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Ammonia (NH3) is easily liquefied by compression at 1 MPa and 25 °C, and has a highest volumetric hydrogen density of 10.7 kg H2 /100L in hydrogen carriers. The volumetric hydrogen density is above 1.5 times of liquid hydrogen at 0.1 MPa and -253 °C. The vapor pressure of liquid NH3 is similar to propane. Moreover it has a high gravimetric hydrogen density of 17.8 mass%. NH3 is burnable substance and has a side as an energy carrier which is different from other hydrogen carriers. The heat of formation of NH3 is 30.6 kJ/molH2. The value is about 1/10 of heat of combustion for hydrogen.
NH3 has advantages as a hydrogen energy carrier for fuel cell vehicles, solid oxide fuel cell (SOFC), gas turbines of electric power plants and engines of air crafts, ships and trucks. Liquid NH3 is one of the most promising methods for storing and transporting hydrogen.
However NH3 is deleterious substance based on the high vapor pressure. NH3 is classified as a polar molecule and form ammine complex with the materials having ionic bonding such as halides. Much attention has been given to these complexes because of the low vapor pressure compared with NH3. In this work, the relation between the equilibrium pressure at plateau region (plateau pressure) and the electronegativity of cation or anion in the metal halides or borohydrides was clarified. The plateau pressure decreases with increase in the electronegativity of cation and decrease in the electronegativity of anion. We found that those halides composed of higher electronegativity cations and lower electronegativity anions form thermodynamically more stable ammine complexes which have lower plateau pressure.