It is a real pleasure to introduce this scientific and technical capsule by highlighting the top-end results achieved recently at the Institut National de la Recherche Scientifique - Énergie, Matériaux et Télécommunications, and published earlier this year in Science under the title “Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells” (M. Lefèvre, E. Proietti, F. Jaouen and J.-P. Dodelet, Science 324 (2009), 71). The international research team, led by French Professor Jean-Pol Dodelet, has been involved in the development of non-platinum, iron-based catalysts for PEM fuel cells at INRS-EMT near Montreal, Canada, for many years. Further evidence is given today that consistent efforts and talent are paying back.
Michel Lefèvre, Frédéric Jaouen and their colleagues have successfully synthesized iron-nitrogen catalysts exhibiting an activity for the cathodic oxygen reduction reaction (ORR) that has proved quite comparable with that of commercial platinum supported on carbon black. As in the case of platinum and platinum alloys, the compound’s microstructure is of utmost importance regarding the final activity: therefore, work has focused toward both a fundamental understanding of the relation between microstructure and activity, and the optimization of the catalyst’s preparation based on this knowledge. The overall rate of ORR reaction, i.e. volumetric activity of the Fe-N compound was eventually improved more than 35 times compared with the previous best non-precious metal catalyst (and within 10% of the best Pt-based catalysts to date), by increasing the number of active sites for ORR per unit volume thanks to proper choice of the reactants, reaction method and subsequent thermal conditions. Results are very close to the Department of Energy’s 2010 technical target for PEM fuel cells’ non-precious metal catalyst activity.
Prior to this work, the team had achieved promising ORR activities with iron-based catalysts synthesized by impregnation of microporous carbon black with a soluble Fe precursor (iron acetate, for example) and nitrogen-containing complexing agents like phenanthroline. Hat treatment in ammonia vs. neutral atmosphere was also investigated. It was hypothesized that the coordinating bridges between Fe and the C-N-C bounds formed in the carbon black micropores during heat treatment in NH3 were responsible for the catalytic activity. Results would also show that only micropores created during the heat treatment in ammonia might host active sites, and not pristine micropores present in carbon black material. In the current work, researchers at INRS have filled the pores with a Fe-N containing material (including the acetate Fe(II) precursor and a suitable N-bearing complexing agent) by using planetary ball-milling instead of impregnation. Due to lower thermodynamic limitations, ball-milling allows filling more pores than does close-to-equilibrium impregnation while hardly affecting carbon’s microstructure. The modified carbon black support was then pyrolised in argon and/or ammonia atmosphere. The most active catalyst contained 1 wt% of Fe content. At 0.9 V, the current density of a cathode made with a catalyst loading of 5.3 mg/cm², i.e., 90 µg/cm² of Fe, is 30-40 mA/cm², which is equivalent to a commercial Pt-based cathode with a Pt loading of 400 µg/cm². Losses at current densities > 100 mA/cm² arise from excessive mass transport limitations due to electrode thickness at high Fe loadings.
Being able to replace platinum by iron at the cathode of a PEM fuel cell means that, since most of the platinum loading is currently required for the sluggish ORR and iron is the cheapest among metals the related cost of the catalyst could drop in a terrific way. Maybe a major technical breakthrough in the world of fuel cells! I hope to hear follow-on news from this high-profile research field very soon… Durability tests and further optimization of the density of Fe active sites in the carbon support are still necessary. The complete set of results is available in the Science paper Science 324 (2009), 71. Dr.
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