Enhancement of Electrical Conductivity and Mechanical Properties of Polymer Nanocomposites used for Proton Exchange Membrane Fuel Cell Bipolar Plates

F. Mighri, N. Athmouni, S. Elkoun
Laval University,
Canada

Keywords: polymer nanocomposite, through-plane resitivity, PEMFC, bipolar plates

Summary:

Proton exchange membrane fuel cell (PEMFC) technology is considered as one of the most promising future technologies for industrial and transportation applications. Bipolar plates (BPPs), which comprise one of the major parts of PEMFC stack, are ones of the main components that got more research focus. Thus, the development of appropriate and low cost functional materials for BPPs becomes a key factor for PEMFCs commercialization. Multiwall Carbon Nanotubes (MWCNT) are considered as interesting fillers due to their unique combination of high aspect ratio, good mechanical properties, thermal conductivity, and also their excellent electrical conductivity. A small amount of MWCNT can significantly improve the electrical and mechanical properties of a polymeric matrix, however, due to their intrinsic strong Van der Waals interactions, they have a strong tendency to agglomerate. Thus, to improve their dispersion and their good interfacial interaction with polymers, several methods have been proposed, such as polymer grafting on their surface. Due to its low melt viscosity, polybutylene terephthalate (PBT), an engineering thermoplastic obtained by the polymerization reaction of cyclic butylene terephthalate oligomer (c-BT), has an excellent injection and compression molding processability. Thus, PBT can be easily used as a compatibilizer between the MWNCT and the polymeric matrix in order to improve MWCNT dispersion and to facilitate the nanocomposite processability in bipolar plates, which was the main objective of this study. We showed how the functionalization of MWCNT by nitric acid (HNO3) and their pre-dispersion into PBT improved the through-plane electrical conductivity and mechanical properties of co-continuous morphology polyvinylidene fluoride (PVDF)/poly (ethylene terephthalate)(PET)/carbon black (CB)/graphite (GR)/MWCNT nanocomposites. In the first processing step, when MWCNT were functionalized with HNO3 then pre-mixed with PBT by using a Brabender internal mixer, scanning electron microscopy (SEM) characterization showed no aggregations of MWCNT inside the PBT matrix due to their improved interfacial interactions and chemical compatibility. Differential scanning calorimetry (DSC) characterization revealed that HNO3-functionalized MWCNT increased the crystallization temperature of PBT and acted as nucleation sites inside the PBT matrix, leading to better MWCNT dispersion. In a second step, when PBT/(HNO3-functionalized MWCNT) mixture was added in small quantities to (PET/PVDF)/(CB/GR) composites by using twin-screw extrusion process, it decreased significantly their through-plane resistivity and enhanced their impact and flexural properties. Its synergistic effect also led to the best proton exchange membrane fuel cell (PEMFC) bipolar plate prototypes with enhanced mechanical properties and electrical through-plane conductivity, and also with smoother surface.