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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Remarkable Reinforcement Effect in Sulfonated Aromatic Polymers as Fuel Cell Membrane Junpei Miyake,† Masato Kusakabe,§ Akihiro Tsutsumida,∥ and Kenji Miyatake*,†,‡ †
Clean Energy Research Center and ‡Fuel Cell Nanomaterials Center, University of Yamanashi, 4-4-37 Takeda, Kofu, Yamanashi 400-8510, Japan § Material Solutions Research Institute, Kaneka Corporation, 5-1-1 Torikai-nishi, Settu, Osaka 566-0072, Japan ∥ Toray Research Center, Inc., 3-3-7 Sonoyama, Otsu, Shiga 520-8567, Japan S Supporting Information *
ABSTRACT: Fluorine-free aromatic ionomers are next generation materials for proton exchange membrane fuel cells (PEMFCs). In addition to high proton conductivity and chemical durability, a membrane must also have high mechanical durability under practical fuel cell operating conditions, where frequent humidity changes are involved. We herein demonstrate that a fluorine-free reinforced aromatic PEM exhibits much improved mechanical durability compared with the parent aromatic PEM under the humidity cycling test conditions. The parent PEM and the reinforcement material are a sulfonated polybenzophenone derivative (SPK, in house) and a nonwoven fabric (NF, composite of glass and PET fibers), both of which do not contain any fluorine atoms. Because the compatibility between the SPK and the reinforcement materials is high, an almost void-free, dense, homogeneous, and tough reinforced PEM is attainable even with thin membrane thickness (18 μm), leading to a reasonably high fuel cell performance. The reinforcement material improves in-plane dimensional stability and mitigates crack propagation during frequent humidity changes, resulting in high durability (more than 18 000 cycles) in the wet−dry cycling test. The advantages of this fluorine-free reinforced PEM, unlike typical reinforced PEMs (e.g., Gore-SELECT consisting of a perfluorosulfonic acid ionomer and a microporous expanded polytetrafluoroethylene support layer), include versatility in molecular design, enabling further improvement in performance and durability of PEMFCs with lower cost. KEYWORDS: aromatic ionomers, reinforcement, proton exchange membranes, fuel cells, wet−dry cycling durability
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INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) have attracted much attention for their applications in residential cogeneration systems, electric vehicles, and portable devices, with their intrinsic advantages such as high efficiency and low pollution levels. Recently, fuel cell vehicles (FCVs) have been commercialized; however, further improvement in the performance, durability, and cost effectiveness of the fuel cell systems and materials is essential for their widespread dissemination.1,2 Proton exchange membranes (PEMs), as one of the key components in PEMFCs, are required to have high proton conductivity, durability, and gas barrier properties. The benchmark PEM is perfluorosulfonic acid (PFSA) ionomer3 (e.g., Nafion); however, the high production cost, high gas permeability, and environmental incompatibility have been intrinsic problems for the PFSA membranes. In addition, the complicated synthetic process has hampered further progress toward the improvement of membrane properties. In contrast, fluorine-free aromatic PEMs, as alternatives, have considerably high flexibility in molecular design and synthesis.4−15 Therefore, various (co)polymers have been proposed in the © XXXX American Chemical Society
literature; however, none of these has fulfilled all of the required properties, in particular, proton conductivity (at low humidity) and durability (oxidative, mechanical), simultaneously. We have developed a semiblock copolymer composed of a sulfonated benzophenone group as the hydrophilic component (SPK, Figure 1a).16 The SPK membrane was more oxidatively durable compared with Nafion under operando fuel cell (open circuit voltage hold test) conditions. Furthermore, the SPK membrane was highly proton-conductive even under high temperature and low humidity conditions and exhibited fuel cell performance comparable to that of Nafion over a wide range of humidity. However, in order to attain such high performance, the ion exchange capacity (IEC) of the SPK membrane needs to be high (>2.5 mequiv g−1 compared to 0.9 mequiv g−1 for Nafion). Since such a high IEC membrane shows large water uptake and swelling under hydrated Received: December 28, 2017 Accepted: February 20, 2018 Published: February 20, 2018 A
DOI: 10.1021/acsaem.7b00349 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
Figure 1. (a) Chemical structure of SPK. Photographic images of (b) SPK membrane, (c) nonwoven fabric (NF), and (d) NF-reinforced SPK membrane.
Figure 2. Cross-sectional SEM images of (a) SPK, (b) NF-reinforced SPK, and (c) NF-reinforced SPK (at higher magnification) membranes.
