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 steam is present in the feedstock of the cells, frequently mixed with other gases, such as hydrogen or CO₂ . An increase in the steam concentration in the feed mixture is considered accountable for the electrode oxidation and the deactivation of the device. However, direct experimental evidence 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 H₂ O environments by using near-ambient-pressure X-ray photoelectron and absorption spectroscopies. Changes in the surface oxidation state and composition of NiGDC in response to the ambient gas are observed. It is revealed that, in the mbar pressure regime and at intermediate temperature conditions (500-700 °C), steam acts as an oxidant for nickel but has a dual oxidant/reductant function for doped ceria.