineering, PSU–DUT Joint Center for Energy Research, S chool of Chemical Engineering, Dalian University of Technology, Dalian 116024, China. ‡

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of

Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China. Corresponding Authors

Fax: +86 0411 87986024; Tel: +86 0411 87986024 * Email: [email protected] (G. Wang); [email protected] (J. Qiu)

ABSTRACT: Layered double hydroxides (LDHs) are a class of anionic clay materials. When calcined at a certain temperature range, LDHs decompose into corresponding mixed metal oxides (Ox), which are regarded as promising electrode materials for capacitive deionization (CDI). We have developed a simple yet efficient strategy for in-situ assembly of the MgAl-Ox nanosheets on graphene (G). The as-obtained MgAl-Ox/G nanohybrids with numerous channels for the access of anion exhibited high surface area and high electrical conductivity. When used as 1

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the anode of CDI cell, MgAl-Ox/G nanohybrids exhibited excellent cycling stability and high electrosorption capacity due to its the memory effect and oxidation resistant property.

INTRODUCTION The lack of clean, fresh water caused by the growing population and worsening environmental pollution brings many problems to humans1,2. Currently known desalination technologies mainly include thermal processes, reverse osmosis, and electrodialysis3. In recent years, capacitive deionization (CDI) has been reported as a promising technology for saltwater desalination4-6. Non-Faradaic reaction and pseudocapacitance (Faradaic reaction) are two fundamental mechanisms for electrochemical water desalination. Non-Faradaic reaction is based on the formation of electric double-layer capacitors which is the most important process for CDI technology7,8. Faradaic reaction means the redox reactions reacted on the surface of and within the carbon electrodes which has been extensively examined by researchers recently. These reactions may generate byproducts or destory the performance and long-term stability of the electrode9. However, others researchers think that it can help to form charged species and improve desalination performance through pseudocapacitive/intercalation effects10. In contrast to membrane, thermal treatments or reverse osmosis (RO) , CDI has the advantages of low-pressure operation, high energy efficiency, minimized energy consumption and maintenance costs, easy regeneration of electrodes, and environmental friendliness11-13. 2


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Electrode materials, as the core part of CDI technology, attract the attention of many researchers. A great number of studies have been conducted on the preparation of CDI electrodes14,15. At present, the development of novel electrode materials remains the most pressing issue. Meanwhile, various carbon materials with high specific surface areas, excellent electronic conductivity, superior chemical and electrochemical stability, and low propensity for scaling and bio-fouling have been achieved and used as CDI electrodes16,17. However, the loss of salt capacity and performance stability during the operation of a CDI cell increases due to the oxidation of the carbon anode in an aqueous solution18. Layered double hydroxides (LDHs), as a class of anionic clay materials, features low cost, high stability, and high versatility in both composition and morphology19,20. LDHs

can

be

described

using

the

general

formula

[M1-xIIMxIII(OH)2]x+[(An-)x/n]x-·mH2O, where MII and MIII are divalent and trivalent metal cations, An- is the interlayer anion, x is the molar ratio of MIII/(MII + MIII), and m is the molar amount of water21. When calcined in a certain temperature range, MgAl-LDH decomposes into corresponding mixed metal oxides (Ox). The calcined products allow the regeneration of the original layered structure by absorbing anions into the interlayer from the aqueous solution, this process is known as the “memory effect”22,23. However, when an external electrostatic field was added on the surface of Ox, the “memory effect” can be prevented, which means Ox will not transform to LDH(24). Ox materials have been used as potential ion exchangers/adsorbents for electrodes for electrosorption and removal of toxic anions from contaminated 3



