Reversible hydrogen storage in Ti-Zr-codoped NaAlH4 under realistic operation conditions

Article abstract of DOI:10.1016/j.jallcom.2010.02.162

Ti-Zr-codoped NaAlH₄ exhibits improved hydrogen desorption and reabsorption properties compared with sole Ti- or Zr-doped alanate. This contribution aims on reversible hydrogen storage in such material under realistic operation conditions. Results on isothermal dehydrogenation-rehydrogenation cycles at 125 °C and desorption at 4 bar hydrogen back-pressure are presented, proving NaAlH₄ to be a suitable hydrogen material in combination with proton exchange membrane fuel cells.

Full text of DOI:10.1016/j.jallcom.2010.02.162

Journal of Alloys and Compounds 496 (2010) L38–L40
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Letter
Reversible hydrogen storage in Ti–Zr-codoped NaAlH4 under realistic operation
conditions
Thomas Schmidt∗, Lars Röntzsch
Fraunhofer Institute for Manufacturing Engineering and Applied Materials Research, Winterbergstr. 28, 01277 Dresden, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Ti–Zr-codoped NaAlH4 exhibits improved hydrogen desorption and reabsorption properties compared
with sole Ti- or Zr-doped alanate. This contribution aims on reversible hydrogen storage in such material
under realistic operation conditions. Results on isothermal dehydrogenation–rehydrogenation cycles at
125 ◦C and desorption at 4 bar hydrogen back-pressure are presented, proving NaAlH4 to be a suitable
hydrogen material in combination with proton exchange membrane fuel cells.
Received 11 February 2010
Accepted 23 February 2010
Available online 3 March 2010
Keywords:
Hydrogen storage
© 2010 Elsevier B.V. All rights reserved.
Alanate
Sodium aluminum hydride
Codoping
Ti
Zr
Realistic operation conditions
PEM fuel cell
1. Introduction
was observed both for desorption and reabsorption of hydrogen by
different authors [11,12].
Since the discovery of the catalytic effect of Ti halide precursors
[1], NaAlH4 has become a promising candidate for the solid-state
storage of hydrogen. This finding was followed by several papers
dealing with different approaches to improve the activity of Ti
catalysts [2]. Several Ti-containing compounds, such as organic pre-
cursors [3], TiH2 [4] and Ti nanoparticles [5] have been proven to
catalyze hydrogen desorption of NaAlH4 efficiently. In this respect,
recent findings indicate that Ti(0) is the actual catalyst [6,7].
Zr is in the same group in the periodic table as Ti, thus, a sim-
ilar catalytic effect can be anticipated. However, there are only
few publications dealing with Zr catalysts for dehydrogenation and
rehydrogenation of NaAlH4. From a systematic screening of a mul-
titude of transition metal chlorides for dehydrogenation of NaAlH4
it turned out that ZrCl4 is less efficient than different Ti chlorides
but much more efficient than all other catalyst precursors tested
[8]. In addition, organometallic Zr additives were successfully used
for the dehydrogenation of NaAlH4. Furthermore, it was demon-
strated that thecombinationofZrand Ti catalyst precursorshelpsto
improve the desorption kinetics [9]. X-ray diffraction studies were
carried out in order to investigate the influence of the dopants Ti
and Zr on the lattice parameters of NaAlH4 [10]. A synergistic effect
Doped NaAlH4 has several promising properties as reversible
hydrogen storage material for technical use, such as cycle sta-
bility, rather low absorption and desorption temperatures from
100 to 150 ◦C [2] and the possibility of its direct synthesis start-
ing from NaH and Al via reactive milling [13]. More importantly,
the dehydrogenation of NaAlH4 exploiting the exhaust heat of a
high-temperature proton exchange membrane (HT-PEM) fuel cell,
working at 1.2 bar has been tested [14]. However, for practical
applications a fuel cell back-pressure of several bar is required in
order to reach a sufficient performance, in particular for mobile
applications [17].
In this contribution, dehydrogenation of doped NaAlH4 is pre-
sented for the first time at a constant hydrogen back-pressure of 4 bar
and a temperature of 150 ◦C in order to mimic realistic operation
conditions in combination with a HT-PEM fuel cell. For this pur-
pose, NaAlH4 was ball-milled with Zr, TiCl4 and both in combination
(codoping). In addition, dehydrogenation and rehydrogenation of
hydrogen storage material were studied under isothermal condi-
tions, being favourable for technical processes.
2. Experimental
2.1. Powder preparation
All samples were prepared and handled in a glovebox (MBraun) under Ar to
prevent unwanted oxidation. For Zr-doping, 5 mol% Zr powder (Chemetall, 97.8%
purity) with a particle size of 2–3 ␮m was intermixed with NaAlH4 powder (Aldrich,
∗
Corresponding author. Tel.: +49 351 2537413; fax: +49 351 2554496.
E-mail address: thomas.schmidt@ifam-dd.fraunhofer.de (T. Schmidt).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.02.162

