Electroorganic CO₂ Fixation Via Paired Electrolysis and Nanoscale Electrocatalysis

Abstract

Sustainable energy driven electroorganic synthesis provides a green and inexpensive alternative to traditional synthetic approaches. For instance, the electroreduction of many classes of organic molecules enables the synthesis of carboxylic acids via coupling with CO2 (electrocarboxylation, EC) that presents a route towards pharmaceuticals and industrially relevant precursors. Historically, the development of EC was performed in undivided systems with sacrificial metal anodes (e.g., Mg). Upon dissolution, metal anodes produce Mg2+ ions that may interact with active species produced on a cathode, significantly affecting the reaction mechanism. Despite the high yields of target products achieved on a lab scale, the sacrificial-anode-based systems showed many significant drawbacks preventing them from industrial implementation. To this end, during my Ph.D. program, I aimed to develop an alternative sacrificial anode-free system. Thus, I studied the system and catalyst design for EC paired with useful anodic processes. In this thesis I discuss the feasibility of EC coupling with sustainable anodic processes (oxygen evolution reaction, ammonia or urea oxidation reactions) and suggest suitable system design. Next, I show the electrocatalyst screening on bulk metal electrodes for the EC of organohalides and imines in the absence of sacrificial metal anodes. Then I elucidate how surface chemistry and morphology of metal nanoparticles affect the EC of organohalides and imines and how stable Au, Cu, and Pd nanoparticles are during these reactions. Finally, I propose EC in a pressurized electrochemical cell towards high EC selectivity even at high organic precursor concentrations. In Chapter 3, I focus on the EC coupling with oxygen evolution reaction, ammonia oxidation reaction, and urea oxidation reaction. I discuss the disadvantages of the conventional method, perform membrane optimization and the screening of reaction conditions in the new proposed system. Extensive electrochemical results allowed me to propose the changes in the reaction mechanism in detail. I concluded this chapter with the discussion of the feasibility and energetical benefits of the anodic N-waste treatment (ammonia and urea oxidation reactions). This chapter shows that anion exchange membrane is the only membrane type that allows efficient EC in the absence of sacrificial metal anodes. The experimental observations revealed the need in the optimization of electrocatalyst and system design towards more selective EC at higher precursor concentrations. In Chapter 4, I aimed to evaluate electrocatalytic activity of bulk materials towards EC of organohalides and imines. I mapped out the relationship between cathode materials and activation of a wide range of organohalides. Additionally, I determined suitable substrates for electrode preparation for EC based on the material inactivity within the EC potential window. I demonstrate that even though there are certain trends for material (in)activity in EC, each organic precursor is unique and catalyst selection needs to be performed carefully. The discussion in this chapter highlights the need to further optimize cathode materials towards higher activity of EC and provides a tool – a map – to continue the cathode optimization in that direction. In Chapter 5, I discuss the application of nanomaterials as electrocatalysts in EC. I am evaluating nanoparticle electrocatalytic efficiency and structural stability in EC considering factors such as the surface chemistry of the nanoparticles, their surface morphology, and the nature of the material. I discovered that the surface chemistry of nanoparticles has strong effect on the EC selectivity. The effect of the nanoparticle surface features on the EC outcome varied depending on the organic precursor-nanomaterial combination. I found that Pd nanoparticles show great performance in the EC of imines and Cu nanoparticles with developed surface gave almost quantitative faradaic efficiencies for the EC of organohalides. Pd and most of the Au nanoparticles showed good stability in EC, while Cu nanoparticles underwent severe structural degradation. In Chapter 6, I show the feasibility of EC at high concentrations of organic precursor with the correct CO2/RBr ratio in the reaction mixture that was achieved using a pressurized electrochemical cell.