Correlation Studies of Electrochemical and Crystallographic Properties on SOFC and Soec Related Materials By Simultaneous EIS and XRD in-Operando Measurements
Authors: Napolitano Federico , Montenegro-Hernández Alejandra , Basbus Juan Felipe , Arce Mauricio Damian , Mogni Liliana Veronica , Serquis Adriana 
Published: February 2020

The complexity of the process involved in the development of new technological devices requires the use of complementary characterization techniques in order to understand the whole phenomena. A usual approach to study materials for devices with non-ambient working conditions is to perform the experiment simulating the same environment (temperature, atmosphere, pressure, magnetic field, etc.). Although being a reasonable approach, this could fall short when real working conditions are more complex [1, 2]. Nowadays, the implementation of in-operando characterization techniques is a major field of capabilities development at world-wide large facilities [3-5] and, in lesser scale, at laboratory level [6].

The situation afore described is particularly applicable to the field of high temperature electrochemical devices such as Solid Oxide Fuel Cells (SOFC) and Solid Oxide Electrolyzer Cells (SOEC) where the different materials involved in the device are exposed to extreme environments and, moreover, could exists a strong correlation between different properties. For example, it is well known the dependence of the crystallographic parameters with several relevant electrochemical properties such as electrical conductivity, chemical compatibility, electrode reaction performance, etc [7, 8]. The simultaneous characterization of such properties in-operando condition allows gaining insight into some still not well understood phenomena.

In this work, we present the simultaneous crystallographic and electrochemical characterization through X-ray Diffraction and Electrochemical Impedance Spectroscopy, respectively, of three different materials: a proton conductor electrolyte, an O2 electrode material, and a symmetrical (H2 and O2) electrode material. In order to study these materials we use a home-made sample holder specifically designed to study the electrical resistivity and the electrode polarization resistance evolution simultaneously with structural properties, under in-situ or in-operando conditions.

Firstly, we followed a second order phase transition found on the proton conductor electrolyte material BaCe0.4Zr0.4Y0.2O3-
δ at around 400°C under wet oxidizing and reducing atmospheres and its influence on its ionic conductivity. This phase transition (related to a crystallographic distortion coming from the hydration/dehydration process) affects the lattice parameters and, therefore, the ionic transport properties.  The utilization of the in-operando sample holder allow us to get direct information about how this structural distortion affects the transport properties.

Secondly, we studied the electrochemical performance of Nd2NiO4+
δ electrode material and its crystallographic parameters under an in-operando condition through the simultaneous EIS and XRD measurements of a half cell applying several electrode potentials. While their ionic conductivity combines the migration of interstitial oxygen (δ) in the rock-salt layer and the migration of oxygen vacancies in the perovskite layer, it is actually dominated by the former due to the higher mobility of these defects. Therefore, the O-content play a fundamental role in the O2 electrode activity (specially working under anodic polarization) and, moreover, is the driven force of a first order phase transition from an orthorhombic (Fmmm) to a tetragonal (I4/mmm) symmetry at 505 °C.

Lastly, we also followed the second order phase transition exhibited by the symmetrical electrode La0.4Sr0.6Ti0.5Co0.5O3 at around 300 °C (in both oxidizing and reducing atmospheres), passing from a rhombohedral perovskite structure (R-3c) to the ideal cubic perovskite (Pm-3m) as temperature is increased. This phase transition is mediated by the rigid rotation of the oxygen octahedra (around a-a-a- direction, in Glazer’s notation) [9].

The correlation between the electrochemical properties and structural parameters on each system is discussed.

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