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Polymer dehalogenation-enabled fast fabrication of N, S-codoped carbon materials for superior supercapacitor and deionization applications Yingna Chang, Guoxin Zhang, Biao Han, Haoyuan Li, Cejun Hu, Yingchun Pang, Zheng Chang, and Xiaoming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08181 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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ACS Applied Materials & Interfaces

Polymer dehalogenation-enabled fast fabrication of N, S-codoped carbon materials for superior supercapacitor and deionization applications Yingna Chang,a Guoxin Zhang,a,d,* Biao Han,a Haoyuan Li,a Cejun Hu,b Yingchun Pang,a Zheng Chang,a,* and

Xiaoming Suna,b,c

a. State Key Laboratory of Chemical Resource Engineering, College of Science, Beijing University of Chemical Technology, Beijing, 100029, China. Email: [email protected] b. College of Energy, Beijing University of Chemical Technology, Beijing, 100029, China. c. Beijing Advanced Innovation Centre for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China. d. College of Electrical Engineering and Automation, Shandong University of Science and Technology, Qingdao 266590, China. Email: [email protected]

KEYWORDS.

Dehalogenation;

carbon

materials;

codoping;

supercapacitor;

capacitive deionization

ABSTRACT. Doped carbon materials (DCM) with multiple heteroatoms hold broad interests in electrochemical catalysis and energy storage but require several steps to fabricate, which greatly hinder their practical applications. In this study, a facile 1 ACS Paragon Plus Environment


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strategy is developed to enable the fast fabrication of multiply doped carbon materials via room-temperature dehalogenation of polyvinyl dichloride (PVDC) promoted by KOH with the presence of different organic dopants. A N, S-codoped carbon material (NS-DCM) is demonstratively synthesized using two dopants (dimethylformamide for N doping and dimethylsulfoxide for S doping). Afterwards, the precursive room-temperature NS-DCM with intentionally overdosed KOH is submitted to inert annealing to obtain large specific surface area and high conductivity. Remarkably, NS-DCM annealed at 600 °C (named as 600-NS-DCM), with 3.0 at.% N and 2.4 at.% S, exhibits a very high specific capacitance of 427 F g-1 at 1.0 A g-1 in acidic electrolyte and also keeps ~60% of capacitance at ultrahigh current density of 100.0 A g-1.

Furthermore,

capacitive

deionization

(CDI)

measurements

reveal that

600-NS-DCM possesses a large desalination capacity of 32.3 mg g-1 (40.0 mg L-1 NaCl), and very good cycling stability. Our strategy of fabricating multiply doped carbon materials can be potentially extended to the synthesis of carbon materials with various combinations of heteroatom doping for broad electrochemical applications.

1. Introduction Doped carbon materials (DCM) have gained tremendous attention due to their broad applications in electrochemical energy conversion1-7 and storage8-14. Because of their low cost and high performance, DCM hold great potentials in practical applications. Meanwhile, their highly controllable surface properties enabled by heteroatom doping and functionalization have brought about nearly unlimited 2 ACS Paragon Plus Environment

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opportunities to continuously enhance their electrochemical performance.15 For instance, a N-doped ordered carbon material developed by Huang et al shows a capacitance as high as 855 F g-1, and consequently, it delivers a very high specific energy of 41.0 Wh kg-1 in aqueous electrolyte, which is highly comparable to these lead-acid batteries.12 As proved by many studies, heteroatom doping has the capability of activating stable carbon lattices, and good cooperative effects towards the activation of carbon matrix can be achieved if doping with multiple heteroatoms.16-18 For instance, nitrogen and sulfur codoped carbon spheres with a specific surface area of only ~400 m2 g-1 exhibit outstanding specific capacitance (295 F g-1 at 0.1 A g-1) and rate capability (247 F g-1 at 10 A g-1).18 However, the synthesis procedures of multiply doped carbon materials (MDCM) still required tedious steps and strict control on byproduct wastes. Therefore, the clean, scalable, and facile production of MDCM with good electrochemical performance is still challengeable.

