Heat of adsorption for hydrogen in microporous high-surface-area materials.

The limited resources of fossil fuels will soon require a change to renewable energies. The ideal energy carrier for mobile applications is hydrogen, but the problem of an adequate and safe storage system is still unsolved. One of the possibilities for hydrogen storage is physisorption in porous materials. The big advantage of molecular hydrogen storage is that short refuelling times can be realized due to the extremely fast kinetics. Additionally, no extra heat management is needed as the heat of adsorption is lower than for other storage processes, for example in metal hydrides. On the other hand, owing to this low heat of adsorption, a cryosystem is needed to reach high storage capacities. Microporous materials possessing a high specific surface area (SSA), for example carbon nanotubes, activated carbon, zeolites, and coordination polymers or metal–organic frameworks (MOFs), show a high hydrogen uptake at low temperatures, typically 77 K. Characteristic for this cryo-adsorption in porous materials is that the maximum hydrogen uptake at high pressures depends linearly on the SSA of the material. At low pressures distinct differences in the hydrogen uptake exist for the different materials. Therefore, the pressure to reach, for example, 80% of the maximum storage capacity, differs strongly between the materials. For all materials the hydrogen uptake decreases with increasing temperature. The strength of this decrease as well as the hydrogen uptake at low pressures are governed by the heat of adsorption. Typically, the isosteric heat of adsorption is calculated from the adsorption isotherms measured at 77 K and 87 K since these temperatures can easily be realized by liquid nitrogen and liquid argon, respectively. This small temperature range leads to a very high uncertainty in the heat of adsorption. Only in a few publications is the isosteric heat of adsorption determined with higher accuracy from several isotherms measured at various temperatures. Herein, we present hydrogen adsorption isotherms measured over a wide temperature (77–298 K) and pressure ACHTUNGTRENNUNG(0–20 bar) range. This allows the determination of the heat of adsorption for a wide range of surface coverage with very high accuracy. For the first time different microporous materials have been investigated systematically and their heats of adsorption are correlated to the structures of the materials. Two activated carbon samples, Norit R0.8 and Takeda 4A, with BET SSAs of 1384 mg 1 and 397 mg 1 respectively, and four different metal–organic frameworks, MOF-5, Cu-BTC, MIL-53 and MIL-101, with SSAs between 902 mg 1 and 3293 mg 1 have been investigated. The hydrogen uptake was measured with an automated Sieverts’ apparatus (PCTPro2000, HyEnergy, USA). Figure 1 shows the dependence of the hydrogen uptake on the pressure as an example for Cu-BTC (for other materials, see the Supporting Information). For all materials the isotherms at