Low-temperature hydrogenation of Mg-Ni-Nb2O5 alloy processed by high-pressure torsion

Article abstract of DOI:10.1016/j.jallcom.2021.160309

Powder mixtures with a nominal composition Mg-5 wt%Ni-2 wt%Nb₂O₅ are used to produce ultrafine-grained bulk material by high-pressure torsion (HPT) processing. Samples subjected to 10 and 20 revolutions in torsion under a pressure of 3 GPa developed an ultrafine grain size, mainly in the 100?200 nm range. The hydrogenation properties of these HPT-processed materials have been characterized at temperatures in the interval of 373–723 K. The hydrogenation experiments clearly show faster kinetics of hydrogenation and higher storage capacities in the HPT materials compared to the initial mild-milled powder mixture. The storage capacities have been limited to about 5.5 wt% H₂ by the formation of a substantial amount of MgO in the materials after several hydrogenation cycles. However, as a matter of practical relevance, the production of bulk pieces of the Mg-Ni-Nb₂O₅ alloy having an ultrafine grain structure allows handling of the material without special precautions against ignition, as well as its storage in contact with air for several months with no evident deterioration of its fast activation in posterior hydrogenation treatments.

Full text of DOI:10.1016/j.jallcom.2021.160309

Journal of Alloys and Compounds 878 (2021) 160309
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Low-temperature hydrogenation of Mg-Ni-Nb2O5 alloy processed by
high-pressure torsion
M. Osorio-Garcíaa, K. Suárez-Alcántarab, Y. Todakac, A. Tejeda-Ochoad, M. Herrera-Ramírezd,
O. Hernández-Silvaa, F. Cruz-Gandarillae, J.G. Cabañas-Morenoa,⁎
a Programa de Nanociencias y Nanotecnología, Centro de Investigación y de Estudios Avanzados (Cinvestav-IPN), Av. Instituto Politécnico Nacional 2508,
San Pedro Zacatenco, Deleg. Gustavo A. Madero, CP 07360 Ciudad de México, Mexico
b Instituto de Investigaciones en Materiales-Unidad Morelia de la Universidad Autónoma de México (IIM-UNAM), Antigua Carretera a Pátzcuaro 8701,
Ex-Hacienda San José de la Huerta, CP 58190 Morelia, Michoacán, Mexico
c Department of Mechanical Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
d Centro de Investigación en Materiales Avanzados (CIMAV), Laboratorio Nacional de Nanotecnología, Av. Miguel de Cervantes Saavedra 120,
Complejo Industrial Chihuahua, CP 31136 Chihuahua, Chihuahua, Mexico
e Instituto Politécnico Nacional, ESFM, Unidad Prof. ALM, Zacatenco, CP 07738 Ciudad de México, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Powder mixtures with a nominal composition Mg-5 wt%Ni-2 wt%Nb2O5 are used to produce ultrafine-
grained bulk material by high-pressure torsion (HPT) processing. Samples subjected to 10 and 20 revolu-
tions in torsion under a pressure of 3 GPa developed an ultrafine grain size, mainly in the 100−200 nm
range. The hydrogenation properties of these HPT-processed materials have been characterized at tem-
peratures in the interval of 373–723 K. The hydrogenation experiments clearly show faster kinetics of
hydrogenation and higher storage capacities in the HPT materials compared to the initial mild-milled
powder mixture. The storage capacities have been limited to about 5.5 wt% H2 by the formation of a
substantial amount of MgO in the materials after several hydrogenation cycles. However, as a matter of
practical relevance, the production of bulk pieces of the Mg-Ni-Nb2O5 alloy having an ultrafine grain
structure allows handling of the material without special precautions against ignition, as well as its storage
in contact with air for several months with no evident deterioration of its fast activation in posterior hy-
drogenation treatments.
Received 16 February 2021
Received in revised form 15 April 2021
Accepted 4 May 2021
Available online 11 May 2021
Keywords:
Metal hydrides
Mg alloys
MgH2
Hydrogen storage
High-pressure torsion (HPT)
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
dehydrogenation kinetics [2–5]. SPD processes include but are not
limited to high-pressure torsion (HPT) and equal channel angular
Severe plastic deformation (SPD) processes have proved useful to
fabricate bulk nanostructured and ultrafine-grain materials with
novel properties [1]. The very large plastic deformations applied in
the SPD process makes possible the subdivision of the grains in
metallic and ceramic materials, and the attainment of grain sizes
under 1 µm and even in the nanometric range. Research in hydrogen
storage can take advantage of the SPD processes in producing
compact materials with good heat transfer properties and an ultra-
fine grain structure, which can translate into good hydrogenation/
pressing (ECAP) [1]. In the HPT process, the sample is compressed
between anvils while being subject to deformation in torsion; re-
finement of the microstructure can be achieved by controlling the
number of revolutions and the pressure imposed on the anvils. In the
ECAP process, the materials are pressed into a special die through
two channels with equal cross-section, usually intersecting at an
angle of 90° or 120° [1,6]. Heating of the material during ECAP
processing is frequently needed; for example, temperatures between
523 K and 823 K have been used for Mg-based materials [7,8]. In this
case, microstructure refinement can be achieved by controlling the
number of passes through the die [6]. Recently, other processing
techniques such as cold-rolling, accumulative roll bonding, re-
petitive corrugation, straightening or hot-extruding, and surface
mechanical attrition have also been applied to hydrogen storage
alloys with encouraging results [9–16].
⁎
Corresponding author.
E-mail addresses: mayara.osorio@cinvestav.mx (M. Osorio-García),
karina_suarez@iim.unam.mx (K. Suárez-Alcántara), todaka@me.tut.ac.jp (Y. Todaka),
armando.tejeda@cimav.edu.mx (A. Tejeda-Ochoa),
martin.herrera@cimav.edu.mx (M. Herrera-Ramírez),
fcruz@ipn.mx (F. Cruz-Gandarilla), jcabanasm@cinvestav.mx (J.G. Cabañas-Moreno).
https://doi.org/10.1016/j.jallcom.2021.160309
0925-8388/© 2021 Elsevier B.V. All rights reserved.

