Mitigation of Hydrogen Capacity Losses during Pressure Cycling of the Li3N-H System by the Addition of Nitrogen

We report PCT measurements, reaction paths, and thermodynamic modeling of prolonged pressure cycling of Li3N in mixed hydrogen/nitrogen atmospheres. For 20 mol % N-2 in H-2 mixtures, we observe a significant enhancement of the reversible capacity (from similar to 3 to similar to 10 wt % H after 516 cycles at 528 K) as compared with cycling without nitrogen additives. We attribute this enhancement to the reaction of nitrogen with liquid lithium during cycling as the Gibbs free energy of formation of Li3N (Delta G degrees = -98.7 kJ/mol) is more negative than that of LiH (Delta G degrees = 50.3 kJ/mol). We propose that the mitigation of hydrogen capacity losses is due to the partial destabilization of the LiH phase that tends to accumulate during cycling and the liquid Li formation.