Reversible storage of hydrogen in destabilized LiBH4

Article abstract of DOI:10.1021/jp040769o

Destabilization of LiBH₄ for reversible hydrogen storage has been studied using MgH₂ as a destabilizing additive. Mechanically milled mixtures of LiBH₄ + 1/2MgH₂ or LiH + 1/2MgB ₂ including 2-3 mol % TiCl₃ are shown to reversibly store 8-10 wt % hydrogen. Variation of the equilibrium pressure obtained from isotherms measured at 315-400?°C indicate that addition of MgH₂ lowers the hydrogenation/dehydrogenation enthalpy by 25 kJ/(mol of H ₂) compared with pure LiBH₄. Formation of MgB₂ upon dehydrogenation stabilizes the dehydrogenated state and, thereby, destabilizes the LiBH₄. Extrapolation of the isotherm data yields a predicted equilibrium pressure of 1 bar at approximately 225?°C. However, the kinetics were too slow for direct measurements at these temperatures. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp040769o

3
719
2005, 109, 3719-3722
Published on Web 02/12/2005
Reversible Storage of Hydrogen in Destabilized LiBH4
John J. Vajo* and Sky L. Skeith
HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, California 90265
Florian Mertens
General Motors Research and DeVelopment Center, 30500 Mound Road, Warren, Michigan 48090
ReceiVed: December 21, 2004
Destabilization of LiBH
additive. Mechanically milled mixtures of LiBH
are shown to reversibly store 8-10 wt % hydrogen. Variation of the equilibrium pressure obtained from
isotherms measured at 315-400 °C indicate that addition of MgH lowers the hydrogenation/dehydrogenation
enthalpy by 25 kJ/(mol of H ) compared with pure LiBH . Formation of MgB upon dehydrogenation stabilizes
the dehydrogenated state and, thereby, destabilizes the LiBH . Extrapolation of the isotherm data yields a
4
for reversible hydrogen storage has been studied using MgH
2
as a destabilizing
1
1
4
+ /
2
MgH
2
or LiH + /
2
MgB
2
including 2-3 mol % TiCl
3
2
2
4
2
4
predicted equilibrium pressure of 1 bar at approximately 225 °C. However, the kinetics were too slow for
direct measurements at these temperatures.
Currently, reversible hydrogen storage materials are being
sought that will yield gravimetric storage system capacities of
Here we show that LiBH4 can be reversibly dehydrogenated
and rehydrogenated with a reduced reaction enthalpy by the
addition of MgH2. Dehydrogenation of a LiBH4 + /2MgH2
mixture reacts according to
1
>6-9 wt % hydrogen. These capacities have been deemed
necessary for commercial development of vehicles powered by
H2/O2 proton exchange membrane fuel cells. Because it is
unlikely that capacities of >6 wt % can be obtained in transition
metal-based materials, intense interest has developed in low-Z
1
1
LiBH + / MgH T LiH + / MgB + 2H
2
(1)
4
2
2
2
2
1
- 2-4
complex hydrides such as alanates (AlH4 ),
amides
which has a maximum hydrogen capacity of 11.4 wt %.
Formation of MgB2 stabilizes the dehydrogenated state and
effectively destabilizes the LiBH4.
-
5-10
- 11-13
(NH2 ),
and borohydrides (BH4 ).
Use of complex
hydrides for hydrogen storage is challenging because of both
kinetic and thermodynamic limitations. Kinetically, the revers-
ible formation of complex hydrides is slow, although, Bogdanovic´
and Schwickardi showed that dramatic improvement in reaction
Details regarding sample preparation and the temperature
ramp hydrogenation and dehydrogenation experiments have
16
been described in detail elsewhere. Briefly, all materials were
2
rates are possible with appropriate catalysts. Thermodynami-
obtained commercially, used without further purification, and
handled under an inert atmosphere. Approximately 1.2 g
cally, the stability of a complex hydride must lie within a
specific range for the hydrogenation/dehydrogenation phase
boundary to occur at practical pressures and temperatures. For
example, except for NaAlH4, most low-Z alanates appear to be
1
1
mixtures of LiBH4 + /2MgH2 or LiH + /2MgB2 were
16
mechanically milled for 1 h as described previously. Typically,
2
or 3 mol % TiCl3 was included as a catalyst during milling.
1
4
too unstable. In contrast, the borohydrides LiBH4 and NaBH4
Isotherm measurements were performed manually in the same
Sieverts apparatus used for the temperature ramp experiments.
