Electrocatalysis of the hydrogen-evolution
reaction by electrodeposited amorphous cobalt
selenide
fi
lms
†
Azhar I. Carim,
a
Fadl H. Saadi,
bc
Manuel P. Soriaga
cf
and Nathan S. Lewis
*
acde
Using an electrochemical method under ambient conditions, crystal-
lographically amorphous
fi
lms of cobalt selenide have been deposited
from aqueous solution onto planar Ti supports. These
fi
lms have been
evaluated as electrocatalysts for the hydrogen-evolution reaction. In
0.500 M H
2
SO
4
, the cobalt selenide
fi
lms required an overpotential of
135 mV to drive the hydrogen-evolution reaction at a benchmark
current density of
10 mA cm
2
. Galvanostatic measurements indi-
cated stability of the electrocatalytic
fi
lms for >16 h of continuous
operation at
10 mA cm
2
. The facile preparation method, and the
activity of the cobalt selenide
fi
lms, suggest that electrodeposited
metal chalcogenides are potentially attractive earth-abundant elec-
trocatalysts for the hydrogen-evolution reaction.
The development of a technology capable of e
ffi
ciently
producing molecular hydrogen from water is a critical step
toward the realization of a sustainable, renewable, and carbon
–
neutral source of energy. The electrochemical half-reaction for
this process, known as the hydrogen-evolution reaction (HER),
typically exhibits sluggish kinetics at most cathodes, but is
e
ff
ectively catalyzed by noble metals such as Pt and Pd, which
provide high cathodic current densities at modest over-
potentials.
1,2
Earth-abundant materials capable of catalyzing
the HER are being vigorously sought to reduce the cost and
increase the scalability of water-splitting systems.
3
–
6
Catalysts
that operate e
ff
ectively in acidic aqueous solutions are of
particular interest to these e
ff
orts because acidic electrolytes are
compatible with existing proton-exchange membranes and
minimize the e
ffi
ciency losses that can result from the forma-
tion of a pH gradient across the membrane.
7,8
E
ff
ective catalysis of the HER in alkaline media has been
demonstrated using earth-abundant transition metal alloys,
most notably Ni-based alloys such as Ni
–
Mo. These highly active
materials o
en reach a current density of
10 mA cm
2
,a
benchmark value for photoelectrochemical water-splitting
systems,
3
with overpotentials <100 mV at pH
$
14, but are
typically unstable in acidic media due to corrosive dissolution,
at least when anodic current is passed through the material.
6,9
–
12
Ni
–
Mo
–
N, Co
0.6
Mo
1.4
N
2
, MoB, Mo
2
C, Ni
2
P, Ni
5
P
4
and CoP have
exhibited stable catalysis of the HER in strongly acidic aqueous
electrolytes, but of these materials only the phosphides have
been demonstrated to produce current densities of
10 mA
cm
2
with overpotentials <200 mV.
13
–
18
Recently, transition
metal chalcogenide materials, the prototypical example of
which is MoS
2
, have attracted signi
cant attention as HER
electrocatalysts in acidic media.
19,20
Accordingly, signi
cant
e
ff
ort has been devoted to optimizing and engineering MoS
2
,
and to a lesser extent MoSe
2
,WS
2
and WSe
2
, to improve their
catalytic performance, resulting in overpotentials of <200 mV
for e
ff
ecting HER current densities of
10 mA cm
2
for certain
preparations of MoS
2
.
21
–
24
These e
ff
orts include maximization of
the areal exposure of active crystallographic sites;
21,22,25,26
deposition of the active material onto structured (non-planar)
supports;
21,26,27
developing improvements in the conductivity of
the materials;
21,22,27
and alloying with other metals to promote
intrinsic activity.
28,29
The binary Co chalcogenides have not been investigated as
thoroughly as their Mo analogues for catalysis of the HER, but
several catalytically active preparations have been reported.
