In-operando spatially resolved probing of solid oxide electrolysis/fuel cells

The reactions occurring at the oxygen electrodes of solid oxide fuel and electrolysis cells (SOFC/SOEC - SOC) were investigated, both with conventional techniques and with advanced in situ techniques, in order to study the reaction mechanisms and the surface evolution of the electrode materials under realistic operating temperatures and oxygen partial pressures. For this purpose, model (La,Sr)(Co,Fe)O3 (LSCF), (La,Sr)FeO3 (LSF) and La(Ni,Fe)O3 (LNF) electrodes were produced with pulsed laser deposition (PLD) and characterized using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), scanning photoemission microscopy (SPEM) and high temperature scanning probe microscopy (SPM) with the additional functionality of Kelvin probe force microscopy (KPFM). In particular, XPS and SPEM represent novel tools with respect to solid state electrochemical characterization, as they only recently have been reaching relevant operating conditions in terms of obtainable temperatures and oxygen pressures in the experimental chambers. KPFM is a less established technique with respect to SOC studies, being used mostly at low temperatures in corrosion science and for the study of semiconducting devices, but has a great potential and was optimized for the desired operating parameters during this work, obtaining promising results.

The influence of the experimental conditions on the surface exchange, as measured by EIS on model thin film electrodes produced by PLD, was the subject of the first main study. The influence of current constriction, current collector material and design and the purity of the gases proved to be most important with respect to Rsurf. However, other parameters were also evaluated, such as the stoichiometry of the thin films and their geometric area, but showed negligible effects with respect to the aforementioned parameters. The results succeeded in reproducing the scatter of three orders of magnitude present in literature data for the PLD, and resulted in a set of useful guidelines for measuring the intrinsic electrode materials performance and avoiding the influence of external artifacts.

In the second main study, the oxygen electrode reactions were studied under polarization, obtaining current-voltage profiles in varying oxygen partial pressures ranging between atmospheric oxygen content (210 mbar) and 10-1 mbar at 600 °C. These studies were integrated by surface chemistry characterization performed with XPS in an oxygen content between 1 mbar and 10-2 mbar at 600 °C, and with the added benefit of lateral spatial resolution of the surface chemistry with SPEM in 2.6-5∙10-2 mbar oxygen at 600 °C. The surface chemistry characterization allowed an interpretation of the surface behavior, both in terms of degradation and with respect to the oxygen reactions, and for the first time a correlation between the electrode overpotential and the surface potential was deduced. Furthermore, the outcome of the studies of the electrode reactions under polarization also allowed the identification of the most probable reaction pathway for the oxygen incorporation.

SPEM was also used to investigate, with lateral spatial resolution, the surface chemistry and the electrical potential profiles in distributed electrodes deposited on thin electrolytes, in an attempt to contemporarily study the evolution of the surface chemistry and the distribution of the electric potentials in the LSCF electrodes and the GDC electrolyte under externally applied potential differences. The sample was designed as a model system which could replicate the composite nature of technological SOC electrodes. The overpotential distribution that was experimentally determined between the electrodes and the electrolyte was compared with finite element modelling simulations, showing good correspondence between the simulated values and the measured ones.

In order to approach the real operating conditions for the study of SOC materials, SPM and KPFM were performed in a specially designed microscope at temperatures of up to 600 °C in atmospheres ranging from pure N2 to pure O2 on a model sample, consisting of two isolated LNF electrodes on an MgO substrate. The sample could be used as a high temperature capacitor in order to evaluate the spatial resolution of KPFM in the relevant conditions, as well as the quality of the obtainable signal and the stability of commercial probes in more demanding operating conditions than the ones usually present in SPM setups. The results were very promising, and KPFM could represent a useful technique in future studies of SOC materials in realistic operating conditions, combining topographic characterization with chemical and electrostatic distributions across the sample surface.