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
In 2004, Skripnyuk et al.[6] reported that the application of ECAP
to a ZK60 magnesium alloy produced an improvement in the ki-
netics of hydrogenation (formation of MgH2) and dehydrogenation
(MgH2 decomposition). Comparison of high-energy ball milling
(HEBM), ECAP, and ECAP + HEBM processed ZK60 alloy, indicated
that ECAP was at least as beneficial to the hydrogenation/ dehy-
drogenation kinetics as was prolonged HEBM [6]. The authors ex-
plained the effect due to the refinement of the grain structure
achieved by ECAP processing with the concomitant acceleration in
the diffusion and nucleation processes required for MgH2 formation
and decomposition [6].
Following the initial work of Skripnyuk et al. [6], a number of
reports have focused on the application of diverse plastic deforma-
tion processes to Mg-based alloys with the same goal of improving
their hydrogen storage properties. In a survey on plastic deformation
techniques applied to Mg-based alloys intended for hydrogen sto-
rage (2004–2021), we located the following relevant reports: HPT
[5,13,17–44]; ECAP [6–8,11,14,44–60]; cold-rolling [5,9,10,13,14,32,34,
38,39,55–79]. Other processes less frequently used are: friction stir
processing [80], fast forging [81–83], hot extrusion [84], and surface
mechanical attrition [15,16,40]. More recently, a combination of
processing techniques has gained some interest, for example, cold-
rolling and ball milling [5,75,79], ECAP and accumulative roll-
bonding or cold rolling [55–60]. In some cases, mixed processing
techniques may have advantages as compared with single proces-
sing. However, there are some advantages of the HPT process over
other plastic deformation processes, including the fact that it is
usually applied at room temperature, very large plastic deformations
can be reached, and it can be easily applied to the compaction of
powders [18,20–22,26–30,35–37,43]. The present work makes use of
short-time ball milling (henceforth, “mild milling”) and HPT for
processing of a Mg-rich powder mixture.
between these two additions in ball-milled material. The following
results seem to indicate that there is indeed an advantage in their
combined application.
In the present work, we extensively tested the hydrogenation of
Mg-Ni-Nb2O5 mild-milled powder mixtures, which have been fur-
ther processed by HPT with the purpose to obtain a bulk (con-
solidated), fine-grained material with improved hydrogen storage
properties at temperatures lower than 573 K. Simultaneously; we
used to our advantage the fact that the bulk materials fabricated by
HPT can be manipulated in ambient conditions with overall low
detriment of the hydrogen storage properties. This is particularly
convenient compared to handling fine Mg powders, which can burn
spontaneously on exposure to air, unless proper passivation is
applied [91,92].
2. Experimental
Based on their well-known effects as catalysts in hydrogen storage
applications, fine powders of Ni and Nb2O5 were chosen as additions
to commercially pure Mg powders, in nominal quantities of 5 and
2 wt%, respectively. All materials were acquired in powder form: Mg,
100 µm (Riedel de Haen, cat. # S030195); Ni, <1 µm (Sigma-Aldrich,
cat. #268283); Nb2O5 (Sigma-Aldrich, cat. #203920).
Mixing of the Mg, Ni, and Nb2O5 powders was carried out by
short term (45 min) milling under N2 atmosphere in a planetary ball
mill, Pulverisette 5 (FRITSCH), run at a rotational speed of 250 rpm
with a ball-to-powder ratio of 5:1. The milling media (5 mm dia-
meter balls) and vials were made of stainless steel. Notice that these
are considered “mild” milling conditions since the milling operation
was intended to disperse the original fine powders of Ni and Nb2O5
onto the surface of the much larger Mg particles, but not to the point
of considerable fracture of the latter and the creation of new, re-
active magnesium surfaces. The mild-milled powders were trans-
ferred to plastic containers in a glove box filled with commercial
grade N2 gas. These powders remained in the plastic containers for
several months before being used in the HPT experiments described
below. This delay between the production of powder mixtures and
their HPT processing was dictated by the fact that each one of these
processes was carried out in different laboratories and countries.
Pre-compacted disks, with dimensions of approximately 10 mm
diameter and 1 mm thickness, were produced at room temperature
from the mild-milled powders by uniaxial pressing under 1.05 GPa
applied for 5 s. The pre-compacted disks were placed in an HPT
machine (at Toyohashi University of Technology, Japan) and sub-
jected to 10 or 20 revolutions at room temperature under a pressure
of 3 GPa, using a rotational speed of 0.2 rev/min. The literature on
HPT of magnesium indicates that the application of 5 or more re-
volutions yields basically homogeneous microstructures in most
cases [41,42,93]. It should be noted that no special precautions were
taken to keep the materials from contact with the atmosphere
during or after these operations.
Mild-milled powders and HPT processed disks were used to de-
termine the kinetics of hydrogenation, while pressure - composition -
temperature (PCT, also known as pressure-composition isotherms
PCI) curves [94] were measured only for the latter. Kinetics experi-
ments were performed in an in-house constructed Sievert´s type
apparatus, while PCT data were obtained using a Quantachrome iSorb
HP1 instrument under the conditions described below. The details of
the Sievert´s equipment are fully described in reference [95]. The
hydrogen gas used in all experiments was of chromatographic purity.
Samples used in these experiments had a weight of about 0.5 g.
