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

Abstract

The effective diffusivity of O 2 -N 2 in porous media was measured at high temperatures ͑650-800°C͒ using an electrochemical concentration cell. Porous membranes having total porosity between 29 and 48 vol% were fabricated from Sr-doped LaMnO 3 ͑LSM͒ with 20 to 30 wt% carbon added as a pore former. The O 2 -N 2 effective binary diffusivity, D O 2 -N 2 eff , at 800°C increased from ϳ0.016 to ϳ0.12 cm 2 /s with increasing open porosity between 15 and 44 vol%. The D O 2 -N 2 eff 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 O 2 K eff and D N 2 K eff , at 800°C were an order of magnitude higher than the effective binary diffusivity, D O 2 -N 2 eff . Thus, O 2 -N 2 transport through the porous membranes was governed by the effective binary diffusivity, D O 2 -N 2 eff . The effects of O 2 -N 2 effective binary diffusivity, D O 2 -N 2 eff, 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 O 2 -N 2 eff was investigated between 650 and 800°C, which was found to be in accord with the Chapman-Enskog model.The contribution of concentration polarization to the total polarization at both the anode and cathode of solid oxide fuel cells ͑SOFCs͒ can be substantial. 1,2 The effects of concentration polarization are most pronounced in electrodes of low porosity and at high current densities. In electrode-supported SOFCs, the contribution of ohmic polarization ͑IR drop͒ of a thin electrolyte to the total overpotential of the cell is small as compared to that of an electrolytesupported SOFC. In fact, anode-supported SOFC have been reported wherein the contribution of the electrolyte ohmic polarization ͑IR drop͒ is less than that of the combined polarization losses at the two electrodes. Thus, in the case of electrode-supported SOFCs, the maximum current density attainable is largely limited by polarization losses at the electrodes. In electrode-supported SOFCs, one of the two electrodes, either anode or cathode, is fabricated of a thickness large enough to function as a support and thereby lend mechanical integrity. However, this does not mean that concentration polarization of the support electrode is necessarily greater than that of the thin electrode. For example, anode-supported SOFCs are routinely fabricated with anodes consisting of NiO and YSZ composites that are subsequently reduced in situ resulting in highly porous Ni and YSZ cermets. Measurements conducted on anode-supported SOFCs show that overpotential at the thick anode can often be less than that at the thin cathode. Typically, cathodes are fabricated from an electrocatalyst such as Sr-doped LaMnO 3 or Sr-doped LaCoO 3 ....

Three-Dimensional Reconstruction of Porous LSCF Cathodes

“…This is within the range of 1.5-7.8 previously published for various porous cathodes with ϳ30% porosity. 18,19 The increasing tortuosity is caused by pore coarsening. [20][21][22][23] Coarsening causes higher standard deviation in tortuosity caused by the variance in the pore size distribution.…”

Flow and Transport in Tight and Shale Formations: A Review

“…Malek and Coppens [43] studied the effects of surface roughness on Knudsen regime diffusion in porous media. Knudsen diffusion is a result of collisions of gas molecules with the pore walls, rather than intramolecular collisions, so we always consider that Knudsen diffusion and molecular diffusion compete with one another by a "resistances in series" approach [44]. Welty et al [45] presented the different types of diffusion as shown in Figure 9.…”

Active Cathodes for Super-High Power Density Solid Oxide Fuel Cells Through Space Charge Effects

“…(2) Activation polarization -that associated with the occurrence of the overall electrochemical cathodic reaction of charge transfer. The former is relatively small, as long as the cathode is thin and has sufficient porosity [14]. The latter is the dominant one, and depends upon a number of microstructural and intrinsic -fundamental, parameters.…”

“…Using model developed earlier [22], equation (1) and values of various parameters from the literature [15,22,231, the electrode polarization resistance can be calculated. Figure 1 shows the calculated polarization resistance as a function of grain size and neck size at 800°C for an electrode using YSZ as the ionic conductor in the composite cathode, and the following parameters: pg (YSZ grain resistivity) = 14 Q.cm, p,, (resistivity of the space charge layer) = 2,250 n.cm, pgb (grain boundary core resistivity) = 10 Llcm, h (space charge layer thickness) = 2 nm, S,b (grain boundary core thickness) = 0.4 nm, and R,,= 2 Q.cm2.…”