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How to accurately determine the uptake of hydrogen in carbonaceous materials
International Journal of Hydrogen Energy 29 (2004) 1271 – 1276
www.elsevier.com/locate/ijhydene
How to accurately determine the uptake of hydrogen in carbonaceous materials Chao Zhang∗ , XueSheng Lu, AnZhong Gu Institute of Refrigeration and Cryogenics Engineering, School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200030, People’s Republic of China Received 6 July 2003; received in revised form 30 October 2003; accepted 15 December 2003
Abstract A volumetric apparatus for adsorption measurement of hydrogen in carbonaceous materials aimed at hydrogen adsorption storage was constructed. The performance of the apparatus was assessed by helium and hydrogen adsorption in one kind of vapor-grown graphitic nano8ber (GNF). The bulk gas amounts determined by the equations of state of the Modi8ed-BenedicWebb-Rubin (MBWR), the ideal gas and the Soave-Redlich-Kwong (SRK) under the conditions of present study are compared. Two di@erent methods of processing experimental data to determine the residual volume are studied. Experimental results and theoretic analysis showed that the volumetric apparatus and the data processing method described in this paper could accurately determine the Gibbs excess adsorption amount of hydrogen in carbonaceous materials. Although the vapor-grown GNF used in the present study did not show a signi8cant storage capacity of hydrogen, the obtained results would provide favorable reference data for the development of the carbonaceous material for hydrogen adsorption storage. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen adsorption; Adsorption capacity; Carbonaceous material; Adsorption storage
1. Introduction Hydrogen is a renewable and environmentally friendly energy source, and it has been considered an ideal energy medium for replacing fossil fuels such as oil and coal. The challenge we face is how to transport and store hydrogen safely and eCciently. There are currently four main methods for hydrogen storage such as liquid hydrogen, compressed hydrogen, metal hydrides and hydrogen physisorption. The hydrogen physisorption has been considered as the most promising hydrogen storage method for meeting the goals of the Department of Energy of the United States (DOE) hydrogen plan for fuel cell powered vehicles [1]. The DOE hydrogen plan requires system weight eCciency of 6:5 wt% and volumetric density of 62 kg H2 =m3 [1,2]. Recently, graphitic nano8bers (GNFs) and carbon nanotubes have been claimed to have phenomenally high capacities to
∗
Corresponding author. Tel./fax: +86-21-6293-2602. E-mail address: [email protected] (C. Zhang).
adsorb hydrogen at ambient temperature and pressure close to 10 Mpa [1–6]. Especially, Rodriguez and co-workers have reported the uptake of hydrogen in GNFs of 40 wt% [3]. So more and more researchers have paid their attention on hydrogen adsorption storage in carbonaceous nano-materials. However, good storage capacity of hydrogen adsorption has hardly recurred and large discrepancies have been evident on the reported uptakes of hydrogen in various carbonaceous materials [7]. The reasons for those could be two-fold: experimental errors and little characterization of the materials. One of the biggest obstacles in experiments is the unavailable amount of materials. Usually, researchers have been working with extremely small amount of materials, which limit the accuracy of the experimental measurements. Furthermore, minute amount of impurities will also signi8cantly alter the experimental results. In the current work, we construct a volumetric apparatus for the measurement of hydrogen adsorption in carbonaceous materials. We evaluate the performance of the apparatus with one kind of vapor-grown GNF and discuss
0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2003.12.001
1272
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276
di@erent methods available to determine the uptake of hydrogen in carbonaceous materials. 2. Experimental details 2.1. Material used The material used in the present study is one kind of vapor-grown GNF, which was prepared by the decomposition of argon : acetylene : hydrogen = 5:4 : 1:2 : 1 gas Oow over nickel catalyst at 670◦ C. The transmission electron microscopy (TEM) image of the samples is illustrated in Fig. 1. The sample was puri8ed with KOH solution to wash out graphitic fragments, amorphous carbon and nickel catalyst grains. The TEM image of the puri8ed sample is illustrated in Fig. 2. For adsorption measurement, about 4:610 g of GNF were 8lled into sample cell and were evacuated at 120◦ C for 4 h prior to the adsorption measurement. The residual volume of the sample cell full of GNFs, i.e., the volume of the sample cell subtracted by the volume occupied by GNFs grains, was determined from helium adsorption.
