UCLA

With the surge of intermittent, renewable electricity, the storage of excessive electricity and reduction of CO2 or N2 into value-added chemicals is of great significance for a sustainable society. One viable route that fulfills such a target is to construct a hybrid inorganic-biological system that converts electricity into chemical energy and reduces CO2/N2 into commodity chemicals. In this general approach, water is split into H2 and O2 by renewable electricity and the yielded H2 is consumed by microbes as a reducing equivalent for CO2/N2 reduction. The throughput of this system is limited by the poor gas solubility in the aqueous environment. The control and design of the gas environment in these hybrid systems is crucial to achieving high throughput and sustainable reactions. To this end, my thesis has focused on combining perfluorocarbon (PFC) nanoemulsions with the hybrid system to enhance H2 or O2 gas solubility and optimize reaction performance. My first research project focused on combining PFC nanoemulsions with a hybrid biological inorganic CO2 fixation system to enhance the faradaic efficiency of acetic acid production (Chapter 2). PFC nanoemulsions are biologically inert and have reported high H2 gas solubilities. We determined that the nanoemulsions are compatible with the acetogenic microbe strain Sporomusa ovata and the electrochemical water-splitting catalysts used in the hybrid CO2 fixation system. When 2.5 % (v/v) PFC nanoemulsion was combined with the hybrid system we observed approximately 90 % or greater faradaic efficiency at all tested current densities. After 4 days, we were able to achieve an average acetic acid titer of 6.4 � 1.1 g�L−1 (107 mM), which equates to one of the highest reported productivities of 1.1 mM�h−1. With this observed enhancement, we decided to explore the change in the gas environment made by the addition of PFC nanoemulsions. We performed experiments to understand the mechanism of the observed enhancement when PFC nanoemulsions were added to the hybrid CO2 fixation system (Chapter 3). Experiments of flow cytometry were performed using fluorescently tagged PFC nanoemulsions to probe the association of the S. ovata with the nanoemulsions. We found a non-specific binding interaction between the nanoemulsion and the bacteria that could result in an increase in the local H2 concentration or an increase in the rate of H2 transfer to the microbes. Rotating disk electrode (RDE) experiments were performed as an electrochemical surrogate to understand the H2 transfer kinetics. Our experiments revealed that there was 3.5 times increase in the kinetic current density with the addition of the nanoemulsion. Additionally, we found that the local H2 concentration was 1.2 times the calculated bulk H2 concentration. This suggests that the addition of the PFC nanoemulsions resulted in a local increase in H2 concentration and an increase in the rate of transfer of the reducing equivalent to the microbe. We theorized that the addition of PFC nanoemulsions to other hybrid systems could result in similar enhancement and an ability to control the gas environment. With the success of the PFC nanoemulsion in the hybrid CO2 fixation system, my research shifted to focus on using the nanoemulsions with a hybrid N2 fixation system to enhance ammonia production and self-sufficiency (Chapter 4). The hybrid N2 fixation reaction utilizes Xanthobacter autotrophicus, a microaerobic bacterial strain that reduces N2 to NH3 through its nitrogenase enzyme. This has previously been accomplished through a precise supplied gas mixture, since the electrochemically generated O2 is poorly soluble in the aqueous solution. We added the PFC nanoemulsions to this system to utilize the electrochemically generated O2 and enhance the throughput of the N2 fixation system. Our experiments demonstrated that the use of PFC nanoemulsions drastically reduced the loss of cell viability in a supplied anaerobic atmosphere and led to a 3-fold increase in Faradaic efficiency of the system during 5-days of running. We performed mechanistic studies to understand the O2 gas-PFC nanoemulsion environment within the hybrid system (Chapter 5). Confocal microscopy images of the PFC nanoemulsions interacting with the microbes revealed binding and complete coverage of the microbe surface by the PFC nanoemulsions. Experiments of RDE were performed to ascertain the O2 transfer kinetics between the PFC nanoemulsions and the microbes. We found that there was a 20-fold increase in the kinetic O2 reduction rate when the PFC nanoemulsions were added. Assuming a Langmuir adsorption model of the PFC nanoemulsion binding to the microbe, we were able to use flow cytometry to determine the maximum number of nanoemulsions that could adsorb to the microbe surface and the equilibrium constant for the adsorption. Our results showed that the number of nanoemulsions that could bind to a given microbe or particle is dependent on the size of the particle, but the equilibrium constant is more influenced by the surface charge, with the -COOH functionalized microspheres having the largest binding affinity. In conclusion, my thesis research was able to integrate PFC nanoemulsions with hybrid biological inorganic systems to enhance throughput and alter the gas environment in the systems. The ability to understand and alter the gas environment using these PFC nanoemulsions can allow for other aqueous systems with poor gas transfer rates or gas restrictions to function with enhanced benefits. This dissertation also serves as groundwork for continued research in customizing the PFC nanoemulsion to achieve the optimal binding and gas environment.