The only technologically viable method to store hydrogen is to liquefy it. Unfortunately, this cannot be scaled to match the enormous demands of a hydrogen economy, principally because hydrogen requires impractical cryogenic temperatures for liquefaction. One major alternative is to store hydrogen within metal-organic framework materials (MOFs). These are formed by organic molecules that symmetrically align around a metal ion resulting in crystalline materials with large cavities that adsorb gases. One of the first of these materials to be studied for hydrogen storage is MOF-5, which is built around zinc ions and benzodicarboxylate ions. Now, a group led by Matthew R. Hartings at American University has made a major technological leap in the processing of these materials by three-dimensionally (3D) printing a composite of acrylonitrile butadiene styrene (ABS) and MOF-5.

“An engineer can’t necessarily press a powder into a shape and be able to incorporate that shape into a larger structure or device. MOFs are nice when you are just studying them on their own. But, if we want to deploy them in other technologies, we need to have a way to process them. That’s where the incorporation into 3D printing polymers comes in. Surely, I could have just injection-molded structures from these composites as well. But, 3D printing has the added benefit of dictating the precise geometries that I find interesting and useful,” Hartings says in a communication to MRS.

The group produced a range of composites with MOF mass percentages ranging from 1% to 10%. Larger amounts of MOF led to clumping and undesirable phase separation. The measured hydrogen capacity per gram of printed material was found to be 1.15 times greater for ABS-10% MOF-5 than pure ABS.  Furthermore, the specific H₂ capacity is found to be 6.1 × 10⁻⁴ mass % of H₂ at 60.7 kPa and 23°C by the MOF within the ABS. This value is comparable to the absolute adsorption of H₂ by pure MOF-5 at room temperature and similar H₂ pressure showing that the polymer does not affect the hydrogen retention of MOF-5. The two materials also show similar kinetics—exhibiting diffusion on the order of 10^-4 per sec. This work is published in a recent issue of Polymers Advanced Technologies.

The MOF-5 was found to have undergone degradation due to humidity even before its incorporation into the composite, which may have led to a lowering of its hydrogen capacity. Thus, the results are expected to improve with phase pure MOF-5. Hartings and his research team have already resolved this issue, “Our next paper, which should be out soon, addresses this shortcoming and shows how we have dealt with humidity. In that paper, we show that you can place an ABS-MOF composite in water for a week without the MOF degrading.”

Others in the MOF community have responded positively to this work. Gregory Peterson at the US Army Edgewood Chemical Biological Center says, “The ability to process MOFs into new shapes via additive manufacturing has important implications for a wide range of applications. For example, this method allows us to build composite structures into shapes that are otherwise inaccessible through traditional processing techniques. This may lead to important innovations not only in gas storage, but filtration, sensing, electronics, and other areas.”

Read the abstract in Polymers Advanced Technologies.