T. Schmidt, L. Röntzsch / Journal of Alloys and Compounds 496 (2010) L38–L40
L39
Fig. 1. Hydrogen reabsorption and desorption kinetics of NaAlH4, doped with Zr,
TiCl4 and Zr + TiCl4.
Fig. 2. X-ray diffraction patterns of NaAlH4 + 2.5 mol% Zr + 2.5 mol% TiCl4 prior to
and after cyclization.
technical grade, 90% purity). For Ti-doping, 5 mol% TiCl4 (Aldrich, >99% purity) as
inexpensive and liquid Ti precursor was added to the NaAlH4 powder using a syringe.
Both combinations were individually ball-milled under Ar for 24 h in a Fritsch P6
with 600 rpm and a ball-to-powder ratio of 20:1. To prepare codoped NaAlH4, sam-
ples with equal concentrations of Ti and Zr were ball-milled for 2 h with 400 rpm,
thus, the catalyst concentration amounts to 2.5 mol% for each Ti and Zr. To prepare
the sample with each 1 mol% Ti and Zr the formerly produced sample was ball-milled
together with the respective amount of pure NaAlH4 for 4 h at 600 rpm.
to NaAlH4 (reverse reaction of (1)) starts at a pressure of >70 bar.
Dehydrogenation of Na3AlH6 to NaH (2) is restricted to maximum
pressures of a few bar. Detailed kinetic and thermodynamic inves-
tigations will be presented elsewhere.
For both hydrogen absorption and desorption, the Zr–Ti-
codoped system achieved faster kinetics than the storage material
with pure Zr or Ti as catalyst. This in qualitative agreement with
the findings in Ref. [12], whereas we achieved faster kinetics (espe-
cially for hydrogenation) with lower catalyst loading under milder
conditions (lower hydrogenation pressure and lower dehydrogena-
2.2. Analysis
The crystal X-ray diffraction analysis (XRD) was carried out in a D5000 Siemens
using Co K␣ radiation in the scanning range of the diffraction angle 40◦ ≤ 2ꢀ ≤ 100◦.
The samples were covered with a capton foil to avoid sample contamination with
moisture and oxygen.
The morphology of the specimens has been analyzed using an EVO 50 ZEISS
scanning electron microscope (SEM).
2.3. Kinetics
The rehydrogenation/dehydrogenation kinetics and cycle stability of the sam-
ples were studied using a magnetic suspension balance (Rubotherm). Therefore,
hydrogen desorption and reabsorption can be investigated at constant hydrogen
pressures in the range from 10−5 to 200 bar (flow-through mode), i.e. covering the
operating pressure range of most PEM fuel cell systems.
3. Results and discussion
The doped NaAlH4 samples were activated by a first des-
orption step. Thereafter, the hydrogenation and dehydrogenation
properties of the hydrogen storage material were investigated in
dependence of the catalyst under isothermal conditions, being
favourable for technical processes. The catalytic effect of Zr was
compared with TiCl4 as benchmark catalyst precursor. The effect
of codoping NaAlH4 with both Zr and Ti on the kinetics of hydro-
gen sorption and the influence of the catalyst concentration was
investigated (cf. Fig. 1).
In accordance with the thermodynamic data of the reaction
equilibria between NaAlH4, Na3AlH6 and NaH [15,16], two steps
can be observed during both absorption and desorption (Scheme 1).
At a constant temperature of 125 ◦C rehydrogenation of Na3AlH6
Fig. 3. SEM micrographs (BSE mode) of NaAlH4 + 2.5 mol% Zr + 2.5 mol% TiCl4 prior
to (a) and after (b) cyclization.
Scheme 1. Decomposition reactions of NaAlH4 to NaH.

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T. Schmidt, L. Röntzsch / Journal of Alloys and Compounds 496 (2010) L38–L40
hydrogen desorption at 4 bar starts at higher temperature than in
the case of 1 bar hydrogen pressure and proceeds slower. There is
a characteristic step in the desorption curves of the systems with
TiCl4 and the Zr–TiCl4 mixture at 4 bar and temperatures below
150 ◦C, indicating a temperature range where the desorption reac-
tion nearly stops at the stage of Na3AlH6. The codoped system is
superior to the other catalysts over the whole temperature range.
About 4 wt% hydrogen were desorbed within 5 h.
4. Conclusions
Codoping NaAlH4 with Zr and TiCl4 considerably improves the
kinetics for hydrogen desorption and reabsorption, compared to
sole doping with Zr or TiCl4. Reversible hydrogen storage of 4 wt%
via pressure variation between 1 and 100 bar at a constant tem-
perature of 125 ◦C with reasonable kinetics was demonstrated. It
was shown for the first time that about 4 wt% of hydrogen could
be released during dehydrogenation of NaAlH4 against a hydro-
gen pressure of 4 bar, which will be needed for PEM fuel cells in
automotive applications.
Fig. 4. Dehydrogenation of NaAlH4 at 4 bar in dependence of the catalytic system
in comparison with the dehydrogenation of Zr–TiCl4-codoped NaAlH4 at 1 bar.
Acknowledgements
tion temperature, respectively). The codoped NaAlH4 reversibly
stored about 4 wt% hydrogen at a constant temperature of 125 ◦C
with a total catalyst amount of only 2 mol%. Values of >4 wt%
reversible hydrogen release were achieved after 5 h of dehydro-
genation, and hydrogen uptake of 4 wt% after 1 h was demonstrated
for this sample. Within the investigated range the catalyst con-
centration in the codoped system has only a minor effect on the
sorption kinetics of NaAlH4. However, the storage system with
2 mol% catalyst stored 0.4 wt% hydrogen more than the sample with
5 mol%.
The reversibility was confirmed by XRD analysis (cf. Fig. 2).
In the representative XRD pattern of codoped NaAlH4 prior to
dehydrogenation and after three cycles of dehydrogenation and
rehydrogenation Ti could not be detected directly, probably due
to an overlap with the Al peaks of the finely dispersed catalyst.
However, NaCl was found which is formed during the reduction
of TiCl4. As expected from Fig. 1, Na3AlH6 has not been detected,
thus, indicating a low concentration. The XRD patterns of the other
samples we investigated provide similar results and are therefore
not presented.
The authors would like to acknowledge financial support of
the Fraunhofer Attract program. We thank M. Eckardt and V.
Pacheco for assistance with XRD measurements and T. Richter and
S. Kalinichenka for SEM analysis.
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