Operando Analysis of a Solid Oxide Fuel Cell in Environmental Transmission Electron Microscopy

Solid oxide fuel cells (or SOFC) are a class of solid-state electrochemical conversion devices that produce electricity directly from oxidizing a fuel gas. They consist in an anode-cathode duet separated by a solid electrolyte, i.e. an oxide conducting material. The anode is fed with hydrogen or other fuels whereas the cathode is in contact with air, meaning oxygen. Overall, a SOFC operates thanks to the combined action of two external stimuli: a gaseous environment and temperature.<br/>Owing to the recent advances in in situ and operando transmission electron microscopy (TEM), we have set up an experiment to operate a SOFC inside an environmental TEM to identify how the device microstructure determines its electrical properties.<br/>An elementary anode-electrolyte-cathode sandwich was prepared by Focused Ion Beam (FIB), and mounted on a heating and biasing microelectromechanical (MEMS) based specimen holder (DENSsolutions) inserted a FEI Titan ETEM.<br/>Our sample is made as a standard SOFC: the cathode is based on strontium-doped lanthanum manganite (LSM), the electrolyte is yttria-stabilized zirconia (YSZ) and the anode is a NiO cermet; both electrodes were co-sintered with YSZ. NiO was first reduced to Ni, leaving pores in the structure due to the volume loss and hence enabling the penetration of the fuel to the triple phase junctions Ni/YSZ/porosity at the anode side. For practical reasons, we have used a single chamber configuration where the anode and cathode were exposed simultaneously to the oxidant and reducing gases. Thanks to a difference in the catalytic activity between the electrodes, O₂ should reduce at the cathode, while H₂ should oxidize at the anode, thus leading to a voltage difference between the two terminals and detected by the biasing system of the holder.<br/>The reduction of NiO was then first performed under a forming gas N₂:H₂ in the ratio 20:1 under 15 mbar up to 750°C (N₂ was constantly used as a mixing vector gas for safety reasons due to the need of mixing O₂ and H₂ in the single-chamber configuration). The O₂ to H₂ ratio was then increased to trigger the operation of the cell. A small quantity of oxygen was introduced into the microscope up to about 16 mbar at 600°C, a temperature sufficient to promote the dissociation of O₂ molecules in the ETEM.<br/>At this point, the variation of current was correlated to the evolution of the gas fuel composition and the anode microstructure. The latter was followed by conventional and high resolution imaging, diffraction work and EELS (Electron Energy-Loss Spectroscopy).<br/>The system was then cycled several times by decreasing and re-increasing the O₂ concentration in the gas flow and correlations between microstructure, gas composition, and cell voltage were positively established, as it will be discussed at the conference. Results were further confirmed by macroscopic ex situ tests in an oven using the same materials [1].<br/>The operation of a SOFC in a single chamber configuration was demonstrated thanks to operando ETEM. Nevertheless, the pressure and flow limits with this instrument have restricted its operation to a transient working state. This operando experiment opens numerous perspectives to investigate possible failure pathways affecting SOFCs, like poisoning of active sites or coarsening of the Ni catalyst [2].<br/>[1] Q. Jeangros, M. Bugnet T. Epicier, C. Frantz, S. Diethelm, D. Montinaro, E. Tyukalova, G. Pivak, J. Van herle, A. Hessler-Wyser, M. Duchamp, under review (submitted 2021/10).<br/>[2] The authors acknowledge the French microscopy network METSA (www.metsa.fr) for funding and CLYM (Consortium Lyon-St-Etienne de Microscopie, www.clym.fr) for the ETEM access. The FIB preparation was performed at the Facilities for Analysis, Characterization, Testing and Simulations (FACTS, Nanyang Technological University NTU). Additional support was provided by the INSTANT project (France-Singapore MERLION program 2019-2020) and the start-up grant M4081924 at NTU.