of 82
Supporting Information
ACS Energy Letters
1
Supporting Information:
Asymmetric Bipolar Membrane for High Current Density
Electrodialysis Operation with Exceptional Stability
Éowyn Lucas
1
, Justin C. Bui
2,3
, T. Nathan Stovall
4,5
, Monica Hwang
1
, Kaiwen Wang
1
,
Emily R. Dunn
6
, Ellis
Spickermann
1
, Lily Shiau
1
, Ahmet Kusoglu
5
, Adam Z. Weber
5
,
Alexis T. Bell
2,3
,
Shane Ardo
7
, Harry A. Atwater
1*
, Chengxiang Xiang
1*
1
Division of Engineering and Applied Science, California Institute of Technology,
1200
E California Blvd, Pasadena, CA 91125, United States.
2
Department of Chemical and Biomolecular Engineering, University of California,
Berkeley, University Avenue and, Oxford St, Berkeley, CA 94720, United States.
3
Chemical Sciences Division, Lawrence Berkeley National Laboratory,
1 Cyclotron Rd,
Berkeley, CA 94720, United States.
4
Department of Chemistry, University of California, Berkeley,
University Avenue and,
Oxford St, Berkeley, CA 94720, United States.
5
Energy Technologies Area, Lawrence Berkeley National Laboratory,
1 Cyclotron Rd,
Berkeley, CA 94720, United States.
6
Division of Chemistry and Chemical Engineering, California Institute of Technology,
1200 E California Blvd, Pasadena, CA 91125, United States.
7
Department of Chemistry, Department of Chemical & Biomolecular Engineering, and
Department of Materials Science & Engineering, University of California, Irvine,
260
Aldrich Hall, Irvine, CA
92697,
U
nited
S
tates
.
These authors contributed equally
*
Correspondence to
haa@caltech.edu
and
cxx@caltech.edu
Supporting Information
ACS Energy Letters
2
Table of Contents
S1. Materials and Methods
................................
................................
................................
......
3
S2. Bipolar Membrane
(BPM)
Experimental Design and Analysis
................................
.......
10
S3. B
PM
Stability
................................
................................
................................
.....................
23
S4. BPM Scaling and Testing in Electrodialysis Stack
................................
.........................
26
S5. Characterization and Analysis of GrOx Loading
................................
............................
31
S6. Cell and Membrane Temperature Model
................................
................................
.........
38
S7. Computational Methods
................................
................................
................................
...
41
S7.1 Ion Transport
................................
................................
................................
...............
41
S7.2 Charge Transport
................................
................................
................................
........
45
S7.3 Homogeneous Kinetics and Electric Field
-
Enhanced Water Dissociation
.............
47
S7.4 Boundary Conditions
................................
................................
................................
..
52
S7.5 Numerical Methods
................................
................................
................................
.....
53
S8.
Table of Parameters Employed in Model
................................
................................
........
55
S9.
Mesh Dependence Study
................................
................................
................................
.
58
S10.
Conservation Studies
................................
................................
................................
.....
58
S11.
Analytic Hyperbolic Tangent Distributions in Model
................................
....................
61
S12.
Titration Details
................................
................................
................................
...............
63
S13.
Supplemental Figures on Ion Transport
................................
................................
.......
65
S14.
Supplemental Figures on Water Dissociation Catalysis Simulations
.........................
67
S15.
Sensitivity to Catalyst Layer Properties
................................
................................
........
72
S16. Water Electrolysis with GrOx BPM
................................
................................
................
77
References
................................
................................
................................
..............................
78
Appendix 1: Variable Definitions
................................
................................
...........................
