The sulfonated polynaphthoyleneimide polymer (co-PNIS70/30) was prepared by copolymerization of 4,4′-diaminodiphenyl ether-2,2′-disulfonic acid (ODAS) and 4,4’-methylenebisanthranilic acid (MDAC) with ODAS/MDAC molar ratio 0.7/0.3. High molecular weight co-PNIS70/30 polymers were synthesized either in phenol or in DMSO by catalytic polyheterocyclization in the presence of benzoic acid and triethylamine. The titration reveals the ion-exchange capacity of the polymer equal to 2.13 meq/g. The membrane films were prepared by casting polymer solution. Conductivities of the polymer films were determined using both in- and through-plane geometries and reached ~96 and ~60 mS/cm, respectively. The anisotropy of the conductivity is ascribed to high hydration of the surface layer compared to the bulk. SFG NMR diffusometry shows that, in the temperature range from 213 to 353 K, the 1H self-diffusion coefficient of the co-PNIS70/30 membrane is about one third of the diffusion coefficient of Nafion® at the same humidity. However, temperature dependences of proton conductivities of Nafion® and of co-PNIS70/30 membranes are nearly identical. Membrane–electrode assemblies (MEAs) based on co-PNIS70/30 were fabricated by different procedures. The optimal MEAs with co-PNIS70/30 membranes are characterized by maximum output power of ~370 mW/cm2 at 80 °C. It allows considering sulfonated co-PNIS70/30 polynaphthoyleneimides membrane attractive for practical applications.
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.
A model of the proton system in one-dimensional water is proposed, which takes into account the Coulomb interaction between protons, quantum tunneling of protons between the nearestneighboring positions, and the interaction of protons with the channel walls. Cases of extremely confined channels in a hydrophilic and hydrophobic medium are studied, various approximations of the proposed model are considered, and their analogy with the corresponding electronic systems is indicated. Almost all protons of water molecules are found to be localized on broken bonds near the walls of the hydrophilic channel so that one-dimensional water can be considered as a molecular gas tightly bound to the channel walls. The mechanical motion of such water media and the diffusion of water molecules are determined by the interaction of water molecules with hydrophilic walls. Most likely, the water flow through such channels will be significantly hampered. Conversely, almost all hydrogen bonds at the center of the channel are occupied by protons in the channel with strongly hydrophobic walls, while the bonds with the walls are considerably weakened, and the mechanical behavior of one-dimensional water becomes similar to that of a solid. Onedimensional water in this form moves as a whole medium through the channel, while the hydrodynamic boundary condition on the channel walls (the velocity vanishing at the boundary) will be violated. This leads to a sharp increase of water flow through the channel compared to the classical hydrodynamic calculations of the water flow. It is assumed that one-dimensional water in this case reveals antiferromagnetic behavior.
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