1. 氫能源 Hydrogen energy 材料系 蔡文達 教授 Dec 30 th 2008

2. Overview of hydrogen energy

3. Energy Consumption Passenger vehicles are major consumption of fossil fuel Energy consumption is outpacing production Over view

4. Energy Consumption Over view

5. Pollution of Fossil Fuel Fossil fuel burning has produced approximately three-quarters of the increase in CO2 from human activity over the past 20 years.Fossil fuel In the United States, more than 90% of greenhouse gas emissions come from the combustion of fossil fuels. Combustion of fossil fuels also produces other air pollutants, such as nitrogen oxides, sulfur dioxide, volatile organic compounds and heavy metals.greenhouse gasnitrogen oxidessulfur dioxidevolatile organic compoundsheavy metals Global fossil carbon emission by fuel typecarbon Sources of greenhouse gases Over view

6. Global warming Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade) Northern Hemisphere ice trends Relationship between [CO 2 ] and temperature Over view

7. Greenhouse Effect Over view

8. Renewable energy Renewable energy is energy generated from natural resources—such as sunlight, wind, rain, tides and geothermal heat—which are renewable (naturally replenished).energynatural resources In 2006, about 18% of global final energy consumption came from renewables wind turbinesMonocrystalline solar cell Over view

9. Hydrogen Energy If the energy used to split the water were obtained from renewable or Nuclear power sources, and not from burning carbon-based fossil fuels, a hydrogen economy would greatly reduce the emission of carbon dioxide and therefore play a major role in tackling global warming.renewable Nuclear powercarbon dioxideglobal warming 2H 2 O → O 2 + 4H + +4e - 2H + + 2e - → H 2 Over view

10. H2H2 Hydrogen is the only chemical energy carrier that has the potential to used without generating pollutants to the atmosphere. Environmentally friendly. Hydrogen fueled heat engines can be optimized by the manufacturer to operate at much higher thermal efficiencies than heat engines powered with traditional hydrocarbon fuels. Efficient combustion. Clean, Renewable and Sustainable. “ The choice for the future.” Why hydrogen ? Over view

12. Building Hydrogen Economy Over view

14. H 2 production Hydrogen is commonly produced by extraction from hydrocarbon fossil fuels via a chemical path. Hydrogen may also be extracted from water via biological production in an algae bioreactor, or using electricity (by electrolysis), chemicals (by chemical reduction) or heat (by thermolysis)biological productionbioreactor electricityelectrolysischemical reductionheatthermolysis Biological production : Biohydrogen can be produced in an algae bioreactor. In the late 1990s it was discovered that if the algae is deprived of sulfur it will switch from the production of oxygen, i.e. normal photosynthesis, to the production of hydrogen.Biohydrogenalgae bioreactorphotosynthesis Fig. An algae bioreactor for hydrogen production. Over view

15. H 2 production Electrolysis : Hydrogen can also be produced through a direct chemical path using electrolysis. With a renewable electrical energy supply, such as hydropower, wind turbines, or photovoltaic cells, electrolysis of water allows hydrogen to be made from water without pollution. electrolysiswater Chemical production : By using sodium hydroxide as a catalyst, aluminum and its alloys can react with water to generate hydrogen gas. aluminum Al + 3 H 2 O + NaOH → NaAl(OH) 4 + 1.5 H 2 Solar Energy Fig. Photoelectrochemical cell Over view

16. H 2 storage High pressure gas cylinders (up to 800bar) Liquid hydrogen in cryogenic tanks(at 21 K) Fig. Liquid hydrogen tank for a hydrogen car Fig. gas cylinders Over view

17. H 2 storage Adsorbed hydrogen on materials with a large specific surface area (T<100 K) : carbon materials or zeolite Adsorbed on interstitial sites in a host metal (at ambient pressure and temperature) : metal hydride Chemically bond in covalent and ionic compounds (at ambient pressure, high activity) : complex metal hydride Fig. Hydrogen in metal matrix Fig. Carbon nanotube Over view

18. H 2 utilization (Fuel cell) A fuel cell is an electrochemical conversion device. It produces electricity from fuel (on the anode side) and an oxidant (on the cathode side), which react in the presence of an electrolyte.electrochemicalfuel Fig. Direct-methanol fuel cell Fig. Scheme of fuel cell Over view

19. H 2 on-board vehicle application A hydrogen vehicle is a vehicle that uses hydrogen as its on-board fuel for motive power. The term may refer to a personal transportation vehicle, such as an automobile, or any other vehicle that uses hydrogen in a similar fashion, such as an aircraft.hydrogen Fig. Hydrogen station Over view

20. Introduction of hydrogen storage

21. Hydrogen Storage Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in transportation applications. The major bottleneck for commercializing fuel- cell vehicles is on-board hydrogen storage. The goal is to pack H 2 as close as possible. Hydrogen Storage implies the reduction of an enormous volume of hydrogen gas. Compression of H 2 gas. What is Hydrogen Storage ?

