1. Proceeding
of
Fuel
Cells
Final
Project
MAE
528
Fall
2014,
Miami,
Florida,
USA
The
Mechanics
of
Metal
Hydride
Hydrogen
Storage
Systems
for
Portable
Applications
Jordan
Suls
Department
of
Mechanical
Engineering
University
of
Miami

2.
Introduction
With
the
escalating
needs
for
sustainable
energy
in
today’s
society,
many
researchers
turn
to
hydrogen
as
the
hope
for
a
clean,
renewable
energy
source.
The
main
problems
being
faced
with
this
energy
source
is
the
need
for
efficient
and
cost-­‐
effective
methods
for
production,
storage
and
utilization
of
this
gas.
The
development
of
safe
and
reliable
hydrogen
storage
technologies
is
one
major
barrier
that
must
be
overcome
to
achieve
the
implementation
of
hydrogen-­‐
based
fuel
systems
into
today’s
society.
One
of
the
main
priorities
is
finding
a
way
to
supply
hydrogen
in
portable
applications
ranging
from
cars
to
mobile
phones.
Therefore,
the
focus
of
this
research
will
be
on
finding
practical
hydrogen
storage
techniques
that
can
be
utilized
with
a
PEM
fuel
cell
to
provide
the
necessary
energy
for
a
wide
range
of
mobile
applications.
Currently,
there
are
three
main
methods
being
employed
for
hydrogen
storage,
each
having
its
own
limitations.
Hydrogen
can
be
stored
as
a
high
pressure
compressed
gas,
which
involves
the
use
of
large,
heavy
tanks.
The
size
and
weight
involved
in
storing
compressed
hydrogen
gas
make
this
method
undesirable
for
mobile
applications.
Cryogenic
liquid
hydrogen
storage
has
a
greater
volumetric
storage
density
than
compressed
hydrogen
gas
but
further
complicates
the
system
needed
for
storage.
The
process
of
liquefying
the
hydrogen
gas
and
insulating
the
tank
requires
energy
and
a
greater
cost.
The
operating
conditions
needed
for
this
method
only
make
it
another
impractical
application
for
mobile
uses.
Finally,
there
are
hydrogen
storage
materials,
such
as
high
surface
area
carbon-­‐based
materials
and
metal
hydride

3. alloys.
Hydrogen
gas
can
be
stored
in
materials
either
by
adsorption
or
absorption.
Adsorption
is
the
process
of
hydrogen
attaching
to
the
surface
of
a
material
as
a
gas
(H2)
or
as
atoms
(H).
In
absorption,
the
hydrogen
gas
dissociates
into
hydrogen
atoms
that
are
then
incorporated
into
the
solid
lattice
framework
of
the
material.
The
process
of
absorption
with
the
use
of
metal
hydrides
is
particularly
interesting
because
it
offers
the
ability
to
store
large
amounts
of
hydrogen
at
low
pressure
and
temperature.
Figure
1
gives
an
approximate
operational
temperature
for
several
different
hydrogen
storage
methods
(some
of
which
are
outside
the
scope
of
this
research)
[1].
Figure
1:
Temperature
Requirements
for
Different
Hydrogen
Storage
Methods
[1]
The
potential
presented
by
metal
hydrides
in
their
gravimetric
and
volumetric
storage
capability
makes
this
method
the
most
appealing
for
the
ability
to
be
used
in
mobile
applications.
Similarly,
the
required
operating
conditions
are
easy
to
manage
and
maintain.
Metal
Hydrides
Overview
A
metal
hydride
consists
of
finely
ground
powders
that
absorb
large
quantities
of
hydrogen
gas
by
dissociating
the
gas
into
hydrogen
ions.
Metal
hydrides
use
hydrogenation
to
absorb
hydrogen
gas
into
the
lattice
of
the
metal
hydride.
Similarly,
they
use
dehydrogenation
to
release
the
stored
hydrogen
ions
from
the
surface
to
produce
hydrogen
gas
again
[2].