81
Supporting Information
ACS Energy Letters
3
S1. Materials and Methods
Materials:
Nafion 212 (50 μm, Fuel Cell Store), Nafion 211 (25 μm, Fuel Cell Store),
Nafion 115 (127 μm, Fuel Cell Store), PiperION A15R (15 μm, Versogen), PiperION 20
(20 μm, Versogen), PiperION 60 (60 μm, Versogen), Fumasep FAB
-
PK
-
130 (110
-
140
μm, Fuel Cell Store
), Fumasep FKB
-
PK
-
130 (110
-
140 μm, Fuel Cell Store), Nafion D520
(5 wt% Ionomer, Fuel Cell Store, IonPower), graphene oxide paste (30 g/L, Graphene
Supermarket), sodium chloride (NaCl, Sigma Aldrich), sodium hydroxide (NaOH, Pellets,
Macron Chemi
cals), hydrochloric acid (HCl, 1.0 M and 0.1 M, J. T. Baker), potassium
hydroxide (KOH, pellets, Sigma
-
Aldrich). All membranes were received in dry form,
pretreated according to manufacturer’s instructions before use, and stored in DI water
(CEMs) or 1 M N
aOH (AEMs). All chemicals were used as received.
Catalyst ink
: Catalyst inks were made by first diluting graphene oxide paste (Graphene
Supermarket) from 30 g/L to 10 g/L. The dilute graphene oxide dispersion was then mixed
with Nafion D520 in a 1:1 volume ratio. The final ink solution was sonicated for at least
10
minutes prior to use.
BPM fabrication:
First, a piece of purchased Nafion membrane (NR212, NR211,
NR115), precut into a 1.5x1.5 cm square and soaked in DI water for at least 1 h, was
placed on a glass slide and patted dry with a Kim wipe. The membrane with then taped
to the glass slide on all
4 sides with Kapton tape. GrOx catalyst ink was then spin coated
onto the Nafion membrane at 3000 rpm for 30 s. Next, the Nafion membrane with GrOx
was placed in an oven at 100°C for 2 min. This process of spin coating and heating was
repea
ted if more layers, i.e., greater mass loading, was desired. Finally, the Nafion
membrane with GrOx was rewetted with a few drops of DI water, sandwiched with the
desired thickness of PiperION membrane, and pressed firmly between gloved fingers,
Supporting Information
ACS Energy Letters
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taking care to press out any air pockets. All membranes were tested directly after
assembly. The same methods were used for fabrication of both the 1 cm
2
and 6 cm
2
active area BPMs.
Membrane conductivity measurements:
The conductivity of the all AEMs and CEMs
used in this work were measured using a four
-
point probe on a Lake Shore FastHall
Station. All measurements were taken from
-
10 to 10 V on fully hydrated membranes.
These measurements gave an in
-
plane conductivity,
however, as the membranes are
isotropic, this is equivalent to the through plane conductivity.
Measuring mass loading of GrOx:
To determine the mass loading of GrOx ink spin
coated onto Nafion, the Nafion membranes taped to glass slides were weighed before
and after spin coating using a Sartorius CP Series electronic microbalance. Before
weighing, the Nafion taped to a glass slide
, was dried at 100°C for 10 min so that the
measurements would not be affected by a change in hydration after the GrOx ink was
added and heat treated. After the GrOx was spin coated onto the Nafion and heated, a
Kim wipe was
used to remove excess GrOx ink from the tape and glass. The final loading
amount was calculated based on the exposed Nafion area within the tape border.
Electrodialysis cell design/assembly: Figure S1
shows a schematic of the
electrodialysis cell used for testing the BPMs in this work. The cell consisted of, from left
to right in schematic, an anode, an anolyte chamber, a CEM, a dilute chamber, an AEM,
an acid chamber, a BPM (1 cm
2
active area), a base chamber, a CEM, a catholyte, and
a cathode. Both the anode and cathode consisted of Ni foil with copper tape as leads.
Aqueous 1 M NaOH with used as both the anolyte and the catholyte and was recirculated
through both chambers at ~10
mL/min. Aqueous 3 M N
aCl was recirculated at ~5 mL/min
Supporting Information
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through the dilute chamber and aqueous 0.5 M NaCl was flowed through the acid and
base chambers at 0.2 mL/min. Both CEMs used in the cell stack were Nafion N324 (280
μm, Fuel Cell Store) and the AEM was Fumasep FAB
-
PK
-
130 (130 μm, Fuel Cell Store).
