Abstract:

SrFeO_{3- δ} is a widely studied cathode material for solid oxide fuel cells owing to its good mixed electronic and oxygen ionic conductivity and considerable catalytic activity. However, SrFeO_{3- δ} usually has a tetragonal structure at room temperature, and the tetragonal-to-cubic phase transition at high temperatures produces volume change, which poses the risk of structural deterioration. In addition, its catalytic activity toward oxygen reduction needs to be improved further. In this study, fluorine substitution of oxygen was employed to modulate the lattice structure of SrFeO_{3- δ} as a cathode material and improve its various properties. F-ion-doped SrFeF _x O_{3- x - δ} (SFF _x , x = 0, 0.125, and 0.25) materials were synthesized via the sol-gel process. The effects of F ion doping on the lattice structure, thermal expansion coefficient, electrical conductivity, oxygen surface exchange coefficient, oxygen bulk diffusion coefficient, polarization resistance, electrode reaction kinetics, and cell performance were investigated through X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical characterization techniques. The results show that the partial substitution of O ions by F ions (x = 0.25) can stabilize the cubic structure of SrFeF _x O_{3- x - δ} at room temperature owing to a decreased tolerance factor, decreased iron valence, and increased Fe ion size. Both cell parameters and cell volume increase with F ion doping content. The F ion doping decreases the thermal expansion coefficient but increases the electrical conductivity of SFFx. With F ion doping, the oxygen surface exchange coefficient (K _ex) and bulk diffusion coefficient (D _chem) of the material improve, which is correlated with the effective reduction of the polarization resistance (R _p) of the materials by F substitution. By monitoring the impedance change of each component process with a change in oxygen partial pressure, the electrode reaction kinetics study reveals that F ion doping considerably enhances the dissociation process of oxygen molecules on the surface of SFF _x . For materials with an F-doping amount of x = 0.25, the R _p value is 0.508, 0.173, 0.077, 0.039, and 0.023 Ω·cm² at 650, 700, 750, 800, and 850 °C, respectively. Single cells were constructed using SFF _x and Sm_0.2Ce_0.8O_{2- δ} (1∶1) as the cathode, La_0.9Sr_0.1Ga_0.8Mg_0.2O_{3- δ} (300 μm) as the electrolyte, La_0.4Ce_0.6O_{2- δ} as the buffer layer, and NiO-Gd_0.1Ce_0.9O_{2- δ} as the anode. Humidified H₂ and air were fed as the fuel and oxidant, respectively. The cell tests show that the F ion doping significantly increases the peak power density of the cells. For materials with x = 0.25, the maximum power density can reach 446 and 962 mW·cm^-2 at 700 °C and 850 °C, respectively.

Key words: solid oxide fuel cells, cathode, perovskite, fluorine doping, electrode reaction kinetics