Influence of the Nb2O5 distribution on the electrochemical hydrogenation of nanocrystalline magnesium

Article abstract of DOI:10.1016/j.jallcom.2006.08.151

Nanocrystalline Mg powder without and with 2 mol% Nb₂O₅ catalyst was studied as an electrode material for electrochemical hydrogen charging in a 6 M KOH electrolyte. A strong influence of the compaction parameters, the current density and the catalyst on the hydrogenation behavior was observed. The addition of graphite and PTFE to the Mg/Nb₂O₅ electrodes improves the charging kinetics as well as the hydrogen content. The hydrogen contents achieved in Mg with Nb₂O₅, however, show a broad scatter. It was concluded that the catalyst distribution influences the upper limit of the storage capacity as well as the oxidation process at the surface during preparation. Since the addition of Nb₂O₅ was observed to reduce the hydrogen overpotential of Mg depending on the catalyst distribution, it is assumed that the catalyst influences the electrode reactions. Therefore, hydrogenation was investigated for different Nb₂O₅ distributions at different current densities in detail.

Full text of DOI:10.1016/j.jallcom.2006.08.151

Journal of Alloys and Compounds 434–435 (2007) 753–755
Influence of the Nb O distribution on the electrochemical
2
5
hydrogenation of nanocrystalline magnesium
a,∗
a
a
b
b
D. Zander , L. Lyubenova , U. K o¨ ster , T. Klassen , M. Dornheim
a
Department of Biochemical and Chemical Engineering, University of Dortmund, D-44221 Dortmund, Germany
b
Institute for Materials Research, GKSS Research Centre, D-21502 Geesthacht, Germany
Available online 6 October 2006
Abstract
Nanocrystalline Mg powder without and with 2 mol% Nb
2
5
O catalyst was studied as an electrode material for electrochemical hydrogen charging
in a 6 M KOH electrolyte. A strong influence of the compaction parameters, the current density and the catalyst on the hydrogenation behavior
was observed. The addition of graphite and PTFE to the Mg/Nb electrodes improves the charging kinetics as well as the hydrogen content.
The hydrogen contents achieved in Mg with Nb , however, show a broad scatter. It was concluded that the catalyst distribution influences the
upper limit of the storage capacity as well as the oxidation process at the surface during preparation. Since the addition of Nb was observed to
reduce the hydrogen overpotential of Mg depending on the catalyst distribution, it is assumed that the catalyst influences the electrode reactions.
Therefore, hydrogenation was investigated for different Nb distributions at different current densities in detail.
2006 Elsevier B.V. All rights reserved.
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©
Keywords: Magnesium; Nanocrystalline; Catalyst; Hydrogen absorbing materials; Electrochemical reactions
1
. Introduction
on the hydrogenation behavior at low current densities. With
decreasing current density the storage capacity as well as the
During the last decades investigations were conducted on the
kinetics of Mg/Nb2O electrodes increased significantly up to
5
improvement of hydrogen storage electrodes for battery appli-
cations [1–3]. Nanocrystalline Mg-based metal hydrides are
considered to be important candidates for safe energy storage
andtransportmaterial. Butduetostillhighsorptiontemperatures
and unknown cycling life commercialization in mobile storage
systems, metal/hydride batteries and other clean applications
slowed down. Recently it has been published that for hydrogena-
tion from the gas phase the design of improved nanocrystalline
Mg alloys can proceed by the addition of metal oxides, e.g.
1 wt.% H2 at a charging time of 30 min. Since the hydrogen
overpotential of Mg, which is a measure of the hydrogen evolu-
tion at the electrode surface, was observed to be reduced by the
addition of Nb2O , it is assumed that the catalyst influences the
5
electrode reactions. The aim of this paper is to present further
results on the influence of the Nb2O distribution on electro-
5
chemical hydrogenation of nanocrystalline Mg.
2
. Experimental
Nb2O , as catalysts [4–7]. However these studies have focused
5
only on the hydriding characteristics of nanocrystalline Mg with
the addition of metal oxides in the gas phase. In order to verify
whether such an addition improves also electrochemical hydro-
genation, nanocrystalline Mg powder without and with 2 mol%
Mg powder without and with 2 mol% Nb O as a catalyst was prepared
by milling MgH2 (Goldschmidt AG, 95% purity, the rest being magnesium)
2
5
using a planetary ball mill (Fritsch P5) with a ball to powder weight ratio of
◦
4
00 g/40 g and subsequent desorption of the MgH2 powders at 300 C. 2 mol%
Nb2O5 powder was added after milling MgH2 for 20 h and blended by milling
for additional 100 h. The observed hydrogen content after thermal desorption
Nb2O catalyst as an electrode material for electrochemical
5
hydrogen charging processes electrolyte was studied in a 6 M
KOH [8,9]. These investigations revealed a strong influence of
the compaction parameters, the current density and the catalyst
was 0.15 wt.% H . The powder was used to design a proper electrode for elec-
trochemical hydrogenation which provides good mechanical stability and good
2
conductivity to allow the dissociation and adsorption of hydrogen at the sur-
face as well as diffusion into the material. The Mg electrodes were prepared by
compaction at different pressures without any additions as well as with 5 wt.%
graphite and 5 wt.% PTFE. Since Mg is highly reactive in oxygen the preparation
of the powders, including milling and electrode preparation, was performed in
a glove box under a continuously purified Ar atmosphere. Cathodic hydrogena-
∗
Corresponding author. Tel.: +49 231 755 3015; fax: +49 231 755 5978.
E-mail address: daniela.zander@bci.uni-dortmund.de (D. Zander).
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.08.151

