TY - JOUR
T1 - Effect of nitroxide spin probes on the transport properties of Nafion membranes
AU - Überrück, Till
AU - Neudert, Oliver
AU - Kreuer, Klaus Dieter
AU - Blümich, Bernhard
AU - Granwehr, Josef
AU - Stapf, Siegfried
AU - Han, Songi
N1 - Funding Information:
The authors are grateful for the support from the Deutsche Forschungsgemeinschaft (DFG – Project: BL231/49-1) and the exchange network ACalNet (funded by DAAD). Additional thanks are expressed to G. Portale (BM 26 at ESRF-Grenoble) for recording the SAXS patterns and Igor Moudrakovski (MPI Solid-State-Research) for proofreading.
Publisher Copyright:
© the Owner Societies.
PY - 2018
Y1 - 2018
N2 - Nafion is the most common material used as a proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still under debate. Overhauser Dynamic Nuclear Polarization (ODNP) has proven to be 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 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.
AB - Nafion is the most common material used as a proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still under debate. Overhauser Dynamic Nuclear Polarization (ODNP) has proven to be 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 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.
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U2 - 10.1039/c8cp04607g
DO - 10.1039/c8cp04607g
M3 - Article
C2 - 30320331
AN - SCOPUS:85055601867
SN - 1463-9076
VL - 20
SP - 26660
EP - 26674
JO - Physical Chemistry Chemical Physics
JF - Physical Chemistry Chemical Physics
IS - 41
ER -