www.advmat.de www.MaterialsViews.com
COMMUNICATION
Interlocking Membrane/Catalyst Layer Interface for High Mechanical Robustness of Hydrocarbon-Membrane-Based Polymer Electrolyte Membrane Fuel Cells Keun-Hwan Oh, Hong Suk Kang, Min-Ju Choo, Duk-Hun Jang, Dongyoung Lee, Dai Gil Lee, Tae-Ho Kim, Young Taik Hong, Jung-Ki Park,* and Hee-Tak Kim*
The use of hydrocarbon (HC) ionomers in proton exchange membrane fuel cells (PEMFCs) as a membrane to replace the commonly used perfluorosulfonic acid (PFSA) ionomers has been widely explored in efforts to lower costs, raise fuel efficiency, and enable easier manufacturing of membrane/electrode assemblies (MEAs).[1–7] Examples include sulfonated poly(ether ether ketone) (SPEEK),[8,9] sulfonated poly(arylene ether sulfone) (SPAES),[10,11] and sulfonated polyimide (SPI).[12,13] However, for the practical use of HC membranes for PEMFCs, attaining a tight interface between the HC membranes and the PFSA-ionomer-based catalyst layer (CL) to address the problem of interfacial delamination and consequent poor cell performance has proved challenging.[14,15] The poor interface is attributed to the incompatibility between the HC membrane and the PFSA ionomer of the CL.[16] The high glass-transition temperature (Tg) of the HC membrane is another factor contributing to the weak interface during thermal lamination of the membrane and the CL; lamination temperatures usually selected for HC membranes are lower than Tg of HC membranes due to the thermal degradation of the PFSA ionomer in CL above the Tg, and thus the HC membrane is too rigid to produce conformal contact with CL during thermal lamination. Many previous reports have focused on the use of HC ionomers as a CL ionomer instead of the PFSA ionomer in efforts to solve the interfacial problem.[17–22] Although these
Dr. K.-H. Oh, H. S. Kang, Prof. J.-K. Park Graduate School of EEWS Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305–701, South Korea E-mail: [email protected] M.-J. Choo, D.-H. Jang, Prof. J.-K. Park, Prof. H.-T. Kim Department of Chemical and Biomolecular Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305–701, South Korea E-mail: [email protected] D. Lee, Prof. D. G. Lee School of Mechanical Aerospace & Systems Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305–701, South Korea Dr. T.-H. Kim, Dr. Y. T. Hong Center for Membrane Korea Research Institute of Chemical Technology Daejeon 305–600, South Korea
DOI: 10.1002/adma.201500328
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
established methods could significantly improve interfacial bonding strength between the HC ionomer and the CL, the power performance of the HC-ionomer-based CL is too poor for practical application due to slow oxygen transport through the HC ionomer, the oxygen permeability of which was two orders of magnitude lower than that of Nafion ionomer.[23] The other approach is chemical modification of the membrane or electrode ionomer materials by incorporating partially or fully fluorinated monomers for minimum interfacial resistance; the fluorinated membranes and the catalyst layer made of Nafion can provide better adhesion when both chemical components of the CL and membrane are similar;[16,24,25] the durability of these interfaces, however, was not assessed. In addition, the cost of fluorinated monomer is relatively high, thus making fluorinated copolymers less attractive. On the other hand, it was reported that surface-fluorination of a HC membrane with F2 gas, which leads to anisotropic swelling behavior of the membrane, forms a more stable electrode interface.[26] In spite of these efforts, the perception in the technology sector of current PEMFCs is that the interfacial bonding of the HC membrane is not sufficient for practical applications. In this study, as a novel strategy for tight interfacial bonding of HC membranes and PFSA-ionomer-based CL, we propose a physical interlocking interface that tightly binds the two surfaces without any chemical treatment of the HC membrane or the CL. As schematically illustrated in Figure 1, a micropatterned surface with regularly arrayed pillars was fabricated for the SPAES membrane, and a flat and uniform Nafion layer was formed on a Nafion-based CL. At an enhanced temperature where Nafion is softened but SPAES is not, thermal lamination allows protrusion of the hard SPAES pillars into the soft Nafion layer, forming an interlocking interface akin to Lego blocks. The Nafion layer thereby acts as an interfacial bonding layer (IBL) that links the CL to the patterned SPAES (P-SPAES). In particular, the interlocking interface is well tightened owing to the characteristic of HC ionomers of larger expansion with hydration in comparison with PFSA ionomers.[27–29] The difference in expansion generates huge normal force exerted on the pillar/hole interface, resulting in a higher peel strength. Indeed, the interlocking interface exhibited much higher interfacial bonding strength than a conventional flat interface and wet/dry cycling durability was enhanced by 4.7 times. The master pattern of a regular array of micrometer-sized pillars was formed on a silicon wafer by conventional lithography, and the inverse of the silicon master pattern was transferred
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. Schematic diagram of the fabrication of the pillar P-SPAES membrane and its working principle of interlocking effects (following the arrows from top left): a–c) the PDMS mold was obtained from conventional soft lithography and d) the pillar P-SPAES membrane was generated by a solution casting technique (see details in the Experimental Section); e) lamination of P-SPAES with IBL-coated CL; f) interlocking effects between pillar P-SPAES and Nafion IBL. This interlocking effect is attributed to the different swelling indices between SPAES and Nafion ionomer.
