Rheological Study of Micro Porous Layer and Its Effect on Transport Properties in a PEMFC

Abstract

Technical challenges related to catalyst flooding in a polymer exchange membrane fuel cell (PEMFC) have led to various studies to improve water management through evolutionary gas diffusion layer (GDL) design[1] [2].It has been well established that a GDL consisting of a macro-porous substrate coated with a hydrophobic microporous layer (MPL) has an indispensable role in gas and water transport [3]. However, a fundamental understanding of the MPL role and its optimal composition and structure are still lacking. Currently, the most widely used MPL in a PEMFC is made from ink dispersion that consists of carbon, PTFE, and solvent. Before MPL optimization is possible, the desired properties of the MPL ink dispersion and its interaction with the macro-porous substrate needs to be studied. In this study, we used rheological measurements as an evaluation tool to study a baseline MPL ink with Acetylene Black (AB) and PTFE. Specifically, we performed intensive rheological experiments to study the following three properties of the MPL ink: 1) flow properties, like viscosity, through continuous ramp rate, 2) viscoelastic properties through cyclical sweeping method, and 3) transient properties through recovery and relaxation measurement. Our preliminary results show that the ink stability, size of the primary agglomerate, viscosity, viscoelastic property, and creep recovery are strongly influenced by carbon to PTFE ratio and the weight percentage of the solid content. Built on the foundation of our previous work, we expand our study to include two types of carbon materials, AB and Carbon Nano Tube, three PTFE/Carbon ratios (20%, 30%, and 40%), and two solvent ratios (IPA rich and water rich) in this study. Standard Toray-H-060 substrate with 20% PTFE is used as the baseline macro-porous substrate and the MPL ink is applied directly onto the substrate by rod coating. The GDL is then dried and sintered accordingly. Both ex-situ evaluations, such as surface roughness, hydrophobicity, coating quality, and MPL penetration, and in-situ fuel cell testing are performed to study the effect of MPL on transport and performance under both wet and dry conditions. Our results provide new insights to enhance transport properties in a PEMFC by optimizing the design of MPL ink. Detail experimental results will be presented at the conference.