Herein, we develop a facile strategy for the fast fabrication of MDCM via in situ doping halogenated polymer (such as polyvinyl dichloride, PVDC)-defunctionalizing carbon materials. The synthetic procedure can be done in one step by simply milling the mixture of halogenated polymer, dehalogenation agent, and dopants. As a demonstration, N, S-codoped carbon material (RT-NS-DCM) is synthesized via the dechlorination reaction between PVDC and KOH using two common solvents of dimethylformide and dimethylsulfoxide as N and S sources, respectively. After further annealing to obtain large specific surface area and high conductivity, the resulted 600-NS-DCM achieves heteroatom doping contents of 3.0 at.% N and 2.4 3 ACS Paragon Plus Environment


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at.% S. Remarkably, it delivers a very high capacitance of 427 F g-1 at 1.0 A g-1, and also supplies 251 F g-1 (~60% of initial capacitance) at ultrahigh current density of 100.0 A g-1. Furthermore, capacitive deionization (CDI) measurements find that 600-NS-DCM achieves an absorption capacity as high as 32.3 mg g-1 in 40.0 mg L-1 NaCl and also can effectively work in concentrated salt solutions.

2. Experimental section

2.1 Materials

Polyvinyldichloride (PVDC) was obtained from Solvay (American company) through

Alibaba.

Other

reagents,

including

dimethylformide

(DMF),

dimethylsulfoxide (DMSO), sodium chloride (NaCl), concentrated sulfuric acid (H2SO4), and potassium hydroxide (KOH) of A.R. grade were all purchased from Beijing Chemical Works without any treatment.

2.2 Preparation of N, S-codoped carbon materials

Typically, for the synthesis of N, S-codoped carbon materials at room temperature, 1.00 g PVDC, 2.00 g KOH together with 1.69 mL DMSO and 10.00 mL DMF were added into a ZrO2 ball mill jar. The mixture was ball milled at 30 Hz for 2 hrs, and then the black slurry was collected and dried at 60 °C for 6 hrs to obtain solid products. The dried products were then purified by water for removing excess KOH and those by-products such as KCl. After further drying, clean codoped carbon materials (designated as RT-NS-DCM) were collected. In order to explore the 4 ACS Paragon Plus Environment

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potentials of DCM in capacitive applications, high temperature annealing of unwashed RT-NS-DCM was performed at 500/600/700 °C in inert atmosphere, followed by washing and drying. The resulted products were named as 500/600/700-NS-DCM, respectively. For comparison, control samples were synthesized following the protocol of 600-NS-DCM but with the absence of either DMF or DMSO, and correspondingly, the resulted control samples were named as 600-N-DCM and 600-S-DCM, respectively.

2.3 Characterizations

Scanning electron microscopy (SEM, Zeiss SUPRA55, with EDX apparatus). High resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). Powder X-ray diffraction (XRD, Shimadzu XRD-6000). Raman spectroscopy (LabRAM ARAMIS, excited with a 532 nm laser). X-ray photoelectron spectroscopy (XPS, Thermo Electron ESCALAB 250). N2 adsorption/desorption isotherms (Quantachrome Autosorb-1CVP, measured at 77 K). The samples were preprocessed in a vacuum at 200 °C for 3 hrs. The desorption branches were used to calculate specific surface areas according to the Brunauer–Emmett–Teller (BET) method and pore size distributions based on the Barrett–Joyner–Halenda (BJH) method. The micropore area was calculated through t-plot of quantity of adsorbed N2 versus multilayer absorption thickness. Thermogravimetry (TG) and differential thermal analysis (DTA) (NETZSCH STA 449F3, heated at 10 °C min-1 up to 800 °C).

2.4 Electrochemical characterizations 5 ACS Paragon Plus Environment


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Supercapacitor measurements were conducted using an electrochemical workstation (Chenhua CHI 660E) in three-electrode system. Typically, the slurry containing 85.0 wt.% NS-DCM, 10.0 wt.% carbon black, and 5.0 wt.% polytetrafluoroethylene (PTFE, Aldrich; 60 wt.% dispersion in water) binder was cast-coated onto a glassy carbon electrode (1.0 cm in diameter). The electrode was fully dried using a NIR lamp and 80°C oven. The loading mass of working electrode was ~1.0 mg cm-2. The measurements were respectively carried out in three aqueous electrolytes (6.0 M KOH, 1.0 M NaCl, or 1.0 M H2SO4), with Pt as counter electrode and Hg/HgO (or Ag/AgCl) as reference electrode. The gravimetric specific capacitances (Cs, F g-1) were calculated depending on the charge/discharge curves obtained with different current streams as shown in the equation (1):19

Cs =

I × ∆t M ×V

(1)

where I is the current density (A g-1), ∆t is the charging/discharging duration time, M equals to the loading mass of active materials, and V is the potential window (eliminating IR drop). Electrochemical impedance spectroscopy (EIS) was conducted at a 10 mV AC amplitude from 10 mHz to 100 kHz.

