Performance of DMFC with SS 316 bipolar/end plates

This work mainly emphasizes the development of new materials and design for a bipolar/end plate in a direct methanol fuel cell (DMFC). According to the DOE requirements, preliminary studies show that SS 316 (Stainless Steel 316) is a suitable candidate. Several flow field designs were studied and a modified serpentine design was proposed. SS 316 end plates were fabricated with an intricate modified serpentine flow field design on it. The performance of a single stack DMFC with SS 316 end plates were studied with different operational parameters. A long-term test was carried out for 100 h with recycling the methanol and the contaminants in the MEA were characterized. The stack efficiency is found to be 51% and polarization losses are discussed. SS 316 with low permeability resulted in an increased pressure drop across the flow field, which increased the fuel cell performance. The use of SS 316 as bipolar plate material will reduce the machining cost as well as volume of the fuel cell stack.     

Development of cylindrical PEM fuel cells with semi-cylindrical cathode current collectors

Proton exchange membrane (PEM) fuel cells are attractive because of advantages such as low-temperature operation, no emission of harmful gases and high efficiency. However, the bipolar plates used in the state-of-the-art planar architecture are costly and increase the dead weight of the cell. In addition, the flow channels in the planar fuel cell increase the difficulty in removing the water produced in the cathode during cell operation. Cylindrical PEM fuel cells, on the other hand, do not require bipolar plates and there is no need for precisely machined flow channels. Thus, cylindrical PEM fuel cells are cheap, efficient in water management, and possess higher volumetric and gravimetric power density compared to planar PEM fuel cells. The design of a cylindrical fuel cell is very simple, but the fabrication of the same is fairly complex. In this work, a novel cathode current collector design for cylindrical PEM fuel cell has been developed. The cell performance was limited by low open circuit voltage and high ohmic resistance. The open circuit voltage of the cell is increased from 0.85 V to 0.95 V using an acrylic based adhesive to seal the membrane edges. The contact resistance of the cell is reduced from 75 mOhm to 50 mOhm by increasing the contact pressure on the membrane electrode assembly and it is further reduced to 30 mOhm by gold coating the current collectors. Furthermore, a cumulative 40% increase in peak power has been achieved from the optimization of cathode rib width and hydrogen flow rate. The optimized cell delivered a current density of 400 mA/cm² at 0.6 V and peak power of 2 W, which is appreciable considering the fact that the cell is air-breathing and operated with very minimal subsystems.     

Development of micro direct methanol fuel cells for high power applications

A micro direct methanol fuel cell (μDMFC) with active area of 1.625 cm² has been developed for high power portable applications and its electrochemical characterization carried out in this study. The fragility of the silicon wafer makes it difficult to compress the cell for good sealing and hence to reduce contact resistance in the Si-based μDMFC. We have instead used very thin stainless steel plates as bipolar plates with the flow field machined by photochemical etching technology. For both anode and cathode flow fields, widths of both the channel and rib were 750 μm, with a channel depth of 500 μm. A gold layer was deposited on the stainless steel plate to prevent corrosion. This study used an advanced MEA developed in-house featuring a modified anode backing structure with a compact microporous layer. Maximum power density of the micro DMFC reached 62.5 mW cm⁻² at 40 °C, and 100 mW cm⁻² at 60 °C at atmospheric pressure, which almost doubled the performance of our previous Si-based μDMFC.     

Effect of flow field design on performances of high temperature PEM fuel cells: Experimental analysis

Among all types of fuel cells, attention is being drawn lately on high temperature Polybenzimidazole (PBI) PEM because their operative temperature range (120-180 °C) increases the tolerance to carbon monoxide. This feature allows working with low quality hydrogen produced by hydrocarbon reformation. Most of the literature on PBI PEM deals with membrane and MEA related issues, however, cell efficiency and specially, commercial feasibility are conditioned by other fuel cell components as bipolar plates. In the present study the focus is on the effect of the flow field geometry of high temperature PBI PEM composite bipolar plates on the overall performance of the cell. For this purpose, three different channel geometries are studied: two serpentine flow fields and parallel channels flow field. Results show that serpentine geometry yields higher performance though it introduces higher pressure drop along the cell as well.     

