of 14
SUPPORTING INFORMATION
FOR:
Decoupling H
2
(g) and O
2
(g) Production in Water
Splitting by a Solar
-
Driven
V
3+/2+
(aq,H
2
SO
4
)|KOH(aq) Cell
Alec Ho
1
, Xinghao Zhou
2
,
Lihao Han
1
,
Ian Sullivan
1
,
Christoph Karp
1
,
Nathan S.
Lewis
1, 3, 4*
, Chengxiang Xiang
1*
1
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
CA 91125, USA
2
Division of Engineering and Applied Science, Department of Applied Physics and Materials
Science, California Institute of Technology, Pasadena, C
A 91125, USA
3
Beckman Institute Molecular Materials Research Center, California Institute of Technology,
Pasadena, CA 91125, USA
4
Kavli Nanoscience Institute, California Institute of Technology, Pasadena, CA 91125, USA
*To whom correspondence should
be addressed: nslewis@caltech.edu, cxx@caltech.edu
Experimental Procedures
Materials
All chemical reagents were used without further purification. The anolyte was
prepared with 2.5 M KOH(aq) (>=85%, Macron). For the catholyte, a solution of 0.36 M
V
4+
was prepared from VOSO
4
-
xH
2
O (97%, x=3.41, Sigma
-
Aldrich) in 2.0 M H
2
SO
4
(aq)
by diluting ultrapure, concentrated sulfuric acid H
2
SO
4
(93
-
98%, J.T. Baker) in ultrapure
water (18.2 M
Ω
cm resistivity). 0.36 M V
2
(SO
4
)
3
in 2.0 M H
2
SO
4
(aq) was obtained by
reduc
ing V
4+
to V
2+
, followed by oxidizing V
2+
to V
3+
by bubbling with oxygen (O
2
, Air
Liquide) for < 30 min. The cathodic solution was deoxygenated in the cathodic half
-
cell
for at least 10 min with in
-
house nitrogen (N
2
) before the charging process. Mo
2
C (99.
5%,
Strem) catalyst powder was used for H
2
(g) production from V
2+
in H
2
SO
4
.
Redox Cell
The electrochemical cell consisted of two 3
-
necked glass half cells, an O
-
ring, a gasket,
and a bipolar membrane with a geometric area of 6.0 cm
2
(fumasep FBM, FuMA
-
Tec
h
GmbH). The electrode in the cathodic half
-
cell was a carbon fiber cloth with a geometric
area of 6.0 cm
2
(FuelCellsEtc), and the electrode in the anodic half
-
cell was a Ni mesh with
a geometric area of 6.0 cm
2
(Sigma Aldrich).
Electrochemical charging
experiments
During charging and discharging, H
2
(g) production was measured with a differential
pressure manometer (HD755, Extech). EC
-
lab was used to measure the charge of the cell.
Beginning with solutions of 50.0 mL 0.36 M V
2
(SO
4
)
3
, 2.0 M H
2
SO
4
(aq) (pH =
-
0.16),
varying degrees of charge were passed to obtain solutions of various ratios of V
2+
/V
3+
in
H
2
SO
4
(aq). 6.0 mL of the partially charged solution was added to a flask and then purged
with N
2
(g). The flask was sealed to a compartment that contained 0.5
0 g Mo
2
C powder,
with the compartment also connected to a differential pressure manometer. To proceed with
the measurement, the catalyst powder was mixed with the partially charged solution and
the suspension was stirred at 400 rpm. While the stirring of t
he catholyte influenced the
initial hydrogen release rate, the overall hydrogen collected after 5 min was independent
of the rate of stirring. The manometer measured the amount of H
2
(g) produced over time,
recording data every 1 s.
Bipolar membrane measur
ements
The current density
vs.
voltage behavior of the bipolar membrane was measured with
a four
-
point measurement configuration.
1
Various current densities were applied to the cell
using two Pt mesh electrodes, one as the cathode and one as the anode, and two Ag/AgCl
reference electrodes were used to measure the potential near the membrane.
Ion crossover measurements
The
concentr
ations of V
3+/2+
in the anolyte before and after a continuous operation of
10 mA cm
-
2
for 24 hours were determined using inductively coupled plasma mass
spectroscopy (Agilent 8800 ICP
-
MS Triple Quard), and the concentrations of K
+
in the
catholyte and SO
4
2
-
in the anolyte were determined using ion chromatography (IC, Dionex
ICS
-
2000 Ion Chromatography System). Table S2 lists the concentration of V
3+/2+
with
known concentrations (1, 10, 50, 100, 500
μ
M
)
as well as the measured concentration in
the anolyte samples. The five measured concentrations were very close to the known
values
,
confirming the accuracy of the measurement for the vanadium concentration in the anolyte
samples before and after operating
for 24 hours.