water25,26. MgAl-LDH, as the representation of LDHs, possesses the characteristic of LDHs. MgAl-Ox is characterized by a homogeneous dispersion of metal cations and high specific surface areas. The composites obtained by combining LDHs and graphene sheets via a hydrothermal process exhibit superior capacitive performance27. Yasin. et al used ZrO2 nanofibers/activated carbon composite as electrode to conduct CDI test and electrosorption capacity could reach 16.35 mg g-1 at 1.2 V28. When TiO2/ZrO2 nanofibers/nitrogen co-doped activated carbon29 was used as electrode, the electrosorption capacity was 3.98 mg g-1. Li. et al prepared nitrogen-doped hollow mesoporous carbon spheres and found the electrode presented excellent cycle stability over 20 adsorption-desorption cycle for CDI test30. In the present work, we reported the preparation of porous MgAl oxide/graphene hybrids (MgAl-Ox/G) for CDI using a friendly one-step hydrothermal method. The reaction of the double metal hydroxide with graphite oxide resulted in the formation of hexagonal rings of MgAl-layered double hydroxide that were anchored to the graphene oxide (GO) layers. The interaction between the acidic groups on GO and the basic hydroxide layers induced the chemical etching of hexagonal platelets, leading to the formation of the hexagonal rings of MgAl-LDH. After calcination at a high temperature, MgAl-LDH decomposed to mixed metal oxides (MgAl-Ox), which could uptake chloride ions. Two-dimensional graphene was discovered in 2004, and it has since been studied widely31. The as-prepared MgAl-Ox/G exhibited better desalination performance than MgAl-Ox when it was used as anode material. This result is expected to facilitate the development of a novel direction in CDI application. 4


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Dioxide/activated carbon (AC) composites are common electrode materials for CDI32. In our study, an asymmetric electrode was prepared by assembling MgAl-Ox with AC. As we know, the surface of AC can adsorb anions and cations at the same time without voltage. When AC was regarded as cathode with negative charge, anions on the surface of AC electrode were rejected to the solution because of fundamental laws of charges repel, which is known as co-ion expulsion phenomenon. However, electrode modification is one effective way to prevent co-ion expulsion. In our work, we treated AC with nitric acid (AC-HNO3) to introduce carboxyl groups. The introduction of negtively charged functional groups prevents the adsorption of anions on the surface of modified AC electrode and enhances cation selectivity. Modification the surface charge of the electrodes will prevent the movement of co-ions to enhance the desalination performance, charge efficiency, and cyclic stability of CDI33. The modified electrode was used as the cathode material, and it exhibited good cation selectivity. Carboxylic groups were grafted onto the surface of AC to enhance the CDI performance by reducing co-ion expulsion34. EXPERIMENTAL PROCEDURES Material Preparation All the chemicals were purchased commercially, and all the reagents were used directly without further purification. GO was prepared with the Hummers method35. For the synthesis of MgAl-LDH, Al(NO3)3·9H2O (0.005 mol, 1.875 g) and Mg(NO3)2·6H2O (0.01 mol, 2.56 g) were dissolved in deionized water by ultrasonication for 30 min. Urea (0.06 mol, 3.6 g) was then added to the solution. The 5



resulting suspension was transferred into a 100 mL Teflon-lined stainless-steel autoclave. MgAl-LDH was aged via hydrothermal reaction at 120 °C for 24 h. The solid product was collected by centrifugation, washed until a pH of 7 was reached, and dried at 80 °C overnight. To obtain MgAl-Ox, the product was calcined at 400 °C in a nitrogen flow with a heating rate of 2 °C min−1 for 5 h. To avoid the co-ion expulsion effect, we treated AC with nitric acid. AC powder was treated with 6 M HNO3 solution at 65 °C for 5 h. The obtained product was collected by filtration, washed until a pH of 7 was reached, and dried at 80 °C for 12 h. Carboxyl groups were introduced on the surface of AC. Consequently, the surface charge of the electrode changed. To enhance the electrical conductivity of the electrode materials, we introduced graphene by a hydrothermal route. The synthetic procedure is described as follows. Exactly 0.2 g of dried GO was dispersed in 100 mL deionized water and sonicated for 1 h. Subsequently, Al(NO3)3·9H2O (0.005 mol, 1.875 g) and Mg(NO3)2·6H2O (0.01 mol, 2.56 g) were dissolved in the above suspension by ultrasonication for 30 min. Urea (0.06 mol, 3.6 g) was then added to the suspension. Through hydrothermal reaction, washing, drying, and calcination as the method to prepare MgAl-Ox, MgAl-Ox/G was obtained. Figure 1a shows the schematic of the synthesis of MgAl-Ox/G composites. Electrochemical Test The electrochemical measurements for MgAl-LDH, MgAl-Ox MgAl-LDH/G and 6


