Modeling And Parametric Study Of Electrical Contact Resistance In PEM Fuel Cells

A fuel cell is an electrochemical device that converts the chemical energy of a fuel and an oxidant into electricity. Interest has been growing in proton exchange membrane (PEM) fuel cells due to their unique characteristics of low operation temperature, low emission and quick startup. However, various irreversible losses existing in an operating PEM fuel cell affect its performance and reduce its efficiency. Ohmic loss is one of the main losses in normal fuel cell operation. The contribution from the electrical contact resistance to the ohmic loss has been reported to be approximately equal to that from the proton conduction resistance in the membrane.This paper investigates the electrical contact resistance at the interface between the bipolar plate (BPP) and the gas diffusion layer (GDL), which dominates the total contact resistance in a PEM fuel cell. The contact between a graphite BPP and a carbon paper GDL microscopically occurs between a BPP asperity and a carbon fiber in the GDL. Mathematical models are developed for the microscale contact force and contact resistance. The pressure at the BPP/GDL interface is the sum of the contact forces at all micro-contact spots divided by the nominal contact area. Each contact spot contributes a certain resistance and the overall contact resistance at the interface is modeled as all the resistances in parallel. The following contributions have been made by this research:(1) Numerical models are developed for the microscale contact force and contact resistance based on finite element analysis (FEA) of the microscale contact response and Holm's formula for the contact resistance. The design of experiment (DOE) technique and multiple regression analysis are employed to facilitate the calculation of the overall contact force and contact resistance at the interface. Results show that a more reasonable estimate of the contact resistance versus pressure relationship is achieved by including the material anisotropy and the bending behavior of the carbon fibers in the GDL;(2) Analytical models are developed for the overall contact force and contact resistance at the BPP/GDL interface. An analytical model for the micro-contact responses is proposed using beam bending theory and Hertz contact theory. The overall contact resistance versus pressure relationship is then obtained by probability theory. The model result compares favorably with the existing experimental data;(3) A parametric study is carried out to evaluate factors that affect the relationship between the contact resistance and the pressure at the BPP/GDL interface. Factors investigated include the material properties and surface parameters of the BPP and the carbon fibers in the GDL. Practical advices are given for the manufacturing of the two contact bodies to achieve a lower contact resistance and therefore improve the performance of PEM fuel cells.The analytical models for the contact responses can be extended for contacts between metal BPPs and carbon paper GDLs. This research also provides useful insights for further studies on contacts between BPPs and carbon cloth GDLs.