Large entropy derived from low-frequency vibrations and its implications for hydrogen storage

Abstract

Adsorption and desorption are driven by the energy and entropy competition, but the entropy effect is often ignored in hydrogen storage and the optimal adsorption strength for the ambient storage is controversial in the literature. This letter investigated the adsorption states of the H2 molecule on M-B12C6N6 (M = Li, Na, Mg, Ca, and Sc) and analyzed the correlation among the zero point energy (ZPE), the entropy change, and the adsorption energy and their effects on the delivery capacities. The ZPE has large correction to the adsorption energy due to the light mass of hydrogen. The computations show that the potential energies along the spherical surface centered at the alkali metals are very flat and it leads to large entropy (∼70 J/mol·K) of the adsorbed H2 molecules. The entropy change can compensate the enthalpy change effectively, and the ambient storage can be realized with relatively weak adsorption of ΔH = −12 kJ/mol. The results are encouraging and instructive for the design of hydrogen storage materials.