氫能源

Hydrogen energy材料系 蔡文達 教授

March 18th 2010

二十一世紀前五十年 人類將面臨之十大問題

ENERGY

WATER

FOOD

ENVIRONMENT

POVERTY

TERRORISM & WAR

DISEASE

EDUCATION

DEMOCRACY

POPULATION

Greenhouse Effect

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Global warming – mean surface temperature 1850-2006

Needs of New Type of Energy -- 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

wind turbines Monocrystalline solar cell

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Types of Energy

Biomass Fossil energy Electricity

Hydropower Natural gas Coal Nuclear energy Wind Geothermal

Renewable energy Biomass Solar energy Batteries Fuel cells Wind energy Hydrogen energy Wide and tidal power

Fossil energy

Hydrocarbons Remains of dead plants and animals

Coal Oil Gas

Electricity Hydropower Natural gas Coal Nuclear energy Wind Geothermal

Efficiency

Types of Energy

Biomass Fossil energy Electricity

Hydropower Natural gas Coal Nuclear energy Wind Geothermal

Renewable energy Solar energy Batteries Fuel cells Wind energy Hydrogen energy Wide and tidal powe

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To achieve comparable driving range may require larger amount of H2.

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.

氫能經濟的意義 由來：

1970 年：「 Hydrogen Economy （氫能經濟）」首次由美國通用汽車公司提出。

2000 年：美國通用汽車公司在國家石油化學與煉製協會的年會上說：「我們的長期遠景是氫能經濟」。

意義：主要為描繪未來氫取代石油成為支撐全球經濟之主要能源後，整個氫能源生產、輸送、貯存及使用之市場運作體系。

Building Hydrogen Economy

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• Overview of hydrogen energy

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

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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.

Clean , Renewable and Sustainable .

“ The choice for the future .”

Why hydrogen ?

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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 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.

Fig. An algae bioreactor for hydrogen production.

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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

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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

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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. Hydrogen in metal matrixFig. Carbon nanotube

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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

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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

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• Introduction of hydrogen storage

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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 H2 as close as possible.

Hydrogen Storage implies the reduction of an enormous volume of hydrogen gas. Compression of H2 gas.

What is Hydrogen Storage ?

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Reversible on-board vs. Regenerable off-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.

On-board

Off-board

Vehicular hydrogen storage approaches:

Definitions

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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 TankFuel Cell Stacks

Air Pump

Power Control Unit

Hydrogen Filler Mouth

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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

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To achieve comparable driving range may require larger amount of H2.

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.

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 H2 storage system targets

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6 wt% 9 wt%

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Current approaches include:1. High pressure H2 cylinders (Gas)2. Cryogenic and liquid hydrogen (Liquid)

3. High surface area sorbents (Solid)4. Metal hydrides (Solid)

Hydrogen Storage

Methods

Conventional Storage

Advanced Solid Materials Storage

Increasing H2 density by Pressure and Temp. control.

Using little additional material to reach high H2 density.

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The basic hydrogen storage method and phenomena.

1

2

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Gravimetric density

Volumetric density

Working Temp.

Pressure

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.

Hydrogen Storage

Methods

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1. HP H2 CylindersIntroduction…

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.

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1. HP H2 Cylinders

Volumetric density of compressed H2 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/m3.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…

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2. Liquid H2 StorageIntroduction…

Primitive phase diagram for hydrogen.

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.

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2. Liquid H2 StorageIntroduction…

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.

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3. High Surface Area SorbentsIntroduction…

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

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3. High Surface Area Sorbents

Factor 1Temp. 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.

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3. High Surface Area Sorbents

Factor 2Micropore 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 3Specific Surface Area

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Where is Hydrogen

Interplanar spacing

Inner surface

External surface

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.

H2 Molecules

Hydrogen Storage Active Materials

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Other Possible Sorbents

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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.

4. Metal hydridesIntroduction…

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Alloys

Solidsolutions

Intermetalliccompounds

others

AB5 AB2 AB A2B

OtherAB3,A2B7

A2B17,etc.

