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Microporous activated carbon aerogels via a simple subcritical drying route for CO2 capture and hydrogen storage
Microporous and Mesoporous Materials 179 (2013) 151–156
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Microporous activated carbon aerogels via a simple subcritical drying route for CO2 capture and hydrogen storage Calum Robertson, Robert Mokaya ⇑ School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
a r t i c l e
i n f o
Article history: Received 22 April 2013 Received in revised form 29 May 2013 Accepted 31 May 2013 Available online 6 June 2013 Keywords: Activated carbon aerogel Subcritical drying Microporous CO2 capture Hydrogen storage
a b s t r a c t The successful synthesis of carbon aerogels, via a simple subcritical drying route and subsequent activation to high surface area carbons with attractive properties for gas storage is demonstrated. The route generates highly microporous carbon aerogel with a surface area of 508 m2/g and pore volume of 0.68 cm3/g wherein micropores account for 80% (407 m2/g) of surface area. The carbon aerogel is dominated by micropores of size <15 Å with a broad distribution of pores centered at 8 and 12 Å. Chemical activation of the carbon aerogel with KOH generates activated carbon aerogels with surface area of 915–1980 m2/g and pore volume up to 2.03 cm3/g. Activation at 600, 700 or 800 °C (and KOH carbon ratio of 2, 4 or 5) yields activated carbon aerogels with micropore size distribution centred at ca. 8 and 13 Å (i.e., similar to that of the starting carbon aerogel) but with a large increase in pore volume arising from the micropores with the effect that pores of size <15 Å already present in the starting CA aerogel are retained and enhanced in the activated carbon aerogels; the proportion of microporosity rises from 80% to 87%. The activated carbon aerogels exhibit high CO2 uptake of 2.7–3.0 mmol/g at 25 °C and 1 bar, and store between 3.5 and 4.3 wt% hydrogen at 196 °C and 20 bar. The hydrogen storage density of the carbons is high (up to 16.2 lmol H2 m 2) with small micropores favouring high density. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The storage of energy related gases in porous materials is currently an area of intense research interest as typified by the need for sequestration of CO2, which is generated by the burning of fossil fuels, and the drive towards cleaner energy sources such as hydrogen [1–3]. The development of solid state adsorbents that can efficiently sequester polluting green house gases such as CO2 would certainly convey economical and environmental benefits and contribute to reduction in our reliance on fossil fuels [1–4]. There is intense focus on developing simple and cheap synthesis methodologies that deliver highly porous materials with appropriate properties for use as solid state gas stores [2]. In this regard, porous carbons are attractive due to their chemical inertness, versatility and low density. Carbon aerogels, in particular, are porous materials with a low bulk density and a high internal surface area. Carbon aerogels may be derived from a variety of precursors but are typically prepared via three sequential steps; (i) sol–gel polymerization of molecular precursors into an organic gel, (ii) drying of the organic gel and (iii) carbonization of the organic gel to generate the final carbon aerogel. The sol–gel polymerization (of for example resorcinol with formaldehyde) step typically occurs in
⇑ Corresponding author. E-mail address: [email protected] (R. Mokaya). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.05.025
aqueous solution wherein molecular reagents combine to transform into cross-linked organic gels. The conditions of the sol–gel polymerisation, such as concentration of molecular precursors, the pH and curing time and temperature can vary the structure of the organic gel. During the sol–gel step, the process of curing and gelation is important as it enables covalent cross-linking between precursor polymer chains, which allow solidification of the gel once the solvent is removed. In the case of resorcinol–formaldehyde (RF) derived gels, the curing or ‘‘aging’’ step involves placing the solidifying gel in dilute acid, which promotes the crosslinking by increasing the rate of the gel condensation/polymerisation reaction. The organic gels derived from the sol–gel step are then supercritically dried (to aerogels) [5–8] or freeze-dried (to cryogels) [9–12] and then carbonised at medium to high temperature under inert conditions to generate the final carbon aerogel. The use of special drying conditions (supercritical or freeze drying) in the preparation of carbon aerogels is considered necessary so as to allow retention of porosity in the organic gels while conventional air-drying is thought to lead to collapse of the weak gel network, which on carbonization generates lowly porous carbons [13–18]. It is thought that air-drying is unfavorable due to the formation of a liquid–vapor meniscus arising from surface tension of the liquid, which causes a build up of capillary pressure within the pores of the gel and a collapse of the pores and porosity due to inherent weakness of the tenuous links within the gel. The
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disadvantage of having to use drying techniques such as supercritical drying is that it is cumbersome and requires specialised equipment which necessarily increases the cost of preparing the carbon aerogels. It is therefore desirable to develop synthesis processes to carbon aerogels that avoid the use of supercritical drying methods but without compromising porosity. On the other hand, activation of low porosity carbons may be used to generate highly porous carbon materials. There are two main activation routes; physical activation, which involves subjecting the carbon precursor to high temperature in the presence of an oxidizing gas, such as carbon dioxide [19] or steam, [20] while chemical activation involves the impregnation of an activating reagent onto a carbon precursor, which is then heated to high temperature in an oxygen free environment. In chemical activation, the precursors are mixed with chemical activating agents such as KOH, NaOH or ZnCl2 etc., and then subjected to heat treatment in inert atmosphere at various temperatures [21–26]. Here we report on a very simple method for the formation of carbon aerogels and their subsequent activation to generate carbon materials with very attractive CO2 storage capacity. As far as we are aware the use of such activated carbon aerogels for CO2 storage has not yet been explored. In particular, we simplify the carbon aerogel synthesis route by using subcritical drying under ambient conditions wherein no extra drying steps [27–30] or sol–gel additives or modifications are needed [31–34]. We also report on the hydrogen uptake capacity of the carbon aerogels and discuss the effect of porosity and in particular microporosity on gas uptake. 2. Experimental 2.1. Material synthesis The carbon aerogels were synthesized using a simplified method as follows; [15] 12.3 g of resorcinol and 17.9 g of formaldehyde were added to 15 ml of water under stirring, followed by the addition of 0.44 g (0.007 mol) of glacial acetic acid. The resulting mixture was heated at 80 °C for 72 h during which time a gel was formed. The gel was washed with acetone and dried overnight in an oven at 100 °C. Finally the gel was carbonized at 1050 °C under nitrogen for 3 h to generate the carbon aerogel designated as CA. For the chemical activation, the CA carbon aerogel was thoroughly mixed with KOH at carbon/KOH weight ratio of 1/4. The mixture was then heat treated in a horizontal furnace under a nitrogen flow at 700, 800 or 900 °C for 1 h with a heating ramp rate of 3 °C/min. In all cases, the resulting mixture was washed three times with 2 M HCl at room temperature to remove any inorganic salts, and then with distilled water until neutral pH was achieved. Finally, the resultant activated carbon aerogel was dried in an oven at 120 °C for 3 h. The activated carbon aerogels were denoted as 14ACA-X, where X is the activating temperature. We also prepared two more samples activated at 800 °C and a carbon/KOH weight ratio of 1/2 (designated as 12ACA-800) or 1/5 (designated as 15ACA-800). 2.2. Material characterization Thermogravimetric analysis was performed using a TA Instruments SDT Q600 analyzer under flowing air conditions. For porosity analysis, each sample was pre-dried in an oven and then degassed overnight at 200 °C under high vacuum. The textural properties were determined by nitrogen sorption at 196 °C using a Micromeritics ASAP 2020 volumetric sorptometer. The surface area was calculated by using the BET method applied to adsorption data in the relative pressure (P/P0) range of 0.06–0.22. The total pore volume was determined from the amount of nitrogen
adsorbed at P/P0 = 0.99. The micropore surface area and micropore volume were determined via t-plot analysis. The pore size distribution was determined by a non-local density functional theory (NLDFT) method using nitrogen adsorption isotherms. CO2 uptake was measured using a TA Instruments SDT Q600 analyzer at atmospheric pressure (1.0 atm). Prior to uptake measurements, the carbon samples were heated to 250 °C under static nitrogen and then cooled to 25 °C under a flow of nitrogen. The samples were then purged with CO2 (50 mL min 1) for 3 h, and then the CO2 uptake was determined repeatedly at 25 °C. Between measurements, desorption of CO2 was undertaken by heating the sample to 250 °C under nitrogen gas flowing at the rate of 100 mL min 1. Hydrogen uptake measurements were performed using high-purity hydrogen (99.9999%) over the pressure range of 0–20 bar with an intelligent gravimetric analyzer (IGA-003, Hiden). The carbon samples were dried in an oven for 24 h at 80 °C overnight and then placed in the analysis chamber and degassed at 200 °C and 10 10 bar for 4–6 h before analysis. Then the hydrogen uptake isotherms were measured at 196 °C (under a liquid nitrogen bath). The hydrogen uptake was corrected for buoyancy effect with carbon density of 1.5 g/cm3 and hydrogen density of 0.04 g/cm3.