All hydrogen capacities are reported with respect to full
hydrogenation, but not including the weight of the catalyst.
X-ray analyses were conducted on samples sealed in glass
capillary tubes using a Philips PW3040/60 X’Pert Pro diffrac-
tometer.
1
1
are too stable. For LiBH4, partial decomposition to LiH + B
3
+
/2H2 can yield 13.6 wt % hydrogen. However, the standard
enthalpy for this reaction is ∼67 kJ/(mol of H2) and thus, if
reversible, an equilibrium pressure of 1 bar would require a
temperature of >400 °C.15 Recently, we have pursued increasing
the equilibrium pressure of strongly bound hydrides such as
LiH and MgH2 using additives that stabilize the dehydrogenated
state.16 Stabilizing the dehydrogenated state reduces the enthalpy
for dehydrogenation, thereby increasing the equilibrium hydro-
gen pressure. Using this approach, the thermodynamic properties
of reversible hydrogen storage material systems can potentially
be tuned to an extent finer than would be possible with
individual materials.
1
Hydrogenation and dehydrogenation of a LiH + /2MgB2 +
.03TiCl3 mixture are shown in Figure 1. During heating at 2
0
°
1
C/min, hydrogen uptake from an initial hydrogen pressure of
00 bar begins at 230-250 °C. For the initial cycle, hydrogen-
ation is relatively slow; uptake of only 2.5 wt % hydrogen occurs
after 2 h at 300 °C. Upon heating to 350 °C additional hydrogen
uptake occurs, reaching >9 wt %. On the second and third
cycles, the uptake is faster and 9 wt % uptake occurs in
approximately 2 h at 300 °C. Desorption, into an initially
*
Corresponding author. E-mail: vajo@hrl.com.
1
0.1021/jp040769o CCC: $30.25 © 2005 American Chemical Society

3720 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Letters
Figure 2. X-ray diffraction pattern from a mixture of LiBH
4
+ 1/
2 2
MgH
+
0.02TiCl after milling, dehydrogenation, and rehydrogenation. Curve
3
a, after milling. Curve b, after dehydrogenation at 450 °C. Curve c,
after rehydrogenation at 350 °C in 100 bar of hydrogen. The broad
feature seen at 15-40° 2θ originates from the glass wall of the capillary
Figure 1. Hydrogenation and dehydrogenation of milled LiH +
2
1
/
MgB
2 3
with 3 mol % TiCl . (a) Hydrogen uptake during heating in
1
00 bar of hydrogen. Curve a shows the temperature profile. Curve b
2 2
tubes. Peaks locations for MgH , LiCl, LiH, and MgB were obtained
shows the initial uptake of hydrogen. Curves c and d show uptake during
the second and third cycles, respectively. (b) Desorption following
hydrogenation into a closed evacuated volume. Curve a shows the
temperature profile. Curves b and c, respectively, show the wt % of
desorbed hydrogen following the initial and second hydrogenation
cycles that are shown in panel (a).
from ICDD cards 00-012-0697, 01-074-1972, 00-009-0340, and 00-
038-1369, respectively. Peak locations for LiBH
ref 11.
4
were obtained from
evacuated volume, begins at ∼270 °C. Two desorption steps
are evident at 270-340 and 380-440 °C, respectively. After
heating to 450 °C, desorption of 8 wt % hydrogen was obtained.
The desorption kinetics for the first two cycles are nearly
identical. Final pressures were 5-6 bar and may be limited by
equilibrium (see Figure 3 below). The increase in hydrogen
pressure during desorption appears to be important for achieving
dehydrogenation, according to reaction 1, and good reversibility.
It was observed that dehydrogenation under dynamic vacuum
resulted in formation of Mg metal, not MgB2, and loss of
capacity upon subsequent rehydrogenation. The effect of
hydrogen pressure during desorption needs to be investigated
further. Mass spectrometric analysis of the desorbed gas using
a residual gas analyzer (RGA) attached to the Sieverts apparatus
Figure 3. Absorption/desorption isotherms at 315-450 °C for the
1
1
LiBH
4
+ /
2
MgH
2
system. Several samples starting with LiH + /
2 2
MgB
+
0.02TiCl
3
were used for these data. Each sample was hydrogenated
and dehydrogenated through one cycle, as shown in Figure 1, before
collecting the isotherm data. Filled symbols show absorption. Open
symbols, at 400 °C only, show desorption. Pressures used for the van’t
Hoff plot in Figure 4 are indicated at 4 wt %.