Stable Metastable

Multiphase Quasiccrystalline Amorphous Nanocrystalline

4. Metal hydridesIntroduction…

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.

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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.

Solid solution

Hydride phase

How to form Metal Hydrides

The thermodynamic aspects of hydride formation from gaseous hydrogen are described here.

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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.

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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.

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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%).

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.

Alanates

Borohydrides

Table selected complex hydrides

Hydride H2 ( wt % ) Source

LiAlH4 10.5 Commercially available

NaAlH4 7.5 Commercially available

KAlH4 5.8 As described in J. Alloys Compd., 353 (2003) 310

Mg(AlH4)2 9.3 As described in Inorg. Chem., 9 (1970) 325

Ca(AlH4)2 7.7 As described in Inorg. Nucl. Chem., 1 (1955) 317

LiBH4 18.5 Commercially available

NaBH4 10.6 Commercially available

Mg(BH4)2 14.9 As described in Inorg. Chem., 11 (1972) 929

Ca(BH4)2 11.4 Synthetic procedure to be developed

Al(BH4)3 16.9 As described in J. Am. Chem. Soc., 75 (1953) 209

Complex Hydrides

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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 χ

P. The charge transfer becomes smaller with increasing value of χP, which makes ionic bond weaker.

χp of cation Mn+ ↑, ionic bond weaker, thermal desorption temperature↓

Fig. The desorption temperature Td as a function of the Pauling electronegativity χp.

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Complex Hydrides

H2 storage in Mg2Ni alloy

Ball-milling vial

Mg powder

Ni powderL

Milling steel balls

Diameter: 5/16 inch, 2.10g

Ball to powder ratio, BPR= 5:1 , 10:1

In 1atm N2 glove box

＋

Ball-milling powder for 5, 10, 15, 20, 25hr in SPEX 8000

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4. Metal hydridesExamples…

Experimental method - preparation of Mg2Ni

Powders Milling time (hr) BPR Hydrogenation density

Mg2Ni, Ni 10 10 1.57 wt.%Mg2Ni 15 10 2.76 wt.%Mg2Ni 20 10 2.89 wt.%

Table hydrogen capacities measured at 300 psi H2 and 573 K with different ball-milling conditions.

0 2 0 4 0 6 0 8 0 1 0 0

T im e (m in )

0

1

2

3

4

H2

Ab

sorp

tion

(w

t.%

)

10 h r15 h r

T h eoretica l H 2 a b sorp tio n d en sity o f M g 2N i

20 h r

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.

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V(BH4)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 hydridesExamples…

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4. Complex hydridesExamples…

Ball-milling vial

NaBH4 powder

VCl3 powder

Milling steel balls

Diameter: 5/16 inch, 2.10g

Ball to powder ratio, BPR= 35:1

In 1atm N2-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(BH4)3

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4. Metal hydridesExamples…

Aluminum Hydride

Aluminum hydride or alane, AlH3, is potentially an attractive storage material due to the large amount of hydrogen that can be contained in a relatively small, light-weight package. AlH3 contains 10 % H by weight and has a theoretical H density of 148 g/L, which is more than double the density of liquid H2.

SEM micrographs of α-AlH3 showing large cuboids 50-100 microns in diameter.

Crystal structure of α-AlH3 (R-3c) showing the H atoms in an octahedral coordination around the Al.

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4. Metal hydridesExamples…

Aluminum Hydride

SEM micrographs of α-AlH3 showing large cuboids 50-100 microns in diameter.

Crystal structure of α-AlH3 (R-3c) showing the H atoms in an octahedral coordination around the Al.

Theoretically, based on thermodynamic considerations, AlH3 will decompose to H2 and Al at room temperature. However, due apparently to the presence of an oxide surface layer, it exhibited slow H2 evolution rates below 150 °C. Recently, freshly synthesized, nanoscale AlH3 has been shown to decompose at less than 100 °C without the need of a dopant or ball milling. In addition, the total H2 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)

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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.

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Thank you for your kind attention.

Department of Materials Science and Engineering

National Cheng Kung UniversityCorrosion Prevention Laboratory