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Synthesis and Supercapacitor Application of Alkynyl Carbon Materials Derived from CaC2 and Polyhalogenated Hydrocarbons by Interfacial Mechanochemical Reactions Yingjie Li, Qingnan Liu, Wenfeng Li, Hong Meng, Ying-zhou Lu, and Chunxi Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13610 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 16, 2017

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ACS Applied Materials & Interfaces

Synthesis and Supercapacitor Application of Alkynyl Carbon Materials Derived from CaC2 and Polyhalogenated Hydrocarbons by Interfacial Mechanochemical Reactions Yingjie Li †,‡, Qingnan Liu †,‡, Wenfeng Li †,‡, Hong Meng ‡, Yingzhou Lu ‡, Chunxi Li *,†,‡ †

State Key Laboratory of Chemical Resource Engineering, and ‡ College of Chemical Engineering,

Beijing University of Chemical Technology, Beijing 100029, P. R. China.

ABSTRACT: The discovery of new carbon materials and the reactive activation of CaC2 are challenging subjects. In this study, a series of alkynyl carbon materials (ACMs) were synthesized by the interfacial mechanochemical reaction of CaC2 with four typical polyhalogenated hydrocarbons. Their properties and structures were characterized, and their electrochemical performances were examined. The reaction was rapid and efficient arising

from

the

intense

mechanical activation

of

CaC2.

The

ACMs

are

micro–mesoporous materials with distinct layered structure, specific graphitization degree, and clear existence of sp-C. In addition, the ACMs exhibit high specific capacitance in the range of 57–133 F g−1 and thus can be ideal candidates for active materials used in supercapacitors. The results may imply an alternative synthesis of carbon allotropes, as well as an efficient approach for the activation of CaC2. KEYWORDS: alkynyl carbon materials, CaC2, polyhalogenated hydrocarbons, mechanochemistry, supercapacitors 1. INTRODUCTION The design and discovery of new carbon materials (CMs) with unique structures and properties has long been of interest in the field of materials science.1-2 Fullerenes, carbon nanotubes (CNTs), and graphene have been successively discovered in the past few decades,3-5 and have attracted significant attention, with respect to their synthesis, 1 ACS Paragon Plus Environment


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characterization, structure–property relationship, and diverse applications. These CMs have extended the categories of available zero-, one-, and two-dimensional sp2-hybridized CMs with highly conjugated structures, which render several unique properties to CMs, with tremendous potential for applications,6-8 particularly in electrochemistry.9-10 Recently, the synthesis of alkynyl carbon materials (ACMs) as CMs has been reported and attracted wide spread concerns. Although their presence was predicted decades ago,11 they were first synthesized and characterized in 2010 by Li et al. via the Cu-catalyzed cross-coupling reaction of hexaethynylbenzene under mild conditions.12-13 ACMs are predominantly composed of sp- and sp2-hybridized carbon atoms, which are connected by a highly conjugated planar structure; thus, ACMs are considered to be next-generation CMs demonstrating prospective applications. Meanwhile, CaC2, serving as an ideal source of alkynyl, has attracted increasing attention in synthetic reactions,14 particularly for the synthesis of CMs.15-18 However, the reactivity of CaC2 is predominantly limited by its crystalline structure,19 which makes it more difficult to synthesize CaC2-drived CMs. Huang et al.15-16 have synthesized a series of CMs in an autoclave by using CaC2 and chlorinated methanes. Simultaneously, Wang et al.17-18 have synthesized CaC2-drived CMs via etching CaC2 using fresh chlorine at medium to high temperature and investigated the application of these CMs in supercapacitors. However, these synthesis processes suffer from many problems, e.g., high equipment requirement and energy consumption, low carbon yield, less environmental friendliness, and thus limited practical use. Most importantly, the alkynyl groups may be absent in these CMs due to their instability at high temperatures,20 which has not been elaborated wherein. Thus, preserving the alkynyl during the activation of the CaC2 is the key to synthesize the ACMs. On the other hand, mechanochemistry (MC), a historical chemical technology, has been gradually developing and has currently become an important research area.21 MC demonstrates notable chemical effects and some attractive advantages, such as solvent independence, mild operating conditions, low 2 ACS Paragon Plus Environment

