pubs.acs.org/Langmuir © 2010 American Chemical Society
Hierarchical Carbon Foams with Independently Tunable Mesopore and Macropore Size Distributions Adam F. Gross* and Andrew P. Nowak HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, California 90265 Received February 23, 2010. Revised Manuscript Received April 28, 2010 Hierarchical carbon foams with independently tunable mesopore and macropore size distributions were formed in a high internal phase emulsion (HIPE) template. The HIPE consists of an internal oil phase that controls the macropore dimensions and an aqueous resorcinol-formaldehyde precursor solution external phase that directs the mesopore size distribution. Once the emulsion is formed, the precursor solution is cured, fluid elements are extracted from the monolith via solvent exchange, and then the sample is pyrolyzed to create a hierarchical open-cell foam consisting of macropores with mesoporous carbon xerogel walls. Both mesopore and macropore size distributions may be independently tuned by changing the synthesis parameters. These samples have a peak in the mesopore size distribution that may be tuned to between 5 and 8 nm and macropore average diameters that may be tuned to between 0.7 and 2.1 μm. Furthermore, the 0.7 and 2.1 μm average diameter macropores have 0.18 and 0.53 μm diameter macropore windows between adjacent pores, respectively. Pore volumes up to 5.26 cm3/g and electrical conductivities as high as 0.34 S/cm are observed after 1200 °C carbonization of the framework. These foams may have potential applications as 3-D current collectors in batteries and as fuel cell catalyst supports.
Introduction Many porous materials may be formed from emulsion-templated solutions.1,2 Emulsions are an advantageous tool for structural control because of their ease of fabrication, wide range of possible pore sizes, and scalability. Furthermore, porous silica, metal oxides, carbons, and metals have been templated with emulsions and these materials demonstrate the flexibility of this technique to form solid materials.1 We are particularly interested in forming porous materials in high internal phase emulsions (HIPEs) that have greater than 74% internal phase volumes, which is the free space occupied by packed uniform spheres.3,4 At higher than 74% internal phase volume, the droplets become abutted and the degree of interconnection between resulting pores increases. By forming solid materials in the continuous phase of a HIPE, an open cell foam monolith consisting of polymers, silica, or carbons may be produced.1-3,5-7 Furthermore, when the solid material formed in the external phase of the HIPE is mesoporous silica, a hierarchical macroporous and mesoporous foam with increased surface area and pore volume is obtained.6,7 Our goal is to produce hierarchical macroporous carbons with mesoporous carbons walls made from inexpensive and easily removable fluidic precursors templated in HIPEs where both the mesopore and macropore dimensions may be independently tailored by altering the synthetic composition. Porous carbon materials with hierarchical macropores and mesopores have potential applications as 3-D current collectors,
electrodes, and catalyst supports.1,2,8-10 The carbon framework provides electrical conductivity, and the macropore dimensions and interconnectivity provide control of mass transport and pore volume.1 Furthermore, the mesopore structure influences the surface area, which is important for catalyst supports, and enables additional ion-transport channels in batteries. Finally, the carbon structure is chemically inert and will survive a variety of reaction conditions. There are many templating approaches that produce hierarchical porous carbons with different degrees of pore structure tunability. A porous carbon may be formed by carbonizing a HIPE in which the continuous phase consists of a polymerized styrene monomer.5 This approach produces porous carbons with nanometer length scale pores in the walls that are induced as a result of densification during carbonization; however, no peak in the mesopore size distribution was shown because this approach does not actively template mesopores during synthesis. Additionally, hierarchical foams may be formed from agglomerated colloids of a carbonaceous precursor.11,12 The packed colloids form macropores whereas surfactant templating in the colloids, or their densification during carbonization, produces mesopores. Pluronic surfactants may also be mixed with water and phenolic resins to form a macroporous and mesoporous hierarchical carbon structured by phase separation during hydrothermal treatment.13 In another approach used by many groups, a resin may be polymerized around solid micrometer-sized polymer or silica colloids and then carbonized to form a hierarchical carbon.1,2,8-10,14-18 Macropores
*Corresponding author. E-mail:
[email protected]. (1) Zhang, H; Cooper, A. I. Soft Matter 2005, 1, 107. (2) Yuan, Z.; Su, B. J. Mater. Chem. 2006, 16, 663. (3) Cameron, N. R. Polymer 2005, 46, 1439. (4) Barbetta, A.; Cameron, N. R.; Cooper, S. J. Chem. Commun. 2000, 221. (5) Wang, D.; Smith, N. L.; Budd, P. M. Polym. Int. 2005, 54, 297. (6) Hu, Y.; Nareen, M.; Humphries, A.; Christian, P. J. Sol-Gel Sci. Technol. 2010, 53, 300. (7) Carn, F.; Colin, A.; Achard, M. F.; Deleuze, H.; Sellier, E.; Birot, M.; Bakov, R. J. Mater. Chem. 2004, 14, 1370. (8) Yu, J.-S.; Kang, S; Yoon, S. B.; Chai, G. J. Am. Chem. Soc. 2002, 124, 9382. (9) Chai, G. S.; Yoon, S. B.; Yu, J. S.; Choi, J. H.; Sung, Y. E. J. Phys. Chem. B 2004, 108, 7074. (10) Lee, K. T.; Lytle, J. C.; Ergang, N. S; Oh, S. M.; Stein, A. Adv. Funct. Mater. 2005, 15, 547.
