Development of porous materials for hydrogen storage

Transcription

1 Development of porous materials for hydrogen storage Shinji Oshima, Osamu Kato, Takeshi Kataoka, Yoshihiro Kobori, Michiaki Adachi Hydrogen & New Energy Research Laboratory Nippon Oil Corporation 8, Chidoricho, Naka-ku, Yokohama, JAPAN ABSTRACT: To achieve hydrogen storage of more than 5 mass%, we are focusing on porous materials that consist of light elements. At WHEC 15, we reported that KOH-activated bamboo charcoal showed 0.79 mass% hydrogen uptake at 9.5 MPa and 303K. After examining various carbon materials, we found that carbonized and KOH-activated polyacrylonitrile fibers showed 1.0 mass% hydrogen uptake at 9.5 MPa and 303K. When the pressure was raised to 35 MPa, this material showed 1.5 mass% hydrogen uptake at 303K. Besides porous carbon, other materials, such as coordination polymers, were examined. Since these materials contain elements other than carbon, different adsorption phenomena may be expected. Although the values of their hydrogen uptakes are still lower than those of carbon materials, a coordination polymer which showed 0.38 mass% hydrogen uptake at 9.5 MPa and 303K was revealed to give an adsorption density of 47kg/m 3 at 0.1 MPa and 77K, the highest value reported for a coordination polymer. KEYWORDS: Hydrogen Storage, Porous Carbon, Coordination Polymer 1. Introduction In the effort to combat environmental problems, expectation of the arrival of a hydrogen energy society is growing. In the JHFC project[1](japan Hydrogen & Fuel Cell Demonstration Project), some hydrogen refueling stations have already been installed, especially in the Metropolitan area in Japan, and this project is coming into the second phase, which includes an expansion of the demonstration area. But there are many problems that must be solved to realize a true hydrogen energy society. Hydrogen storage is thought to be the greatest technical barrier. Hydrogen storage materials can be applied to fuel cell vehicles; stationary hydrogen storage tanks for hydrogen refueling stations; and hydrogen transport trailers from central hydrogen production facilities to hydrogen refueling stations. Our current minimum target for hydrogen storage materials is 5 mass% at 35 to 40 MPa, and to realize this target we are focusing on porous materials. Considerable attention has been paid to carbon materials because of characteristics such as lightness and the diversity of structures. When we overview this research field, we can find many papers that reported high hydrogen uptake of carbon materials[2-6]. But we think that there are few reports which passed a crosscheck in two or more research organizations. For example, there was a report that graphitic nanofibers (GNFs) showed a hydrogen uptake of 65 mass% at ambient temperature, but other researchers have not reported this level of uptake. In the case of carbon nanotubes, there are various reports and the situation is complicated. But the values of hydrogen uptake in some reports are too high to believe. At WHEC 15, we reported on an interesting observation with KOH-activated bamboo charcoal, where the material showed a considerable hydrogen uptake while specific surface area was low[7]. This observation suggests a structure suitable for hydrogen adsorption and prompted us to continue further research with carbon materials. We also examined other materials, such as coordination polymers, in the expectation that materials including elements other than carbon may exhibit unique behavior on hydrogen adsorption. 2. Experimental section 2.1 Material used (1) Preparation of carbonized and KOH-activated polyacrylonitrile fibers (KOH-activated PAN) Polyacrylonitrile fibers were cut into small pieces. They were carbonized by heating them to an appropriate temperature (773K to 1173K) at the rate of 5K/min in nitrogen atmosphere. Next, a concentrated KOH solution containing KOH two to seven times the weight of the carbonized PAN was poured into the carbonized PAN in a nickel boat, and the pieces were dried thoroughly and heated to an appropriate temperature (873K to 1173K) in nitrogen atmosphere. After the KOH activation process, they were washed with HCl aqueous solution and pure water, successively, and dried under vacuum at 423K for more than 4h. 1/7

