Determination of effective binary diffusivity in anode of solid oxide fuel cell based on Fick’s law

Abstract

It is important for the improvement of the performance and durability of solid oxide fuel cells (SOFCs) to clarify gas transfer phenomena in their porous electrodes. This study proposes two methods, an oxygen sensor method and an electrochemical impedance method, for the in-situ measurement of the effective binary diffusivity of an H2 – H2O system in the anode of an anode-supported SOFC, which is composed of a YSZ (Y2O3 stabilized ZrO2) electrolyte and an Ni-YSZ anode. By these methods, the effective binary diffusivity is determined based on Fick’s law by using hydrogen gas concentrations at two positions, the inside and surface of the anode. When the oxygen sensor method is used, a local hydrogen gas concentration in the anode is determined with a sensor made of a platinum wire coated with YSZ. On the other hand, when the electrochemical impedance method is used, a mean hydrogen gas concentration in the anode adjoining the electrolyte is determined by analyzing the measured electrochemical impedance spectra. The effective binary diffusivities determined by these methods are almost the same under the same conditions. And also they are in agreement with effective binary diffusivities calculated using the porosity and volume-averaged pore diameter obtained by analyzing the anode cross-section in an SEM image. Therefore, it is judged that both of these proposed methods enable the in-situ measurement of the hydrogen gas diffusivities in the anode when the SOFC is in operation. Furthermore, it is shown that nitrogen gas, including fuel gas, may work to inhibit the transfer of hydrogen gas.

Keywords

• solid oxide fuel cell
• anode
• effective binary diffusivity
• fick’s law
• hydrogen gas concentration
• oxygen sensor
• electrochemical impedance
• concentration overvoltage