A magnetic resonance and electrochemical study of the role of polymer mobility in supporting hydrogen transport in perfluorosulfonic acid membranes

Abstract

Perfluorosulfonic acid (PFSA) materials have been used in polymer electrolyte membrane fuel cells (PEMFCs) as electrolyte materials due to their mechanical durability and high proton conductivity. To understand the fundamental chemistry at a molecular level in material performance properties, we have developed and validated method for evaluating local dynamics using ¹⁹F double-quantum solid-state nuclear magnetic resonance (ssNMR) spectroscopy. The local dynamics information can be separated and analyzed in terms of fluorine interactions with respect to the different temperatures and hydration levels. The polymer side chain is proven to be more locally mobile which is reflected by the lower apparent dipolar coupling constant (D_app) compared to the backbone. This observation agrees with the micro-phase separation morphology evolution. In the current study, different types of PFSA materials were explored and compared. The dynamics investigation of the PFSA materials has been conducted at various conditions. In operando membrane performance analyses were performed in parallel at Ballard Power Systems. PFSA membranes were prepared into membrane electrode assemblies (MEAs), with catalyst layers and gas diffusion layers. From the cyclic voltammetry measurements, the H₂ crossover values were extracted. These data reveal a strong correlation between the proton conductivity and the site-specific PFSA side chain local dynamics. Moreover, a correlation was drawn between increasing side chain mobility (lower D_app), and increased H₂ permeability. The link between the fundamental dynamics study and this key PFSA performance analysis provides insight into proton transport mechanisms.