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(11) Carriazo, D.; Pico, F.; Gutierrez, M. C.; Rubio, F.; Rojo, J. M.; del Monte, F. J. Mater. Chem. 2010, 20, 773. (12) Zou, C.; Wu, D.; Li, M.; Zeng, Q.; Xu, F.; Huang, Z.; Fu, R. J. Mater. Chem. 2010, 20, 731. (13) Huang, Y.; Cai, H.; Feng, D.; Gu, D.; Deng, Y.; Tu, B.; Wang, H.; Webley, P. A.; Zhao, D. Chem. Commun. 2008, 2641. (14) Zhao, J.; Cheng, F.; Yi, C.; Liang, J.; Tao, Z.; Chen, J. J. Mater. Chem. 2009, 19, 4108. (15) Wang, Z.; Kiesel, E. R.; Stein, A. J. Mater. Chem. 2008, 18, 2194. (16) Lukens, W. W.; Stucky, G. D. Chem. Mater. 2002, 14, 1665. (17) (a) Baumann, T. F.; Satcher, J. H. Chem. Mater. 2003, 15, 3745. (b) Baumann, T. F.; Satcher, J. H. J. Non-Cryst. Solids 2004, 350, 120. (18) Woo, S. W.; Dokko, K.; Sasajima, K.; Takei, T.; Kanamura, K. Chem. Commun. 2006, 4099.
Published on Web 05/21/2010
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form after removal of the colloids, and their dimensions are adjusted by changing the initial colloid diameter. Furthermore, very uniform macropores were created by first forming a colloidal crystal (instead of using agglomerated colloids) and then using this ordered solid as a template for the carbonaceous material.2,8-10,14,17-19 An additional level of order in hierarchical carbons may be induced by arranging smaller colloids around larger colloids in a templating colloidal crystal.2,18,19 Mesoporosity is induced without a template in many of these colloid-templated carbons as a result of carbonization and densification of the resin. In a few reports, the mesopore dimensions were more precisely controlled when larger colloids were surrounded by a mesoporous carbon instead of a resin.16,17 One drawback with solid colloidal particle templating is that it requires the initial synthesis of particles and creating porosity in these materials may necessitate subsequent etching with hydrofluoric acid if silica-based materials are used. Finally, a macroporous polymer foam may be infiltrated with mesoporous silica in order to form a hierarchical carbon.20 The macropore size is controlled by the initial polymer foam pore dimensions, and the mesopore dimensions are controlled by the synthesis parameters of the mesoporous silica. Mesoporous carbon is formed through the vapor deposition of a precursor on the silica, carbonization of the precursor, and the removal of the silica template with HF. Our goal is to fabricate hierarchical porous carbon monoliths with independently tunable mesopore and macropore dimensions without templating from solid materials. By using an oil-in-water soft matter template, we are able to form an open-cell hierarchical carbon foam using inexpensive and easily removable templating materials. We template our materials with a high internal phase emulsion formed by the high-speed mixing of a silicone oil internal phase and an aqueous continuous phase. The aqueous phase contains a resorcinol-formaldehyde precursor solution that enables the tuning of mesoporosity with synthetic variables.21,22 Additionally, the macropore dimensions are tuned with oil viscosity.23 A carbon foam is fabricated by blending the oil and aqueous phases, thermally curing the precursor solution, removing the oil phase via solvent extraction, exchanging water for organic solvent, drying the monolith, and finally carbonizing the sample. This process results in a hierarchical macroporous foam made from mesoporous carbon xerogel. Our material differs from carbonized polymer HIPEs because the meosporosity is controlled by the xerogel synthetic variables rather than being a byproduct of polymer densification during pyrolysis. We believe that our material is the first hierarchical carbon monolith formed from all fluidic precursors with tunable pore size mesoporous xerogel walls. In this study, the composition of the aqueous and oil phases of the HIPE are varied to demonstrate independent tuning of the mesopore and macropore pore size distributions followed by the characterization of the resulting foams. Finally, we measure the electrical conductivity of the carbon foam monoliths to determine if these materials could be candidates for fuel cell and battery applications.
