The hydrogen storage capacities of several carbon nano-materials evaluated by using the PCT and gravimetric method.
The hydrogen is a clean fuel source, which produces water vapor as the only exhaust gas when it is burnt with oxygen. The chemical energy density of hydrogen (142 MJ/kg) is at least three times larger than that of other chemical fuels. When the hydrogen is electrochemically burnt using a fuel cell system, the efficiency can reach 50~60%, twice as much as the thermal process because the efficiency of the direct process of electron transfer from oxygen to hydrogen in a fuel cell system is not limited by the Carnot efficiency in . However, the hydrogen volume is 3000 times higher than that of gasoline at room temperature and atmosphere because it is a molecular gas. Therefore, on-board hydrogen energy storageneed compact,light, safe and affordable containment.The condensation of a monolayer of hydrogen on a solid leads to amaximum of 1.3x10–5 mol/m2 of adsorbed hydrogen. For automotive applications, the US DOE required a hydrogen storage capacity ofgreater than 6.5 wt% and ambient temperatures for hydrogenrelease and moderate storage pressures for industrialapplications. Hydrogen storage in carbon materials is a very attractive field since high gravimetric storage capacities may be possible owing to the low specific weight and high specific surface area of carbon. The reversibly adsorbed quantity ofhydrogen on nanostructure graphitic carbon amounts to 1.5 mass% per 1000 m2/g of specific surface area at 77 K(liquid nitrogen temperature) in . On active carbon with the specific surface area of1315 m2/g, 2 mass% of hydrogen was reversibly adsorbed at a temperatureof 77 K in .Carbon materials with different nanostructures areavailable for hydrogen storage, e.g. carbon nanofibers (CNF), graphite nanofiber (GNF), carbon nanohorns, multiwalledcarbon nanotubes (MWNT), and single-walled carbonnanotubes (SWNT).
Since the excellent 6 to 8 wt% hydrogen storage using carbon nano-materials at room temperature and with atmospheric pressure was first reported in , several studies on hydrogen storage using SWNT, MWNT, GNF, active carbon, and active carbon fibers etc., have been conducted in [4,5].The hydrogen adsorption of 4.2 wt% (0.5-H/C) at 100 bars and at room temperature was observed using SWNT synthesized through the arc electric discharge method in .A hydrogen adsorption of approximately 3 wt% at 3~100 MPa and room temperature was reported using a well-aligned SWNT bundle in . The hydrogen storage of SWNTs measured using volumetric method, however, showed scattered capacities within the range of 0.03~4 wt% at room temperature because of introduction of some error during the measurement in [7,8].As listed in Table 1, SWNTs in more accurate volumetric measurements showed low capacity within the range of 0.14~0.43 wt% and results for MWNTs and graphite powder were less than 0.04 wt% in . Despite the large volume of data from studies conducted on SWNTs, MWNTs, GNFs, etc., as potential hydrogen storage materials, these data are scattered and are thus inconclusive.
|SWNT||Carbon nanotechnology Inc.||298 K / 80 bars||0.43 wt%|
|SWNT||MTR, Ltd(20~40% purity)||298K/ 80bars||0.14wt%|
|MWNT||ground core, Strem Chemicals, Inc||298K/80bars||<0.04wt%|
|aligned MWNT bundle||ferrocenepyrolysis||300K/145bars||1.0~3.3wt%|
|aligned MWNT bundle||ferrocene/acetylene pyrolysis||300K/145bars||3.5~3.7wt%|
|Commercial ACF||A-20(Osaka Gas Chemicals Co. Ltd) /FT300-20(Kuraray Chemical Co., Ltd)_||298K/ 80bars||0.35~0.41wt%|
|vitreous carbon||80-200μm, 99.5% purity|
(Goodfellow Cambridge, Ltd.)
|298K/ 80bars||< 0.04wt%|
|Graphite powder||200μm, 99.997% purity|
(Goodfellow Cambridge Ltd.)
|298K/ 80bars||< 0.04wt%|
Typical carbon materials, such as active carbon, active carbon fiber, and graphite powder, were also investigated as potential materials for hydrogen storage. Purified SWNT (285 m2/g) and saran carbon (1600 m2/g) with a high BET surface area were also reported to have a hydrogen adsorption of approximately 0.04 and 0.28 H/C, respectively, at 0.32 MPa and 80 K in .Large hydrogen adsorption was also observed by a micro porous zeolite and active carbons at 77 K under atmospheric pressure in .In the case of highly porous carbon (AX-21 carbon), very high hydrogen adsorption of 5.3 wt% (0.64 H/C) was observed at 77 K and 1 MPa in .However, active carbon materials with very high surface areas showed very low capacities at room temperature.This may be due to the very low levels of the effective pore size for hydrogen storage in spite of their high surface areas. The hydrogen storage capacity of materials surface greatly depended on the adsorption potential energy between the materials and hydrogen molecules. But too high adsorption potential energy may give to irreversible storage with chemisorptions of hydrogen molecule. The potential fields from opposite walls may overlap so that the attractive force acting on hydrogen molecules is greater than that on an open flat surface. Therefore, in micro porous carbon materials the pores with a width not exceeding a few hydrogen molecules may be more effective pores for hydrogen storage because of the dynamic diameter of hydrogen molecule with 0.41 nm, in .