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MgAl-Ox/G electrodes were carried out in three electrode system. The samples were used as working electrode and Ag/AgCl electrode was treated as the reference electrode. The counter electrode was platinum foil and the test was conducted in NaCl solution of 1 mol L-1. The cyclic voltammograms (CV), the electrochemical impedance spectroscopy (EIS) and the galvanostatic charge/discharge tests were conducted on a CHI 660D electrochemistry workstation. CDI Test The preparation of CDI electrodes involved several steps, including mixing, coating on graphite plate, drying, cutting, and assembling. The specific steps are as follows. Active material, carbon black, and binder were mixed with a quality proportion of 8:1:1. The binder (0.1 g) was dissolved in N,N-dimethyl acetamide. Continuous stirring was performed until the binder was completely dissolved in the solution. Then, carbon black (0.1 g) was added and stirred uniformly. Finally, the active material (0.8 g) was added, and the final black sticky mixture was mixed by ultrasonic agitation. The prepared slurry was coated on graphite foil with 300 µm applicator. The graphite foil coated with slurry was dried at 80 °C for 24 h to remove the organic solvent. The dried electrode slice was cut into the size of 5 cm × 6 cm. The anode material (0.1 g) and cathode material (0.1 g) were prepared according to the method above. They were assembled into the CDI module, and the gap between two electrodes, which were separated with silicone rubber spacers, was 150 µm. A CDI test was conducted in the NaCl aqueous solution with an initial concentration of 500 mg

7



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L−1 with a single-pass CDI apparatus (Figure 1b). The NaCl solution was pumped into the CDI cell (Figure 1c) by a peristaltic pump at a constant rate of 10 mL min−1 at room temperature. A voltage of 1.0 V was applied with the power supply during the process of adsorption for 10 min. When the voltage was removed or the reverse voltage was applied, the process of desorption began. Adsorption capacity and charge efficiency are two important indexes to evaluate the adsorption process of CDI. Adsorption capacity indicates the ion quality adsorbed by each gram of electrode. This parameter is important in comparing various types of electrodes and different test conditions. Charge efficiency refers to the proportion of charge used for ion adsorption in electrode charge. The adsorption capacity of NaCl and charge efficiency were calculated as follows:

(1)

(2)

where Γ (mg g−1) is the adsorption capacity; Φ (mL min−1) is the flow velocity of the solution; C0 and Ct (mg L−1) are the initial and equilibrium concentrations of NaCl in the solution, respectively; m (g) is the mass of the electrode material; and t (min) is the time for adsorption. Λ is the charge efficiency, F (96485 C mol−1) is the Faraday constant, I (A) is the real-time current of the adsorption process, and MNaCl (58.5 g mol−1) is the molar mass of NaCl. Material Characterization

8


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The morphologies of the samples were characterized by a transmission electron microscope. The crystalline structure of the samples was analyzed using powder X-ray diffraction (XRD, D/MAX-2400) using Cu Ka radiation (λ= 0.154 nm). The specific surface area and pore size distribution were calculated by N2 adsorption–desorption isotherms with an ASAP 2020 (Micromeritics). Before adsorption, the samples were outgassed for 6 h at 100 °C under a vacuum of 10−6 Torr. The specific surface area was calculated from the Brunauer−Emmett−Teller (BET) plot of the nitrogen adsorption isotherm. Thermal analysis (STA 449 F3, NETZSCH) of the products was conducted at a heating rate of 10 °C min−1, under air atmosphere, from 30 °C to 800 °C.

RESULTS AND DISCUSSION Structural Characterization Transmission electron microscopy (TEM) was performed to examine the morphological differences of MgAl-LDH, MgAl-Ox, MgAl-LDH/G and MgAl-Ox/G. The size and shape of the LDH were controlled by the concentration of the NH3H2O parameter36. The size and shape of the LDH also determined its performance. As-synthesized MgAl-LDH was shaped in hexagonal form with a lateral size of ca. 2 µm (Figure 2a), which matched well with that in the literature in terms of size and uniformity37-39. After calcination at 400 °C under the heating rate of 2 °C min−1, MgAl-LDH lost water molecules between the layers, and CO32- transformed to CO2. However, the structure of the calcined product did not collapse, and the hexagonal rings were maintained integrally (Figure 2b). MgAl-LDH/G was also prepared by a 9



