Crystalline Nickel, Cobalt, and Manganese Antimonates as
Electrocatalysts
for the
Chlorine Evolution Reaction
Ivan A. Moreno-Hernandez
1
, Bruce
S. Brunschwig
2
, Nathan S.
Lewis
1,2,3
1
Division
of Chemistry and Chemical
Engineering,
127-72, California Institute of Technology,
Pasadena, CA
91125, USA
2
Beckman Institute Molecular Materials
Research
Center, California Institute of Technology,
Pasadena, CA
91125, USA
3
Kavli Nanoscience Institute, California Institute of Technology, Pasadena,
CA 91125, USA
*Correspondence
to: nslewis@caltech.edu
S1
Electronic
Supplementary
Material
(ESI)
for
Energy
&
Environmental
Science.
This
journal
is
©
The
Royal
Society
of
Chemistry
2019
Supplementary Information
Materials and Methods
Chemicals
All chemicals
were used as received, including antimony(III)
chloride
(SbCl
3
, Alfa
Aesar, ACS,
99.0% min), tin(IV) chloride hydrate (SnCl
4
·xH
2
O,
Alfa Aesar, 98%),
sodium
chloride (NaCl, Macron Chemicals, ACS grade), potassium iodide
(KI, EMD Millipore, ACS
grade), sodium thiosulfate pentahydrate
(Na
2
S
2
O
3
, Alfa Aesar,
ACS
grade), 1.0 M
hydrochloric
acid (1.0
M HCl(aq), Fluka Analytical), multielement standard
solution 1 for ICP (Sigma
Aldrich, TraceCERT), sulfuric acid (H
2
SO
4
(aq), Fischer Scientific, TraceMetal
grade, 93-98%),
sodium hydroxide (NaOH, Macron
Chemicals,
ACS
grade), antimony standard for ICP
(Sigma
Aldrich, TraceCERT), potassium chloride
(KCl, Macron Chemicals, ACS grade),
and gallium-
indium eutectic (Alfa Aesar, 99.99%). Deionized water with
a resistivity of 18.2
MΩ-cm
was
obtained from
a Millipore deionized water system.
Sample Preparation
A previously
described
spray
pyrolysis
procedure
was
used
to
deposit
conductive
films
of
antimony-doped
tin
oxide
(ATO).
1,
2
The
process
consisted
of
spraying
a 0.24
M
SnCl
4
solution
in
ethanol
doped
with
3 mol%
SbCl
2
onto
a quartz
microscope
slide
heated
at 550
°C
on
a hot
plate.
The
thickness
of
the
ATO
film
was
adjusted
by
controlling
the
duration
of
the
spray.
ATO
films
with
a sheet
resistance
of
5- 10
Ω
sq
-1
,
as
determined
from
four-point
probe
measurements,
were used
for subsequent
experiments.
Metallic
films
of
Ni,
Co,
Mn,
Sb,
NiSb
2
,
CoSb
2
,
and
MnSb
2
were
deposited
onto
the
ATO
substrates
with
an AJA
Orion
sputtering
system.
The
ATO
substrates
were
partially
S2
covered
with
Kapton
tape
to
prevent
complete
coverage
of
the
ATO
with
the
catalyst
films,
to
form
a direct
contact
between
the
ATO
and
the
working
electrode
wire.
The
metallic
films
were
co-sputtered
from
four
metallic
targets
in
an
Ar
plasma:
Antimony
(ACI
Alloys,
99.95%),
Nickel
(ACI
Alloys,
99.95%),
Cobalt
(ACI
Alloys,
99.95%),
and
Manganese
(ACI
Alloys
99.95%).
The
chamber
pressure
was
< 10
-7
Torr
prior
to
the
depositions.
A
chamber
pressure
of
5 mTorr
was
sustained
during
the
depositions
with
an
Ar
flow
rate
of
20
sccm.
The
samples
were
not
intentionally
heated
during
the
deposition
process.
The
power
applied
to
the
metal
targets
was
varied
to
obtain
similar
transition
metal
loadings
and
a stoichiometry
close
to 2:1
Sb:M
in
MSb
x
samples.
The
actual
stoichiometry
and
loading
of
Ni,
Co,
Mn,
and
Sb
was
determined
by
dissolving
in
1.0
M
H
2
SO
4
(aq)
films
deposited
on
glass,
and
then
using
the
concentration
of
the
metals
as determined
by
ICP-MS,
the
areas
of
the
samples
dissolved,
and
the
amount
of
H
2
SO
4
(aq) used during the dissolution to
obtain
the total loading.