TiO2 nanotubes could reproducibly store up to about 2 wt% H2 at room temperature and 60 bars in . However, only 75% of the H2 is physisorbed and can be reversibly released upon pressure reduction. Approximately 13% is weakly chemisorbed and can be released at 70 °C as H2, and 12% is bonded to oxide ions and released only at temperatures above 120 °C as H2O. The sorption of hydrogen between the layers of the multilayered wall of nanotubular TiO2was also investigated in the temperature range of -195 to 200 °C and at pressures of 0 to 6 bar and it got a 1~2.5 wt% hydrogen sorption at 1 bar and temperatures in the range 80 to 125 °C, in . The hydrogen storage capacity of 0.83 wt% using ZnO nanowires with the mean diameter of 20 nm was found under the pressure of about 3.03 MPa and at room temperature, and about 71% of the stored hydrogen can be released under ambient pressure at room temperature, in . And also hydrogen storages using MoS2 nanotube and TiS2 nanotube were investigated in [15,16]. So we need the study on the effective pore size of materials with appropriate adsorption potential energy for hydrogen storage at room temperature rather than large surface area of materials.
Carbon fibers have been used in a variety of fields as high-performance and functional materials. Woven or nonwoven carbon fibers are used as absorbed materials because of better adsorption capacity than conventional activated granular and powder carbon materials. They are being applied to gas separation and liquid adsorption. Saran carbons showed higher the hydrogen storage capacity than that of other active carbon materials, in . Poly(vinylidene fluoride) (PVdF) also can be used in obtaining meso porous carbon similar to saran polymers, in . The carbonization of PVdF can also produce a polyacetylene or carbyne structure and its pore size may be smaller than that of saran carbon due to the small size of the fluorine atom. Nano-sized fibers may be more helpful in obtaining carbon materials with a well-defined pore structure compared to the carbonization of micro-sized polymer fibers. Because thinner fibers are expected to be more desirable for those separation and adsorption applications, there has been growing interest in electrospinning for producing ultrafine fibers.
The recent electrospinning process for polymer or metal oxide sole gel solutions is a powerful method for producing ultrafine fibers within the range of a few to a few hundred nanometers in diameter, core-shell nanostructure nanofibers, etc., which cannot be easily obtained using traditional methods. There is a growing interest in the electrospinning process of polymer or metal oxide sol-gel solution because of their several potential applications such as ultrasensitive gas sensors, polymer electrolytes for lithium ion polymer battery, dye-sensitized solar cell etc., in [18~22]. Thus, such electrospun polymeric nanofibers can be used as effective precursors for carbon nanofibers.In addition, a transition metal would promote carbonization and graphitization of polymeric precursor, which was verified for Kapton films by various research groups, in [23~25]. Recent reports showed the effects of a transition metal on the carbonization behavior of the electrospun polyimide nanofiber, PAN nanofibers and PVdF nanofibers, resulting in graphite nanofiber (GNF), in [26,27]. And also electrospinning of metal oxide sol-gel solution provide metal oxide nanofibers with various morphologies after calcinations. They are also expected to have some hydrogen storage because of higher adsorption potential energy between the metal oxide materials and hydrogen molecules although they had much lower surface area than those of electrospun polymer based carbon nanofibers.
In this chapter, the preparation of carbon nanofibers, graphite nanofiber, and metal oxide nanofibers such as titanium oxide and lithium titanate nanofiber through heat-treatment of electrospun precursor nanofibers, their structural properties such as surface area and pore size, and morphologies were investigated. And their hydrogen storage capacities discussed with their pore size and surface area.
2. Results and discussion
2.1. Morphology and crystalline structure of CNFs, GNFs, and lithium titanate nanofiber
Polyacrylonitrile (PAN) typically has been used for preparation of carbon fiber with high performance. Carbonizations of PVdF or saran polymers also give to mesoporous or microporous carbon materials. Ultrafine structure of electrospun PVdF nanofibers is expected to be suitable for the formation of pores which are effective for hydrogen adsorption, compared to PVdF films or microfibers. So, micro porous carbon nanofibers as hydrogen storage materials were prepared through the carbonization of as-electrospun PAN and PVdF nanofibers.As-electrospun PAN or PVdF-based nanofibers were prepared from the typical electrospinning of the polymer solution containing several contents of iron (III) acetylacetonate (IAA) on the weight of the polymer as a catalyst for graphitization. Carbon nanofibers (CNF) and graphite nanofiber (GNF) were prepared from carbonization after stabilization of them, in [28~31].
