BY AMY MAST
A multidisciplinary University of Michigan research group exploring more efficient materials for hydrogen fuel cells has been awarded a $1.2 million Department of Energy (DOE) grant aimed at isolating and developing “best-in-class” hydrogen storage technology.
The project investigates man-made compounds, called metal-organic frameworks, or MOFs (pictured). MOFs are constructed from metal ions and organic molecules (called linkers), and are being researched for many possible applications. These structures act like sponges; their porosity and high surface area make them a potentially highly efficient way to capture and store molecular species such as CO2 or gaseous fuels such as hydrogen. The pores in most MOFs are about a nanometer in diameter (for scale, a sheet of copy paper is 100,000 nanometers thick).
Finding MOFs that can store hydrogen at high densities could eventually pave the way for a market-ready hydrogen powered-vehicle; such a car’s only emission would be water. The right MOF, in concert with many other research innovations, would make a hydrogen-powered vehicle’s user experience similar to a traditionally powered one. Drivers would be able to visit a station and pump hydrogen into a MOF-filled tank; when the driver hits the accelerator, hydrogen gas released from the tank would power a fuel cell. An important goal is to achieve driving ranges that are comparable to today’s conventional, gasoline-powered vehicles.
The UM team’s efforts to develop such road-ready materials began in 2012. That first phase of the project required a cooperative team including a computer scientist (Mike Cafarella), a chemist (Antek Wong-Foy), a mechanical engineer (ME Assistant Professor Don Siegel), and postdoc Jacob Goldsmith. Most known MOFs are included in a larger database of known organic crystalline materials called the Cambridge Structure Database (CSD). The CSD contains over 600,000 entries, most of which aren’t MOFs and aren’t relevant for hydrogen storage. So Goldsmith wrote a program that combed the CSD for MOFs with the best possible combination of promising characteristics for hydrogen storage.
Within about a year, Dr. Goldsmith’s routines identified about 4,000 MOFs, and the team began further studying the characteristics of each until just a handful of promising MOFs remained. The exploratory research described above was initiated with the aid of a $40,000 seed grant from the University of Michigan’s Energy Institute’s Partnerships for Innovation in Sustainable Energy Technology (PISET) program. Additional support came from the DOE Hydrogen Storage Engineering Center of Excellence, which has sponsored Siegel’s hydrogen-related research since 2009.
The next phase of the project will be funded by the DOE Fuel Cell Technologies office, and aims to synthesize and more completely characterize the promising MOFs identified earlier. Moving forward, the team will maintain its multi-disciplinary nature, and include new participants from the U-M Department of Chemistry (Adam Matzger) and Ford Motor Company. The group aims to explore MOFs that, at least on paper, could meet the DOE’s 2017 targets for hydrogen storage systems. “We have identified interesting materials that in some cases have been totally overlooked in the search for compounds that store gases,” said Don Siegel, the project’s Principal Investigator and an Assistant Professor of Mechanical Engineering.
To synthesize the targeted MOFs, team members first combine the relevant metals and linkers in a solvent; the MOF then precipitates out of solution and must be “activated” to remove the solvent and make it amenable to gas adsorption. It’s a delicate, often frustrating process; the more surface area a MOF has, the more generally delicate it is. Some of the most promising MOFs simply fall apart once the solvent is removed. The group is working together on experimental and computational techniques to address these and other limitations. “There will be a feedback loop between computation and experiments as we explore these leads – experimental data will be used to refine the computational models, which will result in increasingly accurate suggestions for promising compounds,” Siegel said.
The group’s MOF database is publicly accessible at http://esms-lab.engin.umich.edu/MOF_Search_Query.php.