[N <i> _x </i> H _{3 <i>x</i> +1} ⁺ ][BH ₄ ⁻ ] ( <i>x</i>  = 1−5): role of polynuclear superalkalis in chemical hydrogen storage

Ammonium borohydride, [NH4+][BH4−], despite its high gravimetric hydrogen density, can not be used for hydrogen storage due to its instability against dehydrogenation. In this paper, we have replaced NH4+ with polynuclear NxH3x+1+ cations and studied the resulting [NxH3x+1+][BH4−] complexes for x = 2–5 using density functional theory. NxH3x+1+ are superalkali cations, whose vertical electron affinity is lower than the ionisation energy of alkali cations. Their binding energy, dehydrogenation energy and enthalpy, being higher than that of [NH4+][BH4−], increase with an increase in x. Thus, these complexes are more stable than [NH4+][BH4−], which comes at the cost of slightly decreased gravimetric hydrogen density. The enhanced stability of [NxH3x+1+][BH4−] complexes is a consequence of the lower vertical electron affinity, i.e. the superalkali properties of NxH3x+1+ cations. Further, their stability against the loss of ammonia suggests a possible route of synthesis of these complexes. Thus, the superalkalis play an important role in the design of more stable and novel borohydrides for hydrogen storage.