Supporting Information
Graphene oxide as a water dissociation
catalyst in the bipolar membrane
interfacial layer
Michael B. McDonald and Michael S. Freund*
Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2,
Canada
*E-mail: [email protected]
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1. Schemes of BPMs in electrodialysis and water electrolysis applications
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Figure SI-1: Scheme of a BPM under applied reverse bias sufficient to effect WD,
depicting reactions, and electron and ion transport under conditions for a)
electrodialysis, concentrating NaOH(aq) and HClO4(aq) (orange circles) in the
compartments adjacent to the BPM, from NaClO4(aq) feed solution in the
compartments between the electrodes and auxiliary ion-exchange membranes; b)
electrodialysis, producing water (orange circle) in the compartments adjacent to the
BPM, from HClO4(aq) and NaOH(aq) feed solutions in the compartments between the
anode and auxiliary anion-exchange membrane, and the cathode and auxiliary
cation-exchange membrane, respectively; c) water electrolysis in NaClO4(aq) , where
charges generated at the gas-evolving electrodes are neutralized by the WD
products to maintain solution pH, and; d) water electrolysis with a pH gradient
(anode in NaOH(aq) and cathode in HClO4(aq)), where charges are the primary
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reactants at the gas-evolving electrodes and are replaced by the WD products to
maintain the pH gradient.
2. Images of ion-exchange membranes
Figure SI-2: FE-SEM images of a) Neosepta AHA and; b) Nafion NR-211.
3. Membrane resistance
For individual ion-exchange membranes, J-E curves were collected in 1 M NaClO4,
HClO4, and/or NaOH with applied current steps between 0 mA and 50 mA (0 and 30
mA-cm-2) until the measured potential reached steady-state. The curve is ohmic, and
so a linear regression analysis of the data was used to give the slope; the inverse
slope is the membrane resistance.
For BPMs, resistance to co-ion leakage was measured from the inverse of the slope
of the J-E curve from 0 ma-cm-2 to the beginning of the limiting region, where ion
exclusion dominates the J-E profile. H+/OH- conductivity is measured from the
inverse slope of the J-E curve from values relevant for electrodialysis operation
(~100 mA-cm-2), through higher values collected (~120 mA-cm-2). At these current
densities, the overall relationship is not necessarily linear but is approximated to
yield an average resistance across electrodialysis-relevant current densities.
Example: for the AEL-CEL BPM, regression analysis of the J-E curve between 96-120
mA-cm-2 gives a slope of 15.496 mA-cm-2-V-1.
15.496 mA-cm-2-V-1/1000 mA-A-1 = 0.0155 A-cm-2-V-1
1/0.0155 A-cm-2-V-1 = 65 Ω-cm2
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4. Permselectivity
The maximum flux of co-ions was estimated from the current density at EWD, Jco-ion,
on the J-E curve for that membrane, as larger current densities are carried by OH/H+ transport and so it is assumed for simplicity that the leakage rate of co-ions
does not increase significantly above this current density.
Example:
Jco-ion from AEL|CEL control sample (Table 1) = 0.25 mA-cm-2 = 2.5E-4 A-cm-2
Jco-ion/2F = 2.5E-4 A-cm-2/[2(96 485.34 C-mol-1)] = 1.30E-9 mol-s-1-cm-2 = 1.3 nmols-1-cm-2.
5. Maximum product purity
This calculation is done using Equation 8 from ref. [6], with the simplification that
assumes the limiting current density magnitude does not change as a function of
product concentration in an electrodialysis cell.
[co-ion] = [H+ (OH-)]
Jco-ion
d
-2
2(100 mA-cm – Jco-ion)
% acid (base) = {1 – ([co-ion]/[H+ (OH-)])} x 100
Example: To produce the same 98% sulfuric acid (18.4 M) as is done with industrial
methods, using electrodialysis with AEL|CEL:
[co-ion] = 18.4 M
0.25 mA-cm-2
d= 0.023 M
-2
-2
2(100 mA-cm – 0.25 mA-cm )
% acid (base) = {1 – (0.023/18.4)} = 99.875%
6. EWD, RWD, and E(100)
The potential at the on-set of WD, EWD, was determined from the J-E curve at the
intersection of the ohmic region (>1 mA-cm-2) and the limiting current density
region.
The resistance of WD and WD product (OH- and H+) transport, RWD, was determined
from the inverse slope of the J-E curve from values relevant for electrodialysis
operation (~100 mA-cm-2), through higher values collected (~120 mA-cm-2).
The potential at 100 mA-cm-2, E(100), was determined from reading the potential of
J-E curve at 100 mA-cm-2.
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7. Correlation of resulting GO film thickness to deposition conditions
Figure SI-3: Resulting GO IL thickness on Neosepta AHA as a function of spin-coat
rate and starting GO solution concentration.
8. Correlation of permselectivity, RWD and E(100) to GO deposition conditions
Figure SI-4: Resulting permselectivity of GO-BPMs as a function of a) thickness and;
b) spin-coat rate and starting GO solution concentration.
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Figure SI-5: Resulting RWD of GO-BPMs as a function of a) thickness and; b) spincoat rate and starting GO solution concentration.
Figure SI-6: Resulting E(100) of GO-BPMs as a function of a) thickness and; b) spincoat rate and starting GO solution concentration.
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