New catalyst helps combine fuel cell, battery into one device

Key is the 'bifunctionality index'

Rendering of an underwater drone
Regenerative fuel cells, with high round-trip efficiencies like that produced with a catalyst developed in the lab of Vijay Ramani, are well suited for submersibles, drones, and spacecraft, as well as for off-grid energy storage. (Submersible rendering image: Shutterstock)

A single device that both generates fuel and oxidant from water and, when a switch is flipped, converts the fuel and oxygen into electricity and water, has a host of benefits for terrestrial, space and military applications. From low environmental impact to high energy density, developing efficient unitized regenerative fuel cells, or URFCs as they are called, has been in researchers’ sights for years now.

But to truly be efficient, an URFC needs bifunctional catalysts. This means, in electrolyzer mode, catalysts should facilitate the breakdown of water into hydrogen and oxygen, and, in fuel cell mode, facilitate their recombination into water. Now, working in the lab of Vijay Ramani, the Roma B. & Raymond H. Wittcoff Distinguished University Professor, a team of researchers has found an excellent bifunctional catalyst for the oxygen electrode.

Headshot of Vijay Ramani

Their work was published in the journal Proceedings of the National Academy of Sciences.

“Unlike the hydrogen electrode, wherein platinum is an effective bifunctional catalyst, it is very challenging to identify a suitable catalyst for the oxygen electrode due to the sluggish kinetics of oxygen reduction and oxygen evolution,” said Pralay Gayen, currently working at Intel, who was a postdoctoral research associate in Ramani’s lab at the McKelvey School of Engineering at Washington University in St. Louis and served as the paper’s first author.

Sulay Saha, a postdoctoral research associate in Ramani’s laboratory, and Gayen’s research was guided by first principles — taking into account the fundamental properties of different substances before heading to the lab to test potential catalysts.

Along with former undergraduate researcher and co-author Xinquan Liu, the team ultimately identified and developed Pt-Pyrochlore, a composite of platinum and a lead ruthenate pyrochlore, which yielded high bifunctionality.

The “bifunctionality index” is a measure of catalyst’s ability to facilitate both the forward and reverse direction of a reaction. “We want the index to be low,” said Kritika Sharma, a PhD engineering student. “Zero, ideally.” This new catalyst has a bifunctionality index of 0.56 volts — very low compared with other catalysts reported. When used in a URFC device developed by the laboratory, the catalyst enabled a round-trip energy efficiency (RTE) of 75% — the highest reported round-trip efficiency in this type of URFC.

With such high efficiency, the URFCs developed are well suited for applications such as submersibles, drones, spacecrafts and space stations, as well as for off-grid energy storage.

The McKelvey School of Engineering at Washington University in St. Louis promotes independent inquiry and education with an emphasis on scientific excellence, innovation and collaboration without boundaries. McKelvey Engineering has top-ranked research and graduate programs across departments, particularly in biomedical engineering, environmental engineering and computing, and has one of the most selective undergraduate programs in the country. With 140 full-time faculty, 1,387 undergraduate students, 1,448 graduate students and 21,000 living alumni, we are working to solve some of society’s greatest challenges; to prepare students to become leaders and innovate throughout their careers; and to be a catalyst of economic development for the St. Louis region and beyond.

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