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Activated carbon with optimum pore size distribution for hydrogen storage
Carbon 99 (2016) 289e294
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Activated carbon with optimum pore size distribution for hydrogen storage Govind Sethia 1, Abdelhamid Sayari* Department of Chemistry, Centre for Catalysis Research and Innovation (CCRI), University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2015 Received in revised form 12 December 2015 Accepted 13 December 2015 Available online 15 December 2015
A series of nitrogen containing activated carbons (NAC-1.5-y) with surface areas and pore volumes in the range of 526e2386 m2/g and 0.26e1.16 cm3/g, respectively were prepared by treatment of a nitrogenrich carbon with KOH (KOH to carbon weight ratio ¼ 1.5) at different temperatures (y) in the range of 550e700 C. The prepared samples were used to delineate the role of pore sizes and nitrogen content in hydrogen adsorption at 77 K. The activated carbons showed high hydrogen uptake, due to increased porosity (>0.364 nm). The optimized carbon material NAC-1.5-600, with high nitrogen content (22.3 wt %) and large volume of ultra-micropore centered at 0.59 nm in size, showed hydrogen uptake of 2.94 wt% at 77 K and 1 bar. The adsorption capacity was found to be linearly dependent on ultra-micropore volume (0.5e0.7 nm), but not linearly associated with the total surface area nor the total pore volume. Nonetheless, the surface area of larger pore material (NAC-1.5-700) was found to be favorable for hydrogen adsorption at high partial pressure. The data obtained indicated that the occurrence of nitrogen, oxygen and hydrogen-containing species has hardly any effect on hydrogen uptake at 77 K. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Activated carbon KOH activation H2 adsorption
1. Introduction The decreasing oil reserves and growing environmental awareness created the need for more eco-friendly fuels and zero emission vehicles. Hydrogen as an alternative to fossil fuel, has been recognized as an attractive energy carrier for both stationary and locomotive applications. Although hydrogen has the highest gravimetric energy density of 120 kJ/g, about three times the energy density of gasoline, and it does not generate any greenhouse gas emissions upon oxidation, the on-board storage of hydrogen is still a major challenge [1,2]. The US Department of Energy set the goal of 5.5 wt% gravimetric reversible hydrogen storage, for systems to be used in automotive applications [3,4]. To a large extent the success of on-board hydrogen storage applications rests on the development of efficient, low-cost materials, including adsorbents with high reversible uptake and stability [5]. Many hydrogen storage methods including physisorption, chemisorption, spillover, metal hydrides, liquid hydrogen, and high-pressure hydrogen have been proposed for hydrogen storage.
* Corresponding author. E-mail address: [email protected] (A. Sayari). 1 Present address: Advanced Analytical Sciences, Reliance R&D Center, Reliance Industries Limited, Thane-Belapur Road, Navi Mumbai, 400701, India. http://dx.doi.org/10.1016/j.carbon.2015.12.032 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
However, they are still far from large-scale implementation due to economic and/or technical challenges [4,5]. Hydrogen adsorption in porous materials offers several advantages over other methods, namely, fast adsorption and desorption kinetics, no need for energy to release hydrogen, and high hydrogen uptake at low temperature and moderate pressures. Among the various adsorbents like metalorganic frameworks (MOFs), zeolites and others, carbon materials offer many advantages such as heat resistance, chemical stability, reversibility, low-cost and availability [6e8]. Additionally, lightweight carbons with large porosity are a preferred choice to achieve the gravimetric hydrogen storage target. Various carbon materials for hydrogen adsorption have been reported, including activated carbons (AC), carbon aerogels, graphene, and carbon nanotubes [1,9e11]. Despite several advantages of carbon materials, the main drawback that may limit their application in hydrogen storage is their low heat of adsorption. The optimum heat of adsorption required for maximum delivery at 298 K is 15.1 kJ/mol [12]. Poorly-polarizable hydrogen molecules adsorb on the surface of carbons by weak van der Waals forces. Therefore, the narrower the pores, the stronger the hydrogen-tosurface interactions, and the higher the adsorption capacity. Hence, among the many proposed carbons, materials with ultramicroporosity are of significant importance in hydrogen storage due to their enhanced heat of adsorption [13].
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Recently, a number of microporous carbon materials based on natural and synthetic precursors were found to exhibit excellent hydrogen adsorption capacity (Table 1) [14e20]. The organic precursors were transformed into porous activated carbons by carbonization followed by KOH activation. To the best of our knowledge the highest hydrogen uptakes at 77 K and 1 bar reported for any natural and synthetic material-derived AC are 3.28 wt% from hemp stem [20] and 2.85 wt% from petroleum pitch [15]. Among the wide range of pore sizes in carbon materials, several researchers stressed the importance of small micropores (1 nm) and showed that such pores are efficient for hydrogen storage, whereas pores above 1 nm do not contribute much toward hydrogen adsorption at 77 K [13,21e26]. Both experimental and theoretical studies suggested that pores with 0.5e0.7 nm width are ideal for hydrogen adsorption at 77 K due to the overlap of potential fields from both sides of the pore walls [13,21e26]. Adsorption is a surface phenomenon and it is obvious that the surface composition, particularly the nitrogen content may play an important role in the hydrogen adsorption by activated carbons. Nonetheless, recent reports regarding the role of surface composition in hydrogen adsorption are contradictory [15,26e32]. Taking into account the data available in the literature, and considering the importance of micropore volume and surface area from pores with 0.5e0.7 nm diameter, and the conflicting findings regarding the effect of surface composition on hydrogen adsorption at 77 K, we successfully optimized the carbon activation procedure and developed strictly microporous AC materials with high nitrogen content, and large volume from pores around 0.59 nm wide for ultra-high hydrogen uptake. The effects of ultra-micropores and chemical composition were discussed in light of the experimental data obtained. 2. Experimental 2.1. Materials Trimethylsilyl imidazole and chloroacetonitrile were obtained from Alfa Aesar, hydrochloric acid (38 wt%), ammonia (28 wt%) and potassium hydroxide were purchased from SigmaeAldrich. Hydrogen (99.999%), nitrogen (99.9993%), and helium (99.999%) were supplied by Linde Canada. 2.2. Synthesis of activated carbons A three-step method was used for the preparation of activated carbons, starting by the synthesis of carbon precursor (CP). The CP was synthesized using a reported procedure [33]. A mixture of trimethylsilyl imidazole (4.6 g) and chloroacetonitrile (4.9 g) was stirred at room temperature for 24 h under inert atmosphere to form 1,3 bis(cynomethyl imidazolium) chloride, a white solid material. This material was washed with diethyl ether and dried under vacuum before further use as carbon precursor. The second step
