Hydrogen absorption-desorption characteristics of titanium-cobalt-manganese alloys

Abstract

The hydrogen absorption and desorption characteristics of Ti-Co-Mn alloys were investigated. Alloys of composition TiCo 1− x Mn 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 0.95Mn 0.05H 1.4, TiCo 0.9Mn 0.1H 1.4, TiCo 0.8Mn 0.2H 1.6 and TiCo 0.5Mn 0.5H 1.7 (hydrogen content 1.3, 1.3, 1.4 and 1.6 wt.% respectively) under a hydrogen pressure of 30 atm at room temperature. The dissociation pressures of these hydrides are dependent on the manganese content x (the manganese partially substitutes for the cobalt) and the value of log P increases gradually with increasing x. In addition, with increasing x, the width of the plateau in the isotherms becomes greater than that of TiCo hydride and the hydrogen content increases. The enthalpy change on hydride formation determined from the dissociation isotherms for the TiCo 0.5Mn 0.5−H system is −11.2 kcal (mol H 2) −1; this value is smaller than that of the TiCo-H system. The temperature required to produce a dissociation pressure of 1 atm is 90 °C, which is lower than the temperature required for TiCo (130 °C). The desorption rate of hydrogen for TiCo 0.5Mn 0.5 is greater than that for TiCo, and a value of 9.9–11.4 kcal mol −1 was obtained for the apparent activation energy of hydrogen desorption. For TiCo 0.5Mn 0.5, only two cycles under mild conditions are needed for the activation treatment so the alloy can be easily 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 0.5Mn 0.5 proved to be suitable for use as a hydrogen storage material.