The synthesis of sulfur-doped graphite nanostructures by direct electrochemical conversion of CO2 in CaCl2NaClCaOLi2SO4

Accepted Manuscript The synthesis of sulfur-doped graphite nanostructures by direct electrochemical conversion of CO2 in CaCl2 NaCl CaO Li2SO4 Liwen Hu, Zhikun Yang, Wanlin Yang, Meilong Hu PII:

S0008-6223(18)31183-7

DOI:

https://doi.org/10.1016/j.carbon.2018.12.049

Reference:

CARBON 13752

To appear in:

Carbon

Received Date: 8 October 2018 Revised Date:

12 December 2018

Accepted Date: 15 December 2018

Please cite this article as: L. Hu, Z. Yang, W. Yang, M. Hu, The synthesis of sulfur-doped graphite nanostructures by direct electrochemical conversion of CO2 in CaCl2 NaCl CaO Li2SO4, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2018.12.049. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Abstract

The electrochemical synthesis of sulfur-doped nanostructured graphite including graphene and carbon nano-tube via direct electrochemical conversion of CO2 in CaCl2-NaCl-Li2SO4 has been realized at a relative low temperature (675℃). The

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results indicate that sulfur can successfully incorporate into carbon matrix and the graphitization degree can also be enhanced by introducing sulfur. In addition to SO42-, Li+ will also have positive effect on improving the graphitization of carbon deposition. Aggregated carbon particles are included in all samples while ultrathin

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graphite sheet, wire-like carbon, carbon nano-tube and interwoven carbon chains can be obtained in some cases at different electrolytic conditions. This work opens up a

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novel way to fabricate heteroatom-doped nanostructured carbons from carbon dioxide.

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The synthesis of Sulfur-Doped graphite nanostructures by direct

electrochemical

conversion

of

CO2

in

CaCl2-NaCl-CaO-Li2SO4

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Liwen Hu*1, Zhikun Yang, Wanlin Yang, Meilong Hu* College of Materials Science and Engineering, Chongqing University, Chongqing

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400044, PR China

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Abstract

The electrochemical synthesis of sulfur-doped nanostructured graphite including graphene and carbon nano-tube via direct electrochemical conversion of CO2 in CaCl2-NaCl-CaO-Li2SO4 has been realized at a relative low temperature (675℃). The

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results indicate that sulfur can successfully incorporate into carbon matrix and the graphitization degree can also be enhanced by introducing sulfur. In addition to SO42-, Li+ will also have positive effect on improving the graphitization of carbon deposition.

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Aggregated carbon particles are included in all samples while ultrathin graphite sheet,

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wire-like carbon, carbon nano-tube and interwoven carbon chains can be obtained in some cases at different electrolytic conditions. This work opens up a novel way to fabricate heteroatom-doped nanostructured carbons from carbon dioxide. 1.

Introduction

Due to the rising concentrations of atmospheric carbon dioxide (CO2) and subsequent changes in climate, including temperature and precipitation extremes, the

1 * Corresponding author.

Tel: 86-23-65112631. E-mail address: [email protected] (Liwen Hu)

ACCEPTED MANUSCRIPT capture and conversion of CO2 has been a research hotspot where many novel technologies about the utilization of CO2 have been proposed [1, 2]. It has been found that CO2 is a low-cost carbon containing resource and is the feedstock for the

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productions of urea, salicylic acid, polycarbonate, carbamates and inorganic compounds [3, 4]. Among these carbon containing products, ordered porous carbons are highly attractive, as they have widespread application and various allotropes [5].

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Solely the carbon is able to create sp, sp2 and sp3 bonding, since only carbon has no

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inner p-electrons [6]. However, despite availability of C1 chemistry and abundance from atmospheric or effluent gases, CO2 has not been recognized as feedstock for producing graphite. The reason is that CO2 is chemically stable and its conversion requires sufficient energy. An electrochemical way is well suited to reduce this most

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stable state of carbon as electrons can be regarded as strong reducing agents. The electrochemical reduction of CO2 in aqueous solution has been well documented, but the applications are limited by the low solubility of the CO2 gas, as well as the

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competing reaction of water decomposition [7]. More recently, ionic liquids were

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investigated for electrochemical reduction of CO2 at room temperature owing to their wide electrochemical window and high solubility of CO2 [8], however, the high cost of ionic liquids remains an impediment to their widespread adoption. Molten carbonates are low cost electrolytes with high ionic conductivity and low vapor pressure [9, 10], and have been investigated extensively as alternatives for electrochemical reduction of CO2 [11-13]. However, the resulting products were almost exclusively amorphous carbon [14], CNT and Small-Diameter CNT with the

ACCEPTED MANUSCRIPT aid of metal catalyst [15, 16]. There are two traditional and usually complementary ways in which porous carbon properties can be easily controlled including the design of pore architecture

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and the incorporation of specific functional groups onto the carbon surface and within the carbon matrix [17, 18]. Up to now, tremendous progress has been made in syntheses of nanoporous carbons with novel morphologies by electrochemical

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conversion of CO2 in molten salt [19-23], exciting opportunities remain for

