Designing electrolyzers to elucidate governing phenomena involved in the electroreduction of CO2 operating in bulk-neutral pH
Keywords: Chemical Engineering.

The accelerating need to mitigate global emissions of CO2 has driven significant advancements in designing systems and strategies that transition towards a sustainable future. However, a growing global population continues to rely on fossil-derived sources for power, chemicals, agriculture, and transportation sectors. Several strategies and technologies are being developed to mitigate greenhouse gas emissions that exacerbate global climate change. Of these, electrochemical carbon dioxide reduction (CO2R) exhibits promise for utilizing captured carbon to produce industrially relevant products, especially if coupled to point sources. The performance of CO2 electrolyzers is tied to a combination of coexisting phenomena with varying weighted contributions, which change under different operating regimes. This dissertation aims to elucidate governing factors and provide understanding for high-rate CO2 reduction in vapor-fed systems operating in bulk-neutral pH electrolyte. The adoption of gas diffusion electrodes (GDEs) brings forth new formulations and environments to study, providing increased reaction rates due to enhanced mass transport. Most strategies for targeting multi-carbon product formation rely on utilizing Cu within the electrode, derived from early work from Hori and co-workers. Several advancements in materials discovery, component analysis and configuration, and system design have led to higher reaction rates and selectivity towards these products. However, a lot of the current strategies for high-rate CO2R require highly alkaline pH systems that are expensive to maintain and lead to large anodic CO2 crossover. Conversely, the use of bulk-neutral pH electrolyte leads to lower selectivity due to increased rates of hydrogen evolution reaction. To explore this, we design electrolyzers using advanced manufacturing paired with a thin catalyst-GDE for improved understanding and performance in bulk-neutral pH systems. We observe high selectivity towards multi-carbon products (> 85%) at > 200 mA cm−2 due to a high local pH induced by the catalyst environment. Upon increased current density, mass transport greatly affects product formation based on potential and inlet CO2 flow rate within the mixed-control regime. Both experimental and modeling approaches help uncover underlying mass transport effects including temperature, hydroxide formation, and partial penetration into the diffusion media. To further uncover effects on the local reaction environment in both foil and gas diffusion electrodes, we tune the electrode surface area of catalysts by synthesizing a rough Cu nanoflower morphology shown to produce multi-carbon products when performing carbon monoxide reduction. Starting with foil, these catalyst nanostructures are translated to the GDE architecture to understand the interplay of catalyst surface and surrounding microenvironment on current-potential relationships. Like on Cu foil electrodes, the intrinsic activity of Cu does not appear to change based on nanostructuring GDEs. Furthermore, we observe lower overpotentials with increased catalyst area at a constant current density, uncovering a semi-linear relationship between the ratio of roughness factors and potential difference when activation-controlled. We then create a foil-GDE hybrid reaction environment by synthesizing a deterministic electrode using a two-photon printing technique. This electrode consists of a perfectly distributed grid, representing an ordered structure with uniform pore size. This results in geometric current densities operating between previous foil and GDEs in comparison, with probable product activity zones based on increased rates of HER and slow CO2 diffusion in aqueous phase. These approaches towards understanding changing reaction environments in combination with materials design can aid in the development of systems with increased energy efficiency without sacrificing selectivity. Our efforts alongside others investigating CO2R have provided compelling results towards commercialization. Several techno-economic assessments suggest selectivity, cell potential, electricity price, and anodic CO2 cross-over are among the most influential on production price; this is often compared to market price. However, these analyses do not consider differences in technology readiness level or cost class. In the final part of this dissertation, we present an energetics-first approach for analyzing these systems. Our results show a small threshold for cell potential is permitted in order to compare with ethylene production from conventional cracking technologies, thus requiring new formulations for operating in bulk-neutral pH. We assess the prospects of various anode reaction pairs for sustainable ethylene production. Finally, we conclude and provide outlook and considerations for the future development of these systems. With both experimental and modeling approaches, this dissertation evaluates the underlying effects and prospects for high-rate electrochemical CO2 reduction operating in bulk-neutral pH