Transcript 氫能源

氫能源
Hydrogen energy
材料系 蔡文達 教授
Dec 30th 2008
• Overview of hydrogen energy
Overview
Energy Consumption
Passenger vehicles are major
consumption of fossil fuel
Energy consumption is
outpacing production
Overview
Energy Consumption
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
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
Overview
Greenhouse Effect
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
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
Overview
Why hydrogen ?
Clean , Renewable and Sustainable .
H2
“ The choice for the future .”
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
Overview
Building Hydrogen Economy
Overview
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 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.
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
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
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
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
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
• Introduction of hydrogen storage
Hydrogen Storage
What is
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 fuelcell 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.
Hydrogen Storage
Vehicular hydrogen
storage approaches:
Definitions
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.
Off-board
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.
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 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 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.
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.
Hydrogen Storage
US DOE H2 storage
or
system targets
6 wt%
Gasoline
Hydrogen
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.
Hydrogen Storage
Hydrogen
Storage
Methods
Current approaches include:
1. High pressure H2 cylinders
Conventional
Storage
2. Cryogenic and liquid hydrogen
Increasing H2 density by Pressure and Temp. control.
Advanced Solid
Materials Storage
3. High surface area sorbents
4. Metal hydrides
Using little additional material to reach high H2 density.
Hydrogen Storage
The basic hydrogen storage
method and phenomena.
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.
Gravimetric
density
1
2
3
4
Hydrogen
Storage
Methods
Volumetric Working Pressure
density
Temp.
Hydrogen Storage
1. HP H2 Cylinders
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 hydrogenpowered 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.
Hydrogen Storage
Volumetric density of compressed H2 as a
function of gas pressure.
1. HP H2 Cylinders
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.
Hydrogen Storage
Primitive phase diagram for hydrogen.
2. Liquid H2 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.
Hydrogen Storage
LH2 tank system
2. Liquid H2 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.
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
Hydrogen Storage
Factor 1
Temp. and Pressure
3. High Surface Area
Sorbents
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.
Hydrogen Storage
Factor 2
Micropore Density
3. High Surface Area
Sorbents
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.
Hydrogen Storage
Where is
Hydrogen
H2 Molecules
External surface
Inner surface
Hydrogen Storage
Active Materials
Interplanar spacing
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.
Hydrogen Storage
Other Possible
Sorbents
Hydrogen Storage
4. Metal hydrides
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.
Hydrogen Storage
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).
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.
Hydrogen Storage
4. Metal hydrides
Brief Category
Introduction…
Alloys
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.
Solid
solutions
AB5
Intermetallic
compounds
AB2
AB
others
A2B
Other
AB3,A2B7
A2B17,etc.
Stable
Multiphase
Metastable
Quasiccrystalline Amorphous Nanocrystalline
Hydrogen Storage
Metal
Hydrides
How to form
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.
Hydrogen Storage
The thermodynamic aspects of
How to form
hydride formation from gaseous
hydrogen are described here.
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.
Solid
solution
Hydride
phase
Hydrogen Storage
Metal
Hydrides
The lattice structure is that of a How to form
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.
Hydrogen Storage
Metal
Hydrides
How to form
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 1.351.82 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.
Hydrogen Storage
Metal
Hydrides
About
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 1.351.82 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.
Hydrogen Storage
Metal
Hydrides
About
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.
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%).
Hydrogen Storage
The method for improving
hydrogen storage capacity
Complex
Hydrides
Destabilization
of LiBH
Although
the storage
density
promising,
one of the
4 withisMgH
2!
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 onboard 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.
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
Hydride
Alanates
Borohydrides
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
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.
χp of cation Mn+ ↑, ionic bond weaker,
thermal desorption temperature↓
Fig. The desorption temperature Td as a
function of the Pauling
electronegativity χp.
