Durability of Newly Developed Polyphenylene-Based Ionomer Membranes in Polymer Electrolyte Fuel Cells: Accelerated Stress Evaluation

Abstract

Perfluorosulfonic acid (PFSA) membranes such as Nafion® have been widely used as the electrolyte for polymer electrolyte fuel cells (PEFCs) due to their high proton conductivity and chemical stability. However, their fully fluorinated chemical structure leads to high production cost, low glass transition temperature and high gas permeability, which tend to impede the widespread application of PEFCs. To address these problems, sulfonated hydrocarbon (HC) membranes have been researched over the past decade. In our previous study,1 we investigated a sulfonated benzophenone poly(arylene ether ketone) semiblock copolymer (SPK-bl-1, SPK, Figure 1a)2 and a phenylene poly(arylene ether ketone) semiblock copolymer (SPP-bl-1, SPP, Figure 1b)3 membrane under accelerated open circuit-voltage (OCV) conditions appropriate for FCVs. The results indicate that a simple hydrophilic structure that does not include ketone groups leads to increased chemical stability versus radical attack decomposition.On the other hand, an SPP membrane quickly decomposed when subjected to a Fenton’s reagent test, which may be attributed to the presence of heteroatoms in the hydrophobic structure. Based on these results, we developed a new hydrocarbon membrane that consists solely of phenylene groups, which is designated a sulfonated poly(phenylene) quinquephenylene (SPP-QP, Figure 1c).4 In the present research, we focus on the durability of the SPP-QP membrane, in comparison with previous results for HC membranes (SPP and SPK) under identical accelerated OCV conditions.1 The properties of the membranes are listed in Table 1. We have designated these combinations of MEA components as cell types 1, 2 and 3 cells, as listed in Table 2. The membrane designations, e.g., “SPP-QP type #” (Figure 2), simply refer to the same membrane tested in different cell configurations. During the durability evaluations, the exhaust water from both sides of the cells was collected and then analyzed by ion chromatography (IC). After the durability evaluations, the post-test membrane electrode assemblies (MEAs) were analyzed with nuclear magnetic resonance (NMR) spectroscopy. The cell voltages were continuously measured during the accelerated OCV stress evaluation, and the hydrogen leak current density and the composition of the water that was drained from each cell were measured every 200 h by use of LSV and IC, respectively, as shown in Figures 2a–2c. The cell voltage for the SPP-QP type 1 cell decreased significantly after 700 h. In the case of SPP-QP type 2 and type 3 cells, the cell voltages maintained high values for over 1000 h, as well as those for the SPP cell and SPK cell (Figure 2a). The hydrogen leak current density also behaved nearly the same for each of the cells, except for the SPP-QP type 1 cell, which reached ca. 100 times higher H2 leakage at 800 h than those of the others (Figure 2b). The emitted concentration values of sulfate decreased in the order SPK » SPP > SPP-QP type 1 > SPP-QP type 2 ≈ SPP-QP type 3(Figure 2c). The intrinsic chemical stabilities of the SPP-QP, SPP and SPK membranes were evaluated with Fenton’s reagent. These results indicated that the SPP-QP membranes had much higher intrinsic chemical stability compared to those of SPP and SPK membranes, which was reflected in the results of the accelerated OCV stress evaluation. We also studied approaches to mitigate the mechanical stress by use of GDLs5 and gaskets in order to evaluate the chemical stability of the membranes under accelerated conditions in MEAs.6 Acknowledgement This work was partially supported by funds for the “Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)” Project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. The authors are grateful to Panasonic Corporation for kindly providing the experimental GDL as soft GDL. Also, the authors are grateful to Honda R&D Co., Ltd. for kindly providing the accelerated OCV stress-evaluation protocol. References1) R. Shimizu, J. Tsuji, N. Sato, J. Takano, S. Itami, M. Kusakabe, K. Miyatake, A. Iiyama, and M. Uchida, J. Power Sources, 367, 63 (2017).2) T. Miyahara, T. Hayano, S. Matsuno, M. Watanabe, and K. Miyatake, ACS applied materials & interfaces, 4, 2881 (2012).3) J. Miyake, T. Mochizuki, and K. Miyatake, ACS Macro Letters, 4, 750 (2015).4) J. Miyake, R. Taki, T. Mochizuki, R. Shimizu, R. Akiyama, M. Uchida, and K. Miyatake, Science Advances, 3, eaao0476 (2017).5) H. Ishikawa, T. Teramoto, Y. Ueyama, Y. Sugawara, Y. Sakiyama, M. Kusakabe, K. Miyatake, and M. Uchida, J. Power Sources, 325, 35 (2016).6) R. Shimizu, K. Otsuji, A. Masuda, N. Sato, M. Kusakabe, A, Iiyama, K. Miyatake, and Makoto Uchida, J. Electrochem. Soc, 166 (7) F3105-F3110 (2019) Figure 1