Ca₃Co₄O_9−δ: A Thermoelectric Material for SOFC Cathode

The misfit compound [CoO 2 ][Ca 2 CoO 3−δ ] 0.62 is well-known for its good potentialities in the field of thermoelectric oxides combining good electronic transport, high Seebeck coefficient, and low thermal conductivity. Its 2D-crystal structure can be regarded as a natural intergrowth between electronic-conducting Co 3+ /Co 4+ hexagonal layers and oxygen deficient Co 2+ /Co 3+ rock-salt layers with low thermal conductivity. Their lacunar character suggests a possible anionic conductivity. We took advantage of this model for application as a SOFC cathode material. Additional advantages appear from the good chemical and mechanical adaptability (TEC = 9−10 × 10 −6 °C −1 ) with intermediate temperature electrolyte, namely, CGO. The manufactured symmetrical cells show a good electrode/electrolyte adherence, stable after long-time experiments. Our promising preliminary electrochemical tests show a rather low electrode overpotential (4Ω·cm 2 ) for ∼40 μm thick layers with a rather dense microstucture. The porosity and electric performances are improved in the composite with 30 wt % CGO (∼1 Ω·cm 2 ). In general, from polarization experiments versus temperature and oxygen pressure, we found two distinct processes, frequency-separated, that is, HF, charge transfer at the TPB with intrinsic O 2− diffusion; LF, gas transfer/oxygen dissociation. This latter is largely fastened in the CGO/Ca 3 Co 4 O 9−δ, reminiscent of the existing but limiting ionic mobility in the single phase of the title compound.