4. Metal
hydrides
typically
use
iron,
titanium,
manganese,
nickel
and
chromium
alloys
but
new
research
is
exploring
new
complex
materials
such
as
alanates,
amides
and
borohydrides
[2].
The
focus
for
metal
hydrides
is
the
thermodynamic
and
kinetic
properties
of
the
materials
used.
The
kinetics
of
the
reaction
between
the
metal
hydride
and
the
hydrogen
gas
can
influence
the
rate
at
which
hydrogen
gas
is
absorbed
and
desorbed.
Faster
reactions
translate
to
shorter
refuel
times
of
the
metal
hydride,
which
is
beneficial
in
mobile
applications
[3].
Similarly,
the
thermodynamics
of
the
reactions
can
determine
which
metal
hydride
materials
can
be
used
in
a
PEM
fuel
cell
system.
The
enthalpy
and
entropy
can
change
the
temperature
at
which
dehydrogenation
occurs.
The
implementation
of
additives
and
catalysts
are
used
to
ensure
that
the
necessary
temperature
and
pressure
for
the
hydrogenation/dehydrogenation
reactions
are
within
reasonable
ranges
for
the
PEM
fuel
cell.
These
processes
will
be
further
discussed
in
following
sections
[3].
Process
of
Hydrogen
Absorption
The
process
involved
with
hydrogen
absorption
into
the
metal
lattice
of
the
hydrides
consists
of
four
steps.
First,
the
hydrogen
molecules
are
attracted
to
the
metal
surface
by
Van
Der
Waal
forces
and
form
a
physisorbed
state.
Next,
before
the
hydrogen
can
diffuse
through
the
metal,
the
hydrogen
gas
must
dissociate
into
two
hydrogen
atoms.
A
chemisorbed
state
is
formed
as
the
hydrogen
atoms
form
new
bonds
at
the
metal’s
surface.
Finally,
the
chemisorbed
hydrogen
atoms
can
jump
to

5. subsurface
layers
and
diffuse
at
the
interstitial
sites.
These
four
steps
are
shown
below
in
Figure
2
[4].
Figure
2:
Dissociation
and
Diffusion
Processes
of
Hydrogen
in
Metal
Hydrides
[4]
For
diffusion,
hydrogen
atoms
form
a
metal-­‐hydrogen
(M-­‐H)
solid
solution,
which
is
the
referred
to
as
the
α-­‐phase.
The
formation
of
this
α-­‐phase
leads
to
an
expansion
of
the
metal
lattice.
As
the
pressure
increases
as
diffusion
progresses,
the
nucleation
of
a
hydrideβ-­‐
phase
occurs.
The
process
of
hydrogen
diffusion
is
demonstrated
in
Figure
3
[4].
Figure
3:
Formation
of
a
Hydride
Phase
as
Hydrogen
Diffuses
[4]
Metal
Hydride
Compositions

6. There
are
three
main
types
of
metal
hydride
materials:
light,
intermetallic,
and
complex
metal
hydrides.
Light
metal
hydrides
usually
consist
of
Li,
Be,
Na,
Mg,
B
or
Al.
The
advantage
of
this
type
is
the
light
weight
of
the
materials,
which
allows
for
better
gravimetric
storage
density
[3].
The
three
most
common
forms
of
intermetallic
metal
hydride
compounds
are
AB2,
AB5
and
Ti-­‐based
body
centered
cubic
(BCC)
alloys.
Typically,
AB2
type
metal
hydrides
are
composed
of
Ti-­‐Zr-­‐Mn-­‐V
or
Ti-­‐Zr-­‐Cr-­‐Fe
alloys
and
are
derived
from
Laves
phase
crystal
structures.
The
hydrogen
storage
capacity
for
this
type
of
metal
hydrides
is
generally
in
the
1.5
to
1.9
wt.%
range.
The
advantages
of
this
type
of
intermetallic
metal
hydrides
are
low
cost,
relatively
fast
kinetics
and
long
lifespans.
AB5
are
mostly
metal
alloys
of
Mischmetal
(Mm)
and
nickel.
The
hydrogen
storage
capacity
is
lower
than
that
of
AB2
metal
hydrides,
usually
maxing
out
at
1.5
wt.%.
The
advantage
of
AB5
metal
hydrides
is
their
better
volumetric
energy
storage
and
cyclic
durability.
These
factors
allow
for
AB5
metal
hydrides
to
generally
be
more
suitable
as
a
reversible
hydrogen
storage
material
for
small-­‐scale
mobile
applications,
such
as
a
mobile
phone
fuel
cell
system
that
would
replace
common
Lithium
Ion
batteries.
The
Ti-­‐based
BCC
alloys
exhibit
the
best
hydrogen
storage
capacity
of
the
3
types,
although
they
have
limited
practical
applications
due
to
the
high
cost
of
this
material.
The
vanadium
in
the
Ti-­‐based
alloys
is
what
increases
the
cost
but
is
essential
in
enhancing
the
hydrogen
absorption
capacity
[3].
New
research
is
exploring
the
more
complex
metal
hydrides,
along
with
the
inclusion
of
catalysts
to
accelerate
the
kinetics
of
the
reactions
between
the
hydride
materials
and
hydrogen.
Ahluwalia
et
al.
focus
on
the
development
of
new
class
of

