Development of an Isotopic Oxygen Exchange Setup Operating Under High Pressure: Preliminary Measurements Performed on Ln2NiO4+d (Ln=La, Pr, Nd, MIEC) Oxides

Abstract

MIEC (Mixed Ionic Electronic Conductors) are commonly used as oxygen electrodes in SOC (Solid Oxide Cells) either for fuel cell or high-temperature steam electrolysis applications. In addition to a high electronic conductivity, the electrodes require both high oxygen diffusivity (D) and surface exchange (k) coefficient in the operating temperature range to be considered as candidates.The two parameters D and k can be directly determined thanks to the IEDP (Isotopic Exchange Depth Profiling) method: a dense material is annealed during controlled time under enriched 18O atmosphere, prior to the recording of the 18O/(18O+16O) profile inside the ceramic using a TOF-SIMS analyser. The tracer coefficients for the oxygen diffusion (D*) and surface exchange (k*) can be determined by fitting this experimental 18O penetration profile: it is achieved by using the solution for Fick’s second law proposed by Crank [1], when applied to the diffusion into a semi-infinite medium.To date, the reported measurements in the literature have been performed at ambient pressure and even below [2]. However, in order to reach a higher efficiency at the system level or for a ‘Power-to-Gas’ application, it would be helpful to operate the Solid Oxide Electrolysers at high pressures, between 10 to 60 bars [3,4]. A preliminary study is therefore needed, which consists in assessing the oxygen reduction/oxidation and the transport properties of the electrodes materials when they are submitted to such high pressure. In order to address these questions, an innovative setup able to operate up to a total pressure of 50 bars has been developed in collaboration between the ICMCB-CNRS and the CEA-LITEN. Its technical characteristics will be described in the first part of the article.This setup has been used to characterize the properties of nickelate dense pellets with the K2NiF4-type structure. These materials, whose main defects and charge carriers are interstitial oxygens, have been developed for several years as an alternative to the more conventional oxygen deficient perovskites [5]. Oxygen exchanges have been performed in the range 500 < T°C < 700, under P (air) = 30 bars (equivalent to p(O2) ~ 6.5 bars. Compared to atmospheric condition, the oxygen over-stoichiometry is as expected significantly increased after the oxygen exchanges, whatever the nickelate. Preliminary results (D* and k*) obtained on Ln2NiO4+ d (Ln=La, Pr, Nd) MIEC oxides will be presented and discussed.