It should be mentioned that, at present, no standard practice is in
general use to activate those materials intended for hydrogen sto-
rage applications. However, the activation procedure may influence
the subsequent response of the material to hydrogenation and de-
hydrogenation testing [9]. In our case, the conditions of temperature
Most of the above-listed works deal with the formation and
decomposition of MgH2 from pure Mg or commercially available Mg
alloys. In contrast, a few others are focused on the production of Mg-
Ni hydrides (mainly Mg2NiH4) [8,17,19,21,29,33,50,70,82,83]. But
within these reports one can hardly find evidence of a remarkable
change in the kinetics of MgH2 formation at low temperatures, say
below 523 K. Notably, Edalati et al. [31] reported the hydrogenation
of 99.9% pure Mg at 423 K; this remarkable result was obtained in
samples processed by HPT for a total of 10 revolutions.
An important issue regarding most of the previous work on SPD
of Mg-based materials for hydrogen storage applications is that their
compositions did not include the best known catalysts [2,4,85] for
this application. Some works have used cold rolling to incorporate
transition metals [64] or oxides [61] into magnesium or magnesium
hydride. However, Vincent et al. reported that cold rolling might not
be an effective way to incorporate catalysts to magnesium hydride
for hydrogen storage [64]. On the other hand, in the work of Revesz
et al. [30] and Gajdics et al. [43], Mg - 5 wt% Nb2O5 powder mixtures
(with or without carbon nanotubes) were processed by HPT, and the
hydrogenation properties were determined only at the relatively
high temperature of 573 K. In the present work, we have in-
corporated two catalysts materials which are well known for cata-
lytic effect on the hydrogenation/dehydrogenation of Mg and MgH2,
namely, Ni and Nb2O5. The addition of a few wt% Ni has been shown
to improve the kinetics of hydrogenation, both by acting as a catalyst
for the splitting of the hydrogen molecule [2,4,86] and also by way of
the formation of the Mg2Ni phase and, subsequently, the corre-
sponding mixed hydride Mg2NiH4, with an accelerating effect on the
kinetics of MgH2 formation and decomposition [82,83]. In turn,
niobium oxides seem to play the role of hydrogen “pumps” and
dispersing agents during the hydrogenation and dehydrogenation
processes [85,87–89]. To the extent of our knowledge, these two
additions have not been used together in materials subjected to HPT,
although Simchi et al. [90] have reported a possible synergetic effect
2

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
and pressure used in the activation stage are reported as
Supplementary material S1.
specimens for longer times (up to 4–5 h) at the maximum activation
temperature of 623 K did not change the hydrogen content in any
significant way. After that, the furnace of the equipment was turned
off and the temperature in the chamber decreased (Fig. 1a), while
the hydrogen uptake was basically unchanged because the hydrogen
gas pressure in the chamber was maintained at 2.5 MPa. After the
temperature had dropped to room temperature, the hydrogen
pressure was reduced to 0 gauge pressure and the specimen was
again heated up to 623 K (step 5). At this temperature, dehy-
drogenation occurred very rapidly (Fig. 1b), so that in about 10 min
or less the amount of hydrogen released was essentially the same as
the maximum uptake recorded during the previous hydrogena-
tion step.
This activation process is not particularly severe in terms of
pressure and temperature conditions, but it is time consuming since
it involves natural cooling. The effect of an activation process is ty-
pically explained in two ways: the first one involves the breaking up
of the surface oxides thus providing a path for hydrogen gas to
contact the bare surface of the alloy [2,9,75,86], while a second way
involves the nucleation of the hydride phase [2,86]. Both ways are
enhanced by SPD, as many reports have previously indicated [e.g.,
5,13,14,37,44,45]. In our HPT processed-materials, we observed that
the activation stage led to extensive crumbling of the Mg-Ni-Nb2O5
disks and thus to path-opening for hydrogen diffusion.
In the PCT experiments, the samples were degassed by heating to
623 K under a dynamic vacuum for a total time of 2 h. After this, the
sample was cooled down to room temperature, and the calibration
for void volume with ultrahigh purity helium was performed.
Hydrogenation (and dehydrogenation) reactions were performed by
a stepped increase (decrease) of the hydrogen pressure from 0.001 to
1.5 MPa gauge pressure (1.5–0.001 MPa). The attainment of equili-
brium conditions dictated the time employed at each step, namely,
no change in set pressure superior to 10 Pa for 250 s or a maximum
time of 240 min per step. Under these conditions, one hydrogena-
tion/dehydrogenation cycle lasted about 5–7 days. PCT cycles were
recorded at 523, 548, 573, 598, and 623 K.
The kinetics of hydrogenation were measured at 2.5 MPa (gauge)
hydrogen pressure and temperatures in the range 373–623 K. All
dehydrogenations steps were performed at 623 K and 0 MPa gauge
pressure, i.e., equal to atmospheric pressure at 1920 m above sea
level (approximately 81 kPa hydrogen pressure). This high tem-
perature was used to ensure that no remanent hydride was present
before proceeding to the next hydrogenation cycle. In the hydro-
genation part of the cycle, the first step was to fix the initial pressure
in the apparatus; then, the sample was heated up to the test tem-
perature with a heating ramp of 10 °C/min. After reaching the test
temperature, the isothermal condition was kept for 1.5–2 h, de-
pending on how fast saturation conditions were established. Unless
otherwise stated, a total of 6 cycles were performed, each at a
temperature 50 K lower than the previous one. A final hydrogenation
at 2.5 MPa and 623 K was applied after the previous cycles, and the
samples were cooled down to room temperature in their hydro-
genated state. Samples were then transferred from the Sieverts type
reactor to an argon-filled glove box having less than 10 ppm oxygen
and humidity, and without exposure to the atmosphere by utilizing a
closing valve on the sample holder. As stated above, the HPT disks
were routinely handled in air prior to their use in these experiments.
Typical results of an activation run (steps 3–4 in Table 1 in
Supplementary material S1) are shown in Fig. 1(a). As shown in this
figure, hydrogenation of the specimen started after about 15 min
into the heating stage, when the temperature in the sample chamber
was about 425 K. Above roughly 450 K a decreasing rate in the hy-
drogenation kinetics can be observed followed by a still lower rate
starting at roughly 600 K. From there on, the quantity of hydrogen in
the material steadily increased until a maximum amount was
reached (in this case, about 3.8 wt%) after about 2 h. Keeping the
Structural and compositional characterization of the materials
was performed by X-ray diffraction (XRD), scanning electron mi-
croscopy (SEM), and scanning transmission electron microscopy
(STEM) and energy-dispersive spectrometry in the SEM.
