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New Materials For High Pressurized Hydrogen Storage
Prof. Dr. M. Oktay Alnıak1* , Yüksel Palacı2 and İbrahim Güneş3
1Bahcesehir University, Faculty of Engineering İstanbul, Turkey, 2Teknoser Teknik Seramik ve Kompozitleri San. Ve Tic. Ltd. Sti. , ,
3Istanbul University , Electrical Engineering Dept. İstanbul, Turkey,
*Bahcesehir University, Faculty of Engineering İstanbul, Turkey,
Abstract
Main Energy sources coal, petroleum and natural gas are the fossil fuels we use today. They are going to exhaust is a fact that we are going to face in the near future. On the other hand as the fossil fuels pollute the environment makes the hydrogen stand as an alternative energy source to the fossil fuels. Due to the unsolved problems in hydrogen’s production, storage and converting into electrical energy, extensive usage of Hydrogen is delaying. Hydrogen storage stands on an important point in the development of Hydrogen energy Technologies. Hydrogen is volumetrically low energy concentration fuel. Hydrogen energy, to meet the energy quantity necessary for the nowadays technologies and to be accepted economically and physically against fossil fuels, Hydrogen storage technologies must be developed. Today the most common method in hydrogen storage is high pressure tanks. Hydrogen is stored as liquid and gaseus phases. Liquid hygrogen phase can be stored by using composite tanks
Some materials used for storage will be investigated for high pressure for Hydrogen, low carbon steel, stainless steels, can be used for storage, but products will be heavy for transportation purposes some kind of Aluminium alloys are good for liner but strength of material is not good enough. Composite materials can also be used for storage, besides new materials weren’t to be used so far will be investigated by means of this paper.
Meeting the security standards, low weight and high hydrogen transportation capacity, composite materials development and economical production became important. Researches development current aspect shows that target is reachable and high expectation in liquid hydrogen storage, usage, transportation could be possible. In order to reach the purpose developments of technologies of hydrogen getting importance through out the world. The proposed paper aimed to gain determination and selection of the high technology, resistance to high pressure new materials.
Keywords: New Materials, Hydrogen Storage, Hydrogen Energy, Hydrogen Transportation.
1. INTRODUCTION
Hydrogen storage has been widely analyzed, not only by the scientific community, but also at the political and industrial levels in lands as wide apart in interests as the USA, Iceland or Germany. In the former, a comprehensive set of requirements has been drafted, in order to steer research towards practical systems, with the potential to be sold to customers all over the country. These requirements are divided into short and long term, and have been described in a DOE (Department of Energy) document [1],
The most stringent requirements are set for the year 2015, From this set of requirements, the most difficult to meet are the ones regarding usable specific energy and usable energy density, as well as cycle life and fuel cost. In this, the different hydrogen storage techniques, which will be compared shortly, show their very different characteristics. Whereas pressure and cryogenic tanks have no problem surviving more than 1500 cycles, hydrogen storage systems based on chemicals very often show deactivation due to agglomeration of particles or oxidation. It is also very difficult to estimate the cost of these systems, since the corresponding fuel cell as well as the ancillary equipment (reformer) may or may not be added into the equation. As the cost of liquefaction or compression of hydrogen has to be taken into account as well, the comparison grows ever more difficult.
A first phase contemplates the usage of these vehicles in closed fleets, as delivery vans, taxi or the like. In this case, the requirements regarding fuel storage are not as demanding, because the distances to be traveled are not so long, and city traffic conditions are likely to be the everyday situation. The vehicles are probably always going to be near a fuelling station and / or come back to it to stay overnight. The next phase will be introduction to the general public, which may accept some minor disadvantages because of the ecological reasons behind hydrogen. The last phase will see real competition between hydrogen and gasoline or diesel propulsion, thus determining the most stringent requirements in capability of the storage system.
It has to be said that at present no storage system meets DOE’s targets [2].
Table.1.1. Classification of hydrogen storage systems.
