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
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
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].
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
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
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
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].
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
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
Δ𝐻 = 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).
𝐿𝑖𝐵𝐻! → 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.
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:
𝐿𝑖𝐵𝐻! + !!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].
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
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].
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].
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].
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
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
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
hydride technology advances, so does the ability to safely and efficiently implement
hydrogen-‐based electrical systems into society.
References
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