Nb₁₄C₁₂ ⁺ metcubane: a new class of metal-carbide clusters for hydrogen storage

While studying the reactions of hydrocarbons with niobium clusters, we find an exceptionally inert cluster that consists of 14 niobium and 12 carbon atoms. Using infrared photodissociation spectroscopy, we have identified that this Nb₁₄C₁₂ ⁺ cluster adopts a hollow cubic cage structure of O _h symmetry, a class of new clusters that we term "metcubanes". This "metcubane" Nb₁₄C₁₂ ⁺ consists of 6 face-centred Nb atoms and 8 vertex Nb atoms, and twelve C atoms sitting on the 12 edges of the cube. Each carbon atom bonds to four Nb atoms, granting Nb₁₄C₁₂ ⁺ a unique cage structure that contrasts to the body-centred cubic (bcc) bulk niobium and face-centred cubic (fcc) NbC solid. This subnanometer Nb₁₄C₁₂ ⁺ cluster promises a high hydrogen uptake, allowing 42 H₂ on the surface and 8 H atoms in the hollow cage, namely, a gravimetric density of 6.4 wt% and a volumetric density of 120 g L^-1, with an average binding energy of -0.15 eV. The finding of exceptional stability and remarkable hydrogen uptake establishes Nb₁₄C₁₂ ⁺ as a new class of stable metal carbides for hydrogen storage, presenting a promising way to advance hydrogen infrastructure and hasten the transition to a sustainable, carbon-neutral economy.