was a carbonization process. The white solid was placed in an alumina boat, transferred into a tubular furnace and then heated under N2 flow (100 cm3/min) at a rate of 10 C/min, with a holding time of 2 h at the final temperature. Materials obtained after carbonization at 400 and 600 C were named as CP-400 and CP600, respectively. The last step was an activation process to generate porosity in the carbonized material. The activation procedure was reported elsewhere [34]. Briefly, the non-porous CP400 material (vide infra) was thoroughly mixed with KOH (KOH/CP400 weight ratio 1.5) and heated under N2 flow (100 cm3/min) at a rate of 3 C/min up to a temperature in the range of 550e700 C, and held at the final temperature for 1 h. The activated samples were then thoroughly washed and dried in an oven at 80 C overnight. The activated carbons thus synthesized were denoted as NAC-1.5-y, where NAC stands for nitrogen-doped activated carbon, 1.5 is the KOH/CP-400 weight ratio and y is the activation temperature in C. 2.3. Characterization The textural properties of the synthesized AC materials were determined based on nitrogen adsorption/desorption measurements at 77 K using a Micromeritics ASAP 2020 apparatus. The surface area was calculated using the BET method within the relative pressure (P/Po) range of 0.05e0.20. The total pore volume was determined from the amount of nitrogen adsorbed at P/Po as close as possible to 1. The pore size distribution (PSD) was determined using the density functional theory (DFT) model with slit pore geometry. The carbon, nitrogen, hydrogen contents of the carbon samples were determined by elemental analysis using a Vario EL III instrument. The surface analysis was achieved by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra spectrometer. More details may be found elsewhere [34]. Transmission electron microscopy (TEM) images were taken using a JEOL JEM2100F instrument. 2.4. Hydrogen adsorption measurements Hydrogen adsorption isotherms were measured at 77 K using a static volumetric system (ASAP 2020). The hydrogen adsorption capacity, as weight percent of hydrogen adsorbed per gram of adsorbent, was determined from the adsorption isotherms. Prior to adsorption measurements, the samples were activated in situ by heating under vacuum at 200 C for several hours to remove the adsorbed moisture and clean the surface. 3. Results and discussion Fig. 1 shows the nitrogen adsorption/desorption isotherms of the prepared carbons. All AC samples exhibited type I isotherms according to the IUPAC classification. The non-activated carbons
Table 1 Hydrogen storage at 77 K and 1 bar over KOH-activated carbons derived from natural and synthetic precursors. Activated carbons a
NAC-1.5-600 ACT-850 PBA CAC1 Ch800/700/3 C2-1/4-700 CAC4 AC8b a b
Precursor (natural/synthetic)
H2 uptake (wt%)
Reference
1,3 bis(cynomethyl imidazolium) chloride Polythiophene Petroleum residue Corncob Chitosan Cellulose Corncob Hemp stem (Cannabis sativa L.)
2.96 2.41 2.84 2.85 2.95 2.50 3.21 3.28
This work [14] [15] [16] [17] [18] [19] [20]
Highest reported hydrogen uptake for any activated carbon derived from synthetic materials. Highest reported hydrogen uptake for any activated carbon derived from natural materials.
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Fig. 1. N2 adsorptionedesorption isotherm at 77 K for N-doped activated and nonactivated carbons. (A color version of this figure can be viewed online.)
Fig. 2. DFT pore size distribution for N-doped activated carbons. (A color version of this figure can be viewed online.)
showed very low nitrogen uptake, indicating that no pores larger than the kinetic diameter of N2 molecules (0.364 nm) occurred in such materials (Table 2). The adsorption isotherms for materials activated in the range of 550e650 C, present a sharp uptake at very low relative pressure (P/P0 < 0.01), which indicates that the microporosity of the samples is mainly composed of ultramicropores. The materials also exhibited almost flat isotherms in the range P/P0 > 0.05, indicative of insignificant contribution from wider pores towards the total porosity. However, for NAC-1.5-700 there was a broader isotherm knee (Fig. 2), indicating the presence of micropores and narrow mesopores with broad pore size distribution. The textural properties of the carbons are given in Table 2. As seen, increasing the activation temperature brought about an increase in the BET surface area, the total pore volume, and the average pore diameter. The changes in the shape of the PSD (Fig. 2) with increasing activation temperature reflect changes in the pore structure of activated carbons. In particular, the PSD widened and shifted towards larger pores as the activation temperature increased. TEM images were taken for the carbon precursor (CP-400) and KOH activated carbon (NAC-1.5-600). As shown in Fig. S1, CP-400 appears as a layered non porous material, whereas NAC-1.5-600 shows a highly porous structure (Fig. S2). The chemical composition of materials prepared at different temperatures at a constant KOH/CP-400 ratio of 1.5 is presented in Table 3. As can be seen, there is a decrease in the content of foreign elements with increasing activation temperature, and materials
Table 3 Chemical composition of N-doped activated and non-activated carbon materials [34]. Samples
CP-400 CP-600 NAC-1.5-550 NAC-1.5-600 NAC-1.5-650 NAC-1.5-700 a b
Chemical composition (wt%) Ca
Ha
Na
Ob
C/N
56.9 61.9 60.3 56.8 64.5 70.7
2.8 2.1 2.9 2.6 2.2 1.4
29.7 27.3 25.2 22.3 18.0 11.9
6.8 NA 10.1 9.2 8.3 NA
1.9 2.3 2.4 2.6 3.6 5.9
Determined from elemental analysis. Determined from XPS measurements.
activated at 600e700 C showed significant decrease in nitrogen, oxygen and hydrogen contents. The surface properties of the activated carbons were investigated using XPS and the resulting spectra are presented in Fig. S3. The N1s signal was deconvoluted into 3 to 5 components. The corresponding binding energies of ca. 398, 399, 400, 401 and 403 eV were attributed to pyridine, AreNH2 (or eC]NH), pyrrole, quaternary nitrogen and pyridine N-oxide respectively (Table S1) [34]. The non-activated carbon showed three peaks, whereas four to five peaks were observed for materials activated with KOH at different temperatures. The XPS spectra clearly show that significant chemical changes occurred during the KOH activation. The amount of each type of surface nitrogen was
Table 2 Textural properties of activated and non-activated N-doped carbons [34]. Samples
CP-400 CP-600 NAC-1.5-550 NAC-1.5-600 NAC-1.5-650 NAC-1.5-700 a b c d
H2 uptake (wt%)
0.03 0.63 1.47 2.96 2.43 2.42
BET surface area (m2/g)
11 18 526 1317 1342 2386
Total pore volume (cm3/g)
0.01 0.02 0.26 0.64 0.65 1.16
Maxima of the PSDs calculated by DFT model with slit pore geometry. Determined using probe molecules (H2, N2, Ar) with different sizes. Volume of pores with less than 0.7 nm width. Volume of pores with less than 2 nm width.
