Graphitic carbons with ordered mesostructure and high surface areas (of great interest in applications such as energy storage) have been synthesized by a direct triblock-copolymer-templating method. Pluronic F127 is used as a structure-directing agent, with a low-molecular-weight phenolic resol as a carbon source, ferric oxide as a catalyst, and silica as an additive. Inorganic oxides can be completely eliminated from the carbon. Small-angle XRD and N2 sorption analysis show that the resultant carbon materials possess an ordered 2D hexagonal mesostructure, uniform bimodal mesopores (about 1.5 and 6 nm), high surface area (∼1300 m2/g), and large pore volumes (∼1.50 cm3/g) after low-temperature pyrolysis (900 °C). All surface areas come from mesopores. Wide-angle XRD patterns demonstrate that the presence of the ferric oxide catalyst and the silica additive lead to a marked enhancement of graphitic ordering in the framework. Raman spectra provide evidence of the increased content of graphitic sp2 carbon structures. Transmission electron microscopy images confirm that numerous domains in the ordered mesostructures are composed of characteristic graphitic carbon nanostructures. The evolution of the graphitic structure is dependent on the temperature and the concentrations of the silica additive, and ferric oxide catalyst. Electrochemical measurements performed on this graphitic mesoporous carbon when used as an electrode material for an electrochemical double layer capacitor shows rectangular-shaped cyclic voltammetry curves over a wide range of scan rates, even up to 200 mV/s, with a large capacitance of 155 F/g in KOH electrolyte. This method can be widely applied to the synthesis of graphitized carbon nanostructures.
Electrochemical double-layer capacitors (EDLCs) or supercapacitors have high potential for energy storage devices with high power densities. Their capacity depends on the available surface area of the porous materials accessible to the electrolyte.1, 2 The advantages of ordered mesoporous carbon for such a system include high surface area, large pore volume, and uniform pore size distribution.3 Increasing the graphitization of ordered mesoporous carbon, preserving as far as possible their high surface areas and pore volumes, is expected to enhance their electrical conductivity. Up until now, all efforts towards developing graphitic ordered mesoporous carbon have proceeded from the basis of nanocast carbon.4–6 The synthesis steps include preparation of mesoporous silica hard templates, filling of the carbon source and/or catalyst inside the silica mesochannels, carbonization and graphitization at high temperatures (and sometimes high pressure), and finally removal of the silica template. To achieve graphitization of ordered mesoporous carbon frameworks, four methods are considered feasible: 1) Choosing an appropriate, easily graphtiized carbon source, such as mesophase pitch,7, 8 polyaromatic hydrocarbons,9, 10 aromatic molecules,10 aniline,11 or acrylonitrile.12–14 As an example, Ryoo and co-workers9 first reported fabrication of nanocast mesoporous graphitic carbons by filling Al-containing mesoporous silica (Al-SBA-15) mesochannels with acenaphthene, converting the acenaphthene in situ to mesophase pitch, and then graphitizing it in a special alloy autoclave at 900 °C and under vacuum. 2) High-temperature (>900 °C) chemical vapor deposition (CVD).15–17 N-Doped mesoporous carbons with graphitized pore walls were synthesized using SBA-15 as a hard template and styrene and acetonitrile as carbon sources via a CVD method. Graphitized ordered mesoporous carbons were obtained after pyrolysis and carbonization at temperatures above 900 °C.15 3) High-temperature (>2000 °C) and high-pressure treatment.18 4) In situ catalytic graphitization of carbon precursors to form graphitized mesoporous carbon.19 These methods can be combined to synthesize ordered mesostructures with a high degree of graphitization. Fuertes et al.18, 19 reported a high-temperature carbonization method (2300–2600 °C) to prepare graphitized nanocast mesoporous carbon by using SBA-15 as a hard template, polypyrrole as a carbon source, and FeCl3 as a polymerization and graphitization catalyst. The graphitized mesoporous carbon has a relatively high surface area (>1000 m2/g) and bimodal pore system (3 and 10 nm). The former is inherited from the silica pore walls and is uniform, while the large mesopores originate from voids caused by the partially unoccupied silica template and is non-uniform. Electrodes in EDLCs made by graphitized mesoporous carbon exhibit extremely high current density, showing much better performance than those formed of amorphous carbon, which can be attributed to the open nanopores and high electric conductivity.19
