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PMMA interpenetrating polymer networks
Electrochimica Acta 148 (2014) 203–210
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical performance of porous carbon by carbonizing PF/PMMA interpenetrating polymer networks Jun Zhang a , Xiaofeng Zhong a , Hongbiao Chen a, *, Yong Gao a , Huaming Li a,b, * a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, and Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 August 2014 Received in revised form 8 October 2014 Accepted 8 October 2014 Available online 13 October 2014
Porous carbon was synthesized by the direct carbonization of full interpenetrating polymer networks (IPNs) of phenol-formaldehyde resin (PF) and poly(methyl methacrylate) (PMMA). The PF/PMMA IPNs were prepared by simultaneous polymerization of PF prepolymer as well as methyl methacrylate and ethylene glycol dimethacrylate monomers in dimethylformamide. During the followed carbonization process, the PF polymeric networks tended to form carbon matrix (carbon precursor) while the PMMA decomposed into gaseous products, leaving pores in carbon matrix (pore-former). The obtained porous carbon has an interconnected pore structure with a high BET surface area of 865 m2 g1 and an average pore size of 4.4 nm. Such porous carbon shows outstanding capacitive performance (252 F g1), good rate capacitive behavior, and excellent cycling stability. Due to its unique pore structure, the as-prepared porous carbon shows promise as an electrode material for supercapacitors. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Interpenetrating polymer networks porous carbon electrochemical supercapacitors
1. Introduction Porous carbon materials have attracted substantial attention in recent years due to their remarkable properties such as high specific surface area, large pore volume, tunable pore size, low density, high conductivity, and high chemical stability [1–9]. These properties make them desired materials for absorbents [2,10], catalyst supports [11,12], and electrode materials for electrochemical capacitors [12–15] and fuel cells [16,17]. So far, various methods including activation process, template method, organic aerogel carbonization, and polymer blend carbonization have been developed to prepare porous carbon materials. Among these approaches, activation process [18] by virtue of simplicity and scalability has frequently been employed for the preparation of porous carbon materials. However, the resulting carbon materials usually have non-uniform pore sizes and isolated non-interconnected pores. Careful regulation of the activation conditions is therefore needed to control the pore size. In contrast, the template synthesis method involving hard and soft templates provide an opportunity for synthesizing porous carbon materials with well-defined pore structures and uniform
* Corresponding author. Tel.: +86 731 58298572; Fax: +86 731 58293264. E-mail addresses: [email protected] (H. Chen), [email protected] (H. Li). http://dx.doi.org/10.1016/j.electacta.2014.10.023 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
pore sizes. Various inorganic materials such as mesoporous silica materials [19,20], silica nanoparticles [21], zeolites [22–24], metalorganic frameworks [25,26], and anodic alumina membranes [27] have been successfully used as hard templates. Although the precise control of pore size and pore structures is desirable, the hard template method, however, has several limitations such as the sacrificial use of hard templates and difficulties of synthesis of different types of templates. In addition, the inorganic template removal process usually employs hazardous chemicals such as HF or NaOH, which is a drawback in industrial applications. In order to overcome these limitations, efforts have been directed toward soft template method, which is based on a sequence of self-assembly of carbon precursors with the soft templates, stabilization of the precursor framework by polymerization and finally the thermal removal of the templates. There are three key requirements for the successful synthesis of porous carbon materials using soft templates: a) interactions between soft templates and carbon precursors, b) ability of carbon precursors to cross-link, and c) different thermal stability between carbon precursors and soft templates. Typically applied carbon precursors are phenol-formaldehyde prepolymer resins (PF resin) derived from acid-catalyzed or base-catalyzed polymerization of phenol and formaldehyde [28]. The most common used soft templates are amphiphilic block-copolymers such as Pluronic triblock copolymers (PEO-b-PPO-b-PEO) [29–31], polystyrene-b-poly(4-
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vinylpiridine) (PS-b-P4VP) [32], and PS-b-PEO [33]. So far only hydrogen-bonding interaction between soft template and PF resin has been exploited for the production of mesoporous carbon materials. Recently, organic aerogel carbonization as well as polymer blend carbonization has been proposed as an alternative easy, hard template-free methods for the preparation of porous carbon materials. The carbonization of organic aerogels prepared by the sol-gel technique, followed by supercritical drying, has been regarded as a facile way to synthesize carbon foams with welldeveloped mesoporosity [34]. Although the supercritical drying process relieves the large capillary forces generated during the drying process, and makes it possible to preserve the highly crosslinked and porous structure, it unusually, however, requires expensive equipments and high operating cost. Polymer blend carbonization involves carbonizing a polymer composite composed of a carbon precursor polymer and a decomposable polymer or a small organic compound that is pyrolyzed to generate pores [35,36]. So far this synthesis method has been successfully used to prepare porous carbons with various pore sizes [35] as well as porous carbon nanofibers [37]. In previous studies, phase-separated polymer blends of PF resin (carbon precursor) and thermoplastic polymers pore-former) such as poly(vinyl butyral) [35], PEO [36], poly(methyl methacrylate) [38], PS [39], or polyethylene [37], have been chosen to prepare porous carbon materials by pyrolysis of thermoplastic polymer and carbonization of PF resin. However, macro-phase separation often occurs in these polymer blends during the processes of PF resin curing as well as carbonization due to the weak molecular interactions between carbon precursor and pore-forming agent, leading to the resulting carbon materials with broad pore-size distribution, isolated pore structure, small pore volume, and low surface area. Latter studies have focused on reactive blend system based on PF resin and small organic acids, in which the PF resin networks as well as the covalent linkages between PF resin and organic acids through the esterification reaction of acid and hydroxymethyl groups (PF resin) are simultaneously formed [40]. In this way, further phase separation during the carbonization process can be eliminated and the controlled pore generation is expected to achieve. However, this method can only produce mesoporous carbon materials with pore size of around 2 nm together with considerable isolated pores. The chemical reactions between pore-forming agents and carbon precursors play a key role in the success of the porous carbon materials synthesis. In addition, most researches are only limited to PF resin and small organic acids, thus making this method less attractive. Herein, we presented a novel method for the preparation of porous carbon materials with controlled pore size as well as interconnected pore structure. This method involves direct
carbonization of the interpenetrating polymer networks (IPNs), which are composed of two polymeric networks that are at least partially interlaced on a polymer scale but not covalently bonded to each other. During the carbonization process, one of two polymeric networks with high cross-linking density tends to form carbon matrix (carbon precursor) while the other with relatively low cross-linking density decomposes into gaseous products, leaving pores in the carbon matrix (pore-former). In the IPNs, the chain segment movement is hindered by chain-chain interactions, cross-links, and inter-network entanglements, resulting in the nanophase separation with dual phase continuity [41]. After completion of carbonization, porous carbon materials with interconnected pore structure as well as controlled pore size that range from mesopore to macropore are produced. This method can effectively solve these existing problems associated with the conventional synthesis strategies, including a) tedious procedures for the preparation and removal of hard templates in the template method; b) harsh conditions for drying in the sol-gel method; and c) uncontrolled pore sizes and isolated pore structure in the polymer blend technique. In the present study, full IPNs of PF resin and poly(methyl methacrylate) (PMMA) were prepared by simultaneous polymerization. As expected, the resulting porous carbon materials after carbonization exhibited high surface area and enhanced electrochemical performance for supercapacitor. This simple and straightforward method, which dispenses with the need for hard templates or harsh conditions for organic aerogel drying, is cheap and robust. As such, it represents a prospective method for the synthesis of porous carbon materials. 2. Experimental 2.1. Preparation of PF/PMMA IPNs and porous carbons PF prepolymer resins (1.0 g, 50 wt%), methyl methacrylate (480 mg, 4.8 mmol) and ethylene glycol dimethacrylate (20 mg, 0.10 mmol) as well as 2,20 -azobisisobutyronitrile (8.0 mg, 49 mmol) were dissolved in 2.0 mL of dimethylformamide (DMF) in a 10 mL round-bottom flask. The flask was sealed, then stirred and purged with nitrogen for 30 min. Subsequently, the mixture was polymerized at 80 C for 6 h. A brown polymeric gel was obtained after polymerization. The gel was transferred to a ceramic boat followed by curing the PF prepolymer and evaporating the solvent at 160 C for 4 h in an oven. The final product is full PF/PMMA interpenetrating polymer networks (IPNs). The resultant IPNs were pressed under a pressure of 10 MPa and then grinded into fine powders. Finally, the porous carbon was fabricated by carbonizing the IPN powders under N2 atmosphere for 2 h. The final temperature and heating rate of the furnace were
Fig. 1. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of C-PF, C-PB and C-IPN.