Capacitive deionization (CDI) measurements were following the previous paper published by our group.20 For a typical electrode fabrication, 80.0 wt.% NS-DCM, 15.0 wt.% carbon black, and 5.0 wt.% PTFE were fully mixed in ethanol, followed by sonication for 1 hr. Resulted slurry with ~100 mg solids was casted on a graphite sheet (6 cm×7 cm×0.2 cm), and then dried at 80°C overnight to obtain a solid 6 ACS Paragon Plus Environment

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electrode. The loading mass of electrode materials was ~2.4 mg cm-2. CDI measurements were carried out using a home-made CDI set-up consisting of a CDI cell, a peristaltic pump, a conductivity meter and an external power supply, which had been carefully described in our previous publication.20 100.0 mL NaCl aqueous solution circulated in the CDI set-up at a flow rate of 25.0 mL min-1. A series of NaCl concentrations (40, 100, 500 and 1000 mg L-1) and applied voltages (1.2, 1.4 and 1.6 V) were selected to evaluate CDI performance of electrode materials. The NaCl concentrations were calculated from the conductivities in situ collected using a conductivity meter (Leici Type 308F). The desalination capacities (W, mg g-1) were calculated according to the equation (2):20

W=

(C 0 −C ) × V m

(2)

where V is the total solution volume, and m is the total mass of electrode materials. C0 and C are the initial and final NaCl concentrations, respectively. The time-dependent accumulated ion removal rates (U, mg g-1 min-1) were given in the equation (3):20

U=

W t

(3)

where t is the operation time.

3. Results and discussions

Polymer

dehalogenation

is

a

newly

developed

strategy

for

the

time-/cost-effective fabrication of carbon materials, which can ensure the 7 ACS Paragon Plus Environment


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room-temperature synthesis of carbon materials with high carbon contents.5-6, 21 But the defunctionalization of halogenated polymers such as polyvinyldichloride (PVDC) is hardly done even under high-temperature treatment. As confirmed by TG profile of pure PVDC in Figure S1A, heating PVDC to over 800℃ only results in a weight loss of ~50% while the initial Cl content in PVDC is ~73%, and no obvious defunctionalization of PVDC can be seen (Figure S1B). However, the defunctionalization of PVDC can be rapidly promoted at room temperature using strong alkaline such as KOH.21 Moreover, the previous reports have confirmed that N in DMF solvent could be extracted and efficiently doped into the as-formed carbon materials.21 As far as we know, DMF with amide N is normally stable, but can be decomposed into dimethylamine with secondary amine under strong alkaline promotion.

The

successful

doping

of

N

using

dopant

of

secondary

amine-dimethylamine into carbon lattice suggests that the carbon sites during or after dehalogenation are highly reactive for coupling adjacent atoms.21 Encouraged by the applicable N doping under mild conditions, therefore, we may safely envision plenteous possibilities for the room-temperature fabrication of MDCM that will not be limited by single type of heteroatoms. As demonstration, two common solvents of DMF and DMSO, which are rarely used as heteroatom sources because most of the outer electrons of their N and S atoms are sharing with C or O atoms, were selected as N and S sources for synthesizing the N, S-codoped carbon materials during PVDC dehalogenation at room temperature (named as RT-NS-DCM). The SEM image (Figure 1A) reveals amorphous morphology. EDX element mapping (Figure 1B) 8 ACS Paragon Plus Environment

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verifies the substantial presence and uniform distribution of N and S in addition to C and O in RT-NS-DCM. Meanwhile, Raman spectrum in Figure 1C shows that a clear graphitization band is located at ~1583 cm-1 and also a disorder band at ~1354 cm-1, confirming the successful fabrication of carbonaceous materials at room temperature. XRD profile in Figure 1D further evidences the as-formed carbon to be amorphous, which is in agreement with SEM image. In addition, the overwhelming presence of XRD signals for KCl is observed for unwashed products (Figure 1D), which evidences that the dehalogenation reaction is probably following the formula (1).