More recently, several groups reported separately on direct triblock-copolymer-templating synthesis of ordered mesoporous carbons.20–22 These carbon materials possess the same positive topological mesostructure as mesoporous silica. The mesostructures include Iad, p6mm, Imm, Fdm and Fmm, the pore sizes range from 3 to 40 nm, the Brunauer–Emmet–Teller (BET) surface areas are between 500 and 2500 m2/g, and the macroscopic morphologies vary from film to fiber to sphere to “single crystal” to monolith.23–28 Phenolic resins are generally used as the carbon sources, which are denoted as non-graphitizing carbons. Characteristic of amorphous carbon is observed after carbonization at 1400 °C, suggesting a difficult graphitization of the ordered mesoporous carbon framework.23 Graphitization can not be achieved at 900 °C with the addition of single nickel, iridium and silica catalyst.29–31
Herein, we report the direct triblock-copolymer-templating synthesis of ordered mesoporous carbon with graphitic domains, using phenolic resins as a carbon source, silica as an additive, and iron oxide as a catalyst. The resulting carbon materials, after treatment at 900 °C, show ordered mesostructure with uniform bimodal mesopores (about 1.5 and 6.0 nm), high surface areas (∼1300 m2/g) and large pore volumes (1.50 cm3/g), and partially graphitic domains. All surface areas derive from the mesopores, which are expected to be highly accessible for electrolytes. Our synthetic strategy is based on the combination of a soft-templating approach, successfully employed to obtain amorphous ordered mesoporous carbon, and the use of catalysts to create graphitic domains (catalytic graphitization). The key issues for obtaining carbonaceous material with an ordered mesostructure and a graphitic nature include the silica additive, metal content, and graphitization temperature.
Ordered mesoporous carbon with graphitic domains can be synthesized using low-molecular-weight resol as a carbon source, an iron compound as a catalyst, silica as an additive, and triblock copolymer Pluronic F127 as a structure-directing agent via the evaporation induced self-assembly (EISA) route. The as-made membranes are russet colored, transparent, and uniform, suggesting that no macrophase separation occurs. The first carbonization products (900 °C) are carbon-iron compound/silica composites (GMCFeSi), which are uniform black powders. Thermogravimetric (TG) analysis of the GMCFeSi-2.3–900 sample at up to 1000 °C in air (Supporting Information Figure S1) shows that the main weight loss occurs between 300 and 700 °C and that no significant weight loss takes place at temperatures above 700 °C. This demonstrates that the carbon and inorganic contents of the composite are about 42 and 58 wt.%, respectively. After etching away the silica components using NaOH solution, a black GMCFe-2.3–900 composite can be obtained. The inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis shows that the atomic iron content in the composite is 2.3% and negligible silicon can be detected. A further treatment by nitrohydrochloric acid can almost entirely remove iron compounds and obtain pure carbon GMC-2.3–900. The TG result shows that the ash content in GMC-2.3–900 is undetectable, suggesting that all of the iron and silica compounds are dissolved and completely removed (SI Figure S1).
The small-angle X- ray diffraction (XRD) patterns of the as-made GMCFeSi-2.3 and GMCFeSi-2.3–900 composites exhibit well-resolved diffraction peaks, indicating that the ordered mesostructure is formed via self-assembly of the triblock copolymer and retained upon carbonization (Figure1a). The two diffraction peaks for the carbonaceous composite GMCFeSi-2.3–900 can be indexed to the 10 and 20 reflections of an ordered 2D hexagonal mesostructure with a space group of p6mm, similar to the ordered mesoporous carbon FDU-15–900.22 The cell parameters of the sample GMCFeSi-2.3 before and after carbonization are 15.8 and 11.8 nm, reflecting the small structural shrinkage (∼25%), much less than that of FDU-15–900 (41%). After basic and acidic dissolution, the derived products for the GMCFe-2.3–900 and GMC-2.3–900 samples both exhibit similar small-angle XRD patterns to GMCFeSi-2.3–900, revealing the same 2D hexagonal mesostructure and a similar cell parameter of 11.8 nm. The mesostructure is preserved well after removal of the silica and iron compounds due to mild treatment by NaOH and nitrohydrochloric acid at low temperatures.
