Fluidic Carbon Precursors for Formation of Functional Carbon under Ambient Pressure Based on Ionic Liquids

Abstract

A new strategy for the preparation of functional porous carbons is developed via direct, ambient-pressure, thermal pyrolysis of task-specific ionic liquids (ILs). The simple synthesis lies in the synergistic use of the negligible volatility of the ILs and incorporation of the crosslinkable nitrile groups in the anions. The resulting carbon materials at 800 °C retain an extremely high content of nitrogen (up to 18 at%). Carbon materials are central to many important applications because of their wide availability and superior physicochemical properties, such as electric and thermal conductivities, chemical stabilities, and low densities.1-3 For example, they have been extensively used as electrode materials for batteries,4, 5 fuel cells,6, 7 and supercapacitors8-10 as well as effective supports for catalysis,11-13 separation,14 and gas storage.15 Conventionally, carbon materials can be synthesized by the pyrolysis and physical or chemical activation of low-vapor-pressure polymeric precursors derived from either synthetic (e.g., polyacrylonitrile (PAN) and phenolic resins) or natural (e.g., coal, pitch, and shell nuts) polymeric sources at elevated temperatures.16, 17 However, carbonization of polymeric precursors usually breaks down the polymeric chain and generates considerable volatile species, such as H2O, CO2, and low-molecular-weight organic compounds, resulting in the formation of cracks14 and/or foams, and thus rendering the difficulty of fabricating high-quality carbon nanocomposites and uniform carbon coatings. Although nonpolymeric precursors can be used to prepare carbon materials under high pressure and explosive conditions, carbon yields are normally very poor because of their vaporization during high-temperature pyrolysis.18 These deficiencies prompted us to recently develop a new strategy for synthesis of carbon materials from task-specific ionic liquids (TSILs) composed of nitrile-functionalized imidazolium-based cations.19 The efficient formation of carbon materials lies in exploitation of the negligibly low vapor pressure of ILs and the unique precursor-controlled thermolysis properties,20, 21 in which imidazolium-based IL cations act as vehicles for introduction of crosslinkable nitrile groups and IL anions, such as Cl− and bis(trifluoromethylsulfonyl)imide ([Tf2N]−), serve as porogens. Another unique feature is the high mobility of liquid carbon precursors at elevated temperatures, providing great opportunities in the development of advanced functional carbon materials, such as the formation of uniform carbon films. In last two decades, various applications of ILs, including catalysis,22 chemical synthesis,23-25 electrochemistry,26, 27 separation,28, 29 and advanced materials30-32 have been developed for their intrinsic properties such as negligible vapor pressure, high thermal, chemical, and electrochemical stabilities, and wide liquid temperature ranges. The properties of ILs can be easily tailored by the selection or modification of either cation or anion components and synthesized for specific tasks. Compared to cations, typically based on imidazolium, anions are more easily designed to bear multiple crosslinkable components, for example, three nitrile groups in [C(CN)3]−, which are expected to be more favorable to the formation of 3D-connected frameworks at elevated temperatures than those from cations. In addition, pyrolysis of a polynitrile-containing network will induce nitrogen-rich carbon/carbonaceous materials. Herein, we describe several examples of TSILs consisting of [C(CN)3]− to demonstrate the feasibility of this new concept towards advanced functional carbon materials. High carbonization yields (up to ca. 44 wt%) and high nitrogen contents (up to ca. 18 at%) can be achieved for the carbon materials derived from these new molecular precursors. One of the most widely used synthetic carbon precursors is PAN.17 The nitrile groups in PAN are the key to its high carbon yield and can undergo crosslinking reactions under pyrolytic conditions. By introducing the nitrile functionality into the anion of ILs, we seek to develop new TSILs for the formation of porous carbons. Many ILs contain anions with nitrile moieties. Figure 