Preparation and properties of carbon nanotube/polypropylene nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

This study aims at the fabrication of lightweight and high performance nanocomposite bipolar plates for the application in polymer electrode membrane fuel cells (PEMFCs). The thin nanocomposite bipolar plates (the thickness <1.2 mm) consisting of multiwalled carbon nanotubes (MWCNTs), graphite powder and PP were fabricated by means of compression molding. Three types of polypropylene (PP) with different crystallinities including high crystallinity PP (HC-PP), medium crystallinity PP (MC-PP), low crystallinity PP (LC-PP) were prepared to investigate the influence of crystallinity on the dispersion of MWCNTs in PP matrix. The optimum composition of original composite bipolar plates was determined at 80 wt.% graphite content and 20 wt.% PP content based on the measurements of electrical and mechanical properties with various graphite contents. Results also indicate that MWCNTs was dispersed better in LC-PP than other PP owing to enough dispersed regions in nanocomposite bipolar plates. This good MWCNT dispersion of LC-PP would cause better bulk electrical conductivity, mechanical properties and thermal stability of MWCNTs/PP nanocomposite bipolar plates. In the MWCNTs/LC-PP system, the bulk electrical conductivities with various MWCNT contents all exceed 100 S cm⁻¹. The flexural strength of the MWCNTs/LC-PP nanocomposite bipolar plate with 8 phr of MWCNTs was approximately 37% higher than that of the original nanocomposite bipolar plate and the unnotched Izod impact strength of MWCNTs/LC-PP nanocomposite bipolar plates was also increased from 68.32 J m⁻¹ (0 phr) to 81.40 J m⁻¹ (8 phr), increasing 19%. In addition, the coefficient of thermal expansion of MWCNTs/LC-PP nanocomposite bipolar plate was decreased from 32.91 μm m⁻¹ °C⁻¹ (0 phr) to 25.79 μm m⁻¹ °C⁻¹ (8 phr) with the increasing of MWCNT content. The polarization curve of MWCNTs/LC-PP nanocomposite bipolar plate compared with graphite bipolar plate was also evaluated. These results confirm that the addition of MWCNTs in LC-PP leads to a significant improvement on the cell performance of the nanocomposite bipolar plate.     

Preparation and properties of functionalized multiwalled carbon nanotubes/polypropylene nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

Multiwalled carbon nanotubes (MWCNTs) are covalently modified with different molecular weights 400 and 2000 poly(oxyalkylene)-amine bearing the diglycidyl ether of bisphenol A (DGEBA) epoxy (POA400-DGEBA and POA2000-DGEBA) oligomers. The oxidized MWCNTs (MWCNTs-COOH) are converted to the acid chloride-functionalized MWCNTs, followed by the reaction with POA-DGEBAs to prepare the MWCNTs/POA400-DGEBA and MWCNTs/POA2000-DGEBA. FTIR, thermogravimetric analysis (TGA) and high resolution X-ray photoelectron spectra (XPS) reveal that the POA-DGEBAs are covalently attached to the surface of MWCNTs. The morphology of MWCNTs/POA-DGEBA is observed by TEM. The POA400-DGEBA coated on the MWCNTs is thicker and more uniform. However, the coating of POA2000-DGEBA on the MWCNTs shows a worm-like bulk substance and the MWCNT surface is bare. In addition, the flexural strength and the bulk electrical conductivity of the MWCNTs/polypropylene nanocomposite bipolar plates are measured 59% and 505% higher than those of the original composite bipolar plates by adding 8 phr of MWCNTs/POA400-DGEBA. The maximum current density and power density of the single cell test for the nanocomposite bipolar plate with 4 phr MWCNTs/POA400-DGEBA are 1.32 A cm⁻² and 0.533 W cm⁻², respectively. The overall performance confirms the functionalized MWCNTs/polypropylene nanocomposite bipolar plates prepared in this study are suitable for PEMFC application.     