Firstly polyacrylonitrile (PAN) solutions for electrospinning wereprepared by dissolvingPAN (Mw 150,000, polyscience) in N,N’-dimethylacetamide.The PAN solutions contained0 wt%, 2 wt%, 5wt%, and 7.5 wt%of iron (III) acetylacetonate (IAA) on the weight of thepolymer as a catalyst for graphitization. As-electrospun PAN-based nanofibers were stabilized by heating them at a rate of 1 oC/min up to 260 oC, and by holding themfor 2 hrs under air atmosphere. Carbonization was performedat a given temperature within the range of 900 to 1500 oCunder nitrogen atmosphere. The samples were kept for 1 hr sequentially at 400 and600 oC andthen heated up to final temperature at arate of 3 oC/min. As-electrospun PVdF nanofibers were also obtained from electrospinning of 11 wt% PVdF solution (Kynar 761) in acetone/N,N’-dimethylacetamide(=7/3, wt. ratio) mixture. They were slightly dehydrofluorinated (DHF) in the methanol/ N, N’-dimethylacetamide (=9/1 wt. ratio) solution containing 10 ml of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) at 50 oC for 5 hours. They were also highly dehydrofluorinated in the methanol/DBU (=1/2 wt. ratio). A PVdF solutions for GNFs were prepared by dissolving 11 wt% ofPVdF in 100 ml of acetone/N,N’-dimethylacetamide(=7/3, wt. ratio) mixture containing 25 ml of 1,8-diazabicyclo[5.4.0]undec-7-ene for partial dehydrofluorination and also contained 5.5 wt% of IAA basedon the weight of the polymer as a catalyst for graphitization. As-electrospun PVdF nanofibers for GNFs were chemically dehydrofluorinated with a 4 MaqueousNaOH solution containing 0.25 mmole of tetrabutylammoniumbromide at 70oC for 1 h. Carbonization was performed to induce micropore structureswithout a further activation process at a given temperaturewithin the range 800~1800 oC under a nitrogen atmosphere.The samples were heated at a rate of 3 oC/min and weremaintained for 1 h at the final temperature.
Figure 1 shows the SEM images of electrospun PAN- and PVdF-based CNFs. In the case of as-electrospun PAN-based nanofiber with a diameter of about 90 nm the fiber diameter hardly changed during the carbonization at 1300 oC, while that with a diameter ofabout 240 nm remarkably shrank to 110 nm and showed roughened surfaces. Solvent evaporation during the electrospinningprocessgreatlyhas an effect on internal structure of the resulting PAN nanofibers. Thin fiber is denser and has a higher orientation than thick fiber because it is formed at a much higher draw ratio and with a much faster solvent evaporation during the electrospinning process. Oxidation and carbonization of the above as-electrospun PAN nanofibers were carried out under a tensionless condition or with a slight tension. Therefore, the dimensions of dense, highly oriented ultrathin fibers are thought to hardly change during carbonization and the PAN-based CNFs showed very smooth surfaces. In the case of as-electrospun PVdF nanofibers, it is also necessary to make the nanofibers infusible to maintain their fibrous shape through dehydrofluorination (DHF) treatment before the carbonization. The effect of DHF treatment in the carbonization of PVdF polymer has been reported in previous studies, in . The structure of the PVdF-based CNFs greatly depends on the DHF condition, in . The carbon nanofibers carbonized after a high DHF treatment had very smooth surfaces and dense, nonporous structures in Figure 1(c), while slightly DHF treatment gave to the micro porous carbon nanofibers with granular-shaped surfaces, and with an internal structure consisted of 20 to 30-nm carbon granules after carbonizationat above 800 oC, as shown in Figure 1(d). The pore structure became dense with the increase of carbonization temperature, indicating the formation of micro pores at higher temperature. The slightDHF treatment of PVdF nanofibers induced inhomogeneous structures consistingof the DHF-treated, amophorous region and the non-reacted crystalline region. The non-reactedcrystalline region melted at a high temperature while the dehydrofluorinated region maintained its fibrous shape during thecarbonization. The onset temperature and the amount of volume reduction during carbonization differed between the non-reacted and reacted regions. Thus, relatively large pores were produced between these regions.
As shown in Figure 2, the PAN-based CNFs had disordered, amorphous carbon structures with
Catalytic graphitization using volatile hydrocarbon fractions during the carbonization may be helpful in increasing the carbon yieldand in forming effective ultra micro pores for hydrogen storage. For these purpose, as-electrospun PAN and PVdF nanofiberscontaining IAA were carbonized to induce catalytic graphitization within the range 800~1800 oC under a nitrogen atmosphere. PAN based graphite nanofibers (GNF) with a diameter of 150-300 nm were prepared by carbonization at 900~1500 oC after stabilization of as-electrospun PAN nanofibers containing 2, 5, and 7.5 wt% of IAA at air atmosphere, respectively. Figure 3 and 4 shows SEM and TEM images of PAN-based GNF. White spots were observed on the surface of the GNF fibers (1100 oC), indicating the development of graphite crystal structures centered on the Fe catalyst. This catalytic graphitization was accelerated at above 1300 oC. The TEM image around the white spot on the surface of the GNF showed a well-ordered graphite structure similar to natural graphite.