A self-pumping and self-breathing micro direct methanol fuel cell with polymer bipolar plates

A passive micro direct methanol fuel cell (DMFC) for reducing volume and parasitic power is designed and fabricated using several integrated technologies. New bipolar plates with tapered channels at the anode and a pillar array at the cathode are first applied to a passive micro-DMFC. The substrate of the bipolar plates made of acrylonitrile butadiene styrene (ABS) is hot embossed with two molds, fabricated by UV-LIGA and micro machining. To make the bipolar plates conductive and hydrophilic, a nickel layer is electroplated on the ABS plates, and three PDDA/PSS bi-layers are self-assembled onto the nickel layer. The bipolar plates are produced using hot embossing, a low cost, highly accurate batch process. A single cell is assembled to verify the self-pumping function, and it can generate a peak power density of 7.4 mW cm⁻² with a 3 M methanol solution. The fuel cell is verified to work in three different orientations. When the fuel cell is placed horizontally, the self-pumping rate is about 0.1-0.15 mL h⁻¹. And the fuel cell can work through self-pumping for 5 h under this condition.     

The effect of pH and halides on the corrosion process of stainless steel bipolar plates for proton exchange membrane fuel cells

Stainless steel is attractive as material for bipolar plates in proton exchange membrane fuel cells, due to its high electrical conductivity, high mechanical strength and relatively low material and processing cost. Potentiostatic and potentiodynamic tests were performed in H₂SO₄ solutions on AISI 316L stainless steel bipolar plates with etched flow fields. The effect of pH and presence of small amounts of fluoride and chloride on the corrosion rate and interfacial contact resistance of the stainless steel bipolar plate were investigated. The tests performed in electrolytes with various pH values revealed that the oxide layer was thinner and more prone to corrosion at pH values significantly lower than the pH one expects the bipolar plate to experience in an operating proton exchange membrane fuel cells. The use of solutions with very low pH in such measurements is thus probably not the best way of accelerating the corrosion rate of stainless steel bipolar plates. By use of strongly acidic solutions the composition and thickness of the oxide layer on the stainless steel is probably altered in a way that might never have happened in an operating proton exchange membrane fuel cell. Additions of fluoride and chloride in the amounts expected in an operating fuel cell (2 ppm F⁻ and 10 ppm Cl⁻) did not cause significant changes for neither the polarization- nor the contact resistance measurements. However, by increasing the amount of Cl⁻ to 100 ppm, pitting was initiated on the stainless steel surface.     

Development of titanium bipolar plates fabricated by additive manufacturing for PEM fuel cells in electric vehicles

Bipolar plates (BPs) are one of the main parts of proton exchange membrane (PEM) fuel cell stacks, which constitute a significant percentage of a PEM fuel cell system in terms of cost, weight, and structural strength. Although frequently used graphite BPs have low density, high conductivity, and high corrosion resistance, machining the desired flow channels on these plates is challenging. On the other hand, BPs made of various materials rather than graphite can be also fabricated by additive manufacturing methods. These methods can be considered as a reasonable alternative to conventional machining for the fabrication of graphite BPs in PEM fuel cells regarding material cost, fabrication of flow channels, and some post-processes in which the large-scale manufacturing of graphite BPs is more complex. This study offers a comparison of formed stainless-steel, additive manufactured titanium and machined composite graphite plates having the same flow-field geometry as a bipolar plate. In addition, titanium BPs are coated with gold and their performances are compared. Among the cells tested, the highest peak power of 639 mWcm^?2 is measured from the cell with 450 nm gold coated titanium BP, whereas those of the cell with conventional graphite and stainless-steel BP are only around 322 mWcm^?2 and 173 mWcm^?2, respectively. Moreover, a new titanium bipolar plate design providing high specific power density is also presented.     