RT (CH 2CCl2 ) n + 2nKOH  → C2 n + 2nKCl + 2nH 2O

(1)

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Figure 1 (A) SEM image and (B) element mapping of RT-NS-DCM, (C) Raman spectra of RT/600-NS-DCM, and (D) XRD profiles of RT/600-NS-DCM and unwashed RT-NS-DCM samples.

XPS technique was applied to reveal the composition and functionality of RT-NS-DCM, as shown in Figure 2. Firstly, RT-NS-DCM has a high carbon content of 76.6 at.% (Figure 2A), in which the carbon species indicated by the C1s peak at ~285 eV takes the dominant proportion (Figure 2B), revealing the high completeness of PVDC dehalogenation and carbonization under KOH promotion. It is consistent with little Cl residual in the products. Secondly, the signals for N at ~400 eV and S at ~168 eV can be clearly observed in Figure 2A. According to the detailed deconvolutions of N1s and S2p spectra (Figure 2C/D), the N species is mainly taking pyrrolic form, and S species is mainly taking sulfone form.

In order to extend the N, S-codoped carbon materials in capacitive applications, high temperature annealing was performed on unwashed RT-NS-DCM prior to electrode fabrication. According to the annealing temperature of 500/600/700 °C, the products were named as 500/600/700-NS-DCM, respectively. In Figure 1D (XRD profile) and Figure 1C (Raman spectrum), 600-NS-DCM shows little variation relative to RT-NS-DCM, which may be ascribed to the high completeness of defunctionalization and carbonization at room temperature. Similar trends of XRD profiles and Raman spectra of 500/700-NS-DCM in Figure S2C/D are also observed.


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A

B

500k

40k

C1s~76.6%

400k

Cl2p ~3.4%

N1s ~4.7%

200k

S2p ~0.6%

100k 0 700

Intensity / cps

Intensity / cps

300k

30k 20k

~68.5% ~25.4%

10k

~6.1%

RT-NS-DCM

0 600

500

400

300

200

100

294

Binding energy / eV

D Raw Fitted Baseline Pyrrolic N Pyridinic N Graphitic N

4.0k

288

285

282

279

276

2.0k

1000 900

Intensity / cps

N1s

291

Binding energy / eV

C 6.0k

C1s

Raw Fitted Baseline C-C&C=C C-O/-N/-S C=O/=N Sat. peak

O1s~14.7%

Intensity / a.u.

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


800

S2p

Raw Fitted Baseline C-SOx-C

700 600 500

404

402

400

398

Binding energy / eV

396

174

172

170

168

166

164

Binding energy / eV

Figure 2 XPS measurement on RT-NS-DCM: (A) element survey, (B) C1s spectrum, (C) N1s spectrum, and (D) S2p spectrum.

Figure 3A (SEM image of 600-NS-DCM) typifies that high temperature annealing transforms RT-NS-DCM into 3D porous structure, and NS-DCM materials annealed at 500 and 700 °C also show the similar morphology (Figure S2A/B). Uneven contrast is observed through the enlarged view of TEM image in Figure 3B, which arises from the porous structure of 600-NS-DCM. HRTEM image in Figure 3C shows that 600-NS-DCM possesses a large content of micropores, which is related to the activation of intentionally overdosed KOH in unwashed RT-NS-DCM; meanwhile, both amorphous and graphitic domains are presenting. As indicated by BET measurements (Figure 3D-F), increasing annealing temperature from 500 to 700 11 ACS Paragon Plus Environment


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°C leads the significant increasing of specific surface area (SSA), micropore area and pore volume, because KOH activation usually begins to happen at 400 °C and thereafter gets intense. In our case, 700-NS-DCM does not present so much difference in SSA (760 m2 g-1) and micropore area (611 cm2 g-1) to 600-NS-DCM (SSA of 699 m2 g-1 and micropore area of 594 cm2 g-1), which is possibly due to the insufficient KOH for activation at 700 °C. Herein, the optimized temperature for activating carbon is about 600 °C after balancing porosity and yield, as other common reports.22

Figure 3 (A) SEM image, (B) TEM image, and (C) HRTEM image of 600-NS-DCM, (D) N2 adsorption/desorption curves, (E) specific surface areas and micropore areas, and (F) pore size distributions of 500/600/700-NS-DCM.

