CU-Boulder team develops more efficient isothermal solar-thermal water splitting technique for H2 production
A University of Colorado Boulder team has developed a new solar-thermal water-splitting (STWS) system for the efficient production of hydrogen. A paper on their work is published in the journal Science.
STWS cycles have long been recognized as a desirable means of generating hydrogen gas (H2) from water and sunlight, the team notes. Two-step, metal oxide–based STWS cycles generate H2 by sequential high-temperature reduction of a metal oxide catalyst (releasing oxygen atoms) and cooler conditions in which the catalyst is reoxidized by oxygen from water (freeing up hydrogen molecules for collection as hydrogen gas). The temperature swings between reduction and oxidation steps have hobbled STWS’ overall efficiency, however, because of thermal and time losses that occur during the frequent heating and cooling of the metal oxide. The cycling can also limit catalyst lifetime.
The CU-Boulder team showed that these temperature swings are unnecessary by developing an approach that allows the redox cycle to be driven isothermally, using pressure swings. Isothermal water splitting (ITWS) at 1350 °C using the “hercynite cycle” exhibits H2 production capacity >3 and >12 times that of hercynite and ceria, respectively, per mass of active material when reduced at 1350 °C and reoxidized at 1000 °C.
We have designed something here that is very different from other methods and frankly something that nobody thought was possible before. Splitting water with sunlight is the Holy Grail of a sustainable hydrogen economy.—CU-Boulder Professor Alan Weimer, research group leader
One of the key differences between the CU method and other methods developed to split water is the ability to conduct two chemical reactions at the same temperature, said Associate Professor Charles Musgrave, first author. Conventional theory holds that producing hydrogen through the metal oxide process requires heating the reactor to a high temperature to remove oxygen, then cooling it to a low temperature before injecting steam to re-oxidize the compound in order to release hydrogen gas for collection.
The more conventional approaches require the control of both the switching of the temperature in the reactor from a hot to a cool state and the introduction of steam into the system. One of the big innovations in our system is that there is no swing in the temperature. The whole process is driven by either turning a steam valve on or off.—Charles Musgrave
With the new CU-Boulder method, the amount of hydrogen produced is entirely dependent on the amount of metal oxide (a combination of iron, cobalt, aluminum and oxygen) and how much steam is introduced into the system. One of the designs proposed by the team is to build reactor tubes roughly a foot in diameter and several feet long, fill them with the metal oxide material and stack them on top of each other.
The CU-Boulder system would use mirror arrays to concentrate sunlight onto a single point atop a central tower up to several hundred feet tall. The tower gathers the heat to 1,350 °Celsius, then delivers it into the reactor containing the metal oxides. A working system to produce a significant amount of hydrogen gas would require a number of the concentrating towers from several acres of mirrors surrounding each tower.
Despite the discovery, the commercialization of such a solar-thermal reactor is likely years away, the researchers said.
With the price of natural gas so low, there is no incentive to burn clean energy. There would have to be a substantial monetary penalty for putting carbon into the atmosphere, or the price of fossil fuels would have to go way up.—Alan Weimer, also the executive director of the Colorado Center for Biorefining and Biofuels (C2B2)
C2B2 is an arm of the Colorado Energy Research Collaboratory involving CU-Boulder, the Colorado School of Mines, Colorado State University and the National Renewable Energy Laboratory in Golden.
The research was supported by the National Science Foundation and by the US Department of Energy (DOE).
Christopher L. Muhich, Brian W. Evanko, Kayla C. Weston, Paul Lichty, Xinhua Liang, Janna Martinek, Charles B. Musgrave, and Alan W. Weimer (2013) Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle. Science 341 (6145), 540-542 doi: 10.1126/science.1239454
Martin Roeb and Christian Sattler (2013) Isothermal Water Splitting. Science 341 (6145), 470-471 doi: 10.1126/science.1241311