Improved H₂ Storage in Zeolitic Imidazolate Frameworks Using Li⁺, Na⁺, and K⁺ Dopants, with an Emphasis on Delivery H₂ Uptake

We use grand canonical Monte Carlo simulations with first principles based force fields to show that alkali metal (Li +, Na +, and K + )-doped zeolitic imidazolate frameworks (ZIFs) lead to significant improvement of H 2 uptake at room temperature. For example, at 298 K and 100 bar, Li-ZIF-70 totally binds to 3.08 wt % H 2, Na-ZIF-70 to 2.19 wt % H 2, and K-ZIF-70 to 1.62 wt % H 2, much higher than 0.74 wt % H 2 for pristine ZIF-70. Thus, the dopant effect follows the order of Li-ZIF > Na-ZIF > K-ZIF, which correlates with the H 2 binding energies to the dopants. Moreover, the total H 2 uptake is higher at lower temperatures: 243 K > 273 K > 298 K. On the other hand, delivery H 2 uptake, which is the difference between the total adsorption at the charging pressure (say 100 bar) and the discharging pressure (say 5 bar), is the important factor for practical on-board hydrogen storage in vehicles. We show that delivery H 2 uptake leads to Na-ZIF-70 (1.37 wt %) > K-ZIF-70 (1.25 wt %) > Li-ZIF-70 (1.07 wt %) > ZIF-70 (0.68 wt %), which is different from the trend from the total and excess uptake. Moreover, the delivery uptake increases with increasing temperatures (i.e., 298 K > 273 K > 243 K)! To achieve high delivery H 2 uptake at room temperature, the large free volume of ZIFs is required. We find that higher H 2 binding energy needs not always lead to higher delivery H 2 uptake.