XRD was performed in a D2 Phaser diffractometer (Bruker) using
Cu Kα radiation (wavelength of 1.540598 Å). The hydrogenated ma-
terials were placed in a dedicated sample holder, in which they were
covered with a Kapton tape for protection against ambient oxygen
and moisture exposure.
For SEM-STEM-EDS examination used was made of a JSM 7800
FEG-SEM (JEOL, Ltd.) operated at different voltages. In addition to
conventional imaging with secondary and backscattered electrons,
this equipment was also operated in STEM mode at 30 kV and bright
field images were taken at different magnifications from specimens
previously obtained by Focused Ion Beam machining on selected
locations of the HPT disks. Characterization by EDS was perfomed
with a SDD detector (JEOL Ltd.) operated with the Genesis software
(EDAX Corp.).
Density determination of HPT disks was carried out according to
ASTM standard B962-17 [96].
3. Results
3.1. Processing of materials
Fig. 2(a,b) illustrate the microstructure of the mild-milled pow-
ders. The backscattered electron image in Fig. 2(a). Fig. 1(a) clearly
shows the distribution of the Ni and Nb2O5 particles on the surface
of the much larger Mg particles; some of the catalysts particles form
agglomerates (Fig. 2b) with sizes that can reach a few μm.
Fig. 1. Plots of temperature, pressure and calculated hydrogen capture/release in a
typical activation process applied in the present study. (a) Hydrogenation. (b)
Dehydrogenation.
3

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
XRD patterns from the mild-milled powder mixture and the
as-processed HPT disks show weak signals from Mg(OH)2
(Supplementary material S2).
In the consolidated HPT materials, there is clear evidence of grain
refinement, as shown in the micrographs in Fig. 3. The STEM mi-
crographs in this figure show the microstructure of the HPT material
subjected to 10 revolutions under 3 GPa at locations near the center
and the edge of the disk, respectively. At both sites, we observe
distorted grains with sizes smaller than about 1 µm, and mostly in
the 100−200 nm range. This is the type of microstructure that is
usually reported in Mg-based materials that have undergone large
plastic deformations under HPT conditions [22,31,38,39]. Notice that
torsional deformation of a solid disk is not expected to occur
homogeneously over the whole volume [22,31,93]; however, it has
been frequently found [41,42,93] that metallic disks undergoing
more than about 5 revolutions under a high pressure show clear
evidence of heavy plastic deformation all over their volume.
3.2. Hydrogenation experiments
After proper activation, we performed experiments designed to
explore the hydrogenation properties of the HPT materials. In each of
these experiments, the hydrogenation kinetics was measured con-
secutively, with a single specimen, at temperatures of 623, 573, 523,
473, 423 and 373 K, and finally again at 623 K, under 2.5 MPa of
hydrogen pressure. Within the precision of our experiments (0.1 –
0.2 wt% H2), the first and last of the hydrogenation runs at 623 K
coincided fairly well. Independently of the hydrogenation tempera-
ture, the specimens were dehydrogenated at 623 K to ensure that all
the hydrogen captured in the previous hydrogenation step was
completely released before the next hydrogenation step. Unless
otherwise indicated, this dehydrogenation step was omitted after
the last hydrogenation step at 623 K, so that the samples could be
later characterized in the hydrogenated state.
Fig. 4(a) illustrates the data obtained in one series of such hy-
drogenation cycles, in this case for a specimen previously subjected
to 10 revolutions under a pressure of 3 GPa. For comparison, Fig. 4(b)
shows the data corresponding to the mild-milled powders (no HPT
processing). In the HPT material, the maximum storage capacity is
practically invariant (5.2–5.4 wt%) for hydrogenation temperatures
from 623 down to 523 K. However, as the temperature is further
lowered, the capacity decreases too, as indicated in the summary of
results included in Table 1. In this way, at a hydrogenation tem-
perature of 473 K, the hydrogen storage capacity became about
4.6 wt% (Fig. 4a). Notice that the initial hydrogenation kinetics (the
initial slope of the curves) is very similar at all temperatures down to
423 K, with a value of the order of 0.25 wt% H/minute.
On the other hand, the mild-milled powders (Fig. 4b) display a
hydrogen storage capacity which decreases regularly with the hy-
drogenation temperature (Table 1), eventually reaching only about
0.5 wt% at 373 K. In addition, although the initial hydrogenation ki-
netics are similar at the different temperatures, their value (about
0.15 wt% H/minute) is considerably lower than the one measured for
the HPT material. The above results have been confirmed in a
number of experiments, with some variations in the maximum
hydrogen storage capacities and kinetics determined in different
specimens.
The maximum storage capacity in the HPT materials was mainly
dictated by the hydrogenation temperature, but not on the specific
cycling procedure that was followed in our tests, as illustrated by the
results shown in Fig. 4(a). In this case, the specimen was re-hydro-
genated at 623 K after the series of six previous runs at decreasing
temperatures, and it is observed that the last hydrogenation at 623 K
produced practically the same storage capacity than the first one at
the same temperature (Table 1).
Fig. 2. (a, b) SEM micrographs (BSE signal) at different magnifications of the mild-
milled Mg-Ni-Nb2O5 powder mixture. Bright dots denote Ni or Nb2O5 powders, while
the gray large particles are Mg powders. (c) SEM micrograph (BSE signal) of the
fracture surface of a consolidated HPT disk (inset). The approximate direction of the
applied pressure (3 GPa) is indicated. The exact location on the disk of the cross-
section was not recorded.
