A parameter study of 20 solid oxide electrolysis cells was carried out to systematically investigate long-term degradation each over 1,000 h under variation of temperature, humidity and current density. The influence of operating temperature was investigated between 750 and 850 °C, the humidity ranged from 40 % to 80 % H 2 O, and the current density varied between open circuit voltage (OCV) and 1.5 A•cm-2. The progress of degradation was monitored insitu by electrochemical impedance spectroscopy. Five different contributions to the spectra were identified by calculating the distribution of relaxation times and separated via a complex non-linear square fitting routine. The present work focuses on the degradation of the fuel electrode. From SEM analysis, Ni depletion and an increased pore fraction close to the electrode/electrolyte interface was derived, which is correlated with an increased ohmic resistance of the cells. This unidirectional transport of Ni away from the fuel electrode/electrolyte interface leads to an effective electrolyte extension and is the main source of degradation. Ni depletion is shown to be driven by current density and its extent is shown to be dependent on the complex interplay between the operating parameters current density, anode overpotential, humidity and temperature. It is particularly pronounced for pH 2 O larger than 0.8 atm and temperatures above 800 °C. Furthermore, the fuel electrode electrochemistry also exhibits degradation in the high-frequency region around 10 4 Hz.

The aim of the present study is the measurement and understanding of sulfur poisoning phenomena in nickel/gadolinium-doped ceria (CGO) based solid oxide fuel cells (SOFC) operating on reformate fuels. The sulfur poisoning behavior of commercial, high-performance electrolyte-supported cells (ESC) with Ni/Ce0.9Gd0.1O2−δ (CGO10) anodes operated with different fuels was thoroughly investigated by means of current–voltage characteristics and electrochemical impedance spectroscopy and compared with Ni/Yttria-stabilized zirconia (YSZ) anodes. Various methane and carbon monoxide containing fuels were used in order to elucidate the underlying reaction mechanism. The analysis of the cell resistance increase in H2/H2O/CO/CO2 fuel gas mixtures revealed that the poisoning behavior is mainly governed by an inhibited hydrogen oxidation reaction at low current densities. At higher current densities, the resistance increase becomes increasingly large, indicating a particularly severe poisoning effect on the carbon monoxide conversion reactions. However, the ability of Ni/CGO anodes to convert carbon monoxide even at H2S concentrations up to 20 ppm was demonstrated, while this was not possible for Ni/YSZ. The sulfur poisoning behavior of Ni/CGO in reformate fuels was fully reversible for short exposure times. From methane steam re-forming experiments, it is deduced that the Ni surface is blocked and, thus, the water-gas shift reaction is fully deactivated as well. However, electrochemical CO oxidation on the CGO surface was shown to be still active. The present results clearly demonstrate that the high sulfur tolerance of Ni/CGO not only is limited to H2/H2O fuel systems but also extends to CO-containing gases.

Within a joint project of German Aerospace Center (DLR) and Forschungszentrum JÜLICH solid oxide fuel cells have been electrochemically characterized and the influence of the current density and fuel gas humidity on the long-term stability of electrolysis operation have been investigated. In order to observe the time- dependent change in electrochemical behaviour of the fuel electrode supported cells, in-situ analysis with a special focus on impedance spectroscopy was utilized. DRT-analysis was applied to validate assumptions made for equivalent circuit fitting and to improve fitting quality and reliability. Four rate limiting processes (electrolyte, cathode, anode and mass transport) could be separated and it was possible to track their development during the 1000 h experiments, showing their individual degradation behaviour under the applied operating conditions. This information was used for a dependence study, while additionally post mortem analysis methods were applied.

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Solid Oxide Cells exhibit the unique feature to be able to run in reversible operation and the capability to run with hydrocarbons. Due to the high temperature of the process, solid oxide cells exhibits very high efficiency in electrolysis mode. This makes it very attractive for Power-to-X. At the German Aerospace Center, the SOC are investigated from cell to stack level in order to characterize and understand the behavior, identify a safe operating window and extend it by identifying strategy of operation and propose alternative materials to mitigate degradation issues. At the system level, for the first time, the coupling of High Temperature Electrolysis process with heat from Concentrated Solar Power plant is investigated in order to achieve very high efficiency. Moreover, within the next two years, a research platform will be built up to investigate the co-electrolysis process at high temperature and high pressure for efficient use of CO2.