7. hydrides
such
as
destabilized
hydrides
(especially
borohydrides
and
lithium
hydrides),
amide/imide
materials,
off-­‐board
regenerable
materials
(i.e.
AlH3
and
LiAlH4)
and
alanates
[3].
The
scope
of
this
research
is
limited
to
on-­‐board
reversible
metal
hydrides
that
offer
the
potential
to
achieve
the
guidelines
set
by
the
U.S.
Department
of
Energy,
which
is
discussed
later
on.
Because
of
this,
the
materials
considered
in
this
research
will
be
a
more
commonly
researched
sodium
alanate
(NaAlH4),
an
Mg-­‐based
metal
hydrides
(MgH2),
a
borohydrides
(LiBH4),
and
a
high-­‐
pressure
metal
hydride
(Ti1.1CrMn).
Metal
Hydride
Parameters
of
Performance
There
are
certain
factors
that
determine
the
effectiveness
of
the
metal
hydride
as
a
hydrogen
storage
system.
Volumetric
and
gravimetric
storage
densities
are
crucial
because
they
quantify
the
amount
of
hydrogen
that
can
be
stored
as
a
function
of
volume
and
weight,
respectively.
A
large
gravimetric
storage
density
is
harder
to
achieve
due
to
the
large
weight
associated
with
the
metal
hydride
materials
[5].
In
the
consideration
of
a
metal
hydride
storage
system
for
a
car,
a
higher
volumetric
and
gravimetric
storage
density
allows
for
the
driver
to
travel
farther
distances
without
refueling.
Another
key
parameter
is
the
rate
of
hydrogenation/dehydrogenation.
The
rate
of
hydrogen
absorption
into
the
metal
hydride
determines
the
time
it
would
take
to
refuel
the
metal
hydride.
Dehydrogenation
involves
the
desorption
of
hydrogen
from
the
metal
hydride
and
plays
a
crucial
role
in
providing
the
fuel
cell
with
sufficient
amounts
of
fuel
[5].

8. The
optimum
hydrogen
storage
should
also
contain
the
following
properties;
high
reversibility,
limited
energy
loss
during
charging
and
discharging,
high
stability
against
oxygen
gas
and
moisture
to
ensure
long
life
cycles,
and
high
safety.
[3]
Table
1
gives
the
U.S.
Department
of
Energy
guidelines
for
metal
hydride
storage
in
fuel
cell
cars.
The
target
values
are
rough
estimates
of
the
required
metal
hydride
performance
to
be
able
to
compete
with
the
current
automobile
standards.
Current
metal
hydride
materials
are
unable
to
reach
these
standards
but
further
research
is
being
performed
to
attempt
to
meet
the
guidelines
by
improving
the
interaction
between
the
metal
hydrides
and
hydrogen
by
including
additives
and
catalysts.
Table
1:
U.S.
Department
of
Energy
Guidelines
[6]
Thermodynamics
and
Kinetics
It
is
important
that
the
metal
hydride
storage
system
is
compatible
with
the
PEM
fuel
cell
being
used.
The
main
consideration
is
the
operating
temperature
and

