Materials for the Hydrogen Economy (2007) Episode 8 docx

186 Materials for the Hydrogen Economy and has been used to create permeation barrier coatings on steels. One consideration is the temperature required for the process relative to heat treatment temperatures of steels. The treatment temperature of the MANET-II ferritic-martensitic steel limits the process temperature to about 750˚C, which in turn limits the amount of alumi - num and depth of diffusion of aluminum in the surface. For this case, hot dipping was chosen as a preferred method. 41 Permeation reduction factors of up to 10,000, or 10 4 , have been realized with the best coatings based on aluminized steels. Ferritic-martensitic steels that were alu - minized had the Fe 2 Al 5 phase predominant in the layer sequence, while a 316L steel had FeAl 3 and FeAl 2 as the main aluminide phases. 25 The best permeation barrier resulted from an external alumina lm of about 1 micron in thickness grown on the aluminide layers. 25 The vacuum evaporation process and polymer slurry process are quite new rela- tive to the others and have the potential to provide more control in the processing, in the case of the vacuum evaporation technique, or greatly reduce the cost and environmental concerns of pack aluminizing with the polymer slurry methods. The vacuum evaporation process allows one to diffuse other elements than Al into the steels or to deposit FeAl coatings directly onto the surface of the steels, with Al dif - fusion occurring to help bond the deposited coating. 39 This process is referred to as enclosed vacuum evaporation (EVE) coating tech- nology and is applicable to a variety of coating and substrate materials, with a unique capability of producing smooth and uniform coatings on the inner surface of small- diameter, high aspect ratio cylindrical components or other conned geometries. Atomic Percent Aluminum 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 1600 1538°C 1400 1394°C 1200 1000 912°C 800 770°C 600 400 Fe Weigt Percent Aluminum Al L 660.452°C 1310°C (_Fe) (aFe) 1232 1169°C ~1160°C 1102°C FeAl Fe 3 Al Fe 2 Al 5 FeAl 3 655°C (Al) ¡ Temperature °C FIGURE 8.1  AlFe phase diagram showing intermetallic phases, such as FeAl 3 , Fe 2 Al 5 , FeAl 2 , and FeAl, which can form as separate layers in aluminized steel. 5024.indb 186 11/18/07 5:53:06 PM Hydrogen Permeation Barrier Coatings 187 The technique has been used to deposit reproducible coatings on the inner surface of tubes as small as 10 mm in diameter and in lengths up to 3.8 m. For larger-diameter tubes or pipes in which radiant heating of the substrate from the source lament is impractical, separate resistive or inductive heating of the substrate to the desired temperature is used. Figure 8.2 shows the inner surface of a steel tube coated with a FeAl alloy for hydrogen barrier testing. The deposition is rapid, and substrate tem - perature rise can be controlled to avoid de-tempering alloys. The polymer slurry method for aluminizing steel surfaces is straightforward and simple, and should be low cost since the raw materials and processing steps are also low cost. Aluminum ake of 1 to 2 microns in diameter is blended with a preceramic polysiloxane polymer and heated in air or nitrogen to 700 to 800˚C for several hours to allow the aluminum to diffuse into the steel and to allow an external Si-Al-O lm to form from reactions between the siloxane backbone and the Al. As with all alumi - nizing reaction–diffusion coatings, a series of aluminide layers form on the surface of the steel, as shown in gure 8.3. The outermost layer of alumina is the hydrogen permeation barrier, while the aluminum-rich layers provide additional aluminum for alumina formation in oxidizing environments, as required to maintain the external oxide layer. The advantages of aluminizing steels go beyond hydrogen barrier formation, however, as such surface treatments also provide additional corrosion protection. The fusion materials community continues to study these processing methods and may continue to be the main driving force for research in this area until hydrogen infrastructure issues become more important. 27 FIGURE 8.2  FeAl coating on the inner diameter of a 316SS tube that was deposited using the EVE technique. 