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Energy, Environmental, and Catalysis Applications

Membrane/Electrode Interface Design for Effective Water Management in Alkaline Membrane Fuel Cells Segeun Jang, Min Her, Sungjun Kim, Jue-Hyuk Jang, Ji Eon Chae, Jiwoo Choi, Mansoo Choi, Sang Moon Kim, Hyoung-Juhn Kim, Yong-Hun Cho, Yung-Eun Sung, and Sung Jong Yoo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08075 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Membrane/Electrode Interface Design for Effective Water Management in Alkaline Membrane Fuel Cells Segeun Jang#,†, Min Her§,║,†, Sungjun Kim§,║,†, Jue-Hyuk Jang‡, Ji Eon Chae‡, Jiwoo Choi¶, Mansoo Choi¶, Sang Moon Kim⊥, Hyoung-Juhn Kim‡, Yong-Hun ChoO, Yung-Eun Sung§,║*, and Sung Jong Yoo‡*

# Department

of Mechanical Engineering, Hanbat National University, Daejeon 34158, Republic

of Korea § Center

for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of

Korea ║ School

of Chemical and Biological Engineering and Institute of Chemical Processes (ICP),

Seoul National University, Seoul 08826, Republic of Korea ‡ Fuel

Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792,

Republic of Korea ¶ Department

of Mechanical and Aerospace Engineering, Seoul National University, Seoul

08826, Republic of Korea ⊥

Department of Mechanical Engineering, Incheon National University, Incheon 22012,

Republic of Korea O Department

of Chemical Engineering, Kangwon National University, Samcheok 24341,

Republic of Korea †These

authors contributed equally to this work

KEYWORDS: dual-side patterning, anion-exchange membrane, alkaline membrane fuel cell, membrane-electrode assembly, water management

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ABSTRACT

The recent development of ultra-thin anion-exchange membranes and optimization of their operating conditions have significantly enhanced the performance of alkaline-membrane fuel cells (AMFCs); however, the effects of the membrane/electrode interface structure on the AMFC performance have not been seriously investigated thus far. Herein, we report on a highperformance AMFC system with a membrane/electrode interface of novel design. Commercially available membranes are modified in the form of well-aligned line arrays of both the anode and cathode sides by means of a solvent-assisted molding technique and sandwich-like assembly of the membrane and polydimethylsiloxane molds. Upon incorporating the patterned membranes into a single cell system, we observe a significantly enhanced performance of up to ~35% compared with that of the reference membrane. The enlarged interface area and reduced membrane thickness from the line-patterned membrane/electrode interface result in improved water management, reduced ohmic resistance, and effective utilization of the catalyst. We believe that our findings can significantly contribute further advancements in AMFCs.


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INTRODUCTION The development of clean and highly efficient power sources with economic competitiveness has become essential in the light of increasing concerns about our environment and the implementation of stringent environmental policies such as the regulation of CO2 emission.1 Consequently, fuel cells, and in particular, proton-exchange membrane fuel cells (PEMFCs) have attracted considerable attention because of their high efficiency, zero pollution (even purifying polluted atmospheric air), and their utility in applications ranging from portable power sources to transportation vehicles.2-4 However, the high cost of PEMFCs still hinders their further commercialization and practical use.5 Meanwhile, as an attractive alternative to PEMFCs, alkaline-membrane fuel cells (AMFCs) have recently attracted considerable interest in the fuel cell research community. Intrinsically, the noncorrosive alkaline environment of AMFCs allows the use of less expensive non-platinum-group metal (non-PGM) catalysts such as silver6-8, metal oxides9-15, and metal/nitrogen-doped carbon16, along with relatively inexpensive metal-based stack systems.17 Despite these economic advantages, the lower performance of AMFCs relative to PEMFCs is still a challenge to overcome for achieving market viability of AMFCs.18 In AMFCs, performance loss mainly results from the two following factors17-24: (a) relatively slow kinetics of the hydrogen oxidation reaction (HOR) in alkaline environments relative to that of acidic media, and (b) difficulty of balancing water management in the system during operation. The water management issue, as a decisive factor in the AMFC performance, can be explained by studying the electrochemical reactions that occur in AFMCs.23 At anode: 2 H2 + 4 OH− → 4 H2O + 4 e−

