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Cite This: ACS Appl. Nano Mater. 2019, 2, 3857−3863
Engineered Membrane−Electrode Interface for Hydrocarbon-Based Polymer-Electrolyte-Membrane Fuel Cells via Solvent-VaporAnnealed Deposition Keun-Hwan Oh† and Insung Bae*,†,‡ †
Corporate R&D, LG Chem, 188 Munji-ro, Yuseong-gu, Daejeon 34122, Republic of Korea Department of Advanced Materials and Chemical Engineering, Hannam University, 1646 Yuseong-daero, Yuseong-gu, Daejeon 34054, Republic of Korea
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‡
S Supporting Information *
ABSTRACT: We present a facile and simple method to fabricate a threedimensional (3D) interface between a hydrocarbon-based polymer membrane and an electrode of a membrane−electrode assembly via solvent-vapor-annealed deposition (SVAD). SVAD not only increases the membrane proton conduction with nanophase-separated morphology but also reduces the interfacial resistance between the membrane and electrode with formation of nanoscale 3D interfaces. The enlarged interfacial area improves the power performance of fuel cells, originating from reduced interfacial resistances and increased electrochemical active surface area of the catalyst layer (CL) by ionomer impregnation into the tortuous nanopores of the nearest CL. Furthermore, the effect of the engineered 3D interface is investigated by measuring the mechanical durability by the wet− dry cycle and peel strength. KEYWORDS: polymer-electrolyte-membrane fuel cell, hydrocarbon membrane, proton-exchange membrane, solvent-vapor-annealed deposition, membrane−electrode assembly
1. INTRODUCTION A proton-exchange-membrane fuel cell or polymer-electrolytemembrane fuel cell (PEMFC) has been worth considering as a strong candidate for ecofriendly technology for a sustainable energy conversion and electric power source for automotive and stationary applications.1−3 Perfluorosulfonic acid (PFSA) ionomers like Nafion have been used because of their high proton conductivities and mechanical stabilities. However, they have significant difficulties such as limited operating temperature, high fuel crossover, and cost inefficiency.4 As an alternative membrane to the PFSA ionomers, hydrocarbon (HC) block copolymers, such as sulfonated poly(ether ether ketone), sulfonated poly(arylene ether ketone) (SPAEK), and sulfonated polyimide, have been intensively studied for their high proton conduction, low gas permeability, and cost efficiency.5−11 To realize a practical use of the fuel cell, the development of a robust interface between the membrane and catalyst layer (CL) is a critical issue in achieving a high electrochemical active surface area (ECSA) and reliable stability of a membrane−electrode assembly (MEA). Many previous reports have suggested improving the interfacial contact between the PFSA membranes and CLs with alternative fabrication techniques such as membrane patterning, direct inkjet printing, and spray coating.12−19 Jeon et al. defined shape-controlled patterns of the membrane to enhance the power performance © 2019 American Chemical Society
of PEMFCs by increasing the specific membrane surface area for higher platinum utilization.15 Moreover, Klingele et al. reported the direct deposition of proton-exchange membrane onto the electrode surface, enabling high-performance PEMFC with lower ohmic, charge-transfer, and mass-transport resistances due to an enlarged contact area between the membrane and CL.19 However, the high glass transition temperature of HC membranes gives rise to a difficulty in morphological deformation of the membranes, resulting in a weak interface between the membrane and electrode during thermal lamination for the fabrication of the MEA.20 In efforts to resolve the interfacial problem of the HC-based MEA, there have been studies on the development of the robust interface between the HC membranes and CL.21−24 The HC polymers were applied in the CL as electrode ionomers to enhance the interfacial affinity of the HC membrane to the CL.21,22 However, the low permeability of oxygen gas for the HC membrane prevents the oxygen reduction reaction at the cathode CL for a consequent decrease of the power performance.25 Recently, the PFSA interlayers as a mechanical fastener have been introduced for the robust interlocked Received: April 16, 2019 Accepted: May 15, 2019 Published: May 15, 2019 3857
