Immobilization of Polyoxometalates within a Polymer Matrix Ionomer for Enhanced Durability and Performance in Proton-Exchange Membrane Water Electrolysis
Proton-exchange membrane water electrolysis (PEMWE) is one of the most pivotal technologies in searching for clean and sustainable hydrogen production. It has the ability to produce high-purity hydrogen at high current densities within a compact system design, which makes it a great promise as a solution for future hydrogen-based energy systems[1, 2]. Developing high-performing ionomers for electrode integration is crucial for optimizing the performance and long-term durability of PEMWE systems [3]. Currently, perfluorosulfonic acid (PFSA)--based ionomers are regarded as standard choices for electrochemical devices due to their chemical durability and excellent proton conductivity. However, their widespread use is limited on a daily basis for several key drawbacks, such as environmental concerns related to their fluorinated nature, high cost, and the possibility of degradation by reactive oxygen species like hydroxyl radicals generated from hydrogen peroxide byproducts[4]. This PFSA limitation accelerated the search for alternative fluorine-free ionomers, such as hydrocarbon-based ionomer materials, which provide low cost, improved performance, and reduced environmental impact. Polyoxometalates (POMs), especially lacunary heteropolyacids (HPAs), are potential candidates for their strong proton mobility and ability to reduce chemical degradation through scavenging hydroxyl radicals[5]. However, HPA's usages are still limited due to their high water solubility, which, if improperly immobilized within a solid matrix, might lead to induced migration, clustering, or leaching. In response, this study investigates the synthesis of next-generation proton-conducting ionomers using polymer polyoxometalates (PolyPOMs). In order to create a stable hybrid ionomer matrix, a novel synthetic technique is employed in which a lacunary HPA—more precisely, 11-silicotungstic acid, a Keggin-structured POM—is covalently immobilized at maximal loading onto a polymer backbone and then copolymerized with an organic co-monomer. By efficiently immobilizing the ordinarily water-soluble POM inside the ionomer, this technique allows it to serve as a radical scavenger as well as a proton-conducting moiety. A higher concentration of HPA should theoretically improve proton transport and chemical degradation resistance against oxygenated radicals like •OH and •OOH without introducing the leaching problems often seen with mobile radical scavengers like cerium or manganese [6]. Additionally, the inclusion of the polymer backbone offers superior chemical and thermal stability, which qualifies the ionomer for the demanding water electrolysis operating conditions. This study primarily focuses on the synthesis, structural optimization, and assessment of its essential transport characteristics so that the synthesized ionomer can be incorporated into membranes and electrodes in PEMWE systems. A suite of different advanced analytical techniques is used to determine the structural integrity of the synthesized ionomers. A detailed understanding of the ionomer's molecular architecture and morphology is thoroughly obtained via Fourier Transform Infrared Spectroscopy (FTIR), Small Angle X-ray Scattering (SAXS), Wide Angle X-ray Scattering (WAXS), and Solid-State Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Using linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), the electrochemical performance of the produced ionomers will be investigated. This study seeks to identify key structure-property correlations that govern proton conductivity and stability. The findings will guide the logical design of advanced ionomers for PEMWE systems that meet the current and future operational and regulatory requirements. References: [1] G. A. Lindquist, Q. Xu, S. Z. Oener, and S. W. Boettcher, “Membrane Electrolyzers for Impure-Water Splitting,” Joule , vol. 4, no. 12, pp. 2549–2561, Dec. 2020, doi: 10.1016/j.joule.2020.09.020. [2] J. E. Park et al. , “High-performance proton-exchange membrane water electrolysis using a sulfonated poly(arylene ether sulfone) membrane and ionomer,” J. Membr. Sci. , vol. 620, p. 118871, Feb. 2021, doi: 10.1016/j.memsci.2020.118871. [3] “Advanced Science - 2023 - Kim - Hydrocarbon Ionomeric Binders for Fuel Cells and Electrolyzers.” [4] S. Siracusano, V. Baglio, F. Lufrano, P. Staiti, and A. S. Aricò, “Electrochemical characterization of a PEM water electrolyzer based on a sulfonated polysulfone membrane,” J. Membr. Sci. , vol. 448, pp. 209–214, Dec. 2013, doi: 10.1016/j.memsci.2013.07.058. [5] J. L. Horan et al. , “Copolymerization of Divinylsilyl‐11‐silicotungstic Acid with Butyl Acrylate and Hexanediol Diacrylate: Synthesis of a Highly Proton‐Conductive Membrane for Fuel‐Cell Applications,” ChemSusChem , vol. 2, no. 3, pp. 226–229, Mar. 2009, doi: 10.1002/cssc.200800237. [6] A. R. Motz et al. , “Heteropoly acid functionalized fluoroelastomer with outstanding chemical durability and performance for vehicular fuel cells,” Energy Environ. Sci. , vol. 11, no. 6, pp. 1499–1509, 2018, doi: 10.1039/C8EE00545A.