Multinuclear Solid State Nuclear Magnetic Resonance Investigation of Water Penetration in Proton Exchange Membrane Nafion-117 by Mechanical Spinning

Abstract

(1)H, (17)O, and (19)F solid state NMR spectroscopies have been used to investigate water penetration in Nafion-117 under mechanical spinning. It is found that both (1)H and (17)O spectra depend on the orientation of the membrane with respect to the magnetic field. The intensities of the side chain (19)F spectra depend slightly on the orientation of membrane with respect to the magnetic field, but the backbone (19)F spectra do not exhibit orientation dependence. By analyzing the orientation dependent (1)H and (17)O spectra and time-resolved (1)H spectra, we show that the water loaded in Nafion-117, under high spinning speed, may penetrate into regions that are normally inaccessible by water. Water penetration is enhanced as the spinning speed is increased or the spinning time is increased. In the meantime, mechanical spinning accelerates water exchange. It is also found that water penetration by mechanical spinning is persistent; i.e., after spinning, water remains in those newly found regions. While water penetration changes the pores and channels in Nafion, (19)F spectra indicate that the chemical environments of the polymer backbone do not show change. These results provide new insights about the structure and dynamics of Nafion-117 and related materials. They are relevant to proton exchange membrane aging and offer enlightening points of view on antiaging and modification of this material for better proton conductivity. It is also interesting to view this phenomenon in the perspective of forced nanofiltration.

Characterization of Heterogeneous Solvent Diffusion Environments in Anion Exchange Membranes

“…In addition, individual water and methanol resonances were resolved even at slower 1 kHz MAS spinning, and for the ATMPP (IEC = 1.48) membrane static PFG NMR diffusion experiments revealed a biexponential decay for the water environment with diffusion rates on the same order of magnitude as those obtained using PFG HRMAS NMR. This does not fully dismiss the effects of MAS centrifugation as the membranes, as the multiple broad overlapping resonances observed for the water or methanol environments in the TMAC 6 PPC 6 membranes (Figure 1) are very similar in appearance to those reported during the Nafion study, 36 and warrant further investigations.…”

“…There are concerns about the possibility of the free-solvent environments arising from the penetration of water and methanol into the polymer membrane due to centrifugal forces. A recent study showed that water migrates into Nafion membranes during MAS spinning, probing polymer environments not normally accessible to water penetration . This water penetration occurred mainly at high MAS speeds (12 kHz) or extended periods of spinning.…”

“…A recent study showed that water migrates into Nafion membranes during MAS spinning, probing polymer environments not normally accessible to water penetration. 36 This water penetration occurred mainly at high MAS speeds (12 kHz) or extended periods of spinning. For the AEM membranes described in this manuscript, the MAS speeds were always maintained at 4 kHz or below, and there were no observed spectral changes over extended MAS (e.g., weeks), or following elevated temperature studies.…”