30
–
33
Nanocrystalline thin
lms of CoS
2
and CoSe
2
synthesized by
treating metallic Co
lms with S vapor at 550
C or Se vapor at
450
C have achieved a current density of
4mAcm
2
at
overpotentials of
220 mV and 200 mV, respectively, in N
2
-
dearated 0.5 M H
2
SO
4
. Nanoparticle
lms of CoSe
2
fabricated by
a
Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA 91125, USA. E-mail: nslewis@caltech.edu
b
Division of Engineering and Applied Sciences, California Institute of Technology,
Pasadena, CA 91125, USA
c
The Joint Center for Arti
cial Photosynthesis, California Institute of Technology,
Pasadena, CA 91125, USA
d
Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA
e
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125,
USA
f
Department of Chemistry, Texas A&M University, College Station, TX 77843, USA
†
Electronic supplementary information (ESI) available: Detailed experimental
methods, X-ray di
ff
raction data, X-ray photoelectron spectra, calculation of
turnover frequencies. See DOI: 10.1039/c4ta02611j
Cite this:
J.Mater.Chem.A
,2014,
2
,
13835
Received 25th May 2014
Accepted 11th June 2014
DOI: 10.1039/c4ta02611j
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treating CoO nanoparticles (initially formed by pyrolysis of an
ink containing Co(NO
3
)
$
6H
2
O at 600
C) with Se vapor at 450
C
required an overpotential of
210 mV to e
ff
ect a current density
of
10 mA cm
2
in 0.5 M H
2
SO
4
. Amorphous
lms of Co
–
S,
composed of Co and S in a
1 : 1.4 atomic ratio, produced
via
electrodeposition followed by thermal annealing at 300
C
required an overpotential of
160 mV to reach
10 mA cm
2
in
phosphate bu
ff
er at pH
¼
7, but rapidly deactivated in 0.5 M
H
2
SO
4
. Ni/NiO/CoSe
2
nanocomposites fabricated utilizing
several hydrothermal, solvothermal and annealing steps at
temperatures between 180 and 250
C have been shown to reach
10 mA cm
2
with an overpotential <100 mV in Ar-saturated 0.5
MH
2
SO
4
, but are unstable in such media. Methods that employ
solution-based techniques for catalyst synthesis and assembly
under ambient conditions are preferable to high temperature
and/or vapor-based techniques because solution-based tech-
niques require relatively low energy inputs and provide
increased compatibility with substrates. We describe herein an
electrochemical method for the preparation of catalytically
active, amorphous cobalt selenide
lms, from fully oxidized
precursors in aqueous solution under ambient conditions.
The cobalt selenide
lms were electrodeposited from an
aqueous solution of Co(C
2
H
3
O
2
)
2
and SeO
2
using potentiostatic
electrolysis, and were then conditioned in 0.500 M H
2
SO
4
using
galvanostatic electrolysis in the HER regime (see the ESI
†
for
detailed Experimental methods). The cobalt selenide
lms were
characterized by scanning electron microscopy (SEM), energy-
dispersive X-ray spectroscopy (EDS), Raman spectroscopy,
powder X-ray di
ff
raction (XRD) and X-ray photoelectron spec-
troscopy (XPS).
34,35
Fig. 1a shows a scanning electron micro-
graph of an electrochemically prepared
lm. The
lms were
mesoporous and contained pores estimated to be
20 nm in
diameter. Measurements made using EDS indicated a Co : Se
atomic ratio of 1 : 2. The Raman spectrum (Fig. 1b) displayed no
signatures of crystalline CoSe
2
nor of Se,
36,37
but contained a
broad band centered at
233 cm
1
indicative of the stretching
of Se
–
Se bonds in glassy selenium comprised of close-packed
chains in which the local microscopic structure resembles that
of crystalline, trigonal Se.
38
The Raman spectrum also exhibited
a less intense, lower frequency feature, centered at
166 cm
1
,
consistent with expectations for the bending of the Se
–
Se bonds
in the close-packed chains as well as the presence of disordered
CoSe.
39,40
No signals were observed from oxygenated cobalt
species.
41
–
43
The Raman spectrum was consistent with the XRD
pattern obtained for the
lm (Fig. S1
†
), which only exhibited
re
ections that corresponded to the Ti substrate and did not
show re
ections that corresponded to any crystalline cobalt
selenide species. High-resolution XPS analysis of the Co 2p
region (Fig. S2a
†
) displayed an intense Co 2p
3/2
line at 781.5 eV
which was not consistent with expectations for CoSe
2
.