one-pot hydrothermal reaction in the GO solution with proper concentration. Figure 2c shows that MgAl-LDH combined well with graphene nanosheets. Brucite-like layers showed a positive charge, and the reduced GO exhibited a negative charge. This result was due to the electrostatic interactions between MgAl-LDH and graphene, which were attached together firmly. There are a lot of oxygenated groups on the surface of GO. These groups can provide active anchoring sites for metal ions, which facilitated nucleation and growth of MgAl-LDHs. This is benefit for MgAl-LDHs to crystal with the substrate, graphene nanosheets40. The prepared MgAl-LDH/G retained the thin hexagonal platelets with a lateral size of ca. 2 µm. This result indicated that the introduction of graphene did not influence the nucleation and growth of MgAl-LDH nanosheets. After calcination in the same condition, the product maintained its regular hexagonal structure and homogeneous size (Figure 2d). The combination of graphene and MgAl-LDH built a three-dimensional structure to prevent the restacking of both GO and MgAl-LDH. Graphene is hydrophobic and usually suffers from irreversible agglomeration in water due to the strong van der Waals interactions between neighboring sheets, which significantly reduces the surface area and isn’t beneficial for the adsorption. The exfoliated GO nanosheets can provide a lot of nucleation sites for MgAl-LDHs to grow and simultaneously prevent the stacking of LDHs platelets41. Figure 3a illustrates the XRD patterns of MgAl-LDH, MgAl-Ox, MgAl-LDH/G and MgAl-Ox/G. The pattern of MgAl-LDH shows diffraction peaks at 2θ = 11.4°, 22.9°, 34.3°, 34.8°, 38.3°, 46.8°, and 58.7°, all of which fit well with the characteristic peaks of the standard compound42. The peaks of MgAl-Ox/G are in good agreement with those of MgAl-Ox, and GO shows no characteristic peaks (2θ = 10.0°) and the peak of 2θ = 26.0° appeared. This result indicates that GO was completely reduced to 10


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graphene. After calcination at 400 °C, the peaks of MgAl-Ox and MgAl-Ox/G showed two broad peaks, whereas MgAl-LDH did not show characteristic peaks. This outcome indicated the decomposition of the original MgAl-LDH into the mixed oxide of magnesium and aluminum because of calcination. By hydrothermal hydrolysis of urea GO was reduced to graphene, which simultaneously provided a substrate for in-situ growth of these LDH nanoplatelets. The introduction of GO enhanced crystallinity of MgAl-LDH.

The results of the thermal analysis of the products are illustrated in Figure 3b. MgAl-Ox showed a weight loss of 15% for further decomposition after being heated to 800 °C. The weight loss of MgAl-Ox/G was 25%, which meant that the mass fraction of graphene was about 10%. The 50% mass loss of MgAl-LDH/G includes carbon burning and the loss of water and gas in the interlayer of MgAl-LDH. MgAl-Ox and MgAl-Ox/G were further investigated by N2 adsorption/desorption measurement to determine its surface area and porosity (Figure 3c, d). MgAl-LDH doesn’t have microporous structure. So it doesn’t have any contribution to specific surface area. The specific surface area of MgAl-LDH/G is only 31 cm3 g−1 (Figure S1). For MgAl-Ox and MgAl-Ox/G, a typical IV isotherm with hysteresis loops (P/P0 > 0.4) indicates the presence of a mesoporous structure. The BET specific surface area measured by N2 gas adsorption for MgAl-Ox was 137 cm3 g−1. The average pore radius was in the range of 3–5 nm. After combining with graphene, specific surface area of MgAl-Ox/G increased to 241 cm3 g−1, which improved the utility of MgAl-Ox for CDI. This is mainly because porosity of the MgAl-Ox/G increased after the calcination due to the release of

11



gases. The pore size showed a narrow distribution below 15 nm, and the average pore radius was in the range of 3–5 nm. MgAl-Ox/G was deemed to have been obtained by incorporating and assembling two-dimensional inorganic nanosheets on graphene with high conductivity. Electrochemical Properties Electrochemical properties of MgAl-LDH, MgAl-LDH/G, MgAl-Ox and MgAl-Ox/G have been shown in Figure S2. Almost symmetrical triangles are shown in galvanostatic charge/discharge curves of MgAl-LDH, MgAl-Ox, MgAl-LDH/G and MgAl-Ox/G. This exhibited that is obtained without obvious voltage drop related to the internal resistance during the changing of polarity that can be captured in the curves, which suggested the fast transmission of ions in the porous structure. The curve of MgAl-Ox and MgAl-Ox/G exhibited nearly ideal rectangle at the rate of 2 mV s-1, 5 mV s-1, 10 mV s-1, 20 mV s-1, 50 mV s-1, 100 mV s-1. This indicates the adsorption mechanism based on electric double-layer . CDI Tests The above results showed that MgAl-Ox and MgAl-Ox/G were prepared successfully. MgAl-Ox and MgAl-Ox/G were used as anodes in the following experiments. We evaluated the performance of these two materials by comparing their adsorption capacities and charge efficiencies. To investigate the modification of electrode materials to improve their desalination performance, we assembled three asymmetric CDI cells using original and modified electrode materials. The three pairs of electrodes were (1) MgAl-Ox versus AC, where MgAl-Ox was used as the anode 12


