A team at Northwestern University is developing a material so porous that if you were able to unfold a gram of it, you could go from end zone to end zone.

And farther, in fact.

"Think of it this way," Northwestern Professor Omar Farha told Tech Briefs. "We can fit the area of 1.3 football fields into a sample with the mass of a single M&M."

Such a sweet, adsorbent material enables the onboard storage of hydrogen and methane -- an important capability for next-generation vehicles.

The new materials also could be a breakthrough for the gas storage industry at large, said Farha, lead researcher on the sponge project, because many industries and applications require the use of compressed gases.

The ultraporous structure is a metal-organic framework called NU-1501. The "MOF," built from self-assembling organic molecules and metal ions, form multidimensional, highly crystalline, porous frameworks. Almost like Tinkertoys , says Farha, the metal clusters act as nodes, and the organic molecules hold the nodes together.

The U.S. Department of Energy has made targets for developing the next generation of clean energy automobiles.

In an edited interview with Tech Briefs below, Prof. Farha explains how his team's new adsorbent materials can help scientists and engineers reach those goals.

Tech Briefs: How do you envision this kind of material being used to support next-gen clean-energy vehicles?

Prof. Omar Farha: The transportation, storage, and operations of hydrogen-powered vehicles require high pressure compression (i.e., 700 bar), which is both costly and unsafe. They construct the tanks on these vehicles from expensive carbon fiber-reinforced composite vessels. Using adsorbent materials can reduce the compression pressure while maintaining the maximum deliverable capacities of the fuel.

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Both the size and weight requirements for the on-board fuel tank are important design parameters to make these vehicles one step closer to large scale commercialization. Therefore, it is crucial to optimize both the volumetric and gravimetric deliverable capacities of the fuel itself within adsorbent materials as concurrent objectives rather than separate ones.

The metal-organic framework (MOF) adsorbent materials we developed balance both volumetric and gravimetric uptakes for hydrogen and methane, which pave the way as future storage materials for clean energy vehicles.

Tech Briefs: Any other possible applications?

Prof. Omar Farha: Another application of porous materials that we find exciting is the use of metal-organic framework composite materials to destroy deadly nerve agents, a class of chemical warfare agents.

Tech Briefs: Why is a porous material so important for hydrogen storage?

Prof. Omar Farha: Porosity in adsorbents like MOFs is crucial for gas storage because the more porous the material, the more available space there is to accommodate gas molecules.

Tech Briefs: How is the material able to achieve a kind of maximum porosity?

Prof. Omar Farha: We synthesized the NU-1501 material from metal trimers composed of aluminum and iron, both of which are inexpensive and abundant materials. The underlying topology and orientation of the metal node to the organic linkers permit these MOFs to be highly porous. Relative to other ultra-porous MOFs, NU-1501-Al exhibits high gravimetric and volumetric surface areas, rather than just one or the other, which is rare.

Tech Briefs: What inspired you to go with a metal-organic framework to achieve this?

Prof. Omar Farha: In this new study, we built upon our initial work synthesizing a MOF called NU-1500 , which is based on a designable network. We constructed NU-1500 from 6-connected organic linkers and metal trimers of iron, aluminum, chromium, or scandium. These initial materials exhibited good volumetric gas sorption properties; however, they had relatively small pore sizes and pore volumes, limiting their gravimetric performance. It was the relationship between the volumetric uptake and gravimetric uptake in these materials that motivated us to reach out to collaborators who could computationally model the structure-property relationships of similar MOFs. They modeled the gas sorption behavior of many MOFs with similar topologies, pore sizes, and organic linkers.

From their work, we identified a new type of MOF not previously synthesized that exhibited ideal gravimetric and volumetric gas storage performance for methane and hydrogen. We then synthesized these MOFs in the laboratory and measured their methane and hydrogen gas storage capacities under various conditions.

Tech Briefs: How can these kind of adsorbent materials help to relieve some of the danger and costs associate with hydrogen storage?

Prof. Omar Farha: The current technology in hydrogen-powered vehicles requires high pressure compression (700 bar or 10,000 Psi for H2) to operate. This pressure is 300 times greater than the pressure in car tires. It is expensive to accomplish this task because of hydrogen’s low density, and it can be unsafe because it is also highly flammable. Developing new adsorbent materials which can store hydrogen gas onboard vehicles at much lower pressures can help us reach Department of Energy targets for developing the next generation of clean energy automobiles. To meet these goals, it is crucial that engineers optimize both the size and weight of the on-board fuel tank.

Tech Briefs: What needs to happen for hydrogen-powered vehicles to be used in a mainstream way?

Prof. Omar Farha: The highly porous materials in this study balance both the volumetric (size) and gravimetric (mass) deliverable capacities of hydrogen and methane and bring us one step closer to attaining these targets. We can store tremendous amounts of hydrogen within the pores of the MOFs and deliver them to the engine of the vehicle at lower pressures than needed for current fuel cell vehicles. To put the surface areas of the materials we designed here at Northwestern in perspective, think of it this way: we can fit the area of 1.3 football fields into a sample with the mass of a single M&M. These MOFs can therefore store more hydrogen than conventional adsorbent materials at much safer pressures and at much lower costs.

A one-gram sample of the Northwestern material (with a volume of six M&Ms) has a surface area that would cover 1.3 football fields. (Credit: Northwestern University)

I think research investigating the cost of large-scale synthesis, long-term utility of these materials, and vehicle operation efficiency over time are important for the future general implementation of these materials into hydrogen-powered vehicles.

Tech Briefs: What’s next? What are you working on now?

Prof. Omar Farha: We are focusing our efforts on developing better materials for hydrogen storage. In particular, we are investigating materials with greater volumetric capacity at cryogenic conditions and exploring materials with higher capacities for storing hydrogen at room temperature.

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