The search for cheap, Earth abundant materials for solar cells, and for photo-electrodes for water splitting, calls for detailed investigations of the efficiency of light absorption in materials and nanostructures. Theoretical frameworks and efficient computer simulations are used to help interpret a growing body of complex measurements, and to predict optimal systems for harvesting sun light.
The conversion of water to oxygen and hydrogen molecules is essential for a variety of renewable energy technologies. Nickel–iron (NiFe) oxyhydroxide is an important, earth-abundant electrocatalyst for the oxygen evolution reaction. A combined experimental and computational study of pure Ni oxyhydroxide and mixed NiFe oxyhydroxide thin films elucidates the chemistry governing their different electrochemical and optical properties. The Ni and Fe oxidation states in each system are assigned as a function of applied potential based on quantum-mechanical calculations, cyclic voltammetry, and UV-visible spectroscopy. In the more catalytically active NiFe system, oxidation to Fe4+ coincides with the onset of oxygen evolution. Synergy between experiment and theory provides a detailed, atomistic understanding of this robust catalyst.
"Characterization of NiFe oxyhydroxide electrocatalysts by integrated electronic structure calculations and spectroelectrochemistry",
Zachary Goldsmith, Aparna Harshan, James Gerken, Márton Vörös, Giulia Galli, Shannon Stahl, Sharon Hammes-Schiffer,
Proc. Nat. Acad. Sci. USA114, pp 3050-3055 (2017)
Light absorbers for water oxidation
The desirable properties of water-splitting photoanode
and/or photocathode materials include: (i) Efficient
absorption of visible light. The optimum value of
the band gap should be larger than 1.9 eV and
smaller than 3.1 eV, so as to fall within the visible range of the solar spectrum. (ii) High chemical stability in the dark
and under illumination. (iii) Band edge positions
that straddle the water reduction and oxidation potentials.
We are studying the opto-electronic properties of metal oxide and nitride semiconductors that are promising, stable materials for water oxidation, in particular WO3 and solid solutions of copper tungstanates and molibdates, BiVO4, and Ta3N5.
"Simultaneous Enhancements in Photon Absorption and Charge Transport of BiVO4 Photoanodes for Solar Water Splitting",
Tae Woo Kim, Yuan Ping, Giulia Galli, and Kyoung-Shin Choi
Nature Comm.6, 8769 (2015)
"Optoelectronic properties of Ta3N5: A joint theoretical and experimental study",
Juliana M. Morbec, Ieva Narkeviciute, Thomas F. Jaramillo, and Giulia Galli,
Phys. Rev. B90, 155204 (2014)
"Tungsten Oxide Clathrates for Water Oxidation: a First Principles Study",
Y.Ping, Y.Li, F.Gygi and G.Galli,
Chem. Mater.24, 4252 (2012)
"Thermally Stable N2-intercalated WO3 Photoanodes for Water Oxidation",
Qixi Mi, Yuan Ping, Yan Li, Bingfei Cao, Bruce S. Brunschwig, Peter G. Khalifah, Giulia Galli, Harry B. Gray, and Nathan S. Lewis,
J. Am. Chem. Soc.134, 18318 (2012)
Functionalized Si surfaces
Semiconductor/liquid interfaces are promising platforms for solar fuels production, and hence for solar energy storage. The efficiency of such systems depends critically on the alignment of the semiconductor band edges with the Nernst potentials for fuel production, e.g., with the reduction and oxidation half-reactions involved with water-splitting and/or CO2 reduction. We have carried out first principles calculations of functionalized Si surfaces to understand and interpret spectroscopic measurements. We have also investigated the effect of water on these surfaces and how band edges are shifted by solvation effects.
"Interfacial Effects on the Band Edges of Functionalized Si Surfaces in Liquid Water",
Tuan Anh Pham, Donghwa Lee, Eric Schwegler, and Giulia Galli,
J. Am. Chem. Soc.136, 17071 (2014)
"Combined Theoretical and Experimental Study of Band-Edge Control of Si through Surface Functionalization",
Y. Li, L.E. O’Leary, N.S. Lewis, and G. Galli,
J. Phys. Chem. C117, 5188 (2013)
The design of optimal interfaces between photoelectrodes and catalysts is a key challenge in building photoelectrochemical cells to split water. Using first-principles mechanical calculations, we investigated the structural and electronic properties of tungsten trioxide (WO3) surfaces interfaced with an IrO2 thin film. We found that, upon full coverage of WO3 by IrO2, the two oxides form undesirable Ohmic contacts. However, our calculations predicted that if both oxides are partially exposed to water solvent, the relative position of the absorber conduction band and the catalyst Fermi level favors charge transfer to the catalyst and hence water splitting. We propose that, for oxide photoelectrodes interfaced with IrO2, it is advantageous to form rough interfaces with the catalyst, e.g., by depositing nanoparticles, instead of sharp interfaces with thin films. Our study highlights the importance of catalysts and interface morphology in designing optimal photoelectrochemical cells.
"Energetics and solvation effects at the photoanode-catalyst interface: Ohmic contact versus Schottky barrier",
Yuan Ping, William Goddard III and Giulia Galli,
J. Am. Chem. Soc. Comm.137, 5264 (2015)