3. Results/discussion 3.1. Porosity and textural properties The carbon aerogel (CA) in this study was synthesised from gellation and then carbonisation of a resorcinol–formaldehyde resin. The thermal stability and combustion properties of the starting CA carbon aerogel were evaluated using thermogravimetric analysis (TGA) in static air. The TGA curve is shown in Fig. 1. The CA carbon aerogel is stable at temperatures under 400 °C; minor mass loss observed below 120 °C is ascribed to evaporation of adsorbed water. A significant mass loss step occurs in the temperature range 400–600 °C, which is the combustion of carbon. The CA carbon aerogel is completely burnt off in air leaving no residual mass. Fig. 1 also shows the TGA curves of the carbon aerogel following activation. The thermal properties of the activated carbon aerogels are broadly similar to that of CA, except that they appear to start burning off at slightly lower temperature and thus have a broader combustion temperature range. As the extent of activation increases (i.e., increase in KOH/carbon ratio at constant temperature), the carbon burn off temperature slightly shifts to higher values and the burn off temperature zone reduces indicating that the nature of carbon in the samples becomes more uniform with respect to thermal stability. The porosity of the carbon aerogel sample before and after activation was probed using nitrogen sorption analysis. The nitrogen adsorption–desorption isotherms are shown in Fig. 2 and the textural properties are summarised in Table 1. The isotherm of the CA sample is type I and exhibits virtually no hysteresis between adsorption and desorption branches at relative pressure up to P/ P0 = 0.9. The type I nature of the isotherm, with significant nitrogen uptake at relative pressure (P/P0) below 0.1, indicates that the subcritical drying route generated a carbon aerogel with a mainly microporous structure. However, the isotherm of sample CA also shows some significant adsorption at P/P0 above 0.9, which we attribute to the presence of larger mesopores and macropores that are typical for carbon aerogels. Sample CA has a surface area of 508 m2/g and pore volume of 0.68 cm3/g, which is higher or comparable to textural values for similar materials [35]. The surface area and pore volume generated by the subcritical drying is similar to that known for carbon aerogels prepared via more complicated routes that include supercritical drying [5–8]. The pore size
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100 CA 14ACA-700 14ACA-800 14ACA-900
Mass (%)
80
60
40
20
0 0
200
400
600
Temperature (°C) 100
12ACA-800 14ACA-800 15ACA-800
Mass (%)
80
60
40
20
0 0
200
400
600
800
Temperature (°C) Fig. 1. Thermal gravimetric (TGA) curves of carbon aerogel (CA) before and after activation. See experimental section for sample designation.
Volume adsorbed (cm3/g, STP)
1400 CA 12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
1200 1000 800 600 400 200 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po) Fig. 2. Nitrogen sorption isotherms of carbon aerogel (CA) before and after activation. See experimental section for sample designation.
distribution of sample CA, determined via a DFT model using nitrogen adsorption data, is shown in Fig. 3 (and Supporting Fig. S1); Fig. 3 indicates the presence of micropores of size between 5 and 16 Å with pore size maxima at 8 and 12 Å. This indicates that the subcritically prepared carbon aerogel is predominantly
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microporous, which is confirmed by the high micropore surface area of 407 m2/g representing 80% of the total surface area of 508 m2/g. It is interesting that sample CA possesses no pores in the size range 16–100 Å (Fig. 3 and Supporting Fig. S1). However, as expected for a low density carbon aerogel, sample CA has a significant proportion of pores of size above 100 Å (Supporting Fig. S1). The presence of these larger pores contributes to a high pore volume of 0.68 cm3/g wherein the micropore volume at 0.20 cm3/g is only 30% of the total pore volume. Thus subcritical drying favours the dominant formation of micropores and some larger mesopores and macropores compared to supercritical drying wherein microporosity is not dominant and there is a broader range of pores [5–8]. The overall picture that emerges from the porosity of the CA aerogel is that subcritical drying engenders the formation of micropores and larger mesopores and macropores, but with microporosity dominating the surface area available. We have previously shown that activation of carbonaceous materials that already possess significant microporosity is an excellent way of generating high surface area activated carbons with a high proportion of microporosity [36,37]. This has already been demonstrated for carbide-derived carbons [36] and zeolite-templated carbons [37] wherein activation enhanced the microporosity associated with already existing pores. The present CA carbon aerogel is therefore an excellent candidate for activation to highly microporous activated carbons given that it already possesses a high proportion of micropores of size between 5 and 16 Å (Fig. 3). The nitrogen sorption isotherms of the activated carbon aerogels are shown in Fig. 2 and indicate that KOH activation leads to activated aerogel materials that, compared to the starting CA aerogel, retain the general shape of the isotherm but with a significant increase in the amount of nitrogen adsorbed. The amount of nitrogen adsorbed at low relative pressure (i.e., P/P0 below 0.05) increases after activation. The general shape of the isotherm, with a plateau at P/P0 between 0.05 and 0.9 and sharp sorption ‘knee’ at P/P0 below 0.05, is retained for all activated aerogels except for sample 14ACA-900, which was activated at 900 °C. This indicates that the microporosity of the CA aerogel is retained and enhanced after activation. The isotherm of sample 14ACA-900 has a slight broadening of the sorption ‘knee’, which extends up to P/P0 = 0.25. The slight widening of the isotherm ‘knee’ to cover the P/P0 range between 0.05 and 0.25 is an indication of the creation of large micropores and mesopores in sample 14ACA-900. On the other hand, the isotherms of all the activated carbons indicate higher adsorption at P/P0 above 0.9 compared to the starting CA aerogel, which is typical of activated carbon aerogels [5–8]. Overall, however, it is clear from the nitrogen sorption isotherms that the activated carbon aerogels are still predominantly microporous. Thus the aim of increasing the microporosity in the subcritically dried CA aerogel is achieved in the activated aerogels. The pore size distribution of all the activated carbon aerogels is given in Fig. 3 and the pore size is summarised in Table 1. Similar to the starting CA aerogel, all the activated carbon aerogels (except for sample 14ACA-900) possess a broad distribution of micropores centred at ca. 8 and 13 Å. Sample 14ACA-900, in addition to pores centred at ca. 8 and 13 Å, also possesses larger 24 Å pores. In all cases, KOH activation results in a significant increase in pore volume arising from micropores of size ca. 8 and 13 Å, and in particular the 8 Å pores. Thus the micropores of size <15 Å already present in the starting CA aerogel are retained and enhanced in the activated carbon aerogels. The pore size of the pores centred at ca. 13 Å is, however, slightly broadened with a small proportion of ‘new’ pores in the range 15–20 Å (for sample 12ACA-800 and 14ACA-700) and 15–30 Å (for sample 14ACA-800 and 15ACA800). On the other hand, sample 14ACA-900 possesses a significant proportion of ‘new’ pores of size 18–45 Å (centered at ca. 24 Å). As
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Table 1 Textural properties and gas (CO2 and hydrogen) uptake capacity of carbon aerogels before (CA) and after (ACA-X) activation with KOH.
