Publication date: 10th April 2024
The oxygen reduction in a Solid Oxide Fuel Cell is a complex process that is greatly influenced by the type of material used, its microstructural characteristics, and the operating conditions.
Using Kröger and Vink's notation, the overall reaction can be written as ½O2(g)+2e′+VO..→OOx. This complex reaction involves several steps: diffusion of molecular oxygen, dissociation of molecular oxygen at the electrode surface, diffusion of oxygen or partially ionized atoms at the solid surface or their incorporation into the solid, charge transfer, diffusion of ions into the solid, etc… with different time scales. Whereas gaseous diffusion is a slow process, ionic diffusion in solids is rapid.
Here the calcium cobaltite Ca3Co4O9+δ (CCO), which can be viewed as an emerging electrode for Solid Oxide Fuel Cells or Solid Electrolyser Cells [1-4], was studied as a model SOFC air electrode material. In order to increase the density of triple-phase boundaries between the gas and the two materials, a CCO powder with small grain size was prepared using a citrate route. For screen printed 50/50 wt % CCO (citrate)/CGO composite electrode deposited on a symmetrical cell with a CGO electrolyte, an ASR of 0.35 Ω.cm2 was achieved at 700°C, under air. To go further in the understanding of the oxygen reduction reaction, the cell was then carefully studied at variable temperatures and under variable oxygen partial pressures, combining calculation of the distribution function of relaxation times (DFRT) and complex nonlinear least squares fitting (CNLS) of impedance spectra. This study revealed that this composite is primarily limited by the adsorption dissociation of oxygen on the CCO and CGO surface. Based on this result, the performances of the electrode were improved by partial substitution of calcium with strontium for which better kinetics had been evidenced for oxygen exchange at the surface [5]. A specific surface resistance of 0.15 Ω.cm2 was reached at 700°C.
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