Effect of Surface Ion Conductivity of Anion Exchange Membranes on

Aug 24, 2016 - Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae, Kofu 400-0021, Japan ... Phone: +81-55-220-8707...
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Effect of Surface Ion Conductivity of Anion Exchange Membranes on Fuel Cell Performance Masanori Hara,† Taro Kimura,‡ Takuya Nakamura,‡ Manai Shimada,‡,§ Hideaki Ono,‡ Shigefumi Shimada,‡ Kenji Miyatake,*,†,∥ Makoto Uchida,† Junji Inukai,*,† and Masahiro Watanabe† †

Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae, Kofu 400-0021, Japan Interdisciplinary Graduate School of Medicine and Engineering and ∥Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8510, Japan § Takahata Precision Japan Co., Ltd., 390 Maemada, Sakaigawa, Fuefuki, Yamanashi 406-0843, Japan ‡

S Supporting Information *

ABSTRACT: Anion conductivity at the surfaces of two anion-exchange membranes (AEMs), quaternized ammonium poly(arylene ether) multiblock copolymer (QPE-bl-3) and quaternized ammonium poly(arylene perfluoro-alkylene) copolymer (QPAF-1), synthesized by our group was investigated using current-sensing atomic force microscopy under purified air at various relative humidities. The anion-conducting spots were distributed inhomogeneously on the surface of QPE-bl-3, and the total areas of the anion-conducting spots and the current at each spot increased with humidity. The anion-conductive areas on QPAF1 were found on the entire surface even at a low humidity. Distribution of the anion-conducting spots on the membrane was found to directly affect the performance of an AEM fuel cell.

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ammonium groups as AEMs (QPE-bl).9,19,20 Recently, a novel AEM containing perfluoro-alkylene chain in the hydrophobic group of the main chain, a quaternized ammonium poly(arylene perfluoro-alkylene) copolymer (QPAF), was synthesized.21 QPAF showed higher durability and flexibility than those of QPE-bl, with a comparable anion conductivity. To improve the conductivity of both PEMs and AEMs, understanding the ion-transport mechanism is important. Inside of a PEM, phase-separated structures are proposed to be organized, in which hydrophilic groups in a hydrophobic matrix form spherical-shaped clusters, interconnected by narrow ionic channels;2,22,23 these interconnected hydrophilic clusters in the phase-separated structure serve as protonconductive networks for the proton transport. Coordination of the phase-separated structures has been attempted to improve proton conductivity. Block copolymerization is an effective approach for developing phase separation.2,17,24−26 The

INTRODUCTION Fuel cells have attracted considerable attention as promising sources of clean power owing to their high efficiency and low emission.1,2 Among them, proton-exchange-membrane fuel cells (PEMFCs) are commercialized and used in stationary fuel cells and fuel cell automobiles. In recent years, anionexchange-membrane fuel cells (AEMFCs) have been actively studied because of their potential use of non-precious metal catalysts and the enhancement of oxygen reduction reaction kinetics.3−6 Several research groups reported demonstrations of AEMFCs with non-precious metals such as nickel, cobalt, and silver.7−12 To achieve higher performance and durability of AEMFCs, enhanced properties of the anion-exchange membranes (AEMs) are required, such as high stability at high temperatures under alkaline conditions and higher anion conductivity. To improve the stability and conductivity of AEMs, various types of main-chain structures and cationic groups in polymers have been examined.3−6,13−16 In our study, a series of poly(arylene ether) (PE) multiblock copolymers, used also in proton-exchange membranes (PEMs),17,18 were applied to the main chains and functionalized with quaternary © XXXX American Chemical Society

