Evaluation of Ln2CuO4 (Ln: La, Pr, Nd) oxides as cathode materials for IT-SOFCs

Abstract In this work an evaluation of Ln2CuO4 (Ln: La, Pr, Nd) cuprates is presented in terms of their possible application as cathode materials in Solid Oxide Fuel Cells working in an intermediate temperature range (600–800 °C). The materials were synthesized by a soft chemistry EDTA-based method. Structural and microstructural characterization, measurements of transport properties and oxygen nonstoichiometry, determination of chemical stability in relation to Ce0.8Gd0.2O1.9 electrolyte, as well as mass relaxation studies for determination of oxygen diffusion coefficient D and surface exchange coefficient K in Pr2CuO4 and measurements of electrochemical properties of electrolyte-supported cells with Pr2CuO4 cathodes were performed. Pr2CuO4 and Nd2CuO4 oxides were found to possess tetragonal I4/mmm structure, in case of La2CuO4, orthorhombic Bmab one was observed. High temperature XRD data revealed anisotropic thermal expansion of Pr2CuO4, smaller along c-axis. Based on reduction experiments performed on TG apparatus, the initial slight oxygen excess δ present in the compounds at room temperature was evaluated: La2CuO4.03(2), Pr2CuO4.05(2) and Nd2CuO4.04(2). Additionally, it was observed that up to 900 °C in air the oxygen content in the materials does not change considerably. Pr2CuO4 exhibits activated character of the electrical conductivity as a function of temperature, with values relatively high for cuprates, exceeding 100 S cm−1 in 500–800 °C range, as well as negative values of Seebeck coefficient from room temperature (RT) up to 800 °C. Electrical conductivities of La2CuO4 and Nd2CuO4 are much lower in the temperature range of interest. Using mass relaxation technique it was possible to simultaneously evaluate D and K in 600–900 °C temperature range. Activation energy of D was calculated to be 0.13(2) eV. Results of chemical stability studies in relation to Ce0.8Gd0.2O1.9 electrolyte revealed moderately good chemical compatibility of Pr2CuO4 oxide, while La2CuO4 and Nd2CuO4 stability proved to be insufficient. Using custom-made electrolyte-supported SOFC cells with Pr2CuO4-based cathode, it was possible to obtain power densities exceeding 0.1 W cm−2 at 800 °C and 0.25 W cm−2 at 900 °C.