Teams at Helmholtz Zentrum Berlin (HZB) and TU Darmstadt have produced a cost-effective fuel-cell catalyst material consisting of iron-nitrogen complexes embedded in tiny islands of graphene only a few nanometers in diameter. The FeN4 centers provide excellent catalytic efficiency, approaching that of platinum.For their synthesis process, they devised a simple and feasible way to reduce the contribution of inorganic metal species in the catalyst material—in some cases even down to zero. The presence of inorganic species interferes with the oxygen reduction reaction (ORR) activity of metal and nitrogen-doped carbon catalysts. A paper on their work is published in the Journal of the American Chemical Society.
|Nano-island of graphene in which iron-nitrogen complexes are embedded. The FeN4 complexes (shown in orange) are catalytically active. Image: S. Fiechter/HZB. Click to enlarge.|
Iron-nitrogen complexes in graphene (known as Fe-N-C catalysts) have been achieving levels of activity comparable to Pt/C catalysts for several years already. However, most approaches for preparing the materials lead to heterogeneous compounds containing various species of iron compounds such as iron carbides or nitrides besides the intended FeN4 centers, said Sebastian Fiechter of HZB.
HZB researchers had already developed a new preparation method to produce an inexpensive catalyst material from organometallic compounds such as iron or cobalt porphyrin. The metal-N-C catalysts developed at HZB held the world record for the highest density of catalytically active centers of various nitro-metallic compounds up to about 2011. However, it remained unclear as to which inorganic compounds influenced the catalytic efficiency.
The highlight in the current work is the purification process (a combination of thermal treatment with a subsequent etching step) by which the proportion of metallic compounds that interfere with catalytic activity can be substantially reduced, even for catalysts that are highly heterogeneous.
Ulrike Kramm, who has since become a junior professor at TU Darmstadt, was successful in purifying several catalysts to such an extent that all the iron present in the graphene layers was exclusively in form of FeN4 complexes. The scientists thereby disproved the hypothesis debated among experts by which improvement in the activity of the FeN4 centres only resulted from promoters, as they are known, such as iron nanoparticles.
|Credit: ACS, Kramm et al. Click to enlarge.|
To check this hypothesis, we employed numerous complex measurement techniques like Mößbauer spectroscopy, electron paramagnetic resonance spectroscopy and X-ray absorption spectroscopy at BESSY II. These enabled us to precisely survey the atomic structure of the catalytic centers.
The purification process enables us now to create catalysts having exclusively FeN4 centers. This allows us to subsequently select compounds to be added afterwards as promoters that further improve the activity level or stability of these catalysts.—Ulrike Kramm
Ulrike I. Kramm, Iris Herrmann-Geppert, Jan Behrends, Klaus Lips, Sebastian Fiechter, and Peter Bogdanoff (2016) “On an Easy Way To Prepare Metal–Nitrogen Doped Carbon with Exclusive Presence of MeN4-type Sites Active for the ORR” Journal of the American Chemical Society 138 (2), 635-640 doi: 10.1021/jacs.5b11015