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electrode and AC was used as the cathode electrode (MgAl-Ox//AC); (2) MgAl-Ox versus AC-HNO3, where MgAl-Ox was used as the anode electrode and AC-HNO3 was used as the cathode electrode (MgAl-Ox//AC-HNO3); (3) MgAl-Ox/G versus AC, where MgAl-Ox/G was used as the anode electrode and AC-HNO3 was used as the cathode electrode (MgAl-Ox/G//AC-HNO3). In each single-pass CDI test, NaCl aqueous solution with an initial concentration of 500 mg L−1 at a constant rate of 10 mL min−1 was pumped into each CDI cell at room temperature. The CDI test of the asymmetric cell (MgAl-Ox//AC) was processed under the voltage of 1.0 V/0 V. The asymmetric cell exhibited no decrease in its adsorption capacity over 40 cycles, thereby demonstrating that the process could be repeated well (Figure 4a). By contrast, the adsorption capacity of the symmetric AC electrode decreased rapidly (Figure 4b), which indicated that the AC electrode did not have excellent cycling stability because of the oxidation at the anode via the electrochemical reaction of the carbon electrodes in the aqueous solution, leading to the formation of oxide layers at the carbon surface17, the result was the co-ion expulsion effect. Figure 4c shows the excellent stability of the hybrid electrodes compared with that of pure AC. For example, the number of cycles taken by the hybrid electrode in operating a CDI cell was maintained at 40 cycles while the capacity of the symmetric AC electrode decreased rapidly from 3.2 mg g−1 to 1.8 mg g−1. Oxidation reaction on AC anode caused capacity decline When AC was treated as anode, oxidation reaction occurred easily. However, positive charge exists on the surface of MgAl-Ox. So the oxidation reaction on the surface of MgAl-Ox occurred slowly. The asymmetric cell led to a significant improvement in performance stability for long-term operations owing to its surface charge, which contributed to the reduction of co-ion expulsion. MgAl-Ox, as the anode in the asymmetric cell, 13



enhanced anion adsorption due to its memory effect.

Our previous work provided a simple method to treat AC with nitric acid (AC-HNO3) to introduce carboxyl groups and thus effectively avoid the co-ion expulsion effect34,43. AC was treated in 6 M HNO3 at 65 °C for 5 h to obtain AC-HNO3. The treatment introduced carboxyl groups on the surface of the AC, which made the surface of the AC negatively charged. As the surface of MgAl-Ox was positively charged, the CDI test could be carried out under the supply voltage of 1.0 V/−1.0 V for the process of adsorption and desorption. Each process was conducted for 10 min, and other conditions were kept unchanged. The as-prepared asymmetric cell (MgAl-Ox//AC-HNO3) exhibited excellent cycling stability and adsorption capacity that was maintained around 5 mg g−1 after 40 cycles. The CDI cyclic pattern is shown in Figure S3a. For comparison, we prepared the AC//AC-HNO3 asymmetric cell for the CDI test. From the curves in Figure S3b, we can see that the shape of the curves makes a difference. Desorption happened before adsorption, thereby indicating the existence of the co-ion expulsion effect. This phenomenon proved the positive charge on the surface of MgAl-Ox. Graphene exhibits a perfect hybrid structure and good electrical conductivity. However, strong van der Waals forces exist in the laminar space, and they cause the materials to reunite easily. This effect greatly reduces the specific surface area and conductive performance of graphene44-46. To enhance the electrical conductivity of electrode materials, we combined graphene with MgAl-Ox in building a 3D structure that could benefit energy storage and electrosorption47,48. Intercalated MgAl-LDH 14


