Fabrication of Anion-Exchange Polymer Layered Graphene

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Fabrication of Anion-Exchange Polymer Layered GrapheneMelamine Electrodes for Membrane Capacitive Deionization Xiaoyu Gu, Yonghong Deng, and Chaoyang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01685 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 12, 2016

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ACS Sustainable Chemistry & Engineering

Fabrication of Anion-Exchange Polymer Layered Graphene-Melamine

Electrodes

for

Membrane

Capacitive Deionization Xiaoyu Gu,† Yonghong Deng,*, ‡ and Chaoyang Wang*,†

†

Research Institute of Materials Science, South China University of Technology, No. 381

Wushan Road, Tianhe District, Guangzhou 510640, China ‡

Department of Materials Science & Engineering, South University of Science and Technology

of China, No. 1088 Xueyuan Road, Xili, Nanshan District, Shenzhen 518055, China

*To whom correspondence should be addressed. E-mail: [email protected] (CY Wang); [email protected] (YH Deng)

KEYWORDS: Graphene, N-doped, Anion-exchange, Membrane capacitive deionization, Melamine

ACS Paragon Plus Environment

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ABSTRACT A novel nitrogen-doped reduced graphene sponge composite (NRGS) is fabricated by using melamine sponge to restrain the aggregation of graphene sheets during reduction. The anion-exchange polymer layered graphene composites (A-NRGS) are prepared by coating the surface of NRGS electrode with cross-linked poly(vinyl alcohol) with quaternization modification (C-qPVA). With the help of melamine sponge to suppress the agglomerate of graphene sheets, the NRGS exhibits a unique three dimensional (3D) interconnected porous structure with abundant nitrogen doping of 5.2%. Its specific surface area is up to 241 m2/g. In addition, the enhanced wettability of A-NRGS composites favors the diffusion of ion from the electrolyte to electrode. Therefore, A-NRGS composites have excellent electrochemical capacity (184 F/g). The membrane capacitive deionization (MCDI) performance for A-NRGS electrode (11.3 mg/g) is higher than that of pristine reduced graphene oxide (RGO) (6.2 mg/g) and NRGS (8.6 mg/g) electrodes. All the results demonstrate that A-NRGS composites can be a promising candidate for CDI and other electrochemical applications.

Entry for the Table of Contents

NRGS

Anion-exchange polymer layered graphene-melamine electrodes were facilely fabricated for GO-Sponge

PMF Sponge

membrane capacitive deionization.


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

INTRODUCTION Fresh water supply has been an impending challenge in modern civilization, as water pollution

and contamination become increasingly serious due to the rapid development of modern industry.1,2 Desalination of seawater is an effective ways to produce safe drinking water in order to solve water crisis. Capacitive deionization (CDI) is an electrosorption water treatment based on the formation of the electrical double layers (EDL), which has drawn extensive attention in the water treatment processes because of its energy saving, non-polluting, and environmental benignity.1,3 At the electrolyte-electrode interface, ionic species with opposite charges are electrosorbed to perforated electrodes under direct electric fields.4 Membrane CDI (MCDI) is a specific CDI technology by introducing ion-exchange membranes into the CDI electrodes, can effectively minimized

the

desorption

due

to

the

electrostatic

interaction

between

opposite-charged ions in the solution.5 Many researches have demonstrated that the electrosorptive capacity (qe) of MCDI is superior to CDI without the impact of co-ions effect, which also means that MCDI is more energy efficiency.6,7 However, the electrical resistivity might increase significantly because of existing of the interfaces between the membranes and electrodes, which would influence the diffusion of ions and limit the total ion adsorption amount.8 To overcome this problem, Choi’s group reported a layered carbon electrode with ion-exchange resins for MCDI application. The resistance between the membranes and carbon electrodes decreased remarkably. It demonstrated that qe of this membrane-modified carbon electrode was improved 19%.9 In relation to this working principle, the electrodes made up of porous carbon materials with high surface area and appropriate pore size distribution would be an ideal material in the CDI applications. Therefore, porous carbon materials,10-17 carbon nanotube composites,18-25 and


