Investigation on the Impact of Carbon Porosity in the Microporous Layer of PEM Fuel Cells

Abstract

In recent years, the PEMFC has seen tremendous progress in terms of reliability, durability and performance. The cathode performance in particular, greatly influences the overall performance. Issues such as water management remains a huge issue that must be addressed particularly in cases under high current density operating conditions as the occurrence of flooding is prevalent. The microporous layer (MPL) is pivotal in improving water management in PEMFCs. The layer which generally consists of carbon particles and a hydrophobic agent, functions as an intermediate layer between the catalyst layer and the more porous substrate layer. In MPL research, different types of carbon particles (BLACKPEARLS, XC-72R, acetylene black, graphite, etc.) have been compared and key relationships between the carbon porosity and overall PEMFC performance have been reported. However, it is difficult to delineate the impact of porosity from parameters such as particle size and agglomeration packing which are intrinsic to each type of carbon. This study focuses on the application of porous carbons that have been systematically modified in terms of porosity in the MPL to establish a clearer comparison. The first approach involves the modification of the porosity of carbon black to different degrees through physical activation. Carbon black particles were activated under CO2 ­for different durations to induce different levels of porosity. In the second approach, colloid-imprinted carbons (CIC) were synthesized with various pore sizes and used to study the impact of pore size in carbons on overall MPL performance in PEMFCs. Both approaches utilize the same type of carbon material but with varying porosities. The wettability of the carbon materials will also be studied as it also impacts the interaction between water and the carbon surface. To fabricate the MPL, the carbon was made into a slurry before being applied on a carbon fiber paper followed by sintering. The newly fabricated gas diffusion layer (GDL) was incorporated in a membrane electrode assembly (MEA) before being tested in a single cell PEMFC. The Pt loadings on the anode and cathode were 0.04 mgPt/cm2 and 0.4 mgPt/cm2 respectively. Polarization tests were used to study the electrochemical properties of the GDL during operation. Bulk powder samples were also characterized using N2 sorption, x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The wettability and through-plane resistance of the fabricated GDLs were also examined. This study attempts to elucidate the impact of the porosity of carbons in the MPL on the mass transport losses in PEMFC. By establishing a further understanding on the effect of carbon porosity, improved microporous layer designs through material selection and design can be achieved leading to greater PEMFC operational robustness. Synthesis and characterization results of the porous carbon samples and the fabricated GDL along with their electrochemical results will be presented.