Chemists at Cornell University have discovered a class of nonprecious metal derivatives—transition metal nitrides (TMNs)—that can catalyze the oxygen reduction reaction (ORR) in alkaline fuel cells about as well as platinum, at a fraction of the cost.
The researchers, led by Héctor D. Abruña, the Émile M. Chamot Professor in the Department of Chemistry and Chemical Biology in the College of Arts and Sciences, published their findings in an open-access paper in the journal Science Advances.
Hydrogen fuel cells hold great promise for future automotive transportation because of their higher overall energy efficiency and potential zero carbon emissions when compared to internal combustion engines. However, the use of costly platinum (and other precious metals) for accelerating the sluggish oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) has precluded their widespread deployment for electric vehicle applications. Although extensive research efforts have been devoted to minimizing Pt usage and enhancing its intrinsic activity via alloying and nanostructuring, replacing Pt with nonprecious metals or oxides represents a more promising strategy.
Compared to PEMFCs in which the catalyst is estimated to contribute about 40% of the fuel cell stack cost, the primary advantage of anion exchange membrane fuel cells (AEMFCs) is that they enable the use of nonprecious transition metal–based ORR catalysts because of the improved catalyst stability in alkaline electrolytes. In an effort to facilitate the ORR kinetics in alkaline medium, a broad spectrum of nonprecious catalysts has been extensively studied, including metal-nitrogen-carbon, transition metal oxides, and perovskites. In particular, transition metal oxides, especially Co-Mn spinel oxides, have exhibited a power density of over 1 W cm−2 in membrane electrode assemblies (MEAs). Unfortunately, the low intrinsic electrical conductivity of those semiconducting spinel oxides has prevented further improvements in their ORR activity.
An ideal ORR electrocatalyst should have an active surface responsible for catalyzing the ORR process and a conductive bulk to facilitate the charge transfer. Thus, developing conductive nonprecious catalysts represents a viable and attractive approach to circumvent the conductivity challenges and enhance ORR performance.
… Here, we report on a group of nonprecious TMNs as potential ORR catalysts in alkaline medium.
Schematic synthesis procedure of metal nitrides (MxN) supported on high–surface area carbon via nitridation with ammonia. HMT, hexamethylenetetramine. Zeng et al.
A class of compounds derived from cobalt, manganese, iron and other transition metals, TMNs conduct electricity and, when exposed to air, tend to form a thin oxygen-based outer shell that provides a surface for catalyzing chemical reactions. After synthesizing a family of TMNs with conductive nitride cores and reactive oxide shells, the team tested each candidate catalyst in a model hydrogen fuel cell.
Manganese- and iron-based candidates made strong showings. But the cobalt nitride catalyst was “the clear winner,” Abruña said, with near identical efficiency to platinum while costing 475 times less as of 2 February.
The carbon-supported cobalt nitride (Co3N/C) catalyst exhibited a half-wave potential of 0.862 V and achieved a record-high peak power density among reported nitride cathode catalysts of 700 mW cm−2 in alkaline membrane electrode assemblies.
Hydrogen fuel cells are enormously powerful, enabling you to run at an efficiency that simply does not exist for more traditional engines. People recognize that fuel cells are the way to go. The trick is designing stable and affordable catalysts that make it all possible.
Funding for this research was provided in part by the Center for Alkaline Based Energy Solutions, an Energy Frontier Research Center supported by the US Department of Energy.
Rui Zeng et al. (2022) “Nonprecious transition metal nitrides as efficient oxygen reduction electrocatalysts for alkaline fuel cells” Science Advances doi: 10.1126/sciadv.abj1584