Novel functionalized carbon nanotubes as cross-links reinforced vinyl ester/nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

In this study, the novel functionalized multi-walled carbon nanotubes (MWCNTs) 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 is found that the MWCNTs-POAMA exhibited better dispersion in the vinyl ester matrix than those of pristine MWCNTs. Moreover, the results demonstrate that the mechanical and electrical properties of the vinyl ester nanocomposite bipolar plate are improved dramatically. The ultimate flexural strength, unnotched impact strength, in-plane electrical conductivity and contact resistance of the MWCNTs-POAMA/vinyl ester nanocomposite bipolar plate are increased by 45%, 90%, 315% and 28%, respectively. In addition, the maximum current and power densities of the single fuel cell test using the MWCNTs-POAMA/vinyl ester nanocomposite bipolar plates is 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 polymer electrolyte membrane fuel cell applications can be achieved.     

Electrical and thermal conductivities of novel metal mesh hybrid polymer composite bipolar plates for proton exchange membrane fuel cells

This study prepares novel metal mesh hybrid polymer composite bipolar plates for proton exchange membrane fuel cells (PEMFCs) via inserting a copper or aluminum mesh in polymer composites. The composition of polymer composites consists of 70 wt% graphite powder and 0-2 wt% modified multi-walled carbon nanotubes (m-MWCNTs). Results indicate that the in-plane electrical conductivity of m-MWCNTs/polymer composite bipolar plates increased from 156 S cm⁻¹ (0 wt% MWCNT) to 643 S cm⁻¹ (with 1 wt% MWCNT) (D.O.E. target >100 S cm⁻¹). The bulk thermal conductivities of the copper and aluminum mesh hybrid polymer composite bipolar plates (abbreviated to Cu-HPBP and Al-HPBP) increase from 27.2 W m⁻¹ K⁻¹ to 30.0 W m⁻¹ K⁻¹ and 30.4 W m⁻¹ K⁻¹, respectively. The through-plane conductivities decrease from 37.8 S cm⁻¹ to 36.7 S cm⁻¹ for Cu-HPBP and 22.9 S cm⁻¹ for Al-HPBP. Furthermore, the current and power densities of a single fuel cell using copper or aluminum mesh hybrid polymer composite bipolar plates are more stable than that of using neat polymer composite bipolar plates, especially in the ohmic overpotential region of the polarization curves of single fuel cell tests. The overall performance confirms that the metal mesh hybrid polymer composite bipolar plates prepared in this study are promising for PEMFC application.     

Electromagnetic-carbon surface treatment of composite bipolar plate for high-efficiency polymer electrolyte membrane fuel cells

Polymer electrolyte membrane (PEM) or proton-exchange membrane fuel cell systems are environmentally friendly power sources for many applications. Bipolar plates are essential components of a PEM fuel cell. Recently, composite bipolar plates have received considerable interest due to their superior performance. The most important properties of bipolar plates are electrical resistance and contact resistance, which are largely dependent on the surface morphology of the bipolar plate, because low electrical resistance improves the efficiency of PEM fuel cells. In this study, a selective surface preparation technology is developed using an electromagnetic field and carbon black (electromagnetic-carbon surface treatment). The carbon black is heated by an electromagnetic field on the surface of the bipolar plate with a high rate of temperature rise. The non-electrically conducting surface resin is removed, without damaging the carbon fibre to give a low electrical resistance. It is found that the surface-treated composite bipolar plate has a lower electrical resistance than those of conventional composite bipolar plates, and that the electromagnetic-carbon surface treatment can be applied for production of the composite bipolar plates in a fast and efficient process.     

