Hydrogen absorption-desorption characteristics of Ti-Co-Fe Alloys

Abstract

The hydrogen absorption and desorption characteristics of Ti-Co-Fe alloys were investigated. Alloys of composition TiCo 1− x Fe x ( x = 0.05 − 0.5) were found to have the same cubic structure as TiCo and they reacted readily with hydrogen to form the hydrides TiCo 0.95Fe 0.05H 1.2, TiCo 0.7Fe 0.3H 1.2 and TiCo 0.5Fe 0.5H 1.2 (hydrogen contents, 1.1 wt.%) under a hydrogen pressure of 30 atm at room temperature. The dissociation pressures of these hydrides are dependent on the iron content x (the iron partially substitutes for the cobalt) and the value of log P H 2 increases gradually with increasing x. The enthalpy change on hydride formation, determined from the dissociation isotherms for the TiCo 0.5Fe 0.5-H system, is −10.1 kcal (mol H 2) −1; this value is smaller than those (−13.8 kcal (mol H 2) −1 and −11.2 kcal (mol H 2) −1 respectively) of the TiCo-H and TiCo 0.5Mn 0.5-H systems. The temperature required to produce a dissociation pressure of 1 atm is 70 °C, which is lower than the temperatures required for TiCo (130 °C) or TiCo 0.5Mn 0.5 (90 °C). A value of 6.2 kcal mol −1 was obtained for the apparent activation energy of hydrogen desorption. For TiCo 0.5Fe 0.5, only two cycles under mild conditions are needed for the activation treatment so the alloy can easily be activated. The hydrogen absorption-desorption cycles were repeated 30 times but no variation in the hydrogen absorption-desorption capacity was observed. The hydride of TiCo 0.5Fe 0.5 proved to be suitable for use as a hydrogen storage material.