By performing annealing, partial functionalities can be eradicated from carbon matrix, leading to the formation of highly carbonized materials with less heteroatom doping. As shown in Figure 4A and Figure S3, increasing annealing temperature 12 ACS Paragon Plus Environment

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shows obvious positive effect on further carbonization, resulting in the carbon contents of 81.4 at.% for 600-NS-DCM and 88.3 at.% for 700-NS-DCM. The N contents show little variation and stably keep at ~3.0 at.%. But the S contents sharply drops from 2.4 at.% (600-NS-DCM) to 0.8 at.% (700-NS-DCM). As revealed by the deconvolutions of C1s spectra in Figure 4B, the C1s peaks are dominantly 285 eV-centered. The N configurations of 500/600-NS-DCM are dominated by the pyrrolic N, similar to that of RT-NS-DCM. While at 700 °C, the pyrrolic N will be gradually converted into the six-atom ringed pyridinic and graphitic N (Figure 4C).5-6, 23-24

The S configurations of 500/600-NS-DCM go through similar variation of

stability by keeping minimal change of the ratios of C-S-C and C-S(=O)-C, but due to the relatively poor stability of S species at high temperature, 700-NS-DCM shows a low S content (Figure 4D).

B 90

500-NS-DCM 600-NS-DCM 700-NS-DCM

80

Intensity

Atomic percentage / %

A

20

Raw Fitted Baseline C-C/C=C C-O/C-N/C-S C=O/C=N/C=S

700-NS -DCM 600-NS -DCM 500-NS -DCM

10 0 C

O

N

290

S

288

286

284

282

Binding energy / eV Raw

12.0k 10.0k 8.0k

700-NS-DCM

Fitted

Baseline Pyridinic N Pyrrolic N Graphitic N

600-NS-DCM

6.0k 500-NS-DCM

4.0k 406 404 402 400 398 396 394 392

Binding energy / eV

D 4k Raw C-S-C

Intensity / a.u.

C Intensity / cps

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


3k

2k

Fitted Baseline C-SOx-C

700-NS-DCM

600-NS-DCM

1k 500-NS-DCM

174

171

168

165

162

159

Binding energy / eV



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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Figure 4 XPS measurements of 500/600/700-NS-DCM: (A) bar chart of element analysis, (B) C1s spectra, (C) N1s spectra and (D) S2p spectra.

Therefore, after balancing the SSA, porosity, carbonization degree and heteroatom doping level, 600-NS-DCM was firstly selected as electrode material to explore capacitive applications. After a three-electrode electrochemical system was set up, the measurements in different electrolytes of 6.0 M KOH, 1.0 M NaCl or 1.0

M H2SO4 were respectively conducted and compared. Usually, the enclosed area of CV profile is one clear indicator for qualitative measurement of capacitance. As revealed in Figure 5A, the CV profile of 600-NS-DCM collected in 1.0 M H2SO4 shows the largest enclosed area, meaning that the highest capacitance can be expected in acid electrolyte. The largest area is probably related to the pseudocapacitance provided by S and N species when absorbing H+ ions.12 The second largest area is present in 6.0 M KOH and followed by the area collected in 1.0 M NaCl. The smallest capacitance achieved in neutral electrolyte is probably due to Na+ and Cl- which are hard to provide any pseudocapacitance.25 According to the galvanic charge/discharge profiles in Figure 5B, 600-NS-DCM shows very high specific capacitances of 427 F g-1 at 1.0 A g-1 and 334 F g-1 at 10.0 A g-1 in 1.0 M H2SO4, and both the initial capacitance and rate capability of 600-NS-DCM are among the best capacitive performance ever reported as listed in Table S1.13-14, 21, 26-36 600-NS-DCM also performs well in both 6.0 M KOH and 1.0 M NaCl, respectively exhibiting 328