DFT pore sizea (nm)
NA <0.364b 0.54 0.59 0.63 0.74
DFT cumulative pore volume (cm3/g) Ultra-microporesc
Microporesd
NA NA 0.12 0.27 0.21 0.0
NA NA 0.16 0.43 0.50 0.69
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calculated using the deconvoluted N1s XPS spectra (Fig. S3) and are given in Table S1. Detailed discussion regarding the effect of activation conditions on textural properties and chemical composition of carbons can be found in our earlier publication [34]. Fig. 3 shows the hydrogen adsorptionedesorption isotherms of the activated carbons at 77 K and up to 1 bar pressure, which indicated that the hydrogen adsorption was completely reversible. Table 2 summarizes the H2 adsorption capacities of activated and non-activated carbon materials. Non-activated (CP-400, CP-600) and activated (NAC-1.5-y; y ¼ 550e700) samples showed H2 adsorption capacity ranging from 0.03 to 2.96 wt%, at 77 K and 1 bar. The high hydrogen uptake for NAC-1.5-600 is associated with the formation of large number of ultra-micropores (0.7), whose cumulative volume as calculated by DFT increased from 0.12 to 0.27 cm3/g on increasing activation temperature from 550 to 600 C (Table 2). The porosity of carbons activated at 550 and 600 C consisted mostly of two groups of ultra-micropores with diameters around of 0.54 and 0.59 nm, respectively (Fig. 2). NAC-1.5-550 exhibited a surface area and total pore volume of 526 m2/g and 0.26 cm3/g, respectively (Table 2). Increasing the activation temperature to 600 C was accompanied by a sharp increase in surface area (1317 m2/g) and pore volume (0.64 cm3/g) due to increased porosity, particularly in the ultra micropore region (Figs. 4 and 5). Figs. 4 and 5 show the hydrogen uptake at 77 K and the porosity (pore volume and surface area) of the ACs, as a function of activation temperature. From Figs. 4 and 5, it is inferred that hydrogen uptake is strongly associated with the ultra-micropore volume (0.7 nm), and the ultra-microporosity is the key factor for hydrogen adsorption. NAC-1.5-600 and NAC-1.5-650 exhibited almost identical total pore volume (0.64 cm3/g vs 0.65 cm3/g) and BET surface area (1312 m2/g vs 1342 m2/g). However, they showed 21.8% difference in hydrogen uptake (2.96 vs 2.43). The significant decrease in hydrogen uptake is due to the occurrence of a broader PSD with larger micropores (0.63 vs. 0.59 nm) (Fig. 2) together with 28.6% decrease in ultra-micropore volume (0.21 vs 0.27 cm3/g) (Table 1). The decrease in H2 uptake for materials prepared at higher activation temperature (NAC-1.5-650 and NAC-1.5-700) is associated with the smaller ultra-micropore volume (Table 2) and broader PSD (Fig. 2). This indicates that pores with diameters in the range of 0.56e0.70 nm are crucial for hydrogen adsorption and pre-
Fig. 4. Hydrogen uptake and pore volume as a function of activation temperature for NAC-1.5-y materials. (A color version of this figure can be viewed online.)
Fig. 5. Hydrogen uptake and surface area as a function of activation temperature for NAC-1.5-y materials. (A color version of this figure can be viewed online.)
Fig. 3. Hydrogen adsorption isotherms at 77 K for activated and non-activated carbons. (A color version of this figure can be viewed online.)
dominantly involved in hydrogen uptake at 77 K. Among the prepared samples, NAC-1.5-600 showed the highest hydrogen adsorption capacity because of the large ultra-micropore volume from the pores centered at 0.59 nm. This finding is in line with the literature, which also indicated that the pore width of 0.6 nm could be optimum for hydrogen storage [13,35,36]. Narrow pores possess stronger surface-H2 interactions due to overlap of potential fields from both sides of the pores. The stronger the interactions in the ultra-micropores result into higher heat of adsorption, and therefore exerts a stronger influence on the hydrogen storage capacity of the materials [13,26,36]. Both the interaction energy and the hydrogen uptake decrease with pore widening. Similar trend was observed for CO2 adsorption on ACs, where CO2 adsorption capacity and heat of adsorption decreased with increasing pore diameters [34]. Sevilla et al. [18] also reported that both pore size and functional group content determine the heat of hydrogen adsorption in ACs. Moreover, Yushin et al. [13] reported that high heat of hydrogen adsorption in ACs is attributed to the considerably stronger carbon-H2 interactions in the smaller micropores and the possible role of pore shape, degree of
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disorder and surface composition. Based on theoretical calculation, Garcia Blanco et al. [37] reported that the heat of hydrogen adsorption in activated carbons decreases at any pressure with increasing pore sizes. This indicates that the attractive contributions of gasegas interactions are stronger in the smaller pores where a more compact adsorbate structure can be formed. The hydrogen adsorption isotherms (Fig. 3) indicated that in the low pressure region, NAC-1.5-650 has higher hydrogen uptake than NAC-1.5-700. Moreover, NAC-1.5-650 showed a cumulative pore volume of 0.21 cm3/g of ultra-micropores with diameters 0.63 nm, whereas NAC-1.5-700 did not have ultra-micropores (Table 3). It may thus be inferred that the presence of ultramicropores in NAC-1.5-650 is responsible for the higher hydrogen uptake in the low pressure region. The hydrogen adsorption isotherms also indicated that although NAC-1.5-650 exhibits higher adsorption in the low pressure region than NAC-1.5-700, the uptake difference between the two materials decreased with increasing pressure. As seen in Table 1, at 1 bar, both NAC-1.5-650 and NAC-1.5-700 showed similar hydrogen uptakes (2.43 vs 2.42 wt%). In spite of the lack of ultra-micropores in NAC1.5-700, the comparatively superior increase in hydrogen uptake with increasing pressure up to 1 bar (Fig. 3) is attributed to the cumulative effect of large specific surface area from micro and mesopores (Fig. 5). The porosity increased with activation temperature as NAC-1.5-700 exhibited significantly higher surface area (2386 vs 1342 m2/g) and pore volume (1.16 vs 0.65 cm3/g), albeit with broader PSD (Table 2). Fig. 5 shows that although the hydrogen uptake by activated carbons at 77 K and 1 bar is not linearly dependent on the surface area from wider pores, the latter plays an important role in hydrogen adsorption. To some extent, the large surface area compensates for the lack of ultra-micropores and enhances the hydrogen adsorption capacity of NAC-1.5-700 (Figs. 4 and 5). Sevilla et al. [18] also reported compensatory effect of higher surface area and larger pores of activated carbons for hydrogen adsorption. Using the analysis of variance test (ANOVA), Tellez-Juarez et al. [29] reported that the surface area controls hydrogen adsorption at 77 K and higher pressure. From the above discussion and Figs. 3e5 it could be concluded that for activated carbons at 77 K, ultramicropores are the most critical factor for hydrogen storage at low pressure and the total surface area is responsible for hydrogen adsorption at higher pressure. The excellent hydrogen uptake of NAC-1.5-600 was compared with the hydrogen adsorption capacities of ACs originating from natural and synthetic precursors. Literature data for ACs that yielded the best known performances under identical conditions are listed in Table 1. To the best of our knowledge, the hydrogen adsorption capacity of NAC-1.5-600 at 77 K and 1 bar is one of the highest uptakes reported so far, for any synthetic material-derived activated carbon (Table 1). The hydrogen adsorption isotherm (Fig. 3) of the non-activated CP-600 carbon at 77 K leveled off at a low pressure of 100 mmHg, whereas the hydrogen uptake of activated carbons showed a steady increase up to 1 bar. The hydrogen uptake for CP-600 is attributed to the presence of ultra-micropores (<0.364 nm), which are accessible to only a single layer of hydrogen molecules. The critical pore diameter required for more than one layer of hydrogen adsorption should be ca. 0.56 nm (double of the kinetic diameter of hydrogen molecule i.e. 0.28 nm). The steady increase in hydrogen adsorption of activated carbons indicated that higher uptakes were expected at higher pressures. This behavior could be explained by the development of pores with hierarchical diameters (Fig. 2), in the ACs during the chemical activation. In contrast, the porosity in CP-600 was almost entirely due to ultra-micropores with diameters less than 0.364 nm.
