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January 2014

A Tipping Point for Solar Fuels

A surface-bound molecule splits water with light as the predominant energy source

Ralph L. House

Graphics depicting a mesoporous film with ultrathin, conformal TiO2. The structures are decorated with a chromophore-catalyst assembly that does the job of both light absorption and water splitting. Artwork credited to Tom Celano of the Center for Solar Fuels.

A working "artificial leaf" for solar water splitting has been constructed, based for the first time on a dye-sensitized photoelectrosynthesis cell. The cell converts about 4% of the photons into electrons, which is four times the efficiency of plant photosynthesis. The energy from about 85% of these electrons can be stored as hydrogen. Developed at the Center for Solar Fuels with collaborators from the Center for Catalytic Hydrocarbon Functionalization and North Carolina State University, the cell uses a light-absorbing (a.k.a. chromophore) and catalytic molecule bound into a molecular assembly that is tethered to a semiconducting oxide (TiO2). Light stimulates the assembly, providing the energy needed to split water (2H2O --> O2 + 4H+ + 4e-) at the electrode's surface. Until now, the electrons usually recombined with the assembly in a process called back electron transfer (BET) rather than being converted to fuel, killing cell efficiency.

The core/shell advantage. The key to overcoming BET was to construct a "core/shell" structure made up of a thin (~3.6-nanometer) TiO2 layer, constructed one atomic layer at a time over conductive tin-doped indium oxide cores (nanoITO). Compared to previous films that were micrometers thick, the electron transit time through the shell to reach the conducting nanoparticle core was significantly reduced, solving the BET problem.

Sticking it out. The ultra-thin film was also used to stabilize a molecular catalyst on the surface of nanoITOBefore this discovery, surface-bound catalysts were only stable in acidic solutions (<pH 4) where the rate of water splitting is very low. Use of atomic layer deposition shielded the bonds that held the molecules to the surface enabling them to withstand basic solutions, up to pH 11, where water splitting is a million times faster.

With limited usable sunlight each day, this cell holds the key to storing solar energy for use on massive scales. The next step will be to combine these two exciting results to simultaneously increase the rate of electron capture and overall surface binding stability.

More Information

Alibabaei L, MK Brennaman, MR Norris, B Kalanyan, W Song, MD Losego, JJ Concepcion, RA Binstead, GN Parsons, and TJ Meyer. "Solar Water Splitting in a Molecular Photoelectrochemical Cell." Proceedings of the National Academy of Sciences of the United States of America 110(50):20008-20013. DOI: 10.1073/pnas.1319628110

Vannucci AK, L Alibabaei, MD Losego, JJ Concepcion, B Kalanyan, GN Parsons, and TJ Meyer. 2013. "Crossing the Divide between Homogeneous and Heterogeneous Catalysis in Water Oxidation." Proceedings of the National Academy of Sciences of the United States of America 110(52):20918-20922. DOI: 10.1073/pnas.1319832110

Acknowledgments

The Center for Solar Fuels (UNC-EFRC), an Energy Frontier Research Center funded by Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences supported the molecular design and synthesis, as well as cell design and construction. The Center for Catalytic Hydrocarbon Functionalization, an EFRC funded by DOE, Office of Science, Office of Basic Energy Sciences, supported data collection and device characterization. Operation of the atomic layer deposition technique involved personnel at North Carolina State University, funded by the Research Triangle Solar Fuels Institute, the DOE Office of Energy Efficiency and Renewable Energy and the National Science Foundation.

About the author(s):

  • Ralph L. House. A member of the Center for Solar Fuels (UNC-EFRC) 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 helping lead the construction of an electrochemical bioreactor and is the UNC-EFRC Liaison for External Outreach and Collaboration.

Building a Better Leaf

Nano-sphere with molecular light absorbers and catalysts photosynthesizes, using light to create fuel

The assembly begins with a conductive core (gray) thinly coated with semiconducting titanium dioxide. The shell has light-absorbing chromophores (blue) attached to catalysts (green), and together, they drive solar-driven water splitting, making fuel synthesis a reality.

To heat more homes and power more industries, solar cells should be more like plant leaves and store the energy they produce for later use. The challenge is to create a solar cell that is both efficient and economic enough to be accessible on massive scales. Scientists built such a nanosized assembly that converts 4.4% of photons absorbed into hydrogen—four times more efficiently than a typical plant produces carbohydrates. The new assembly begins with a conductive core thinly coated with semiconducting titanium dioxide. The shell has light-absorbing chromophores attached to catalysts, and together, they drive solar-driven water splitting, making fuel synthesis a reality. A related shell can also be applied on top of surface-bound catalysts, stabilizing them on surfaces in environments where water splitting is a million times faster. The research has implications in green chemistry and sensors as well as solar fuels. The work was done by the Center for Solar Fuels (led by the University of North Carolina) with collaborators from the Center for Catalytic Hydrocarbon Functionalization (led by the University of Virginia) and North Carolina State University.

More Information

Alibabaei L, MK Brennaman, MR Norris, B Kalanyan, W Song, MD Losego, JJ Concepcion, RA Binstead, GN Parsons, and TJ Meyer. "Solar Water Splitting in a Molecular Photoelectrochemical Cell." Proceedings of the National Academy of Sciences of the United States of America 110(50):20008-20013. DOI: 10.1073/pnas.1319628110

Vannucci AK, L Alibabaei, MD Losego, JJ Concepcion, B Kalanyan, GN Parsons, and TJ Meyer. 2013. "Crossing the Divide between Homogeneous and Heterogeneous Catalysis in Water Oxidation." Proceedings of the National Academy of Sciences of the United States of America 110(52):20918-20922. DOI: 10.1073/pnas.1319832110

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.