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energy consumption, and environmental friendliness.22-23 Recently, MC has been applied in materials synthesis and has demonstrated inestimable potential applications,24-26 particularly in the synthesis of CMs.27-28 Thus, MC may be beneficial for synthesizing ACMs via CaC2 activation without the destruction of the alkynyl group.29 Based on this background, a series of ACMs were synthesized via the mechanochemical reaction between CaC2 and four typical polyhalogenated hydrocarbons (PHHCs) in the absence of solvents under ambient temperature. The mechanochemical reaction process was investigated, and the unique structures of the ACMs, as well as their electrochemical performances, were investigated. This study can hopefully provide a new member of CMs to the carbon family while simultaneously revealing the reactive activation mechanism of CaC2. 2. EXPERIMENTAL SECTION 2.1. Synthesis and characterization of ACMs Scheme 1 summarizes the mechanochemical method utilized for the synthesis of the ACMs, which mainly involves two steps. First, a mixture of CaC2 (100 mesh particles) and PHHC with a specific molar ratio was added into a stainless steel pot (containing approximately 250 g stainless steel balls with four different diameters), followed by sealing the pot airtight. Then, the planetary ball mill was operated at 530–600 rpm for 4 h under vacuum conditions and ambient temperature. In this study, CCl4, CCl2=CCl2, C6Cl6, and C6Br5CH2CH2C6Br5, representing four typical PHHCs, were used as reactants. Second, the resultant mixture was immersed in dilute nitric acid, filtered using a microfiltration membrane, and washed with ultrapure water. The resulting ACMs were obtained by vacuum drying at 100 °C for 5 h, and the halogen ion content in the filtrate was analyzed by ICS-900 ion chromatography. The ACMs were comprehensively characterized by scanning electron microscopy (SEM), X-ray energy-dispersive spectroscopy (EDS), high-resolution transmission electron microscopy (HR-TEM), N2 3 ACS Paragon Plus Environment


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adsorption–desorption isotherm analysis, Raman spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Please check the Experimental Section in Supplementary Information (SI) for details.

Scheme 1. Mechanochemical Synthesis of ACMs (ACM-3 as representative).

2.2. Electrochemical performance measurements of ACMs The electrochemical performance of ACMs was examined by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS), as well as long cycling performance. All electrochemical investigations were conducted on a CHI 660E electrochemical workstation. CV tests were investigated between −0.8 and 0 V (vs. Hg/HgO) at scan rates ranging from 1 to 30 mV s−1. GCD tests were conducted at the same potential range at current densities ranging from 0.1 to 2 A g−1. EISs were characterized at open-circuit potential in the frequency range from 100 kHz to 0.01 Hz with a amplitude of 5 mV. Details related to the fabrication of ACM electrodes and the calculation methods for gravimetric specific capacitance (Cm, F g−1) have been provided in the Experimental Section in SI. 3. RESULTS AND DISCUSSION 3.1. Mechanochemical reaction process Table 1 summarizes the results obtained from the mechanochemical reactions for synthesizing ACMs. The four mechanochemical reactions can reach deep level, as evidenced by the very high yields of carbon and dehalogenation degrees of PHHCs. Notably, by a control experiment, approximately 9.5 wt% of insoluble impurities are observed in CaC2, and Si and Fe are detected in the EDS spectra of the ACMs (Figure S1 4 ACS Paragon Plus Environment

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and Table S1), affording experimental carbon yields greater than the theoretical ones. Even if the insoluble impurities in CaC2 are ignored for the four reactions, the carbon yields are still high (>91%) (Table S2). On the other hand, as compared to CCl4 and CCl2=CCl2, aryl halides require higher mechanical energy (600 rpm), attributed to their lower reactivity. Overall, this process can be considered as a green, cost-effective, and efficient synthetic route of CMs, and provides a viable approach for the activation of CaC2. Table 1. Mechanochemical reaction processes and results Samples ACM-1 ACM-2 ACM-3 ACM-4

Experimental conditions 530 rpm, 4 h, CaC2/CCl4 (mol/mol)=3.0:1 530 rpm, 4 h, CaC2/C2Cl4 (mol/mol)=3.0:1 600 rpm, 4 h, CaC2/C6Cl6 (mol/mol)=4.5:1 600 rpm, 4 h, CaC2/C14H4Br10 (mol/mol)=7.5:1

ACM yield (%)a

Dehalogenation degree (%)

Carbon content (weight/atom, %)b

131.2

95.9

71.3/81.3

141.4

100

71.7/79.4

126.3

100

73.5/83.9

120.8

98.1

78.7/88.9

a: calculated carbon yield including the insoluble impurities of CaC2; b: calculated from the EDS results of the ACMs.

