Supp
lementary
Information for:
ELECTRO
CATALYSIS OF THE
HYDROGEN
-
EVOLUTION
REACTION BY
ELECTRODEPOSITED
AMORPHOUS
COBALT SELENIDE
FILMS
A
ZHAR
I.
C
ARIM
1
,
F
ADL
H.
S
AADI
2
,
3
,
M
ANUEL
P.
S
ORIAGA
3,
6
AND
N
ATHAN
S.
L
EWIS
1
,
3
-
5
1
Division of Chemistry and Chemical Engineering
2
Division of Engineering and Applied Sciences
3
The Joint Center for Artificial Photosynthesis
4
Beckman Institute
5
Kavli Nanoscience Institute
California Institute of Technology
Pasadena, CA 91125
6
Department of Chemistry
Texas A&M University
College Station, TX 7784
3
*Corresponding Author:
nslewis@caltech.edu
Electronic
Supplementary
Material
(ESI)
for
Journal
of
Materials
Chemistry
A.
This
journal
is
©
The
Royal
Society
of
Chemistry
2014
S
2
S1.
Contents
Th
is
document
contains
detailed
description
s
of the experimental
procedure
s
used in this
work
(Section S2),
X
-
ray diffraction data (Section S3) and
X
-
ray photoelectron spectra
(Section
S4)
for electrochemically prepared cobalt selenide film
s
,
calculations of the turnover frequencies
of such films for the electrochemical hydrogen
-
evolution reaction (Section S5)
and a list of
associated references (Sect
ion S6)
.
S
3
S2.
Experimental Methods
Materials and Chemicals
:
H
2
(g) (
99.999%
,
Air Liquide
),
H
2
SO
4
(
J.T. Baker, A.C.S.
Reagent
),
Ti
foil (99.
7+
%, 0.
127
mm thick,
Sigma
-
Aldrich
),
S
eO
2
(99.999%, Acros Organics),
Co(C
2
H
3
O
2
)
2
·
4
H
2
O
(
98%, Strem Chemicals
) and
LiCl
(
≥
99.0%, Sigma
-
Aldrich
) were used as
received
.
H
2
O with a resistivity
≥
18.2 MΩ cm
-
1
(Barnsted
Nanopure
System
) was used
throughout.
Cobalt Selenide
Electrodeposition
:
Cobalt selenide films were
prepared on Ti substrates
via
electrodeposition.
The
sealed, single
-
compartment electrochemical cell
equipped
was
with a
graphite
-
rod counter
electrode
(Alfa Aesar,
99%
)
and
a
Ag/AgCl reference electrode
(3 M KCl;
Bioanalytical Systems
)
that were collectively
controlled by a Bio
-
Logic SP
-
200 potentiostat
.
Squares
~
2 cm
x
2
cm
in dimension
were cut from the Ti foil and
were then
sealed into a
n
O
-
ring compression
cell
that
confined
the
contact
region
between the electrolyte
, an aqueous
solution of
0.065
M Co(C
2
H
3
O
2
)
2
,
0.035
M SeO
2
and
0.200
M LiCl (pH = 4.7),
and the Ti foil
,
to
a
circular area
of 0.1 cm
2
.
An e
xternal e
lectrical contact to the
Ti foil
was made using an
alligator clip
.
Electrodeposition was effected by potentiostatically biasing the Ti at
a potential
of
-
0.45 V vs.
the
Ag/AgCl
reference electrode
for 8 h
at room temperature
.
After deposition,
the Ti
foil pieces were
removed from the compression cell
and
rinsed
first with 0.5
00
M H
2
SO
4
and
then
with
H
2
O
.
T
he area of the Ti substrate
that had not been
covered by the
electrodeposited
cobalt selenide
was
then covered
with nitrocellulose
-
based nail polish
, to provide
electrical
insulation
.
Electrochemical
Conditioning
and
Analysis
:
A single
-
compartment cell
equipped
with a
graphite
-
rod counter electrode and a saturated
calomel electrode
(SCE
; CH Instruments
)
and
controlled by a Bio
-
Logic SP
-
200 potentiostat was used for conditioning and analysis of the
S
4
electrodeposited
material
.
An O
-
ring compression seal was u
sed
to
mount the
Ti
substrate
that
support
ed
the electrodeposit
ed material
in the cell
.
All
experiments
were performed
using an
aqueous
solution of
0.5
00
M H
2
SO
4
that was continuously sparged
with H
2
(g) and stirr
ed
using
a
magnetic stir bar. The potential of
a
reversible hydrogen electrode (RHE)
rel
ative
to the SCE was
determined by measuring the potential of
a
Pt foil
(
which
was
annealed in a
H
2
-
air flame
immediately prior to use)
in the
H
2
(g)
-
saturated
0.5
00
M H
2
SO
4
electrolyte
.
All
quoted
potentials
are
referenced against RHE unless otherwise
noted.
Prior to voltammetric experiments and
physical characterization, e
lectrodes were preconditioned by
galvanostatic
electrolysis
at a
current density of
-
10 mA cm
-
2
for 1 h
.
The CoSe mass loading was determined to be 3 mg cm
-
2
.
The u
ncompensated cell
resistance (
R
u
) was determined
from
a
single
-
point electrochemical
impedance
measurement obtained by applying a sine
-
wave modulated potential with amplitude
of 20 mV at a modulation frequency of
100 kHz
centered at the open
-
circuit potential of the cell
.
A
l
l
subsequent
measurements
were
corrected for an uncompensated resistance of 85%
of the
value of
R
u
.
Voltammetric data w
ere
recorded at a scan rate of 1 mV s
-
1
.
The e
lectrochemical
stability
of the cobalt selenide films
was assessed
using both galvanostatic and
accelerated
degradation
technique
s
.
