Understanding electro-catalysis in Solid Oxide Cells (SOCs) for designing electrodes

Kurzfassung

Beyond the fundamentals of the operation of high temperature Solid Oxide Cells (SOCs), the key performance factors are linked to the functionality of both air and fuel electrodes. Nowadays, the common approach for developing electrodes lies in the investigation of new phases with novel compositions showing improved electrocatalytic performances towards the specific electrochemical processes. Nonetheless, as all functional materials, the final performance of the electrodes stays not only in the intrinsic electro-catalytic performance of the considered phase, but also in the technology implemented for the coating and the resulting microstructure of the electrode. As changing a phase composition may change the mechanism of the electrochemical processes, the implementation of new phases in SOCs and their optimization in terms of performances, requires a deep understanding of the electro-catalytic phenomena to design and manufacture efficient electrodes. Indeed, if historically composite electrode materials made of separate ionic and electronic conductors, for example nickel zirconia cermet as anode, or LaxSr1-xMnO3-δ (LSM)/zirconia as cathode materials have been widely used as fuel and air electrode respectively in SOCs, materials development over the last 20 years have led to the development of an increased number of Mixed Ionic and Electronic Conductors (MIEC), able to provide better functionality like electronic conductivity in the case of the air electrode or resilience towards events like redox cycles characteristic of practical operation. Shifting from composite materials with single properties for each phase to MIEC has redefined the localization of active sites of the functional electrolyte. In this paper, we present the results of a combined experimental and electro-kinetic modeling aiming at understanding fundamental physico-chemical and transport processes in SOCs. In the case of LaxSr1-xTiO3-δ (LST)-Ce1-xGdxO2-α (CGO) novel perovskite anodes a detailed multi‐step heterogeneous chemical and electrochemical reaction mechanism was established and modeling results were thoroughly compared to electrochemical measurements. It was found that heterogeneous chemistry at LST surface has capacitive behavior that alters the impedance spectra. In addition, surface charge-transfer reaction, which describes partial oxygen ionization, caused impedance feature and is rate-limiting at high temperature. Comparable investigations have been performed on the system LaxSr1-xCo1-yFeyO3-δ (LSCF)-Ce1-xGdxO2-α (CGO) as cathode to identify main rate limiting processes (see Fig. 1 Right), which are (i) gas conversion (low frequencies), (ii) electrochemical oxygen reduction on the LSCF surface (intermediate frequencies) and (iii) charge-transfer of double negatively charged oxygen through two-phase boundary between LSCF and CGO (high frequencies). Analyzing the microstructural features governing the models, provides useful indications for optimizing further the electrodes. Moreover this approach would allow the development of operating strategies for increasing cell’s