44
XPS
analysis of the Se 3d region (Fig. S2b
†
) revealed the presence of
Se in both the Se
0
and Se
2
oxidation states. Thus, the cumu-
lative data suggest that the
lms were composed of disordered
CoSe in a polymeric Se matrix.
Fig. 2a presents the cathodic polarization behavior of a
cobalt selenide
lm, as well as that of a bare titanium substrate,
in a three-electrode voltammetric con
guration in an aqueous
solution of 0.500 M H
2
SO
4
that was saturated with 1 atm of
H
2
(g). The overpotentials required to produce current densities
of
10 mA cm
2
and
25 mA cm
2
at a cobalt selenide
lm
were 135
2 mV and 160
3 mV (mean
s.d.), respectively,
based on six independently prepared
lms. The Ti substrate
exhibited negligible current densities throughout the interro-
gated potential window. Ti is an ine
ff
ective HER catalyst and
thus provides a clean background for interrogating the HER
activity of other materials. Fig. 2b presents a Tafel plot (over-
potential
versus
log[|current density|]) derived from the vol-
tammetric data of Fig. 2a. A Tafel slope of 62 mV dec
1
and an
exchange current density of 6.4
10
2
mA cm
2
were derived
from the region between 110 and 180 mV in overpotential. The
62 mV dec
1
Tafel slope is inconsistent with the predicted
values for the HER in acidic solution at room temperature
characteristic of the Volmer (
120 mV dec
1
), Heyrovsky
(
40 mV dec
1
) or Tafel (
30 mV dec
1
) elementary reac-
tions.
45,46
Slopes of
60 mV dec
1
may be observed when either
the Heyrovsky or the Tafel reaction is rate-limiting if the
adsorption of hydrogen atoms onto the electrode requires an
activation energy.
47
The
60 mV dec
1
Tafel slope is consistent
with the value observed for several amorphous molybdenum
sul
de catalysts.
34,35
Turnover frequencies (TOFs) exhibited by
the cobalt selenide
lms for the HER were derived from the
voltammetric data by a method previously reported for an
Fig. 1
Representative (a) scanning electron micrograph and (b) Raman
spectrum of an electrochemically prepared cobalt selenide
fi
lm.
13836
|
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,2014,
2
,13835
–
13839
This journal is © The Royal Society of Chemistry 2014
Journal of Materials Chemistry A
Communication
Published on 24 July 2014. Downloaded by California Institute of Technology on 18/09/2014 15:49:55.
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amorphous molybdenum sul
de
lm (see the ESI
†
for calcula-
tion details).
35
A plot of the TOF as a function of HER over-
potential is presented in Fig. S3.
†
TOF values were estimated to
be 1 s
1
at an overpotential of 100 mV and 5 s
1
at an over-
potential of 150 mV.
The stability of the as-prepared CoSe
lms during electro-
chemical hydrogen evolutio
n was assessed by two indepen-
dent experimental protocols. First, a current density of
10
mA cm
2
was maintained galvanostat
ically and the electrode
potential was monitored as a function of time (Fig. 3a). Over
16 h of continuous operation, the overpotential required to
maintain this current density increased by
5mV.Separately,
an accelerated degradation study
21,32
was performed wherein
the electrode potential was swept repeatedly between 0.100
and
0.175 V
vs.
RHE for 1000 full cycles. Voltammograms
recorded before and a
er cycling (Fig. 3b) revealed that the
overpotential required to achieve current densities of
10 mA
cm
2
and
25 mA cm
2
increased by less than 5 mV in both
cases. The catalytic stability of the CoSe
lms during the HER
in 0.5 M H
2
SO
4
is consistent with that displayed by poly-
crystalline cobalt diselenide
lmspreparedbytreating
metallic cobalt
lms with Se vapor, but is in contrast to that
displayed by electrode
posited amorphous Co
–
S
lms which
quickly dissolve under these conditions.
31,32
These observa-
tions are consistent with the fact that in solutions with pH < 4,
Se is cathodically stable until at least
0.4 V
vs.
