Impact of position and number of boron atom substitution on hydrogen uptake capacity of Li-decorated pentalene

Abstract

We have performed an ab initio and density functional theory study of the hydrogen adsorption on a lithium (Li)-decorated pentalene (C₈H₆Li₂) complex. The C₈H₆Li₂ complex can interact with a maximum of two hydrogen molecules with a H₂ uptake capacity of 3.36 wt%. The effect of the number and position of boron atom substitution in the C₈H₆Li₂ complex on the H₂ uptake capacity is also studied. Two and four carbon atoms are substituted by boron atoms in the C₈H₆Li₂ complex. Two different structures are considered for each of the two and four boron atom substitutions. It is found that boron substitution in the C₈H₆Li₂ complex enhances the binding energy of Li to the substrate. Four boron atom substitution at different positions also affects the H₂ uptake capacity. The two structures of two boron-substituted complexes (C₆B₂H₆Li₂) show the same H₂ uptake capacity viz. 6.63 wt%. Unlike the two boron-substituted complexes, two different structures of the four boron-substituted complexes (C₄B₄H₆Li₂) show different H₂ uptake capacity which is found to be 9.81 wt% and 6.76 wt%. The temperature and pressure range for energetically favourable H₂ adsorption on these complexes is predicted using the Gibbs free energy corrected H₂ adsorption energy. Various interaction energies are calculated for all the maximum H₂-adsorbed complexes using the many-body analysis approach. The H₂ desorption temperature for these complexes is predicted using the Van't Hoff equation and is found to be in the range of 25 K to 115 K. Molecular dynamics simulations for all these complexes are performed which show that these complexes can not bind a single H₂ molecule at ambient conditions during the simulations. However, H₂ adsorption on these complexes is energetically favourable at low temperature.