DOI: 10.1021/acsanm.9b00706 ACS Appl. Nano Mater. 2019, 2, 3857−3863
Article
ACS Applied Nano Materials
Figure 1. (a) Schematic illustration of SVAD and cross-sectional SEM images of the (b) pristine MEA and (c) SVAD-MEA. 2.2. Synthesis of Sulfonated Poly(arylene ether ketone) (SPAEK) Block Copolymer. The simple condensation reaction was applied for the synthesis of SPAEK block copolymers.6−8 First, for the synthesis of hydrophilic blocks, in a 500 mL round-bottom flask with an overhead stirrer and Dean−Stark traps under a N2 flow, 8.470 g of DFBP (38.82 mmol), 8.221 g of HQS (36.01 mmol), 9.955 g of potassium carbonate (72.03 mmol), 40 mL of benzene, and 52.47 mL of anhydrous DMSO were mixed together, after which the mixture was heated at 140 °C for 4 h, followed by 180 °C for 20 h. For the synthesis of hydrophobic blocks, 1.310 g of DFBP (6.00 mmol), 3.575 g of BHF (10.20 mmol), 0.059 g of potassium carbonate (0.4 mmol), 20 mL of benzene, and 57.65 mL of anhydrous DMSO were added successively to the above hydrophilic block oligomers. The reaction mixture was heated at 140 °C for 4 h, followed by 180 °C for 20 h. The highly viscous SPAEK block copolymer solution was diluted with DMSO (40 mL) and slowly poured into excess water/ethanol for precipitation. The collected polymer was filtered, thoroughly washed with deionized water several times to remove residual solvent, and dried overnight at 80 °C in a vacuum oven. The synthesis details of the SPAEK block copolymer are described in Figure S1. 2.3. SVAD of the SPAEK Membrane and Preparation of the CLs and MEAs. The SPAEK (8 wt % in DMSO) was cast onto a poly(ethylene terephthalate) (PET) substrate, dried at 50 °C for 3 h, and subsequently annealed under vacuum at 100 °C for 12 h. The resultant membrane was immersed in a 1.0 M sulfuric acid solution at 80 °C for 24 h and then rinsed with deionized water to remove the residual acidic materials. After that, the membrane was dried in a vacuum oven at 80 °C for 4 h. For SVAD, the 26 μm SPAEK membrane was peeled off from the PET substrate, placed on the prepared CL in a closed jar, and exposed to 10 mL of DMSO at 60 °C for 18 h. After SVAD, the sample was removed from the jar and heated at 80 °C for 24 h in a vacuum oven to evaporate the residual solvent.
interfaces between the HC membrane and CL containing the PFSA ionomer.23,24 The tight interfacial bonding with the engineered structures of the HC membranes such as micropillar arrays and the interconnected ball and joint features improved the mechanical durability of the MEA with improved interfacial bonding. However, these interfacial enhancements only focused on the development of mechanical durability but cannot achieve the increased performance of the strengthened HC-based fuel cells. In this work, we present a simple and effective process for the engineered three-dimensional (3D) interface between an HC membrane and an electrode with the developed nanophase-separation morphology of the HC block copolymer via the solvent-vapor-annealed-deposition (SVAD) method. Compared to the pristine MEA, the MEA fabricated via SVAD (SVAD-MEA) gives a significant 42% increased current density at 0.6 V because of its reduced ohmic, charge-transfer, and mass-transport resistances. Furthermore, the impregnated structure of the HC copolymers into the tortuous pores of the CL enables robust interfacial contact with nanoscale 3D structures, increasing catalyst utilization. The improved interfacial properties were investigated by measuring the hydrogen crossover current and adhesion strength.
