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We report a study of the hydrogen storage properties of materials that result from ball milling Ca(BH 4 ) 2 and MNH 2 (M = Li or Na) in a 1:1 molar ratio. The reaction products were examined experimentally by powder X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS), and temperature-programmed desorption (TPD). The Ca(BH 4 )/LiNH 2 system produces a single crystalline compound assigned to LiCa-(BH 4 ) 2 (NH 2 ). In contrast, ball milling of the Ca(BH 4 )/NaNH 2 system leads to a mixture of NaBH 4 and Ca(NH 2 ) 2 produced by a metathesis reaction and another phase we assign to NaCa(BH 4 ) 2 (NH 2 ). Hydrogen desorption from the LiCa(BH 4 ) 2 (NH 2 ) compound starts around 150 °C, which is more than 160 °C lower than that from pure Ca(BH 4 ) 2 . Hydrogen is the major gaseous species released from these materials; however various amounts of ammonia form as well. A comparison of the TGA/DSC, STMBMS, and TPD data suggests that the amount of NH 3 released is lower when the desorption reaction is performed in a closed vessel. There is no evidence for diborane (B 2 H 6 ) release from LiCa(BH 4 ) 2 (NH 2 ), but traces of other volatile boron−nitrogen species (B 2 N 2 H 4 and BN 3 H 3 ) are observed at 0.3 mol % of hydrogen released. Theoretical investigations of the possible crystal structures and detailed phase diagrams of the Li−Ca−B−N−H system were conducted using the prototype electrostatic ground state (PEGS) method and multiple gas canonical linear programming (MGCLP) approaches. The theory is in qualitative agreement with the experiments and explains how ammonia desorption in a closed volume can be suppressed. The reduced hydrogen desorption temperature of LiCa(BH 4 ) 2 (NH 2 ) relative to Ca(BH 4 ) 2 is believed to originate from intramolecular destabilization.

This study examines the practical prospects and benefits for using interstitial metal hydride hydrogen storage in "unsupported" fuel cell mobile construction equipment and aviation GSE applications. An engineering design and performance study is reported of a fuel cell mobile light tower that incorporates a 5 kW Altergy Systems fuel cell, Grote Trilliant LED lighting and storage of hydrogen in the Ovonic interstitial metal hydride alloy OV679. The metal hydride hydrogen light tower (mhH 2 LT) system is compared directly to its analogue employing highpressure hydrogen storage (H 2 LT) and to a comparable diesel-fueled light tower with regard to size, performance, delivered energy density and emissions. Our analysis indicates that the 5 kW proton-exchange-membrane (PEM) fuel cell provides sufficient waste heat to supply the desorption enthalpy needed for the hydride material to release the required hydrogen. Hydrogen refueling of the mhH 2 LT is possible even without external sources of cooling water by making use of thermal management hardware already installed on the PEM fuel cell. In such "unsupported" cases, refueling times of ~ 3 -8 hours can be achieved, depending on the temperature of the ambient air. Shorter refueling times (~ 20 minutes) are possible if an external source of chilled water is available for metal hydride bed cooling during rapid hydrogen refueling. Overall, the analysis shows that it is technically feasible and in some aspects beneficial to use metal hydride hydrogen storage in portable fuel cell mobile lighting equipment deployed in remote areas. The cost of the metal hydride storage technology needs to be reduced if it is to be commercially viable in the replacement of common construction equipment or mobile generators with fuel cells.

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