9. pressure
of
the
fuel
cell.
Rate
and
amount
of
hydrogen
absorption/desorption
are
dependent
on
the
temperature
of
the
system.
PEM
fuel
cells
generally
operate
at
50
to
100°C
and
near
ambient
pressures.
Increasing
the
operating
temperature
of
the
fuel
cell
can
generate
significant
problems
and
efficiency
losses.
Therefore,
it
is
necessary
to
find
materials
that
react
with
gaseous
hydrogen
at
lower
temperatures.
This
can
be
calculated
by
finding
the
enthalpy
and
entropy
of
the
reactions.
Equation
(1)
demonstrates
the
dependence
of
temperature
on
enthalpy
and
entropy
Δ 𝐻 = 𝑇Δ𝑆
(1)
Züttel
et
al.
[7]
approximated
that
the
entropy
of
most
metal
hydride-­‐hydrogen
reactions
is
130
J
K-­‐1
mol-­‐1.
If
the
entropy
can
be
estimated
and
the
desired
temperature
is
known,
the
necessary
enthalpy
can
be
found
[8].
An
example
of
this
process
is
discussed
by
Alapatti
et
al
[8].
for
the
reaction
of
a
LiBH4
metal
hydride.
The
reaction,
which
demonstrates
the
desorption
of
hydrogen,
is
shown
in
equation
(2).
𝐿 𝑖𝐵𝐻! → 𝐿𝑖𝐻 + 𝐵 +
!
!
𝐻!
(2)
Here,
the
entropy
is
estimated
to
be
95 ≤ ∆𝑆 ≤ 140 𝐽 𝐾!!
𝑚𝑜𝑙!!
.
If
the
desired
temperature
is
between
50
and
150°C,
then
the
enthalpy
must
be
between
30
and
60
kJ/mol.
If
the
enthalpy
is
above
60
kJ/mol,
then
the
amount
of
hydrogen
delivered
to
the
fuel
cell
will
be
small
unless
the
temperature
is
increased.
If
the
enthalpy
is
less
than
the
30
kJ/mol,
the
reaction
will
not
be
easily
reversible
[8].
Therefore,
the
metal
hydride
will
not
be
able
to
easily
absorb
hydrogen
gas.
To
change
the
enthalpy
of
the
reaction
between
metal
hydride
materials
and
hydrogen
gas,
the
concept
of
destabilization
is
explored
by
new
research.

10. Destabilization
involves
the
inclusion
of
additives
that
form
compounds
or
alloys
in
the
dehydrogenated
state.
These
compounds
help
stabilize
the
dehydrogenated
state
and
therefore
destabilize
the
hydrogenated
state.
The
effect
of
these
additives
can
be
demonstrated.
Consider
the
reaction
shown
in
equation
(2).
In
the
estimations
of
Alapatti
et
al.,
by
adding
MgH2
to
LiBH4,
the
total
reaction
becomes:
𝐿 𝑖𝐵𝐻! +
!
!
𝑀𝑔𝐻! → 𝐿𝑖𝐻 +
!
!
𝑀𝑔𝐵! + 2𝐻!
(3)
The
formation
of
MgB2
stabilizes
the
right
side
of
the
equation
(the
dehydrogenated
state),
which
decreases
the
enthalpy
of
the
reaction
from
69
kJ/mol
to
44
kJ/mol
[9].
This
allows
the
reaction
to
take
place
at
significantly
lower
temperatures
(around
250°C
less).
The
downside
of
using
additives
is
that
the
hydrogen
storage
capacity
of
the
metal
hydride
is
marginally
decreased
[9].
Research
for
metal
hydrides
is
focused
on
finding
new
additives
that
can
reduce
the
enthalpy
of
the
reaction
while
maintaining
the
desirable
volumetric
and
gravimetric
energy
density.
Improving
Metal
Hydride
Kinetics
By
introducing
catalysts
or
dopants,
the
kinetics
involved
with
the
hydrogenation/dehydrogenation
rates
can
be
theoretically
improved
in
hopes
to
achieve
the
standards
set
by
the
DOE.
Consider
one
of
the
more
promising
potential
metal
hydrides,
sodium
alanate
(NaAlH4).
The
Van’t
Hoff
plot
shown
in
Figure
2
shows
that
at
a
dissociation
pressure
of
0.1
MPa,
sodium
alanate
is
one
of
the
few
metal
hydride
materials
that
do
not
require
a
high
temperature
for
hydrogen
desorption
[6].

11.
Figure
4:
Van’t
Hoff
plot
for
various
metal
hydride
materials
[6]
In
the
1990’s,
it
was
hypothesized
that
sodium
alanate
could
not
be
a
practical
hydrogen
storage
material
due
to
the
slow
kinetics
and
high
temperature
requirements.
This
changed
when
Bogdanovic
and
Schwickardi
doped
the
sodium
alanate
with
small
amounts
of
a
titanium
catalyst
[8].
The
new,
doped
sodium
alanate
was
able
to
achieve
rehydrogenation
under
much
milder
conditions
(just
above
100°C).
The
experiments
run
showed
that
a
highly
dispersed
Ti
in
the
Al
surface
improved
the
hydrogen
uptake
and
release
processes
[10].
TiAl3
is
most
likely
to
form
during
the
dehydrogenation
process.
It
was
concluded
that
Ti
doping
can
effectively
lower
the
dissociation
pressure
for
hydrogen
absorption
[8].
A
similar
experiment
was
performed
by
Schuth
et
al.
[6]
to
study
the
affects
of
ball-­‐milling
on
NaAlH4
with
catalyst
TiCl3
under
various
conditions.
In
Figure
3,
it
is
apparent
that
the
volume
capable
of
being
desorbed
from
the
metal
hydride