Experimental Section All chemicals were purchased from Aldrich and used as received. Resorcinol-formaldehylde precursor solutions were (19) Chai, G. S.; Shin, I. S.; Yu, J. S. Adv. Mater. 2004, 16, 2057. (20) Alvarez, S.; Fuertes, A. B. Mater. Lett. 2007, 61, 2378. (21) Al-Muhtaseb, S. A.; Ritter, J. A. Adv. Mater. 2003, 15, 101. (22) Li, W.; Lu, A.; Weidenthaler, C.; Sch€uth, F. Chem. Mater. 2004, 16, 5676. (23) Mason, T. G.; Wilking, J. N.; Meleson, K.; Chang, C. B.; Graves, S. M. J. Phys.: Condens. Matter 2006, 18, R635–R666.
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synthesized by mixing a 2:1:0.002 molar ratio of formaldehyde (37 wt % solution in water) to resorcinol (99%) to sodium carbonate (99%) in deionized water.22 The resorcinol-formaldehyde precursor solutions contained 30 or 40 wt % organic content (formaldehyde þ resorcinol mass). The resorcinol-formaldehyde precursor solutions were stirred until they dissolved and allowed to rest for 1 h before use in carbon foam fabrication. Carbon foams were fabricated by dissolving 0.360 g of sodium dodecylbenzenesulfonate surfactant (technical grade) in 12.5 g of a resorcinol-formaldehyde precursor solution that was then combined with 37.5 g of silicone oil (100 and 1000 cP viscosity, Fluka brand DC 200). The mixture was transferred to a Waring laboratory blender and blended for 10 min in all experiments. A white viscoelastic emulsion was obtained and transferred in equal fractions to three 60 mL Nalgene polypropylene jars. The jars were sealed and aged for 72 h at 85 °C, which resulted in bright orange resorcinol-formaldehyde foam monoliths formed in the shape of the Nalgene containers. The foams were then soaked in chloroform to extract the silicone oil. The chloroform bath was poured off and refilled with fresh solvent twice, and there was at least a 4 h soak time during each of the first two soaking cycles and 8 h during the third cycle. The foams then were then removed from chloroform and placed in an acetone bath to exchange solvent for water in the monolith. The acetone bath was poured off and refilled two times with at least 2 h between each soaking cycle. Finally, the foam monoliths were removed from the acetone bath, allowed to dry in air, and heated in a tube furnace under flowing nitrogen from room temperature to 800 °C at 2.6 °C/min and maintained at 800 °C for 6 h to pyrolyze the resorcinol-formaldehyde gel. In one case, a foam was pyrolyzed under flowing nitrogen and heated from room temperature to 1200 °C at 2 °C/ min and maintained at 1200 °C for 6 h in order to measure the effect of different pyrolysis conditions. There are no significant chemical hazards encountered during the preparation of the carbon foams described in this study. Samples carbonized at 800 °C are denoted throughout this article on the basis of the organic loading of the xerogel and the viscosity of the silicone oil. For example, a 40%/1000 sample was made by blending a 40 wt % organic resorcinol-formaldehyde precursor solution with 1000 cP silicone oil. The water/oil volume ratios of the 40%/100, 40%/1000, and 30%/1000 foams were 23.5:76.5, 23.5:76.5, and 23.9:76.1, respectively. The ratio is slightly higher for the 30%/1000 emulsions as compared to that of the 40%/100 and 40%/1000 emulsions because of the lower density of the 30 wt % resorcinol-formaldehyde precursor solution. If the sample was carbonized at a temperature other than 800 °C, then the carbonization temperature is listed after the organic loading of the xerogel and the viscosity of the silicone oil. The sample pore size, surface area, and pore volume were characterized with N2 adsorption and Hg intrusion at Micromeritics Analytical Services (Norcross, GA). N2 data were analyzed using the Brunner-Emmett-Teller (BET) and BarrettJoyner-Halenda (BJH) methods.24 Hg intrusion experiments were sensitive to pores between 3 nm and 360 μm in diameter. SEM images were acquired on a Hitachi S-4800. Electrical conductivity was measured with an HP 34401A multimeter using a four-wire probe. Current and voltage probes were placed on opposite faces of a carbon foam monolith, and the resistance was measured. This resistance was converted to conductivity by inverting the resistance value and dividing by the thickness of the monolith.