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F g-1 and 242 F g-1 at 1.0 A g-1. Remarkably, 600-NS-DCM maintains ~60% of capacitance at ultrahigh current density of 100.0 A g-1 in 1.0 M H2SO4, achieving the specific capacitance of 251 F g-1. The rate capability in 1.0 M H2SO4 is the best over the other two electrolytes (Figure 5C), which is possibly ascribed to the smallest resistance of charge transferring when absorbing H+ ions with the smallest ionic radius, as revealed by the smallest semicircle of Nyquist plot (Figure S4). While those control samples doped with single heteroatom N or S show much lower capacitances in three electrolytes, as shown in Figure S5, confirming the importance of N and S codoping in DCM. The capacitive

behaviours of 500-NS-DCM and 700-NS-DCM are similar to that of 600-NS-DCM, but their specific capacitances are relatively low (Figure S6). Moreover, cycling

stability tests are performed as shown in Figure 5D. 600-NS-DCM shows very good cycling stability at current densities of both 1.0 A g-1 and 10.0 A g-1, obtaining almost 90% of capacitance retention after 1000 cycles in 1.0 M H2SO4. The good cycling stability can be also observed in the other two

electrolytes: the capacitance retention at 10.0 A g-1 was 91% in 1.0 M NaCl and 80% in 6.0 M KOH, respectively.



B

A

1.0

1 M H2SO4

2 0

6 M KOH 1 M NaCl 1 M H2SO4

0.8

Potential / V

Current / mA

4

1 M NaCl 6 M KOH

-2

10 A/g

0.6 0.4 0.2

1 A/g

-4 0.0

C

-0.5

0.0

0.5

Potential / VRHE 6 M KOH 1 M NaCl 1 M H2SO4

-1

300

200 400 600 800

200

100

0

20

40

60

Time / s

Time / s

D

500 400

0

1.0

400

Capacitance / F g-1

-1.0

Capacitance / F g

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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300

200

10 A/g 1 M H2SO4 1 M NaCl 6 M KOH

100

1 A/g 1 M H2SO4

0

0

20

40

60

80

Currrent density / A g-1

100

0

200

400

600

800

1000

Cycle number / n

Figure 5 (A) CV profiles, (B) GCD curves, (C) rate capability curves, and (D) Cycling stability tests of 600-NS-DCM measured in different electrolytes of 6.0 M KOH, 1.0 M NaCl, and 1.0 M H2SO4.

600-NS-DCM with porous structure, large SSA and ultrahigh capacitance should be promising electrode materials for CDI. The desalination performance of 600-NS-DCM was tested in NaCl solution using a home-made CDI set-up. The conductivity was in situ measured to monitor the change in solution concentration. The influence of applied voltage was firstly studied in 40 mg L-1 NaCl, as shown in Figure 6A. To exclude physical adsorption, applied voltage is not supplied in the first 20 min. During this period, it is observed that the conductivity of each experiment engages very limited increase, which is probably related to trace amount of ions remaining after annealing and washing processes. Once applied voltage is supplied, the conductivity of each experiment rapidly decreases and then reaches to a plateau 16 ACS Paragon Plus Environment

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within 80 min. When the CV profiles of 600-NS-DCM are collected using two-electrode testing system (Figure S7), no obvious faradaic current can be observed in between 1.4 to 1.6 V in NaCl solution, indicating that the operating voltage of 1.6 V for our CDI measurements is safe to avoid the splitting of water, which is mainly due to the largely expanded stable window of neutral electrolyte. With the higher applied voltage supply, the larger ion removal rate (U) and higher desalination capacity (W) are accordingly obtained. U plotting with W at different applied voltages are respectively shown in Figure 6B. For instance, 600-NS-DCM with 1.6 V applied voltage has a desalination capacity of 32.3 mg g-1 and a maximum ion removal rate of 0.56 mg g-1 min-1, which exceed most of the currently reported values for CDI applications as listed in Table S2.20, 33, 37-53

