Molybdenum Carbide-Derived Chlorine-Doped Ordered Mesoporous

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Molybdenum Carbide-derived Cl-doped Ordered Mesoporous Carbon with Few-layered Graphene Walls for Energy Storage Applications Zongkui Kou, Beibei Guo, Yufeng Zhao, Shifei Huang, Tian Meng, Jie Zhang, Wenqiang Li, Ibrahim Saana Amiinu, Zonghua Pu, Min Wang, Min Jiang, Xiaobo Liu, Yongfu Tang, and Shichun Mu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14440 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on January 6, 2017

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ACS Applied Materials & Interfaces

Molybdenum Carbide-derived Cl-doped Ordered Mesoporous Carbon with Few-layered Graphene Walls for Energy Storage Applications Zongkui Koua, Beibei Guoa, Yufeng Zhaob*, Shifei Huangb, Tian Menga, Jie Zhanga, Wenqiang Lia, Ibrahim Saana Amiinua, Zonghua Pua, Min Wanga, Min Jianga, Xiaobo Liua, Yongfu Tangb and Shichun Mua* a

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of

Technology, Wuhan 430070, PR China. b

Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China.

a

*Corresponding author. Tel: +86 27 87651837. E-mail: [email protected] (Shichun Mu).

b

*Corresponding author. Tel: +86 335 8387743. E-mail: [email protected] (Yufeng Zhao).

Abstract:

In this work, we propose a one-step process to realize the in-situ evolution of molybdenum

carbide (Mo2C) nanoflakes into ordered mesoporous carbon with few-layered graphene walls (OMG) by chloridization and self-organization, and simultaneously the Cl-doping of OMG (OMG-Cl) by modulating chloridization and annealing processes is fulfilled. Benefiting from the improvement of electroconductivity induced by Cl-doping, together with large specific surface area (1882 cm2 g-1) and homogeneous pore structures, as anode of LIBs, OMG-Cl shows remarkable charge capacity of 1305 mAh g-1 at current rate of 50 mA g-1 and fast charge-discharge rate within dozens of seconds (a charge time of 46 s), as well as retains a charge capacity of 733 mAh g-1 at a current rate of 0.5 mA g-1 after 100 cycles. Furthermore, as promising electrode material for supercapacitors, OMG-Cl holds the specific capacitance of 250 F g-1 in 1M H2SO4 solution and 220 F g-1 at a current density of 0.5 A g-1 in 6M KOH solution, which are ~ 40% and 20% higher than those of undoped OMG electrode, respectively. The high capacitive performance of OMG-Cl material

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can be due to the additional fast Faradic reactions induced from Cl-doping species.

Key words:

Molybdenum carbide, Chlorine-doped ordered mesoporous carbon, few-layered graphene

wall, Supercapacitor, Li ion battery

Introduction In response to energy conservation and emissions reduction, the application of high performance energy storage devices (ESDs) must be expanded to a wider range, particularly to electric vehicles (EVs) and smart grids.

1

Due to their convenience of synthesis and regulation of structures, along with high specific surface

area (SSA), ordered mesoporous carbon (OMC), particularly OMC with few-layered graphene walls (OMG) has been playing a vital role in prospective ESDs.

2-4

To tread on the heels of this development towards

meeting ESDs requirements, advanced OMG electrode with superior capacity, excellent rate performance, and good structural stability are still urgently required.

Beyond all doubts, templating is one of the most popular pathways to synthesis of OMG. 5-7 For example, ordered mesoporous carbon or element-doped materials can also be by soft-templating methods hard-templating methods.

12, 13

8-11

or

Despite the high possibility to preserve the structural ordering and higher

graphitic degree of the resulting carbon networks by using the templating strategy, 14-16 this method is complex and expensive as a result of the multi-steps, harsh processing conditions and templates requirements. The latest research finding indicated that hierarchical porous carbons (i.e. micro and mesoporous carbons) could also be prepared from interpenetrating polymer networks without any template.17 So, high-powered OMC electrodes with superior structures based on a simple or template-free method are still urgently needed. Interestingly, high surface area micro- and mesoporous carbon can be simply obtained by abstracting the


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non-carbon atoms from crystalline carbide which is noted for the carbide-derived carbon (CDC) method. 17 By adjusting experimental configurations, various carbon microstructures such as amorphous carbon, graphene sheets, ordered graphite and graphite ribbons can be obtained.

18, 19

In theory, such a simple method can also

be used for fabricating OMG network, but has not been previously reported.

More unfortunately, OMG has few active sites and insufficient electrical conductivity which pull down their electrochemical performance.

