The catalytic effect of Nb2O5 on the electrochemical hydrogenation of nanocrystalline magnesium

Article abstract of DOI:10.1016/j.jallcom.2005.06.063

Nanocrystalline Mg powder without and with 2 mol% Nb₂O ₅ catalyst was studied in a 6 M KOH electrolyte as electrode material for electrochemical hydrogen charging processes. Since the hydrogen overpotential of Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced by the addition of Nb ₂O₅, it is assumed that the catalyst influences the electrode reactions. Considering this assumption hydrogenation was studied at different current densities. The storage capacity as well as the kinetic of Mg/Nb₂O₅ electrodes increased significantly up to 1 wt.% H₂ at a charging time of 30 min with decreasing current density. The storage capacity of nanocrystalline Mg powder showed only minor changes to lower hydrogen contents with decreasing current density.

Full text of DOI:10.1016/j.jallcom.2005.06.063

Journal of Alloys and Compounds 413 (2006) 298–301
The catalytic effect of Nb2O5 on the electrochemical
hydrogenation of nanocrystalline magnesium
D. Zandera,∗, L. Lyubenova , U. Koster , M. Dornheim , F. Aguey-Zinsou , T. Klassen
a
a
b
b
b
¨
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
Received 15 May 2005; received in revised form 25 June 2005; accepted 28 June 2005
Available online 8 February 2006
Abstract
Nanocrystalline Mg powder without and with 2 mol% Nb2O5 catalyst was studied in a 6 M KOH electrolyte as electrode material for
electrochemical hydrogen charging processes. Since the hydrogen overpotential of Mg, which is a measure of the hydrogen evolution at the
electrode surface, was observed to be reduced by the addition of Nb2O5, it is assumed that the catalyst influences the electrode reactions.
Considering this assumption hydrogenation was studied at different current densities. The storage capacity as well as the kinetic of Mg/Nb2O5
electrodes increased significantly up to 1 wt.% H2 at a charging time of 30 min with decreasing current density. The storage capacity of
nanocrystalline Mg powder showed only minor changes to lower hydrogen contents with decreasing current density.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Magnesium; Nanocrystalline; Nb2O5; Catalyst; Electrochemical; Hydrogenation; Metal hydrides; Hydrogen storage
1. Introduction
Nb2O5 [4–7] as catalysts. Furthermore, these nanocrystalline
Mg alloys exhibit very good cycling properties [4]. However,
these studies have focused only on the hydriding charac-
teristics of nanocrystalline Mg with the addition of metal
oxides in the gas phase. In order to verify whether such
an addition improves also electrochemical hydrogenation,
nanocrystalline Mg powder without and with 2 mol% Nb2O5
catalyst as an electrode material for electrochemical hydro-
gen charging processes electrolyte was studied in a 6 M KOH
[8]. This first investigation revealed a strong influence of the
compaction parameters and the current density, but no influ-
ence of the catalyst on the hydrogenation behavior at high
current densities. In addition it was reported that the addition
of graphite and PTFE to the Mg/Nb2O5 electrodes improves
the charging kinetic as well as the hydrogen content.
The aim of this paper is to present further results on the
influence of Nb2O5 on electrochemical hydrogenation of
nanocrystalline Mg in particular at current densities lower
than 50 mA/g. The mechanisms by which metal oxides
improve hydrogen absorption as well as desorption are still
unclear and should be investigated also by electrochemical
investigations in some detail.
Desirable properties of hydrogen storage materials
include high hydrogen capacity, high number of hydrogena-
tion/dehydrogenation cycles, low temperature of dissociation
and microstructural stability. Hydrogen storage can proceed
from the gas phase as well as during cathodic charging.
Whereas, bulk Mg cannot be used as hydrogen storage
material due to the early formation of a Mg-hydride layer
which acts as a hydrogen barrier, nanocrystalline Mg-based
hydrides are considered to be an important candidate for safe
energy storage and transportation [1]. But due to still high
sorption temperatures and unknown cycling life commercial-
izationinmobilestoragesystems, metal/hydridebatteriesand
other clean applications have been slowed down [2,3].
Recently, it has been published that for hydrogenation
fromthegasphasethedesignofimprovednanocrystallineMg
alloys can proceed by the addition of metal oxides, e.g. V2O5,
∗
Corresponding author.
E-mail addresses: daniela.zander@bci-dortmund.de,
daniela.zander@bci.uni-dortmund.de (D. Zander).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.06.063

