Transparent Bipolar Membrane for Water Splitting ... - ACS Publications

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Transparent Bipolar Membrane for Water Splitting Applications Sakineh Chabi,† Andrew G. Wright,‡ Steven Holdcroft,*,‡ and Michael S. Freund*,† †

Department of Chemistry, Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, United States Department of Chemistry, Simon Fraser University, Burnaby, BC V5A 1S6, Canada

‡

S Supporting Information *

ABSTRACT: This study describes the use of a benzimidazoliumbased anion exchange membrane for creating bipolar membranes and the assessment of their suitability for solar-driven water splitting. Bipolar membranes were prepared by laminating anion exchange membrane with Nafion NR-211 membrane without modification of the interface. Under acidic and basic conditions, proton and hydroxide ion conductivities of 103 and 102 mS cm−1 were obtained for Nafion and benzimidazolium-based membranes, respectively. The fabricated bipolar membranes have an average thickness of 90 μm and show high transmittance, up to 75% of the visible light. The findings suggest that the two membranes create a sharp hydrophilic interface with a space charge region of only a few nanometers, thereby generating a large electric field at the interface that enhances water dissociation. KEYWORDS: bipolar membrane, transparent membrane, interfacial layer, water splitting, artificial photosynthesis

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INTRODUCTION Solar-driven water splitting (WS), or artificial photosynthesis (AP), is an environmentally friendly technology currently being developed to generate hydrogen. Photoelectrochemical water splitting allows direct production of hydrogen without unwanted byproducts such as carbon monoxide and carbon dioxide generated from steam reforming of natural gas.1,2 The efficiency of a solar-driven water splitting cell was evaluated in terms of (1) the photocurrent generated, (2) the required voltage for WS into hydrogen and oxygen gas, (3) the yield and purity of the product, and (4) the cost of catalysts and/or photoabsorbers.3−5 A fully integrated membrane-based architecture shown in Figure 1, which separates anolyte from catholyte, is an optimal design because the distance between the components, and in turn the ionic and electronic resistance of the system, can be minimized.6 The main requirements of a membrane in such a system include light transparency, mechanical and chemical stability, and ionic and electronic conduction.7−9 Currently, membrane materials used in water splitting electrolyzers include cation exchange membranes (CEM) such as Nafion as well as anion exchange membranes (AEM). These membranes provide physical separation between anolyte and catholyte, thereby separating products while maintaining efficient ionic conduction. However, use of a single membrane restricts the operation to acidic (CEM) or basic (AEM) conditions which in turn dictates and limits the selection of photoelectrode materials and catalysts that can be used. Further, monopolar membrane cells may develop a pH gradient and thus voltage change and instability over time. Mallouk’s © XXXX American Chemical Society

Figure 1. Schematic of a membrane-embedded AP system. The device uses two different semiconductors to produce the >1.23 V necessary to electrolyze water. The anode material absorbs higher energy light (blue light), allowing lower energy light to be absorbed by the cathode. Catalysts distributed along the semiconductor surface facilitate the reactions at low overpotential. Protons are transported via a transparent ion exchange membrane. Reproduced with permission from ref 10. Copyright 2011 Royal Society of Chemistry.

group studied this effect in detail in commercial membranes such as Neosepta AEM11 and Nafion membrane12 cells. Received: March 28, 2017 Accepted: July 18, 2017

A

DOI: 10.1021/acsami.7b04402 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 2. Chemical structures of PBI polymer (a), Nafion (b), and an image of the bipolar membranes produced (c).

Table 1. Physical Properties of the Membranes Used in This Study membrane

thickness (μm)

IEC (mmol/g)

water uptake (wt %)

tensile strength (MPa)

density (g/cm3)

ionic group

ionic conductivitya (mS cm−1)

Nafion PBI

25 ± 3 55 ± 3

1.0 2.5

47 80

28 64.725

1.96 1.84

sulfonic acid benzimidazolium

70 (ref 24) 10 (ref 25)

a

The ionic resistance of both membranes was measured in an in-plane direction at ambient temperature.

investigated through polarization experiments and optical characterization.

