Abstract
The objectives of this research are the preparation and characterization of multiwalled carbon nanotubes (MWCNTs) and functionalized MWCNTs/polymer nanocomposite bipolar plates for use in proton exchange membrane fuel cells (PEMFCs). There are four parts in this dissertation. The first part of this dissertation investigates the novel functionalized multi-walled carbon nanotubes (MWCNTs) which are used as cross-links between MWCNTs-vinyl ester interfaces to achieve homogeneous dispersion and strong interfacial bonding for developing fully integrated MWCNTs-vinyl ester nanocomposite bipolar plates. POAMA (i.e. poly(oxyalkylene)-amines (POA) bearing maleic anhydride (MA)) are grafted onto the MWCNTs by amidization reaction, forming MWCNTs-POAMA. In the MWCNTs-POAMA/vinyl ester nanocomposites, MWCNT-POAMAs react with vinyl ester and become part of the cross-linked structure, rather than just a separate component. It was found that MWCNTs-POAMA exhibited better dispersion in the vinyl ester matrix than those of pristine MWCNTs. Moreover, results demonstrate that the mechanical and electrical properties of the vinyl ester nanocomposite bipolar plate are improved dramatically. The ultimate flexural strength and bulk electrical conductivity of the MWCNTs-POAMA/vinyl ester nanocomposite bipolar plate are increased from 28.54 MPa to 41.44 MPa and 156 S cm-1 to 643 S cm-1, exhibiting 45 and 315 % improvement, respectively. The gas tightness of all of composite bipolar plates in this study was no leak. In addition, the maximum current and power densities of the single fuel cell test using the MWCNTs-POAMA/vinyl ester nanocomposite bipolar plates were enhanced from 1.03 to 1.23 A cm-2 and from 0.366 to 0.518 W cm-2, respectively, which suggested that a higher electron transfer ability for PEMFC applications can be achieved. The second part of this dissertation investigates a novel one-step preparation of functionalized multi-walled carbon nanotubes (MWCNTs) by free-radical modification. MA-POA, i.e. Maleic anhydride (MA) grafting molecular weight 400 and 2,000 poly (oxyalkylene)-amines (POA400 and POA2000), was attached onto the MWCNTs, forming MWCNTs/MA-POA400 and MWCNTs/MA-POA2000. The functionalized MWCNTs, especially MWCNTs/MA-POA2000, exhibited higher solubility than the pristine MWCNTs in organic solvents and showed well dispersion in the vinyl ester matrix. Furthermore, this study also investigated the mechanical, electrical and single fuel cell properties of functionalized MWCNT nanocomposite bipolar plates for use in polymer electrolyte membrane fuel cells. The flexural strength of the nanocomposite bipolar plates containing 2 wt% MWCNTs/MA-POA2000 was increased from 28 MPa to 48.33 MPa, exhibiting 73% improvement. In addition, the bulk electrical conductivity of the nanocomposite bipolar plates was 780 % (from 156 to 1340 S cm-1) higher than those of the original composite bipolar plates by adding only a small quantity (1 wt%) of MWCNTs/MA-POA2000. The maximum current density and power density of the single cell tests of the nanocomposite bipolar plate with 1 wt% MWCNTs/MA-POA2000 were enhanced from 1.03 to 1.32 A cm-2 and from 0.392 to 0.587 W cm-2, respectively. The overall performance confirms the MWCNTs/MA-POA2000 nanocomposite bipolar plates prepared in this study are suitable for PEMFC application. The third part of this dissertation studies the fabrication of lightweight and high performance nanocomposite bipolar plates for the application in proton exchange membrane fuel cells (PEMFCs). Three types of polypropylene (PP) with different crystallinities including high crystallinity PP (HC-PP)、medium crystallinity PP (MC-PP) and low crystallinity PP (LC-PP) were prepared. The optimum composition of original composite bipolar plates was determined with 80 wt% graphite content and 20 wt% PP content based on the measurements of electrical and mechanical properties with various graphite contents. Results indicated that MWCNTs was dispersed better in LC-PP than other PP owing to enough dispersed regions in nanocomposite bipolar plates. The good MWCNT dispersion of LC-PP causes better bulk electrical conductivity and mechanical properties of MWCNTs/PP nanocomposite bipolar plates. In the MWCNTs/LC-PP system, the bulk electrical conductivity of 4 wt% MWCNTs/LC-PP nanocomposite bipolar plates increases from 160 S cm-1 (0 wt%) to 548 S cm-1, exhibiting 242 % improvement. The flexural strength of the MWCNTs/LC-PP nanocomposite bipolar plate with 8 wt% of MWCNTs was 29.46 MPa, approximately 37 % higher than that of the original nanocomposite bipolar plate. The single cell performance of MWCNTs/LC-PP nanocomposite bipolar plate was also evaluated. The maximum current density and power density of the single cell tests of the nanocomposite bipolar plate with 4 wt% MWCNTs/LC-PP were 1.19 Acm-2 and 0.533 Wcm-2, respectively. The four part of this dissertation investigates the preparation of aminated mullti-walled carbon nanotubes (MWCNTs-NH2) attached with polypropylene grafted maleic anhydride (PP-g-MA compatibilizer) by ring-opening reaction. The prepared MWCNTs/PP-g-MA was introduced to the PP/composite bipolar plates, to achieve a high compatibility and good adhesion between carbon nanotubes and PP matrix via PP-g-MA chains. Replacement of amine-terminated groups by PP-g-MA in the MWCNTs leads to the grafting of long copolymer chains to the MWCNTs, and improves the dispersion of MWCNTs. Due to the strong reinforcing properties of MWCNTs, the electrical and mechanical properties of the PP composite bipolar plates were enhanced. The effect of incorporating MWCNTs/PP-g-MA on the morphology, electrical and mechanical properties of the PP-based composite bipolar plates was studied. The resulting PP composite bipolar plates with 1, 2, and 4 wt% of MWCNTs/PP-g-MA demonstrate the flexural strength and the bulk electrical conductivity were improved by 56.3, 68.5, and 70.9 % and by 282, 425, and 473 %, respectively. Comparing with the maximum power density of the graphite bipolar plates (0.614 W cm-2), the MWCNTs/PP-g-MA PP nanocomposite bipolar plates (0.586 W cm-2) are suitable for bipolar plates of PEMFCs.