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Table 1 Synthesis conditions and properties of porous carbon. Sample
PF/PMMA (mg/mg)
SBET (m2 g1)
Daa (nm)
Vtotalb (cm3 g1)
Vmicc (cm3 g1)
Vmic/Vtotal (%)
Cgd (F g1)
C-PF C-PB C-IPN
100/0 50/50 50/50
576 601 865
1.6 9.4 4.4
0.23 0.42 0.50
0.21 0.22 0.31
91.3 52.4 62.0
165 172 252
a b c d
Average pore diameter. Total pore volume. Micropore volume. The gravimetric capacitance calculated from GC curves at a current density of 0.5 A g1 in a three electrode system.
800 C and 4 C min1, respectively. The product was denoted as C-IPN, standing for carbon prepared from IPNs. For comparison, porous carbons were also prepared from PF/PMMA polymer blends and the pure PF resins, the as-synthesized carbons were named as C-PB and C-PF (for synthesis details see Supporting Information, SI). 2.2. Characterization The morphology and structure of these materials were observed by scanning electron microscopy (SEM, JEOL 3600LV) and transmission electron microscopy (TEM, JEOL JEM-2100). The crystallographic information of the carbon was investigated by powder X-ray diffraction (XRD, Rigaku D/Max 2500PC). Raman spectra were collected on a Renishaw inVia Raman spectrometer. The textural properties were characterized by N2 sorption measurements at 77.3 K (Micromeritics TriStar II 3020). The specific surface area was obtained by Brunauer-Emmett-Teller (BET) method. The pore size distribution (PSD) was calculated from the adsorption branches of the isotherms using the Barrett-JoynerHalenda (BJH) model. The total pore volume (Vtotal) was estimated from the adsorbed amount at a relative pressure p/p of 0.99. Micropore volume (Vmicro) was calculated using the t-plot method. 2.3. Electrochemical measurements The working electrodes were typically fabricated by mixing the porous carbon as the active material (80 wt%), carbon black (10 wt %) and polytetrafluoroethylene (PTFE, 10 wt%) in ethanol and then coated onto the nickel foam current collectors (1 cm 1 cm) with a spatula. The premade electrodes were pressed under a pressure of 10 MPa for 5 min and finally dried at 130 C for 12 h in an oven. The active material loading on each electrode was approximately 4 mg. The symmetric two-electrode supercapacitor was assembled with nearly identical (both weight and size) electrodes, the electrolyte (6 M KOH) and a glassy fibrous separator. The capacitive performance of the single electrode was studied on a CHI760D electrochemical workstation (CH Instruments Inc. Shanghai, China) using a standard three-electrode system with
platinum wire and Hg/HgO electrodes as counter and reference electrodes, respectively, in 6 M KOH electrolyte at 25 C. Cyclic voltammetry (CV), galvanostatic charge-discharge (GC) technique and alternating current impedance were employed in the electrochemical investigations. CV tests of individual electrode were carried out between–1.0 and 0 V (vs. Hg/HgO). GC was performed at different current density varying from 0.5 to 20 A g1 in the same potential range as the CV tests. CV, GC and cycling tests of the assembled supercapacitor were performed in a twoelectrode configuration, in which the counter and reference electrodes are nearly identical with the work electrode. The specific capacitance based on GC was calculated using the equation [42]: Cg = I/(mdV/dt), where I is the constant current and m the mass of active materials, and dV/dt is calculated from the slope obtained by fitting a straight line to the discharge curve from the end of the voltage drop to the end of the discharge process. The capacitance responding frequency was calculated according the formula: C =1/(2pfZ00 ), where f is frequency in Hz and Z00 is the imaginary part of the impedance. 3. Results and discussion 3.1. Preparation and characterization of porous carbons In the present study, full IPNs of PF and PMMA (the mass ratio of PF/PMMA is 50/50) were prepared by the simultaneous polymerization process, in which the PF network was obtained by thermal curing of PF prepolymer, while the free radical copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDA) in the presence of 2,20 -azobisisobutyronitrile (AIBN) led to the formation of PMMA network. During the followed carbonization process, the PF network, named carbon precursor, tends to form carbon matrix, while the PMMA network, named poreformer, decomposes completely and leaves pores in the carbon matrix. For comparison, PF/PMMA blend with the same composition was prepared by mixing PF prepolymer with powder PMMA (MMA/EGDA copolymer) followed by curing PF prepolymer under identical conditions. In addition, PF prepolymer was also cured under the same condition, giving pure PF.
Fig. 2. SEM images of C-IPN (a), C-PB (b), and C-PF (c).
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Fig. 3. XRD patterns (a) and Raman spectra (b) of the three carbons, and TEM images (c, d) of C-IPN.
In order to investigate the carbon yield of PF as well as the decomposition behavior of PMMA, thermalgravimetric analysis (TGA) was initially performed. As expected, pure PF shows a gradual weight decrease at 200–700 C and give an adequate
carbon residue (60 wt%) (Fig. S1, see SI), indicating that PF is an excellent carbon precursor. Conversely, the powder PMMA can be completely decomposed at around 500 C due to low cross-linking density (the molar ratio of MMA/EGDA is 98/2), suggesting that the
Fig. 4. The CV curves tested at a scan rate of 20 mV s1 (a), the GC curves tested at a current density of 0.5 A g1 (b), the Nyquist plots in the frequency range of 100 kHz to 10 mHz in KOH electrolyte at 25 C (c), and frequency response of the gravimetric capacitance of the three carbons (d).