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prevented the restacking of the graphene nanosheets effectively. The prepared MgAl-Ox/G was used as the anode of the CDI cell, and AC was acidized by nitric acid to obtain the asymmetric cell (MgAl-Ox/G//AC-HNO3). The CDI performance and electrosorption capacity under the same test condition are shown in Figure 5a. Figure 5b depicts the adsorption/desorption curves for the tested materials over 12 consecutive cycles. The electrosorption capacity reached 13.6 mg g−1, which exceeded that of the MgAl-Ox electrode, and remained at 13 mg g−1 after 12 cycles. Hardly any change was observed from cycle to cycle, thereby suggesting that the electrode materials exhibited excellent regeneration capability and chemical stability. The comparison of the three asymmetric cells can be described as follows. The desalination curves of the three types of electrodes are shown in Figure 5c. When the adsorption voltage was applied, the conductivity of the NaCl solution decreased quickly and recovered gradually with the completion of adsorption. The same quick response was observed during the period of desorption. The excellent desalination performance of MgAl-Ox/G further established that MgAl-Ox/G offers great promise as an electrode material for CDI applications. Charge efficiency, which reflects energy losses, is one of the most important parameters to evaluate CDI electrode feasibility. Charge efficiency is an effective way to illustrate the double layer formed at the interface between the surface electrode materials and the solution. The charge efficiency of the MgAl-Ox/G//AC-HNO3 cell was 88.7% (MgAl-Ox//AC-HNO3: 78.5%; MgAl-Ox//AC: 50.5%), as shown in Figure 5d and calculated according to Eq. (2). The charge efficiency of the MgAl-Ox/G//AC-HNO3 cell was higher than 15



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that of the others because of the rational design of graphene and MgAl-Ox. The combination of MgAl-Ox and graphene resulted in a perfect 3D structure, enhanced electrical conductivity of the electrode, and reduced reunion of graphene. The high adsorption capacity and charge efficiency proved the positively synergetic interaction of MgAl-Ox and graphene. The test results showed the feasibility of using MgAl-Ox/G as an electrode in CDI. CDI Ragone plot, as a novel concept, can evaluate salt adsorption capacity in CDI49. The plot exhibits the deionization capacity (mg g-1), rate (mg g-1 s-1) and time (s). Figure 6a shows the CDI Ragone plot of MgAl-Ox//AC-HNO3 cell and MgAl-Ox/G//AC-HNO3 cell.

It

reflected

intuitively

that

compared

with

MgAl-Ox//AC-HNO3 cell, MgAl-Ox/G//AC-HNO3 cell exhibited a higher capacity and rate . Parameter Investigation In the study, we explored the influence of different test conditions on electrosorption capacity. MgAl-Ox/G was used as the anode material, and AC-HNO3 was used as the cathode material. Based on the as-prepared asymmetric electrode, a series of CDI tests was performed. To examine the desalination performance under different voltages, we conducted the CDI tests under the voltages of 0.6, 0.8, 1.0, 1.2, and 1.4 V. When the voltage was applied, the adsorption process began with the aid of the electric field. Figure 6b shows that the conductivity of the NaCl solution decreased with the application of the positive voltage. Furthermore, a large voltage equated to a considerable decrease 16


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in conductivity, which indicated a satisfactory desalination performance. However, when the voltage exceeded 1.4 V, the shape of the curve changed obviously. A shoulder peak appeared before the adsorption process, as observed in the curve. Excessive current accelerated the anodic oxidation reaction on the surface of the electrode and strengthened the co-ion expulsion effect, which mainly explained the appearance of the shoulder peak. Therefore, the working voltage should generally not exceed 1.2 V. When calcined in a certain temperature range, MgAl-LDH decomposed into corresponding mixed metal oxides by losing interlayer water, anion, and hydroxyl. As long as the structure was not damaged, the product retained its memory effect. When placed in an aqueous solution containing anionic material, the calcined products could regenerate to the original layered structure by absorbing anions into the interlayer. As the anode material, MgAl-Ox affected the desalination performance directly through its structure. The calcining temperature could affect the structure of MgAl-Ox as an important factor. A series of tests was conducted to calculate the desalination capacity of different products that were calcined under different temperatures. In the study, MgAl-LDH/G was prepared via the urea-hydrolyzed hydrothermal reaction. The as-prepared composites were calcined at 300 °C, 400 °C, 500 °C, 600 °C, and 800 °C and then labeled as MgAl-Ox/G-300, MgAl-Ox/G-400, MgAl-Ox/G-500, MgAl-Ox/G-600, and MgAl-Ox/G-800, respectively. These materials were used as the anode and assembled into CDI electrodes with AC treated with nitric acid. The tests were conducted in 500 mg L−1 NaCl solution under the 17