In the consolidated HPT materials, the location of the catalysts
reflects the prior position of Mg particle-to-particle contacts, as il-
lustrated in the SEM (backscattered electrons) image of the fracture
surface of an HPT disk (see inset in this figure). In this image, the
bright catalysts particles are embedded in regions having the darker
grey color; this points to a lower average atomic number in the
chemical composition of such regions compared to the ones having a
lighter tone of grey. We interpret these observations as an indication
of the existence of magnesium oxides and hydroxides on the surface
of the Mg particles before HPT consolidation since such are the ex-
pected products due to exposure of magnesium to atmospheric air.
4

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
Fig. 3. STEM micrographs from a specimen processed by HPT (10 revolutions, 3 GPa), at locations near the center and near the edge of the disk. Sections taken approximately
perpendicular to the disk surface and the approximate direction of the applied pressure is indicated by an arrow. The exact location on the disk of the cross-section was not
recorded.
Table 1
Hydrogenation data from Fig. 4(a,b). HPT sample subjected to 10 revolutions
under 3 GPa.
Temperature (K)
Max. storage capacity (wt%)
HPT specimen
Mild-milled powder
623
573
523
473
423
373
623
5.4
5.2
5.2
4.6
4.2
2.8
5.4
5.4
4.6
3.8
2.2
1.2
0.5
4.9
Fig. 5. Hydrogenation curves at 473 K (after activation at 623 K) of the HPT material
subjected to 20 revolutions under 3 GPa.
Fig. 4. Hydrogenation curves at different temperatures. (a) HPT material subjected to
10 revolutions under 3 GPa. (b) Mild-milled powder mixture.
second run at 473 K. Notice that this is basically the same value re-
ported at 473 K in Table 2 for a sample which underwent a very
different series of hydrogenation cycles.
As further evidence of the reproducible effect of temperature on
the hydrogen storage capacity, Fig. 5 shows the results of a series of
seven hydrogenation runs performed at 473 K (after the same acti-
vation stage shown in S1) on a sample previously subjected to 20
revolutions under a pressure of 3 GPa. It is seen in this figure that the
storage capacity was stabilized at a value of about 4.5 wt% from the
Table 2
Approximate quantitative phase analysis (in wt%) from the XRD patterns of Fig. 7(a,b).
Sample
MgH2
MgO
Mg
Mg2NiH4
HPT specimen
Mild-milled powder mixture
58.0%
56.4
35.5
29.0
3.0
10.5
3.5
4.1
5

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
Fig. 6. (a) PCT curves at different temperatures of the HPT material subjected to 10 revolutions under 3 GPa. (b) van’t Hoff plot of the plateau pressure versus temperature data.
3.3. PCT experiments
3.4. Phase analysis
Fig. 6(a) displays the results of PCT tests performed on HPT
material after having undergone two full cycles at 623 K. The results
in this figure were obtained from consecutive runs on the same
specimen, in descending order from the highest temperature. Due to
time constraints, the lowest temperature used in these tests was
548 K. Notice that both PCT plots corresponding to a given tem-
perature display the existence of the same plateau during the for-
mation and decomposition of MgH2, i.e., there is no significant
hysteresis between the hydrogenation and dehydrogenation pro-
cesses. Notice also that the maximum storage capacities determined
in these tests are consistent with the results of the hydrogenation
tests in Fig. 4(a).
The measured hydrogen storage capacities fall short of the value
expected (~ 7.12 wt% H) according to the nominal composition of the
material due to the existence of magnesium oxide in samples which
have undergone several hydrogenation cycles. In Fig. 7(a) and (b) we
show typical XRD patterns from the mild-milled Mg-Ni-Nb2O5
powder mixture and an HPT sample, respectively, after the sequence
of hydrogenation treatments indicated in Fig. 4. In this case, the
phases found in both hydrogenated materials after the last hydro-
genation at 623 K are MgH2, MgO, Mg, and possibly Mg2NiH4 (the
existence of a small quantity of the mixed hydride Mg2NiH4 is sug-
gested by the diffraction peak at about 39°); apparently, the amount
of Nb2O5 was too small to be detected in these hydrogenated sam-
ples. The amount of each phase, as calculated by Rietveld analysis of
the XRD data, is presented in Table 2. The main differences between
these two samples involve a slightly larger amount of MgO in the
HPT material (35.5 wt% versus 29.0 wt% in the mild-milled powders)
and a higher percentage of unreacted Mg in the hydrogenated
powder mixture (10.5 wt% versus 3.0 in the HPT sample). Notice,
however, that the quantity of each of these phases from the XRD
patterns should be taken as approximate values, since the XRD peaks
from the MgO phase are rather broad and those from the Mg2NiH4
phase are barely visible.
From the data in Fig. 6(a), plateau pressures corresponding to
different temperatures were fitted to van’t Hoff´s equation, ln (Peq
/
P0) = (∆H0/RT) – (∆S0 /R), as applied to the reaction Mg+H2 ↔ MgH2.
In this equation, Peq and P0 are, respectively, the equilibrium hy-
drogen pressures at temperature T and at the reference state (taken
as 101.3 kPa), and ∆H0 and ∆S0 are the standard enthalpy and en-
tropy changes of the reaction. The plot of ln peq versus 1/T in Fig. 6(b)
can be reasonably adjusted to a straight line, whose slope is equal to
∆H0/R, and the calculated value of ∆H0 is approximately −78.5 kJ/
moleH2. This value falls within the interval of values normally re-
ported for the reaction between Mg and H2, e.g., [97]. This result
indicates that HPT processing did not alter the thermodynamics of
MgH2 formation.
Comparison of the XRD patterns in Fig. 7(a) and (b) with the ones
from the mild-milled powder mixture and the same mixture after
HPT processing before any hydrogenation treatments (see S2), the
Fig. 7. XRD patterns of hydrogenated (a) mild-milled powder mixture and (b) HPT material (subjected to 10 revolutions under 3 GPa). The broad peaks at angles lower than about
28° are produced by the kapton film used to protect the sample from the environment.