A solution to these problems lies in combining a CEM and AEM to create a bipolar membrane (BPM), Figure S1. Use of BPM in fuel cells,13 CO2 reduction,12,14 CO2 extraction and regeneration,15,16 and water electrolysis and AP applications17−21 has shown promising results by (1) allowing operation of redox reactions simultaneously under different pH conditions and (2) maintaining a pH gradient across the membrane, thereby reducing the total overpotential of the system.22 Additionally, flexibility in the choice of pH allows use of inexpensive, earth abundant catalysts such as nickel under optimum pH conditions, thereby lowering the cost of an AP device. Thus, BPM-based systems offer opportunities to reduce energy consumption of electrochemical processes such as the oxygen evolution reaction because both the pH and the catalyst materials of the anolyte can be chosen independently from the conditions of the catholyte.14,18 Typically, an extreme pH gradient is ideal for operating a BPM-based device. Under neutral pH or mild pH gradients co-ion leakage may become problematic, resulting in pH changes and thus energy loss in the device. For BPMs to operate with minimum voltage losses, a thin hydrophilic interface must be present between the CEM and AEM, consisting of a space charge region generating an electric field and weak acid or base to facilitate water dissociation. An ideal interface must provide (1) rapid orientation of water molecules in the electric field, (2) fast dissociation into protons and hydroxide ions, and (3) efficient migration of the ions toward the bulk of the ion-exchange membranes.17 To achieve these conditions, both the chemical structure of the interface, such as the nature of the fixed charge groups, and the physical features of this region, including the thickness of this layer, should be optimized. Optical transparency is also an important criterion for membrane-based AP systems.7,9 Indeed, light absorption by the membrane can result in decreased current and voltage7 because any photons absorbed by the membrane reduces the number and/or energy of the light-induced carriers. The need for transparency imposes restrictions on the thickness of the membrane and the angle of the incident light that may be employed. Herein, a new bipolar membrane is introduced that addresses these issues. The performance of this new system is

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EXPERIMENTAL SECTION

Membrane Preparation. Nafion NR-211 (Ion Power) and hexamethyl-p-terphenyl poly(benzimidazolium), referred to as PBI here, were used to make BPMs. The PBI (iodide form) was exchanged to the OH− form by soaking the membrane in 1 M KOH (SigmaAldrich) solution for 48 h at room temperature. The membrane was then rinsed with water several times and soaked in water for a few days prior to use. Nafion membranes were converted to the H+ form by soaking in 1 M H2SO4 (Fisher Scientific) for 24 h, followed by rinsing several times with water. After rinsing, the membrane was soaked in water prior to use. All solutions were prepared with Milli-Q water (18 MΩ cm resistivity). The BPMs were prepared by manually laminating PBI and Nafion membranes without using any binder or applying pressure or heat. First, a wet Nafion membrane was placed on a glass slide or directly on the opening of the half-cell, and then a wet PBI membrane was placed on top of it. The two membranes readily adhered together due to the electrostatic interactions. The thicknesses of the membranes were then measured using a micrometer, which has accuracy to 1 μm (Marathon). Measurements. Electrochemical measurements were performed using Solartron 1287 electrochemical system in a four probe DC measurement configuration. A custom-made four probe cell was used for the electrochemical tests. The geometric area of the cell opening was 2.7 cm2, defining the cross-sectional area of the membrane used in the measurements. Membranes were clamped between two glass flanges in a glass cell. Both sides of the cell were filled with either 0.5 M H2SO4 or 1 M KOH for the measurement of Nafion or PBI, respectively. For BPM measurements, the PBI membrane was faced toward basic electrolyte, and the Nafion membrane was exposed to acid electrolyte. Ag/AgCl (1 M KCl) reference electrodes were used to measure the voltage drop across the membrane. The reference electrodes were located inside Luggin capillaries to minimize the electrolyte resistance. Platinum foil electrodes, 4 cm2, were used as working and counter electrodes, producing the current through the cell. The scan rate used in I−V experiments was 1 mA s−1. UV/vis measurements were carried out using Jasco V-650 spectrophotometer. An Accumet Basic AB-15 pH meter was used to collect pH data.

