Hydrogen Storage Capabilities of the Low-⁃Lying Ca₂B₄ Clusters

The structural feature and electronic property of Ca₂B₄, as well as its potential for hydrogen storage, have been studied using density functional theory. The first, second, and fourth low-⁃lying isomers Ca₂B₄ 01, Ca₂B₄ 02, and Ca₂B₄ 04 have high stabilities in thermodynamics and can adsorb 12, 12, and 10 H₂ molecules with respective H₂ gravimetric uptake capacity of 16.3%, 16.3%, and 14.0%, which far exceeds the target (5.5%) proposed by the US department of energy (DOE). The average absorption energies per H₂ molecule are in the range of 0.58-⁃4.21 eV for Ca₂B₄ 01(H₂)₁₂, 0.54-⁃3.69 eV for Ca₂B₄ 02(H₂)₁₂, and 0.10-⁃0.12 eV for Ca₂B₄ 04(H₂)₁₀. Born-⁃Oppenheimer molecular dynamic (BOMD) simulations indicate Ca₂B₄ 01 and Ca₂B₄ 02 are promising candidates for adsorbing hydrogen, but Ca₂B₄ 04 is not. The results of hydrogen adsorption energies with Gibbs free energy correction indicate that 12 H₂ molecules on Ca₂B₄ 01 and Ca₂B₄ 02 are energetically favorable with a wide range of temperatures at 101 325 Pa.