voltage of 1.0 V/−1.0 V. Figure 6c shows the desalination curves of the five materials. With the increase of temperature, the adsorption capacity increased first and then decreased, eventually reaching a peak (13.6 mg g−1) at 400 °C. As the temperature continued to rise, the desalination performance degenerated to a minimum at 800 °C. This change was caused by the transformation of the MgAl-Ox/G structure, which was achieved by changing the calcination temperature. The structure of the product was characterized by XRD and TEM to specifically analyze the influence of calcination at different temperatures. Figure S4 shows that when the calcination temperature ranges from 300 °C to 600 °C, two wide peaks appear in the XRD diffraction pattern. These peaks match well with the MgAl-Ox characteristic peaks of the standard compound. When the calcination temperature rises to 800 °C, these two wide peaks disappear, thereby indicating the change in the microstructure of the compound. The turning point of the calcination temperature is 400 °C. Thus, the microstructure of the products calcined at 300 °C, 400 °C and 500 °C should be investigated. When the calcination temperature was lower than °C, the hexagonal rings did not collapse, and their integrity was maintained (Figures S5a,b). When the temperature reached 500 °C, the hexagonal structure was no longer integral, and some fragments appeared (Figure S5c). The TEM images show that excessive temperature damaged the structure of the material. When placed in the aqueous solution containing anionic material, the products could no longer absorb anions. At the same time, the original layered structure could not be recovered. The desalination performance was improved greatly by modifying the electrode 18


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materials. First, the AC was treated with nitric acid to introduce carboxyl groups on the surface and thereby avoid the co-ion expulsion effect effectively. Second, the introduction of graphene enhanced conductivity. Previous test results exhibited that the introduction of graphene made an undeniable contribution to the increase of the adsorption capacity. The introduction of graphene enhanced the conductivity of the material and consequently benefited the charge migration and the formation of an electric double layer. To enhance the conductivity of the material, GO was reduced to graphene with a one-step hydrothermal method followed by calcination. Graphene, as a pervasive carbon material, features electrochemical stability, good conductivity, and quick charge and discharge response. The composition of graphene and MgAl-Ox results in a three-dimensional porous structure, which accelerates the mass transfer rate of electrolytes. However, the content of graphene can affect the desalination performance. To investigate this aspect, we adjusted the content of graphene by changing the amount of the added graphite oxide. Graphite oxide was added into a 100-mL solution with mass values of 0.05, 0.1, 0.2, and 0.3 g. These materials were used as the anode and then combined with the AC treated with nitric acid to assemble asymmetric CDI electrodes. The tests were conducted in 500 mg L−1 NaCl solution under the voltage of 1.0 V/−1.0 V. The adsorption curve indicated that the adsorption capacity increased first and then decreased with the increase of the content of graphene (Fig ure 6d). When the mass of the added graphite oxide was 0.2 g, superior desalination performance was achieved. The introduction of graphene enhanced the conductivity 19



Page 20 of 34

of MgAl-Ox. By contrast, an excessive amount of graphene can reunite and even impede the improvement of desalination performance. CONCLUSIONS We used a simple yet efficient strategy for the in situ assembly of MgAl-Ox nanosheets, which we then combined with graphene successfully. The as-obtained MgAl-Ox/G nanohybrids showed a high surface area, numerous channels for the access of anions, and high electrical conductivity. The introduction of graphene enhanced the conductivity and prevented the restacking phenomenon. The prepared MgAl-Ox/G nanohybrids were used as the anode of the CDI cell. AC treated with nitric acid was used as the cathode to effectively avoid the co-ion expulsion effect. The MgAl-Ox/G//AC-HNO3 cell

exhibited

excellent

cycling

stability

and

high

electrosorption capacity. The electrosorption capacity reached 13.6 mg g−1 and remained at 13 mg g−1 after 12 cycles. The charge efficiency of the asymmetric electrode reached 88.7%. A series of CDI tests was conducted to investigate the optimum electrode performance. The result showed that the effective working voltage was at the range of 0.6–1.2 V. Calcination at a high temperature transformed MgAl-LDH into MgAl-Ox. When calcined at 400 °C, which was the turning point of the calcination temperature, the compound maintained hexagonal rings integrally and exhibited optimal desalination performance. The introduction of graphene improved the adsorption capacity by enhancing conductivity. A series of MgAl-Ox/G was prepared by changing the content of graphene. Finally, we found that the product with 10% graphene exhibited the maximum adsorption capacity. 20


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ASSOCIATED CONTENT Supporting Information adsorption/desorption regeneration curves of MgAl-Ox//AC-HNO3 cell and AC// AC-HNO3 cell; XRD patterns of MgAl-Ox/G calcined at different temperatures; TEM images of MgAl-Ox/G calcined at different temperatures.