20

As reported, the doping of heteroatoms, such as boron, nitrogen and

sulfur into the carbon lattice can greatly modify the electronegativity of carbon atoms attached to heteroatoms and electrical properties of carbon materials. Consequently, much effort has been contributed to preparation of heteroatom-doped OMG which couple the high mesoporosity with unique electronic configuration arising from the doping of hetero-elements. For example, Cheng et al synthesized mesoporous carbon with uniform boron dopants and enhanced interfacial capacitance by 1.5-1.6 times in contrast to undoped mesoporous carbon. 21 In addition, nitrogen doped ultramicroporous carbon spheres with regular porous structures can also possess the excellent electrochemical performances.

22

Obviously, heteroatomic doping can remarkably

enhance physicochemical properties of OMG framework, e.g. electrical conductivity and specific active sites, predominantly contributing to higher electrochemical performance for ESDs. 23

In this work, we conduct a one-step process to obtain OMG by chlorination and self-organization without templates at a relatively low temperature (700 °C) via using molybdenum carbide (Mo2C) nanoflakes as precursor, and simultaneously to realize the in-situ Cl-doping of OMG (OMG-Cl) (Scheme 1). The resulting OMG network is present by ordered and interconnected mesopores, which possess dual-modal pore size distribution centered at ~2 and ~3.5 nm, uniform porous structure and large specific surface area. Importantly, OMG-Cl is first performed as anode of LIBs and electrode materials for supercapacitors. It is demonstrated



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that Cl-doping can greatly stimulate electrochemical capacity of OMG network which is comparable to most heteroatom-doped OMG electrode materials. Moreover, to clarify the intrinsic causes of the elevated electrochemical activity for OMG-Cl, contact angle, zeta potential and cyclic voltammetry analyses are carried out.

Scheme 1 Template-free self-organization of Cl-doped ordered mesoporous carbon with few-layered graphene walls. ① Crystal model of pristine Mo2C; ② The corresponding schematic of Reaction 1 in Supplementary Information; ③ The rearrangement of reserved carbon atoms into few-layered graphene nanoflakes; ④ The self-organization of few-layered graphene flakes into OMG framework with the aid of Cl2 gas molecules; ⑤ The doping of Cl atoms into OMG framework to form Cl-doped OMG.

Experimental Methods Preparation of OMG and OMG-Cl. In this experiment, 2 g of commercial Mo2C nanoflakes with average size of < 0.5 µm (that is obtained from Sigma-Aldrich Co. LLC) was placed into a small porcelain


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boat and dried at 80 °C for 8 h. Next, the samples were heated to set temperature (700 ℃). Subsequently, Cl2 gas generated by a typical chemical reaction between KMnO4 powder and excess of concentrated hydrochloric acid solution (HCl 36 wt %) (mole ratio of 1: 8) was passed into the tube furnace at a certain rate of 15 sccm (the rate was detected by a mass flowmeter) to etch Mo2C flakes into amorphous carbon networks. Next, the temperature is held at 700 °C for 4 h to enable further transformation from unordered carbon frameworks to OMG framework with the aid of residual Cl2 gas and Ar gas (Scheme. 1). When dosage amount of Cl2 gas was controlled at 22.1 mol by a mass flowmeter, the outcome was OMG, but further dosage reached up to 36.9 mol, Cl-doped OMG was obtained. Additional experimental details are contained in Supplementary Information.

Physical characterization. Field emission transmission electron microscope (TEM) and high resolution TEM (HRTEM) were performed on a JEM 2100F. Field emission scanning electron microscope (FESEM) images were recorded on JEOL. X-ray diffraction (XRD) analysis was taken with a Rigaku diffractometer using a Cu Kα X-ray source carried out at 45 kV, at 2 º/min from 10° to 90°. X-ray photoelectron spectroscopy (XPS) analysis was implemented to analyze the surface chemical composition of the samples on a VG Scientific ESCALAB 210 with Mg KR radiation at 14 kV. Raman spectroscopy was applied using a RENISHAW Raman microscope with Ne-He excitation operated at a wavelength of 633 nm. Fourier Transform Infrared Spectroscopy (FTIR) was conducted on a spectrometer (Nicolet MAGNA-IR 560, USA). N2 adsorption–desorption isotherms were measured by a Micromeritics ASAP 2020 Brunauer–Emmett–Teller (BET) analyzer. The pore volume and pore size were calculated by the Barrett−Joyner−Halenda (BJH) method. The zeta potentials of our samples were investigated at 25 ℃ by a zeta potential analyzer (Malvern, Zetasizer Nano, ZS90). Wettability of the undoped and Cl-doped OMG was evaluated by measuring the contact angle of water, 1M H2SO4, and 6M KOH droplets on the sample electrode films using a microliter syringe at ambient



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temperature.

