Ambient-Temperature Hydrogen Storage

Hydrogen that is created using renewable energies could be a sustainable, environmentally friendly fuel. However, storing and transporting H₂ is difficult due to its low density. Usually, either high pressures or very low temperatures are required to achieve usable storage densities. This limits the adoption of hydrogen as a fuel—especially in vehicles, where space is severely limited.

Adsorbing hydrogen into other materials could be a promising alternative to compression or cryogenic storage. However, finding materials that can adsorb sufficient amounts of hydrogen at ambient temperature and low pressures is challenging. Metal–organic frameworks (MOFs) are one class of possible candidates. However, physisorption in MOFs does not lead to sufficient storage densities. Chemical interactions with exposed metal nodes could improve hydrogen binding and storage.

Jeffrey R. Long, University of California, Berkeley, and Lawrence Berkeley National Laboratory, Berkeley, CA, USA, and colleagues have investigated the hydrogen adsorption properties of the vanadium-based metal–organic framework V₂Cl_2.8(btdd) (H₂btdd = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i])dibenzo[1,4]dioxin). The MOF was synthesized from VCl₂(tmeda)₂ (tmeda = N,N,N‘,N‘-tetramethylethylenediamine) and H₂(btdd) in the presence of dimethylformamidium trifluoromethanesulfonate in a dimethylformamide (DMF) solution.

The team measured the MOF’s hydrogen adsorption properties and studied the interactions between the MOF and H₂ using powder neutron- and X-ray diffraction, variable-temperature infrared (IR) spectroscopy, and density functional theory (DFT) calculations. They found that the MOF reaches a capacity of 0.26 mmol/g at 1.2 bar and 298 K. At 100 bar and 298 K, the total volumetric hydrogen uptake is 10.7 g/L. According to the researchers, the MOF provides a high density of vanadium sites that can take part in backbonding interactions with H₂. Optimizing materials with such interactions could lead to viable H₂ storage solutions.