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graphene,26-29 were intensively investigated. Among the carbon-based species, graphene with outstanding electrical, mechanical, optical, and photovoltaic properties has been proved to have excellent CDI performance.26-29 However, the aggregation of graphene during the reduction process of graphene oxide (GO) would generate the low conductivity and specific surface area.30-32 To date, different strategies were applied to restrain the aggregation of graphene and improve the CDI performance.33-36 Yan and Sun’s group reported a novel sponge (polyurethane, PU)-templated graphene-based CDI electrode with high surface area.37 Our group has fabricated 3D graphene/metal oxide hybrids,38 graphene/conducting polymers composites,39 3D macroporous graphene architectures,40 and mesoporous graphene sheets,41 to be used as CDI electrodes. In this work, we have prepared nitrogen-doped reduced graphene sponge composite (NRGS) based on poly(melamine-formaldehyde) (PMF) sponge-template, the surface of which is covered by quaternized poly (vinyl alcohol) (PVA) for membrane capacitive deionization. Unique graphene macroporous structure throughout the entire melamine sponge is well-preserved after the annealing process, and its electrochemical capacity is enhanced due to the contribution of nitrogen doping. More importantly, the obtained graphene composite exhibits excellent electrosorption capacity of 11.3 mg/g in MCDI application. The detailed preparation process of NRGS electrode for CDI applications is illustrated in Figure 1.


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NRGS Electrical power + PMF Sponge Graphite paper

Conductivity meter

GO-Sponge Outlet

Inlet Anion Cation

NRGS Anion exchange Pump

membrane

Figure 1. Schematic diagram of the fabrication of NRGS electrodes for CDI applications.



EXPERIMENTAL SECTION Materials. PVA, (2,3-epoxypropyl) trimethylammonium chloride (EPTMAC) with purity ≥

95%, graphite powder, were obtained from J&K Chemical (China). PMF sponges were provided by Guangzhou GreenCARE Co. Ltd. (China). All other chemicals were of analytical grade and used as received. Water with resistivity ≥ 18.0 Mcm was used in all experiments. Preparation of NRGS Composites. GO was produced from graphite according to our previous work.41 GO aqueous dispersion (2 mg/mL) was formed by ultrasonication for 2 h. PMF sponge was soaked into the dispersion to absorb GO sheets. To make GO distribute well in the PMF sponge, the soaking-extruding process were repeated more than 5 times.42 The sponge then was dried in vacuum for 12 h to remove water. Different contents of GO sheets in the sponges


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can be tuned by increasing the number of the adsorption-drying cycle.43 The GO contents can be calculated by weight difference the sponges before and after GO adsorption. Finally, NRGS composite was obtained after the GO-sponge was annealed from 30 to 800 °C with a heating rate of 3 oC/min under argon atmosphere. The samples are denoted by GO-S-n and NRGS-n before and after annealing, respectively, where n represents the number of the adsorption-drying cycle. Unless specifically noted, GO-S refers to GO-S-3 and NRGS refers to NRGS-3, respectively. Pristine reduced graphene oxide (RGO) was also fabricated independently without the PMF sponge template.

Fabrication of NRGS and A-NRGS Electrodes. NRGS composite (80 wt%), acetylene black (10 wt%), and polytetrafluoroethylene (PTFE, 10 wt%) were ground for 2-3 h. The NRGS electrode was obtained by casting and pressing the grinding mixtures onto a nickel foam plate. In this work, A-NRGS electrode was prepared by casting cross-linked quaternized PVA (C-qPVA) solution onto NRGS electrode, which provided anion-exchange functional groups and acted as an anion-exchange membrane. Therefore, it is an asymmetric CDI system that NRGS electrode has no cation-exchange membrane. qPVA was synthesized and cross-linked by glutaraldehyde (GA) according to a previously reported method.44 The detailed experimental procedures can be found in Support Information (SI). The degree of quaternization (Dq) of qPVA and the degree of cross-linking (Dc) of C-qPVA can be calculated according to Equations S1 and S2.


