Reduced Surface Area for the Oxygen Reduction Reaction in Porous Electrode via Electrical Conductivity Relaxation

Oxygen reduction reaction (ORR) performance of porous electrodes is critical for solid oxide fuel cells (SOFCs). However, the effects of gas diffusion on the ORR in porous media need further investigation, although some issues, such as nonthermal surface oxygen exchange, have been attributed to gas diffusion. Herein, La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) with various porosity, pore radii, and gas permeability were investigated via the electrical conductivity relaxation method and analysed via the distributed of characteristic time (DCT) model. The ORR is revealed with three characteristic times, which are gas diffusion, oxygen exchange via the surface corresponding to small pores, and oxygen exchange to large pores. Gas diffusion delays the oxygen surface exchange reaction, resulting in a very low chemical oxygen surface exchange coefficient compared with that obtained with dense samples under the assumption that all the surfaces are active for the ORR. Reduced surface area is thus defined to quantitatively represent the gas diffusion effects. The reduced surface area increases with increasing gas permeability, demonstrating the importance of electrode engineering for fast gas transport. Moreover, reduced surface area is suggested for replacing the specific surface area to calculate the electrode polarization impedance via the ALS model.