Violations of Coordination: Exploring Metastable Diborides for Energy Storage

The ideal aeronautical solid-state fuel should possess a high density (more thrust per Newton’s Third Law), high gravimetric heat of combustion (more energy for less weight), and a high volumetric heat of combustion (more room for mission-critical items). In this work, manganese diboride (MnB2) demonstrates a high density of 5.3 g/cm3, a high gravimetric heat of combustion of 38.30 kJ/g, and the highest volumetric heat of combustion of any known fuel of 202.99 kJ /cm3. When compared to the currently used fuel in Space Shuttle rocket boosters and the Space Launch System, aluminum metal, MnB2 represents a 100% increase in density, 23% increase in gravimetric heat of combustion, and 141% increase in volumetric heat of combustion. Calorimetry results were obtained by developing a dispersed burning aid that enables combustion to ignite refractory fuels typically considered troublesome. A model cluster system analyzed by density functional theory shows that the local environment can contribute to the bulk properties even without physical manifestations in the periodic structure. This high enthalpic performance comes from the metastability of MnB2 and demonstrates that transition metals, typically shunned as solid-state fuels, can store potential energy from their high temperature synthesis through ‘over coordination’ and violation of their valence shell.