of 4
Ni
Mo Nanopowders for E
ffi
cient Electrochemical Hydrogen
Evolution
James R. McKone, Bryce F. Sadtler, Caroline A. Werlang, Nathan S. Lewis, and Harry B. Gray
*
Division of Chemistry and Chemical Engineering, the Kavli Nanoscience Institute, and the Beckman Institute, California Institute of
Technology, 1200 E. California Blvd., Pasadena, California 91125, United States
*
S
Supporting Information
ABSTRACT:
Earth-abundant metals are attractive alternatives to the noble
metal composite catalysts that are used in water electrolyzers based on
proton-exchange memb
rane technology. Ni
Mo alloys have been
previously developed for the hydrogen evolution reaction (HER), but
synthesis methods to date have been limited to formation of catalyst
coatings directly on a substrate. We report a method for generating
unsupported nanopowders of Ni
Mo, which can be suspended in common
solvents and cast onto arbitrary substrates. The mass-speci
fi
c catalytic
activity under alkaline conditions approaches that of the most active
reported non-noble HER catalysts, and the coatings display good stability
under alkaline conditions. We have also estimated turnover frequencies per
surface atom at various overpotentials and conclude that the activity enhancement for Ni
Mo relative to pure Ni is due to a
combination of increased surface area and increased fundamental catalytic activity.
KEYWORDS:
nickel, molybdenum, Ni
Mo, hydrogen evolution, HER, cathode, electrolysis, stability
A
lkaline electrolysis is an attractive alternative to proton-
exchange membrane (PEM)-based electrolysis because
the non-noble metal electrocatalysts involved in alkaline
electrolysis are stable and exhibit relatively high activity for
both the hydrogen evolution reaction (HER) and the oxygen-
evolution reaction (OER). Alkaline electrolyzers generally use
Ni-based materials, or steel, as the electrodes and/or electro-
catalysts.
1
Speci
fi
cally, Ni
Mo alloys exhibit high activity and
long-term stability as HER catalysts under alkaline con-
ditions.
2
5
Ni
Mo composites have also recently been mixed
with other elements, such as zinc or nitrogen, to provide
enhanced HER activity and/or stability under neutral or acidic
conditions.
6,7
Electrocatalysts used in modern PEM-based fuel cell and
electrolysis systems are generally synthesized as powders or
colloids, and often supported on a porous, conductive matrix
such as carbon black.
8,9
This mode of synthesis permits facile
processing and attachment of the electrocatalyst to suitable
substrates, such as metallic current-collectors or ion-exchange
membranes. Synthesis of highly processable powders also
facilitates assessment of the maximum attainable mass-speci
fi
c
catalytic activity of the electrocatalyst of interest. In contrast,
the electrochemical behavior of Ni
Mo alloys has generally
been investigated by the synthesis of the active electrocatalyst
directly onto an electrode substrate,
2
5,10
20
confounding
direct characterization of the morphology, composition, and
activity of the electrocatalytically active species.
We describe a method for synthesizing unsupported Ni
Mo
nanopowders that exhibit high catalytic activity for the HER.
The powder is readily processed into colloidal inks and can be
deposited in desired loadings onto substrates. The composition,
morphology, catalytic activity, and stability during hydrogen
evolution of such Ni
Mo nanopowders under aqueous acidic
and alkaline conditions have been evaluated in detail.
A two-step precipitation/reduction process was used to
prepare the Ni
Mo nanopowders (Figure 1; also see
Supporting Information). First, an aqueous solution of nickel
hexammine and ammonium molybdate was prepared at a 6:4
mol ratio of Ni to Mo. Then the mixture was rapidly heated in
diethylene glycol, leading to precipitation of a green mixed Ni
Mo oxide powder. The oxide powder was recovered by
centrifugation, dried, and subsequently reduced under forming
Received:
October 26, 2012
Revised:
November 30, 2012
Published:
December 3, 2012
Figure 1.
