Crown Ether As a Chemical Stabilizer for Enhanced Cerium Stability and Radical Scavenging in Proton Exchange Membranes

Nafion is one of the standard proton exchange membranes that serves as the electrolyte membrane in fuel cells. However, it has been found to chemically degrade over time due to the generation of harmful peroxide and hydroxyl radicals. Attack by such radicals on the Nafion side chains causes polymer chain unzipping, and the membrane loses its mechanical integrity over time [1]. Cerium is proven to be an effective radical scavenger due to rapid transition between its +3 and +4 oxidation states and regenerates swiftly in the fuel cell environment. However, over time, cerium migrates due to the electrochemical potential, high water flux, and ionic diffusion [2] [3]. To resolve this issue, we functionalized a fraction of the side chains of Nafion with crown ether to complex with cerium and thereby prevent its migration. Crown ether has shown the ability to enhance cerium stability in recent studies [4] [5]. Cerium complex formation with crown ether was studied using UV visible spectroscopy, and enhanced retention was verified by testing drop in cerium concentration in the acidic medium, and migration of cerium under an external potential gradient. Membrane durability testing was conducted to verify that the stabilized membrane indeed shows higher lifetime in fuel cell operation. [1] F. D. Coms, “The Chemistry of Fuel Cell Membrane Chemical Degradation,” ECS Transactions, 16(2): 235–255 (2008). [2] S. M. Stewart, D. Spernjak, R. Borup, A. Datye, and F. Garzon, “Cerium Migration through Hydrogen Fuel Cells during Accelerated Stress Testing,” ECS Electrochemistry Letters, 3(4): F19–F22 (2014). [3] Baker, Andrew M., et al. ''Cerium ion mobility and diffusivity rates in perfluorosulfonic acid membranes measured via hydrogen pump operation.'' Journal of The Electrochemical Society 164.12 (2017): F1272. [4] Park, Junghwa, Yongman Park, and Dukjoon Kim. ''Chemical stability enhancement of crown ether grafted sulfonated poly (arylene ether ketone) fuel cell membrane by cerium ion fixation.'' Journal of Polymer Science Part A: Polymer Chemistry 57.2 (2019): 101-109. [5] Tinh, Vo Dinh Cong, and Dukjoon Kim. ''Enhancement of oxidative stability of PEM fuel cell by introduction of HO• radical scavenger in Nafion ionomer.'' Journal of Membrane Science (2020): 118517.