Lawrence Berkeley National Laboratory

Mg(BH₄)₂ is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB₂. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB₂. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB₂ to Mg(BH₄)₂ complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H₂ dissociation, likely at Mg-terminated MgB₂ surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH₄)₂ complexes without persistent B_xH_y intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB₂ are discussed.