Fig. 1. TEM images of the GNF sample.
2.2. Volumetric apparatus and adsorption measurement A schematic diagram of the volumetric apparatus is shown in Fig. 3. The gas passage of the apparatus is the stainless-steel capillary in 1:5 mm inner diameter and 3:0 mm outer diameter that could endure a pressure of 30 MPa. The volumes of the referenced cell and the sample cell are, respectively, 50 and 25 ml. The referenced cell and the sample cell are put into a thermostat which could maintain temperature with precision ±0:05◦ C. The pressure measurement is automated by using Fluke 2620 connected to a personal computer with an IEEE-488 interface. The MPM480 pressure transducer (Micro Sensor Co., Ltd.) is calibrated by Shanghai Institute of Measurement and Testing Technology with 0.03% of reading accuracy in the pressure range of 0–18 MPa. An activated carbon container is located on the exit of the gas supply to wipe o@ the possible water vapor in gas source. The principle of the volumetric measurement method has been described in detail in Refs. [8–11]. It could be concisely described as n(P; T ) = n(P0 ; T ) + n(P1 ; T; Vrc ) + n(P2 ; T; Vsc ) −n(P; T; Vrc ) − n(P; T; Vsc );
(1)
where P is the pressure when adsorption reaches equilibrium after the valve bv6 is opened, P1 and P2 are, respectively, the initial pressures in the referenced cell and the sample cell when the valve bv6 is closed. Vrc and Vsc are, respectively, the volume of the referenced cell and the residual volume of the sample cell. We apply step-by-step method to
Fig. 2. TEM images of the puri8ed GNF sample.
measure adsorption isotherm, while the pressure is changed step-by-step and the amount of adsorption is summed up at each step. Where P0 is the pressure corresponding to the last adsorption equilibrium and T is the temperature of adsorption. Where n(P; T ) is the cumulative adsorption amount at pressure P; n(P0 ; T ) is the cumulative adsorption amount at last equilibration pressure P0 . n(P1 ; T; Vrc ) and n(P2 ; T; Vsc ) are, respectively, the initial bulk gas amount in the referenced cell at pressure P1 and in the sample cell at pressure P2 ; n(P; T; Vrc ) and n(P; T; Vsc ) are respectively, the bulk gas amount in the referenced cell and in the sample cell at the equilibration pressure P. 2.3. Leakage test and measurement of the residual volume The leakage rate of the experimental system is the key factor to the accurate measurement of the gas adsorption. So it is necessary to assess the leakage rate of the whole system. Here, nitrogen was used in the leakage test. High-purity nitrogen (¿ 99:999% purity) was introduced into the system and high pressure about 13 MPa had been kept for about 48 h. Leakage rate (degressive pressure/initial pressure/day ×100%) was less than 0.08%.
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276 Pressure Gauge
1273
Turbo Pum PT bv 7 to atmosphere
activated carbon container bv 4
bv 5
bv 8
bv 6 bv1
bv2
bv3
referenced cell
H2
He
N2 sample cell
thermostat
Fig. 3. Schematic diagram of the volumetric apparatus.
