UC Berkeley

Climate change is one of the most significant challenges facing humankind. Despite outsized efforts to further foster renewable energy, the lion’s share of energy use is still derived from non-renewable fossil fuels. Through reducing CO2, the primary byproduct of fossil fuel combustion, into value-added fuels using renewable energy sources, we can help close the carbon emission loop, mitigate CO2 emissions, and make our society more sustainable. A significant challenge in artificial photosynthesis, where an artificial process mimics natural photosynthesis to produce solar fuel, is designing a catalyst for CO2 reduction and integrating it into a functional light-harvesting system. Biological microorganisms have evolved to convert CO2 to upgradeable intermediates with exquisite selectivity. Proteins undertake conformational changes to create local microenvironments and promote steric effects, leading to high reaction selectivity. Recent work has proposed an avenue to integrate whole-cell CO2-reducing biocatalysts with solid-state semiconductor light absorbers. Such integration between biology and materials science separates the demanding requirements for catalytic activity and light-capturing efficiency. It provides a route to bridge the high catalytic activity of enzymes in living cells with efficient solar conversion in robust solid-state devices. Directly interfacing CO2-reducing microorganisms with a light-harvesting semiconductor electrode presents a promising line of research to enable solar-powered CO2 reduction, called the “photosynthetic biohybrid system.” To make this approach a viable option for solar fuel generation, the fundamental working principles of the biohybrids must be better understood, and the performances need to be improved as well. Hence, this dissertation centers around the scientific fundamentals of biohybrids and attempts to improve the overall performance of CO2 reduction. After briefly introducing the problem statement and fundamental concepts related to artificial photosynthesis for light-driven CO2 to value-added products, I discuss the prospects and existing works around photosynthetic biohybrid systems, representatively nanowire-bacteria hybrids, in Chapter 1. In Chapter 2, I show how the local microenvironment created on the regularly aligned nanowire array interface influences the dynamic interaction and abio-biotic interface between the nanomaterials and bacteria. I also show how the strategies to form a hospitable environment toward living organisms impact the overall CO2-reducing performance of the system. In Chapter 3, I move to a methanol adaptation of S. ovata with intriguing enhanced metabolic activity with a robustness of the biocatalysts under an electrochemical environment. I illustrate how individual biocatalysts' turnover rates collectively influence the system's performance. I also describe the importance of matching the electron flux of each component in biohybrids, that of semiconductors and biocatalysts, to harmoniously enhance the total flux of CO2-reducing rates. In Chapter 4, I discuss the optimization of photovoltages and photocurrents of silicon nanowire array for the application of biohybrids and photoelectrochemical CO2 reduction performance based upon the strategies to enhance biohybrid systems in the earlier two chapters. Finally, I close Chapter 5 with an outlook on strategies for photosynthetic biohybrids at the time of writing.