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heteroatom-doping of such materials. Heteroatom-doped nanostructured carbons represent one of the most prominent families of materials that are used in energy related applications [24, 25], such as fuel cells, batteries, hydrogen storage or supercapacitors

as

they

possess

enhanced

porosity

and

their

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physicochemical/mechanical characteristics can be easily tailored by introducing of heteroatoms into the sp2-hybridized carbon matrix. Through theoretical analysis, carbon can be doped with B, N, O, S, P, Se or Si [26-30]. The substitutional doping of

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heteroatom with a different number of valence electrons in the honeycomb lattice will

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generally introduce additional states in the density of states of carbon [31]. In comparison to N and B, doping sulfur in carbon materials is still quite rare and represents an emerging field within carbon material research. While nowadays potential need of sulfur doped carbon for energy applications in fuel cells, supercapacitors or batteries is continuously discovered and exploited [32], until only a few years ago, only little had been known about such sulfur-doped carbonaceous species.

ACCEPTED MANUSCRIPT There are two traditional methods for graphite synthesis: (1) directly transforming carbon materials into graphite under high temperature (3000

or higher) and high

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pressure. (2) Catalytic graphitization by chemical reactions between non-graphitized carbon and metal catalysts (i.e. Fe, Co, Ni, Mn, etc.) at a relatively low temperature (about 1000

) [6]. Then in situ doping and post-treatment will be employed to

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introduce heteroatoms sulfur into the graphite framework, which is complicate and

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energy consuming process. In this paper, sulfur-doped graphite nanostructures were prepared by direct electrochemical conversion of CO2 in molten CaCl2-NaCl-Li2SO4 at a relatively lower temperature (about 700

), where CO2 sulfur-doped graphite can

successfully be obtained at a graphite cathode with the generation of O2 at the

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RuO2-TiO2 inert anode. The influences of temperature, concentration of Li2SO4, effect of Li+ and SO42- was evaluated and investigated. The results demonstrated that both Li+ and SO42- will contribute to graphitizing of cathodic products. And

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the crystallization degree of the cathode products enhanced with the increasing

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concentration of Li2SO4. The basic carbon morphologies obtained are quasi-spheres and ultrathin graphite sheet and they can assemble into various structures under different conditions. This work presents a novel way to fabricate heteroatom-doped nanostructured graphite via the electrochemical conversion of CO2 by molten salt electrolysis, which will facilitate the reduction of green-house gas and give insight in preparing advance carbon structures. 2.

Experimental

ACCEPTED MANUSCRIPT The experimental setup was including an alumina crucible containing the molten salt placed in an alumina tube with cooling water circulating around the lid, as shown in Fig.S1. Anhydrous CaCl2, NaCl, LiCl, Na2SO4 and Li2SO4 were of analytical

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purity and were purchased from Sinopharm Chemical Reagent Co., Ltd. The CaO was obtained as previously by calcinatiing of CaCO3 at 1423K for 15 min. The RuO2•TiO2 anode was prepared by mixing TiO2 and RuO2 powder in a mortar with mole ratio of

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2:8 and was then pressed into pellets with a diameter of 20 mm, using a uniaxial

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pressure of 4.15 MPa, which was then sintered at 950℃ for 12h in a muffle furnace. Holes were then drilled in the pellets and platinum wires were then inserted to form the anode. The graphite rod with a diameter of 3 mm was used as the cathode during electrolysis.

About

220

g

anhydrous

CaCl2-NaCl-Li2SO4-CaO

or

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CaCl2-NaCl-Na2SO4-CaO was added into an alumina crucible, which was placed in a sealed vertical tubular reactor. The salt was dried at 300

under vacuum for up to 24

h to remove moisture before it was slowly heated up to the target temperature.

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The sulfur-doped graphite nanostructures were successfully prepared via

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constant current electrolysis carrying out between TiO2•RuO2 anode and graphite rod (3mm

in

diameter)

cathode

in

the

molten

CaCl2-NaCl-Li2SO4-CaO

or

CaCl2-NaCl-Na2SO4-CaO with the utilization of power supply (Prinston 2287). The influence of the concentration of Li2SO4 and temperature were both investigated, and the specific information of the experiment was shown in Table 1. Before each electrolysis experiment, CO2 was continuously bubbled into the melt through an alumina tube for half an hour. The atmosphere of CO2 was maintained over the melt

ACCEPTED MANUSCRIPT continuously. After electrolysis, the electrodes were withdrawn to the upper cooler part of the reactor. The cathodic product was taken out and immersed into 1-7 mol•L-1 HCl solution to remove the residuals or impurities, and then dried in a drying oven at for about 48 hours.

