PNNL

Abstract

The thermodynamic properties of key compounds, Mg(B3H8)2, MgB2H6, MgB10H10, Mg(B11H14)2, Mg3(B3H6)2, and MgB12H12, proposed to be formed in the release of hydrogen from magnesium borohydride Mg(BH4)2 and uptake of hydrogen by MgB2, have been investigated using solid–state density functional theory (DFT) calculations. More accurate treatment of cell–size effects to the entropies were also investigated, in order to improve the accuracy of the thermodynamic properties of complex borohydrides. We find that the zero–point energy corrections can lower the electronic energies of reaction by 20–30 kJ/(mol H2) for these intermediates, while adding the thermal and entropy contributions results in the total decrease up to ~50 kJ/(mol H2). Although our treatment lowers the calculated formation energy of Mg(B3H8)2, it is still too high to explain the experimental observation of B3H8-. We discuss possible reasons for this disparity and propose that the formation of B3H8- and H- in a disordered amorphous phase has a large energy difference compared to the phase–separated Mg(B3H8)2 and MgH2 considered in calculations. Comparison of the experimental and NMR chemical shifts calculated within a DFT approach for known species Mg(BH4)2, Mg(B3H8)2, Mg(B11H14)2, MgB10H10 and MgB12H12 provides validation for predicting the chemical shifts of the other compounds which are yet to be confirmed experimentally. These include MgB2H6 and the proposed tri–anion species Mg3(B3H6)2 that both have favorable thermodynamics for reversible hydrogen storage in Mg(BH4)2 without the formation of MgH2 as a co–product which could phase–separate and inhibit rehydrogenation.