The mesoporous carbon product GMC-2.3–900 exhibits several well-resolved diffraction peaks in the wide-angle XRD pattern (Figure 1b), a sharp 002 peak corresponding to a 0.34 nm interlayer spacing along with clearly observable 100, 101, and 004 reflections characteristic of 2D and 3D graphitic structures.32 Raman spectroscopy is considered to be a solid method for studying carbon phases. A band around 1575 cm−1 (G-band) is the Raman active E2g mode of 2D graphite, which is associated with the vibration in all sp2 bonded carbon atoms in a carbon lattice, such as a graphene layer.33 Another band at around 1355 cm−1 (D-band) is related to the vibration of carbon atoms with dangling bonds in planar terminations of a disordered graphite-like structure. This band is attributed to the A1g mode. The relative intensity of these two lines, R (ID/IG), depends on the type of graphitic materials and reflects the degree of graphitization.34 The partially graphitic mesoporous carbon composite GMC-2.3–900 exhibits a strong G-band signal at 1575 cm−1 and a weak D-band signal at 1325 cm−1 with R = 0.94 in its Raman spectrum, while mesoporous amorphous carbon FDU-15–900, which was synthesized in the absence of metal and silica, shows a higher-intensity broad G-band and a lower-intensity D-band with a R value of 1.20 (Figure2).23 The observed difference in the Raman spectra for both ordered mesoporous carbons indicates the graphitic nature of the carbon synthesized with the addition of silica and ferric compounds.
Together with the above graphitic carbon bands, the XRD diffraction peaks corresponding to γ-Fe2O3 (around 2θ = 30.0°, 43.4°, 35.5°, 53.9°, 57.1°, and 62.8°) also appear for the GMCFe-2.3–900 composite. The mean particle size of ferric oxide is calculated from Scherrer’s equation to be about 26.1 nm. These phenomena demonstrate the presence of partially graphitic carbon and ferric oxide nanoparticles in the composite. The nanoparticle size is clearly larger than the domain size of the ordered mesostructure. This phenomenon implies that the particles anchor on the carbon matrix, similar to metal-containing graphitic structures in metal-loaded carbon aerogels.35 The GMCFeSi-2.3–900 composite shows two broad diffraction peaks around 23° and 43°, corresponding to large amounts of amorphous substance in the matrix, and several narrow peaks assignable to γ-Fe2O3. We attribute the main amorphous substance peak to silica, because the broad diffraction peak at 23°, is not dominant in the GMCFe-2.3–900 pattern after removal of the silica using NaOH solution. The diffractions from graphitic carbon appear to be a superposition of the broad silica reflections and become unclear. These results show that the graphitic structure of the GMCFe-2.3–900 composite is formed as consequence of heat treatment in the presence of silica and metals, and can be preserved upon basic and acidic dissolution of the inorganic compounds. The structural parameters are deduced from an analysis of the XRD patterns of these carbonaceous materials, i.e. (002) plane spacing (d002) and crystallite size perpendicular to the basal plane (Lc).14 The values of d002 in GMCFe-2.3–900 and GMC-2.3–900 are about 0.339 nm, larger than that of graphite (0.335 nm), suggesting a random combination of graphitic and turbostratic stacking.34 In all cases, the 002 diffraction peak shows a tail to the low-angle side and the 100 peak a tail to the high-angle side, further indicative of the co-existence of highly crystalline graphite with turbostratic carbon materials. The Lc values are about 1.54 nm. Thus the height of the stacked graphene sheets for graphitic mesoporous carbon and composites is estimated to be 4–5 sheets. It should be noted that if the calculations are performed on the basis of the width of the narrow spikes observed on the 002 diffraction peak, the average number of graphene sheets in a single domain would be several times higher.14 The calculation of the size of graphitic domains further confirms the stability of the graphitic nature during basic and acidic dissolution.
The structural study is further carried out with the TEM measurements for the partially graphitic mesoporous carbon and composites prepared by using the direct triblock-copolymer-templating method. As shown in Figure3, large domains with ordered stripe-like and hexagonal arrays are observed for the graphitized mesoporous carbon-ferric oxide-silica composite GMCFeSi-2.3–900. The estimated unit cell parameter of the sample is about 11.8 nm, in good agreement with the value determined from the XRD data. Large nanoparticles with sizes of about 30 nm and numerous domains composed of characteristic graphitic carbon nanostructures, namely nanocapsules and nanoribbons, are also displayed. The large ferric oxide nanoparticles uniform anchor on the carbon matrix. The crystalline graphite shows the (002) lattice fringes. This graphitic structure sometimes appears around metal particles (Figure 3b inset). Similar nanostructures have been reported for graphitic carbons prepared by the carbonization of metal-impregnated polymeric gels.34–36 After the removal of silica, the derived composite GMCFe-2.3–900 shows the similar TEM images as its mother GMCFeSi-2.3–900. Well-arranged pores with large particles in the matrix and graphitic nanocapsules can be observed. Once the ferric oxides are etched, large particles disappear. Graphitic domains are much distinct. These phenomena imply that a certain amount of graphitic domains may be formed around ferric oxides. Large particles may shield the graphitic nanostructures in the carbonaceous composites.
