Impact of Metal Oxide Nanoparticles in Anode Catalyst Layer on Membrane Degradation and Output Performance of PEFC

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The chemical degradation of polymer electrolyte membranes (PEMs) is one of the most serious issues that need to be addressed to achieve a long lifetime for the polymer electrolyte fuel cells (PEFCs). The PEMs such as Nafion (perfulorinated sulfonic acid polymers) are attacked by ·OH radicals generated from the decomposition of H2O2molecules, which are predominantly produced at the Pt/C anode catalyst layer (ACL).1 The addition of metal oxide nanoparticles (NPs) such as CeO2 and MnO2 to PEM or anode side has been introduced as one of the effective solutions to demolish the chemical degradation of PEM by scavenging the ·OH radical through the redox properties of these metal ions.1‒3 Although the lifetime of the PEMs have effectively prolonged, the proton conductivity of both the PEM and the ionomer binder in the cathode catalyst layer (CCL) has dramatically decreased, resulting in an appreciable loss of the output performance,4, 5 i.e., trade-off relationship between the durability and the output performance. To suppress the membrane degradation accompanied with increased output performance, we have proposed a unique concept of addition of metal oxide NPs without releasing the metal ions into ACL. The silica NPs were the first that exhibited such excellent properties.6 In the present research, we offer a new candidate of metal(IV) oxide (MO2) NPs which played a great role in increasing the lifetime of Nafion-PEM much longer than in the case of using silica NPs. The catalyst-coated membranes (CCMs) of the current study have been prepared as reported in our previous work.6 The fabricated CCMs comprised Nafion 211 (NRE-211; 25 µm in thickness) as the PEM, heat-treated Pt/graphitized carbon black (Pt/GCB-HT, TEC10EA50E-HT, 50.2 wt%-Pt) as the cathode catalyst, and Pt supported on carbon black (Pt/C, TEC10E50E, 46.4 wt%-Pt) as the anode catalyst, in which MO2-NPs were incorporated with a changeable volume ratio to the carbon content (VMO 2/VC, ranging from 0 to 0.4). Each CCM was mounted in a single cell (geometric electrode area 29.2 cm2). The chemical stability of the PEM was examined via an accelerated stress test (AST) in which the single cell was maintained at open circuit voltage (OCV) in a 40% humidified H2/air at Tcell = 90 °C and a backpressure of 160 kPa-G.6 As a measure of the durability of the PEM, the lifetime is defined as the time at which the OCV reached 0.85 V (ca. 10% loss).Figure 1 shows the changes of the OCV and the total amount of fluoride (F−) ions emitted during the AST. The OCV of the cell without any metal oxide in the ACL (denoted as Pt/C only in Fig. 1) has rapidly dropped, reaching values below 0.85 V with a very high F− emission rate (FER, slope of the line in Fig. 1b) after ca. 150 h of durability testing. With the addition of silica at VSiO2/VC = 0.2 in the ACL, the lifetime has appreciably increased (ca. 570 h) with a low FER. It is striking for the MO2-based ACL with VMO2/VC =0.2 that the lifetime has been remarkably prolonged, reaching ca. 1650 h (ca. 11 times increase of the Nafion lifetime) together with the lowest FER.Also, it has been found that the use of silica NPs has provided a reduced ohmic resistance (increase in water content of PEM and ionomers) and an effective utilization of Pt cathode catalyst in a single cell, which contributed to increase the output I-V performance.6 A similar effect was observed for the incorporation of MO2 NPs into the ACL. Such an improvement of the output performance accompanied with the remarkable increase in the durability of PEM is quite distinct from the trade-off effect of conventional radical scavengers. The mechanism for suppression of membrane degradation, as well as the changes in the microstructure of CCM, are under progress in our laboratory.This work was supported by funds for the “R&D of novel anode catalyst project” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Endoh, ECS Trans., 16(2), 1229 (2008).Zhao, B.L. Yi, H.M. Zhang, and H.M. Yu, J. Membr. Sci., 346, 143 (2010).Wang, C. Cai, J. Tan, and M. Pan, Int. J. Hydrogen Energy, 46, 34867 (2021).Lin, C. Cao, H. Zhang, H. Huang, and J. Ma, Int. J. Hydrogen Energy, 37, 4648 (2012).H. Wong and E. Kjeang, J. Electrochem. Soc., 166, F128 (2019).Mohamed R. Berber, M. Imran, H. Nishino, and H. Uchida, ACS Appl. Mater. Interfaces, 15, 13219 (2023). Figure 1