to a PDMS mold,[30,31] generating a regular array of cylindrical holes in the mold (pitch: 5.8 µm, diameter of the hole: 2.4 µm, depth of the hole: 3.7 µm), as shown in Figure S1a,b in the Supporting Information. In addition, the SEM image of the PDMS mold with low magnification (Figure S1c, Supporting Information) shows Moiré fringes, which originate from the interaction between a scanning line pattern of the electron microscope and the PDMS hole-arrays at a specific magnification and confirm the well-ordered pattern.[32,33] A P-SPAES membrane was fabricated by casting a 10 wt% SPAES solution in DMAc on the PDMS mold. After the subsequent drying process, the resulting membrane was separated from the mold without destruction or distortion owing to the elastic nature, high chemical stability, and low interfacial energy of PDMS.[34–36] According to the SEM images of the P-SPAES membrane (Figure 2a,b), an array of pillars, the shape of which is the inverse replica of the PDMS mold, was successfully formed on the membrane surface; the height, diameter, and pitch were 3.7, 2.4, and 5.8 µm, respectively. A high degree of regularity in the surface pattern was also confirmed by the generation of periodic rainbow colors (Figure 2c) under white light and of diffraction patterns (Figure 2d) upon the irradiation of a 633 nm laser. On the other hand, Nafion IBL, which eventually connects the P-SPAES membrane and the CL, was formed on the surface of the cathode CL by spraying a 5 wt% Nafion solution in water/isopropyl alcohol (1/1 in volume). As shown in Figure S2 in the Supporting Information, a 4 µm-thick, uniform, and dense Nafion IBL was successfully formed on the surface of the CL. We controlled the degree of solvent evaporation between the spray nozzle and the cathode surface, forming a Nafion gel
2
wileyonlinelibrary.com
that did not penetrate the interior of the CL, but remained on the surface of the CL. Therefore, the Nafion layer does not prevent oxygen transport of the CL. During the lamination of the P-SPAES membrane and the IBL-coated CL at 150 °C, the pillars of the P-SPAES intruded into the Nafion IBL. At the lamination temperature, SPAES behaves as a hard solid, but Nafion is soft enough for permanent deformation, and thus the hard P-SPAES and the soft IBL could form an interlocking structure similar to Lego blocks. Figure 3a shows a SEM image of the P-SPAES/IBL interface, which was partly delaminated for clearer investigation. The formation of the regular holes in the IBL and the insertion of the pillars on the holes are clearly observed, confirming the formation of the interlocking interface during the lamination. The interfacial bonding with the physical interlocking structure is conceptually associated with the friction force generated at the pillar/hole interfaces. The key idea of this approach is the generation of normal force at the pillar/hole interface by exploiting the difference in the degree of volume expansion between the pillars and holes. Since the friction force is proportionally increased with the normal force, the larger difference in expansion could generate a high friction force. We compared the dimensional change of the pillar and hole with the transition from a dry condition (23 °C, RH 0%) to a wet condition (70 °C, RH 100%), simulating typical fuel cell operating conditions. Figure S3 in the Supporting Information compares optical microscopy images of the P-SPAES membrane and the hole-patterned Nafion film under the dry and wet conditions. Switching from dry to wet conditions, the diameter of the holes in Nafion was increased from by 9%, from 2.3 to 2.5 µm,
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 2. a,b) SEM images of pillar P-SPAES membrane: a) top-view and b) tilted-view. The pitch, diameter, and height of the pillars are 3.4, 2.4, and 3.7 µm, respectively. c,d) Optical images of the pillar P-SPAES membrane with c) white-light bulb illumination and d) 633 nm wavelength laser irradiation.
whereas that of the pillar was increased by 35%, from 2.3 to 3.1 µm. This difference in dimensional change is attributed to the larger volume expansion ratio for SPAES (83%) relative to that of Nafion (25%), showing the possibility of normal force generation. The peel strength of the interlocking interface was measured at two different RHs (0 and 50%). At each sample, the peeling test was carried out at 10 times. The peel strength, which is defined as the force required to separate the interface normalized by the width of the specimen, was monitored as a function of peel distance. For the flat SPAES/Nafion interface, the average peel strength was 3.38 ± 0.07 mN mm−1 at RH 0% and 3.07 ± 0.06 mN mm−1 at RH 50% (Figure 3b). To our surprise, the peel strength of the interlocking interface (P-SPAES/ Nafion) was 8-times higher than that of the normal interface at RH 50% (24.6 ± 0.14 mN mm−1) and 4-times higher at RH 0% (12.9 ± 0.08 mN mm−1). The significant increase in peel strength with increasing RH indicates that normal force is responsible for the tight bonding. For a clearer understanding of the generation of the normal force at the pillar/hole interface, an FEM analysis was conducted for a single interlocking structure. The physical parameters for the FEM analysis including kinematic and static friction coefficient, modulus, yield stress, ultimate tensile strength, dimensional change, and Poisson’s ratio are listed in Table S1 in the Supporting Information. The simulated stress distributions with hydration at RH 50% at two difference
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
peeling distances (0 and 2 µm) are displayed in Figure 3c. At zero peeling distance, the generation of significant normal stress along the interface was identified, explaining the large peeling strength. As the pillar is removed from the hole, the stresses are reduced because the interfacial area is reduced. This explains the gradual decrease of the peeling strength with peeling distance shown in Figure 3b. The membrane expansion with hydration, which generates stress at the interface, is regarded as the major cause of interfacial delamination during fuel cell operation.[37] Therefore, the immersion of a MEA into boiling water (hot water test) can serve as an accelerated interfacial durability test. For the hot water test, an MEA that has both an interlocking interface and a normal flat interface was used. The normal interface was delaminated 10 min after the immersion, whereas the interlocking interface was preserved even after 1 h, as shown in Figure S5 in the Supporting Information. It is interesting that the large degree of membrane expansion, which causes delamination of the flat interface, provides a tight interface for the interlocking configuration. To evaluate the effect of the interlocking interface on the cell performance, including long-term durability in particular, we carried out a wet/dry cycling test with a cycle comprising supply of dry (RH 0%) N2 feed for 2 min and supply of wet (RH 100%) N2 feed for another 2 min to both the cathode and anode compartments. Figure 4a,b compares the gavanostatic polarization curves before and after cycles for the
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. a) SEM image of the interlocking structure. b) Peel strength of the laminates of SPAES/Nafion membranes and P-SPAES/Nafion membranes. c) FEM results for a single interlocking structure: i) dry state at static condition; ii) wet state (RH 50%) at static condition; iii) wet state (RH 50%) under peeling at a peeling distance of 2 µm.
P-SPAES-based MEA with an interlocking interface and the SPAES-based MEA with a flat interface, respectively. The initial IV polarization curve is nearly identical to those of the two MEAs, indicating that the interlocking interface does not block proton transport and does not impede the performance of the CL. The SPAES-MEA exhibited a significant loss in power performance after 1050 cycles; the power density at 0.4 V dropped from 323.3 to 20.4 mW cm−2. However, the interlocking interface showed much slower performance decay; the power density at 0.4 V decreased from 331.3 to 202.1 mW cm−2 during 1950 cycles. The decay rate was 4.7-times faster for the flat interface (0.094% per cycle) relative to that of the interlocking interface (0.020%/cycle). Also, the OCV of the P-SPAES cell was 0.954 V at the first cycle and 0.951 V at the 1950th cycle, and hydrogen crossover measured by linear sweep voltammetry was 2.05 mA cm−2 at the first cycle and 2.15 mA cm−2 at the 1950th cycle (see Figure S6, Supporting Information). These results indicate that no significant mechanical membrane degradation such as pin-hole generation occurred during the wet/dry cycling. After the cycling test, the CL of the SPAES MEA was partly detached from the SPAES membrane, as shown in Figure S7a in the Supporting Information. In contrast, the interface of the P-SPAES cell was apparently preserved even after the cycles (Figure S7b, Supporting Information). The difference in the
4
wileyonlinelibrary.com
interfacial stability was also reflected in the impedance spectroscopy results. The impedance measured under N2/H2 for the cathode/anode provides information on the proton transport through the MEA. The bulk resistance (ROhm), which is determined by the intercept at the real axis of the Nyquist plot, includes the contributions from proton transport through the membrane and the membrane/CL interface. Figure 4c displays Nyquist plots of the impedances before and after wet/dry cycles for the SPAES and P-SPAES cell. The initial ROhm of the P-SPAES MEA (62 mΩ cm2) was somewhat smaller than that of the SPAES MEA (87 mΩ cm2), indicating that the interlocking structure provides better interfacial contact at the early stage of the cycling. For the SPAES MEA, the ROhm was significantly increased to 147 mΩ cm2, or by 1.69 times, after 1050 cycles, indicating significant interfacial delamination. In contrast, for the P-SPAES MEA, it was increased to 101 mΩ cm2, or by 1.19 times, after 1950 cycles. The resistance of ionic conduction through the CL (Rcl) can be obtained from a transmission line model. According to the model, a Warburg-like impedance at high frequencies corresponds to ion migration through the catalyst layer, and a subsequent vertical spike at low frequencies to the total capacitance and resistance of the CL.[38,39] The value for Rcl also can be determined from the length of the Warburg-like region (Rcl/3) projected onto Z′-ROhm.[38,39] The Nyquist plots
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. a,b) Galvanostatic polarization curves before and after 1050/1950 wet/dry cycles for the SPAES MEA (a) and the P-SPAES (b). c) Nyquist plots of the impedances for the SPAES MEA and P-SPAES MEA before and after 1050/1950 wet/dry cycles. d,e) Cross-sectional SEM images of the SPAES MEA (d) and the P-SPAES MEA (e) after 1950 wet/dry cycles.
