Structural Evolution of a High-Temperature Oxygen Evolution Catalyst under Transient Working Conditions

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Solid oxide electrolysis cells (SOECs) as an efficient power-to-X (P2X) conversion technology for chemical storage of intermittent renewable energy suffer from poorly understood degradation processes under steady and dynamic operating conditions. In the first project phase a qualitatively new description of the anode/electrolyte interface was gained which was identified as a nano-scale complexion. This finding forms the basis of the following funding period in which we propose to investigate, the air electrode/electrolyte interface of the SOEC under dynamic operating conditions. We will build upon our tightly interwoven multi-modal microscopic, spectroscopic, and first-principle theoretical approach developed during the first project phase and extend it to near-ambient operando and quasi in situ conditions. The outcome of this multi-modal study will rationalize structural factors that influence the catalytic activity and related degradation processes. We will focus on LSM based electrolysis cells, which will be characterized electrochemically under constant and dynamic reaction conditions. The aim of this project is the evaluation of the complexion under reaction conditions in order to deliver evidence for its role in the electrochemical process. Ex situ analysis of the surface and the air electrode/electrolyte interface will be performed by electron microscopy and photoelectron spectroscopy. For the study of the structure and the oxidation states of the cations with correlative environmental scanning electron microscopy (ESEM) and operando X-ray photoelectron spectroscopy special cells will be prepared. The cells will be produced by the deposition of LSM (< 10 nm) on an YSZ substrate. This very thin electrode layer will allow the photoelectrons emanating from the complexion to leave the electrochemically active domain. The formation and stability of the complexion in the thin cells will be studied under constant and dynamic loads. Identical location TEM imaging (ILI) will complement the ex situ analysis and act as a structural basis to subsequent operando experiments. The experimental investigations will be complemented by DFT calculations. Starting from large complexion models of the YSZ/LSM interface, produced by force-field based sampling techniques, we will generate ensembles of DFT tractable model cells where each ensemble is representative of a thin section within the complexion that is parallel to the YSZ/LSM interface. To establish the properties of the complexion as a mixed conductivity (MIEC) of catalytically active interface structure, activation barriers for the hopping processes along the oxide ion conduction pathways will be computed using NEB DFT simulations within the small DFT cell ensembles generated. The optimal MIEC slice will be used to construct slab models representing possible surface configurations at the catalytically active site.