showed only hydrogen, although, low concentrations of species
such as B2H6 or HCl cannot be ruled out.17
Figure 2 shows X-ray diffraction patterns of a LiBH4 +
2MgH2 + 0.02TiCl3 mixture after mechanical milling, and after
1/
LiBH4 and demonstrate its reversible formation. However, some
residual MgB2 remains. Fourier transform IR spectra (data not
shown) also confirmed the reversible formation of LiBH4 with
absorption at characteristic frequencies of 2385, 2289, 2223,
dehydrogenation and rehydrogenation. Mechanical milling
produces a physical mixture of LiBH4 and MgH2. However, at
least a portion of the TiCl3 reacts with the LiBH4 to form LiCl.
No diffraction from any species containing Ti was observed.
After dehydrogenation at 450 °C, LiH and MgB2 are formed.
Upon rehydrogenation at 350 °C, LiBH4 and MgH2 re-form and
-
1
and 1124 cm .
Absorption and desorption isotherms for the LiBH4 +
/2MgH2 system at 315-450 °C are shown in Figure 3. The
1
18
the diffraction peaks from LiCl disappear. The quartet of peaks
isotherms show sloping plateaus from 2 to 8 wt % with
capacities of approximately 10 wt %. Equilibrium pressures
around 25° 2θ together with the peak at 18° distinctly identify

Letters
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3721
Consequently, the variation of equilibrium pressure with tem-
perature should display a lower enthalpy, i.e., a lower slope,
above ∼360 °C. The measured equilibrium pressure at 450 °C
is indeed lower than the pressure extrapolated from lower
21
temperatures. These data points may indicate a transition from
reaction 1 at temperatures below ∼360 °C to reaction 2 at higher
temperatures. However, additional data are necessary to clearly
resolve this transition. At higher capacities, above approximately
8.5 wt %, a second plateau would be expected for the isotherms
at 400 and 450 °C corresponding to hydrogenation/dehydroge-
nation equilibrium of Mg. Thus far, we have not detected a
distinct second plateau. However, the slope of the isotherm at
4
00 °C gives an equilibrium pressure of ∼19 bar at 9 wt %,
Figure 4. Van’t Hoff plots for destabilized LiBH
4
+ 1/
2
MgH
2
, pure
which is close to the expected pressure for MgH2/Mg. Although
two plateaus have not been observed in the isotherm data, the
temperature ramp desorption measurements (see Figure 1b)
show two desorption steps that may correspond to dehydroge-
nation of MgH2 followed by reaction of Mg with LiBH4 to form
MgB2. As mentioned above, dehydrogenation under vacuum
results in formation of Mg, not MgB2. Thus, it appears that a
finite hydrogen pressure, approximately 2 bar after the first
desorption step for the experiment shown in Figure 1b, is
necessary for the subsequent reaction of Mg with LiBH4 to yield
MgB2. Further work is underway to completely understand this
behavior.
Extrapolation of the equilibrium pressures at 315-400 °C to
lower temperatures gives a pressure of 1 bar at 225 °C. Direct
measurements at this temperature have not been possible because
the kinetics are currently too slow. Following work with
NaAlH4, 2-3 mol % TiCl3 was added as a catalyst because
initial experiments without added TiCl3 displayed poor perfor-
mance. However, the catalyst composition and system process-
ing, i.e., milling conditions and particle and crystallite sizes,
must still be optimized to obtain reasonable reaction rates at
low temperatures and to realize the potential of this system for
hydrogen storage. In addition, a temperature of 225 °C for an
equilibrium pressure of 1 bar is higher than desired for most
applications. Ideally, an equilibrium pressure of 1 bar would
occur at e150 °C. Thus, further thermodynamic destabilization
is also required.
LiBH
the absorption isotherms at 4 wt %. A linear fit to the data at 315-400
C indicates a dehydrogenation enthalpy of 40.5 kJ/(mol of H ) and an
equilibrium pressure of 1 bar at 225 °C. Curve b shows an estimate of
to LiH + B.15 Curve c
the behavior for dehydrogenation of LiBH
shows the equilibrium pressure for MgH /Mg from ref 20. Addition of
MgH increases the equilibrium pressure by approximately 10 times
while lowering the enthalpy by 25 kJ/(mol of H ) compared with pure
LiBH
4 2
, and MgH . Curve a shows equilibrium pressures obtained from
°
2
4
2
2
2
2
2
4
.
varied from 4.5 bar at 315 °C to 19 bar at 450 °C. Absorption
and desorption isotherms obtained at 400 °C display a hysteresis
of 2-3 bar. For all of these measurements, the kinetics were
slow and times up to 100 h were necessary to attain equilibrium.