In the case of PVdF-based GNF, a notable grainy structure was observed on the surface and cross-section of the GNFs. The partial dehydrofluorination of PVdF nanofibers induced inhomogeneous structures consisting of a dehydrofluorinated amorphous region and an non-reacted crystalline region. Therefore, carbonization of them produced porous GNFs with a high surface area due to their porous granular surface. Figure 5 showsSEM and TEM images of PVdF-based GNFs. Clusters of Fe catalyst and the development of graphite structures centered on the Fe catalyst are clearly observed in TEM images of PVdF-based GNFs. The size of the Fe catalyst is from a few tens to a few hundreds of nanometers.
Figure 6 and 7 shows XRD patternsof PAN and PVdF-based GNFs. The catalytic graphitization of electrospun PAN nanofibersintensively started to proceed from 900 oC, while PVdF nanofibers intensively started to proceed from 800 oC. The sharp peaks in the PAN-based GNF at 1100 oC were observed at around 26o (002) and 42-46o (100), respectively. New peaks also appeared at 35o and 50o, corresponding to Fe3O4. In the case of PVdF-based GNFs, the GNFs at 800 oC show sharp peaks at approximately 26o and 44o, corresponding to the diffraction of the (002) plane and (100)/ (101) of the graphite structure, respectively. The presence of the (112) peak at 83o is also indicative of a graphite structure. The intensities of these peaks increased and sharpened with the carbonization temperature. New peaks also appeared at 36o and 48o, corresponding to Fe3O4. It was assumed that IAA was converted into Fe3O4
|PAN-based CNF||1300||25.98||"/> 0.370||-|
TiO2 nanofiber was also prepared from typical electrospinning of a mixture solution of titanium tetraisoproxide and polyvinyl acetate (PVAc, Mw 500,000) in N, N’-dimethylacetamide (DMF).As-electrospun TiO2/PVAc nanofiber was calcined at 450oC to completely remove the PVAc component by thermal decomposition and to give to TiO2 nanofiber. As shown in Figure 9, the resulting TiO2 nanofiber showed a smooth surface and internal structure composed of 20- to 50-nm TiO2 granules. Figure 10 indicated this TiO2 nanofiber was composed of typical anatase crystalline. Lithium titanate nanofiber was also prepared by electrospinning using a mixture of LiNO3 andtitanium tetraisoproxide (1:2 mole ratio) instead of titanium tetraisoproxidesimilar to preparation of TiO2 nanofiber.As-electrospun lithium titanate/PVAc nanofibers were calcined at 450oC, 600oC, and 700oC to remove the PVAccomponent by thermal decomposition and to give to lithium titanate nanofibers. As shown in Figure 9, the lithium titanate nanofiber calcined at 450 oC showed lots of wrinkled structure in the surface and it looked like the surface composed of 20 to 60-nm nanorods. Crystalline size of lithium titanate was increased with increase of calcinations temperature. The lithium titanate nanofibers calcined at 600 oC and 700 oC showed the fibrous morphology composed of lithium titanate granules. These lithium titanate nanofibers calcined at above 450oC were typical crystalline structure of LiTi2O4 as show in Figure 10, in [33, 34].
2.2. Specific surface area and pore structure
The electrospun PAN-based CNFs showed the typical adsorption curves very similar to that of nonporous carbon in the nitrogen gas adsorption-desorption isotherms, while the PAN-based GNFs showed the typical curve of micro porous carbon in addition to a hysteresis loop that indicates existence of the meso pore, as shown in Figure 11. The PAN-based CNFs and GNFs had low surface areas within the range of 22~31 m2/g and 60~253 m2/g, respectively, as listed in Table 3. The surface areas of PAN-based GNFs were much higher than the CNFs, but they decreased with increase of carbonization temperature and increased with increase of IAA content. Although this could not be fully explained at present, it may be due to the surface roughness and inhomogeneous structure of the GNFs, which resulted from the induction of the metal catalyst in the GNFs. But they still had much lower surface area compared to common active carbon. Commercial active carbons and active carbon fibers generally have very high surface areas of above 1000m2/g, and SWNT also has a surface area of a few hundred m2/g. They had low storage capacities, however, within the range of 0.35-0.41 wt%, at room temperature, in . So, high hydrogen adsorption by PAN-based CNFs and GNFs with very low surface area may notbe expected. However, if they have effective pores for hydrogen storage when compared to hydrogen molecule sizes of about 0.41 nm, they may show high hydrogen adsorption. The electrospun PAN-based GNFs showed the adsorption curves very similar to that of meso porous carbon in the nitrogen gas adsorption-desorption isotherms in spite of their low surface area and also they had micro pores unlike those in the CNFs in the nitrogen gas adsorption-desorption isotherms. In the case of the PAN-based CNFs the change of the pore volumes with increase of carbonization temperature did not show because of their very low surface area. The micro pore volumes and mesopore volumes of the PAN-based GNFs, however, decreased with increase of carbonization temperature.
