The Influence Of Ti And Lu/Gd Co-doping On The Electrical Properties Of Bi₂O₃ Composite Oxygen Ion Conductors

Solid oxide fuel cell(SOFC)is a device that converts chemical energy stored in fuels(hydrogen,hydrocarbon)and oxidizing agents(such as oxygen)into electrical energy efficiently and environmentally through a series of chemical reactions at high temperature.One of the core components is the electrolyte,and it is very important to improve the conductivity and stability of the electrolyte.At present,the electrolytes of SOFC are mainly fluorite and perovskite structural materials.Fluorite structured Bi₂O₃electrolyte is one of the most important materials.Compared with other traditional electrolytes(8YSZ),the conductivity of Bi₂O₃ has been improved by more than two orders of magnitude.Bi₂O₃ electrolyte has the advantage of extremely high conductivity at medium and low temperature.However,Bi₂O₃ exists in six phases at different temperatures,resulting in a phase transition and a decreased conductivity.Therefore,in order to improve the stability and conductivity of Bi₂O₃ based materials,the following aspects were investigated in this paper:On the basis of single doping,the co-doping method is adopted in this paper.Firstly,Lu and Ti elements with similar ionic radius were selected,and the required samples(abbreviated as"TLSB")were obtained by co-doping Bi₂O₃ with a simple and feasible solid phase sintering method.X-ray diffraction(XRD)and scanning electron microscopy(SEM)were used to analyze the phase and morphology of the powder and ceramic samples.XRD indicated that when the total doping concentration of Lu and Ti is 24 mol.%and remains constant,all the ceramic samples are single cubic fluorite pure phase,and the rest of the doped powder and ceramic pellets have impurities.The SEM results showed that all the ceramic pellet samples were relatively dense.The electrochemical analyzer was used to test the samples,and the results showed that the conductivity improved with the increase of the total doping concentration when the Lu and Ti doping ratio was 2:1.When the total Lu and Ti doping concentration remained unchanged at 24 mol.%,the conductivity first increased and then decreased with the increase of Ti doping concentration,and the conductivity of Bi_0.76Lu_0.16Ti_0.08O_1.5+?reached the highest,which was about 0.56 S?cm^-1 at 700?,and its stability was also relatively good.In order to further improve the conductivity and stability of co-doped Bi₂O₃ based materials.Moreover,Gd and Ti elements with similar ionic radius were selected,and Gd and Ti co-doped Bi₂O₃ samples(abbreviated as"TGSB")were prepared by solid phase sintering method.By changing the doping ratio and total doping concentration respectively,the properties of these two elements after co-doping were studied.XRD indicated that only Bi_0.76Gd_0.12Ti_0.06O_1.5+?(abbreviated as"6T12GSB")ceramic pellet sample was pure phase,and the rest samples were all mixed phase.The SEM results showed that all the samples were relatively dense.From the analysis of electrical performance,it could be seen that the electrical conductivity increased first and then decreased whether the Gd and Ti doping ratio was controlled at 2:1 or the total concentration of Gd and Ti doping was maintained at 18 mol.%.Compared with Ti,Gd was more conducive to the stability of Bi₂O₃ phase,but was not conducive to the improvement of electrical conductivity.When the doping concentration of Gd was greater than 16 mol.%,its stability was better.The highest conductivity of the TGSB series was 6T12GSB,which was consistent with the XRD results.Under the same conditions,the conductivity of TGSB series samples is better than that of TLSB series samples in the low temperature area,but the conductivity at high temperature is not as high as that of TLSB series samples.