Transcript Metal hydride - University of Victoria

Mechanical Engineering Department
University of Victoria
Ferrous Non-Ferrous Materials (Mech473)
Metal hydride overview
By
Ramadan Abdiwe
Experiment
Purpose:

Past work has involved the
development of a mathematical model
that simulates the behaviour inside a
metal hydride tank during absorption
and desorption. The model has been
validated against numerical and
experimental work reported in the
literature. The first goal of the
experiments is to validate the
developed model against in-house
experimental data. The second goal is
to properly characterize the specific
metal hydride alloys being used in the
lab, so that more accurate modelling
can be performed in the future
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Role of Team Members
Dr. Andrew Rowe: Faculty supervisor of the project
Brendan MacDonald: Graduate student in charge of co-ordinating the research and
performing the modelling simulations and experiments
Fred McGuinness: Undergraduate student who is responsible for designing and
manufacturing a metal hydride tank that will allow detailed testing of the behaviour inside the
vessel, specifically through the use of internal instrumentation devices
Tyler Isaacson: Undergraduate student who is responsible for designing a metal hydride tank
which will be used for testing some of the enhanced heat transfer configurations, and also for
writing a user’s manual that describes the test apparatus
Ramadan Abdiwe: Graduate student who is responsible for assisting with the experimental
testing work, specifically through the use of an inert environment glove box which will be used
to fill the tanks designed by Fred and Tyler with the various metal hydride alloys utilized in
the test runs
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Main Problems with fossil fuel

All the fossil fuels we are using are running out and burning them
increases carbon dioxide in atmosphere which increases the
greenhouse effects, causing GLOBAL WARMING

Some fossil fuels contain sulphur and when they burn this becomes
sulphur dioxide, a poisonous gas which reacts with water in the
atmosphere to form sulphuric acid or ACID RAIN
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Properties of hydrogen
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Hydrogen gas is very powerful, It has the highest energy per unit of
weight of any chemical fuel
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Hydrogen is highly abundant element, it is one of the most
common substances on earth
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Hydrogen is environmentally friendly and its oxidation product is
water.
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Hydrogen Production
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Direct Energy Conversion using H2
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Challenges of using hydrogen
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Even though, hydrogen is an
attractive alternative to hydrocarbon
fuels such as gasoline in mobile
applications. However, the storage
of hydrogen in these applications
still remains a problem and and
scientists have to come up with
applicable, light, affordable, and
safe method for storing hydrogen in
such applications.
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Hydrogen storage options

Storage as gas under pressure (250-350 bar)
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Cryogenic storage as liquid hydrogen (temp –253 0C )
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Storage as metallic hydrides
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Carbon adsorption and glass microsphere
storage techniques (under development)
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Compressed hydrogen
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the most straightforward option
at this time
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offers the simplest and least
expensive method for onboard
storage of hydrogen
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The refilling time of compressed
hydrogen tanks is also similar to
that of gasoline tanks.
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Problems

Low energy density (One way to
increase the fuel stored in the container
is to increase pressure, but this requires
more expensive storage containers,
increasing compression costs )

hydrogen has a tendency to leak
because of its small size. Seals and
valves on the containers need to be
designed to prevent leaks. If a fuel cell
vehicle is stored in a closed garage,
hydrogen that has leaked out could
accumulate and increase the risk of
fire or explosion.in addition to the
explosion could happen from cars
accidents on the road
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Liquefied Hydrogen
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Could perform better in an
accident
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high energy density of liquid
hydrogen
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Increase available space
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Reduce environments effect
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Problems
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Does not liquefy until (~-253 0C)
Which cost energy
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40% of energy can be lost(25
percent of LH2 boiled off during
refueling and 1 percent lost per day
for onboard storage.)

requires excellent insulation of
storage containers; otherwise, left
for a period of time, the storage
tanks could become depleted
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Bonded hydrogen (Metal hydride)
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Since heat is required to release the
hydrogen, this method avoids safety
concerns surrounding leakage that
can be a problem with compressed
hydrogen and LH2. In fact, metal
hydrides are one of the safest
methods for storing hydrogen.
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Problems

Heavy weight. (One major obstacle to this method is that the metal
compounds used to attract the hydrogen tend to be very heavy
resulting in only 1.0 to 1.5 percent hydrogen by weight)