RHE, whereas
the reduction of S to H
2
S is thermodynamically possible at
potentials >0 V
vs.
RHE.
48
The
135 mV overpotential for e
ff
ecting current densities of
10 mA cm
2
in 0.500 M H
2
SO
4
exhibited by the cobalt selenide
lms described in this work compares well to the values for
other stable and highly-active catalyst materials in acidic media
that are composed of Earth-abundant elements. Defective and
disordered ultrathin nanosheets of MoS
2
as well as nanosheets
of the 1T-MoS
2
polymorph have been demonstrated to achieve
10 mA cm
2
with overpotentials in the range of 180
–
190 mV.
22
–
24
MoS
2
/reduced graphene oxide composites have
reached current densities of
10 mA cm
2
with an over-
potential of
160 mV.
21
Ni
2
P nanoparticles have been shown to
drive current densities of
10 mA cm
2
with an overpotential of
120 mV.
16
Notably, the cobalt selenide
lms described herein
were prepared entirely at room temperature and ambient
conditions,
via
a facile electrochemical method. Also, as
synthesized, the amorphous Se matrix containing the CoSe may
not act as a good electrical conductor, so the activity reported in
this work may represent a lower bound on the true electro-
catalytic activity of the material.
49
Additionally, high-surface
area substrates were not utilized to improve the geometric
current densities of the CoSe
lms in this work. Mo
2
C sup-
ported on carbon nanotubes and amorphous MoS
x
coated on
N-doped carbon nanotubes have been shown to e
ff
ect current
densities of
10 mA cm
2
with overpotentials of
150 mV and
110 mV, respectively.
50,51
Similarly, nanoparticles of CoSe
2
have
achieved
10 mA cm
2
with an overpotential of 137 mV when
deposited on a high-surface area, porous, three-dimensional
Fig. 2
(a) Cathodic polarization curves of titanium foil and of a cobalt
selenide
fi
lm in 0.500 M H
2
SO
4
saturated with H
2
(g) (inset highlights
behavior at low overpotentials). (b) Tafel plot derived from data in (a).
Fig. 3
(a) Potential applied to a cobalt selenide
fi
lm as a function of
time during galvanostatic electrolysis at
10 mA cm
2
in 0.500 M
H
2
SO
4
(b) Polarization curves of a cobalt selenide
fi
lm acquired before
and after 1000 accelerated full potential cycles between 0.100 V and
0.175 V
vs.
RHE.
This journal is © The Royal Society of Chemistry 2014
J.Mater.Chem.A
,2014,
2
, 13835
–
13839 |
13837
Communication
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electrode (carbon
ber paper).
33
Fabrication of a similar struc-
tured composite with electrochemically prepared cobalt sele-
nide may thus lead to improved performance. Similarly,
deposition with dynamic potential-time waveforms, as well as
alloying with Fe or Ni as has been done to promote the activity
of molybdenum sul
de, could also prove e
ff
ective for increasing
the activity of cobalt selenide-based materials.
28,29
In conclusion, cobalt selenide
lms prepared by a simple
electrochemical method under ambient conditions have
demonstrated signi
cant electrocatalytic activity and stability
for catalysis of the HER in strongly acidic aqueous solution.
This work suggests that electrochemically synthesized metal
chalcogenide materials are promising catalysts for applications
involving the HER, especially for applications involving the
direct production of fuels from sunlight in solar-driven water-
splitting systems.
5,52
Moreover, investigations of deposition on
structured supports, complex potential-time waveforms for
electrodeposition, and alloying with other transition metals, as
have been demonstrated for MoS
2
, may result in further
enhancements of the HER activity of such materials.
27,34
Acknowledgements
This material is based in part upon work performed by the Joint
Center for Arti
cial Photosynthesis, a DOE Energy Innovation
Hub, supported through the O
ffi
ce of Science of the U. S.
Department of Energy under Award Number DE-SC0004993.
The authors gratefully acknowledge Dr S. Mitrovic for assistance
with X-ray photoelectron spectra acquisition, R. Gerhart for
assistance with cell fabrication and Dr J. H. Baricuatro for
insightful discussions. AIC recognizes a Graduate Research
Fellowship from the National Science Foundation for support.
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