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RESULTS AND DISCUSSION Physical Properties of the Membranes. Partially methylated hexamethyl-p-terphenyl-3,3″-poly(benzimidazolium) was used as the AEM. The chemical

B


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transmittance close to 0%.27 Similarly, in the case of Neosepta membrane, it is expected that the polymer fiber content within the membrane negatively impacts the light transmittance of the membrane.28 However, in the case of PBI membrane, no additional reinforcement materials are used, and in fact the robustness of the membrane is due to the mechanical integrity of the PBI polymer itself. Electrochemical Characterization. Ionic transport and electrochemically driven water dissociation in these BPMs were investigated using the four-probe cell shown in Figure S2. The experiments were carried out under reverse bias conditions, Figure 4a, in which the AEM (PBI) faces the anode and the CEM, Nafion, faces the cathode. In a reverse bias configuration, Figure 4c, a depletion layer between the two membranes is maintained, and current only flows as water dissociation takes place, allowing protons and hydroxide to proceed in opposite directions to participate in reduction and oxidation reactions. Because the hydrogen evolution reaction (HER) in Figure 4a consumes protons, the arrival of protons from the CEM allows the bipolar membrane to maintain its initial pH gradient. The same holds for the oxygen evolution reaction (OER), in which hydroxide is consumed but replaced from the AEM. In this study, 1.0 M KOH (pH: 14.0) and 0.5 M H2SO4 (pH: 0.3) were used as electrolytes in the BPM-based cell to provide a pH gradient across the membrane of 13.7. The membrane area, defined by the cell, was 2.7 cm2. Figure 5 shows polarization curves for monopolar membranes, Nafion and PBI, and a BPM using the four-electrode electrochemical cell, where current at the anode and cathode are due to OER and HER, respectively. Schematic illustrations of monopolar membrane and BPM-containing systems are shown in Figure S3, where the voltage is measured across the membrane and is a result of the resistance to proton and hydroxide transport in Nafion and PBI. In the case of the BPM, the voltage also has contributions due to pH gradient, which can be calculated using

structure of the polymer (showing the fully methylated unit) is shown in Figure 2a and contains benzimidazolium in the backbone. The synthesis and study of this polymer has been reported in previous work.23 For simplicity, this membrane is referred to as PBI, which has an ion-exchange capacity (IEC) of 2.5 mmol g−1 in the hydroxide form. Nafion NR-211 was used as the CEM. Table 1 lists pertinent physical properties of the two membranes. While their densities are similar, PBI membranes show higher water uptake and tensile strength compared to Nafion membranes. The BPMs produced consisted of two hydrated membranes laminated together without the presence of added materials or binders. The interface of the hydrated membranes showed good physical compatibility and adhesion due to electrostatic interactions. The total thickness of the BPM was approximately 90 μm. Optical Transparency of the Membranes. Optical transparency of the membrane plays an important role in the efficiency of an AP system. Unlike the natural photosynthetic system where photosystems I and II are laterally distributed within the lipid bilayer structures, the AP system shown in Figure 1 involves semiconductor materials arranged in series. Therefore, photons not absorbed by the first layer must make it through the membrane to the second layer of photoabsorbers. Any photons absorbed by the membrane will not be available to create charge carriers to complete the circuit. To date, only three commercial ion exchange membranes have been used for in AP systems: Nafion 117, Neosepta (AEM form), and Fumasep FBM bipolar membranes,21,26 and poor light transmission poses a significant challenge to their use in an integrated system. Figure 3 compares the performance of these membranes to that of the Nafion-PBI BPM introduced in this work.

