Effect of current density on poisoning rate of Co-containing fuel cell cathodes by chromium

Variation of electrochemical performance of a La_0.58Sr _0.4Co_0.2Fe_0.8O₃ (LSCF) cathode due to chromium gas-phase deposition has been studied at 800°C. The highest degradation rate is observed under open circuit conditions and is related to formation of a SrCrO₄ layer on the surface of an LSCF cathode. This results in an increase in both polarization and ohmic electrode resistance. The degradation rate is 3.7 and 0.5 Ohm cm²/1000 h, accordingly. When external polarization is applied, the degradation rate decreases considerably. It is found that the amount of chromium in the cathode bulk changes nonlinearly at an increase in current density. At 0.2 A/cm², the overall amount of chromium is twice as large as under open circuit conditions, but the degradation rate is three times lower. Herewith, a considerable amount of chromium was found both on the cathode surface and in its bulk. The results of model experiments show that exposure to external electric current leads to migration of chromium cations in the bulk of the porous cathode. It is shown that the growth of a SrCrO₄ layer on the surface of an LSCF cathode and penetration of chromium into the cathode bulk becomes slower at the current density of 0.5 A/cm². Under similar conditions (temperature, current density, and time), the amount of chromium in an LSCF cathode is 2.5-7.3 times larger as compared to that in La_0.65Sr_0.3MnO₃. However, the rate of degradation of electrochemical performance of an LSCF cathode is lower, which points to its higher tolerance towards the presence of chromium. This is due to higher oxygenionic conductivity of an LSCF cathode.