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Fig. 6 presents the hydrogen adsorption capacities of activated carbons as a function of nitrogen, oxygen and hydrogen content. The hydrogen uptake increased with increasing activation temperature up to 600 C, then decreased thereafter, whereas the nitrogen, oxygen and hydrogen content decreased steadily. This indicates that hydrogen adsorption at 77 K does not depend on the occurrence of foreign elements in the activated carbon. Furthermore, instead of the total nitrogen content, Fig. 7 shows the hydrogen uptake versus the content of different nitrogen species as determined by XPS (Table 3). It is seen that the content of all nitrogen species (pyridine, AreNH2 (or eC]NH), pyrrole, and quaternary nitrogen) decreased steadily with increasing activation temperature, whereas the hydrogen uptake increased, then leveled off. Thus, Figs. 6 and 7 show that none of the nitrogen species nor the total nitrogen content correlates with the hydrogen uptake. This indicates that nitrogen content irrespective of N-profile, is not a significant factor for hydrogen adsorption at 77 K. Although the effect of surface composition on hydrogen adsorption by activated carbon has been a matter of heated debate, often with contradictory statements [15,26e32], our results are in line with Zhao et al. [27] finding that the nitrogen content is not a relevant parameter for hydrogen adsorption at 77 K. For practical applications, in addition to high hydrogen adsorption capacity, the adsorbents should have high stability, low energy requirement for regeneration and high tolerance to recycling. To show the regeneration and recyclability of the current carbon adsorbents, we performed four runs of hydrogen adsorption/desorption measurements for NAC-1.5-600 (Fig. S4) and obtained almost identical isotherms without any significant decrease in hydrogen uptake (less than 0.5% loss per cycle), indicating that this material exhibits excellent regenerability and recyclability.
4. Conclusion High nitrogen containing ultra-microporous activated carbon with different pore widths have been prepared and investigated for hydrogen adsorption at 77 K and up to 1 bar. NAC-1.5-600 showed a hydrogen uptake of 2.96 wt%, which is the highest uptake reported so far, for any synthetic material-derived activated carbon. The high uptake for NAC-1.5-600 was attributed to the presence of large pore volume from optimized pores with a diameter of 0.59 nm. From the results, it is also inferred that the ultra-micropore volume primarily
Fig. 6. Hydrogen uptake and elemental composition as a function of activation temperature at KOH/CP-400 ¼ 1.5. (A color version of this figure can be viewed online.)
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Fig. 7. Hydrogen uptake and N-profile as a function of activation temperature at KOH/ CP-400 ¼ 1.5. (A color version of this figure can be viewed online.)
controls hydrogen adsorption at 77 K. Nonetheless, the surface area from larger pores (>1 nm) as in NAC-1.5-700 is also involved in hydrogen adsorption, at higher pressure. The nitrogen, oxygen and hydrogen content has hardly any effect on hydrogen uptake at 77 K. Acknowledgments The financial support of the Natural Sciences and Engineering Council of Canada (NSERC) is acknowledged. A.S. thanks the Federal Government for the Canada Research Chair in Nanostructured Materials for Catalysis and Separation (2001e2015). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2015.12.032 References [1] S.J. Yang, H. Jung, T. Kim, C.R. Park, Recent advances in hydrogen storage technologies based on nanoporous carbon materials, Prog. Nat. Sci. 22 (6) (2012) 631e638. [2] S.J. Yang, J.H. Im, H. Nishihara, H. Jung, K. Lee, T. Kyotani, et al., General relationship between hydrogen adsorption capacities at 77 and 298 K and pore characteristics of the porous adsorbents, J. Phys. Chem. C 116 (19) (2012) 10529e10540. [3] Targets for Onboard Hydrogen Storage Systems for Light-duty Vehicles, 2009, pp. 1e22. http://www1.eere.energy.gov/hydrogenandfuelcells/storage/pdfs/ targets_onboard_hydro_storage_explanation.pdf. [4] M. Sevilla, R. Mokaya, Energy storage applications of activated carbons: supercapacitors and hydrogen storage, Energy Environ. Sci. 7 (4) (2014) 1250e1280. € bel, J. Garche, P.T. Moseley, L. Jo €rissen, G. Wolf, Hydrogen storage by [5] R. Stro carbon materials, J. Power Sources 159 (2) (2006) 781e801. [6] G. Sethia, A. Sayari, Nitrogen-doped carbons: remarkably stable materials for CO2 capture, Energy Fuels 28 (4) (2014) 2727e2731. Linares-Solano, D. Cazorla-Amoro , F. Su s, [7] D. Lozano-Castello arez-García, A. guez LMGAM (Ed.), Advances in hydrogen storage in carbon materials, in: Die Renewable Hydrogen Technologies, Elsevier, Amsterdam, 2013, pp. 269e291. -Beneyto, D. Lozano-Castello , F. Sua rez-García, D. Cazorla-Amoro s, [8] M. Jorda Linares-Solano, Advanced activated carbon monoliths and activated carA. bons for hydrogen storage, Microporous Mesoporous Mater. 112 (1e3) (2008) 235e242. [9] M. Hirscher, Fuels e hydrogen storage j Carbon materials, in: J. Garche (Ed.), Encyclopedia of Electrochemical Power Sources, Elsevier, Amsterdam, 2009, pp. 484e487. [10] Y. Yürüm, A. Taralp, T.N. Veziroglu, Storage of hydrogen in nanostructured carbon materials, Int. J. Hydrogen Energy 34 (9) (2009) 3784e3798.
