Hydrogen storage based on physisorption

Physisorption of molecular hydrogen based on neutral and negatively charged aromatic molecular systems has been evaluated using ab initio calculations to estimate the binding energy, ΔH, and ΔG at 298 (̃77 bar) and 77 K (45 bar) in order to compare calculated results with experimental measurements of hydrogen adsorption. The molecular systems used in this study were corannulene (C₂₀H₁₀), dicyclopenta[def,jkl] triphenylene (C₂₀H₁₀), 5,8-dioxo-5,8-dihydroindeno[2,1-c] fluorene (C₂₀H₁₀O₂), 6-hexyl-5,8-dioxo-5,8- dihydroindeno[ 2,1-c]fluorene (C₂₆H₂₂O₂), coronene (C₂₄H₁₂), dilithium phthalocyanine (Li ₂Pc, C₃₂H₁₆Li₂N₈), tetrabutylammonium lithium phthalocyanine (TBA-LiPc, C₄₈H ₅₂LiN₉), and tetramethylammonium lithium phthalocyanine (TMA-LiPc, C₃₆H₂₈LiN₉). It was found (a) that the calculated term that corrects 0 K electronic energies to give Gibbs energies (thermal correction to Gibbs energy, TCGE) serves as a good approximation of the adsorbent binding energy required in order for a physisorption process to be thermodynamically allowed and (b) that the binding energy for neutral aromatic molecules varies as a function of curvature (e.g., corannulene versus coronene) or if electron-withdrawing or -donating groups are part of the adsorbent. A negatively charged aromatic ring, the lithium phthalocyanine complex anion, [LiPc]^-, introduces charge-induced dipole interactions into the adsorption process, resulting in a doubling of the binding energy of Li ₂Pc relative to corannulene. Experimental hydrogen adsorption results for Li₂Pc, which are consistent with MD simulation results using x-Li₂Pc to simulate the adsorbent, suggest that only one side of the phthalocyanine ring is used in the adsorption process. The introduction of a tetrabutylammonium cation as a replacement for one lithium ion in Li ₂Pc has the effect of increasing the number of hydrogen molecules adsorbed from 10 (3.80 wt %) for Li₂Pc to 24 (5.93 wt %) at 77 K and 45 bar, suggesting that both sides of the phthalocyanine ring are available for hydrogen adsorption. MD simulations of layered tetramethylammonium lithium phthalocyanine molecular systems illustrate that doubling the wt % H₂ adsorbed is possible via such a system. Ab initio calculations also suggest that layered or sandwich structures can result in significant reductions in the pressure required for hydrogen adsorption.