Metal Fluoride Particles to Enhance Durability of Composite Membranes at MT-PEM Fuel Cell Operating Temperatures

Abstract

Fuel cells, as a local, emission-free and versatile system, promise to overcome our dependence on fossil fuels.[1] Polymer electrolyte membrane fuel cells (PEMFCs) are considered one of the most promising technologies among the various kinds of existing fuel cells and offer an attractive alternative for automotive and stationary energy applications. Especially PEMFCs operating at an increased temperature range (MT-PEM) offer an enhanced performance. Operating temperatures between 100 – 130 °C lead to better reaction kinetics, higher tolerance to fuel impurities and to an improved heat, water and power management of the system.[2] However, some issues regarding durability and performance, such as low proton conductivity of PFSA-based membranes, higher membrane degradation and lower long-term stability at increased temperatures, are still unsolved.[3] In previous works we have shown that various metal fluorides implemented into perfluorosulfonic acid (PFSA)-membranes have a positive impact on performance and mechanical stability at operating temperatures above 100 °C.[4] In this work, we describe the modification of lithium fluoride nanoparticles, their influence on membrane durability in single cell tests at enhanced PEMFC operating temperatures and the morphology of the composite membranes, at different temperatures and degree of hydration, by in-situ small-angle X-ray scattering (SAXS). The lithium fluoride modified membrane showed increased cell performance under both standard and harsher cell conditions as well as in various long-term stability tests, such as accelerated OCV tests, load cycles and on-off cycles.One explanation for the performance boost, in addition to the increased mechanical stability of the membrane, would be an increased water uptake and storage capability, especially at low humidity levels during cell operation. We assume that the nanoparticles adsorb water molecules by hydrogen bond formation, which leads to an enhanced proton conductivity even at high temperatures. To confirm this assumption, we applied in-situ SAXS to analyze the water uptake of the modified membranes at various relative humidity and temperatures to understand the structural changes. In addition, we hope to connect different nanoparticle shapes to their influence on water uptake and retention.[1] I. Staffell, D. Scamman, A. Velazquez Abad, P. Balcombe, P. E. Dodds, P. Ekins, N. Shah, K. R. Ward, Energy Environ. Sci. 2019, 12, 463–491.[2] R. E. Rosli, A. B. Sulong, W. R. W. Daud, M. A. Zulkifley, T. Husaini, M. I. Rosli, E. H. Majlan, M. A. Haque, Int. J. Hydrogen Energy 2017, 42, 9293–9314.[3] L. Mazzapioda, S. Panero, M. A. Navarra, Polymers 2019, 11, 914.[4] A. Moszczynska, H. Wolf, M. A. Willert-Porada, Patent WO/2009/014930, 2009.