5024.indb 187 11/18/07 5:53:08 PM 188 Materials for the Hydrogen Economy 8.5 SUMMARY The best hydrogen barrier coatings have been fabricated using aluminized steels produced by a variety of methods, including pack aluminizing, hot dipping, vacuum evaporation, or polymer slurry techniques. Permeation reduction factors of up to 10 4 have been realized in this manner. Titanium-based coatings offer an alternative choice to Al but are not as permeation resistant as the alumina-based methods, and are not as reproducible in fabrication. Much work remains to be done in the general area of hydrogen permeation barriers, particularly in the development of new meth - ods that can provide barriers over large areas for anticipated hydrogen economy infrastructure needs. Low-cost methods and better reproducibility are required. Hydrogen remains an elusive species in this regard, and a perfect solution is appar - ently very challenging. REFERENCES 1. Forcey, K.S. et al., Hydrogen transport and solubility in 316L and 1.4914 steels for fusion reactor applications, Journal of Nuclear Materials, 160, 117–124 (1988). 2. Gibala, R. and R.F. Hehemann, Eds., Hydrogen Embrittlement and Stress Corrosion Cracking, ASM, Metals Park, OH, 1984, p. 324. 3. Honeycombe, R.W.K., Steels: microstructure and properties, in Metallurgy and Materi- als Science, R.W.K. Honeycombe and P. Hancock, Eds., London: Edward Arnold, 1981. FIGURE 8.3  A typical aluminized steel surface following the polymer slurry method at 800˚C in air. Any of the methods mentioned in this section about aluminized lms will pro- duce similar reaction layers. Ease of processing and cost may dictate which method is pre- ferred for a given application. 5024.indb 188 11/18/07 5:53:09 PM Hydrogen Permeation Barrier Coatings 189 4. Hollenberg, G.W. et al., Tritium/hydrogen barrier development, Fusion Engineering and Design, 28, 190–208 (1995). 5. Roberts, R.M. et al., Hydrogen permeability of sintered aluminum oxide, Journal of the American Ceramic Society, 62, 495 (1979). 6. Yu, G.T. and S.K. Yen, Determination of the diffusion coefcient of proton in CVD gamma aluminum oxide thin lms, Surface and Coatings Technology, 166, 195 (2003). 7. Serra, E. et al., Hydrogen permeation measurements on alumina, Journal of the Ameri- can Ceramic Society, 88, 15 (2005). 8. Brimhall, J.L., E.P. Simonen, and R.H. Jones, Data Base on Permeation, Diffusion, and Concentration of Hydrogen Isotopes in Fusion Reactor Materials, Fusion Reactor Materials Semiannual Progress Report, DOE/ER-0313/16, 1994. 9. Forcey, K.S. et al., Hydrogen transport and solubility in 316L and 1.4914 steels for fusion reactor applications, Journal of Nuclear Materials, 160, 117 (1988). 10. Hollenberg, G.W. et al., Tritium/hydrogen barrier development, Fusion Engineering and Design, 28, 190 (1995). 11. Perujo, A. and K.S. Forcey, Tritium permeation barriers for fusion technology, Fusion Engineering and Design, 28, 252 (1995). 12. Mühlratzer, A., H. Zeilinger, and H.G. Esser, Development of protective coatings to reduce hydrogen and tritium permeation, Nuclear Technology, 66, 570 (1984). 13. Yamada-Takamura, Y. et al., Hydrogen permeation barrier performance characteriza - tion of vapor deposited amorphous aluminum oxide lms using coloration of tungsten oxide, Surface and Coatings Technology, 153, 114 (2002). 14. Song, R.G., Hydrogen permeation resistance of plasma-sprayed Al 2 O 3 and Al 2 O 3 - 13wt% TiO 2 ceramic coatings on austenitic stainless steel, Surface and Coatings Tech- nology, 168, 191 (2003). 15. Tazhibaeva, I.L. et al., Hydrogen permeation through steels and alloys with different protective coatings, Fusion Engineering and Design, 51/52, 199 (2000). 16. Forcey, K.S. et al., Formation of tritium permeation barriers by CVD, Journal of Nuclear Materials, 200, 417 (1993). 17. Shan, C. et al., Behaviour of diffusion and permeation of tritium through 316L stainless steel with coating of TiC and TiN + TiC, Journal of Nuclear Materials, 191–194, 221 (1992). 18. Van Deventer, E.H. and V.A. Maroni, Hydrogen permeation characteristics of some Fe-Cr-Al alloys, Journal of Nuclear Materials, 113, 65 (1983). 19. Earwaker, L.G. et al., Inuence on hydrogen permeation through steel of surface oxide layers and their characterisation using nuclear reactions, IEEE Transactions on Nuclear Science, NS-28, 1848 (1980). 20. Fazio, C. et al., Investigation on the suitability of plasma sprayed Fe-Cr-Al coatings as tritium permeation barrier, Journal of Nuclear Materials, 273, 233 (1999). 21. Forcey, K.S., D.K. Ross, and L.G. Earwalker, Investigation of the effectiveness of oxi - dised Fecralloy as a containment for tritium in fusion reactors, Zeitschrift fur Physika- lische Chemie Neue Folge, 143, 213 (1985). 