Optimization design of slotted-interdigitated channel for stamped thin metal bipolar plate in proton exchange membrane fuel cell

The stamped metal bipolar plate is a promising candidate of the traditional graphite plate for proton exchange membrane fuel cells (PEMFCs) due to its advantages, such as low cost, compactness, robustness and high production efficiency. This study proposes a new type of flow configuration, which is called slotted-interdigitated channel, for stamped metal bipolar plates. Numerical simulation of the flow distribution of slotted-interdigitated channels is studied by using three-dimensional computational fluid dynamics (CFD) and the results show the flow distribution is uneven. Consequently, an optimization model, based on a linear analytical model, is proposed to eliminate flow maldistribution. Finally, even flow distribution is obtained according to the optimum results and high fuel cell performance can be achieved.     

Performance of PEMFC stack using expanded graphite bipolar plates

The bipolar plates are in weight and volume the major part of PEM fuel cell stack, and also a significant effect to the stack cost. To develop the low-cost and low-weight bipolar plate for PEM fuel cell, we have developed a kind of cheap expanded graphite plate material and a production process for fuel cell bipolar plates. The plates have a high electric conductivity and low density, and can be stamped directly forming fuel cell bipolar plates. Then, 1 and 10 kW stacks using expanded graphite bipolar plates are successfully assembled. The contact resistance of the bipolar plate is investigated and the electrochemical performances of the fuel cell stacks are tested. Good fuel cell performance is obtained and the voltage distribution among every single cell in the stacks is very uniform.     

Neutron radiography and current distribution measurements for studying cathode flow field properties of direct methanol fuel cells

The influence of the cathode flow field properties on the water distribution and performance of direct methanol fuel cells (DMFCs) was studied. All measurements were performed with DMFC stack cells (A = 314.75 cm²). The local and temporal water distributions in the flow field channels during DMFC operation were visualized by means of through‐plane neutron radiography. Current and temperature distributions were measured simultaneously by the segmented cell technology. Additionally, the time‐dependent current distribution, cell performance and pressure drop were measured. Cathode flow field designs with channel and grid structures were compared. The cathode flow field channels were impregnated by either hydrophobizing or hydrophilizing agents or used as received. It turned out that hydrophobized and partially also untreated flow fields cause large water droplets in the cathode channels. The water droplets cause a blocking of the air flow and consequently a lower and more unstable (fluctuating) performance, less steady current and temperature distributions, and higher pressure drops between cathode inlet and outlet. Because of their two‐dimensional design, grid flow fields are less prone to water accumulations. The best results are achieved with a hydrophilized grid flow field that has a channel depth and width of 1.5 mm each (‘C‐GR15’). Copyright © 2013 John Wiley & Sons, Ltd.     

Performance of the gold-plated titanium bipolar plates for the light weight PEM fuel cells

In this present work, we are attempting to develop a light weight and corrosion resistant bipolar plate for the proton exchange membrane fuel cell. A titanium bipolar plate substrate has been chosen as the base metal due to its low cost, ease of manufacture into stampable bipolar plates, and its light weight. Our approach to obtain a smaller and lighter weight single fuel cell is to coat titanium with a corrosion resistant coating. Gold (Au) was investigated. The cell performance of the gold-plated bipolar plates is close to and even better than the PEM fuel cells with graphite and pure titanium bipolar plates. Gold-plated titanium bipolar plates can be employed to produce fuel cells with light weight, low coating cost and low contact resistance, ideal for portable applications.     

Molybdenum carbide coated 316L stainless steel for bipolar plates of proton exchange membrane fuel cells

Superior corrosion resistance and high electrical conductivity are crucial to the metallic bipolar plates towards a wider application in proton exchange membrane fuel cells. In this work, molybdenum carbide coatings are deposited in different thicknesses onto the surface of 316 L stainless steel by magnetron sputtering, and their feasibility as bipolar plates is investigated. The microstructure characterization confirms a homogenous, compact and defectless surface for the coatings. The anti-corrosion performance improves with the increase of the coating thickness by careful analysis of the potentiodynamic and potentiostatic data. With the adoption of a thin chromium transition layer and coating of a ∼1052 nm thick molybdenum carbide, an excellent corrosion current density of 0.23 μA cm⁻² is achieved, being approximately 3 orders of magnitude lower than that of the bare stainless steel. The coated samples also show a low interfacial contact resistance down to 6.5 mΩ cm² in contrast to 60 mΩ cm² for the uncoated ones. Additionally, the hydrophobic property of the coatings’ surface is beneficial for the removal of liquid water during fuel cell operation. The results suggest that the molybdenum carbide coated stainless steel is a promising candidate for the bipolar plates.     

