Visualising redox-driven metal-oxide interface dynamics in Ni/GDC electrodes via in situ Auger-Meitner electron microscopy.

Abstract

Metal/oxide interfaces play a decisive role in both high-temperature electrochemical energy conversion using solid oxide cells (SOCs) and heterogeneous catalysis. However, their morphological stability under dynamic redox conditions remains poorly understood. In the present work, we investigate model electrodes of Ni/GDC (GDC = Gd-doped ceria = Gd_0.1Ce_0.9O_2-δ ) composites in thin-film and porous form. Electrochemical polarization of these electrodes enables electrochemical control of the oxygen chemical potential in ceria and thus provides direct experimental access to redox-driven interface dynamics independent of the gas-phase composition. While morphological degradation in Ni/GDC is often discussed in the context of electrochemical polarization, the underlying driving forces are more general and arise from changes in local oxygen chemical potential, metal oxidation state, and oxide defect chemistry. In this study, we present a multi-method fundamental analysis that enables well-defined control of the oxygen chemical potential in ceria while simultaneously resolving surface chemistry and morphology of Ni/GDC. By combining the in-house developed Electrochemical oXygen Activity ConTrol (EXACT) method with in situ Auger-Meitner electron microscopy (AMEM), we directly visualize redox-driven interface dynamics over an exceptionally wide effective pO₂ range. The results on the studied Ni/ceria system clearly show that upon consecutive oxidation and reduction of Ni and NiO (redox-cycling), dewetting - one of the main degradation mechanisms in solid oxide cells - is strongly accelerated. Notably, formation of only a thin NiO interface layer is sufficient to trigger this destabilization, without requiring full oxidation of Ni. However, while conditions are kept more reducing than the Ni/NiO threshold, Ni remains morphologically stable, even though the effective pO₂ varies by more than 10 orders of magnitude. These trends are observed for both thin-film model systems and 3D porous Ni/GDC composites. Using near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) on thin-film samples, we demonstrate that our results still hold true when the electrodes are exposed to gas atmospheres such as H₂/H₂O and CO/CO₂. In addition to Ni redistribution, pronounced ceria mobility is observed, including migration of a Ce-oxide species onto Ni under strongly reducing conditions, and Ce enrichment near Ni/NiO triple-phase boundaries under oxidising conditions, thus highlighting the active role of the oxide support in redox-driven morphology evolution.