2 (2) Preparation of carbonized and KOH-activated silk fibers (KOH-activated silk) Silk fiber was used as a carbon precursor. Carbonization and KOH activation were performed using a process similar to that for KOH-activated PAN. (3) Preparation of Cu polymer-a, B A standard synthetic procedure for jungle gym-type coordination polymers was employed[8]. Metal salt solution, dicarboxylic acid solution and diamine solution were mixed, and the resulting slurry was stirred for 1 day. The slurry was then filtered and dried for 1 day under vacuum. (4) Preparation of Zn polymer Metal salt solution, dicarboxylic acid solution and diamine solution were mixed, then placed in a Teflon vessel within an autoclave. The mixture was heated at 393K for 48h and then cooled to room temperature. The resulting precipitate was collected in a centrifuge, then dried for 1 day under vacuum. 2.2 Characterization The specific surface area was measured by the BET method of nitrogen adsorption isotherms at 77K. Before measurement, each sample was degassed under high vacuum for more than 5h at a temperature high enough to desorb water, which had been determined by TG-DTA analysis. 2.3 Measurements of hydrogen adsorption capacity at 9.5 MPa We used a volumetric method to determine hydrogen adsorption capacity under high pressures up to 9.5 MPa. At first a porous material weighing 0.5 g is placed in a sample tube. Then the sample is degassed under high vacuum for more than 2h at an appropriate temperature, which has been determined by TG-DTA analysis. The sample tube is set in a water bath at 303K, and a leak test is performed by He gas at 0.5 MPa. Then the volume of the sample tube is measured by the volumetric method. He gas is introduced from the charge chamber and the sample tube volume is computed from the pressure change. The volume of the charge chamber is about 14.0 ml and the volume of the sample tube is about 10.5 ml. Then we measure hydrogen uptake by measuring the pressure change when we introduce hydrogen from the charge chamber. 3. Results and discussion 3.1 Porous carbon materials The ultramicropore, whose diameter is a little larger than a hydrogen molecule, is likely a suitable site for hydrogen adsorption because of its strong potential field. In our previous work, bamboo charcoal was found to be an ultramicroporous material, which showed a considerable amount of hydrogen adsorption while BET specific surface area is very low. After KOH activation, its pore volume was enlarged, suggesting that many micropores were created by the activation. And the activated sample showed increased hydrogen uptake to 0.79 mass% at 9.5 MPa and 303K. Its BET specific surface area was 1860 m 2 /g. If the relationship between BET specific surface area and hydrogen uptake were the same as that for ordinary activated carbons, the hydrogen uptake would have been about 0.6 mass%. The observed uptake was much higher than the estimated one. We think that the higher value results from two effects of micropores. The first one is increased adsorption sites created by formation of micropores through the activation process, and the second one is an opening effect of closed ultramicropores. In order to raise the hydrogen uptake, many carbon precursors were tested, and fibrous materials, such as PAN and silk, were found to be good precursors in this work. We think there are two primary reasons for the improvement through the use of fiber. The first reason is that active species in the KOH activation process can easily penetrate into the inner sites of the fiber, which has a microscopic structure. And the second reason is that the gas generated in the fiber can easily make its path from inside to outside, leaving footprints that make ultramicropores. The result of KOH activation of carbonized PAN is shown in Figure 1. For comparison, two types of PAN fibers were used, which were purchased from different sources. The yield of KOH activation was found to be closely related to BET specific surface area. As the KOH activation progressed, BET specific surface area was enlarged, which means that pore structure grew during the process. SEM images of KOH-activated PAN-B are shown in Figure 2. It was carbonized at 873K and KOH-activated at 1073K using KOH of four times the weight of the carbonized PAN. The fiber was broken into small pieces or melted and stuck together, while the original fibril structure was preserved as a whole. 2/7

3 Figure 1. Yields of KOH activation of carbonized PAN plotted versus BET specific surface area 50m 10m Figure 2. SEM images of KOH-activated PAN-B Figure 3. Hydrogen uptakes of porous carbon materials at 9.5 MPa and 303K plotted versus BET specific surface area 3/7