Synthetic scheme for generating Ni
Mo nanopowder,
involving an initial precipitation step and subsequent heat treatment
under a reducing atmosphere.
Letter
pubs.acs.org/acscatalysis
© 2012 American Chemical Society
166
dx.doi.org/10.1021/cs300691m
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ACSCatal.
2013, 3, 166
169
gas to generate a black Ni
Mo nanopowder. The initial
precipitation was also successfully performed with water as the
solvent, in which case the method closely resembled the
synthesis of a known ammonium nickel molybdate.
21
Figure 2 shows scanning electron microcope (SEM) and
transmission electron microscope (TEM) images of the Ni
Mo oxide and Ni
Mo nanopowders. The Ni
Mo oxide
exhibited a polydisperse nanoparticulate morphology, with
particle sizes ranging from 50 to 300 nm. The oxide also
exhibited low crystallinity by electron di
ff
raction (Supporting
Information). Upon reduction, the powder retained a nano-
particulate morphology, but exhibited increased crystallinity
(Supporting Information) and porosity (Figure 2d). The
increase in porosity is consistent with the volume contraction
expected upon reduction of the oxidized intermediate to the
putatively metallic Ni
Mo product. Energy dispersive spectro-
scopic analysis integrated in the SEM indicated that, for
powders that had
40% Mo content, the Ni:Mo atomic ratios
in the powders generally were in accord with the Ni:Mo atomic
ratios in the precursor solutions. For precursor solutions with
>40% Mo content, however, the resulting catalyst contained a
lower proportion of Mo than the precursor solution
(Supporting Information).
The hydrogen evolution activity and electrochemical stability
of these Ni
Mo nanopowders were evaluated under alkaline
and acidic conditions by dispersing the Ni
Mo in isopropanol
followed by deposition of the particles onto clean Ti electrodes.
The highest catalytic activities were observed after a second
reduction step had been performed on the deposited
fi
lms,
presumably because of the tendency of the nanopowders to
form surface oxides in air. Indeed, the powders were pyrophoric
in air, and were generally kept wet with water or solvent until
catalytic
fi
lms were generated.
Figure 3 shows the room temperature polarization data
under alkaline conditions for various catalysts, including
fi
lms of
Ni
Mo nanopowders on Ti. For Ni
Mo loadings of 1 mg
cm
2
, <100 mV overpotential,
η
, was required to sustain
cathodic current densities in excess of 10 mA cm
2
. These
activities greatly exceeded those exhibited by the Ti substrate,
by smooth Ni electrodes, or by Ni nanopowder
fi
lms that had
been generated by the same precipitation-reduction. Ni
Mo
fi
lms exhibited similarly high catalytic activities under alkaline
conditions for Mo contents ranging from 10 to 50%
(Supporting Information).
The stability of the deposited Ni
Mo electrocatyst
fi
lms was
evaluated under acidic and alkaline conditions by galvanostatic
control of the electrodes at a current density
J
=
20 mA cm
2
(Figure 4). Under alkaline conditions, the overpotential at
J
=
20 mA cm
2
was stable for 100 h, and in fact
η
decreased
Figure 2.
Scanning electron micrographs (left) and transmission
electron micrographs (right) of Ni
Mo oxide intermediate (a,b) and
Ni
Mo nanopowders (c,d).
Figure 3.
Comparison of HER catalytic activities for various electrodes
in 1 M NaOH solution. The counter electrode was a Ni mesh, and the
reference was Hg/HgO (1 M NaOH). Potentials are reported versus
the thermodynamic potential for hydrogen evolution, which was
measured after the experiments using a clean Pt electrode. The data
are labeled as follows: (a) Ti foil substrate; (b) Smooth Ni wire; (c) Ni
nanopowder on Ti foil (1 mg cm
2
); (d) Ni
Mo nanopowder on Ti
foil (1 mg cm
2
).