First, a current density of
-
10 mA cm
-
2
was
maintained
galvanostatically
and the electrode potential was monitored over the course of 1
6
h. Separ
ately, the
cobalt
selenide films
w
ere
subjected to 1000 full potential cycles between
-
0.175 V and 0.1 V vs. RHE
at a sweep rate of 50 mV s
-
1
.
Voltammograms
at a sweep rate of 1 mV s
-
1
were recorded before
cycling
and after
completion of
the 1000
full potential
cycles
.
Physical
Characterization
:
Scanning electron microscopy (SEM) was conducted
using
a
FEI Nova
NanoSEM
450
at an accelerating voltage of
1
5 kV
with a working distance of 5 mm
and
an in
-
lens secondary electron detector
.
Energy
-
dispersive X
-
ray spectroscopy was perfor
med
S
5
in the SEM at
a
working distance of 12 mm
,
using an
accelerating voltage of 15 kV and an
Oxford Instruments silicon drift detector
.
Raman spectra were obtained with a Renishaw inVia
spectrometer equipped wit
h a Leica
DM 2500M
microscope, a
Leica
N Plan
5
0x objective
(numerical aperture =
0.75
), a
n
1800 lines
mm
-
1
grating
,
and a CCD detector
in a 180 ̊
backscatter geometry
.
A
532 nm diode
-
pumped solid state (DPSS) laser (Renishaw RL532C50)
was used as the excitation source
and
a radiant
flux
of
20
μW
was
incident on the sample
.
X
-
ray
diffraction
(XRD)
p
owder
patterns
were acquired with a Bruker D8
Discover
diffractometer
equipped with a Cu Kα source and a
2
-
dimensional Vantec
detector
.
X
-
ray photoelectron spectra
(XPS)
were acquired with
a Kratos Axis Nova
spectrometer at a base pressure of 10
-
9
torr with
monochromatic Al Kα excitation at 1486.7 eV
.
High
-
resolution spectra were obtained
using
a
pass energy of 40 eV
.
CasaXPS software (CASA Ltd)
was used to fit peaks in the XP spectra
,
and peak fitting
was pe
rformed
assuming a Shir
l
ey background and symmetric Voigt
line
-
shapes
comprised of Gaussian (70%) and Lorentzian (30%) functions. The peak fitting was constrained
to maintain both a 2:3 ratio between the areas of the Se 3d
3/2
and Se 3d
5/2
lines and
to maintain
a
0.85 eV separation between the binding
energies
of these two lines.
S
6
S
3
.
X
-
r
ay Diffraction
25
45
65
85
105
25
45
65
85
105
2
/ degrees
Figure S1
. T
op: Representative
X
-
ray diffraction pattern collected
from a cobalt selenide film
.
Bottom:
Standard lines for
polycrystalline Ti (JCPDS 65
-
9622).
S
7
S
4
.
X
-
ray Photoelectron Spectroscopy
Figure S2 shows high resolution X
-
ray photoelectron spectra of
an
electrochemically
prepared cobalt selenide film in the
Co 2p (a) and Se 3d (b)
regions. The presence of multiple
intensity maxima in the region between ~ 790 and 780 eV in Figure S2a may be due to the
presence of oxidized cobalt species that formed due to air oxidation of the cobalt selenide film
surface prior to loading into the spe
ctrometer. XPS interrogates only the first several nanometers
of the material surface. However, the Raman spectrum (Figure 1b), representative of a much
larger depth, did not indicate the presence of any oxygenated cobalt species.
S
8
Figure S2
. Represent
ative high resolution X
-
ray photoelectron
spectra of an electrochemically prepared cobalt selenide film in the
(a) Co 2p and (b) Se 3d regions.
810
800
790
780
Binding Energy / eV
58
56
54
52
Binding Energy / eV
Se
0
Se
-2
Fit
S
9
S
5
.
Calculation of Turnover Frequencies
Turnover frequencies of the cobalt selenide films for the hydrogen
-
evolution reaction
were calculated
the method
previously reported for an amorphous mo
lybdenum sulfide catalyst
film.
1
The number of surface sites
per unit area
was estimated by calculating the area
occupied
by a CoSe unit in freboldite
to determine the number of surface sites for a planar material
and
then scaling this value
by
the roughness factor of the cobalt selenide film
(13
, determined via
electrochemical capacitance measurements)
. The selenium
-
selenium distance in frebdolite is
0.361 nm. Based on the arrangement of selenium atoms in the frebdolite basal plane, each Se
atom, and thus each CoSe unit, occupies
0.13 nm
2
.
Thus,
freboldite has
approximately 8 x 10
14
surface sites per cm
2
in the basal plane
.
From this, it is estimated that the cobalt selenide films
had approximately 1 x 10
16
surface sites per cm
2
. The
total
nu
mber of hydrogen evolution
turnovers was
analytically derived
from the current density
via the conversion factor
3.1
x 10
15
H
2
s
-
1
mA
-
1
. The turnover frequency is then the
total
number of turnovers divided by the number
of active sites.
Figure S3 presents
a plot of t
urnover frequency as function of
hydrogen
-
evolution
reaction overpotential derived from the data in Figure 2a.
S
10
0.0
0.1
0.2
0.3
0
20
40
60
80
Turnover Frequency / s
-1
HER Overpotential / V
Figure S3
. Turnover frequency
of cobalt selenide films as function
of overpotential for the hydrogen
-
ev
olution reaction in 0.500 M
H
2
SO
4
saturated with H
2
(g) derived from the voltammetric data in
Figure 2a.
S
11
S
6
. References
1.
J. D. Benck, Z. Chen, L. Y. Kuritzky, A. J. Forman and T. F. Jaramillo,
ACS Catal.
,
2012, 2, 1916
-
1923.