The partially graphitized mesoporous carbon-iron-silica composite with the 2D hexagonal p6mm symmetry yields representative type IV isotherms of N2 sorption with a H1 hysteresis loop at 77 K (Figure4). A well-defined step occurs at medium relative pressures, associated with the filling of the mesopores due to capillary condensation. The calculated pore size distribution is narrow, indicating that the graphitic carbon-based composite has a very uniform mesoporous structure. The pore size, BET surface area, and pore volume are 6.4 nm, 271 m2/g, and 0.41 cm3/g, respectively. The t-plot analysis shows that this sample has no micropore surface area (Figure S2, Supporting Information). Almost the total of both the surface area and the pore volume derive from the mesopores. The absence of micropores may be caused by the condensed carbon-inorganic oxide framework. The pore diameter is larger than that of pure carbon FDU-1 5–900 (∼3.1 nm), possibly due to the enhancement of carbon pore wall rigidity upon heating by the addition of inorganic nanoparticles. After leaching of the silica, the mesoporous features become much more pronounced. The GMCFe-2.3–900 composite possesses a high BET surface area of 1169 m2/g, a pore volume of 1.24 cm3/g, and a bimodal pore size distribution with the most common pore sizes of ∼1.5 and 6.4 nm. The primary mesopore size is the same as in its parent GMCFeSi-2.3–900 composite, while the secondary one is much smaller. A distinctly increased adsorption at low relative pressures is observed for sample GMCFe-2.3–900, suggesting small pores (below 2.0 nm). The micropore (below 1 nm) surface area is again negligible on the basis of the t-plot analysis (Figure S2, Supporting Information), and the primary mesopore (∼6.4 nm) surface area is 271 m2/g, inherited from the parent GMCFeSi-2.3–900 composite with the same feature. The secondary mesopores contribute about 77% of the total surface area. Though the practical reality is made more complicated because of the different densities of the carbon and silica components (amorphous carbon: 1.8–2.1 g/cm3 and amorphous silica: 2.2–2.6 g/cm3), the estimation reflects the fact that many voids are present in the pore walls and these secondary mesopores inside the pore walls contribute the majority of the total pore surface area and volume. Upon dissolution of the ferric oxide, the graphitic mesoporous carbon GMC-2.3–900 shows similar nitrogen sorption isotherms to its parent composite GMCFe-2.3–900, demonstrating a similarly high BET surface area (1245 m2/g), large pore volume (1.47 cm3/g), and bimodal mesopores (about 1.5 and 6.4 nm). All surface areas derive from the mesopores. The large number of mesopores, which are highly accessible by electrolytes, and the absence of micropores are believed to be important for EDLC electrode performance.19
Raman spectra for ordered mesoporous carbons with different carbonization temperature are shown in Figure 2. All spectra feature two major peaks (D- and G-band). As mentioned above, the G-band in the sample GMC-2.3–900 prepared at a temperature of 900 °C is more prominent than the D-band. Samples prepared either at a higher temperature (1000 °C) or a lower temperature (750 °C) show a more distinct D-band. This phenomenon indicates the highest content of uniform graphitic structures in the GMC-2.3–900 carbon. To study the effect of temperature on the graphitization of mesoporous carbon, we must trace back to the parent carbon-ferric compound composites, since they show similar XRD, TEM, and N2 sorption results. The small-angle XRD patterns for GMCFe-2.3 composites prepared at temperatures between 750 and 1000 °C show the characteristic patterns of an ordered mesostructure, further indicating high stability of the structure upon high-temperature treatment and etching of silica (Figure S3, Supporting Information). Typical diffraction patterns for non-graphitized carbon are represented by the data obtained for GMCFe-2.3–750 and feature a characteristic broad 002 diffraction peak at 23° and a less intense peak at 43°, which corresponds to the 101 and/or 100 reflections (Figure5a). The diffractions for ferric oxide are almost undetectable, not surprising given the dispersion of tiny particles. The low degree of graphitization may be related to the low heating temperature. A high temperature is believed to be a major factor in achieving full graphitization.15, 17 However, once the temperature reaches 1000 °C, the sample GMCFe-2.3–1000 shows a lower graphitic degree than GMCFe-2.3–900, as evidenced by the relatively distinct tail to the low-angle side for the 002 diffraction peak of graphitic carbon and in accordance with the Raman results. The large difference in the XRD patterns between GMCFe-2.3–900 and GMCFe-2.3–1000 is that the latter composite possesses relatively weak diffractions for ferric oxides and strong ones for metallic iron (peaks at 44.6 and 64.9°), suggesting that a mixture of Fe2O3 and Fe particles coexist inside the carbon matrix and the iron oxide particles are partially reduced by the organic matrix during 1000 °C-carbonization. In this case, the decrease in the graphitic degree for ordered mesoporous carbon after carbonization at 1000 °C may be related to the formation of metallic iron.
