of 14
S
1
Supporting Information
for
:
Catalysis of the oxygen
-
evolution reaction in 1.0 M sulfuric acid by manganese
antimonate
films synthesized via chemical vapor deposition
Jacqueline A. Dowling
†,
, Zachary P. Ifkovits
†,
, Azhar I. Carim
†,‡
,
J
ake
M. Evans
,
Madeleine C.
Swint
, Alexander Z. Ye
, Matthias H. Richter
, Anna X. L
i
, and Nathan S. Lewis
†,‡,
*
Division
of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Beckman
Institute,
California Institute of Technology, Pasadena, California 91125, United States
J. A. D. and Z. P. I. contributed equally.
*
E
mail:
nslewis@caltech.edu
Experimental Methods
Materials and Chemicals
The c
hemical vapor deposition (CVD) precursor
,
bis(ethylcyclopentadienyl)manganese
(98%
-
Mn, STREM) (Mn(EtCp)
2
)
,
was
preloaded in a CVD cylinder
and
used as received.
The
a
tomic layer deposition (ALD) precursor tris(
dimethylamino)antimony (99.99%
-
Sb, STREM)
(TDMA
-
Sb) was loaded
into a new ALD cylinder in a glovebox under N
2
(g)
. TEC 8 fluorine
-
doped tin oxide (FTO, Sigma Aldrich) substrates were used as received. The electrolyte, H
2
SO
4
(
95.0
-
98.0%
,
ACS reagent grade,
JT Baker
)
was diluted to 1
.0
M using 18.2 M
Ω
cm resistivity
water obtained from a Thermo Scientific Nanopure system. In
-
Ga eutectic (99.99%, metals basis,
Alfa Aesar)
,
PELCO conductive Ag paint (Ted Pella, Inc)
, and
Loctite
epoxy
(
EA 9460
)
were used
as received. O
2
(g)
(ultra
-
high purity grade, Airgas) was used to produce O
3
(g)
,
which was used as
a co
-
reactant in chemical vapor deposition. O
2
(g)
(industrial
grade
, Airgas)
was used to
purge the
electrolyte solution during durability experiments.
12.1 M
HCl and
15.6 M
HNO
3
(
ACS
reagent
grade
,
Sigma Aldrich
) were used to clean glassware.
S
2
Sample preparation
Chemical vapor deposition was
performed
using a
Cambridge Nanotech
Savannah S200
Atomic Layer Deposition (ALD) System. After
the
precursors and co
-
reactants were pulsed
,
a
waiting period
allow
ed
the ALD chamber (set at 150 °C) to return to
the
base vacuum pressure
(~0.5 torr
)
at
a
20 s
c
cm N
2
flow rate
. P
recursor cylinder jackets were heated
as noted
(
Table S
1
)
.
A
fter consecutive
rinsing
with isopropyl alcohol, acetone, and H
2
O
,
and drying with N
2
(g)
,
the
TEC8 fluorine
-
doped tin oxide (FTO) substrates were loaded into the ALD chamber
.
P
rior to
deposition, a glass slide was positioned
in
the ALD chamber
on top of the FTO substrate
.
The slide
covered
part of the surface,
allowing for
subsequent
top
-
facing electrical contact directly to the
substrate during
the
fabrication
of electrodes
(Figure S
1
).
To
anneal the
samples, the t
emperature
of
a
Thermolyne muffle furnace
was
increased
from room temperature to 600 °C at a rate of 10 °C
min
-
1
,
and then
was
held for 6 h
at
600 °C
before
being allowed to return
to room temperature.
Table S
1
.
Chemical vapor deposition recipe for Mn
0.63
Sb
0.37
O
x
.
Figure S
1
.
Electrode preparation schematic.
A
glass slide partially cover
ed
the FTO substrate
to
allow
for
subsequent
electrical contact directly to
the
substrate.
S
3
A
nnealed Mn
0.63
Sb
0.37
O
x
samples were cleaved
,
and
In
-
Ga
was
scribed directly onto the
bare FTO substrate
that had been
covered in the ALD chamber
by a glass slide
(Figure S
1
a).
The
sample was affixed to
tinned Cu wire
using
Pelco conductive
Ag
paint (Ted Pella, Inc.)
,
and the
assembly was then
dried in
an
oven
for 1 h
at 95 °
C
.
E
xcept
for
a small area where the catalyst
was exposed
,
the entire electrode
was encased in epoxy
,
and
the entire electrode was
dried in
an
oven
for
~
8 h
at 95 °C. The
exposed
catalyst area (Figure S
1
d) was measured with an optical
scanner (Epson perfection V360)
,
and the
geometric
area
(
0.10
-
0.15 c
m
2
)
was quantified
using
ImageJ software.
