Optimization of Silicon Hybrid Photoelectrodes for Light-Driven CO2 Reduction

Modern energy demands and the threat of climate change necessitate the development of innovative and sustainable strategies for energy production. Renewable energy sources such as wind, hydro, solar, and geothermal offer promising alternatives to the combustion of fossil fuels, but they present challenges due to their reliance on ideal environmental conditions and the logistical difficulty associated with their energy storage. One novel approach to leveraging solar energy harnesses artificial photosynthesis to generate and store energy in the form of chemical bonds. This process can be accomplished via one or more electrochemical reaction(s) at the interface of a hybrid photoelectrode comprised of a visible light-absorbing semiconductor and a catalyst to reduce carbon dioxide. In this work, sonochemical hydrosilylation was first leveraged to study the loading behavior of select Re-bipyridine(CO)₃Cl catalysts on the surface of low-doped p-type silicon (111) before investigating the impact that the loading behavior of these complexes has on the photoelectrochemical performance of the resulting hybrid photoelectrodes. X-ray photoelectron spectroscopy confirmed covalent attachment of the relevant catalysts. ¹H NMR spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy supported the maintenance of the catalysts’ molecular integrity after sonication. Through analysis by inductively coupled plasma mass spectrometry, it was determined that a maximum surface coverage of approximately 0.25 nmol/cm² of Re-bipyridine(CO)₃Cl molecules is achieved by sonochemical hydrosilylation after ca. 5 hr. of sonication. Hybrid photoelectrodes featuring maximum surface coverage were then determined to have catalytic layers that are approximately 3 nm thick. Cyclic voltammetry, linear sweep voltammetry, and controlled potential bulk photoelectrolysis were finally used to confirm the catalytic activity of optimized hybrid photoelectrodes, report a photovoltage for such photoelectrodes of 406 mV, and determine a Faradaic efficiency for CO of 34% +/- 5%.