(1)

At cathode: O2 + 2 H2O + 4 e− → 4 OH−

(2)


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From the above equations, during AMFC operation, it is obvious that the generation and consumption of water molecules occur simultaneously in the anode and cathode sides, respectively. This distinctive water transport phenomenon makes the water management of AMFCs more complex compared to that of PEMFC system where the water molecules are only generated at the cathode side without consumption. From this perspective, recently, several studies have reported high-performance AMFCs realized via the introduction of water management strategies.17,

20-22

Their research objectives were to match the water balance between the two

electrodes through back diffusion with the use of ultra-thin membranes and to determine systemic optimal operating conditions such as relative humidity, temperature, and flow rate to alleviate the water flooding and dehydration problem.20,

21

Because water and ion transport occur at the

catalyst/ionomer and membrane/electrode interfaces during fuel cell operation, the appropriate design of such interfaces is very important in terms of water management.25 However, to the best of our knowledge, no studies have been conducted on the engineering of membrane/electrode interfaces with well-defined structures to assist water management in AMFCs. Against this backdrop, herein, we report on the fabrication of a dual-side patterned membrane for high-performance AMFCs by means of a simple solvent-assisted molding (SAM) technique with a micro-line patterned polydimethylsiloxane (PDMS) mold. With the use of an appropriate solvent and mold for the patterning, commercially available AEMs can be easily modified to have well-defined structures without chemical deterioration. Furthermore, vertically crossed line structures imprinted on both sides of the membrane are expected to generate synergetic effects on the cathodic/anodic reactions and resolve the alignment and mechanical stability issues. In the study, upon incorporating the patterned AEM into the AMFC system, we achieved a significantly higher power density of ~0.901 W cm−2 over that obtained using a non-modified


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membrane (~0.668 W cm−2). This is the highest value of AMFC performance reported using a commercially available AEM, and to the best of our knowledge, the first study introducing a patterned hydrocarbon-based AEM into an AMFC to improve the system performance.

EXPERIMENTAL SECTION Fabrication of line-patterned polymeric mold: An array of ordered micro-line patterned silicon master molds with a 5-µm pitch size and spacing ratio of 1:1 was prepared by means of standard photolithography and a reactive-ion etching process. The patterned silicon master was further treated with C4F8 gas by means of an inductively coupled plasma (ICP) system to reduce the surface energy of the silicon master.26 Subsequently, a polydimethylsiloxane (PDMS) solution (base: curing agent = 10:1) was casted onto the prepared silicon master, and thermal curing of PDMS was conducted in an oven for approximately 2 h at 70°C. Next, the cured micro-line patterned PDMS mold was gently detached from the silicon master. Preparation of one-side and dual-side patterned membranes: First, modification of the surface-wetting property from hydrophobicity to hydrophilicity of the prepared PDMS molds was performed via oxygen plasma treatment (200 mTorr, 10 W) for ~1 min. To form a thin N-methyl-2pyrrolidone (NMP) (Sigma Aldrich) solvent layer on the hydrophilic surface of the PDMS molds, the PDMS molds were immersed in a glass vial containing NMP solvent. Next, the NMP-coated PDMS molds were removed, and the excess solvent was removed with N2 blowing. Subsequently, the Fumapem FAA-3-50 membrane (FuMA-Tech) was placed onto the NMP-coated line-patterned PDMS mold and uniformly covered with the second NMP-coated PDMS mold by conformal contact. During this assembly process, the line arrays of the bottom and top sides of the membrane