of the impedances for before and after cycling exhibit nearly the same impedance shape (12 mΩ cm2), indicating that the values for Rcl are nearly identical and the performance fading is mainly from interface degradation, not from CL degradation. The results strongly support that the large difference in IV polarization after cycling is responsible for the interfacial delamination. Finally, we investigated the cross-section of the MEAs after wet/dry cycling. The interface of S-PAES and Nafion IBL was delaminated (Figure 4d), whereas the interlocking interface was well preserved (Figure 4e), clearly demonstrating that the interface was tightened by the interlocking structure. In conclusion, the interfacial bonding of a hydrocarbon membrane and a perfluorinated-ionomer-based catalyst layer was significantly improved by a physical interlocking interface structure. This structure was realized by the intrusion of micrometer-sized pillars fabricated on a SPAES membrane into a Nafion IBL formed on a catalyst layer. Normal force is generated at the pillar/hole interface owing to the higher expansion with hydration for SPAES than for Nafion, thereby increasing the peel strength of the interface. The improved interfacial bonding enhances the resistance of the hydrocarbon-membrane-based MEA to interfacial delamination during wet/dry cycles. The physical interlocking strategy could allow numerous types of HC membranes that suffer from poor interfacial adhesion to be used in practical applications.
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
Experimental Section Preparation of Polydimethylsiloxane (PDMS) Mold and P-SPAES Membrane: PDMS molds were prepared by replica molding from a silicon wafer having an array of micrometer-pillars on its surface fabricated by conventional photolithography. A mixture of the PDMS prepolymer and an initiator (Sylgard 184, Dow Corning, Midland, MI, USA) at a ratio of 10:1 by weight was degassed by vacuum suction and carefully poured onto the silicon master pattern. After curing at 80 °C for 1 h, the PDMS mold was gently released from the master pattern. The P-SPAES membrane was prepared by a solution-casting technique. Using a doctor blade, we carefully cast a 10 wt% DMAc solution of SPAES onto the PDMS mold, which had been treated by oxygen plasma in advance in order to enhance wetting of the SPAES solution. After completely evaporating residual DMAc at 60 °C for 12 h under vacuum, the replicated SPAES membrane was peeled off the PDMS mold. The molecular weight, ion exchange capacity, and water uptake were 50 000 g mol−1, 1.34 meq g−1 and 93%, respectively. Preparation of CL and IBL: A Pt/C catalyst (46.6 wt% Pt, Tanaka Kikinzoku Kogyo, Japan) was dispersed in a solvent; predetermined amounts of a Nafion (EW 1100 g eq−1, Dupont, USA) dispersion were added to the catalyst dispersion. The weight ratio of the carbon to ionomer was fixed at 1:1 in a dry state. The mixtures underwent ball milling for 2 h and were cast onto a fluorinated ethylene propylene (FEP, Dupont, USA) film using a 190 µm-gap doctor blade, followed by drying at 60 °C for 24 h. The Pt loading level of all the CLs was 0.35 ± 0.03 mg cm−2 and the active area of the CL 2 cm × 2 cm. A CL with 0.35 ± 0.03 mg cm−2 based on a Nafion ionomer was adopted as a cathode for all the MEA samples. For the anode CLs, the same procedure was applied except for the use of SPAES as an ionomer. To fabricate the IBL on the cathode CL, 5 wt% of Nafion dispersion (SE-5112, Dupont)
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com was sprayed onto the cathode CL (five straight times) by using a spray coater (LSC-300, Lithotech., Korea) at 60 °C. The nozzle air pressure and moving speed were 320 kPa and 50 mm s−1, respectively. Physical Characterization and Analysis of the Interlocking Interface: The peel strength was measured for the laminates of Naftion/PSPAES membranes and Nafion/nonpatterned SPAES (NP-SPAES) by using a universal testing machine (Instron 5583) at room temperature and at a strain rate of 20 mm min−1; the assembly of two membranes (60 mm × 13 mm), half of which were laminated at 3.4 MPa and at 150 °C for 3 min, pulled apart by tugging the two unlaminated sides in opposite directions. Finite element modeling (FEM) was conducted for the interlocking structure using the commercial software ABAQUS (Dassault Systems Americas Corp., Waltham, MA, USA). The various physical parameters of Nafion and SPAES such as friction coefficient, modulus, yield stress, ultimate tensile strength, dimensional change, and Poisson’s ratio were experimentally determined and utilized for the FEM analysis. The friction coefficient of Nafion/SPAES is measured by rubbing abrasion test (see Figure S4, Supporting Information). The morphologies of the P-SPAES were investigated by scanning electron microscopy (SEM) (Sirion, FEI) and optical microscopy (OM) (BX51, Olympus). Fabrication, Electrochemical Performance, and Electrochemical Analysis of MEAs: MEAs were fabricated by decal-transfer of the anode and the cathode CLs coated on an FEP film on each side of the membrane at a pressure of 3.4 MPa and at 150 °C for 3 min. The FEP films were peeled off the laminate at room temperature. A single cell was assembled with the MEA, a pair of gas diffusion media (39BC, SGL), a pair of silicon