The systems that are envisioned as probable contenders for the hydrogen storage standard are classified in Table 1-1. The “physical storage” systems shown in the first column hold the hydrogen in its pure form, without resorting to any chemical compounds or absorbing agents. Compressed gas, as well as liquid hydrogen are well understood technologies, which nevertheless prove to have some unresolved issues. Compressed gas tanks have too small capacities [3], giving storage tanks that are both voluminous and heavy. Some advances have being achieved, both in the field of better usage of the volume [4] and increasing the capacity [3]. Liquid hydrogen requires a considerable amount of energy for the liquefaction process (452 kJ · kg-1) [5]. Since the critical temperature of hydrogen is –241 °C, liquid hydrogen systems are open ones: some of the hydrogen evaporates continuously.
This goes on to show that a system with a (relatively) low capacity of 5.6 wt % (undoped) and high reactivity which yields pure hydrogen and can be easily reloaded also with hydrogen could be made to work if hydrogen release temperatures and loading pressures were low enough. Doped sodium alanate is such a system.
2. MATERIALS FOR HIGH PRESSURIZED HYDROGEN STORAGE
2.1. The sodium alanate system
Bogdanovic and Schwickardi showed in 1997 [6] that doping sodium alanate (NaAlH4) with a catalyst would result in a reversible system with the capability of reabsorbing hydrogen to a large extent. Sodium alanate is a light compound with a high hydrogen content. Its thermal decomposition to NaH, which takes place in two steps with an intermediate in the form of Na3AlH6, has been known for many years [7]. However, it had been assumed to be irreversible. The reaction is shown in
It was first observed that solutions of Mg2Cl3AlH4·6THF in THF decompose with evolution of hydrogen and deposition of a mirror of Al much more readily if the MgH2 used for the reaction in is catalytically prepared by means of a Ti instead of a Cr catalyst (Eq. 1-3). This caused the start of a systematic study of the thermal decomposition of complex alanates using different transition metals as catalysts. At the time, the Ti-catalyzed decomposition of LiAlH4 was already known [8].
The first step of the decomposition of NaAlH4 yields 3.7 wt %, the second 1.9 wt % hydrogen. The conditions under which the reaction happens in the right-hand direction (decomposition) are much milder than the thermal process without catalyst: 180 °C for decomposition to NaH and Al versus 240 °C. Rehydrogenation of Ti-doped samples has even been proven to take place, via treatment with high pressure hydrogen, at room temperature [9]. Thus, doped samples show a profound catalytic effect in the de- and rehydrogenation . By comparison, the different synthesis methods of undoped NaAlH4 [10] need high pressures and temperatures, as well as a solvent (THF) or working in the melt.
The reason for the interest in the sodium alanate system lies in the fact that the theoretical amount of hydrogen which can be obtained is 5.6 wt %. Even after doping, capacities of 5 wt % have been demonstrated [11]. This exceeds the percentage obtainable with traditional hydrogen storage alloys by far [46], so that a system for hydrogen storage in transportation becomes a distinct possibility. The obstacles to its being put to use immediately are:
• Excessive temperature to achieve desorption, especially of the 2nd step (150 -180 °C)
• Unsatisfactory absorption kinetics (in some cases a period of up to twelve hours was needed to hydrogenate a sample).
• Arguably, low capacity in relation to DOE’s targets.
These factors (except the last one, which is inherent to the system) were the topic of a project, of which this thesis was a part. The solution was addressed by optimization of the catalyst and the doping procedure. Theoretical work has been done, especially related to the structural aspects of the alanate system [12,13] and its kinetics [9,15,16,17].
Preparation of the doped NaAlH4 hydrogen storage material is in principle possible in three ways:
• Doping of the NaAlH4, either by ball milling or using the solvent-based technique, as mentioned above.
• Ball milling of NaH and Al together with the dopant. This is known as the direct synthesis method. Although this doping method was initially described using the solvent-based technique [66], in this work only milling will be used.
• Doping of a mixture of Na and Al in the presence of hydrogen.
2.2. Catalyst screening and sample cycling
Ti-containing species are grouped under the title "Ti – based". Dopants in which Ti is present in a zero or low oxidation state (Ti powder and TiCl2) were targeted as higher capacity materials, The other effect that was investigated, was increasing of the reaction rates, especially of the hydrogenation reaction by using extremely fine Ti particles (Ti*) instead of the commercial Ti powders as doping agents. As for Fe, a previous work [10] had shown that combinations of Fe and Ti (under the form of Fe(OEt)2 and Ti(OBun)4) showed particularly good characteristics as dopants. This was ascribed to a synergistic effect of Ti and Fe, which was now to be tested using the chlorides instead of the alcoholates. Sc and V chlorides as dopants were used due to the fact that the metals are neighbors of Ti in the Periodic Table.