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were maintained perpendicular to each other. Next, this sandwich-like assembly was placed on a hot plate at ~ 80 °C for 3 h and it placed in a vacuum oven at 100°C and ~0.1 Torr for 24 h to completely remove the NMP solvent. Finally, the dual-side patterned membrane was obtained by gently peeling off the PDMS molds from the membrane. In the case of the single-side patterned membrane, the NMP-coated flat PDMS mold was utilized instead of the line-patterned PDMS mold. Membrane characterization: Scanning electron microscopy (AURIGA, Carl Zeiss) images were recorded for analyzing the morphology of the membranes and catalyst-coated membranes (CCM) with and without patterning process. Parameters ion Exchange Capacity, water Uptake (%), and dimensional stability (%) were measured based on a previously reported paper.2729

Membrane–electrode assembly (MEA) and single-cell preparation: For AMFC application, MEAs were fabricated by the CCM method. The FAA-3-50 based membranes were pre-treated with a 1.0 M KOH (aq) solution for 24 h to be converted into hydroxide form, followed by washing with distilled water. Commercially available PtRu/C (HiSPEC 10000, Johnson Matthey Fuel Cells) and Pt/C (HiSPEC 9100, Johnson Matthey Fuel Cells) were used as hydrogen oxidation reaction and oxygen reduction reaction catalysts, respectively. Each catalyst was dispersed in an aqueous solution of ethanol and iso-propanol with an appropriate amount of FAA3-SOLUT-10 (10 wt.% anion-exchange ionomer solution in NMP, FuMA-Tech) until the ink became homogeneous using a ultrasonic bath. CCMs were prepared by spraying the catalyst inks directly onto the flat or patterned FAA-3-50 membranes, and the precious metal loadings for both electrodes were fixed at 0.4 mg cm-2. The fabricated CCMs were dried at room condition for more than 12 h to evaporate the remaining solvent and pre-treated with 1.0 M KOH (aq) solution for 30


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min to convert the ionomer in the catalyst layers into hydroxide form and rinsed with dionized water prior to single cell application. Then, gas diffusion layers (GDLs, JNTG-30-A3, JNTG), Teflon type gaskets and graphite plates with serpentine-type one-channel flow field were put onto both sides of CCM, and assembled with a torque of ~ 8.5 N m. The active geometric area of MEA is 5 cm2. Electrochemical analysis: Prepared single cell was assembled in a fuel cell test station (CNL Energy). For the AMFC performance evaluation, humidified hydrogen (relative humidity of ~80 %) and oxygen (relative humidity of ~90 %) were made to flow into the anode and cathode, respectively. The flow rate of hydrogen/oxygen was 0.8/1.0 L min-1 and the operating cell temperature was maintained at 60 °C. Single cell polarization curve was obtained at a scan rate of 50 mA s-1 when the cell performance was stabilized under these condition. To characterize the electrochemical properties of the MEA, electrochemical impedance spectroscopy (EIS) (Zennium, Zahner) was mesured at 0.2 A cm−2 and 0.8 A cm−2 with an amplitude of 5–10% of the corresponding current and 0.4 V with an amplitude of 5 mV. The measurement was conducted in the frequency range of 50 mHz to 100 kHz. Other experimental conditions, such as temperature and gas feeding condition, were the same as the case of the single cell polarization. To demonstrate the effect of the patterned interface between the membrane and the electrode on the electrochemically active surface area (ECSA) of the catalyst layer, cyclic voltammetry (CV) experiment ws conducted with voltage range of 0.05 V to 1.20 V at a scan rate of 0.02 V s-1 using Pt/C cathode as working electrode and PtRu/C anode as a counter electrode. The ECSA of the cathode was calculated by integration of the hydrogen amount under the potential desorption peak (Hupd)30, 31.