2. EXPERIMENTAL SECTION 2.1. Materials. Hydroquinonesulfonic acid potassium salt (HQS), 4,4-difluorobenzophenone (DFBP), 9,9-bis(hydroxyphenyl)fluorene (BHF), potassium carbonate, anhydrous dimethyl sulfoxide (DMSO), benzene, anhydrous ethyl alcohol, and sulfuric acid were purchased from Sigma-Aldrich. Nafion (NR211, Dupont) with a thickness of 26 μm was used as the control membrane. 3858
DOI: 10.1021/acsanm.9b00706 ACS Appl. Nano Mater. 2019, 2, 3857−3863
Article
ACS Applied Nano Materials
Figure 2. (a) Impregnation depth of the membrane into the CL with various SVAD times. (b−g) SEM cross-sectional images with various annealing times of 0, 6, 12, 18, 24, and 72 h (scale bars: 1 μm). where L, R, and S represent the distance between the four potential sensing platinum electrodes, measured resistance, and surface area of cross-sectional membranes, respectively. The proton conductivity was measured at 70 °C with various relative humidity (RH) conditions. The cross-sectional morphology of the membrane was analyzed by field-emission scanning electron microscopy (SEM; Hitachi S-4800). The phase-separation morphology of the membrane was investigated by atomic force microscopy (AFM; Park Systems XE7) in noncontact tapping mode and small-angle X-ray scattering (SAXS; Rigaku D/ max-2500) with an operation condition at 40 kV and 70 mA using Cu Kα as an X-ray source (γ = 1.542 Å). A peeling test between the membrane and electrode was performed using the universal test machine (UTM) under ambient conditions at a speed of 5 mm min−1. The pore volume and pore-size distribution of the CL were examined by mercury intrusion porosimetry (MIP). 2.5. Electrochemical Characterization of the MEA. The PEMFC performance of the fabricated MEA was evaluated at 70 °C cell temperature and 50% RH using a test station (Scribner Associates Inc.) with triple serpentine flow plates, glass-fiberreinforced PTFE gaskets, and a gas diffusion layer (10BB, SGL). High-purity hydrogen and air with stoichiometries of 8.6 and 14.5 at 0.2 A cm−2, respectively, were supplied to the anode and cathode sides with ambient pressure. Before the IV polarization measurement, alternate voltage cyclings at 0.6 and 0.3 V were conducted under fuel supply until the current density reached a constant value. Electrochemical impedance spectroscopy measurements were performed
To fabricate the anode and cathode CLs, 1 g of platinum/carbon (46.6 wt % platinum 10F50E, Tanaka Kikinzoku Kogyo, Japan) and 2.4 g of 20 wt % Nafion dispersion (EW 1100 g equiv−1, Dupont, USA) were mixed together in a cosolvent of propanol/water (1/1 weight ratio) and then dispersed with a tip-type ultrasonicator for 10 min. The catalyst slurry was cast onto poly(tetrafluoroethylene) (PTFE) substrate with a 100 μm gap bar, then dried at 40 °C for 30 min, and subsequently heated at 140 °C for 30 min in the atmosphere. The Nafion-to-carbon ratio was fixed at 0.9. To fabricate the MEA with an active area of 25 cm2, the membrane was sandwiched between the anode and cathode, followed by hotpressing at 140 °C under 10 MPa of pressure for 5 min. The platinum loading for each electrode was carefully managed to be within 0.35 ± 0.02 mg cm−2. The pristine MEA was fabricated with the same procedure except for the SVAD process. 2.4. Membrane Characterization. The proton conductivity of the membrane was measured with a four-probe system using a membrane conductivity cell by the alternating-current (ac) impedance spectroscopy method (Bio-Logics, HCP-803). The membrane was prepared with a size of 1 × 4 cm2 and 26 μm thickness and contacted with four platinum electrodes, which supplied specific voltages from the potentiostat and resulted in current. The proton conductivity (σ) of membranes was calculated from the equation σ = L /RS
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DOI: 10.1021/acsanm.9b00706 ACS Appl. Nano Mater. 2019, 2, 3857−3863
Article
ACS Applied Nano Materials
Figure 3. (a) AFM phase images with tapping mode. (b) SAXS and (c) proton conductivity of the pristine MEA and SVAD-MEA (scale bar: 100 nm). with an ac impedance analyzer (Bio-Logics, HCP-803) over the frequency range of 105−0.1 Hz and with an ac amplitude of 10% of the applied current. The ECSA was calculated by integrating the hydrogen desorption area of the cyclic voltammogram. The mechanical durability test with wet−dry cycles was examined by alternating gas supplies of humidified (150% RH) and dry (0% RH) N2 for every 2 min at a flow rate of 1 L min−1. To measure the hydrogen crossover current, linear-sweep voltammetry (LSV) was conducted under 0.2/0.2 L min−1 (H2/N2) and 50% RH as a rate of 1 mV s−1 from 0.1 to 0.6 V.