Whether the residual volume of the sample cell could be accurately determined is the key factor to the determination of the Gibbs excess adsorption amount. Just one presupposition is added that helium adsorption amount at room temperature in graphitic nano8bres is nil [8]. Here, we use helium (¿ 99:999% purity) adsorption to determine the residual volume. The experimental operation process is the same as in measuring hydrogen adsorption isotherm [8–11]. 3. Results and discussion The bulk gas amount in the referenced cell and the sample cell were calculated by equations of state. The gas amount determined by various equations of state may be di@erent even under same conditions, so the equation of state that could accurately determine PVT data of bulk gas must be chosen. The empty sample cell of previously known volume is regarded as the criterion and the equations of state of the ideal gas, the SRK [12] and the 32-term modi8ed Bennedict–Webb–Rubin [13] are used to determine the bulk gas amount. The volume of the empty sample cell could be calculated with Eq. (1) by setting the cumulative adsorption amount to zero. Step-by-step measurement method was not used here. Measurement was carried out under di@erent pressures and prior to each measurement, the whole system was out-gassed, so the initial bulk gas amount in the sample cell was nil. Helium gas was 8rstly introduced into the referenced cell with di@erent initial pressures and then expanded to the sample cell till equilibrium. Fig. 4 shows the calculated volume of the empty sample cell with three di@erent methods at 298 K and di@erent pressures. In fact, the volume of the empty sample cell should be independent of the
pressure and keep unchanged. But the calculated volumes by the ideal gas equation of state and the SRK equation of state are apparently dependent on the pressure. However, the calculated volume of the empty sample cell from the MBWR state equation is independent of the pressure and closest to the real value of the empty sample cell. So, the MBWR state equation could determine accurately PVT data of bulk gas and was chosen to determine bulk gas amount. The results are similar to those of Kiyobayashi et al. [14]. The residual volume of the sample cell full of GNFs was calculated with helium gas adsorption at 298 K. The experimental data were obtained by the step-by-step measurement method described above. We used two di@erent methods to process experimental data. One of the methods is to calculate the residual volume directly by solving Eq. (1) assuming that the cumulative adsorption amount is equal to zero at each step. The second method is to calculate the residual volume by iterative calculation. That is to say, an initial residual volume is assumed and the last result is obtained when the cumulative adsorption amount on the right of Eq. (1) is less than 10−8 mol by iterative calculation. The calculated results are shown in Fig. 5. As clearly shown in the 8gure, the residual volume calculated by the direct calculation method is apparently larger than that by the iterative calculation method. In order to validate which method is correct, we use the residual volume calculated by the two methods to calculate the helium adsorption amount and hydrogen adsorption amount at 298 K. As shown in Fig. 6, the helium and hydrogen adsorption amount calculated with the residual volume by direct calculation is less than zero. It does not accord with the assumption that the helium adsorption amount at room temperature is equal to zero. The helium adsorption amount using the residual volume
1274
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276 28.0 SRK state equation ideal gas state equation MBWR state equation
the empty sample cell volume (ml)
27.6 27.2 26.8 26.4 26.0 25.6 25.2 24.8 24.4 24.0 1
2
3
4
5
6
7
8
9
pressure(MPa)
Fig. 4. The calculated empty sample cell volume.
the residual volume of the sample cell (ml)
26.0 25.6
iterative calculation direct calculation
25.2 24.8 24.4 24.0 23.6 23.2 22.8 22.4 22.0 1
2
3
4
5
6
7
8
9
10
11
12
Pressure (MPa) Fig. 5. The residual volume of the sample cell.
calculated by iterative calculation accords with the assumption that the helium adsorption amount at room temperature is nil. So the residual volume calculated by direct method is dummy. The reason is that the true value of the residual volume may be concealed by the cumulative error of the direct calculation for step-by-step measurement method, and the cumulative error could be removed by iterative calculation. In Fig. 7, the adsorption isotherms of hydrogen and helium at 298 and 273 K are shown. The Gibbs excess amount adsorbed is signi8cantly larger for hydrogen than for
helium. The excess adsorption amount is about 0:06 wt% for hydrogen at 298 K; 0:01 wt% for helium and 0:08 wt% for hydrogen at 273 K. The uptakes of hydrogen here are largely lower than those listed in Ref. [7]. So the vapor-grown GNF used in the present study does not show signi8cant storage capacity of hydrogen. There is another 8nding that adsorption equilibrium only takes about 20 min and does not take a long time described by Huang et al. [15] and Zhu et al. [16]. The reason is not clear at present. Maybe it is because of the purity or the intrinsical adsorption ability of materials used in the present study.