Received: May 14, 2016 Revised: August 22, 2016

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Figure 1. Chemical structures of (a) QPE-bl-3 and (b) QPAF-1. the membranes was calculated using a microbalance (water immersion for 2 h at room temperature and vacuum-drying for 3 h at 80 °C).19−21 OH− conductivity of the membranes in pure degassed water was measured using a four-probe conductivity cell using an impedance spectroscope (Solartron 1255B and 1287, Solartron Inc.).19−21 Ionconducting resistances (R) were determined from the impedance plot obtained in the frequency range of 1−105 Hz. Anion conductivity (σ) was calculated from the equation σ = l/(A × R), where A and l are the conductive area and the distance between two reference electrodes, respectively. Anion conductivity of an AEM in the hydroxide form is difficult to measure because of the presence of carbonate anions from CO2 in air as contaminants.47 Therefore, we carried out the conductivity measurements very carefully as described in our previous paper.21 Moreover, the conductivity of the two membranes was measured under the same conditions and procedures, which allowed a good comparison of the two membranes. TEM and SAXS. For the TEM observations,20,21 AEMs were stained with PtCl42− ions by ion exchange of the quaternized ammonium groups in a PtCl42− aqueous solution, rinsed with deionized water, and dried in a vacuum oven for 12 h. The stained membranes were embedded in an epoxy resin, sectioned to 90 nm thickness with a Leica Ultracut UCT microtome, and placed on copper grids. Images were taken on a Hitachi H-9500 transmission electron microscope, with an accelerating voltage of 200 kV. SAXS profiles were obtained using a Nano-Viewer (Rigaku), and Cu (Kα) was used as an X-ray source.48 The SAXS measurement was performed at 80 °C under a humidified N2 atmosphere of 30%−90% RH. The AEMs in the chloride-ion form were placed in a SAXS cell and exposed to and equilibrated in humidified N2 for 2 h before each measurement. CS-AFM Measurements. Membrane surfaces attached to the glass substrate (substrate side) and exposed to air (air side) during the formation of the membrane were both used for the CS-AFM measurements. QPE-bl-3 and QPAF-1 membranes in the hydroxideion form were pressed with a gas diffusion electrode (GDE), composed of three layersthat is, catalyst layer, microporous layer, and wet-proof carbon paperat 1.0 MPa for 10 min. The GDE was prepared by spraying a catalyst ink containing the Pt catalyst supported on carbon black (TEC10E50, 47.9 mass %-Pt, Tanaka Kikinzoku Kogyo K.K.) and ionomers (IEC = 1.4 mequiv g−1, AS-4, Tokuyama Co.) as a binder on a gas diffusion layer (CFP300, Toray Ind. Inc.) using a pulse−swirl−spray apparatus (PSS, Nordson Co., Ltd.). The mass ratio of binder (dry basis) to carbon black in the ink paste was adjusted to 0.8. The Pt loading of electrodes was 0.5 ± 0.1 mg cm−2. To improve the adhesion of the membranes to the GDE, a thin layer of Tokuyama binder was coated on the GDE. The AFM samples were placed on a temperature-controlled sample stage of the CS-AFM system.

performance of a PEMFC with a multiblock copolymer synthesized in our laboratory was first found to be lower than that with Nafion in spite of its higher conductivity. By introducing a thin layer of Nafion on the synthesized membrane, the cell performance was very much improved;27 not only the ion-conducting paths inside of the membrane were important for improving the cell performance, but those on the surfaces (or the interfaces with the catalyst layers) directly governed the ion transports into/out of the membrane.28 To investigate the proton-conducting properties on a PEM surface, current-sensing atomic force microscopy (CS-AFM) has recently been applied to investigation of Nafion,29−41 then to hydrocarbon-based membranes,42−46 which allowed a direct observation of proton-conducting paths on the surface under controlled conditions. Hydration of the membranes was revealed to have an effect on the distribution of the protonconductive domains on PEM surfaces.30,34,38,39,41,43 However, for AEMFCs, only a few reports have been published on the anion-conducting behavior both inside of the bulk and on the surface.46 In this study, the phase separations and the distributions of the anion-conductive areas of two AEMs with different phaseseparated structures, QPE-bl-320 and QPAF-1,21 were investigated using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) for bulk and using CSAFM for surfaces. Arrangement of the conducting spots at the surface was found to significantly affect the fuel cell performance. The importance of the interfacial structure between a catalyst layer and an AEM was visually explained at the nanoscale level for the first time.