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slightly crosslinked PMMA is an ideal pore-former. Additionally, both IPN and blend give a similar carbon yield of about 25 wt% due to the same compositions. Based on the results of TGA analysis, the as-prepared polymer samples were directly subjected to pyrolysis, in which the carbonization was conducted at 800 C for 2 h in nitrogen atmosphere. The carbonized products are denoted as C-PF, C-PB, and C-IPN, respectively, for pure PF, PF/PMMA blend, and PF/PMMA IPNs. As mentioned previously, the aim of this work is to establish a simple and facile method for the preparation of porous carbon with an interconnected pore structure. Therefore, the porous structures of the obtained carbons were initially characterized by N2 adsorption at 77.3 K. As can be seen from Fig. 1a, the isotherm of C-IPN exhibits a typical type-IV curve with an obvious type-H4 hysteresis loop at p/p = 0.45. In the earlier literatures, the type-H4 loop was ascribed to the narrow slitlike pores [43], the extensively interconnected porous network [44], as well as the network of mesoporous cavity and connecting channels [45–47]. Later experimental results for MCM-41 particles with internal voids of irregular shape and broad size distribution (5–30 nm) also revealed the existence of hysteresis behavior of the type-H4 [48]. This would suggest that typeH4 hysteresis loops may merely arise from the presence of large mesopores embedded in a matrix with interconnected channels of much smaller size. In our case, the chain segments movement of IPNs is hindered by the inter-network entanglements, chain-chain interactions, and cross-links, resulting in the nanophase separation with dual phase continuity [41], whose domain size is around several hundreds of nanometer. After completion of carbonization, carbon with interconnected pore structure along with pore size that ranges from micropore to mesopore is obtained, in which the latter is derived from the decomposition of PMMA pore-former. This has been demonstrated by the pore size distribution (PSD) of C-IPN presented in Fig. 1b, which shows a bimodal PSD with average pore size centered at around 1.9 nm and 17 nm, respectively, as calculated
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by BJH method, but the PSD of mesopore is much broader. On the other hand, the isotherms of C-PB and C-PF, however, exhibit a typical type-I curve, which is an indication of a microporous structure only [43]. It is noteworthy that a sharp inflection is clearly observed in the isotherm of C-PB at p/p = 0.96, which is indicative of macropore [49]. The synthesis conditions and properties of obtained carbons are summarized in Table 1. As can be seen, the specific surface areas of samples from C-IPN, C-PB, to C-PF decrease from 865, 601, to 576 m2 g1, and the pore volumes decrease from 0.50, 0.42, to 0.23 cm3 g1. Clearly, the C-IPN possesses the highest specific surface area as well as the highest pore volume, indicating that IPNs carbonization is a prospective method for the synthesis of porous carbons. The SEM observation reveals that the C-IPN possesses a continuous sponge-like branched framework and fully interconnected macropores ranging from tens to hundreds of nanometers (Fig. 2a). As a comparison, the morphology change of C-PB can clearly be seen, in which isolated pores ranged from 1 to 3 mm are dispersed in the carbon matrix (Fig. 2b). For the C-PF, only carbon blocks are observed and no pores exist in the matrix (Fig. 2c). The XRD patterns of the obtained carbons are shown in Fig. 3a. As can be seen, all carbons show two characteristic diffraction peaks centered at 2u = 23.3 and 43.7. The diffraction peak at 23.3 can be indexed to the (002) reflection of carbon materials, which is shifted from 26 (characteristic for graphite [50]), indicating that the carbons possess well-developed graphitic stacking. The value of d002 is about 0.389 nm, larger than that of graphite (0.335 nm), also implying a random combination of graphitic and turbostratic stacking [13]. On the other hand, the small peak at 43.7 demonstrates the formation of interlayer condensation of the carbons, suggesting that graphitic structure was developed to a slight extent. These results demonstrate that a partial graphite-like microstructure is constructed even at a moderate carbonization temperature of 800 C.
Fig. 5. CV curves tested at scan rates of 5–300 mV s1 (a), the the correlation of specific capacitances with current densities (b), and the GC curves tested at 0.5–20 A g1 (c, d) of C-IPN electrode.
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Fig. 6. CV curves at 10–500 mV s1 (a, b), GC discharge curves of 0.5–10 A g1 (c), and cycling performance (d) of C-IPN based supercapacitor.
Supporting evidence comes from Raman spectroscopic analysis. As shown in Fig. 3b, all carbons exhibit two bands at around 1340 cm1 (D band) and 1586 cm1 (G band). The disorder mode band (D band) is related to the presence of disordered carbon structures, and corresponds to the breathing modes of rings or k-point phonons of A1 g symmetry. The G band refers to the graphite in-plane vibration of sp2-hybridized carbon atoms with E2 g symmetry [51,52]. The intensity ratio between Raman D and G bands (ID/IG) is known to be an indicator of the degree of graphitization, and the smaller the ID/IG ratio, the higher the degree of graphitization. In our case, the ID/IG peak ratio is around 0.98, 1.00, and 0.99 for C-IPN, C-PB, and C-PF, suggesting slight degree of graphitization. This is in good agreement with the TEM study presented in the following section. The porous structure and graphite crystallite structure of C-IPN were further studied by TEM technique. The TEM image (Fig. 3c) reveals that C-IPN with porous texture has been successfully fabricated. In addition, the localized graphitic structure can be seen in the high-resolution TEM image (Fig. 3d), which will lead to an enhanced conductivity. Clearly, the C-IPN with the characteristics of hierarchical porous, interconnected pore structure, and localized graphitic structures make it more promise as electrode materials for supercapacitors. 3.2. Electrochemical performance: supercapacitors Performances of the as-synthesized carbons as electrode materials for supercapacitors were estimated by a standard three-electrode system and a symmetrical two-electrode configuration in 6 M KOH aqueous solution. The typical cylic voltammograms (CVs) of the three carbon based electrodes at a scan rate of 20 mV s1 in the potential range between –1 V and 0 V (vs. Hg/ HgO) are shown in Fig. 4a. As can be seen, the CV curve of C-IPN electrode has a quasi-rectangular shape, suggesting an ideal double-layer capacitance behavior based on ionic adsorption and
exchange. The good rectangular shape of C-IPN also implies that the electrode has a quick charge and discharge feature [15]. The CV curves of C-PB and C-PF are deviated from rectangular shape, which is mainly determined by the pore texture and surface area. However, the CV of C-PB exhibits a shape more close to rectangle than C-PF, resulting from the facilitated ionic diffusion by macropores in the carbon framework of C-PB. Galvanostatic charge-discharge (GC) curves of these carbon electrodes are perfectly liner and display an isosceles triangle shape (Fig. 4b), indicating the three carbon based electrodes in KOH solution have good coulombic efficiency and good double-layer capacitive performance. The gravimetric capacitance (Cg) values of the three carbons are 165 F g1 for C-PF, 172 F g1 for C-PB, and 252 F g1 for C-IPN (from the discharge curves at a constant current density of 0.5 A g1). More importantly, the voltage drop at the start of the discharge is just 5.0 mV (vs. Hg/HgO) for C-IPN, respectively, suggesting a low equivalent series resistance and the ability for quick charge propagation [12]. The electrochemical impedance spectroscopy (EIS) was then measured in 6 M KOH solution in the range from 10 mHz to 100 kHz. As shown in Fig. 4c, the Nyquist plot of C-IPN electrode comprises an almost vertical line at low frequency, a nearly 45 diagonal line at intermediate frequency, and a quasi-semicircle in high frequency (inset of Fig. 4c). The vertical line in the low frequency region indicates the characteristics of the pure capacitive behavior of C-IPN. The 45 line at intermediate frequency is related to the characteristic of ion diffusion into the carbon based electrodes, while the quasi-semicircle in the high-frequency region corresponds to the charge-transfer resistance. However, the Nyquist plots of C-PF and C-PB electrodes deviate from vertical line at low frequency, indicating the electrolyte can not easily access the pores. In addition, all these carbons present low internal resistance, which is mainly ascribed to the high electrical conductivity resulting from the local graphitized carbon framework. The capacitance dependence on frequency of the carbon
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based electrodes is then compared. As can be seen in Fig. 4d, the C-IPN electrode shows a slow capacitance drop, indicating fast ion transport and quick ion adsorption on the surface. On the basis of above studies, we focused our further study on the C-IPN based supercapacitors. To evaluate the capacitive performance of C-IPN based electrode as supercapacitors, CV measurements at different scan rates and GC tests at different current densities were thus conducted. As shown in Fig. 5a, with the scan rate increasing from 5 to 300 mV s1, the CV curves of C-IPN become somewhat distorted but still retain a rectangular-like shape, suggesting an excellent high-rate capacitive behavior. The GC curves of C-IPN display a nearly isosceles triangle shape at different current densities (Fig. 5c and d). More importantly, the GC curves still maintain triangle shapes at a high current density of 20 A g1, implying good coulombic efficiency and ideal capacitive behavior. In addition, sudden potential drops are not observed at this high current density, indicating the high charge-discharge efficiency and low equivalent series resistance [12]. The correlation of specific capacitances with current densities ranging from 0.5 to 20 A g1 obtained from GC tests is shown in Fig. 5b, as calculated from the GC discharge curve, even at high current density up to 20 A g1, the specific capacitance retains 171 F g1 (68% of its initial capacitance at 0.5 A g1), indicating excellent rate capability. This superior specific capacitance of C-IPN can be ascribed to the high specific surface area, hierarchical porous structures, and good conductivity. The rate capability and cycling performance of C-IPN based supercapacitor were tested with a two-electrode system. CV tests at different scan rates (0–500 mV s1) and GC tests at different current densities (0.5–10 A g1) were utilized to examine the rate capability of C-IPN based supercapacitor. As shown in Fig. 6a and b, the CV curves possess a typical rectangular shape at relatively low scan rates. However, the CV curve becomes slightly distorted but still retains a rectangular-like shape when the scan rate reaches up to 500 mV s1, indicating an ideal electrochemical capacitive behavior with rapid diffusion and easy transportation of electrolyte ions to the interface of the electrode [53]. Fig. 6c shows the GC discharge curves of the C-IPN supercapacitor. Obviously, the curve at 10 A g1 still maintains the linear shape and no obvious IR drop can be observed, indicating that C-IPN has the characteristics of little internal resistance and high rate capability. The cycling stability of the C-IPN based supercapacitor during a chargedischarge process has been considered as another crucial factor in practical applications. Thus the cycling stability of C-IPN based supercapacitors was investigated in a two-electrode configuration at a current density of 2 A g1. As shown in Fig. 6d and Fig. S2 (see SI), C-IPN exhibits high capacitance retentions, i.e., 97.9% and 93.4% of its initial capacitance after 10000 and 16000 charge-discharge cycles, respectively, showing excellent cycling stability. Such excellent capacitive performances can be ascribed to the continuous porous carbon framework in the prepared carbons.
