Thermal Stability and Mixed Conductivity in Ln_2 Ni_(1-x)Cu_xO_(4+δ) (Ln = La, Pr, Nd) for SOFC Cathodes

摘要

With the aim of reducing the operating temperature of SOFC devices in an intermediate range (600 < t(°C) < 800), we assume that a new breakthrough will result from finding materials having new structures. A series of overstoichiometric A_2BO_(4+δ) oxides that can be expected to be used as future cathodes have been prepared ; they are characterized by a mixed conductivity (both electronic and ionic (O~(2-))), which is required for this purpose. The additional oxygen atoms located in the rock - salt (A_2O_2) layers of the structure, are expected to be mobile on the basis of previous electrochemical experiments performed at room temperature. Moreover, the oxygen excess amount can be modulated by choosing the appropriate rare earth (A) and d - metal cations (B). Nickelates with various rare- earth in the A site (Ln = La, Pr, Nd) and La_2Cu_(0.5)Ni_(0.5)O_(4+δ) phases were prepared using the nitrate - citrate route. Then they were characterized using various techniques in the temperature range 20 < t(°C) < 1000 : XRD, TGA, dilatometry measurements, total conductivity measured by the classical four - probe method. Then the only ionic conductivity was obtained following two ways: the first one is the measurement of the oxygen diffusion coefficient by the SIMS technique using ~(18)O as tracer. This study shows that, in this temperature range, the oxygen diffusion coefficients of these oxides are about ten times higher than those of the best perovskite materials previously reported. The surface exchange coefficients are also significantly enhanced (about two to five times). In order to evaluate the electrochemical performances of these materials used as porous cathodes, a.c. impedance measurements have been performed on a symmetrical cell (cathode/YSZ/cathode). The response was measured under controlled temperature (in the range 600 < T(K) < 900) and oxygen pressure Po_2 (from 1 to 10~(-4) bar). All the results show that these materials seem to be promising for using as SOFC cathodes.