a b c
Sample
Surface area (m2/g)a
Pore volume (cm3/g)b
Pore size (Å)c
CA 14ACA-700 14ACA-800 14ACA-900 12ACA-800 15ACA-800
508 (407) 1083 (947) 1871 (1622) 1980 (1522) 915 (788) 1521 (1312)
0.68 1.22 2.03 1.92 0.99 0.99
8/12 8/13 8/13 8/13/24 8/13 8/13
(0.20) (0.45) (0.77) (0.72) (0.37) (0.62)
CO2 uptake (mmol/g)
CO2 uptake density (lmol/m2)
H2 uptake (wt%)
H2 uptake density (lmol/m2)
3.0 3.0 2.7 2.7 3.0
2.77 1.60 1.36 2.95 1.97
3.5 3.6 4.3 2.7 3.8
16.2 9.6 10.9 14.8 12.5
Values in parenthesis are micropore surface area. Values in parenthesis are micropore volume. Pore size maxima from NLDFT pore analysis.
microporosity. The proportion (80%) of micropore surface area in the starting CA aerogel is retained for sample 14ACA-900 (77%) or improved to 87% for all the other activated carbon aerogels. The proportion of micropore volume (30% for the starting CA aerogel) increases to at least 37% for all the activated carbons. The high proportion of microporosity in the activated carbon aerogels makes them interesting as potential candidates for storage of gases such as CO2 and hydrogen as discussed below.
0.04
Pore Volume (cm3/g)
CA
12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
0.03
0.02
3.2. Gas uptake 0.01
0.00 0
10
20
30
40
50
60
Pore Size (Å) Fig. 3. Pore size distribution curves of carbon aerogel (CA) before and after activation. See experimental section for sample designation.
shown in Supporting Fig. S1, there is also an increase in pore volume arising from pores of size >100 Å after activation. The pore size data suggests that the main effect of KOH activation on the CA aerogel is chiefly to increase the amount of already existing micropores of size between 5 and 15 Å, and larger mesopores of size above 100 Å. Only in sample 14ACA-900 is there a significant formation of ‘new’ pores of size between 15 and 45 Å. This rather differs from the activation of other porous carbons such as carbon-derived carbons [36] and zeolite-templated carbons [37] where there is retention of existing pores but a much greater formation of new larger pores or the activation of disordered carbons or mesoporous carbon where KOH activation mainly generates pores of size <1.5 nm with little or no retention of pre-existing pores [38]. In all cases, the activation process leads to an increase in surface area and pore volume. The surface area rises from 508 m2/g for the CA aerogel to between 915 and 1980 m2/g for the activated carbon aerogels, representing increase of between 80 and 290%. The total pore volume rises from 0.68 cm3/g, by between 46 and 200%, to be in the range 0.99–2.03 cm3/g. The largest increase (290%) in surface area from 508 m2/g to 1980 m2/g is observed for sample 14ACA900, which was activated at the highest temperature, while the lowest increase in surface area and pore volume is observed for sample 12ACA-800 (activated at 800 °C at lower KOH/carbon ratio of 2). It is thus clear that the extent of increase in surface area and pore volume is dependent on the level of activation (i.e., temperature and KOH/carbon ratio). What is however remarkable is that the activated carbon aerogels retain a very high proportion of
The CO2 uptake capacity of the activated carbon aerogels was determined at atmospheric pressure and 25 °C under flowing gas (pure CO2) conditions. Table 1 summaries the amount of CO2 captured at 25 °C, and in all cases the activated carbon aerogels show high uptake of between 2.7 and 3.0 mmol/g. CO2 uptake of ca. 3 mmol/g is on the higher end when compared to previous reports on similar carbonaceous materials [3,39–53]. The relatively similar uptake despite the large variation in textural properties indicates that total surface area and pore volume are not the critical factors in determining CO2 adsorption. The CO2 uptake density (Table 1) reveals that samples that predominantly or exclusively possess smaller pores (<15 Å) exhibit the greatest uptake per unit surface area. The porosity of sample 14ACA-700 and 12ACA-800 with a CO2 uptake density of 2.77 and 2.95 lmol/m2 respectively, is dominated by pores of size 8 and 13 Å; these two carbons do not possess any ‘small’ pores larger than 20 Å. As the presence and proportion of pores in the size range 20–45 Å increases (Fig. 3), the CO2 uptake density decreases to a low of 1.36 lmol/m2 for sample 14ACA900. However, all the activated carbon aerogels possess enough pores in the size range <15 Å and particularly <10 Å), which ensures an attractively high CO2 uptake of ca. 3 mmol/g. The attraction of the present activated carbon aerogels is the simplicity of their formation and a high CO2 uptake of 3 mmol/g. It is worthwhile to note that, unlike the present carbon aerogels, most of porous carbon materials that have so far been reported to have high CO2 uptake contain nitrogen as a dopant [41–47]. The presence of nitrogen is thought to generate basic sites onto which CO2 molecules are readily adsorbed thus increasing overall uptake [41–47]. Nevertheless, despite being N-free, our activated aerogels have uptake that is comparable or similar to that of nitrogen enriched carbons. For example, nitrogen-doped carbons derived from melamine–formaldehyde resins adsorb 2.25 mmol/g of CO2 under conditions similar to those utilised in this study, [48] while glucose-derived amine-rich carbons manage an uptake of only of 1 mmol/g under similar ambient conditions [49]. The CO2 uptake of our N-free activated carbon aerogels is similar to the 3.1 mmol/ g (under flow conditions at 25 °C and 1 bar) achieved by N-doped carbon monoliths derived from resorcinol–formaldehyde as carbon precursor (with amino acid L-lysine as catalyst) [44]. Our samples, thus highlight the importance of microporosity and pore size in
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4
H2 uptake (wt%)
determining CO2 uptake of carbon materials. Similar trends that highlight the importance of microporosity in determining CO2 uptake have previously been demonstrated for some N-free carbons [50–53]. Sevilla and co-workers obtained uptake of up to 4.8 mmol/g (at 1 bar and 25 °C) for biomass-derived activated carbons that possessed a large number of narrow micropores of size <10 Å [51]. On the other hand, based on the analysis of narrow micropore size distribution of several activated carbons prepared under different conditions, Wei and co-workers suggested a linear relationship between the volume of micropores in the range 3.3– 8.2 Å and CO2 adsorption at 0 °C and 1 bar, and that narrow micropores of ca. 5.5 Å contributed most, [52] while Gogotsi and co-workers investigated a variety of carbide-derived carbons and observed a linear correlation between pore volume and CO2 uptake at a 1 bar and that pores smaller than 8 Å contributed the most to the CO2 uptake [53]. The CO2 uptake as a function of time is shown in Fig. 4. The rate of uptake proceeds rapidly in the first few minutes and then gradually slows to a smooth increase as it approaches equilibrium. The maximum uptake is attained in ca. 60 min, which is comparable to what has previously been observed for porous carbons under similar conditions [48]. The sample with the largest pores (14ACA-900) appears to have the slowest uptake rate while smaller pore samples (e.g., 12ACA-800) have faster kinetics even though the maximum uptake is similar for both samples. This suggests that smaller micropores favour faster uptake of CO2. The hydrogen uptake isotherms of all the activated carbon aerogels are shown in Fig. 5 and the hydrogen storage capacity at 20 bar is summarized in Table 1. All samples have uptake isotherms that are completely reversible with no hysteresis, and no saturation is achieved even at 20 bar, suggesting that higher hydrogen adsorption capacity can be obtained at higher pressures. The hydrogen uptake of the activated carbon aerogels at 196 °C and 20 bar (2.7–4.3 wt%) is consistent with their surface area [54]. The hydrogen uptake is, as expected, highest for samples with the largest surface area; the highest hydrogen storage capacity (4.3 wt% at 20 bar) was observed for the activated carbon aerogel (14ACA-900) that has the highest surface area, while sample 12ACA-800 with the lowest surface area of 915 m2/g had the least hydrogen uptake of 2.7 wt%. A plot of the hydrogen uptake (at 20 bar) as a function of surface area (Supporting Fig. S2) shows that the uptake generally increases with surface area although the relationship is not linear. For example it appears that sample 14ACA800 has a lower than expected hydrogen uptake. A closer inspection of the relationship between surface area and hydrogen uptake
