Structure and Dynamics of Cu and Cu-Ag Nanocrystal Catalysts during Electrochemical CO2 Reduction

In recent years, the CO2 reduction reaction (CO2RR) has been a popular topic in the field of electrocatalysis for its potential to help in mitigating climate change effects. As renewable energy sources such as solar and wind are rising in the market, there is a growing need for energy storage due to the intermittency of production. Renewable energy can be stored in batteries, but chemical bonds offer energy densities that are orders of magnitude higher. CO2 reduction is one of the electrocatalytic reactions that can store electrons in chemical bonds, simultaneously decreasing the amount of harmful greenhouse gas and creating useful fuels and chemical feedstocks. However, this vision is only possible if new catalysts for CO2 reduction are developed, because currently the efficiency and selectivity of the reaction are not high enough to allow for an industrially-viable technology. Due to the fundamental restrictions between the bonding strengths of CO2RR intermediates, complex nano-engineered catalysts are particularly well suited for achieving substantially better catalytic selectivity as compared to state-of-the-art Cu bulk materials. Creating high-energy facets with low-coordination atoms as well as alloying Cu with other metals are among widely pursued strategies, and for such objectives, we find small (<10 nm) synthetically-tunable nanocrystals to be promising candidates. That said, industrially-viable catalysts must not only be efficient and selective but also stable. Ironically, the very properties that offer favorable catalytic performance also render the material prone to morphological restructuring, namely sintering, under the operating conditions. We believe that in order to prevent sintering in the future, it needs to be studied first, so we directly focus on this issue by systematically probing the reaction conditions during electrocatalysis. We hope that this topic will be further investigated as the complexity of the experimental parameters calls for efforts of the same magnitude as to what has been done to understand the CO2RR selectivity and reaction mechanism. Chapter 1 discusses the opportunities and challenges of electrocatalysis in the 21st century. We draw some analogies to other fields where an understanding of the theoretical limits as well as an ambitious pursuit of chemical reaction control were needed to create industrially-viable technologies. We then motivate the need of for a fundamental understanding of morphological changes occurring during electrocatalysis in light of these considerations. Chapter 2 describes the synthesis, characterization, and thermodynamic understanding of different morphologies of Cu-Ag bimetallic nanocrystals. Cu and Ag do not alloy, so the bulk Cu-Ag materials used for catalysis possess monometallic domains of a specified size, but with nano-colloidal synthesis, it is possible to bring Cu and Ag into a more intimate contact which may give rise to a change in material properties. As such, we have synthesized a new structure, the nanocrescent, and its formation, based on thermodynamic principles, is explained. Chapter 3 presents the catalytic performance of Cu-Ag bimetallic nanocrystals for CO2RR and compares it with that of physical mixtures of monometallic Cu and Ag particles. This chapter illustrates precisely why an understanding of sintering and its prevention are crucial, as the studied structures undergo a complete morphological restructuring and can no longer be confidently distinguished as distinct particles. Nevertheless, we observe a significant shift in catalytic selectivity as compared to pure Cu nanocrystals. Cu-Ag materials decrease the activity towards undesired H2 production and increase the efficiency of oxygenates formation. Chapter 4 focuses directly on the issue of electrochemical sintering by studying the behavior of Cu nanocrystals under the standard conditions of CO2RR as well as a range of control experiments. We hypothesize some possible driving factors that could lead to the morphological restructuring and aim at distinguishing between them by changing variables such as the gas environment or pH. Ligand presence is probed with spectroscopic techniques, and morphological structures are imaged with electron microscopy. The presented set of evidence demonstrates that CO, a CO2RR intermediate, plays an important role in the sintering process by changing nanoparticle surface properties, leading to the formation of larger single-crystal facets