Preparation of the working electrode. The working electrodes for Li ion anode materials were prepared by mixture of 70 wt % OMG samples, 10 wt% polyvinylidene fluoride binder dissolved in N-methyl-2-pyrrolidinone, and 20 wt % acetylene black (Super-P). After brushing the above slurry on a Cu foil, the obtained electrodes were dried at 80 °C under vacuum for 12 h to eliminate the solvent before pressing. Then the electrode (coated Cu foil) was cut into small rounds and dried at 80 °C for 24 h before assembling coin cells. The Li/OMG network cells were fabricated under 1 ppm oxygen and water vapor in an argon-filled glovebox where lithium metal as counter/reference electrode, a Celgard 2400 membrane separator and 1 M LiPF6 electrolyte solution dissolved in a mixture of ethylene carbonate and dimethyl carbonate (1:1 v/v) were used. Finally, CR2032 (3 V) coin-type cells can be achieved for electrochemical measurements.

The working electrodes for supercapacitor were made up of 85 wt % OMG, 10 wt % acetylene carbon black, and 5 wt % PTFE binder (60% suspension in water). The prepared slurry was coated onto a nickel foam for tests in 6 M KOH and a platinum mesh for tests in 1 M H2SO4 with a size of 1 × 1 cm and a loading amount of 4 mg cm-2, and dried at 120 °C under vacuum for 12 h to remove the solvent.

Electrochemical measurements. Galvanostatic charge-discharge cycles for Li ion anode materials were recorded on a LAND CT2001A electrochemical workstation at different current densities of 50 mA g-1 to 10 A g-1 between 3.0 and 0.01 V versus Li+/Li at room temperature.

Electrochemical investigations for supercapacitor were recorded on a three-electrode system where the platinum wire was used as counter electrode, and Hg/HgO (0.1 M KOH) was a reference electrode in 6 M KOH while the reference electrode was substituted by Hg/Hg2Cl2 (SCE) in 1 M H2SO4. The frequency range


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of 105 Hz to 10-2 Hz and an ac modulation of 5 mV were performed to acquire electrochemical impedance spectroscopy (EIS) spectra. Galvanostatical charge-discharge tests were taken in 6 M KOH at different densities by cycling equipment (Land CT2001A, China). The capacitance was calculated by the following equation:

C=

I m

dV dt

(1)

where I (A) is the discharge current, m (g) is the mass of active materials, and (dV)/(dt) (V s-1) is the gradient of discharge curves.

Results Synthesis and characterization of OMG-Cl framework. The self-organisation technique is employed for the synthesis of OMG network consisted of few-layered graphene by direct chloridizing commercial Mo2C nanoflakes (see details in the experimental methods). Scheme. 1 presents the synthetic mechanism for OMG framework. As Cl2 gas reacts with Mo2C (Reaction. S1), Mo atoms in Mo2C nanoflakes are gradually removed via forming volatile MoCl5 gaseous molecules. When the mass to mole ratio of Mo2C and Cl2 gas reaches 2g: 22 mol, Mo atoms are completely reacted. 17 Importantly, for the sake of principle of lowest energy, the remaining C atoms are preferentially arranged into a graphene-like honeycomb structure. Interestingly, increasing the amount of Cl2 gas to 36.9 mol, the redundant Cl atoms not only can induce the formation of carbon network structure and but be automatically doped into this network.

After Cl2 etching reaction, it can be found that pristine lamellate Mo2C (Fig. S1) is evolved into carbon networks with high SSA (1991 m2/g) and mesoporous structures (Fig. S2), which also can be verified by the emergence of new diffraction peak (002) of OMG network (Fig. S3). Carbon (10) peak centered at 42.7° can