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EXPERIMENTAL SECTION

Water Uptake and Ion Conductivity. The molecular structures of QPE-bl-3 and OPAF-1 are shown in Figure 1a,b, respectively. The synthesis of the membranes was reported elsewhere.20,21 The number of repeating units in the hydrophobic block (X) and the hydrophilic (Y) block for QPE-bl-3 was X = 5 and Y = 10. The ratio of the hydrophobic units (m) and the two different hydrophilic units (n and o) for QPAF-1 (Figure 1b) was m:n:o = 1.00:0.48:0.62. The ionexchange capacity (IEC) of the membranes obtained from titration was 1.80 mequiv g−1 for QPE-bl-3 and 1.18 mequiv g−1 for QPAF-1. The membranes were formed by solution casting on a glass substrate.20,21 Water uptake [(mass of the hydrated membrane − mass of the dry membrane)/(mass of the dry membrane) × 100%] of B

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Langmuir CS-AFM measurements were carried out using a commercial AFM system (SPM-5500, Agilent) equipped with a home-made environmental chamber43−45 under a purified (CO2-free) air atmosphere at 40 °C and at 50% and 70% RH. A Pt/Ir-coated silicon tip (NanoWorld) was used for the CS-AFM measurements. The morphological and current images were obtained at the contact mode, with a contact force of 10 nN on membrane surfaces and a sample voltage of 2.0 V between the AFM tip and the GDE. A schematic representation of the CS-AFM measurement is shown in Figure S1. Before measurements, humidified air was supplied to the environmental chamber (dead volume = 500 mL) at 100 mL min−1 for 2 h. During the AFM measurements, the flow rate was reduced to 10 mL min−1. To analyze the current images, the threshold value was set at a background current of 0.5 pA. To ensure that there was no tip damaging on the surface, we obtained the first and second scanned images at the same position for each CS-AFM measurement. The tip voltage was kept at 2.0 V during the image acquisition. Fuel Cell Performance. Commercial 50 wt % Pt-loaded catalysts (Pt/CB, TEC10E50, Tanaka Kikinzoku Kogyo K.K.) were used for both the cathode and the anode. QPE-bl-3 and QPAF-1 membranes were used for membrane electrode assemblies (MEAs). Catalystcoated membranes (CCMs) were prepared as previously described in the case of GDEs. The catalyst pastes were prepared from Pt-loaded catalysts with QPAF-1 ionomers (IEC: 1.19 mequiv g−1) as the binder. The mass ratio of binder (dry basis) to carbon black (QPAF-1 ionomer/carbon) was adjusted to 0.8. The catalyst paste was directly sprayed onto the electrolyte membrane to prepare a CCM using a pulse−swirl−spray apparatus and then dried under an ambient condition. The active geometric area of the electrode was 4.41 cm2, and the Pt loading on both the cathode and the anode was 0.2 ± 0.05 mg Pt cm−2. The MEA was prepared by sandwiching a CCM between two gas diffusion layers (25BC, SGL Carbon Group Co., Ltd.) at 1.0 MPa for 3 min. The MEA was assembled into a single-serpentinepatterned cell (Takahata Precision Japan Co., Ltd.) consisting of two carbon separator plates. I−E curves were measured for the cell performance test using a fuel cell evaluation system (Netuden Kougyou Co., Ltd.) at Tcell = 40 °C under an ambient pressure with humidified H2 and O2 supplied to the anode and the cathode, respectively, as reported in our previous papers.20,21 Gases were humidified at 100% RH. The gas flow rate was 100 mL min−1. The high-frequency resistance (HFR) of the cell with the QPE-bl-3 and QPAF-1 membranes was measured with an FC impedance meter (Kikusui Electronics Co.) at 5.0 kHz after obtaining the Cole−Cole plot of a cell.

Figure 2. (a) Water uptake and (b) OH− conductivity of QPE-bl-3 (filled circles) and QPAF-1 (open circles).

layer interface, affecting the AEMFC performance should be understood by a different method. Bulk Structures of QPE-bl-3 and QPAF-1. Phaseseparated structures of the QPE-bl-3 and QPAF-1 membranes were investigated using TEM. Figure 3a shows a cross-sectional

