Understanding the Operating Mechanisms of Mixed Ionic Electronic Conductors: From Synchrotron-Based 3D Reconstruction to Electrochemical Characterization and Modeling

Mixed Ionic Electronic Conductors (MIEC) are increasingly used as oxygen electrode of Solid Oxide Cells (SOCs). The main advantage of these materials lies in their electrochemical reactivity which is not restricted to the Triple Phase Boundary lines (TPBls) but can extend to the whole MIEC surface. This results in a significant decrease in activation overpotentials. Several studies have been dedicated to cathodic polarization (fuel cell mode) to understand the MIEC reactive mechanisms. In anodic polarization (electrolysis mode), much fewer studies are available and the electrochemical process occurring remains unclear. Moreover, the role of the electrode microstructure in the reactive mechanisms has to be clarified. Thanks to a coupled modelling and experimental approach, the present study intends to analyze the predominant reaction mechanisms in both polarizations and to investigate the impact of electrode microstructure on the cell performance.

The electrode material is of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ compound. In order to obtain relevant morphological properties, a 3D reconstruction was performed by X-ray nano-holotomography at the European Synchrotron Radiation Facility (ESRF) on the Nano-Imaging beamline ID16A. This new beamline has been designed to investigate large volumes of materials with a high spatial resolution. The obtained 3D reconstruction of a sample prepared with a plasma Focused Ion Beam (PFIB) presents a large field of view of 50 µm and a voxel size of 25 nm (Fig. 1). These characteristics allow being representative of the porous medium and thus computing accurately all the electrode morphological properties such as the specific surface area, TPBls, tortuosity factors, etc…

These morphological properties are introduced as input data in a specific electrochemical model developed in both anodic and cathodic polarizations. The model takes into account the oxygen reactivity by surface reactions at the gas/MIEC interface as well as the direct oxidation of oxygen adsorbates at the TPBls. This model has been validated on the basis of experimental results performed on symmetrical cells (with a three electrode set-up) (Fig. 2). Simulations have allowed determining the rate-limiting steps in each polarization. As expected in fuel cell mode, the reactive pathway is found to be controlled by the oxygen adsorption and incorporation followed by solid state diffusion. In electrolysis mode, it appears that the mechanism is governed by the direct oxidation at the TPBls followed by the adsorbates diffusion on the surface of MIEC particles. In addition, the specific role of microstructure on the electrode performance has been investigated in both polarizations.

The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207 and 621173.