Introduction: is in throughout many technical applications. Hydrogen is

Introduction:

This paper will primarily discuss
the results of a paper by J. Graetz et. al. entitled “Kinetics and
thermodynamics of the aluminum hydride polymorphs”, which examines AlH­3
polymorphs.  The primary reason that this
topic is of interest to researchers is for its use as an effective hydrogen
storage system 1.  In fact, AlH3­
has a higher density of hydrogen in it that even liquid hydrogen with 10% of
its mass being hydrogen 1.  The
importance of hydrogen storage quickly becomes clear when considering how
frequently used the element is in throughout many technical applications.  Hydrogen is used industrially in a wide range
of applications including the production of oil, fatty foods, fertilizers,
silicon chips, and items that are manufactured in a protective atmosphere.  Also, hydrogen offers a potential green
energy source for the future in the form of hydrogen fuel cells, which is the
primary use of interest for Graetz 2.

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Of particular importance in the work
discussed in this paper is of how hydrogen can be obtain from AlH3
due to species decomposition, why this decomposition reaction occurs, and how
it can be controlled.  The decomposition
of these hydrides is important in their use as a storage material because in
order to use the material as a method of hydrogen storage, the process of
extracting hydrogen gas from the material must be very well controlled and
reliable, and this decomposition reaction is determined both by thermodynamics
and kinetics.  By causing decomposition
of the material, it is possible to collect and use the hydrogen gas that
evolves from the decomposition reaction.

Theory and Background:

 There are a number of important thermodynamic
concepts that need to be understood in order to understand the discussion of
AlH3 that occurs in Graetz’s paper. 
The most basic concepts that are needed to be understood are enthalpy
and entropy.  Enthalpy describes the
amount of energy in a system while entropy is a mathematical way of measuring
the disorder of a system 3.  Together,
these two thermodynamic quantities define a third thermodynamic quantity called
Gibbs free energy, which is a variable that indicates the equilibrium behavior
of a system, and it can be obtained via the following equation:

                                                                              (1)

where

 is
the Gibbs Free Energy change,

 is
the enthalpy change in a system, T is the temperature, and

 is
the enthalpy change in a system 3.  If
physical systems depended only on thermodynamics, all systems held under
isothermal and isobaric conditions would always move towards a minimum Gibbs
free energy 3.  In this way, Gibbs free
energy provides a sort of preference for how a system “wants” to be. 

Perhaps the most important concept
to understand is phase stability and the thermodynamics of reactions.  For any two phases to exist in thermodynamic
equilibrium, they must have the same value of Gibbs free energy 3.  This goes for both solid phases and gas
phases.  Furthermore, this rule also
applies to reactions.  For all reactions,
there is a point at which the Gibbs free energy will reach a minimum value and
this minimum value means that the reaction will be at equilibrium and no change
in reactant or product concentrations would be expected to occur 3.  In the case of a system such as Aluminum
Hydride decomposition, it is critical to understand what drives the
decomposition reactions and how phase equilibrium behavior is dictated by
thermodynamics considering AlH3 has multiple different possible
phases, and each of these phases have different Gibbs free energies 1.

In all solids, there exists a
thermodynamic gas-solid phase equilibrium, but usually this is so heavily skewed
towards the solid phase that there is little to no gas phase present 3.  This is true in the case of AlH3
in that the formation of gas phase AlH3 is not critical, but there
are other gas phase reactions that must satisfy equilibrium.  For AlH3 the more important
consideration is the hydrogen reaction equilibrium.  As specified by Graetz, the decomposition of
Aluminum oxide can be represented by the following equation:

                                                                                   (1)

For this equation, there is an
equilibrium hydrogen pressure at which the reaction will not proceed in any
direction.  This can be described by the
equilibrium constant K 3.  This is an
important factor in thinking about how the AlH3 system behaves
because the equilibrium behavior of reaction 1 indicates a very high H2
of 10,000 bar is expected when looking solely at thermodynamic considerations
1.  For context, this is around a
Gigapascal of pressure.  This would
indicate that there is a strong tendency for AlH3 to decompose.  Mathematically two equations are needed to
represent the equilibrium conditon.

                                                                                     (2)

                                                                                              (3)

where

 is
Gibbs Free energy, R is the gas constant, T is temperature, K is the
equilibrium constant, and

, is the partial pressure of
hydrogen.  Equation 3 assumes that the
other components specified in equation 1 are pure substances 3.  Equations 2 and 3 together show that if Gibbs
free energy is low then reaction will naturally shift towards the products and
attempt to create a high pressure of hydrogen. 
Another important concept related is the enthalpy of formation and Gibbs
free energy of formation, which provide both enthalpy and the Gibbs free energy
at 298K required to make a substance from its constituent elements 3. 

 

Salient Results

The work done by Gaetz et al. provides
critical data for thermodynamics involved in the decomposition of AlH3,
in both the alpha and gamma phases.  This
particular paper was concerned with both the thermodynamics and kinetics of the
decomposition described by equation 1, and as such needed a number of different
experimental techniques to characterize material behavior.  The first step in this characterization was
to examine all of the AlH3 powders using SEM and x-ray diffraction
1.  This was done to verify the
structure of the materials before looking at their thermodynamic
properties.  This is important because
for the three materials that were examined, a stabilized

-AlH3, an unstabilized

-AlH3, and a

-AlH3, all three had different
grain sizes and were from two different suppliers.  The average grain size ranged from over 200
nm to only 140 nm 1.  The XRD data also
confirmed that the supplied material was in the desired phases 1. 

