Article pubs.acs.org/JPCC
Hydrogen Storage in High Surface Area Carbons with Identical Surface Areas but Different Pore Sizes: Direct Demonstration of the Effects of Pore Size Eric Masika and Robert Mokaya* School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, U. K. ABSTRACT: We present experimental data that directly shows the effect of pore size on hydrogen uptake in high surface area porous carbons. A direct study of the influence of pore size has been made possible by comparing the uptake capacity of porous carbons with identical surface areas but with different pore sizes and pore size distributions. A variety of synthesis methods have been used to prepare carbon materials with similar surface areas with pore sizes ranging from the micropore range (12 Å) to supermicropore/lower mesopore (23 Å) and lower mesopore (31 Å) range. This allowed a simple and straightforward analysis of the influence of pore size without any changes in total surface area. The pore size essentially defines the hydrogen uptake with no apparent regard to the similar surface areas. The excess and total hydrogen uptake (at −196 °C and 20 bar) of carbons with identical surface areas of 3340 m2/g, increased from 3.7 and 5.4 wt % (31 Å sample), to 4.8 and 6.3 wt % (23 Å sample) and to 6.3 and 7.3 wt % for a 12 Å sample. The excess hydrogen storage density (μmol H2·m−2) decreases linearly with pore size from 9.5 at 12 Å to 7.3 at 23 Å and 5.5 at 31 Å. Thus at a surface area of 3340 m2/g, a change of pore size from 31 to 12 Å improves the excess hydrogen storage by a staggering 70%. The pore size effect has general applicability; for carbons with similar surface areas of 2770 m2/g, the excess and total hydrogen storage was 1.7 and 3.0 wt % for a 28 Å sample and increased to 5.6 and 6.4 wt % for and 15 Å sample. In this case, a change of pore size from 28 to ca. 15 Å results in a more than 3-fold increase in excess hydrogen storage. Therefore, to improve hydrogen storage capacity of carbons, we need to increase the surface area, but with pores of the right size. A high surface area and pore volume associated with large pores cannot compensate for “unfavorably” sized pores.
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(MOFs),6,7 and nanostructured carbon materials (carbon nanotubes, carbon fibers, nanoporous carbons)8,9 have been performed. Nanostructured carbon materials have received much attention due to their low density, high surface area, and chemical stablility.8,9 Besides carbon nanotubes (CNTs) and carbon nanofibers, nanoporous carbon materials (activated carbon, microporous carbon and mesoporous carbon)10 have been studied extensively as hydrogen storage media. It is now well established that the hydrogen sorption capacity of porous carbons (and other types of porous materials) generally increases with surface area.10−13 A number of studies indicate that surface area associated with microporosity,11−14 and the presence of micropores below 1 nm,13,14 is crucial for hydrogen storage. Recently, there have been studies that have closely analyzed the dependence of hydrogen uptake on the pore size of solid state materials.13−17 The general observation from these studies is that smaller pores are more efficient for hydrogen storage. However, all these previous studies have been performed on materials where both the surface area and
INTRODUCTION Hydrogen is an ideal alternative to fossil fuels from an environmental point of view because its combustion does not generate pollutants and carbon dioxide (a green house gas).1,2 One of the main obstacles to the use of hydrogen as a transportation fuel or in power generation is storage and transport. There are essentially four ways to store hydrogen: liquefaction, compressed gas, metal hydrides and adsorption on porous materials.1−3 The high energy cost for liquefaction and the low energy efficiency due to continuous boil-off of hydrogen make liquid systems unfeasible for on-board hydrogen storage. The compressed gas method involves high pressure gas cylinders that engender safety issues. Thus chemisorption (in hydrides) and physisorption (in porous materials) are attractive. To date, hydrides that store significant amounts of hydrogen require undesirable thermal treatment to release hydrogen.4 On the other hand, take-up and release of hydrogen via physisorption methods can be achieved via simple changes in pressure. Furthermore, physisorption is preferred to chemisorption as the former is reversible, exhibits fast kinetics, and involves only a small amount of energy for adsorption and desorption. Extensive studies on physisorption of hydrogen on porous solids, such as zeolites,5 metal−organic frameworks © 2012 American Chemical Society
Received: October 10, 2012 Revised: November 9, 2012 Published: November 19, 2012 25734
dx.doi.org/10.1021/jp3100365 | J. Phys. Chem. C 2012, 116, 25734−25740