22. Hydrogen Storage Reversible on-board vs. Regenerable off-board System that bind H 2 with low binding energy (less than 20-25 kJ/mol H 2 ) can undergo relatively easy charging and discharging of hydrogen under moderate conditions that are applicable. While in stronger bonds (typically in excess of 60-100 kJ/mol H 2 ), once the hydrogen is released, recharging with H 2 under operating conditions convenient at a refueling station is problematic. On-board Off-board Vehicular hydrogen storage approaches: Definitions

23. Hydrogen Storage Reversible on-board The on board storage media require hydrogen in liquid or gaseous form under different pressures, depending on specifications of the on-board technology. “Reversible” on-board ? because these methods may be recharged with hydrogen on-board the vehicle, similar to refueling with gasoline today. Hydrogen Tank Fuel Cell Stacks Air Pump Power Control Unit Hydrogen Filler Mouth

24. Hydrogen Storage Current analysis activities is to optimize the trade-off among… Weight, volume, cost, as well as life-cycle cost, energy efficiency, and environmental impact analyses. Hydrogen Tank Hydrogen Filler Mouth The technical challenge is… Storing sufficient hydrogen while meeting all consumer requirements without compromising passenger or cargo space. Reversible on-board

25. Hydrogen Storage To achieve comparable driving range may require larger amount of H 2. On a weight basis, hydrogen has nearly three times the energy content of gasoline. However, on a volume basis the situation is reversed and hydrogen has only about a quarter of the energy content of gasoline. Why Challenge? For the successful commercialization and market acceptance of hydrogen powered vehicles, the performance targets developed are based on achieving similar performance and cost levels as current gasoline fuel storage systems for light-duty vehicles. Gasoline or Hydrogen.

26. Gasoline or Hydrogen The 2015 targets represent what is required based on achieving similar performance to today’s gasoline vehicles (greater than 300 mile driving range) and complete market penetration. US DOE H 2 storage system targets Hydrogen Storage 6 wt%9 wt%

27. Hydrogen Storage Current approaches include: 1.High pressure H 2 cylinders 2.Cryogenic and liquid hydrogen 3.High surface area sorbents 4.Metal hydrides Hydrogen Storage Methods Conventional Storage Advanced Solid Materials Storage Increasing H 2 density by Pressure and Temp. control. Using little additional material to reach high H 2 density.

28. Hydrogen Storage The basic hydrogen storage method and phenomena. 1 2 3 4 Gravimetric density Volumetric density Working Temp. Pressure At ambient temp. and atmospheric pressure, 1 kg of H 2 gas has a volume of 11 m 3 ! Work must be applied to increase H 2 density. Hydrogen Storage Methods

29. Hydrogen Storage 1. HP H 2 Cylinders Introduction… 70 MPa H 2 storage cylinders ? The most common storage system is high pressure gas cylinders. Carbon fiber-reinforced composite tanks for 350 bar and 700 bar compressed hydrogen are under development and already in use in prototype hydrogen- powered vehicles. The cost of high-pressure compressed gas tanks is essentially dictated by the cost and the amount of the carbon fiber that must be used for structural reinforcement for the composite vessel.

30. Hydrogen Storage 1. HP H 2 Cylinders Volumetric density of compressed H 2 as a function of gas pressure. The safety of pressurized cylinders is a concern. Industry has set itself a target of a 110 kg, 70 MPa cylinder with a gravimetric storage density of 6 wt% and a volumetric density of 30 kg/m 3. The relatively low hydrogen density together with the very high gas pressures in the system are important drawbacks of this technically simple method. The volumetric density increases with pressure and reaches a maximum above 1000 bar, depending on the tensile strength of the material. Introduction…

31. Hydrogen Storage 2. Liquid H 2 Storage Introduction… Primitive phase diagram for hydrogen. Liquid H 2 only exists between the solid line and the line from the triple point at 21.2 K and the critical point at 32 K. Liquid hydrogen (LH 2 ) tanks can, in principle, store more hydrogen in a given volume than compressed gas tanks, since the volumetric capacity of liquid hydrogen is 0.070 kg/L (compared to 0.039 kg/L at 700 bar). Key issue with LH 2 tanks are hydrogen boil-off, the energy required for hydrogen liquefaction, as well as tank cost.