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D. Zander et al. / Journal of Alloys and Compounds 434–435 (2007) 753–755
Fig. 1. SEM of Mg-powder with (a) inhomogenous and (b) homogenous Nb2O5 catalyst distribution.
◦
tion was carried out in a 6 M KOH electrolyte at 25 C and a current density
between i = 50 and 5 mA/g. Further electrochemical measurements such as deter-
mining the overvoltage were performed under the same electrolyte conditions.
The potential was measured and plotted against the potential of a Hg/HgO elec-
trode. Thehydrogencontentwasmeasuredbyahydrogendeterminator(RH-404,
Leco). Morphology, catalyst distribution and microstructure of the Mg powder
with and without Nb2O5 were investigated by X-ray diffraction and SEM.
as well as the kinetic of Mg/Nb2O electrodes increased sig-
nificantly up to 1 wt.% H2 at a charging time of 30 min with
decreasing current density.
The upper limit and the scattering of the storage capac-
ity might be influenced by the catalyst distribution. Therefore,
5
Mg/Nb2O electrodes with a homogenous catalyst distribution
5
were hydrogenated at different current densities (Fig. 3) in com-
parison to the inhomogenous Nb2O distribution (Fig. 2). Fig. 3
3
. Results and discussion
5
exhibits that the results for the Mg/Nb2O electrodes with a
5
homogeneous catalyst distribution show a higher accuracy in
comparison to the broad scattering in the other case. But as
compared to the inhomogeneous catalyst distribution the homo-
geneous distribution results not only in a better reproducibility
of the hydrogen content but also to a reduced kinetic. This might
be due to a change in the overpotential with the catalyst distri-
bution and/or accelerated oxidation processes by the catalyst at
the surface during preparation.
Microstructural investigations by SEM (Fig. 1) of desorbed
MgH2/2 mol% Nb2O powder showed a Mg particle size of
5
5
–10 ␮m and a Nb2O particle size of 1–4 ␮m. The distribution
5
of the Nb2O catalyst of the investigated Mg/Nb2O electrodes
5
5
varied from inhomogenous to homogenous. The grain size of
the Mg/Nb2O powder was found to be 50–200 nm [9].
5
As reported earlier [9] a strong influence of the Nb2O cata-
5
lyst was observed with decreasing current densities. The results
Fig. 2) for the Mg electrodes show a high accuracy in compar-
Potentiodynamic polarization experiments (Fig. 4) gave addi-
tional information regarding the surface reactions at the elec-
trode, specially regarding the hydrogen overvoltage of the
(
ison to a broad scattering for the Mg/Nb2O electrodes (grey
5
shaded area). The observed hydrogen contents varied between
.25 and 1.0 wt.% for the Mg/Nb2O electrodes. However, a
5
strong influence of the catalyst was observed with decreasing
current densities for the upper hydrogen charging limit. While
the storage capacity of about 0.4 wt.% H2 of the Mg-electrode
changes only by a small amount to lower hydrogen contents of
investigated electrodes. In general it was observed that Nb2O5
0
decreases the hydrogen overvoltage η of Mg compacted at
2.5
2
a compaction pressure of 6.2 N/mm . As reported earlier [8]
no significant influence of the addition of graphite as well as
PTFE on the hydrogen overvoltage was observed. The reduced
0
.35 wt.% with decreasing current density, the storage capacity
Fig. 2. Influence of the current density after charging for 30 min on inhomoge-
nous distributed Nb2O5 in Mg/Nb2O5-electrodes compacted with a suspension
of graphite and PTFE at a compaction pressure of 6.2 N/mm .
Fig. 3. Influence of the current density after charging for 30 min on homogenous
distributed Nb2O5 in Mg/Nb2O5-electrodes compacted with a suspension of
graphite and PTFE at a compaction pressure of 6.2 N/mm .
2
2