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276
1275
excess adsorption amount(gH2 /gC,wt%)
0.1
0.0
-0.1
-0.2
-0.3 direct calculation helium iterative calculation helium direct calculation hydrogen iterative calculation hydrogen
-0.4
-0.5 0
2
4
6
8
10
12
10
12
Pressure (MPa) Fig. 6. Adsorption isotherm at 298 K. 0.10
excess adsorption amount (gH2 /gC,wt %)
0.09 298K hydrogen excess adsorption amount 298K helium excess adsorption amount 273K hydrogen excess adsorption amount 273K helium excess adsorption amount
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 0
2
4
6
8
Pressure (MPa) Fig. 7. Hydrogen and helium adsorption isotherm at 298 and 273 K.
4. Conclusion We have constructed a volumetric apparatus for adsorption isotherm measurement aimed at hydrogen adsorption storage in carbonaceous materials. The performance of the apparatus has been assessed by helium and hydrogen adsorption in one kind of vapor-grown GNF. It is proved that the MBWR state equation is more suitable to determine the bulk gas amount than the equations of state of the ideal gas and the SRK under the present conditions. Two di@erent
methods of processing experimental data, the direct calculation and the iterative calculation, have been compared. The results obtained by the iterative calculation are closer to the presupposition that the helium adsorption amount is nil at ambient temperature. We conclude that the volumetric apparatus and the methods of data processing described above could provide accurate measurement of gas adsorption in carbonaceous materials. Although the vapor-grown GNF used in present study does not show a signi8cant storage capacity of hydrogen, the obtained results would provide
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C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276
useful information for the development of the carbonaceous material for hydrogen storage. Acknowledgements This work is subsidized by the Special Funds of the Science and Technology Development of Shanghai City in China (#0116nm044).
[7]
References
[9]
[1] Hynek S, Fuller W, Bentley J. Hydrogen storage by carbon sorption. Int J Hydrogen Energy 1997;22(6):601–10. [2] Dillion AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997;386:377. [3] Chambers A, Park C, Terry R, Barker K, Rodriguez N. Hydrogen storage in graphite nano8bers. J Phys Chem B 1998;102(22):4253–6. [4] Zhu HW, Ci LJ, Chen A, Mao ZQ, Xu CL, Xiao X, Wei BQ, Liang J, Wu DH. Hydrogen uptake in multi-walled carbon nanotubes at room temperature. In: Mao ZQ, Veziroglu TN, editors. Proceedings of the 13th World Hydrogen Energy Conference, Beijing, China. Beijing: China International Conference Center for Science and Technology, International Hydrogen Association; 2000. p. 560. [5] Browning DJ, Gerrard ML, Laakeman JB, Mellor IM, Mortimer RJ, Turpin MC. Investigation of the hydrogen storage capacities of carbon nano8bres prepared from an Ethylene precursor. In: Mao ZQ, Veziroglu TN, editors. Proceedings of the 13th World Hydrogen Energy Conference, Beijing, China. Beijing: China International Conference Center for Science and Technology, International Hydrogen Association; 2000. p. 554. [6] Gupta BK, Awasthi K, Srivastava ON. New carbon variants: graphitic nano8bres and nanotubules as hydrogen storage
[8]
[10]
[11]
[12]
[13] [14]
[15] [16]
