Hydriding/dehydriding behavior of Mg2CoH5 produced by reactive mechanical milling

Abstract Complex Mg 2 CoH 5 hydride was obtained by a combined procedure that included a milling stage of a 2Mg–Co mixture under argon followed by reactive mechanical alloying (RMA) under hydrogen, both at room temperature. During RMA, MgH 2 is produced at short milling times (10 h) and Mg 2 CoH 5 (50 wt%) after 90 h. Improvement in the yield and the formation times could be associated with both refinement of microstructure and enhancement of intermixing of Mg–Co during pre-milling stage. DSC studies of Mg 2 CoH 5 phase produced by RMA show that the starting decomposition temperature is about 205 °C. Absorption and desorption PCIs were determined under static (300 °C) and dynamic (230–330 °C) conditions. An important hysteresis and two plateaus were observed and correlated with formation/decomposition of Mg 2 CoH 5 (high-pressure plateau) and Mg 6 Co 2 H 11 (low-pressure plateau) hydrides. For comparing hydrogen sorption kinetics, Mg 2 CoH 5 (65 wt%) was also obtained by a sintering method at 410 °C and 6.0 MPa of hydrogen pressure. Absorption was very fast in the temperature range of 150–350 °C, independently of synthesis procedure. However, desorption curves showed a better behavior for RMA powders. MgCo was observed after decomposition of Mg 2 CoH 5 under particular thermal treatments, while MgCo 2 phase was not detected. The results of this study reinforce the idea that kinetics factors related with atomic mobility play a key role in the formation of Mg–Co intermetallics.