This work is focused on La 0.6 Sr 0.4 CoO 3-δ (LSC) infiltrated La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) oxygen electrode for high temperature steam electrolysis aimed at efficient hydrogen production. In this respect, first the chemical and structural stability of both LSCF and LSC materials are investigated as a function of temperature under air and oxygen. The electrochemical performance of LSC infiltrated LSCF oxygen electrode is then investigated for steam electrolysis and compared with conventional LSCF electrode. The symmetrical half-cell as well as single cell containing LSCF oxygen electrode with and without LSC infiltration are characterized using electrochemical impedance spectroscopy in the temperature range 700-900 • C. It is observed that the symmetrical cell as well as single cells with LSC infiltrated LSCF electrode performs better than the conventional LSCF electrode. The degradation experiments were performed with the symmetrical cells under polarizations. Post-test analysis using SEM-EDX was performed to investigate the changes of electrode and electrode/electrolyte interface microstructures.

This work is focused on the degradation effect of chrome poisoning on LSCF cathodes for high temperature SOFCs aimed at an increase of the cell durability. Therefore the electrochemical performance of single cells is investigated at open cell voltage (OCV) and polarizations of 0.5, 0.75 and 1 A*cm -2 as well as at temperatures of 900 and 700°C in presence/absence of a chrome source. To identify the cathode related processes of the impedance spectra without anode contributions, symmetrical half cells were tested as well. It is observed that in presence of a chrome source the ohmic as well as the polarization resistances increase. At a temperature of 900 °C and a humidity of 3.5 %, the polarization resistance shows a stronger increase at current densities above 0.75 A*cm -² whereas the polarization resistance remains linear. Furthermore, measurements at 700 °C confirmed that also the operation temperature serves as a factor for accelerated chrome related cell degradation.

No abstract

In future there will be a strong demand for large capacity rechargeable batteries to store electrical energy (e.g. from renewable power sources) in long-term stationary applications  [1]. A high temperature metal / metal oxide battery can be built up by combining solid oxide fuel cell (SOFC) technology and a metal/metal oxide storage system [2]. Such a type of battery promises charging and discharging capacities of more than 250 W/cm2 [3]. Requirements for a reversible working solid oxide cell (SOC) are a high performance, minimum internal resistance of the cell, and long-term stability at operating conditions. In the present work the performance of solid oxide cells (SOC) operating in fuel cell and steam electrolysis mode over a temperature range of 650-900 °C and as a function of humidity were studied. Results presented were obtained from single SOCs, with an active area of 16 cm2 and button cells with an active area of 0.5 cm2. The SOCs investigated were anode substrate cells (ASC), with nickel-YSZ-cermet steam/hydrogen electrodes, yttria-stabilized zirconia (YSZ) electrolytes, and lanthanum strontium iron cobalt perovskite (LSCF) air electrodes. Current-voltage measurements were coupled with electrochemical impedance spectroscopy (EIS), in order to identify the different loss terms in cell behaviour during the fuel cell and electrolysis mode. EIS measurements are conducted under practical load conditions in SOCs in both modes. The cells show stable current-voltage curves during cycling between fuel cell and electrolysis mode at short cycling times between 2.5 h and 5 h. Measurements at different humidity show that high electrical-to-hydrogen energy conversion efficiencies are achieved and the amount of steam content is the limiting factor for the electrolysis mode. During electrolysis mode remarkable high current densities around -1.3 A/cm2 were achieved at a cell voltage of 1.3 V and a temperature of 800 °C. Below 50 % steam content, however, a strong efficiency loss was observed. It is also well known that the degradation of the SOC during steam electrolysis is still a limiting factor for the long term application [4]. Hence the focus of interest was also the degradation of the air electrode. Increasing the current density and elongating the duration of electrolysis experiments resulted frequently in a very fast delamination of the LSCF electrode.

In the institute for fundamental electrochemistry IEK-9 at the Forschungszentrum Jülich GmbH the electrochemical properties of solid oxide cells are electrochemically characterized at a variety of sizes, configurations and operating conditions. In various projects different aspects of the complete cell and the reversible oxygen electrode (ROE) are studies. Both electrode supported cells of 5 x 5 cm² and 20 mm diameter are characterized by galvanostatic DC methods and electrochemical impedance spectroscopy (EIS). Also electrolyte supported symmetric cells are studied using a configuration with reference electrodes to investigate the reversible air electrode in detail. The main focus lies on the use of EIS to try and understand the different fundamental (electrochemical) reactions within the electrode process. Measurements are conducted on 5 x 5 cm² electrode supported cells comprising a 0.6 mm thick Ni/8YSZ fuel electrode, a 10 µm thick 8YSZ electrolyte, a 10 µm thick GDC diffusion barrier layer and a 50 µm thick LSFC air electrode. The impedance of the cells shows a continuous increase when progressing from fuel cell to electrolysis conditions (see Figure 1A). At high currents densities under both positive and negative current densities an inductive loop appears in the low frequency range. Due to the high currents involved and the use of a power booster during the measurements the EIS data are limited to ~10 kHz in the high frequency range. Therefore the EIS data do not directly show a high frequency intercept with the real axis and the Ohmic resistance has to be determined using a fitting procedure. Similar measurements were also performed on 20 mm round or button cells of the same composition and microstructure. These cells show similar behavior (see Figure 1B). The overall shape of the impedance spectra is comparable for both cell types although the contributions of the underlying processes seem to vary with the cell size. Here the high frequency intercept can be determined but the measured data have to be corrected for an inductive contribution originating in the measurement setup. To separate the contribution of the ROE from the total cell resistance, measurements on electrolyte supported cells with symmetrical CGO/LSCF electrodes and a platinum reference electrode are performed. Also here a continuous transformation of the EIS spectra can be observed when going from oxygen reduction in the fuel cell mode to oxygen evolution in the electrolysis mode. By combining the data of all three different cell geometries an attempt is made to separate contributions from both electrodes and the influence of cell size on the total cell resistance. Figure 1