After the material was
confirmed to be correct, the researchers performed DSC tests on all three
materials to characterize the decomposition reaction for all three which can be
seen in Figure 1. 
They found values of enthalpy of the decomposition reaction very similar
to literature with a measured enthalpy of approximately -10 kJ/mol and a
measured Gibbs free energy of 46.4 kJ/mol at 298K 1.  The negative enthalpy of the reaction
indicates that the reaction will become more spontaneous at higher temperature,
and the positive Gibbs free energy indicates that the reaction will not be
spontaneous at room temperature 3.  The
DSC curves also show a distinct phase transformation that occurs in the gamma
phase before decomposition begins 1. 
This is a phase transition from gamma to alpha phase and due to its
absorption of energy is endothermic 1. 
The addition of this step complicates the decomposition process of gamma
aluminum hydride at high temperatures by making it a two-step decomposition at
lower temperatures  1.

Figure 1: DSC curves for the three
investigated materials

The other focus of this
work was characterizing the rate at which this decomposition reaction occurs
because this research is targeting a fuel cell application where controlling
the rate of hydrogen evolution is critical to power generation.  This was done by placing a samples in an
evacuated chamber at number of temperatures within the range of interest,
30-140 ?C
1.  The decomposition was performed
isothermally, meaning that the temperature of the chamber was held
constant.   While this involves more of the kinetics of
the decomposition reaction, the rate and favorability of the reaction is still
related to thermodynamic concepts such as the driving force and the Gibbs free
energy of the reaction.  Through these
experiments, the researchers found that there are three distinct phases in the
decomposition reaction, an induction period, a period of rate acceleration, and
a period of rate decay and that this is indicative of nucleation limited
reaction 1.  In this case, it is the
nucleation of the decomposition product aluminum phase which is limiting the
reaction 1.   Further experiments were
done to investigate the materials hydrogen release rate by thermal cycling
intermittently the results of which can be seen in Figure
2:

Figure 2: Hydrogen evolution behavior during
intermittent heating

What’s notable here is that the rate of hydrogen
decomposition follows similar trends during intermittent heating as during
prolonged heat exposure.  In practice
this means that the rate of hydrogen evolution can be controlled during the
lifetime of the hydrogen storage material. 
The researchers also verified that the hydrogen decomposition rate can
be directly controlled with temperature which makes sense because the negative
enthalpy of the decomposition reaction means that increasing the temperature
would increase the driving force of the reaction 1.  This allows for direct control over the rate
of hydrogen production based on applied temperature and life time of the
material. 

Applications

The major results of this work are vital in the
application of AlH­3 in fuel cells and in particular for fuel cells
in cars.  This work shows that the rate
of hydrogen as a result of AlH­3 decomposition works similarly when
fully decomposed at one time or when cycled. 
This is critical for a fuel cell because the power demands on a fuel
cell are almost never constant.  They are
cyclic based on product use, so this study confirms that AlH­­3 will
work as a storage material that is stable and useable.  The other result of particular interest for
fuel cell applications is the highly predicable hydrogen evolution rate changes
with respect to temperature 1.  This
allows for the potential to highly accurately control how much hydrogen is
being produced at any time by varying the temperature and is based on the
thermodynamic driving force.  In the
context of an automotive fuel cell, this allows for easy modulation of power
output to meet the variable demands of a car. 
Ultimately, this means that for a 100 kg sample of AlH3, the
evolution rate of H2 can be highly controlled in a range from 0-1 g
H2/s.  For context, this meets
the standards for a 50 kW fuel cell 1. 
One of the salient results of this work as well is that some AlH3
samples has drastically different decomposition rates than others based on
their enthalpies of formation and Gibbs free energy 1.  Both the alpha and gamma AlH3
showed effective rates of H­2 decomposition, but the third tested
material, the stabilized alpha AlH3 measured around for orders of
magnitude less hydrogen evolution 1. 
This is important when designers and engineers are considering how to
manufacture fuel cells for new automotive applications.  It’s critical for the correct material to be
selected in order to maximize the controlled production of hydrogen. 

                Another
result that engineers must take into account is the thermodynamic stability of AlH­3
at low temperatures.  Materials such as
gamma aluminum hydride appear at first to be very suitable for fuel cell
applications because of their very high decomposition rates, but because the
formation enthalpy of this phase is smaller than that of the alpha phase, the
driving force on the AlH­3­ decomposition reaction is higher for the gamma
phase 1.  This prompts less low
temperature stability, which mean that it is harder to safely store hydrogen
for an extended period of time. 
Graphically, this can be seen in Figure 3:

 

Figure 3: Decomposition of alpha and gamma
AlH­3 at two different temperatures

As seen in Figure 3, even at
low temperatures the gamma phase still shows significant decomposition over the
course of about 10 days.  Over this
period of time, the gamma phase aluminum hydride has lost almost half of
hydrogen content at 29 ?C,
which is close to ambient temperature 1. 
This is in comparison to the alpha phase which shows almost no
decomposition at these low temperatures. 
Fuel cell engineers need to take into account both the ability for the
material to provide hydrogen quickly when needed and to be stable when no
hydrogen is desired.  This work lays the
foundation for the use of these materials as hydrogen storage materials by
further quantifying basic thermodynamic properties involved in their
decomposition, and it has been suggested that aluminum hydrides may make a
viable alternative to lithium batteries for a range of applications including
small devices and wearables 4. 

Conclusions

This paper has effectively probed the potential of
Aluminum Hydride as a potential storage material for fuel cell
applications.  AlH3 was
identified as an attractive material to store hydrogen because it has a very
high hydrogen density, with approximately 10% of it’s mass being hydrogen.  It has now been established the degree to
which the preferable thermodynamics of AlH3 decomposition can cause
controlled and reliable hydrogen production at a range which is suitable for
large fuel cells.  It has also shown that
the phase and composition of the AlH3 is highly impactful for the
decomposition reaction to occur rapidly.