Results 1 - 10 of 476
Results 1 - 10 of 476. Search took: 0.024 seconds
|Sort by: date | relevance|
[en] The morphological changes that can be induced in a dry ionomer by application of a strong electric field have been studied by means of computer simulation. The internal energy of the membrane at first slowly decreases with increasing field, but then rapidly increases after a certain threshold field is reached. This effect is interpreted as the reorganization of interacting head group dipoles in response to the external perturbation. The resulting morphology contains continuous channels of hydrophilic material capable of facilitating proton conduction. Upon removal of the poling field, the system does not return to its original morphology, but retains the anisotropic structure of the poled material. The poled structure appears to be thermodynamically stable, as confirmed by calculations of the Helmholtz energy of the original and poled samples.
[en] Conductive electrochemical AFM images demonstrating the complex nature and structure of Nafion surface conductivity are presented. Nanoscale regions with high currents determining the overall total membrane current can be distinguished from majority domains with lower currents and non-conductive areas. The different conductive domains form ordered structures and show a specific dynamic behaviour. These observations were compared to the structural and electrical models in the literature. None of the models is able to explain all aspects of the current images. The existence of inverted micelles seems to be quite probable since the formation of agglomerates like chains and larger ordered clusters is clearly visible. This aspect is best described by the model of Schmidt-Rohr and Chen. In addition, the highly dynamic behaviour and distribution of conductive channels of Nafion leading to the formation of new current pathways also indicates the formation of different meso-phases with a high local fluctuation rate. The other discussed models also predict structural features which are in agreement with our observations like the formation of super-structures and agglomeration of fibers. The structural characterisation reflects the situation at or near the membrane surface and might differ from the bulk structure since the surface energy may have a large influence on the formation of structures during the membrane solidification process. The quite large dynamics of conductivity changes of Nafion reflected in the formation of new current pathways even at room temperature leads to the assumption that the internal structure of Nafion is subject to significant changes due to humidity and temperature variations. The local variation of individual structures may reflect the variation of concentration of hydrophilic and hydrophobic groups during membrane solidification. The minimization of surface free energy during a self-assembling process is essential for the formation of different phases and subsequent structures like chains, etc. as well as higher order clustering.
[en] Highlights: • A catalytic approach is used to optimize proton conductivity of LCNO electrolytes. • Exsolution of Ni nanoparticles is achieved by heating under reducing atmosphere. • Ni nanocatalysts significantly improve the proton conductivity of LCNO electrolytes.
[en] Procedure was developed for measuring the diffusion coefficient of hydrogen in its mixture with nitrogen in the temperature range 450–600°C under atmospheric pressure. This is done in an electrochemical cell based on a solid-oxide electrolyte with proton conductivity of composition BaCe0.7Zr0.1Y0.2O3–δ. The procedure makes it possible to trace how the hydrogen diffusion coefficient varies with temperature and hydrogen concentration in nitrogen. It is shown that the hydrogen diffusion coefficient grows with increasing hydrogen concentration, its temperature dependence is of power-law type with n = 1.5, and it is in satisfactory agreement with the theoretical temperature dependence
[en] Relations between morphology and transport sensitively govern proton conductivity in perfluorsulfonate ionomers (PFSIs) and thus determine useful properties of these technologically important materials. In order to understand such relations, we have conducted a broad systematic study of H+-form PFSI membranes over a range of uniaxial extensions and water uptakes. On the basis of small-angle X-ray scattering (SAXS) and 2H NMR spectroscopy, uniaxial deformation induces a strong alignment of ionic domains along the stretching direction. We correlate ionic domain orientation to transport using pulsed-field-gradient 1H NMR measurements of water diffusion coefficients along the three orthogonal membrane directions. Intriguingly, we observe that uniaxial deformation enhances water transport in one direction (parallel-to-draw direction) while reducing it in the other two directions (two orthogonal directions relative to the stretching direction). We evaluate another important transport parameter, proton conductivity, along two orthogonal in-plane directions. In agreement with water diffusion experiments, orientation of ionic channels increases proton conduction along the stretching direction while decreasing it in the perpendicular direction. These findings provide valuable fodder for optimal application of PFSI membranes as well as for the design of next generation polymer electrolyte membranes.
[en] Compounds of formula Li1+xMIIIxTi2-x(PO4)3 with MIII = Cr, Fe and x = 0 and 0.05 have been prepared at soft temperatures using the Pechini synthesis method, based on sol-gel chemistry. The structural and microstructural characterization by X-ray diffraction and Scanning Electron Microscopy (SEM), shows that all of them crystallize in a NASICON-type structure with similar lattice parameters. Doping with Fe and Cr, causes an increase of the density of the samples after sinterization what clearly improves the ionic conductivity of the original material, LiTi2(PO4)3 until values of 9x10-4 S cm-1 at room temperature in the chromium-doped material. (Author)
[en] Highlights: • Nafion modified carbon dots (NCDs) with multi-functional groups were successfully synthesized. • The aggregation structure of the Nafion matrix was adjusted by NCDs. • NCD Nafion composite PEMs present a 5–10 times increase in proton conductivity compared to recast Nafion. • The methanol permeability of the NCD Nafion composite PEMs is effectively decreased. - Abstract: It is of great necessity to achieve a proton exchange membrane (PEM) with high proton conductivity and low methanol permeability for the practical applications. Nafion modified carbon dots (NCDs) with multi-functional groups were successfully synthesized via the pyrolysis of citric acid (CA) with the existence of Nafion. The modification process is motivated by the hydrophilicity-to-hydrophobicity transformation of CA as well as the non-covalent hydrophilic-hydrophobic interaction during the pyrolysis procedure. Multi-functionalized NCDs with moderate hydrophilicity influence the aggregation structure of Nafion matrix of the composite membranes and effectively enhance the high-temperature water retention ability. Both the proton conductivity and the methanol resistance ability of the composite PEMs are significantly enhanced. 0.5-NCD-0.5 Nafion composite PEM presents a 5–10 times increase in proton conductivity and 50% percent decrease in methanol permeability than that of recast Nafion.
[en] TlH2PO4, a hydrogen-bonded ferroelectric, was investigated by AC impedance measurements at various temperatures. Both the real and the imaginary impedance values showed anomalous behaviors at the ferroelastic-paraelastic transition. On the complex impedance plane, the Cole-Cole plots evolved into two regions with increasing temperature. The low- frequency region shows Warburg impedance (n ∼ 0.5), as in the case of surface conduction in solid electrolytes. The high frequency conduction mechanism appears to be proton ionic transport in the bulk. The proton conduction for TlH2PO4 consists of two-phase elements, corresponding to a glass-like surface region and a normal bulk region.
[en] The proton conductivity of a dense coordination polymer (CP) was investigated under high-pressure conditions. Impedance measurements under high pressures revealed that the proton conductivity of the CP decreased more than 1000-fold at pressures of 3–7 GPa and that the activation energy for proton conduction almost doubled compared with that at ambient pressure. A synchrotron X-ray study under high pressure identified the amorphization process of the CP during compression, which rationally explains the decrease in conductivity and increase in activation energy. This phenomenon is categorized as reversible pressure-induced amorphization of a dense CP and is regarded as a demonstration of the coupling of the mechanical and electrical properties of a CP