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RESULTS AND DISCUSSION Water Uptake and Ion Conductivity of QPE-bl-3 and QPAF-1. Figure 2a shows the water uptake of QPE-bl-3 and QPAF-1 at room temperature as a function of IEC.20,21 With the increase in the IEC, the water uptake increased on both membranes. At IEC < 1.0 mequiv g−1, the water uptake of QPE-bl-3 (filled circles) and QPAF-1 (open circles) was similar. At IEC > 1.0 mequiv g−1, the water uptake of QPAF-1 increased very much with respect to the IEC and became higher than that of QPE-bl-3 at a high IEC. Figure 2b shows the in-plane OH− conductivity of QPE-bl-3 and QPAF-1 as a function of temperature. The OH− conductivity of QPE-bl-3 (1.80 mequiv g−1) was similar to that of QPAF-1 (1.18 mequiv g−1) in pure water in spite of the lower IEC of QPAF-1.20,21 Interestingly, as seen in Figure 2a, the water uptake in QPE-bl-3 (1.80 mequiv g−1) and QPAF-1 (1.18 mequiv g−1) was the same, which is approximately 50%. The result suggests the importance of water uptake on conductivity of AEMs. It should be noted that the bulk conductivity was measured using impedance spectroscopy. Therefore, the OH− transport through the membrane surface, or the membrane/catalyst

Figure 3. TEM images of (a) QPE-bl-3 and (b) QPAF-1.

TEM image of a PtCl42− ion-exchanged QPE-bl-3 membrane. The quaternized ammonium groups forming hydrophilic clusters were stained and observed as dark domains, whereas the hydrophobic moieties were observed as the bright ones. Both hydrophilic and hydrophobic regions inside of QPE-bl-3 had a wide distribution of 10−20 nm.20 The TEM image revealed comparatively uniform phase-separated hydrophobic C

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coated AFM tip contacted with the air side of the QPE-bl-3 surface in an environmental chamber filled with purified air at 40 °C and at 50% and 70% RH are shown in Figure 5a. The

and hydrophilic domains in QPE-bl-3. As shown in Figure 3b for the QPAF-1 membrane, spherical hydrophobic and hydrophilic domains were much smaller, approximately 2 nm in diameter.21 This phase-separated fine structure in QPAF-1 could be partly due to the random distribution of hydrophobic (perfluoro-alkylene) and hydrophilic (ammonium-substituted phenylene) groups in a QPAF-1 polymer molecule. SAXS profiles were obtained for QPE-bl-3 and QPAF-1 in a chloride-ion form at 80 °C and at relative humidities (RHs) between 30% and 90%. The scattered intensity with respect to the scattering vector (q) is shown in Figure 4. For QPE-bl-3

Figure 5. I−V curves obtained on a PtIr-coated AFM tip located on a membrane surface at 40 °C and 70% RH (red line) and 50% RH (black line) under a purified air condition. (a) QPE-bl-3. (b) QPAF-1.

membrane was stable, showing no indication of decomposition between the tip bias voltages of −3.0 and 3.0 V. Before the I−V measurements, the AFM chamber was purged with a feed gas for 2 h. In our setup of the CS-AFM measurement, OH− ions formed at the cathode (GDE) are transferred to the anode through an AEM and reduced to oxygen by the oxygen evolution reaction (OER) on the anode (AFM tip). The OER current detected at the AFM tip increased with increasing bias voltage. At a higher relative humidity, the current on the AFM tip became larger because of the higher anion conductivity of the membrane and because of the higher reaction rates on the GDE and the AFM tip (Figure S1). To understand the OH− formation mechanism, we replaced the purified air by pure nitrogen humidified at 70% RH for the I−V measurements. The I−V curves with nitrogen were approximately identical to those with air. Therefore, OH− ions were formed mainly by water electrolysis. Figure 5b shows the I−V curves obtained on the air side of QPAF-1 at 40 °C and at 50% and 70% RH. The current detected on the AFM tip increased with increasing bias voltage and relative humidity, as the same as on QPE-bl-3. The current detected on QPAF-1 was larger than that on QPE-bl-3 at the two humidities. CS-AFM measurements on the air side of QPE-bl-3 were carried out under purified air. The morphological and current images of the membrane surface in an area of 1 × 1 μm2 are shown in Figure 6 at the temperature of 40 °C and at the humidities of 50% and 70% RH. The morphological image of the membrane at 50% RH (Figure 6a) shows that the surface was flat, with corrugations of only 10−20 nm in the scanned area. The corrugation of the membrane surface at 70% RH (Figure 6b) was slightly larger than that at 50% RH, probably because of water swelling. The current images were simultaneously obtained at the tip bias voltage of 2.0 V. The bright areas represent conductive regions, and the dark areas

Figure 4. SAXS intensity of (a) QPE-bl-3 and (b) QPAF-1 at 80 °C and 30%−90% RH as a function of q.