Some of the metals used for hydrides are very expensive. There are
less expensive options but they are impractical for use in fuel cell
vehicles as these cheaper metals require extremely high temperatures
to release the hydrogen
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Vehicles requirements of H2
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A modern, commercially available car optimized for
mobility and not prestige with a range of 400km burns
about 24 kg of petrol in a combustion engine; to cover the
same rage by electric car with a fuel cell 4kg hydrogen is
needed (Louis Schlapbach & Andreas Zuttel)
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Volume of 4kg hydrogen compacted in different ways,
with size relative to the size of a car
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Background of metal hydride
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the last century, scientists discover
that Pd metal occluded large
amount of H2 at ambient pressure
and temperature. However, it was
not very useful because of issues
associated with the cost and low
capacity of hydrogen storage. The
recent discovery of hydrogen sorption
by intermetallic compounds created
great hopes and stimulated research
and development worldwide of using
the metal hydrides as a new
alternative for storing and delivering
pure hydrogen that can be very useful
for fuel cell technology
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Mechanism of H2 movement through
the metal crystal lattice

The H2 molecule is first
weakly physisorbed on the
surface and then dissociatively
chemisorbed as strongly
bound, individual H-atoms

the size of the hydrogen atoms
is lighter and smaller than the
metal atoms; therefore, they
diffuse quickly from the
surface into the periodic sites
in the metal crystal lattice
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hydrogen adsorption/ dissociation and hydride formation
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PCT curves
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Basically, PCT is determined by keeping an alloy sample at
constant temperature and measuring the pressure change as
hydrogen is absorbed. The hydride/dehydride cycling causes a change
of the intermetallic compounds volume that cause cracking for the
particles. This cracking of the particles cause increasing of the surface
area, which leads to an increase of the hydrogen reactivity

Most metal have high attraction for hydrogen and there are also few
have poor attraction for hydrogen and the reaction between the metal
and hydrogen can be exothermic or endothermic respectively. The
process of absorption and desorption is best illustrated by the
pressure compostion-tempreture profiles (PCT curves)
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Thermodynamic Behavior of M-H Reaction

The thermodynamic behavior of
metal hydride formation is
illustrated in the figure beside,
which is a PCT curve. The line of
the upper pressure represents the
absorption process and the lower
line represents the desorption
process and the flat part of both
lines is called Plateau. And the
difference
in
equilibrium
pressures between the absorption
and desorption reactions is called
Hystersis
Schematic isothermal pressure-composition hysteresis loop
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
The chemical reaction associated with absorption and desorption is as
follows:
M + (x/2) H2  MHx + Heat
The heat of the reaction in the absorption process could be very useful
in using metal hydride for heat pumps and thermal storage. But this
heat will be a problem when using metal hydride in vehicles. Also it
should be mentioned that higher heat of reactions, have lower
equilibrium pressures at a given temperature. The fundamental
connection between pressure and temperature is given by the Van’t
Hoff equation
lnP = ΔH/RT - ΔS/R
R is the gas constant. It should be mention that ΔH vary widly from
metal to metal because it is a measure of the strength of the M-H
bond. Where ΔS doesn’t vary much
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Metal hydrides material
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Hydrogen is a highly
reactive element and has
been shown to form
hydrides and solid
solutions with thousands
of metals and alloys.
Figure beside shows the
family tree of hydriding
alloys and complexes
Family tree hydriding alloys and complexes
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Metal hydride elements
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For PEM fuel cell vehicular applications
the range is 0-100 0C and 1-10 atm. This is
based on the possibility of using the waste
heat from the fuel cell to release the
hydrogen from the metal hydride. In the
last century Pd was used to store H2.
However, it is no longer used because it is
very expensive, doesn’t hold much
hydrogen, and requires heating above 100
0C to release that hydrogen. Today
Vanadium (V) and Niobium (Nb)
are well known elements for storing
hydrogen in the range of practical
applications
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Van’t Hoff lines (desorption) for elemental hydrides. Box
indicates 1–10 atm, 0–100°C ranges
Metal Hydride Alloys
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Scientists have been required to combine strong hydride forming elements
(A) with weak hydride elements (B) to form alloys (especially intermetallic
compounds) that have the desired intermediate thermodynamic affinities for
hydrogen
there are some conditions should be considered of choosing these alloys as
metal hydrides:
1-Temperature and pressure of hydride and dehydride
2-Hydrogen storage capacity of the alloys
3-Rate of absorption and desorption
4-Ease of activation
5-Poisoning by impurities
6-Cost and availability
Typically there are three type of bonding between metals and hydrogen:
Ionic, Covalent, and Metallic
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Metallic hydride
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Ionic, Covalent types of bonding are not very practical in mobile
applications because they require extremely high temperature in order to
liberate H2 . Where as the metallic type bond offers the necessary
behavior for hydrogen storage systems (In the metallic hydrides, the
hydrogen acts an electron accepter, the hydrogen atom accept the
electron from the conduction band of the metal and fill its first orbital )