3
2 12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
1
0 0
5
10
15
20
Pressure (bar) Fig. 5. Hydrogen uptake at -196 °C for activated carbon aerogels prepared at various KOH/carbon ratio and activation temperature. See experimental section for sample designation.
(Supporting Fig. S2) reveals that the hydrogen storage capacity of the activated carbon aerogels does not fit into the Chahine rule (i.e., 1 wt% hydrogen stored per 500 m2/g of carbon) [55], due to higher than expected uptakes per given surface area. The hydrogen uptake density of the activated carbon aerogels given in Table 1 varies between 9.6 and 16.2 lmol H2 m 2. All the activated carbon aerogels, expect for sample 14ACA-900, outperform the Chahine rule (i.e., 1 wt% per 500 m2/g of carbon corresponding to a density of 10 lmol H2 m 2). We have previously observed similar behaviour for KOH activated carbide-derived carbons, [36] KOH activated zeolite-templated carbons, [37] and biomass-derived activated carbons [21]. It is clear that the hydrogen storage density of the activated carbon aerogels depends on the proportion of pores in the lower micropore range. Small micropores of size ca. 8 Å are known to be the most effective for hydrogen storage [56–63]. The large mesopores and macropores of the activated aerogels (Supporting Fig. S1) do not contribute much to the hydrogen storage. This is supported by the fact that samples 14ACA-700 and 12ACA-800, which have no pores larger than 20 Å have the highest storage density of 16.2 and 14.8 H2 m 2 respectively.
4. Conclusion
CO2 uptake (mmol/g)
3.0 2.5 2.0 1.5 12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
1.0 0.5 0.0 0
20
40
60
80
100
120
140
160
180
Time (minutes) Fig. 4. CO2 uptake at 25 °C and 1 bar under flowing gas conditions for activated carbon aerogels prepared at various KOH/carbon ratio and activation temperature. See experimental section for sample designation.
We have demonstrated the successful synthesis of carbon aerogels via a simple subcritical drying route and their subsequent activation to high surface area carbons with attractive gas storage capacity. The subcritical drying method, wherein an acetone washed gel is dried overnight at 100 °C prior to carbonisation, is simple and avoids the more cumbersome conventional supercritical or freeze drying routes. A highly microporous carbon aerogel with a surface area of 508 m2/g and pore volume of 0.68 cm3/g wherein micropores contribute 80% of the surface area (i.e., micropore surface area of 407 m2/g) is generated. The surface area of the carbon aerogel is dominated by micropores of size <15 Å with a broad micropore size distribution centered at 8 and 12 Å. Chemical activation of the carbon aerogel with KOH generates activated carbon aerogels with surface area between 915 and 1980 m2/g and pore volume of up to 2.03 cm3/g with the extent of the increase in textural properties dependent on the KOH/carbon ratio and activation temperature. Activation at 600, 700 or 800 °C (and KOH carbon ratio of 2, 4 or 5) generates activated carbon aerogels with micropore size distribution centred at ca. 8 and 13 Å, which is similar to that of the starting carbon aerogel, but with a large increase in pore volume arising from
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the 8 and 13 Å micropores. Thus the micropores of size <15 Å already present in the starting CA aerogel are retained and enhanced (with respect to their pore volume and surface area) in the activated carbon aerogels. The pore size data therefore suggests that the main effect of KOH activation is chiefly to increase the amount of already existing micropores of size less than 15 Å. Remarkably, the proportion of microporosity in the activated carbon aerogels increases from an already high of 80–87%. All the activated carbon aerogels exhibit attractive CO2 uptake of between 2.7 and 3.0 mmol/g at 25 °C and 1 bar under flowing pure CO2 conditions. The activated carbon aerogels also store between 3.5 and 4.3 wt% hydrogen at -196 °C and 20 bar, and the amount of hydrogen adsorbed correlates well with the surface area. The hydrogen storage density of the carbons is high and ranges between 9.6 and 16.2 lmol H2m 2 with the presence of small micropores favouring high density. Taking into account the gas storage capacities of these materials, as well as the simplicity of their synthesis procedure, it can be concluded that the activated carbon aerogels constitute promising adsorbents for CO2 and hydrogen. Acknowledgement This research was funded by the University of Nottingham. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2013. 05.025. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Microporous activated carbon aerogels via a simple subcritical drying route for CO2 capture and hydrogen storage Calum Robertson, Robert Mokaya ⇑ School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom
a r t i c l e
i n f o
Article history: Received 22 April 2013 Received in revised form 29 May 2013 Accepted 31 May 2013 Available online 6 June 2013 Keywords: Activated carbon aerogel Subcritical drying Microporous CO2 capture Hydrogen storage
a b s t r a c t The successful synthesis of carbon aerogels, via a simple subcritical drying route and subsequent activation to high surface area carbons with attractive properties for gas storage is demonstrated. The route generates highly microporous carbon aerogel with a surface area of 508 m2/g and pore volume of 0.68 cm3/g wherein micropores account for 80% (407 m2/g) of surface area. The carbon aerogel is dominated by micropores of size <15 Å with a broad distribution of pores centered at 8 and 12 Å. Chemical activation of the carbon aerogel with KOH generates activated carbon aerogels with surface area of 915–1980 m2/g and pore volume up to 2.03 cm3/g. Activation at 600, 700 or 800 °C (and KOH carbon ratio of 2, 4 or 5) yields activated carbon aerogels with micropore size distribution centred at ca. 8 and 13 Å (i.e., similar to that of the starting carbon aerogel) but with a large increase in pore volume arising from the micropores with the effect that pores of size <15 Å already present in the starting CA aerogel are retained and enhanced in the activated carbon aerogels; the proportion of microporosity rises from 80% to 87%. The activated carbon aerogels exhibit high CO2 uptake of 2.7–3.0 mmol/g at 25 °C and 1 bar, and store between 3.5 and 4.3 wt% hydrogen at 196 °C and 20 bar. The hydrogen storage density of the carbons is high (up to 16.2 lmol H2 m 2) with small micropores favouring high density. Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction The storage of energy related gases in porous materials is currently an area of intense research interest as typified by the need for sequestration of CO2, which is generated by the burning of fossil fuels, and the drive towards cleaner energy sources such as hydrogen [1–3]. The development of solid state adsorbents that can efficiently sequester polluting green house gases such as CO2 would certainly convey economical and environmental benefits and contribute to reduction in our reliance on fossil fuels [1–4]. There is intense focus on developing simple and cheap synthesis methodologies that deliver highly porous materials with appropriate properties for use as solid state gas stores [2]. In this regard, porous carbons are attractive due to their chemical inertness, versatility and low density. Carbon aerogels, in particular, are porous materials with a low bulk density and a high internal surface area. Carbon aerogels may be derived from a variety of precursors but are typically prepared via three sequential steps; (i) sol–gel polymerization of molecular precursors into an organic gel, (ii) drying of the organic gel and (iii) carbonization of the organic gel to generate the final carbon aerogel. The sol–gel polymerization (of for example resorcinol with formaldehyde) step typically occurs in
