Überrück, Till, Oliver Neudert, Klaus-Dieter Kreuer, Bernhard Blümich, Josef Granwehr, Siegfried Stapf, and Songi Han. “Effect of Nitroxide Spin Probes on the Transport Properties of Nafion Membranes.” Physical Chemistry Chemical Physics 20, no. 41 (2018): 26660–74.
Nafion is the most common material used as proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still debated. Overhauser Dynamic Nuclear Polarization (ODNP) has proven a useful tool for probing hydration dynamics and interactions within 5–8 Å of protein and soft material surfaces. Recently it was suggested that ODNP can also be applied to analyze surface water dynamics along Nafion’s inner membrane. Here we interrogate the viability of this method for Nafion by carrying out a series of measurements relying on 1H nuclear magnetic resonance (NMR) relaxometry and diffusometry experiments with and without ODNP hyperpolarization, accompanied by other complementary characterization methods including small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA) of hydration, and proton conductivity by AC impedance spectroscopy. Our comprehensive study shows that the commonly used paramagnetic spin probes—here, stable nitroxide radicals—for ODNP, as well as their diamagnetic analogues, reduce the inner membrane surface hydrophilicity, depending on the location and concentration of the spin probe. This heavily reduces the hydration of Nafion, hence increases the tortuosity of the inner membrane morphology and/or increases the activiation barrier for water transport, and consequently impedes water diffusion, transport, and proton conductivity.