gaskets, and a pair of graphite blocks with a triple serpentine flow field for the reactants. The flow fields for the anode and cathode reactants were mirror images. The IV polarization curves of the single cells were obtained with a fuel cell test station (SMART II, Won-A tech, Korea). We tried to exclude the influence of water flooding by operating the cells at a high flow rate of 500 sccm for humidified hydrogen (RH 100%) and of 1500 sccm for humidified air (RH 100%), which correspond to the stoichiometry of H2/Air = 14.3/18.0 at 0.2 A cm−2. The IV polarization curve of each cell was obtained at 70 °C without any back pressure after daily measurements of three IV curves for three successive days as a break-in. The IV curves did not vary after the second day. The electrochemical impedance values were measured using an AC impedance analyzer (1400 FRA and 1470E, Solartron); the AC amplitude was 10 mV and the frequency range was from 105 to 10−1 Hz. The hydrogen crossover was determined from the limiting current observed in linear sweep voltammetry (from 0.05 to 0.70 V) under an H2/N2 (anode/cathode) atmosphere. After initial evaluation of the cell performance of the given PEMFCs, humidity cycle tests were conducted to examine the effect of the interlocking structures on the cell performance under repeated hydration and dehydration cycles. For this experiment, non-humidified and fully humidified nitrogen gas was alternately fed every 2 min at a flow rate of 1000 cm3 s−1. The humidification was controlled by (dis)connection to a humidifier. During these repeated cycles, the polarization curve and open-circuit voltage (OCV) were obtained at every 150 humidity cycles.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements K.-H.O. and H.S.K. contributed equally to this work. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Code No. NRF-2012R1A1A2042558),
6
wileyonlinelibrary.com
the WCU program (EEWS) at KAIST (Code No. R-31–2008–000–1005–0), and the KRICT OWN Project. Received: January 21, 2015 Revised: February 14, 2015 Published online:
[1] M. A. Hickner, H. Ghassemi, Y. S. Kim, B. R. Einsla, J. E. McGrath, Chem. Rev. 2004, 104, 4587. [2] K. D. Kreuer, J. Membr. Sci. 2001, 185, 29. [3] K. Goto, I. Rozhanskii, Y. Yamakawa, T. Otsuki, Y. Naito, Polym. J. 2008, 41, 95. [4] T. Higashihara, K. Matsumoto, M. Ueda, Polymer 2009, 50, 5341. [5] C. H. Park, C. H. Lee, M. D. Guiver, Y. M. Lee, Prog. Polym. Sci. 2011, 36, 1443. [6] R. Devanathan, Energy Environ. Sci. 2008, 1, 101. [7] S. Y. Kim, S. Kim, M. J. Park, Nat. Commun. 2010, 1, 88. [8] P. Xing, G. P. Robertson, M. D. Guiver, S. D. Mikhailenko, K. Wang, S. Kaliaguine, J. Membr. Sci. 2004, 229, 95. [9] K.-H. Oh, D. Lee, M.-J. Choo, K. H. Park, S. Jeon, S. H. Hong, J.-K. Park, J. W. Choi, ACS Appl. Mater. Interfaces 2014, 6, 7751. [10] F. Wang, M. Hickner, Y. S. Kim, T. A. Zawodzinski, J. E. McGrath, J. Membr. Sci. 2002, 197, 231. [11] B. Bae, T. Yoda, K. Miyatake, H. Uchida, M. Watanabe, Angew. Chem., Int. Ed. 2010, 49, 317. [12] J. Fang, X. Guo, S. Harada, T. Watari, K. Tanaka, H. Kita, K.-I. Okamoto, Macromolecules 2002, 35, 9022. [13] X. Guo, J. Fang, T. Watari, K. Tanaka, H. Kita, K.-I. Okamoto, Macromolecules 2002, 35, 6707. [14] H.-Y. Jung, K.-Y. Cho, K. A. Sung, W.-K. Kim, M. Kurkuri, J.-K. Park, Electrochim. Acta 2007, 52, 4916. [15] H.-Y. Jung, K.-Y. Cho, K. A. Sung, W.-K. Kim, J.-K. Park, J. Power Sources 2006, 163, 56. [16] M. Sankir, Y. S. Kim, B. S. Pivovar, J. E. McGrath, J. Membr. Sci. 2007, 299, 8. [17] K.-H. Oh, W.-K. Kim, K. A. Sung, M.-J. Choo, K.-W. Nam, J. W. Choi, J.-K. Park, Int. J. Hydrogen Energy 2011, 36, 13695. [18] K. A. Sung, W.-K. Kim, K.-H. Oh, J.-K. Park, Electrochim. Acta 2009, 54, 3446. [19] W.-K. Kim, K. A. Sung, K.-H. Oh, M.-J. Choo, K. Y. Cho, K.-Y. Cho, J.-K. Park, Electrochem. Commun. 2009, 11, 1714. [20] S. Sambandam, V. Ramani, Electrochim. Acta 2008, 53, 6328. [21] E. B. Easton, T. D. Astill, S. Holdcroft, J. Electrochem. Soc. 2005, 152, A752. [22] S. Holdcroft, Chem. Mater. 2013, 26, 381. [23] A. Ayad, J. Bouet, J. F. Fauvarque, J. Power Sources 2005, 149, 66. [24] Y. S. Kim, M. J. Sumner, W. L. Harrison, J. S. Riffle, J. E. McGrath, B. S. Pivovar, J. Electrochem. Soc. 2004, 151, A2150. [25] K. B. Wiles, C. M. de Diego, J. de Abajo, J. E. McGrath, J. Membr. Sci. 2007, 294, 22. [26] C. H. Lee, S. Y. Lee, Y. M. Lee, S. Y. Lee, J. W. Rhim, O. Lane, J. E. McGrath, ACS Appl. Mater. Interfaces 2009, 1, 1113. [27] K. A. Mauritz, R. B. Moore, Chem. Rev. 2004, 104, 4535. [28] S. Kaliaguine, S. D. Mikhailenko, K. P. Wang, P. Xing, G. Robertson, M. Guiver, Catal. Today 2003, 82, 213. [29] L. Carrette, K. A. Friedrich, U. Stimming, Fuel Cells 2001, 1, 5. [30] H. S. Kang, H.-T. Kim, J.-K. Park, S. Lee, Adv. Funct. Mater. 2014, 24, 7273. [31] H. S. Kang, S. Lee, J.-K. Park, Adv. Funct. Mater. 2011, 21, 4412. [32] S.-H. Kim, S. Y. Lee, G.-R. Yi, D. J. Pine, S.-M. Yang, J. Am. Chem. Soc. 2006, 128, 10897.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
COMMUNICATION
[33] H. S. Kang, S. Lee, S.-A. Lee, J.-K. Park, Adv. Mater. 2013, 25, 5490. [34] T. W. Lee, O. Mitrofanov, J. W. P. Hsu, Adv. Funct. Mater. 2005, 15, 1683. [35] W. M. Choi, O. O. Park, Microelectron. Eng. 2003, 70, 131. [36] J. K. Koh, Y. Jeon, Y. I. Cho, J. H. Kim, Y.-G. Shul, J. Mater. Chem. A 2014, 2, 8652.