6

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
latter with no evident signal from the MgO phase, reveals the large
fraction of magnesium (and some Mg(OH)2) converted to MgO in the
hydrogenated materials.
submicron particle sizes, concentrations of the order of 0.5–2 wt%
and a homogeneous dispersion provide considerable kinetic ad-
vantages to the hydrogen storage properties of Mg-based sys-
tems [2,4,85].
4. Discussion
As already mentioned in the Introduction, Revesz et al. [30] and
Gajdics et al. [85] investigated the effect of HPT on the hydrogen
storage properties of Mg - 5 wt% Nb2O5 powder mixtures; the former
also introduced 5 wt% of carbon nanotubes, while the latter worked
with mixtures with and without carbon nanotubes. The work re-
ported in [](30,43) involved a more energetic milling operation than
the one used in the present work, a number of revolutions in HPT
fixed at 3, and the hydrogenation properties were evaluated only at
573 K. From their observations, Gajdics et al. [43] concludes that
Nb2O5 particles act as nucleation sites for new phases and for the
recombination of hydrogen atoms.
For their Mg - 5 wt% Nb2O5 powder mixture subjected to high-
energy milling followed by HPT, Gajdics et al. [43] reported an initial
hydrogen sorption rate of 24.7 × 10−4 wt%•s−1 at 573 K; this value is very
close to the one found for our Mg - 5 wt% Ni – 2 wt% Nb2O5 powder
mixtures only subjected to mild-milling, namely, ~ 25.0 × 10−4 wt%•s−1
at the same temperature. However, it is significantly lower than the
sorption rate measured for our mild-milled + HPT samples, namely,
41.2 × 10−4 wt%•s−1 in the range of 473–623 K. This comparison between
the work of Gajdics et al. [43] and ours would seem to indicate that the
main contribution to the enhanced hydrogenation kinetics in our
samples comes from the addition of Ni, with Nb2O5 playing a
secondary role.
Transition metals, and particularly nickel, have been used for
many years as catalysts in Mg-based hydrogen storage materials
[e.g., 2,4,86,92]. Since nickel has a very low solubility in magnesium
in the solid state [e.g., 98], it is to be expected that intensive plastic
deformation (as imparted by, for example, high-energy milling or
SPD) and heating will promote the formation of Mg-Ni intermetallic
compounds, particularly Mg2Ni, in Mg-based materials. This is
indeed the case found in most of the investigations concerning
Mg-Ni mixtures or alloys intended for hydrogen storage [e.g.,
2,17,19,21,29,33,51,62,82,99,100]. Moreover, a number of investiga-
tions have presented experimental evidence of enhanced (de)hy-
drogenation kinetics in Mg-Mg2Ni mixtures [e.g., 83,100–102]. It has
been speculated [100–102] that in mixture or composite containing
a dispersion of Mg2Ni (or Mg2NiH4) in Mg (or MgH2), the lower
temperatures required for the (de)hydrogenation in the Mg2Ni –
Mg2NiH4 system serve to expedite the diffusion of hydrogen and
magnesium and promote de nucleation of MgH2 (in magnesium) or
magnesium (in MgH2).
In our initial HPT specimens, DXR reflections from Ni were clearly
visible (Supplementary material S2), indicating that its dissolution in
Mg and the formation of Mg2Ni was minimally induced by the HPT
process. However, in the DRX patterns from a mild-milled mixture
and an HPT specimen, both subjected to several hydrogenation cy-
cles, Fig. 7(a) and (b), we detected (at a diffraction angle of about
39°) what would be a pair of closely situated diffraction peaks from
the Mg2NiH4 phase [103]. Nevertheless, such meager experimental
evidence of the occurrence of this phase can be further substantiated
by the DRX pattern from a similarly treated HPT which underwent a
final dehydrogenation process, as shown in Supplementary material
S4. In the dehydrogenated material it is much easier to recognize the
XRD peaks belonging to the Mg2Ni phase. Since it is commonly ac-
cepted that the dehydrogenation of the mixed hydride Mg2NiH4
produces the Mg2Ni phase and hydrogen [103], this demonstrates
that Mg2NiH4 should be present in the hydrogenated specimens
corresponding to Fig. 7(a) and (b). Moreover, a more gradual for-
mation of Mg2Ni would be expected in HPT specimens subjected to
lower temperatures during the hydrogenation cycles, at it was the
case of the HPT specimen repeatedly hydrogenated at 473 K (Fig. 4).
In this case, we speculate that the Mg2NiH4 / Mg2Ni phases were
4.1. The effect of HPT processing
Fig. 4(a) and (b) clearly indicate that HPT processing of mild-
milled Mg-5 wt%Ni-2 wt%Nb2O5 powder mixtures resulted in faster
hydrogenation and higher storage capacity compared to the prop-
erties found in the powder mixture which only underwent the mild
milling process, particularly at temperatures lower than 623 K. The
improved hydrogenation properties have been explained [e.g.,
[5,29,51] in terms of the refinement of the grain structure and the
generation of crystalline defects (dislocations and grain boundaries)
in the severely deformed material; both hydrogen diffusivity and
nucleation rates of Mg and/or MgH2 are assumed to increase con-
siderably because of such microstructural changes. In our HPT ma-
terials, we confirmed the occurrence of grain refinement (grains
with sizes smaller than 1 µm) and high dislocation densities due to
HPT processing, as illustrated in Fig. 2. The beneficial effects of heavy
plastic deformations on the kinetics of de/hydrogenation have been
reported for many years in materials having undergone prolonged
and/or high-energy milling, which may be considered a SPD process
applied to powder material [13,14,91,92].