Effect of carbon fillers on properties of polymer composite bipolar plates of fuel cells

Bipolar plates are major components of fuel cell (FC) stacks and they make up a large portion of the stack volume and cost. In order to reduce their weight and fabrication cost, polymer composite materials with various carbon conducting fillers are tested for use as composite bipolar plates for FCs. The composite materials are prepared by using graphite with a small vol.% of carbon black (CB), multi-walled carbon nanotubes (MWNTs) or carbon fibres (CF) in an epoxy resin. The electrical conductivity and flexural properties of the composites are measured as a function of the carbon conductive filler content. The highest electrical conductivity is observed at a total conducting filler content of 75 vol.%. The addition of a small amount of hybrid conducting filler enhances the electrical conductivity up to certain threshold, viz. 5 vol.% of CB, 2 vol.% of MWNTs, and 7 vol.% of CF. Above these thresholds, the electric conductivity decreases with increasing filler content, due to the lack of sufficient resin to bind the fillers tightly. The hybrid filler system has better properties than the single filler system. The experimental results indicate that there is an optimum composition range with respect to electrical conductivity and mechanical properties.     

Effect of surface wettability of polymer composite bipolar plates on polymer electrolyte membrane fuel cell performances

The effects of fluoropolymer based additive at different additive/binder and additive/filler ratios on surface wettability, conductivity and mechanical properties of polymer composite bipolar plates are investigated in this study. Fuel cell performance tests are performed at different feed flow rates by using composite bipolar plates containing organic based hydrophobic and inorganic based hydrophilic additives to investigate the effect of surface wettability properties on polymer electrolyte membrane fuel cell (PEMFC) performance. The conductivity of the composite materials decreases with the increase in additive/filler ratios, due to a decrease in the amount of conductive filler in the composite structure, whereas conductivity increases with the increase in additive/binder ratios due to a decrease in the amount of nonconductive binder. The surface hydrophobicity gets stronger with increasing fluoropolymer/filler and fluoropolymer/binder ratio amounts, related to the hydrophobic properties of both filler and fluoropolymer. In all feed flow rates, at low current densities, the single cells exhibit almost the same performance. At intermediate and high current densities, polymer composite without any additives shows higher performance than the bipolar plates containing organic or inorganic based additives. Current and power densities show maxima at the bipolar plate contact angle of 80°.     

Cr and Zr/Cr nitride CAE-PVD coated aluminum bipolar plates for polymer electrolyte membrane fuel cells

In this work, two nitride coatings deposited on aluminum-based bipolar plates via cathodic arc evaporation physical vapor deposition (CAE-PVD) have been evaluated using two different techniques. The coating materials, a multi-layer chromium-zirconium nitride (ZrN–CrN) and a monolayer chromium nitride (CrN) have been exposed to electrochemical polarization tests for corrosion resistance simulating the typical environment in the anode and cathode sides in polymer electrolyte membrane fuel cells (PEMFC). Besides, two 3-cell PEMFC stacks, one per each coating material, have been formed. The migration of metal cations toward both the gas diffusion layers (GDL) and catalyst layers have been analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), after 100 h of continuous operation of the stacks. Results have shown that the two coatings applied over the Al-plates satisfy the corrosion resistance requirements in the short-term tests performed at the two stacks. Moreover, results obtained from electrochemical polarization tests have revealed that the CrN-coating confers a good corrosion resistance to the Al-based metal plate, achieving values of corrosion potential and corrosion current two orders of magnitude lower than the ones obtained for the Al alloy as-received.     

Synergy effects of hybrid carbon system on properties of composite bipolar plates for fuel cells

A hybrid carbon system of graphite powder (GP) and continuous carbon fibre fabric (CFF) is used for an epoxy composite to improve the electrical conductivity, mechanical properties and mouldability of a composite bipolar plate. These improvements are achieved simultaneously by inserting several layers of CFF into the GP/epoxy composite to enhance the mechanical properties and in-plane conductivity. The electrical properties, flexural strength and mouldability of the composite plates are measured as a function of conducting filler content and number of CFF layers. The composites show improved electrical conductivity, flexural properties and mouldability. Composites with 70-75 vol.% carbon fillers have the highest electrical conductivity with reasonable flexural properties. These results suggest that the poor mouldability and low through-plane electrical conductivity of the continuous fibre composite bipolar plate, as well as the weak flexural properties of GP composites, can be overcome by incorporating a GP/CFF hybrid system.     