⇑ Corresponding author. E-mail address: [email protected] (R. Mokaya). 1387-1811/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2013.05.025
aqueous solution wherein molecular reagents combine to transform into cross-linked organic gels. The conditions of the sol–gel polymerisation, such as concentration of molecular precursors, the pH and curing time and temperature can vary the structure of the organic gel. During the sol–gel step, the process of curing and gelation is important as it enables covalent cross-linking between precursor polymer chains, which allow solidification of the gel once the solvent is removed. In the case of resorcinol–formaldehyde (RF) derived gels, the curing or ‘‘aging’’ step involves placing the solidifying gel in dilute acid, which promotes the crosslinking by increasing the rate of the gel condensation/polymerisation reaction. The organic gels derived from the sol–gel step are then supercritically dried (to aerogels) [5–8] or freeze-dried (to cryogels) [9–12] and then carbonised at medium to high temperature under inert conditions to generate the final carbon aerogel. The use of special drying conditions (supercritical or freeze drying) in the preparation of carbon aerogels is considered necessary so as to allow retention of porosity in the organic gels while conventional air-drying is thought to lead to collapse of the weak gel network, which on carbonization generates lowly porous carbons [13–18]. It is thought that air-drying is unfavorable due to the formation of a liquid–vapor meniscus arising from surface tension of the liquid, which causes a build up of capillary pressure within the pores of the gel and a collapse of the pores and porosity due to inherent weakness of the tenuous links within the gel. The
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disadvantage of having to use drying techniques such as supercritical drying is that it is cumbersome and requires specialised equipment which necessarily increases the cost of preparing the carbon aerogels. It is therefore desirable to develop synthesis processes to carbon aerogels that avoid the use of supercritical drying methods but without compromising porosity. On the other hand, activation of low porosity carbons may be used to generate highly porous carbon materials. There are two main activation routes; physical activation, which involves subjecting the carbon precursor to high temperature in the presence of an oxidizing gas, such as carbon dioxide [19] or steam, [20] while chemical activation involves the impregnation of an activating reagent onto a carbon precursor, which is then heated to high temperature in an oxygen free environment. In chemical activation, the precursors are mixed with chemical activating agents such as KOH, NaOH or ZnCl2 etc., and then subjected to heat treatment in inert atmosphere at various temperatures [21–26]. Here we report on a very simple method for the formation of carbon aerogels and their subsequent activation to generate carbon materials with very attractive CO2 storage capacity. As far as we are aware the use of such activated carbon aerogels for CO2 storage has not yet been explored. In particular, we simplify the carbon aerogel synthesis route by using subcritical drying under ambient conditions wherein no extra drying steps [27–30] or sol–gel additives or modifications are needed [31–34]. We also report on the hydrogen uptake capacity of the carbon aerogels and discuss the effect of porosity and in particular microporosity on gas uptake. 2. Experimental 2.1. Material synthesis The carbon aerogels were synthesized using a simplified method as follows; [15] 12.3 g of resorcinol and 17.9 g of formaldehyde were added to 15 ml of water under stirring, followed by the addition of 0.44 g (0.007 mol) of glacial acetic acid. The resulting mixture was heated at 80 °C for 72 h during which time a gel was formed. The gel was washed with acetone and dried overnight in an oven at 100 °C. Finally the gel was carbonized at 1050 °C under nitrogen for 3 h to generate the carbon aerogel designated as CA. For the chemical activation, the CA carbon aerogel was thoroughly mixed with KOH at carbon/KOH weight ratio of 1/4. The mixture was then heat treated in a horizontal furnace under a nitrogen flow at 700, 800 or 900 °C for 1 h with a heating ramp rate of 3 °C/min. In all cases, the resulting mixture was washed three times with 2 M HCl at room temperature to remove any inorganic salts, and then with distilled water until neutral pH was achieved. Finally, the resultant activated carbon aerogel was dried in an oven at 120 °C for 3 h. The activated carbon aerogels were denoted as 14ACA-X, where X is the activating temperature. We also prepared two more samples activated at 800 °C and a carbon/KOH weight ratio of 1/2 (designated as 12ACA-800) or 1/5 (designated as 15ACA-800). 2.2. Material characterization Thermogravimetric analysis was performed using a TA Instruments SDT Q600 analyzer under flowing air conditions. For porosity analysis, each sample was pre-dried in an oven and then degassed overnight at 200 °C under high vacuum. The textural properties were determined by nitrogen sorption at 196 °C using a Micromeritics ASAP 2020 volumetric sorptometer. The surface area was calculated by using the BET method applied to adsorption data in the relative pressure (P/P0) range of 0.06–0.22. The total pore volume was determined from the amount of nitrogen
adsorbed at P/P0 = 0.99. The micropore surface area and micropore volume were determined via t-plot analysis. The pore size distribution was determined by a non-local density functional theory (NLDFT) method using nitrogen adsorption isotherms. CO2 uptake was measured using a TA Instruments SDT Q600 analyzer at atmospheric pressure (1.0 atm). Prior to uptake measurements, the carbon samples were heated to 250 °C under static nitrogen and then cooled to 25 °C under a flow of nitrogen. The samples were then purged with CO2 (50 mL min 1) for 3 h, and then the CO2 uptake was determined repeatedly at 25 °C. Between measurements, desorption of CO2 was undertaken by heating the sample to 250 °C under nitrogen gas flowing at the rate of 100 mL min 1. Hydrogen uptake measurements were performed using high-purity hydrogen (99.9999%) over the pressure range of 0–20 bar with an intelligent gravimetric analyzer (IGA-003, Hiden). The carbon samples were dried in an oven for 24 h at 80 °C overnight and then placed in the analysis chamber and degassed at 200 °C and 10 10 bar for 4–6 h before analysis. Then the hydrogen uptake isotherms were measured at 196 °C (under a liquid nitrogen bath). The hydrogen uptake was corrected for buoyancy effect with carbon density of 1.5 g/cm3 and hydrogen density of 0.04 g/cm3.