Outdoor charging experiments
The vanadium redox cell was placed in series with a 16% efficient polycrystalline
photovoltaic module (Sundance Solar) affixed to a dual
-
axis smart solar tracker (Fuel Cell
Store). The illumination intensity was
measured with a photodiode (Thorlabs). Fresh
catholyte was charged over the course of a day to 85
-
86% charge capacity, and all 50 mL
of the charged solution was combined with Mo
2
C to generate H
2
(g). The discharged
solution was passed through a 0.45
μ
m syringe filter (PALL) followed by a 0.2
μ
m syringe
filter (PALL) and reused in the system.
Pressurization experiments
In the pressurization experiment apparatus, a stainless
-
steel tube was used as the
reaction vessel. A plastic tube closed off at one en
d contained Mo
2
C catalyst and another
portion of the system had 25.0 mL of catholyte solution. The chamber was connected to
the rest of the apparatus, and the whole apparatus was filled with hydrogen gas to various
pressures (1 atm, 10 atm and 100 atm). Af
ter pressurization, the hydrogen tank was
detached, the valve to the pressure gauge was temporarily closed, and the apparatus was
rotated upside down to allow the catholyte to mix with the catalyst. This rotation, along
with some shaking of the apparatus,
was repeated several times to ensure sufficient mixing.
The apparatus was then rotated back upright, and the valve to the pressure gauge was
opened and the resulting pressure from the reaction was read. The headspace of the
apparatus was estimated to be 10
.4 mL, based on measuring the amount of water needed
to fill the chamber.
Figure S1
. A schematic illustration of the full process flow.
Figure S
2
. Current density of the V
3+/2+
(aq,H
2
SO
4
)|KOH(aq
) cell as vanadium was charged
at various potentials.
Figure S
3
.
Outdoor setup of the V
3+/2+
(aq,H
2
SO
4
)|KOH(aq) cell coupled with a
photovoltaic cell affixed to a solar tracker.
Ph
o
t
o
v
o
l
t
a
i
c
C
e
l
l
Ph
o
t
o
d
i
o
d
e
So
l
a
r
Tr
a
c
k
e
r
Ca
t
h
o
d
e
An
o
d
e
St
i
r
B
a
r
s
Bu
r
e
t
t
e
Ca
t
h
o
l
y
t
e
An
o
l
y
t
e
Bi
p
o
l
a
r
Me
m
b
r
a
n
e
Figure S
4
.
Hydrogen collection from V
2+
stirred with the Mo
2
C
catalyst.
Figure S
5
.
Pressurization experiment apparatus. With the vent valve closed and the gauge
valve and tank/chamber valve open, the hydrogen tank was opened and adjusted so the
pressure gauge read the desired pressure. The tank/chamber valve and
hydrogen tank were
then closed, and the vent valve was opened to vent the intermediate chamber. The hydrogen
tank was then disconnected, and then the apparatus connected to the reaction chamber was
flipped to allow catalyst to mix with catholyte, before be
ing flipped back upright to read
the resulting pressure.
P
r
es
s
u
r
e
G
au
g
e
H
y
dr
o
g
e
n T
ank
Reac
tio
n
C
h
am
b
er
V
ent
V
alv
e
T
an
k
/
C
h
am
b
er
V
alv
e
G
au
g
e
V
alv
e
Figure S
6
.
Schematic of catalyst introduction to the catholyte in the pressurized
stainless
-
steel
apparatus reaction chamber. The catalyst only mixed with the catholyte (purple) when
the apparatus
was flipped upside down, causing hydrogen to be produced and measured by
the pressure gauge. The apparatus was flipped right side up after the catalyst was mixed
with the catholyte.
Flip
c
h
am
b
er
u
p
s
id
e
d
o
w
n
to
m
ix
,
th
en
f
lip
b
ac
k
u
p
r
ig
h
t
Table S
1
.
Bipolar membrane potential loss and crossover rates
comparison
pH
Authors
Electrolytes
V
membrane, loss
Crossover
rates
Ref
Extreme pHs
Vermaas,
Wiegman, Nagaki
& Smith
1 M H
2
SO
4
(pH= 0)
/1 M KOH (pH= 14)
19 mV @
10 mA cm
-
2
N.A
2
Reiter, White &
Ardo
1 M H
2
SO
4
(pH= 0)
/1 M KOH (pH= 14)
58 mV
@
10
mA cm
-
2
N.A.
3
Ho, Zhou, Han,
Sullivan, Karp,
Lewis & Xiang
2.0 M H
2
SO
4
(pH =
-
0.16)
/ 2.5 M KOH (pH =
14.21)
81 mV@
10 mA cm
-
2
f
H
+=93.9%
f
OH
-
=98.2%
(@ 10 mA
cm
-
2
)
This
work
Ho, Zhou, Han,
Sullivan, Karp,
Lewis & Xiang
2.0 M H
2
SO
4
,
0.36 M V
2
(SO
4
)
3
(pH
=
-
0.16)
/ 2.5 M KOH (pH =
14.21)
18 mV@
10 mA cm
-
2
f
H
+=92.1%
f
OH
-
=99.5%
(@ 10 mA
cm
-
2
)
This
work
Near
-
neutral pHs
Vermaas,
Wiegman, Nagaki
& Smith
1 M KP
i
(0.55 M
KH
2
PO
4
, 0.45 M
K
2
HPO
4
) (pH=7)
797 mV
@
10 mA cm
-
2
N.A.
