The development of sustainable energy technologies is critically needed to reduce greenhouse gas emissions and mitigate anthropogenic climate change. The electrochemical conversion of carbon dioxide (eCO2R) is a promising technology to produce valuable, low-carbon fuels and chemicals with renewable electricity, thereby facilitating renewable energy penetration into the transportation and industrial sectors. However, significant improvements in energetic efficiency and reactor engineering are needed for eCO2R to become commercially competitive. In this thesis, I present a series of works which utilize a flow electrolyzer to provide fundamental insights into catalyst design and eCO2R reaction mechanism for multi-carbon chemical (C2+) production at industrially relevant reaction rates. In addition, I show that decoupling eCO2R through the CO intermediate enables sustainable operation in alkaline conditions, which enhance C2+ formation, as well as the production of heteroatomic products.
I begin by presenting a general techno-economic analysis of a CO2 electrolyzer system which integrates capital and operating costs for electrochemical conversion and product separation. I find that more desirable C2+ products such as ethylene and alcohols could become cost-competitive if electrolyzer performance can be dramatically improved. This model is extended to a two-step process, where required performance metrics for acetic acid and ethylene production are provided, and a life-cycle analysis is conducted.
Next, I present the synthesis and incorporation of a highly-porous copper catalyst into a three-compartment flow electrolyzer for eCO2R to C2+ products. The high porosity provides excellent bubble management, enabling stable operation at current densities up to 650 mA/cm2. Then, an electrolyte study is conducted showing that alkaline conditions greatly enhance C2+ selectivity relative to neutral electrolytes. However, the inevitable formation of carbonates with alkaline electrolyte motivates the two-step conversion of CO2 through the CO intermediate.
Motivated by the previous study, I then present the electrochemical conversion of CO (eCOR) in a flow electrolyzer in alkaline electrolyte. I show that compared to eCO2R under identical conditions, eCOR has a significantly higher C2+ selectivity, and that acetate becomes a significant product. The formation of acetate is attributed to a higher surface pH during eCOR facilitating hydroxide attack of a surface intermediate, as supported through isotopic labeling.
Based on the preceding hypothesis of electrolyte incorporation into eCOR products, I then present the electrolysis of CO in the presence of ammonia, leading to the formation of acetamide with up to 40% selectivity at 300 mA/cm2. DFT calculations suggest acetamide is formed through nucleophilic amine attack of a ketene intermediate, in competition with hydroxide attack leading to acetate. The presence of additional amines leads to their respective acetamides, providing valuable insight into the eCOR mechanism and demonstrating the concept of electrochemical heteroatomic product formation from a CO2 feedstock.
|Commitee:||Xu, Bingjun, Yan, Yushan, Rosenthal, Joel|
|School:||University of Delaware|
|School Location:||United States -- Delaware|
|Source:||DAI-B 82/3(E), Dissertation Abstracts International|
|Subjects:||Chemical engineering, Environmental engineering, Alternative Energy|
|Keywords:||Electrocatalysis, Electrochemistry, Sustainable energy|
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