materials. In: Mao ZQ, Veziroglu TN, editors. Proceedings of the 13th World Hydrogen Energy Conference, Beijing, China. Beijing: China International Conference Center for Science and Technology, International Hydrogen Association; 2000. p. 487. Lamari DF, Malbrunot P, Tartaglia GP. Review of hydrogen storage by adsorption in carbon nanotubes. Int J Hydrogen Energy 2002;27(2):193–202. Keller JU, Dreisbach F, Rave H, et al. Measurement of gas mixture adsorption equilibria of natural gas compounds on microporous sorbents. Adsorption 1999;5:199–214. Li M. The characteristic study of methane adsorption on AX-21 activated carbon. Master Dissertation, Chemical Engineering Institute of TianJin University, Tianjin, 1998 [in Chinese]. Yang XD. A study of supercritical methane storage by adsorption. Doctoral dissertation, Institute of Refrigeration and Cryogenic Engineering of Shanghai Jiaotong University, Shanghai, 2001 [in Chinese]. Zheng QR. A study of hydrogen storage by adsorption on multi-walled carbon nanotube. Doctoral dissertation, Institute of Refrigeration and Cryogenic Engineering of Shanghai Jiaotong University, Shanghai, 2002 [in Chinese]. Zhou L, Zhou Y. Determination of compressibility factor and fugacity coeCcient of hydrogen in studies of adsorptive storage. Int J Hydrogen Energy 2001;26(6): 597–601. Johnson JK, John AZ, Keith EG. The Lennard-Jones equation of state revisited. Mol Phys 1993;78(3):591–618. Kiyobayashi T, Takeshita HT, Tanaka H. Hydrogen adsorption in carbonaceous materials—how to determine the storage capacity accurately. J Alloys Compounds 2002;330(332): 666–9. Huang WZ, Zhang XB, Tu JP, Kong FZ. The e@ect of pretreatments on hydrogen adsorption of multi-walled carbon nanotubes. Mater Chem Phys 2003;78:144–8. Zhu HW, Li CH, Li XS. Hydrogen storage by platelet-carbon 8bers at room temperature. Mater Lett 2003;57:32–5.
www.elsevier.com/locate/ijhydene
How to accurately determine the uptake of hydrogen in carbonaceous materials Chao Zhang∗ , XueSheng Lu, AnZhong Gu Institute of Refrigeration and Cryogenics Engineering, School of Mechanical and Power Engineering, Shanghai Jiaotong University, Shanghai 200030, People’s Republic of China Received 6 July 2003; received in revised form 30 October 2003; accepted 15 December 2003
Abstract A volumetric apparatus for adsorption measurement of hydrogen in carbonaceous materials aimed at hydrogen adsorption storage was constructed. The performance of the apparatus was assessed by helium and hydrogen adsorption in one kind of vapor-grown graphitic nano8ber (GNF). The bulk gas amounts determined by the equations of state of the Modi8ed-BenedicWebb-Rubin (MBWR), the ideal gas and the Soave-Redlich-Kwong (SRK) under the conditions of present study are compared. Two di@erent methods of processing experimental data to determine the residual volume are studied. Experimental results and theoretic analysis showed that the volumetric apparatus and the data processing method described in this paper could accurately determine the Gibbs excess adsorption amount of hydrogen in carbonaceous materials. Although the vapor-grown GNF used in the present study did not show a signi8cant storage capacity of hydrogen, the obtained results would provide favorable reference data for the development of the carbonaceous material for hydrogen adsorption storage. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen adsorption; Adsorption capacity; Carbonaceous material; Adsorption storage
1. Introduction Hydrogen is a renewable and environmentally friendly energy source, and it has been considered an ideal energy medium for replacing fossil fuels such as oil and coal. The challenge we face is how to transport and store hydrogen safely and eCciently. There are currently four main methods for hydrogen storage such as liquid hydrogen, compressed hydrogen, metal hydrides and hydrogen physisorption. The hydrogen physisorption has been considered as the most promising hydrogen storage method for meeting the goals of the Department of Energy of the United States (DOE) hydrogen plan for fuel cell powered vehicles [1]. The DOE hydrogen plan requires system weight eCciency of 6:5 wt% and volumetric density of 62 kg H2 =m3 [1,2]. Recently, graphitic nano8bers (GNFs) and carbon nanotubes have been claimed to have phenomenally high capacities to
∗
Corresponding author. Tel./fax: +86-21-6293-2602. E-mail address: [email protected] (C. Zhang).