22. Shen, J N. et al., Effect of alumina lm prepared by pack cementation aluminizing and thermal oxidation treatment of stainless steel on hydrogen permeation, Yuanzineng Kexue Jishu/Atomic Energy Science and Technology, 39, 73 (2005). 23. Aiello, A. et al., Hydrogen permeation through tritium permeation barrier in Pb-17Li, Fusion Engineering and Design, 58/59, 737 (2001). 24. Glasbrenner, H., A. Perujo, and E. Serra, Hydrogen permeation behavior of hot-dip aluminized MANET steel, Fusion Technology, 28, 1159 (1995). 25. Forcey, K.S., D.K. Ross, and C.H. Wu, Formation of hydrogen permeation barriers on steels by aluminising, Journal of Nuclear Materials, 182, 36 (1991). 5024.indb 189 11/18/07 5:53:10 PM 190 Materials for the Hydrogen Economy 26. Fukai, T. and K. Matsumoto, Surface modication effects on hydrogen permeation in high-temperature, high-pressure, hydrogen-hydrogen sulde environments, Corrosion (Houston), 50, 522 (1994). 27. Konys, J. et al., Status of tritium permeation barrier development in the EU, Fusion Science and Technology, 47, 844 (2005). 28. Glasbrenner, H. et al., Corrosion behaviour of Al based tritium permeation barriers in owing Pb-17Li, Journal of Nuclear Materials, 307–311, 1360 (2002). 29. Glasbrenner, H. et al., Development of a Tritium Permeation Barrier on F82H-mod, in Sheets and on MANET Tubes by Hot Dip Aluminising and Subsequent Heat Treatment, Forschungszentrum Karlsruhe GmbH, Karlsruhe, Germany, 1998, p. 30. 30. Glasbrenner, H. et al., The Formation of Aluminide Coatings on MANET Stainless Steel as Tritium Permeation Barrier by Using a New Test Facility, Vol. 2, Elsevier, Lisbon, 1997, p. 1423. 31. Iordanova, I., K.S. Forcey, and M. Surtchev, Structure and composition of aluminized layers and RF-sputtered alumina coatings on high chromium martensitic steel, Materi- als Science Forum, 321–324, 422 (2000). 32. Iordanova, I., K.S. Forcey, and M. Surtchev, X-ray and ion beam investigation of alu - mina coatings applied on DIN1.4914 martensitic steel, Nuclear Instruments and Meth- ods in Physics Research, Section B: Beam Interactions with Materials and Atoms, 173, 351 (2001). 33. Forcey, K.S. et al., Use of aluminising on 316L austenitic and 1.4914 martensitic steels for the reduction of tritium leakage from the NET blanket, Journal of Nuclear Materi- als, 161, 108 (1989). 34. Yao, Z.Y. et al., Hot dipping aluminized coating as hydrogen permeation barrier, Acta Metallurgica Sinica, 14, 435 (2001). 35. Aiello, A., A. Ciampichetti, and G. Benamati, An overview on tritium permeation bar - rier development for WCLL blanket concept, Journal of Nuclear Materials, 329–333, 1398 (2004). 36. Aiello, A. et al., Qualication of tritium permeation barriers in liquid Pb-17Li, Fusion Engineering and Design, 69, 245 (2003). 37. Benamati, G. et al., Development of tritium permeation barriers on Al base in Europe, Journal of Nuclear Materials, 271/272, 391 (1999). 38. Perujo, A., K.S. Forcey, and T. Sample, Reduction of deuterium permeation through DIN 1.4914 stainless steel (MANET) by plasma-spray deposited aluminum, Journal of Nuclear Materials, 207, 86 (1993). 39. Knowles, S.D. et al., Method of Coating the Interior Surface of Hollow Objects , U.S. Patent 6,866,886 , 2005. 40. C.H. Henager, Jr., Low-cost aluminide coatings using polymer slurries, personal com - munication, 2006. 41. Serra, E., H. Glasbrenner, and A. Perujo, Hot-dip aluminium deposit as a permeation barrier for MANET steel, Fusion Engineering and Design, 41, 149 (1998). 5024.indb 190 11/18/07 5:53:11 PM 191 9 Reversible Hydrides for On-Board Hydrogen Storage G. J. Thomas CONTENTS 9.1 Introduction 191 9.2 Hydride Properties and Hydrogen Capacity 192 9.3 Alanates 197 9.4 Borohydrides 200 9.5 Destabilized Borohydrides 201 9.6 Nitrogen Systems 202 9.7 Other Materials 204 9.8 Summary 205 References 205 9.1 INTRODUCTION In concept, reversible hydrides offer a direct means of storing hydrogen on-board fuel cell vehicles and would be compatible with a hydrogen-based transportation fuel infrastructure. A tank, or perhaps more accurately a storage system, containing an appropriate hydride material would remain xed on a vehicle and could be refueled simply by applying an overpressure of hydrogen gas. Once lled, the hydrogen gas would remain at the equilibrium pressure for the particular hydride material, chang- ing only with temperature changes induced in the storage tank. When hydrogen was needed, it would be released endothermically, using the waste heat from the fuel cell (or internal combustion engine [ICE]) to supply the required energy. This approach offers certain advantages over high-pressure compressed gas tanks and cryogenic liquid