In-Operando Observations of Ni-YSZ Patterned Fuel Electrodes Under SOFC and SOEC Operations

Abstract

Solid oxide cells (SOCs), both solid oxide fuel cell (SOFC) and solid oxide electrolysis cell (SOEC), have being attracting more and more attention due to its high energy conversion efficiency and fuel flexibility. However, degradation of fuel electrodes after long-term operation remains as one of the main challenges for their commercial application. In particular, migration and coarsening of nickel (Ni) in Ni - yttria-stabilized zirconia (Ni-YSZ) fuel electrodes have strong impacts on the decreases in both active three-phase boundary (TPB) and Ni network connectivity, which result in performance deterioration. One of the mechanisms proposed as an explanation of Ni migration is Ni transport via volatile Ni(OH)x species, so-called evaporation-condensation mechanism [1]. On the other hand, Jiao and Shikazono [2] proposed a model to explain Ni spreading on YSZ surface based on the variation of Ni-YSZ wettability which is modified by the oxygen adsorption. Nevertheless, further investigations are required to understand the mechanisms of Ni migration. In the present study, Ni-film patterned electrodes which can provide well defined TPB configurations are used to investigate the Ni-YSZ degradation under SOFC, SOEC and sequential SOFC and SOEC operations, i.e. reversible operation (RSOC). The edge of sputtered Ni-film contacting with open YSZ surface can be treated as active TPB during operation. Confocal laser scanning microscopy (CLSM, Keyence, VK-X 1000) is used, which enabled in-operando observation of the real-time local morphological change at active TPB [3]. The in-operando potentiostatic experiment was carried out inside a high temperature chamber which was kept at 800oC with 4% H2O-4% H2-92% N2 as fuel gas and 30% O2-70% N2 as oxidant as shown in Fig. 1(a). The correlations between the Ni morphological changes and cells’ electrochemical performances were investigated. The Ni phase dynamically spread and de-percolated only under the SOFC operation, while such a phenomenon was not observed during the SOEC operation, as shown in Figs. 1(c), (d) and (e). Ni coarsening occurred regardless of the operation modes. The competition between Ni-film spreading and breaking-up finally determined the complicated local morphological change and corresponding cell performance variation. The time variations of active TPB and current density showed good correlation as shown in Fig. 1(b). Real-time in-operando observations revealed different Ni migration under SOFC, SOEC and RSOC operations, which may provide new insights for the understanding of Ni-YSZ fuel electrode degradation.Fig. 1. (a) Schematic illustration of in-operando setup. (b) Correlation between current density and Ni morphological evolution. (c)-(e) CLSM images and 3D surface height contour maps of the yellow-frame-enclosed areas after SOFC, SOEC and RSOC modes, respectively.