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Electrochemical measurements. To investigate the electrochemical performance of the NRGS and A-NRGS electrode, galvanostatic charge/discharge (GC) tests, cyclic voltammetry (CV) analysis, and electrochemical impedance spectroscopy (EIS) were carried out in NaCl aqueous solutions (1 M). The pH value of aqueous electrolyte is neutral. All electrochemical experiments were measured using a three-electrode cell system and more detailed information of CV, GC, and EIS tests can be found in Supporting Information. The three-electrode method was usually used in the electrochemical tests about CDI study. The reference electrode has a very stable potential, which can guarantee the stability of potential in the working electrode. Maybe it is appropriate to use the three-electrode cell system to prevent polarization.

Electrosorptive capacity measurement. The CDI experiments of A-NRGS were performed in a continuous recycle system as described in our pervious works.39,40 The system consists of a CDI cell, a constant flow pump (Longer BT100-1L), a potentiostat (RS1302DQ), a conductivity meter (DDS 307) and a water tank, as shown in Figure S1. The neutral NaCl solution of 150 mL was pumped into the CDI cell by the constant flow pump and the flow rate was maintained at 10 mL/min. The concentration and conductivity calibration curve of NaCl was shown in Figure S2. The electrosorptive capacity (qe, mg/g) and salt removal efficiency (η) of the electrodes were obtained with Equations 1 and 2, respectively:45,46

qe 

 C 0  Ce V m

(1)

where C0 (mg/mL) and Ce (mg/mL) are the initial and final NaCl concentrations, respectively. V


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(mL) is the total volume of the NaCl solution and m (g) is the total mass of the electrodes.

  % 

C 0  Ce 100 C0

(2)

Characterization. The morphology of the obtained materials was observed by scanning electron microscopy (SEM, Zeiss EVO18) and the operating voltage is 10 KV. Wide-angle X-ray diffraction (XRD) tests were measured on an X’pert PRO diffractometer with a Cu Kα radiation (wavelength 0.154 nm). Fourier-transform infrared spectroscopy (FT-IR) was applied on a spectrometer (NICOLET 6700). The specific surface area and pore size distribution were calculated from the N2 adsorption-desorption isotherms (NovaWin2, Quantachrome). X-ray photoelectron spectroscopy (XPS) was examined by a theta probe spectroscope (ESCA Axis Ultra DLD).

 RESULTS AND DISCUSSION Morphology of NRGS. To minimize the restacking effects inside graphene sheets during preparation, unique graphene macroporous structure throughout the entire melamine sponge has been well-preserved. Figure 2a-d shows the photographs of PMF and GO-absorbed sponges with different dipping and drying processes. We can see that PMF sponge shows a general trend of colour from white to dark while the size and shape of GO-absorbed sponges maintain well after three dipping processes. From SEM image of GO-S in Figure 2e, GO sheets are uniformly distributed in the gap of PMF sponge. The


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sponge-shaped graphene block is obtained by the carbonization of the PMF sponge and the thermal reduction of GO into RGO after annealing, as displayed in Figure 2f. Different from polyurethane (PU)-templated graphene composites with a complete pyrolysis, NRGS exhibits macroporous structure with well-preserved PMF sponge framework without seriously deformation by the carbonization treatment.37 The surface of the sponge is covered by flexible graphene sheets from Figure 2g. The well-preserved 3D PMF sponge network would suppress the agglomeration of graphene, leading to faster ion diffusion and an effective double-layer capacitance.

a

c

b

d

e

f

100 μm

20 μm

2 μm

h

i

j

g

CA=130o

CA=35o

30 μm

Figure 2. (a-d) Photographs of PMF sponges with different times of the dipping-drying process. From left to right: PMF, GO-S-1, GO-S-2 and GO-S-3 composites. SEM photos of (e) GO-S-3,


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and (f,g) NRGS-3 composites with different magnifications. (h) SEM image of A-NRGS-3 electrode. Contact angle images of water droplets on (i) NRGS-3 and (j) A-NRGS-3 electrodes coated with C-qPVA polymer.

