Preparation And Characterization Of Pd-YSZ Composite Cathode Of Solid Oxide Fuel Cells

IT-SOFCs avoid the problems such as the limitation of the materials, the preparation and the stability of the cells, which always disturb traditional SOFCs. However, the electrochemical performance drops due to the activation loss of cathode at low working temperature. So the cathode is a limiting component for a further step forward. In order to improve the electrochemical performance of IT-SOFCs, a new cathode Pd-YSZ composite cathode will be investigated in this paper.YSZ paste is printed on the electrolyte disk by screen printing at the first step, then Pd solution was impregnated into YSZ coating then sintered at proper temperature, and Pd-YSZ composite cathode was gained. SEM pictures indicate that the metal phase, the electrolyte phase and the gas phase in the composite cathode are all like networks, and those networks intersect to form large and consecutive boundary areas which we called three-phase boundary(TPB) areas. In the past, the three-phase boundary areas in pure noble metal electrode were restricted at the interface of metal and electrolyte, but Pd-YSZ composite cathode extends the TPB into the bulk of the cathode. The extended and consecutive TPB will improve cathode properties, such as the high efficiency of the charge transfer reaction due to the rapid surface exchange kinetics, high oxygen vacancy concentration. What's more, composite make the electrode and electrolyte connect each other more tightly, and the YSZ will prevent fine Pd grains to grow up.The electrochemical performance of the composite electrode is evaluated by AC impedance. The AC impedance spectroscopy indicates that the electrochemical performance of Pd-YSZ composite cathode is good due to larger TPB areas and Pd's high activity towards oxygen reduction. The Re of the composite cathode which impregnated Pd 8 times and dispersed by an ultrasound then sintered at 750℃is only 0.201Ωcm2 at the work temperature of 750℃, and the Re will drop to 0.112Ωcm2 at 800 ℃. The most important and interesting result is that the activation energy for the oxygen reduction on Pd-YSZ cathode is 138.94 kJ/mol, which is significantly lower than 160-230 kJ/mol for the reaction on conventional oxide cathodes such as LSM and LSCF.