3. Results/discussion 3.1. Porosity and textural properties The carbon aerogel (CA) in this study was synthesised from gellation and then carbonisation of a resorcinol–formaldehyde resin. The thermal stability and combustion properties of the starting CA carbon aerogel were evaluated using thermogravimetric analysis (TGA) in static air. The TGA curve is shown in Fig. 1. The CA carbon aerogel is stable at temperatures under 400 °C; minor mass loss observed below 120 °C is ascribed to evaporation of adsorbed water. A significant mass loss step occurs in the temperature range 400–600 °C, which is the combustion of carbon. The CA carbon aerogel is completely burnt off in air leaving no residual mass. Fig. 1 also shows the TGA curves of the carbon aerogel following activation. The thermal properties of the activated carbon aerogels are broadly similar to that of CA, except that they appear to start burning off at slightly lower temperature and thus have a broader combustion temperature range. As the extent of activation increases (i.e., increase in KOH/carbon ratio at constant temperature), the carbon burn off temperature slightly shifts to higher values and the burn off temperature zone reduces indicating that the nature of carbon in the samples becomes more uniform with respect to thermal stability. The porosity of the carbon aerogel sample before and after activation was probed using nitrogen sorption analysis. The nitrogen adsorption–desorption isotherms are shown in Fig. 2 and the textural properties are summarised in Table 1. The isotherm of the CA sample is type I and exhibits virtually no hysteresis between adsorption and desorption branches at relative pressure up to P/ P0 = 0.9. The type I nature of the isotherm, with significant nitrogen uptake at relative pressure (P/P0) below 0.1, indicates that the subcritical drying route generated a carbon aerogel with a mainly microporous structure. However, the isotherm of sample CA also shows some significant adsorption at P/P0 above 0.9, which we attribute to the presence of larger mesopores and macropores that are typical for carbon aerogels. Sample CA has a surface area of 508 m2/g and pore volume of 0.68 cm3/g, which is higher or comparable to textural values for similar materials [35]. The surface area and pore volume generated by the subcritical drying is similar to that known for carbon aerogels prepared via more complicated routes that include supercritical drying [5–8]. The pore size
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100 CA 14ACA-700 14ACA-800 14ACA-900
Mass (%)
80
60
40
20
0 0
200
400
600
Temperature (°C) 100
12ACA-800 14ACA-800 15ACA-800
Mass (%)
80
60
40
20
0 0
200
400
600
800
Temperature (°C) Fig. 1. Thermal gravimetric (TGA) curves of carbon aerogel (CA) before and after activation. See experimental section for sample designation.
Volume adsorbed (cm3/g, STP)
1400 CA 12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
1200 1000 800 600 400 200 0 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/Po) Fig. 2. Nitrogen sorption isotherms of carbon aerogel (CA) before and after activation. See experimental section for sample designation.
distribution of sample CA, determined via a DFT model using nitrogen adsorption data, is shown in Fig. 3 (and Supporting Fig. S1); Fig. 3 indicates the presence of micropores of size between 5 and 16 Å with pore size maxima at 8 and 12 Å. This indicates that the subcritically prepared carbon aerogel is predominantly
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microporous, which is confirmed by the high micropore surface area of 407 m2/g representing 80% of the total surface area of 508 m2/g. It is interesting that sample CA possesses no pores in the size range 16–100 Å (Fig. 3 and Supporting Fig. S1). However, as expected for a low density carbon aerogel, sample CA has a significant proportion of pores of size above 100 Å (Supporting Fig. S1). The presence of these larger pores contributes to a high pore volume of 0.68 cm3/g wherein the micropore volume at 0.20 cm3/g is only 30% of the total pore volume. Thus subcritical drying favours the dominant formation of micropores and some larger mesopores and macropores compared to supercritical drying wherein microporosity is not dominant and there is a broader range of pores [5–8]. The overall picture that emerges from the porosity of the CA aerogel is that subcritical drying engenders the formation of micropores and larger mesopores and macropores, but with microporosity dominating the surface area available. We have previously shown that activation of carbonaceous materials that already possess significant microporosity is an excellent way of generating high surface area activated carbons with a high proportion of microporosity [36,37]. This has already been demonstrated for carbide-derived carbons [36] and zeolite-templated carbons [37] wherein activation enhanced the microporosity associated with already existing pores. The present CA carbon aerogel is therefore an excellent candidate for activation to highly microporous activated carbons given that it already possesses a high proportion of micropores of size between 5 and 16 Å (Fig. 3). The nitrogen sorption isotherms of the activated carbon aerogels are shown in Fig. 2 and indicate that KOH activation leads to activated aerogel materials that, compared to the starting CA aerogel, retain the general shape of the isotherm but with a significant increase in the amount of nitrogen adsorbed. The amount of nitrogen adsorbed at low relative pressure (i.e., P/P0 below 0.05) increases after activation. The general shape of the isotherm, with a plateau at P/P0 between 0.05 and 0.9 and sharp sorption ‘knee’ at P/P0 below 0.05, is retained for all activated aerogels except for sample 14ACA-900, which was activated at 900 °C. This indicates that the microporosity of the CA aerogel is retained and enhanced after activation. The isotherm of sample 14ACA-900 has a slight broadening of the sorption ‘knee’, which extends up to P/P0 = 0.25. The slight widening of the isotherm ‘knee’ to cover the P/P0 range between 0.05 and 0.25 is an indication of the creation of large micropores and mesopores in sample 14ACA-900. On the other hand, the isotherms of all the activated carbons indicate higher adsorption at P/P0 above 0.9 compared to the starting CA aerogel, which is typical of activated carbon aerogels [5–8]. Overall, however, it is clear from the nitrogen sorption isotherms that the activated carbon aerogels are still predominantly microporous. Thus the aim of increasing the microporosity in the subcritically dried CA aerogel is achieved in the activated aerogels. The pore size distribution of all the activated carbon aerogels is given in Fig. 3 and the pore size is summarised in Table 1. Similar to the starting CA aerogel, all the activated carbon aerogels (except for sample 14ACA-900) possess a broad distribution of micropores centred at ca. 8 and 13 Å. Sample 14ACA-900, in addition to pores centred at ca. 8 and 13 Å, also possesses larger 24 Å pores. In all cases, KOH activation results in a significant increase in pore volume arising from micropores of size ca. 8 and 13 Å, and in particular the 8 Å pores. Thus the micropores of size <15 Å already present in the starting CA aerogel are retained and enhanced in the activated carbon aerogels. The pore size of the pores centred at ca. 13 Å is, however, slightly broadened with a small proportion of ‘new’ pores in the range 15–20 Å (for sample 12ACA-800 and 14ACA-700) and 15–30 Å (for sample 14ACA-800 and 15ACA800). On the other hand, sample 14ACA-900 possesses a significant proportion of ‘new’ pores of size 18–45 Å (centered at ca. 24 Å). As
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Table 1 Textural properties and gas (CO2 and hydrogen) uptake capacity of carbon aerogels before (CA) and after (ACA-X) activation with KOH.