Luggin
capillaries holding Ag/AgCl reference electrodes (CHI111, CH Instruments) were
placed in the acid and base chambers to allow for the most direct measurement of the
voltage across the BPM.
1
Chronopotentiometry:
After the electrodialysis cell described above was assembled,
potentiostat (Biologic SP 300, Biologic SP 200, Kiethley 2400) leads were attached in a
four
-
point measurement configuration so that a current could be applied across the full
cell and the resu
lting voltage could be measured directly across the BPM.
Chronopotentiometry measurements were used to obtain all reported data for all
polarization curves. For each point, a chosen current was applied across the anode and
cathode and h
eld steady for 5
-
20 min or until the voltage measured across the BPM
reached steady state. The current was then increased to the next value and the process
was continued until all desired current measurements were performed. The reported
voltage values are
averages of the voltage collected over the steady state region for each
chronopotentiometry step.
Electrochemical Impedance Spectroscopy (EIS):
EIS measurements were performed
in the same electrodialysis cell as the chronopotentiometry measurements. For each BPM
tested, measurements were started at 500 mA cm
-
2
and stepped down through each
desired current density. For each step, the current was held for 1 min, then scanned from
600 kHz to 20 Hz with an amplitude of 5
-
10% of the current, recording every 0.5 sec
.
Nyquist plots were then fitted using EIS Spectrum Analyzer software
.
Supporting Information
ACS Energy Letters
6
Faradaic efficiency:
The same five chamber electrodialysis cell was used for collecting
acid and base samples to measure the Faradaic efficiency at various current densities.
Aqueous 0.5 M NaCl was flowed at 5 mL/min through the acid and base chambers and
the desired current
was applied across the cell until the voltage stabilized (usually 10
-
20
min). Samples were then collected in 20 mL vials from the acid and base chamber. The
current was then increased to the next desired value and the process repeated.
Once the
samples were collected, the H
+
and OH
-
activity was evaluated via pH probe
measurements or pH titration. Titration was used for more pH values > 12 and < 2. All pH
titration measurements and the subsequent calculation of theoretical H
+
/OH
-
concentration and Faradaic efficiency were performed as reported in É Lucas
et al
.
1
Low vacuum SEM:
All SEM images were obtained using the low vacuum mode on an
FEI Nova NanoSEM 450. A spot size of 5.0 and a voltage of 10.00 kV was used for most
images. For the BPM cross
-
sectional images, the membrane was embedded in resin and
cut using a microtome. For
cross sections of just the Nafion with a GrOx CL, the
membranes were sliced using a razor blade. ImageJ was used to evaluate membrane
and CL thickness from these SEM cross sections.
Optical Microscopy:
All optical microscope images were obtained using a Nikon Eclipse
LV100D
-
U. Images of GrOx dispersions were taken during the BPM fabrication process,
while Nafion and GrOx
-
coated Nafion remained taped to glass slides before they we
rewetted and sandwiched
with the AEM.
AFM:
An Asylum AFM in AC Air Topography mode was used for topological and
roughness measurements of the membrane and GrOx layer surface. As with the optical
Supporting Information
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microscope images, AFM was performed on Nafion and GrOx
-
coated Nafion prior to the
samples being rewetted and sandwiched with the AEM.