Herein, A-NRGS electrodes are successfully employed as electrodes for membrane capacitive deionization by the introduction of positively charged quaternized poly (vinyl alcohol) groups. The synthesis chemical reaction equation for cross-linked qPVA (C-qPVA) is shown in Scheme S1. According to the Dq calculated by Equation S1, it is found that samples qPVA-3 has the largest Dq value as shown in Table S1. It is because that KOH plays a role in the quaternization of PVA by opening the epoxy bonding. However, KOH crystals would be generated on the surfaces of the qPVA membrane due to the existence of excessive KOH.44 The hydrophilicity of PVA increases by introducing quaternary ammonium groups and PVA may swell seriously in water. Therefore, GA is used to cross-link the qPVA membrane to improve its stability in the aqueous solution. Table S2 shows the Dc of C-qPVA, and the weight ratio of qPVA keep 4 wt% in all conditions. We can find that sample C-qPVA-1 has the largest Dc value when the weight ratio of GA is 0.4 wt%. However, hydrophilicity of the C-qPVA membranes with excess GA content would decrease due to the consumption of quaternary ammonium groups. It is found that C-qPVA-2 membrane with Dc value of 4.97 does not dissolve after being immersed into a water bath for 24 h. It suggests that the crosslinking of qPVA and GA has occurred, and C-qPVA-2 membrane with 4.97 Dc value is strong enough to be an ion-exchange membrane for MCDI performance.


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As demonstrated in Figure 2h, the coating layer shows high adhesion to the NRGS electrode. The C-qPVA membrane is uniform and the membrane thickness is approximate 20 μm. It is known that the surface wettability always affect the CDI performance. As depicted in Figure 2i and 2j, the NRGS electrode has a contact angle of 130°, which is much larger than that of the A-NRGS electrode (35°). The enhanced wettability of the A-NRGS electrode can be ascribed to the increasing hydrophilicity of C-qPVA with introducing quaternary ammonium groups, which can increase the electrolyte diffusion rate and the contact area of the solvent.47 Therefore, the A-NRGS electrode would have a higher CDI performance.39 As shown in Figure 3a, the residual weight of PMF sponge is 16.04% due to the unique chemical structure compared to PU sponge. We can see that the TG curves of GO-S-n composites are similar to those of GO and the PMF sponge, and the residual weight of Gr-S-n composites increase with the increased number of dipping process. According to the loading calculation, the GO loading in GO-S-n composites is 118.7, 141.4, 147.1%, respectively. It also can be confirmed by the SEM images of GO-S-n composites in Figure S3a-c, which show that the amount of GO sheets anchored in PMF network obviously increases with increasing times of the dipping processes. From Figure S3d-S3f, graphene is uniformly distributed in the PMF sponge network and porous NRGS network is maintained well with a slightly shrinkage due to calcination. As depicted in Figure 3b, GO-S-3 composites exhibit (001) peak at 2=10.3o and a broad peak at 22.5o for the PMF sponge.48 Both of the pristine RGO and NRGS-n composites show a similar XRD pattern of (002) peak at 2=25o, indicating that the embedding of the PMF sponge has no


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significant influence on the hierarchical structure of NRGS composites and most of the oxygen-containing groups have been removed. It can be also proved from the FT-IR observation in Figure S4, which shows that peaks located at 800 cm-1 corresponded to the presence of triazine-based groups in NRGS composites disappear.49 Therefore, it is proved that the graphene-based NRGS composites are generated after the carbonization of the PMF sponge. 100

a

b

80 60

GO GO-S-1 GO-S-2 GO-S-3 PMF

40 20 0

46.36%

16.04%

100 200 300 400 500 600 700 800 o Temperature ( C)

10

c

d Intensity (a.u.)

Intensity (a.u.)

GO-S-3 NRGS-1 NRGS-2 NRGS-3 RGO

Intensity (a.u.)