(Figure 4a), the curves were nearly identical at different RHs. Significant peaks were seen neither on the curves, except for a small shoulderlike hump indicated by an arrow in the figure around 0.2 nm−1, nor on the d spacing of 30 nm. This small, broad peak might indicate the formation of some cluster arrangement with a large distribution in size, resulting in a low periodicity. Therefore, the SAXS profiles were consistent with the TEM image (Figure 3a), with large hydrophilic and hydrophobic clusters and wide size distributions. The repeating units in QPE-bl-3 were twice the size of those observed using TEM because the sum of diameters of a hydrophobic and a hydrophilic cluster was observed on the SAXS profiles as peaks. For QPAF-1 (Figure 4b), the SAXS profile with a peak around 1 nm−1, or the d spacing of 6 nm, was observed. The peak shifted to a lower q value with increasing humidity; the ionomer clusters became larger with humidity in the case of QPAF-1. The repeating units in QPAF-1 observed using SAXS were larger than those expected from the TEM image obtained in vacuum (Figure 3b) because of the water swelling and expansion of the membrane. The influence of water was thus larger in QPAF-1 than that in QPE-bl-3, which was not contradictory to the water uptake behavior mentioned above (Figure 2a). Surface Conductivity on QPE-bl-3 and QPAF-1. The current−voltage (I−V) curves obtained at the apex of a Pt/IrD

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Figure 6. Topography (upper row) and current images (lower row) on the air side of QPE-bl-3 at 40 °C and at 50% (left column) and 70% RH (right column) under purified air conditions. Tip bias voltage = 2.0 V.

Figure 7. Topography (upper row) and current images (lower row) on the air side of QPAF-1 at 40 °C and at 50% (left column) and 70% RH (right column) under purified air conditions. Tip bias voltage = 2.0 V.

represent nonconductive or low-conductive regions. The OH− conducting spots were about ten to several tens of nanometers in diameter and distributed inhomogeneously on the

membrane surface. The size of the spots increased, and the current became larger at a higher humidity. Correlation E

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Langmuir between the topographic image and the current image was not observed. In our previous paper on PEMs, we reported that the distribution of proton-conductive regions on the membrane surface was dependent on the casting and pretreatment conditions.45 To investigate the effect of the casting process on the distribution of ion-conductive regions on AEMs, CSAFM images of both surfaces of QPE-bl-3, air and substrate sides, were compared. Morphological and current images obtained using AFM on the substrate side of QPE-bl-3 at 40 °C and at 70% RH are shown in Figure S2a,c, respectively. The topographic image shows that the membrane surface of the substrate side was also very flat, with the maximum height difference of only 20 nm in the measured area. In the current image, the anion-conductive areas on the substrate side were more uniformly distributed than those on the air side (Figures 6d and S2d). On the substrate side, the relationship between the morphology and conductivity was not seen either. As shown in Figure S2, the current detected at the conductive areas was larger on the air side than that on the substrate side. On the other hand, the ratio of the conductive areas observed on the substrate side was larger than that on the air side. However, the difference in CS-AFM images on the two sides was much smaller when compared with that observed on PEMs,45 probably because of swelling and reformation of the membrane during the preparation of QPE-bl-3 in a hydroxideion form, similarly reported on a PEM during pretreatment.45 The small variance observed on the surfaces on an AEM image should imply insignificant effects on the membrane performance during power generation because of the expected absorption of the hot water produced at the cathode. Figure 7 shows the morphological and current images of the air side of QPAF-1 at 40 °C and at 50% and 70% RH. The air side of QPAF-1 was flat, and the corrugation of the membrane surface also increased with relative humidity. The current images show that the OH− conductive area was distributed more homogenously on QPAF-1 than on QPE-bl-3 (Figure 6). The current increased as the relative humidity increased. The morphological and current images on the substrate side of QPAF-1 at 40 °C and 70% RH are shown in Figure S3a,c, respectively. The topographic image (Figure S3a) shows that the membrane surface of the substrate side had a smaller corrugation than that of the air side (Figures 7b and S3b) and that the anion-conductive areas on the substrate side (Figure S3c) were more uniformly distributed than those on the air side (Figures 7d and S3d). The current detected at the conductive areas on the air side was larger than that on the substrate side (Figure S3), as the same as on QPE-bl-3. It is interesting to note that conductive spots were larger in number and distributed uniformly on the air side on the PEM prepared in our group45 when a hydrophobic substrate of poly(ethylene terephthalate) was used for casting. In the present study, the surface of a glass substrate was hydrophilic, and uniform arrangements of the conductive spots were observed on the substrate side. This might indicate the importance of the hydrophilicity of substrate surfaces on the membrane preparation process. In this study, however, the membranes were immersed in an alkaline solution and in deionized water before the AFM measurements, so the casting process showed smaller influences. The cross-sectional current profiles are depicted in Figure 8 along the yellow lines drawn in Figures 6d and 7d. The anionconducting spots on QPE-bl-3 are seen to aggregate themselves

