Proof-of-concept thermoelectric oxygen sensor exploiting oxygen mobility of GdBaCo₂O_5+δ

In this paper, we demonstrate a proof-of-concept oxygen sensor based on the thermoelectric principle using polycrystalline GdBaCo₂O_5+δ, where 0.45 < δ < 0.55 (GBCO). The lattice oxygen in layered double perovskite oxides is highly susceptible to the ambient oxygen partial pressure. The as-synthesized GBCO sample processed in ambient conditions shows a pure orthorhombic phase (P_mmm space group) and a δ-value close to 0.5 as confirmed by x-ray diffraction Rietveld refinement. The x-ray photoelectron spectroscopy (XPS) shows a significant Co³⁺ oxidation state in non-octahedral sites in addition to Co³⁺ as well as Co⁴⁺ in octahedral sites. The insulator-to-metal transition (MIT) is observed at 340 K as seen from resistivity and Seebeck coefficient. The Seebeck coefficient shows a large change of 10–12 μV/K with a time constant of ∼20 s at 300 K, when the gas ambience is changed from 100% oxygen to nitrogen and vice versa. The diffusion of oxygen in the GdO_δ planes leads to the hole doping, which is a dominant factor for a large change observed in the Seebeck coefficient. This is also evident from the higher fraction of oxidized Co⁴⁺ as seen from XPS measurements. The interfacial grain boundary in addition to the oxygen diffusion contributes to the change in Seebeck. The change in Seebeck coefficient is minimal in the metallic state due to an insignificant increase in the carrier concentration, but the response is fairly well and reproducible for stoichiometry δ = 0.5 ± 0.05 below MIT. This principle shall be of significant importance in designing oxygen sensors operational at room as well as cryogenic temperatures.