Preparation And High Temperature Electrochemical Properties Of Cu 2 4 Nanocomposite Fibers

Mixed ionic- electronic conducting（MIECs） materials play important roles in developing the intermediate temperature solid oxide fuel cells（IT-SOFCs）. A2BO4-type MIECs cathode materials attracted great attention because of their high conductivity,promising oxygen surface exchange coefficient（K *） and oxygen diffusion coefficient（D *） and good thermal match properties with the electrolyte. For the material Ln2CuO4 with the element Cu in B-site（except La2CuO4）, the structure can be considered as the overlap of the fluorite structure Ln-O layer with Cu-O plane along the c axis. The oxygen ions have high conductivity along the fluorite structure Ln-O layer layer.Therefore, the material can achieve improved oxygen ion-electronic mixed conductivity with appropriate dopants.In this thesis, Pr₂CuO₄ nanofibers were synthesized by electrospinning technology,and the thermo-decomposition procedures, phases and microstructure morphology evolutions were followed by TG, XRD, FT-IR and SEM, respectively. The results of SEM showed that the oxide nanofiber is made up by nano particles of 400-500 nm in diameter, the thickness of the cathode is 24 μm with a porosity of 71.3%. The nanofiber cathode exhibited a low polarization resistance of 0.38 Ω cm2 at 700 oC in air, which was 32% smaller than the powder cathode with the same composition. The polarization results showed that the current density of fibers cathode was 105 mA cm-2 under the overpotential of 33 mV. Combined with the test results of oxygen partial pressure and AC impedance, it is found the charge transfer process was the rate limiting step for the oxygen reduction reaction（ORR） on the nanofiber cathode. The enhancement of the cathode performance is probably due to the fine microstructure of the PCO nanofiber cathode, but not to the change of the oxygen reduction mechanism.In order to further improve the electrochemical properties of Pr₂CuO₄ nanofiberscathode, Pr₂CuO₄-Ag（PCO-Ag） and Pr₂CuO₄-CGO（PCO-CGO） nanofibers were synthesized by electrospinning technology. The results showed that the nanofiber cathode with 5 wt. % Ag exhibited a low polarization resistance（Rp） of 0.22 Ω cm2 at700 oC in air, about 42% reducing compared with PCO nanofiber cathode. This result was due to Ag composite improved the electronic conductivity of the cathode, and accelerated the charge transport process. Similarly, the Rp value of PCO-10%CGO cathode reached 0.14Ω cm2 at 700 oC, about 60% less than the Rp of PCO. This result proved that the electrochemical properties of PCO-CGO nanofiber cathode were effectively improved.The Pr_2-xCuO₄（x =0.0-0.4） powders were prepared by glycinenitrate process（GNP）, and the effects of Pr-deficiency on the electrochemical properties were studied.Pr-deficiency was found to increase the concentration of oxygen vacancy in the material,and the electrochemical properties could be improved. The Rp of Pr1.8CuO4 powders cathode was 0.25 Ω cm2 at 700 oC, reduced about 55% compared to the stoichiometric PCO powder cathode. The Pr_2-xCuO₄（x = 0-0.4） nanofibers were further synthesized by electrospinning method, and the Rp value was 0.19 Ω cm2 at 700 oC. This value was only 50% that of PCO nanofiber cathode. The ORR rate limiting step was charge transfer process for both the powder and the fiber cathodes. Thus, it was proved that the modification of the cathode microstructure can improve the cathode performance, but have no effect on the oxygen reduction reaction mechanism.