4 Hydrogen uptakes of prepared porous carbon materials were then measured at 9.5 MPa and 303 K, and the relationship between BET specific surface area and the amount of hydrogen uptake is shown in Figure 3. Hydrogen uptakes of ordinary porous carbon materials, such as activated carbon and activated carbon fibers, increased proportionally to specific surface area, and KOH-activated PAN-A,B showed tendencies similar to ordinary porous carbon materials. This means that the adsorption sites should increase in proportion to the specific surface area, which marked an obvious contrast with our previous work in which activated bamboo charcoal showed a higher hydrogen uptake than the value expected from the specific surface area. However, KOH-activated PAN-A,B showed higher hydrogen uptake than that of bamboo charcoal, whose uptake was 0.79 mass%. This suggests that the PAN-derived material s overwhelmingly vast BET specific surface area gives rise to the high uptake of hydrogen. Many synthetic parameters were tested, including carbonizing temperature, KOH activation temperature and the weight ratio of KOH/carbon, and two synthetic conditions showed the best hydrogen uptake (Table 1). Runs 2 and 3 were carried out at the same conditions, and both results fall within the experimental error, indicating fairly good reproducibility of the preparation. Table 1. Effect of the synthetic parameter of KOH-activated PAN-B on adsorption property No. Carbonizing temperature (K) KOH activation temperature (K) Weight ratio of KOH/carbon BET specific surface area Hydrogen uptake at 9.5 MPa and 303K (mass%) (m 2 /g) Silk fiber was tested as another fibrous precursor, and it also showed good adsorption properties. The relationship between BET specific surface area and hydrogen uptake is shown in Figure 3. KOH-activated silk showed adsorption properties similar to KOH-activated PAN-B. The best value of hydrogen uptake, 1.12 mass% at 9.5 MPa and 303K, was observed for a sample carbonized at 1173K and activated with KOH of seven times the weight of the carbonized silk. (a) (b) Figure 4. Nitrogen adsorption isotherm of porous carbon materials at 77K 4/7

5 In order to clarify the structure of pores of KOH-activated materials in detail, nitrogen adsorption measurement at 77K was performed as shown in Figure 4. The isotherm was measured below 10-7 of relative pressure, and the microporous structure was clarified. KOH-activated bamboo adsorbed nitrogen only at the low relative pressure, which means it consist only of micropores. In contrast, KOH-activated PAN and KOH-activated silk showed significant adsorption at both low and high relative pressure regions. It is well recognized that the relative pressure at which adsorption takes place and pore width are closely related. For instance, the relationship between slit width and relative pressure where condensation takes place was calculated by DFT method for slit type pores, indicating that gas condensation takes place at relative pressures 0.25 and 0.6 for slits of 2.6 nm and 4.6 nm, respectively. Both KOH-activated PAN and KOHactivated silk consist not only of micropores but also mesopores. However, the nitrogen uptake at 10-6 to 10-5 of relative pressure caused by ultramicropores is lower than KOH-activated bamboo, indicating the PAN- and silk-derived carbon samples contain fewer ultramicropores. On the contrary, total hydrogen uptakes of both samples were higher than the bamboo-derived one because of their overwhelmingly vast BET specific surface areas. The hydrogen adsorption isotherms of KOH-activated silk and KOH-activated bamboo at 20K are shown in Figure 5. This measurement uses hydrogen as a probe molecule, which is smaller than nitrogen. This result is consistent with nitrogen adsorption isotherms, and the ultimate adsorption was shown to reach up to 15 mass% for KOH-activated silk. Figure 5. Hydrogen adsorption isotherms of porous carbon materials at 20K 3.2 Hydrogen adsorption capacity at 35 MPa Hydrogen adsorption capacity at pressures higher than 9.5 MPa has to be measured in order to know the adsorption behavior in practical conditions. We are planning to use hydrogen storage materials under high pressure, for example 35 MPa, so an apparatus was devised to measure hydrogen adsorption capacity at that pressure. The apparatus is shown in Figure 6. First, porous carbon material was placed in a sample cell, while a reference cell was kept empty. 35 MPa hydrogen gas was introduced into both sample cell and reference cell, then all valves were closed. Valve 2 was gradually opened, and the volume of hydrogen gas running out from the sample cell was measured by a gas meter. Using a similar method, the volume of hydrogen gas in the reference cell was measured and hydrogen uptake was calculated from the difference between the two cells. The dead volume of the porous carbon material was taken into account in this calculation, assuming that the apparent density of carbon matrix is 2.26 g/cm 3. Hydrogen uptakes of porous carbons at 35 MPa and 303K are shown in Figure 7. Scattering of the plots may be the result of insufficient measurement accuracy. For comparison, commercial activated carbons were measured, and activated carbon A showed about 1 mass% hydrogen uptake at 35 MPa and 303K. In contrast, the KOH-activated PAN synthesized in our work showed about 1.5 mass% hydrogen uptake at 35 MPa and 303K, which is much higher than that of commercial activated carbons. 5/7

