Modeling Triple Phase Boundary (TPB) in Solid Oxide Fuel Cell (SOFC) Anode

Abstract

The idea of having the triple phase boundary (TPB) is extensively used in the fuel cell literature, especially with respect to solid oxide fuel cells (SOFC). The TPB concept indicates that the hydrogen oxidation reaction and the oxygen reduction reaction, which produce electric current, can actually occur at special sites, called "triple phase boundaries" where the gaseous fuel phase, ion conducting phase: electrolyte, and electron conducting phase, come into contact. Recent study shows that despite the common assumption about TPB, it is not just a point, but a zone that consists of two lines. The kinetic reaction often introduces a significant limitation to fuel cell performance. Therefore, understanding, characterizing, and optimizing the TPB content in fuel cells provides excellent opportunities for performance improvement. Studying the kinetics of the reaction that takes place at the triple phase boundary is one aspect of this paper. It includes the study of all kinds of chemical and electrochemical reactions as well as their reaction rates, the surface species, and the electrochemical parameters, such as reaction rate constants and conductivity. A mathematical model is developed to describe a simplified anodic solid oxide fuel cell (SOFC) system, Ni/ H2--H 2O/YSZ, and its reaction occurring in the vicinity of the triple phase boundary (TPB). The model incorporates coupled diffusion, migration and reaction phenomena of the chemical components in the gas phase, Ni particle and zirconia solid state. The kinetic constants necessary for the simulations are estimated on the basis of literature data.