Published: Jan. 25, 2022 By

Justin Tran and Kent Warren pose in front of lab equipment
Tran and Warren in the Weimer lab. Photo courtesy the researchers.

Hydrogen has long been seen as a possible renewable fuel source, held out of reach for full-scale adoption by production costs and inefficiencies. Researchers in the Weimer Group are working to address this by using solar thermal processing to drive high-temperature chemical reactions that produce hydrogen and carbon monoxide, which can be used to synthesize liquid hydrocarbon fuels.

Postdoctoral research associate Kent Warren and graduate student Justin Tran of the Weimer Group are co-authors with Melvin E. and Virginia M. Clark Professor Alan Weimer on “A thermochemical study of iron aluminate-based materials: a preferred class for isothermal water splitting” published in Energy & Environmental Science earlier this month.

“This will result in a seismic shift in research directions for solar thermal water splitting,” Weimer said.

Warren, Tran and Weimer believe that low-cost iron aluminate-based oxides may improve performance over current methods of thermochemical H2 production, as they remain effective under less favorable conditions expected in large-scale production systems where implementing wide temperature changes and using excess steam is avoided to improve the process’ efficiency.

“There is a prevailing consensus in the solar thermochemistry community that, in order to produce an appreciable hydrogen yield under an isothermal operating configuration, prohibitive amounts of steam are required,” Warren said. “We conclusively demonstrated that, for the first time, this concern can be mitigated with proper active material selection. My hope is that this work not only helps rewrite this narrative, but also encourages other research labs and institutions to consider thermochemical water-splitting as a more viable alternative to other green hydrogen technologies such as water electrolysis.”

The researchers came to this conclusion by establishing the thermodynamic equilibrium behavior of iron aluminate-based oxides, then compared their findings to other materials subjected to similar methods by other researchers.

“We demonstrate that iron aluminate-based oxides can isothermally outperform other candidates, even when said candidates are exposed to more favorable temperature-swing conditions,” Warren said.

Warren cited his ten-year fascination with solar thermochemistry as inspiration for his work on this project, going back to his time as an undergraduate at Valparaiso University and later as a graduate research assistant at the University of Florida under Associate Professor Jonathan Scheffe, who is a former graduate student of Weimer’s.

“In 2019, I was offered a postdoctoral position to work with Professor Weimer on breaking the world record of solar-to-hydrogen conversion efficiency,’ which I eagerly accepted,” Warren said. Before I undertook that challenge, however, I needed to ensure that we were operating with the ideal material composition under conditions most favorable for practical applications.

Prior to Warren’s arrival at CU Boulder, Weimer had performed some preliminary work on iron aluminate-based oxides.

“That was the natural starting point,” Warren said. I did not expect to learn that this class of materials exhibits such favorable thermodynamic properties under such adverse operating conditions.

Graduate research assistant Justin Tran was responsible for gaining insight into the workings and mechanism of the iron aluminate-based materials during the characterization process. He developed phase diagrams and ran Rietveld refinement to help the group thermochemically characterize them. 

“I'm inspired to work in this topic because of the potential to efficiently produce clean fuel, having a higher theoretical efficiency than competing processes,” Tran said. “This field still has a lot of room to grow and I'm excited to be part of that.”

Warren believes their research serves as the foundation for the development of a prototype-scale reactor that will be evaluated with CU Boulder’s high-flux solar simulator facility.

“The goal is to establish a world record solar-to-hydrogen conversion efficiency – the key metric for benchmarking our technology against other pathways to green hydrogen,” Warren said.

Tran expressed hope that their work will bring renewed interest to thermochemical fuel production, particularly isothermal operation.

This work shows that with the proper material choice, we can efficiently produce clean, sustainable fuels,” Tran said.

Tran’s position with the Weimer Group is funded by a National Science Foundation Graduate Research Fellowship Program. Parts of this research project are included in a CHEN 4530 senior capstone design project. It is sponsored by OMC Hydrogen, a startup interested in developing commercial green hydrogen processing, and is supported by the BOLD Center.