D. Zander et al. / Journal of Alloys and Compounds 413 (2006) 298–301
299
2. Experimental
Mg powder without and with 2 mol% Nb2O5 as a cata-
lyst was prepared by milling MgH2 (Goldschmidt AG, 95%
purity-the rest being magnesium) using a planetary ball mill
(Fritsch P5) with a ball to powder weight ratio of 400 g/40 g
and subsequent desorption of the MgH2 powders at 300 ◦C.
Two molar percent Nb2O5 powder was added after milling
MgH2 for 20 h and blended by milling for additional 100 h.
The powder was used to design a proper electrode for elec-
trochemical hydrogenation which provides good mechanical
stability and good conductivity to allow the dissociation and
adsorption of hydrogen at the surface as well as diffusion
into the material. The Mg electrodes were prepared by com-
paction 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, includ-
ing milling and electrode preparation, was performed inside
a glove box under a continuously purified Ar atmosphere.
Cathodic hydrogenation was carried out in a 6 M KOH elec-
trolyte at 25 ◦C and a current density between i = 50 mA/g
and i = 5 mA/g. Further electrochemical measurements such
as determining the overvoltage were perfomed under the
same electrolyte conditions. The hydrogen content was mea-
sured by a hydrogen determinator (RH-404, Leco). Morphol-
ogy and microstructure of the Mg powder with and without
Nb2O5 were investigated by X-ray diffraction, SEM and
TEM. To prevent the Mg powder from oxidation the TEM
samples were dispersed in toluene in a glove box, dropped
on a copper grid and inserted immediately in the TEM.
Fig. 1. Mg-powder with Nb2O5 catalyst (SEM micrograph).
was 0.15 wt.% H2; this explains the existence of nanocrys-
talline MgH2 particles as seen by TEM. The grain size of
the Nb2O5 catalyst of Mg/Nb2O5 powder was found to be
50–200 nm.
The main electrode parameters influencing the cathodic
charging are given by the compaction, the electrochemical
surface reaction and the chosen catalyst. As reported earlier
[8] during hydrogen charging of Mg and Mg/Nb2O5 elec-
trodes a strong influence of the (1) compaction parameters
and (2) the current density on the hydrogenation behavior but
(3) no influence of the catalyst on the hydrogenation at a high
currentdensityof50 mA/g, achargingtimeof3 hat25 ◦Cand
different porosities was observed. Recent results [5,6] on the
influence of different Nb2O5 contents on the gaseous hydro-
gen reaction kinetics of nanocrystalline Mg showed that the
catalytic effect of Nb2O5 is improved in absorption as well
as desorption. Therefore, it was assumed that the observed
influence of the current density on the Mg/Nb2O5 electrode
and the independence of the addition of the catalyst on the
hydrogenation indicate that the catalyst plays a major role
regarding the electrochemical processes at the electrode sur-
face. In order to reveal this assumption, the influence of the
current density on Mg and Mg/Nb2O5 electrodes compacted
with a suspension of graphite and PTFE at a compaction pres-
sure of 6.2 N/mm2 was studied (Fig. 3). A strong influence of
3. Results and discussion
Microstructural investigations by SEM (Fig. 1) and TEM
(Fig. 2) of desorbed MgH2 powders without and with 2 mol%
Nb2O5 powder showed a Mg particle size of 5–10 ␮m. X-ray
as well as electron diffraction of MgH2 without and with cat-
alyst after thermal desorption at 300 ◦C (Fig. 2c) revealed the
precipitation of nanocrystalline MgH2 within the Mg parti-
cles. The observed hydrogen content after thermal desorption
Fig. 2. Mg-powder (a) without and (b) with Nb2O5 catalyst (TEM micrographs) and (c) electron diffraction pattern of Mg-powder with Nb2O5 catalyst.