[37] C. H. Park, C. H. Lee, M. D. Guiver, Y. M. Lee, Prog. Polym. Sci. 2011, 36, 1443. [38] M. C. Lefebvre, R. B. Martin, P. G. Pickup, Electrochem. Solid-State Lett. 1999, 2, 259. [39] D. Malevich, B. R. Jayasankar, E. Halliop, J. G. Pharoah, B. A. Peppley, K. Karan, J. Electrochem. Soc. 2012, 159, F888.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
7
COMMUNICATION
Interlocking Membrane/Catalyst Layer Interface for High Mechanical Robustness of Hydrocarbon-Membrane-Based Polymer Electrolyte Membrane Fuel Cells Keun-Hwan Oh, Hong Suk Kang, Min-Ju Choo, Duk-Hun Jang, Dongyoung Lee, Dai Gil Lee, Tae-Ho Kim, Young Taik Hong, Jung-Ki Park,* and Hee-Tak Kim*
The use of hydrocarbon (HC) ionomers in proton exchange membrane fuel cells (PEMFCs) as a membrane to replace the commonly used perfluorosulfonic acid (PFSA) ionomers has been widely explored in efforts to lower costs, raise fuel efficiency, and enable easier manufacturing of membrane/electrode assemblies (MEAs).[1–7] Examples include sulfonated poly(ether ether ketone) (SPEEK),[8,9] sulfonated poly(arylene ether sulfone) (SPAES),[10,11] and sulfonated polyimide (SPI).[12,13] However, for the practical use of HC membranes for PEMFCs, attaining a tight interface between the HC membranes and the PFSA-ionomer-based catalyst layer (CL) to address the problem of interfacial delamination and consequent poor cell performance has proved challenging.[14,15] The poor interface is attributed to the incompatibility between the HC membrane and the PFSA ionomer of the CL.[16] The high glass-transition temperature (Tg) of the HC membrane is another factor contributing to the weak interface during thermal lamination of the membrane and the CL; lamination temperatures usually selected for HC membranes are lower than Tg of HC membranes due to the thermal degradation of the PFSA ionomer in CL above the Tg, and thus the HC membrane is too rigid to produce conformal contact with CL during thermal lamination. Many previous reports have focused on the use of HC ionomers as a CL ionomer instead of the PFSA ionomer in efforts to solve the interfacial problem.[17–22] Although these
Dr. K.-H. Oh, H. S. Kang, Prof. J.-K. Park Graduate School of EEWS Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305–701, South Korea E-mail: [email protected] M.-J. Choo, D.-H. Jang, Prof. J.-K. Park, Prof. H.-T. Kim Department of Chemical and Biomolecular Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305–701, South Korea E-mail: [email protected] D. Lee, Prof. D. G. Lee School of Mechanical Aerospace & Systems Engineering Korea Advanced Institute of Science and Technology (KAIST) Daejeon 305–701, South Korea Dr. T.-H. Kim, Dr. Y. T. Hong Center for Membrane Korea Research Institute of Chemical Technology Daejeon 305–600, South Korea
DOI: 10.1002/adma.201500328
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
established methods could significantly improve interfacial bonding strength between the HC ionomer and the CL, the power performance of the HC-ionomer-based CL is too poor for practical application due to slow oxygen transport through the HC ionomer, the oxygen permeability of which was two orders of magnitude lower than that of Nafion ionomer.[23] The other approach is chemical modification of the membrane or electrode ionomer materials by incorporating partially or fully fluorinated monomers for minimum interfacial resistance; the fluorinated membranes and the catalyst layer made of Nafion can provide better adhesion when both chemical components of the CL and membrane are similar;[16,24,25] the durability of these interfaces, however, was not assessed. In addition, the cost of fluorinated monomer is relatively high, thus making fluorinated copolymers less attractive. On the other hand, it was reported that surface-fluorination of a HC membrane with F2 gas, which leads to anisotropic swelling behavior of the membrane, forms a more stable electrode interface.[26] In spite of these efforts, the perception in the technology sector of current PEMFCs is that the interfacial bonding of the HC membrane is not sufficient for practical applications. In this study, as a novel strategy for tight interfacial bonding of HC membranes and PFSA-ionomer-based CL, we propose a physical interlocking interface that tightly binds the two surfaces without any chemical treatment of the HC membrane or the CL. As schematically illustrated in Figure 1, a micropatterned surface with regularly arrayed pillars was fabricated for the SPAES membrane, and a flat and uniform Nafion layer was formed on a Nafion-based CL. At an enhanced temperature where Nafion is softened but SPAES is not, thermal lamination allows protrusion of the hard SPAES pillars into the soft Nafion layer, forming an interlocking interface akin to Lego blocks. The Nafion layer thereby acts as an interfacial bonding layer (IBL) that links the CL to the patterned SPAES (P-SPAES). In particular, the interlocking interface is well tightened owing to the characteristic of HC ionomers of larger expansion with hydration in comparison with PFSA ionomers.[27–29] The difference in expansion generates huge normal force exerted on the pillar/hole interface, resulting in a higher peel strength. Indeed, the interlocking interface exhibited much higher interfacial bonding strength than a conventional flat interface and wet/dry cycling durability was enhanced by 4.7 times. The master pattern of a regular array of micrometer-sized pillars was formed on a silicon wafer by conventional lithography, and the inverse of the silicon master pattern was transferred
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 1. Schematic diagram of the fabrication of the pillar P-SPAES membrane and its working principle of interlocking effects (following the arrows from top left): a–c) the PDMS mold was obtained from conventional soft lithography and d) the pillar P-SPAES membrane was generated by a solution casting technique (see details in the Experimental Section); e) lamination of P-SPAES with IBL-coated CL; f) interlocking effects between pillar P-SPAES and Nafion IBL. This interlocking effect is attributed to the different swelling indices between SPAES and Nafion ionomer.