Results and Discussion Carbon foams were synthesized as described in the Experimental Section from a HIPE template consisting of an external (24) Lowell, S.; Shields, J. E.; Thomas, M. A.; Thommes, M. Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density; Springer: Dordrecht, The Netherlands, 2006.
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phase resorcinol-formaldehyde precursor solution and an internal oil phase. Carbon xerogel is formed in the external phase, and the mesoporosity is controlled by the resorcinol-formaldehyde precursor solution composition. The water fraction of the resorcinolformaldehyde precursor solution acts as a porogen and templates the mesopores. Additionally, the organic fraction and amount of catalyst in the resorcinol-formaldehyde precursor solution control the micropore structure and shrinkage during drying and pyrolysis. (Smaller amounts of catalyst relative to resorcinol create microporosity in the carbon particles that make up the xerogel and better resist shrinkage during drying.21) Although the cured resorcinol-formaldehyde gel from the continuous phase of the HIPE provides structural rigidity in the carbon foam, the oil phase creates regions where the resorcinolformaldehyde precursor solution is excluded to produce macropores.3 Additionally, the oil droplets are abutted, which results in windows forming between the macropores and creates an opencell foam. We refer to these openings between macropores as “macropore windows” to differentiate them from the larger macropores. A conceptual diagram of the HIPE is shown in Figure S1. The HIPE that templates the carbon foam is formed using high shear mixing, and both the macropore and macropore window dimensions are controlled by the oil viscosity. Higher oil viscosity results in increased viscous resistance against oil droplet deformation and separation into larger droplets. Thus a lower interfacial surface area is created between oil and water for a given amount of mechanical energy deposited into the system (with the same mixing speed used in all experiments).25 This reduced surface area results in larger droplets, larger macropores, and larger macropore windows. We varied both the oil viscosity and resorcinolformaldehyde precursor solution composition to demonstrate control of both mesopore and macropore textural properties. Sample Fabrication. The monolithic structures of an assynthesized resorcinol-formaldehyde HIPE-templated foam and of a subsequently 800 °C pyrolyzed carbon foam are shown in Figure S2. The foams take the shape of the cylindrical jars in which the emulsions are cured and maintain this shape with some shrinkage during carbonization. SEM images of the carbon foams are shown in Figures 1 (top) and 2 and demonstrate that the foam retained the HIPE structure through carbonization. The macropores reproduce the morphology of the templating silicone oil phase with macropore windows resulting from abutted droplets. Similar openings between macropores are also observed in colloidally templated carbons because adjacent templates are touching.2,8-14,17,18 The higher-magnification image of the macropore wall in Figure 1 (bottom) clearly shows the hierarchical structure of the mesoporous carbon xerogel forming the walls of the macropores. SEM images in Figures 2 and S3 of the 30%/ 1000 and 40%/1000 materials show larger macropores than observed in the 40%/100 sample and demonstrate that the macropore size may be tuned with the silicone oil viscosity. Higher-resolution images of the xerogel structure, such as in Figure 1 (bottom), show no discernible differences for all three samples. Additionally, multiple SEM images for each sample were used to calculate the mean macropore diameter by averaging the diameters of at least 60 macropores. When 100 cP silicone oil is used in the 40%/100 foam, the macropores have a mean diameter of 0.7 μm, whereas both the 30%/1000 and 40%/1000 samples have a mean macropore diameter of 2.1 μm. This represents a 3-fold increase in the macropore mean diameter from a 10-fold increase in oil viscosity. To determine if the macropore pore (25) Meleson, K.; Graves, S.; Mason, T. G. Soft Mater. 2004, 2, 109.
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Figure 1. SEM images of a 40%/100 carbon foam showing the HIPE-templated structure of the carbon foam. Scale bars are shown in the images. Macropores and macropore windows formed from abutted silicone oil droplets are observed in the top image. This image is indistinguishable from a HIPE-templated polymer, demonstrating that the original emulsion structure is retained.3 The mesoporous xerogel structure of the walls is observed under higher magnification (bottom).