The doping procedure also evolved, giving another variable with which to play in order to attain the desired optimization of the hydrogen storage characteristics.
2.2.1. Ti-based doping agents
· Ti(OBun)4 and TiCl3 as reference doping agents :
Since Ti(OBun)4 was the first doping agent for the sodium alanate hydrogen storage system it became a standard to evaluate other doped samples. However, after Sandrock et al. found that organic residues might turn up not only in the sample but also in the evolved gas when dehydrogenating [18], a new standard was needed.
· Al - Ti alloy as a doping agent :
An alloy with 2 weight % Ti in Al was kindly made available by the Max Planck Institute for Iron Research in Düsseldorf 4. This alloy was milled in a SPEX mill for 3 h together with NaH, and Al (with a ratio of 2 mol Ti to 100 mol of NaH).
The milled sample was hydrogenated (at 120 bar and over 150 °C for > 24 h) and then dehydrogenated. The first step of dehydrogenation (NaAlH4 to Na3AlH6 and Al) worked, and hydrogen evolved. [19]
· TiCl2 as a doping agent
TiCl2 was used as a doping agent for NaAlH4, since it was expected that it would yield a higher storage capacity in comparison to TiCl3, because for the reduction of Ti (II) to the zerovalent stage only 2 mol NaAlH4 / mol TiCl3 (Eq. 2-1) are needed.
The rehydrogenations were carried out at 120 °C and 120-100 bar.
TiCl2 did not seem to be suitable as a dopant for hydrogen storage materials on NaAlH4 basis [19].
2.2.2. Alternatives to Ti based dopants
· VCl4 as a doping agent
The samples were doped with VCl4 using the "wet" method described in the Appendix. Thus, VCl4 (2 mol %) was added to 4 g of NaAlH4 suspended in toluene and the mixture was stirred for 12 h at room temperature.
The sample doped with VCl4 showed (Fig. 2-7) not only slower dehydrogenations, but its hydrogen storage capacity decreased rapidly with cycling. Thus, VCl4 is inferior to Ti(OBun)4 as a dopant for NaAlH4 [19].
· ScCl3 as a doping agent
The doping method for the NaAlH4 with ScCl3 was milling in a SPEX 8000 mill for 4.3 h, with a doping level of 2 mol %. A comparison of the hydrogenation rates of TiCl3- and ScCl3-doped samples [19].
· FeCl2 as a doping agent
NaAlH4 was doped with 4 mol % of FeCl2 (Aldrich, 97 %) by ball milling of the mixture in the Retsch MM 200 mill for ca. 4 h. [19].
3. SUMMARY AND CONCLUSION
A screening of doping agents for the doped NaAlH4 hydrogen storage system was undertaken in this thesis. Parallel to it, new methods for the synthesis of doped NaAlH4 via ball milling were developed, and a part of the investigations into the catalysis mechanism of the de- and rehydrogenation of doped NaAlH4, as well as into the nature of the catalyst were carried out.
In the screening phase, alternative catalysts to Ti(OBun)4 were investigated: TiCl3, TiCl2, Ti powder, Ti*, Al-Ti alloy (2 wt% Ti), FeCl3 / TiCl3, FeCl2, VCl4 and ScCl3. In the course of this work, ScCl3 proved to be the best dopant for the NaAlH4. Measurements with this dopant revealed high rates of rehydrogenation coupled with acceptable storage capacities. Most of the others proved to yield catalysts for the de- and rehydrogenation of NaAlH4 when used as dopants, although either the capacity or the rehydrogenation rate performance were disappointing.
Hydrogen storage materials based on doped NaAlH4 have been produced which very closely match almost all of the requirements of the automobile industry (see Introduction). In order to obtain a higher system capacity, new materials other than doped NaAlH4 have to be found. This may need a paradigm change from low temperatures (around 80 °C) to higher ones, in order to make use of Mg based hydrides or Li – N systems [93]. Together with boron based hydrogen storage materials [14], these are the most promising candidates for high capacity, small volume hydrogen storage systems.
4. References
[1] U. S. Department of Energy, 2005, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/freedomcar_targets_explanations.pdf