Measurement of O 2 ­ N 2 Effective Diffusivity in Porous Media at High Temperatures Using an Electrochemical Cell

The effective diffusivity of O 2 -N 2 in porous media was measured at high temperatures (650-800°C) using an electrochemical concentration cell. Porous membranes having total porosity between 29 and 48 vol% were fabricated from Sr-doped LaMnO 3 (LSM) with 20 to 30 wt% carbon added as a pore former. The O 2 -N 2 effective binary diffusivity, D eff O2-N2 , at 800°C increased from ∼0.016 to ∼0.12 cm 2 /s with increasing open porosity between 15 and 44 vol%. The D eff O 2 -N 2 exhibited a nonlinear dependence on open porosity and increased dramatically for samples with greater than 35 vol% open porosity. The estimated effective Knudsen diffusivities of O 2 and N 2 , D eff O2K and D eff N2K , at 800°C were an order of magnitude higher than the effective binary diffusivity, D eff O2-N2 . Thus O 2 -N 2 transport through the porous membranes was governed by the effective binary diffusivity, D eff O2-N2 . The effects of O 2 -N 2 effective binary diffusivity, D eff O2-N2 , on concentration polarization of cathodes for solid oxide fuel cells were assessed. The nonlinear behavior of the O 2 -N 2 effective diffusivity as a function of open porosity indicates that a critical amount of porosity in the cathode is necessary to ensure that the overpotential due to concentration polarization is small. The temperature dependence of D eff O2-N2 was investigated between 650 and 800°C, which was found to be in accord with the Chapman-Enskog model.