Cyclic voltammograms of the graphitic ordered mesoporous carbon GMC-2.3–900 are presented in Figure6. The CV curves maintain a typical rectangular shape at all voltage sweep rates from 5 to 200 mV/s, which indicates the good accessibility of the ions to the electrochemically active surface, and the excellent capacitive behavior of graphitic ordered mesoporous carbon even in quick charge–discharge operations. The capacitances of different scan rates calculated from the data at -0.4 V in Figure6. GMC-2.3–900 possesses a capacitance of 155 F/g at the slow sweep of 5 mV/s and can maintain most of its capacitance at high voltage sweep rates, for example, 130 F/g at 100 mV/s and 112 F/g at 200 mV/s. The result indicates solvated ions can diffuse fast enough into the mesopores of GMC-2.3–900 even at the voltage sweep rate of 200 mV/s. The capacitance is comparable to the best ordered mesoporous carbon materials.37 and far superior to graphitized CMK-3 materials derived from anthracence.38 The high capacitance can be attributed to the bimodal mesopores with large pore sizes which can accelerate the kinetic process of the ion diffusion in the electrodes2, 39, 40 and the graphitic pore walls which lead to excellent electric conductivity.41
3.1. The Role of Ferric Oxide
The iron compound is a well-known and efficient catalyst for graphitization of organic aerogels.36 In the current synthesis, Fe(NO3)3·9H2O was added to the triblock copolymer solution. Liquid-crystal phases of a transition metal nitrate salt/triblock copolymer system can be formed because of the hydrogen bonding interaction between the [Fe(H2O)x]3+ and ethoxy (EO) groups of Pluronic F127, which has been well established by Dag and co-workers.42–44 The transition metal ions are homogenously distributed into the hydrophilic domains of the ordered mesophases. During the heat treatment, the triblock copolymers are decomposed with reservations of ferric oxides inside the carbonaceous matrix. Catalytic graphitization for phenolic resins occurs as expected at high temperatures in the presence of a metal catalyst. For comparison, a blank sample carbon-silica composite MCSi-900 without Fe was also synthesized. This composite shows a diffraction peak at 2θ of about 23°, together with a weak broad diffraction peak at about 43° in the wide-angle XRD pattern (Figure 5b), suggesting the co-existence of amorphous silica and carbon. After removal of the silica, the XRD pattern for MC-900 shows indistinct change. The Raman spectrum displays two broad peaks attributed to the D-band and the G-band, with a relative ratio R of 1.20. These results give evidence for amorphous carbon frameworks in MC-900, which may be related to the phenolic resins carbon precursor and the low calcination temperature. Therefore, the presence of Fe compounds plays a vital role for graphitization of ordered mesoporous carbon.
