UC Berkeley

Electrochemistry builds the bridge between renewable electricity and matter. As a promising branch of green chemistry, electrochemistry allows the transformation and utilization of carbon dioxide (CO2) under mild operating conditions per the needs of carbon capture utilization and storage (CCUS). Interestingly, with the increase in electron numbers in the electrochemical CO2 reduction, multiple products such as carbon monoxide (2e-), methanol (6e-), methane (8e-), ethylene (12e-), ethanol(12e-), and propanol(18e-) can be generated. As a double-edged sword, the broad spectrum of CO2 electroreduction products ensures a large market size of downstream products but also puts forward great challenges for the electrocatalytic theory and process of CO2 reduction. Catalysts play an instrumental role in activating the surface adsorbed molecules and modulating the pathway for molecular transformation, which determines the reaction kinetics, and catalyst selectivity, and therefore is regarded as one of the core components in the CO2 electrolyzer. Suitable electrocatalysts, such as Au, Ag, and Cu, were found to be necessary to overcome the chemical inertness of CO2 and minimize overpotentials. Specifically, Au and Ag are promising catalyst candidates for carbon monoxide (CO) generation while Cu is suitable to catalyze the carbon-carbon (C-C) coupling and generate multicarbon products in CO2 electroreduction. Understanding and designing active catalytic sites for efficient conversion of CO2 to multicarbons products (C2+) through effective catalytic theory is of importance to the community.This dissertation is to present a systematic study of rational design strategy in CO2 electroreduction. In particular, it focuses on how the tandem strategy can facilitate practical CO2 electrolysis. Within this scope, the methodology, universality of the tandem strategy, structure and phase property of the bimetallic catalysts, and corresponding CO2 transformation mechanism are discussed in detail. Chapter 1 provides an overview of the CO2 electrolysis. It introduces the background, utilization method, and the current progress and remaining challenges of the CO2 reduction reaction with an emphasis on the corresponding catalyst design strategies. Moreover, the extension of CO2RR from benchtop to industrialization is also briefly discussed. Chapter 2 demonstrates the feasibility and universality of tandem strategy in CO2RR under industrial relevant current densities with Cu-Ag bimetallic catalyst on a gas diffusion electrode (GDE). The significant enhancement of multicarbon production rates can be attributed to the efficient Cu-Ag tandem strategy. Meanwhile, the increased intrinsic activity suggests the existence of new mechanisms in a CO-enriched local environment enabled by the tandem strategy. As the combination of Cu and Ag has been proved to be a good tandem CO2RR electrocatalyst, chapter 3 focuses on the interaction and structural evolution of Cu-Ag nano-alloy under electrochemical CO2RR conditions. It was found that Cu would gradually leach out from nano-alloy during electrolysis, leading to a phase separation process and the formation of thermodynamically stable inter-doped Cu- and Ag-rich grains. Further electrochemical Pb-UPD and Operando high-energy-resolution X-ray absorption spectroscopy (XAS) methods were utilized to reveal the surface property and dynamic evolution of Cu-Ag nano-alloy during CO2RR. On the other hand, chapter 4 switches the focus from the exploration of tandem catalysts to the understanding of molecular transformation pathways in the tandem system. Inspired by the Wood-Ljungdahl pathway in natural CO2 fixation, the asymmetric coupling process in CO2RR was experimentally validated in aqueous CO2 electrolysis by conducting isotope-labeled co-reduction experiments on the Cu surface where 13CH3I and 12CO were co-fed externally as the methyl and the carbonyl sources, respectively. Cu-Ag nanoparticle assembly was further employed as a practical system to achieve asymmetric C-C coupling for CO2 electroreduction. Chapter 5 closes this dissertation by summarizing the overall Ph.D. research and provides an outlook for the future research direction on electrochemical CO2RR.