to a PDMS mold,[30,31] generating a regular array of cylindrical holes in the mold (pitch: 5.8 µm, diameter of the hole: 2.4 µm, depth of the hole: 3.7 µm), as shown in Figure S1a,b in the Supporting Information. In addition, the SEM image of the PDMS mold with low magnification (Figure S1c, Supporting Information) shows Moiré fringes, which originate from the interaction between a scanning line pattern of the electron microscope and the PDMS hole-arrays at a specific magnification and confirm the well-ordered pattern.[32,33] A P-SPAES membrane was fabricated by casting a 10 wt% SPAES solution in DMAc on the PDMS mold. After the subsequent drying process, the resulting membrane was separated from the mold without destruction or distortion owing to the elastic nature, high chemical stability, and low interfacial energy of PDMS.[34–36] According to the SEM images of the P-SPAES membrane (Figure 2a,b), an array of pillars, the shape of which is the inverse replica of the PDMS mold, was successfully formed on the membrane surface; the height, diameter, and pitch were 3.7, 2.4, and 5.8 µm, respectively. A high degree of regularity in the surface pattern was also confirmed by the generation of periodic rainbow colors (Figure 2c) under white light and of diffraction patterns (Figure 2d) upon the irradiation of a 633 nm laser. On the other hand, Nafion IBL, which eventually connects the P-SPAES membrane and the CL, was formed on the surface of the cathode CL by spraying a 5 wt% Nafion solution in water/isopropyl alcohol (1/1 in volume). As shown in Figure S2 in the Supporting Information, a 4 µm-thick, uniform, and dense Nafion IBL was successfully formed on the surface of the CL. We controlled the degree of solvent evaporation between the spray nozzle and the cathode surface, forming a Nafion gel
2
wileyonlinelibrary.com
that did not penetrate the interior of the CL, but remained on the surface of the CL. Therefore, the Nafion layer does not prevent oxygen transport of the CL. During the lamination of the P-SPAES membrane and the IBL-coated CL at 150 °C, the pillars of the P-SPAES intruded into the Nafion IBL. At the lamination temperature, SPAES behaves as a hard solid, but Nafion is soft enough for permanent deformation, and thus the hard P-SPAES and the soft IBL could form an interlocking structure similar to Lego blocks. Figure 3a shows a SEM image of the P-SPAES/IBL interface, which was partly delaminated for clearer investigation. The formation of the regular holes in the IBL and the insertion of the pillars on the holes are clearly observed, confirming the formation of the interlocking interface during the lamination. The interfacial bonding with the physical interlocking structure is conceptually associated with the friction force generated at the pillar/hole interfaces. The key idea of this approach is the generation of normal force at the pillar/hole interface by exploiting the difference in the degree of volume expansion between the pillars and holes. Since the friction force is proportionally increased with the normal force, the larger difference in expansion could generate a high friction force. We compared the dimensional change of the pillar and hole with the transition from a dry condition (23 °C, RH 0%) to a wet condition (70 °C, RH 100%), simulating typical fuel cell operating conditions. Figure S3 in the Supporting Information compares optical microscopy images of the P-SPAES membrane and the hole-patterned Nafion film under the dry and wet conditions. Switching from dry to wet conditions, the diameter of the holes in Nafion was increased from by 9%, from 2.3 to 2.5 µm,
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 2. a,b) SEM images of pillar P-SPAES membrane: a) top-view and b) tilted-view. The pitch, diameter, and height of the pillars are 3.4, 2.4, and 3.7 µm, respectively. c,d) Optical images of the pillar P-SPAES membrane with c) white-light bulb illumination and d) 633 nm wavelength laser irradiation.
whereas that of the pillar was increased by 35%, from 2.3 to 3.1 µm. This difference in dimensional change is attributed to the larger volume expansion ratio for SPAES (83%) relative to that of Nafion (25%), showing the possibility of normal force generation. The peel strength of the interlocking interface was measured at two different RHs (0 and 50%). At each sample, the peeling test was carried out at 10 times. The peel strength, which is defined as the force required to separate the interface normalized by the width of the specimen, was monitored as a function of peel distance. For the flat SPAES/Nafion interface, the average peel strength was 3.38 ± 0.07 mN mm−1 at RH 0% and 3.07 ± 0.06 mN mm−1 at RH 50% (Figure 3b). To our surprise, the peel strength of the interlocking interface (P-SPAES/ Nafion) was 8-times higher than that of the normal interface at RH 50% (24.6 ± 0.14 mN mm−1) and 4-times higher at RH 0% (12.9 ± 0.08 mN mm−1). The significant increase in peel strength with increasing RH indicates that normal force is responsible for the tight bonding. For a clearer understanding of the generation of the normal force at the pillar/hole interface, an FEM analysis was conducted for a single interlocking structure. The physical parameters for the FEM analysis including kinematic and static friction coefficient, modulus, yield stress, ultimate tensile strength, dimensional change, and Poisson’s ratio are listed in Table S1 in the Supporting Information. The simulated stress distributions with hydration at RH 50% at two difference
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
peeling distances (0 and 2 µm) are displayed in Figure 3c. At zero peeling distance, the generation of significant normal stress along the interface was identified, explaining the large peeling strength. As the pillar is removed from the hole, the stresses are reduced because the interfacial area is reduced. This explains the gradual decrease of the peeling strength with peeling distance shown in Figure 3b. The membrane expansion with hydration, which generates stress at the interface, is regarded as the major cause of interfacial delamination during fuel cell operation.[37] Therefore, the immersion of a MEA into boiling water (hot water test) can serve as an accelerated interfacial durability test. For the hot water test, an MEA that has both an interlocking interface and a normal flat interface was used. The normal interface was delaminated 10 min after the immersion, whereas the interlocking interface was preserved even after 1 h, as shown in Figure S5 in the Supporting Information. It is interesting that the large degree of membrane expansion, which causes delamination of the flat interface, provides a tight interface for the interlocking configuration. To evaluate the effect of the interlocking interface on the cell performance, including long-term durability in particular, we carried out a wet/dry cycling test with a cycle comprising supply of dry (RH 0%) N2 feed for 2 min and supply of wet (RH 100%) N2 feed for another 2 min to both the cathode and anode compartments. Figure 4a,b compares the gavanostatic polarization curves before and after cycles for the
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
Figure 3. a) SEM image of the interlocking structure. b) Peel strength of the laminates of SPAES/Nafion membranes and P-SPAES/Nafion membranes. c) FEM results for a single interlocking structure: i) dry state at static condition; ii) wet state (RH 50%) at static condition; iii) wet state (RH 50%) under peeling at a peeling distance of 2 µm.
P-SPAES-based MEA with an interlocking interface and the SPAES-based MEA with a flat interface, respectively. The initial IV polarization curve is nearly identical to those of the two MEAs, indicating that the interlocking interface does not block proton transport and does not impede the performance of the CL. The SPAES-MEA exhibited a significant loss in power performance after 1050 cycles; the power density at 0.4 V dropped from 323.3 to 20.4 mW cm−2. However, the interlocking interface showed much slower performance decay; the power density at 0.4 V decreased from 331.3 to 202.1 mW cm−2 during 1950 cycles. The decay rate was 4.7-times faster for the flat interface (0.094% per cycle) relative to that of the interlocking interface (0.020%/cycle). Also, the OCV of the P-SPAES cell was 0.954 V at the first cycle and 0.951 V at the 1950th cycle, and hydrogen crossover measured by linear sweep voltammetry was 2.05 mA cm−2 at the first cycle and 2.15 mA cm−2 at the 1950th cycle (see Figure S6, Supporting Information). These results indicate that no significant mechanical membrane degradation such as pin-hole generation occurred during the wet/dry cycling. After the cycling test, the CL of the SPAES MEA was partly detached from the SPAES membrane, as shown in Figure S7a in the Supporting Information. In contrast, the interface of the P-SPAES cell was apparently preserved even after the cycles (Figure S7b, Supporting Information). The difference in the
4
wileyonlinelibrary.com
interfacial stability was also reflected in the impedance spectroscopy results. The impedance measured under N2/H2 for the cathode/anode provides information on the proton transport through the MEA. The bulk resistance (ROhm), which is determined by the intercept at the real axis of the Nyquist plot, includes the contributions from proton transport through the membrane and the membrane/CL interface. Figure 4c displays Nyquist plots of the impedances before and after wet/dry cycles for the SPAES and P-SPAES cell. The initial ROhm of the P-SPAES MEA (62 mΩ cm2) was somewhat smaller than that of the SPAES MEA (87 mΩ cm2), indicating that the interlocking structure provides better interfacial contact at the early stage of the cycling. For the SPAES MEA, the ROhm was significantly increased to 147 mΩ cm2, or by 1.69 times, after 1050 cycles, indicating significant interfacial delamination. In contrast, for the P-SPAES MEA, it was increased to 101 mΩ cm2, or by 1.19 times, after 1950 cycles. The resistance of ionic conduction through the CL (Rcl) can be obtained from a transmission line model. According to the model, a Warburg-like impedance at high frequencies corresponds to ion migration through the catalyst layer, and a subsequent vertical spike at low frequencies to the total capacitance and resistance of the CL.[38,39] The value for Rcl also can be determined from the length of the Warburg-like region (Rcl/3) projected onto Z′-ROhm.[38,39] The Nyquist plots
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 4. a,b) Galvanostatic polarization curves before and after 1050/1950 wet/dry cycles for the SPAES MEA (a) and the P-SPAES (b). c) Nyquist plots of the impedances for the SPAES MEA and P-SPAES MEA before and after 1050/1950 wet/dry cycles. d,e) Cross-sectional SEM images of the SPAES MEA (d) and the P-SPAES MEA (e) after 1950 wet/dry cycles.