The electropun PVdF-based CNFs prepared after a slight DHF-treatment showed typical curves of microporous carbon in the nitrogen gas adsorption-desorption isotherms. They showed high surface areas of 414~1300 m2/g. BET surface area rapidly decreased with increase of carbonization temperature, as shown in Table 3.Micro pore volume at 1500 oC greatly decreased while meso pore volume continuously increased with an increase of carbonization temperature. However, the PVdF-based CNF at 1800 oC showed a very high surface area of 1300 m2/g and a high volume (1.767 cm3/g) of only ultra- or super micro pores. The PVdF-based CNFs prepared after high DHF-treatment showed very low surface area and adsorption curves similar to those of nonporous carbon in nitrogen gas adsorption-desorption isotherms. They did not have the micro pores. In the case of PVdF-based GNF nitrogen adsorption–desorption isotherms were the type II showing a hysteresis loop that indicates the existence of meso pores, as shown in Figure 11.They showed high surface areas of 377~473 m2/g but still have low surface areas compared to typical active carbon. The BETsurface area and micro pore volume decreased with increasing carbonization temperature, as listed in Table 3. Decreases in the surface area and micro pore volume are thought to be due to densification of the porous structure with increasing carbonizationtemperature. Nitrogen adsorption–desorption isotherms for the LiTi2O4 nanofiber calcined at 450 oC were also the type II showing a hysteresis loop that indicates the existence of meso pores, as shown in Figure 10. However, the TiO2 nanofiber and the LiTi2O4 nanofibers calcined at 450 oC have very low surface are of about 50 m2/g, as listed in Table 4. The surface area of the LiTi2O4nanofibers calcined at 700 oC decreased with increase of crystalline size.
The pores in carbon materials are classified by their size into macro pores (>50 nm), meso pores (>2-50 nm), and micro pores (<2 nm) according to IUPAC. Micro pores are further divided into super micro pores with a size of 0.7~2 nm and ultra micro pores ofless than 0.7 nm, in .Unfortunately, it is difficult to exactly analyze the ultra micro pore size distribution and volume in porous carbon through the nitrogen gas adsorption-desorption isotherms measurement when compared to the kinetic diameter of hydrogen molecules. In the case of direct observations of pores on the surface of carbon materials by scanning tunneling microscopy (STM/AFM), the problem is how to differentiate the pores from other surface defects, such as depressions and trenches. The net ultra micro pore volume of carbon material cannot obtain from the pore analysis of small area on the surface by STM. The oxidation and carbonization of PAN precursor fiber for making carbon fiber usually accompany with the release of NH3, HCN, N2 gases, etc., resulting in the formation of pores within the carbon fiber structure, in . So the preparation of carbon fiber with high tensile strength requires the removal of pore structure by heat treatment at high temperature. The carbonization of the electrospun PVdF nanofibers is also usually accompanied by the release of HF, H2, F2 and other gases, resulting in the formation of pores within the carbon fiber structure. In addition, micro and meso pores are generated through the carbonization of PVdF nanofibers after partial dehydrofluorination. The calcinations of as-electrospun lithium titanate/PVAc nanofibers or TiO2/PVAc nanofibers may also produce the pore structure by evaporation of thermally decomposition product of PVAc. Therefore, we assume the generation of ultra micro pores and super micro pore during the carbonization and calcinations of the electrospun polymeric nanofibers and metal oxide nanofiber precursors. These pore structures became dense with the increase of carbonization temperature. Therefore, increase of carbonization temperature might bring out the increase of ultra- and super micro pore volume instead of loss of large pores.
|Samples||Carbonization temperature(oC)||Total pore volume(cm3/g)||IAA|
|Surface area(m2/g)||Pore Size Distribution(cm3/g)|
|< 1 nma||1~2 nma||2~4 nmb||4~10 nmb|
|PVdF- CNF||low DHF||800||-||0||967||0.146||0.052||0.075||0.070|
|Pore Size Distribution(cm3/g)|
2.3. Hydrogen storage capacities
The hydrogen storage capacity of electrospun nanofibrous materials in this chapter was evaluated through the gravimetric method using magnetic suspension balance (MSB, Rubotherm), as show in Figure 12.First, the blank test chamber containing samples was evacuated, to remove the impurities and water, at 150 oC/10-6 torr for 6 hrs. The weights of the sample basket and samples were then measured at 10-6 torr/25 oC (±0.5 oC) and at a He gas atmosphere of 10 bars, respectively. It was assumed that He gas was not adsorbed by the nanofiber samples in this condition. The weight difference between the vacuum and the 10 bars He gas, which indicates buoyancy due to the He gas, was used to determine the volume of the nanofiber samples, as follows:
where W2 is the weight difference of the samples between in the vacuum and at the specific H2 pressure,
In the case of monolayer condensation of hydrogen on carbon absorbents, theoretical quantity of absorption is 1.3×10-5 mol/m2, in[1,13], and the quantity of reversible hydrogen absorption is known to proportional to specific surface area of absorbents.Commercial activecarbons and active carbon fibers generally have very high surface areas of above 1000 m2/g, and SWNT alsohas a surface area of afew hundred m2/g. They showed low storage capacities, however, within the range of 0.35~0.41 wt%, at room temperature, in . Low storage capacity of carbon materials at ambient temperature is due to too low absorption potential between carbon and hydrogen. If the tendency that hydrogen is going to escape from carbon absorbent is smaller than absorption potential, hydrogen will be absorbed as condensed phase by whole micro pore. It is predicted there are the optimum pore size and pore geometry for hydrogen absorption. Therefore, when the kinetic diameter of hydrogen molecule (0.41 nm) is considered, ultra micro pores (< 0.7 nm) are expected by doing important contribution for hydrogen storage by means of nanocapillary mechanism and superposing of potential on the pore wall substantially. Hydrogen adsorption of the CNFs with very low surface area may not be expected because of their low surface area. If they have ultra micro pores, however, which may be effective for hydrogen storage,they will show hydrogen adsorption.