exhibited decreased power density because of the difficulty in filling the narrow trenches with catalyst particles.13,14 However, the nanoscale 3D interface of the SVAD-MEA was successfully achieved by impregnation of the SPAEK copolymer along the nanosize porous architecture of the CL. Using MIP, the poresize distribution of the CL was investigated and 50 nm size pores were observed in the CL. The penetration interlayer of the 3-μm-thick SPAEK membrane by solvent vapor annealing showed the wavy and winding film morphology formed along with the porous electrode structure, compared to the pristine SPAEK membrane exhibiting a flat interface (Figure S3). An impregnation depth of the SPAEK into the electrode of the SVAD-MEA was controlled from 0 to 6.8 μm with various annealing times from 0 to 72 h (Figure 2a). The SPAEK polymers were annealed and began to impregnate into the nanopores of the CL by locating in saturated DMSO vapor, generating the robust interface between the membrane and electrode, as shown in the cross-sectional SEM images of Figure 2. The annealing time was optimized at 18 h, representing 1-μm-thick impregnation depth to enlarge the interfacial contact area without preventing gas and water transport. After 72 h, however, the SPAEK was impregnated across the CL, which reduced the porosity of the electrode, disturbing mass transport (Figure S4). Overall, the SPAEK membrane successfully formed a nanoscale 3D interface with the CL by SVAD but not in the case of conventional approaches such as lithography and surface patterning.13−15 In addition to the interfacial morphology between the membrane and electrode, the nanostructure of the membrane was controlled by SVAD, characterized by tapping-mode AFM, as shown in Figure 3a. Although Nafion exhibits a uniform and
3. RESULTS AND DISCUSSION The schematic illustration of an interface between the membrane and electrode is shown in Figure 1a. The SPAEK copolymer was synthesized via one-pot condensation, and membranes were prepared by uniform film casting of the homogeneous SPAEK solution in DMSO onto a glass substrate, followed by sulfonation (Figure S1). The thickness of the membranes was carefully controlled with 26 μm for the reliability of the SVAD method and comparison with Nafion (NR211). To fabricate the nanoscale 3D interface, the SPAEK membrane on the cathode electrode was annealed at 60 °C for 18 h by saturated DMSO vapor in a closed jar, giving the chain mobility of the copolymer. The fabrication process details for the SVAD-MEA are schematically illustrated in Figure S2. The annealed SPAEK could be easily impregnated into the porous structure of the CL and have a larger interfacial contact area between the SPAEK and CL with 3D interfaces, compared to the interface of the pristine MEA, as observed in the SEM images (Figure 1b,c). In the conventional patterned structure of the membrane, the MEA with nanosize membrane patterns 3860
DOI: 10.1021/acsanm.9b00706 ACS Appl. Nano Mater. 2019, 2, 3857−3863
Article
ACS Applied Nano Materials
Figure 4. (a) Polarization curves, (b) impedance plots, (c) schematic illustration of oxygen reduction reaction, and (d) cyclic voltammograms of the pristine MEA and SVAD-MEA. The inset of part b is the equivalent circuit used to model the impedance spectra.
membrane were 6.45 and 1.83 mS cm−1, which are higher than those of the pristine SPAEK membrane (3.76 and 0.59 mS cm−1) under 50% and 30% RH, respectively. The wellconnected and integrated hydrophilic proton channels may help the proton to improve the conduction under low RH conditions because of a high density of −SO3H groups of the SPAEK.26 This can also be observed in the nanostructured morphology and proton conductivity of the Nafion membrane, as shown in Figures S5b and S6, respectively. Nafion showed a smaller Bragg distance of 10.7 nm for the bicontinuous nanophase-separated morphology and higher proton conductivities of 27.97 and 12.2 mS cm−1 under 50% and 30% RH, respectively, compared with the SPAEK membranes. Single-cell polarization data of the pristine MEA and SVADMEA under 50% RH are shown in Figure 4a. (The polarization results of the MEAs under 32% RH are shown in Figure S7.) In the ohmic polarization regime (