After
the
metal
films
were
deposited
on
ATO,
the
films
were
annealed
in
a muffle
furnace
(Thermolyne
F48020-80)
to
form
the
crystalline
oxides.
2
Unless
otherwise
specified,
the
temperature
was
increased
to
750
°C
at
a ramp
rate
of
10
°C
min
-1
,
was
held
at
750
°C
for
6 h,
and
then
allowed
to
return
to
room
temperature
without
active
cooling.
RuTiO
x
films
with
the
same
molar
loading
(~
1.5
μmol
cm
-2
)
as
the
MSb
2
O
x
films
were
prepared
by
drop
casting
4
μL
cm
-2
of a 0.11
M RuCl
3
and
0.26
M
TiCl
4
solution
in
ethanol
onto
ATO,
followed
by
drying
on
a
hot
plate
at
400
°C.
3
The
RuTiO
x
was
annealed
at
500
°C
for
1 h in a muffle
furnace.
3
The
samples
were
cleaved
into
pieces
that
had
exposed
ATO
regions,
and
In-Ga
eutectic
was
scribed
on
the
ATO.
The
electrode
support
consisted
of
a tinned
Cu
wire
that
was
threaded
through
a
glass
tube.
The
Cu
wire
was
coiled
and
bonded
to
the
ATO
substrate
by
use
of
Ag
paint
(SPI,
Inc).
The
contact
was
allowed
to
dry
for
at
least
2 h at
room
temperature
or
for
15
min
at
85
°C
S3
in
an
oven.
Hysol
9460
epoxy
was
used
to
insulate
the
Cu,
ATO,
and
In-Ga
from
the
electrolyte
and
to define
the
geometric
electrode
area.
The
epoxy
was
allowed
to
cure
for
> 12
h at
room
temperature
or
for
2 h at 85
°C
in
an
oven.
The
electrode
area
and
a calibration
ruler
was
imaged
with
an optical
scanner
(Epson
Perfection
V360),
and
the
electrode
area
was
quantified
with
ImageJ software. Electrode areas were between 1 and 40 mm
2
unless otherwise specified.
Materials Characterization
X-ray diffraction (XRD) data
were
collected
with a Bruker
D8
Discover
instrument.
The
Cu
Kα
(1.54
Å) x-ray beam was
generated
with a tube current of
1000
μA
and
a tube voltage of
50 kV, and was detected with a Vantec-500 2-dimensional detector. The incident beam
was
collimated with
a 0.5 mm diameter mono-capillary
collimator. A
calibrated visible
laser
was
used to align
the
sample with the x-ray
beam. XRD data were collected in coupled
θ-2θ
mode,
with four scans collected every 20°
from
a
2θ
theta
range of 20°
– 80°. The x-ray radiation was
collected for 1
h for each
scan, corresponding
to 4 h per sample. The 2-dimensional
signal
was
integrated to obtain a 1-dimensional scan
with an angular resolution
of 0.01°
2θ.
The
x-ray
diffraction peaks
were
analyzed using Bruker
EVA software with
reference
patterns of SnO
2
for
the ATO substrate, in addition to reference patterns
for
monoclinic
Sb
2
O
4
, orthorhombic Sb
2
O
4
,
NiSb
2
O
6
, CoSb
2
O
6
, MnSb
2
O
6
, RuO
2
and TiO
2
obtained from the Crystallography Open Database
or literature.
4, 5
Scanning-electron microscopy
(SEM) images were
collected
using immersion
mode with an accelerating voltage of 10 kV on a
Nova
nanoSEM 450 (FEI)
instrument.
X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS)
scans were collected using a
Kratos
Axis
NOVA (Kratos Analytical, Manchester, UK) at a background pressure of <10
-9
Torr.
The x-ray
S4
source consisted of a monochromatic Al
kα
beam with
an
energy
of 1486.6 eV. Survey scans
were collected at
1.0 eV
resolution, and high-resolution scans were collected at 0.05 eV
resolution. The binding energy of the
scans was corrected against the adventitious C 1s peak
with
a constant offset to obtain an adventitious
C 1s peak energy of 284.8 eV.
The M 2p spectra of Ni,
Co, and Mn were
fit
using
previously reported fitting parameters.
6
The
reported peak separations,
FWHM ratios, and
relative peak
areas
were
used to fit the collected M
2p spectra. However, in
most cases the
peak shapes could
not be fit adequately without shifting the peaks towards more
positive binding energies.
