Abstract
We analyze thermodynamic stability and decomposition pathways of LiBH4 nanoclusters using grand-canonical free-energy minimization based on total energies and vibrational frequencies obtained from density-functional theory (DFT) calculations. We consider (LiBH4)n nanoclusters with n = 2 to 12 as reactants, while the possible products include (Li)n, (B)n, (LiB)n, (LiH)n, and Li2BnHn; off-stoichiometric LinBnHm (m ≤ 4n) clusters were considered for n = 2, 3, and 6. Cluster ground-state configurations have been predicted using prototype electrostatic ground-state (PEGS) and genetic algorithm (GA) based structural optimizations. Free-energy calculations show hydrogen release pathways markedly differ from those in bulk LiBH4. While experiments have found that the bulk material decomposes into LiH and B, with Li2B12H12 as a kinetically inhibited intermediate phase, (LiBH4)n nanoclusters with n ≤ 12 are predicted to decompose into mixed LinBn clusters via a series of intermediate clusters of LinBnHm (m ≤ 4n). The calculated pressure-composition isotherms and temperature-pressure isobars exhibit sloping plateaus due to finite size effects on reaction thermodynamics. Generally, decomposition temperatures of free-standing clusters are found to increase with decreasing cluster size due to thermodynamic destabilization of reaction products.
Highlights
We analyze thermodynamic stability and decomposition pathways of Li2B2H4 and (LiBH4) nanoclusters using grandcanonical free-energy minimization based on total energies and vibrational frequencies obtained from density-functional theory (DFT) calculations
Hydrogen storage systems should simultaneously possess several characteristics such as safety, high gravimetric and volumetric densities, fast reversible hydrogen release and uptake under moderate pressures and temperatures matched to the operating conditions of proton exchange membrane (PEM) fuel cells
We present a detailed computational study of the decomposition thermodynamics of small LiBH4 nanoclusters using DFT-based ground state structure prediction algorithms and total DFT free energies, including vibrational contributions
Summary
We analyze thermodynamic stability and decomposition pathways of LiBH4 nanoclusters using grandcanonical free-energy minimization based on total energies and vibrational frequencies obtained from density-functional theory (DFT) calculations. Complex metal hydrides emerged as feasible high-density hydrogen storage materials after Bogdanović and Schwickardi demonstrated reversible (de)hydrogenation reactions in transition metal doped sodium alanate (NaAlH4)[2]. Due to its high thermodynamic stability and slow kinetics, hydrogen release from bulk LiBH4 requires very high temperatures that are incompatible with proton-exchange membrane (PEM) fuel cells[3,4,5]. The canonical example of a destabilized reaction is LiBH4 +MgH2 →LiH +MgB2 +(5/2)H2, where the addition of MgH2 leads to the formation of MgB2 as a low-energy product phase. This significantly lowers the reaction enthalpy relative to the decomposition reaction of the pure compound, LiBH4 →LiH +B +(3/2)H2. A complete picture of the size-dependent thermodynamics properties of LiBH4 nanoparticles and their decomposition products is not yet available
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