Kinetic Investigation of the Ti-H System by a Molten Salt Electrochemical Technique

Abstract

Electrochemical kinetic investigation of a Ti-H system was conducted in LiCl-KCl-LiH melts at 723 K. Apparent chemical diffusion coefficients of hydrogen including phase transformations, D H Ј , were estimated for ␣, ␣ ϩ ␤, ␤, and ␤ ϩ ␦ phases by chronopotentiometry. Chemical diffusion coefficients of absorbed hydrogen D H in ␣, ␤, and ␦ phases were obtained by ac impedance spectroscopy and that in ␦ phase was estimated by potential-step chronoamperometry. Obtained results were compared with reported values to confirm the validity of the electrochemical technique for kinetic investigation of M-H systems in both cases, with and without phase transformations.The thermodynamic and kinetic properties of metal-hydrogen ͑M-H͒ system have been extensively investigated for scientific and industrial interests. Especially, the hydrogen diffusion behavior in metals at high temperatures is worth investigating for energy-related applications. When considering hydrogen energy systems, which include hydrogen absorbing alloys and hydrogen permeation membranes, higher operating temperature has the advantage of achieving higher overall energy efficiency. Furthermore, investigation of hydrogen diffusion behavior in elemental metals and their alloy at high temperatures is very important to understanding the hydrogen embrittlement mechanism of structural materials.Hydrogen diffusion behavior in metals has been investigated by several methods, such as nuclear magnetic resonance ͑NMR͒, 1-5 quasi-elastic neutron scattering ͑QNS͒ measurement, 6-8 and internal friction measurement. 9 They give the self-diffusion coefficient of hydrogen in metals, which corresponds to jump frequency of hydrogen atoms. The gas-volumetric method, 10-12 nuclear physics, 13,14 Gorsky effect measurement, 15,16 and the electrochemical method [17][18][19][20][21][22] give the chemical diffusion coefficient of hydrogen in metals, which is crucial when considering practical applications.

Novel electrochemical reactions related to electrodeposition and electrochemical synthesis

“…From these relationships, the thermodynamic quantities of all phase states of TiH x were evaluated, which are well consistent with those reported by other authors using conventional thermochemical methods [22,24]. On the other hand, for the kinetic investigations, the diffusion coefficients of hydrogen in TiH x with various hydrogen contents were estimated by chronopotentiometry, potential-step chronoamperometry and AC impedance spectroscopy [25]. The obtained values are in agreement with reported values.…”

“…In the case of the Ti-H system, the thermodynamic and kinetic properties have been investigated precisely in the LiCl-KCl-LiH and LiBr-KBr-CsBr-LiH melts at 523-773 K [22][23][24][25]. For the thermodynamic investigation, galvanostatic electrolysis was conducted on titanium electrode intermittently to absorb hydrogen according to the following reaction,…”

Infrared Spectroscopy of Molten LiCl-KCl-LiH

“…[8][9][10] Related with these applications, there have been electrochemical studies which deal with the anodic 2,11-13 and cathodic [14][15][16] hydrogen electrode reaction in molten LiCl-KCl-LiH systems. However, there have been few structural studies which deal with the ionic interactions in the melts.…”

Electrochemical Impedance Spectroscopy Study of a Hydrogen Electrode Reaction at a Zn Electrode in a Molten LiCl−KCl−LiH System

“…8 The electrochemical reaction of the Hion also provides a precise and convenient means to investigate both thermodynamic and kinetic behaviors of metal-hydride systems at high temperatures. [9][10][11] Related with these applications, there have been a number of studies that deal with the anodic 2,12-14 and cathodic [15][16][17] hydrogen electrode reaction in molten LiCl-KCl-LiH systems. However, kinetic parameters such as the exchange current density and symmetry factor have not been evaluated so far.…”