Since MSb
2
O
x
samples are chemically
different than MO
x
or M(OH)
2
samples, we tentatively assign the shifted
peak shapes to
M(II)
in a MSb
2
O
6
lattice. For example,
while the peak position of NiSb
2
O
x
is similar to other Ni(II) species,
the peak shape could not
be
adequately fit with Ni 2p spectra of Ni oxide or hydroxide species. We introduced
an
addition
peak shape that consisted
of the Ni(OH)
2
spectra shifted 1.0 eV more
positive.
7
We tentatively
assign this
peak
shape to
Ni(II) in the
NiSb
2
O
6
lattice. The
XP spectrum
of Sb
3d
3/2
was used to
determine the oxidation state of the
surface
Sb on MSb
2
O
x
samples. Literature
values of Sb 3d
3/2
peak binding energies
for oxidation states of
3
+
, 3
+
/5
+
, and
5
+
are 539.5 eV,
540.1 eV, and
540.4
eV respectively, for
a C 1s peak
binding energy
of 284.8
eV.
8
Electrochemical Testing
NaCl was used to make 4.0 M aqueous
solutions, and 1 M HCl(aq) was
used to
adjust the
pH to 2 as
measured by a
pH
probe. A saturated calomel
electrode (SCE) was calibrated with a
normal hydrogen
electrode
(NHE). The NHE consisted
of a
platinum
disk (CH
Instruments)
submerged in a
H
2
saturated 1.0 M
sulfuric acid
electrolyte, with H
2
(g)
bubbled underneath
the
Pt disk
to ensure saturation.
The
potential
of
the SCE
was
0.244 V vs. NHE. Electrochemical
measurements were
collected
in a
two-compartment
cell with the compartments
separated using
S5
a Nafion N424
membrane. The cathode compartment was
filled
with
0.1 M NaOH(aq),
and
the
anode compartment was filled with 4.0 M NaCl(aq) adjusted to pH = 2 with HCl(aq).
After 48 h
chronopotentiometry experiments, the pH of
the electrolyte was usually 2.05 – 2.10. The OER
acidifies solutions, and
the observations are consistent with
the observed
increase in
pH
arising
from minor
leakage of NaOH through the
Nafion
N424 membrane. The electrolyte was replaced
after 48 h to prevent the pH from increasing. The
working
and reference electrodes were placed
in the anode compartment, and the
counter
electrode
was placed in
the cathode compartment.
The working, reference,
and counter electrodes
consisted of the sample, an
SCE, and a
carbon
rod or Ni wire, respectively. The
anode compartment was saturated with Cl
2
(aq) by applying
~
10 V for at least 30 min between the counter electrode and a second
working electrode that
consisted of a
graphite
rod.
The
saturation
of the
electrolyte with
Cl
2
(aq)
did
not substantially
affect the activity of
the
electrocatalysts
or
the pH of the electrolyte.
However,
this step was
implemented
to establish and maintain
a well-defined potential based on the Nernst equation,
which requires that Cl
2
(aq) is
present in the electrolyte. Cyclic
voltammograms were collected at
a scan
rate
of 10 mV s
-1
unless
otherwise specified. Electrochemical data were collected using a
digital
potentiostat (SP-200, Bio-Logic). The thermodynamic potential for
chlorine evolution
was calculated to be 1.331 V vs. NHE in 4.0 M NaCl(aq).
9
The roughness
factor (RF) of the TMAs was determined by comparing the
electrochemically active surface area
of bare ATO substrates and TMAs, as determined
from
impedance measurements. Impedance
measurements
were collected in 4.0 M NaCl(aq)
adjusted
to pH = 2 with
the
electrolyte additionally
saturated
with Cl
2
(aq).
Electrodes
were
held at
1.660
V vs. NHE for 15 s
prior
to impedance measurements, which were collected at the same potential
with a frequency range of 20 Hz – 20 kHz, with a sinusoidal wave amplitude of 10 mV. The
S6
impedance data were
fit
with a circuit model consisting of a resistor
in series
with a parallel
component consisting of a constant phase
element
and another resistor.
10
The capacitance was
obtained by using a
formula previously reported
for the analysis of this
circuit.
