Thermodynamic Investigation of Ti-H System by Molten Salt Electrochemical Technique

Abstract

There exist numerous investigations on thermodynamic properties of metal-hydrogen, MH, systems both from scientific and industrial interests. Thermodynamic data on MH systems can be derived from pressure-composition-temperature (PCT) measurement, electrode potential-composition-temperature (ECT) measurement, or calorimetry. Among them, ECT measurement is the most direct for determination of thermodynamic data because the relative partial molar Gibbs energy is directly measured. Therefore, this approach has been utilized to estimate thermodynamic properties of MH systems (M ϭ Pd, 1-3 Pd-based alloys, 4-7 and Ni-based alloys 8,9 ). In addition, it only needs very simple apparatus and it is possible to change the hydrogen pressure, even if it is extremely low, instantaneously by controlling the electrode potential. However, the electrochemical approach has a critical disadvantage by its limited temperature range as long as aqueous solutions are used for the electrolyte, so the methods utilizing PCT measurement have been widely employed at high temperatures until now.Concerning the hydrogen source for the electrochemical investigations at high temperatures, H Ϫ ion is produced by dissolution of a saline hydride like LiH in molten salts, such as LiCl-KCl, 10 LiFNaF-KF, 11 and LiI-KI 12 eutectic melts. It is stable in the molten salts and its half-cell reaction, represented by Eq. 1, has been investigated by several research groups 10-14In 1991, Takenaka and Ito 13 found that a small amount of hydrogen was absorbed in an iron electrode by electrochemical anodic oxidation of H Ϫ ion in LiCl-KCl-LiH melts at 673-737 K. By utilizing similar electrochemical hydrogen absorbing reactions in molten salts, it is possible to investigate MH systems at high temperatures. In addition, the use of H Ϫ ion never deprives the advantages of the electrochemical approach. For instance, Ito and Nohira 14 reported that the Ni |H 2 |H Ϫ electrode potential was 0.50 V (vs. Li ϩ /Li) in a molten LiCl-KCl-LiH (2.0 mol % LiH added) system at 673 K. When the Nernst equations applies for this system, hydrogen pressure is equivalent to 3.2 ϫ 10 Ϫ8 atm at an electrode potential of 0 V (vs. Li ϩ /Li), which is the cathodic limit of this melt. It is very difficult for conventional vacuum equipment to achieve such low pressure values.In the present study, the authors have measured detailed ECT relationships of the Ti-H system (TiH x ; 0 < x < 1.8) at 673-773 K. In these experimental conditions, there exist three single phases (␣, ␤, and ␦) and two coexisting phases (␣ ϩ ␤ and ␤ ϩ ␦) according to the phase diagram 15 shown in Fig. 1. The Ti-H system is one of the systems whose thermodynamic properties have been extensively investigated. Nevertheless, few thermodynamic investigations in the high-hydrogen-concentration region were conducted than in the low-hydrogen-concentration region. In this temperature range, Liaw et al. 16 also measured thermodynamic quantities in ␣, ␣ ϩ ␤, and ␤ phases of the Ti-H system by electrochemical methods using LiClKCl-LiH...

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.…”

Thermoelectric Power of M-H Systems in Molten Salts and Application to M-H Thermogalvanic Cell

“…The corresponding metal hydride electrodes were prepared by galvanostatic electrolysis in LiCl-KCl-LiH melts at 673 K. The hydrogen contents were estimated from the total quantities of applied charge, considering the current efficiency as 100%. 12,13 The electrochemically formed ZrH 0.8 and TiH 0.2 was characterized to be in the (␣ ϩ ␦) and (␣ ϩ ␤) coexisting phase regions, respectively, by X-ray diffraction. 12 The reference electrode was an Al-Li alloy in the (␣ ϩ ␤) coexisting phase state.…”

“…12,13 The electrochemically formed ZrH 0.8 and TiH 0.2 was characterized to be in the (␣ ϩ ␦) and (␣ ϩ ␤) coexisting phase regions, respectively, by X-ray diffraction. 12 The reference electrode was an Al-Li alloy in the (␣ ϩ ␤) coexisting phase state. The equilibrium potential of this electrode is determined by the following reversible reaction [14][15][16] Al͑␣ ͒ ϩ Li ϩ ϩ e Ϫ AlLi͑␤ ͒ ͓4͔…”

Novel electrochemical reactions related to electrodeposition and electrochemical synthesis

“…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,…”

“…) and stable open-circuit potentials were measured. Since the hydrogen contents (x in TiH x ) were estimated by assuming 100% current efficiency [22], electrode potential-composition-temperature (E-C-T) relationships were obtained as shown in Fig. 4.…”