Hydrogen Storage
4. Metal hydrides
H2 storage in
Mg2Ni alloy
Examples…
Experimental method - preparation of Mg2Ni
In 1atm N2 glove box
Milling steel balls
＋
Mg powder
Ni powder
L Diameter: 5/16 inch, 2.10g
Ball to powder ratio,
BPR= 5:1 , 10:1
Ball-milling powder
for 5, 10, 15, 20, 25hr
in SPEX 8000
Ball-milling
vial
Hydrogen Storage
4. Metal hydrides
Mg2Ni
Mg
Ni
Examples…
Relative Intensity
15hr,BPR=5
20hr,BPR=10
15hr,BPR=10
10hr,BPR=10
Mg2Ni can be prepared by ballmilling Mg and Ni powder over
10 hr.
Prolonging milling time and
enlarging ball-to-powder ratio
are able to increase the
crystallinity of Mg2Ni powders,
reducing the particle size as
well as grain size.
5hr,BPR=10
10
20
30
40
50
60
70
Diffraction Angle(2)
80
Fig. X-ray diffraction patterns
of as-milled powders
with different ball-milling
conditions.
Powders
Milling time (hr)
BPR
Hydrogenation density
Mg2Ni, Ni
Mg2Ni
Mg2Ni
10
15
20
10
10
10
1.57 wt.%
2.76 wt.%
2.89 wt.%
4
H2 Absorption (wt.%)
Hydrogen Storage
Table hydrogen capacities measured at 300 psi H2 and 573 K with
different ball-milling conditions.
Theoretical H2 absorption density of Mg2Ni
3
20 hr
15 hr
10 hr
2
1
0
0
20
40
60
Time (min)
80
100
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.
Fig Hydrogen absorption rate
among 10, 15, 20 hr ballmilled powders
Hydrogen Storage
4. Complex hydrides
Examples…
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
Hydrogen Storage
4. Complex hydrides
Examples…
Experimental method - preparation of V(BH4)3
In 1atm N2-filled glove box
Milling steel balls
Diameter: 5/16 inch, 2.10g ＋
NaBH4 powder
VCl3 powder
Molar ratio= 2:1
Ball to powder ratio, BPR= 35:1
High energy ball-milling
mixed powder in SPEX
8000 for 5, 10 hr
Ball-milling
vial
Hydrogen Storage
4. Complex hydrides
Examples…
NaCl
Relative Intensity
VCl3 + 3NaBH4 → V(BH4)3 + 3NaCl
No diffraction peaks of starting
materials
5 hr
10 hr
V(BH4)3 is disordered
Infrared spectroscopy is helpful to
identify B-H bond of V(BH4)3
NaBH4
VCl3
20
30
40
50
60
70
80
Diffraction angle (2)
Fig. XRD patterns of ball-milled NaBH4 and VCl3 for 10 hr
Weight change
Thermal decomposition of
ball-milled V(BH4)3 + NaCl
10 hr
10 hr at -15 oC
1
250
200
150
100
0.98
50
0
100
200
Temperature (oC)
Hydrogen Storage
1.02
0
Time (min)
Dehydrogenation amount = 0.5 wt. %
Fig. Thermogravimetric change of ball-milled NaBH4 and VCl3 for 10 hr
at different temperature (at 2.5×10-4 mbar)
Hydrogen Storage
Aluminum Hydride
4. Metal hydrides
Examples…
SEM micrographs of α-AlH3
showing large cuboids 50-100
microns in diameter.
Crystal structure of α-AlH3 (R3c) showing the H atoms in
an octahedral coordination
around the Al.
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.
Hydrogen Storage
Aluminum Hydride
4. Metal hydrides
Examples…
SEM micrographs of α-AlH3
showing large cuboids 50-100
microns in diameter.
Theoretically, based on thermodynamic considerations,
structure
of α-AlH3 (RAlH3 will decompose to H2 and Al atCrystal
room
temperature.
3c) showing
atoms in
However, due apparently to the presence
of the
anHoxide
an octahedral coordination
surface layer, it exhibited slow H2 evolution
around the Al.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)
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.
END
Thank you for your kind attention.
Department of Materials Science and Engineering
National Cheng Kung University
Corrosion Prevention Laboratory