6 Reference cell Hydrogen Valve 3 Valve 4 Sample cell Gas meter Valve 1 Valve 2 Filter Quartz glass wool Hydrogen adsorbing material Figure 6. Apparatus suitable for measuring volumetric hydrogen adsorption capacity Figure 7. Hydrogen uptakes of porous carbons at 35 MPa and 303K 3.3 Coordination polymers In the expectation that materials containing elements other than carbon may show different adsorption phenomena, in addition to porous carbon, other materials such as coordination polymers were examined. When elements comprising the pore walls are different, the potential field felt by a hydrogen molecule in the pores must be different, even if they are of the same size. And coordination polymers can have a good variety and controllability of structure, which cannot be achieved with carbon materials. Three types of coordination polymers were synthesized, and adsorption properties were measured (Table 2). They are the same type of coordination polymers, which have jungle-gym-like open frameworks. Cu polymer- A, B consists of Cu 2+ ions, dicarboxylic acid, and diamine. Zn polymer has Zn 2+ ions instead of Cu 2+ ions. The hydrogen uptakes of these three coordination polymers at 9.5 MPa and 303K were lower than the values seen with porous carbon materials. Further development is necessary to raise hydrogen uptake to a practical level. But an interesting phenomenon was observed in these materials regarding adsorption density. The volumetric hydrogen density at 0.1 MPa and 77K in the pore is thought to be an index of affinity with hydrogen storage material and hydrogen gas. In past literature, many coordination polymers were investigated[8-10], and the highest value of volumetric hydrogen density at 0.1 MPa and 77K in the pore was 45kg/m 3, reported by M.P. Suh. Both Cu polymer-a and Zn polymer exceeded the highest value in past literature at the point of affinity. 6/7

7 Table 2. Hydrogen uptakes of coordination polymers Pore volume H 2 uptakes at 9.5 (cm 3 /g) MPa and 303K H 2 uptakes at 0.1 MPa and 77K (mass%) Volumetric H 2 density at 0.1 MPa and 77K (kg/m 3 ) 1) (mass%) Cu polymer - A Cu polymer - B Zn polymer ) Calculated by (H 2 uptake at 0.1 MPa and 77K) / (pore volume) 4. Conclusion Fibrous materials were found to be good precursors for synthesizing porous carbon materials. KOH-activated PAN showed 1.0 mass% hydrogen uptake at 9.5 MPa and 303K. When the pressure was raised to 35 MPa, this material showed 1.5 mass% hydrogen uptake at 303K. Porous carbon derived from silk fiber was also tested, showing 1.1 mass% hydrogen uptake at 9.5 MPa and 303K. The higher hydrogen uptake was thought to be caused by its overwhelmingly vast BET specific surface area. Although many carbon materials have been examined, it was very difficult to reach the target value, so other materials, such as coordination polymers, were examined. A coordination polymer showed 0.38 mass% hydrogen uptake at 9.5 MPa and 303K. It also showed a volumetric adsorption density of 47kg/m 3 at 0.1MPa and 77K, which is the highest value reported for a coordination polymer. 5. Acknowledgements The authors are grateful to Professor K. Kaneko and Dr. H. Tanaka of Chiba University for the nitrogen adsorption measurements at 77K and the hydrogen adsorption measurements at 20 K. We would like to thank Professor S. Kitagawa and Dr. M. Higuchi of Kyoto University for the synthesis of coordination polymers. References: [1] [2] A.C.Dillon, K.M. Jones, T.A. Bekkedahl, C.H.Kiang, D.S.Bethune, M.J.Heben, Nature 386,377 (1997). [3] A.C.Dillon, M.J.Heben, Appl.Phys. A 72, 133 (2001). [4] M.Hirscher, M.Becher, M.Haluska, U.Dettlaff-Weglikowska, A.Quintel, G.S.Duesberg, Y.M.Choi, P.Downes, M.Hulman,S.Roth, I.Stepanek, P.Bernier, Appl.Phys. A 72, 129 (2001). [5] A.Chambers, C.Park, R.T.K.Baker, N.M.Rodriguez, J.Phys.Chem.B 102, 4253 (1998). [6] C.C.Ahn, Y.Ye, B.V.Ratnakumar, C.Witham, R.C.Bowman, Jr., B.Fultz, Appl.Phy.Lett. 73, 3378 (1998). [7] S.Oshima, Y.Kude, O.Kato, Y.Kobori, 15th World Hydrogen Energy Conference, Yokohama, Japan June 2004 P [8] H.Chun, D.N.Dybtsev, H.Kim, K.Kim, Chem. Eur. J. 11, 3521 (2005). [9] E.Y.Lee, S.Y.Jang, M.P.Suh, J. Am. Chem. Soc. 127, 6374 (2005). [10] B.Chen, N.W.Ockwig, A.R.Millward, D.S.Contreas, O.M.Yaghi, Angew. Chem. Int. Ed. 44, 2 (2005). 7/7