10
The
formula
is
shown below:
퐶
퐷퐿
=
[
푄
0
(
1
푅
푠
+
1
푅
퐶푇
)
(
푎
‒
1
)
]
1
푎
Where
Q
0
and
a
are
the
parameters associated with the constant phase
element,
R
s
is the series
resistance,
R
ct
is the
charge-transfer resistance, and
C
DL
is the determined double-layer
capacitance. The impedance data
were
fit using EC-Lab software by constraining all variables to
positive values, and
using the Randomize +
Simplex method
for
at least
10,000 iterations. The
fitting process was repeated at
least four times to ensure that
the
best
fit was obtained. Table S5
shows examples
of impedance data
collected
for the
electrocatalysts
studied herein. ATO
substrates prepared by a spray deposition method
have previously been determined from atomic
force microscopy measurements
to have
a RF
= 1.32.
2
The
capacitance
of ATO electrodes was
determined with
impedance measurements, and divided by the projected
area of the electrodes to
determine the geometric-area normalized capacitance. The geometric-area normalized
capacitance of ATO was 14.4 ± 1.6
μF
cm
-2
, which corresponds to an electrochemical surface
area normalized capacitance of 11 ± 1
μF
cm
ox
-2
after
dividing
by the RF
= 1.32 of ATO.
The
roughness factor of the TMAs was determined by dividing the geometric-area
normalized
capacitance of the
TMAs by the electrochemical surface area normalized capacitance
of ATO
(11
μF
cm
ox
-2
).
Inductively-coupled plasma mass spectrometry
S7
An Agilent 8000 Triple
Quadrupole
Inductively Coupled
Plasma Mass Spectrometer
(ICP-MS) system was used to determine the
concentration of various ions in aqueous samples.
Calibration solutions were prepared
by diluting antimony and
multielement
standard solutions
(Sigma Aldrich) with 18.2
MΩ
cm resistivity water. The concentration
of various
ions was
determined from a linear fit of the counts per second
of each standard solution versus
the
known
concentration. The mass loading of the
TMAs
was
determined
by depositing
the MSb
2
(M =
Ni,
Co, Mn) layers on glass slides that
were
then
cut into ~ 1 cm
2
pieces. The projected area
of the
pieces
was
determined with a calibrated
optical scanner and
ImageJ
software. The MSb
2
layers
were dissolved
in 10 mL of 1.0 M H
2
SO
4
(aq)
for >
100 h, and samples from
these solutions
were
diluted with water and analyzed with ICP-MS.
The loading of the
catalyst
layer was determined
using the concentration of
M and Sb, the volume of 1.0
M H
2
SO
4
(aq), and the projected area of
the MSb
2
layers.
The dissolution
of
species
from TMAs films under chlorine evolution
conditions was determined
by collecting
40
μL
samples of electrolyte from
a cell operating
at
100 mA cm
-2
with an
initial 5 mL volume of 4.0 M NaCl(aq), pH
= 2 electrolyte
in the anode
compartment, and
diluting
these
samples to 5 mL with 18.2
MΩ
cm resistivity water. For
RuTiO
x
samples, 1 mL
of the electrolyte
was collected from a 7 mL cell, and electrolyte
was
replenished after every sample was taken. The 1 mL samples were diluted to
10 mL with 18.2
MΩ
cm
resistivity water. The dissolution studies for RuTiO
x
could
only be conducted for ~ 20 h,
because the expected Ru dissolution product,
RuO
4
, is a volatile compound that
escapes the
anode compartment in conjunction
with the evolved Cl
2
(g),
resulting
in an
underestimate
of the
Ru dissolution rate.
11
ICP-MS measurements of Ru in the
collected samples after 48 h of the
initial measurements verified the
volatility of the dissolved Ru. The amount of M and Sb lost
was determined from the concentration,
volume of the cell, and electrode area. The amount of M
S8
and Sb removed
from the cell after each sample
was collected was
taken into account when
determining the amount of metals
lost
over
time during chronopotentiometry measurements.
Chlorine Faradaic
Efficiency and Oxygen
Evolution Reaction Activity
The faradaic efficiency towards chlorine evolution was determined
using an established
iodometric titration technique.
12-14
A two-compartment cell separated by a Nafion
N424
membrane and with an 8 mL anode compartment
was used for this
study. The anode
compartment
was completely filled
with
4.0 M NaCl(aq), pH
= 2 electrolyte.
Electrodes
consisting of RuTiO
x
or TMAs were operated at 100 mA cm
-2
for
10 minutes. The electrolyte
was transferred to a 25 mL Erlenmeyer flask containing 0.3 g of KI, and 0.2 mL of glacial acetic
acid was
added to the
solution. The
resulting yellow solution was titrated with
0.01 M
NaS
2
O
3
(aq) using a
10 mL burette,
and
starch
solution
was
added near
the endpoint.