VBPM =

RT [H +]cathode ln [H +]anode nF

(

) = 0.059ΔpH as well as a compo-

nent due to the resistance to water dissociation (analogous to charge transfer resistance). Polarization data were measured in a current density range similar to that expected for solar flux conditions, i.e., up to ∼20 mA cm−2. As shown in the inset of Figure 5, Nafion and PBI membranes support large currents with minimal overpotential (0.8 V, current increases linearly with voltage, associated with field-enhanced water dissociation. According to EFE theory, when the established electric field across the BPM is large enough, an electroneutral region forms at the interface of BPM that enhances the rate of water dissociation. At V < 0.8 V, the region shown in the inset, a low current associated with H+ and OH− exists, and the I−V curve is more resistive because the field at the interface is less.32,33 The lower membrane voltage at low current density is likely due to nonideal selectivity of the membrane. However, above 0.8 V, the I−V curve becomes ohmic, suggesting the formation of co-ion depleted zone. D


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Figure 7. (a) Stability comparison at 10 mA cm−2 between the Nafion-PBI and Fumase FBM bipolar membranes. (b) pH−t curve of Nafion-PBI contained cell during 4 h of experiment. (red) pH gradient of the cell, (purple) pH values of the cathode, and (blue) pH values of the anode.

s−1 V−1 for protons and hydroxide, respectively, in our calculations.45 The resistance of the transition region (Rtr) can be calculated using Rtr = RBPM − (RPBI + RNAF). This provides an Rtr value of ∼17.25 Ω cm2 and leads to a transition region thickness of about 9 nm. A description of this calculation is provided in the Supporting Information. These findings are consistent with the production of a very thin transition region resulting in low voltage losses in the system and enhanced water dissociation. The stability of the Nafion-PBI-based cell was investigated by operating it at a constant current of 10 mA cm−2 for an operating time of 13 h. Typically, nonstable membrane-based devices show significant voltage change within the first 6−7 h of the operation.11,12 Figure 7 presents a stability comparison between PBI-Nafion bipolar membrane and Fumasep bipolar membrane. As shown, the voltage of the Fumasep FBM was stable throughout the 13 h of operation with no change in the membrane voltage. Similarly, Nafion-PBI membrane exhibited stable voltage after long-term operation of the cell at constant current. The small fluctuations that appeared in the V−t graphs of both membranes could be related to the bubble removal and/or nonideal contact of the PBI-Nafion interface in some locations. Unlike Fumasep FBM, which is a single sheet (of two ideally connected membranes), PBI-Nafion bipolar membranes were manually laminated; thus, it is possible that some locations of the BMP (e.g., the edges) are not ideally connected. The good stability of BP-contained cell is evidence that the pH gradient is maintained across the BPM. The results of the stability test (Figure 7a) are consistent with the stable pH gradient recorded for the PBI-Nafion-contained system (Figure 7b). As shown, after 4 h of continuous operation, the pH gradient of the overall cell remained constant and stable. The total charge passed after 4 h of operation at constant current of 10 mA cm−2 is C = 10 × 10−3 A cm2 × 4 × 3600 C cm−2 = 144 C. The expected oxygen volume at the anode may 1000TρC be calculated from O2 (volume, mL) = 4FM ,21 where T is

voltage) is the sum of four contributions: (i) voltage drop across the IL where the CEM and AEM are in intimate contact and the concentration of mobile ions is markedly reduced, (ii) the Donnan potential at both solution−membrane interfaces (cathode and anode side), (iii) the iR drop across the membrane caused by the limited mobility of the counterions (proton and hydroxide ions for CEM and AEM, respectively, and (iv) the iR drop in the solution (which is negligible).40−42 The electrochemical performance of the BP membrane was investigated by conducting I−V measurement and also galvanostatic tests. From Figure 6, Vmembrane of about 0.87 and 0.94 V was obtained at current density of 5 and 8 mA cm−2, respectively. Given the fact, that WD occurs at the bulk interface of PBI-Nafion, and no external materials (i.e. catalyst), modification process, or any binder was applied in the interface; this performance indicates a very good physical and chemical compatibility between the two membranes. Indeed, upon simply interfacing the hydrated PBI with hydrated Nafion, the two membranes readily adhered due to the strong electrostatic interaction between them. Thus, the low voltage drop across the PBI-Nafion bipolar membrane is due to both good conductivity of the individual membranes and the good interface between the two membranes. Table S1 compares the voltage drop across the membrane for various catalyst-free and catalyst-assisted BPM cells.14,17,18,21 As shown, PBI-Nafion membrane possess good potential for AP applications. Further, the efficiency of a BPM is impacted by the transition region, where the water dissociation takes place. There is a direct relationship between the thickness of the transition region, Figure S4, and the magnitude of voltage drop across the BPM, where a thicker transition forms as the voltage drop across the BPM increases.43 The thickness of the transition region (2λ) can be calculated from the following equation:32 2λ = R trF(C H+μH+ + COH−μOH− )