Experimental investigations of the anode flow field of a micro direct methanol fuel cell

The effect of the anode flow field design on the performance of an in-house fabricated micro direct methanol fuel cell (μDMFC) with an active area 1.0 cm × 1.0 cm was investigated experimentally. Single serpentine and parallel flow fields consisting of micro channels were tested. The experimental results indicated that the serpentine flow field exhibited significantly higher cell voltages than did the parallel flow field, particularly at high current densities. The study of the effect of channel depth of the serpentine flow field suggested that there exists an optimal channel depth for the same channel width and the same open ratio when the same methanol flow rate is supplied; either shallower or deeper channels will lead to a reduction in the cell performance. Finally, it was demonstrated that performance of the μDMFC with the reactants fed by an active means was insensitive to the cell orientations, which is different from conventional DMFCs with larger flow channels reported in the literature.     

Cross-leakage flow between adjacent flow channels in PEM fuel cells

In polymer electrolyte membrane (PEM) fuel cells, serpentine flow channels are used conventionally for effective water removal. The reactant flows along the flow channel with pressure decrease due to the frictional and minor losses as well as the reactant depletion because of electrochemical reactions in the cells. Because of the short distance between the adjacent flow channels, often in the order of 1 mm or smaller, the pressure gradient between the adjacent flow channels is very large, driving part of reactant to flow through the porous electrode backing layer (or the so-called gas diffusion layer)—this cross-leakage flow between adjacent flow channels in PEM fuel cells has been largely ignored in previous studies. In this study, the effect of cross-flow in an electrode backing layer has been investigated numerically by considering bipolar plates with single-channel serpentine flow field for both the anode and cathode side. It is found that a significant amount of reactant gas flows through the porous electrode structure, due to the pressure difference, and enters the next flow channel, in addition to a portion entering the catalyst layer for reaction. Therefore, mixing occurs between the relatively high concentration reactant stream following the flow channel and the relatively low reactant concentration stream going through the electrode. It is observed that the cross-leakage flow influences the reactant concentration at the interface between the electrode and the catalyst layer, hence the distribution of reaction rate or current density generated. In practice, this cross-leakage flow in the cathode helps drive the liquid water out of the electrode structure for effective water management, partially responsible for the good PEM fuel cell performance using the serpentine flow channels.     

Electrochemical behavior of surface treated metal bipolar plates used in passive direct methanol fuel cell

Corrosion resistance performance of SS316L treated by passivation solution was investigated in a simulated environment of the passive direct methanol fuel cell (DMFC). Electrochemical impedance spectroscopic (EIS) test showed that polarization resistance of untreated and treated SS316L were 1191 Ω cm² and 9335 Ω cm², respectively. The above result agreed with the Tafel slope analysis of potentiodynamic polarization curves. Comparing the untreated and treated SS316L in the simulated environment of DMFC anode working conditions, it was observed that the corrosion current density of treated SS316L as estimated by 4000 s potentiostatic test reduced from 38.7 μA cm⁻² to 0.297 μA cm⁻², meanwhile, the current densities of untreated and treated SS316L in cathode working conditions were 3.87 μA cm⁻² and 0.223 μA cm⁻², respectively. It indicated that the treated SS316L should be suitable in both anode and cathode environment of passive DMFCs. The treated SS316L bipolar plates have been assembled in a passive single fuel cell. A peak power density of 1.18 mW cm⁻² was achieved with 1 M methanol at ambient temperature.     

