Electrochemical and Raman study of La0.7Sr0.3Co0.8Fe0.2O3−δ reduction

Abstract

Abstract The reduction of La 0.7 Sr 0.3 Co 0.8 Fe 0.2 O 3 − δ (LSCF7382) by heat-treatment in H 2 atmosphere or by electrochemical polarization has been followed by X-ray diffraction and Raman spectroscopy. The transition from perovskite ABO 3 to brownmillerite A 2 B 2 O 5 occurs at 250–300 °C in 10% H 2 atmosphere and is reversible. The Raman spectrum of brownmillerite shows two bands at 560 and 640 cm − 1 that we attribute to CoO 4 tetrahedron stretching. Above 600 °C the sample starts to decompose irreversibly into Co, La 2 O 3 , SrO, and LaSrFeO 4 . This confirms that Co cations are more reducible than Fe ones. The Raman spectra obtained under cathodic polarization of the LSCF7382/CGO interface at 300 °C and PO 2 = 10 − 4 or 0.1 atm show the same bands as for the A 2 B 2 O 5 structure. This indicates that electrochemical reduction produces Co 2 + in tetrahedral environment, even at low reducing overpotentials. Two reduction steps have been observed by linear sweep voltammetry and open circuit voltage relaxation. In both cases, Raman spectra indicate the presence of Co 2 + in tetrahedral environment. The first reduction step is thus attributed to the reduction of B 4 + within the perovskite phase and the second one to the reduction of Co 3 + to Co 2 + associated to the transition from perovskite to brownmillerite phase. Thermodynamic calculations from published literature support this idea. The association of n-type mobile species (Co 2 + ) with oxygen vacancies is confirmed.