4. Conclusions Hierarchical porous carbon material with an interconnected pore structure was synthesized by direct carbonization of PF/PMMA interpenetrating polymer networks (IPNs). The assynthesized porous carbon material (C-IPN) possesses a partial graphitic structure with a high specific surface area of 865 m2 g1 and an average pore size of 4.4 nm. The C-IPN based electrode for supercapacitor displays a high specific capacitance of 252 F g1 at 0.5 A g1, and possesses superior rate capability, excellent cycling stability. The preparation of porous carbon from PF/PMMA IPNs is cost-effective and timesaving with high carbon yield, which can be
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extended to synthesis other porous carbons such as heteroatomdoped porous carbon materials. Acknowledgment Financial support from Program for NSFC (51072172), International Joint Research Program of Hunan Province (2010WK2009), and Open Project of Hunan Provincial University Innovation Platform (10K066) is greatly acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2014.10.023. References [1] Z. Yang, Y. Xia, R. Mokaya, Enhanced hydrogen storage capacity of high surface area zeolite-like carbon materials, J. Am. Chem. Soc. 129 (2007) 1673. [2] L. Wang, R.T. Yang, Molecular hydrogen and spiltover hydrogen storage on high surface area carbon sorbents, Carbon 50 (2012) 3134. [3] J. Schuster, G. He, B. Mandlmeier, T. Yim, K.T. Lee, T. Bein, et al., Spherical ordered mesoporous carbon nanoparticles with high porosity for lithium– sulfur batteries, Angew. Chem. Int. Ed. 51 (2012) 3591. [4] F. Han, W.C. Li, M.R. Li, A.H. Lu, Fabrication of superior-performance SnO2@ C composites for lithium-ion anodes using tubular mesoporous carbon with thin carbon walls and high pore volume, J. Mater. Chem. 22 (2012) 9645. [5] A. Kim, R. Black, Y.J. Hyun, L.F. Nazar, E. Prouzet, Synthesis of monolithic meso– macroporous silica and carbon with tunable pore size, Chem. Commun. 48 (2012) 4335. [6] C. Zheng, W. Qian, C. Cui, Q. Zhang, Y. Jin, M. Zhao, et al., Hierarchical carbon nanotube membrane with high packing density and tunable porous structure for high voltage supercapacitors, Carbon 50 (2012) 5167. [7] E. Spanakis, M. Pervolaraki, J. Giapintzakis, N. Katsarakis, E. Koudoumas, D. Vernardou, Effect of gold and silver nanoislands on the electrochemical properties of carbon nanofoam, Electrochim. Acta 111 (2013) 305. [8] Y.S. Hu, P. Adelhelm, B.M. Smarsly, S. Hore, M. Antonietti, J. Maier, Synthesis of Hierarchically Porous Carbon Monoliths with Highly Ordered Microstructure and Their Application in Rechargeable Lithium Batteries with High-Rate Capability, Adv. Funct. Mater. 17 (2007) 1873. [9] L.R. Radovic, B. Bockrath, On the chemical nature of graphene edges: origin of stability and potential for magnetism in carbon materials, J. Am. Chem. Soc. 127 (2005) 5917. [10] M.D. Hornbostel, J. Bao, G. Krishnan, A. Nagar, I. Jayaweera, T. Kobayashi, et al., Characteristics of an advanced carbon sorbent for CO2 capture, Carbon 56 (2013) 77. [11] P. Liu, D. Yang, H. Chen, Y. Gao, H. Li, Discrete and dispersible hollow carbon spheres for PtRu electrocatalyst support in DMFCs, Electrochim. Acta 109 (2013) 238. [12] D.C. Guo, J. Mi, G.P. Hao, W. Dong, G. Xiong, W.C. Li, et al., Ionic liquid C16mimBF4 assisted synthesis of poly (benzoxazine-co-resol)-based hierarchically porous carbons with superior performance in supercapacitors, Energy Environ. Sci. 6 (2013) 652. [13] M. Sevilla, A.B. Fuertes, Fabrication of porous carbon monoliths with a graphitic framework, Carbon 56 (2013) 155. [14] J. Zhang, D. Yang, W. Li, Y. Gao, H. Li, Synthesis and electrochemical performance of porous carbons by carbonization of self-assembled polymer bricks, Electrochim. Acta 130 (2014) 699. [15] M. Liu, L. Gan, W. Xiong, F. Zhao, X. Fan, D. Zhu, et al., Nickel-doped activated mesoporous carbon microspheres with partially graphitic structure for supercapacitors, Energy Fuels 27 (2013) 1168. [16] R. Liu, C. Zhao, J. Li, F. Zeng, S. Wang, T. Wen, et al., A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells, J. Power Sources 195 (2010) 480. [17] C. Jiang, J. Ma, A.D. Bonaccorso, J.T. Irvine, Demonstration of high power, direct conversion of waste-derived carbon in a hybrid direct carbon fuel cell, Energy Environ. Sci. 5 (2012) 6973. [18] J. Lee, J. Kim, T. Hyeon, Recent progress in the synthesis of porous carbon materials, Adv. Mater. 18 (2006) 2073. [19] L.C. Sang, A. Vinu, M.O. Coppens, Ordered mesoporous carbon with tunable, unusually large pore size and well-controlled particle morphology, J. Mater. Chem. 21 (2011) 7410. [20] H. Lu, W. Dai, M. Zheng, N. Li, G. Ji, J. Cao, Electrochemical capacitive behaviors of ordered mesoporous carbons with controllable pore sizes, J. Power Sources 209 (2012) 243. [21] Z. Chen, J. Wen, C. Yan, L. Rice, H. Sohn, M. Shen, et al., High-performance supercapacitors based on hierarchically porous graphite particles, Adv. Energy Mater. 1 (2011) 551. [22] A. Garsuch, O. Klepel, Synthesis of ordered carbon replicas by using Y-zeolite as template in a batch reactor, Carbon 43 (2005) 2330.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Synthesis and electrochemical performance of porous carbon by carbonizing PF/PMMA interpenetrating polymer networks Jun Zhang a , Xiaofeng Zhong a , Hongbiao Chen a, *, Yong Gao a , Huaming Li a,b, * a
College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, PR China Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, and Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan Province, PR China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 August 2014 Received in revised form 8 October 2014 Accepted 8 October 2014 Available online 13 October 2014
Porous carbon was synthesized by the direct carbonization of full interpenetrating polymer networks (IPNs) of phenol-formaldehyde resin (PF) and poly(methyl methacrylate) (PMMA). The PF/PMMA IPNs were prepared by simultaneous polymerization of PF prepolymer as well as methyl methacrylate and ethylene glycol dimethacrylate monomers in dimethylformamide. During the followed carbonization process, the PF polymeric networks tended to form carbon matrix (carbon precursor) while the PMMA decomposed into gaseous products, leaving pores in carbon matrix (pore-former). The obtained porous carbon has an interconnected pore structure with a high BET surface area of 865 m2 g1 and an average pore size of 4.4 nm. Such porous carbon shows outstanding capacitive performance (252 F g1), good rate capacitive behavior, and excellent cycling stability. Due to its unique pore structure, the as-prepared porous carbon shows promise as an electrode material for supercapacitors. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Interpenetrating polymer networks porous carbon electrochemical supercapacitors
1. Introduction Porous carbon materials have attracted substantial attention in recent years due to their remarkable properties such as high specific surface area, large pore volume, tunable pore size, low density, high conductivity, and high chemical stability [1–9]. These properties make them desired materials for absorbents [2,10], catalyst supports [11,12], and electrode materials for electrochemical capacitors [12–15] and fuel cells [16,17]. So far, various methods including activation process, template method, organic aerogel carbonization, and polymer blend carbonization have been developed to prepare porous carbon materials. Among these approaches, activation process [18] by virtue of simplicity and scalability has frequently been employed for the preparation of porous carbon materials. However, the resulting carbon materials usually have non-uniform pore sizes and isolated non-interconnected pores. Careful regulation of the activation conditions is therefore needed to control the pore size. In contrast, the template synthesis method involving hard and soft templates provide an opportunity for synthesizing porous carbon materials with well-defined pore structures and uniform
