Diffusion in Sulfonated Co-Polynaphthoyleneimide Proton Exchange Membranes with Different Ratios of Hydrophylic to Hydrophobic Groups Studied Using SFG NMR

Abstract

We use static field gradient (SFG) NMR to determine the self-diffusion coefficients of protons in fluorine-free sulfonated co-polynaphthoyleneimide (co-PNIS) proton exchange membranes with different ratios of hydrophilic to hydrophobic groups. The investigations were carried out in the temperature range from 193 to 355 K. Because there are protons not only in water but also in the polymer framework, 1H NMR diffusion studies of these membranes may suffer from cross-relaxation effects between the different types of protons. To overcome this problem, different methods for measuring proton diffusion coefficients are compared and a suitable strategy for analysis is proposed. It is found that the proton diffusion is practically isotropic and shows two activation regimes, separated by a crossover near 260 K. The activation energy above the crossover is 0.19 eV, which is close to that of Nafion 212. Below the crossover, all co-PNIS membranes studied have very similar diffusion coefficients and the activation energy amounts to 0.46 eV, which is higher than that of Nafion (0.36 eV). Increasing the ratio of hydrophilic to hydrophobic polymer groups leads to faster diffusion in the temperature range from 273 to 355 K. For the co-PNIS membranes with the highest ratio of hydrophilic to hydrophobic groups, the proton diffusivity is about 2.3 times higher than for the Nafion 212 membrane. Unlike for Nafion-type membranes, the diffusion does not depend on the length scale of the experiment, indicating that the morphology of co-PNIS membranes may differ from the channel-like structure of Nafion membranes.

Transport Mechanism in Nafion Revealed by Detailed Comparison of1H and17O Nuclear Magnetic Resonance Diffusion Coefficients

“…17 O-enriched (90%) water ( 1 H 2 17 O) was purchased from Cambridge Isotope Labs, and the Nafion 212 membrane was obtained from Chemours (USA). For cleaning, the standard procedure was applied. , The membranes were boiled in a 3 wt % hydrogen peroxide solution for 1 h, washed with distilled water to eliminate any organic contaminants, and boiled in distilled water for an additional 1 h to ensure that all hydrogen peroxide residue was removed. The final protonation procedure included exposure to 0.5 M sulfuric acid for 1 h, followed by double boiling in distilled water for an additional 1 h. The purified and protonated membrane was cut into 1 cm × 10 cm stripes, which were annealed at 80 °C in a vacuum of 0.1 bar for 20 h and subsequently kept at 25 °C under 0.1 bar for 20 h before they were rolled and inserted into NMR tubes.…”

“…The factor exp(−t m /T 1 ) considers the additional damping of the STE signal S q (t m ) by spin−lattice relaxation. Because we controlled t m and t e in the STE sequence and predetermined T 1 in independent relaxation measurements or removed relaxation effects by performing measurements for different gradient strengths g, 27 diffusion coefficient D was the only free parameter when we fitted S q (t m ) data. To ensure the best possible comparison of the D H and D O values from 1 H and 17 O NMR studies, we used similar q values for both nuclei; i.e., we adjusted evolution times t e to compensate for the different gyromagnetic ratios γ of 1 H and 17 O.…”

“…Many of the results described above were obtained from computer simulations; e.g., large-scale first-principles molecular dynamics simulations of hydrated Nafion have become feasible in recent years . Effective experimental methods for studies of proton migration in PEMs proved to be dc conductivity and NMR diffusivity measurements. − ,,− These approaches showed that proton conduction and diffusion in Nafion are strongly impacted by moisture level λ. To analyze the transport mechanism, proton self-diffusion coefficients D H from 1 H NMR were compared with diffusivities D σ calculated from dc conductivity σ using the Nernst–Einstein relation ,,, where e is the elementary charge, n denotes the concentration of charge carriers, k B is the Boltzmann constant, and T is the temperature.…”

Development of High-Performance Hydrogen-Air Fuel Cell with Flourine-Free Sulfonated Co-Polynaphthoyleneimide Membrane

“…Earlier, it was shown that hydrocarbon polymers of the polynaphthoyleneimide class (further co-PNIS) were notable because of their high transport characteristics [ 32 , 33 ]. The production of proton-exchanged membranes from sulfonated polynaphtoyleneimides (SPNI) based on industrial 1,4,5,8-naphthalene tetracarboxylic acid (NTDA) anhydride with six-membered imide rings was carried out in a number of works [ 34 , 35 , 36 ].…”

Proton Transport, Electroosmotic Drag and Oxygen Permeation in Polytetrafluoroethylene Reinforced Ionomer Membranes and Their Effects on Fuel Cell Performance

“…Theories on the effects of ionomer structure [13][14][15] and environmental factors [16][17][18][19][20] (e.g., temperature, humidity) on the proton transport mechanism are now well established. For instance, when the water content in the membrane increases, hydrogen bonds are formed and broken between water and hydrated hydrogen ions (H 3 O + ), allowing proton transfer via the water bridges.…”