Figure 8. Cross sections of the current images of (a) QPE-bl-3 and (b) QPAF-1 under 70% RH purified air condition.

forming clusters several tens of nanometers in diameter (Figure 8a). The peak current for the clusters on the QPE-bl-3 surface was within the range of between 5 and 13 pA. For QPAF-1 (Figure 8b), the peak current was approximately between 6 and 10 pA, and the clusters were smaller than those observed on QPE-bl-3. The base current for QPE-bl-3 was almost 0 pA (Figure 8a), whereas that for QPAF-1 was approximately 1.5 pA (Figure 8b). On the surface of QPAF-1, the anionconducting current was detected practically at any location because the conducting spots were very homogeneously distributed. Figure 9a,b shows the “pseudo average current density” and the surface allotment ratio of the anion-conductive areas, respectively, analyzed from the current images in Figures 6d and 7d. In this study, a “pseudo current density” is defined as

Figure 9. (a) Pseudo average current density and (b) the ratio of the anion-conductive area on QPE-bl-3 and QPAF-1 at 50% and 70% RH. F

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intersection with the Z″-axis, for the real part of the dielectric constant, was found at the frequency of 5.0 kHz. Therefore, the cell ohmic resistance was obtained at a constant frequency of 5.0 kHz, where the ionic transfer resistance of the bulk membrane was predominately obtained. The mass activity at 0.85 V was 17 and 23 A g-Pt−1 on the cell with QPE-bl-3 and QPAF-1, respectively. Apparently, the cell performance with QPAF-1 was higher than that with QPE-bl-3, especially at a high current density (Figure 10a). The ohmic resistance with QPE-bl-3 was higher only by 20% than that with QPAF-1 (Figure 10b), whereas the ion conductivity of QPE-bl-3 and QPAF-1 was comparable (Figure 2b). The ohmic resistances were approximately 4−5 times higher than those estimated from the hydroxide ion conductivity of the membrane in water at 40 °C (Figure 2b) because of the lower conductivity of AEMs in gases and of the contact resistance between the membrane and the catalyst layers. The current density at 0.75 V, an indication of the cell performance under the practical operating conditions, was 1.8 times higher with QPAF-1 than that with QPE-bl-3. The high performance with QPAF-1 might seem puzzling because both the bulk conductivity (Figure 2b) and the pseudo average current density at the surface (Figure 9a) were comparable with QPAF-1 and QPE-bl-3. The current density at 0.75 V is thought to reflect the overall influence of Pt utilization, ion conductivity, and oxygen/hydrogen gas transport.1,49,50 With QPAF-1 and QPE-bl-3, the ion conductivity was similar as already discussed. The gas transport must be identical because the same GDLs, catalysts, and ionomers were used for the two MEAs. Therefore, it should be the Pt catalysts that must be less effectively used in the MEA with QPE-bl-3. Differences existed in the distribution of the conductive spots on surfaces; on QPAF-1, the spots were distributed very uniformly and nonconductive regions practically did not exist. Therefore, the transported OH− ions through the interface between the QPAF-1 membrane and the catalyst layer should be efficiently supplied to the Pt catalysts in the MEA for power generation. On the other hand, the anion-conducting spots were not uniformly distributed on the QPE-bl-3 surface, and large nonconductive regions existed on the QPE-bl-3 surface. The anion fluxes, therefore, must be inhomogeneous through the interface between QPE-bl-3 and the catalyst layer. At the catalysts located on the nonconductive areas of the QPE-bl-3 surface, it would be difficult to supply the OH− ions, which should lower the Pt utilization. The influence of the ionic conductance at the interface between the membrane and the catalyst layer has been reported for PEMs with a Nafion-coated hydrocarbon-type membrane with a Nafion-based catalyst layer.27 A similar effect was observed even for a Nafion membrane with a Nafion-based catalyst layer.28 In this study, we visually showed the importance of the interfaces with AEMs which directly influenced the performance of an AEMFC.