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120

Table 1 Electrolytic conditions for each sample Melt compositions

Temperature(

SD-1 SD-2

CaCl2-NaCl-Li2SO4(1mol%)-CaO CaCl2-NaCl-Li2SO4(3 mol%)-CaO

725 725

SD-3

CaCl2-NaCl-Li2SO4(5 mol%)-CaO

725

SD-4

CaCl2-NaCl-Na2SO4(3 mol%)-CaO

SD-5

CaCl2-NaCl-Li2SO4(3 mol%)-CaO

SD-6

CaCl2-NaCl-Li2SO4(3 mol%)-CaO

SD-7

CaCl2-NaCl-Li2SO4(3 mol%)-CaO

SD-8

CaCl2-NaCl-LiCl(3 mol%)-CaO

Current(A)

Electrolytic time(h)

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Sample

4 4

0.75

4

725

0.75

4

625

0.75

4

675

0.75

4

775

0.75

4

725

0.75

4

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0.75 0.75

The obtained products on the cathode were characterized by scanning electron microscopy (SEM, JEOL-JSM-7800F, FEI NOVA NANOSEM 400), transmission microscopy

(TEM,

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electron

ZEISS,

LIBRA 200),

high-resolution

Raman

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spectroscopic analysis (HORIBA Jobin Yvon S.A.S HR evolution) with excitation at 532 nm, X-ray di ractometer (XRD, Panalytical X’Pert Powder, Panalytical B.V.) and X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi). 3.

Results and Discussion

The sulfur-doped graphite was prepared by constant current electrolysis in CaCl2-NaCl-Li2SO4-CaO and the effects of the Li2SO4 concentration was firstly examined. Fig.1a shows the relevant cell voltage variation during electrolysis with the

ACCEPTED MANUSCRIPT addition of different concentration of Li2SO4. It is observed that they showed a similar trend where the cell voltage increases at the initial stage and then gradually decreases over electrolysis for each condition. The increase of cell voltage probably due to the

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slower diffusion of reactant after the addition of Li2SO4 and the carbonate ions are diluted to a certain extent, which will lead to slower electrochemical kinetics and stronger electrode polarization. Afterwards, the cell voltage slightly decreased as the

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surface area increased when carbon deposition was in progress. It is also noticed that

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it takes longer time to reach the cell voltage peak with the increasing Li2SO4 concentration which also in accordance with the above mentioned explanation. Compared with reported results, the cell voltage is obviously higher than that without Li2SO4 [20, 22], indicating the higher impedance of the melt after the introduction of

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Li2SO4.

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Fig.1 (a) Cell voltage-time curves conducted in the melts containing 1mol%, 3mol% and 5mol% Li2SO4; (b) XRD patterns of the SD-1, SD-2 and SD-3; (c) Raman spectra of the SD-1, SD-2 and SD-3.

ACCEPTED MANUSCRIPT The XRD and Raman spectra of the carbon nanostructures deposited in melts with different Li2SO4 concentration are demonstrated in Fig.1b. One sharp peak at around at 2θ =26° and the other weak peak at around 2θ =44.5° can be observed in

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Fig.1b, which can be attributed to (002) and (100) diffraction peaks, respectively. The full width at half maximum of graphite (002) peak for SD-1, SD-2 and SD-3 are 0.501°, 0.387° and 0.280°, respectively. The decline of full width at half maximum of

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graphite (002) peak also confirmed the enhanced crystallinity of the graphite

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nanostructure in melts containing higher concentration of Li2SO4 [33]. Fig.1c depict Raman spectra of SD-1, SD-2 and SD-3. As the graph described, all the samples have two peaks located at about 1340 and 1585cm-1 which can be assigned to the D band and G band, respectively. The G-band at about 1588 cm-1 is related to the sp2 (C=C)

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aromatic carbon structure vibrations of a perfect graphite, while the D-band at about 1348 cm-1 is approximately associated with the disorder-induced scattering resulting from imperfections [34, 35]. The integral intensity ratio of ID/IG represent the degree

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of graphitization. The ID/IG is 1.64, 1.017 and 0.577 for SD-1, SD-2 and SD-3,

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respectively. The 2D peak emerges at about 2700 cm-1 and becomes sharper when more Li2SO4 is added, indicating more graphitic crystallites and fewer defects of the electrolytic carbon [36, 37]. The Raman also confirmed that the degree of graphitization is optimized by adding more Li2SO4 into the melt. Fig.2 shows the SEM images of SD-1, SD-2 and SD-3 obtained in the melt containing

different

concentration

of

Li2SO4.

The

graphite

deposited

in

CaCl2-NaCl-Li2SO4(1mol%)-CaO are mainly comprised of aggregated quasi-sphere

ACCEPTED MANUSCRIPT and carbon sheets, while SD-3 contains aggregated quasi-sphere. When the concentration of Li2SO4 increased, the product obtained are basically aggregated quasi-sphere and the size of the quasi-sphere declined. So it is noteworthy that

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aggregated quasi-sphere can be found in all cases, but their content and size changed with electrolytic conditions. With the increasing of Li2SO4 concentration, the quasi-spheres obtained become smaller. The formation of spherical carbon should be

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related to the rapid nucleation of carbon, which has been observed in a lot of papers

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[12].

Fig.2 SEM images of SD-1 (a, b), SD-2(c, d) and SD-3(e, f).

