Effect of Characteristic Lengths of Electron, Ion and Gas Diffusion on Electrode Performance and Electrochemical Reaction Area

Abstract

A precise evaluation of the active reaction area in the electrodes is important to design an effective solid oxide fuel cell (SOFC). A scale analysis and 1-D numerical simulations are conducted to obtain a better understanding of the electrochemical reaction area in SOFC anode. In the scale analysis, the characteristic lengths of the electron, oxide ion and gas diffusions are evaluated from their conservation equations. Relative comparisons of the characteristic lengths show that the transport phenomena in the SOFC anode are primarily governed by the oxide ion conduction under standard operating conditions. The gas diffusion may affect the extent and the location of the active reaction area at high temperature and⁄or low reaction gas concentration conditions. 1-D numerical simulations for an anode provided detailed information such as the electron⁄ion potential distributions, the gas concentration distribution and local reaction rate. It is found that the electrochemical reaction actively occurs at the vicinity of the anode-electrolyte interface. The reaction thickness increases as the characteristic length of the ion conduction is increased resulting in a better power generation performance. The reaction thickness is also increased when the gas diffusion length is short. The cell performance is, however, lowered in this case because the low gas diffusivity yields the increase of the ohmic loss of ion conduction as well as the concentration overpotential.