Long term behaviour of solid oxide electrolyser (SOEC) stacks

Abstract

The successful market introduction and public acceptance of the solid oxide fuel/electrolysis cell technology requires high performance, long-term stability and low costs of the corresponding cells and stacks. However, especially the degradation of SOEC stacks during long-term operation for several thousand hours in electrolysis and reversible SOFC/SOEC mode are not yet well understood. The paper presents the long-term behavior and the degradation results of a 30-cell stack with electrolyte supported cells operated in reversible SOFC/SOEC mode. The stack was supplied by Sunfire GmbH (Dresden/Germany) as part of the German funded project Reversible Solid Oxide Cell. The reversible cycles with duration of 24 h consist of 9 h SOFC operation at 70 % fuel utilization, 9 h SOEC operation at 70 % steam conversion and two switching phases, each with duration of 3 h. The stack is currently being operated for 6300 h under these reversible SOFC/SOEC cycling conditions. After the initial performance test, current-voltage curves and electrochemical impedance spectra were recorded every 2000 h. The electrochemical properties such as open circuit voltages, area specific resistances, power densities, fuel utilization, steam conversion rate and impedance values of the stack and of selected repeat units are presented. The homogeneity among the repeat units is discussed in context with the measured temperature gradients in the stack. The degradation and the change of the resistances, e.g. the ohmic resistances, the polarization resistances of the electrodes and the gas concentration resistances, during the reversible SOFC/SOEC long-term operation are presented and compared to each other. Furthermore, the degradation mechanisms of the different cell and stack components are discussed.