adsorb hydrogen at ambient temperature and pressure close to 10 Mpa [1–6]. Especially, Rodriguez and co-workers have reported the uptake of hydrogen in GNFs of 40 wt% [3]. So more and more researchers have paid their attention on hydrogen adsorption storage in carbonaceous nano-materials. However, good storage capacity of hydrogen adsorption has hardly recurred and large discrepancies have been evident on the reported uptakes of hydrogen in various carbonaceous materials [7]. The reasons for those could be two-fold: experimental errors and little characterization of the materials. One of the biggest obstacles in experiments is the unavailable amount of materials. Usually, researchers have been working with extremely small amount of materials, which limit the accuracy of the experimental measurements. Furthermore, minute amount of impurities will also signi8cantly alter the experimental results. In the current work, we construct a volumetric apparatus for the measurement of hydrogen adsorption in carbonaceous materials. We evaluate the performance of the apparatus with one kind of vapor-grown GNF and discuss
0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2003.12.001
1272
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276
di@erent methods available to determine the uptake of hydrogen in carbonaceous materials. 2. Experimental details 2.1. Material used The material used in the present study is one kind of vapor-grown GNF, which was prepared by the decomposition of argon : acetylene : hydrogen = 5:4 : 1:2 : 1 gas Oow over nickel catalyst at 670◦ C. The transmission electron microscopy (TEM) image of the samples is illustrated in Fig. 1. The sample was puri8ed with KOH solution to wash out graphitic fragments, amorphous carbon and nickel catalyst grains. The TEM image of the puri8ed sample is illustrated in Fig. 2. For adsorption measurement, about 4:610 g of GNF were 8lled into sample cell and were evacuated at 120◦ C for 4 h prior to the adsorption measurement. The residual volume of the sample cell full of GNFs, i.e., the volume of the sample cell subtracted by the volume occupied by GNFs grains, was determined from helium adsorption.
Fig. 1. TEM images of the GNF sample.
2.2. Volumetric apparatus and adsorption measurement A schematic diagram of the volumetric apparatus is shown in Fig. 3. The gas passage of the apparatus is the stainless-steel capillary in 1:5 mm inner diameter and 3:0 mm outer diameter that could endure a pressure of 30 MPa. The volumes of the referenced cell and the sample cell are, respectively, 50 and 25 ml. The referenced cell and the sample cell are put into a thermostat which could maintain temperature with precision ±0:05◦ C. The pressure measurement is automated by using Fluke 2620 connected to a personal computer with an IEEE-488 interface. The MPM480 pressure transducer (Micro Sensor Co., Ltd.) is calibrated by Shanghai Institute of Measurement and Testing Technology with 0.03% of reading accuracy in the pressure range of 0–18 MPa. An activated carbon container is located on the exit of the gas supply to wipe o@ the possible water vapor in gas source. The principle of the volumetric measurement method has been described in detail in Refs. [8–11]. It could be concisely described as n(P; T ) = n(P0 ; T ) + n(P1 ; T; Vrc ) + n(P2 ; T; Vsc ) −n(P; T; Vrc ) − n(P; T; Vsc );
(1)
where P is the pressure when adsorption reaches equilibrium after the valve bv6 is opened, P1 and P2 are, respectively, the initial pressures in the referenced cell and the sample cell when the valve bv6 is closed. Vrc and Vsc are, respectively, the volume of the referenced cell and the residual volume of the sample cell. We apply step-by-step method to
Fig. 2. TEM images of the puri8ed GNF sample.