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ZTC weight ratio of 4. The mixture was then heat treated (heating ramp rate of 3 °C/min) in a horizontal furnace under a nitrogen gas flow to a temperature of 800 °C and held for 1 h. The activated sample was then thoroughly washed several times with 2 M HCl to remove any inorganic salts and then washed with distilled water until neutral pH was achieved. Finally, the activated ZTC was dried in an oven at 120 °C for 3 h and designated as Ac-ZTC. The third sample was prepared via activation of a carbon aerogel. First the aerogel was prepared via the carbonization of a melamine formaldehyde resin (MF) with a metal salt (CaCl2) as porogen. Formaldehyde (37%) and melamine were mixed in water at a molar ratio of 7/1 and the pH of the resulting mixture was adjusted to 8.5 by the addition of Na2CO3. The mixture was then heated at 40 °C with stirring to obtain a clean hexamethylomelamine (MF) solution. A metal salt (CaCl2) was then added at a CaCl2/MF ratio of 2. Condensation of the MF was initiated by adjusting the pH to 4.5 with HCl and heating at 60 °C for 1 h, during which a CaCl2-containing MF resin was formed. The resin was dried and cured at 180 °C for 6 h, and then carbonized in flowing nitrogen at 800 °C for 2 h. The resulting composite was washed with water to remove the salt and dried at 120 °C to generate a carbon aerogel. The carbon aerogel was then activated with KOH (at 800 °C and KOH/ carbon ratio of 4) as described above to generate the final activated carbon aerogel designated as Ac8-CA. Set 2 Samples. The first sample for set 2 was obtained via double activation of cellulose hydrochar materials.19 The cellulose hydrochar was first activated at 600 °C at KOH/ hydrochar ratio of 2. The resulting activated carbon was then reactivated at 700 °C and a KOH/carbon ratio of 4 to generate the final carbon designated as Ac-AC. For the second set 2 sample, we prepared a sample (designated as Ac7-CA) similar to Ac8-CA described above except that the KOH activation step was performed at 700 °C rather than 800 °C. The third set 2 sample was prepared via activation of a mesoporous SBA-15-templated mesoporous carbon. The mesoporous silica SBA-15 was prepared via standard procedures utilizing triblock copolymer as template and tetraethyl orthosilicate as silica source.20 Chemical vapor deposition was used to nanocast a mesoporous carbon using the calcined SBA-15 as previously described.21 The mesoporous carbon was then activated with KOH, at 700 °C and a carbon/ KOH ratio of 4. The full activation process was as described above for other activated samples. The final activated sample was designated as Ac-MC. Material Characterization. Nitrogen sorption isotherms and textural properties of the carbons were determined at −196 °C using nitrogen in a conventional volumetric technique by a Micromeritics ASAP 2020 sorptometer. Before analysis, the samples were evacuated for 12 h at 300 °C under vacuum. The surface area was calculated using the BET method based on adsorption data in the relative pressure (P/Po) range 0.02−0.22 and total pore volume was determined from the amount of the nitrogen adsorbed at P/Po = 0.99. Micropore surface area and micropore volume were obtained via t-plot analysis. Elemental analysis was carried out using a CHNS analyzer (Fishons EA 1108). Hydrogen Uptake Measurements. The excess hydrogen uptake capacity of the carbons was measured by gravimetric analysis with an Intelligent Gravimetric Analyzer, IGA, (Hiden) using 99.9999% purity hydrogen additionally purified by a
pore size vary. Thus, given the known relationship between hydrogen uptake and surface area,10−18 there are always ambiguities in studying the effect of pore size in a set of materials with varying surface area.13,15−17 In fact, no studies have so far studied the effect of pore size on hydrogen storage for materials with identical surface areas. Such a study is difficult because it is not easy to generate a set of high surface area materials wherein the surface areas remain identical but the pore size changes. This is a consequence of the fact that smaller pores tend to generate porous materials with high surface area, and that in general an increase in pore size leads to a decrease in surface area. It would thus be of great interest to be able to prepare a range of high surface area materials with similar surface areas but varying pore sizes. Indeed, a direct investigation of the effects of pore size on hydrogen storage requires that the surface area remain unchanged. Such a study would unambiguously and directly show the effect of pore size on hydrogen storage in high surface area porous materials. In this study, we have used a variety of synthesis methods to generate high surface area carbons that possess identical surface areas but different pore sizes. This has allowed us, for the first time, to directly and with no ambiguity show how changes in pore size affect hydrogen uptake of high surface area materials. We also assess whether a high surface area can compensate for the presence of “unfavorably” sized pores. Our findings provide direct and unambigous evidence that pore size is a critical factor for hydrogen storage in carbon materials.