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Ramirez-Cuesta, The rotational and translational dynamics of molecular hydrogen physisorbed in activated carbon: a direct probe of microporosity and hydrogen storage performance, Carbon 44 (13) (2006) 2724e2738. [26] K.M. Thomas, Hydrogen adsorption and storage on porous materials, Catal. Today 120 (3e4) (2007) 389e398. ndez-Huerta, M.T. Izquierdo, A. Celzard, Hydrogen [27] W. Zhao, V. Fierro, N. Ferna uptake of high surface area-activated carbons doped with nitrogen, Int. J. Hydrogen Energy 38 (25) (2013) 10453e10460. [28] Y. Xia, G.S. Walker, D.M. Grant, R. Mokaya, Hydrogen storage in high surface area carbons: experimental demonstration of the effects of nitrogen doping, J. Am. Chem. Soc. 131 (45) (2009) 16493e16499. rez, V. Fierro, W. Zhao, N. Fern [29] M.C. Tellez-Jua andez-Huerta, M.T. Izquierdo, E. Reguera, et al., Hydrogen storage in activated carbons produced from coals of different ranks: effect of oxygen content, Int. J. Hydrogen Energy 39 (10) (2014) 4996e5002. [30] M. Georgakis, G. Stavropoulos, G.P. Sakellaropoulos, Molecular dynamics study of hydrogen adsorption in carbonaceous microporous materials and the effect of oxygen functional groups, Int. J. Hydrogen Energy 32 (12) (2007) 1999e2004. [31] L. Wang, R.T. Yang, Hydrogen storage properties of N-doped microporous carbon, J. Phys. Chem. C 113 (52) (2009) 21883e21888. [32] C.-C. Huang, H.-M. Chen, C.-H. Chen, J.-C. Huang, Effect of surface oxides on hydrogen storage of activated carbon, Sep. Purif. Technol. 70 (3) (2010) 291e295. [33] X. Zhu, P.C. Hillesheim, S.M. Mahurin, C. Wang, C. Tian, S. Brown, et al., Efficient CO2 capture by porous, nitrogen-doped carbonaceous adsorbents derived from task-specific ionic liquids, ChemSusChem 5 (10) (2012) 1912e1917. [34] G. Sethia, A. Sayari, Comprehensive study of ultra-microporous nitrogendoped activated carbon for CO2 capture, Carbon 93 (2015) 68e80. [35] Y. Gogotsi, R.K. Dash, G. Yushin, T. Yildirim, G. Laudisio, J.E. 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Carbon journal homepage: www.elsevier.com/locate/carbon
Activated carbon with optimum pore size distribution for hydrogen storage Govind Sethia 1, Abdelhamid Sayari* Department of Chemistry, Centre for Catalysis Research and Innovation (CCRI), University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 October 2015 Received in revised form 12 December 2015 Accepted 13 December 2015 Available online 15 December 2015
A series of nitrogen containing activated carbons (NAC-1.5-y) with surface areas and pore volumes in the range of 526e2386 m2/g and 0.26e1.16 cm3/g, respectively were prepared by treatment of a nitrogenrich carbon with KOH (KOH to carbon weight ratio ¼ 1.5) at different temperatures (y) in the range of 550e700 C. The prepared samples were used to delineate the role of pore sizes and nitrogen content in hydrogen adsorption at 77 K. The activated carbons showed high hydrogen uptake, due to increased porosity (>0.364 nm). The optimized carbon material NAC-1.5-600, with high nitrogen content (22.3 wt %) and large volume of ultra-micropore centered at 0.59 nm in size, showed hydrogen uptake of 2.94 wt% at 77 K and 1 bar. The adsorption capacity was found to be linearly dependent on ultra-micropore volume (0.5e0.7 nm), but not linearly associated with the total surface area nor the total pore volume. Nonetheless, the surface area of larger pore material (NAC-1.5-700) was found to be favorable for hydrogen adsorption at high partial pressure. The data obtained indicated that the occurrence of nitrogen, oxygen and hydrogen-containing species has hardly any effect on hydrogen uptake at 77 K. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Activated carbon KOH activation H2 adsorption
1. Introduction The decreasing oil reserves and growing environmental awareness created the need for more eco-friendly fuels and zero emission vehicles. Hydrogen as an alternative to fossil fuel, has been recognized as an attractive energy carrier for both stationary and locomotive applications. Although hydrogen has the highest gravimetric energy density of 120 kJ/g, about three times the energy density of gasoline, and it does not generate any greenhouse gas emissions upon oxidation, the on-board storage of hydrogen is still a major challenge [1,2]. The US Department of Energy set the goal of 5.5 wt% gravimetric reversible hydrogen storage, for systems to be used in automotive applications [3,4]. To a large extent the success of on-board hydrogen storage applications rests on the development of efficient, low-cost materials, including adsorbents with high reversible uptake and stability [5]. Many hydrogen storage methods including physisorption, chemisorption, spillover, metal hydrides, liquid hydrogen, and high-pressure hydrogen have been proposed for hydrogen storage.
* Corresponding author. E-mail address: [email protected] (A. Sayari). 1 Present address: Advanced Analytical Sciences, Reliance R&D Center, Reliance Industries Limited, Thane-Belapur Road, Navi Mumbai, 400701, India. http://dx.doi.org/10.1016/j.carbon.2015.12.032 0008-6223/© 2015 Elsevier Ltd. All rights reserved.
However, they are still far from large-scale implementation due to economic and/or technical challenges [4,5]. Hydrogen adsorption in porous materials offers several advantages over other methods, namely, fast adsorption and desorption kinetics, no need for energy to release hydrogen, and high hydrogen uptake at low temperature and moderate pressures. Among the various adsorbents like metalorganic frameworks (MOFs), zeolites and others, carbon materials offer many advantages such as heat resistance, chemical stability, reversibility, low-cost and availability [6e8]. Additionally, lightweight carbons with large porosity are a preferred choice to achieve the gravimetric hydrogen storage target. Various carbon materials for hydrogen adsorption have been reported, including activated carbons (AC), carbon aerogels, graphene, and carbon nanotubes [1,9e11]. Despite several advantages of carbon materials, the main drawback that may limit their application in hydrogen storage is their low heat of adsorption. The optimum heat of adsorption required for maximum delivery at 298 K is 15.1 kJ/mol [12]. Poorly-polarizable hydrogen molecules adsorb on the surface of carbons by weak van der Waals forces. Therefore, the narrower the pores, the stronger the hydrogen-tosurface interactions, and the higher the adsorption capacity. Hence, among the many proposed carbons, materials with ultramicroporosity are of significant importance in hydrogen storage due to their enhanced heat of adsorption [13].