The formation of partially graphitic carbon nanostructures by catalytic graphitization is a complex process. In general, when carbonaceous materials impregnated with metallic salts are heat-treated under inert atmosphere, metallic oxides are first formed due to the decomposition of the salts, and then reduced by carbon to elemental metal. Finally, at temperatures above 750 °C, the metal or oxide nanoparticles that are involved in the carbon matrix serve as catalysts to convert amorphous carbon into graphitic carbon.34, 45
As mentioned above, the appearance of metallic iron may have a negative effect on the degree of graphitization for ordered mesoporous carbons after carbonization at 1000 °C in the current synthesis. To further elucidate the effect of iron compounds, a series of graphitic mesoporous carbon-Fe compound-silica composites were synthesized. After leaching silica, the carbon-Fe compound composites were studied for graphitization. All samples displayed resolved diffraction peaks in the small-angle region, suggesting ordered mesostructure (Figure S4, Supporting Information). As shown in Figure7, the GMCFe-2.3–900 and GMCFe-3–900 composites show Raman spectra with a relatively high-intensity G-band and low-intensity D-band, indicating a relatively high degree of graphitization. Only ferric oxides can be detected in these two composites (Figure 5b). The composite GMCFe-0.3–900 with a low iron content exhibits a low graphitic degree. This is reasonable because of the catalytic graphitization role played by the ferric compounds. When the iron content increases to 6.3 wt.% in the GMCFe-6.3–900 composite, large metallic Fe particles (because of very sharp diffraction peak at 44.7° and 64.9°, α-Fe)are formed together with ferric oxides in the XRD pattern (Figure 5b). At the same time, the D-band intensity is a little higher than G-band in the Raman spectrum, indicating that the graphitic degree reduces to some extent. As a result, the ferric oxide nanoparticles in the ordered mesoporous carbonaceous matrix are contributed to the catalyst for graphitization.34 Once the metallic oxides are reduced, the metal nanoparticles sinter together. Simultaneously, the immobilization of metal may be improved during in situ high-temperature reduction by organic species and carbon. Large particles are phase-separated from the carbon matrix.29 Consequently, the graphitic degree of the carbon mesostructured framework may be decreased to some extent.
The ferric oxide can be nearly completely dissolved and removed by nitrohydrochloric acid from the partially graphitic carbon matrix. The sample GMC-3–900 shows typical stripe-like pore arrays and a large number of nanocapsules and nanoribbons, similar to GMC-2.3–900, confirming an ordered mesostructure with a graphitic nature (Figure S5, Supporting Information). The N2 sorption isotherms and corresponding pore size distribution curve are also similar to those of GMC-2.3–900 (SI Figure S6). As a result, the graphitic ordered mesoporous carbon GMC-3–900 shows a high surface area (1478 m2/g), bimodal mesopores (1.7 and 5.8 nm) and a large pore volume (1.53 cm3/g, Table 1).
Table 1.. Structural and textual properties of partially graphitic ordered mesoporous carbonaceous materials.
R = ID/IG
a)(n.d.: not detected)
3.2. The Role of Silica
The framework shrinkage of the mesoporous composite GMCFeSi-2.3–900 is less than that of the pure carbon FDU-15–900. In addition, large amounts of secondary mesopores are formed upon the dissolution of silica in the carbon-ferric oxide-silica composite. These phenomena have also been observed in the mesoporous carbon-silica composites MCSi-900 and their derivation MC-900,31 which were prepared by the triconstituent (TEOS, phenolic resol and triblock copolymer F127) co-assembly route.31, 46 Accordingly, a co-assembling route is proposed for the present synthesis. Silica oligomers and low-molecular-weight resols aggregate around the liquid-crystal phases of the transition metal nitrate salt:triblock copolymer system due to hydrogen-bonding interaction. Silica and phenolic resins crosslink around metal:triblock copolymer aggregation while ions are coordinated in the hydrophilic parts after low-temperature polymerization (100 °C). Upon pyrolysis, the triblock copolymers can be eliminated. Silica and carbon form an interpenetrating framework and ferric oxides are presented in the hybrid matrix. The large amount of silica nanoparticles dispersed in the carbon matrix can resist the framework shrinkage at high temperatures and the formation of micropores inside carbon matrix,31, 47 and the mesoporous solids have primary mesopores with a size of ∼6.4 nm. Once silica nanoparticles are etched, plenty of voids are formed inside carbon pore walls and contribute to secondary mesopores. A high surface area (1169 m2/g) can be obtained and all of the surface areas derive from mesopores. The increase in the mesopore surface area is found to be essential for the improvement of the accessibility of electrolyte and the electric capacity in EDLCs.19 At the same time, the mesostructure can be well retained, the primary mesopores remain unchanged and ferric oxides are preserved in the matrix. Ferric oxides can be further removed by acid washing while maintaining the structure and pore properties. Therefore, a graphitic ordered mesoporous carbon with a possible compromise of considerably high mesopore surface area (1245 m2/g) and good electronic conductivity can be expected. It should be noted that the BET surface area and pore volume of GMC-2.3–900 (1245 m2/g, and 1.47 cm3/g) are much less than those of MC-900 (2580 m2/g and 2.16 cm3/g). The reason may be attributed to the graphitized crystalline nature of GMC-2.3–900. The material contains well-stacked graphene layer domains, which can not afford secondary voids inside walls. Consequently, the BET surface area and pore volume reduce.