Some authors have suggested that among the SPD processes, HPT
may not be the most suitable in terms of processing time, i.e., HTP is
a relatively slow production process. In favor of HPT, we observed
that in our particular application, it provides pieces of material with
sizes (a few mm) very suitable to serve as the feeding charge in a
hydride tank. On the other hand, it would be important to determine
the minimum number of revolutions to be applied in an HPT process
to develop adequate hydrogen storage properties in Mg-based ma-
terials. By way of example, Edalati et al. [20] reported hydrogenation
at 300 K of a Mg-50 at%Zr alloy which was given 1000 revolutions in
an HPT operation; however, such a large number of revolutions
would be impractical on an industrial scale. In our case, we tested
HPT samples subjected to 10 or 20 revolutions and did not find a
significant difference among them. However, a thorough micro-
structural characterization of these materials is currently in progress
to address this issue in a more appropriate manner.
Our results show that HPT processing of an Mg-5 wt%Ni-2 wt%
Nb2O5 alloy makes possible hydrogenation at temperatures as low as
423 K with gravimetric storage capacities of at least 4.0 wt% H2. Such
low hydrogenation temperatures have seldom been consistently
reported for the reaction Mg + H2 ↔ MgH2., see, e.g., [4]. Moreover, it
should be noted that the manipulation of our materials has been
carried out in normal air atmosphere, with exposure times of several
months between the preparation of the powder mixtures, the pro-
cessing by HPT, and the hydrogenation tests. This has practical im-
portance, considering that Mg readily reacts with oxygen and water
vapor from the atmosphere, but it would be rather cumbersome to
avoid any exposure during its practical application as a reversible
hydrogen storage medium.
4.2. The effect of Ni and Nb2O5 additions
For many years, Nb2O5 has been considered an effective catalyst
in the (de)hydrogenation of Mg-based materials [2,85,90]. As with
most solid catalysts, there are a number of parameters which in-
fluence the catalytic activity, including particle size, degree of par-
ticle agglomeration, amount/concentration of catalyst, spatial
distribution, chemical nature and behavior, among many others.
Most of these variables have not been fully investigated in the case
of Mg - Nb2O5 systems, but the available evidence suggests that
7

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
gradually produced during the activation step (at 673 K) and the
following hydrogenation cycles 1–3. The observable changes in the
kinetic curves of Fig. 4 for the three first cycles would be related to
the increasing amount of Mg2NiH4 produced with each successive
cycle, until a point was reached when the available nickel was
bonded to Mg2Ni in the dehydrogenated state or to Mg2NiH4 in the
hydrogenated state. As more hydrogenation cycles were applied in
Fig. 4, the hydrogenation behavior showed very little differences
from cycles 4–7 (except for a slight increase in storage capacity in
cycle 7), indicative of no more changes in the amount of Mg2NiH4
being produced during these later hydrogenation cycles. Finally,
notice that the formation of Mg2Ni would occur at a faster rate with
the use of higher temperatures during the hydrogenation cycles.
The amount of nickel in the nominal composition of our mate-
rials would amount to 9.14 wt% of Mg2Ni, reducing the amount of
nominally “free” magnesium from 93.0 wt% to about 88.9 wt%. This
change would not be expected to appreciable modify the maximum
hydrogen storage capacity of our materials. Therefore, the most
relevant effect of the nickel addition would be its catalytic action
through the formation of Mg2Ni and Mg2NiH4, since pure nickel
would have disappear after a few hydrogenation cycles.
It should also be mentioned that small Nb2O5 particles may exert
a stabilizing effect on the grain size of magnesium during the hy-
drogenation cycles, as others researchers have suggested for this and
other compounds [e.g., 27,85]. The grain size refinement induced by
high-energy milling and SPD processes is considered a major cause
of the enhancement of (de)hydrogenation kinetics in many hydrogen
storage systems, including Mg-based systems [e.g., 2,4,5,39,44,77,85,
86,92,99]. However, the hydrogenation cycles may involve tem-
peratures considerably higher than room temperature, inducing the
growth of the magnesium grains or particles, and thus to the elim-
ination of the beneficial effect of grain refinement. Our Mg-Ni-Nb2O5
mixtures have been produced with fine Nb2O5 (and Ni) particles
(see, for example, Fig. 2). Their size has not been noticeably changed
by the posterior HPT processing nor the hydrogenarion treatments.
Therefore, the addition of small Nb2O5 particles in our materials
could also represent a contributor factor in their reproducible hy-
drogenation behavior.
As we mentioned above, it has been proposed that small particles
from different compounds can lead to grain size stabilization. Of
particular relevance to our work, the role of MgO should be con-
sidered. This compound was not intentionally added to our mate-
rials, but it was expected that it would appear in some quantity due
to the exposure for several months of powder mixtures and HPT
disks to the atmosphere. The results of XRD characterization (Fig. 7)
indicate the existence of a large amount of MgO phase in the ma-
terials subjected to several hydrogenation cycles. Several reports
have addressed the effect of MgO on the hydrogen storage behavior
of Mg-based materials [e.g., 2,7,104,105] and concluded that it can
act as a grain growth inhibitor when present as very small particles.
In our materials, however, we do not know to what extent this could
be true. From the SEM image in Fig. 2(c), it is not likely that most of
the MgO in the HPT disks will be present as small, well-dispersed
particles and, therefore, their retarding effect on the grain growth of
magnesium would be much diminished. Some preliminary SEM
observations (Supplementary material S5) seem to indicate that
most of the MgO phase is located as a highly fractured or pulverulent
shell surrounding Mg hydride in the hydrogenated materials.
expected from the nominal composition of our materials (Mg-5 wt%
Ni-2 wt%Nb2O5), assuming the conversion of all the magnesium into
MgH2 (i.e., 7.66 × 0.93 = 7.12 wt% hydrogen). Incidentally, it should be
noticed that similar capacity values are frequently quoted in the
literature for Mg base materials with catalysts additions and in
which the main hydride product is MgH2 [86].