Figure 13 shows the hydrogen storage results of the electrospun PAN-based CNFs and GNFs under several hydrogen pressures and at room temperature. The hydrogen storage was measured after 2 hrs under a specific hydrogen pressure. Their hydrogen adsorption continuously increased even after 2 hrs, and also increased with increase of hydrogen pressure. The dotted line in Figure 13(a) indicates the increased hydrogen storage capacities after 16 hrs. The hydrogen storage capacities of the PAN-based CNFs obtainedfrom carbonization at 1000oC, 1100 oC and 1300oC, and 1500oC were 0.16 wt%, 0.37 wt%, 0.50 wt%, and 0.26 wt%respectively, although they were nonporous carbon with very low surface areas in the nitrogen gas adsorption-desorption isotherms. These may indicate the presence of ultra micro pores that cannot differentiate by using nitrogen gas adsorption-desorption isotherms. Thereduction of hydrogen storage in the CNF obtained from carbonization at 1500 oC was thought to be due to the disappearance ofpore structure including ultra micro pore at high carbonization temperature of above 1300 oC. In the case of the PAN-based GNFs the hydrogen storage capacities increased with increase of carbonization temperature and the content of IAA catalyst, as shown in Figure 13(b), andwere higher than those of the CNFs. The hydrogen storages of PAN-based GNF-5 showed highest capacity of 1.01 wt% at 1300 oC and lowest capacity of 0.14 wt% at 1500 oCsimilar to those of the PAN based CNF samples. Increase of the content of IAA resulted in increase of the hydrogen storage. The hydrogen storage of the PAN-based GNF-7.5 at 1300 oC, however, showed very low storage of 0.32 wt% though it had higher surface area and higher micro-, meso pore volume than those of GNF-2 and GNF-5 at 1300 oC. So, the hydrogen storage of the PAN-based CNF and GNF did not show the correlation with surface area, and micro-, meso pore volume in Table 3.Fe metal catalyst in the GNFs may contribute to the hydrogen adsorption. Figure 14 showed the cycle property about the hydrogen adsorption of the PAN-based GNF-5 (1300oC), which showed highest storage capacity. The GNF-5 (1300 oC) still retained initial hydrogen capacity storages, indicating physisorption of hydrogen. However, about 0.078 wt% of hydrogen did not desorbed under atmosphere and vacuum of 10-6 torr at room temperature. This is thought to be chemisorptions by Fe metal catalyst.
Figure 15 shows the hydrogen storage capacities of the electrospun PVdF-based CNFs. The PVdF-based CNFs (high DHF) also showed some hydrogen absorption although they had no micro pore volumes. The hydrogen absorptions of about 0.3 wt% (100 H2 bar) were observed in the PVdF-based CNFs (high DHF) prepared at 1,300~1500 oC although they have very low specific surface area of 16~33 m2/g. But the PVdF-based CNFs (low DHF) prepared at 800~1300 oC showed the hydrogen storage capacities of only0.05~0.2 wt% in spite of high specific surface area of 865~967 m2/g. The hydrogen adsorption of the carbon nanofibers with high surface areas decreased with the increase of hydrogen pressure. This may be due to the buoyancy effect of hydrogen gasadsorbed on the samples. Hydrogen storage capacities of the electrospun PVdF-based CNF (low DHF) increased with increase of carbonization temperature and showed the maximum value of about 0.39 wt% at 1500 oC in spite of its lowest surface area. And also the PVdF-based CNF (low DHF) carbonized at 1800 oC showed hydrogen storage capacity of 0.39 wt% even though it had highest surface area of 1300 m2/g and highest ultra micro pore volume of 1.767 cm3/g, similar to the activated carbon fibers having 0.35~0.41 wt% hydrogen storage, in . This is thought to be due to the disappearance of the pore structure including ultra micro pores at high carbonization temperatures.Figure 16shows hydrogen storage results for the PVdF-based GNFs under several hydrogen pressures at room temperature. Their hydrogen adsorption increased with increase of carbonization temperature while specific surface area and micro pore volume (< 1 nm) were decreased, but they showed very low storage capacities of about 0.1~0.2 wt% although they have highly graphite crystalline structure.