1 and Figure S1 in the Supporting Information show the chemical structures of typical ILs used in our present investigation (EMIm = 1-ethyl-3-methylimidazolium and BMIm = 1-butyl-3- methylimidazolium). These imidazolium-based ILs were synthesized according to procedures described in the literature.33, 34 The synthesized ILs were either low-melting solids or free-flowing liquids and were dried under vacuum at 80 °C overnight prior to use. Carbonization of ILs was carried out by introducing 0.5 g of a particular IL into an alumina crucible placed within a quartz tube furnace. Under N2 carrier gas flow (100 mL min−1), temperature was controllably ramped at a rate of 10 °C min−1 to a final temperature of 800 °C. After heat treatment for 1 h, the furnace was cooled down to room temperature. Two representative dialkylimidazolium TSILs containing nitrile-functionalized anions. The carbon yields under pyrolytic conditions (N2 carrier gas and 800 °C) were determined via thermal gravimetric analysis (TGA). Figure 2 compares the TGA curves of three selected TSILs containing nitrile-functionalized anions with that of a conventional aprotic IL, [BMIm]Tf2N (Tf2N = bis(trifluoromethylsulfonyl)imide). Significant carbonization yields were achieved with the ILs containing nitrile-functionalized anions (see structures in Figure S1), whereas negligible carbon residue was observed via the direct carbonization of [BMIm]Tf2N. The thermal properties of nitrile-containing TSILs are summarized in Table S1. Most of the ILs have melting points below room temperature with the exception of [BCNIm][C(CN)3] (BCNIm = 1,3-bis(cyanomethyl)imidazolium), which is a pale-brown powder at room temperature (Figure S2). The decomposition temperatures of all TSILs except [BCNIm][C(CN)3] are in the range of 309.7 to 329.0 °C. As listed in Table 1, although the carbonization yields of all nitrile-functionalized TSILs, excluding [BCNIm][C(CN)3], are much lower than the corresponding theoretical values calculated on the basis of overall carbon contents, the numbers agree well with the values calculated from the carbon contents of the anion, [C(CN)3]−. In our previous study, we found IL cations without nitrile functionality gave negligible carbon yield.19 Therefore, the corresponding carbon yields originate mainly from the IL anions. Accordingly, the ILs containing smaller cations (e.g., [EMIm][C(CN)3]) give a higher carbonization yield at 800 °C than those containing larger cations. To confirm this conclusion, we prepared an IL with both cation- and anion-containing nitrile groups, [BCNIm][C(CN)3], which can be crosslinked via both cation and anion. The TGA profile of [BCNIm][C(CN)3] shows a small weight loss of ∼8% at around 210 °C, indicating a structural rearrangement and the formation of a crosslinked intermediate that is stable up to 450 °C. The final carbonization yield at 800 °C reaches as high as 44%, which is much higher than the value calculated only from the carbon content of the anion. Scanning TGA profiles of a) [BMIm]Tf2N, b) [EMIm][C(CN)3], c) [BMIm][C(CN)3], and d) [C10MIm][C(CN)3] under flowing nitrogen (60 mL min−1) with a ramp rate of 10 °C min−1. Heating the TSILs at ∼300 °C results in the formation of partially solidified liquid-solid intermediate and heating at 400 °C gives solid products. Polymerization of [C(CN)3]− in the temperature range of 300–400 °C may follow a similar dynamic cyclotrimerization reaction (Scheme 1) to the condensation of aromatic nitriles,35, 36 cyanamide,37 and acetylenes,38 which is accompanied by the decomposition of the corresponding IL cations. Taking [BMIm][C(CN)3] as an example, although we did not observe crystalline structures of the carbonaceous material derived from [C(CN)3]−-based TSILs at 400 °C, the resulting carbon at 800 °C exhibits graphitic features to some extent. Upon further treatment at 2000 °C, the graphitization degree (g)39 can reach 0.17 (see the X-ray diffraction (XRD) pattern in Figure S4). These results indicate that TSILs are a novel kind of precursors to generate graphitizable carbons. Reaction scheme of the trimerization of a nitrile-containing anion, leading to the formation of an extended framework. The N2-adsorption/desorption isotherms for the carbons derived from various ILs at 800 °C are shown in Figure 3 and Figure S5. Interestingly, the