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be assigned to in-plane diffraction of a graphene sheet. Furthermore, these mesopores with diameter 3-5 nm (see bright zones circled by walls, Fig. S2B) are built by graphene nanosheets (Fig. 1A and B). Fig. 1C discloses a curly carbon plate networks as depicted in step 4 of Scheme 1. The thickness of the pore wall was calculated based on the follow equations: 23 d100 = λ/2sinθ, a = 2×3-1/2d100 and WT = a-D, where WT is the thickness of wall, a is a unit cell parameter, D is pore diameter obtained from the N2 sorption measurements, θ is diffraction angle and λ is wave length of Cu Kα. As a result, the new produced OMG-Cl exhibits the wall thickness of 5.33 nm (Table S1) corresponding to about 16-layer graphene which is more than that from our TEM observations (where the number of graphene layers is 2-5). This suggests that the graphene nanosheets can be exfoliated from the pore walls by sonicating and dispersing OMG network into alcohol during TEM sample preparation. Fig. 1D-F show elemental distribution of C and Cl in the OMG-Cl network in which Cl element is uniformly interspersed in the mesoporous networks of OMG. These findings demonstrate a successful one-step synthesis of OMG and OMG-Cl framework. Whereas for pristine Mo2C nanoflakes with carbon-rich surfaces (Fig. S1A-C), their sharply X-ray diffraction peaks (Fig. S1D) can be well matched with PDF35-0787 of monocrystal Mo2C, indicating that The ordered superstructure of OMG framework was investigated by low-angle x-ray diffraction (Fig. 1G) and non-linear dark-field TEM (Fig. S4). XRD pattern shows characteristic (100), (110), and (200) peaks of hexagonal packing, indicating a perfect pore ordering of the superstructure. 1 Nitrogen adsorption-desorption measurements further suggest a dual-modal pore size distribution (Fig. 1I) centered at ~2 and ~3.5 nm in both OMG and OMG-Cl, which also can be observed by TEM (Fig. 1B). They own similar N2 adsorption-desorption isotherms with a Langmuir hysteresis (Fig. 1H). For OMG-Cl, it exhibits structural features with high SSA (1882 m2 g-1), average pore width (2.64 nm), where the most prominent pores are smaller than 3.5 nm, which are very close to that of OMG (1991 m2 g-1, 2.64 nm, 3.5 nm, respectively). This


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demonstrates that Cl-doping does not have a negative effect on the pore structure of OMG framework. Also, during the process of the carbon atomic rearrangement, few micropores can come into being due to the appearance of Mo atom vacancies (Fig. 1I), conducive to the high-efficiency ion transportation. the raw materials in this work were of perfect single-crystal Mo2C nanoflakes. 25

Figure 1 Morphology and pore structure analysis. (A, B) TEM and HRTEM images of OMG network. Number in Figure 1B represent layer number of “graphene wall”. (C) SEM image of cross-sectional “graphene wall” of OMG network. (D-F) Electron image and elemental mapping of Cl-doped OMG network. (G) Low-angle x-ray diffraction patterns of OMG-Cl and OMG. (H) N2 adsorption-desorption isotherms of



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OMG-Cl framework. (I) Pore size distribution of the samples.

Surface elemental analysis. SEM mapping was taken to prove a successful Cl-doping into OMG network. To further clarify the effect of Cl-doping, we performed spectroscopic analysis to identify surface bonding species in the carbon network. Raman spectroscopy is a powerful tool for probing carbide and its derivative carbon materials,

26

as well as doping state of graphitic carbon materials.

27

As shown in Fig. 2A, pristine

Mo2C has three peaks in the first region of 500-1000 cm-1, 28 making it distinct from its derivatives (OMG and OMG-Cl). The presence of D peak at 1329 cm-1 and G peak at 1592 cm-1 indicates that the surface of Mo2C nanoflakes is carbon-enriched, which is also confirmed by HRTEM images (Fig. S1B). After the conversion reaction, both OMG and OMG-Cl show distinctive peaks including the G-peak at 1589.3 and 1594.7 cm-1 corresponding to the E2g phonon, the dispersive D-peak at 1318.6 and 1322.6 cm-1, and the 2D-peak and D+D′-peak.

29

The blue shift of D peak and G peak of OMG-Cl relative to OMG implies an effective

Cl-doping. 27 Another evidence of Cl-doping is from the decreasing relative intensity of the 2D peak relative to the G peak.

28

The D-peak corresponds to the A1g phonon and arises from the breathing modes of the

hexagonal sp2 carbon rings, requiring the presence of a defect for its activation.

29

In this sense, after

Cl-doping, the ratio of defects in OMG-Cl (ID/IG=1.17) decreases relative to OMG (ID/IG=1.40), which can be evidenced by the higher carbon (10) fingerprint peak of OMG-Cl (Fig. S3).

25, 30

This indicates that the more

chlorine gas is introduced, the more sufficient conversion reaction takes place, which is demonstrated by less residual Mo atoms in the structure (Table S2). FTIR spectroscopy was employed to determine the chemical bonds of OMG and OMG-Cl. As shown in Fig. 2B, they reveal similar surface chemical bonding phases and slightly difference from pristine Mo2C. The peaks (~3430 and 1625 cm-1) are attributed to O-H bond for water molecules. 31, 32 Furthermore, the new wide bands at 1245 cm-1 for OMG and at 1258 cm-1 for OMG-Cl originate from vibrations of C-O-C and C-Cl,


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respectively,

33

which provide another strong evidence for Cl atomic doping. Note that the two tiny bands at

974 and 893 cm-1 for pristine Mo2C are assigned to in-plane bending of C-H and out-of-plane hydrogen wagging. 34

Figure 2 Spectroscopic analysis. (A, B) Raman spectrum and Infrared spectroscopy of the pristine Mo2C, OMG and OMG-Cl. (C-F) C1s spectrum of OMG (C) and OMG-Cl (D), and Cl 2p spectrum of OMG (E) and OMG-Cl (F).