D. Zander et al. / Journal of Alloys and Compounds 434–435 (2007) 753–755
755
way and should clarify the effect of Nb2O on the charging
5
behavior of nanocrystalline Mg in more detail.
4. Conclusion
Nanocrystalline Mg powder without and with 2 mol% Nb2O5
catalyst compacted with a suspension of graphite and PTFE at
2
a compaction pressure of 6.2 N/mm was studied in a 6 M KOH
as an electrode material for electrochemical hydrogen charging
processes electrolyte. A strong influence of the Nb2O cata-
5
lyst on the electrochemical surface reaction was observed with
decreasing current densities.
Nb2O reduces the hydrogen overvoltage of Mg compacted
5
2
at a compaction pressure of 6.2 N/mm . A homogeneous cata-
lyst distribution results not only in a better reproducibility of the
hydrogen content but also to a slower kinetic in comparison
to an inhomogenous distribution. It is shown that the cata-
lyst as well as the catalyst distribution influences significantly
the electrode reactions and the oxidation mechanism in the
electrolyte.
Fig. 4. Influence of the catalyst distribution on potentiodynamic polarization of
the Mg and Mg/Nb2O5-electrodes compacted with a pressure of 6.2 N/mm .
2
hydrogen overvoltage due to the catalyst is assumed to acceler-
ate the hydrogen evolution at the electrode surface and explains
the independence of the addition of the catalyst on the hydrogen
content at 50 mA/g and a charging time of 30 min for both distri-
butions. At lower current densities it is assumed that the Volmer
Acknowledgment
This work was supported by the EU RTN, Project HPRN-
CT-2002-00208.
−
−
reactionH2O + e → H + OH willbecomethespeedlimiting
ad
reaction and the competing hydrogen recombination reactions
References
will be irrelevant for the Mg/Nb2O electrodes. It is assumed
5
that at very low current densities the electrochemical equilib-
rium is moved to the anodic reaction and leads to a lower storage
[
[
[
1] J.J.G. Willems, Philips J. Res. 39 (1984) 1.
2] A. Z u¨ ttel, F. Meli, L. Schlapbach, J. Alloys Compd. 200 (1993) 157.
3] N. Cui, B. Luan, H.K. Liu, H.J. Zhao, S.X. Dou, J. Power Sources 55 (1995)
capacity of the Mg/Nb2O electrodes because of an increased
5
oxidation in the 6 M KOH electrolyte.
163.
In addition Fig. 4 reveals a significant higher hydrogen over-
potential for the homogenous in comparison to the inhomoge-
nous catalyst distribution; thus might explain the lower hydrogen
content and slower kinetic. It is assumed that the differences
of the catalyst distribution and therefore in the hydrogen over-
potential indicate a major influence of the oxidation behavior
[4] Z. Dehouche, T. Klassen, W. Oelerich, J. Goyette, T.K. Bose, R. Schulz, J.
Alloys Compd. 347 (2002) 319.
[
[
[
5] W. Oelerich, T. Klassen, R. Bormann, J. Alloys Compd. 315 (2001) 2372.
6] G. Barkhordarian, T. Klassen, R. Bormann, Scripta Mater. 49 (2003) 213.
7] G. Barkhordarian, T. Klassen, R. Bormann, J. Alloys Compd. 364 (2004)
242.
[8] D. Zander, L. Lyubenova, U. K o¨ ster, T. Klassen, M. Dornheim, Hydrogen
storage materials, in: M. Nazri, G.-A. Nazri, R.C. Young, C. Ping (Eds.),
Mater. Res. Soc. Symp. Proc., vol. 801, Warrendale, 2004, p. BB 3.1.
during the electrode preparation and/or the Nb2O /Mg inter-
5
faces on the hydrogenation. There is evidence for the formation
of an oxide layer at the interfaces during ball milling [10] which
could explain the differences of the electrochemical hydrogena-
tion behavior depending on the catalyst distribution. Further
microstructural and electrochemical investigations are under-
[
9] D. Zander, L. Lyubenova, U. K o¨ ster, M. Dornheim, F. Aguey-Zinsou, T.
Klassen, J. Alloys Compd. 413 (2006) 298.
[
10] O. Friedrichs, A. Fern a´ ndez, Instituto de Ciencia de Materiales de Sevilla,
Centro de Investigaciones Cient ´ı ficas Isla de la Cartuja, Sevilla (Spain),
private communication, 2005.