2
Vargas
-
Barbosa,
Geise, Hickner &
Mallouk
1
.
75 M NaH
2
PO
4
1.75 M K
2
HPO
4
(
pH=6.86)
845 mV
@ 10 mA
cm
-
2
N.A.
4
Sun, Liu, Chen,
Verlarge, Lewis &
Xiang
1 M H
2
SO
4
(pH= 0)
/0.5 M KBi (pH=9.3)
450 mV
@4.36 mA
cm
-
2
f
H
+=97.5%
f
OH
-
=92.3%
(@ 4 mA
cm
-
2
)
1
Zhou, Liu, Sun,
Chen, Verlage,
Francis, Lewis &
Xiang
2.8 M KHCO
3
(pH=8.0)
/1.0 M KOH
(pH=13.7)
508 mV
@
10 mA cm
-
2
f
H
+=>90%
f
OH
-
=>95%
(@ 8 mA
cm
-
2
)
5
Table S2.
ICP
-
MS measurement for vanadium species crossover the bipolar membrane
All
the samples were in 2.5 M KOH, and diluted by 80 times in HNO
3
solution
for
neutralization before the ICM
-
MS characterization
.
Vanadium concentration in the anolyte enhancement was 110.
99
-
101.82 = 9.1
7
μ
M, and
then be converted into
J
V
3+/2+
= 0.226 μ
A cm
-
2
.
Concentration of vanadium
species (
μ
M)
using
measured
calibrat
ion
data
Calibration sample A: 1
μ
M
vanadium species
1.29
Calibration sample B: 10
μ
M
vanadium species
10.95
Calibration sample C: 50
μ
M
vanadium species
49.30
Calibration sample D: 100
μ
M
vanadium species
104.48
Calibration sample E: 500
μ
M
vanadium species
499.15
Anolyte
before
operating at
10 mA cm
-
2
for 24 h
101.82
Anolyte after
operating at 10 mA cm
-
2
for 24 h
110.99
Table S3.
Comparison of overpotentials at an operating current density of 10 mA cm
-
2
in
three different systems
Unit
: mV (unless noted otherwise)
System A: Pt/1.0 M H
2
SO
4
/Nafion/1.0 M H
2
SO
4
/IrO
x
System B: Carbon cloth/2.0 M H
2
SO
4
0.36 M V
2
(SO
4
)
3
/Nafion/1.0. M H
2
SO
4
0.10 M
Ce
3+
/Pt
System C: Carbon cloth/2.0 M H
2
SO
4
0.36 M V
2
(SO
4
)
3
/BPM/2.0 M KOH/Ni mesh
Reference
(1)
Sun, K.; Liu, R.; Chen, Y.; Verlage, E.; Lewis, N. S.; Xiang, C. A Stabilized,
Intrinsically Safe, 10% Efficient, Solar
-
Driven Water
-
Splitting Cell Incorporating Earth
-
Abundant Electrocatalysts with Steady
-
State pH Gradient
s and Product Separation
Enabled by a Bipolar Membrane.
Adv. Energy Mater.
2016,
6
(13), 1600379.
(2)
Vermaas, D. A.; Wiegman, S.; Nagaki, T.; Smith, W. A. Ion Transport Mechanisms
in Bipolar Membranes for (Photo)electrochemical Water Splitting.
Sustainab
le Energy
Fuels
2018,
2
(9), 2006
-
2015.
Sys
tem
A
B
C
C
athode
37
2
66
2
66
M
embrane
~
70
~
70
~
70
A
node
2
70
244
379
Solution
/Electrolyte IR drops
~100
~100
~100
Total voltage
~1
.
70
V
~2
.
37
V
~2
.
3
2
V
(3)
Reiter, R. S.; White, W.; Ardo, S. Communication
-
Electrochemical
Characterization of Commercial Bipolar Membranes under Electrolyte Conditions
Relevant to Solar Fuels Technologies.
J. Electrochem. Soc.
2016,
163
(4), H3132
-
H3134.
(4)
Vargas
-
Barbosa, N. M.; Geise, G. M.; Hickner, M. A.; Mallouk, T. E. Assessing the
Utility of Bipolar Membranes for use in Photoelectrochemical Water
-
Splitting Cells.
ChemSusChem
2014,
7
(11), 3017
-
3020.
(5)
Zhou, X.; Liu, R.; Sun,
K.; Chen, Y.; Verlage, E.; Francis, S. A.; Lewis, N. S.; Xiang,
C. Solar
-
Driven Reduction of 1 atm of CO
2
to Formate at 10% Energy
-
Conversion
Efficiency by Use of a TiO
2
-
Protected III
V Tandem Photoanode in Conjunction with a
Bipolar Membrane and a Pd/C Ca
thode.
ACS Energy Lett.
2016,
1
(4), 764
-
770.