Prediction of a Ca(BH4)(NH2) quaternary hydrogen storage compound from first-principles calculations

We use a combination of density functional theory (DFT) calculations and a Monte Carlo (MC)-based crystal structure prediction tool, the Prototype Electrostatic Ground State (PEGS) method, to search for new hydrogen storage compounds in the Ca-based mixed-amide-borohydride quaternary system. We predict the existence of a new ordered quaternary compound, CaBNH6, whose stoichiometry comes from a 1:1 mixture of Ca(BH4)(2) and Ca(NH2)(2). Our DFT calculations show that CaBNH6 is similar to 12.5 kJ/mol Ca (at T = 0 K) lower in energy than the mixture of 1/2[Ca(BH4)(2) + Ca(NH2)(2)]. DFT phonon calculations of vibrational thermodynamics show that this stability of CaBNH6 [with respect to Ca(BH4)(2) and Ca(NH2)(2)] persists to finite temperatures. The predicted crystal structure contains two formula units of CaBNH6. We have also performed a thermodynamic analysis of hydrogen decomposition of our predicted compound using the Grand Canonical Linear Programming (GCLP) method combined with a large database of DFT energies and vibrational thermodynamics. We find that the thermodynamically preferred decomposition reaction for CaBNH6 involves formation of BN with a low decomposition enthalpy. Though the decomposition enthalpy is low, the kinetic behavior of CaBNH6 decomposition is not yet known. We assert that further experimental investigation of this system is warranted to verify the existence of predicted quaternary compounds in this Ca-B-N-H system, as well as to elucidate their hydrogen release reaction pathways.