In situ near-ambient pressure X-ray photoelectron spectroscopy discloses the surface composition of operating NdBaCo2O5+δ solid oxide fuel cell cathodes

Layered perovskite mixed ionic electronic conductors with formula NdBa1-xCo2O5+δ, (x = 0, 0.05, 0.10) are promising Solid-Oxide Fuel Cell (SOFC) cathodes. This study concentrates on the synthesis and physico-chemical characterization of these materials. Specifically, the surface composition is investigated by in situ near-ambient pressure X-Ray Photoelectron Spectroscopy (XPS) measurements at 800K in 0.5 and 5 mbar O2 up to 2.5 Vcell. Under electrochemical polarization, a chemisorbed molecular O2 species is identified, accompanied by the partial reduction of Ba. The former species is the precursor for the oxygen-reduction mechanism and the latter is the active surface site. This attribution is supported by two pieces of evidence: (i) the surface concentration of Ba oxides anticorrelates with oxygen pressure; (ii) the molecular oxygen species is not affected by the degree of Ba deficiency. Ba deficiency, instead, correlates positively with the surface concentration of BaO and negatively with those of Co oxides and BaO2. Ageing under oxygen-reduction conditions leads to the loss of electrocatalytic performance, owing to the surface enrichment with defective cobalt oxidic phases, exhibiting lower electronic conductivity, and depletion of active Ba. EIS measurements, carried out in parallel with photoelectron spectroscopy, independently confirm the key kinetic impact of the electronation of adsorbed elemental oxygen.