(1)

where F is Faraday’s constant, Rtr is the ionic resistance of the transition region (referred to as IL herein), and CH+ and COH− are the concentrations of proton and hydroxide in the transition region, respectively, assumed to be 10−7 mol/L each because only water molecules exist there. This is a result of the fact that within the IL, no mobile protons or hydroxide exist. μH+ and μOH− are the mobility of the proton and hydroxide ions in the transition region, respectively.39,43 The mobilities of proton and hydroxide in the IL are assumed to be similar to those in bulk water44 and were taken to be 36.23 × 10−8 and 20.64 × 10−8 m2

W

the reaction temperature (295 K), ρ is the O2 density at atmospheric pressure (1.43 g cm−3), C is the total charge passed (144 C here), F is Faraday’s constant (96 500 C mol−1), and Mw is the molecular weight of O2 (31.99 g mol−1). Thus, assuming 100% faradic efficiency, expected O2 volume is about 4.92 mL. Similarly, using Faraday’s law and assuming 100% E


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(4) Wasielewski, M. R. Self-Assembly Strategies for Integrating Light Harvesting and Charge Separation in Artifical Photosynthetic Systems. Acc. Chem. Res. 2009, 42, 1910−1921. (5) Ronge, J.; Bosserez, T.; Martel, D.; Nervi, C.; Boarino, L.; Taulelle, F.; Decher, G.; Bordiga, S.; Martens, J. A. Monolithic Cells for Solar Fuels. Chem. Soc. Rev. 2014, 43, 7963−7981. (6) Gray, H. B. Powering the Planet with Solar Fuel. Nat. Chem. 2009, 1, 7. (7) McFarlane, S. L.; Day, B. A.; McEleney, K.; Freund, M. S.; Lewis, N. S. Designing Electronic/ionic Conducting Membranes for Artificial Photosynthesis. Energy Environ. Sci. 2011, 4, 1700−1703. (8) McKone, J. R.; Lewis, N. S.; Gray, H. B. Will Solar-Driven WaterSplitting Devices See the Light of Day? Chem. Mater. 2014, 26, 407− 414. (9) Spurgeon, J. M.; Walter, M. G.; Zhou, J.; Kohl, P. A.; Lewis, N. S. Electrical Conductivity, Ionic Conductivity, Optical Absorption, and Gas Separation Properties of Ionically Conductive Polymer Membranes Embedded with Si Microwire Arrays. Energy Environ. Sci. 2011, 4, 1772−1780. (10) Spurgeon, J. M.; Walter, M. G.; Zhou, J.; Kohl, P. A.; Lewis, N. S. Electrical Conductivity, Ionic Conductivity, Optical Absorption, and Gas Separation Properties of Ionically Conductive Polymer Membranes Embedded with Si Microwire Arrays. Energy Environ. Sci. 2011, 4 (5), 1772−1780. (11) Hernández-Pagán, E. A.; Vargas-Barbosa, N. M.; Wang, T.; Zhao, Y.; Smotkin, E. S.; Mallouk, T. E. Resistance and Polarization Losses in Aqueous Buffer−membrane Electrolytes for Water-Splitting Photoelectrochemical Cells. Energy Environ. Sci. 2012, 5 (6), 7582. (12) Li, Y. C.; Zhou, D.; Yan, Z.; Gonçalves, R. H.; Salvatore, D. A.; Berlinguette, C. P.; Mallouk, T. E. Electrolysis of CO2 to Syngas in Bipolar Membrane-Based Electrochemical Cells. ACS Energy Lett. 2016, 1, 1149−1153. (13) Unlu, M.; Zhou, J.; Kohl, P. A. Hybrid Anion and Proton Exchange Membrane Fuel Cells. J. Phys. Chem. C 2009, 113, 11416− 11423. (14) 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 CO2 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 Cathode. ACS Energy Lett. 2016, 1, 764−770. (15) Eisaman, M. D.; Alvarado, L.; Larner, D.; Wang, P.; Garg, B.; Littau, K. A. CO2 Separation Using Bipolar Membrane Electrodialysis. Energy Environ. Sci. 2011, 4 (4), 1319−1328. (16) Eisaman, M. D.; Parajuly, K.; Tuganov, A.; Eldershaw, C.; Chang, N.; Littau, K. A. CO2 Extraction from Seawater Using Bipolar Membrane Electrodialysis. Energy Environ. Sci. 2012, 5 (6), 7346. (17) McDonald, M. B.; Ardo, S.; Lewis, N. S.; Freund, M. S. Use of Bipolar Membranes for Maintaining Steady-State pH Gradients in Membrane-Supported, Solar-Driven Water Splitting. ChemSusChem 2014, 7, 3021−3027. (18) Vermaas, D. A.; Sassenburg, M.; Smith, W. A. Photo-Assisted Water Splitting with Bipolar Fuel Devices. J. Mater. Chem. A 2015, 3, 19556−19562. (19) Chen, Y.; Lewis, N. S.; Xiang, C. Operational Constraints and Strategies for Systems to Effect the Sustainable, Solar-Driven Reduction of Atmospheric CO2. Energy Environ. Sci. 2015, 8, 3663− 3674. (20) 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, 3017−3020. (21) Sun, K.; Liu, R.; Chen, Y.; Verlage, E.; Lewis, N. S.; Xiang, C. A Stabilized, Intrinsically Safe, 10% Efficient, Solar-Driven WaterSplitting Cell Incorporating Earth-Abundant Electrocatalysts with Steady-State pH Gradients and Product Separation Enabled by a Bipolar Membrane. Adv. Energy Mater. 2016, 2, 1−7. (22) McDonald, M. B.; Bruce, J. P.; McEleney, K.; Freund, M. S. Reduced Graphene Oxide Bipolar Membranes for Integrated Solar Water Splitting in Optimal pH. ChemSusChem 2015, 8, 2645−2654.

