Available methods to analyze proton conduction mechanisms cannot distinguish between two proton-conduction processes derived from the Grotthuss mechanism. The two mechanistic variations involve structural diffusion, for which water movement is indispensable, and the recently proposed "packed-acid mechanism," which involves the conduction of protons without the movement of water and is typically observed in materials consisting of highly concentrated (packed) acids. The latter mechanism could improve proton conductivity under low humidity conditions, which is desirable for polymer electrolyte fuel cells. We proposed a method with which to confirm quantitatively the packed-acid mechanism by combining (2)H and (17)O solid-state magic-angle-spinning nuclear magnetic resonance (MAS-NMR) measurement and (1)H pulsed-field gradient (PFG)-NMR analysis. In particular, the measurements were performed below the water-freezing temperature to prevent water movement, as confirmed by the (17)O-MAS-NMR spectra. Even without water movement, the high mobility of protons through short- and long-range proton conduction was observed by using (2)H-MAS-NMR and (1)H-PFG-NMR techniques, respectively, in the composite of zirconium sulfophenylphosphonate and sulfonated poly(arylene ether sulfone) (ZrSPP-SPES), which is a material composed of highly concentrated acids. Such behavior contrasts with that of a material conducting protons through structural diffusion or vehicle mechanisms (SPES), in which the peaks in both (2)H and (17)O NMR spectra diminished below water-freezing temperature. The activation energies of short-range proton movement are calculated to be 2.1 and 5.1 kJ/mol for ZrSPP-SPES and SPES, respectively, which indicate that proton conduction in ZrSPP-SPES is facilitated by the packed-acid mechanism. Furthermore, on the basis of the mean-square displacement using the diffusivity coefficient below water-freezing temperature, it was demonstrated that long-range proton movement, of the order of 1.3 μm, can take place in the packed-acid mechanism in ZrSPP-SPES.
Magnesium-sulfur batteries are one of the most promising next-generation battery systems due to their high energy density, low cost, and high level of safety. However, the reaction mechanisms are not well understood, and in particular, the discharge reaction products have not yet been identified. Here we show that zinc blende magnesium sulfide is observed as a reaction product after discharging in magnesium-sulfur batteries. When magnesium reacts electrochemically with sulfur in a sulfone-based magnesium electrolyte, sulfur becomes amorphous consisting of magnesium and sulfur in the cathode. In this study, it has been found that the amorphous material has an unusual local structure, which is not related to the most stable rock salt phase of magnesium sulfide but rather the metastable zinc blende phase. It was indicated that this material realizes the reversibility of magnesium-sulfur batteries.
Diffusional behaviors of probe polystyrenes (PSs) with M w ) 4K, 19K, 29K, and 400K and chloroform as solvent were observed in the same PMMA gel matrix with the diffusing time varied (4-500 ms) by diffusing-time-dependent 1 H NMR in order to elucidate how the same gel network structure is differently detected by the probes with different sizes. The diffusion of the solvent and the smallest PS with M w ) 4K proved to be of a single mode, which is safely ascribed to the sufficiently small size of the probes compared with the network mesh (∼1.5 nm). The other PS molecules that should be larger than or comparable to the mesh size showed a multimode diffusional behavior, which was analyzed as a dual mode diffusion composed of a fast component and a slow one. Thus, those larger PS molecules were capable to "probe" the inhomogeneity for the mesh size distribution of the network. On the other hand, the so-called restricted diffusion or diffusing-time dependence for the diffusion coefficient D was observed only for the largest probe PS with M w ) 400K, and the fast diffusion component and the slow diffusion component with the diffusing time were recognized. This means that the PS chains were entrapped within a dense or an open structure during the measurement time, respectively. On the basis of these diffusion behaviors of probe molecules, the mesh and region sizes were estimated; for the open network structure the region size was scaled to be on the order of micrometers at least, while the mesh size of the dense region was to be larger than 1.5 nm and must be considerably smaller than 30 nm.
In this study, we investigated the physicochemical and electrochemical properties of LiFSI solution comparing with those of LiPF6 in EC/DEC (3/7, v/v), and discuss the difference in ionic conductivity between these electrolyte solutions based on self-diffusion coefficients measured by PFG-NMR. Self-diffusion coefficients of solvent molecules and ions in 1M LiFSI solution are about 1.5 times larger than those of 1M LiPF6 solution. On the other hand, the degree of dissociation of LiFSI estimated by the Nernst-Einstein Equation from measured ionic conductivity and self-diffusion coefficients of Li+ and FSI- is lower than that of LiPF6. Therefore it is strongly suggested that almost the same ionic conductivity of 1M LiFSI solution as that of 1M LiPF6 in EC/DEC (3/7, v/v) is the result of cancelation of these two factors in the opposite direction.
A series of inhomogeneities were introduced to poly(methyl methacrylate) (PMMA) gels by carrying out the polymerization of PMMA network in the presence of various amounts of polystyrenes (PS) (M w ) 400 000). The pulsed-field-gradient stimulated-echo 1 H NMR measurements were performed for the PMMA gel samples with variable diffusion time ∆, which enabled us to investigate the PMMA gel network structure on the basis of the diffusional behavior of the large probe molecule (PS) and a small probe molecule (unreacted monomer) in PMMA gels. The diffusion behavior of the large PS probe molecule proved to significantly dependent on the amount of PS that had been added in the gelation batch. The analysis of PGSE 1 H NMR measurements strongly suggested that the PS diffusion consists of two diffusion components, while it changes to a single mode with increasing the PS concentration. These results were ascribed to the course of the phase separation resulting from development of "open" network structure that was induced by the PS probe molecule in the gelation batch.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.