Weight (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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GO RGO GO-S-3

20

30 40 50 2 (degree)

60

70

NRGS-3 N 1s Pyridine N Graphitic N Pyrrole N

NRGS-3

0

200 400 600 Binding energy (eV)

800 406

404

402 400 398 396 Binding energy (eV)

394

Figure 3. (a) TG curves of PMF, GO and GO-S. (b) XRD patterns of RGO, GO-S-3 and NRGS. (c) XPS spectra of the samples. (d) N 1s peaks of XPS spectrum of NRGS-3.

XPS measurements are carried out to analyze the surface chemistry of the samples to confirm the effective nitrogen doping into graphene after pyrolysis. The XPS wide spectra of all samples are evaluated by deconvolution of the C 1s, O 1s, and N 1s core elements level. Obviously in


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Figure 3c, only the signal of oxygen and carbon atoms appears while no N signal can be observed from GO and RGO in the XPS spectra. Compared with GO-S-3, a more strong N1s peak of NRGS is centered at 398.5 eV (5.2 at.% N), while the oxygen content greatly reduce (9.6 at.% O). More detailed information about atomic percentage can be found in Table S3. Three peaks at 398.5, 399.1 and 401.3 eV can be derived from the N 1s spectrum of the NRGS-3 (Figure 3d), which are attributed to pyridine nitrogen, graphitic nitrogen and pyrrole nitrogen, respectively.49-50 From the results of XRD, FT-IR and XPS, it is concluded that nitrogen doped graphene have been successfully prepared by annealing, as well as reduction of GO and carbonization of PMF sponge. Pore size and distribution, surface area are important for electrode composites.51 The porous structure of the samples is also investigated by a N2 adsorption-desorption measurement. Figure 4 shows the N2 adsorption isotherms, pore size and distribution of RGO, NRGS-3 and GO-S-3. The isotherms of the samples have a representative type-IV profile with a H3-type hysteresis loop, indicating that the mesoporous structure of the tested composites.52 The specific surface area of NRGS-3 and GO-S-3 is 241 and 154 m2/g, respectively, which is much larger than that of RGO (120 m2/g). Detailed results about the porous structure are listed in Table S4. The PMF-sponge template is a versatile approach to fabricate graphene electrodes with large specific surface area by preventing the agglomeration of graphene sheets during reduction.


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3

3

3

150

0.04 0.02 0.00 0

10

20

Pore Diameter (nm)

100

30

50

RGO

0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P0)

3

200 150

3

250

dV/dR (cm /g/nm)

300

3

400

dV/dR (cm /g/nm)

dV/dR (cm /g/nm)

0.06

Volume adsorption (cm /g STP)

a

0.08

200


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



300 200

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b

0.09 0.06 0.03 0.00 0

10

20

30

Pore Diameter (nm)

100 NRGS-3 0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P0)

c

0.06 0.04 0.02 0.00 0

10

20

Pore Diameter (nm)

30

100 50

GO-S-3

0 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P0)

Figure 4. N2 adsorption isotherms, pore size and distribution (inset) of (a) RGO, (b) NRGS-3 and (c) GO-S-3.

Electrochemical Performance. First, CV tests are carried out because they are strongly sensitive to the CDI properties of the fabricated graphene electrodes. From Figure 5a, the current response and CV enclosed area of all A-NRGS-n electrodes are larger than those of the RGO electrode, demonstrating that A-NRGS-n electrodes have larger specific capacitance and better electrosorption performance. Figure S5 shows CV curves of A-NRGS-n, NRGS-n (n=1, 2, 3) and RGO electrodes. The potential window is from -0.5 to 0.5 V. CV curves of A-NRGS-n electrodes remain almost rectangular and no Faradaic reaction is observed with the scan rate varying from 5