12. increased
with
both
the
amount
of
the
catalyst
used
and
the
size
ball
used
in
the
ball-­‐milling
process.
Figure
5:
Volume
of
hydrogen
desorbed
from
a
ball-­‐milled
NaAlH4
with
a
TiCl3
catalyst
under
various
conditions
[6]
Most
catalyst
materials
being
explored
in
current
research
are
titanium,
iron
and
zirconium.
One
major
downside
of
the
inclusion
of
dopants
is
the
reduction
in
the
reversible
hydrogen
storage
capacity.
The
reason
for
this
is
that
the
dopant
adds
weight
to
the
metal
hydride
system.
Doping
levels
are
usually
around
2
to
4
mol%.
The
dopants
add
weight
that
is
not
being
used
to
store
hydrogen
so
the
overall
storage
capacity
is
reduced
[8].
Metal
Hydride
Surface
Structures
A
metal
hydrides
ability
to
dissociate
hydrogen
gas
is
dependent
on
the
materials
surface
structure.
When
considering
Mg-­‐based
metal
hydrides,
improving
the
surface
properties,
with
the
use
of
ball-­‐milling,
is
essential
in
improving
the
rate
of
hydrogen
diffusion
through
the
metal
hydride
layers
[3].

13. As
the
reaction
of
hydrogenation
progresses,
hydrogen
diffusion
occurs
and
the
hydride
layer
grows.
This
creates
an
almost
impermeable
layer,
which
limits
the
rate
of
hydride
formation.
Along
with
forming
a
compact
hydride
layer,
exposure
to
oxygen
can
form
highly
stable
oxide
layers
on
the
hydride,
which
severely
lower
the
hydrogen
absorption
rate
[3].
To
avoid
these
issues,
ball-­‐milling
is
used
to
increase
the
surface
area,
and
create
defects
on
the
surface
and
interior
of
the
hydride.
The
lattice
defects
created
are
areas
of
low
activation
energy
of
diffusion,
which
aids
the
hydrogen
absorption
[11].
The
greater
surface
area
allows
for
larger
surface
contact
with
catalysts,
which
leads
to
faster
kinetics
[11].
The
process
of
ball-­‐milling
can
be
controlled
to
alter
grain
size,
microstructure
or
surface
properties
of
the
material
in
the
hopes
of
achieving
faster
absorption/desorption
times.
Figure
4
shows
the
variation
of
desorption
time
for
unmilled
and
ball-­‐milled
MgH2.
As
one
can
see,
the
ball-­‐milled
MgH2
(white
symbols)
had
a
much
faster
desorption
rate
than
the
unmilled
MgH2
at
the
same
hydrogen
content
[3].

14.
Figure
6:
Hydrogen
desorption
for
unmilled
(black
symbols)
and
ball-­‐milled
(white
symbols)
MgH2
at
a
pressure
of
0.15
bar
[3]
Heat
Management
for
Metal
Hydride
Systems
A
study
performed
by
Sandrock
et
al.
[12]
on
a
one
hundred
gram
bed
of
sodium
alanate
found
that
during
the
rehydrogenation
process,
large
amounts
of
heat
were
produced
during
this
exothermic
reaction.
Within
one
minute,
the
heat
of
the
system
increased
from
155°C
to
234°C,
which
caused
several
problems
such
as
sintering
and
decreased
performance.
The
need
for
a
heat
exchanger
can
further
complicate
the
system
but
severely
increase
the
durability
and
efficiency
of
the
metal
hydride
under
cyclic
operating
[13].

15. Similarly,
since
the
desorption
of
hydrogen
gas
is
an
endothermic
reaction,
the
metal
hydrides
temperature
will
decrease
as
hydrogen
is
released.
This
will
lead
to
a
continuous
reduction
in
the
hydrogen
release
rate
as
the
temperature
drops
[10].
Therefore
the
required
operating
temperature
needs
to
be
maintained
through
the
use
of
a
heat
exchanger.
In
an
ideal
setup,
the
heat
produced
by
the
operations
of
the
PEM
fuel
cell
can
be
enough
to
cause
the
metal
hydride
to
continually
release
hydrogen
gas
until
all
the
hydrogen
is
consumed
[13].
Certain
heat
exchanger
systems
have
been
explored
for
metal
hydride
heating/cooling.
Pasini
et
al.
[1]
describe
the
use
of
a
shell-­‐and-­‐tube
heat
exchanger
with
the
metal
hydride
packed
in
the
shell
and
coolant
flowing
through
the
tubes.
The
schematic
of
such
a
system
is
shown
in
detail
in
Figure
5.
Figure
7:
Fuel
Cell
System
with
Sodium
Alanate
Metal
Hydride
[1]
Pasini
et
al.
[1]
also
explored
a
Ti1.1CrMn
metal
hydride
system
as
a
means
of
comparison
with
the
more
popular
Sodium
Alanate.
The
benefit
of
using
Ti1.1CrMn
is
that
the
desorption
reaction
only
requires
a
temperature
of
85°C.
The
waste
heat
produced
by
a
PEM
fuel
cell
is
enough
to
satisfy
this
condition.
Radiator
fluid
can
be

