A University at Buffalo-led research team has developed an efficient platinum group metal (PGM)-free catalyst for the oxygen reduction reaction (ORR) in PEM fuel cells that consists of atomically dispersed nitrogen-coordinated single Mn sites on partially graphitic carbon (Mn-N-C).

In a paper in Nature Catalysis, the researchers report that the Mn-N-C catalyst exhibits a half-wave potential of 0.80 V versus the reversible hydrogen electrode, approaching that of Fe-N-C catalysts, along with significantly enhanced stability in acidic media.

The advancement could eventually help solve a basic issue with hydrogen fuel cells—i.e., they’re not affordable because most catalysts are made with platinum, which is both rare and expensive.

We haven’t been able to advance a large-scale hydrogen economy because of this issue involving catalysts. But manganese is one of the most common elements in Earth’s crust and it’s widely distributed across the planet. It could finally address this problem.

—lead author Gang Wu, PhD, associate professor of chemical and biological engineering in UB’s School of Engineering and Applied Sciences

Additional authors come from Oak Ridge National Laboratory, Brookhaven National Laboratory, Argonne National Laboratory, Oregon State University, University of Pittsburgh, University of South Carolina, Giner Inc. and Harbin Institute of Technology.

For more than a decade, Wu has been searching for alternative catalysts for hydrogen fuel cells. He has reported advancements in iron- and cobalt-based catalysts; however, each wears down over time, limiting their usefulness, he says.

In previous work, Wu discovered that adding nitrogen to manganese causes internal changes to the metal that makes it a more stable element. In experiments reported in the study, he devised a relatively simple two-step synthesis strategy involving doping and adsorption processes by leveraging the unique properties of ZIF-8 precursors, which has been shown to effectively increase the active-site density.

In the first step of synthesis, Mn ions are combined with Zn ions to prepare Mn-doped ZIF-8 precursors. After carbonization and acid leaching, the derived porous carbon is used as a host to adsorb additional Mn and N sources followed by a subsequent thermal activation.

Schematic of atomically dispersed MnN4 site catalyst synthesis. Li et al.

The result was a catalyst that’s comparable in its ability to split water as platinum and other metal-based alternatives. More importantly, the stability of the catalyst makes it potentially suitable for hydrogen fuel cells. This could lead to wide-scale adoption of the technology in buses, cars and other modes of transportation, as well as backup generators and other sources of power.

Wu plans to continue the research, focusing on improving the catalyst’s carbon microstructure and the method in which nitrogen is added. The goal, he says, is to further enhance the catalyst’s performance in practical hydrogen fuel cells.

The research was supported the UB RENEW Institute, the US National Science Foundation and the US Department of Energy.

Resources

• Jiazhan Li, Mengjie Chen, David A. Cullen, Sooyeon Hwang, Maoyu Wang, Boyang Li, Kexi Liu, Stavros Karakalos, Marcos Lucero, Hanguang Zhang, Chao Lei, Hui Xu, George E. Sterbinsky, Zhenxing Feng, Dong Su, Karren L. More, Guofeng Wang, Zhenbo Wang & Gang Wu (2018) “Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells” Nature Catalysis doi: 10.1038/s41929-018-0164-8