• Plaza-Mayoral, Elena
• Rossmeisl, Jan
• Pascual, Paula Sebastian
• Escudero-Escribano, María

Abstract

The recent energy and environmental crisis has become one of the biggest concerns in society. Our sustainable future requires to replace fossil fuels with clean technologies and renewable sources to accelerate the green transition. Clean energy conversion devices, such as electrolyzers and fuel cells, address this problem by converting renewable electricity into chemicals and fuels and vice versa through electrocatalytic reactions. In this thesis, I have focused on two model electrocatalytic reactions: The electrochemical reduction reaction of CO2 (CO2RR) to hydrocarbons and oxygenates, and the formic acid oxidation reaction (FAOR) for its implementation in direct formic acid fuel cells. Preparation of nanocatalysts with tunable structure and composition is essential to optimize the activity, stability, and selectivity for these key energy conversion reactions. One studied strategy to enhance the performance of these reactions is the preparation of new bi-/multi-metallic nanocatalysts. When combining two or more metals, the adsorption energies of the intermediate species change, modifying the reaction pathways to enhance the electrocatalytic performance.<br/><br/>This thesis investigates the electrodeposition of bimetallic and multimetallic nanostructures using a choline chloride: urea deep eutectic solvent (DES). DES have emerged as an inexpensive, clean, and green alternative to prepare nanomaterials. Additionally, these non-aqueous solvents do not require the addition of surfactant agents as the DES components act as ligand agents and regulate the growth of the deposit. Using this method, I have tested the applicability of choline chloride plus urea DES through the preparation of CuAu nanostructures, followed by CuAg and PdAu nanocatalysts for the CO2RR and FAOR, respectively. I have tuned the size, morphology, and composition of the nanostructures by adjusting the applied potential, bath composition and coverage of the deposits. The characterization of the bimetallic nanostructures has been assessed electrochemically and with ex-situ microscopy and spectroscopy techniques including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and Xray photoelectron spectroscopy (XPS). To assess the intrinsic activity and selectivity under reaction conditions, it is important to estimate the electrochemically active surface area (ECSA) and roughness factor. For that purpose, I have used voltammetric metal underpotential deposition (UPD) to estimate the ECSA and roughness factor (R).<br/><br/>This thesis is divided into two main parts. The first part is dedicated to the preparation of Cu-based bimetallic catalysts for the CO2RR (chapters 3, 4, 5 and 6) and the importance of determining the ECSA of our catalysts to assess their performance. The CO2RR on Cubased catalysts requires high applied overpotentials and suffers from poor selectivity to valuable fuels and chemicals. I started preparing CuAu nanostructures for the CO2RR to CO aiming to evaluate the applicability of the DES to prepare a bimetallic catalyst. Subsequently, we have used the same DES to prepare and characterize bimetallic CuAg nanostructures with variable composition for the CO2RR. Combining copper with silver enhances the product selectivity of copper towards C2+ products and produce more valuable oxygenates products such as ethanol and propanol. Our work shows that the addition of silver mainly suppresses the competing H2 production while the distribution of the C2+ products changes towards oxygenates over ethylene. We have mainly evaluated the performance of polycrystalline disordered bimetallic catalysts. As the CO2RR is a highly structure sensitive reaction, at the end of this part, we also propose a simple electrochemical method to tailor the facet distribution of copper with chloride and measure the preferential orientation of nanostructured copper. This control of the copper surfaces structures can open the door to improve selectivity of copper catalysts for the CO2RR.<br/><br/>The second part of this thesis (Chapters 7 and 8) describes the preparation and characterization of new Pd and PdAu nanostructures from DES and their performance for the FAOR. On Pd-based electrocatalysts, the FAOR is typically hindered by the low catalyst stability. To improve the stability of Pd under reaction conditions while keeping its activity towards FAOR, we have investigated how different Pd-Au combinations affect the electrocatalytic response. Finally, some insights into other promising bimetallic and multimetallic nanostructures prepared by metal electrodeposition from DES for FAOR are included (i.e., PdPt, PdPtAu, PdPtAuAg) as preliminary results.

Topics

• impedance spectroscopy
• morphology
• surface
• silver
• scanning electron microscopy
• x-ray photoelectron spectroscopy
• copper
• Energy-dispersive X-ray spectroscopy
• electrodeposition
• surfactant