Solar Water Oxidation by Composite Cobalt-Based Catalyst/Oxide Semiconductor Photoanodes
Zhong, Diane K.
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Photoelectrochemical (PEC) water splitting is an attractive approach to capturing and storing the earth's abundant solar energy influx. The challenging four-electron water-oxidation half-cell reaction has hindered this technology, giving rise to slow water oxidation kinetics at the photoanode surfaces relative to competitive loss processes. Inspired by nature's photosynthesis, where the tasks of photon absorption, charge separation, and water-oxidation catalysis are performed by separate protein components, division-of-labor strategies for solar water splitting in PEC cells are explored in this thesis. Prototypical alpha-Fe2O3 nanostructured photoanodes are interfaced with cobalt-phosphate water oxidation catalysts (Co-Pi) that can operate at low overpotentials. The resulting composite photoanodes exhibit onset potentials over one hundred millivolts lower than the semiconductor alone, indicating a reduced external bias would be needed to drive overall water splitting. A kinetic bottleneck limiting the performance of such photoanodes was identified to be related to the deposition of thick catalyst layers. This bottleneck was then circumvented by employing a photo-assisted electrodeposition technique to deposit the catalyst, which allows deposition only where visible light generates oxidizing equivalents, and provides a more uniform distribution of catalyst onto semiconductor surfaces. In order to achieve even lower photocurrent onset potentials, the influence of Co-Pi on W:BiVO4 photoanode surfaces was also investigated. Using peroxide as a surrogate substrate revealed that interfacing Co-Pi with W:BiVO4 photoanodes almost completely eliminates losses due to surface electron-hole recombination. The low absolute onset potential achieved with Co-Pi/W:BiVO4 is promising for overall solar water splitting in low-cost tandem PEC cells, and is encouraging for application of this surface modification strategy to other candidate photoanodes. Finally the role of Co-Pi in enhancing the photocatalytic activity of various oxide semiconductors towards water oxidation is examined by a series of PEC, impedance spectroscopy, photoconductive microscopy, and PEC kinetic experiments. All evidence strongly suggests the superior PEC performance of Co-Pi/semiconductor composite photoanodes is a direct result of efficient water oxidation catalysis by the Co-Pi surface electrocatalyst competing with surface electron-hole recombination in the semiconductor.
- Chemistry