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to 50 mV/s, which suggests that the CV behavior belongs to the ideal capacitive electrical double layer.53 Compared with the NRGS-3 electrode, A-NRGS-3 with positively charged ion-exchange membrane displays a more symmetry rectangular CV curve, as shown in Figure 5b. The improved electrochemical performance with highly reversible capacity is attributed to the reasons as following: (1) The macroporous structures and large accessible surface area increase the specific capacitance with the help of PMF sponge template, which means that more ions would be eliminated by A-NRGS electrodes at the same direct voltage; (2) A-NRGS electrodes have high wettability and hydrophilicity by introducing C-qPVA polymer on the surface of NRGS electrode, which can favor the electrode/electrolyte interactions and facilitate the transport of electrons and ions; (3) Since the electronegativity of C is weaker than that of N, doping of graphene sheets with N heteroatoms can significantly improve the capacitive performance of A-NRGS electrodes due to the pseudofaradic reactions involving the surface active groups. 54,55 The GC curves of the A-NRGS-3 electrode at various current densities are shown in Figure 5c. We can find the GC curves of A-NRGS-3 electrodes are linear and symmetric and the potential drop (iR drop) is low. It indicates that the A-NRGS-3 electrodes have an excellent stability in the charge/discharge process. The specific capacitance of A-NRGS-3, A-NRGS-2, A-NRGS-1 electrodes calculated from the GC curve in Figure 5d by Equation S3 are 184, 155, 138 F/g, respectively, while the specific capacitance of RGO is 76 F/g. Under the same electrochemical conditions, longer GC time of A-NRGS electrodes means a higher ion storage capacity, which is attributed to the enlarged specific surface area and abundant nitrogen doping mentioned above.


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As depicted in Figure 5e, the iR drop is lower at a small current density. It increases gradually with increasing the current densities. Furthermore, the iR drop of the A-NRGS-3 electrode is much lower than that of the NRGS-3 and pristine RGO electrodes at the same current density. It

d

0.2 0.0

-0.2

0.4

0.0

0.24

A-NRGS-3 NRGS-3 -0.4

-0.2 0.0 0.2 Potential (V)

200 Time (s)

300

-0.4 0

100

0.18 0.12

0.00

0.0

80

A-NRGS-3 NRGS-3 RGO

0.06 100

0.2

0.4

e

-0.4 0

1.5 A/g 1 A/g 0.8 A/g 0.4 A/g 0.2 A/g

-0.2

-0.4

-0.8

c

0.5 1.0 Current density (A/g)

1.5

200

800

10

f

8 6 4 2

60

0 0 2 4 6 8 10 12 14 z' (Ohm)

40

A-NRGS-3 A-NRGS-2 A-NRGS-1 RGO

20 0 0

400 600 Time (s) -z'' (Ohm)

0.4

-0.4 -0.2 0.0 0.2 0.4 0.6 Potential (V) A-NRGS-3 A-NRGS-2 A-NRGS-1 RGO

b Potential (V)

A-NRGS-3 A-NRGS-2 A-NRGS-1 RGO

-0.5

0.4

-z'' (Ohm)

0.0

Current density (A/g)

a

0.5

iR drop (V)

Current density (A/g)

means that the overall resistance reduces significantly.56

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

40

60 80 z' (Ohm)

100

120

Figure 5. (a) Comparative CV curves of A-NRGS-n (n=1, 2, 3) and RGO electrodes at 10 mV/s. (b) Comparative CV curves of A-NRGS-3 and NRGS-3 at 10 mV/s. (c) GC curves of A-NRGS-3 electrodes at different current densities. (d) GC curves of RGO and A-NRGS electrodes with a 1 A/g current density. (e) The iR drop as function of discharge current density for the A-NRGS-3, NRGS-3, RGO electrodes. (f) Nyquist profiles of the A-NRGS-n (n=1, 2, 3) and RGO electrodes. The inset is the plots in the high frequency region.