A robust design of the forming process parameters of the metallic bipolar plate for proton exchange membrane fuel cells

Metallic bipolar plates are important components of the proton exchange membrane fuel cell. To achieve higher performance of fuel cells, dozens of fine flow channels with the width of approximately 1 mm and a high aspect ratio of over 0.3 are required for bipolar plates. Those functional demands push the forming process of bipolar plates to the limit of material and tools, making the rupture sensitive to even a tiny deviation of the tools' geometric dimensions and material properties. To address that, a robust design method is established in this work to analyze the rupture probability by introducing the stochastic variation of material properties and tools’ geometric dimensions. Utilizing the method, the rupture probability is decreased from 19.14% to 0.18% during the practical forming of metallic bipolar plates by parameters control. The yield rate is also verified to be over 99.5%.     

Development and performance evaluation of a high temperature proton exchange membrane fuel cell with stamped 304 stainless steel bipolar plates

In this work, a high temperature proton exchange membrane fuel cell (HT-PEMFC) with stamped SS304 bipolar plates is successfully developed. Its performance was evaluated under two types of gaskets at different assembly torques and air stoichiometric ratios. The rates of pressure loss at a torque of 7 N-m with 50 Shore A hardness gaskets was 2.0 × 10^?3 MPa min^?1, which is acceptable. The best performance of the developed HT-PEMFC with stamped SS304 bipolar plates was 228.33 mW cm^?2, which approaches the performance of HT-PEMFCs with graphite bipolar plates. The optimal air stoichiometric ratio for the HT-PEMFC with stamped SS304 bipolar plates was 4.0, which is higher than that for proton exchange membrane fuel cells with CNC milled graphite bipolar plates. This is probably because of the deformation of the flow channels under the assembly compression force, which causes an elevated gas-diffusion drag in the flow channels. After the test, it was observed that some products of corrosion reaction formed on the surface of the SS304 bipolar plate. This phenomenon may lead to a decrease in the operating life of the HT-PEMFC.     

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.     

Improved polymer electrolyte fuel cell performance with membrane electrode assemblies using modified metallic plate: Comparative study on impact of various coatings

Bipolar plate is one of the key components of polymer electrolyte membrane fuel cell. In the present study, metallic plates are explored as bipolar plates in comparison to most generally used high-density graphite plates. Among various metals, stainless steel 316L is preferred due to its low cost, high strength, ease of machining and for its corrosion resistance characteristics. However, the challenges associated with metallic plates are high interfacial contact resistance due to passive oxide layer formation and possible corrosion product during operation in chemically harsh environments, which may contaminate the membrane electrode assembly. Three electrically conductive and corrosion resistant coatings namely Titanium Nitrides, Plasma Nitride, and Gold have been coated over the surface of stainless steel 316L metallic plate to overcome these challenges and to explore their impact on fuel cell performance using standard membrane electrode assemblies. These coatings are characterized by X-Ray Diffraction, Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy along with interfacial contact resistance measurements. Further, the coated SS plates have been tested in real time polymer electrolyte membrane fuel cell operation for their use as bipolar plates and their performances have been compared with the fuel cell comprising conventional graphite plates. A cell comprising Titanium Nitride, Gold and Plasma Nitride coated metallic plates exhibit a power density of 430, 720 & 268 mW cm⁻² respectively, at an operating fuel cell potential of 0.6 V. Gold coated metallic plate shows comparable polymer electrolyte membrane fuel cell performance in relation to conventional graphite plate.     

Performance and long-term stability of Ti metal and stainless steels as a metal bipolar plate for a direct methanol fuel cell

In this study, STS 316L (Stainless Steel 316L), STS 430, and Ti metal are investigated as metal bipolar plates for a direct methanol fuel cell (DMFC). The corrosion resistance of these materials is investigated by potentiodynamic and chronoamperometry tests. Their cell performance and long-term stability are then studied under a real fuel cell test. The corrosion resistance of the metal bipolar plates is in the order of Ti > STS 316L > STS 430. However, the results of the real fuel cell test differ from the results of the corrosion resistance. Ti shows the lowest performance due to a sharp performance decrease in ohmic loss regions, while STS 430 shows a lower performance decrease in ohmic loss regions. Although STS 430 has less resistance to corrosion than STS 316L in the simulated environment, STS 430 performs better as a metal bipolar plate for a DMFC than STS 316L, particularly, with regard to cell performance, cell resistance, and durability.