sulfonated poly(arylene ether sulfone)/polyimide nanofiber composite membrane for microbial electrolysis cell application),20 none of them has demonstrated the improvement of mechanical durability under practical fuel cell operating conditions, where frequent humidity changes are involved. For this purpose, there has been one successful report on a less fluorine system (hydrocarbon ionomer/poly(vinylidene fluoride) blend membrane),21 however to the best of our knowledge, there have been no successful reports on completely fluorine-free reinforced systems. The objective of this research is to improve the mechanical durability of our fluorine-free aromatic ionomer membrane (SPK, Figure 1a) in the humidity (wet−dry) cycling test, by introducing a fluorine-free mechanical support layer. For this purpose, we selected a nonwoven fabric (NF), which contains the glass fiber and polyethylene terephthalate (PET) fiber (Figure 1c), to ensure both the strength and flexibility of the mechanical support layer. The reinforcement effect of the NF
conditions, mechanical durability could be another issue under practical fuel cell operating conditions, where frequent humidity changes are involved. One of the effective approaches to improve mechanical durability is to reinforce the ionomer with mechanical support layers. A typical example is the Gore-SELECT membrane, where the PFSA ionomer is reinforced by a microporous expanded polytetrafluoroethylene (ePTFE) layer.17 Nafion XL, an advanced version of the reinforced PFSA membrane, also has a similar configuration including PTFE mechanical support layers.18 These reinforced PFSA membranes exhibit much improved mechanical durability compared to those of nonreinforced PFSA membranes;19 however, such systems become more expensive because of the use of costly PTFE mechanical support layers in addition to a costly PFSA ionomer. Thus, cost-effective fluorine-free systems, i.e., fluorine-free aromatic ionomers reinforced by a fluorine-free mechanical support layer, is favorable. Although there are some reports on such fluorine-free reinforced systems for other purposes (e.g., B
DOI: 10.1021/acsaem.7b00349 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials on the performance and durability of the SPK membrane is discussed.
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RESULTS AND DISCUSSION Preparation of the SPK membrane (Figure 1b) was performed according to the literature.16 By casting the SPK solution (10 Table 1. Physical Properties of SPK and NF-Reinforced SPK Membranes dimensional change ratio (%)c
membrane SPK NFreinforced SPK
thickness (μm) 26 18
a
IECb (mequiv g−1)
water uptake (%)c
inplane
throughplane
2.58 1.69
412 163
64 3
122 140
Figure 5. Crossover current density of NF-reinforced SPK (IEC = 1.69 mequiv g−1) cell during the humidity cycling test at 80 °C.
wt %) with the NF (see the Supporting Information for details), the NF-reinforced SPK membranes (Figure 1d) with IEC of ca. 1.7−1.9 mequiv g−1 were successfully prepared. Figure 2 shows cross-sectional SEM images of the SPK and the NF-reinforced SPK membranes. Both membranes possessed uniform membrane thickness, whose values were in good agreement with the ones obtained by micrometer (Table 1). For the NF-reinforced SPK membrane (Figure 2b), a sandwichlike (triple-layer) structure (i.e., SPK/SPK + reinforcement materials/SPK) was observed. The center layer (ca. 8−10 μm thick), where the reinforcement materials located homogeneously, indicated the good compatibility between the reinforcement materials (i.e., the glass and PET fibers) and the aromatic-type PEM (i.e., SPK). To investigate the interface between the reinforcement materials and the aromatic-type PEM in more detail, Figure 2c shows the SEM image at higher magnification in which cross sections of the glass (ca. 0.5 μm) and PET (ca. 3 μm) fibers were clearly observed. The fibers and SPK seemed to have close contacts at the interfaces. In the center layer, a very small amount of small voids (black dots) was observed near the reinforcement materials. It is unclear whether the small voids (ca. 0.2−0.8 μm) originated from bubbles or detachment of the smaller glass fibers (average diameter = 0.5 μm) during the preparation of the SEM samples. In any cases, it seemed that the amount and the size of the voids were small enough not to affect much on the membrane properties. Overall, in spite of its thin membrane thickness (18 μm), an almost void-free, dense, homogeneous, and tough reinforced PEM was obtained by a simple solution casting method. Table 1 shows other physical properties of the membranes such as IEC, water uptake, and dimensional change ratio of the SPK and NF-reinforced SPK membranes. The IEC of the NFreinforced SPK membrane was 1.69 mequiv g−1, which corresponded to the SPK/NF mass ratio of 68:32. This IEC (1.69 mequiv g−1) was 32% lower than that (2.48 mequiv g−1) of the parent SPK because of the absence of acid groups in NF. Therefore, the water uptake of the NF-reinforced SPK membrane was also reasonably low. The parent SPK membrane exhibited anisotropic swelling with higher dimensional change of through-plane direction (122%) than that of in-plane direction (64%). The anisotropic swelling was more pronounced for the NF-reinforced SPK membrane with through-plane (140%) and in-plane (3%) dimensional changes. A similar tendency was reported with reinforced perfluorinated PEMs, where the PFSA ionomer was reinforced with a microporous expanded polytetrafluoroethylene (ePTFE) layer.22
a Determined by micrometer. bDetermined by acid−base titration. cIn deionized water at 80 °C for 6 h.
Figure 3. Proton conductivity of the membranes at 80 °C as a function of relative humidity.
Figure 4. IR-included H2/O2 polarization curves (solid symbols) and ohmic resistances (open symbols) of the SPK (black) and NFreinforced SPK (red) cells at 80 °C under humidity conditions of (a) 100% and (b) 53% RH.
C
DOI: 10.1021/acsaem.7b00349 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials
Figure 6. Cross-sectional SEM images of (a, b) SPK and (c, d) NF-reinforced SPK membranes after the humidity cycling test.
Figure 3 shows humidity dependence of the proton conductivity of the NF-reinforced SPK membrane at 80 °C. The data of the SPK membrane were also included as a comparison.23 At > 80% RH, the proton conductivity of the NF-reinforced SPK and the SPK membranes were comparable; however, the proton conductivity of the NF-reinforced SPK was slightly lower than that of the SPK membrane at