The above results are also consistent with the Nyquist profiles of EIS spectra, which can be seen in Figure 5f. The slope for A-NRGS-n electrodes at low frequency region is obviously


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higher than that of RGO electrodes, which indicates better capacitive property and faster ionic diffusion into the macroporous graphene composites for A-NRGS-n electrodes. The equivalent series resistance of A-NRGS-3 electrode (Rs, the intersection of Nyquist profiles with the real axis) can be calculated to be 0.03 Ω, much smaller than that of RGO (5.46 Ω), which suggest good conductivity. In addition, the negligible semicircle of A-NRGS-3 electrode indicates low interfacial electron-transfer resistance due to the high wettability of the C-qPVA treated graphene electrode.57 All the results discussed above indicate that the A-NRGS-n electrodes with excellent electrochemical performance are more appropriate as CDI electrode materials in comparison with the pristine RGO and NRGS electrodes. CDI performance. In order to investigate the electrosorption behaviors and selectivity of the anion-exchange layer of A-NRGS electrodes, the MCDI test is measured in the 150 mL NaCl solution under 1.6 V. The initial conductivity is 300 µS/cm. To compensate for the resistance of the CDI system, the potential difference in this test exceeds the threshold of 1.23 V.58 No gas bubbles were observed at the voltage of 1.6 V and no hydrolysis happens in all cases. So, all CDI properties are measured at 1.6 V.40 The electrochemical test and CDI test have some differences. The potential difference of 1.6 V in electrosorption experiments exceeds the threshold of 1.23 V to compensate for the circuit system resistance, while -0.5-0.5 V is an appropriate potential window for graphene electrodes to measure the electrochemical performance. From Figure 6a, the conductivity would decline when all graphene-based electrodes is applied with the voltage, indicating a fast ionic adsorption. The conductivity decreases slowly until no change is observed any further with increasing time, which indicate that saturation is achieved because of the mutual


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repulsion among the ions adsorbed in the electrodes.45 Obviously, the A-NRGS electrodes have larger conductivity change compared to pristine RGO and NRGS electrodes. The electrosorptive capacity of A-NRGS is calculated to be 11.3 mg/g from Equation 2, which is much higher than that of 8.6, 6.2 mg/g for NRGS and RGO, respectively. As discussed above and other reports, the reason of A-NRGS electrode with higher adsorption amount can be attributed to the macroporous structure, abundant nitrogen doping and the selective shielding of the membrane. In comparison with RGO, A-NRGS have protection from PMF sponge during the annealing process, which leads to successful nitrogen doping at the same time. These provide fine surface accessibility which facilitates the A-NRGS electrodes to easily adsorb the ions from the salt solution. On the other hand, the co-ion effect is effectively eliminated in MCDI because cations can be blocked in the bulk solution with the existence of the anion-exchange membrane.5 Therefore, compare to the CDI performance of RGO and NRGS electrode, the salt removal efficiency of A-NRGS can be significantly enhanced.


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a

A-NRGS NRGS RGO

290

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280 270 0

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60 80 100 120 140 Time (min) Electrosorption capacity Removal efficiency

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A-NRGS-3

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0

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3 Cycle

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Removal efficiency (%)

Conducivity (s/cm)

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Electrosorption capacity (mg/g)

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Electrosorption capacity (mg/g)

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12 8

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4 0

0

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400 600 800 1000 Conductivity (s/cm)

0

Figure 6. (a) CDI performance of the RGO, NRGS, and A-NRGS electrodes at 1.6 V. (b) Regenerated electrosorptive capacity of A-NRGS electrode. (c) The desalination efficiency and electrosorption capacity of the A-NRGS electrode as function of the conductivity of the initial NaCl solutions.

The workable electrodes with a good reversibility are important to the long life for the actual CDI application. To investigate the stability and reversibility of the A-NRGS electrodes, consecutive electrosorption and regeneration experiments are carried out by turning off the voltage of the cell. The regeneration cycles of the E-Gr-Fe3O4 electrodes with the absence of an external potential can be found in Figure S6. From Figure 6b, the regenerated electrosorptive capacities of A-NRGS are 11.3, 10.7, 10.3, 9.7 and 9.3 mg/g for the five times, respectively.