Because of the slow kinetics, only a single point was obtained
at 315 °C.
A preliminary van’t Hoff plot (logarithm of the equilibrium
pressure versus the inverse of the absolute temperature) using
absorption equilibrium pressures at 4 wt % (see Figure 3) is
19
shown in Figure 4. From 315 to 400 °C the behavior is linear
with an enthalpy of 40.5 kJ/(mol of H2) and an entropy of 81.3
J/(K mol of H2). At 450 °C (1000/T ) 1.38) the equilibrium
pressure is lower than the pressure predicted on the basis of an
extrapolation of the linear behavior at lower temperatures (see
discussion below). Also shown in Figure 3 are equilibrium
pressures for MgH2/Mg obtained from the IEA/DOE/SNL
2
0
database and an estimate of the equilibrium pressure for
15
dehydrogenation of pure LiBH4 to LiH + B. The enthalpy
for the LiBH4/LiH + B system is estimated to be 67 kJ/(mol of
H2). Compared with pure LiBH4, the hydrogenation/dehydro-
1
In summary, we have shown that addition of /2MgH2 to
LiBH4 yields a destabilized, reversible hydrogen storage material
system with a capacity of approximately 8-10 wt %. The
hydrogenation/dehydrogenation enthalpy is reduced by 25 kJ/
1
genation enthalpy for the LiBH4 + /2MgH2 system is lower
by 25 kJ/(mol of H2) and at 400 °C the equilibrium pressure is
increased from approximately 1 to 12 bar. Alternatively,
extrapolating the linear behavior gives a temperature of 225 °C
for an equilibrium hydrogen pressure of 1 bar. Overall, the
(
mol of H2) compared with pure LiBH4, and the tempera-
ture for an equilibrium pressure of 1 bar is estimated to be 225
C.
°
1
equilibrium pressure indicates that addition of /2MgH2 signifi-
cantly destabilizes LiBH4 for hydrogen storage.
References and Notes
Interestingly, the equilibrium pressure behavior for the LiBH4
(
(
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(
/2MgH2 system crosses the curve for MgH2/Mg at ∼360 °C
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1.
pressures are greater than those for pure MgH2. Thus, in addition
to LiBH4, the MgH2 is also destabilized. In this region the
combined LiBH4 + /2MgH2 system has equilibrium pressures
higher than either individual component. Above 360 °C the
equilibrium pressures, obtained from the isotherms at 4 wt %,
are below the equilibrium pressures for MgH2/Mg. Under these
conditions, the system reacts according to
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(
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1
3
LiBH + / Mg T LiH + / MgB + / H
2
(2)
4
2
2
2
2
(
Because hydrogenation of Mg is exothermic, the enthalpy for
reaction 2 should be less than the enthalpy for reaction 1.
(

3722 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Letters
(
11) Z u¨ ttel, A.; Wenger, P.; Rentsch, S.; Sudan, P.; Mauron, Ph.;
(18) Loss of the diffraction features from LiCl is not fully understood.
However, the LiCl may have dissolved into the LiBH4, which is above its
melting point of 275 °C when it is formed at 300-350 °C.
Emmenegger, Ch. J. Power Sources 2003, 118, 1.
(
12) Nakamori, Y.; Orimo, S. J. Alloy Compd. 2004, 370, 271.
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kinetics and, hence long times necessary to reach equilibrium. Optimizing
the catalyst formulation, which has not been attempted here, could greatly
increase the reaction rates and improve the equilibrium data.
(20) IEA/DOE/SNL Hydride databases available at Hydride Information
Center, Sandia National Laboratories Home Page. http:/hypark.ca.sandi.gov/.
(21) We note that at temperatures above ∼400 °C sublimation of Mg
can affect the system behavior and complicate the measurements. Thus at
450 °C, only a partial isotherm was collected.
5
1.
(
14) Fichtner, M.; Engel, J.; Fuhr, O.; Kircher, O.; Rubner, O. Mater.
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(
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(
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17) Adsorption on the walls of the tubing and in the leak valve leading
(
(22) Interrupting the dehydrogenation after the first desorption should
show Mg by X-ray diffraction. This measurement was not performed but
is planned for future work.
to the RGA could have prevented detection of reactive species at low
concentrations.