The electrochemical CO2 reduction reaction (CO2RR) is envisioned to play a significant role in achieving carbon neutrality while contributing to storing renewable energies. Cu-based materials are among the most promising electrocatalysts. However, 16 different products are obtained from a Cu foil, therefore strategies to make more selective catalysts are needed. This thesis explores Cu/metal oxide interfaces as a mean to direct selectivity towards hydrocarbon products in CO2RR. The aim is to deviate from the scaling relationships which exist on single metal surfaces. Cu/metal oxides hybrid nanocrystals (HNCs) with tunable morphologies and spatial configuration are synthesized via colloidal chemistry. The achieved level of control and tunability is demonstrated to be crucial in understanding the impact of these parameters on the reaction outcome.
First, we start by designing a synthesis procedure for Cu/CeO2-x heterodimers (HDs) and then we study their catalytic behavior toward CO2RR. We show that the Cu/CeO2-x interface promotes CO2RR, particularly methane with around 54% selectivity, which is ~5 times higher than those obtained with a physical mixture of isolated Cu and CeO2-x nanocrystals, thus highlighting the important role played by the intimate bonding at the interface. A miscellanea of characterization techniques indicates the higher extent of ceria reduction in the HDs during CO2RR. DFT elucidates the presence of unique catalytic motifs that enable bidentate adsorption of critical intermediates at both Cu and the CeO2-xO-vacancy site, creating an opportunity for reaction engineering beyond scaling relationship limitations.
Having assessed the importance of the Cu/ceria interface, we investigate the impact of increasing the Cu/CeO2 interfacial area on the CO2RR outcome. We find that increasing the number of ceria domain around copper inhibits CO2RR. This result highlights that a balance between interfacial area and accessibility to the active catalytic sites must be sought after to exploit synergies arising at the interface between different domains.
Finally, we explore Cu/ZnO HNCs as CO2RR pre-catalysts to elucidate the structural and compositional parameters which direct the reaction pathway towards ethanol. Using different sizes of ZnO seeds for the synthesis of Cu/ZnO HNCs, we are able to achieve two different configurations including ZnO decorated Cu core (Cu@ZnO) and Cu decorated ZnO core (ZnO@Cu) HNCs. Upon activation via cathodic voltage, we find that Cu@ZnO HNCs favor methane production while ZnO@Cu exhibits a noticeable selectivity toward ethanol. Operando XAS, XPS, and electron microscopy reveal that both HNCs transform into Cu@CuZn core@shell NCs during the activation step. However, the Zn content in the surface alloy (i.e. degree of alloying) and the presence of surface cationic Zn species are found to be crucial in determining the selectivity of these catalysts, which possibly elucidates some of the contrasting results reported on Cu-Zn across the literature.
Overall, this thesis showcases the importance of well-defined and tunable HNCs as platforms to study structure/properties relations and to advance the current knowledge in CO2RR. Furthermore, the library of nanomaterials accessible through the developed synthesis routes might also be utilized in the future to investigate other catalytic reactions and applications where the hybrid interfaces might be relevant.
EPFL2021