the integrated currents detected on the AFM tip at individual 512 × 512 pixels in a current image, subsequently divided by the scanned area.45 The pseudo current density is, therefore, not the actual current density but expected to be nearly proportional to the actual anion current density through the AEM surface. As shown in Figure 9a, the pseudo average current density at 50% RH on QPAF-1 was three times larger than that on QPE-bl-3, but at 70% RH, the anion conductivity on QPE-bl-3 (45 A cm−2) became comparable to that on QPAF-1 (51 A cm−2). The ratio of the anion-conductive region on the QPE-bl-3 surface increased from 37% to 83%, that is more than doubled, as the humidity increased from 50% to 70% RH. On QPAF-1, the ratio of the anion-conductive area was almost 100% at both 50% and 70% RHs. Therefore, the increase in pseudo average current density on QPE-bl-3 by an order of magnitude (×10) at 70% RH (Figure 9a) should be attributed to the increase both in the surface-conductive area (×2) and in bulk conductivity (×5), whereas that on QPAF-1 should be attributed mainly to the increase in bulk conductivity (×3) because the anion-conductive area did not change (Figure 9b). At a high humidity, the bulk conductivity was measured nearly the same for both QPE-bl-3 and QPAF-1 (Figure 2b), and the ratio of the anion-conducting region was smaller on QPE-bl-3 than that on QPAF-1 by 17% (Figure 9b). Therefore, it is understandable that the pseudo average current density at QPE-bl-3 was smaller than that at QPAF-1 by 12% (Figure 9a). It should be remembered, however, that the distribution of the conductive spots on the QPAF-1 and QPE-bl-3 surfaces was very different (Figures 66−8). Cell Performance with QPE-bl-3 and QPAF-1 Membranes. Figure 10 shows the current density−voltage curves and the ohmic resistances of AEMFCs with H2 and O2 at 100% RH at a cell temperature (Tcell) of 40 °C using membrane electrode assemblies (MEAs) with QPE-bl-3 and QPAF-1. In the Cole−Cole plot with QPE-bl-3 shown in Figure S4, the

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CONCLUSIONS CS-AFM measurements were carried out on the AEMs of QPEbl-3 and QPAF-1 under purified air at different humidities. The anion-conductive areas were clearly imaged on the membrane surfaces. The surface corrugation and the detected current increased with increasing relative humidity for both membranes. The distributions of the anion-conductive regions, inhomogeneous on QPE-bl-3 and homogeneous on QPAF-1, were clearly imaged. The high cell performance of the MEA with a QPAF-1 membrane could be understood by the uniform distribution of the anion-conductive regions on the surface,

Figure 10. (a) I−E curves and (b) ohmic resistances of a single cell with the QPE-bl-3 (black circles) and the QPAF-1 (red circles) membranes used at 40 °C and 100% RH with H2 and O2. G

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which makes an effective interface between the membrane and the catalyst layer for a higher utilization of Pt catalysts.

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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/acs.langmuir.6b01747. Schematic description of the principle of CS-AFM, topography and current images on the substrate and air sides of QPE-bl-3 and of QPAF-1, and Cole−Cole plot of an AEMFC with QPE-bl-3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +81-55-220-8707. Fax: +81-55-220-8707 (K.M.). *E-mail: [email protected]. Phone: +81-55-254-7129. Fax: +81-55-254-7129 (J.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Japanese Science and Technology Agency (JST), CREST, and a Grant-in-Aid for Young Scientists B from the Japan Society for the Promotion of Science (JSPS).

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REFERENCES

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Langmuir

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DOI: 10.1021/acs.langmuir.6b01747 Langmuir XXXX, XXX, XXX−XXX