Even with 1.4 V applied voltage, the desalination capacity still reaches 30.9 mg g-1 and the maximum ion removal rate is 0.51 mg g-1 min-1. Secondly, the cycling stability of 600-NS-DCM was measured by monitoring the conductivity changes over five adsorption/desorption cycles with 1.4 or 1.6 V (Figure 6C). Desorption processes were achieved by applying transposed voltages. For the CDI cycles with both 1.4 V and 1.6 V, the trends of conductivity can be reproducible and the conductivity retentions achieve over 98% for the first four cycles, but the abnormal trends occur at the fifth cycle which is probably related to the detachment of active materials from current collectors after long-term operations. Finally, CDI experiments with higher NaCl concentrations were performed with 1.4 V for broadening the applications of NS-DCM. As shown in Figure 6D and Figure S8A-C, with NaCl concentrations 17 ACS Paragon Plus Environment


increasing, a continual enhancement of desalination capacity can be obtained, and 600-NS-DCM exhibits the desalination capacities of 48.2, 87.2, 140.7 mg g-1 in 100, 500 and 1000 mg L-1 NaCl solutions, respectively. The corresponding maximum ion removal rates are respectively 0.99, 1.53, and 1.60 mg g-1 min-1, as revealed in Figure

S8D. These results prove that the as-formed NS-DCM electrode materials are capable of treating concentrated NaCl solutions.

A

B

100

0.7

90

incr e

0.6

80 70 60

1.2V

50

1.4V

40

U / mg g-1 min-1

Conductivity / µS cm

-1

0V

1.6V

30

asin g

time

0.5 0.4 0.3

1.2 V 1.4 V 1.6 V

0.2 0.1 0.0

-20

0

20

40

60

80

100

Time / min

0

5

10

15

20

25

30

35

W / mg g-1

D

-1

100

80

60

40 1.4 V

1.6 V

20 0

200

400

600

Time / min

800

Desalination capacity / mg g

-1

C

Conductivity / µS cm

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140.7

150

1.4 V 120 87.2 90 48.2

60 30.9 30

100

1000 -1

Initial concentration of NaCl / mg L

Figure 6. CDI performance of 600-NS-DCM measured in 40.0 mg L-1 NaCl: (A) desalination curves at different applied voltages, (B) desalination rate plotting with desalination capacity at different applied voltages, (C) cycling stability tests at applied voltages of 1.4 V and 1.6 V, and (D) desalination capacities in different initial NaCl concentrations at applied voltage of 1.4 V.


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4. Conclusions

In conclusion, we develop a facile synthetic strategy for multiply doped carbon

materials

at

room

temperature

through

in

situ

doping

PVDC-dehalogenated carbon with multiple heteroatom sources. N, S-codoped carbon materials (NS-DCM) are demonstratively fabricated using two common solvents: DMF and DMSO, during the dehalogenation of PVDC promoted by KOH. For electrochemical applications, the room-temperature synthesized NS-DCM are submitted to annealing to gain large SSA and good conductivity. Remarkably, the resulted 600-NS-DCM exhibits a specific capacitance as high as 427 F g-1 (at 1.0 A g-1) and nearly 60% of capacitance retention at ultrahigh current density of 100.0 A g-1 in 1.0 M H2SO4, and very good cycling stability. Meanwhile, it exhibits a high efficiency and long-term stability towards CDI, achieving a desalination capacity of 32.3 mg g-1 in 40.0 mg L-1 NaCl. We believe this clean and time/cost-efficient strategy for the fabrication of multiple heteroatoms-doped carbon materials has wide application prospects in electrochemical energy conversion and storage. ASSOCIATED CONTENT

Supporting Information. TG profile of PVDC and digital image of ball-milled PVDC/DMF, SEM images, Raman spectra, XRD profiles, XPS element survey and electrochemical data of 500/700-NS-DCM, rate capability of 600-N or S-DCM, supportive CDI data, tables for the summary of capacitive and CDI performance of 19 ACS Paragon Plus Environment


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previously reported works. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *[email protected], and *[email protected].

ACKNOWLEDGMENT This work was financially supported by NSFC, 973 Program (2014CB932104), and Beijing Engineering Center for Hierarchical Catalysts, the Fundamental Research Funds for the Central Universities, the Program for Changjiang Scholars and Innovative Research Team in the University, and the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC.

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