New metal-free catalyst holds promise for clean transportation
UvA chemists discover a cheap and effective replacement for platinum in fuel cells
Researchers from the UvA Research Priority Area Sustainable Chemistry have developed a new material that can replace platinum in the oxygen reduction cathode of fuel cells. Their nitrogen-doped carbon catalyst can be made easily on multi-gram scale, using cheap and readily available materials. The results, which offer opportunities for clean future transportation, have recently been published as a communication in 'Chemistry A European Journal'.
Increasing the energy efficiency of engines is the most straightforward solution to today’s transportation challenges. Cars running on proton-exchange membrane fuel cells (PEMFCs) are promising because fuel cells are much more efficient than internal combustion engines (80% vs. ca. 30% energy efficiency, respectively) and do not produce polluting exhaust fumes.
Fuel cells convert fuel directly to electricity in a catalytic process. They have no moving parts and waste very little energy as heat. Moreover, unlike batteries, fuel cells take their fuel from external tanks, making them more compatible with the existing logistics infrastructure.
Not enough platinum
Yet fuel cells also have some practical drawbacks: Although the oxidation of the fuel at the anode is usually fast, the reduction of oxygen at the cathode is a slow reaction. Most of today’s fuel cells use cathodes based on platinum catalysts. Platinum cathodes work nicely, but there is a catch: there is simply not enough platinum to go around. The car industry already consumes over 60% of the annual world platinum supply for use in catalytic converters that remove pollutants from the exhaust fumes. Fuel-cell-powered cars would require ten times as much platinum, which is simply not available.
Thus, any practical future scenario must include the replacement of platinum with a more abundant (and cheaper!) alternative. One exciting alternative to platinum is using nitrogen-doped carbons. These nitrogen-graphite materials contain no metals at all; yet in some cases reduce oxygen as efficiently as platinum. Their synthesis, however, is difficult, and usually done in milligram quantities only.
No platinum and simple synthesis
Now, Dr David Eisenberg from the Research Priority Area Sustainable Chemistry has discovered a new nanostructured nitrogen-doped carbon that can replace platinum in oxygen reduction cathodes. Unlike other nitrogen-doped carbons, this catalyst can be made easily on multi-gram scale, using cheap and readily available materials.
Together with Dr. Ning Yan - who leads the Fuel Cells initiative of the RPA - and project student Wowa Stroek the team developed the new material in collaboration with chemists from the University of Texas in Austin. They discovered that its special multi-walled graphite structure (see micrograph, left) is responsible for the high activity. Surface porosity analysis by technician Norbert Geels revealed that this carbon contains structured micropores, mesopores and macropores. The latter were measured using ultra-high-pressure mercury porosimetry, a technique available in only a handful of labs in the Netherlands.
The new nitrogen-doped carbon, obtained via the sequential pyrolysis and acid-washing of magnesium nitrilotriacetate, (HMgN(CH 2COO) 3), has unique properties. The hierarchical micro/meso/macro porosity, lined with highly graphitic multi-walled shells, offers good mass transport as well as improved electronic conductivity.
Prof. Gadi Rothenberg of the Research Priority Area Sustainable Chemistry presented the remarkable results in a Keynote Lecture at the 1st International Symposium on Chemistry Energy & Materials in Shanghai. By placing the full synthesis protocol in the public domain, the UvA team encourages other scientists to follow-up on this discovery, which opens exciting avenues not only for fuel cell catalysis but also metal-air batteries and biological sensors.
A Simple Synthesis of an N-Doped Carbon ORR Catalyst: Hierarchical Micro/Meso/Macro Porosity and Graphitic Shells, D. Eisenberg, W. Stroek, N.J. Geels, C.S. Sandu, A. Heller, N. Yan and G. Rothenberg, Chem. Eur. J., 2015, published online, DOI: 10.1002/chem.201504568