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Autumn 2015

A Catalytic Cycle Built for Two

Harmonious functioning of nitrogen compounds rivals platinum efficiency in metal-free fuel cell

Michelle A. Harris

Depiction of the nitroxyl molecule (left) and nitrogen oxide (right) cycles working together to reduce oxygen and generate water by interacting with each other and the electrode of the fuel cell.

A major facet of cleaner energy sources is the fuel cell. A burgeoning option for cars and other vehicles, fuel cells are more efficient than standard combustion engines and emit only heat and water. The Center for Molecular Electrocatalysis (CME) has developed an innovative fuel cell design that is cheaper and more accessible than previous designs.

In a fuel cell, electricity is produced by hydrogen losing electrons, a process called oxidation, and oxygen gaining electrons, called reduction. Each process by itself is referred to as a half-reaction. An electrode conveys electrons between the two half-reactions thus creating the functioning fuel cell.

Efficient fuel cells use a catalyst to increase the rate of the half-reactions. A platinum catalyst is the standard used in the oxygen reduction half-reaction of fuel cells. Platinum is a highly efficient catalyst, but the high cost and scarcity of platinum limits the availability of fuels cells and drives the search for an efficient Earth-abundant catalyst. Researchers James Gerken and Shannon Stahl used metal-free catalysts composed primarily of nitrogen, specifically, an organic nitroxyl (R2NO) molecule and inorganic nitrogen oxide (NOx) source, to promote oxygen reduction.

They chose these compounds because they are known to work synergistically as redox couples in a well-known aerobic alcohol oxidation cycle. The CME team modified this cycle by introducing an electrode to isolate its reductive half-reaction.  They achieved sustained electrochemical reduction of oxygen at potentials that are higher than many of the best previously reported molecular catalysts.

Similar catalysts have been explored previously in fuel cell applications. A common limiting factor for such catalysts in the oxygen reduction half-reaction is the creation of hydrogen peroxide. Hydrogen peroxide is a caustic chemical that can degrade parts of the fuel cell and limits the efficiency of the half-reaction. These new fuel cell catalysts do not form hydrogen peroxide and show significant improvement in the energy efficiency of the oxygen reduction reaction.

Compared to other similar catalysts, this harmonious pair operates at a lower overpotential. This means that little energy is wasted, allowing the fuel cell to operate more efficiently.

A unique aspect of this approach is that neither the nitroxyl molecule nor the NOx species are effective when used alone, but they work efficiently in tandem. The NOx species reacts rapidly with oxygen. Conversely, the organic nitroxyl does not react with oxygen, but reacts rapidly at the cathode. The nitroxyl/NOx combination represents an effective partnership to convert oxygen to water, after which they cycle back to their original states and begin the process all over again.

The dual cycle for oxygen reduction, shown in the figure, replaces the role of platinum for reducing oxygen in fuel cells. It also opens up the possibility of optimizing the process, for instance, by finding tailorable catalyst partners to optimize the system further.

"This work is important because it shows that simple molecules can nearly match the efficiency of platinum in the oxygen reduction reaction. The next step is to show that such catalysts exhibit good performance in fuel cells," said Stahl.

By replacing platinum with low-cost molecules, this study begins to address one of the major barriers for wider application of the fuel cell. This work by the CME points toward more efficient fuel cells that are also much more accessible for clean energy needs.

Acknowledgments

This work was funded by the Center for Molecular Electrocatalysis, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

More Information

Gerken JB and SS Stahl. 2015. "High-Potential Electrocatalytic O2 Reduction with Nitroxyl/NOx Mediators: Implications for Molecular ORR and Aerobic Oxidation Catalysis." ACS Central Science. Article ASAP. DOI: 10.1021/acscentsci.5b00163

About the author(s):

  • Michelle A. Harris is a postdoctoral researcher in the Argonne-Northwestern Solar Energy Research (ANSER) Center at Northwestern University under Michael Wasielewski. Her research involves ultrafast spectroscopic studies of charge transfer in DNA hairpins and in donor-acceptor molecules for solar fuels applications. She received her B.S. in integrative biology from the University of Illinois at Urbana-Champaign in 2009. She did her dissertation research under Dewey Holten in the Photosynthetic Antenna Research Center (PARC) and received her Ph.D. in chemistry from Washington University in St. Louis in 2014.

Two Great Catalysts that Work Great Together

Researchers use metal-free materials to speed the troubling side of fuel cell reaction

Electricity produced without gasoline, diesel, or coal. It could change our nation's energy and security landscape, but not if the new approach uses rare metals. That's part of the problem for fuel cells. The cells can use hydrogen generated at solar and wind stations to produce electricity, heat, and water. No fossil fuels necessary. But, the cells won't work without platinum. At the Center for Molecular Electrocatalysis, scientists are looking for alternatives to the highly efficient, precious metal. They combined two simple, inexpensive, metal-free catalysts to speed a fuel cell's slower reaction. Together, the nitrogen-based catalysts, neither of which is effective alone, take protons left from the hydrogen fuel and oxygen to pump out water. This work provides insight into how reactions could be catalyzed without metals or traditional single catalysts. The center is based at Pacific Northwest National Laboratory.

More Information

Gerken JB and SS Stahl. 2015. "High-Potential Electrocatalytic O2 Reduction with Nitroxyl/NOx Mediators: Implications for Molecular ORR and Aerobic Oxidation Catalysis." ACS Central Science. Article ASAP. DOI: 10.1021/acscentsci.5b00163

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.