* Corresponding author. Tel.: +86 731 58298572; Fax: +86 731 58293264. E-mail addresses: [email protected] (H. Chen), [email protected] (H. Li). http://dx.doi.org/10.1016/j.electacta.2014.10.023 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
pore sizes. Various inorganic materials such as mesoporous silica materials [19,20], silica nanoparticles [21], zeolites [22–24], metalorganic frameworks [25,26], and anodic alumina membranes [27] have been successfully used as hard templates. Although the precise control of pore size and pore structures is desirable, the hard template method, however, has several limitations such as the sacrificial use of hard templates and difficulties of synthesis of different types of templates. In addition, the inorganic template removal process usually employs hazardous chemicals such as HF or NaOH, which is a drawback in industrial applications. In order to overcome these limitations, efforts have been directed toward soft template method, which is based on a sequence of self-assembly of carbon precursors with the soft templates, stabilization of the precursor framework by polymerization and finally the thermal removal of the templates. There are three key requirements for the successful synthesis of porous carbon materials using soft templates: a) interactions between soft templates and carbon precursors, b) ability of carbon precursors to cross-link, and c) different thermal stability between carbon precursors and soft templates. Typically applied carbon precursors are phenol-formaldehyde prepolymer resins (PF resin) derived from acid-catalyzed or base-catalyzed polymerization of phenol and formaldehyde [28]. The most common used soft templates are amphiphilic block-copolymers such as Pluronic triblock copolymers (PEO-b-PPO-b-PEO) [29–31], polystyrene-b-poly(4-
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vinylpiridine) (PS-b-P4VP) [32], and PS-b-PEO [33]. So far only hydrogen-bonding interaction between soft template and PF resin has been exploited for the production of mesoporous carbon materials. Recently, organic aerogel carbonization as well as polymer blend carbonization has been proposed as an alternative easy, hard template-free methods for the preparation of porous carbon materials. The carbonization of organic aerogels prepared by the sol-gel technique, followed by supercritical drying, has been regarded as a facile way to synthesize carbon foams with welldeveloped mesoporosity [34]. Although the supercritical drying process relieves the large capillary forces generated during the drying process, and makes it possible to preserve the highly crosslinked and porous structure, it unusually, however, requires expensive equipments and high operating cost. Polymer blend carbonization involves carbonizing a polymer composite composed of a carbon precursor polymer and a decomposable polymer or a small organic compound that is pyrolyzed to generate pores [35,36]. So far this synthesis method has been successfully used to prepare porous carbons with various pore sizes [35] as well as porous carbon nanofibers [37]. In previous studies, phase-separated polymer blends of PF resin (carbon precursor) and thermoplastic polymers pore-former) such as poly(vinyl butyral) [35], PEO [36], poly(methyl methacrylate) [38], PS [39], or polyethylene [37], have been chosen to prepare porous carbon materials by pyrolysis of thermoplastic polymer and carbonization of PF resin. However, macro-phase separation often occurs in these polymer blends during the processes of PF resin curing as well as carbonization due to the weak molecular interactions between carbon precursor and pore-forming agent, leading to the resulting carbon materials with broad pore-size distribution, isolated pore structure, small pore volume, and low surface area. Latter studies have focused on reactive blend system based on PF resin and small organic acids, in which the PF resin networks as well as the covalent linkages between PF resin and organic acids through the esterification reaction of acid and hydroxymethyl groups (PF resin) are simultaneously formed [40]. In this way, further phase separation during the carbonization process can be eliminated and the controlled pore generation is expected to achieve. However, this method can only produce mesoporous carbon materials with pore size of around 2 nm together with considerable isolated pores. The chemical reactions between pore-forming agents and carbon precursors play a key role in the success of the porous carbon materials synthesis. In addition, most researches are only limited to PF resin and small organic acids, thus making this method less attractive. Herein, we presented a novel method for the preparation of porous carbon materials with controlled pore size as well as interconnected pore structure. This method involves direct
carbonization of the interpenetrating polymer networks (IPNs), which are composed of two polymeric networks that are at least partially interlaced on a polymer scale but not covalently bonded to each other. During the carbonization process, one of two polymeric networks with high cross-linking density tends to form carbon matrix (carbon precursor) while the other with relatively low cross-linking density decomposes into gaseous products, leaving pores in the carbon matrix (pore-former). In the IPNs, the chain segment movement is hindered by chain-chain interactions, cross-links, and inter-network entanglements, resulting in the nanophase separation with dual phase continuity [41]. After completion of carbonization, porous carbon materials with interconnected pore structure as well as controlled pore size that range from mesopore to macropore are produced. This method can effectively solve these existing problems associated with the conventional synthesis strategies, including a) tedious procedures for the preparation and removal of hard templates in the template method; b) harsh conditions for drying in the sol-gel method; and c) uncontrolled pore sizes and isolated pore structure in the polymer blend technique. In the present study, full IPNs of PF resin and poly(methyl methacrylate) (PMMA) were prepared by simultaneous polymerization. As expected, the resulting porous carbon materials after carbonization exhibited high surface area and enhanced electrochemical performance for supercapacitor. This simple and straightforward method, which dispenses with the need for hard templates or harsh conditions for organic aerogel drying, is cheap and robust. As such, it represents a prospective method for the synthesis of porous carbon materials. 2. Experimental 2.1. Preparation of PF/PMMA IPNs and porous carbons PF prepolymer resins (1.0 g, 50 wt%), methyl methacrylate (480 mg, 4.8 mmol) and ethylene glycol dimethacrylate (20 mg, 0.10 mmol) as well as 2,20 -azobisisobutyronitrile (8.0 mg, 49 mmol) were dissolved in 2.0 mL of dimethylformamide (DMF) in a 10 mL round-bottom flask. The flask was sealed, then stirred and purged with nitrogen for 30 min. Subsequently, the mixture was polymerized at 80 C for 6 h. A brown polymeric gel was obtained after polymerization. The gel was transferred to a ceramic boat followed by curing the PF prepolymer and evaporating the solvent at 160 C for 4 h in an oven. The final product is full PF/PMMA interpenetrating polymer networks (IPNs). The resultant IPNs were pressed under a pressure of 10 MPa and then grinded into fine powders. Finally, the porous carbon was fabricated by carbonizing the IPN powders under N2 atmosphere for 2 h. The final temperature and heating rate of the furnace were
Fig. 1. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of C-PF, C-PB and C-IPN.