The hydrogen storage capacity of the GNFs and CNFs did not show correlation with surface area or micro- and meso pore volume, as shown in Table 3. The quantity of adsorbed hydrogen on nanostructure graphitic carbon as well active carbon materials at 77 K is proportional to the specific surface area of carbon materials, in .However, because at ambient temperature the thermal motion of hydrogen molecules overcomes van der Waals-type weak physisorption of molecular hydrogen, their hydrogen storage capacities were very low. So the hydrogen adsorption on the GNFs and CNF samples may be influenced by pore structure as well as specific surface area.Therefore, we think that micro- and meso pores that are calculated using the nitrogen gas adsorption–desorption isotherms are not the effective pore for hydrogen storage. The effective pore for hydrogen storage may require small pore size not exceeding 1 nm, when compared to the kinetic diameter of hydrogen molecule of about 0.41 nm. It is assumed that these micro pores are different from the micro pores calculated using nitrogen adsorption–desorption isotherms. Thus, hydrogen adsorptions by the electrospun PAN or PVdF-based CNFs and GNFs may be due to the presence of ultra micro pores rather than micro- and meso pores, even though they have very low surface areas compared to commercially available active carbons and active carbon fibers.
The multilayered TiO2 nanotubes with a surface area of 199 m2/g (a pore size; 8 nm, and a pore volume; 0.70 cm3/g) had known to store a 1~2.5 wt% hydrogen at 1 bar and room temperatures in therange 80 to 125 °C, in . This high storage capacity may resultfrom much higher adsorption potential energy between the multilayered TiO2 nanotubes and hydrogen molecules than those of carbonmaterials in consideration of very low hydrogen storage of typical meso porous carbon materials with high surface area.Figure 17shows hydrogen storage results for the electrospun TiO2 nanofiber and LiTi2O4 nanofibers. The hydrogen storage was measured after 2 hrs under a specific hydrogen pressure. The TiO2 nanofiber and LiTi2O4nanofibers calcined at 450 oC showed high hydrogen storages of 1.11 wt% and 0.74wt% in spite of their low surface area of 49.4 m2/g and 50.2 m2/g, respectively. Their hydrogen absorptions were higher than those of the electrospun CNFs and GNFs. Although we presently cannot determine effective ultra micro pore volumes for hydrogen storage inthe TiO2 nanofiber and LiTi2O4nanofibers, these are thought to be due to higher adsorption potential energy between the metal oxide materials and hydrogen molecules than those of carbon nanofibers. The LiTi2O4 nanofiber showed higher hydrogen storage than TiO2 nanofiber with similar surface area. This is also thought to be due to higher adsorption potential energy of LiTi2O4 nanofiber than TiO2nanofiber.The hydrogen storage of the LiTi2O4nanofiber calcined at 700 oC was greatly reduced to 0.41 wt%. Increase of the calcination temperature resulted in decrease of hydrogen storage with great reduction of surface area, indicating loss of effective pores with increase of LiTi2O4 crystalline size, as shown in Figure 9.Figure 18 showed the hydrogen adsorption/desorption cycle of electrospun LiTi2O4 nanofibers. Their hydrogen adsorption continuously increased even after 2 hrs under hydrogen pressure of 100 bars. The hydrogen storage of the LiTi2O4nanofibers calcined at 450oC and 600oC were about 1.50wt% and 1.23 wt% at the equilibrium state under hydrogen pressure of 100 bars, respectively. However, about 0.06 wt% and 0.054 wt% of hydrogen were not desorbed under atmosphere and vacuum of 10-6 torr at room temperature, respectively. These are thought to be chemisorptions.
2.4. Further work
Active carbon with the same specific surface area reversibly adsorbed 2 mass% hydrogen at a temperatureof 77 K, in [1,2]. The quantity of adsorbed hydrogen on nanostructured graphitic carbon as well active carbon materials at 77 K is proportional to the specific surface area of carbon materials, in . However, because at ambient temperature the thermal motion of hydrogen molecules overcomes van der Waals-type weak physisorption of molecular hydrogen, high surface area and large micro- and meso pores volumes of active carbon does not greatly contribute to the hydrogen adsorption. The effective pores for hydrogen storage are assumed to be ultra micro pores with small pore size not exceeding 1 nm, when compared to the kinetic diameter of hydrogen molecule of about 0.41 nm. The electrospun CNF and GNF were prepared by carbonization without further activation process to induce increase of ultra micro pore through heat-treatment for densification of large pores structure at high temperature. Thus, hydrogen adsorptions results of the electrospun CNFs and GNFs indicated the presence of ultra micro poreseven though they have very low surface area and micro-, meso pores, that are calculated using the nitrogen gas adsorption–desorption isotherm, when compared to commercially available active carbons or active carbon fibers.For automotive and industrial applications, the solid absorbent with hydrogen storage capacity ofgreater than 6.5 wt% and ambient temperatures for hydrogenrelease are presently required. The hydrogen storage capacities of the electrospun CNFs and GNFs, however, still showed the limitations in overcoming this requirement. That is, it may not be possible to increase effective ultra micro pores for above 6.5 wt% hydrogen storage even though we can find more improved process for the CNF and GNF in future.