ACCEPTED MANUSCRIPT In order to further figure out the effect of Li+ ions, CaCl2-NaCl- Na2SO4 (3mol%)-CaO was employed to capture and electrochemical conversion of CO2 into sulfur-doped carbon nanostructures. The XRD and Raman spectra of the cathodic

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product obtained was compared with SD-2 to evaluate the influence of the Li+. As Fig.S3a described, SD-2 and SD-4 both exhibit one sharp peak at around at 2θ =26°, which can be attributed to (002) diffraction peaks. And the full width at half

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maximum of graphite (002) peak for SD-4 and SD-2 are 0.609° and 0.387°, indicating

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the decreased crystallinity of the graphite nanostructure in melts without Li+. Fig. S3b depicts the Raman spectra of SD-2 and SD-4, which both exhibit the characteristic graphitized carbon G peak ∼1580 cm−1, indicative of in-plane sp2-hybridized carbons, and a D mode ∼1350 cm − 1, which corresponds to out-of-plane defective

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sp3-hybridized carbons. A higher D/G peak intensity ratio is observed for SD-4, indicating a greater concentration of sp3 carbon materials in the sample. The ID/IG is 1.017 and 1.417, respectively. This result declares that Li+ ions have positive effect on

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improving the degree of graphitization.

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Fig.3a shows the SEM images of SD-4 sample. It is observed that the fibrous carbon (see in region A) and carbon nano-tubes (see in region B) are the main product in SD-4 sample. And TEM photos showed in Fig.3b and Fig.3c further proved that SD-4 mainly comprised of carbon nano-tubes and curved rod-like structures. The average diameter of carbon nano-tubes is about 20 nm and the diameter of curved rod-like structure is about 100 nm. It is noticed that the SD-4 seems to be more uniform from SEM photo.

Fig.3 SEM (a) and TEM (b, c) images of SD-4

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The effect of SO42- was also evaluated with the utilization of CaCl2-NaCl-LiCl (3

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mol%)-CaO as the electrolyte. Fig.S4a shows the XRD patterns of SD-2 and SD-8, it

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is observed that the intensity of (002) peak of SD-8 is obviously lower than that of SD-2, which indicates the declined crystallinity of the cathodic product. The full width at half maximum of graphite (002) peak for SD-8 and SD-2 are 1.358° and 0.387°, indicating the decreased crystallinity of the graphite nanostructure in melts

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without SO42-. The Raman spectra shown in Fig. S4b exhibit the characteristic graphitized carbon G peak ∼1580 cm−1 and a D mode ∼1350 cm−1, A higher D/G peak intensity ratio is observed for SD-8, indicating a greater concentration of sp3

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carbon materials in the sample. The ID/IG is 1.017 and 1.214, respectively. This result

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proved that SO42- ions also have positive influence on enhancing the degree of graphitization.

The above results demonstrate that both Li+ ions and SO42- ions have positive

and significant effect on improving the degree of graphitization and decreasing the sp3 carbon materials in the sample. The influence of temperature was also investigated and the cell voltage versus time curves are shown in Fig.4a. It is observed that they all shows a similar trend

ACCEPTED MANUSCRIPT where the cell voltage declined sharply at the initial stage, then gradually increased within short time and remain approximately stable. The initial decrease of cell voltage can be ascribed to progressive nucleation and growth of carbon deposit which will

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increase the surface area of the electrode and the next increase of cell voltage was due to the consumption of CO32-, which will lead to stronger concentration polarization. As indicated, the cell voltage slightly decreased with the increasing temperature. The

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reason for this phenomenon is a faster electrochemical kinetics at elevated

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temperature and weaker electrode polarization. The corresponding XRD pattern is shown in Fig.4b, it is observed that SD-5, SD-6, SD-2 and SD-7 all exhibit two distinct diffraction peaks. One sharp peak at around 2θ =26° and the other at around 2θ =44.5°, which can be attributed to (002) and (100) diffraction peaks, respectively.

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The intensity of (002) peak increase with the temperature, which indicates the enhanced degree of graphitization of the cathodic product. The full width at half maximum of graphite (002) peak for SD-5, SD-6, SD-2 and SD-7 are 0.640°, 0.521°,

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0.387° and 0.271°, respectively. The decline of full width at half maximum of

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graphite (002) peak also confirmed the enhanced crystallinity of the graphite nanostructure by elevating the temperature. Fig.4c exhibits Raman spectra of SD-5, SD-6, SD-2 and SD-7. All the samples have two obvious peaks located at around 1340 cm-1 and 1585 cm-1 which can be assigned to the D band and G band, respectively. The integral intensity ratio of ID/IG represent the degree of graphitization. The ID/IG is 1.417, 1.430, 1.017 and 1.475 for SD-5, SD-6, SD-2 and SD-7, respectively. The value of ID/IG do not decrease obviously with increasing temperature

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surface defects, at higher operating temperatures.

ACCEPTED MANUSCRIPT Fig.4 (a) Cell voltage-time curves conducted at different temperatures; (b) XRD patterns of the SD-5, SD-6, SD-2 and SD-7; (c) Raman spectra of the SD-5, SD-6, SD-2 and SD-7.

