A schematic of a URFC with the highly active and highly stable bifunctional oxygen electrocatalyst with low bifunctionality. Image: Vijay Ramani/Washington University in St. Louis.A single device that can use electricity to generate fuel and oxygen from water and, when a switch is flipped, convert that fuel and oxygen back into electricity and water could offer a host of benefits for terrestrial, space and military applications. With low environmental impact and high energy density, such unitized regenerative fuel cells, or URFCs as they are called, have been in researchers’ sights for years now.
But to truly be efficient, an URFC needs bifunctional catalysts. This means, in electrolyzer mode, catalysts that can 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 at Washington University in St. Louis, a team of researchers has found an excellent bifunctional catalyst for the oxygen electrode. They report their work in a paper in the 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,” explained Pralay Gayen, the paper's first author, who was a postdoctoral research associate in Ramani’s lab at the McKelvey School of Engineering but is now working at Intel.
The team'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. This work ultimately led Gayen and his colleagues to identify and develop 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 in Ramani's group. “Zero, ideally.”
The new Pt-Pyrochlore catalyst has a bifunctionality index of 0.56 volts, which is very low compared with other reported catalysts. When used in a URFC device developed in Ramani's lab, this catalyst helped the device achieve a round-trip energy efficiency (RTE) of 75% – the highest reported round-trip efficiency in this type of URFC.
With such high efficiency, this URFC is well suited for use in submersibles, drones, spacecrafts and space stations, as well as for off-grid energy storage.
This story is adapted from material from the Washington University in St. Louis, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.