cation structures of these TSILs also exhibit a profound influence on Brunauer–Emmett–Teller (BET) surface areas. In general, carbonaceous materials obtained from ILs containing bulky cations give high surface areas, suggesting a templating role played by the IL cations during micropore generation (Table 1). For example, simply replacing [BMIm]+ with [EMIm]+ for a fixed anion results in a complete loss of porosity. The dependence of the carbon surface area on the alkyl group of the imidazolium cations is more complicated. The surface areas of carbons derived from [C9MIm][C(CN)3] (C9MIm = 1-methyl-3-nonylimidazolium) and [C10MIm][C(CN)3] (C10MIm = 1-decyl-3-methylimidazolium) are less than that from [C6MIm][C(CN)3] (C6MIm = 1-hexyl-3- methylimidazolium) but greater than that from [BMIm][C(CN)3]. The slight reduction of the surface areas of the carbon materials derived from the ILs containing the imidazoliun cations with longer alkyl groups can be attributed to the partial collapse of pore structure during carbonization at high temperature. The carbon material derived from [BCNIm][C(CN)3] has a slightly lower surface area than that derived from [BMIm][C(CN)3]. This reduction could be due to the formation of a more condensed structure via crosslinking of both cations and anions. The fact that we can use not only non- carbonizable anions but also non-carbonizable cations for manipulation of carbon pore structures demonstrates how precisely our IL-based approach to carbon materials can be tuned for various applications. Nitrogen sorption isotherms of carbon materials derived from a) [EMIm][C(CN)3], b) [BMIm][C(CN)3], c) [C6MIm][C(CN)3], and d) [C10MIm][C(CN)3]. Conditions: ramp rate = 10 °C min−1, temperature = 800 °C, dwell time = 1 h. Energy-dispersive X-ray analysis (EDAX) results for the carbon materials derived from our carbon precursors at 800 °C (Figure S6 and Table S2) reveal very high nitrogen contents (between 11.4 and 17.6 at%), indicating that significant nitrogen functionalities still remain. This observation is consistent with the involvement of cyclotriazine building blocks during the formation of carbon networks. Figure 4 shows the comparison of the X-ray photoelectron spectroscopy (XPS) spectrum of [C6MIm][C(CN)3] pyrolyzed at 400 °C with that of [C6MIm][C(CN)3] pyrolyzed at 800 °C. The nitrogen environments in the carbon material derived from 400 °C pyrolysis are dominated by a pyridinic structural feature. The content of this pyridinic structural unit decreases with the increasing pyrolytic temperature, indicating a concomitant rearrangement of carbon networks under high-temperature conditions. It should be noted that even in the carbon materials derived by high-temperature pyrolysis, the content of the pyridinic nitrogen is unprecedentedly high. It is known that the pyridinic nitrogen in carbon materials is responsible for catalytic sites for oxygen reduction reaction (ORR).40 There has been considerable interest recently in synthesizing carbon materials with a high pyridinic nitrogen content.41-43 Our new method based on crosslinkable nitrile-containing anions in ILs opens up an alternative route towards advanced carbon materials with high pyridinic nitrogen contents. XPS N 1s narrow-scan spectra of carbonaceous materials derived from [C6MIm][C(CN)3] at a) 400 °C for 2 h (N at% = 22.9%) and b) 800 °C for 1 h (N at% = 13.3%). To summarize, we have successfully developed a new strategy for synthesizing carbon materials via fluidic TSIL precursors under ambient pressure. Our method relies on the synergistic use of the negligible vapor pressure of ILs and incorporation of the crosslinkable functional group into the structure of the anion. The structural morphology (e.g., porosity and surface area) of these carbon materials is highly dependent on the structural motif of the corresponding IL cation structures. This novel strategy can be further expanded by pairing various IL cations with nitrile-based anions as well as incorporating other crosslinkable functionalities into IL anions. This research was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. Supporting Information is available online from Wiley InterScience or from the author. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. 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