Carbon Nano-Tube Based Nano-Porous Layer in Membrane-Electrode Assemblies for Polymer Electrolyte Membrane Fuel Cells

Abstract

In the membrane-electrode assemblies(MEAs) for polymer electrolyte membrane fuel cells(PEMFCs), there is a micro-porous layer(MPL) which is placed between gas-diffusion layer(GDL) and catalyst-coated layer(CCL). It roles as a buffer layer between GDL and CCL, where it helps removing the byproduct (Water) and connects electrically to conduct the generated electrons outside of the electrochemical reaction sites. However, commercial MPLs are normally composed of nano-particulated bulk carbon, which are hot-pressed in the manufacturing process. Therefore it is normally brittle and their thickness is micro-scale. This feature sometimes causes the structural breakdown by the byproduct or cracked by repeated pressurization of hydrogen or air. In addition, the micro-thick layer is not proper to remove the byproduct because such thick structure would rather disturb removing the byproduct water from the inside of the CCL.This study therefore developed the alternative of the conventional MPL. It is called “carbon nano-tube sheet” which is composed of well-networked multi-walled carbon nano-tubes. Thus it shows higher electrical conductivity, mechanically stronger, and thinner than conventional MPLs. It was employed as a new MPL and the thickness effect on the electrochemical performance was explored. As a result, the MPL thickness of 0, 15, 30, 100 μm was investigated. The electrochemical polarization characteristics showed that the 15 μm thick MPL has the highest performance, meaning it is the optimized thickness. 0 thick (No MPL) and 100 μm thick MPL showed lower electrochemical performance than that of 15 μm thick MPL. For no MPL case, it is attributed that the contact between CCL and GDL was poor because of no buffer layer, while the case of thicker MPL is attributed to the disturbance of the water removal and poor diffusivity of the reactant into the CCL. Their corresponding electrochemical impedance spectra were also investigated and the results also showed the same effects (Electrical connection and removal of water).