| Increasing global energy demand, as well as the tremendous impact of fossil fuels on nearly all aspects of modern life, gives rising urgency for methods of generating and storing energy in an environmentally friendly, low-cost, widely distributed, and carbon-neutral manner. Of the forms of energy generation that could possibly satisfy all of these requirements, the sun possesses the most potential, with enough energy from sunlight striking the earth in one hour to provide energy for all of humankind for a full year. Two of the greatest obstacles toward harnessing the sun's energy as a replacement for fossil fuels are that sunlight is both diffuse and intermittent, characteristics that are shared by many other solar-derived energy sources, such as wind. In order to overcome this obstacle, methods of converting light and electrical energy into chemical energy are being explored in order to tackle the current environmental crisis. Artificial photosynthesis is one such form of energy storage that does this by mimicking photosynthesis in plants; this uses the energy from sunlight to oxidize water into dioxygen at a photoanode, thereby extracting electrons and protons from H2O to form fuel either by proton reduction to produce H2, or CO2 reduction to generate hydrocarbons, at a photocathode.;In order to store energy via photosynthesis, there are three major steps that must be optimized in order to convert sunlight into chemical energy efficiently: light absorption, charge separation and transport, and catalysis. The studies detailed herein focus on light absorption and catalysis; specifically, the enhancement of light absorption in a photoanode by concentrating light using localized surface plasmon resonance, as well as the modification of photoanodes with a highly efficient heterogenized molecular water oxidation catalyst.;Light scattering phenomena from plasmonic metal nanoparticles are frequently employed in solar cell research to increase their effectiveness by trapping light in thin absorbers that possess high charge carrier collection efficiencies. In photoanodes, these light scattering materials must be combined with the high surface area nanoparticulate oxides used in photoelectrochemical cells, which makes surface plasmon resonant metal nanoparticles excellent candidates. In addition, these must be protected and insulated from the electrolyte in the cell. Toward this end, we synthesized Au SiO2 TiO2 coreshell-shell nanoparticles and aggregates, performed theoretical calculations for further insight into their structure, and incorporated them into dye-sensitized photoelectrochemical cells. We show that this architecture allows the light-absorbing dyes to be anchored in close proximity to the surface of the plasmonic particle, thereby maximizing the effect of localized surface plasmon resonance around the nanoparticle. Additionally, the first demonstration of using a coupled plasmonic system to enhance a photoelectrochemical cell is demonstrated, and is shown to provide large enhancements to overall solar cell efficiency.;For years, Cp*Ir-containing water oxidation catalysts were believed to be highly effective molecular species for water oxidation. Here we show that this is not the case, and that the Cp*Ir family of compounds are precatalysts for the active catalytic species which is formed after oxidative loss of the Cp* moiety. In cases where an oxidatively stable bidentate chelate ligand is present on the iridium precatalyst, the active catalyst remains as a discrete molecular species and retains its homogeneity. Using a variety of characterization techniques, we inferred the structure of the true catalyst to be an iridium dimer bearing a single bidentate chelate ligand per iridium, bridged between iridium atoms by either a mono- or bis- mu-oxo ligand. Electrochemically and chemically driven water oxidation trials both show that this material is a very highly active catalyst for water oxidation, performing the oxygen evolution reaction with nearly zero overpotential.;While this molecular iridium species is a highly active homogeneous catalyst for water oxidation, its true utility in practical applications is as a heterogeneous catalyst. Remarkably, it directly and robustly binds to oxide surfaces without the need for any externally applied potential or linking groups by water elimination, forming a direct bond between the iridium centers in the molecular species and the metal oxide surface via an oxo-bridge. This molecular monolayer grants corrosion protection to the underlying oxide and allows for water oxidation at record low overpotential by combining the versatility of a homogeneous catalyst with the robustness of a heterogeneous one in an entirely new class of auto-grafting catalytic molecules. The ease and versatility of auto-grafting materials can also be extended to water oxidation catalysts containing earth-abundant metals. In concurrent studies, we found a highly active particulate cobalt-phosphine material can adhere to electrode surfaces by a different mechanism, enabling very low-cost, paint-on catalytic materials. |