The existence of a considerable amount of MgO in Mg-based
hydrogenated materials has frequently been explained as due to (i)
residuals of oxygen or water vapor remaining in, and/or leaking into,
the equipment (milling equipment or hydrogenation testing equip-
ment), (ii) its preexistence in the original Mg powders (as MgO, but
more likely, as Mg(OH)2, although they may not exist as well crys-
tallized phases), and (iii) to reaction with the atmosphere before
physical characterization [e.g., [24,27,28,34,64,75,84,92,104,105]. In
our materials, XRD did not show evidence for the existence of
crystalline oxides or hydroxides of magnesium before or after HPT
processing. However, density measurements of the HPT disks turned
out values in the range of 1.880–1.885 g/cm3 for samples subjected
to 10 – 20 revolutions (Supplementary Material S5), which should be
compared to the value of 1.83 g/cm3 calculated by the rule of mix-
tures for the Mg-5 wt%Ni-2 wt%Nb2O5 powders. Since the densities
of MgO and Mg(OH)2 (3.6 and 2.34 g/cm3, respectively) are both
higher than 1.738 g/cm3, for pure Mg, the measured density values of
the HPT material may be used to estimate their oxide or hydroxide
content prior to any hydrogenation treatment. Assuming the ex-
istence of an oxide of stoichiometric composition (and neglecting
any porosity), the measured densities would indicate a MgO content
of approximately 5.0 wt% in the HPT materials before hydrogenation;
in turn, the corresponding Mg(OH)2 content would be about 9.0 wt%.
It would be expected that such contents of MgO or Mg(OH)2 in the
HPT material could be detected by conventional XRD techniques, but
we do not find evidence of their existence, al least in such quantities,
as already mentioned. In any case, the estimates made above of the
possible contents of MgO or Mg(OH)2 in the HPT material (before
any hydrogenation treatments), which are based on the density
measurements, cannot account completely for the large MgO con-
tents calculated from the XRD data in samples subjected to a series
of hydrogenation cycles, as given in Table 2. Notice that magnesium
hydroxide would be expected to decompose during heating ac-
cording to the reaction Mg(OH)2 → MgO + H2O.
At this point, it should be mentioned that there is some dis-
crepancy between the measured values of hydrogen uptake given by
the Sievert´s technique (Table 1) and the hydrogen content de-
termined from the phase analysis by XRD (Table 2). Notice that the
comparison is only made for the last hydrogenation step, since the
XRD measurement was performed at the end of the hydrogenation
cycling. In this particular case, Table 1 indicates a hydrogen uptake in
the last hydrogenation step of 5.4 wt%, while the hydrogen content
that can be calculated from the quantitative phase analysis in Table 2
is about 4.5 wt%. We believe that this discrepancy arises mainly due
to difficulty in correctly quantifying the content of the different
phases from the XRD data. For one, the diffraction peaks from
Mg2NiH4 are barely visible in Fig. 7(a) and (b); however, from the Ni
content found in the initial powder mixtures, we would expect a
higher fraction (by a factor of 2 or 3) of this phase in the hydro-
genated material. On the other hand, it seems that the major pro-
blem in quantification comes from the MgO phase. The XRD peaks in
Fig. 7(a) and (b) are considerable wider than those of the other major
phase, MgH2, which suggests the occurrence of crystal sizes in the
nanometer scale and/or a semi-crystalline structure. Another pos-
sible source of error in the calculations could arise if some nickel is
incorporated in the MgO structure (NiO and MgO have isomorphous
structures and can form extended solid solutions). The insertion of
Ni in MgO would increase considerably the scattering power of the
oxide, resulting in an overestimation of the phase fraction as
pure MgO.
4.3. Hydrogen storage capacity
The maximum hydrogen storage capacity in our HPT materials
was limited to approximately 4.2–5.5 wt% in the temperature in-
terval of 423–623 K. At the lowest hydrogenation temperature used
in our study (373 K), the storage capacity diminished to about 3 wt%.
These values are considerably lower than the theoretical value
8

M. Osorio-García, K. Suárez-Alcántara, Y. Todaka et al.
Journal of Alloys and Compounds 878 (2021) 160309
The data in Table 1 correspond to samples having undergone the
series of cycles shown in Fig. 4(a) (in total 8 hydrogenation steps,
including the activation step described in S1). It should be noticed
that the storage capacity measured in the HPT sample in the first
hydrogenation step, held at 623 K (Fig. 4a and Table 1), is basically
the same as in the last hydrogenation at the same temperature. If the
storage capacity is limited by the amount of MgO existing in the
material, this would suggest that the formation of most of the MgO
phase already occurred during the activation stage and/or during the
first hydrogenation step at 623 K. At this point, we are not able to
explain how exactly all the MgO phase found in the hydrogenated
HPT material is produced. Ongoing experiments are being conducted
to elucidate these observations, which could be used to decrease the
amount of MgO in the hydrogenated materials and thus increase the
hydrogen storage capacity, in the form of MgH2, in our HPT
materials.
Declaration of Competing Interest
The authors declare that they have no known competing fi-
nancial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
MOG and ATO have been supported by graduate student fel-
lowships from CONACYT, México. Hydrogen storage experiments
were conducted at Instituto de Investigaciones en Materiales, UNAM,
Morelia Campus, México, while HPT processing was performed at
Toyohashi University of Technology, Japan. We also acknowledge the
use of experimental facilities at LANE-Cinvestav (SEM), CNMN-IPN
(SEM, STEM) and Universidad Politécnica de Cataluña, Spain (Prof.
J.M. Cabrera´s group). This work has been funded by CONACYT-
México (project PN 2017-6072).
5. Conclusions
Appendix A. Supporting information
High-pressure torsion of mild-milled Mg-5Ni-2Nb2O5 powder
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.jallcom.2021.160309.
•
mixtures has provided with a bulk, ultrafine-grained solid ma-
terial with improved hydrogenation properties. A hydrogen sto-
rage capacity of about 4.0–5.5 wt% was maintained in the
temperature interval 423–623 K with hydrogenation rates of
approximately 0.25 wt% H2 per minute even after several cycles
of (de)hydrogenation.
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