32. Hydrogen Storage 2. Liquid H 2 Storage Introduction… The energy required for liquefy hydrogen, over 30% of the lower heating value of hydrogen, remains a key issue and impacts fuel cost as well as fuel cycle energy efficiency. The large amount of energy necessary for liquefaction and the continuous boil-off of hydrogen limit the use of liquid hydrogen storage system. LH2 tank system To increase the storage capacities of these tanks, ‘Cryo-compresed’ tanks i.e. compressed cryogenic hydrogen or a combination of liquid hydrogen and high pressure hydrogen are developed.

33. Hydrogen Storage 3. High Surface Area Sorbents Introduction… Carbon nanotubes (CNTs), and several other high surface area sorbents (e.g. carbon nanofibers, graphite materials, metal-organic frameworks, aerogels, etc.) are being studied for hydrogen storage. The process for hydrogen adsorption in high surface area sorbents is physisorption, which is based on weak Van der Waals forces between adsorbate and adsorbent. Some factors investigated: Temperature and pressure, micropore density, specific surface area

34. Hydrogen Storage 3. High Surface Area Sorbents Factor 1 Temp. and Pressure Hydrogen adsorption isotherms at room temperature and at 77 K fitted with a Henry type and a Langmuir type equation, respectively (a) for activated carbon, (b) for purified SWCNTs.

35. Hydrogen Storage 3. High Surface Area Sorbents Factor 2 Micropore Density Correlation between the hydrogen storage capacity at 77 K and the pore volume for pores with diameter ＜ 1.3 nm. Relation between hydrogen storage capacity of the different carbon samples and their specific surface area at 298 K. Factor 3 Specific Surface Area

36. Hydrogen Storage Where is Hydrogen Interplanar spacing Inner surface External surface The long path for hydrogen diffusion into interior of CNTs is a challenge. Generally, the H 2 storage capacity under moderate conditions was at or below 1 wt%. Physisorption alone is not sufficient to reach the high capacity at ambient temperature. The big advantages of physisorption for hydrogen storage are the low operating pressure, the relatively low cost of the material involved, and the simple design of the system. The rather small gravimetric and volumetric hydrogen density on carbon are significant drawbacks. H 2 Molecules Hydrogen Storage Active Materials

37. Hydrogen Storage Other Possible Sorbents

38. Hydrogen Storage It’s a chemical compound or form of a bond between hydrogen with a metal. Metals hydrize at certain temperatures and pressures. Magnesium Hydride, MgH 2, stores the largest density of hydrogen but requires high temperature (> 300 °C) to let go of it. 4. Metal hydrides Introduction…

39. Hydrogen Storage Again of the reversible hydrides simple magnesium does best. Magnesium is the world's third most abundant metal. Iron titanium comes next for price. Pretty much everything else is an exotic designer alloy as of now: tens of thousands of dollars per kilo. 4. Metal hydrides Introduction… The temperature at which the metal hydrides release the hydrogen at standard pressure. There's about a 30% penalty to heat the magnesium (30% of the fuel cell keeps the metal hot).

40. Hydrogen Storage 4. Metal hydrides Introduction… Brief Category The most important families of hydride-forming IMC. Element A has a high affinity to hydrogen and element B has a low affinity to hydrogen.

41. Hydrogen Storage How to form Metal Hydrides Hydrogen reacts at elevated temperatures with many transition metals and their alloys to form hydrides. The electropositive elements are the most reactive, i.e. Sc, Yt, lanthanides, actinides, and members of the Ti and Va groups. The binary hydrides of the transition metals are predominantly metallic in character.

42. Hydrogen Storage Pressure composition isotherms for hydrogen absorption in a typical intermetallic compound on the left hand side. The coexistence region is characterized by the flat plateau and ends at the critical temperature T c. Solid solution Hydride phase How to form Metal Hydrides The thermodynamic aspects of hydride formation from gaseous hydrogen are described here.

43. Hydrogen Storage How to form Metal Hydrides The lattice structure is that of a typical metal with hydrogen atoms on the interstitial sites; and for this reason they are also called interstitial hydrides. The type is limited to the composition This type of structure is limited to the compositions of MH, MH 2, and MH 3. The ternary system AB x H n, element A is usually a rare earth or an alkaline earth metal and tends to form a stable hydride. Element B is often a transition metal and forms only unstable hydrides. Some well defined ratios of B:A, where x=0.5, 1, 2, 5, have been found to form hydrides with a hydrogen to metal ratio of up to two.