Figure 2. SEM images of 40%/1000 (top) and 30%/1000 (bottom) carbon foams. Scale bars are shown in the images. The structure of a high internal phase emulsion is observed in both samples.
diameters between samples were distinct, the statistical significance between the means of macropore distributions was measured by applying Welch’s t test for unequal variances for all sample pairs. Differences in the means for each macropore distribution with differing oil viscosity (40%/100 vs 30%/1000 and 40%/100 vs 40%/1000) were both found to be statistically different (two-tailed p < 0.0001) whereas macropore populations between identical oil viscosity samples (30%/1000 vs 40%/1000) were not (two-tailed p = 0.97). Porosimetry. The carbon foams fabricated in this study were characterized with Hg intrusion and N2 adsorption to understand the pore structures. Hg intrusion is most sensitive to pores with diameters >50 nm whereas the small size and gaseous delivery of N2 atoms make their adsorption best suited to characterizing mesopores and micropores.24 Pore size distributions for all 800 °C Langmuir 2010, 26(13), 11378–11383
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Figure 3. Hg intrusion pore size distributions of 800 °C pyrolyzed carbon foams. Samples are indicated by the legend on the graph. Hg intrusion is sensitive to the smallest opening between pores, and this data is indicative of the macropore windows. The 40%/1000 and 30%/1000 carbon foams were templated with higher-viscosity silicone oil than was the 40%/100 sample, and this results in larger macropores and macropore windows. This data demonstrates that the macropore window pore size is insensitive to the composition of the resorcinol-formaldehyde precursor solution and is tunable with silicone oil viscosity.
pyrolyzed sample were derived from Hg intrusion data (Figure S4) and are shown in Figure 3. The peak positions for the 30%/ 1000 and 40%/1000 samples are both at 0.53 μm, and the peak for the 40%/100 sample is at 0.18 μm. Additionally, the pore size distributions for 30%/1000 and 40%/1000 samples overlap almost perfectly and are insensitive to resorcinol-formaldehyde precursor solution composition. We note that these pore dimensions are much smaller than those observed in SEM, which is due to Hg intrusion measuring the smallest openings between pores.24 These openings are the macropore windows seen in Figures 1, 2, and S3, and the macropore window dimensions observed in SEM images are in good agreement with the Hg intrusion data. In addition, the macropore window diameters have the same ∼3fold increase in diameter from a 10-fold increase in oil viscosity observed for macropores in SEM. Because macropore and macropore window dimensions were tuned with silicone oil viscosity but not with the resorcinol-formaldehyde precursor solution, we conclude that the silicone oil viscosity alone mainly controls macropore and macropore window structure. All textural parameters from Hg intrusion are presented in Table 1. All carbon foams have large pore volumes and porosities; however, the 40%/100 sample is the most distinct from the other samples. The 40%/100 sample is templated with a volume of silicone oil that is equal to that used for other samples; however, it is formed from lower-viscosity oil that is sheared into smaller droplets with an increased surface area to volume ratio during HIPE formation. The greater surface area of the smaller oil droplets should produce a carbon foam with greater surface area, which agrees with the data in Table 1. Additionally, the smaller droplets result in the aqueous phase being distributed over a larger interfacial area, which yields thinner xerogel walls. The larger Hg intrusion pore volume of the 40%/100 sample may result from abutted droplets more easily producing macropore windows between thinner walls and creating more open space as compared to the thicker walls in the 40%/1000 and 30%/1000 samples. N2 adsorption BJH pore size distributions calculated from N2 adsorption isotherms (Figure S5) are shown in Figure S6 for all 800 °C pyrolyzed samples. The 40%/100 and 40%/1000 samples have very similar pore size distribution peak positions, and larger pores are found in the 30%/1000 sample as shown by the 1.33-fold larger pore size distribution peak position. This demonstrates that Langmuir 2010, 26(13), 11378–11383
