Karla Banjac
The urgency of climate change demands the simultaneous removal of carbon dioxide from the atmosphere and the transition to renewable energy sources. This aim is realizable through electrochemical reduction of carbon dioxide (CO2RR), which is a promising route for synthesizing renewable carbon-based fuels and chemicals.Among many existing CO2RR catalysts, copper is the only earth-abundant metal that produces energy-rich molecules. Practical application is hindered by its poor product selectivity and modest long-term catalyst stability. To overcome these limitations, an atomistic understanding of the catalytic processes and identification of CO2RR active sites on Cu are needed.Current research implies that the active sites form upon the catalysts' evolution during the reaction. However, fast Cu oxidation hinders their identification in conventional ex situ studies. The following questions remain unanswered: What are the morphology and chemical composition of the active sites? How do they form?This thesis explores the in situ evolution of Cu-based CO2RR catalysts. We study this phenomenon down to the nanoscale through a synergy of in situ microscopy and spectroscopy studies. We employ graphene as a 2D protecting layer on Cu to study the Cu nanoscale structural evolution in situ via electrochemical scanning tunneling microscopy (EC-STM). We use in situ Raman and X-ray photoelectron spectroscopies to study the evolution of the Cu catalysts' oxidation state.First, EC-STM studies reveal the formation of few nanometer-sized cuboids on Cu catalysts as the main structural evolution during CO2RR. We show that this evolution occurs in aqueous electrolytes over a wide range of pH and compositions. Second, we show that metallic Cu is the active phase. Reduction of the Cu oxide, formed either through chemical synthesis or through the exposure of Cu catalysts to air, is unavoidable prior to CO2RR.To understand the nanocuboid formation mechanism, we present a comparative study of the Cu nanocuboids formed upon CO2RR and the Cu2O cubes prepared through electrochemical cycling synthesis. Morphological and chemical composition differences between them allow us to conclude on distinct formation mechanisms. We show that the metallic Cu nanocuboids form during a potential-driven re-organization of atoms in the topmost surface layers.Furthermore, we present EC-STM studies revealing the atomistic details of the Cu nanocuboid formation. By comparing these processes with the multilayer mound growth in Cu homoepitaxy, we show that the nanocuboids are (100) facet multilayers formed upon reduction of Cu oxides. We propose that the nanocuboids are CO2RR active sites, ultimately forming on any Cu catalyst regardless of its initial macroscopic shape and oxidation state.The in situ identification of active sites, presented here, offers a unique framework for the re-interpretation of the existing literature, invites further fundamental theoretical and experimental research, and paves the way for rational catalyst design. Moreover, this thesis further demonstrates the importance of in situ studies across different length scales in understanding key catalytic processes in renewable energy schemes.
EPFL2021