AUTHOR INFORMATION Corresponding Author Fax: +86 0411 87986024; Tel: +86 0411 87986024 * Email: [email protected] (G. Wang); [email protected] (J. Qiu) NOTES The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The authors acknowledge financial support from the Qaidam Salt Lake Chemical Joint Research Fund Project of the National Science Foundation of China and Qinghai Province State People's Government (No. U1507103), the National Science Foundation of China (No. 21336001).

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Figures Captions

Figure 1 (a) Schematic of the synthesis process of MgAl-Ox/G; (b) Schematic diagram of CDI; (c) Structure of CDI cell. Figure 2 TEM images of (a) MgAl-LDH; (b) MgAl-Ox; (c) MgAl-LDH/G and (d) MgAl-Ox/G; Figure 3

(a) XRD patterns of MgAl-LDH/G, MgAl-LDH, MgAl-Ox/G, and

MgAl-Ox; (b) TG curves of MgAl-LDH, MgAl-Ox/G, MgAl-LDH/G and MgAl-Ox; Nitrogen adsorption/desorption isotherms, with the inset showing the pore diameter distribution of (c) MgAl-Ox and (d) MgAl-Ox/G; Figure 4. Regeneration curves of (a) MgAl-Ox//AC cell and (b) AC//AC cell; (c) adsorption capacity of MgAl-Ox//AC cell and AC//AC cell. Figure 5 (a) Regeneration curves of MgAl-Ox/G//AC-HNO3 cell; (b) Adsorption capacity of MgAl-Ox/G//AC-HNO3 cell; (c) Adsorption/desorption curves and (d) current

curves

of

MgAl-Ox//AC

cell, MgAl-Ox

//AC-HNO3

cell,

and

MgAl-Ox/G//AC-HNO3 cell. Figure 6 (a) Kim-Yoon plot for average salt adsorption rate (ASAR) of MgAl-Ox //AC-HNO3 cell; Adsorption/desorption curves with different parameters (b) Voltage, (c) calcination temperature, (d) graphene content.

28


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Figure 1 (a) Schematic of the synthesis process of MgAl-Ox/G; (b) Schematic diagram of CDI; (c) Structure of CDI cell.

29



Figure 2 TEM images of (a) MgAl-LDH; (b) MgAl-Ox; (c) MgAl-LDH/G and (d) MgAl-Ox/G.

30


Page 30 of 34

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Figure 3

(a) XRD patterns of MgAl-LDH/G, MgAl-LDH, MgAl-Ox/G, and

MgAl-Ox; (b) TG curves of MgAl-LDH, MgAl-Ox/G, MgAl-LDH/G and MgAl-Ox; (c) Nitrogen adsorption/desorption isotherms of MgAl-Ox and (d) MgAl-Ox/G, with the inset showing the pore diameter distribution.

31



Page 32 of 34

Figure 4 Regeneration curves of (a) MgAl-Ox//AC cell and (b) AC//AC cell; (c) adsorption capacity of MgAl-Ox//AC cell and AC//AC cell.

Figure 5 (a) Regeneration curves of MgAl-Ox/G//AC-HNO3 cell; (b) Adsorption capacity of MgAl-Ox/G//AC-HNO3 cell; (c) Adsorption/desorption curves and (d) current

curves

of

MgAl-Ox//AC

cell, MgAl-Ox

MgAl-Ox/G//AC-HNO3 cell. 32


//AC-HNO3

cell,

and

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Figure 6 (a) Kim-Yoon plot for average salt adsorption rate (ASAR) of MgAl-Ox //AC-HNO3 cell; Adsorption/desorption curves with different parameters (b) Voltage, (c) calcination temperature, (d) graphene content.

33



TOC

Porous MgAl oxide/graphene hybrids (MgAl-Ox/G) exhibited high capacity and superior cycling stability because of the memory effect and oxidation resistant property.

34


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Graphene Hybrids for

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