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May 2011

Watching Water Splitting in Real Time

A transparent nanocrystalline electrode reveals details of the chemical steps involved in generating solar fuels

Ralph L. House

Mechanism of electrocatalytic water oxidation. Artwork was derived from Chen et al. 2010.

A 12-fold increase in the rate of water splitting was achieved using a transparent nanocrystalline electrode made of indium oxide doped with tin (ITO), instead of a conventional planar electrode. Catalysis was sustained for at least 8 hours, equivalent to ~800 turnovers, at a rate of ~0.027 s-1 and an impressive 95% oxygen yield. This work, conducted by theSolar Fuels and Next Generation Photovoltaics Energy Frontier Research Center in conjunction with the Center for Catalytic Hydrocarbon Functionalization, is perhaps the first demonstration of using a transparent electrode to monitor spectral changes of the catalyst during water splitting, yielding a comprehensive picture of each step in the reaction.

Challenges in electrocatalysis: Using molecular catalysts to split water, technically referred to as water oxidation, into hydrogen and oxygen gas is key for solar fuel applications because it provides a method for storing and generating fuel. Until recently, water oxidation electrocatalysis was largely accomplished using multi-site catalysts that were inefficient and difficult to synthesize. Planar electrodes with low surface areas also prevented spectral monitoring of intermediates so that mechanistic details remained elusive. This left the door open for two major advancements that have recently been demonstrated by Tom Meyer’s group at the Center.

1) A breakthrough in water oxidation catalysis: Facile synthesis of an efficient molecular catalyst that drives water oxidation using a single-site mechanism was created. The ruthenium-based complex is technically referred to as Ru(Mebimpy)-(4,4’-((HO)2OPCH2)2bpy)(OH2)]2+. Ru-OH22+ Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy is 2,2’-bipyridine. The complex can be bound to conducting oxide surfaces such as ITO, which are commonly used as electrodes.

2) Watching water splitting through a transparent, nanocrystalline electrode: The preparation of high surface area, nanocrystalline ITO films, which are both conductive and transparent, were used to spectrally monitor water splitting in real time. The result is meticulous characterization of the catalytic mechanism and key intermediates.

In solar fuel applications, an efficient and robust method for splitting water is a promising way to store and generate fuel in a sustainable manner, and represents one of the major challenges facing the EFRCs. The work described in this article overcomes many of the barriers that hampered this process and is a pioneering effort towards the realization of efficient catalytic water oxidation.

More Information

Chen ZF, JJ Concepcion, JF Hull, PG Hoertz, and TJ Meyer. 2010. "Catalytic water oxidation on derivatized nanoITO." Dalton Transactions 39(30), 6950. DOI: 10.1039/c0dt00362j.

Hoertz PG, ZF Chen, CA Kent, and TJ Meyer. 2010. "Application of high surface area tin-doped indium oxide nanoparticle films as transparent conducting electrodes." Inorganic Chemistry 49(18), 8179. DOI: 10.1021/ic100719r.

Concepcion JJ, JW Jurss, MR Norris, ZF Chen, JL Templeton, and TJ Meyer. 2010. "Catalytic water oxidation by single-site ruthenium catalysts." Inorganic Chemistry 49(4), 1277. DOI: 10.1021/ic901437e.

Acknowledgments

Funding for the electrochemical work came from the Army Research Office. Synthesis of the catalyst was supported by the Solar Fuels and Next Generation Photovoltaics Energy Frontier Research Center. Nanocrystalline ITO fabrication was supported by the Solar Fuels and Next Generation Photovoltaics Energy Frontier Research Center and the Center for Catalytic Hydrocarbon Functionalization. The centers are funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences.

About the author(s):

  • A member of the Solar Fuels Energy Frontier Research Center, Ralph is a Research Associate specializing in the use of multiple spectroscopic techniques to analyze the steps leading to the generation of solar fuels. Ralph is also involved with coordinating projects leading to the fabrication of a prototype solar fuels device and is the UNC-EFRC Liaison for External Outreach and Collaboration.

Great Views with Transparent Electrode

New see-through electrode lets scientists view catalyzed reactions as water splits into fuels

Mechanism of electrocatalytic water oxidation. Artwork was derived from Chen et al. 2010.

Cloudy days and dark nights. Both are challenges for solar cells. Current systems cannot store power for use when clouds appear or the sun goes down, so scientists are working to employ solar energy to create use-any-time fuels. The key is the catalyst, which drives the reactions. The catalyst, coupled with a new transparent crystalline electrode that uses indium and tin, is showing promise. The catalyst can split water 95% of the time and does so continuously for 8 hours with a 12-fold increase in speed over conventional catalytic molecules. The new electrode allows scientists to monitor, via sophisticated instruments, the reactions involved. The Solar Fuels and Next Generation Photovoltaics, led by the University of North Carolina, did the research in conjunction with the Center for Catalytic Hydrocarbon Functionalization. Both centers are DOE Energy Frontier Research Centers.

More Information

Chen ZF, JJ Concepcion, JF Hull, PG Hoertz, and TJ Meyer. 2010. "Catalytic water oxidation on derivatized nanoITO." Dalton Transactions 39(30), 6950. DOI: 10.1039/c0dt00362j.

Hoertz PG, ZF Chen, CA Kent, and TJ Meyer. 2010. "Application of high surface area tin-doped indium oxide nanoparticle films as transparent conducting electrodes." Inorganic Chemistry 49(18), 8179. DOI: 10.1021/ic100719r.

Concepcion JJ, JW Jurss, MR Norris, ZF Chen, JL Templeton, and TJ Meyer. 2010. "Catalytic water oxidation by single-site ruthenium catalysts." Inorganic Chemistry 49(4), 1277. DOI: 10.1021/ic901437e.

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.