Effect of Characteristic Lengths of Electron, Ion, and Gas Diffusion on Electrode Performance and Electrochemical Reaction Area in a Solid Oxide Fuel Cell

摘要

A precise evaluation of the active reaction zone in the electrodes is important to design an effective solid oxide fuel cell (SOFC). A scale analysis and one-dimensional numerical simulations are conducted to obtain a better understanding of the electrochemical reaction zone in a SOFC anode. In the scale analysis, the characteristic lengths of the electron, oxide ion, and gas transports 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 zone at high temperature and/or low reaction gas concentration conditions. The one-dimensional numerical simulations for an anode provided detailed information such as the electric potential of electron- and ion-conducting phases, the gas concentration, and local charge-transfer current distributions. It is found that the electrochemical reaction actively occurs in the vicinity of the anode-electrolyte interface. The effective thickness increases as the characteristic length of the ion conduction is increased resulting in better power generation performance. The effective 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.