of the impedances for before and after cycling exhibit nearly the same impedance shape (12 mΩ cm2), indicating that the values for Rcl are nearly identical and the performance fading is mainly from interface degradation, not from CL degradation. The results strongly support that the large difference in IV polarization after cycling is responsible for the interfacial delamination. Finally, we investigated the cross-section of the MEAs after wet/dry cycling. The interface of S-PAES and Nafion IBL was delaminated (Figure 4d), whereas the interlocking interface was well preserved (Figure 4e), clearly demonstrating that the interface was tightened by the interlocking structure. In conclusion, the interfacial bonding of a hydrocarbon membrane and a perfluorinated-ionomer-based catalyst layer was significantly improved by a physical interlocking interface structure. This structure was realized by the intrusion of micrometer-sized pillars fabricated on a SPAES membrane into a Nafion IBL formed on a catalyst layer. Normal force is generated at the pillar/hole interface owing to the higher expansion with hydration for SPAES than for Nafion, thereby increasing the peel strength of the interface. The improved interfacial bonding enhances the resistance of the hydrocarbon-membrane-based MEA to interfacial delamination during wet/dry cycles. The physical interlocking strategy could allow numerous types of HC membranes that suffer from poor interfacial adhesion to be used in practical applications.
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
Experimental Section Preparation of Polydimethylsiloxane (PDMS) Mold and P-SPAES Membrane: PDMS molds were prepared by replica molding from a silicon wafer having an array of micrometer-pillars on its surface fabricated by conventional photolithography. A mixture of the PDMS prepolymer and an initiator (Sylgard 184, Dow Corning, Midland, MI, USA) at a ratio of 10:1 by weight was degassed by vacuum suction and carefully poured onto the silicon master pattern. After curing at 80 °C for 1 h, the PDMS mold was gently released from the master pattern. The P-SPAES membrane was prepared by a solution-casting technique. Using a doctor blade, we carefully cast a 10 wt% DMAc solution of SPAES onto the PDMS mold, which had been treated by oxygen plasma in advance in order to enhance wetting of the SPAES solution. After completely evaporating residual DMAc at 60 °C for 12 h under vacuum, the replicated SPAES membrane was peeled off the PDMS mold. The molecular weight, ion exchange capacity, and water uptake were 50 000 g mol−1, 1.34 meq g−1 and 93%, respectively. Preparation of CL and IBL: A Pt/C catalyst (46.6 wt% Pt, Tanaka Kikinzoku Kogyo, Japan) was dispersed in a solvent; predetermined amounts of a Nafion (EW 1100 g eq−1, Dupont, USA) dispersion were added to the catalyst dispersion. The weight ratio of the carbon to ionomer was fixed at 1:1 in a dry state. The mixtures underwent ball milling for 2 h and were cast onto a fluorinated ethylene propylene (FEP, Dupont, USA) film using a 190 µm-gap doctor blade, followed by drying at 60 °C for 24 h. The Pt loading level of all the CLs was 0.35 ± 0.03 mg cm−2 and the active area of the CL 2 cm × 2 cm. A CL with 0.35 ± 0.03 mg cm−2 based on a Nafion ionomer was adopted as a cathode for all the MEA samples. For the anode CLs, the same procedure was applied except for the use of SPAES as an ionomer. To fabricate the IBL on the cathode CL, 5 wt% of Nafion dispersion (SE-5112, Dupont)
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5
www.advmat.de
COMMUNICATION
www.MaterialsViews.com was sprayed onto the cathode CL (five straight times) by using a spray coater (LSC-300, Lithotech., Korea) at 60 °C. The nozzle air pressure and moving speed were 320 kPa and 50 mm s−1, respectively. Physical Characterization and Analysis of the Interlocking Interface: The peel strength was measured for the laminates of Naftion/PSPAES membranes and Nafion/nonpatterned SPAES (NP-SPAES) by using a universal testing machine (Instron 5583) at room temperature and at a strain rate of 20 mm min−1; the assembly of two membranes (60 mm × 13 mm), half of which were laminated at 3.4 MPa and at 150 °C for 3 min, pulled apart by tugging the two unlaminated sides in opposite directions. Finite element modeling (FEM) was conducted for the interlocking structure using the commercial software ABAQUS (Dassault Systems Americas Corp., Waltham, MA, USA). The various physical parameters of Nafion and SPAES such as friction coefficient, modulus, yield stress, ultimate tensile strength, dimensional change, and Poisson’s ratio were experimentally determined and utilized for the FEM analysis. The friction coefficient of Nafion/SPAES is measured by rubbing abrasion test (see Figure S4, Supporting Information). The morphologies of the P-SPAES were investigated by scanning electron microscopy (SEM) (Sirion, FEI) and optical microscopy (OM) (BX51, Olympus). Fabrication, Electrochemical Performance, and Electrochemical Analysis of MEAs: MEAs were fabricated by decal-transfer of the anode and the cathode CLs coated on an FEP film on each side of the membrane at a pressure of 3.4 MPa and at 150 °C for 3 min. The FEP films were peeled off the laminate at room temperature. A single cell was assembled with the MEA, a pair of gas diffusion media (39BC, SGL), a pair of silicon gaskets, and a pair of graphite blocks with a triple serpentine flow field for the reactants. The flow fields for the anode and cathode reactants were mirror images. The IV