This
titration method requires 2 mol of NaS
2
O
3
(aq) per mol of Cl
2
(aq). The moles of Cl
2
expected was
calculated using the charge passed during
the galvanostatic measurement, Faraday’s constant
(F,
96485.3389 C mol
-1
), and
the electrons
required
to obtain
Cl
2
(2 mol e
-
per
mol Cl
2
). The
faradaic efficiency was determined by comparing the moles of Cl
2
(aq) detected to
the
moles of
Cl
2
expected.
Measurements were
also collected
for 4.0 M NaCl(aq), pH =2 electrolyte that
had
not been used for electrochemical measurements
as a blank. At least three measurements were
collected per
electrode. In these experiments, some minor
nucleation
of bubbles
on the
epoxy
used to encapsulate the electrodes
and
the Teflon adapter used seal the electrochemical cell
was
observed. Since only dissolved species are detected by the iodometric
technique,
the
faradaic
efficiency measurements represent
a lower limit on the Faradaic efficiency of the electrocatalysts
studied herein.
The generation of
Cl
2
(aq)
was also verified with colorimetric
measurements using
N,N-diethyl-p-phenylenediamine. The
high activity
towards chlorine evolution relative to
S9
oxygen evolution was also verified by collecting
cyclic voltammograms of RuTiO
x
and
TMAs in
pH = 2 H
2
SO
4
(aq) electrolyte.
S10
Supplementary Information Figures
Figure S1.
X-ray diffraction of as-synthesized
NiSb
2
O
x
films on quartz
and ATO, and
NiSb
2
O
x
films after electrochemical operation
in 4.0 M NaCl(aq), pH
= 2.0 electrolyte
at 100 mA cm
-2
for
65 h.
S11
Figure S2.
X-ray diffraction of as-synthesized
CoSb
2
O
x
films
on quartz and ATO, and
CoSb
2
O
x
films after electrochemical operation
in 4.0 M NaCl(aq), pH
= 2.0 electrolyte
at 100 mA cm
-2
for
90 h.
Figure S3.
X-ray diffraction of as-synthesized
MnSb
2
O
x
films on quartz and
ATO, and
MnSb
2
O
x
films
after electrochemical
operation in 4.0
M
NaCl(aq), pH = 2.0 electrolyte at 100
mA cm
-2
for 90 h.
S12
Figure S4.
X-ray diffraction of as-synthesized
RuTiO
x
films on ATO, and
RuTiO
x
films after
electrochemical operation in 4.0 M NaCl(aq),
pH
= 2.0 electrolyte at 100 mA cm
-2
for 90 h.
Figure S5.
Scanning-electron
microscope image of NiSb
2
O
x
: a) before operation, b) after 65 h at
100 mA cm
-2
in 4.0 M
NaCl(aq), pH = 2.0 electrolyte.
S13
Figure S6.
Scanning-electron
microscope image of CoSb
2
O
x
: a) before operation, b) after 90 h at
100 mA cm
-2
in 4.0 M
NaCl(aq), pH = 2.0 electrolyte.
Figure S7.
Scanning-electron
microscope image of MnSb
2
O
x
:
a) before
operation,
b) after 90 h
at 100 mA cm
-2
in 4.0 M NaCl(aq), pH = 2.0 electrolyte.
S14
Figure S8.
Scanning-electron
microscope image of RuTiO
x
: a) before
operation,
b) after 90 h at
100 mA cm
-2
in 4.0 M
NaCl(aq), pH = 2.0 electrolyte.
Figure S9.
Roughness factor determined
from impedance data
collected
at 1 h intervals between
chronopotentiomery stability
tests at
100 mA cm
-2
for a)
NiSb
2
O
x
, b) CoSb
2
O
x
, c) MnSb
2
O
x
, and
S15
d) RuTiO
x
. Comparison between initial
impedance
data and impedance model
fit for e) NiSb
2
O
x
,
f)
CoSb
2
O
x
, g) MnSb
2
O
x
, and h) RuTiO
x
.
Figure S10.
Amount of elements
dissolved from MSb
2
O
x
and
RuTiO
x
electrodes operated at 100
mA cm
-2
in 4.0
M
NaCl,
pH =
2.0 electrolyte:
a)
NiSb
2
O
x
, b) CoSb
2
O
x
, c) MnSb
2
O
x
, d) RuTiO
x
.
S16
Figure S11.
a)
Chronopotentiometry of ATO and
SbO
x
at 100 mA cm
-2
.
b) Chronopotentiometry
of NiO
x
, CoO
x
, and
MnO
x
at 100 mA cm
-2
.
S17