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The morphologies and nanostructures of the graphite obtained under different temperatures are shown in Fig.S5. As shown in Fig.S5a and Fig. S5b, it can be observed that the SD-5 sample mainly contain aggregated nano-particle and

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interwoven carbon chains. The carbon chains are covered with some quasi-sphere

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forming a rough surface. The Fig.S5c and Fig. S5d indicate that the SD-6 consists of graphite platelet, aggregated nano-particle and carbon fibers. And when the temperature elevated to 725

, the main product is aggregated quasi-sphere, as shown

in Fig.S5e and Fig.S5f. And from the SEM images of SD-7, aggregated carbon

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particle, carbon nano-fiber and carbon sheet can be detected. The above results demonstrated that all the samples comprise a fraction of carbon nano-particles. Typical transmission electron microscope (TEM) image of graphite obtained

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under different temperatures are shown in Fig.5. The product obtained at 625

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shown in Fig.5a and Fig.5b display interwoven carbon chains with a diameter of about 50nm and aggregated carbon particles which is in accordance with SEM. When the electrolysis temperature is increased to 675

, the images in Fig.5c and Fig.5d

reveal that carbon fiber and carbon sheet is the main product. The formation of the wire-like carbon products may be related to the connection of the small quasi-spherical particles at higher temperature or lithium intercalation. By further elevating temperature to 725

, aggregate carbon particles and ultra-thin graphite

ACCEPTED MANUSCRIPT sheet encapsulated carbon chain was observed in Fig.5e and Fig.5f. At 775 , the patterns present in Fig.5g and Fig.5h indicate that carbon nano-tube, carbon fiber and graphene are included in sample SD-7. However, they are not stable and will be

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destroyed when HRTEM or selected area electron diffraction (SAED) was employed. Morphological observations in Fig.5 are quite different from those obtained in CaCl2 based molten salt without the addition of Li2SO4[20, 23], where uniform

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morphologies are obtained.

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Fig.5 TEM images of SD-5 (a, b), SD-6 (c, d), SD-2 (e, f) and SD-7(g, h). In order to investigate the sulfur content and sulfur bonding configurations, X-ray photoelectron spectroscopy (XPS) were applied to characterized the SD-5,

ACCEPTED MANUSCRIPT SD-6, SD-2 and SD-7. The survey spectra (Fig. 6a) show two distinct and two weak peaks, assigned to C 1s, O 1s and S 2p, respectively. The C, O and S atom content of samples at different temperature based on XPS analyses are shown in Table S1.

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Detailed analyses of the high-resolution C 1s, O 1s and S 2p XPS spectra of SD-2 are shown in Fig.6b, Fig.6c and Fig.6d, respectively. The high-resolution C 1s depicted in Fig.6b can be deconvoluted into four peaks centered at 284.1, 284.8, 285.6 and

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288.6 eV, assigned to C-S-C, C-C, C-O and C=O, respectively [38]. The O 1s peaks

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located at 531.5, 532.3 and 533.1 eV, assigned to C=O, C-O-C and sulfone, respectively. The main peaks observed in S 2p region (Fig. 6d) at 164.2 and 165.2 eV (S 2p 1/2) are attributed to sulfide groups (C-S-C). The higher-energy doublet at 168.7 and 169.5 eV is assigned to sulfone bridges (C-SOx-C) [39], consistent with the C 1s

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and O 1s spectra. The high-resolution S 2p of SD-5, SD-6 and SD-7 can be seen in Fig. S6, which show similar results. According to the XPS results, the sulfur is successfully incorporated into carbon matrix. The atom percentage of sulfur and

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oxygen group in SD-5, SD-6, SD-2 and SD-7 are presented in Fig.6e and Fig.6f. The

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total sulfur and oxygen percentage decrease at higher temperature, indicating higher temperature is benefit for removing oxygen. When CO2 is bubbled into the melt, CO2 is captured and electro-converted via reaction (1) and reaction (2), (3): O2- + CO2(g) = CO32-

(1)

CO32- + 4e- = C + 3O2-

(2)

CO32- + 2e- = CO + 2O2-

(3)

ACCEPTED MANUSCRIPT The SO42- introduced can mainly be electro-transformed via reactions (4), (5) and (6) at the cathode: (4)

SO42- + 6e- = S + 4O2-

(5)

SO42- + 8e- = S2- + 4O2-

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SO42- + 2e- = SO2 + 2O2-

(6)

It is inferred that S can react with imperfect carbon in the carbon matrix to form

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CS2 gas at a high temperature. The elimination of CS2 gas contributes to the removal

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of imperfect carbon, leading to a higher degree of graphitization [36]. The S2- are also regarded to be responsible for the deoxygenation of graphite, which is benefit to a

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higher degree of graphitization.

Fig.6 (a)XPS spectra of the SD-2 sample; High-resolution XPS at (b) C 1s, (c) O 1s and (d) S 2p regions of the SD-2 deposited in molten CaCl2-NaCl-Li2SO4(3 mol%%)-CaO at 725 °C; (e) and (f) The atom percentage of oxygen and sulfur group in SD-5, SD-6, SD-2 and SD-7.

ACCEPTED MANUSCRIPT The current efficiencies and energy consumption for sulfur doped graphite based on the following formula:

η=

mr mt

(7)

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Where mr(g) is the washed electrolytic carbon obtained at the cathode and mt(g) is the theoretical mass of carbon in different electrolytic conditions, and the mt(g) can

mt =

QM 3It = zF F

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be calculated through equation:

(8)

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Where I is the current employed and t is the electrolytic time; F is the faraday constant; z is the transfer electron number; M is the molar mass of carbon. Then the energy consumption can be calculated as: EC =

UIt mr /1000

(9)

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Where U is cell voltage record during constant current electrolysis. The corresponding current efficiencies and energy consumption for sulfur doped

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graphite are displayed in Fig.7. As displayed in Fig.7a, the current efficiency for SD-2 is higher those of SD-1 and SD-3, indicating the optimum amount of Li2SO4 is

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3 mol%. Compared with SD-2 and SD-4, the current efficiency for SD-2 is obvious higher, demonstrating that the current efficiency can also be optimized by introducing Li+. The current efficiencies for SD-5, SD-6, SD-2 and SD-7 are 86.32%, 58.37%, 47.96% and 23.52%. Apparently, the current efficiency declined with the increasing temperature as more CO will be generated at higher temperature, which is in accordance with other reported investigations [23]. It should be pointed out that the current efficiency was calculated by weighting the mass of deposition on the cathode.