T
-
Peel Tests:
A 1 inch x 7
-
inch Nafion NR212 membrane was soaked in DI water for ~1
hour and then placed onto a glass slide and dried with a Kim wipe. It was then taped onto
the slide via Kapton tape. To deposit the graphene oxide/TiO
2
(Aeroxide
®
P25 TiO
2
,
Thermo Fischer Scientific) catalyst layer, a mother ink of 1 wt % catalyst in water was first
made, and then diluted to 0.5 wt % using a 5 wt% Nafion D520 ionomer solution. This
final ink was then airbrushed using a Testors airbrush. The airbrush was pas
s
ed over the
membrane 10 times to ensure a uniform coating of catalyst. We note that the final loading
of catalyst was higher when performing peel tests when compared to electrochemical
measurements. This was done to ensure the adhesive strength of the BPM
was only a
result of the catalyst, and not AEL/CEL interactions. Finally, the membrane was cut into
a 0.5
-
inch x 6
-
inch strip, and tape was added to the first 1 inch of the membrane to prevent
adhesion to AEL. The AEL was cut into a 0.5
-
inch x 6
-
inch strip
and soaked in 1 M NaOH
for at least one hour. Tape was similarly added to the AEL, and then the AEL was
laminated to the CEL. The membranes were allowed to dry over night with light
compression to ensure the membranes dried flat.
The adhesive strength of the BPM samples were measured via a T
-
peel test using an
Instron Mechanical Testing system with tensile grips (5944). The experiments were
carried out using dry membranes in ambient conditions. The taped ends of the
membranes were
attached to the tensile grips in a T
-
peel geometry. The BPM was then
subjected to a displacement
-
controlled uniaxial tensile test with an extension rate of 6
mm/min and the measured force was recorded. Upon stabilization of the force value the
Supporting Information
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force per width of the BPM was plotted as a function of the extension length. The average
force/width is calculated to quantify the adhesive strength of the BPMs.
Continuum simulation.
The simulation was performed using the COMSOL Multiphysics
v.6.0 software package. The concentration of H
3
O
+
, OH
, Na
+
, Cl
, and of all GrOx surface
species along with the electrostatic potential profile were solved using conservation
equations where Poisson
-
Nernst
-
Planck described mass and charge transport. Crucially,
the rates of net
-
charge
-
generating homogeneous reactions w
ere modified by the Second
Wien effect such that the rate of ion dissociation, i.e., the forward direction, was
substantially enhanced by an electric field.
2
4
Supplementary Methods (
Section S7
-
S15)
provide detailed information regarding the physics and parameters employed in the
simulations.
Porous
Electrode Fabrication
for Compressed MEA Measurements
:
The anode ink
was prepared in a 50 mL centrifuge tube with 0.2 g IrO
2
(Premion
®
, Alpha Aesar), 1 g
18.2 MΩ
-
cm
DI H
2
O, 3.4 g
n
-
propanol and 0.2 g PiperION
-
A5 ionomer suspension (TP
-
85 5% w/w). The acidic cathode ink was prepared using 0.2 g 46.3% Pt/C (TEC10V50E,
Tanaka), 1 g DI H
2
O, 3.4 g
n
-
propanol and 0.2 g Nafion
®
D521 (5 wt%). In the case of the
anion
-
exchange membrane water electrolyser (AEMWE), the alkaline cathode ink was
prepared with 0.2 g PiperION
-
A5 (5 wt%) and was otherwise identical to the acidic ink.
The inks were sonicated in an ice bath for ~1 h, or until fully dispersed. The anode and
catho
de gas diffusion layers
(GDLs) were stainless steel (15FP3, Bekaert Bekipor
®
) and
Toray 120 5% PTFE, respectively. Following sonication, the catalyst inks were slowly
airbrushed (Iwata) onto 30.25 cm
2
cutouts of their respective GDLs in a serpentine
pattern. Following each layer of deposited ink, the GDL was placed on a hot plate (80
°C)
Supporting Information
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to ensure full solvent evaporation between airbrush passes. Between each pass the GDL
was rotated 90° to minimize edge effects. The airbrushed porous transport electrodes
(PTEs) had final loadings of ~1.9 (+/
-
0.2) mg/cm
2
and ~1.6 (+/
-
0.2) mg/cm
2
for the anode
and cathode, respectively. For each electrode, additional ionomer (diluted to 2 w/w % in
ethanol) was then sprayed on top of the catalyst layer until the mass of additional ionomer
was 15
-
25% of the mass of the catalyst. Finally, the PTEs we
re cut in
to 5 cm
2
squares.