efficiency, the estimated hydrogen volume after 4 h of operation is about 9.82 mL. Regarding product crossover, e.g., O2 permeability into cathode compartment, it is of note to mention that this feature is strongly affected by the relative humidity (RH), membrane thickness, and membrane structures. For example, PBI membrane (OH− form) showed oxygen permeability of 2.4 × 10 −12 and 0.3 × 10 −12 mol cm−1 s−1, at 70 and 90% RH, respectively.46 For Nafion 211 membrane, oxygen permeability of 0.25 × 10−12 and 0.27 × 10−12 mol cm−1 s−1 has been reported at 70% and 90% RH, respectively.46 Other reports21 indicate values of about 0.45 × 10−12 and 2 × 10−12 mol cm−1 s−1 for oxygen permeability in Fumasep FBM and Nafion 117, respectively. Because in this study a 90 μm-thick bipolar membrane, PBI-Nafion, was used, it is expected that this membrane performs similarly in terms of crossover.

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CONCLUSION In conclusion, the use of a benzimidazolium-based anion exchange membrane for making BPMs was explored, and the suitability of the new BPM system for artificial photosynthesis applications was assessed. The findings reveal that the two membranes create a very good interface in terms of both physical connection and electrostatic interaction. The strongly bonded interface between CEM and AEM produces a large electric field that enables fast water dissociation near the thermodynamic potential without any need for additional water dissociation catalyst layers. It is expected that the new transparent BPM may form the basis for efficient membranebased AP applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04402. Bipolar membrane configuration, four probe cell, bipolar membrane voltage drop data, and calculation details (Figures S1−S4 and Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: msfreund@fit.edu. *E-mail: [email protected]. ORCID

Sakineh Chabi: 0000-0002-5578-3984 Michael S. Freund: 0000-0003-1104-2292 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSF under the NSF CCI Solar Fuels Program under Grant CHE-1305124 and by the Natural Sciences and Engineering Research Council of Canada.

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REFERENCES

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