Influence of configuration and microstructure on performance of La₂NiO_4+δ intermediate-temperature solid oxide fuel cells cathodes

The influence of sintering temperature on the polarization resistance of screen printed La₂NiO_4+δ (LNO) cathodes is studied. The electrode dispersion is measured on symmetrical cells with a 100 mm 3% yttriadoped zirconia (TZ3Y) electrolyte and screen-printed yttria-doped ceria (YDC) barrier layers. The asreceived commercial LNO powder was used in the formulation of the ink. For cathodes prepared following this procedure the optimum sintering temperature is 1000 C. Analysis of the impedance spectra shows clearly a Gerischer dispersion (chemical impedance), with an activation energy of 124 kJ mol^-1 for the exchange rate parameter KG. The addition of a dense LNO layer between electrolyte and porous electrode lowers the ASR by ∼35%, showing a significant change in the oxygen transfer mechanism. A tentative model, based on a global two step oxygen exchange mechanism, is presented. There are also strong indications that surface diffusion of mono atomic oxygen is a major transport path. The electronic conductivity of LNO is too low at the intended operating temperature of ∼600 C, resulting in an additional resistance to the apparent electrolyte resistance.