Hydrogen cycling in γ-Mg(BH4)2 with cobalt-based additives

Abstract

Magnesium borohydride (Mg(BH₄)₂) is an attractive candidate as a hydrogen storage material due to its high hydrogen content and predicted favorable thermodynamics. In this work we demonstrate reversible hydrogen desorption in partially decomposed Mg(BH₄)₂ which was ball milled together with 2 mol% Co-based additives. Powder X-ray diffraction and infrared spectroscopy showed that after partial decomposition at 285 °C, amorphous boron-hydride compounds were formed. Rehydrogenation at equivalent temperatures and hydrogen pressures of 120 bar yielded the formation of crystalline Mg(BH₄)₂ in the first cycle, and amorphous Mg(BH₄)₂ with other boron–hydrogen compounds upon the third H₂ absorption. Reversibility was observed in the samples with and without Co-based additives, although the additives enhanced hydrogen desorption kinetics in the first cycle. X-ray absorption spectroscopy at Co K-edge revealed that all the additives, apart from Co₂B, reacted during the first desorption to form new stable species.