16. heated
with
the
waste
heat
of
the
fuel
cell
and
used
to
keep
the
Ti1.1CrMn
system
at
the
desired
temperature.
And
for
cooling,
the
same
shell-­‐and-­‐tube
heat
exchanger
can
be
used
during
hydrogen
absorption.
The
new
system
design
is
shown
in
Figure
6.
Figure
8:
Fuel
Cell
System
with
Ti1.1CrMn
Metal
Hydride
[1]
For
a
sodium
alanate
system,
on-­‐board
hydrogen
combustion
is
required
to
heat
the
metal
hydride
to
the
necessary
temperature
for
desorption
(130°C).
The
disadvantage
of
the
Ti1.1CrMn
system
is
a
large
decrease
in
the
theoretical
hydrogen
storage
capacity
(1.9-­‐2.0
wt.%)
when
compared
to
sodium
alanate
(5.5
wt%)
[1].
Conclusions
With
the
consideration
of
the
effectiveness
of
metal
hydrides
as
a
hydrogen
storage
method
in
portable
applications,
there
are
currently
no
available
materials
that
meet
the
guidelines
set
in
place
by
the
DOE
for
on-­‐board
hydrogen
storage.
Although
that
conclusion
can
definitively
be
made,
the
field
of
metal
hydrides
is
one
in
need
of
further
research.
Metal
hydrides
offer
the
potential
to
surpass
other
hydrogen
storage
methods
but
are
greatly
understudied.
There
are
several
nascent

17. areas
of
research
that
are
still
relatively
untouched
such
as:
complex
metal
hydrides,
catalyst
and
methods
of
doping,
thermodynamic
and
kinetic
properties
of
metal
hydrides
and
heat
exchanger
systems
for
metal
hydrides.
What
can
be
determined
from
this
study
is
that
certain
metal
hydride
types
are
simply
not
feasible
for
the
mobile
applications.
The
biggest
deterrent
for
researchers
is
a
lower
hydrogen
storage
capacity,
because
this
obstacle
is
hard
to
overcome.
Due
to
this,
intermetallic
hydrides
can
be
considered
undesirable
because
they
possess
a
minimal
hydrogen
storage
capacity
and
a
high
cost.
As
for
light
and
complex
hydrides,
the
most
common
problem
faced
is
the
high
desorption
temperature.
For
example,
Mg-­‐based
metal
hydrides
have
a
hydrogen
storage
capacity
around
7.6
wt.%
but
need
a
temperature
of
around
300°C
to
efficiently
desorb
hydrogen.
The
solution
for
this
problem
can
found
in
three
different
areas
of
study.
One
is
improving
the
thermodynamics
of
the
metal
hydride-­‐hydrogen
reaction
with
the
use
of
catalysts.
The
second
is
the
integration
of
a
heat
exchanger
system
with
on-­‐board
hydrogen
combustion
to
raise
the
temperature
of
the
metal
hydride
system.
The
third
is
the
development
of
a
PEM
fuel
cell
with
a
higher
operating
temperature
and
comparable
efficiency,
such
that
there
is
more
waste
heat
present
to
provide
the
metal
hydride.
In
light
of
the
advancements
and
achievements
in
metal
hydride
studies
thus
far,
there
is
a
clear
potential
for
the
development
of
hydride
materials
that
exhibit
high
reversible
hydrogen
storage
capacity
at
reasonable
temperatures.
Similarly,
as
the
improvement
in
vehicle
design,
PEM
fuel
cells
and
manufacturability
of
hydride
materials
continue,
the
field
of
metal
hydrides
will
continue
to
progress.
As
metal

18. hydride
technology
advances,
so
does
the
ability
to
safely
and
efficiently
implement
hydrogen-­‐based
electrical
systems
into
society.

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