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Recently, a number of microporous carbon materials based on natural and synthetic precursors were found to exhibit excellent hydrogen adsorption capacity (Table 1) [14e20]. The organic precursors were transformed into porous activated carbons by carbonization followed by KOH activation. To the best of our knowledge the highest hydrogen uptakes at 77 K and 1 bar reported for any natural and synthetic material-derived AC are 3.28 wt% from hemp stem [20] and 2.85 wt% from petroleum pitch [15]. Among the wide range of pore sizes in carbon materials, several researchers stressed the importance of small micropores (1 nm) and showed that such pores are efficient for hydrogen storage, whereas pores above 1 nm do not contribute much toward hydrogen adsorption at 77 K [13,21e26]. Both experimental and theoretical studies suggested that pores with 0.5e0.7 nm width are ideal for hydrogen adsorption at 77 K due to the overlap of potential fields from both sides of the pore walls [13,21e26]. Adsorption is a surface phenomenon and it is obvious that the surface composition, particularly the nitrogen content may play an important role in the hydrogen adsorption by activated carbons. Nonetheless, recent reports regarding the role of surface composition in hydrogen adsorption are contradictory [15,26e32]. Taking into account the data available in the literature, and considering the importance of micropore volume and surface area from pores with 0.5e0.7 nm diameter, and the conflicting findings regarding the effect of surface composition on hydrogen adsorption at 77 K, we successfully optimized the carbon activation procedure and developed strictly microporous AC materials with high nitrogen content, and large volume from pores around 0.59 nm wide for ultra-high hydrogen uptake. The effects of ultra-micropores and chemical composition were discussed in light of the experimental data obtained. 2. Experimental 2.1. Materials Trimethylsilyl imidazole and chloroacetonitrile were obtained from Alfa Aesar, hydrochloric acid (38 wt%), ammonia (28 wt%) and potassium hydroxide were purchased from SigmaeAldrich. Hydrogen (99.999%), nitrogen (99.9993%), and helium (99.999%) were supplied by Linde Canada. 2.2. Synthesis of activated carbons A three-step method was used for the preparation of activated carbons, starting by the synthesis of carbon precursor (CP). The CP was synthesized using a reported procedure [33]. A mixture of trimethylsilyl imidazole (4.6 g) and chloroacetonitrile (4.9 g) was stirred at room temperature for 24 h under inert atmosphere to form 1,3 bis(cynomethyl imidazolium) chloride, a white solid material. This material was washed with diethyl ether and dried under vacuum before further use as carbon precursor. The second step
was a carbonization process. The white solid was placed in an alumina boat, transferred into a tubular furnace and then heated under N2 flow (100 cm3/min) at a rate of 10 C/min, with a holding time of 2 h at the final temperature. Materials obtained after carbonization at 400 and 600 C were named as CP-400 and CP600, respectively. The last step was an activation process to generate porosity in the carbonized material. The activation procedure was reported elsewhere [34]. Briefly, the non-porous CP400 material (vide infra) was thoroughly mixed with KOH (KOH/CP400 weight ratio 1.5) and heated under N2 flow (100 cm3/min) at a rate of 3 C/min up to a temperature in the range of 550e700 C, and held at the final temperature for 1 h. The activated samples were then thoroughly washed and dried in an oven at 80 C overnight. The activated carbons thus synthesized were denoted as NAC-1.5-y, where NAC stands for nitrogen-doped activated carbon, 1.5 is the KOH/CP-400 weight ratio and y is the activation temperature in C. 2.3. Characterization The textural properties of the synthesized AC materials were determined based on nitrogen adsorption/desorption measurements at 77 K using a Micromeritics ASAP 2020 apparatus. The surface area was calculated using the BET method within the relative pressure (P/Po) range of 0.05e0.20. The total pore volume was determined from the amount of nitrogen adsorbed at P/Po as close as possible to 1. The pore size distribution (PSD) was determined using the density functional theory (DFT) model with slit pore geometry. The carbon, nitrogen, hydrogen contents of the carbon samples were determined by elemental analysis using a Vario EL III instrument. The surface analysis was achieved by X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra spectrometer. More details may be found elsewhere [34]. Transmission electron microscopy (TEM) images were taken using a JEOL JEM2100F instrument. 2.4. Hydrogen adsorption measurements Hydrogen adsorption isotherms were measured at 77 K using a static volumetric system (ASAP 2020). The hydrogen adsorption capacity, as weight percent of hydrogen adsorbed per gram of adsorbent, was determined from the adsorption isotherms. Prior to adsorption measurements, the samples were activated in situ by heating under vacuum at 200 C for several hours to remove the adsorbed moisture and clean the surface. 3. Results and discussion Fig. 1 shows the nitrogen adsorption/desorption isotherms of the prepared carbons. All AC samples exhibited type I isotherms according to the IUPAC classification. The non-activated carbons
Table 1 Hydrogen storage at 77 K and 1 bar over KOH-activated carbons derived from natural and synthetic precursors. Activated carbons a
NAC-1.5-600 ACT-850 PBA CAC1 Ch800/700/3 C2-1/4-700 CAC4 AC8b a b
Precursor (natural/synthetic)
H2 uptake (wt%)
Reference
1,3 bis(cynomethyl imidazolium) chloride Polythiophene Petroleum residue Corncob Chitosan Cellulose Corncob Hemp stem (Cannabis sativa L.)
2.96 2.41 2.84 2.85 2.95 2.50 3.21 3.28
This work [14] [15] [16] [17] [18] [19] [20]
Highest reported hydrogen uptake for any activated carbon derived from synthetic materials. Highest reported hydrogen uptake for any activated carbon derived from natural materials.
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291
Fig. 1. N2 adsorptionedesorption isotherm at 77 K for N-doped activated and nonactivated carbons. (A color version of this figure can be viewed online.)