ACCEPTED MANUSCRIPT Therefore, the weight loss will be found in the melt and during washing process. The energy consumption for each electrolytic condition was also calculated and shown in

Fig.7b. The minimum energy consumption is 44.06 kWh for 1 Kg of carbon obtained

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for SD-5 with the corresponding highest current efficiency of 86.32%. Apparently, the current efficiency declines and energy consumption increases with the increasing temperature as more CO will be generated at higher temperature. Moreover, the

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current efficiency decreases and energy consumption increases with the addition of

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SO42-, as S and S2- will generate and they can react with imperfect carbon in the

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carbon matrix to form CS2 gas at a high temperature [22].

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Fig.7 Current efficiency (a) and energy consumption (b) calculated by experimental data under different electrolytic conditions. Afterwards, the intrinsic mechanism and reasons for the improvement of crystallinity of the graphite nanostructure by introducing Li+, SO42- or increasing temperature has shown in Fig.8. It is known that CO2 can be captured by O2- ions and

ACCEPTED MANUSCRIPT converted into CO32- when bubbled into the melt and it is easier for CO32- to be reduced at the cathode. Fig.8 shows the electrochemical reduction process of CO32- in CaCl2-NaCl-CaO-Li2SO4. CO32- can be electrochemical reduction to carbon and CO

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at the same time with the generation of O2- ions at as the above demonstrated equation (2) and (3).

Considering that amorphous carbons usually bond some oxygen atoms on their

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surfaces, the generated O2- ions will contribute to the defects and lower crystallinity.

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From the equation (2) and (3), it is observed that less O2- ions will be generated when CO was produced. Furthermore, the CO gas at the cathode can accelerate the diffusion of O2- ions at the cathode, meaning less O2- ions will accumulate at the cathode. In previous work, we have found that the reduction potential of CO will decrease with

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the increasing temperature. So the graphitization degree can be enhanced by increasing temperature.

When a large number of O2- ions are generated at the cathode, the metal ions will

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migrate to the cathode to form metal oxides and then precipitated, which has been

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systematically investigated previously, [40] in order to keep it neutral. By introducing Li+, the O2- ions can bond to Li+ which is the most stable metal oxide in this system. The SO42- introduced can mainly be electro-transformed via following reactions:

SO42- + 2e- = SO2 + 2O2-

SO42- + 6e- = S + 4O2SO42- + 8e- = S2- + 4O2-

ACCEPTED MANUSCRIPT It is inferred that S and S2- can also react with O in carbon matrix to form SO2 gas at high temperature, leading to a higher degree of graphitization. The S are also regarded to be responsible for the removal of imperfect carbon in the carbon matrix to

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form CS2 gas at a high temperature, which is benefit to a higher degree of

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graphitization.

Fig. 8 The scheme of intrinsic mechanism for the improvement of crystallinity of the

4.

Conclusions

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graphite nanostructure by introducing Li+, SO42- or increasing temperature.

The sulfur doped graphite was successfully synthesized by capture and

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electrochemical conversion of CO2 in CaCl2-NaCl-Li2SO4. The temperature, concentration of Li2SO4 and the melt composition play important roles for controlling

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and shaping the microstructure of the cathodic products. The graphite microstructures with enhanced crystallinity can be obtained by the introduction of Li2SO4. Both Li+ and SO42- have been found to be positive in improving the graphitization of carbon deposition. In addition, the degree of graphitization is optimized by increasing the concentration of Li2SO4 or elevating the temperature. Aggregated carbon particles are the main product in all samples while ultrathin graphite sheet, wire-like carbon, carbon nano-tube and interwoven carbon chains can be obtained in some cases by

ACCEPTED MANUSCRIPT changing electrolytic conditions. In summary, the degree of graphitization, current efficiency and energy consumption could all be improved by the introduction of Li2SO4. The electrolytic synthesis of sulfur doped graphite microstructures offers a

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commercial competitive way to produce heteroatom-doped nanostructured carbons for wide applications.

Supplementary information

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Supporting information and chemical compound information are available in the

Acknowledgements

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online version of the paper.

The authors are grateful to the National Natural Science Foundation of China (No. 51804056) and the Fundamental Research Funds for the Central Universities (Project

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No. 2018CDXYCL0018).

ACCEPTED MANUSCRIPT References [1] L.H. Ziska, L.L. McConnell, Climate Change, Carbon Dioxide, and Pest Biology: Monitor, Mitigate, Manage, J. Agric. Food Chem. 64(1) (2016) 6-12.