Figure 4.
Potential vs log(time) plot for Ni
Mo nanopowder
fi
lms on
Ti electrodes with the noted mass loadings in the noted electrolytes.
Electrodes were poised galvanostatically at
20 mA cm
2
.
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2013, 3, 166
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167
approximately linearly with log(time) over the time period.
This aging e
ff
ect, which has been observed previously, was
attributed to the dissolution of residual molybdenum not
incorporated into an alloy phase in the electrocatalyst.
2,4
Under acidic conditions, the
η
required to pass
20 mA
cm
2
was also initially <100 mV. However, in acid,
η
increased
(linearly) with log(time), and the performance degraded
rapidly after
7 h under galvanostatic conditions. This behavior
is in accord with expectations for a continuous, slow corrosion
of the catalyst coating, which eventually led to dissolution of a
large fraction of the electrocatalytic material. Greatly enhanced
stability in acidic media has been reported recently for Ni
Mo
materials mixed with carbon and nitrogen,
6
albeit at much
higher overpotentials to obtain similar
J
values to those
reported herein. The hydrogen evolution activity data for this
and several recently reported catalyst systems are compared in
Table 1.
Figure 5 shows the current densities produced at constant
overpotentials of 100 and 200 mV, respectively, by various Ni
Mo loadings on Ti. The relationship between the mass loading
and the current density was well described by a power law, for
example,
J
=
14.45
m
0.86
at
η
= 100 mV, where
J
is the current
density in mA cm
2
and
m
is the mass loading in mg cm
2
(blue line in Figure 5). The observed power law is consistent
with attenuation of the marginal increase in activity with
increased mass loading due to diminished transport of reactant
species through porous
fi
lms of increasing thickness.
For comparison, current densities
J
=
1000 mA cm
2
have
been observed at
η
= 80 mV for highly optimized Ni
Mo
electrodes at
20 mg cm
2
mass loading and at 70
°
Cin30wt
% KOH(aq).
5
The mesh geometry, electrolyte concentration,
and increased temperature presumably all contributed to the
very high observed activity for this Ni
Mo cathode. In the
present system, mass loadings >10 mg cm
2
were di
ffi
cult to
obtain because of poor adhesion of such nanoparticle
fi
lms to
the Ti substrate. The adhesion could likely be improved,
however, by the use of polymer binders or mesh substrates that
have similar surface redox chemistry and thermal expansion
characteristics as the catalytic coating.
Measurement of mass-speci
fi
c catalytic activities enables an
estimation of the activity of Ni
Mo nanopowders on a per-
surface-atom basis, given a series of approximations regarding
particle composition and morphology. The total surface area of
0.1 mg of spherical, 5 nm diameter particles with a density of
9.5 g mol
1
(based on the weighted-average density of 6:4 Ni/
Mo metals) is
130 cm
2
, implying that the roughness factor,
γ
,
for a 0.1 mg cm
2
sample is
130. Assuming that the
nanoparticle surfaces exhibit the weighted-average lattice
constants of their bulk Ni and Mo components, 0.1 mg of
material contains 0.4
μ
mol of surface atoms. Hence for
fi
lms
with low mass loading, the turnover frequencies can be
estimated as 0.05 s
1
at
η
= 100 mV and 0.36 s
1
at
η
= 200
mV.
22
Commercially available samples of pure Ni, as well as of
metallurgically prepared Ni
Mo alloys with Mo loadings of 1,
4, and 12%, respectively, were also evaluated electrochemically
for their activities toward the HER. Samples of these materials
were cut and polished to produce a smooth surface, chemically
etched to remove surface polishing damage (Supporting
Information), and tested for electrocatalytic hydrogen evolution
activity under alkaline conditions (Figure 6). The data showed
a clear monotonic trend, in which increasing Mo loading
resulted in decreased overpotentials required to obtain a
speci
fi
ed current density. Notably, assuming surface roughness
factors of 1, the data for 0 and 12% Mo imply turnover
frequencies at
η
= 100 mV of 0.03 and 0.2 s
1
per surface atom,
respectively. These values are within an order of magnitude of
those estimated for the Ni
Mo nanopowders, and support the
notion that alloying Mo into Ni increases the fundamental
hydrogen evolution activity of Ni metal. This further implies
that both enhanced fundamental reactivity and increased
surface area are operative in the observed catalytic activity of
Ni
Mo nanopowder.