Fig. 2. DFT pore size distribution for N-doped activated carbons. (A color version of this figure can be viewed online.)
showed very low nitrogen uptake, indicating that no pores larger than the kinetic diameter of N2 molecules (0.364 nm) occurred in such materials (Table 2). The adsorption isotherms for materials activated in the range of 550e650 C, present a sharp uptake at very low relative pressure (P/P0 < 0.01), which indicates that the microporosity of the samples is mainly composed of ultramicropores. The materials also exhibited almost flat isotherms in the range P/P0 > 0.05, indicative of insignificant contribution from wider pores towards the total porosity. However, for NAC-1.5-700 there was a broader isotherm knee (Fig. 2), indicating the presence of micropores and narrow mesopores with broad pore size distribution. The textural properties of the carbons are given in Table 2. As seen, increasing the activation temperature brought about an increase in the BET surface area, the total pore volume, and the average pore diameter. The changes in the shape of the PSD (Fig. 2) with increasing activation temperature reflect changes in the pore structure of activated carbons. In particular, the PSD widened and shifted towards larger pores as the activation temperature increased. TEM images were taken for the carbon precursor (CP-400) and KOH activated carbon (NAC-1.5-600). As shown in Fig. S1, CP-400 appears as a layered non porous material, whereas NAC-1.5-600 shows a highly porous structure (Fig. S2). The chemical composition of materials prepared at different temperatures at a constant KOH/CP-400 ratio of 1.5 is presented in Table 3. As can be seen, there is a decrease in the content of foreign elements with increasing activation temperature, and materials
Table 3 Chemical composition of N-doped activated and non-activated carbon materials [34]. Samples
CP-400 CP-600 NAC-1.5-550 NAC-1.5-600 NAC-1.5-650 NAC-1.5-700 a b
Chemical composition (wt%) Ca
Ha
Na
Ob
C/N
56.9 61.9 60.3 56.8 64.5 70.7
2.8 2.1 2.9 2.6 2.2 1.4
29.7 27.3 25.2 22.3 18.0 11.9
6.8 NA 10.1 9.2 8.3 NA
1.9 2.3 2.4 2.6 3.6 5.9
Determined from elemental analysis. Determined from XPS measurements.
activated at 600e700 C showed significant decrease in nitrogen, oxygen and hydrogen contents. The surface properties of the activated carbons were investigated using XPS and the resulting spectra are presented in Fig. S3. The N1s signal was deconvoluted into 3 to 5 components. The corresponding binding energies of ca. 398, 399, 400, 401 and 403 eV were attributed to pyridine, AreNH2 (or eC]NH), pyrrole, quaternary nitrogen and pyridine N-oxide respectively (Table S1) [34]. The non-activated carbon showed three peaks, whereas four to five peaks were observed for materials activated with KOH at different temperatures. The XPS spectra clearly show that significant chemical changes occurred during the KOH activation. The amount of each type of surface nitrogen was
Table 2 Textural properties of activated and non-activated N-doped carbons [34]. Samples
CP-400 CP-600 NAC-1.5-550 NAC-1.5-600 NAC-1.5-650 NAC-1.5-700 a b c d
H2 uptake (wt%)
0.03 0.63 1.47 2.96 2.43 2.42
BET surface area (m2/g)
11 18 526 1317 1342 2386
Total pore volume (cm3/g)
0.01 0.02 0.26 0.64 0.65 1.16
Maxima of the PSDs calculated by DFT model with slit pore geometry. Determined using probe molecules (H2, N2, Ar) with different sizes. Volume of pores with less than 0.7 nm width. Volume of pores with less than 2 nm width.
DFT pore sizea (nm)
NA <0.364b 0.54 0.59 0.63 0.74
DFT cumulative pore volume (cm3/g) Ultra-microporesc
Microporesd
NA NA 0.12 0.27 0.21 0.0
NA NA 0.16 0.43 0.50 0.69
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calculated using the deconvoluted N1s XPS spectra (Fig. S3) and are given in Table S1. Detailed discussion regarding the effect of activation conditions on textural properties and chemical composition of carbons can be found in our earlier publication [34]. Fig. 3 shows the hydrogen adsorptionedesorption isotherms of the activated carbons at 77 K and up to 1 bar pressure, which indicated that the hydrogen adsorption was completely reversible. Table 2 summarizes the H2 adsorption capacities of activated and non-activated carbon materials. Non-activated (CP-400, CP-600) and activated (NAC-1.5-y; y ¼ 550e700) samples showed H2 adsorption capacity ranging from 0.03 to 2.96 wt%, at 77 K and 1 bar. The high hydrogen uptake for NAC-1.5-600 is associated with the formation of large number of ultra-micropores (0.7), whose cumulative volume as calculated by DFT increased from 0.12 to 0.27 cm3/g on increasing activation temperature from 550 to 600 C (Table 2). The porosity of carbons activated at 550 and 600 C consisted mostly of two groups of ultra-micropores with diameters around of 0.54 and 0.59 nm, respectively (Fig. 2). NAC-1.5-550 exhibited a surface area and total pore volume of 526 m2/g and 0.26 cm3/g, respectively (Table 2). Increasing the activation temperature to 600 C was accompanied by a sharp increase in surface area (1317 m2/g) and pore volume (0.64 cm3/g) due to increased porosity, particularly in the ultra micropore region (Figs. 4 and 5). Figs. 4 and 5 show the hydrogen uptake at 77 K and the porosity (pore volume and surface area) of the ACs, as a function of activation temperature. From Figs. 4 and 5, it is inferred that hydrogen uptake is strongly associated with the ultra-micropore volume (0.7 nm), and the ultra-microporosity is the key factor for hydrogen adsorption. NAC-1.5-600 and NAC-1.5-650 exhibited almost identical total pore volume (0.64 cm3/g vs 0.65 cm3/g) and BET surface area (1312 m2/g vs 1342 m2/g). However, they showed 21.8% difference in hydrogen uptake (2.96 vs 2.43). The significant decrease in hydrogen uptake is due to the occurrence of a broader PSD with larger micropores (0.63 vs. 0.59 nm) (Fig. 2) together with 28.6% decrease in ultra-micropore volume (0.21 vs 0.27 cm3/g) (Table 1). The decrease in H2 uptake for materials prepared at higher activation temperature (NAC-1.5-650 and NAC-1.5-700) is associated with the smaller ultra-micropore volume (Table 2) and broader PSD (Fig. 2). This indicates that pores with diameters in the range of 0.56e0.70 nm are crucial for hydrogen adsorption and pre-
Fig. 4. Hydrogen uptake and pore volume as a function of activation temperature for NAC-1.5-y materials. (A color version of this figure can be viewed online.)
Fig. 5. Hydrogen uptake and surface area as a function of activation temperature for NAC-1.5-y materials. (A color version of this figure can be viewed online.)
