CGO-Infiltrated Ni/YSZ SOEC Fuel Electrodes for Increased Carbon Tolerance During CO2 Electrolysis

Abstract

For the solid oxide electrolysis technology to be a viable path for future utilization of CO2 and production of carbon-based e-fuels and chemicals based on renewable energy supply, long-term durability and high performance is required. Two of the major degradation issues for SoA SOEC Ni/YSZ fuel electrode are Ni-migration/coarsening, and carbon deposition during CO2 electrolysis. Infiltration of the electrodes has previously been shown to increase performance and durability for steam electrolysis, by increasing the triple phase boundary (TPB) reaction sites, thereby decreasing the local fuel electrode over-potential. It also provides a quick and easy way to introduce new catalysts into the electrode structure to improve the electrochemical/catalytical properties [1]. The objective of this study is to investigate the SOEC fuel electrode infiltration aiming to decrease Ni-coarsening/migration and increase the carbon tolerance. 3-5% weight load of CGO is infiltrated to the fuel electrode in order to determine the effects of the CGO loading on the carbon deposition threshold and durability of the cells. The infiltrated single cells are tested under galvanostatic mode with 58% CO2 utilization at -0.5 A/cm2 or 29% CO2 utilization at -1 A/cm2 at 750 °C. The degradation is monitored by electrochemical impedance spectroscopy (EIS) during operation. Equivalent circuit modelling, using the model L-RΩ-Rion-RTPB-Gox-Rdiff-Rconv is performed to elucidate details of the cell degradation occurring during operation. The threshold in conversion, which leads to carbon deposition is mapped out tracking the iV curves as described in Figure 1. Before initiating the threshold and durability tests, the initial performance of the cells are evaluated using EIS and iV-curves in different gas compositions (H2/H2O or CO/CO2 at fuel electrode, air or O2 at oxygen electrode) at temperatures between 700-850 °C. Scanning electron microscopy (SEM) is used for characterization of any changes in the microstructure such as fuel electrode particle size distribution and Ni-Ni network quality . The results from EIS and iV curve analyses during operation have been correlated with the SEM post-test findings to determine the effect of infiltration on the cells’ performance and durability.Figure: iV curves recorded during ramping up the current to conditions of the subsequent durability tests. The lines depict measurements for non-infiltrated cells. The values for utilization given are the utilizations at the maximum current density for the tests. The tests were performed at 750 °C with 90:10 ratio of CO2:CO supplied to the fuel electrode, and 50 l/h O2 supplied to the oxygen electrode. The sudden bend in the green and line indicate a threshold for degradation, as fuel starvation is not occuring at this degree of utilization. Identifying this bend in a series of iV measurements are used for mapping the threshold for Carbon deposition, in regards to the local partial pressure of O2 at the TPB. The grey ellipsis marks the region where Carbon deposition threshold is speculated to be located.[1] Tong, X., Hendriksen, P. V., Hauch, A., Sun, X., & Chen, M. (2020). An Up-scalable, Infiltration-Based Approach for Improving the Durability of Ni/YSZ Electrodes for Solid Oxide Cells. Journal of The Electrochemical Society, 167(2), 024519. https://doi.org/10.1149/1945-7111/ab6f5c Figure 1