The Cl-atomic doping was quantitatively investigated by XPS analysis. After the thermal chlorination, for OMG network, the Mo 3d and 3p peaks at 228-242 eV and 392-427 eV gradually disappears (Fig. S5A). 35 Consequently, the Cl 2p peak evolves with the enhanced C 1s peak. For C1s spectra of Mo2C (Fig. S5B), five peaks centered at 282.7 eV (C-Mo), 284.5 eV (C=C), 285.5 eV (C-C or C-H), 287.2 eV (C-O-C) and 287.2 eV (C=O) present different bonding states of C.

36

Compared with Mo2C, the C1s spectrum of OMG (Fig. 2C)



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only contains four peaks without C-Mo (282.7 eV), further indicating successful conversion from Mo2C to OMG network. In addition, for OMG-Cl, the appearance of C-Cl peak at 286.6 eV (Fig. 2D) demonstrates Cl-doping (1.38 at%, from Table S2 we can deduce the residual Cl2 amount physically adsorbed on/in pore surfaces) in the OMG network. Through comparing Mo 3d5/2 peaks of both OMG and pristine Mo2C (Fig. S5), no changes can be found for Mo-O bands at 232.6 eV and 235.6 eV apart from the substitution of Mo-C band (231.2 eV) by Mo-Cl (231.8 eV),

37

making clear a Cl2 gas substitution reaction process. Fig. 2E and F

present Cl 2p spectra of OMG and OMG-Cl. The two peaks at 200.6 and 202.2 eV are owed to Cl-C bond. 38 The atomic percent of Cl atoms bonded to C atoms increases in OMG-Cl (Table S2), manifesting the effective Cl doping. The difference between Cl-Mo at 198.2 eV for OMG and Cl-Cl at 198.6 eV for OMG-Cl also supports the Cl atomic doping reaction process.


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Figure 3. Galvanostatic charge-discharge profile, cyclic performance as well as Coulombic efficiency of the OMG (A, B) and OMG-Cl (C, D) electrodes at a low current rate of 50 mA g-1 between 3.0 and 0.01 V versus Li+/Li. Rate capabilities and galvanostatic charge-discharge profiles of the OMG (E, G) and OMG-Cl (F, H) electrodes across various current densities from 0.1 to 10 A g-1. (I) A Li/OMG network cell measured at a high current rate of 0.5 A g-1 for 100 cycles after the rate capability measurement.

Li ion storage capacity of OMG-Cl network. We investigated the electrochemical performance of OMG and OMG-Cl as anode material of LIBs (Fig. 3). Compared with pristine Mo2C nanoflakes (Fig. S6), the reversible capacity and cyclic performance of the obtained OMG framework are immensely enhanced at a current density of 50 mA g-1. The Coulombic efficiency increases from 42.6 % for the undoped OMG (Fig. 3B)



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to 59.2 % for the Cl-doped OMG (Fig. 3D) in the first cycle. It is convinced that, the higher initial Coulombic efficiency strongly depends on the lower BET surface area of active materials,

39

because the irreversible

reduction of the electrolyte leads to form a surface passivation layer on the active materials (namely, the ‘solid electrolyte interphase’.). However, lower BET surface area OMG-Cl (1882 m2 g-1) (Fig. 1H) comparing with undoped OMG (1991 m2 g-1) (Fig. S2D) suggests that the enhanced initial Coulombic efficiency may be owing to the role of Cl-doping. The OMG-Cl electrode (Fig. 3B) exhibits charge capacities of 1305 mAh g-1 in the first cycle and 1153 mAh g-1 after 20 cycles, which outperform that of the undoped OMG electrode (849 mAh g-1 in the first cycle and 897 mAh g-1 after 20 cycles) (Fig. 3D). As a result, the capacity retention can reach up to 88.4 % for the OMG-Cl, which is higher than N-doped or B-doped graphene anodes that have been widely studied. 39

Another important advantage of OMG-Cl as anode of LIBs is the fast charge and discharge performance. As shown in Fig. 3E-H, OMG network are first charged/discharged at 0.1 A g-1 for 10 cycles, and then the current density increases step by step to 10 A g-1, for 10 cycles at every current density. Obviously, both OMG and OMG-Cl electrodes present very large capacity at a current density of 0.1 A g-1, with similar capacity fluctuations observed in pristine Mo2C at higher current rates (Fig. S7). OMG-Cl can be charged to 741 mAh g-1 in about 90 min at a current density of 0.5 A g-1. At the same current density (5 A g-1), the reversible capacity of 250 mAh g-1 is present for OMG-Cl within charge time of ~ 3min (Fig. 3H), which is superior to 181 mAh g-1 for undoped OMG (Fig. 3G). Moreover, OMG-Cl electrode can be fully charged rapidly in dozens of seconds. At a very high current rate of 10 A g-1, corresponding to a charge time of ∼46 s (78 C) for OMG-Cl, the reversible capacity of 128 mAh g-1 can be achieved. All aboved results are far better than those of Mo2C (∼62 mAh g-1 at 10 A g-1, Fig. S6C and D) and some graphitic carbon materials that OMG-Cl is a superb anode of LIBs.