300
D. Zander et al. / Journal of Alloys and Compounds 413 (2006) 298–301
Fig. 3. Influence of the current density after charging for 30 min on
Mg/Nb2O5-electrodes compacted with a suspension of graphite and PTFE
at a compaction pressure of 6.2 N/mm2.
Fig. 4. Potentiodynamic polarization of the Mg and Mg/Nb2O5-electrodes
without and with C/PTFE compacted with a pressure of 6.2 N/mm2.
alyst is assumed to accelerate the hydrogen evolution at
the electrode surface and explains the surprising indepen-
dence of the addition of the catalyst on the hydrogen content
as well as the charging kinetics at 50 mA/g and a charg-
ing time of 3 h. Earlier it was assumed that current den-
sities of 50 mA/g lead to the preferred recombination of
Had and a reduced adsorption of hydrogen [8]. At lower
current densities (Fig. 3) it is assumed that the Volmer reac-
tion H3O+ + e− → Had + H2O will become the speed limiting
reaction and the competing Tafel reaction (Had + Had → H2)
or Heyrowski reaction (Had + H3O+ + e− → H2 + H2O) will
be irrelevant for the Mg/Nb2O5 electrodes. It is likely that
due to the high hydrogen overvoltage of the Mg elec-
trodes and the resulting oxidation at the surface the influ-
ence of the current density on the storage capacity becomes
only minor. It is assumed that at very low current den-
sities <10 mA/g the electrochemical equilibrium is moved
to the anodic reaction and leads to a lower storage capac-
ity of the Mg/Nb2O5 electrodes because of an increased
oxidation in the 6 M KOH electrolyte. The electrochemi-
cal investigations showed the major influence of the Nb2O5
catalyst on the electrochemical surface reactions and there-
fore on the storage capacity as well as on the kinet-
ics. Further microstructural and electrochemical investiga-
tions are underway and should clarify the effect of Nb2O5
on the charging behavior of nanocrystalline Mg in more
detail.
the catalyst was observed with decreasing current densities.
The results for the Mg electrodes show a high accuracy in
comparison to the broad scattering for the Mg/Nb2O5 elec-
trodes (grey shaded area). The observed hydrogen contents
scattered between 0.25 and 1.0 wt.% for the Mg/Nb2O5 elec-
trodes. It is assumed that the catalyst distribution influences
the upper limit and the scattering of the storage capacity as
well as the oxidation process at the surface during prepara-
tion. However, a 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 minor to lower hydro-
gen contents of 0.35 wt.% with decreasing current density,
the storage capacity as well as the kinetic of Mg/Nb2O5 elec-
trodes increased significantly up to 1 wt.% H2 at a charging
time of 30 min with decreasing current density. Measure-
ments at a limiting value of i ≤ 10 mA/g, however, indicate
a change of the electrochemical processes at the electrode
surface.
Potentiodynamic polarization experiments (Fig. 4) gave
some more information regarding the surface reactions at the
electrode, speciallyregardingthehydrogenovervoltageofthe
investigated electrodes. Depending on the hydrogen overpo-
tential and the current density three competing reactions can
occur at the electrochemical surface:
H3O+ + e− → Had + H2O Volmerreaction
Had + Had → H2 Tafelreaction
4. Conclusion
Had + H3O+ + e− → H2 + H2O Heyrowskireaction
Nanocrystalline Mg powder without and with 2 mol%
Nb2O5 catalyst compacted with a suspension of graphite and
PTFE at a compaction pressure of 6.2 N/mm2 was studied
in a 6 M KOH as an electrode material for electrochemical
hydrogen charging processes electrolyte. A strong influence
of the Nb2O5 catalyst on the electrochemical surface reaction
It was observed that Nb2O5 decreases the hydrogen over-
voltage η2.5 of Mg compacted at a compaction pressure
of 6.2 N/mm2. No significant influence of the addition of
graphite as well as PTFE on the hydrogen overvoltage was
observed. The reduced hydrogen overvoltage due to the cat-

D. Zander et al. / Journal of Alloys and Compounds 413 (2006) 298–301
301
was observed with decreasing current densities. Whereas,
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current density. The hydrogen overpotential of Mg is much
higher than for Mg/Nb2O5. It is shown that the catalyst influ-
ences significantly the electrode reactions and the oxidation
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Acknowledgement
ThisworkwassupportedbytheEURTN—ProjectHPRN-
CT-2002-00208.