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the composition of the aqueous phase, and not the templating oil, mainly controls mesopore pore size distribution peak positions. We note that the oil viscosity has some effect on mesopore structure as shown by comparing the 100 and 1000 cP oiltemplated samples in Figure S6. The 40%/100 sample has some mesopore volume above 20 nm, but the pore volumes of the 40%/ 1000 and 30%/1000 samples are negligible in this region. This may be explained by the thinner macropore walls in the 40%/100 sample having less internal volume as compared to that in the 40%/1000 and 30%/1000 samples. There will be more xerogel material exposed to the oil-water interface in the 40%/100 sample, and with fewer surrounding carbon particles on the interface, some surface pores may have diameters greater than 20 nm. The calculated N2 adsorption textural parameters of all samples are presented in Table 2. All foams are microporous as well as mesoporous. The presence of micropores is confirmed by the t-plot micropore volume derived from the N2 adsorption data (Table 2). This micropore volume is formed during pyrolysis in the carbon particles that make up the mesopore walls.26 Thus, these carbon foams have two levels of hierarchy: (1) the microporous carbon particles that make up the mesoporous xerogel and (2) the mesoporous xerogel composing the walls of the macropores. The data in Tables 1 and 2 demonstrates that the resorcinolformaldehyde precursor solution controls the mesopore diameter but has little effect on the macropore and macropore window structure. Although the silicone oil viscosity controls the macropore volume and size, it minimally affects the peak position of the mesopore pore size distribution. Thus, the mesopore and macropore/macropore window dimensions in these hierarchical carbon foams may be independently adjusted. Furthermore, the macropore and macropore window average diameters should be tunable between 0.7 and 2.1 μm and 0.18 and 0.53 μm, respectively, by incorporating oil into the HIPE with the appropriate viscosity between 100 and 1000 cP (with 200, 350, and 500 cP currently available). Larger or smaller macropores and macropore windows than those made in this study are likely accessible using silicone oils with viscosity greater than 1000 cP or less than 100 cP. The mesopore size should also be tunable over a wider range of values than those shown in this study by using alternative aerogel or xerogel materials as discussed later in this article. We note that the mesopore pore size distribution peak positions, pore volumes, and surface areas are significantly smaller than those observed in neat xerogels made with the same recipe. (These materials have also been referred to as aerogels in the literature, and they have sample compositions and processing conditions that are similar to those of our materials.22,27) The mesopore textural parameters may have been altered by the curing temperature profile or the presence of surfactant. The carbon foams in this study were cured for 3 days at 85 °C in contrast to the 1 day at room temperature, 1 day at 50 °C, and 1 day at 90 °C previously used for a neat xerogel in ref 22. When a neat 40 wt % organic xerogel control solution was cured for 3 days at 85 °C and solvent exchanged with chloroform and acetone in an identical manner to that for the carbon foams, we obtained the same pore volume of 1.39 cm3/g and a similar surface area of 641 m2/g as found in a traditionally cured sample.28 However, the peak position in the pore size distribution increased from 25 to 51 nm. Thus, the different curing temperature profile does alter (26) Gavalda, S.; Gubbins, K. E.; Hanzawa, Y.; Kaneko, K.; Thomson, K. T. Langmuir 2002, 18, 2141. (27) Wu, D.; Fu, R.; Dresselhaus, M. S.; Dresselhaus, G. Carbon 2006, 44, 675. (28) Gross, A. F.; Vajo, J. J.; Van Atta, S. L.; Olson, G. L. J. Phys. Chem. C 2008, 112, 5651.
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Gross and Nowak Table 1. Calculated Textural Parameters of Carbon Foams from Hg Intrusion Data
sample
Hg intrusion surface area (m2/g)
Hg intrusion pore volume (cm3/g)
foam density (g/cm3)
skeletal density (g/cm3)
porosity (%)
Hg intrusion pore size distribution peak position (μm)
40%/100 40%/1000 30%/1000 40%/100 1200 °C
155 102 118 254
4.37 3.27 3.22 5.26
0.197 0.263 0.262 0.166
1.41 1.88 1.68 1.34
86 86 84 87
0.18 0.53 0.53 0.15
Table 2. Calculated Textural Parameters of Carbon Foams from N2 Adsorption Data
sample
N2 adsorption surface area (m2/g)
N2 adsorption single-point adsorption total pore volume (cm3/g)
N2 adsorption t-plot micropore volume (cm3/g)
N2 adsorption Vmicropores/Vtotal
N2 adsorption BJH pore size distribution peak position (nm)
40%/100 40%/1000 30%/1000 40%/100 1200 °C
187 174 247 593
0.193 0.144 0.249 0.577
0.033 0.035 0.042 0.068
0.17 0.24 0.17 0.12
5.3 5.7 7.6 7.6