Electrochemical
measurements
Electrochemical analysis was performed using a
B
iologic SP
-
200 potentiostat
with
EC
-
Lab software.
Experiments were performed in a
two
-
compartment, three
-
electrode configuration
in a
50 mL pyrex flask.
The potential of the
SCE reference
electrode
(
CHI150, CH Instruments
)
relative
to
a
reversible hydrogen
electrode (
RHE) was measured by
monitoring the
voltage
difference between the SCE and
an
unannealed
Pt mesh electrode
in 1
.0
M H
2
SO
4
(aq)
that was
exposed to a continuous stream
of
H
2
(g)
.
The
Ti
-
mesh counter electrode was isolated from the
working and reference electrodes using a porous glass frit (porosity of 10
-
20 μm, Ace glass). All
glassware
,
as well as the Pt mesh
,
were
cleaned by
immersion
for several
h
ours
in a freshly
prepared 3:1
(
by volume
)
solution
of
12
.1 M
HCl
(aq)
and
15.6
M
HNO
3
(aq)
,
followed by rinsing
with H
2
O before
use
for electrochemistry.
During all
experiments, the 1
.0
M H
2
SO
4
(aq)
electrolyte
solution (50 mL) was continuously stirred
and maintained at room temperature
. Water
-
saturated
O
2
(g)
was
continuously bubbled into the
solution during electrochemistry
.
The electrochemical durability experiment was
performed
under galvanostatic conditions
(
J
=
10 mA cm
-
2
) for seven 24 h intervals (168 h total).
All electrochemical data
are
presented
S
4
using
the IUPAC con
vention
. At 24 h intervals during durability tests at 10 mA cm
-
2
, three
voltammetr
ic
cycles
were
recorded
at a scan rate of
ν
= 40
mV s
-
1
,
with an initial potential,
E
0
=
1.04 V vs. RHE and
an
upper
-
limit potential of
E
1
= 1.94 V vs. RHE. Impedance data were
collected at open circuit (~1.4 V vs RHE)
by applying a
10 mV
amplitude
sinusoidal wave
and
measuring
10 data points per decade at frequencies that ranged from 10 Hz to 10 kHz. Impedance
data were fit to a
modified
Randles
circuit
(
R
1
+
Q
2
/
R
2
) with EC
-
lab software to determine the
solution resistance (
R
1
)
,
the
capacitance
of a
constant
-
phase element
(
a “leaky” capacitor,
Q
2
)
, and
the charge
-
transfer resistance
associated with
the
OER kinetics
(
R
2
)
.
The u
ncompensated
resistance
correction was set to equal
90% of the solution resistance (10
-
15
Ω)
measured by
impedance.
The value of
Q
2
was used to evaluate
the
electrochemically active surface area (ECSA)
as well as the
change in surface
roughness during the durability experiment. For ECSA
calculations, the roughness factor
(
RF
)
of the
TEC 8 FTO substrate
was
assumed to be equal to
that of antimony
-
doped tin oxide (ATO) (
RF
= 1
.32).
1
T
he geometric area
-
normalized capacitance
of ATO (0.0254 mF cm
-
2
) was divided by the roughness factor to determine the capacitance
normalized to the electrochemical surface area (0.0192 mF cm
-
2
).
1
0.2 mL a
liquots
of
electrolyte were
taken during
galvanostatic operation
and were diluted
by 5 mL of 5%
w/w
HNO
3
(aq)
(
0.8 M
)
before analysis
by
inductively coupled plasma mass
spectrometry (ICP
-
MS) using an Agilent 8800 Triple Quadrupole ICP
-
MS system. Standards of
known concentration were
produced from
10
μ
g
mL
-
1
Mn and
999
±
2
μ
g mL
-
1
Sb standards
(Sigma Aldrich
) via
serial dilution with 5%
w/w HNO
3
(aq)
(
0.8 M).
E
udiometr
ic measurements indicated
>
97% Farad
a
ic efficiency for O
2
production
for
a
Mn
0.63
Sb
0.37
O
x
electrode
during 93 h
of continuous operation at
J
=
10 mA cm
-
2
.
S
5
Materials characterization
Ellipsometric
analysis was used to measure the g
rowth rates of individual oxides on Si
substrates
.
D
ata
were
collected using
65, 70, and 70 ° angles
across a wavelength range of 380
890
nm
with a
J.A. Woolam Co.
ellipsometer
. Ellipsometry data
were
analyzed
using the
CompleteEASE software
package
.
The
stoichiometry
of the catalyst material
was
determined
by dissolving deposited
unannealed metal films on Si substrates
for several days
in 10 mL of 1.0 M H
2
SO
4
(aq)
. The
concentration of dissolved metals
(Sb and Mn)
was then determined
using ICP
-
MS.
