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Gwangju, South Korea

Shin E.-C.,Chonnam National University | Kim Y.-H.,Chonnam National University | Kim S.-J.,Mirae SI Co. | Park C.-N.,Chonnam National University | And 2 more authors.
Journal of Physical Chemistry C

Frequency domain analysis of the hydrogen storage kinetics was performed by Fourier transformation of the point relaxation data monitored for a pressure-composition isotherm (PCI) of Mg/MgH2 system at 325 C. The correlated hydrogen flux and the pressure relaxation under constant-volume conditions in a typical Sievert's type setup leads to the presence of the characteristic inductive loop, the size of which corresponds to R parameter for RC transmission line model for diffusion in the spherical geometry, while the theoretical dc limit resistance may not indicate the diffusion contribution only. The kinetic information was shown equivalent to that of time-domain analysis based on the diffusion problem of a solute in a limited amount into solid state from a well-stirred medium [J. Phys. Chem. C 2013, DOI: 10.1021/jp401286b]. The spectra further develop into the oscillating trajectories originating from the truncation in the sampling. The essential experimental features can be simulated by a RL two-rail transmission line model. A parametric analysis of the preoscillation spectral range using ideal lumped elements provided information on the exponential relaxation constants, nucleation/surface reaction in the two-phase region, and the sampling periods as well as the diffusivity. The pneumatochemical immittance spectroscopy (PnIS) developed in the present work allows a fully automated hydrogen storage kinetic analysis with visualized R and C parameters for the estimation of chemical diffusivity as a function of storage state using the relaxation data measured for PCI by conventional Sievert's apparatuses. © 2013 American Chemical Society. Source

Kim Y.-H.,Chonnam National University | Shin E.-C.,Chonnam National University | Kim S.-J.,Mirae SI Co. | Park C.-N.,Chonnam National University | And 2 more authors.
Journal of Physical Chemistry C

Point relaxations for the pressure-composition isotherms were analyzed as a solid-state diffusion problem of hydrogen solutes from the gas phase provided in a limited amount by a pulse-like supply. For the large differential hydrogen storage, typical in the PCI plateau region, represented by small effective volume ratio λ parameter in the diffusion equation, the relaxation occurs much faster than in the galvanostatic or potentiostatic relaxations for the same chemical diffusivity values. The analysis was successfully applied to Mg/MgH2 system and the apparent chemical diffusivity and the hydrogen self-diffusivity were evaluated as a function of the state of hydrogen storage, in an analogous manner to the well-known electrochemical titration techniques. The increase in the apparent chemical diffusivity and hydrogen self-diffusivity in the plateau region with the increasing state of storage can be consistently explained by the lower hydrogen self-diffusivity in MgH2 than in Mg solid solution. The hydrogen self-diffusivity variation in the single phase MgH2 region suggests the vacancy mechanism. The method is named as pneumatochemical intermittent titration technique (PnITT). © 2013 American Chemical Society. Source

Shin E.-C.,Chonnam National University | Seo H.-H.,Chonnam National University | Kim J.-H.,Chonnam National University | Ahn P.-A.,Chonnam National University | And 9 more authors.
Polymer (United Kingdom)

Percolating carbon conduction network is not only a long-time pursued scientific topic but also becomes more and more technologically important. Yet no established diagnostic tool is available. A simple two-wire transmission line (TL) model with symmetric boundary condition is suggested as working equivalent circuit model for the percolating carbon network in the insulating matrix such as polymer materials. Spatially distributed shunt capacitors of TL network are determined by the spacing between percolating backbone pathways of resistors. Deviation from the ideal RC TL behavior of the real network due to the non-uniform geometry, dangling network, random distribution of carbon fillers between the backbone pathways, etc. can be represented by modified yet parametric TL models featured by the generalization of the shunt capacitors to constant phase elements. © 2012 Elsevier Ltd. All rights reserved. Source

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