Electrical properties of carbon-based polypropylene composites for bipolar plates in polymer electrolyte membrane fuel cell (PEMFC)

An investigation is made of the electrical properties of polypropylene/graphite (PP/G) composites as prospective replacements for the traditional graphite bipolar plate in proton-exchange membrane fuel cells. The composites have relatively low electrical conductivities, i.e., up to 28 S cm⁻¹ at 90 wt.% G. Combination of G with carbon black (CB) is an effective way to develop higher conductivity composites. The conductivity reaches 35 S cm⁻¹ by combination of 25 wt.% CB and 55 wt.% G to 20 wt.% PP. This is five times the value at 80 wt.% G and 20 wt.% PP (7 S cm⁻¹). Two methods are mainly adopted for the preparation of composites, namely, melt compounding and solution blending. Solution blending of PP with conductive fillers followed by moulding of the dried powder leads to higher conductivities compared with those of melt-compounded composites. The combination of conjugated conducting polymers such as polyaniline (PANi) with the PP, G, and CB is also investigated. It is found that composites containing PANi have lower conductivities than those of the neat composites. This decrease in conductivity is attributed to the poor thermal stability of PANi.     

Development and characterization of a miniature PEM fuel cell stack with carbon bipolar plates

This paper details the fabrication and testing of a fuel cell stack using a novel manufacturing approach for creating carbon bipolar plates. The fundamental fabrication techniques have been described in a previous contribution [1]. The initial paper characterized a single cell. In this work, the fabrication techniques are utilized to fabricate a three-cell fuel cell stack. Operating data in different operating conditions is measured and presented. Information on cell performance at different operating temperatures and pressures is included. The methodology for the fuel cell characterization is presented.     

Nitrogen plasma-implanted titanium as bipolar plates in polymer electrolyte membrane fuel cells

Nitrogen plasma immersion ion implantation (PIII), a non-line-of-sight surface treatment technique suitable for bipolar plates in polymer electrolyte membrane fuel cells, is conducted at low and high temperature to improve the corrosion resistance and conductivity of titanium sheets. X-ray photoelectron spectroscopy (XPS) shows that high-temperature (HT) nitrogen PIII produces a thick oxy-nitride layer on the titanium surface. This layer which provides good corrosion resistance and high electrical conductivity as verified by electrochemical tests, inductively coupled plasma optical emission spectroscopy, and interfacial contact resistance (ICR) measurements renders the materials suitable for polymer electrolyte membrane fuel cells. In comparison, the low-temperature (LT) PIII titanium sample exhibits poorer corrosion resistance and electrical conductivity than the untreated titanium control.     

High nitrogen stainless steel as bipolar plates for proton exchange membrane fuel cells

To investigate the applicability of high nitrogen (HN) austenitic stainless steel as bipolar plates for proton exchange membrane fuel cells (PEFCs), the polarization tests were carried out in synthetic solutions (0.05 M SO₄²⁻ (pHs 2.3, 4.3 and 5.5) +2 ppm F⁻) at 353 K. Interfacial contact resistance between the stainless steel and gas diffusion layer was measured before and after polarization. A single cell employing the HN stainless steel as bipolar plates was operated for 1000 h at 0.5 A cm⁻² (12.5 A). The single cell exhibited voltage drop of 17 mV during the operation. Corrosion products were scarcely detected for the HN stainless steel bipolar plate, as confirmed by scanning electron microscopy. After the polarization tests and single cell operation, XPS analyses were carried out to examine the resulting surface states. In the synthetic solutions to pH 4.3, the passive films mainly consisted of oxides enriched with Cr. When the solution pH was 5.5, on the other hand, the films were mainly composed of Fe-oxides. After the single cell operation for 1000 h, it was found that the passive films of the rib surface for the gas inlet part was mainly composed by Fe-oxides. On the other hand, the passive films for the ribs from center to gas outlet part were mainly made up of Cr-oxides. By combining the simulated and real operation environments, it is believed that the corrosion resistive Cr-oxides passive layer of the HN stainless steel obtained by the presence of nitrogen incorporated into the stainless steel could contribute to the maintenance of the higher cell voltage during the extensive cell operation.     