Scheme 2 summarizes the pathway proposed for mechanochemical reactions.22, 30-31 First, the lattice structure, which limits the reactivity of CaC2, is destroyed under strong mechanical stress, and ultrafine CaC2 is considered as plasma containing [C≡C]2− and Ca2+. Second, exposed [C≡C]2− with high surface energy and reactivity can behave as a nucleophile and attack halogens on PHHC. Third, halogen atoms are successively substituted by alkynyl groups, affording alkynyl-linked PHHC. The reactivity of partially substituted PHHC is still high, attributed to the electron-withdrawing effect of the alkynyl group.32-33 Besides, increasing the reaction system temperature will further promote the reaction. Thus, once the mechanochemical reactions are triggered, they will proceed in a rapid, comprehensive manner, similar to a click reaction.29, 34 Finally, all halogens on the 5 ACS Paragon Plus Environment


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PHHC are substituted by the alkynyls, and the ACMs are formed through nucleophilic substitution along with the mineralization of organic halogen atoms to inorganic halogen ions.

Scheme 2. Proposed Mechanochemical Reaction Pathway for the Formation of the ACMs.

3.2. Characterization of ACMs As shown in Figure 1 a–d, ACMs are aggregates of carbon nanoparticles (CNs) with a micro–mesoporous structure. Figure S2 shows additional SEM images of ACMs at different magnifications. Some differences exist among samples. First, as shown in Figure S3, the CNs in ACM-2 (7~20 nm) have smaller diameters and narrower size distribution than those in ACM-1 (7~30 nm), ACM-3 (15~41 nm), and ACM-4 (14~44 nm) in sequence. Second, the morphology and pore structure of ACM-4 and ACM-3 appear to be more developed than those of ACM-1 and ACM-2 in sequence, attributed to the differences of the PHHC precursors. The aromatic ring provides a rigid brace for the pore structure of the ACMs,35 and an increasing halogen atoms is helpful to form a highly cross-linked structure. In addition, the morphology of ACM-1 may be subjected to a partial structure rearrangement during the reaction.20, 36-38 As shown in Figure 1e, ACMs exhibit type IVa isotherms, with an obvious H4 hysteresis loop at a relative pressure P/P0 of 0.4–1.0, often associated with slit-like pores attributed to CN agglomeration.39-40 In addition, pronounced N2 uptake at low P/P0 is observed, 6 ACS Paragon Plus Environment

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demonstrating the existence of micropores in the ACMs. Furthermore, the specific surface areas (SSA) for ACM-1, ACM-2, ACM-3, and ACM-4 are 404.2, 359.6, 486.3, and 712.2 m2 g−1, respectively, as obtained by the Brunauer–Emmett–Teller (BET) method. Table S3 summarizes the SSA and pore parameters obtained from the N2 adsorption isotherms of the ACMs. High SSA is beneficial to a material used in adsorption, supercapacitor, catalyst support, etc. As can be observed from Figure 1f, ACMs exhibit mesopores ranging from 2 to 10 nm and micropores centered at 0.7 and 1.2 nm. Hence, ACMs are micro–mesoporous, which is consistent with the SEM observation.

Figure 1. SEM images of (a) ACM-1; (b) ACM-2; (c) ACM-3; and (d) ACM-4. (e) N2 adsorption–desorption isotherms, and (f) pore size distribution obtained by the density functional theory.

Figure 2a–d and Figure S4 show the HR-TEM images of the ACMs. Two carbon structures are clearly observed for ACMs: disordered and highly graphitic. Both types of carbon are interconnected via strong carbon–carbon bonds, resulting in a stable structure, as well as micro- or mesopores, for ACMs. In addition, fast Fourier transform (FFT) patterns obtained from the corresponding HR-TEM images also demonstrate the 7 ACS Paragon Plus Environment


