Temperature Dependent Mechanical Behavior of Perfluorosolfonic Acid Membranes for Electrochemical Applications

Abstract

Perfluorosulfonic acid (PFSA) membranes are the prototypical solid-electrolyte used in electrochemical devices such as polymer-electrolyte membrane fuel cells (PEMFCs) and electrolyzers [1]. To be commercially viable, polymer-electrolyte membranes (PEMs) in PEMFCs and electrolyzers must perform over a long operational time in both dry and wet environments. The ionic conductivity of PFSA membranes can be optimized by changing thickness, material (ionomer chemistry), and pretreatment to increase the ionic conductivity, and reduce power losses, thus improving cell efficiency. However, any effort to improve the transport functionality tends to undermine the membrane’s mechanical stability. [2] Therefore, a key challenge in PEM design is to improve transport of ions without compromising mechanical integrity and characterize the factors influencing mechanical stability of these materials in controlled environments. Most past studies on the mechanical properties of PFSA membranes investigated only tension [2], although there are some exceptions [3][4]. Compression testing, however, is critical for understanding membrane behavior in electrochemical devices. For example, in fuel cells, hydration-related swelling of a constrained membrane generates partially reversed cyclic stresses, whereas in electrolyzers, PEMS undergo static compression as high as 40 MPa. Cyclic stresses combined with a harsh chemical environment make the PEMs susceptible to failure such as pinhole formation as well as crack initiation and propagation. High static stresses make creep response a great concern. To investigate how temperature and hydration state affect the membrane’s mechanical behavior and properties under compression in-situ, a compression setup with temperature control was used. PFSA membranes of varying chemistry, equivalent weight, thickness (from 50 to 200 µm), and thermal pretreatments were tested. Our results show that the mechanical behavior of PFSA membranes is different under compression than tension. Uniaxial compression and creep tests indicate that the PFSA membranes exhibit time-dependent nonlinear response over long periods of time. This work demonstrates the importance of studying the mechanical properties of these membranes under compression, which is more relevant for the various stress-states the membrane undergoes during operation. Odgaard, M., Advanced Fluoride-Based Materials for Energy Conversion, in Advanced Fluoride-Based Materials for Energy Conversion. 2015, Elsevier. p. 325-374.Kusoglu, A. and A.Z. Weber, New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chemical Reviews, 2017. 117(3): p. 987-1104.Kusoglu, A., et al., Effect of compression on PFSA-ionomer morphology and predicted conductivity changes. Journal of Membrane Science, 2012. 421(0): p. 283-291.Kusoglu, A., et al., Role of Mechanical Factors in Controlling the Structure–Function Relationship of PFSA Ionomers. Macromolecules, 2012. 45(18): p. 7467-7476.