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EXPERIMENTAL SECTION Material Synthesis. To obtain high surface area carbon materials, we employed several synthesis methods resulting in two sets of samples with three samples each. Set 1 Samples. The three samples in set 1 were prepared using three different methods as follows. For the first sample, 0.6 g of zeolite 13X was dried in an oven at 300 °C for 24 h and thereafter impregnated with furfuryl alcohol (FA), which was then polymerized under argon at 80 °C for 24 h and then at 150 °C for 8 h. The zeolite/FA composite was then pyrolyzed at 700 °C under argon for 3 h. The resulting carbon/zeolite composite was placed in a flow through furnace and exposed to chemical vapor deposition (CVD) with ethylene gas (ratio 1:3 in Ar by volume) at 700 °C for 3 h. The gas flow was then switched to argon only and the temperature of the furnace raised to 900 °C and maintained for 3 h in a flow of argon only, followed by cooling to room temperature under argon. The carbon/zeolite composite was washed with 10% hydrofluoric (HF) acid for 24 h and then concentrated hydrochloric (HCl) acid for 6 h at 60 °C to remove the zeolite framework. The resulting carbon, designated as ZTC-X, was oven-dried at 120 °C. For the second sample, 0.5 g of zeolite CBV720 in an alumina boat was placed in a flow-through tube furnace and heated (at a ramp rate of 5 °C/min) to 800 °C under nitrogen flow. With the furnace at 800 °C, CVD was performed for 3 h by switching the gas flow to nitrogen saturated with acetonitrile. After the 3 h CVD step, the gas flow was switched back to nitrogen only and the temperature raised to 900 °C and maintained for 3 h. The resulting zeolite/carbon composite was washed with 10% hydrofluoric (HF) acid several times, followed by refluxing at 60 °C in concentrated hydrochloric acid (HCl) to remove the zeolite framework. The resulting zeolite templated carbon (ZTC) was then activated by thoroughly mixing with KOH in an agate mortar at a KOH/ 25735
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Figure 1. Nitrogen sorption isotherms (left) and pore size distribution curves (right) of set 1 porous carbon materials: (a) ZTC-X; (b) Ac-ZTC; (c) Ac8-CA.
Table 1. Textural Properties and Hydrogen Uptake of Porous Carbon Materials sample ZTC-X Ac-ZTC Ac8-CA Ac-AC Ac7-CA Ac-MC
surface area (m2/g)a 3332 3350 3343 2722 2782 2772
(2837) (2570) (803) (2502) (1258) (1066)
pore volume (cm3/g)b 1.66 1.81 2.65 1.23 2.04 2.10
(1.18) (1.17) (0.36) (1.06) (0.58) (0.50)
pore size (Å)c 12 23 31 12/20 (15)d 13/27 28
H2 uptake (wt %)e,f 7.3 6.3 5.4 6.4 3.2 3.0
(6.3) (4.8) (3.7) (5.6) (1.9) (1.7)
H2 uptake density (μmol H2 m−2)e,f 11.0 9.4 8.1 11.8 5.8 5.4
(9.5) (7.2) (5.5) (10.3) (3.4) (3.1)
a
Values in parentheses are the micropore surface area. bValues in parentheses are the micropore volume. cValues shown are the main pore size. Value in parentheses is the average pore size. eHydrogen uptake at −196 °C and 20 bar. fValues in parentheses are the excess hydrogen uptake at −196 °C and 20 bar. d
molecular sieve filter. The hydrogen uptake measurements were performed at −196 °C (in a liquid nitrogen bath) over the pressure range 0−20 bar. The samples were outgassed (10−10 bar) under heating at 200 °C overnight before measurement. The hydrogen uptake data were rigorously corrected for the buoyancy of the system and samples. The carbon density used in the buoyancy corrections was determined from helium sorption data obtained using the IGA at a pressure of up to 20 bar at 273 K. The total hydrogen uptake was calculated from the measured excess capacity by taking into account the pore volume of the samples and the density of compressed hydrogen at the prevailing temperature and pressure.22 In all cases the hydrogen uptake (excess or total) is reported as weight percent (wt %) of carbon sample weight.