40-42

demonstrating

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To evaluate the cycling stability of OMG-Cl electrode, we investigated their reversible charge capacity at a moderate current rate of 0.5 A g-1 for 100 cycles (Fig. 3I). We notice that the charge capacity (733 mAh g-1) for OMG-Cl is superior to that of the undoped OMG (400 mAh g-1) and the pristine Mo2C nanoflakes (146 mAh g-1). After 100 cycles, OMG-Cl electrode can reach up to 100 % capacity retention, which demonstrate excellent cycling stability of OMG-Cl in the process of lithium intercalation/de-intercalation.

Capacitive properties of OMG-Cl networks. The capacitive properties of OMG networks were evaluated using a conventional 3-electrode system in aqueous 6M KOH and 1M H2SO4 electrolytes, respectively. For both OMG and OMG-Cl coated working electrodes, they nearly rectangular cyclic voltammetry (CV) curves are present at 10 mV s-1 (Fig. S8A) in the 6M KOH electrolyte, representative of an ideal efficient electronic double layer capacitor (EDLC). For OMG-Cl electrode in 1M H2SO4 electrolytes, the CV curve can be deconvoluted into (i) a nearly rectangular EDLC-like curve, albeit with a substantially lower charging/discharging current, and (ii) a set of symmetric Faradaic charging/discharging peaks (Fig. 4A). In (ii), the charging peaks are centered at ~0.24V and ~0.44 V. Consistently with the CV results, all galvanostatic charge/discharge tests (the CC test in Fig. S9) show symmetric features with fairly linear slopes. At a current density of 0.5 A g-1, A relatively higher specific capacitance of 250 F g-1 (1M H2SO4) is obtained for OMG-Cl, versus 179 F g-1 for OMG (Fig. 4B). This exhibits that Cl-doping can increase ~40% capacitive activity of OMG networks in an acidic electrolyte. Similarly, OMG-Cl has a higher specific capacitance of 220 F g-1 with 20% increase in 6M KOH solution, comparing with that of undoped OMG (183 F g-1) (Fig. S8B). Also, OMG-Cl continuously provides a well-behaving CC curve and high capacitance, achieving 249 F g-1 (1M H2SO4) and 224 F g-1 (6M KOH) at 10 A g-1. These values outstrip that of 3D holey graphene and bear comparison with OMC. 3, 15



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Figure 4. (A) Cyclic voltammetry of the first cycle in 1M H2SO4 electrolyte. (B) Rate capacity. (C) Symmetric electrochemical cell devices retain > 100 % after 10000 charge-discharge files. (D) Complex-plane plots of AC impedance. Inset shows phase angle versus frequency.

For practical applications, we further evaluated their stability under cyclic loading. The symmetric electrochemical cell withstood 10,000 cycles between -0.2 V and 0.9 V in 1M H2SO4 electrolyte retained 109 % for OMG-Cl and 108 % for OMG (Fig. 4C). A similarly cycled device between -1.0 and 0 V in a 6M KOH electrolyte leads to 116% capacitance remaining for OMG-Cl and 103% for OMG (Fig. S8C). The enhanced capacitance could be due to the good wettability and activation of the electrode, that is, the gradual diffusion


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of the electrolyte ions into the open pores/graphene layers will result in increasing active sites of the OMG electrode and thus the electrochemical performance. 43

Discussion Motivation on the improvement of Li storage capacity. All the results indicate that OMG-Cl is an outstanding anode of LIBs at various current rates. The unique property can be explained as follows:

First, the Li storage mechanism of the obtained OMG and OMG-Cl is different from the pristine Mo2C nanoflakes. Fig. 5A presents the initial four continuous CV profiles of the pristine Mo2C at 0.2 mV s-1 between 0.01 and 3.00 V. Remarkably, there is a substantive distinction between the first and following cycles. The first cycle presents an irreversible cathodic peak at ~ 1.39 V, due to the decomposition of the electrolyte and the formation of the SEI film on the surface of the electrode. Within the subsequent cycles, two redox couples at ~ 1.21/1.44 V and 0.40/1.06 V are found, which is related to Li+ conversion or alloying. Therefore, the Li-storing mechanism of Mo2C is described as follows: 44