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RESULTS AND DISCUSSION Figure 1 displays the procedure for fabricating a dual-side patterned AEM. The procedure consists of three steps: (a) preparation of PDMS molds with micro-line arrays for replicating the silicon master, (b) formation of a N-methyl-2-pyrrolidone (NMP) coating layer onto the O2 plasma-treated PDMS surface, and (c) imprinting vertically crossed line structures on both sides of the membrane by means of the SAM technique. Here, we remark that it is difficult to soften the hydrocarbon-based AEM surface by applying heat and pressure to imprint the structures32, and therefore, we developed a simple and facile method to modify the AEM by using the SAM technique and the sandwich-like assembly. The key elements of this patterning method include the selection of a suitable solvent and consideration of the wetting property of the PDMS mold. First, the solvent should have good compatibility with the AEM, i.e., high solubility with the AEM, and it should minimize swelling of the PDMS mold for conformal contact between the AEM and the PDMS molds to preserve the high pattern fidelity.33, 34


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Figure 1. Schematic of fabrication process of dual-side patterned membrane using SAM technique. Among the many different types of organic solvents, the NMP solvent shows a high solubility for AEM and low swelling ratio for PDMS (~1.03).35 This means that NMP is a suitable solvent that meets the requirements of the SAM technique. However, as a polar solvent, NMP exhibits non-wetting properties on hydrophobic PDMS surfaces. Thus, the PDMS surface was first subjected to O2 plasma treatment to make it more hydrophilic. During the sandwich-like assembly of the AEM and the NMP-coated PDMS molds, the elastomeric PDMS mold spontaneously achieved conformal contact with the surface of the AEM, and the excess NMP solvent and a small amount of dissolved AEM were squeezed out. The remaining NMP solvent in the assembly dissolved a thin layer of the AEM surface, and the resulting gel-like viscous flow filled the inside of the PDMS molds via capillary force.34 The softened or slightly dissolved surface layer of the


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AEM became solidified as the residue of the NMP solvent evaporated, and finally, a negative copy of the PDMS molds was formed. Figures 2a-c show the representative surface scanning electron microscopy (SEM) images of the (a) flat reference, (b) anode-side 5-µm line-patterned AEM (P5-A), and (c) dual-side 5-µm line-patterned AEM (P5-D). In comparison with the flat reference, the P5-A and P5-D specimens exhibit uniform well-defined line arrays on the membrane without any defects. In particular, P5D image acquired at a tilted angle, the imprinted line arrays on the two sides of the membrane vertically intersect. This strategy of perpendicular crossing of the line direction was introduced to avoid the alignment and mechanical stability issues of dual-side patterning. To examine the thickness change in the membrane after SAM, we obtained cross-sectional SEM images of the reference, P5-A, and P5-D membranes (Figure S1). The average thickness of the membrane decreased as the line structures were imprinted onto the membrane to 50 μm for the reference, 46.7 μm for P5-A, and 43.8 μm for P5-D; the reason for this reduced thickness is that a small amount of dissolved AEM is squeezed out during the molding process. Next, characterization of the membrane properties such as ion conductivity, ion exchange capacity (IEC), water uptake, and dimensional stability, which considerably affect the performance and stability of the AMFC, was performed (Table 1). From the table, we note that the results show comparable values regardless of the patterning process, thereby indicating that our strategy for modifying the membrane surface morphology does not affect the membrane chemical properties.


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Table 1. Properties of prepared membranes. Conductivity at 40°C

Ion exchange capacity

[mS cm-1]

[meq g-1]

Reference

22.31

P5-A P5-D

Membrane

[%]

Dimension stability (xy-axis, %)

1.59

57.5

19.7

23.28

1.60

57.7

22.2

23.34

1.62

58.8

20.8

Water uptake

To elucidate the effects of the patterned membranes on the AMFC performance, we fabricated a membrane–electrode assembly (MEA) with the three kinds of membranes (flat reference, P5-A, and P5-D). Figures 2d-f show the catalyst-coated membranes (CCMs) obtained from the disassembled single cells after electrochemical characterization. The catalyst layers appear well covered along the patterned surface without any voids or defects even after single-cell operation. This result also confirms that the thickness of the membranes is reduced by the patterning process and that the line-grooved feature of the catalyst layer results in an enlarged interfacial area between the membrane/catalyst layer, which is approximately ~2 times that of the flat surface (calculated by use of the geometrical dimension of the line patterns). The single cell polarizations for the MEAs with the flat reference and patterned membranes are shown in Figure 3a. Both the P5-A (0.841 W cm−2) and P5-D (0.901 W cm−2) MEAs exhibit improved performances over the reference MEA (0.668 W cm−2), and this improved performance can be attributed to both the enlarged membrane/electrode interface and the micro-sized patterned structure between the membrane and the catalyst layer. As shown in Figure S2, each type of MEA shows good reproducibility for the AMFC performance.


