Hydration effect on properties of the La2-x A x Ni1-y Fe y O4+δ (A=Ca, Sr) cathode materials for H+-SOFCs

Highlights: • Effect of air humidity on properties of La_2-xA_xNi_1-yFeyO_4+δ was observed at T ≤ 700 °C. • Thermogravimetric analysis suggested hydration of the samples in wet air below 700 °C. • (La/A)O layer of La_2-xA_xNi_1-yFe_yO_4+δ expanded with increasing air humidity. • The impedance studies showed that hydration affected the surface exchange process. • The results indicated the presence of proton conduction in La₁.₄Sr₀.₆Ni_0.5Fe_0.5O_4+δ. -- Abstract: The effect of air humidity on the transport properties of the La_2-xA_xNi_1-yFe_yO_4+δ (A=Ca, Sr; x = 0.5, 0.6 and y = 0.4, 0.5) oxides was studied in the temperature range of 25–950 °C. Total conductivity of the samples decreased with increasing relative humidity (RH), most significantly in the range of 250–550 °C. Thermogravimetric analysis in dry and wet air indicated that the observed changes in conductivity could be attributed to the hydration of the samples. The high-temperature X-ray powder diffraction (HT-XRPD) results for La_1.4Sr_0.6Ni_0.5Fe_0.5O_4+δ (LSNF0605) suggested that the hydration led to the insertion of hydroxide ions into the interstitial sites resulting in the expansion of the rock-salt type layers and shrinkage of the perovskite layers. The Electrochemical Impedance Spectroscopy (EIS) measurements for the LSNF0605/Ce_0.8Sm_0.2O_2-δ (SDC) and LSNF0605/La_28−zW_4+zO_54+1.5z (LWO) symmetrical cells showed that the hydration affected the surface exchange process in LSNF0605. The activation energy of surface exchange decreased with the increase in RH for the LSNF0605/LWO cell indicating that water could participate in the surface exchange. The activation energy of the charge transfer – ionic diffusion process in the LSNF0605 electrode for the LSNF0605/LWO cell possessed smaller values compared to that for the LSNF0605/SDC cell in wet air, suggesting the presence of proton conduction.