and Ac7-CA), zeolite templated carbon (Ac-ZTC) and mesoporous carbon (Ac-MC) was removed during the KOH activation step. Figure 1 shows the nitrogen sorption isotherms of set 1 samples. The sorption isotherms of the carbons vary in terms of the sharpness of their knee. Sample ZTC exhibits a type I isotherm with virtually all adsorption below relative pressure (P/Po) = 0.2. ZTC-X is thus highly microporous, which is consistent with the presence of micropore channels arising from the zeolite-like structural ordering generated by templating with zeolite 13X. The isotherm of sample Ac-ZTC is similar to that of ZTC-X at very low relative pressure (P/Po < 0.02), but then deviates as it thereafter has a gentler adsorption knee and significant adsorption up to P/Po = 0.5. The isotherm of the activated carbon aerogel (Ac8-CA) is similar to that of Ac-ZTC up P/Po = 0.3, after which adsorption continues almost linearly up to P/Po ∼ 1. The isotherm of sample Ac8-CA exhibits a very broad adsorption knee. Thus the isotherms of the three set 1 samples appear to be similar at very low relative pressure (P/Po < 0.2) but differ significantly at higher relative pressure. The differences in the isotherms are due to an increase in pore size and broadening of pore size distribution as the adsorption knee broadens. The pore size distribution (PSD) of set 1 carbons, determined via a NLDFT model using nitrogen adsorption data, is shown in Figure 1, and the pore size is summarized in Table 1. Sample ZTC-X is highly microporous with a pore size of 12 Å and hardly any pores larger than 20 Å. This is a reflection of the high zeolite-like pore channel ordering that is replicated from the zeolite 13X template. The Ac-ZTC sample is dominated by pores of size 23 Å, and a very small proportion of pores of size ca. 12 Å. The
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RESULTS AND DISCUSSION Set 1 Samples. Several carbon synthesis methods including hard template carbonization (using zeolites or mesoporous silicas), aerogel formation and chemical activation were used either singly or together so as to generate the materials required for this study. The key properties desired during the synthesis for each set of samples were high (and identical) surface areas and different pore sizes. We have previously shown that the presence of nitrogen has an influence on the hydrogen storage capacity of high surface area carbons. For this study, we therefore ensured that carbons prepared with melamine (sample Ac8-CA and Ac7-CA) and acetonitrile (Ac-ZTC and Ac-MC) as carbon precursors did not contain any N. Elemental analysis confirmed that for these carbons, which were all chemically activated with KOH, the N content was below 0.2 wt %. Clearly, any N present in the carbon aerogels (Ac8-CA 25736
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PSD of sample Ac-ZTC is also rather wider than that of ZTC-X with the presence of pores as large as 30 Å. Sample Ac-ZTC therefore mainly contains supermicropores and small mesopores. Sample Ac8-CA, on the other hand, contains pores with an average size of 31 Å, and a minor proportion of 13 Å pores. The PSD is much wider, and pores as large as 40 Å are present in sample Ac8-CA. Overall, therefore, it is clear that the predominant pore size of the three set 1 samples increases from 12 Å for ZTC-X to 23 Å for Ac-ZTC and 31 Å for Ac8-CA. The surface area of the samples is given in Table 1 and confirms that, notwithstanding the differences in nature of sorption isotherm (Figure 1), we were successful in synthesizing carbons with high and identical surface areas of ca. 3340 m2/g. The measured surface areas, 3332 m2/g for ZTC-X, 3350 m2/g for Ac-ZTC, and 3343 m2/g for Ac8-CA, are essentially identical as the very small variations are well within experimental error. As far as we know, this is the first time that it has been possible to prepare such high surface area carbons that have identical surface areas but significantly different pore sizes and pore size distributions that extend from the micropore to the mesopore range. The differences in pore size distribution are also reflected by the proportion of micropore surface area in the samples. The highly microporous ZTC-X sample has 85% (i.e., 2837 m2/g) of surface area arising from micropores. The proportion of micropore surface area decreases to 77% (2570 m2/g) for Ac-ZTC and to a lowly 24% (803 m2/g) for Ac8-CA. The pore volumes of the samples increase at larger pore sizes, as shown in Table 1, rising from 1.66 cm3/g for ZTC-X to 1.81 and 2.65 cm3/g