With continual growing concerns of the effects of climate change on our society, various strategies are necessary to achieve global net negative carbon emissions to combat climate change. One such approach is to reduce our reliance on fossil fuels which continues to be our primary source not only for electricity and transportation, but as an important source for producing chemical products such as plastics. Photoelectrochemistry provides one promising pathway for this by directly transforming solar energy into chemical energy in the form of chemical bonds. Photoexcited charges can be used to upgrade abundant chemicals such as water into hydrogen and oxygen, CO2 into ethylene or ethanol, and even oxidize biomass such as glycerol into glyceric acid. Despite considerable research and progress made for these reactions in electrocatalysis, enabling this process photoelectrochemically has thus been hindered by low selectivity, stability, and activity. Photoelectrochemical (PEC) systems require the co-development of both light absorbers and co-catalysts, adding a layer of complexity and an innumerable number of handles that can affect overall performance. Thus, a generalizable framework for designing PEC systems for a variety of target reactions becomes crucial and this dissertation focuses on principles for designing more efficient PEC systems.

To introduce PEC systems for modern applications, the history and fundamental concepts for photoelectrochemistry will first be introduced in the first chapter. The current state of the field will also be surveyed to highlight which directions of research are still in its nascent stage. In the second chapter, a general approach to designing efficient photocathodes is shown using metal nanoparticles on silicon nanowire arrays for CO2 reduction. The effect of catalyst selection on performance will also be highlighted to emphasize its importance as a handle in PEC design. In the third chapter, a perovskite driven PEC device will be used to convert CO2 into multicarbons at very low potentials, allowing for the demonstration of bias-free C2 generation when coupled with a BiVO4 or TiO2 photoanode. The photovoltage required for such sluggish reactions is discussed as well as the effect of photocurrent flux on selectivity for CO2 reduction which is an often-neglected consideration. In the fourth chapter, the focus turns to the photoanode and offers an alternative reaction, glycerol oxidation, as a strategy to reduce overall photovoltage requirements for bias-free systems, and demonstrates how these design principles can effect bias-free current densities over 5 mA/cm2 for hydrogen evolution and glycerol oxidation. Finally, in the fifth chapter, the findings of the previous chapters will be summarized and an outlook for the development of PEC systems will be provided. By the end of this dissertation, the reader will gain an appreciation of the challenges facing PEC development and the framework for tackling such challenges. While the concepts here focus specifically on hydrogen evolution, CO2 reduction, and glycerol oxidation, the hope is that these concepts will provide guidance for any additional target reactions that will inevitably appear in the future.