The catalytic role of silica for the graphitization of ordered mesoporous carbon should also be considered. It has been reported that addition of TEOS to a phenolic resin carbon facilitates the formation of finely divided particles and turbostratic graphite.48 At the same time, the silica particles can separate carbon particles in the mesoporous composite, which may offer immobilization sites for ferric oxides and inhibit in situ reduction to metallic Fe by the organic matrix at higher temperatures.49 Ferric oxides are the main product in the C-Fe2O3-SiO2 composite after carbonization at 900 °C even with a high metal content (about 3 wt.%). As a result, a high graphitic degree for GMC-2.3–900 and GMC-3–900 can be obtained by the ferric oxide catalyst and silica additive.
We have demonstrated a simple direct triblock-copolymer-templating route to preparing ordered mesoporous carbons with partially graphitic structure and high surface areas at low temperature by using phenolic resol as a carbon source, ferric oxide as a catalyst, and silica as an additive. The synthetic strategy is based on the combination of the soft-templating approach for amorphous ordered mesoporous carbon and the use of catalysts to create graphitic domains (catalytic graphitization). The silica component plays important roles in the elimination of micropores inside the carbon matrix, inhibition of reduction of ferric oxide to metallic iron, and catalytic graphitization. Metal content and temperature are key issues for successful graphitization. The silica and ferric oxide can be successively removed by mild base and acid washing. The resulting partially graphitic carbon materials show ordered mesostructure uniform bimodal mesopores (about 1.5 and 6 nm), high surface areas (∼1300 m2/g), and large pore volumes (∼1.50 cm3/g). Negligible micropores are detected and all surface areas and pore volumes derive from the mesopores, which are highly accessible by electrolyte. The graphitic ordered mesoporous carbon material exhibits superior capacitive behavior over a wide range of scan rates, even up to 200 mV/s, with a large capacitance of 155 F/g.
5. Experimental Section
Chemicals: Poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) Pluronic F127 (EO106PO70EO106, Mw = 12600) was purchased from Acros Chemical Inc. Phenol (C6H5OH, 99.98 wt.%), formalin (HCHO, 37.0–40.0 wt.%), hydrogen chloride (HCl, 36.0–38.0 wt.%), sodium hydroxide (NaOH, minimum 96.0 wt.%), ethanol (C2H5OH, minimum 99.7 wt.%), tetraethoxysilane (TEOS, 98 wt.%) and Fe(NO3)3·9H2O (98.5 wt.%) were obtained from Shanghai Chemical Company. All chemicals were used as received without any further purification. Water used in all syntheses was distilled and de-ionized.
Synthesis of resol precursor: Resol was prepared from phenol and formaldehyde in a base-catalyzed process.23 In a typical procedure, phenol (8.0 g) was melted at 42 °C in a flask and mixed with NaOH aqueous solution (1.7 g, 20 wt.%) with stirring. After 10 min, formalin (14.1 g) was dropped in. The mixture was further stirred at 70 °C for 1 h, cooled to room temperature, and titrated by an HCl solution (2 M) till neutral (pH 7.0). Water was removed by vacuum evaporation at 48 °C. The final product was dissolved in ethanol (20 wt.%).
