article · Canadian Journal of Chemistry
Theoretical density functional theory calculations have examined how alkali cations influence hydrogen vacancy stability and release mechanisms in solid-state hydrogen storage materials based on lithium, sodium, and potassium ammonium compounds. In lithium ammonium, a cooperative stabilisation effect consistently favours double hydrogen removal across all evaluated hydrogen chemical potentials without phase crossovers. In contrast, sodium and potassium ammonium compounds exhibit thermodynamic crossovers where the preferred defect state switches between single and double vacancies depending on the chemical potential. Temperature and pressure mapping suggests that the release transition in potassium ammonium can occur under experimentally accessible high-vacuum conditions at room temperature, whereas sodium ammonium faces structural barriers requiring impractical conditions. Furthermore, removing hydrogen triggers an electronic shift from metallic behaviour to narrow p-type semiconducting states, establishing foundational thermodynamic guidelines to control dehydrogenation pathways in metal amides.
Developing efficient solid-state hydrogen storage materials is crucial for the transition towards clean energy systems. By identifying how different alkali metals alter the mechanisms and energy barriers of hydrogen release, this research clarifies which chemical compositions can release hydrogen under practical working conditions. Such insights guide the rational design of safer, more effective storage media without relying on inefficient trial-and-error laboratory experiments.
The findings provide computational guidelines that could assist materials scientists and hydrogen storage developers in selecting suitable metal amides for solid-state storage systems. Because the results are based purely on first-principles theoretical modelling, the work represents early-stage fundamental research. Physical synthesis, experimental validation under real-world operating conditions, and subsequent device engineering will be necessary before these compounds can be deployed in commercial hydrogen storage technologies.
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In this theoretical study, we employ first-principles density functional theory calculations using the generalized gradient approximation to systematically investigate the structural, electronic, and thermodynamic properties of alkali–ammonium compounds XNH 4 (X = Li, Na, K) intended for solid-state hydrogen storage applications. The clear aim of this study is to elucidate how the chemical identity of the alkali cation governs the progressive hydrogen removal mechanisms via single (1H) and double (2H) vacancy formations. The Gibbs free energy changes (Δ G) were evaluated as a function of the hydrogen chemical potential ( μ H ), incorporating zero-point energy corrections and entropic terms at room temperature. Our findings reveal that LiNH 4 exhibits a strong cooperative stabilization effect, consistently favoring full 2H removal across the entire studied μ H range without any phase crossover. Conversely, NaNH 4 and KNH 4 demonstrate unique thermodynamic crossovers where defect preference flips between 1H and 2H states at specific threshold chemical potentials. Pressure–temperature mappings indicate that the transition in KNH 4 is experimentally reachable under realistic high-vacuum conditions (1.8 × 10 −7 atm at 300 K), whereas NaNH 4 requires extreme, unfeasible conditions due to an inherent lattice structural bottleneck. Electronic structure analysis reveals an ionic-to-covalent bonding transition, showing that hydrogen release induces an evolution from a metallic ground state toward a narrow p-type semiconductor behavior with distinct band gap openings (1.37 eV for LiNH 2 , 0.80 eV for NaNH 2 , and 0.93 eV for KNH 2 ). These comprehensive calculations provide quantitative principles for optimizing dehydrogenation and tuning release pathways in complex metal amides.
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DOI: 10.1139/cjc-2026-0060
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