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Table 1 Synthesis conditions and properties of porous carbon. Sample
PF/PMMA (mg/mg)
SBET (m2 g1)
Daa (nm)
Vtotalb (cm3 g1)
Vmicc (cm3 g1)
Vmic/Vtotal (%)
Cgd (F g1)
C-PF C-PB C-IPN
100/0 50/50 50/50
576 601 865
1.6 9.4 4.4
0.23 0.42 0.50
0.21 0.22 0.31
91.3 52.4 62.0
165 172 252
a b c d
Average pore diameter. Total pore volume. Micropore volume. The gravimetric capacitance calculated from GC curves at a current density of 0.5 A g1 in a three electrode system.
800 C and 4 C min1, respectively. The product was denoted as C-IPN, standing for carbon prepared from IPNs. For comparison, porous carbons were also prepared from PF/PMMA polymer blends and the pure PF resins, the as-synthesized carbons were named as C-PB and C-PF (for synthesis details see Supporting Information, SI). 2.2. Characterization The morphology and structure of these materials were observed by scanning electron microscopy (SEM, JEOL 3600LV) and transmission electron microscopy (TEM, JEOL JEM-2100). The crystallographic information of the carbon was investigated by powder X-ray diffraction (XRD, Rigaku D/Max 2500PC). Raman spectra were collected on a Renishaw inVia Raman spectrometer. The textural properties were characterized by N2 sorption measurements at 77.3 K (Micromeritics TriStar II 3020). The specific surface area was obtained by Brunauer-Emmett-Teller (BET) method. The pore size distribution (PSD) was calculated from the adsorption branches of the isotherms using the Barrett-JoynerHalenda (BJH) model. The total pore volume (Vtotal) was estimated from the adsorbed amount at a relative pressure p/p of 0.99. Micropore volume (Vmicro) was calculated using the t-plot method. 2.3. Electrochemical measurements The working electrodes were typically fabricated by mixing the porous carbon as the active material (80 wt%), carbon black (10 wt %) and polytetrafluoroethylene (PTFE, 10 wt%) in ethanol and then coated onto the nickel foam current collectors (1 cm 1 cm) with a spatula. The premade electrodes were pressed under a pressure of 10 MPa for 5 min and finally dried at 130 C for 12 h in an oven. The active material loading on each electrode was approximately 4 mg. The symmetric two-electrode supercapacitor was assembled with nearly identical (both weight and size) electrodes, the electrolyte (6 M KOH) and a glassy fibrous separator. The capacitive performance of the single electrode was studied on a CHI760D electrochemical workstation (CH Instruments Inc. Shanghai, China) using a standard three-electrode system with
platinum wire and Hg/HgO electrodes as counter and reference electrodes, respectively, in 6 M KOH electrolyte at 25 C. Cyclic voltammetry (CV), galvanostatic charge-discharge (GC) technique and alternating current impedance were employed in the electrochemical investigations. CV tests of individual electrode were carried out between–1.0 and 0 V (vs. Hg/HgO). GC was performed at different current density varying from 0.5 to 20 A g1 in the same potential range as the CV tests. CV, GC and cycling tests of the assembled supercapacitor were performed in a twoelectrode configuration, in which the counter and reference electrodes are nearly identical with the work electrode. The specific capacitance based on GC was calculated using the equation [42]: Cg = I/(mdV/dt), where I is the constant current and m the mass of active materials, and dV/dt is calculated from the slope obtained by fitting a straight line to the discharge curve from the end of the voltage drop to the end of the discharge process. The capacitance responding frequency was calculated according the formula: C =1/(2pfZ00 ), where f is frequency in Hz and Z00 is the imaginary part of the impedance. 3. Results and discussion 3.1. Preparation and characterization of porous carbons In the present study, full IPNs of PF and PMMA (the mass ratio of PF/PMMA is 50/50) were prepared by the simultaneous polymerization process, in which the PF network was obtained by thermal curing of PF prepolymer, while the free radical copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDA) in the presence of 2,20 -azobisisobutyronitrile (AIBN) led to the formation of PMMA network. During the followed carbonization process, the PF network, named carbon precursor, tends to form carbon matrix, while the PMMA network, named poreformer, decomposes completely and leaves pores in the carbon matrix. For comparison, PF/PMMA blend with the same composition was prepared by mixing PF prepolymer with powder PMMA (MMA/EGDA copolymer) followed by curing PF prepolymer under identical conditions. In addition, PF prepolymer was also cured under the same condition, giving pure PF.
Fig. 2. SEM images of C-IPN (a), C-PB (b), and C-PF (c).
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Fig. 3. XRD patterns (a) and Raman spectra (b) of the three carbons, and TEM images (c, d) of C-IPN.
In order to investigate the carbon yield of PF as well as the decomposition behavior of PMMA, thermalgravimetric analysis (TGA) was initially performed. As expected, pure PF shows a gradual weight decrease at 200–700 C and give an adequate
carbon residue (60 wt%) (Fig. S1, see SI), indicating that PF is an excellent carbon precursor. Conversely, the powder PMMA can be completely decomposed at around 500 C due to low cross-linking density (the molar ratio of MMA/EGDA is 98/2), suggesting that the
Fig. 4. The CV curves tested at a scan rate of 20 mV s1 (a), the GC curves tested at a current density of 0.5 A g1 (b), the Nyquist plots in the frequency range of 100 kHz to 10 mHz in KOH electrolyte at 25 C (c), and frequency response of the gravimetric capacitance of the three carbons (d).
