Effects of impurities and local behavior characterization using active thermography on solid oxide electrolysis cells

Ambitious policies are needed from every country to drastically curtail the anthropogenic emission of greenhouse gases responsible for the on-going modification of the earth's climate. A quick transition towards a society fully based on renewable materials and energy sources is necessary. Power-to-gas via solid oxide electrolysis cells is a promising solution to accommodate the variability inherent in renewable electricity production. Despite evidence of a high sensitivity to impurities during electrolysis, however, the literature available on the effects of contaminants in the reactant is rather sparse.

This thesis therefore aims to identify and determine the effects of the potential harmful impurities to which an SOEC can be exposed. The effects of SO2 and HCl, the two most critical impurities identified for Ni-YSZ electrodes, on the performance and durability were investigated using EIS and the analysis of the DRT. The results indicate that the operation in electrolysis reduces the tolerance to impurities. Exposure to HCl and SO2 deteriorated the charge transfer processes at the triple phase boundary, whereas only SO2 limited the catalytic reactions. Durability testing performed in galvanostatic operation with impurities showed a voltage runaway behavior after approximately 200 h. Shorter experiments may thus greatly underestimate the impact of impurities. A low-frequency pseudo-inductive hook appeared in the electrochemical impedance spectra during the poisoning experiments, which was attributed to the Ni-YSZ electrode. Two processes were identified as possible causes: 1) a two-step reaction mechanism involving an intermediate species, or 2) the development of an electronic conductivity in the electrolyte. The former appeared to be the most likely explanation.

This thesis also proposes and assesses the use of active thermography as a novel method to characterize in operando the local behavior of SOCs, thereby overcoming the limitation of typical measurements that only provide spatially averaged values. The proposed method consists of stimulating the solid oxide cell with a current perturbation and analyzing the local thermal response using Fourier transforms. As a preliminary step and to support experimental work, a numerical parametric investigation was performed on lock-in thermography results using a pseudo 2-D model. Analysis of the 1st harmonic revealed that, in addition to electrochemical reactions and the Joule effect, the modulation of the local reactant composition also contributed to the thermal response. Analysis of the 2nd harmonic allowed the visual local non-uniformities not visible in the 1st harmonic. Lock-in thermography measurements performed on the developed test bench were in good agreement with the numerical simulations, thus validating the feasibility of the method. Lock-in thermography was found to be more adapted to SOC applications than pulse thermography, as a lower noise level can be achieved while maintaining a reasonably low current modulation. When CO2 was added to the reactant, the thermal response was modified, thereby demonstrating the sensitivity of active thermography to the reactant composition. Lock-in thermography measurements taken during an SO2-poisoning experiment evidced a spatially non-uniform deactivation process.By combining these results with DRT analysis, a detailed degradation mechanism was proposed, thereby demonstrating the complementary and added benefits of active thermography.