polarization curves of the single cells were obtained with a fuel cell test station (SMART II, Won-A tech, Korea). We tried to exclude the influence of water flooding by operating the cells at a high flow rate of 500 sccm for humidified hydrogen (RH 100%) and of 1500 sccm for humidified air (RH 100%), which correspond to the stoichiometry of H2/Air = 14.3/18.0 at 0.2 A cm−2. The IV polarization curve of each cell was obtained at 70 °C without any back pressure after daily measurements of three IV curves for three successive days as a break-in. The IV curves did not vary after the second day. The electrochemical impedance values were measured using an AC impedance analyzer (1400 FRA and 1470E, Solartron); the AC amplitude was 10 mV and the frequency range was from 105 to 10−1 Hz. The hydrogen crossover was determined from the limiting current observed in linear sweep voltammetry (from 0.05 to 0.70 V) under an H2/N2 (anode/cathode) atmosphere. After initial evaluation of the cell performance of the given PEMFCs, humidity cycle tests were conducted to examine the effect of the interlocking structures on the cell performance under repeated hydration and dehydration cycles. For this experiment, non-humidified and fully humidified nitrogen gas was alternately fed every 2 min at a flow rate of 1000 cm3 s−1. The humidification was controlled by (dis)connection to a humidifier. During these repeated cycles, the polarization curve and open-circuit voltage (OCV) were obtained at every 150 humidity cycles.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements K.-H.O. and H.S.K. contributed equally to this work. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (Code No. NRF-2012R1A1A2042558),
6
wileyonlinelibrary.com
the WCU program (EEWS) at KAIST (Code No. R-31–2008–000–1005–0), and the KRICT OWN Project. Received: January 21, 2015 Revised: February 14, 2015 Published online:
[1] M. A. Hickner, H. Ghassemi, Y. S. Kim, B. R. Einsla, J. E. McGrath, Chem. Rev. 2004, 104, 4587. [2] K. D. Kreuer, J. Membr. Sci. 2001, 185, 29. [3] K. Goto, I. Rozhanskii, Y. Yamakawa, T. Otsuki, Y. Naito, Polym. J. 2008, 41, 95. [4] T. Higashihara, K. Matsumoto, M. Ueda, Polymer 2009, 50, 5341. [5] C. H. Park, C. H. Lee, M. D. Guiver, Y. M. Lee, Prog. Polym. Sci. 2011, 36, 1443. [6] R. Devanathan, Energy Environ. Sci. 2008, 1, 101. [7] S. Y. Kim, S. Kim, M. J. Park, Nat. Commun. 2010, 1, 88. [8] P. Xing, G. P. Robertson, M. D. Guiver, S. D. Mikhailenko, K. Wang, S. Kaliaguine, J. Membr. Sci. 2004, 229, 95. [9] K.-H. Oh, D. Lee, M.-J. Choo, K. H. Park, S. Jeon, S. H. Hong, J.-K. Park, J. W. Choi, ACS Appl. Mater. Interfaces 2014, 6, 7751. [10] F. Wang, M. Hickner, Y. S. Kim, T. A. Zawodzinski, J. E. McGrath, J. Membr. Sci. 2002, 197, 231. [11] B. Bae, T. Yoda, K. Miyatake, H. Uchida, M. Watanabe, Angew. Chem., Int. Ed. 2010, 49, 317. [12] J. Fang, X. Guo, S. Harada, T. Watari, K. Tanaka, H. Kita, K.-I. Okamoto, Macromolecules 2002, 35, 9022. [13] X. Guo, J. Fang, T. Watari, K. Tanaka, H. Kita, K.-I. Okamoto, Macromolecules 2002, 35, 6707. [14] H.-Y. Jung, K.-Y. Cho, K. A. Sung, W.-K. Kim, M. Kurkuri, J.-K. Park, Electrochim. Acta 2007, 52, 4916. [15] H.-Y. Jung, K.-Y. Cho, K. A. Sung, W.-K. Kim, J.-K. Park, J. Power Sources 2006, 163, 56. [16] M. Sankir, Y. S. Kim, B. S. Pivovar, J. E. McGrath, J. Membr. Sci. 2007, 299, 8. [17] K.-H. Oh, W.-K. Kim, K. A. Sung, M.-J. Choo, K.-W. Nam, J. W. Choi, J.-K. Park, Int. J. Hydrogen Energy 2011, 36, 13695. [18] K. A. Sung, W.-K. Kim, K.-H. Oh, J.-K. Park, Electrochim. Acta 2009, 54, 3446. [19] W.-K. Kim, K. A. Sung, K.-H. Oh, M.-J. Choo, K. Y. Cho, K.-Y. Cho, J.-K. Park, Electrochem. Commun. 2009, 11, 1714. [20] S. Sambandam, V. Ramani, Electrochim. Acta 2008, 53, 6328. [21] E. B. Easton, T. D. Astill, S. Holdcroft, J. Electrochem. Soc. 2005, 152, A752. [22] S. Holdcroft, Chem. Mater. 2013, 26, 381. [23] A. Ayad, J. Bouet, J. F. Fauvarque, J. Power Sources 2005, 149, 66. [24] Y. S. Kim, M. J. Sumner, W. L. Harrison, J. S. Riffle, J. E. McGrath, B. S. Pivovar, J. Electrochem. Soc. 2004, 151, A2150. [25] K. B. Wiles, C. M. de Diego, J. de Abajo, J. E. McGrath, J. Membr. Sci. 2007, 294, 22. [26] C. H. Lee, S. Y. Lee, Y. M. Lee, S. Y. Lee, J. W. Rhim, O. Lane, J. E. McGrath, ACS Appl. Mater. Interfaces 2009, 1, 1113. [27] K. A. Mauritz, R. B. Moore, Chem. Rev. 2004, 104, 4535. [28] S. Kaliaguine, S. D. Mikhailenko, K. P. Wang, P. Xing, G. Robertson, M. Guiver, Catal. Today 2003, 82, 213. [29] L. Carrette, K. A. Friedrich, U. Stimming, Fuel Cells 2001, 1, 5. [30] H. S. Kang, H.-T. Kim, J.-K. Park, S. Lee, Adv. Funct. Mater. 2014, 24, 7273. [31] H. S. Kang, S. Lee, J.-K. Park, Adv. Funct. Mater. 2011, 21, 4412. [32] S.-H. Kim, S. Y. Lee, G.-R. Yi, D. J. Pine, S.-M. Yang, J. Am. Chem. Soc. 2006, 128, 10897.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2015, DOI: 10.1002/adma.201500328
COMMUNICATION
[33] H. S. Kang, S. Lee, S.-A. Lee, J.-K. Park, Adv. Mater. 2013, 25, 5490. [34] T. W. Lee, O. Mitrofanov, J. W. P. Hsu, Adv. Funct. Mater. 2005, 15, 1683. [35] W. M. Choi, O. O. Park, Microelectron. Eng. 2003, 70, 131. [36] J. K. Koh, Y. Jeon, Y. I. Cho, J. H. Kim, Y.-G. Shul, J. Mater. Chem. A 2014, 2, 8652.
[37] C. H. Park, C. H. Lee, M. D. Guiver, Y. M. Lee, Prog. Polym. Sci. 2011, 36, 1443. [38] M. C. Lefebvre, R. B. Martin, P. G. Pickup, Electrochem. Solid-State Lett. 1999, 2, 259. [39] D. Malevich, B. R. Jayasankar, E. Halliop, J. G. Pharoah, B. A. Peppley, K. Karan, J. Electrochem. Soc. 2012, 159, F888.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
7