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slightly crosslinked PMMA is an ideal pore-former. Additionally, both IPN and blend give a similar carbon yield of about 25 wt% due to the same compositions. Based on the results of TGA analysis, the as-prepared polymer samples were directly subjected to pyrolysis, in which the carbonization was conducted at 800 C for 2 h in nitrogen atmosphere. The carbonized products are denoted as C-PF, C-PB, and C-IPN, respectively, for pure PF, PF/PMMA blend, and PF/PMMA IPNs. As mentioned previously, the aim of this work is to establish a simple and facile method for the preparation of porous carbon with an interconnected pore structure. Therefore, the porous structures of the obtained carbons were initially characterized by N2 adsorption at 77.3 K. As can be seen from Fig. 1a, the isotherm of C-IPN exhibits a typical type-IV curve with an obvious type-H4 hysteresis loop at p/p = 0.45. In the earlier literatures, the type-H4 loop was ascribed to the narrow slitlike pores [43], the extensively interconnected porous network [44], as well as the network of mesoporous cavity and connecting channels [45–47]. Later experimental results for MCM-41 particles with internal voids of irregular shape and broad size distribution (5–30 nm) also revealed the existence of hysteresis behavior of the type-H4 [48]. This would suggest that typeH4 hysteresis loops may merely arise from the presence of large mesopores embedded in a matrix with interconnected channels of much smaller size. In our case, the chain segments movement of IPNs is hindered by the inter-network entanglements, chain-chain interactions, and cross-links, resulting in the nanophase separation with dual phase continuity [41], whose domain size is around several hundreds of nanometer. After completion of carbonization, carbon with interconnected pore structure along with pore size that ranges from micropore to mesopore is obtained, in which the latter is derived from the decomposition of PMMA pore-former. This has been demonstrated by the pore size distribution (PSD) of C-IPN presented in Fig. 1b, which shows a bimodal PSD with average pore size centered at around 1.9 nm and 17 nm, respectively, as calculated
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by BJH method, but the PSD of mesopore is much broader. On the other hand, the isotherms of C-PB and C-PF, however, exhibit a typical type-I curve, which is an indication of a microporous structure only [43]. It is noteworthy that a sharp inflection is clearly observed in the isotherm of C-PB at p/p = 0.96, which is indicative of macropore [49]. The synthesis conditions and properties of obtained carbons are summarized in Table 1. As can be seen, the specific surface areas of samples from C-IPN, C-PB, to C-PF decrease from 865, 601, to 576 m2 g1, and the pore volumes decrease from 0.50, 0.42, to 0.23 cm3 g1. Clearly, the C-IPN possesses the highest specific surface area as well as the highest pore volume, indicating that IPNs carbonization is a prospective method for the synthesis of porous carbons. The SEM observation reveals that the C-IPN possesses a continuous sponge-like branched framework and fully interconnected macropores ranging from tens to hundreds of nanometers (Fig. 2a). As a comparison, the morphology change of C-PB can clearly be seen, in which isolated pores ranged from 1 to 3 mm are dispersed in the carbon matrix (Fig. 2b). For the C-PF, only carbon blocks are observed and no pores exist in the matrix (Fig. 2c). The XRD patterns of the obtained carbons are shown in Fig. 3a. As can be seen, all carbons show two characteristic diffraction peaks centered at 2u = 23.3 and 43.7. The diffraction peak at 23.3 can be indexed to the (002) reflection of carbon materials, which is shifted from 26 (characteristic for graphite [50]), indicating that the carbons possess well-developed graphitic stacking. The value of d002 is about 0.389 nm, larger than that of graphite (0.335 nm), also implying a random combination of graphitic and turbostratic stacking [13]. On the other hand, the small peak at 43.7 demonstrates the formation of interlayer condensation of the carbons, suggesting that graphitic structure was developed to a slight extent. These results demonstrate that a partial graphite-like microstructure is constructed even at a moderate carbonization temperature of 800 C.
Fig. 5. CV curves tested at scan rates of 5–300 mV s1 (a), the the correlation of specific capacitances with current densities (b), and the GC curves tested at 0.5–20 A g1 (c, d) of C-IPN electrode.
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Fig. 6. CV curves at 10–500 mV s1 (a, b), GC discharge curves of 0.5–10 A g1 (c), and cycling performance (d) of C-IPN based supercapacitor.
Supporting evidence comes from Raman spectroscopic analysis. As shown in Fig. 3b, all carbons exhibit two bands at around 1340 cm1 (D band) and 1586 cm1 (G band). The disorder mode band (D band) is related to the presence of disordered carbon structures, and corresponds to the breathing modes of rings or k-point phonons of A1 g symmetry. The G band refers to the graphite in-plane vibration of sp2-hybridized carbon atoms with E2 g symmetry [51,52]. The intensity ratio between Raman D and G bands (ID/IG) is known to be an indicator of the degree of graphitization, and the smaller the ID/IG ratio, the higher the degree of graphitization. In our case, the ID/IG peak ratio is around 0.98, 1.00, and 0.99 for C-IPN, C-PB, and C-PF, suggesting slight degree of graphitization. This is in good agreement with the TEM study presented in the following section. The porous structure and graphite crystallite structure of C-IPN were further studied by TEM technique. The TEM image (Fig. 3c) reveals that C-IPN with porous texture has been successfully fabricated. In addition, the localized graphitic structure can be seen in the high-resolution TEM image (Fig. 3d), which will lead to an enhanced conductivity. Clearly, the C-IPN with the characteristics of hierarchical porous, interconnected pore structure, and localized graphitic structures make it more promise as electrode materials for supercapacitors. 3.2. Electrochemical performance: supercapacitors Performances of the as-synthesized carbons as electrode materials for supercapacitors were estimated by a standard three-electrode system and a symmetrical two-electrode configuration in 6 M KOH aqueous solution. The typical cylic voltammograms (CVs) of the three carbon based electrodes at a scan rate of 20 mV s1 in the potential range between –1 V and 0 V (vs. Hg/ HgO) are shown in Fig. 4a. As can be seen, the CV curve of C-IPN electrode has a quasi-rectangular shape, suggesting an ideal double-layer capacitance behavior based on ionic adsorption and
exchange. The good rectangular shape of C-IPN also implies that the electrode has a quick charge and discharge feature [15]. The CV curves of C-PB and C-PF are deviated from rectangular shape, which is mainly determined by the pore texture and surface area. However, the CV of C-PB exhibits a shape more close to rectangle than C-PF, resulting from the facilitated ionic diffusion by macropores in the carbon framework of C-PB. Galvanostatic charge-discharge (GC) curves of these carbon electrodes are perfectly liner and display an isosceles triangle shape (Fig. 4b), indicating the three carbon based electrodes in KOH solution have good coulombic efficiency and good double-layer capacitive performance. The gravimetric capacitance (Cg) values of the three carbons are 165 F g1 for C-PF, 172 F g1 for C-PB, and 252 F g1 for C-IPN (from the discharge curves at a constant current density of 0.5 A g1). More importantly, the voltage drop at the start of the discharge is just 5.0 mV (vs. Hg/HgO) for C-IPN, respectively, suggesting a low equivalent series resistance and the ability for quick charge propagation [12]. The electrochemical impedance spectroscopy (EIS) was then measured in 6 M KOH solution in the range from 10 mHz to 100 kHz. As shown in Fig. 4c, the Nyquist plot of C-IPN electrode comprises an almost vertical line at low frequency, a nearly 45 diagonal line at intermediate frequency, and a quasi-semicircle in high frequency (inset of Fig. 4c). The vertical line in the low frequency region indicates the characteristics of the pure capacitive behavior of C-IPN. The 45 line at intermediate frequency is related to the characteristic of ion diffusion into the carbon based electrodes, while the quasi-semicircle in the high-frequency region corresponds to the charge-transfer resistance. However, the Nyquist plots of C-PF and C-PB electrodes deviate from vertical line at low frequency, indicating the electrolyte can not easily access the pores. In addition, all these carbons present low internal resistance, which is mainly ascribed to the high electrical conductivity resulting from the local graphitized carbon framework. The capacitance dependence on frequency of the carbon
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based electrodes is then compared. As can be seen in Fig. 4d, the C-IPN electrode shows a slow capacitance drop, indicating fast ion transport and quick ion adsorption on the surface. On the basis of above studies, we focused our further study on the C-IPN based supercapacitors. To evaluate the capacitive performance of C-IPN based electrode as supercapacitors, CV measurements at different scan rates and GC tests at different current densities were thus conducted. As shown in Fig. 5a, with the scan rate increasing from 5 to 300 mV s1, the CV curves of C-IPN become somewhat distorted but still retain a rectangular-like shape, suggesting an excellent high-rate capacitive behavior. The GC curves of C-IPN display a nearly isosceles triangle shape at different current densities (Fig. 5c and d). More importantly, the GC curves still maintain triangle shapes at a high current density of 20 A g1, implying good coulombic efficiency and ideal capacitive behavior. In addition, sudden potential drops are not observed at this high current density, indicating the high charge-discharge efficiency and low equivalent series resistance [12]. The correlation of specific capacitances with current densities ranging from 0.5 to 20 A g1 obtained from GC tests is shown in Fig. 5b, as calculated from the GC discharge curve, even at high current density up to 20 A g1, the specific capacitance retains 171 F g1 (68% of its initial capacitance at 0.5 A g1), indicating excellent rate capability. This superior specific capacitance of C-IPN can be ascribed to the high specific surface area, hierarchical porous structures, and good conductivity. The rate capability and cycling performance of C-IPN based supercapacitor were tested with a two-electrode system. CV tests at different scan rates (0–500 mV s1) and GC tests at different current densities (0.5–10 A g1) were utilized to examine the rate capability of C-IPN based supercapacitor. As shown in Fig. 6a and b, the CV curves possess a typical rectangular shape at relatively low scan rates. However, the CV curve becomes slightly distorted but still retains a rectangular-like shape when the scan rate reaches up to 500 mV s1, indicating an ideal electrochemical capacitive behavior with rapid diffusion and easy transportation of electrolyte ions to the interface of the electrode [53]. Fig. 6c shows the GC discharge curves of the C-IPN supercapacitor. Obviously, the curve at 10 A g1 still maintains the linear shape and no obvious IR drop can be observed, indicating that C-IPN has the characteristics of little internal resistance and high rate capability. The cycling stability of the C-IPN based supercapacitor during a chargedischarge process has been considered as another crucial factor in practical applications. Thus the cycling stability of C-IPN based supercapacitors was investigated in a two-electrode configuration at a current density of 2 A g1. As shown in Fig. 6d and Fig. S2 (see SI), C-IPN exhibits high capacitance retentions, i.e., 97.9% and 93.4% of its initial capacitance after 10000 and 16000 charge-discharge cycles, respectively, showing excellent cycling stability. Such excellent capacitive performances can be ascribed to the continuous porous carbon framework in the prepared carbons.