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There is little change in the MCDI performance, so the good repeatability of A-NRGS electosorption can be realized with free secondary pollution and free driving energy. The negligible decline during the operation cycle could have been caused by the incomplete desorption: a small portion of adsorbed ions remain inside the porous electrode in the desorption process, leading to higher ionic density at the surface of the electrodes. 59 On the other hand, active sites for ion adsorption also decrease due to consumption of the Faradaic reactions between functional groups on the A-NRGS electrode.15 So the A-NRGS electrodes have a better desalination performance. The number of adsorbed ions would gradually decease because of the limited ion adsorption capacity of the electrode.60 The influence of the initial salt concentrations on the electrosorption capacities of the A-NRGS electrode is shown in Figure 6c. The electrosorption capacities are 8.07, 11.3, 12.8, 14.1, 14.8 mg/g and the desalination efficiency are 23.4, 11, 7.4, 5.1 and 4.3% with increasing the initial NaCl concentrations from 100, 300, 500, 800 to 1000 s/cm, respectively. Apparently, the total electrosorption capacity is enhanced and the desalination efficiency is reduced with increasing the ion concentrations.46 The cause is that the overlapping influence of the EDLs on the decrease of micropores for the ionic resistance at the high ion concentration, and therefore electrosorptive capacities increase.61 On the other hand, the excessive ion would not be electro-adsorbed by the vacancy at the high ion concentration, which are inversely proportional to the removal efficiency.62 In fact, A-NGRS composites prepared in this paper has a relatively higher capacitance and electrosorption capacity than other graphene-based electrodes which are used in CDI performance63-65. However, limited to the existing testing conditions, we cannot


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obtain the corresponding current response during CDI experiments. Therefore, we cannot calculate the current efficiency. In addition, the electrosorptive capacity of A-NRGS is much higher than that of NRGS and RGO, which further convincingly confirm the selective shielding of the membrane since cations can be blocked in the bulk solution with the existence of the anion-exchange membrane. In fact, A-NGRS composites prepared in this paper has a relatively higher capacitance and electrosorption capacity than other graphene-based electrodes which are used in CDI performance63-65.

 CONCLUSIONS Graphene composite NRGS was prepared on the basis of the PMF sponge-template, which could restrain the aggregation of graphene sheets during reduction, and lead to successful nitrogen doping after pyrolysis. Then a novel nitrogen-doped A-NRGS composite was fabricated, the surface of which was coated by C-qPVA polymer as anion-exchange membrane for MCDI application. The synergistic effect from a combination of sponge and C-qPVA polymer endowed A-NRGS with excellent electrochemical performance because of high surface surface area, abundant nitrogen doping, increased hydrophilicity and reduced co-ion effect. Compare with RGO and NRGS electrode, the obtained A-NRGS composite exhibits excellent electrosorption capacity of 11.3 mg/g in MCDI application, as well as good cycling stability and regeneration performance. The results demonstrate that A-NRGS composites not only are promising candidates in CDI application, but also can be applied in a variety of applications like energy storage s and supercapacitors.


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

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ASSOCIATED CONTENT

Supporting Information Schematic of the CDI device, SEM photos of GO-S, FT-IR spectra of GO, GO-S, NRGS and PMF, CV curves of A-NRGS and RGO electrodes. This material is available free of charge via the Internet at http://pubs.acs.org. 

AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] (CY Wang); [email protected] (YH Deng). Notes The authors declare no competing financial interest. 

ACKNOWLEDGMENTS

CW thanks the fund supporting of NSFC (21274046 and 21474032). 

REFERENCES

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Entry for the Table of Contents

Fabrication of Anion-Exchange Polymer Layered Graphene-Melamine Electrodes for Membrane Capacitive Deionization Xiaoyu Gu, Yonghong Deng,* and Chaoyang Wang*

GO-Sponge

PMF Sponge

NRGS

Anion-exchange polymer layered graphene-melamine electrodes were facilely fabricated for membrane capacitive deionization.


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Fabrication of Anion-Exchange Polymer Layered Graphene

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