a b c
Sample
Surface area (m2/g)a
Pore volume (cm3/g)b
Pore size (Å)c
CA 14ACA-700 14ACA-800 14ACA-900 12ACA-800 15ACA-800
508 (407) 1083 (947) 1871 (1622) 1980 (1522) 915 (788) 1521 (1312)
0.68 1.22 2.03 1.92 0.99 0.99
8/12 8/13 8/13 8/13/24 8/13 8/13
(0.20) (0.45) (0.77) (0.72) (0.37) (0.62)
CO2 uptake (mmol/g)
CO2 uptake density (lmol/m2)
H2 uptake (wt%)
H2 uptake density (lmol/m2)
3.0 3.0 2.7 2.7 3.0
2.77 1.60 1.36 2.95 1.97
3.5 3.6 4.3 2.7 3.8
16.2 9.6 10.9 14.8 12.5
Values in parenthesis are micropore surface area. Values in parenthesis are micropore volume. Pore size maxima from NLDFT pore analysis.
microporosity. The proportion (80%) of micropore surface area in the starting CA aerogel is retained for sample 14ACA-900 (77%) or improved to 87% for all the other activated carbon aerogels. The proportion of micropore volume (30% for the starting CA aerogel) increases to at least 37% for all the activated carbons. The high proportion of microporosity in the activated carbon aerogels makes them interesting as potential candidates for storage of gases such as CO2 and hydrogen as discussed below.
0.04
Pore Volume (cm3/g)
CA
12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
0.03
0.02
3.2. Gas uptake 0.01
0.00 0
10
20
30
40
50
60
Pore Size (Å) Fig. 3. Pore size distribution curves of carbon aerogel (CA) before and after activation. See experimental section for sample designation.
shown in Supporting Fig. S1, there is also an increase in pore volume arising from pores of size >100 Å after activation. The pore size data suggests that the main effect of KOH activation on the CA aerogel is chiefly to increase the amount of already existing micropores of size between 5 and 15 Å, and larger mesopores of size above 100 Å. Only in sample 14ACA-900 is there a significant formation of ‘new’ pores of size between 15 and 45 Å. This rather differs from the activation of other porous carbons such as carbon-derived carbons [36] and zeolite-templated carbons [37] where there is retention of existing pores but a much greater formation of new larger pores or the activation of disordered carbons or mesoporous carbon where KOH activation mainly generates pores of size <1.5 nm with little or no retention of pre-existing pores [38]. In all cases, the activation process leads to an increase in surface area and pore volume. The surface area rises from 508 m2/g for the CA aerogel to between 915 and 1980 m2/g for the activated carbon aerogels, representing increase of between 80 and 290%. The total pore volume rises from 0.68 cm3/g, by between 46 and 200%, to be in the range 0.99–2.03 cm3/g. The largest increase (290%) in surface area from 508 m2/g to 1980 m2/g is observed for sample 14ACA900, which was activated at the highest temperature, while the lowest increase in surface area and pore volume is observed for sample 12ACA-800 (activated at 800 °C at lower KOH/carbon ratio of 2). It is thus clear that the extent of increase in surface area and pore volume is dependent on the level of activation (i.e., temperature and KOH/carbon ratio). What is however remarkable is that the activated carbon aerogels retain a very high proportion of
The CO2 uptake capacity of the activated carbon aerogels was determined at atmospheric pressure and 25 °C under flowing gas (pure CO2) conditions. Table 1 summaries the amount of CO2 captured at 25 °C, and in all cases the activated carbon aerogels show high uptake of between 2.7 and 3.0 mmol/g. CO2 uptake of ca. 3 mmol/g is on the higher end when compared to previous reports on similar carbonaceous materials [3,39–53]. The relatively similar uptake despite the large variation in textural properties indicates that total surface area and pore volume are not the critical factors in determining CO2 adsorption. The CO2 uptake density (Table 1) reveals that samples that predominantly or exclusively possess smaller pores (<15 Å) exhibit the greatest uptake per unit surface area. The porosity of sample 14ACA-700 and 12ACA-800 with a CO2 uptake density of 2.77 and 2.95 lmol/m2 respectively, is dominated by pores of size 8 and 13 Å; these two carbons do not possess any ‘small’ pores larger than 20 Å. As the presence and proportion of pores in the size range 20–45 Å increases (Fig. 3), the CO2 uptake density decreases to a low of 1.36 lmol/m2 for sample 14ACA900. However, all the activated carbon aerogels possess enough pores in the size range <15 Å and particularly <10 Å), which ensures an attractively high CO2 uptake of ca. 3 mmol/g. The attraction of the present activated carbon aerogels is the simplicity of their formation and a high CO2 uptake of 3 mmol/g. It is worthwhile to note that, unlike the present carbon aerogels, most of porous carbon materials that have so far been reported to have high CO2 uptake contain nitrogen as a dopant [41–47]. The presence of nitrogen is thought to generate basic sites onto which CO2 molecules are readily adsorbed thus increasing overall uptake [41–47]. Nevertheless, despite being N-free, our activated aerogels have uptake that is comparable or similar to that of nitrogen enriched carbons. For example, nitrogen-doped carbons derived from melamine–formaldehyde resins adsorb 2.25 mmol/g of CO2 under conditions similar to those utilised in this study, [48] while glucose-derived amine-rich carbons manage an uptake of only of 1 mmol/g under similar ambient conditions [49]. The CO2 uptake of our N-free activated carbon aerogels is similar to the 3.1 mmol/ g (under flow conditions at 25 °C and 1 bar) achieved by N-doped carbon monoliths derived from resorcinol–formaldehyde as carbon precursor (with amino acid L-lysine as catalyst) [44]. Our samples, thus highlight the importance of microporosity and pore size in
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4
H2 uptake (wt%)