measure adsorption isotherm, while the pressure is changed step-by-step and the amount of adsorption is summed up at each step. Where P0 is the pressure corresponding to the last adsorption equilibrium and T is the temperature of adsorption. Where n(P; T ) is the cumulative adsorption amount at pressure P; n(P0 ; T ) is the cumulative adsorption amount at last equilibration pressure P0 . n(P1 ; T; Vrc ) and n(P2 ; T; Vsc ) are, respectively, the initial bulk gas amount in the referenced cell at pressure P1 and in the sample cell at pressure P2 ; n(P; T; Vrc ) and n(P; T; Vsc ) are respectively, the bulk gas amount in the referenced cell and in the sample cell at the equilibration pressure P. 2.3. Leakage test and measurement of the residual volume The leakage rate of the experimental system is the key factor to the accurate measurement of the gas adsorption. So it is necessary to assess the leakage rate of the whole system. Here, nitrogen was used in the leakage test. High-purity nitrogen (¿ 99:999% purity) was introduced into the system and high pressure about 13 MPa had been kept for about 48 h. Leakage rate (degressive pressure/initial pressure/day ×100%) was less than 0.08%.
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276 Pressure Gauge
1273
Turbo Pum PT bv 7 to atmosphere
activated carbon container bv 4
bv 5
bv 8
bv 6 bv1
bv2
bv3
referenced cell
H2
He
N2 sample cell
thermostat
Fig. 3. Schematic diagram of the volumetric apparatus.
Whether the residual volume of the sample cell could be accurately determined is the key factor to the determination of the Gibbs excess adsorption amount. Just one presupposition is added that helium adsorption amount at room temperature in graphitic nano8bres is nil [8]. Here, we use helium (¿ 99:999% purity) adsorption to determine the residual volume. The experimental operation process is the same as in measuring hydrogen adsorption isotherm [8–11]. 3. Results and discussion The bulk gas amount in the referenced cell and the sample cell were calculated by equations of state. The gas amount determined by various equations of state may be di@erent even under same conditions, so the equation of state that could accurately determine PVT data of bulk gas must be chosen. The empty sample cell of previously known volume is regarded as the criterion and the equations of state of the ideal gas, the SRK [12] and the 32-term modi8ed Bennedict–Webb–Rubin [13] are used to determine the bulk gas amount. The volume of the empty sample cell could be calculated with Eq. (1) by setting the cumulative adsorption amount to zero. Step-by-step measurement method was not used here. Measurement was carried out under di@erent pressures and prior to each measurement, the whole system was out-gassed, so the initial bulk gas amount in the sample cell was nil. Helium gas was 8rstly introduced into the referenced cell with di@erent initial pressures and then expanded to the sample cell till equilibrium. Fig. 4 shows the calculated volume of the empty sample cell with three di@erent methods at 298 K and di@erent pressures. In fact, the volume of the empty sample cell should be independent of the
pressure and keep unchanged. But the calculated volumes by the ideal gas equation of state and the SRK equation of state are apparently dependent on the pressure. However, the calculated volume of the empty sample cell from the MBWR state equation is independent of the pressure and closest to the real value of the empty sample cell. So, the MBWR state equation could determine accurately PVT data of bulk gas and was chosen to determine bulk gas amount. The results are similar to those of Kiyobayashi et al. [14]. The residual volume of the sample cell full of GNFs was calculated with helium gas adsorption at 298 K. The experimental data were obtained by the step-by-step measurement method described above. We used two di@erent methods to process experimental data. One of the methods is to calculate the residual volume directly by solving Eq. (1) assuming that the cumulative adsorption amount is equal to zero at each step. The second method is to calculate the residual volume by iterative calculation. That is to say, an initial residual volume is assumed and the last result is obtained when the cumulative adsorption amount on the right of Eq. (1) is less than 10−8 mol by iterative calculation. The calculated results are shown in Fig. 5. As clearly shown in the 8gure, the residual volume calculated by the direct calculation method is apparently larger than that by the iterative calculation method. In order to validate which method is correct, we use the residual volume calculated by the two methods to calculate the helium adsorption amount and hydrogen adsorption amount at 298 K. As shown in Fig. 6, the helium and hydrogen adsorption amount calculated with the residual volume by direct calculation is less than zero. It does not accord with the assumption that the helium adsorption amount at room temperature is equal to zero. The helium adsorption amount using the residual volume
1274
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276 28.0 SRK state equation ideal gas state equation MBWR state equation
the empty sample cell volume (ml)
27.6 27.2 26.8 26.4 26.0 25.6 25.2 24.8 24.4 24.0 1
2
3
4
5
6
7
8
9
pressure(MPa)
Fig. 4. The calculated empty sample cell volume.