Mo C + xLi + xe ↔ 2Mo + Li C

(2)

However, for OMG and OMG-Cl, the first cycle shows an irreversible cathodic peak at around 1.24 V (Fig. 5B and C), which also could correspond to the generation of SEI film on electrodes. With cycles, no obvious peaks occur for OMG. So, the intercalation and deintercalation processes for OMG can be expressed as: 45

6C + xLi + xe ↔ Li C (x = 2.3 ~ 3.0)

(3)

Significantly, we notice that the plot of OMG-Cl has a redox couple located at around 1.37/2.56 V, which



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could be attributed to Cl-containing species. 46 Therefore, OMG-Cl shows better capacity as anode of LIBs than undoped OMG.

Second, Cl doping simultaneously enhances the electrical conductivity and electrochemical activity of OMG networks in the high-rate electrochemical process. This hypothesis can be demonstrated by the EIS measurements. The obtained Nyquist plots are interpreted by means of a proper electric equivalent circuit (Fig. 5D). It reveals that the OMG-Cl electrode has much lower electrolyte resistances (RΩ = 1.3 Ω) and charge transfer resistances (RCT = 39.4 Ω) than that of the undoped OMG (RΩ = 1.8 Ω, RCT = 122.4 Ω) and the pristine Mo2C nanoflakes (RΩ = 2.8 Ω, RCT = 199.3 Ω).

Third, the enhanced uneven surface morphology (e.g. the folding or scrolling, Fig. 1A and B) and the excellent mesoporous structure also play a key role in improving the electrochemical performance. Futhermore, the topological defects formed in the doping process can be favorable for Li storage in the OMG-Cl network, thus improving the capacity of the OMG-Cl framework. 33

Last but not least, the OMG framework preserves the advantageous characteristics of OMC with few-layered graphene, such as a carbon network with ultrathin “graphene wall”, large surface area, uniform porous structure and chemical stability. These advantages enable OMG-Cl to be beneficial to fast electron and ion transfer and thus lead to high capacity, excellent rate capability and cycle performance for practical application towards LIBs.


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Figure 5 Cyclic voltammogram of the pristine Mo2C (A), OMG (B) and OMG-Cl (C) in the range of 0.01–3.0 V vs. metallic lithium at a scan rate of 0.2 mV s-1. (D) Nyquist plots recorded for cells based on our samples as the working electrodes before charge-discharge cycling.

Real inducement on improvement of capacitive storage. As reported, the doping into sp2 carbon materials can improve their capacitive performance, particularly for acidic supercapacitors. 5, 20, 43 In our work, Cl-doping in OMG network also shows a 40% increase in capacitive performance. However, it is still not clear for the improved capacitive behavior for OMG networks by Cl-doping. We suggest three possible reasons at least to respond the increasing capacitive performance: 1) the change of charge distribution on the surface of OMG network induced by Cl-doping; 2) The wettability improvement induced by Cl-doping; 3)



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Cl-induced pseudo-faradic reactions and active π electrons into OMG network. To confirm the mechanism, detailed tests and analysis were performed.

First, the surface charge was investigated by electrokinetics methods which primarily measured the external surface charge of the particles, especially for granular porous carbon.

47

The acidity or basicity of

OMG surfaces and the isoelectric point (IEP = pH value for zeta potential = 0) can be probed by testing the zeta potential as function of pH. The variation of the zeta potential of the pristine Mo2C and as-synthesized OMG and OMG-Cl dependence of pH is shown in Fig. 6A. The IEP of pristine Mo2C is 3.2. After a conversion process, IEP of OMG sharply shifts to higher pH value = 10.6. This demonstrates a big change from negative to positive charge on the surface of our samples. Importantly, OMG-Cl shows similar IEP of 10.2 to undoped OMG, which indicates that Cl-doping cannot influence the surface charge distribution. Hence, the immanent cause of the capacitance improvement for the OMG-Cl electrode is not from the surface charge distribution.

Second, the electrolyte wettability and flowability through the mesoporous structure can affect capability to store charges. 48 The surface wettability of carbon materials can be identified by testing their contact angles. Because of the existence of surface oxygen-containing moieties which can be clarified by spectroscopy analysis (Fig. 2 and Fig. S5), the pristine Mo2C nanoflakes show super wettability against deionized water, acidic or alkaline electrolytes (more details is shown in videos of Video I, II and III). Fig. 6B exhibits the contact angle measurement for OMG and OMG-Cl. It can be seen that when water is chosen as droplet, OMG-Cl presents similar wettability to undoped OMG. When water droplet is substituted by 1M H2SO4 electrolyte, no obvious change can also be found for the acidic wettability (contact angle is 133° for OMG-Cl and 131° for OMG) before and after Cl-doping. Interestingly, when the 6M KOH electrolyte droplet is


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selected, comparing with OMG, OMG-Cl presents a slightly wettability decreasing, relative to the distribution of positive charges on the surface of OMG frameworks (that had been demonstrated by IEP tests, Fig. 6A). Obviously, for the droplet after staying on the surface for 5 min, the contact angle has a negligible change. Therefore, Cl-doping cannot transform surface wettability in the whole pH range.