Preparation and properties on the graphite/polypropylene composite bipolar plates with a 304 stainless steel by compression molding for PEM fuel cell

Graphite/polymer composites have high corrosion resistance, low contact resistance and low fabrication cost but low cell efficiency and mechanical strength. This study examined the electrical and mechanical properties of graphite/polypropylene composite bipolar plates. Carbon nanotubes (CNTs) were used to improve the electrical properties of the graphite/PP composites. Although the electrical properties increased when excess conducting filler was added to the composite, the mechanical strength decreased significantly. 304 stainless steel (304 SS) plates with different thicknesses were used as the support material of a graphite/PP composite bipolar plate. The 304 SS-supported graphite/PP composite bipolar plate had an optimum CNTs/graphite/PP composite composition of 1.2, 83 and 17 wt.%, respectively. The flexural strength of the 304 SS-supported graphite/PP composites increased from 35 to 58 MPa with increasing 304 SS thickness from 0.5 to 1 mm. The power density of the graphite bipolar plate and 304 SS-supported graphite/PP composite bipolar plate were 968 and 877 mW cm⁻², respectively. The 304 SS complemented the mechanical strength of the graphite/PP composite bipolar plate as well as the cell efficiency.     

Crossover effects of the land/channel width ratio of bipolar plates in polymer electrolyte membrane fuel cells

The crossover effect of the land/channel width ratio of bipolar plates in polymer electrolyte membrane fuel cells is experimentally investigated in this study. To isolate the effect of the land/channel width ratio, three different types of bipolar plates of a fixed sum and channel width are specially prepared. With three different bipolar plates, measurements are taken of electrochemical performance, inlet pressure, and hydrogen crossover rate. When the stoichiometric ratio of hydrogen is 1.5, the standard type of bipolar plate, BP2 (land width = 0.75 mm, channel width = 1.05 mm) show the best performance. However, according to increasing stoichiometric ratio of hydrogen, BP3 (land width = 1.12 mm, channel width = 0.68 mm) has the best performance, especially at the medium and high current range. For the crossover rate, the biggest amount of hydrogen gas crossover to the cathode in BP3. This is because of the anode inlet pressure caused by the largest land/channel ratio of BP3.     

Conductive amorphous carbon-coated 316L stainless steel as bipolar plates in polymer electrolyte membrane fuel cells

Amorphous carbon (a-C) film about 3 μm in thickness is coated on 316L stainless steel by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The AFM and Raman results reveal that the a-C coating is dense and compact with a small size of graphitic crystallite and large number of disordered band. Interfacial contact resistance (ICR) results show that the surface conductivity of the bare SS316L is significantly increased by the a-C coating, with values of 8.3–5.2 mΩ cm² under 120–210 N/cm². The corrosion potential (E_corr) shifts from about −0.3 V vs SCE to about 0.2 V vs SCE in both the simulated anode and cathode environments. The passivation current density is reduced from 11.26 to 3.56 μA/cm² with the aid of the a-C coating in the simulated cathode environment. The a-C coated SS316L is cathodically protected in the simulated anode environment thereby exhibiting a stable and lower current density compared to the uncoated one in the simulated anode environment as demonstrated by the potentiostatic results.     

