Abstract

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 H2 uptake at room temperature. For example, at 298 K and 100 bar, Li-ZIF-70 totally binds to 3.08 wt % H2, Na-ZIF-70 to 2.19 wt % H2, and K-ZIF-70 to 1.62 wt % H2, much higher than 0.74 wt % H2 for pristine ZIF-70. Thus, the dopant effect follows the order of Li-ZIF > Na-ZIF > K-ZIF, which correlates with the H2 binding energies to the dopants. Moreover, the total H2 uptake is higher at lower temperatures: 243 K > 273 K > 298 K. On the other hand, delivery H2 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 H2 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 H2 uptake at room temperature, the large free volume of ZIFs is required. We find that higher H2 binding energy needs not always lead to higher delivery H2 uptake.