Zero
-
gap BPM Water Electrolyzer Measurements:
Zero
-
gap BPM measurements were
performed in a 5 cm
2
Fuel Cell Technology cell, using serpentine graphite and titanium
flow fields at the cathode and anode, respectively. The cell was assembled by placing a
10 mil ETFE gasket onto the titanium flow field followed by the anode PTE. The BPM
(GrO
x
BPMs fabricated as described previously
;
pristine BPMs were fabricated via the
lamination of 20 μm Versogen PiperION
-
A20
-
HCO
3
and Nafion
®
212 membranes) was
then placed AE
L side down atop the anode PTE, followed by 10 and 2 mil ETFE gaskets.
Lastly, the cathode PTE was placed on the BPM followed by the graphite flow field. The
cell was tightened to 50 inch
-
pounds in a star pattern to ensure uniform compression.
Electrochemi
cal measurements were performed using a Biologic VSP 300 Potentiostat
and VMP3B
-
10 10A/20V Biologic current booster.
The cell was fed deionized H
2
O heated to 60 °C and delivered to both the anode and
cathode at 75 mL/min. The cell temperature was held at 55 °C using a resistive heating
element and monitored via a thermocouple inserted into the back plate of the cathode.
Chronopotentiometry measureme
nts were performed by first conditioning the cell by
holding 50 mA for 2 min, or until the potential equilibrated.
The conditioning was continued
by stepping the cell in 1
-
min intervals from 50 to 500 mA in 50 mA increments, 50
0 to
Supporting Information
ACS Energy Letters
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1000 mA in 100 mA increments, 1000 to 2500 mA in
500 mA increments, and finally
holding the cell at 5000 mA for 3 min. Then, the cell was stepped down in current from
5000 to 1000 mA in 500 mA increments, 1000 to 500 mA in 100 mA increments, and then
from 500 to 50 mA in 50 mA increments where each curre
nt was held for a 1
-
min interval.
The data in the reported polarization curves were obtained from the step
-
down process
by averaging the last 5 s of each current hold. Cells containing the pristine BPM were
conditioned by holding the cell at 50 mA for 1 mi
n, then stepping to 100 and 150 mA in
30 s increments. The reported polarization curve was then acquired by stepping the cell
down from 150 to 50 mA in 50 mA increments and each current being held for 30 s. Higher
currents and longer hold times were not ac
hievable for the BPM without a catalyst layer
due to rapid degradation.
S
2
. Bipolar Membrane Experimental Design and Analysis
Figure S1
: (A) Cross section schematic of electrodialysis cell designed for direct testing
of bipolar membranes. (B) Image of actual flow cell, showing Luggin capillaries, reference
electrodes, anode, cathode, and flow channels. Luggin capillaries with Ag/AgCl refe
rence
electrodes are implemented to allow for direct measurement of the BPM voltage without
interference from electrolyte resistance. The tips of the capillary tubes are placed
approximately 0.1 mm from the surface of the BPM. The BPM active area
in the custom
cell is 1 cm
2
. The AEM, CEMs, anode, and cathode all have an active area of 4 cm
2
.
Supporting Information
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11
To accurately understand the performance of bipolar membranes (BPMs) for
electrodialysis, it is important to be able to directly measure the voltage across the BPM
without interference from electrolyte resistance or redox reactions.
