Schematic of the TPX/metal hydride composite for hydrogen storage.
Schematic of the TPX/metal hydride composite for hydrogen storage.

Composite materials that combine polymers with fillers based on metal compounds are a potentially attractive option for hydrogen storage. Storing hydrogen as a gas or liquid requires high pressures and/or low temperatures but amide-hydride composites can store hydrogen chemically in a solid state. The downside is that amide-hydride composites tend to lose their hydrogen-storage capacities after repeated cycles. Among the culprits are oxygen and water, which react with amides and hydrides to form system-contaminating oxides or hydroxides.

Now a team in Germany has come up with an alternative – a composite in which the component polymer, in this case a type of polyolefin, protects the metal hydride from exposure to air and moisture [Cao et al., Materials Today Energy 10 (2018) 98-107].

“We embedded a complex metal hydride inside a polymer with the aim of hindering direct contact with the air and maintaining hydrogen storage cyclability,” explain first authors of the study, Hujun Cao and Prokopios Georgopanos. “In this way, side reactions that would occur between the hydride and O2/H2O are prevented.”

The researchers from the Institutes of Materials Research and Polymer Research of Helmholtz-Zentrum Geesthacht, respectively, together with colleagues from Deutsches Elektronen-Synchrotron (DESY), the University of Hamburg, and Helmut Schmidt University, believe this is the first time that the polymer, polymethylpentene or TPX™, has been used in a hydrogen storage material.

In the novel composite, TPX is paired up with Mg(NH2)2-nLiH to improve hydride stability. A simple four-step process, in which Mg(NH2)2 and LiH undergo ball-milling before mixing with TPX in solution, is used to synthesize the composite. The composite shows no sign of oxidation reactions after air exposure for 90 minutes, according to the researchers. After 12 hours, although there is some deterioration in hydrogen storage capabilities, the TPX/metal hydride composites perform much better, exhibiting higher storage capacities and stability, than pure metal hydrides.

“Our research solves three major problems,” say Cao and Georgopanos. “Firstly, the metal hydride is protected. Secondly, the final air-stable polymer/hydride composite is safe to handle for short periods of time in normal atmospheric conditions, reducing safety risks. Finally, scale up and production costs of hydrogen storage tanks based on this material will be significantly reduced.”

The researchers believe that these attributes render the TPX/metal hydride composite potentially attractive for future applications.

“This composite is quite promising for real hydrogen storage applications because of its excellent cyclability, better handling, and lower production costs,” point out Cao and Georgopanos. “After fabrication, the composite can be easily transferred and packed into hydrogen tanks without the need of an inert atmosphere.”