44. Hydrogen Storage How to form Metal Hydrides The maximum amount of hydrogen in the hydride phase is given by the number of interstitial sites in the IMC. As a general rule, it can be stated that all elements with an electronegativity in the range of 1.35- 1.82 do not form stable hydrides (hydride gap). More general is the Miedema model: the more stable an intermetallic compound is, the less stable the corresponding hydride and vice versa. Because of the phase transition, metal hydrides can absorb large amounts of hydrogen at a constant pressure. One of the most interesting features of metallic hydrides is the extremely high volumetric density of hydrogen atoms present in the host lattice.

45. Hydrogen Storage About Metal Hydrides The maximum amount of hydrogen in the hydride phase is given by the number of interstitial sites in the IMC. As a general rule, it can be stated that all elements with an electronegativity in the range of 1.35- 1.82 do not form stable hydrides (hydride gap). More general is the Miedema model: the more stable an intermetallic compound is, the less stable the corresponding hydride and vice versa. Because of the phase transition, metal hydrides can absorb large amounts of hydrogen at a constant pressure. One of the most interesting features of metallic hydrides is the extremely high volumetric density of hydrogen atoms present in the host lattice.

46. Hydrogen Storage About Metal Hydrides The highest volumetric hydrogen density reported is about 150 kg/m 3 in Mg 2 FeH 6 and Al(BH 4 ) 3. Both hydrides belong to the complex hydrides family. Metal hydrides are very effective at storing large amounts of hydrogen in a safe and compact way, but the gravimetric hydrogen density is shown to less than about 3 wt%. It remains a challenge to explore the properties of lightweight metal hydrides. Complex hydrides? Group 1,2, and 3 light metals, e.g. Li, B, and Al, give rise to a large variety of metal-hydrogen complexes. They are especially interesting because of their light weight and the number of hydrogen atoms per metal atom, which is two in many cases.

47. Hydrogen Storage Complex Hydrides The main difference between the complex and metallic hydrides is the transition to an ionic or covalent compound upon hydrogen absorption. The hydrogen in the complex hydrides is often located in the corners of a tetrahedron with B or Al in the center. Tetrahydroborates M(BH 4 ), and the tetrahydroaluminates M(AlH 4 ) are useful storage materials. The compound with the highest gravimetric hydrogen density at RT known is LiBH 4 (18 wt%).

48. Hydrogen Storage Complex Hydrides The method for improving hydrogen storage capacity Destabilization of LiBH 4 with MgH 2 ! Although the storage density is promising, one of the major issues with many metal hydrides, due to the reaction enthalpies involves (e.g. ~40 kJ/mol H2), is thermal management during refueling. Approximately 0.5-1 MW of heat must be rejected during recharging on- board vehicular systems. Reversibility and durability of these materials also needs to be demonstrated. Issues with handling, pyrophoricity, and exposure to air, humidity and contaminants also need to be addressed.

49. Complex light metal hydrides : AMH 4 (A= alkali or alkali earth metal, M= third group elements) Unlike classic interstitial metal hydrides, the alanates desorb and absorb hydrogen through chemical decomposition and recombination reactions. Alanates Borohydrides Table selected complex hydrides HydrideH 2 ( wt % )Source LiAlH 4 10.5Commercially available NaAlH 4 7.5Commercially available KAlH 4 5.8As described in J. Alloys Compd., 353 (2003) 310 Mg(AlH 4 ) 2 9.3As described in Inorg. Chem., 9 (1970) 325 Ca(AlH 4 ) 2 7.7As described in Inorg. Nucl. Chem., 1 (1955) 317 LiBH 4 18.5Commercially available NaBH 4 10.6Commercially available Mg(BH 4 ) 2 14.9As described in Inorg. Chem., 11 (1972) 929 Ca(BH 4 ) 2 11.4Synthetic procedure to be developed Al(BH 4 ) 3 16.9As described in J. Am. Chem. Soc., 75 (1953) 209 Complex Hydrides Hydrogen Storage

50. Material design of metal borohydride M(BH 4 ) n or alanate M(AlH 4 ) n. Charge transfer from M n+ to [BH 4 ] - is a key feature for the stability of M(BH 4 ) n, which can be estimated by value of Pauling electronegativity χ P. The charge transfer becomes smaller with increasing value of χ P, which makes ionic bond weaker. χ p of cation M n+ ↑, ionic bond weaker, thermal desorption temperature↓ Fig. The desorption temperature T d as a function of the Pauling electronegativity χ p. Hydrogen Storage Complex Hydrides

51. H 2 storage in Mg 2 Ni alloy Ball-milling vial Mg powder Ni powder L Milling steel balls Diameter: 5/16 inch, 2.10g Ball to powder ratio, BPR= 5:1, 10:1 In 1atm N 2 glove box ＋ Ball-milling powder for 5, 10, 15, 20, 25hr in SPEX 8000 Hydrogen Storage 4. Metal hydrides Examples… Experimental method - preparation of Mg 2 Ni

52. Fig. X-ray diffraction patterns of as-milled powders with different ball-milling conditions. Hydrogen Storage 4. Metal hydrides Examples… Mg 2 Ni can be prepared by ball- milling Mg and Ni powder over 10 hr. Prolonging milling time and enlarging ball-to-powder ratio are able to increase the crystallinity of Mg 2 Ni powders, reducing the particle size as well as grain size.

