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Owing to its mixed ionic and electronic conductivity and high thermochemical stability, La 0.6 Sr 0.4 FeO 3-δ (LSF64) is an attractive electrode material in solid oxide fuel/electrolysis cells (SOFCs/SOECs). Well defined thin film microelectrodes are used to compare the electrochemical properties of LSF64 in oxidizing and reducing conditions. The high electronic sheet resistance in hydrogen can be overcome by the use of an additional metallic current collector. With the sheet resistance being compensated, the area specific electrode resistance is similar in humidified hydrogen and oxygen containing atmospheres. Analysis of the chemical capacitance and the electrode resistance for current collectors on top and beneath the LSF64 thin film allow mechanistic conclusions on active zones and bulk defect chemistry. Cyclic gas changes between reducing and oxidizing conditions, performed on macroscopic LSF64 thin film electrodes with top current collector, reveal a strong degradation of the surface kinetics in synthetic air with very fast recovery in reducing atmosphere. Additional in-situ high-temperature powder XRD on LSF64 demonstrates the formation of small amounts of iron oxides in humidified hydrogen at elevated temperatures. Solid oxide fuel cells (SOFCs) are in the process of gaining more and more commercial success as highly efficient power generation systems. They transform the chemically bound energy of a fuel to electrical energy. Solid oxide electrolysis cells (SOECs) are the counter parts of SOFCs as they use electrical energy, e.g. excess energy from the grid, to form fuel by electrolysis. Owing to their very high efficiencies, also SOECs are promising devices in future energy technologies. 1Currently Ni/YSZ cermet electrodes are the standard electrodes in SOF/ECs for reducing conditions, but they are known to suffer from several problems, like sulfur poisoning (in SOFC operation), sintering, redox cycle stability etc.2 To find an alternative to Ni/YSZ could therefore be favorable for both SOFCs and SOECs.Electrodes in SOE/FCs have to meet numerous requirements: thermochemical stability over a wide oxygen partial pressure range, high catalytic activity for oxygen exchange reactions, compatibility with adjacent cell components, sufficiently high electronic and ionic conductivity, etc. Perovskite-type oxides such as LaMnO 3 , LaCoO 3 and LaFeO 3 based materials are used in state-of-the-art SOF/EC electrodes in oxygen atmosphere. Often they are mixed ionic and electronic conductors (MIECs), which is advantageous in SOF/ECs as the whole electrode surface area may become active in the oxygen exchange reaction. Employing such MIECs also in reducing atmosphere could be highly attractive. However, Sr-doped LaMnO 3 and LaCoO 3 are only stable under comparatively high oxygen partial pressures and thus not suited for the use in hydrogen. Fe 4+ states can be interpreted as electron holes (h • ) and those determine the electronic conductivity at high oxygen partial pressure as they are the majority mobile charge carrie...

Comparative (electro)catalytic, structural, and spectroscopic studies in hydrogen electro-oxidation, the (inverse) water-gas shift reaction, and methane conversion on two representative mixed ionic–electronic conducting perovskite-type materials La0.6Sr0.4FeO3−δ (LSF) and SrTi0.7Fe0.3O3−δ (STF) were performed with the aim of eventually correlating (electro)catalytic activity and associated structural changes and to highlight intrinsic reactivity characteristics as a function of the reduction state. Starting from a strongly prereduced (vacancy-rich) initial state, only (inverse) water-gas shift activity has been observed on both materials beyond ca. 450 °C but no catalytic methane reforming or methane decomposition reactivity up to 600 °C. In contrast, when starting from the fully oxidized state, total methane oxidation to CO2 was observed on both materials. The catalytic performance of both perovskite-type oxides is thus strongly dependent on the degree/depth of reduction, on the associated reactivity of the remaining lattice oxygen, and on the reduction-induced oxygen vacancies. The latter are clearly more reactive toward water on LSF, and this higher reactivity is linked to the superior electrocatalytic performance of LSF in hydrogen oxidation. Combined electron microscopy, X-ray diffraction, and Raman measurements in turn also revealed altered surface and bulk structures and reactivities.

A solid oxide electrolysis cell (SOEC) with a model-type La0.6Sr0.4FeO3−δ thin-film cathode (working electrode) on an yttria-stabilized zirconia electrolyte and a porous La0.6Sr0.4Co0.2Fe0.8O3−δ counterelectrode was operated in wet argon gas at the cathode. The hydrogen formation rate in the cathode compartment was quantified by mass spectrometry. Determination of the current as well as outlet gas composition revealed the electrochemical reduction of some residual oxygen in the cathodic compartment. Quantitative correlation between gas composition changes and current flow was possible. At 640 °C a water-to-hydrogen conversion rate of ca. 4 % was found at −1.5 V versus a reversible counterelectrode in 1 % oxygen. Onset of hydrogen formation could already be detected at voltages as low as −0.3 V. This reflects a fundamental difference between steam electrolysis and electrolysis of liquid water: substantial hydrogen production in a SOEC is already possible at pressures much below ambient. This causes difficulties in determining the cathodic overpotential of such a cell.Graphical Abstract

Mixed conducting cathodes for solid oxide electrolysis cells (SOECs) offer a promising alternative to the nowadays used Ni/YSZ cermet. Here, the water splitting kinetics of mixed conducting perovskite-type La0.6Sr0.4FeO3-δ (LSF) thin film electrodes was investigated in humid reducing atmospheres at 600 – 650 °C. Under equilibrium conditions an area specific surface resistance of ca. 15 Ωcm² was obtained on freshly prepared electrodes. Upon cathodic polarization of more than 20 mV a strong decrease of the surface resistance was observed. This acceleration of the water splitting kinetics was accompanied by the formation of metallic iron particles on the LSF surface, which was observed by means of near-ambient pressure XPS experiments.