Identification of Degradation Parameters in SOC Using In-Situ and Ex-Situ Approaches

Abstract

Solid oxide cells (SOC) have been comprehensively tested under fuel cell (FC) and electrolysis (EC) modes over the past decades. Recent SOC activities focus on upscaling the applications to MW scales with target operation for several 10,000 hours. These long lifetimes require new approaches for durability testing. In the present study, the influence of different operating parameters on degradation were studied by long-term cell testing (in situ treatment). Based on the results, accelerating parameters for ex situ treatment were identified, which is a cheaper and time saving approach as compared to conventional cell/stack long-term testing. Two commercial SOC cell designs were studied: Anode supported cells (ASC) and Electrolyte supported cells (ESC). These cells are composed of LSCF cathodes with a CGO barrier layer, YSZ electrolyte and Ni based fuel electrodes. In case of ASCs, the fuel electrode consists of Ni-YSZ cermet and of Ni-CGO in case of ESCs. A comparison to field operated stacks was performed to understand the different degradation mechanisms observed under real, long operation.Single cells were characterized in detail and then operated over 1000 h either in FC or EC mode, and with different steam contents in hydrogen to the fuel electrode. After these conventional tests (in situ), the fuel electrode of fresh cells were exposed to similar temperature and gas compositions for 1000 h but without application of current loads (ex situ), thus studying the effect of temperature and steam on durability of the fuel electrode. After the in-situ and ex-situ treatments, the cells were characterized in detail through deconvolution of EIS spectra using distributions of relaxation times (DRT) and complex least squares approach (CNLS) fitting using appropriate equivalent circuit models (ECM). In addition, the microstructure of the cells were studied after the in-situ and ex-situ operation and compared to the reference cells. For this purpose, secondary electron imaging was done at low accelerating voltage (1keV) using Zeiss Ultra field emission microscope and a quantification of the different phases was performed. The results give insight into the effect of the different operating parameters for the different cell configurations. The two cell types respond differently in terms of degradation under long term tests with highly humidified hydrogen fuel under in situ (under current load) vs. ex situ conditions. In FC mode, the ESCs seem to degrade less in ex situ tests with high steam partial pressures to the fuel electrode as compared to the in situ tests operated under similar steam conditions, particularly regarding the polarization resistance. The different aggravated operation parameters (temperature, steam composition, current density) have differing impact on the cell microstructure as well. For instance, in ESCs, during ex situ treatment with high steam content to the fuel electrode and high temperatures (900º C), there is severe agglomeration of the CGO particles in the fuel electrode. Furthermore, agglomerates of Ni particles are engulfed by the CGO within the functional layer (~3-15 microns from the electrolyte). A similar coverage of the Ni particles by the CGO is observed also at 850 ºC although the Ni particles are finer in size. A similar effect was also observed for the Ni particles after in situ tests. Figure 1