The University of Colorado Boulder’s Weimer Lab has introduced an efficient and economical method to use renewable energy to produce fuel, opening doors to clean and sustainable energy sources for a wide array of industries, including transportation, steelmaking and ammonia production.

The groundbreaking study, detailed in the high-impact journal Joule, outlines a thermochemical process using solar energy to derive either hydrogen gas from water or carbon-neutral fuels from water and carbon dioxide. The new paper marks the first exploration of running this process at elevated pressure, said Kent Warren, one of the paper’s lead authors and a research associate in the Department of Chemical and Biological Engineering. 

Their findings indicated that for specific materials, elevating the pressure not only accelerated the reaction rate but also significantly increased the amount of fuel produced.

“This work is, thus far, the most significant accomplishment of my professional career,” he said.

All of the paper’s authors are affiliated with CU Boulder. Professor Al Weimer is the principal investigator, and Warren and PhD student Justin Tran are the first authors. Other authors include Dragan Mejic, instrument shop supervisor; Robert L. Anderson, senior professional research associate; Lucas Jones; Dana S. Hauschulz, fabrication advisor; and Carter Wilson, an undergraduate research assistant. 

In contrast to electrolysis, an alternative method attracting commercial attention for the production of green hydrogen, the researchers used heat – not electricity – to split water. Warren said the thermochemical process has the potential to be more economically viable. The method eliminates the need for scarce, rare-earth-element-containing materials and, unlike electrolysis, can rely on well-established engineering principles to be easily scaled.

The researchers demonstrated that, by simply elevating pressure, low-cost CU Boulder-developed iron-aluminate materials can more than double hydrogen production, a notable feat considering such yields are nearly 1,000 percent greater than what the current benchmark thermochemical approach can achieve.

The same process can also be used to split carbon dioxide into carbon monoxide. It’s significant because hydrogen and carbon monoxide combined form syngas, the building block for gasoline, diesel and other liquid hydrocarbon fuels. Since carbon dioxide is sourced from the atmosphere or industrial emitters, the resulting fuel – when used – is carbon neutral, contributing only as much emissions to the atmosphere as required for its production.

“The way I like to think about it is some day when you go to the pump you’ll have, for example, unleaded, super unleaded and ethanol options, and then an additional option being solar fuel, where the fuel is derived from sunlight, water and carbon dioxide,” Warren said. “Our hope is that it will be cost-competitive to the fuels sourced from the ground.”

This research was supported by Shell Oil and the National Science Foundation.

Photo caption: From left to right, Justin Tran, Professor Al Weimer and Kent Warren stand in the Weimer Lab.

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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 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.

���n 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. 

���'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 �ٳ󲹳�.”

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.

�հ�����’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.

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