hydrogen systems—it is inherently stable with regard to hydrogen release, it can operate at a low or moderate gas pressure, and it could eliminate some of the energy costs of compression or liquefaction. It also has the potential to achieve volumetric hydrogen densities much higher than those of compressed gas and even liquid hydrogen. In practice, however, the use of hydrides for on-board hydrogen storage is much more complicated than is described above, and a number of issues arise when one attempts to choose a material and design a storage system. These issues arise because (1) many hydride materials do not meet minimal on-board storage requirements for 5024.indb 191 11/18/07 5:53:12 PM 192 Materials for the Hydrogen Economy weight and volume density, and (2) there are some fundamental material properties that determine system performance that are competing against one another; that is, they affect system requirements in opposing directions. Thus, a hydride-based storage system will likely be a design with numerous trade-offs in terms of capacity, kinetics, and thermal requirements. Furthermore, there are a host of other issues beyond capac - ity and kinetic performance, such as hydride–dehydride cycling-induced changes or degradation in performance, response of the material to impurities in the incoming hydrogen gas, evolution of impurities from the bed affecting the fuel cell, and, impor - tantly, cost of the material and its impact on the fuel supply system cost. The development of storage materials with properties that can encompass all of the required performance attributes for on-board hydrogen storage will be an extremely challenging task and likely require a multidisciplinary approach. In 2003, the U.S. Department of Energy (DOE) launched a concerted effort to develop high- capacity materials that have the potential to meet the hydrogen storage system per - formance targets established by the DOE and FreedomCAR and Fuel Partnership 1 a government/industry collaboration. This hydrogen storage initiative has spawned a considerable level of effort over the last few years, and it is in this arena that this chapter will focus. There have been many review articles on metal hydrides, and a few of the more recent ones are referenced here. 2–10 This review will attempt to cover only fairly recent studies on high hydrogen capacity hydrides that (1) have been demonstrated to be reversible or, at the least, partially reversible and (b) have the potential for exhib - iting other properties (e.g., kinetics, operating temperatures) suitable for on-board hydrogen storage applications. This area of materials research and development is very active at the present time, so that it is likely that not all of the relevant work will be included. The author apologizes for any omissions. 9.2 HYDRIDE PROPERTIES AND HYDROGEN CAPACITY Hydrides can be loosely categorized by their chemical binding—metallic, covalent, ionic, or complex—between the host elements and hydrogen. Intermetallic alloys form a large class of hydrides, generally with metallic bonds, that can be further subcategorized by the ratio of the alloying constituents A and B. Thus, for example, one refers to LaNi 5 H 6 as an AB5 hydride. An online database of hydride properties, hydpark, 11 is largely organized along these lines. From an on-board hydrogen stor- age perspective, however, it is the nature of the chemical bond that is key because it determines the thermodynamic stability of the hydride, the hydrogen stoichiometry of the material, and the mechanisms for hydrogen absorption and release. In 2001, Schlapbach and Zuttel published a paper on hydrogen storage 4 and included a plot of volumetric and gravimetric hydrogen densities in a variety of materials. Figure 9.1 shows a similar plot that also includes corresponding values for compressed gas and liquid hydrogen, as well as the DOE and FreedomCAR and Fuel Partnership targets for on-board storage systems. One can see that there are a number of materials that contain hydrogen concentrations well above the system tar - gets, with some having more than twice the density of liquid hydrogen. In addition to 5024.indb 192 11/18/07 5:53:13 PM Reversible Hydrides for On-Board Hydrogen Storage 193 mixing material properties with system properties, the plot also includes many dif- ferent material types, such as solid reversible hydrides, liquid and solid nonreversible chemical systems, and, for comparison, a few liquid and gaseous fuels (e.g., octane, methane, etc.) plotted in terms of their hydrogen