dc.description.abstract
Rising global standards of living and the consequences of unchecked climate change pose a major challenge for the chemical industry. Large-scale industrial processes are expected to increase their output to satisfy consumer demand while simultaneously conserving resources, improving energy efficiency and becoming more sustainable. The majority of the chemical industry as well as energy storage and conversion technologies rely on heterogeneous catalysis to either lower energy barriers and thus energy usage or transform environmentally harmful pollutants into less harmful forms. Thus, continued development of more efficient catalysts is imperative for the future of mankind. In the past decade, a novel type of catalyst, called exsolution catalyst, has gained increased attention. This type of catalyst features highly active, strongly anchored nanoparticles at the surface of an oxide support. In contrast, to classical deposition techniques, the formation of the particles is done in situ by a deliberate partial reduction of the oxidic support structure - often called parent oxide - which releases the reduced metals as particles on its surface. Compared to ex situ preparation routes, catalyst preparation via exsolution offers more control over particle size and density, and provides a more uniform distribution as well as increased stability. This work focuses on combining this exsolution effect with electrochemistry by manufacturing solid oxide cells utilising mixed ionic electronic conducting thin films with exsolvable cations as the working electrode on an oxide ion conducting electrolyte. Upon applying a bias voltage, the working electrode is electrochemically polarised, which is equivalent to a reduction or an oxidation of the mixed conducting electrode, for negative (cathodic) or positive (anodic) voltages, respectively. Thus, an applied bias voltage serves as a control parameter, which not only eliminates the need of the initial reduction process, but also allows increased control over the activity state of the exsolved particles. A combination of electrochemical methods (e.g. electrochemical impedance spectroscopy (EIS) and DC measurements) and surface sensitive spectroscopy (i.e. near ambient pressure X-ray photoelectron spectroscopy; NAP-XPS), partly performed in situ, allowed decoupling of intertwined processes taking place at the perovskite surface. The thesis consists of three chapters, with each chapter dealing with a separate subtopic on the subject of electrochemical switching of the oxidation and thus activity state of exsolution catalysts: The first topic presented in this thesis deals with the impact of deposition parameters in pulsed laser deposition on the electrochemically controlled exsolution behaviour of the ferrite based perovskite La0.6Sr0.4FeO3-δ (LSF). Therein, deposition temperature, laser fluence and the electrolyte (i.e. the substrate on which the thin film working electrode is grown) were varied. The obtained thin film LSF electrodes were tested electrochemically for their exsolution behaviour. Furthermore, the thin films were analysed by various techniques to determine the effect of surface morphology (atomic force microscopy; AFM), lattice strain (X-ray diffraction; XRD) and stoichiometry (inductively coupled plasma mass spectrometry; ICP-MS). The results suggest that the introduction of lattice stresses has a greater positive effect on the ease of exsolution particle formation than A-site deficiency. Second, the electrochemical switching between catalytically active (metallic) and inactive (oxidic) state of iron particles was investigated. The particles were exsolved from the thin film mixed conducting model electrodes LSF and Nd0.6Ca0.4FeO3-δ (NCF) and their electrochemical switching behaviour was studied in humid hydrogen atmospheres with varying H2:H2O ratios. Recorded I-V curves show that the transition between two activity states exhibits a hysteresis-like behaviour and NAP-XPS successfully linked this to the oxidation and reduction of iron particles. Moreover, the impact of the perovskite parent oxide proved negligible, but through an extensive, comparative discussion on the level of oxygen chemical potentials within the involved phases, the surrounding atmosphere and the overpotential applied to the oxide electrode were identified as the main driving forces. Moreover, a 'kinetic competition' between gas atmosphere and oxygen chemical potential in the mixed conducting electrode is suggested and possible ways of how this process could take place are discussed. Thirdly, electrochemical switching of Ni0-Fe0 bimetallic exsolution catalysts was explored. The addition of 3 mol% Ni on the B-site of NCF yielded the parent oxide Nd0.6Ca0.4Fe0.97Ni0.03O3-δ (NCFNi). On the one hand, in the case of bimetallic exsolutions, and with voltage as a control parameter, it was possible to reversibly switch between three different activity states, namely bimetallic Ni0-Fe0, pure Ni0 and the inactive oxides with pure Ni0 proving to result in the highest surface activity. On the other hand, through smart application of a so-called protection voltage it was possible to solely exsolve Ni from the lattice, while the formation of bimetallic exsolutions could be prevented. Ni nanoparticles appear to exhibit a different switching behaviour compared to their iron counterpart described above, which is attributed to a different kinetic interplay between electrochemical driving force and atmosphere. The elementary process responsible is thought to be oxygen transport through nickel oxide, which is much slower than through iron oxide. NAP-XPS measurements confirm that particularly large Ni nanoparticles do not fully oxidise even at extremely high anodic overpotentials. Smaller Ni particles showed this behaviour to a much lesser extent, suggesting a particle size effect, which also supports limited oxygen transport kinetics to be responsible for the significantly more difficult switchability of Ni.
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