A team at Washington University in St. Louis (WUSL) has developed a bifunctional catalyst for the oxygen electrode in a unitized regenerative fuel cell (URFC). A paper on their work is published in Proceedings of the National Academy of Sciences (PNAS).
Unitized regenerative fuel cells (URFCs) combine the functionalities of both electrolyzers and fuel cells in a single cell/stack, enabling lower weight, volume, and capital cost. URFCs theoretically offer a number of benefits for terrestrial, space and military applications, including low environmental impact and high energy density.
However, 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.
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.—Pralay Gayen, first author
The team ultimately identified and developed Pt-Pyrochlore (Pt-Pb2Ru2O7-x), a composite of platinum and a lead ruthenate pyrochlore, which yielded high bifunctionality.
The bifunctionality index (BI) is a measure of catalyst’s ability to facilitate both the forward and reverse direction of a reaction. The index should be low, said co-author 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 would be well suited for applications such as submersibles, drones, spacecrafts and space stations, as well as for off-grid energy storage.
Pralay Gayen, Sulay Saha, Xinquan Liu, Kritika Sharma, Vijay K. Ramani (2021) “High-performance AEM unitized regenerative fuel cell using Pt-pyrochlore as bifunctional oxygen electrocatalyst” Proceedings of the National Academy of Sciences 118 (40) e2107205118; doi: 10.1073/pnas.2107205118