53. PowdersMilling time (hr)BPRHydrogenation density Mg2Ni, Ni10 1.57 wt.% Mg2Ni15102.76 wt.% Mg2Ni20102.89 wt.% Table hydrogen capacities measured at 300 psi H 2 and 573 K with different ball-milling conditions. Fig Hydrogen absorption rate among 10, 15, 20 hr ball- milled powders Prolonging milling time from 10 to 20 hr increases hydrogen capacity over 1 wt% to around 2.9 wt%. Hydrogen absorption rate was improved obviously by prolonging milling time, especially for as-milled powder for 20 hr performing the best absorption rate. Hydrogen Storage

54. V(BH 4 ) 3 possesses theoretical hydrogen density 12.7 wt.% and 4.4 wt.% with NaCl which is the product of ball milling process. Nevertheless the observed weight loss is only 0.1 wt.% now. The desorbed temperature approximately 127 ºC is maybe the crucial factor. Mechano-chemical activation synthesis 1. High energy ball milling 2. reaction in solid state instead of in solvent 4. Complex hydrides Examples… Hydrogen Storage

55. 4. Complex hydrides Examples… Ball-milling vial NaBH 4 powder VCl 3 powder Milling steel balls Diameter: 5/16 inch, 2.10g Ball to powder ratio, BPR= 35:1 In 1atm N 2 -filled glove box ＋ High energy ball-milling mixed powder in SPEX 8000 for 5, 10 hr Molar ratio= 2:1 Experimental method - preparation of V(BH 4 ) 3 Hydrogen Storage

56. Fig. XRD patterns of ball-milled NaBH 4 and VCl 3 for 10 hr 4. Complex hydrides Examples… VCl 3 + 3NaBH 4 → V(BH 4 ) 3 + 3NaCl No diffraction peaks of starting materials V(BH 4 ) 3 is disordered Infrared spectroscopy is helpful to identify B-H bond of V(BH 4 ) 3 Hydrogen Storage

57. Fig. Thermogravimetric change of ball-milled NaBH 4 and VCl 3 for 10 hr at different temperature (at 2.5×10-4 mbar) Dehydrogenation amount = 0.5 wt. % Hydrogen Storage

58. 4. Metal hydrides Examples… Aluminum Hydride Aluminum hydride or alane, AlH 3, is potentially an attractive storage material due to the large amount of hydrogen that can be contained in a relatively small, light-weight package. AlH 3 contains 10 % H by weight and has a theoretical H density of 148 g/L, which is more than double the density of liquid H 2. SEM micrographs of α-AlH 3 showing large cuboids 50-100 microns in diameter. Crystal structure of α-AlH 3 (R- 3c) showing the H atoms in an octahedral coordination around the Al.

59. Hydrogen Storage 4. Metal hydrides Examples… Aluminum Hydride SEM micrographs of α-AlH 3 showing large cuboids 50-100 microns in diameter. Crystal structure of α-AlH 3 (R- 3c) showing the H atoms in an octahedral coordination around the Al. Theoretically, based on thermodynamic considerations, AlH 3 will decompose to H 2 and Al at room temperature. However, due apparently to the presence of an oxide surface layer, it exhibited slow H 2 evolution rates below 150 °C. Recently, freshly synthesized, nanoscale AlH 3 has been shown to decompose at less than 100 °C without the need of a dopant or ball milling. In addition, the total H 2 yield with the fresh material approaches the theoretical value of 10 wt%. G. Sandrock et al., Appl. Phys. A, 80, 687 (2005) J. Graetz et al., J. Phys. Chem. B 109, 22181 (2005)

60. Summary The materials science challenge of hydrogen storage is to understand the interaction of hydrogen with other elements better, especially metals. Hydrogen production, storage, conversion has reached a technological level, although plenty of improvements and new discoveries are still possible.

61. END Thank you for your kind attention. Department of Materials Science and Engineering National Cheng Kung University Corrosion Prevention Laboratory