Evaluation of coated metallic bipolar plates for polymer electrolyte membrane fuel cells

Metallic bipolar plates for polymer electrolyte membrane (PEM) fuel cells typically require coatings for corrosion protection. Other requirements for the corrosion protective coatings include low electrical contact resistance, good mechanical robustness, low material and fabrication cost. The authors have evaluated a number of protective coatings deposited on stainless steel substrates by electroplating and physical vapor deposition (PVD) methods. The coatings are screened with an electrochemical polarization test for corrosion resistance; then the contact resistance test was performed on selected coatings. The coating investigated include Gold with various thicknesses (2 nm, 10 nm, and 1 μm), Titanium, Zirconium, Zirconium Nitride (ZrN), Zirconium Niobium (ZrNb), and Zirconium Nitride with a Gold top layer (ZrNAu). The substrates include three types of stainless steel: 304, 310, and 316. The results show that Zr-coated samples satisfy the DOE target for corrosion resistance at both anode and cathode sides in typical PEM fuel cell environments in the short-term, but they do not meet the DOE contact resistance goal. Very thin gold coating (2 nm) can significantly decrease the electrical contact resistance, however a relatively thick gold coating (>10 nm) with our deposition method is necessary for adequate corrosion resistance, particularly for the cathode side of the bipolar plate.     

The electrical and corrosion properties of carbon nanotube coated 304 stainless steel/polymer composite as PEM fuel cell bipolar plates

In this study, the contact resistance and corrosion resistance of three different types of plates used as PEM fuel cell bipolar plates are investigated, viz. (1) 304 stainless steel without carbon nanotube treatment that was then sandwiched between polymer composites, (2) CNT particles placed on the surface of the 304 stainless steel that was then sandwiched between polymer composites, and (3) direct CNTs coated 304 stainless steel that was then sandwiched between polymer composites. Both treated and untreated 304 stainless steel covered with the polymer composites exhibited good corrosion resistance. The results showed that the highest improvement of the contact resistance was accomplished by the direct deposition of CNTs on the 304 stainless steel insert. The results of the potentiodynamic and potentiostatic measurements also showed that direct CNTs deposition on the 304 stainless steel insert did not degrade the corrosion performance under PEM fuel cell operating conditions.     

Development of bipolar plates with different flow channel configurations for fuel cells

Bipolar plates include separate gas flow channels for anode and cathode electrodes of a fuel cell. These gases flow channels supply reactant gasses as well as remove products from the cathode side of the fuel cell. Fluid flow, heat and mass transport processes in these channels have significant effect on fuel cell performance, particularly to the mass transport losses. The design of the bipolar plates should minimize plate thickness for low volume and mass. Additionally, contact faces should provide a high degree of surface uniformity for low thermal and electrical contact resistances. Finally, the flow fields should provide for efficient heat and mass transport processes with reduced pressure drops. In this study, bipolar plates with different serpentine flow channel configurations are analyzed using computational fluid dynamics modeling. Flow characteristics including variation of pressure in the flow channel across the bipolar plate are presented. Pressure drop characteristics for different flow channel designs are compared. Results show that with increased number of parallel channels and smaller sizes, a more effective contact surface area along with decreased pressured drop can be achieved. Correlations of such entrance region coefficients will be useful for the PEM fuel cell simulation model to evaluate the affects of the bipolar plate design on mass transfer loss and hence on the total current and power density of the fuel cell.     

Polypyrrole-modified hydrophobic carbon nanotubes as promising electrocatalyst supports in polymer electrolyte membrane fuel cells

As an alternative to oxidative acid treatment, a hydrophobic graphitized carbon nanotube (CNT) was functionalized with 1-4 nm thick polypyrrole (PPy) prior to application as catalyst supports in polymer electrolyte membrane (PEM) fuel cells. Unlike oxidative acid treatment, the PPy coating method converts the hydrophobic surface of a CNT to a hydrophilic one without creating defects on the surface of the CNT. As a result, Pt nanoparticles deposited on the PPy-coated CNTs showed an improved distribution, which significantly enhanced the fuel cell performance while preserving the intrinsic properties of the CNTs, i.e., resistance to electrochemical carbon corrosion. An additional advantage of PPy coating is that it prevents Pt nanoparticles from agglomerating on the CNT surface. These results indicated that PPy-coated CNTs are a promising catalyst support to improve both the performance and durability of PEM fuel cells.