Effect of hydrogen pressure and temperature on the reaction kinetics between Fe-doped Mg and hydrogen gas

The effect of hydrogen pressure and of reaction temperature on the phase transformation from Mg to MgH2 by exposure of Fe doped metallic Mg to hydrogen gas has been studied with the purpose of evaluating the importance of operative parameters on the hydrogen storage characteristics of this system. Two temperatures have been investigated and, for each temperature, two values of the hydrogen gas pressure. The gas pressure has been adjusted in order to induce the same thermodynamic driving force in samples processed at different temperatures, with the aim of separating the effect of temperature from the one of gas pressure. Experimental results show that the reaction mechanism is independent from the value of the operative parameters which, instead, influence strongly the absolute value of the kinetics constant. Sample microstructure during phase transformation is affected in a major way by the reaction temperature, with the gas pressure playing a minor role.

Research highlights▶ Phase transformation from Mg to MgH2, in milled powders containing 5 wt% of Fe. ▶ Effect of hydrogen pressure and of reaction temperature. ▶ Reaction mechanism is independent from the value of the operative parameters. ▶ Reaction rate constant is strongly influenced by reaction temperature. ▶ Nucleation process appears to be less efficient at higher temperatures.