Finally, the Cl-induced pseudo-faradic reaction for OMG networks can be considered. Although similar SSA and pore structures are present for both OMG and OMG-Cl, OMG-Cl shows better capacitive performance than undoped OMG. This is mainly because Cl-containing groups have reversible redox reaction with ions from electrolytes. The redox peaks at ~0.24 V and 0.44V for OMG networks (Fig. 4A) strongly suggest that the former redox potential is related to the chlorine doping,

47

and the later redox potential is contributed to

oxygen-containing group. 48 Thus, comparing with undoped OMG, a more obvious current peak at ~0.24 V for OMG-Cl, indicates that the enhanced capacitance can benefit from the doped chlorine atoms. However, as noted above OMG-Cl has the higher capacitive performance in 1M H2SO4. In the light of the theory of Andreas and Conway,

51

the pseudo-faradic contribution of chlorinated functionalities is less efficient in

alkaline than in acid media, and nearly negligible at neutral pH, which is also evidenced by the CV results (Fig. 4A and Fig. S8A). So, Cl-doping leads to only a 20% increase in capacitance of OMG network under alkaline media. In addition, electrochemical impedance spectroscopy (Fig. 4D) shows that OMG-Cl has the lowest equivalent series resistance of ~0.49 ohms in 1M H2SO4 electrolyte, better than that of undoped OMG (0.88 ohms). A similar result is also obtained in 6M KOH electrolyte (Fig. S8D). This result might be attributed to electroconductivity improvement due to active π electrons induced by Cl-doping, 1 which is conducive to fast electron transfer. The >45° (positive) phase angle of OMG networks (OMG and OMG-Cl; inset of Fig. 4D) at relatively high frequencies confirms their capacitive behavior at fast rates.


52

The inset image in Fig. 4D also


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shows phase angles growing closer to 90° from medium to high frequencies, suggesting enhancing capacitive behavior from OMG to OMG-Cl, according to the research conclusion reported by Zheng et al. 52

Figure 6 (A) Zeta potential curves vs. pH of the pristine Mo2C, as-obtained OMG and OMG-Cl. (B) Photographs of droplets resting on as-obtained OMG and OMG-Cl for different time in water, 1M H2SO4 and 6M KOH.

Conclusions


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In summary, as high-performance electrode material for energy storage devices, Cl-doped ordered mesoporous carbon with few-layered graphene walls (OMG-Cl) is synthesized by a Cl2 gas etching reaction to remove Mo atoms and simultaneously inducing the rearrangement of the reserved C atoms. Owing to its Li storage features, ultrahigh SSA and open pore structure, enhanced electrical conductivity, heteroatomic defects and ultrafast ion diffusion, OMG-Cl is also employed as advanced anode material for LIBs. In addition, as electrode material for supercapacitors, it also exhibits an excellent capacitive nature, which can be more assigned to the redox reaction between Cl-containing groups with various ions of electrolytes. We believe that for OMG-Cl materials their excellent properties including high capacities, very short charge time (low to tens of seconds) as well as the low cost and easy facile synthesis technique will develop a chance for commercializing high-performance energy storage devices to drive power vehicles.

ASSOCIATED CONTENT Supporting Information

Synthetic method；Supplementary Figures (S1-S9) and Tables (S1); References (1-6)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Shichun Mu); [email protected] (Yufeng Zhao). Notes The authors declare no competing financial interest.

Acknowledgements The authors express thanks to Xiaoqing Liu and Tingting Luo for HR-TEM measurement support, in the Materials Analysis Center of Wuhan University of Technology. Also, the authors thank Dr. Jun Wan and Prof. Chaocan Zhang for Contact Angle measurement support, in the School of Materials Science and Engineering, Wuhan University of Technology.

Funding: This work is jointly supported by the National Key Research and Development Program of China



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(2016YFA0202603), the National Natural Science Foundation of China (No. 51372186 and No. 51672204), and State Key Laboratory of Advanced Technology for Materials Synthesis and Processing of Wuhan University of Technology (2017-KF-4).

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Table of Contents


Molybdenum Carbide-Derived Chlorine-Doped Ordered Mesoporous

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