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Abstract

In the quest of reducing CO₂ emissions and limiting climate change, the field of electrocatalysis has gained significant interest due to its potential for the sustainable production of energy and chemicals. Inspired by the natural CO₂ metabolism, one promising way for converting intermittent renewable electricity directly into valuable fuels and chemicals is the electrochemical reduction of CO₂ (CO₂RR). Copper catalysts are uniquely capable of converting CO₂ into hydrocarbons and alcohols in significant amounts but suffer from low selectivity and stability. Therefore, nanostructured Cu-based materials with controlled size and shape have been investigated to improve the CO₂RR, e.g., via bimetallic or potential pulse approaches. However, the catalytic properties of these materials are usually only explored with ex situ characterization methods resulting in misleading conclusions. This work aims to enhance the current understanding of the catalyst-adsorbate system toward the production of energy-dense C₂₊ products by applying (sub-second) time-resolved in situ and operando spectroscopic and diffraction techniques during CO₂RR. Here, the implementation of a bimetallic Cu-Ag nanocatalyst enhanced the selectivity of the CO₂RR toward C₂₊ liquid products such as ethanol and acetaldehyde. Operando Ag K-edge X-ray absorption spectroscopy (XAS) and surface-enhanced Raman spectroscopy (SERS) revealed the formation of important Ag-Cu binding sites under reaction conditions which altered the CO binding to Cu correlating with the observed selectivity effect. Furthermore, the evolution of characteristic adsorbates, including OH_ad and COad, was monitored during alternating pulsed potential CO₂RR using time-resolved operando SERS. It was found that the oxidative formation of cationic Cu species and an optimized catalyst surface coverage of OH_ad and CO_ad play a crucial role in enhancing ethanol selectivity. These results were also confirmed by studies of a bimetallic Cu-ZnO nanocatalyst during pulsed CO₂RR, with the selectivities being modulated by changing the anodic potential. Time-resolved operando XAS, SERS, and X-ray diffraction uncovered the dynamic interplay between Cu, Zn, and CuZn composition, as well as the adsorption behavior of CO_ad and OH_ad. These results emphasize the importance of oxides and hydroxide coverage for enhancing the ethanol selectivity, which can be tuned through the oxidation of Cu- or Cu-Zn-based materials using potential pulses. This thesis contributes to the fundamental mechanistic understanding of CO₂RR, which makes a significant contribution to the advancement of the CO₂RR field.