Gases are a 500-billion-dollar input to nearly every sector of the global economy. However, separating, transporting, and storing gases requires energy-intensive compression that accounts for nearly 10% of global energy consumption. With this in mind, Omar Farha, chemist and research professor at Northwestern University and president and CSO of NuMat Technologies, has dedicated his career to creating new frontiers in the area of gas compression and storage technologies.
The Institute for Sustainability and Energy at Northwestern (ISEN) recently sat down with Professor Farha to learn a bit more about the ways in which his research is changing the face of sustainability and energy.
ISEN: How did you end up at Northwestern?
Before coming to Northwestern, I did my graduate work at UCLA making redox and bioactive materials. I then came here to work for Professor Joseph Hupp, who was the chair of the Department of Chemistry at the time. As a postdoc, I was really looking for a creative space to explore my own ideas. Professor Hupp and Northwestern gave me that opportunity.
After I arrived, I quickly started working on porous materials—specifically for gas storage. At the time, there were a lot of blind spots in the field, but it was taboo for scientists to talk about them. Not only did I want to talk about these problems, but I wanted to look for solutions. During my postdoc, my colleagues and I developed a method for repurposing an old technology, and that method is now being used by every professor in the field of porous materials in labs around the world.
Why did you specifically choose to work in the field of energy?
I wanted to do something in my lifetime that would make a difference. I wanted to leave something behind for my kids and future generations. The energy sector needs a lot of help. It’s an area that has the potential to move in a very positive direction or a very negative direction. How are we going to decrease our carbon footprint? How can we bring new energy storage technologies into the field? Not only are these questions incredibly relevant, but I also like trying to answer them.
Tell me more about your research. What makes it unique?
Our lab deals a lot with materials known as metal-organic frameworks (MOFs), which are tiny, porous, crystalline structures with extremely high internal surface areas. They’re great for storing gas. When you make a porous material like a MOF, you need “guest molecules”—typically in the form of a solvent—to help create the pores in the solid structure. When the guest molecules are removed from the material, they leave behind “holes” in the solid structure. In the past, scientists would try to extract the solvent molecules by using heat and a vacuum, but that can create surface tension and may cause the solid structures to collapse when the solvent is removed. At Northwestern, we tried something different. We replaced the organic solvent liquid with liquid carbon dioxide. When you heat up liquid CO2 slightly above room temperature, it can be removed from the pores easily without creating that negative pressure inside the cavity. What you’re left with is a MOF with a solid structure and pores completely intact. Scientists have been talking theoretically about making these types of materials since the ‘90s, but we’re able to successfully and efficiently do it. And now we’re creating MOFs with unprecedented surface areas.
What do MOFs have to do with energy and gas storage?
To understand MOFs, you have to understand how gas molecules behave. Let’s look at a simplified example. On a basic level, gas molecules are attracted to surfaces. When a balloon is filled with helium, the helium molecules move to the balloon’s wall, while the majority of the space inside remains relatively empty. But if this empty space contained additional surfaces for the helium molecules to attach to, the balloon could absorb more of the gas.
What we’re doing with MOFs is “filling the empty space” with a porous material. The fact that MOFs have such large surface areas means the gas molecules have more surfaces to interact with. The gases essentially fill the holes in the MOF. It's really remarkable to witness the storage capacity of these materials.
What sort of storage capacity do MOFs typically have?
Several years ago, we developed a MOF called NU-100, which still holds the record for hydrogen storage efficiency. Right now, this material has an excess storage capacity of 99.5 milligrams of hydrogen per one gram of the MOF. In other words, one gram of NU-100 has an internal surface area exceeding the surface area of a soccer field.
In fact, at Northwestern we’ve developed the most porous materials to-date: NU-109 and NU-110. NU-110 has a surface area of 7,000 square meters per gram, which means that if we “unfolded” the molecules of a single gram of this material, it would cover an area bigger than one and-a-half football fields. That’s significant because it means the gas molecules have a lot of surface to interact within a very condensed space.
What practical uses are there for this work?
We’ve done a lot of work with hydrogen, which is really the Holy Grail in terms of gas storage, since the only byproduct of hydrogen fuel cells is water. It’s completely free of greenhouse gases. The Argonne-Northwestern Solar Energy Research Center (ANSER) is using sunlight to split water molecules and get hydrogen, which is fantastic. But at the end of the day, you still need an efficient storage material once the gas is produced.
Let’s look at the case of hydrogen cars. Imagine using ANSER’s work to make fuel from water, and the only emission coming out of your car’s tailpipe is water. The idea isn’t too farfetched. Toyota and other companies have shown that hydrogen fuel cell cars work. But because they don’t currently use an adsorbent material [note: "adsorption" is a process by which a gas adheres itself to the surface of a material], the car storage tanks are large, and the gas is maintained at extremely high pressure. The hope is that we can use porous materials like MOFs to store the gas at one-fifth or one-seventh the amount of pressure in much more conformal tanks.
The same is true for natural gas storage. CNG [“compressed natural gas”] cars exist, but I want us to move to ANG—“adsorbed natural gas” cars—that use MOFs to store gas much more efficiently and at much lower pressure.
The power of this technology goes beyond cars. Right now, they transport natural gas as liquefied natural gas. To get the gas to a liquid state, you have to cool and compress it which costs a lot of money and is very energy-intensive. The question is, can these MOF materials circumvent this liquification process? Can gas be adsorbed in one location, transported, and then desorbed in another location? This would save money and energy. This is a huge breakthrough considering natural gas could be a really important bridge to clean energy in the United States.
Are there applications for this technology beyond the energy sector?
Definitely! In addition to simply storing gases, MOFs can capture and help filter them. We’re currently working on a project with the U.S. Department of Defense (DOD) that involves capturing and disabling nerve agents for human protection. We’ve developed a MOF that captures nerve agent molecules and is also a catalyst that disables their harmful properties. When the DOD finds nerve gases in a warehouse that they want to dispose of in a safe, environmentally-friendly manner, we hope that our materials can help in that regard. We’re also working on formulating the catalyst to be used inside gas masks and fabrics. We’re even working on stabilizing enzymes that hopefully one day can be used as a potential antidote to nerve gas exposure.
This all sounds promising. What sorts of barriers are there to getting these discoveries more widely adopted?
Aside from public policies, such as those that encourage consumers to purchase hydrogen fuel cell cars, there are also technical barriers to commercialization. First, it’s a question of maturity—is the material stable, and does it work? Second, is it scalable? Third, how much does it is cost? Not everyone can afford to be an early adopter, but if we can produce a car that utilizes hydrogen and costs about the same as a gasoline-powered car but with zero emissions, most consumers would be onboard with that.
At the end of the day, we are making very complex materials to do complex tasks but with very simple components. This makes it affordable, scalable, and cost effective—regardless of what sector you’re working in.
Where do you find inspiration for your work, and where do you see it going?
I always tell our students and colleagues: “We shouldn’t make something just because it’s cute.” In other words, unless we’re making a material that’s solving a problem and that has practical application, I’m not interested in it. As a scientist, I’m interested in basic research, but I’m also interested in advancing our knowledge to make the world better for everyone. That’s why I co-founded NuMat.
I’m lucky to be at Northwestern. I’m surrounded by colleagues who are doing beautiful and inspiring work. We collaborate with engineers, chemists, biologists, material scientists, and so on. It’s a very unique place without boundaries.