content. One can notice some trends in material properties from the plot. First, the inter - metallic hydrides (plotted in bright red), such as LaNi 5 H 6 , generally have low gravi- metric hydrogen density and are clustered toward the left-hand side of the graph. But these materials are also relatively dense, and so they can have high volumetric hydrogen densities, often greater than 100 g H 2 /l. A few elemental hydrides (also plotted in bright red), such as MgH 2 and TiH 2 , are also shown. Although they may exhibit high hydrogen densities, they tend to be heavy as well and, in many cases, have strong covalent hydrogen bonds resulting in more stable structures. Their higher stability means that higher temperatures are required to release the hydro - gen. For example, MgH 2 must be heated above 300°C in order to release hydrogen at a signicant rate. The highest-capacity materials (shown in blue on the plot) lie in the upper-right- hand quadrant and above the system target lines. This chapter will be limited to a discussion of these materials only because they are the materials that are of greatest interest for hydrogen storage applications. Also, the chapter will not cover histori - cal developments and will largely be concerned with research published within the last several years. One additional limitation in scope is that only material proper - ties related to the thermally induced release of hydrogen will be described. Other hydrogen release reactions, such as hydrolysis, that generate an oxide or hydroxide by-product that must be processed off-board will not be discussed. The solid, reversible hydride materials plotted in blue in gure 9.1 generally con - tain an Al–H complex anion (alanates), a B–H complex anion (borohydrides), or N–H groups (amides, imides). The plot also includes some nonreversible materials, such as ammonia borane, NH 3 BH 3 , that have very high hydrogen capacities that can be released by thermolysis, but must be regenerated through a chemical process. This means that the spent fuel must be removed and processed externally. These “chemical hydride” materials will also not be discussed in this chapter. In intermetallic systems, hydrogen absorption (desorption) is relatively straight - forward and occurs through (1) molecular dissociation (recombination) at the metal surface and (2) atomistic diffusion of the hydrogen through the solid. Grochala and Edwards 8 refer to these materials as interstitial hydrides since the hydrogen resides in the interstices of the metal lattice. In contrast, hydrogen dissociation in complex hydrides generally occurs through the formation of intermediate compounds. The reverse processes, the re-formation of the hydride phases, as well as hydrogen trans - port mechanisms, are generally not well understood in these materials. An addi - tional issue with the high hydrogen capacity materials is that they are often quite stable and require high temperatures for hydrogen release. The desired form of a reversible hydride reaction for on-board storage may be written as M X H Y + heat ≡ XM(s) + Y/2H 2 (g) (9.1) 5024.indb 193 11/18/07 5:53:13 PM 194 Materials for the Hydrogen Economy where M is a single element or combination of elements. The heat term on the left side of the reaction indicates that the dissociation of the hydride coupled with the release of hydrogen is endothermic, and conversely, formation of the hydride by reacting the element(s) with gaseous hydrogen is exothermic. From the perspective of a vehicular hydrogen storage system then, the material remains stable on-board the vehicle at low or moderate hydrogen pressures until heat is applied. The preferred source of this heat is waste heat from the fuel cell or ICE, so that there would be no energy penalty for releasing the hydrogen. Ideally then, the operating temperature range of the storage material should lie within the operating temperature range of the fuel cell or ICE coolant loop. It should also be noted from equation 9.1 that heat must be dissipated when refu- eling the tank (recharging the spent hydride), and for refueling rates equivalent to lling conventional gas tanks, the cooling power could, for the more stable hydride materials, exceed the capacity of on-board coolant systems. In these cases, there would be an energy cost borne by the off-board refueling facility. The thermodynamic parameters of the reaction quantify the energy require- ments. The Gibbs free energy of formation per mole of hydride at constant tempera- ture, T, is ∆G f = ∆H f – T∆S (9.2) Hydrogen Densities of