Is Steam an Oxidant or a Reductant for Nickel/Doped‐Ceria Cermets?

Nickel/doped-ceria composites are promising electrocatalysts for solid oxide fuel and electrolysis cells. Very often in the feedstock of the cells there is steam, frequently mixed with other gases, such as hydrogen or CO2. Increase of the steam concentration in the feed-mixture is considered accountable for the electrode oxidation and the deactivation of the device. However, direct experimental evidences of the steam interaction with nickel/doped-ceria composites, with adequate surface specificity, are lacking. Herein we explore in situ the surface state of nickel/gadolinium-doped ceria (NiGDC) under O2, H2 and H2O environments, using ambient pressure X-ray photoelectron and absorption spectroscopies. Changes of the surface oxidation state and composition of NiGDC in response to the gas ambient are observed. It is revealed that in the mbar pressure regime and at intermediate temperature conditions (500-700 oC), steam acts as an oxidant for nickel, but has a dual oxidant/reductant function for doped-ceria.