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 N x H3x+1 + cations and studied the resulting [N x H3x+1 +][BH4 −] complexes for x = 2–5 using density functional theory. N x H3x+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 [N x H3x+1 +][BH4 −] complexes is a consequence of the lower vertical electron affinity, i.e. the superalkali properties of N x H3x+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.