for Ac-ZTC and Ac8-CA, respectively. The proportion of micropore volume decreases in the order 71%, 65%, and 14% for ZTC-X, Ac-ZTC, and Ac8-CA, respectively. Thus sample ZTC-X is microporous whereas Ac8-CA is essentially mesoporous and Ac-ZTC is a supermicropore/lower mesopore sample. A number of studies have argued that the hydrogen storage of porous materials, especially at cryogenic temperature, is directly proportional to the surface area. A widely accepted relationship, the Chahine rule, states that in general there is 1 wt % hydrogen adsorption for every 500 m2/g of surface area, which is equivalent to a hydrogen density of 10 μmol H2 m−2.18 Most previous studies on high surface area porous materials (carbons or metal organic frameworks)7−15 have observed a roughly linear relationship between hydrogen uptake and surface area because the studied structures were mainly microporous so as to ensure high surface area. In a departure from that norm, we here consider the hydrogen uptake at similar surface areas but different pore sizes. Figure 2 shows the total hydrogen uptake of the set 1 samples as a function of pressure at −196 °C. First we note that the three carbons exhibit similar (shape-wise) hydrogen uptake isotherms, which show complete reversibility (i.e., no hysteresis) and no saturation at 20 bar. It is clear that the hydrogen uptake of the three samples varies significantly despite their identical surface areas in the order ZTC-X > Ac-ZTC > Ac8-CA. The hydrogen storage capacities (total and excess) at 20 bar are given in Table 1. The hydrogen uptake at 20 bar varies between 5.4 and 7.3 wt %, whereas the excess storage is in the range 3.7−6.3 wt %. We note that hydrogen uptake of 7.3 wt % (6.3 wt % excess) for the zeolite 13X templated ZTC-X sample is the highest ever reported for a carbon material,8−11 and surpasses the 6.9 wt % previously reported for a zeolite EMC-2 templated carbon,14a 7.03 wt % for a polypyrrole-derived
Figure 2. Hydrogen uptake isotherms, at −196 °C, of set 1 porous carbon materials.
activated carbon,23 and 7.08 wt % for a doubly activated carbon.24 To properly consider the effect of pore size on hydrogen storage, it is important to take into account the varying pore volume of the three carbons. This is taken into account by considering only the excess hydrogen uptake, which was obtained by excluding any hydrogen in the space (pore volume) of the carbons.22 The excess storage gives the amount of hydrogen adsorbed on the carbons above that which would have been present in the same void space (or pore volume) at the same temperature and pressure if there was no interaction between the hydrogen and carbon surface (i.e., zero energy of interaction). The excess hydrogen adsorption therefore allows a direct comparison of the amount of hydrogen adsorbed on different surfaces without interference from the associated void space or pore volume. The different pore volumes of the samples in this study are therefore factored out by only considering the excess hydrogen storage. It is thus remarkable that as the pore size changes from 31 to 23 Å, the excess hydrogen uptake increases by 30% from 3.7 wt % (sample Ac8CA) to 4.8 wt % (Ac-ZTC) whereas a decrease in pore size from 23 to 12 Å is accompanied by a 70% increase in uptake to 6.3 wt % (sample ZTC-X). A change in pore size of the carbons from 23 to 31 Å improves the hydrogen uptake by 31% from 4.8 to 6.3 wt %. These changes in excess hydrogen uptake very clearly illustrate the importance of pore size rather than simply high surface area on hydrogen uptake. As shown in Table 1, the excess hydrogen storage density (μmol H2 m−2) of the carbons decreases from 9.5 at 12 Å to 7.2 at 23 Å and 5.5 at 31 Å. Indeed, a semiquantitative plot of excess hydrogen storage density as a function of pore size gives a linear relationship as shown in Figure 3. Set 2 Samples. The nitrogen sorption isotherns of set 2 samples are shown in Figure 4. The doubly activated carbon (Ac-AC) exhibits an isotherm that is typical for a highly microporous material with all adsorption below P/Po = 0.2. The sorption isotherms of the other two samples are similar to that of Ac-AC up to P/Po = 0.2, but with adsorption continuing up to much higher relative pressure. Overall, the isotherms of the activated aerogel (Ac7-CA) and activated mesoporous carbon (Ac-MC) are quite similar. As indicated in Table 1, all three 25737
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Figure 3. Hydrogen uptake density as a function of pore size of set 1 porous carbon materials.
