The desired high power density from polymer electrolyte membrane fuel cells (PEMFCs) can only be achieved by speeding up the otherwise slow reaction steps at their low operating temperatures (80 °C) through catalysis. For the oxygen-reduction reaction (ORR), non–precious metal catalysts (NPMCs), which are potentially less expensive and more abundant, have been outperformed by Pt-based catalysts, which exhibit high activity as the native metal. For metals such as Co and Fe, improved performance will require a robust method for increasing the reactivity of the metal ion through ligation.

...Our present approach, which introduces a new material (pore filler) and replaces impregnation with planetary ball-milling, has elevated the catalytic activity of an Fe-based NPMC by a factor of >35 relative to the previous best reported activity for Fe-based catalysts (and within 10% of the best Pt-based catalysts). Furthermore, these NPMCs may also present opportunities for ORR in direct alcohol, formic acid, and alkaline fuel cells.

—Lefèvre et al. (2009)

Iron-based catalysts for the oxygen-reduction reaction in polymer electrolyte membrane fuel cells have been too sluggish compared to platinum catalysts, in part because they have a comparatively low number of active sites per unit volume.

Rather than the more conventional wet-impregnation method to load iron into carbon supports, the INRS team used an intense dry-mixing method to react carbon, ferrous acetate, and phenanthroline, and then subjected the product to heat treatments and subsequent reaction with ammonia to produce microporous carbon–supported iron-based catalysts with active sites believed to contain iron cations coordinated by pyridinic nitrogen functionalities in the interstices of graphitic sheets within the micropores.

The researchers found that the greatest increase in site density was obtained when a mixture of carbon support, phenanthroline, and ferrous acetate was ball-milled and then pyrolyzed twice, first in argon, then in ammonia.

Longevity is an issue; after extended use in a fuel cell, the iron catalyst’s high initial activity decreases significantly.

The best NPMC in this work has a much higher initial activity, but less stability, than those prepared by Bashyam and Zelenay according to a nonpyrolytic method based on a cobalt salt and polypyrrole deposited on carbon black. Continued research must now focus on improving the stability of these NPMCs and optimizing their cathode mass-transport properties.

—Lefèvre et al. (2009)

In a commentary appearing in the Science issue, Hubert A. Gasteiger of MIT and M. Markovic of Argonne National Laboratory note that:

Little is currently known about the stability of novel non-precious metal oxygen reduction catalysts and their ability to survive in hostile electrochemical environments. Thus, the next challenge for non-precious metal catalysts will be to overcome the twofold loss in current density at constant potential over 100 hours reported by Lefèvre et al., either related to electrode design or to the catalyst itself. Little is known about the molecular processes that might lead to Fe-N bond breaking, and our insight into the stability of the active sites is based only on trial and error. However, recent durability data on Co/N/C-based catalysts are promising, and the fact that nitrogen-coordinated transition metals are used successfully by nature gives hope that detailed studies will lead to more durable catalysts.

Four years ago, neither Fe/N/C-based catalysts with Pt/C-like turnover frequencies, nor de-alloyed PtM catalysts with fourfold higher mass activity than Pt/C, nor surfaces with 100-fold higher turnover frequencies than Pt/C were known. Despite a number of remaining challenges, these recent successes bring us closer to completing our quest to put PEM fuel cell technology on the road.

Resources

• Michel Lefèvre, Eric Proietti, Frédéric Jaouen, and Jean-Pol Dodelet (2009) Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. Science 324 (5923), 71. doi: 10.1126/science.1170051

• Hubert A. Gasteiger and Nenad M Markovic (2009) Just a Dream—or Future Reality? Science Vol. 324. no. 5923, pp. 48 - 49 doi: 10.1126/science.1172083

• Mitch Jacoby, “Low-Cost Iron For Fuel Cells”, Chemical and Engineering News, 6 April 2009, Vol. 87, No. 14, p8.