An insight into the electrochemical performance of La0.5−xPrxBa0.5CoO3−δ as cathodes for solid oxide fuel cells: study of the O2-reduction reaction

Abstract

New LT-SOFC cathodes with La_0.5−xPr_xBa_0.5CoO_3−δ (0 ≤ x ≤ 0.5) compositions were studied. The La-rich compounds exhibit an inter-growing cubic and tetragonal structure whereas the Pr-rich show a tetragonal structure. Cathode polarization resistances, estimated from electrochemical impedance (EIS), take values around 0.15 and 0.04 Ω cm² in air at 600 and 700 °C, respectively. The O₂-reduction mechanism and its kinetic coefficients, O-ion diffusion (D_chem) and O-surface exchange (k_chem), were studied by applying the ALS model for macro homogenous porous electrodes to the EIS data, in combination with microstructural parameters obtained from three-dimensional tomography using focused ion beam-scanning electron microscopy (3D FIB-SEM). The diffusion coefficients for samples with mixing of phases take values of D_chem ∼ 10⁻⁶ cm² s⁻¹, at 700 °C in air, whereas the pure tetragonal phases present lower values D_chem ∼ 10⁻⁸ cm² s⁻¹. On the contrary the k_chem for all samples takes values ranging between 0.4 and 1 × 10⁻⁴ cm s⁻¹ at 700 °C in air. In addition, the origin of time evolution of polarization at 700 °C in air was evaluated by combining the EIS spectra as a function of time and the ex situ sample characterization of fresh and tested samples by 3D FIB-SEM tomography and inductively coupled plasma-optical emission spectrometry (ICP-OES). Little changes of microstructures were detected, whereas an increasing of the amount of a water-soluble Ba surface species in comparison with total cation surface segregation was observed for all samples after the ageing. This may constitute the main reason causing the polarization resistance degradation. However, the analysis of time evolution shows, that contrary to the situations reported for SrO segregation in Sr-based perovskites, the D_chem is the coefficient mainly affected for compositions with inter-grown phases. At the same time, both D_chem and k_chem decreases with time when only the tetragonal phase is present.