the pore structure and probably contributes to differences in morphology, but it cannot explain the decrease in the pore volume, surface area, or pore size distribution peak position found in Table 2. Another possible reason for the differences in mesopore morphology is that our samples contain 2.8 wt % sodium dodecylbenzenesulfonate surfactant in the resorcinolformaldehyde precursor solution, which is not present in neat carbon xerogel syntheses. Equal and lower surfactant levels are known to densify aerogels/xerogels and result in lower pore volumes, pore diameters, and surface areas than observed in unmodified materials.27,29 Thus, the large amount of surfactant in the HIPEs is likely the primary cause of the morphological changes that we observe in the xerogel framework. In the HIPEs, a significant fraction of surfactant is isolated at the oil-water interface and is not available to alter xerogel structure. Because of uncertainty about the distribution of surfactant between the oil/ water interface and the aqueous phase, we chose not to run a control experiment using only surfactant and our resorcinolformaldehyde precursor solution because the interpretation of results would be not be meaningful. In addition to the previously discussed samples, we attempted to form a 30%/100 material for this study and obtained a resorcinol-formaldehyde foam that partially collapsed after solvent exchange and ambient drying. We measured a Hg intrusion pore volume of only 1.30 cm3/g in the 30%/100 carbonized foam, and the macropore walls buckled during drying as shown in the SEM image in Figure S7. Because lower organic loading xerogels contain less material to resist shrinkage and because the macropore walls are thinner in samples made with 100 cP silicon oil, it is understandable that the 30%/100 sample collapsed. This indicates that our method of forming carbon foams is limited by the strength of the xerogel walls. We believe our method could overcome these limitations and produce macroporous carbon foams with a larger range of densities and average mesopore diameters by incorporating supercritically dried carbon aerogel formulations or by reducing the amount of surfactant during HIPE fabrication.27,30 Supercritically dried carbon aerogel formulations may allow the formation of lower-density carbon foams because of their lower organic content and gentler drying procedure. Additionally, reducing the surfactant concentration may allow larger mesopores. According to ref 27, less surfactant in our xerogel formulation should increase the peak position of (29) Worsley, M. A.; Satcher, J. H.; Baumann, T. F. J. Non-Cryst. Solids 2010, 356, 172. (30) Liu, N.; Zhang, S.; Fu, R.; Dresselhaus, M. S.; Dresselhaus, G. J. Appl. Polym. Sci. 2007, 104, 2849.
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the mesopore size distribution up to values as large as 25 nm. This approach will work only if the surfactant preferentially migrates to the oil-water interface and sufficient surfactant is present to enable HIPE formation. A 40%/100 sample was pyrolyzed at 1200 °C where partial graphitization and sintering of the framework occur to determine the maximum possible electrical conductivity of our materials.21 After 1200 °C pyrolysis, the sample monoliths remained intact and SEM images in Figure S8 shows that the macropore, macropore window, and xerogel structure were all maintained. Elevated firing temperatures are known to degrade the surface area and pore volume of xerogels and we characterized the porosity of this foam to quantify any reduced porosity.31-33 The macropore textural characteristics are shown in Table 1, and a comparison of the macropore pore size distributions from 40%/100 foams pyrolyzed at 800 and 1200 °C is shown in Figure S9. The 40%/100 1200 °C sample shows a reduced macropore window diameter, which is expected because of the sintering of the carbon framework, and a slightly decreased foam density, which has been observed in another report on high-temperature-fired porous carbons.34 However, the sample showed an unexpected large increase in macropore volume and surface area. We hypothesize that closed macropores originally formed around nonabutted oil droplets and may have opened as a result of the 1200 °C treatment of the carbon framework. The mesopore textural data for the 40%/100 1200 °C sample is shown in Table 2, and a comparison of the mesopore pore size distributions from 40%/100 foams pyrolyzed at 800 and 1200 °C is shown in Figure S10. The increase in the peak position of the mesopore size distribution and the reduced micropore volume fraction are both expected from the sintering of carbon particles that template the mesopores.21,31-33 However, the vastly increased surface area and pore volume do not agree with known trends for surfactant-free carbon aerogels and xerogels.31,33 The 40%/100 1200 °C sample is different from materials in previous reports on sintering effects because it contains surfactant in the synthesis solution, which results in denser networks of carbon particles forming the mesoporous framework.27,29 The more closely packed carbon particles may form inaccessible pores, and the modifications in the carbon framework that occur at 1200 °C may enable (31) Zanto, E. J.; Al-Muhtaseb, S. A.; Ritter, J. A. Ind. Eng. Chem. Res. 2002, 41, 3151. (32) Gross, J.; Alviso, C. T.; Pekala, R. W. Mater. Res. Soc. Symp. Proc. 1996, 431, 123 (also available at http://www.osti.gov/bridge/product.biblio.jsp?osti_id=231318) . (33) Lin, C.; Ritter, J. A. Carbon 2000, 38, 849. (34) Wiener, M.; Reichenauer, G.; Hemberger, F.; Ebert, H.-P. Int. J. Thermophys. 2006, 27, 1826.