The conventional metal hydride alloy families to be described here are
the AB5, AB2, and AB intermetallic compounds
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AB5 Intermetallic compounds
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AB5 intermetallic compounds have
a hexagonal crystal structure and
also have an extraordinary
versatility because many different
elements species can be substituted
(at least partially) into the A and B
lattice sites. Element A mostly is
one or more lanthanides with
atomic number between 57-71 and
element B is mostly based on Ni
with other substitutional elements
such as Co, Al, Mn, Fe, Cu,
Sn, Si [2]. Figure beside show the
PCT properties of some of AB5
family alloys
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Van’t Hoff lines for various AB5 hydrides
PCT and cost properties of selected AB5
hydrides
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
There are some advantages of this type of alloy compared to the other
conventional ones. These advantages are: low hysteresis, easy to
activate; (doesn’t require any heat), good tolerance to impurities such
as O2 or H2O, and good intrinsic kinetics. On the other hand, there are
some disadvantages due to relatively low hydrogen capacity (~1.3 wt %)
and also high cost of the raw material compared to AB2 type alloys
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AB2 Intermetallic compounds
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There is a large number of this
type of alloys (~500 alloys). AB2
alloys have two types of crystal
structures: hexagonal and cubic.
Elements A in this family are
often IVA group (Ti, Zr, Hf)
and/or rare earth series (at atomic
number from 57 to 71) and B
elements are from elements with
atomic number 23 to 26 such as
V, Cr, Mn, Fe. Figure beside
show the properties of some of
AB2 alloys
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PCT and cost properties of selected AB2
hydrides
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There are two main advantages in this type of alloys: they have higher
capacity compared to AB5 and lower cost especially if A elements are
mainly Ti (because Zr and V are very expensive in comparison to Ti).
However, this type of alloy suffers from difficulty in activation, and
sensitivity to impurities
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AB Intermetallic compounds
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Fig.7 and table.4 show the properties
of the most popular composition in
this family: TiFe, TiFe 0.85 Mn 0.15 ,
and TiFe 0.8 Ni 0.2. . In summary,
Tife-based AB alloys have good PCT
properties, good H-capacities and
low raw materials costs, but there are
problems associated with activation,
gaseous impurities and upper
plateau instabilities, which make
them unattractive for mobile storage
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PCT and cost properties of selected TiFe-type
hydrides
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A2B Intermetallic compounds
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Various crystal structures are possible. In one subfamily, A is typically of
the group IVA elements Ti, Zr or Hf and B is a transition metal, typically
Ni and the family is based on Mg2Ni. H-capacity and cost properties of
Mg2Ni are attractive, but desorption temperatures are too high for most
applications
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Problems of metal hydrides
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Low mass density is the general
weakness of all known metal
hydrides working near room
temperature. There are intermetallic
compounds and alloys can form
hydrides up to 9 mass% hydrogen
but they are not reversible within the
required range of temperature and
pressure. Table beside shows the
mass density of the well-known
compounds and alloys
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Intermetallic compounds and their hydrogen-storage properties
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The other common problem of
storing hydrogen is the impurity
and the most problematic
impurities are: CO, CO2, NH3,
H2S, CH4 and O2. These
impurities reduce the storage
capacity and also could cause
poisoning, retardation, or
reaction. Table beside shows the
effect of impurities on metal
hydride
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Poisoning = Rapid loss of H-capacity with cycling
Retardation = Reduction in absorption/desorption kinetics without significant
loss in the ultimate capacity
Reaction = Bulk corrosion was leading to irreversible capacity loss
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Practical Problems


There are also some practical problems with using metal hydrides; the
container (tank) is one of these problems. During the hydriding cycle, the
particles of the hydrides become very small which leads to entrainment of
the hydride fines in the gas streams. Therefore, the container system must
provide for these small particles and resulting expansion.
most of the hydrides powder (beds) have poor thermal conductivity.
Therefore, the most important parameter of designing metal hydrides
containers is the thickness of the hydride bed, and most studies done in this
field conclude that in order to have the highest reaction, the heat resistance of
the hydride bed should be decreased and the best way to design containers
with small hydride bed resistance is to keep the bed thickness as small as
possible .
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Conclusion

With the present status of technology, the three main options of storing hydrogen
are: compressed gas at high pressure (300-500 bar), liquid hydrogen (-253 0C),
and metal hydride.

One of the major issues of using metal hydrides in vehicular applications is the
weight of the storage system. Therefore, for the metal hydride to appear as
suitable option, the weight has to be lowered drastically.

Heat resistance of the metal hydride powder (bed) affect the hydriding /
dehydriding cycles and control the rate of hydrogen uptake or removal and this is
due to the low conductivity of the powder and also the poor heat transfer between
the particles and the wall of the container. Therefore, the best way to design
containers with small hydride bed resistance is to keep the bed thickness as small
as possible
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The End