Oxidation stages of Ni electrodes in solid oxide fuel cell environments

Nickel is the most commonly used anode for solid-oxide fuel cells (SOFC) due to its fast kinetics and low price. A leading cause of degradation in Ni electrodes is oxidation. Here we use operando ambient-pressure X-ray photoelectron spectroscopy (XPS) to chemically characterize the Ni electrode of a fuel cell anode during oxidation in a H2/H2O atmosphere. We find three different stages of Ni oxidation in the model SOFC. In the first two stages, the Ni exposed to the gas remains metallic but the Ni at the interface with the zirconia electrolyte is oxidized. In the third oxidation stage, we find that Ni transforms to NiOOH, a phase not previously considered in the SOFC literature. We show that the transformation between Ni and NiOOH is reversible and is initiated at the Ni/gas interface. In addition we find that NiOOH stores charge, as evidenced by the stable discharge plateau (voltage) measured as this oxyhydroxide phase reduces to metallic Ni.