Films from
the same
deposition
batch were annealed
,
assembled
into electrodes (Figure S1)
,
and
subjected to
galvanostatic
testing
(Figure 2
, Figure 3)
.
Samples of the anolyte were taken periodically during
galvanostatic testing
and assessed via ICP
-
MS to quantify
the amount of
material
that had
dissolved into the electrolyte
.
The values from this ICP
-
MS analysis were normalized by the
concentrations
measured
when
the annealed metal films
were fully dissolved
, as described above,
to yield
the
percentages
of metals in the film that had dissolved at any point in time
.
The g
eometric area of the Si
substrates was measured with an optical scanner (using the
same procedure used to define
the
electrode area)
,
to determine the
area
-
normalized mass
-
loading
(23.53 μg Mn cm
-
2
, 32.3
0
μg Sb cm
-
2
). A mass loading of 0.42 μmol Mn cm
-
2
corresponded to
an
~100 nm thick
oxide
(Table S
2
).
Metal
Metal fraction
(%)
Mass loading
(μg cm
-
2
)
Mass loading
(μmol cm
-
2
)
Oxide thickness
(nm)
Sb
63%
32.3
0
0.27
35.7
Mn
37%
23.53
0.43
65.1
Total metal
100%
55.83
0.70
~100
Table S
2
.
M
ass loading and metal fraction
of a
s
-
prepared Mn
0.63
Sb
0.37
O
x
catalysts
determined by
ICP
-
MS
analysis of unannealed films digested
in 1
.0
M H
2
SO
4
(aq)
for several days
.
The o
xide
thickness
was
based on individual oxide growth rates
deter
mined from ellipsometry
and
the
total
subcycles
used during film growth
(Scheme 1).
S
6
Scanning
-
electron micrographs (SEMs) were obtained with a FEI Nova NanoSEM 450 at
an accelerating voltage of 10.00 kV with a working distance of 5 mm and an in
-
lens secondary
electron detector
.
Micrographs were acquired with a resolution of 688 pixels μm
-
1
over ~ 2 μm
2
areas. Energy dispersive X
-
ray (EDX) spectroscopy was performed in the SEM using an
accelerating voltage of 15.00 kV and a working distance of 5 mm
, with an
Oxford Instruments X
-
Max silicon drift detector. Spectra were collected in the range
of 0 to 10 keV
,
and quantitative
compositions
of the deposited material
were
obtained
using the “INCA” software package (Oxford
Instruments).
C
ompositions
were reported for
the average of
n
= 6 independent measurements
from different locations on the sample.
X
-
ray diffraction (XRD) data
were
collected using a Bruker D8 Discover diffractometer
with a Cu K
α
source and a two
-
dimensional Vantec detector. XRD data
were
acquired from the
FTO substrate in a Bragg
-
Brentano geometry. Grazing incidence X
-
ray diffraction (GIXRD)
data
were
collected from Mn
y
Sb
1
-
y
O
x
on a FTO substrate with the X
-
rays directed at a grazing angle
ω
= 0.3° above the plane of the sample surface
, with
the detector swept throughout the entire 2
θ
range.
X
-
ray photoelectron spectroscopy (XPS) was performed using a Kratos Axis Ultra system
with a base pressure of
3
× 10
-
9
Torr in the analysis chamber. A 150 W monochromatic Al K
α
source was used to irradiate the sample with X
-
rays (1486.6 eV).
F
or maximum depth sensitivity
,
the
hemispherical analyzer
was
oriented for detection along the sample surface normal. The data
were analyzed using CasaXPS software. A Shirley background was used for Mn spectra and a U 2
Tougaard background was used
to more accurately ca
pture the background signal
for Sb, O, and
C spectra. All peaks were referenced to adventitious C at a binding energy of 284.8 eV. The Mn
2p
3/2
peak was fit to standards of pure oxides
,
to estimate the Mn oxidation state and the
S
7
contribution of multiple oxidation states to the peak.
2
Mn 3p spectra were used in
conjunction
with
the
Mn 2p
3/2
emissions to
more accurately determine the Mn oxidation state.
2
6
The Sb 3d
5/2
peak
overlap
ped
the
O 1s
emission
, so
the
Sb oxidation state
was determined from
the Sb 3d
3/2
peak,
which has no overlap with O
.
The Sb
3d
3/2
peak was
fit with literature standards for
the
Sb
oxidation state
,
and the Sb 3d
5/2
contribution was calculated by constraining the spin
-
orbit peak
splitting (
Δ
Sb 3d = 9.38 eV),
taking
the full width at half max
imum
(FWHM) to be equivalent
between the peaks,
constraining the peak
area ratio
to be
3d
5/2
:3d
3/2
=
(
3:2)
,
and assuming
that the
remaining signal was due to O 1s.