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existence of the graphite structure. The layered structure of ACM-1 is presumably attributed to partial structural rearrangement, caused by the high energy density of intermediate monomers,41 while those of the other ACMs are attributed to the planar structure of PHHCs (double bond and aromatic ring here). Furthermore, lattice fringes of the planes were measured to be 3.57, 3.61, 3.67 and 3.66 Ǻ for ACM-1, 2, 3, and 4, respectively; these values are greater than that of graphene (3.35 Ǻ). This result is attributed to the strong mutual repulsion of alkynyl groups on the adjacent layers.11, 41-42 As can be observed in the XRD patterns shown in Figure 2e, two strong reflections at around 26° and 44°, as well as some weak reflections are observed; the former is attributed to the (002) and (100) reflections of graphite (PDF 41-1487), while the latter is attributed to the reflections of SiC (PDF 49-1428), C0.14Fe1.86 (PDF 44-1289), and FeSi (PDF 38-1397). These results indicated that a certain content of carbon in ACMs is present in a well-ordered graphitic structure, which is in good agreement with the results obtained from HR-TEM. Simultaneously, the existential states of the impurity elements are determined, indicating that the impurity elements present in the ACMs cannot be removed by acid washing due to their chemical bonding with ACMs. In addition, the crystallinity calculated from the XRD patterns decreases from ACM-1 to ACM-4 (Table S4). This result indicates that the structural defect increases from ACM-1 to ACM-4, attributed to the increasing rotation effect of the alkynyl group, irregular cross-linking between monomers, as well as mechanical disruption during the reaction. Thus, the (002) reflections shift to lower angles and become broader from ACM-1 to ACM-4 with increased lattices pacing and decreased crystallinity.41, 43 In addition, the structural defect of the ACMs is justified by Raman spectra (Figure 2e). Four bands are observed in the Raman spectra: G bands at around 1580 cm−1, attributed to the C–C bond stretching modes for typical graphite; D bands at around 1335 cm−1 and D’ bands at around 1620 cm−1, attributed to defects and disorder structures of carbonaceous solids; and G’ band at around 2660 cm−1, attributed to double resonance 8 ACS Paragon Plus Environment

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inter-valley Raman scattering. Furthermore, these bands are observed when defects are present in the crystalline carbon structure and provide further evidence for the formation of graphitic structures.44-45 The intensity ratio between D and G bands (ID/IG) is also calculated (Table S4) as ID/IG precisely reflects the graphitization of CMs.46 The results reveal that the graphitization of ACMs follows the order of ACM-2 > ACM-3 > ACM-4 >ACM-1. This demonstrates that the ACM-1 made from only sp3-C contented PHHC possesses more defects and lower graphitization than those made from the sp2-C dominant ones. In addition, for the ACMs made from the sp2-C dominant PHHCs (ACM-2 to ACM-4 here), the introduction of aromatic rings and the increasing halogen atoms in PHHCs provide more opportunities for the increasing defects and decreasing graphitization, which is in agreement with the XRD results.

Figure 2. HR-TEM images and the corresponding FFTs of (a) ACM-1; (b) ACM-2; (c) ACM-3; and (d) ACM-4. (e) XRD patterns of ACMs. (f) Raman spectra of ACMs.

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Figure 3. XPS spectra of ACMs: (a) survey scans, (b) narrow scans of carbon fitted with the combination of Lorentzian and Gaussian function.

XPS spectra were recorded to investigate the structural characterization of ACMs. As shown in Figure 3a, ACMs are mainly composed of carbon element. The C 1s peak at 284.8 eV shows essentially identical binding energies for the C 1s orbital, and the O 1s peak at 532 eV is attributed to the chemical adsorption of air in ACMs.12 The high-resolution asymmetric C 1s XPS spectra of the ACMs were corrected by a Shirley background and fitted with a combination of Lorentzian and Gaussian functions. As shown in Figure 3b, the C 1s peak is mainly deconvoluted into four sub-peaks at around 284.6, 285.1, 285.5 and 286.5 eV, attributed to the C 1s orbital of C–C (sp2), C–C (sp), C–C (sp3), and C–O, respectively.12, 47-48 The area ratio of sp2/sp/sp3 for ACM-1 is 9.2/4/1, 10 ACS Paragon Plus Environment

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confirming that the transformation of C–C (sp) to C–C (sp2) occurs because of structural rearrangement during the mechanochemical reaction. The area ratios of sp2/sp for ACM-2 and ACM-3 are 1/2 and 1/1, respectively, confirming ACM-2 and ACM-3 structures shown in Scheme 2. The area ratio of sp2/sp/sp3 for ACM-4 is 6/5/1, confirming that the PHHC precursor is connected to the others via the alkynyl group. These results provide tangible evidence for the proposed pathway of the mechanochemical reactions discussed above. 3.3. Electrochemical performance of ACMs As shown in Figure 4a, four ACM electrodes exhibit typical rectangular CV curves, indicating representative capacitive behavior and low contact resistance.49-50 Even at a high scan rate of 30 mV s−1, near-rectangular-shaped CV curves are still observed for ACM electrodes (Figure S5), demonstrating highly reversible systems with excellent rate capabilities. Furthermore, all ACM electrodes basically exhibit symmetrical triangular GCD curves (Figures 4b and S6), indicative of outstanding capacitive performance.51-52 As calculated by GCD curves (0.1 A g−1), the Cm values for ACM-1, ACM-2, ACM-3, and ACM-4 are 88.7, 56.9, 96.7, and 133.4 F g–1, respectively. The results are in agreement with their order of the SSABET values, demonstrating that SSA plays a key role for their capacitance performance.53 Furthermore, in the GCD method, the Cm values gradually decreased with increasing current density; this trend is similar to that observed with the increase of the scan rate in the CV method (Table S5). Considering that ACMs exhibit relatively low SSA values as compared with other carbon electrode materials, their capacitance values are quite high (Table S6), being comparable to those of other CMs reported previously.54-55 This result features the structural advantages of the ACMs. In addition, the long cycling performance of the ACM electrodes was further investigated by GCD experiments. As shown in Figure 4c, the Cm values do not decline notably after 1000 cycles (>93% of the original values), indicating that ACM electrodes exhibit excellent cycling stabilities. Further, the cycling stability can be remained above 91% 11 ACS Paragon Plus Environment