Synthesis of graphitized ordered mesoporous carbon: Fe(NO3)3·9H2O (0.11 g), and triblock copolymer F127 (1.6 g) were dissolved in ethanol (6.0 g) at 40 °C under stirring to obtain an orange color solution. To it, resol ethanolic solution (5.0 g, 20 wt.%) was added to form mixture A. Mixture B was produced by adding TEOS (2.08 g), into the HCl solution (1.0 g, 0.2 M) under stirring at 40 °C for 30 min. Then, mixture B was poured into A. Upon stirring for 30 min, the mixture was transferred into multiple dishes. The dishes were placed in a hood for 6 h at room temperature to evaporate ethanol, and subsequently in an oven at 100 °C for 24 h to polymerize phenolic resins. The powders were scraped from dishes, placed into a tubular furnace, and carbonized at 800–1000 °C for 2 h under a nitrogen atmosphere. The graphitic mesoporous carbon-Fe (compounds)-silica products (GMCFeSi-x-y) were repeatedly washed with a NaOH solution (4 M) at 40 °C to remove the silica component. The resulting graphitic mesoporous carbon-Fe (compounds) products were named as GMCFe-x-y, wherein x denotes the atomic Fe percent in the GMCFe composite, and y represents the carbonization temperature. Finally, the pure carbon materials were obtained by treating GMCFe with nitrohydrochloric acid in order to remove the iron-containing particles. The carbon samples were labeled as GMC-x-y. For simplification, we used the same x, which was the atomic iron mass content in the graphitic mesoporous carbon-Fe product after removal of silica, for the materials synthesized from the same initial batch. By adjusting the amount of Fe(NO3)3·9H2O, a series of materials with Fe content ranging from 0 to 6.3 was obtained.
Electrochemical Measurements: Electrical measurements were performed with an electrochemical analyzer, CHI 910B (Shanghai Chenhua Limited Co.) under ambient conditions in KOH (6 M) aqueous solution, using a three-electrode system, with the graphitic mesoporous carbon as the working electrode, a platinum wire as the counter electrode and an Hg/HgO electrode (0.052 V vs. the normal hydrogen electrode, NHE) as the reference electrode. To prepare the working electrode, the graphitic mesoporous materials were ground with acetylene black (10 wt.%), and polytetrafluoroethylene (PTFE, 5 wt.%) and then pressed onto nickel foam that served as a current collector. The capacity was calculated with respect to graphitic ordered mesoporous carbon.
Characterization: The X-ray diffraction (XRD) measurements were carried out on a Rigaku Dmax-3C diffractometer with Cu Kα radiation (40 kV, 40 mA, λ = 0.15405 nm). The d spacing values were calculated by the formula d = λ/2sinθ, and the unit cell parameters were calculated from the formula a0 = 2d10/√3. The sizes of metallic Fe and Fe2O3 particles and graphitic domains (L) were estimated according to the Scherrer equation: size = kλ/Bcosθ, where B was the peak width (full width at half-height in radians) and k was a constant based on the diffraction peak in wide-angle XRD patterns. For nanoparticle sizes, k was 0.89. Following Knox et al.,50k of 0.84 was used for evaluation of the stacking height of the graphitic domains from the width of the 002 reflection. Transmission electron microscopy (TEM) experiments were conduced on a JEOL 2011 microscope operated at 200 kV. The samples for TEM measurements were suspended in ethanol and supported onto a holey carbon film on a Cu grid. Raman spectra were collected on a Super LabRam microscopic Raman spectrometer, using a He-Ne laser with an excitation wave length of 633 nm. Weight losses and the associated temperature were determined by thermogravimetry (TG) analysis with a Mettler Toledo TG/SDTA 851e apparatus. Samples were heated from room temperature to 1000 °C at a rate of 10 °C/min in air flow. The metal ion concentrations were quantified using inductively coupled plasma-atomic emission spectroscopy (ICP-AES, VISTA-MPX). N2 adsorption-desorption isotherms were measured at 77 K with a Quantachrome NOVA 4000e analyzer. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas (SBET). By using the Barrett-Joyner-Halenda (BJH) model, the pore volumes and pore size distributions were derived from the adsorption branches of isotherms. The micropore volumes (Vm) were calculated from the V-t plot method.The t values were calculated as a function of the relative pressure using the de Boer equation, t (nm) = [0.1399/(log(p0/p) + 0.0340)]1/2. Vm was obtained using the equation of Vm (cm3/g) = 0.001547I, where I represents the Y intercept in the V-t plot.
Supporting Information is available from the Wiley Online Library or from the author.
This work was supported by NSF of China (20873086 and 21073122), Shanghai Sci. & Tech. and Edu. Committee (08JC1417100, 0852nm0090, 10XD1403300, and S30406), the Fok Ying Tung Education Fund (121013) and the program for New Century Excellent Talents in Universities (NCET-07–0560).