the residual volume of the sample cell (ml)
26.0 25.6
iterative calculation direct calculation
25.2 24.8 24.4 24.0 23.6 23.2 22.8 22.4 22.0 1
2
3
4
5
6
7
8
9
10
11
12
Pressure (MPa) Fig. 5. The residual volume of the sample cell.
calculated by iterative calculation accords with the assumption that the helium adsorption amount at room temperature is nil. So the residual volume calculated by direct method is dummy. The reason is that the true value of the residual volume may be concealed by the cumulative error of the direct calculation for step-by-step measurement method, and the cumulative error could be removed by iterative calculation. In Fig. 7, the adsorption isotherms of hydrogen and helium at 298 and 273 K are shown. The Gibbs excess amount adsorbed is signi8cantly larger for hydrogen than for
helium. The excess adsorption amount is about 0:06 wt% for hydrogen at 298 K; 0:01 wt% for helium and 0:08 wt% for hydrogen at 273 K. The uptakes of hydrogen here are largely lower than those listed in Ref. [7]. So the vapor-grown GNF used in the present study does not show signi8cant storage capacity of hydrogen. There is another 8nding that adsorption equilibrium only takes about 20 min and does not take a long time described by Huang et al. [15] and Zhu et al. [16]. The reason is not clear at present. Maybe it is because of the purity or the intrinsical adsorption ability of materials used in the present study.
C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276
1275
excess adsorption amount(gH2 /gC,wt%)
0.1
0.0
-0.1
-0.2
-0.3 direct calculation helium iterative calculation helium direct calculation hydrogen iterative calculation hydrogen
-0.4
-0.5 0
2
4
6
8
10
12
10
12
Pressure (MPa) Fig. 6. Adsorption isotherm at 298 K. 0.10
excess adsorption amount (gH2 /gC,wt %)
0.09 298K hydrogen excess adsorption amount 298K helium excess adsorption amount 273K hydrogen excess adsorption amount 273K helium excess adsorption amount
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 -0.01 0
2
4
6
8
Pressure (MPa) Fig. 7. Hydrogen and helium adsorption isotherm at 298 and 273 K.
4. Conclusion We have constructed a volumetric apparatus for adsorption isotherm measurement aimed at hydrogen adsorption storage in carbonaceous materials. The performance of the apparatus has been assessed by helium and hydrogen adsorption in one kind of vapor-grown GNF. It is proved that the MBWR state equation is more suitable to determine the bulk gas amount than the equations of state of the ideal gas and the SRK under the present conditions. Two di@erent
methods of processing experimental data, the direct calculation and the iterative calculation, have been compared. The results obtained by the iterative calculation are closer to the presupposition that the helium adsorption amount is nil at ambient temperature. We conclude that the volumetric apparatus and the methods of data processing described above could provide accurate measurement of gas adsorption in carbonaceous materials. Although the vapor-grown GNF used in present study does not show a signi8cant storage capacity of hydrogen, the obtained results would provide
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C. Zhang et al. / International Journal of Hydrogen Energy 29 (2004) 1271 – 1276
useful information for the development of the carbonaceous material for hydrogen storage. Acknowledgements This work is subsidized by the Special Funds of the Science and Technology Development of Shanghai City in China (#0116nm044).
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