Hydrogen physisorptive storage in metal-organic frameworks (MOFs)

Abstract

Generation of power from fossil fuels produces large amounts of carbon dioxide which is a major contributor to global warming and many other environmental problems. Alternative sources of energy are available but fossil fuels remain the main source of energy for on-board energy generation. One alternative source that has gained great popularity in recent years has been hydrogen. The characteristics which it possesses make it an ideal alternative to fossil fuels for on-board energy generation and also for stationary (portable) power applications. Widespread use of hydrogen as an energy carrier can only be realised once a reliable storage method which is safe and affordable becomes available. Such a storage medium should have sufficient hydrogen storage capacity, fast kinetics, and be capable of delivering hydrogen gas to a fuel cell at ambient or near-ambient conditions. This study investigated the storage of hydrogen using metal-organic frameworks (MOFs). MOFs are a class of porous inorganic-organic crystalline materials that store hydrogen by adsorption. What makes MOFs such an attractive option for hydrogen storage is that they have exceptionally high surface areas and porosity, as well as tunable pore sizes and internal surfaces. MOFs have provided successful hydrogen storage at cryogenic temperatures (77 K) and a major challenge is to reach high storage capacities at ambient temperatures and acceptable pressures. The HySA Infrastructure Centre of Competence (CoC), one of three centres of competence established by the Department of Science and Technology (DST) to implement the National Hydrogen and Fuel Cells Strategy, is tasked with R&D on hydrogen generation, storage and distribution. A major part of the hydrogen storage R&D is the development of MOFs for hydrogen storage with the ultimate aim of using the developed MOFs for practical applications, including fuel cell vehicles and portable power applications, using metals that promote the beneficiation of South African mineral resources. The MOFs that were investigated in this study are zinc-based MOF (Zn-MOF, MOF-5), zirconium-based MOF (Zr-MOF, UiO-66) and chromium-based MOF (Cr-MOF, MIL-101). These MOFs were chosen because they were envisaged as potential hydrogen storage options. MOF-5 in particular was seen as an ideal starting point for this study as it has been reported to have high surface area, permanent porosity, good thermal stability and a high synthetic yield. In addition, MOF-5 is amongst the most widely investigated MOFs for hydrogen storage. In this study, MOF-5 with high crystallinity and good morphology was synthesised using either N,N-diethylformamide (DEF) or dimethylformamide (DMF) as solvent. As DMF is a cheaper solvent than DEF the synthesis conditions were optimised for the DMF-synthesised MOF-5 and detailed analyses performed. It was discovered at some point that MOF-69c was produced as a product instead of MOF-5. Successful transition from MOF-69c to MOF-5 was achieved by employing a simple heat treatment. Due to the moisture sensitivity of MOF-5 which results from the weak interaction of Zn metal with O atoms in the framework, MOF-5 and other MOFs with different metal centres were synthesised in an attempt to overcome the moisture sensitivity problem. This led to the investigation of Zr-MOF (UiO-66) as it had the best results and had been reported to be moisture stable. Zr-MOF was synthesised using a solvothermal synthesis procedure and a microwave-assisted synthesis approach. The solvothermal synthesis was optimised using formic acid as a modulator and the microwave-assisted synthesis provided a rapid synthesis technique. The optimised synthesis produced octahedral-shaped Zr-MOF crystals of high crystallinity, with excellent moisture and thermal stability. As the synthesis of MOF-5 and Zr-MOF employed DMF, an environmentally harsh solvent, the need to pursue a greener MOF synthesis led to the investigation of Cr-MOF (MIL-101). A successful modulated synthesis of Cr-MOF using formic acid instead of HF as modulator, and water as the reaction solvent was achieved. The synthesised MOFs were characterised and their hydrogen adsorption capacity measured at 77 K and at pressures up to 1 bar. It was found that MOF-5 had a maximum hydrogen uptake of 1.60 wt% (DEF-synthesised MOF-5) and 1.40 wt% (DMF-synthesised MOF-5) whilst MOF-69c had a maximum uptake of 1.90 wt%. Zr-MOF had a maximum hydrogen uptake of 1.50 wt% and a maximum hydrogen uptake of 1.92 wt% was achieved for the Cr-MOF. Hydrogen uptake was found to be related to the quality of the MOF crystals and also directly related to the surface area, pore volume and/or pore size of the particular MOF.