Comprehensive Analysis of Critical Factors Determining Limiting Current of PEMFC: O 2 and H + Transport Resistance without Cathode Humidification

Abstract

The limiting current in proton exchange membrane fuel cells is widely considered to be determined by the oxygen diffusion resistance, despite the fact that the oxygen reduction reaction occurs by the combination of three constituents: the electron, proton, and oxygen molecule. This study inclusively analyzes how both proton conduction and oxygen diffusion influence the limiting current. To examine the effect of the oxygen transport constraint on the limiting current, the oxygen transport resistance from the gas flow channel to the active sites in the catalyst layer is divided into molecular and non-molecular diffusion resistance. This is achieved by a mathematical treatment of the limiting current measured at various diluted oxygen feeds without humidification. The proton transport resistance of the membrane and the catalyst layer at various relative humidity is quantitatively obtained from a complex capacitance analysis using electrochemical impedance spectroscopy. The results indicate that proton conduction exhibiting a low water content in the membrane and the ionomer can control the utilization of the catalyst layer and the oxygen diffusion length, thus affecting the limiting current.