Hydrogen adsorption and kinetics in MIL-101(Cr) and hybrid activated carbon-MIL-101(Cr) materials

Since the last 15 years, porous solids such as Metal-Organic Frameworks (MOFs) have opened new perspectives for the development of adsorbents for hydrogen storage. Among all MOF materials, the chromium (III) terephthalate-based MIL-101(Cr) is a very stable one which exhibits a good uptake capacity of hydrogen (H2). In this study, syntheses were carried out in soft conditions without hydrofluoric acid as usually reported in literature. Moreover, activated carbon (AC) was introduced in the preparation of the MOF-based adsorbents to create hybrid materials with large specific surface areas (AC-MOF). Hydrogen storage capacities were assessed at 77 K up to 100 bar, and the measurements of adsorption isotherms were performed using both volumetric and gravimetric methods. The experimental data were shown to be in good agreement. A maximal excess hydrogen uptake of 67.4 mol kg−1 (13.5 wt.%) has been reached by the hybrid AC-MOF adsorbent at 77 K under 100 bar. The hydrogen storage capacity was so shown to be greatly enhanced by AC addition, as a maximal value of only 41.1 mol kg−1(8.2 wt.%) was reported for the pristine MIL-101(Cr), under the same conditions. Finally, hydrogen adsorption kinetics were examined at 77 K using experimental transient adsorption curves obtained using volumetric method, and the Linear Driving Force (LDF) model was tested for their interpretation. According to this model, diffusion coefficients could be correctly estimated only in a very low pressure range. However, for high pressures, the quasi-equilibrium assumption is not valid at the initial adsorption times, making the LDF model no more applicable for accurate determination of the average effective diffusivities. To our knowledge we present the first measurement of the adsorption kinetics of hydrogen in a hybrid carbon MOF composite material. Moreover, the adsorption performances reported in this work are the best ones achieved until now by MIL-101(Cr) doping using carbonaceous materials.