This thesis focuses on two main topics in electrochemistry: (1) studying electrochemical energy storage systems and (2) developing electrochemical energy conversion pathways by the reduction of carbon dioxide to useful organic products. The first half of Chapter One provides an introduction to electrochemical energy storage technologies. Different categories of aqueous redox flow battery systems suitable for large-scale energy storage are discussed in detail. At the end of the first half of the introduction, a brief introduction to the lithium-sulfur battery is presented. The rest of Chapter One discusses the electrochemical carbon dioxide reduction.
Among the different redox flow battery technologies, the all-iron redox flow battery is an attractive solution for large-scale energy storage because of the low-cost and eco-friendliness of iron-based materials. A major challenge for realizing a continuously operable all-iron redox flow battery is the parasitic evolution of hydrogen at the iron electrode during the charging step. Results and discussion presented in Chapter Two provide insights for minimizing hydrogen evolution in the all-iron battery system. At a given bulk concentration of iron (II), pH of the electrolyte, temperature, concentration of additives, and current density are recognized as key factors affecting the coulombic efficiency of the battery. Elevation of pH near the electrode surface during electrodeposition plays a significant role in hindering hydrogen evolution. Thus, electrolyte flow rates drastically influence the coulombic efficiency of the all-iron redox flow battery. By operating at 60 °C and a pH of 3 with ascorbic acid and ammonium chloride, we could achieve a coulombic efficiency of 98%. This value of coulombic efficiency is among the highest values reported for the iron electrode of the all-iron flow battery.
Chapter Three discusses the kinetics of electrodeposition of iron and the evolution of hydrogen during the charging of the all-iron redox flow cell. We could verify that the kinetics of iron deposition improved with the higher activity of iron (II), lowering the rate of the hydrogen evolution reaction (HER). Further, the increase of the pH near electrode surface results in increased coulombic efficiency at the higher charging current densities. We show that from 0 to 80% state-of-charge and charging at 30 and 40 mA/cm2, a steady coulombic efficiency of 98 ± 1% could be observed due to improved kinetics at the higher concentration of iron (II) or increased surface pH at raised current densities.
Chapter Four delivers a critical analysis of studies of the lithium-sulfur battery. The lithium-sulfur battery is a promising technology that has the prospect of doubling the energy density of lithium-ion batteries due to the high specific capacity of the sulfur electrode. Further, sulfur is also an expensive material and has the potential to lead to a cost-effective battery. In the present study, we attempted to understand the use of the USC invented mixed-conduction membrane to address the issue of the polysulfide shuttle. Initial cycling data showed that about 20% improved coulombic efficiency and 11 to 16% improved material utilization with this novel membrane. Further, by using different layers and different porosity of mixed conduction membranes, we attempted to validate the intercalation process of lithium ion to facilitate charge transfer kinetics of the lithium-sulfur cell. Further, the properties of Ketjen Black as a cathode matrix material for improving polysulfide retention and increasing material utilization are discussed. And the results presented in Chapter Four with a mixture of carbon materials point to approaches for achieving better utilization with improved rates of charge and discharge using cathodes based on Ketjen Black.
Chapters Five, Six and Seven discuss the bio-catalytic pathways for the reduction of carbon dioxide to formate using the enzyme formate dehydrogenase and redox co-factors. Electrochemical analysis of NAD+/NADH and measurement of enzyme activity is challenging, and Chapter Five presents a simple electrochemical detection of NAD+/ NADH using an unmodified carbon fiber microelectrode as an accurate, convenient and low-cost detection technique for studying enzymatic reactions. Diffusion-limited current measurements of reduction and oxidation of NAD+ and NADH enables investigation of NAD-dependent enzyme activity and indirect analysis of concentrations of substances like formate and ethanol which are substrates for NAD-dependent enzymes. Further, this method was validated for the in-situ detection of electrochemically-generated NADH. This fast, in-situ determination of NAD+ and NADH can be used in bio-electrochemical applications and be further developed to a biosensor.
Chapter Six describes results on the bio-catalytic reduction of carbon dioxide using commercially-available formate dehydrogenase enzyme. We demonstrate an efficient and continuous conversion of carbon dioxide to formate using formate dehydrogenase enzyme derived from Candida boidinii yeast and an electrochemically-generated artificial co-factor, methyl viologen radical cation. Continuous regeneration of this artificial cofactor could be achieved at –0.44 V vs. the Normal Hydrogen Electrode (NHE) leading to a significantly lower overpotential for the reduction of carbon dioxide to formate. Thus, the electrochemically-regenerated methyl viologen radical cation works efficiently as a cofactor to support the proton-coupled electron transfer to the carbon dioxide molecule. With a special electrochemical reactor assembled with three chambers separated by anion and cation exchange membranes, we demonstrate the generation and accumulation of formate and the evolution of molecular oxygen. The proton-exchange membrane allows for the accumulation of formate by preventing loss to electrochemical re-oxidation at the oxygen evolution electrode while the anion-conducting membrane improves the utilization of the cofactor by suppressing its loss by cross-over of the oxygen electrode. This novel three-chamber reactor has been shown as a proof-of-concept for efficient and continuous conversion of carbon dioxide into formate.
In Chapter Seven, we discuss various factors in improving the efficiency of the bio-catalytic carbon dioxide reduction including the role of thermodynamics, kinetics, mass transport and pH balance of the system, and in-situ formate separation on the coulombic efficiency. We also discuss the performance improvements resulting from various cell configurations.
Chapter Eight summarizes the future directions for research on all of the projects discussed in the thesis.
|Commitee:||Prakash, Surya, Shing, Katherine|
|School:||University of Southern California|
|School Location:||United States -- California|
|Source:||DAI-B 80/09(E), Dissertation Abstracts International|
|Subjects:||Chemistry, Chemical engineering|
|Keywords:||Carbon dioxide reduction, Electrochemistry, Energy conversion, Energy storage, Lithium-sulfur, Redox flow battery|
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