A research team from the Technical University of Denmark (DTU) has discovered a new type of iron and carbon-based catalyst, which is stable and active in both acidic and alkaline media, and may even eliminate the need for platinum in catalysts and thus revolutionize the proton exchange membrane (PEM) fuel cell industry.
A Proton Exchange Membrane Fuel Cell (PEMFC) transforms hydrogen and oxygen into electricity with heat and water as byproducts. PEMFCs are energy efficient as well as environmentally friendly and they are among other things used for energy production in hydrogen cars, in backup generators and for the production heat and energy for private households. However, the PEM technology still hasn’t gained a commercial breakthrough yet, primarily due to cost: catalysts in electrodes are made of platinum, a rare and very expensive precious metal. The recent discovery may lead to the substitution of platinum with a composite of iron carbide and carbon, making the PEMFCs more commercially attractive.
“Many good catalysts oxidize and dissolve in the acid environment of a PEM fuel cell. The same should happen to catalysts of cementite, also known as iron carbide (Fe3C), but in our case the iron carbide is protected by a thin layer of graphene, a very stable form of carbon,” says associate professor Jens Oluf Jensen, Head of PEM research at DTU Energy Conversion.
Graphene has an extremely simple crystalline structure that repeats itself in a hexagonal two-dimensional grid pattern like chicken wire, and just like chicken wire the graphene structure can be rolled up; in this case the hollow tubes of the graphene are called carbon nanotubes.
Carbon nanotubes are not an unknown phenomenon. In fact long strands of nanotubes often appears in several chemical reactions where they are undesirable, as they prevent the chemical reactions and are hard to get rid of. Strands of carbon nanotubes also appear when fossil fuels are converted to hydrogen.
The Danish research team discovered that instead of being a troublesome nuisance the carbon nanotubes can in fact be used constructively to enclose a catalyst of iron carbide as long as the carbon layer is very thin. By doing this, the nanotubes actively protect the catalyst from the surrounding acid in the electrolyte which should dissolve it, while allowing the reactions of the catalyst to take place without hindrance through the “grid”.
Chemically, the iron carbide catalyst is composed of hollow microspheres consisting of uniform Fe3C nanoparticles encapsulated by the thin layer of graphene. The graphene layers stabilize the metallic nanoparticles without depriving them of their catalytic activity, resulting in the catalyst exhibiting high activity and stability in both acidic and alkaline electrolytes.
The process is described by the team consisting of visiting Chinese researcher Yang Hu, Jens Oluf Jensen, Wei Zhang, Lars N. Cleemann, Wei Xing, Niels J. Bjerrum and Qingfeng Li in the latest issue of the respected scientific journal Angewandte Chemie (see footnote).
Associate professor Jens Oluf Jensenexplains how it was a spontaneous and quite unexpected reaction in another study conducted by guest researcher Yang Hu and associate professor of the department Qingfeng Li, which led them on to this new way to synthesize the catalyst. The discovery was followed up by a number of test runs supported by the Danish Council for Strategic Research (4M Centre), the Danish National Research Foundation (PROCON) and the ForskEL research program (CatBooster) where the encapsulated catalysts of iron carbide were submerged in an 85°C hot acid bath for a longer period. The iron carbide was not dissolved, and experiments have shown that the catalyst activity is not affected in alkaline environments neither as long as the catalysts are protected by the nanotubes.
“We still need to evaluate the new type of catalysts into a real fuel cell and test it for a longer time, which we intend to do during 2014, but we have started the process of patenting the method, and we have several more scientific papers on the way,” says Associate Professor Qingfeng Li.
Hu, Y., Jensen, J., Zhang, W., Cleemann, L., Xing, W., Bjerrum, N., & Li, Q. (2014). Inside Back Cover: Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic Layers as Oxygen Reduction Catalysts (Angew. Chem. Int. Ed. 14/2014) Angewandte Chemie International Edition, 53 (14), 3749-3749 DOI: 10.1002/anie.201401098