Materials 0 50 100 150 200 0 5 10 15 20 25 30 Hydrogen mass density (wt. %) Hydrogen volume density (gH 2 /L) 100 liquid hydrogen 700 bar 350 bar CH 4 (liq) C 2 H 5 OH C 8 H 18 C 3 H 8 C 2 H 6 NH 3 CH 3 OH Mg 2 NH 4 LaNi 5 H 6 FeTiH 1.7 MgH 2 KBH 4 NaAlH 4 NaBH 4 LiAlH 4 LiBH 4 AlH 3 TiH 2 CaH 2 NaH 2015 system targets 2010 system targets NH 3 BH 3 (3) NH 3 BH 3 (2) NH 3 BH 3 (1) Mg(OMe) 2 .H2O 11M aq NaBH 4 hexahydrotriazine decaborane LiNH 2 (2) LiNH 2 (1) FIGURE 9.1  Plot of hydrogen weight fraction and hydrogen volume density for some rep- resentative hydrogen storage materials. For comparison, the current 2010 and 2015 DOE/ FreedomCAR and Fuel Partnership targets for system weight and system volume densities are indicated by the dashed lines. The densities of compressed hydrogen at ambient temperature and liquid hydrogen at 20K are also shown. 5024.indb 194 11/18/07 5:53:15 PM Reversible Hydrides for On-Board Hydrogen Storage 195 where ∆H f is the formation enthalpy of the hydride and ∆S is the change in entropy of the system when the hydride is formed. When ∆G f is negative, the reaction is favored and heat is released as the hydride is formed. For an ideal gas, the hydrogen overpressure, P, in equilibrium with the hydride at temperature T, can then be expressed in the form 12 RTln(P/P o ) = ∆G f = –∆H f + T∆S (9.3) where R is the gas constant and P o is the pressure at standard conditions, that is, 1 atm of pressure. The enthalpy is expressed as the formation energy per mole of hydrogen. The entropy change is the difference between the entropy of hydrogen gas and the congurational and vibrational entropy of the hydrogen in the solid, and is generally considered to have roughly the same value for most hydrides. One can then readily see from the equation that the more stable a hydride is (larger ∆H f ), the lower the equilibrium pressure is at a given temperature. The equation also shows that a plot of lnP vs. 1/T for a given hydride is a straight line with a slope of ∆H f /R, the familiar van’t Hoff plot. Graphically, the Y-intercept at 1/T → 0 corresponds to an equilibrium pressure at innite temperature. Current research toward developing high hydrogen capacity materials for on- board storage applications is largely concentrated on reducing the energy require - ments, either by modifying the material to reduce the enthalpy of formation, ∆H f , of the hydride phase, or through altering the reaction pathway to hydrogen dissociation or recombination. This stems largely from the on-board need to use the waste heat from an “engine” (e.g., a fuel cell or ICE) to supply the required energy for hydrogen release from the hydride. Roughly speaking, this means that the “operating window” in temperature and pressure for a hydride storage system lies between room tempera - ture and 100°C, and ~1 and 100 bars. An examination of the hydpark database 11 indi- cates that the bulk of experimental values for ∆S range from about 95 to 130 J/mol H 2 . A simple calculation, then, using equation 9.3 would show that ∆H f should be in the range of about 20 to 40 kJ/mol H 2 . The lower enthalpy values would actually be preferred in order to reduce the cooling power requirements during rehydriding. Even lower ∆H f materials, e.g., ~15 kJ/mol H 2 , could be used with higher-pressure containers. However, materials with formation enthalpies higher than the upper limit could not be used as storage materials because the operating temperatures would be too high for on-board systems as they are currently envisioned. Concurrently, the hydrogen kinetics of absorption and release must also be improved to meet minimal performance standards through, for example, the devel - opment of effective catalysts or the formation of very small particle sizes (e.g., nanoscale materials). Of course, an added requirement for small particle sizes is that their small dimensions be maintained through repeated hydride–dehydride cycling. The kinetic requirements for hydrogen release in a storage material are dictated by the needs of the vehicle driver and the fuel cell power. For example, a 100-kW peak power fuel cell with about 42% fuel efciency would need 2 g H 2 /sec to produce full power when demanded by the vehicle driver. Furthermore, this hydrogen delivery rate must be available through nearly all of the range of the hydride composition. 5024.indb 195 11/18/07 5:53:16 PM [...]... effects As with other techniques that may improve the thermodynamics, there is an accompanying loss in capacity The reversible hydrogen content for the LiBH4 –MgH2 couple was found to be in the range of 8 to 10 wt%, rather than 18. 