UC Irvine

Burgeoning global energy demand coupled with continually increasing greenhouse gas emissions prompts reassessment of our entire energy infrastructure in order to mitigate climate change and ensure sustainable use for future generations. An archetypal shift away from fossil fuel dependence towards renewable, carbon neutral energy resources, is crucial in achieving this goal. A promising approach towards the realization of a carbon neutral energy economy is the capture and conversion of carbon dioxide (CO2) into chemical fuels using renewable electricity. Utilization of fuels generated from CO2 and renewable electricity makes the process effectively “carbon neutral”, as the CO2 produced upon combustion can be directly converted back into fuel. As an additional benefit, carbon neutral fuels can be directly incorporated into legacy infrastructure. Existing methods for carbon dioxide capture and conversion into fuels are highly inefficient however, making the approach prohibitively expensive. These inefficiencies must be addressed in order to promote carbon neutrality within our current energy economy. This dissertation describes the implementation and investigation of Lewis-acidic interactions to promote energy-efficient electrochemical CO2 capture and conversion into chemical fuels. Both intra- and inter-molecular interactions are shown to increase the efficiency of each process through leveling of the overall energy landscape. Utilization of these interactions to lower crucial high energy intermediates was observed to inhibit undesirable side reactions, including decomposition; resulting in further improvements to efficiency. This thesis is broken up into two parts, focusing on the topics of electrochemical CO2 capture and electrochemical CO2 reduction. Part I discusses topics in electrochemical CO2 capture and concentration (eCCC) processes, which includes chapters 1 and 2. Part II discusses electrochemical CO2 reduction reactions, and includes chapters 3, 4, and 5. Chapter 1 introduces electrochemical CO2 capture and concentration (eCCC) topics, with a specific focus on systems that utilize redox-carrier species. Relevant equations and thermodynamic considerations for efficient eCCC process are discussed in detail. Additionally, reported examples of eCCC systems utilizing redox-carriers are discussed and evaluated. Chapter 2 describes the utilization of intermolecular hydrogen-bonding interactions in quinone-based systems for electrochemical CO2 capture applications. Addition of alcohols to solutions of 2,3,5,6-tetrachloro-1,4-benzoquinone (TCQ) is shown to shift reduction and CO2 binding to milder potentials, permitting their utilization under aerobic conditions. Thermodynamic evaluation of hydrogen-bonding interactions with TCQ aided in the selection of appropriate alcohol additives that permit TCQ reduction at mild potentials without detrimental effects on CO2 binding affinity. A DMF solution of TCQ containing ethanol was observed to successfully capture, release, and concentrate CO2 from simulated flue gas streams at potentials amenable to aerobic environments. Chapter 3 introduces electrochemical CO2 reduction topics. The chapter primarily focuses on thermodynamic considerations necessary to develop catalysts that can address the challenges of product selectivity and overpotential that are currently facing the field. The utilization of cooperative binding or stabilizing secondary interactions are discussed in detail with relevant examples from the literature. Chapter 4 describes the development of an SNS pincer ligand scaffold (SNScrown) that features an aza-crown ether functionality. The SNScrown ligand allows placement of group I or II metal cations in close proximity to a transition metal center. The synthesis and characterization of palladium and rhodium SNScrown complexes featuring bound sodium or barium cations is reported. The reactivity of the palladium complexes towards CO2 and proton reduction was investigated. It was found that the presence and charge of a bound alkali or alkaline earth metal cation has little to no effect on the electronic structure or CO2 reactivity of the palladium center, but has profound effects on proton reduction and decomposition under catalytic conditions. Incorporation of Na+ or Ba2+ into the crown ether cavity is observed to result in significant inhibition of proton reduction, while hindering dimerization, which serves as an important decomposition pathway. As a result, the Ba2+ complex displays increased CO2 product selectivity under catalytic conditions during controlled potential electrolysis, compared to an identical experiment without an incorporated group I or II metal cation. Chapter 5 describes the investigation of proximal cation effects on rhenium bipyridine complexes featuring crown ether-like functionality within the ligand framework (Bpycrown). The synthesis, characterization, and CO2 reactivity of rhenium carbonyl Bpycrown complexes (Re(Bpycrown)(CO)3Cl) is reported. Unlike the SNScrown complexes, incorporation of group I and II metal cations into the crown ether cavity was found to significantly affect the electronic structure and CO2 reactivity of the Bpycrown complexes. Both metal and bipyridine-based reductions were shown to shift anodically with increasing charge of the metal in the crown. Additionally, CO2 reactivity of the complexes shifts to more positive potentials with increasing charge of the bound cation; however, increasing cation charge also appears to correlate with decreased catalytic rates.