Investigating Impacts of Interfacial Electrode Phenomena on the Carbon Dioxide Reduction Reaction

Abstract

The carbon dioxide reduction (CDR) reaction presents an outstanding issue for the scientific community, with its proper implementation moving humanity towards more sustainable living. Prior to this, CDR needs to be improved further, with fundamental studies laying the groundwork for future large-scale applications. A detailed understanding of how CDR is affected by a variety of methods is required to determine the ideal approach for a host of real-world scenarios. This thesis explores the impacts of different methods to improve CDR, both directly through catalyst modification as well as indirectly through electrolyzer and anodic reaction optimization. Chapter 2 explores the impact of organic ligand modifiers on a Cu electrode surface. Directing selectivity of Cu, a notoriously unselective CDR catalyst, is a widely investigated challenge for the community. Using different organic ligands, a heterogeneous Cu surface was modified and its CDR performance evaluated, providing insight into the effect of this approach. In doing so, a specific class of ligands, those featuring a benzylthiol moiety, were discovered to direct CDR selectivity towards formate. Comparison with standard samples and investigation of the surface uncovered a selective decomposition of these ligands to deposit CuxS active sites that were responsible for the observed selectivity. This highlights the organic modifier approach as a potential method to direct CDR selectivity, while also reiterating the importance of proper investigation for the root cause of observed catalytic changes. Chapter 3 investigates another method for directing CDR selectivity, the modification of the physical electrode with microstructured features. Recent years have seen the rise of nano- and micro- structured electrodes for use in the CDR, and perhaps surprisingly, reported impacts on the selectivity of CDR in doing so. While numerous explanations have been put forth in the literature, little experimentally validated evidence has been provided. This chapter sheds light on the source of the selectivity alteration observed with structured electrodes by coupling CDR performance measurements with an in situ technique to observe the local pH at the electrode surface. The CDR selectivity on microstructured electrodes was discovered to be entirely determined by the evolution of the local pH at the electrode surface, which itself sets the availability of protons for parasitic reaction to the hydrogen evolution reaction. Chapter 4 studies an approach to improve the activity of an alternative anodic half-reaction for CDR electrolyzers, the hydrogen oxidation reaction (HOR). Use of the HOR as the anodic half reaction would enable a decrease in the total cell potential of CDR electrolyzers, but the solubility limit of H2 in aqueous solutions limits the viability of this approach. With the aid of microporous water, investigation of the solubility enhancement of H2 was performed and its effect on the HOR studied. While still below that which would be required for implementation to a CDR electrolyzer, the HOR activity enhancement highlights this as a potentially viable subject of future research. Chapter 5 presents work towards development of a novel electrolyzer device with two individually addressable working electrodes with fluid flow between them. The developed device allows for high collection efficiency between the two electrodes owing to the forced mass transport from the generator to the collector electrode. Validation of the device with performance benchmarks in aqueous and organic solutions with standard reference molecules demonstrated that the device could operate under a variety of conditions. With the device characterized, it can be leveraged for unique future applications, such as the tandem electrolysis of CDR as well as for the oxygen evolution reaction (OER) in seawater conditions. Chapter 6 interrogates the water-catalyst interface of an attractive OER catalyst, La3+ modified Co3O4. Using ambient pressure X-ray photoelectron spectroscopy (APXPS), a series of Co3O4 OER catalysts with variable La3+ concentrations were studied to confirm the previously reported enhanced OER activity of these catalysts. The APXPS studies revealed an increasingly hydroxylated surface that upon introduction of water lead to deconstruction of the Co3O4 lattice. This lattice deconstruction is consistent with the improved activity, as OER requires the shuttling of protons to and from the catalyst surface and the presence of hydroxylated species promotes this. In the context of CDR, this study evaluates a useful catalyst that can be used to enhance the anodic half reaction and increase the performance of an electrolyzer.