High-pressure experimental data collection, analysis and modelling of hydrogen storage on nanoporous materials

The number of materials reported in the literature as possible hydrogen storage media grows by the day. Each of the different materials, including microporous polymers, metal/covalent organic frameworks and nanoporous carbons claims to possess the necessary requirements to comply with the US Department of Energy (DOE) storage targets. These targets, which range from price to adsorbent capacity, operating conditions and density, make it difficult to compare and assess the proper storage material for a hydrogen-powered mobile application. Comparing adsorption with other storage methods is also important and, so far, none of the known materials have reached the gravimetric and volumetric targets which have been put forth by the DOE. We report the experimental acquisition of high-pressure volumetric hydrogen sorption data (up to 20 MPa) collected at different temperatures in a number of adsorbents, including porous carbons, polymers of intrinsic microporosity and metal-organic frameworks. This experimental data have been analysed and, because of the inherent saturation in hydrogen adsorption in nanoporous systems, a maximum capacity for every different material has been found, which can serve as a baseline for comparison with other adsorptive systems or storage methods. The isosteric heat of adsorption (an essential measure of the strength of interaction between the adsorbate gas and the adsorbent) is also calculated from the experimental data using the Clausius-Clapeyron approximation, the Clausius equation and the virial equation. The use of the different methods to calculate the isosteric heats of adsorption is related to the validity of the assumptions present in the Clausius-Clapeyron and virial equations, which assume perfect gas behaviour and negligible bulk phase molar volume and this might not be the case at high pressures. Capacities and isosteric heats determined using Grand Canonical Monte Carlo molecular simulations are compared with the values obtained from the experimental adsorption data. NMR experimental data for high-pressure capacity of adsorbents will also be used and compared with both simulation and experimental volumetric adsorption data. Finally, hydrogen capacities and storage properties will be compared with other conventional hydrogen storage methods, which are gas compression at 35 and 70 MPa and liquefaction at 20 K. We hope this work can help to clarify which materials can meet the necessary requirements for a hydrogen-storage system in a mobile application and highlight the advantages of using nanoporous adsorbents for hydrogen storage.