We describe herein two recent projects utilizing a diverse set of NMR measurements to shed light on the relationship between structure and transport properties in two very different classes of electrolytes. (i) in collaboration with University of Tennessee (R. Elgammal, T. Zawodzinski), we investigated the deep eutectic solvent system glycerol/choline chloride (ChCl) using fast field cycling broadband 1H relaxometry, pulsed field gradient diffusion, variable pressure 2H relaxation, 13C relaxation, and 2D homonuclear NOESY. Generally, translational and rotational diffusion of glycerol are more rapid than those of choline, but the rates of glycerol and choline local motions become more similar at higher ChCl content. These results taken together show that the addition of ChCl makes it easier to disrupt glycerol packing and suggest that the glycerol H-bonding network is disrupted as choline is added, but primarily in regions where there is intimate mixing of the two components. (ii) in collaboration with the University of South Carolina ( L. Murdock, B. Benicewicz), we investigated proton transport in polybenzimidazole (PBI) membranes with high phosphoric acid (PA) content. One set of membranes was prepared by the so-called PPA process with molar ratio of 30 PA per PBI repeat unit, and the other set by a thermal treatment process yielding a dense PBI film and then reacidified to a level of 18 PA per PBI repeat unit. The two kinds of materials exhibit comparable proton conductivity despite the nearly factor of two lower acid concentration in the latter. Proton self-diffusion coefficients for membranes were measured by pulsed field gradient NMR. The results show significantly enhanced proton diffusivity for the dense and re-acidified membrane, which is consistent with its high conductivity despite lower acid content compared to the original PPA-processed membranes. We have also performed solid state magic angle spinning cross-polarization one-dimensional 13C{1H}, 31P{1H} and two-dimensional 31P{1H} and 13C{1H} HETCOR NMR measurements, which yield additional information on structural differences between the membranes and how these differences may affect transport properties. In particular, the results demonstrate a more rigid glassy structure of the polymer and stronger coupling between phosphate moieties and the polymer backbone in the modified membrane, which may enhance proton transport via the Grotthuss mechanism.

Insights into structure and ion transport in novel electrolytes by nuclear magnetic resonance

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ACS Fall 2022

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