RI PT

[2] E.A.G. Schuur, A.D. McGuire, C. Schädel, G. Grosse, J.W. Harden, D.J. Hayes, and et al., Climate change and the permafrost carbon feedback, Nature 520 (2015) 171.

SC

[3] F.D. Meylan, V. Moreau, S. Erkman, CO2 utilization in the perspective of

M AN U

industrial ecology, an overview, J. CO2 Util. 12 (2015) 101-108.

[4] W.Y. Gao, H. Wu, K. Leng, Y. Sun, S. Ma, Inserting CO2 into Aryl C-H Bonds of Metal-Organic Frameworks: CO2 Utilization for Direct Heterogeneous C-H Activation, Angew. Chem. Int. Edit. 128(18) (2016) 5562-5566.

TE D

[5] S. Chandrasekaran, P.G. Campbell, T.F. Baumann, M.A. Worsley, Carbon aerogel evolution: Allotrope, graphene-inspired, and 3D-printed aerogels, J Mater. Res. 32(22) (2017) 4166-4185.

EP

[6] W. Kiciński, M. Szala, M. Bystrzejewski, Sulfur-doped porous carbons: Synthesis

AC C

and applications, Carbon 68 (2014) 1-32. [7] M. Gattrell, N. Gupta, A. Co, A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper, J Electroanal. Chem. 594(1) (2006) 1-19. [8] B.A. Rosen, A. Salehi-Khojin, M.R. Thorson, W. Zhu, D.T. Whipple, P.J.A. Kenis, R.I. Masel, Ionic Liquid–Mediated Selective Conversion of CO2 to CO at Low Overpotentials, Science 334 (2011) 643–644. [9] H. Yin, X. Mao, D. Tang, W. Xiao, L. Xing, H. Zhu, and et al., Capture and

ACCEPTED MANUSCRIPT electrochemical conversion of CO2 to value-added carbon and oxygen by molten salt electrolysis, Energ. Environ. Sci. 6(5) (2013) 1538-1545. [10] H.V. Ijije, R.C. Lawrence, G.Z. Chen, Carbon electrodeposition in molten salts:

RI PT

electrode reactions and applications, RSC Adv. 4(67) (2014) 35808-35817. [11] J. Ge, L. Hu, W. Wang, H. Jiao, S. Jiao, Electrochemical Conversion of CO2 into Negative Electrode Materials for Li-Ion Batteries, ChemElectroChem 2(2) (2015)

SC

224-230.

M AN U

[12] J. Ge, S. Wang, L. Hu, J. Zhu, S. Jiao, Electrochemical deposition of carbon in LiCl–NaCl–Na2CO3 melts, Carbon 98 (2016) 649-657.

[13] W. Weng, L. Tang, W. Xiao, Capture and electro-splitting of CO2 in molten salts, J. Energ. Chem. (2018) In press.

TE D

[14] M.A. Hughes, J.A. Allen, S.W. Donne, Carbonate Reduction and the Properties and Applications of Carbon Formed Through Electrochemical Deposition in Molten Carbonates: A Review, Electrochim. Acta 176 (2015) 1511-1521.

EP

[15] A. Douglas, C.L. Pint, Review—Electrochemical Growth of Carbon Nanotubes

AC C

and Graphene from Ambient Carbon Dioxide: Synergy with Conventional Gas-Phase Growth Mechanisms, ECS J. SOLID STATES C. 6(6) (2017) M3084-M3089. [16] A. Douglas, R. Carter, M. Li, C.L. Pint, Toward Small-Diameter Carbon Nanotubes Synthesized from Captured Carbon Dioxide: Critical Role of Catalyst Coarsening, ACS Appl. Mater. Inter. 10(22) (2018) 19010-19018. [17] A. Stein, Z. Wang, M.A. Fierke, Functionalization of Porous Carbon Materials with Designed Pore Architecture, Adv. Mater. 21(3) (2009) 265-293.

ACCEPTED MANUSCRIPT [18] Z. Li, S. Dai, Surface Functionalization and Pore Size Manipulation for Carbons of Ordered Structure, Chem. Mater. 17(7) (2005) 1717-1721. [19] L. Hu, Y. Song, J. Ge, S. Jiao, J. Cheng, Electrochemical Metallurgy in

RI PT

CaCl2-CaO Melts on the Basis of TiO2·RuO2 Inert Anode, J Electrochem. Soc. 163(3) (2016) E33-E38.

[20] L. Hu, Y. Song, J. Ge, J. Zhu, S. Jiao, Capture and electrochemical conversion of

SC

CO2 to ultrathin graphite sheets in CaCl2-based melts, J Mater. Chem. A 3(42) (2015)

M AN U

21211-21218.

[21] B. Deng, X. Mao, W. Xiao, D. Wang, Microbubble effect-assisted electrolytic synthesis of hollow carbon spheres from CO2, J Mater. Chem. A 5(25) (2017) 12822-12827.

TE D

[22] L. Hu, Y. Song, J. Ge, J. Zhu, Z. Han, S. Jiao, Electrochemical deposition of carbon nanotubes from CO2 in CaCl2–NaCl-based melts, J Mater. Chem. A 5(13) (2017) 6219-6225.

