A device with dual capability — generating fuel and oxidant from water and converting fuel and oxygen into electricity — with a single switch operation, has an array of advantages for space, terrestrial and military applications.
Starting from environmental impact to high energy density, creating efficient unitized regenerative fuel cells (URFCs) has been under the ambit of researchers for a long time.
However, to be truly efficient, a URFC requires bifunctional catalysts. This implies, in electrolyze mode, the catalyst should enable the breakdown of water into hydrogen and oxygen, and in fuel cell mode, must ensure 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.
The study 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.
Pralay Gayen, Study First Author and Postdoctoral Research Associate, McKelvey School of Engineering, Washington University in St. Louis
Sulay Saha, a postdoctoral research associate in Ramani’s laboratory, and Gayen’s research was mentored by the first principles, which considers the basic characteristics of different substances before resulting in the laboratory for testing potential catalysts.
In association with former undergraduate researcher and study co-author Xinquan Liu, the team finally identified and developed Pt-Pyrochlore, a composite of platinum and a lead ruthenate pyrochlore, which expressed high bifunctionality.
The “bifunctionality index” is the determination of the ability of a catalyst to facilitate both the forward and reverse direction of a reaction.
We want the index to be low. Zero, ideally.
Kritika Sharma, PhD Engineering Student, Washington University in St. Louis
The new catalyst holds a bifunctionality index of 0.56 volts, which is much less when compared with other reported catalysts. When the catalyst is applied in the URFC built by the laboratory, it facilitated 75% round-trip energy efficiency (RTE), which is the highest recorded round-trip efficiency in this type of URFC.
The efficiency qualifies the URFCs for applications such as drones, submersibles, spacecraft and space stations including off-grid energy storage.
Gayen, P., et al. (2021) High-performance AEM unitized regenerative fuel cell using Pt-pyrochlore as bifunctional oxygen electrocatalyst. Proceedings of the National Academy of Sciences. doi.org/10.1073/pnas.2107205118.