UC Irvine

This Thesis provides the seminal work towards the development of new technologies for ionic power generation and solar saltwater desalination. Central to the approach is the transformation of optically-inactive ion-exchange membranes used in electrochemical technologies into optically-active power-producing membranes. Chapter 1 provides a brief description of anthropogenic emissions and human induced global warming and climate change. It is desired that these human induced calamities can be mitigated by replacing fossil fuel energies with solar energy conversion technologies. Chapter 2 presents studies on a model system of optically-active membranes. Included is the synthesis of an exemplary visible-light absorbing photoacid dye and its covalent attachment to a nanoporous membrane. Basic materials characterization and more specialized photophysical techniques were used to characterize the dyes dissolved in solution and the dyes covalently bound to membranes. The study suggests that ion transport initiated by photoacids bonded to mesoporous materials will require careful molecular engineering to enable efficient light-to-ionic energy conversion.

Chapter 3 shows the first demonstration of photovoltaic action from a covalently sensitized ion-exchange membrane. Photoelectrochemical measurements quantified the photocurrent and photovoltage production and control studies suggested that the power production is a result of photo-generated ions produced from the photoacid dye. Results were consistent with protons being transported against a pH gradient opposite to the thermodynamically favored direction; albeit, no net power was generated because of a rapid and continuous ion crossover across the membrane due to use of a single monopolar ion-exchange membrane with a large pH gradient across it. The arrangement of these optically-active cation-exchange membranes adjacent to anion-exchange membranes is the focus of Chapter 4. The bipolar membranes showed enhanced photovoltaic efficiencies as compared to the monolithic counterpart and analogies from the physics of solar cells was used to elucidate the mechanism of photovoltaic action. The obtained photovoltages are more than half that needed to desalinate the sodium chloride in sea water to potable water and all that needed to convert moderately saline brackish water to potable water. The Thesis concludes with Chapter 5, which presents the ongoing solar-energy conversion optimization tactics for these devices.