4 wt% for the formula value, or the 13.6 wt% hydrogen released when the borohydride decomposes to LiH, even though this hydrogen capacity, coupled with the lower enthalpy,... density-functional theory (DFT).57 They found the stable structures for each of the compounds, laying the ground work for further work in this area Experimentally, however, little progress has been made from the standpoint of reversible hydrides. 58 64 LiBH4 was studied by Zuttel et al. 58 as a potentially new hydrogen storage material They found that they could release about 13.5 wt% hydrogen from the compound... formed, and the amount of hydrogen released, although not as high as expected (about 70% of the expected level), was not unreasonable However, the temperature was much higher than would be expected based simply on the estimated enthalpy 5024.indb 203 11/ 18/ 07 5:53:22 PM 204 Materials for the Hydrogen Economy Another experimental study on the same material system was performed by Pinkerton et al. ,88 who found... layers using either the anode or cathode as the supporting structure 3 Stability: The operation of SOFCs requires the cathode and the anode to be porous for gas transport; therefore, the electrolyte is exposed to both the air and the fuel at elevated temperature The electrolyte must remain chemically phase stable in these environments, along with thermal and mechanical stability during thermal cycling... exceed the current 2010 FreedomCAR and Fuel Partnership system targets The actual energy densities for a specific system would depend not only on the hydrogen weight and volume densities of the hydride, but also on a number of other factors, including system design, the other materials used to fabricate the system components, thermal requirements (including the hydride materials enthalpy and thermal... in the dehydrogenated state should have lower enthalpies than the borohydride or the alanate alone The potential yields are higher than with LiH For the borohydride case, the expected reaction pathway is LiBH4 + 2 LiNH2 ≡ Li3BN2 + 4 H2 which would yield 11.9 wt% hydrogen The calculated enthalpy for forming Li3BN2 is 23 kJ/mol H2, considerably less than for the pure borohydride phase (69 kJ/mol H2) The. .. thickness at 500 and 600°C In the IT regime, the electrode overpotential, particularly cathode overpotential, is of the order of the ASR of the electrolyte Therefore, a maximum allowed electrolyte thickness is directly determined by the cell design 5024.indb 213 11/ 18/ 07 5:53:32 PM 214 Materials for the Hydrogen Economy Figure 10.2  Area-specific resistance vs electrolyte thickness for YSZ, CGO, and LSMG... significantly, from ~69 kJ/mol H2 for the borohydride decomposition alone (to LiH and H2) down to ~45 kJ/mol H2 when the borohydride reacted with MgH2 to form MgB2 and LiH The lower enthalpy value results in an equilibrium hydrogen overpressure of 1 bar at ~200°C, as compared to ~400°C for the borohydride 5024.indb 201 11/ 18/ 07 5:53:21 PM 202 Materials for the Hydrogen Economy This striking result, however,... conductivity), the packing density of the hydride, and the maximum operating pressure of the system (also dependent on the enthalpy) The hydrogen release rate estimate was based on a 5-kg hydrogen system capacity Table 9.1 Material Properties for Reversible Hydrides in Hydrogen Storage Systems Gravimetric hydrogen density Volumetric hydrogen density >60 g H2/l Enthalpy of hydride formation 15–40 kJ/mol H2 Hydrogen. .. follow on their recommendations Perhaps the greatest obstacles to overcome with this method are the very slow rates of hydrogen absorption and release The hydrogen kinetics are now limited by the reaction rates for forming the destabilized compound, and generally speaking, chemical reaction rates for forming compounds in the solid state are slower by orders of magnitude than interstitial hydrogen transport . important. 27 FIGURE 8. 2  FeAl coating on the inner diameter of a 316SS tube that was deposited using the EVE technique. 5024.indb 187 11/ 18/ 07 5:53: 08 PM 188 Materials for the Hydrogen Economy 8. 5 SUMMARY The. in capacity. The reversible hydrogen content for the LiBH 4 –MgH 2 couple was found to be in the range of 8 to 10 wt%, rather than 18. 4 wt% for the formula value, or the 13.6 wt% hydrogen released. dissociation of the hydride coupled with the release of hydrogen is endothermic, and conversely, formation of the hydride by reacting the element(s) with gaseous hydrogen is exothermic. From the perspective