Nickel catalyst enables high-performance fuel cell free of precious metals

A high acid environment is great for a snappy hydrogen oxidation reaction - the reaction at the heart of a clean-energy fuel cell. The problem is the only catalysts that won't dissolve in the high acid of traditional fuel cells are precious metals - platinum, palladium and the like - and they are very expensive.

To advance fuel cell technologies and lower their cost, the Abruña and Muller Groups at Cornell and other researchers in the Center for Alkaline-based Energy Solutions(CABES) work on fuel cells in alkaline, or nonacidic, environments. They have developed a nonprecious metal catalyst - nickel coated with carbon - that works well in alkaline media, maintaining a strong hydrogen oxidation reaction activity.

In tests, paired with a nonprecious metal based oxygen reduction reaction catalyst, also developed at CABES, it achieves a power density that surpasses the U.S. Department of Energy (DOE) benchmark. This represents a major step toward alkaline fuel cells that use inexpensive commodity metals such as nickel and cobalt in applications including generators, transportation and alternative electricity sources in remote areas.

"This has the potential to be transformative in the application of fuel cells broadly defined because it steps away from the need for precious metals," said Héctor D. Abruña, the Émile M. Chamot Professor in the Department of Chemistry and Chemical Biology in the College of Arts and Sciences. "It has the performance metrics people have been looking for in very inexpensive materials."

Abruña is co-corresponding author, with David Muller, the co-director of the Kavli Institute at Cornell for Nanoscale Science and the Samuel B. Eckert Professor of Engineering in Applied and Engineering Physics in the Cornell Duffield College of Engineering, of "Unveiling the Sensitivity and Significance of Ni Oxidation State for Alkaline Hydrogen Oxidation Electrocatalysis," published March 18 in the Proceedings of the National Academy of Sciences. The co-first authors are Qihao Li, postdoctoral researcher in the Abruña Group, and Schuyler Zixiao Shi, doctoral student.

"An alkaline medium allows you to use nonprecious metals - nickel, iron, cobalt, manganese - which are 500 to 1,000 times less expensive than precious metals like platinum and palladium, so that the cost issue becomes irrelevant," Abruña said. "But it means you have to develop catalysts that can operate in alkaline media, have high performance and exhibit long-term durability during operation."

The sluggish hydrogen oxidation reaction in alkaline media has been a barrier to eliminating precious metal group catalysts from fuel cells, the researchers wrote. Nickel has shown promise in an alkaline environment, but on its own it oxidizes very quickly, meaning it stops being active.

In the study, the researchers examined the state of nickel catalysts in detail, using microscopic and spectroscopic techniques. Expertise from the Muller Groupplayed a key role in revealing that a metallic nickel surface is crucial for effectively catalyzing the hydrogen oxidation reaction. The formation of a nickel hydroxide, which theoretical calculations revealed, indicated a strong interaction between the nickel surface and graphene, resulting in a tightly sealed carbon shell that protects the nickel surface.

"With what we developed, we can coat the nickel with a carbon layer from graphene," Abruña said. "The layer is less than a couple of nanometers; three to four atoms thick."

That's thin enough that electrons can interact and tunnel through to carry out the hydrogen oxidation reaction, but thick enough that it prevents oxidation of the nickel.

Comparative images from Muller's group show nickel at atomic resolution during the reaction, first without the protective carbon, and then coated with carbon. The coated nickel had oxygen only on the surface but not in the bulk, demonstrating the effectiveness of the carbon coating.

The proof of effectiveness came when the researchers tested the power density of the cell, which achieved 1 watt per square centimeter - an exceptional performance and a milestone, Abruña said. It surpasses the DOE target for precious metal group loading in fuel cells, giving a comparable performance to that of fuel cells with precious metal.

This means that alkaline fuel cell technology can be deployed broadly because the cost of the catalyst, a significant component in traditional fuel cells at a few grams per unit, is out of the equation.

Now work needs to be done to improve the durability of alkaline fuel cells. The DOE stability metric for fuel cells is 15,000 hours of operation. "We are at about 2,000, which while not 15,000, is within striking distance," Abruña said. Engineering changes will make up the difference, but the core chemistry for this reaction is in place.

In the future, this technology will be useful in automotive applications, Abruña said. More near-term, it can be used in stationary and mobile generators, and decentralized electricity systems.

The study's co-authors are: Colin R. Bundschu, Ph.D '25; Christopher Pollock, staff scientist at the Cornell High-Energy Synchrotron Source; Andrés Molina Villarino,, Ph.D. '23; Mihail R. Krumov, Ph.D. '25; and Rui Zeng, Ph.D. '22.

Support for the study came from the National Science Foundation and the U.S. Department of Energy, with facility support from the Cornell Center for Materials Research.

Kate Blackwood is a writer for the College of Arts and Sciences.