Indrek Kivi

Effect of Cell Geometry on the Electrochemical Parameters of Solid Oxide Fuel Cell Cathodes

A series of electrochemical impedance experiments has been carried out in order to investigate the effect of cell composition and geometry on the determination of electrochemical characteristics of strontium-doped lanthanum cobaltite (La 0.6 Sr 0.4 CoO 3-δ ) cathodes. The impedance responses at different electrode potentials of the half-cell and symmetric single-cell setups are compared and analyzed by the equivalent circuit modeling method. The deconvolution of impedance spectra for single cells has been achieved by a differential impedance real part vs ac frequency plot analysis method. The results indicate that the three-electrode half-cell configuration is more suitable for fundamental research of material parameters at different electrode potentials (measured vs Pt|O 2 reference electrode), whereas the simpler two-electrode single-cell setup allows the estimation of cathode performance in approximate working conditions.

Optimization of the Cathode Composition for the Intermediate-Temperature SOFC

Electrochemical impedance, cyclic voltammetry, and chronoamperometry have been used for characterization of the medium-temperature half-cells Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖La 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 1), Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖Pr 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 2), and Ce 0 . 8 Gd 0 . 2 O 1 . 9 ‖Gd 0 . 6 Sr 0 . 4 CoO 3 - δ (Sys 3) at various electrode potentials and temperatures. Analysis of the impedance data shows that the kinetically mixed process is probable, characterized by the slow electron transfer to an adsorbed and thereafter dissociated oxygen atom O a d s , as well as by slow mass transfer of electroactive species inside the cathode or O a d s at the internal surface of the porous cathode. The total polarization resistance increases with rising the atom mass of the A-site cation in the porous perovskite structure. The values of activation energy decrease slightly with increasing negative polarization and in the order of half-cells Sys 3 > Sys 2 > Sys 1. The transfer coefficient α c over 0.5 indicates the deviation of the mainly charge-transfer-limited process toward the mass-transfer-limited process in the porous cathode with decreasing temperature. The electrochemical characteristics of half-cells Sys 2 and Sys 1 are stable under repetitive potential and thermocycling during long operation times (t > 1200 h for Sys 2 and t > 4800 h for Sys 1).

Influence of humidified synthetic air feeding conditions on the stoichiometry of (La1-xSrx)yCoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes under applied potential measured by electrochemical in situ high-temperature XRD method

The solid oxide fuel cell symmetrical cells with porous (La1-xSrx)yCoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ electrodes for intermediate temperature applications have been studied under electrochemical polarization and synthetic air + H2O vapor (so called moisturized cathode gas) feeding conditions using high-temperature electrochemical in situ X-ray diffraction method, chronoamperometry, cyclic voltammetry, and impedance spectroscopy methods. Changes in the lattice parameters and electrochemical activity of La0.6Sr0.4CoO3−δ, (La0.6Sr0.4)1.01CoO3−δ, (La0.6Sr0.4)0.99CoO3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ were calculated depending on the temperature (T), electrode potential (E), and oxygen partial pressure (pO2) applied. Influence of H2O vapor in synthetic air on (La1-xSrx)yCoO3−δ parameters was irreversible and continued expansion of (La1-xSrx)yCoO3−δ lattice observed in H2O vapor + synthetic air feeding conditions has been observed continuously after changing wet cathode gas to dry oxygen. Nearly, reversible behavior of La0.6Sr0.4Co0.2Fe0.8O3−δ lattice has been established and for La0.6Sr0.4Co0.2Fe0.8O3−δ, the cell volume and polarization resistance started to decrease after changing humidified synthetic air to dry synthetic air in the cathode compartment. For the slightly cationic deficient (La0.6Sr0.4)0.99CoO3−δ, the cathode structure is more stable and the electroreduction of the oxygen was faster. Detailed comparison of experimental data demonstrates that the dependence of the crystallographic parameters on the electrode potential and temperature applied is in a good agreement with the electrochemical impedance spectroscopy data, indicating that the electrocatalytic activity of the cathode decreases with the rise of Fe ion concentration in B-position of LSCF cathode.