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Figure 2. Scanning electron microscopy (SEM) micrographs of the surfaces of the (a) flat , (b) one-side patterned (P5-A), and (c) dual-side patterned (P5-D) membranes; cross-sectional SEM micrographs of the prepared catalyst-coated membranes with the (d) flat, (e) one-side patterned (P5-A), and (f) dual-side patterned (P5-D) membranes To make clearthe effects of the patterned membrane/electrode interface on the enhanced AMFC performance, EIS analysis for three different regions (0.2 A cm−2, 0.8 A cm−2, and 0.4 V near limiting current density) and cyclic voltammetry (CV) experiments were conducted. Figures 3b-d present the EIS results of the tested MEAs. The fitted curve was obtained based on the equivalent circuit of Figure S3 and exists only at 0.2 A cm-2 (parameters are listed in Table S1). As shown in Table S1, the ohmic resistances of the MEAs with patterned membranes (0.0860 Ω cm−2 for P5-A and 0.0818 Ω cm−2 for P5-D) are less than that of the flat membrane (0.0930 Ω cm−2). Interestingly, the P5-A MEA exhibits very similar ohmic resistance to MEA consisting of the 30-μm flat membrane (0.0849 Ω cm−2, Figure S4) despite the use of a thicker membrane (46.7


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μm). This result suggests that the enlarged interfacial area owing to the patterned surface can improve junction adhesion of the electrode to the membrane and successfully reduce the interfacial contact resistance.36 Figure S5a shows the iR-corrected polarization curves of the MEAs using flat membrane with different thickness, the limiting current density considerably increased from ~1.43 A cm-2 to ~1.83 A cm-2 as the membrane thickness decreased from 50 μm to 30 μm. This is because the improved water management in AMFC due to the increased back diffusion by using the thinner membrane (shorter diffusion pathway). Therefore, limiting current density is closely related to the water balance of AMFC. As shown in Figure 3a and Figure S4b, the MEAs with patterned membranes exhibit more than 40% higher limiting current densities (~2.03 A cm−2 for P5-A and ~2.09 A cm-2 for P5-D), compared with the MEA with the flat reference membrane (~1.43 A cm−2). This result does imply better water management of patterned MEAs than flat reference MEA. In addition, given the facts that the MEAs with 46.7 and 43.8 μm patterned membranes have similar limiting current value each other (2.03 A cm-2 and 2.09 A cm-2) and these values are higher than that of MEA with the thinner 30 μm flat membrane (1.83 A cm-2), the improved water balance in the patterned MEA is mainly due to the structural characteristics of the anode on the patterned interface. To substantiate the enhanced water management in patterned MEAs, EIS was conducted at middle- (0.8 A cm-2) and high-current region (near limiting current density, 0.4 V). Generally, at high current density, a tail arc in the low-frequency region in EIS is related to the mass transfer of polymer electrolyte membrane-based fuel cell.37 At a current density of 0.8 A cm-2 (Figure 3c), it is notable that the flat reference MEA shows a noticeable capacitive semicircle in the low-frequency region (below 1 Hz), whereas the patterned MEAs do not. This tail arc below 1 Hz is correlated to the mass transfer in AMFC, because all the three types of MEAs show this tail at 0.4 V near limiting current density (Figure 3d). Therefore, in addition to the


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smaller tail arcs of patterned MEA than flat reference at 0.4 V, the absence of the noticeable tail arc at 0.8 A cm-2, unlike flat reference MEA, demonstrates the improved water management of the patterned MEAs. The improved water balance in the P5-A and P5-D MEAs is related not only to the increased back diffusion by the thinner membrane thickness due to SAM but also the structural characteristics of the anode, wherein the array of ordered micro-lines is extended toward the gas diffusion layer (GDL) and the membrane/electrode interface for water and ion transport is relatively extended.


