Iron-based catalysts for the oxygen reduction reaction (ORR) in PEM fuel cells have been poorly competitive with platinum catalysts, partly because of a comparatively lower number of active sites per unit volume.
The INRS team, led by Professor Jean-Pol Dodelet, produced 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.
They 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 first in argon, then in ammonia. The current density of a cathode made with the best Fe-based electrocatalyst reported can equal that of a Pt-based cathode with a Pt loading of 0.4 mg/cm2 at a cell voltage of 0.9 V.
The new material – described recently in Science, and reported in the MIT Technology Review – produces 99 A/cm3 at 0.8 V, a key measurement of catalytic activity. This is 35 times better than the best non-precious metal catalyst so far, and close to the US Department of Energy’s goal of 130 A/cm3 for fuel cell catalysts. It also matches the performance of typical platinum catalysts, says Dodelet.
The INRS researchers’ key insight was finding a way to increase the number of active catalytic sites within the material – with more sites for chemical reactions, the overall rate of the reactions in the material increases.
In previous work, the researchers had shown that heating carbon black to high temperatures in the presence of ammonia and iron acetate created gaps just a few atoms wide in the carbon. Nitrogen atoms bind to opposite sides of these tiny gaps, and an iron ion bridges these atoms, forming an active site for catalysis.
To increase the number of these sites, the researchers used a commercially available form of carbon that already has a large number of similarly narrow pores. Filling these pores with a nitrogen-and-iron-containing material and then heating the mixture resulted in the much improved reaction rates.
The catalyst is designed to work in PEM fuel cells. The cathode reaction that combines protons and oxygen to produce water is difficult to perform: in conventional fuel cells, Pt is used in both electrodes, but 10 times as much is needed on the water-producing cathode. The new catalyst replaces Pt on the cathode, eliminating almost all of the platinum.