Multiscale Modelling of Solid Oxide Cells Validated on Electrochemical Impedance Spectra and Polarization Curves

Abstract

Solid Oxide Cells (SOCs) are high temperature energy-conversion devices which have attracted a growing interest in the recent years. Indeed, this technology presents a high efficiency and a good reversibility in fuel cell (SOFC) and electrolysis (SOEC) modes. Thanks to its flexibility, SOCs can offer technical solutions for the development of a clean hydrogen economy. Nevertheless, SOCs durability is still insufficient for large scale commercialization. Therefore, it is still required to improve the SOCs lifetime by maintaining high performances. For this purpose, it is necessary to better understand the impact of global operating conditions on the local processes taking place in the cell components. Besides, the role of the electrode microstructure on the reaction mechanism is still not precisely understood. From this point of view, the modelling can be an efficient tool to unravel and better analyze all the microscopic processes involved in the cell operation.In this context, a physical-based model has been proposed to investigate the impact of operating conditions on the electrodes reaction mechanisms and cell performances. This model takes into account (i) a 3D representation of the electrode microstructure [1], (ii) a description of the reaction mechanisms in full elementary steps [2,3] and (iii) the SOC geometry with the gas flow configuration [4,5]. This multiscale model has been developed considering a typical cell composed of a dense electrolyte in Y0.148Zr0.852O1.926 (8YSZ) sandwiched between an O2 electrode in La0.6Sr0.4Co0.2Fe0.8O3- d-Ce0.8Gd0.2O2-δ (LSCF-GDC) and an H2 electrode made of Ni and YSZ (Ni-YSZ). The model has been validated using a specific experimental setup which was developed to measure the local polarization curves along the cell length. For this purpose, a specific design of the interconnect has been proposed in order to probe the local current density on the standard studied cells (Fig. 1a) [6]. It has been found that the model is able to reproduce accurately the global and local polarizations curves in SOFC and SOEC modes (Fig. 1a and 1b). The stationary model has been also extended to compute electrochemical impedance spectra by keeping the full description of the reaction mechanisms in elementary steps. This dynamic model, which is able to compute the impedance diagrams at Open Circuit Voltage (OCV) and under polarization, has been compared to the experimental data. As shown in Fig. 1c and 1d, a reasonable agreement has been found between the measurements and the simulations without any fitting.The validated stationary and dynamic model has been used to analyze the cell operation in electrolysis and fuel cell modes. The activated reaction pathways associated with the elementary steps in the active layers have been investigated depending on the position along the cell length. The different contributions arising in the impedance spectra have been also identified and discussed.