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Figure 3. (a) Polarization curves and (b-d) EIS spectra of the MEA with flat membrane (reference) and patterend membrane (P5-A and P5-D)


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To verify the solely structural effect of the dual-side patterned MEA on the cathode reaction , we plotted an iR-free polarization curve by eliminating ohmic drops, which are affected by the different membrane thickness of reference (~ 50 μm ), P5-A (~ 46.7 μm ), and P5-D (~ 43.8 μm) MEAs (Figure S5). The dual-side patterned P5-D MEA exhibits higher performance than that of the anode-side-patterned P5-A MEA. This can be explained by the increase in the electrochemically active surface area (ECSA) due to the enlarged interface area of the patterned surface on the cathode side where the oxygen reduction reaction occurs. From the cyclic voltammograms in Figure 4, it can be observed that the ECSA of the cathode Pt/C on the patterned surface (45.3 m2 gPt−1) is 12.4% higher than that on the flat surface (40.3 m2 gPt−1). This enhanced electro-catalytic activity from the cathode structure further enhances the P5-D MEA performance. Additionally, to investigate the effect of micro-patterning on oxygen mass transfer, MEA with the cathode-side patterned membrane (P5-C) was fabricated and compared with the flat reference MEA. The single polarization curves at hydrogen-oxygen (Figure S6a) and hydrogen-clean air (Figure S6b) operating condition of the two types of MEAs. Then, we calculated the oxygen gain (Figure S6c) which provide insight into the degree of oxygen mass transfer resistance within a fuel cell.38 The P5-C MEA exhibits higher limiting current density (~1.66 A cm-2) and slightly lower oxygen gain than those of the flat reference MEA. However, considering the enhancement effect of the thinned membrane to water management and oxygen reduction reaction (Figure S2 and R3 in Table S1), the patterning on the cathode side is not effective as the patterning on the anode side.


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Figure 4. Cyclic voltammograms of the Pt/C cathode on the flat surface (reference) and patterned surface (P5-D) using PtRu/C anode as counter electrode.

CONCLUSION In summary, we presented a novel methodology for improving the AMFC performance by designing a membrane/electrode interface with an (anode/dual-side)-patterned AEM. Based on the simple SAM technique and sandwich-like assembly of the membrane and the PDMS molds, we successfully imprinted well-defined line structures on a commercially available hydrocarbonbased AEM. The enlarged membrane/electrode interface and the existence of micro-sized line arrays brought about improved junction adhesion of the electrodes to the membrane, led to the effective usage of broader catalytic active sites, and most importantly, enhanced the mass transport of the reactants, including hydroxide ions (OH−) and water. The effects of the patterned AEM are shown as a schematic in Figure 5. Our study can significantly contribute to further research on AMFCs as follows: 1) This study is the first investigation of membrane/electrode interface engineering for AMFC application via the introduction of patterned AEMs. 2) We demonstrate


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that the modified AEM can significantly assist ion conduction through the cell and enhance electrode performance via improved water balance and broad utilization of active sites. 3) Our solvent-assisted molding approach can easily be applied to other hydrocarbon-based AEMs using appropriate solvents and molds, and it can be synergistically utilized with other approaches such as those involving highly active catalysts and highly conductive AEMs.

Figure 5. Schematic of the effects of the dual-side pattern in the anion exchange membrane.

ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section, Figures S1−S6 showing cross-sectional SEM images of the (pristine/modified) membranes, equivalent circuit, EIS spectra, and iR-corrected polarization curves, respectively, and Table S1 listing the fitting parameters for the equivalent circuit. (MS Word)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. J. Yoo), [email protected] (Y.-E. Sung) Author Contributions † These authors contributed equally to this work Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Institute for Basic Science (IBS-R006-A2) of the Republic of Korea, the Global Frontier R&D Program on Center for Multiscale Energy System funded by National Research Foundation of Korea (2016M3A6A7945505), the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science and ICT (2018M1A2A2061975), and a National Research Foundation of Korea grant (2019R1C1C1004462)


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