1
Luggin
capillaries with
reference electrodes can be implemented into H
-
Cells to measure the BPM voltage as
close to the surface of the membrane as possible. However, in a simple H
-
Cell
configuration, equilibrium at each applied current density cannot be reached
as the acid
and base concentration will continue to increase (especially directly at the surface of the
BPM) for the entire duration the bias is applied. To overcome these challenges for
electrochemical testing of BPMs, we designed a custom electrodialysis
cell with
embedded Luggin capillaries that also allows electrolyte to be flowed through each
chamber (
Figure S1
). Furthermore, to guarantee equilibrium during experiments, fresh
solution was continuously flowed through each chamber of the electrodialysis cell and the
acid/base chambers were agitated using small magnetic stir bars with a plate placed
under the cell.
All BPM
-
ED electrochemical measurements were performed with constant
flow conditions; 0.5 M NaCl was flowed to refresh the system at a constan
t rate of 0.2
mL/min throughout all experiments.
Using accurate pumps to set specific flow rates,
Equations S1
-
S8
were used to calculate the theoretical concentration of H
+
and OH
-
in
the acid and base chambers of the electrodialysis cell. In
Equations S1 to S8
,
+
is
generation rate of H
+
(mol s
-
1
),
푎푝푝푙푖푒푑
is the applied current (A),
is the number of moles
of electrons transferred per mole of H
+
generated (mol electron (mol H
+
)
-
1
),
is the
Faraday constant (96,485 C (mol electron)
-
1
),
0
.
5
푁푎퐶푙
is the volumetric flow rate for 0.5
M NaCl (L s
-
1
),
[
+
]
푎푑푑푒푑
is the concentration of
H
+
added by water dissociation in BPM
(mol L
-
1
),
[
+
]
푝푟푒푠푒푛푡
is the concentration of H
+
present in the supplied water stream (mol
Supporting Information
ACS Energy Letters
12
L
-
1
),
[
+
]
푒표푟푒푡푖푐푎푙
is the theoretical total concentration of H
+
(mol L
-
1
),
푂퐻
is the
generation rate of OH
-
(mol s
-
1
),
[
푂퐻
]
푎푑푑푒푑
is the concentration of OH
-
added by water
dissociation in the BPM (mol L
-
1
),
[
푂퐻
]
푝푟푒푠푒푛푡
is the concentration of OH
-
present in the
supplied ocean water (mol L
-
1
), and
[
]
푒표푟푒푡푖푐푎푙
is the theoretical total concentration
of OH
-
(mol L
-
1
).
+
=
푎푝푝푙푖푒푑
푛퐹
(S1)
[
+
]
푎푑푑푒푑
=
+
0
.
5
푁푎퐶푙
(S2)
[
+
]
푝푟푒푠푒푛푡
=
10
6
.
85
(S3)
[
+
]
푒표푟푒푡푖푐푎푙
=
[
+
]
푎푑푑푒푑
+
[
+
]
푝푟푒푠푒푛푡
(S4)
푂퐻
=
푎푝푝푙푖푒푑
푛퐹
(S5)
[
푂퐻
]
푎푑푑푒푑
=
푂퐻
0
.
5
푁푎퐶푙
(S6)
[
푂퐻
]
푝푟푒푠푒푛푡
=
10
13
.
71
10
6
.
85
(S7)
[
]
푒표푟푒푡푖푐푎푙
=
[
푂퐻
]
푎푑푑푒푑
+
[
푂퐻
]
푝푟푒푠푒푛푡
(S8)
It is also important to note that since the concentration of acid and base increases
as current density is
increased
, the solution conductivity increases as well and therefore
the solution iR drop cannot be accurately calculated directly from the initial salt
conductivity (
Figure S7
). As equilibrium can be achieved in the custom electrodialysis
cell by flowing and sti
rr
ing the acid and base chamber solutions,
Equation S9
can be
implemented to calculate the ohmic contribution from the acid and base solutions
. In
Equation S9
,
푠표푙푢푡푖표푛
is the voltage contribution from solution resistance (V), J is the
current density (mA/cm
2
),
푠표푙푢푡푖표푛
is the conductivity of NaCl, HCl, or NaOH based on
the current density and flow rate (mS cm
-
1
), and d is the distance of the Luggin capillary