determining CO2 uptake of carbon materials. Similar trends that highlight the importance of microporosity in determining CO2 uptake have previously been demonstrated for some N-free carbons [50–53]. Sevilla and co-workers obtained uptake of up to 4.8 mmol/g (at 1 bar and 25 °C) for biomass-derived activated carbons that possessed a large number of narrow micropores of size <10 Å [51]. On the other hand, based on the analysis of narrow micropore size distribution of several activated carbons prepared under different conditions, Wei and co-workers suggested a linear relationship between the volume of micropores in the range 3.3– 8.2 Å and CO2 adsorption at 0 °C and 1 bar, and that narrow micropores of ca. 5.5 Å contributed most, [52] while Gogotsi and co-workers investigated a variety of carbide-derived carbons and observed a linear correlation between pore volume and CO2 uptake at a 1 bar and that pores smaller than 8 Å contributed the most to the CO2 uptake [53]. The CO2 uptake as a function of time is shown in Fig. 4. The rate of uptake proceeds rapidly in the first few minutes and then gradually slows to a smooth increase as it approaches equilibrium. The maximum uptake is attained in ca. 60 min, which is comparable to what has previously been observed for porous carbons under similar conditions [48]. The sample with the largest pores (14ACA-900) appears to have the slowest uptake rate while smaller pore samples (e.g., 12ACA-800) have faster kinetics even though the maximum uptake is similar for both samples. This suggests that smaller micropores favour faster uptake of CO2. The hydrogen uptake isotherms of all the activated carbon aerogels are shown in Fig. 5 and the hydrogen storage capacity at 20 bar is summarized in Table 1. All samples have uptake isotherms that are completely reversible with no hysteresis, and no saturation is achieved even at 20 bar, suggesting that higher hydrogen adsorption capacity can be obtained at higher pressures. The hydrogen uptake of the activated carbon aerogels at 196 °C and 20 bar (2.7–4.3 wt%) is consistent with their surface area [54]. The hydrogen uptake is, as expected, highest for samples with the largest surface area; the highest hydrogen storage capacity (4.3 wt% at 20 bar) was observed for the activated carbon aerogel (14ACA-900) that has the highest surface area, while sample 12ACA-800 with the lowest surface area of 915 m2/g had the least hydrogen uptake of 2.7 wt%. A plot of the hydrogen uptake (at 20 bar) as a function of surface area (Supporting Fig. S2) shows that the uptake generally increases with surface area although the relationship is not linear. For example it appears that sample 14ACA800 has a lower than expected hydrogen uptake. A closer inspection of the relationship between surface area and hydrogen uptake
3
2 12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
1
0 0
5
10
15
20
Pressure (bar) Fig. 5. Hydrogen uptake at -196 °C for activated carbon aerogels prepared at various KOH/carbon ratio and activation temperature. See experimental section for sample designation.
(Supporting Fig. S2) reveals that the hydrogen storage capacity of the activated carbon aerogels does not fit into the Chahine rule (i.e., 1 wt% hydrogen stored per 500 m2/g of carbon) [55], due to higher than expected uptakes per given surface area. The hydrogen uptake density of the activated carbon aerogels given in Table 1 varies between 9.6 and 16.2 lmol H2 m 2. All the activated carbon aerogels, expect for sample 14ACA-900, outperform the Chahine rule (i.e., 1 wt% per 500 m2/g of carbon corresponding to a density of 10 lmol H2 m 2). We have previously observed similar behaviour for KOH activated carbide-derived carbons, [36] KOH activated zeolite-templated carbons, [37] and biomass-derived activated carbons [21]. It is clear that the hydrogen storage density of the activated carbon aerogels depends on the proportion of pores in the lower micropore range. Small micropores of size ca. 8 Å are known to be the most effective for hydrogen storage [56–63]. The large mesopores and macropores of the activated aerogels (Supporting Fig. S1) do not contribute much to the hydrogen storage. This is supported by the fact that samples 14ACA-700 and 12ACA-800, which have no pores larger than 20 Å have the highest storage density of 16.2 and 14.8 H2 m 2 respectively.
4. Conclusion
CO2 uptake (mmol/g)
3.0 2.5 2.0 1.5 12ACA-800 14ACA-700 14ACA-800 14ACA-900 15ACA-800
1.0 0.5 0.0 0
20
40
60
80
100
120
140
160
180
Time (minutes) Fig. 4. CO2 uptake at 25 °C and 1 bar under flowing gas conditions for activated carbon aerogels prepared at various KOH/carbon ratio and activation temperature. See experimental section for sample designation.
We have demonstrated the successful synthesis of carbon aerogels via a simple subcritical drying route and their subsequent activation to high surface area carbons with attractive gas storage capacity. The subcritical drying method, wherein an acetone washed gel is dried overnight at 100 °C prior to carbonisation, is simple and avoids the more cumbersome conventional supercritical or freeze drying routes. A highly microporous carbon aerogel with a surface area of 508 m2/g and pore volume of 0.68 cm3/g wherein micropores contribute 80% of the surface area (i.e., micropore surface area of 407 m2/g) is generated. The surface area of the carbon aerogel is dominated by micropores of size <15 Å with a broad micropore size distribution centered at 8 and 12 Å. Chemical activation of the carbon aerogel with KOH generates activated carbon aerogels with surface area between 915 and 1980 m2/g and pore volume of up to 2.03 cm3/g with the extent of the increase in textural properties dependent on the KOH/carbon ratio and activation temperature. Activation at 600, 700 or 800 °C (and KOH carbon ratio of 2, 4 or 5) generates activated carbon aerogels with micropore size distribution centred at ca. 8 and 13 Å, which is similar to that of the starting carbon aerogel, but with a large increase in pore volume arising from
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the 8 and 13 Å micropores. Thus the micropores of size <15 Å already present in the starting CA aerogel are retained and enhanced (with respect to their pore volume and surface area) in the activated carbon aerogels. The pore size data therefore suggests that the main effect of KOH activation is chiefly to increase the amount of already existing micropores of size less than 15 Å. Remarkably, the proportion of microporosity in the activated carbon aerogels increases from an already high of 80–87%. All the activated carbon aerogels exhibit attractive CO2 uptake of between 2.7 and 3.0 mmol/g at 25 °C and 1 bar under flowing pure CO2 conditions. The activated carbon aerogels also store between 3.5 and 4.3 wt% hydrogen at -196 °C and 20 bar, and the amount of hydrogen adsorbed correlates well with the surface area. The hydrogen storage density of the carbons is high and ranges between 9.6 and 16.2 lmol H2m 2 with the presence of small micropores favouring high density. Taking into account the gas storage capacities of these materials, as well as the simplicity of their synthesis procedure, it can be concluded that the activated carbon aerogels constitute promising adsorbents for CO2 and hydrogen. Acknowledgement This research was funded by the University of Nottingham. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2013. 05.025. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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