Fig. 3. Hydrogen adsorption isotherms at 77 K for activated and non-activated carbons. (A color version of this figure can be viewed online.)
dominantly involved in hydrogen uptake at 77 K. Among the prepared samples, NAC-1.5-600 showed the highest hydrogen adsorption capacity because of the large ultra-micropore volume from the pores centered at 0.59 nm. This finding is in line with the literature, which also indicated that the pore width of 0.6 nm could be optimum for hydrogen storage [13,35,36]. Narrow pores possess stronger surface-H2 interactions due to overlap of potential fields from both sides of the pores. The stronger the interactions in the ultra-micropores result into higher heat of adsorption, and therefore exerts a stronger influence on the hydrogen storage capacity of the materials [13,26,36]. Both the interaction energy and the hydrogen uptake decrease with pore widening. Similar trend was observed for CO2 adsorption on ACs, where CO2 adsorption capacity and heat of adsorption decreased with increasing pore diameters [34]. Sevilla et al. [18] also reported that both pore size and functional group content determine the heat of hydrogen adsorption in ACs. Moreover, Yushin et al. [13] reported that high heat of hydrogen adsorption in ACs is attributed to the considerably stronger carbon-H2 interactions in the smaller micropores and the possible role of pore shape, degree of
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disorder and surface composition. Based on theoretical calculation, Garcia Blanco et al. [37] reported that the heat of hydrogen adsorption in activated carbons decreases at any pressure with increasing pore sizes. This indicates that the attractive contributions of gasegas interactions are stronger in the smaller pores where a more compact adsorbate structure can be formed. The hydrogen adsorption isotherms (Fig. 3) indicated that in the low pressure region, NAC-1.5-650 has higher hydrogen uptake than NAC-1.5-700. Moreover, NAC-1.5-650 showed a cumulative pore volume of 0.21 cm3/g of ultra-micropores with diameters 0.63 nm, whereas NAC-1.5-700 did not have ultra-micropores (Table 3). It may thus be inferred that the presence of ultramicropores in NAC-1.5-650 is responsible for the higher hydrogen uptake in the low pressure region. The hydrogen adsorption isotherms also indicated that although NAC-1.5-650 exhibits higher adsorption in the low pressure region than NAC-1.5-700, the uptake difference between the two materials decreased with increasing pressure. As seen in Table 1, at 1 bar, both NAC-1.5-650 and NAC-1.5-700 showed similar hydrogen uptakes (2.43 vs 2.42 wt%). In spite of the lack of ultra-micropores in NAC1.5-700, the comparatively superior increase in hydrogen uptake with increasing pressure up to 1 bar (Fig. 3) is attributed to the cumulative effect of large specific surface area from micro and mesopores (Fig. 5). The porosity increased with activation temperature as NAC-1.5-700 exhibited significantly higher surface area (2386 vs 1342 m2/g) and pore volume (1.16 vs 0.65 cm3/g), albeit with broader PSD (Table 2). Fig. 5 shows that although the hydrogen uptake by activated carbons at 77 K and 1 bar is not linearly dependent on the surface area from wider pores, the latter plays an important role in hydrogen adsorption. To some extent, the large surface area compensates for the lack of ultra-micropores and enhances the hydrogen adsorption capacity of NAC-1.5-700 (Figs. 4 and 5). Sevilla et al. [18] also reported compensatory effect of higher surface area and larger pores of activated carbons for hydrogen adsorption. Using the analysis of variance test (ANOVA), Tellez-Juarez et al. [29] reported that the surface area controls hydrogen adsorption at 77 K and higher pressure. From the above discussion and Figs. 3e5 it could be concluded that for activated carbons at 77 K, ultramicropores are the most critical factor for hydrogen storage at low pressure and the total surface area is responsible for hydrogen adsorption at higher pressure. The excellent hydrogen uptake of NAC-1.5-600 was compared with the hydrogen adsorption capacities of ACs originating from natural and synthetic precursors. Literature data for ACs that yielded the best known performances under identical conditions are listed in Table 1. To the best of our knowledge, the hydrogen adsorption capacity of NAC-1.5-600 at 77 K and 1 bar is one of the highest uptakes reported so far, for any synthetic material-derived activated carbon (Table 1). The hydrogen adsorption isotherm (Fig. 3) of the non-activated CP-600 carbon at 77 K leveled off at a low pressure of 100 mmHg, whereas the hydrogen uptake of activated carbons showed a steady increase up to 1 bar. The hydrogen uptake for CP-600 is attributed to the presence of ultra-micropores (<0.364 nm), which are accessible to only a single layer of hydrogen molecules. The critical pore diameter required for more than one layer of hydrogen adsorption should be ca. 0.56 nm (double of the kinetic diameter of hydrogen molecule i.e. 0.28 nm). The steady increase in hydrogen adsorption of activated carbons indicated that higher uptakes were expected at higher pressures. This behavior could be explained by the development of pores with hierarchical diameters (Fig. 2), in the ACs during the chemical activation. In contrast, the porosity in CP-600 was almost entirely due to ultra-micropores with diameters less than 0.364 nm.
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Fig. 6 presents the hydrogen adsorption capacities of activated carbons as a function of nitrogen, oxygen and hydrogen content. The hydrogen uptake increased with increasing activation temperature up to 600 C, then decreased thereafter, whereas the nitrogen, oxygen and hydrogen content decreased steadily. This indicates that hydrogen adsorption at 77 K does not depend on the occurrence of foreign elements in the activated carbon. Furthermore, instead of the total nitrogen content, Fig. 7 shows the hydrogen uptake versus the content of different nitrogen species as determined by XPS (Table 3). It is seen that the content of all nitrogen species (pyridine, AreNH2 (or eC]NH), pyrrole, and quaternary nitrogen) decreased steadily with increasing activation temperature, whereas the hydrogen uptake increased, then leveled off. Thus, Figs. 6 and 7 show that none of the nitrogen species nor the total nitrogen content correlates with the hydrogen uptake. This indicates that nitrogen content irrespective of N-profile, is not a significant factor for hydrogen adsorption at 77 K. Although the effect of surface composition on hydrogen adsorption by activated carbon has been a matter of heated debate, often with contradictory statements [15,26e32], our results are in line with Zhao et al. [27] finding that the nitrogen content is not a relevant parameter for hydrogen adsorption at 77 K. For practical applications, in addition to high hydrogen adsorption capacity, the adsorbents should have high stability, low energy requirement for regeneration and high tolerance to recycling. To show the regeneration and recyclability of the current carbon adsorbents, we performed four runs of hydrogen adsorption/desorption measurements for NAC-1.5-600 (Fig. S4) and obtained almost identical isotherms without any significant decrease in hydrogen uptake (less than 0.5% loss per cycle), indicating that this material exhibits excellent regenerability and recyclability.
4. Conclusion High nitrogen containing ultra-microporous activated carbon with different pore widths have been prepared and investigated for hydrogen adsorption at 77 K and up to 1 bar. NAC-1.5-600 showed a hydrogen uptake of 2.96 wt%, which is the highest uptake reported so far, for any synthetic material-derived activated carbon. The high uptake for NAC-1.5-600 was attributed to the presence of large pore volume from optimized pores with a diameter of 0.59 nm. From the results, it is also inferred that the ultra-micropore volume primarily
Fig. 6. Hydrogen uptake and elemental composition as a function of activation temperature at KOH/CP-400 ¼ 1.5. (A color version of this figure can be viewed online.)
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G. Sethia, A. Sayari / Carbon 99 (2016) 289e294
Fig. 7. Hydrogen uptake and N-profile as a function of activation temperature at KOH/ CP-400 ¼ 1.5. (A color version of this figure can be viewed online.)
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