On the other hands, the hydrogen storage results inTiO2 nanofiber and LiTi2O4 nanofibers gave to some encouragement for overcoming hydrogen storage target. They showed 1.2~1.5 wt% hydrogen in spite of very low surface area of about 50m2/g. These were higher than those of the electrospun CNFs and GNFs because the TiO2 nanofiber and LiTi2O4 nanofibers are thought to be due to higher adsorption potential energy between the metal oxide materials and hydrogen molecules than those of carbon nanofibers. So, future works for the high hydrogen storage capacity need new solid absorbent materials with a high ultra micro pore volume, high surface area, and appropriate hydrogen adsorption potential energy for only reversible physisorption.
Therefore, further work for this purpose will be tried to prepare new nanostructures metal oxide nanofibers with high reversible physisorption at ambient temperature, which are controlled to a high surface area and high ultra micro pore volumes.
The hydrogen storage capacities of electrospun nanofibrous materials were discussed in view of their pore size, surface area, and adsorption potential energy for hydrogen molecules. Carbon nanofibers (CNF) and graphite nanofibers (GNF) were prepared through the carbonization of the electrospun PAN- and PVdF-based nanofibers. The TiO2 nanofiber and LiTi2O4 were also prepared through typical electrospinning of precursor solutions.
The hydrogen storage capacities of the PAN-based CNFs prepared by carbonization at 1000 oC, 1100 oC and 1300oC, and 1500oC were 0.16 wt%, 0.37 wt%, 0.50 wt%, and 0.26 wt% respectively, although they were nonporous carbon with very low surface areas of about 22~32 m2/g in the nitrogen gas adsorption-desorption isotherms. The PVdF-based CNFs (high DHF) prepared by carbonization at 1300oC and 1500oC showed very low surface area of about 16~26 m2/g without the micro pores and showed theadsorption curves similar to those of nonporous carbon in nitrogen gas adsorption-desorption isotherms. However, they also stored the hydrogen of 0.30 wt% and 0.33 wt%, respectively.
The PAN-based GNFs had some micro-, meso pores and higher surface areas of 60~253 m2/g than the CNFs though they were still much lower surface area compared to common active carbon. Their surface area decreased with increase of carbonization temperature and increased with increase of IAA content. Although this could not be fully explained at present, it may be due to the surface roughness and inhomogeneous structure of the GNFs, which resulted from the induction of the metal catalyst in the GNFs. The hydrogen storage capacities of them increased with increase of carbonization temperature and the content of IAA catalyst, andwere higher than those of the CNFs. The hydrogen storages of PAN-based GNF-5 showed highest capacity of 1.01 wt% at 1300 oC and lowest capacity of 0.14 wt% at 1500 oC.
The PVdF-based CNFs (low DHF) showed typical curves of micro porous carbon in the nitrogen gas adsorption-desorption isotherms. They had high surface areas of 414~1300 m2/g and stored the hydrogen of 0.04-0.39 wt%. The PVdF-based CNF (low DHF) at 1800 oC showed the hydrogen storage of only 0.38 wt% in spite of high surface area of 1300 m2/g and a high volume (1.767 cm3/g) of only ultra- or super micro pores. Nitrogen adsorption–desorption isotherms for the PVdF-based GNFs prepared from carbonization at 800~1800oC were the type II showing a hysteresis loop that indicates the existence of meso pores. They had high surface areas of 377~473 m2/g but showed very low storage capacities of about 0.1~0.2 wt% although they have highly graphite crystalline structure.
The above hydrogen storage capacities of the GNFs and CNFs did not show any correlations with surface area or micro- and meso pore volume calculated using nitrogen adsorption–desorption isotherms. Because at ambient temperature the thermal motion of hydrogen molecules overcomes van der Waals-type weak physisorption of molecular hydrogen, their hydrogen storage capacities were very low. So the hydrogen adsorption on the GNFs and CNF samples may be influenced by pore structure as well as specific surface area.Therefore, micro- and meso pores that are calculated using the nitrogen gas adsorption–desorption isotherms are not thought to be the effective pore for hydrogen storage. Theeffective pore for hydrogen storage may require small pore size not exceeding 1 nm, when compared to the kinetic diameter of hydrogen molecule of about 0.41 nm.Thus, hydrogen adsorptions by the electrospun PAN or PVdF-based CNFs and GNFs may be due to the presence of ultra micro pores rather than micro- and meso pores, even though they have very low surface areas compared to commercially available active carbons and active carbon fibers.
The TiO2 nanofiber and LiTi2O4nanofibers calcined at 450 oC showed high hydrogen storages of 1.11 wt% and 0.74wt% in spite oftheir low surface areas of 49.4 m2/g and 50.2 m2/g, respectively. Their hydrogen storages were higher than the electrospun CNFs and GNFs.Although we presently cannot determine the effective ultra micro pore volumes for hydrogen storage, their high hydrogen adsorptions are thought to be due to higher adsorption potential energy than those of carbon nanofibers. The hydrogen storage of LiTi2O4 nanofiber was higher than that of TiO2 nanofiber with similar surface area, indicating higher adsorption potential energy of LiTi2O4nanofiber than that of TiO2 nanofiber. So, the high hydrogen storage capacity need new solid absorbent materials with a high ultra micro pore volume, high surface area, and appropriate hydrogen adsorption potential energy for only reversible physisorption at ambient temperature.