A team from the University of Buffalo (UB), Purdue University, Oak Ridge National Laboratory (ORNL), Argonne National Laboratory (ANL), the University of Pittsburgh and Carnegie Mellon University (CMU), with colleagues from other institutions, has developed a highly durable and active Fe–N–C catalyst for proton-exchange membrane fuel cells.

The stability improvement demonstrated by the catalyst represents a critical step in developing viable Fe–N–C catalysts to overcome the cost barriers of hydrogen fuel cells for numerous applications. A paper on their work is published in Nature Energy.

Nitrogen-coordinated single atom iron sites (FeN₄) embedded in carbon (Fe–N–C) are the most active platinum group metal-free oxygen reduction catalysts for proton-exchange membrane fuel cells. However, current Fe–N–C catalysts lack sufficient long-term durability and are not yet viable for practical applications. Here we report a highly durable and active Fe–N–C catalyst synthesized using heat treatment with ammonia chloride followed by high-temperature deposition of a thin layer of nitrogen-doped carbon on the catalyst surface.

We propose that catalyst stability is improved by converting defect-rich pyrrolic N-coordinated FeN4 sites into highly stable pyridinic N-coordinated FeN4 sites. The stability enhancement is demonstrated in membrane electrode assemblies using accelerated stress testing and a long-term steady-state test (>300 h at 0.67 V), approaching a typical Pt/C cathode (0.1 mg_Pt cm⁻²).

—Liu et al.

The best fuel cell catalysts currently come from the platinum-group metal family. While efficient and durable, these metals are expensive because they are rare. As a result, scientists are seeking less costly alternatives.

One such alternative has been iron-based catalysts. Iron is appealing because it is abundant and inexpensive. However, it does not perform as well as platinum, especially because it lacks the durability to withstand the highly corrosive and oxidative environments inside fuel cells.

To overcome this barrier, the research team bonded four nitrogen atoms to the iron. Researchers then embedded the material in a few layers of graphene with accurate atomic control of local geometric and chemical structures.

The resulting structure is a vastly improved catalyst. The research team reported the catalyst:

• Is believed to be the most efficient iron-based catalyst produced to date, exceeding the DOE’s 2025 target for electric current density.

• Achieved a durability rating that approaches platinum group catalysts.

All this points to the iron-based catalyst’s potential to make fuel cells, particularly hydrogen fuel cells, much more affordable for commercial use, said lead author Gang Wu, PhD, professor of chemical and biological engineering in the UB School of Engineering and Applied Sciences. Researchers are planning follow-up studies to further improve the catalyst.

The study was supported the US Department of Energy and the US National Science Foundation. UB and Giner have filed joint patent applications naming Wu and two co-inventors.

Resources

• Liu, S., Li, C., Zachman, M.J. et al. (2022) “Atomically dispersed iron sites with a nitrogen–carbon coating as highly active and durable oxygen reduction catalysts for fuel cells.” Nat Energy doi: 10.1038/s41560-022-01062-1