4. Conclusions Hierarchical porous carbon material with an interconnected pore structure was synthesized by direct carbonization of PF/PMMA interpenetrating polymer networks (IPNs). The assynthesized porous carbon material (C-IPN) possesses a partial graphitic structure with a high specific surface area of 865 m2 g1 and an average pore size of 4.4 nm. The C-IPN based electrode for supercapacitor displays a high specific capacitance of 252 F g1 at 0.5 A g1, and possesses superior rate capability, excellent cycling stability. The preparation of porous carbon from PF/PMMA IPNs is cost-effective and timesaving with high carbon yield, which can be
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extended to synthesis other porous carbons such as heteroatomdoped porous carbon materials. Acknowledgment Financial support from Program for NSFC (51072172), International Joint Research Program of Hunan Province (2010WK2009), and Open Project of Hunan Provincial University Innovation Platform (10K066) is greatly acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta.2014.10.023. References [1] Z. Yang, Y. Xia, R. Mokaya, Enhanced hydrogen storage capacity of high surface area zeolite-like carbon materials, J. Am. Chem. Soc. 129 (2007) 1673. [2] L. Wang, R.T. Yang, Molecular hydrogen and spiltover hydrogen storage on high surface area carbon sorbents, Carbon 50 (2012) 3134. [3] J. Schuster, G. He, B. Mandlmeier, T. Yim, K.T. Lee, T. Bein, et al., Spherical ordered mesoporous carbon nanoparticles with high porosity for lithium– sulfur batteries, Angew. Chem. Int. Ed. 51 (2012) 3591. [4] F. Han, W.C. Li, M.R. Li, A.H. Lu, Fabrication of superior-performance SnO2@ C composites for lithium-ion anodes using tubular mesoporous carbon with thin carbon walls and high pore volume, J. Mater. Chem. 22 (2012) 9645. [5] A. Kim, R. Black, Y.J. Hyun, L.F. Nazar, E. Prouzet, Synthesis of monolithic meso– macroporous silica and carbon with tunable pore size, Chem. Commun. 48 (2012) 4335. [6] C. Zheng, W. Qian, C. Cui, Q. Zhang, Y. Jin, M. Zhao, et al., Hierarchical carbon nanotube membrane with high packing density and tunable porous structure for high voltage supercapacitors, Carbon 50 (2012) 5167. [7] E. Spanakis, M. Pervolaraki, J. Giapintzakis, N. Katsarakis, E. Koudoumas, D. Vernardou, Effect of gold and silver nanoislands on the electrochemical properties of carbon nanofoam, Electrochim. Acta 111 (2013) 305. [8] Y.S. Hu, P. Adelhelm, B.M. Smarsly, S. Hore, M. Antonietti, J. Maier, Synthesis of Hierarchically Porous Carbon Monoliths with Highly Ordered Microstructure and Their Application in Rechargeable Lithium Batteries with High-Rate Capability, Adv. Funct. Mater. 17 (2007) 1873. [9] L.R. Radovic, B. Bockrath, On the chemical nature of graphene edges: origin of stability and potential for magnetism in carbon materials, J. Am. Chem. Soc. 127 (2005) 5917. [10] M.D. Hornbostel, J. Bao, G. Krishnan, A. Nagar, I. Jayaweera, T. Kobayashi, et al., Characteristics of an advanced carbon sorbent for CO2 capture, Carbon 56 (2013) 77. [11] P. Liu, D. Yang, H. Chen, Y. Gao, H. Li, Discrete and dispersible hollow carbon spheres for PtRu electrocatalyst support in DMFCs, Electrochim. Acta 109 (2013) 238. [12] D.C. Guo, J. Mi, G.P. Hao, W. Dong, G. Xiong, W.C. Li, et al., Ionic liquid C16mimBF4 assisted synthesis of poly (benzoxazine-co-resol)-based hierarchically porous carbons with superior performance in supercapacitors, Energy Environ. Sci. 6 (2013) 652. [13] M. Sevilla, A.B. Fuertes, Fabrication of porous carbon monoliths with a graphitic framework, Carbon 56 (2013) 155. [14] J. Zhang, D. Yang, W. Li, Y. Gao, H. Li, Synthesis and electrochemical performance of porous carbons by carbonization of self-assembled polymer bricks, Electrochim. Acta 130 (2014) 699. [15] M. Liu, L. Gan, W. Xiong, F. Zhao, X. Fan, D. Zhu, et al., Nickel-doped activated mesoporous carbon microspheres with partially graphitic structure for supercapacitors, Energy Fuels 27 (2013) 1168. [16] R. Liu, C. Zhao, J. Li, F. Zeng, S. Wang, T. Wen, et al., A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells, J. Power Sources 195 (2010) 480. [17] C. Jiang, J. Ma, A.D. Bonaccorso, J.T. Irvine, Demonstration of high power, direct conversion of waste-derived carbon in a hybrid direct carbon fuel cell, Energy Environ. Sci. 5 (2012) 6973. [18] J. Lee, J. Kim, T. Hyeon, Recent progress in the synthesis of porous carbon materials, Adv. Mater. 18 (2006) 2073. [19] L.C. Sang, A. Vinu, M.O. Coppens, Ordered mesoporous carbon with tunable, unusually large pore size and well-controlled particle morphology, J. Mater. Chem. 21 (2011) 7410. [20] H. Lu, W. Dai, M. Zheng, N. Li, G. Ji, J. Cao, Electrochemical capacitive behaviors of ordered mesoporous carbons with controllable pore sizes, J. Power Sources 209 (2012) 243. [21] Z. Chen, J. Wen, C. Yan, L. Rice, H. Sohn, M. Shen, et al., High-performance supercapacitors based on hierarchically porous graphite particles, Adv. Energy Mater. 1 (2011) 551. [22] A. Garsuch, O. Klepel, Synthesis of ordered carbon replicas by using Y-zeolite as template in a batch reactor, Carbon 43 (2005) 2330.
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