Ab-initio CALPHAD modeling of NaNH₂BH₃ for solid-state H₂ storage applications

Sodium amidoborane (NaAB) represents a promising solid-state hydrogen storage material, yet fundamental questions about its stability and decomposition mechanisms remain unresolved. This study employs density functional theory and CALPHAD modeling to investigate the thermodynamic properties and decomposition pathways of NaAB and its intermediate, Na2AB. Our comprehensive analysis reveals that, contrary to previous assumptions regarding dihydrogen bond stabilization, NaAB exists only in a metastable state at room temperature. The compound follows a stepwise decomposition pathway, transitioning to Na2AB at 89.3 °C with initial hydrogen release (∼3.81 wt%), followed by gradual, irreversible decomposition to NaH and BN at temperatures up to 310 °C. This behavior is governed by the kinetic hindrance of BN formation, which explains the extended hydrogen evolution observed experimentally. Thermodynamic calculations demonstrate that NaH formation is favorable only alongside BN, while gas-phase analysis confirms that borazine and ammonia emissions stem from synthesis impurities rather than intrinsic decomposition. By elucidating these fundamental relationships, this work reconciles conflicting interpretations and provides practical insights for the development of advanced solid-state hydrogen storage materials.