Table 1. Collected HER Catalysis Data
catalyst
loading (mg cm
2
)
electrolyte
temperature (
°
C)
η
(mV)
J
(mA cm
2
)
reference
Ni
Mo nanopowder
1.0
2 M KOH
25
70
20
this work
Ni
Mo nanopowder
3.0
0.5 M H
2
SO
4
25
80
20
this work
Ni
Mo nanopowder
13.4
2 M KOH
25
100
130
this work
Ni
Mo on Ni
20
30 wt % KOH
70
80
1000
Brown and Mahmood
5
Ni
Mo on Ni
40
1 M KOH
40
110
400
Xiao, et al.
23
Ni
Mo nitride nanosheets
0.25
0.1 M HClO
4
25
200
3.5
Chen, et al.
6
Pt on Carbon
0.28
0.5 M H
2
SO
4
25
50
20
Li, et al.
24
amorphous MoS
x
(10
17
sites cm
2
)
0.5 M H
2
SO
4
25
200
10
Benck, et al.
25
MoS
2
on reduced graphene oxide
0.28
0.5 M H
2
SO
4
25
150
10
Li, et al.
24
MoS
x
on graphene-coated Ni foam
8
0.5 M H
2
SO
4
25
200
45
Chang, et al.
26
Figure 5.
Current density versus mass loading (log
log plot) for Ti
substrates coated with various quantities of Ni
Mo nanopowder at the
noted overpotentials in 2 M KOH solution. The associated lines are
fi
t
to the data by power laws attributed to attenuation of increased
catalytic activity with increased mass loading. Open and closed circles
are data from two di
ff
erent sets of electrodes.
ACS Catalysis
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ACSCatal.
2013, 3, 166
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168
In conclusion, unsupported nanopowders of Ni
Mo alloy
can be readily prepared and processed into
fi
lms that have
various mass loadings. The resulting non-noble electrocatalysts
exhibit high mass-speci
fi
c activities for the HER under alkaline
conditions. The coatings are very stable under hydrogen
evolution conditions in alkaline electrolytes, but degrade after
operation for a few hours under acidic conditions. The
exceptionally high activity is due in part to high porosity in
the
fi
lms, but Ni
Mo nanopowders also exhibit enhanced per-
surface-atom activity as compared with Ni, as corroborated by
electrochemical polarization measurements on well-de
fi
ned
samples of metallurgical alloys.
ASSOCIATED CONTENT
*
S
Supporting Information
Materials, synthetic methods, and characterization details are
included as Supporting Information. Additionally included is a
spreadsheet (Excel format) demonstrating the calculations for
turnover frequency per surface atom. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*
E-mail: hbgray@caltech.edu.
Notes
The authors declare no competing
fi
nancial interest.
ACKNOWLEDGMENTS
This work was supported by the National Science Foundation
Powering the Planet
Center for Chemical Innovation (CHE-
0802907). We thank Carol M. Garland for assistance with
transmission electron microscopy. J.R.M. acknowledges the
Department of Energy, O
ffi
ce of Science for a graduate research
fellowship. C.A.W. thanks the Caltech Summer Undergraduate
Research Fellowship (SURF) program for support during the
summer of 2012. B.F.S. acknowledges the Beckman Institute
for a postdoctoral fellowship.
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ACS Catalysis
Letter
dx.doi.org/10.1021/cs300691m
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ACSCatal.
2013, 3, 166
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169