Additional Data
Figure S
2
.
(A)
Cyclic voltammograms acquired
with a
Mn
0.63
Sb
0.37
O
x
electrode
in 1
.0
M
H
2
SO
4
(aq)
at 24 h intervals
by
interrupting a
continuous galvanostatic hold
of Mn
0.63
Sb
0.37
O
x
at
J
= 10 mA cm
-
2
.
(B)
OER overpotential to
obtain
J
=
10 mA cm
-
2
derived f
ro
m the
cyclic
voltammetric
data in
(A)
.
T
he IR
-
compensation
was
~
14 mV.
Larger
overpotentials were observed in
the first
voltammogram
acquired in each interval relative to the second and third
voltammograms
, and this
effect
increased
at later
time
intervals
of the experiment
(Figure S2B)
.
Figure S2
S
8
Figure S
3
.
Replicate of Mn
0.63
Sb
0.37
O
x
dissolution measurements
during
an
OER durability test.
(A)
The
time
-
average
d
OER overpotential was
η = 706 ± 21
mV at
J
=
10 mA cm
-
2
over
176 h in
1
.0
M H
2
SO
4
(aq)
. ICP
-
MS
measurements of the
corrosion products in the electrolyte during the
analogous 176
h durability test in relative (B) and absolute terms (C, D). The corresponding bar
graphs in panels (E
-
G) compare metal dissolution rates in various regimes:
0
-
168 h, 0
-
48 h, and
48
-
168 h.
Error bars represent the standard error of the dissolution rate given by linear regression
over the specified time regime.
Figure S3
Bubble in counter
electrode
interrupted circuit
S
9
Figure S
4
. (A) Catalyst composition as determined by EDX and ICP
-
MS. Representative SEMs
of Mn
0.63
Sb
0.37
O
x
(B) before and (C) after OER
at
J
= 10 mA cm
-
2
for 168 h in 1
.0
M H
2
SO
4
(aq)
.
Catalyst composition as determined by EDX after OER was
Mn
0.
49
Sb
0.
51
O
x
.
Figure S
5
.
(A) Impedance
data
and (B) roughness factor
for
Mn
0.63
Sb
0.37
O
x
taken at 24
h intervals
during OER at
J
= 10 mA cm
-
2
for 168 h in 1
.0
M H
2
SO
4
(aq).
The
TEC 8 FTO substrate roughness factor was assumed to be
equal to
that of
ATO (
RF
= 1.32).
1
According to preestablished methods, the geometric area
-
normalized capacitance of ATO (0.0254
mF cm
-
2
) was divided by the roughness factor to determine the capacitance normalized to the
electrochemical surface area (0.0192 mF cm
-
2
).
1
C)
EDX
EDX
ICP
-
MS
B)
500 nm
C
)
R
s
R
c
t
CPE
S
10
Figure S
6
.
XP spectra of Mn
0.63
Sb
0.37
O
x
(A
-
C)
before and
(D
-
F)
after OER at
J
= 10 mA cm
-
2
for
168 h in 1
.0
M H
2
SO
4
(aq).
(A
, D
) Mn 2p spectra; (B
, E
) Mn 3p spectra; (C
, F
) Sb 3d, O 1s spectra.
Raw data (black dots)
were fit by peak models as described in experimental methods. The sum of
all fit peaks (e
nvelope) is denoted by a red line in all panels.
S
11
Figure S
7
.
O
xidation state
derived from analysis of
XP spectra
of
Mn
0.63
Sb
0.37
O
x
before and after
OER at
J
= 10 mA cm
-
2
for 168 h in 1
.0
M H
2
SO
4
. (A) Mn oxidation states
;
(B)
derived
Mn
oxidation states from
the
3p and 2p peaks. (C) Sb oxidation state
s
derived from the
3d
peak.
Mn 2p
Mn 3p
Sb 3d
Before OER
2.9
2.8
3.2
After
OER
3.2
3.4
5.0
Table S
3
.
Oxidation
state of
Mn
0.63
Sb
0.37
O
x
derived from analysis of
XP spectra
before and after
OER at
J
= 10 mA cm
-
2
for 168 h in 1
.0
M H
2
SO
4
(aq)
.
S
12
Figure S
8
.
Electrochemical activity and stability of Mn
0.63
Sb
0.37
O
x
during OER at
J =
100 mA
cm
-
2
for 9 h in 1
.0
M H
2
SO
4
(aq)
.
Figure
S
8
presents a magnified view of the same data
depicted
in
Figure 3.
(A) Chronopotentiometric response. (B) Cyclic voltammograms collected both
initially and
after
8 h
of
the
galvanostatic
hold. (C)
C
orrosion products
in the anolyte
, as
determined by
ICP
-
MS
.
Figure S8