Tuning the surface of ceria for high temperature electrochemical CO₂ reduction in solid oxide electrolysis cells

The catastrophic effect of anthropogenic CO₂ emissions on climate presents us with one of the biggest challenges of our era, i.e. the defossilization of the energy infrastructure and the chemical industry. Electrolysis and co-electrolysis of water/steam and carbon dioxide in high temperature (600 – 900 °C) solid oxide electrolysis cells (SOECs) are considered a cornerstone for the sustainable energy system of the future. They enable an efficient conversion of electrical energy from (intermittent) renewable sources and a sustainable production of fuels and chemicals, which may be long-term stored. In particular for CO₂ electrolysis, electrochemical CO₂ reduction in SOECs is now on the verge of commercialization and is expected to reach industrial-scale within the next decade. In this context, ceria (CeO₂) has drawn significant attention as a promising electrocatalyst for the cathode side. On the pathway towards the implementation of ceria into technological electrodes on a large scale, the optimization of its intrinsic electrocatalytic activity is crucial and requires an in-depth atomistic understanding of the reaction mechanism.

The aim of this work is to investigate how appropriate tuning of ceria surfaces affects its electrochemical activity towards CO₂ reduction, through a systematic study of the levers that can promote the reaction rate. The electrochemical properties of ceria are analyzed by employing model electrodes produced by pulsed laser deposition (PLD) in the form of thin films. The possibility offered by PLD to unambiguously define the geometry and surface area of the model system is highly convenient for fundamental studies, as it enables an accurate interpretation of the critical phenomena at play in the electrochemical reaction. In this thesis, the discussion on the effect of ceria surface modification on its electrochemical properties develops in two parts. In the first part, the correlation between the introduction of dopants in the ceria lattice and its electrochemical performance is examined. The doped and undoped surfaces of ceria are shown to have the same dependence of the reaction rate on the equivalent oxygen partial pressure at the working electrode, revealing a mild sensitivity of the electrode performance on doping. In the second part, the termination of the ceria surface is engineered in order to probe its role in enhancing the electrocatalytic activity of ceria. The trend of activities observed for the different surface orientations unveils a strong correlation with the adsorption energy of the CO₂ molecule, and highlights the similarities between the electrocatalytic behavior of ceria towards CO₂ reduction and H₂O splitting.

Through the second part of the dissertation, a number of limitations of the chosen current collection approach, i.e. Pt grids patterned through lift-off photolithography, emerge. Metal patterns are typically the current collector of choice for thin film studies employing electrocatalyst layers with limited electronic conductivity, as the one in this work. However, the placement of this type of current collector can heavily influence the analysis of the electrochemical performance, by introducing uncertainties and misinterpretation of the data obtained from the electrochemical measurements. Consequently, a third part of the thesis is devoted to the development of an alternative current collection strategy, consisting of an epitaxially grown mixed ionic-electronic conductor (MIEC) oxide layer underlying the ceria film. Two materials are identified as promising candidates, namely the perovskite materials Nb-doped strontium titanate (STN) and strontium vanadate (SVO), whose feasibility as oxidic current collectors for thin film studies on the electrocatalytic activity of ceria towards CO₂ reduction and H₂O splitting is assessed.