A multi-institutional research team led by materials scientists from Pacific Northwest National Laboratory (PNNL) has designed a highly active and durable catalyst that doesn’t rely on costly platinum group metals (PGM) to spur the necessary chemical reaction.
The new catalyst contains cobalt interspersed with nitrogen and carbon. When compared to a similarly structured catalyst made from iron—another promising, well-studied platinum substitute—the team found that the cobalt catalyst achieved a similar reaction but with four times the durability. The research is published in Nature Catalysis.
The development of catalysts free of platinum-group metals and with both a high activity and durability for the oxygen reduction reaction in proton exchange membrane fuel cells is a grand challenge. Here we report an atomically dispersed Co and N co-doped carbon (Co–N–C) catalyst with a high catalytic oxygen reduction reaction activity comparable to that of a similarly synthesized Fe–N–C catalyst but with a four-time enhanced durability.
The Co–N–C catalyst achieved a current density of 0.022 A cm−2 at 0.9 ViR-free (internal resistance-compensated voltage) and peak power density of 0.64 W cm−2 in 1.0 bar H2/O2 fuel cells, higher than that of non-iron platinum-group-metal-free catalysts reported in the literature.
Importantly, we identified two main degradation mechanisms for metal (M)–N–C catalysts: catalyst oxidation by radicals and active-site demetallation. The enhanced durability of Co–N–C relative to Fe–N–C is attributed to the lower activity of Co ions for Fenton reactions that produce radicals from the main oxygen reduction reaction by-product, H2O2, and the significantly enhanced resistance to demetallation of Co–N–C.—Xie et al.
Proton exchange membrane—or PEM—fuel cells are typically envisioned to be paired with hydrogen for multiple applications across different sectors, including transportation, stationary and backup power, metals manufacturing, and more. These highly efficient, clean energy conversion devices require very active catalysts for the chemical reaction—the oxygen reduction reaction, or the “lifeblood” that makes a fuel cell efficiently function.
Platinum group metals serve as the most productive catalyst material for PEM fuel cells, but they account for about half of the fuel cell cost.
Scientists are studying transition metals such as iron as a promising alternative to platinum, but they have found that they quickly degrade in the acidic PEM fuel cell environment.
Cobalt is a transition metal that is—relative to platinum—inexpensive and abundant. Previous studies had shown that cobalt is far less active than iron-based catalysts.
We knew that the configuration of cobalt with nitrogen and carbon was key to how effectively the catalyst reacts and that the active site density was critically important for performance. Our goal was to really improve the reaction activity of cobalt-based catalysts.PNNL materials scientist Yuyan Shao, who led the study
The team immobilized cobalt-based molecules in the micropores of zeolitic imidazolate frameworks, which served as protective fences to decrease the cobalt atoms’ mobility and prevent them from clustering together. They then used high-temperature pyrolysis to convert the atoms to catalytically active sites within the framework.
A two-step encapsulation and ligand-exchange approach effectively introduces CoN4 complexes into the ZIF-8 micropores. Subsequent one-step pyrolysis produces atomic CoN4 sites dispersed into porous carbon. Xie et al.
Within this structure, they discovered that the density of the active sites significantly increased, in turn increasing the reaction activity. This, in fact, achieved the highest activity in fuel cells reported for non-iron, platinum group metal-free catalysts to date.
The team also found the cobalt-based catalyst to be much more durable than the iron-based catalyst synthesized using the same approach. They discovered, for the first time, significant differences in demetallation, where metal ions are leached out of the catalyst and that catalyst then loses activity. They also found that oxygen radicals from hydrogen peroxide, a byproduct of oxygen reduction in fuel cells, attack the catalysts and cause performance loss.
In the end, we were able to not only improve the activity of the cobalt-based catalyst, but we significantly improved the durability. Our further investigation led us to discover the mechanisms that typically degrade these types of catalysts.—Yuyan Shao
Along with PNNL, researchers from Washington University in St. Louis; Argonne National Laboratory; Los Alamos National Laboratory; Oak Ridge National Laboratory; University at Buffalo, The State University of New York; the University of Pittsburgh; and Northern Illinois University collaborated on the research.
The study is supported by the US Department of Energy’s (DOE's) Hydrogen and Fuel Cell Technologies Office through the Electrocatalysis Consortium, or ElectroCat. The scientific team used capabilities at three DOE Office of Science user facilities: The Environmental Molecular Sciences Laboratory at PNNL, the Advanced Photon Source at Argonne National Laboratory, and the Center for Nanophase Materials at Oak Ridge National Laboratory—along with the University of Pittsburgh Center for Research Computing and the Extreme Science and Engineering Discovery Environment, supported by the National Science Foundation.
Xie, X., He, C., Li, B. et al. “Performance enhancement and degradation mechanism identification of a single-atom Co–N–C catalyst for proton exchange membrane fuel cells.” Nat Catal 3, 1044–1054 doi: 10.1038/s41929-020-00546-1