EP

[23] L. Hu, Y. Song, S. Jiao, Y. Liu, J. Ge, H. Jiao, and et al., Direct Conversion of

AC C

Greenhouse Gas CO2 into Graphene via Molten Salts Electrolysis, ChemSusChem 9(6) (2016) 588-594.

[24] L.F. Chen, Z.H. Huang, H.W. Liang, H.L. Gao, S.H. Yu, Three-Dimensional Heteroatom-Doped Carbon Nanofiber Networks Derived from Bacterial Cellulose for Supercapacitors, Adv. Func. Mater. 24(32) (2014) 5104-5111. [25] J. Zhang, L. Dai, Heteroatom-Doped Graphitic Carbon Catalysts for Efficient Electrocatalysis of Oxygen Reduction Reaction, ACS Catal. 5(12) (2015) 7244-7253.

ACCEPTED MANUSCRIPT [26] L. Yang, S. Jiang, Y. Zhao, L. Zhu, S. Chen, X. Wang, and et al., Boron-Doped Carbon Nanotubes as Metal-Free Electrocatalysts for the Oxygen Reduction Reaction, Angew. Chem. Int. Edit. 50(31) (2011) 7132-7135.

RI PT

[27] Z. Wang, L. Qie, L. Yuan, W. Zhang, X. Hu, Y. Huang, Functionalized N-doped interconnected carbon nanofibers as an anode material for sodium-ion storage with excellent performance, Carbon 55 (2013) 328-334.

SC

[28] Y. Yan, Y.X. Yin, S. Xin, Y.G. Guo, L.J. Wan, Ionothermal synthesis of

M AN U

sulfur-doped porous carbons hybridized with graphene as superior anode materials for lithium-ion batteries, Chem. Commun. 48(86) (2012) 10663-10665. [29] J.W. Jeon, R. Sharma, P. Meduri, B.W. Arey, H.T. Schaef, J.L. Lutkenhaus, and et al., In Situ One-Step Synthesis of Hierarchical Nitrogen-Doped Porous Carbon for

TE D

High-Performance Supercapacitors, ACS Appl. Mater. Inter 6(10) (2014) 7214-7222. [30] V.I. Zaikovskii, K.S. Nagabhushana, V.V. Kriventsov, K.N. Loponov, S.V. Cherepanova, R.I. Kvon, and et al, Synthesis and Structural Characterization of

EP

Se-Modified Carbon-Supported Ru Nanoparticles for the Oxygen Reduction Reaction,

AC C

J Phys. Chem. B 110(13) (2006) 6881-6890. [31] J. Tang, J. Liu, N.L. Torad, T. Kimura, Y. Yamauchi, Tailored design of functional nanoporous carbon materials toward fuel cell applications, Nano Today 9(3) (2014) 305-323.

[32] J. Song, T. Xu, M.L. Gordin, P. Zhu, D. Lv, Y.B. Jiang, and et al., Nitrogen-Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High-Areal-Capacity Sulfur Cathode with Exceptional Cycling

ACCEPTED MANUSCRIPT Stability for Lithium-Sulfur Batteries, Adv. Func. Mater. 24(9) (2014) 1243-1250. [33] S. Liu, Y. Cai, X. Zhao, Y. Liang, M. Zheng, H. Hu, and et al., Sulfur-doped nanoporous carbon spheres with ultrahigh specific surface area and high

RI PT

electrochemical activity for supercapacitor, J Power Sources 360 (2017) 373-382. [34] H.L. Poh, P. Šimek, Z. Sofer, M. Pumera, Sulfur-Doped Graphene via Thermal Exfoliation of Graphite Oxide in H2S, SO2, or CS2 Gas, ACS Nano 7(6) (2013)

SC

5262-5272.

M AN U

[35] M. Varga, T. Izak, V. Vretenar, H. Kozak, J. Holovsky, A. Artemenko, and et al, Diamond/carbon nanotube composites: Raman, FTIR and XPS spectroscopic studies, Carbon 111 (2017) 54-61.

[36] Z. Chen, Y. Gu, L. Hu, W. Xiao, X. Mao, H. Zhu, D. Wang, Synthesis of

TE D

nanostructured graphite via molten salt reduction of CO2 and SO2 at a relatively low temperature, J Mater. Chem. A 5(39) (2017) 20603-20607. [37] M.S. Dresselhaus, G. Dresselhaus, R. Saito, A. Jorio, Raman spectroscopy of

EP

carbon nanotubes, Phys. Rep. 409(2) (2005) 47-99.

AC C

[38] W. Li, D. Yang, H. Chen, Y. Gao, H. Li, Sulfur-doped carbon nanotubes as catalysts for the oxygen reduction reaction in alkaline medium, Electrochim. Acta 165 (2015) 191-197.

[39] Z. Chen, B. Deng, K. Du, X. Mao, H. Zhu, W. Xiao, D. Wang, Flue-Gas-Derived Sulfur-Doped Carbon with Enhanced Capacitance, Advanced Sustainable Systems 1(6) (2017) 1700047. [40] M. Gao, B. Deng, Z. Chen, M. Tao, D. Wang, Cathodic reaction kinetics for CO2

ACCEPTED MANUSCRIPT capture and utilization in molten carbonates at mild temperatures, Electrochem.

AC C

EP

TE D

M AN U

SC

RI PT

Commun. 88 (2018) 79-82.