氫能源 Hydrogen energy

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

2. Overview of hydrogen energy

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

4. Overview Energy Consumption

5. Overview 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. • 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. Sources of greenhouse gases Global fossil carbon emission by fuel type

6. Overview 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 [CO2] and temperature

7. Overview Greenhouse Effect

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

9. Overview 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. 2H2O → O2 + 4H+ +4e- 2H+ + 2e- → H2

10. H2 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. Overview Why hydrogen ? Clean , Renewable and Sustainable . “ The choice for the future .”

11. Overview

12. Overview Building Hydrogen Economy

13. Overview

14. Overview H2 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 production : Biohydrogen can be produced in an algaebioreactor. 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. Fig. An algae bioreactor for hydrogen production.

15. Overview H2 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. • Chemical production : By using sodium hydroxide as a catalyst, aluminum and its alloys can react with water to generate hydrogen gas. • Al + 3 H2O + NaOH → NaAl(OH)4 + 1.5 H2 Solar Energy Fig. Photoelectrochemical cell

16. Overview H2 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

17. Overview H2 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. Carbon nanotube Fig. Hydrogen in metal matrix

18. Overview H2 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. Fig. Direct-methanol fuel cell Fig. Scheme of fuel cell

19. Overview H2 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. Fig. Hydrogen station

20. Introduction of hydrogen storage

21. Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in transportationapplications. The major bottleneck for commercializing fuel-cell vehicles is on-board hydrogen storage. The goal is to pack H2as close as possible. Hydrogen Storage implies the reduction of an enormous volume of hydrogen gas. Compression of H2 gas. Hydrogen Storage What is Hydrogen Storage ?

22. Hydrogen Storage Definitions Vehicular hydrogen storage approaches: Reversible on-board vs. Regenerable off-board On-board System that bind H2 with low binding energy (less than 20-25 kJ/mol H2) 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 H2), once the hydrogen is released, recharging with H2 under operating conditions convenient at a refueling station is problematic. Off-board

23. Hydrogen Storage Reversible on-board Hydrogen Filler Mouth Hydrogen Tank Fuel Cell Stacks Air Pump Power Control Unit The on board storage media require hydrogen in liquidor 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.

24. Hydrogen Storage Reversible on-board Hydrogen Filler Mouth Hydrogen Tank The technical challenge is… Storing sufficient hydrogen while meeting all consumer requirements without compromising passenger or cargo space. 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.

25. Hydrogen Storage Why Challenge? Gasoline or Hydrogen. 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. To achieve comparable driving range may require larger amount of H2. 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.

26. Gasoline or Hydrogen Hydrogen Storage US DOE H2 storage system targets 6 wt% 9 wt% 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.

27. Hydrogen Storage Hydrogen Storage Methods Current approaches include: Conventional Storage • High pressure H2 cylinders • Cryogenic and liquid hydrogen • High surface area sorbents • Metal hydrides Increasing H2 density by Pressure and Temp. control. Advanced Solid Materials Storage Using little additional material to reach high H2 density.

28. Gravimetric density Volumetric density Working Temp. Pressure 1 2 3 4 Hydrogen Storage The basic hydrogen storage method and phenomena. Hydrogen Storage Methods At ambient temp. and atmospheric pressure, 1 kg of H2 gas has a volume of 11 m3 ! Work must be applied to increase H2 density.

29. 1. HP H2 Cylinders Hydrogen Storage Introduction… 70 MPa H2 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. Volumetric density of compressed H2 as a function of gas pressure. 1. HP H2 Cylinders Hydrogen Storage Introduction… The volumetric density increases with pressure and reaches a maximum above 1000 bar, depending on the tensile strength of the material. 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/m3. The relatively low hydrogen density together with the very high gas pressures in the system are important drawbacks of this technically simple method.

31. Primitive phase diagram for hydrogen. 2. Liquid H2 Storage Hydrogen Storage Introduction… Liquid H2 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 (LH2) 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 LH2 tanks are hydrogen boil-off, the energy required for hydrogen liquefaction, as well as tank cost.

32. LH2 tank system 2. Liquid H2 Storage Hydrogen 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. 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. 3. High Surface Area Sorbents Hydrogen Storage 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. 3. High Surface Area Sorbents Hydrogen Storage 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. Factor 2 Micropore Density 3. High Surface Area Sorbents Hydrogen Storage Correlation between the hydrogen storage capacity at 77 K and the pore volume for pores with diameter ＜ 1.3 nm. Factor 3 Specific Surface Area Relation between hydrogen storage capacity of the different carbon samples and their specific surface area at 298 K.

36. H2 Molecules Inner surface Interplanar spacing External surface Hydrogen Storage Active Materials Where isHydrogen Hydrogen Storage The long path for hydrogen diffusion into interior of CNTs is a challenge. Generally, the H2 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.

37. Other Possible Sorbents Hydrogen Storage

38. 4. Metal hydrides Hydrogen Storage Introduction… It’s a chemical compound or form of a bond between hydrogen with a metal. Metals hydrize at certain temperatures and pressures. Magnesium Hydride, MgH2, stores the largest density of hydrogen but requires high temperature (> 300 °C) to let go of it.

39. 4. Metal hydrides Hydrogen Storage 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). 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.

40. 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. Brief Category 4. Metal hydrides Hydrogen Storage Introduction…

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. The thermodynamic aspects of hydride formation from gaseous hydrogen are described here. Hydrogen Storage How to form Metal Hydrides 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 Tc. Hydride phase Solid solution

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, MH2, and MH3. The ternary system ABxHn, 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/m3 in Mg2FeH6 and Al(BH4)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(BH4), and the tetrahydroaluminates M(AlH4) are useful storage materials. The compound with the highest gravimetric hydrogen density at RT known is LiBH4 (18 wt%).

48. 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. The method for improving hydrogen storage capacity Hydrogen Storage Complex Hydrides Destabilization of LiBH4 with MgH2!

49. Hydrogen Storage Complex Hydrides • Complex light metal hydrides : AMH4 (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. Table selected complex hydrides Alanates Borohydrides

50. Hydrogen Storage Complex Hydrides • Material design of metal borohydride M(BH4)n or alanate M(AlH4)n. • Charge transfer from Mn+ to [BH4]- is a key feature for the stability of M(BH4)n, which can be estimated by value of Pauling electronegativity χP. The charge transfer becomes smaller with increasing value of χP, which makes ionic bond weaker. χpof cation Mn+ ↑, ionic bond weaker, thermal desorption temperature↓ Fig. The desorption temperature Td as a function of the Pauling electronegativity χp.