These interactions are typically within a range of -3 to -7 kJ/mol due to the low polarizability of the H2 molecule. Although stronger framework-H2 interactions usually increase the adsorption capacity of H2 at ambient temperature, extremely strong interactions may reduce the usable H2 capacity. The usable H2 capacity is defined as the difference in H2 uptake between the H2 charging and discharging conditions. The H2 discharging pressure is proposed to be 2-5 bar to create a pressure gradient to feed H2 from the higher-pressure storage tank to the fuel cells. The optimal binding strength that can maximize the usable capacity was originally estimated with a single-site Langmuir adsorption model for simplicity, where the difference in the surface coverage of H2 at the storage and discharge conditions was calculated as a function of the Gibbs free energy of adsorption to find the maximum value. This simple model for economically viable operating conditions (100-170 bar storage and 2-5 bar discharge at (sub)ambient temperatures of 223-298 K) suggests that the optimal binding enthalpy is in the range of -15 to -25 kJ/mol with either the adsorption entropy assumed to be approximately -8R or an empirical positive correlation between enthalpy and entropy. Note that according to this calculation, the optimal adsorption enthalpy is averaged across the entire adsorbent, which is assumed to have a homogenous surface and obeys a single-site Langmuir model. Due to the inherently weak framework-H2 interactions, the primary focus of research has been to increase the magnitude of adsorption enthalpy.