samples have virtually identical surface areas of ca. 2770 m2/g. The lower (compared to that of set 1) surface area of set 2 samples, and the fact that the porosities of sample Ac7-CA and Ac-MC are quite similar allowed us to (1) ascertain whether the effect of pore size, as discussed above for set 1, is applicable at other surface area levels and (2) confirm that, for any given surface area, carbons with similar pore sizes will exhibit similar hydrogen uptakes regardless of their preparation method. The PSD of set 2 samples in Figure 4 indicates that although Ac-AC is highly microporous with an average pore size of ca. 15 Å and hardly any pores above 20 Å, the other two samples are mainly mesoporous. Although both samples have a small proportion of pores in the micropore range, their main pore sizes are relatively similar at 27 and 28 Å for Ac7-CA and Ac-MC, respectively. Samples Ac7-CA and Ac-MC have higher pore volumes (2.04 and 2.1 cm3/g respectively) compared to that of Ac-AC (1.23 cm3/g) due to their larger pore sizes. Differences in PSD are also reflected in the microporosity of the samples; 92% of surface area of Ac-AC arises from micropores, whereas for Ac7-CA and Ac-MC the proportion is much lower at 45 and 38% respectively. The effect of pore size is clearly evident from the hydrogen uptake data of set 2 samples in Figure 5 and Table 1. Despite the similar surface areas, the total hydrogen storage capacity of Ac-AC at 6.4 wt % is at least twice as much as that of the other two samples: 3.2 and 3.0 wt % for Ac7-CA and Ac-MC,
Figure 5. Hydrogen uptake isotherms, at −196 °C, of set 2 porous carbon materials.
respectively. The difference in excess hydrogen uptake is even greater: 5.6 wt % for Ac-AC compared to 1.9 and 1.7 wt % for Ac7-CA and Ac-MC, respectively. It is remarkable that our data show that, at a constant surface area of ca. 2770 m2/g, changing the pore size of a carbon from 28 to ca. 15 Å engenders a more than 3-fold increase in hydrogen uptake. The excess hydrogen storage density (μmol H2 m−2) decreases from 10.3 at ca. 15 Å to 3.1 at 28 Å. It is also noteworthy that despite having a lower surface area (ca. 2770 m2/g), sample Ac-AC has higher hydrogen uptake than both Ac-ZTC and Ac8-CA, which have a surface area of ca. 3340 m2/g. This indicates that pore size can be more important than total surface area. Therefore, to improve hydrogen storage capacity of carbons, we need to increase the surface area, but with pores of the right size. A comparison between the hydrogen uptake of the mesoporous Ac8-CA sample, which has a high surface area and pore volume (2.65 cm3/g), and that of the microporous sample Ac-AC with lower surface area and pore volume indicates that a high surface area associated with large pores cannot compensate for “unfavorably” sized pores.
Figure 4. Nitrogen sorption isotherms (left) and pore size distribution curves (right) of set 2 porous carbon materials. 25738
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CONCLUSIONS A variety of carbon synthesis methods, including hard template carbonization (with zeolites or mesoporous silicas), aerogel formation, and chemical activation have been used either singly or in combination to generate a range of high surface area porous carbons with similar surface areas but different pore sizes and pore size distributions. Two sets of carbons, each with three samples of surface area 3340 or 2770 m2/g, but with pore sizes ranging between 12 and 31 Å were prepared. These sets of carbons allowed a simple and straightforward analysis of the influence of pore size without any ambiguities associated with changes in total surface area. Differences in pore volume were accounted for in the analysis by comparing the excess hydrogen storage that excludes any hydrogen that would occupy the pore space at the prevailing conditions. It was found that the pore size essentially defines both the total and excess hydrogen uptake notwithstanding the identical surface areas. The excess and total hydrogen uptake (at −196 °C and 20 bar) of carbons with identical surface areas of 3340 m2/g, increased from 3.7 and 5.4 wt % (31 Å sample), to 4.8 and 6.3 wt % (23 Å sample), and to 6.3 and 7.3 wt % for a 12 Å sample. The excess hydrogen storage density (μmol H2·m−2) of the carbons decreases linearly with pore size from 9.5 at 12 Å to 7.3 at 23 Å and 5.5 at 31 Å. The pore size effect has general applicability; for carbons with similar surface areas of 2770 m2/g, the excess and total hydrogen storage was 1.7 and 3.0 wt % for a 28 Å sample and increased to 5.6 and 6.4 wt % for a 15 Å sample representing a staggering 3-fold increase in excess hydrogen storage. This work is the first successful attempt at comparing the effect of pore size on the hydrogen uptake capacity of high surface area porous materials that have identical surface areas.
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was funded by the University of Nottingham and the EPSRC. REFERENCES
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