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after 5000 cycles, as exemplified by the test results of ACM-4 (Figure S7). For revealing why ACS exhibit excellent electrochemical behavior, EIS measurements were conducted over a frequency range of 100 kHz to 0.01 Hz, and the results are shown in Figure 4d, Figure S8, and Table S7. As shown in Figure 4d, Nyquist plots of supercapacitors with the four ACM electrodes exhibit straight lines and semicircles in the low- and high-frequency regions, respectively. High slope values in the low-frequency regions indicate near-ideal electric-double-layer capacitor behavior, small x-intercepts of the Nyquist plots indicate low equivalent series resistances (ESRs), and insignificant semicircles in the high-frequency regions indicates low charge-transfer resistance.56-57 These results indicate the excellent electrochemical performance of the ACM electrodes. Further, the lower ESRs and the shorter Warburg regions for the sp2-C PHHC-derived ACMs (Table S7), especially for the ACM-3 and ACM-4, demonstrate higher electrical conductivities and charge-discharge rates,58 owing to the efficient ion and electron transfer resulted from the highly conjugated carbon frameworks of the ACMs.50 In total, the superior electrochemical performance observed for the ACM electrodes, including high Cm values, good electrical conductivities, excellent rate capabilities, and outstanding cycling stabilities, is attributed to their unique structures. The optimized pore structures for ACMs provide short distance for diffusion and enhance the transport of ions and electrons from the electrolyte to the electrodes. In addition, the excellent interconnected framework of ACMs can significantly improve electrical conductivities of the electrodes, which are beneficial to their rate capabilities and cycling stabilities.


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Figure 4. Electrochemical performance of ACM electrodes. (a) CV curves of the ACM electrodes at a scan rate of 1 mV s−1; (b) GCD curves of the ACM electrodes at a current density of 0.1 A g−1; (c) long cycling performance of the ACM electrodes at a current density of 1 A g−1; (d) Nyquist plots of the ACM electrodes obtained from EIS measurements in the frequency range from 100 kHz to 0.01 Hz at a amplitude of 5 mV.

4. CONCLUSION In this study, a series of ACMs were efficiently synthesized by the mechanochemical reaction of CaC2 with four PHHCs at ambient temperature for the first time. In these reactions, CaC2 was highly activated by strong mechanical stress, resulting in a high reaction degree and ACM yields. These processes can provide a significant insight into the reactive activation of CaC2. ACMs are micro- or mesoporous material with distinct layered structures, specific graphitization degree, and alkynyl content. The as-prepared ACMs are unique as they exhibit tunable composition and structure, and superior 13 ACS Paragon Plus Environment


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electrochemical performance, such as high specific capacitance, good electrical conductivities, excellent rate capabilities, and outstanding cycling stabilities. This study provides the mechanochemical synthetic route for a new type of carbon materials, and the ACMs can serve as ideal candidates for electrode materials in supercapacitors. In addition, it provides a perspective approach toward the reactive activation of CaC2, as well as assistance for the renaissance of CaC2 chemistry. ASSOCIATED CONTENT Supporting Information Experimental

section,

material

preparation

details,

electrode

fabrication,

and

supplementary results and discussion. AUTHOR INFORMATION Corresponding Author *Tel. & Fax: +86 010-64410308. E-mail: [email protected] (Chunxi Li). First Author E-mail: [email protected] Note The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors would like to acknowledge Prof. Huaihe Song for reviewing this manuscript and providing valuable comments. Qingnan Liu and Wenfeng Li equally contributed to this study. The authors are grateful for the partial financial support from National Natural Science Foundation of China (21376011). 14 ACS Paragon Plus Environment

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