Operando measurements, reactor design, and kinetic analyses for electrochemical CO2 reduction
Keywords: Engineering.
Materials Science and Engineering.

As greenhouse gas emissions continue to rise, along with the probability of potentially devastating or catastrophic climate change scenarios, so does the need for technologies and processes to reduce or eliminate these emissions. Among them, electrochemical CO2 reduction (CO2R) has the potential to turn the major driver of global warming into a renewable feedstock for conversion to valuable chemicals and fuels. However, grand challenges remain before CO2R processes can be commercialized, including the low activity of CO2R catalysts, their low selectivity, and their low durability. An incomplete understanding of the reaction mechanism of CO2R, the degradation mechanisms of catalysts, and how these depend on the local reaction environment and the catalyst itself are hindering efforts to improve the performance of CO2R catalysts. This dissertation describes four projects designed to address these key unknowns to solve the above grand challenges. First, we developed an electrochemical flow reactor and metal thin film synthesis procedure for operando attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The operando capability was used to probe how the local reaction environment affects the activity and selectivity of nanostructured Au thin film catalysts. We demonstrated the impact of electrode potential, electrolyte flow rate, and thin film morphology on the CO2R activity and the selectivity to CO versus the hydrogen evolution reaction (HER). Our results show that complex interactions between the catalyst and local reaction environment must be considered for designing CO2R electrolyzers. Second, we compiled and analyzed CO2R datasets from the literature for various catalysts and CO2R products. Although we could rationalize many of the reported strategies for improving CO2R activity and selectivity, we highlighted that many gaps remain in the reported wealth of CO2R data. These gaps must be filled to narrow down plausible CO2R mechanisms for all catalysts towards distinct products, and to understand how CO2R pathways are affected by the reaction parameters and local reaction conditions. We identified future opportunities for experiments needed to address these unknowns. Third, we designed a model system to identify and study the poorly understood degradation mechanisms of CO2R catalysts. CO2R experiments demonstrated a surprisingly strong relationship between carbon-based supports and the degradation pathway of Au nanoparticles. These results, when supplemented with a review of the literature, suggested that different degradation processes occur simultaneously, driven by processes or reaction conditions at the electrode-electrolyte interface. Nanobubble formation likely drives the migration of particles on the support, whereas particle coalescence is likely controlled by adsorbates, electrocatalytic activity, and the electric double-layer structure. Multiple classes of experiments are suggested to probe different processes or factors relevant to catalyst degradation. Fourth, we revisited Tafel analysis for studying the intrinsic kinetics of the CO2R reaction and determining its rate-determining step (RDS) on Au catalysts. In particular, several reaction models were developed, and an accessible fitting procedure based on Microsoft Excel's Solver that includes error estimates for fitted kinetic parameters was used to determine the most likely reaction mechanism of CO2R on Au catalysts; however, based on the trends of the analyzed experimental data and data compiled from the literature, we concluded that current assumptions about mass transport effects are insufficient to correct for them and to accurately extract kinetic parameters. In summary, the projects described in this dissertation aimed to understand the reaction mechanism of CO2R, the degradation mechanisms of CO2R catalysts, and how these depend on the catalyst and reaction environment. As our understanding of the CO2R mechanism and the degradation of catalysts improves, we will be able to rationally design CO2R electrolyzers for the efficient conversion of CO2 to valuable fuels and chemicals—simultaneously helping reduce our greenhouse gas emissions while still providing the energy security and chemicals we rely on