Ca3Co4O9 + δ, a growing potential SOFC cathode material: Impact of the layer composition and thickness on the electrochemical properties

Abstract

The thermoelectric material Ca3Co4O9+δ (CCO), with an electronic conductivity of σe=240S·cm−1 at 650°C and a good chemical and mechanical compatibility with the standard Ce0.9Gd0.1O1.95 electrolyte (CGO, TEC: 9–10·10−6K−1), was recently identified as a potential cathode material for solid oxide fuel cells. In this contribution the electrochemical properties of a series of CCO-CGO composite cathodes were studied as function of composition and layer thickness in a symmetrical cell set-up. The cathodes were applied on both sides of a CGO electrolyte by screen-printing. The cathode thickness was controlled through repeated drying-screen-printing cycles and the cathode compositions varied from 80‐20 to 30-70wt.% of CCO/CGO. The lowest area specific resistance (ASR) was obtained for the CCO50–50 composition with 3 layer applications (21μm thickness) with an ASR of 0.5Ω·cm2 at 700°C. All electrode impedances could be modelled successfully with an LR(RQ)(RQ)G(RC) circuit yielding a pseudo-χCNLS2 of 1–8·10−7, which indicates an excellent fit (‘G’ denotes a Gerischer element). The fit-parameters showed quite consistent behaviour as function of temperature, composition and cathode thickness. The CNLS-analysis of thickness dependence showed that almost the entire cathode layer is electrochemically active. The oxygen reduction reaction is mainly governed by three processes, diffusion process at high frequency being the most limiting step.