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previously inaccessible mesopores to be measured with N2 adsorption. This pore-opening mechanism may be responsible for the increased mesopore surface area and volume of the 40%/100 1200 °C sample. Conductivity Measurements. The electrical conductivities of our samples were measured to determine if these carbon foams could be candidates for use in battery electrode materials and fuel cell electrodes. The electrical conductivities of the 40%/100, 40%/ 1000, 30%/1000, and 40%/100 1200 °C foams were 0.066 ( 0.010, 0.068 ( 0.006, 0.097 ( 0.010, and 0.337 ( 0.072 S/cm, respectively (errors are two standard deviations). The highest conductivity of the 40%/100 1200 °C sample is understandable in terms of the higher firing temperature. Elevated-temperature pyrolysis leads to graphitization of the carbon framework and thus increased electrical conductivity.21,33 Even though the skeletal density decreased as a result of 1200 °C firing (Table 1), any graphitization in the carbon particles forming the xerogel would increase the overall conductivity. Furthermore, this result is reasonable by taking into account that porous carbons with unchanged densities after increased pyrolysis temperatures still show increased electrical conductivity.34 Between the 800 °C pyrolyzed samples, the superior conductivity of the 30%/1000 sample is difficult to explain because of a lower skeletal density and equal overall density as compared to those of the 40%/1000 sample (Table 1). The 30%/1000 sample is mainly distinct from the 40%/100 and 40%/1000 samples in that it has larger mesopores, which indicates larger carbon particles making up the structure of this carbon foam according to ref 27. We could not verify the larger carbon particle sizes from SEM, and any ideas we present are purely speculative. If the particle size is indeed larger in the 30%/1000 sample, then this may result in larger surface areas between adjoining carbon particles and better electrical contact. This conductivity result requires more study in the future to discern the mechanism behind this observation. Our goal in this study was to template hierarchical carbon foams with completely fluidic precursor species. We avoided the use of colloidal templates because they require additional syntheses and may be expensive. Although the carbon foams described in this study require multiple solvent-exchange steps that are not necessarily faster than dissolving colloidal templates with hydrofluoric acid, they significantly reduce potential chemical hazards associated with fabrication. To increase the relevance of these materials, we are pursuing xerogel compositions that
Langmuir 2010, 26(13), 11378–11383
Article
do not require solvent exchange,27,30 that have a greater range of mesopore dimensions, and that use easier to remove macropore templating oils.
Conclusions Hierarchical carbon foams with independently tunable mesopore and macropore/macropore window size distributions were formed by fluidic templating in high internal phase emulsions. No solid colloidal templates or hydrofluoric acid etching was used in this study. The carbon foams are open cellular structures that form monoliths in the shape of the synthesis container. Both the macropore and the macropore window average diameters may be increased by 3-fold with a 10-fold increase in the oil phase viscosity, and the mesopore peak position in the pore size distribution could be shifted by 1.33-fold by lowering the organic loading in the resorcinol-formaldehyde precursor solution by 25%. Furthermore, the mesopore diameters should be independently tunable to any value in between those observed in this study by changing the organic loading of the resorcinol-formaldehyde precursor solution. The macropore diameter may be shifted to between 0.7 and 2.1 μm and the macropore diameter may be shifted to between 0.18 and 0.53 μm on the basis of the availability of silicone oils with viscosities between 100 and 1000 cP. Pore volumes as large as 5.26 cm3/g and electrical conductivities of up to 0.34 S/cm were observed. Although this study focused on carbon materials, this method of templating foams with independently tunable mesopore and macropore dimensions is applicable to many xerogel and aerogel materials produced from fluidic precursor solutions incorporated in high internal phase emulsions. Acknowledgment. We thank Joanna A. Kolodziejska and Robert E. Doty for the collection of SEM images. Supporting Information Available: Conceptual diagram of the HIPE used for carbon foam fabrication. Images of resorcinol-formaldehyde and carbon foam monoliths. SEM images under identical magnification of carbon foams. Hg cumulative intrusion data for all samples. N2 adsorption isotherms for all samples. SEM images of 30%/100 and 40%/100 1200 °C carbon foams. N2 adsorption BJH pore size distributions for all samples. Hg intrusion pore size distributions for 40%/100 and 40%/100 1200 °C carbon foams. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la1007846
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