In this thesis the experimental method is used to study the characteristics of a DMFC when a heterogeneous carbon fiber bipolar plate is applied to it. The first main study is about the effect of the different structures of the carbon fiber bunch on the fuel cell performance. Additionally, a high temperature hot-pressing process is performed to change the inner molecular structure so that the hydrogen ion can be blocked to avoid the lateral migration between two adjacent cells. Finally, the two techniques are applied to make our new portable DMFC stack. The bipolar plates with the sawtooth or non-sawtooth carbon fiber bunches have been used in making our DMFC stack. The experimental results display that the performances of the two structures both are better than the traditional graphite bipolar plate. However, the performance of DMFC with the sawtooth bipolar plate is much better than that without sawtooth, especially in high current density. When carbon fiber bunches with sawtooth use at anode and cathode of bipolar plates, the performance can be enhanced and its power density 27.6% higher than that without sawtooth. During our study we also found that part of hydrogen ions can laterally migrate to its adjacent cathode and do not directly cross to its opposite cathode, when the banded type MEA are used to multiple cell stack. Therefore, the performance cannot be performed well due to this type ion transfer. In order to block the lateral migration, the narrow area of the membrane between two adjacent electrodes is pressed with a high temperature hot-pressing device. After a short time hot-press between two adjacent electrodes, the hydrogenion migration phenomenon reduced, and the performance had been improved about 10% higher than that without hot-press. Finally, a double layer 2x6-cell flat type DMFC is made. This 12-cell stack is composed of each electrode area 0.5x5cm2, two sheets of membrane for 6-cell using Nafion 117, the anode catalyst Pt-Ru loading 4mg/cm2, and cathode catalyst Pt loading 4mg/cm2, the methanol concentration 3M, air-breathing, and operating in room temperature. The output power of the cell can reach an average power density 8.0mW/cm2 and total power 240mW with our handmade stack. If the performance of each fuel cell is more uniform, we expect that total power can reach 480 mW. The power level should be satisfied for any kind mobile phone.

In this paper a proton exchange membrane fuel cell (called PEMFC) stack was developed to power or charge 3C products without any voltage transformer. PEMFC stacks made with traditional bipolar plates to generate a high voltage are usually by accumulating multiple single fuel cells together. The design with traditional heavy and large bipolar plates is inconvenient for 3C products to generate a high voltage in a finite volume. To solve this problem, a heterogeneous carbon fiber bunch unipolar plate is adopted to replace traditional bipolar plates, and a special membrane electrode assembly (called MEA) with multiple sets of banded electrodes is used to replace a traditional MEA that is made with only a set electrodes. With this new design, the fuel cell voltage can easily increase in a layer. The designed stack can provide multiple voltages and currents by proper series and/or parallel connections. The variable voltage 16-cell fuel cell is composed of 4-layer 4-banded type MEAs and 5 heterogeneous carbon fiber bunch bipolar plates. The 16-cell stack is divided into 4 sets. Each set of 4 series connection cell is arranged in a line in 4 different layers. The 4-cell sets can connect by series/parallel on the two ends of the stack. The total volume of the 16-cell stack is 385cm3 and its weight is 365g. The new design can power or charge certain 3C products directly. If 2 sets of 4-cell fuel cells are connected in series, the stack can provide 2A at 3.6V. With the above 2 sets of 2*4-cell connected in parallel, the stack can provide 3.5A at 3.6V. If the 4 sets of 4-cell are all connected in series, the stack can provide 1.8 A at 7.2V. These voltages and currents derived from these stacks can power or charge a mobile phone, a photo camera and a video camera directly. If a higher voltage or current are needed, two or more 16-cell stacks can be connected in series XI or parallel. Then notebooks or any other 3C products in which higher power are needed can be driven.

Direct Methanol Fuel Cells (DMFCs) are electrochemical energy conversion devices that convert chemical energy into electrical energy. The bipolar plate component of the DMFC is required to be fluid impermeable to prevent fuel leakage and electrically conductive to collect the electrons produced within the cell. Graphite possesses the properties of high electrical conductivity, low weight and resistance to corrosion that make it an attractive material for bipolar plates. However, the poor mechanical properties of graphite lead to prohibitive machining costs and increased production times. The objective of this research is to develop an indirect laser sintering (LS) process, involving the laser sintering of graphite powders mixed with a phenolic resin binder which offers the advantage of complex part production and testing of prototype bipolar plates in short times. Due to the nature of the indirect LS process, the as-produced (green part) plates are porous and possess low electrical conductivities (< 0.1 S.cm-1). This research describes a viable method to rapidly fabricate and test multiple graphite bipolar plate designs using indirect LS. This process involved identifying and selecting suitable graphite powder and binder systems based on their thermal and electrical properties and developing a post process heat treatment method for achieving electrical conductivity of 250 S/cm for LS graphite parts which exceeds the DOE target of 100 S/cm for bipolar plate materials. The post processing also covered a method of infiltration using cyanoacrylate which was capable of rendering porous brown parts fluid impermeable and suitable for use in DMFCs. The cyanoacrylate infiltrated LS graphite parts were characterized for flexural strength and electrical and thermal conductivities and bipolar plates were made and evaluated in a DMFC test stand. Various flow field designs including plates with varying channel and rib widths and triangular, elliptical and rectangular flow field cross sections were fabricated using indirect LS and their respective polarization curves were compared to commercially machined graphite plates. The fuel cell tests show the improvement in mass transport performance could be due to improved methanol distribution and water removal characteristics of triangular and elliptical cross sectional channels over rectangular channels of equivalent dimensions. / text

The performance of a new cathode flow field plate located on a PEM fuel cell was compared to an industry standard and optimal serpentine design provided from literature. Results were successfully collected through a fuel cell module integrated with the 3D computational fluid dynamics package ANSYS Fluent. Contour plots showing a cathode catalyst layer comparison of local current density, oxygen molar concentrations, water content, and the pressure inside of the flow channels were compared with both PEM fuel cell configurations. The new flow field plate/pattern was shown to distribute more mass species of oxygen, more evenly, to the reaction site given the same boundary conditions, thus contributing to more ideal local current density. The net-power was determined for both fuel cells which included the pump work-in and power-out from each fuel cell. The new flow field plate was shown, through computational power performance results, to outperform the conventional flow pattern by up to 2.4% when excluding the effects of pump work, and still upheld a positive gain when factoring in this value. With an additional 18 corners for improved water management due to the effects of wall adhesion, the new bipolar plate was proven to become a new competitor in PEM fuel cell technology. Furthermore, this thesis gives further insight on PEMFC digital prototyping.

The development, design, and modeling of a rapid continuous processing scheme is developed to economically manufacture conductive polymer composite bipolar plates for fuel cells. Bipolar plates are required to possess several important properties for fuel cell operation, with the most sought after being electrical conductivity and mechanical strength. The polymer composite material generated at Virginia Tech is based on material generated by a wet-lay process and uses polyethylene terepthalate (PET) or polyphenylene sulfide (PPS) as the binder, although PPS is mainly used. In order to reach sufficient conductivity for use in generating bipolar plates, the polymer is doped with high levels of conductive graphite particles in the range of 70-80 wt%. The polymer system is reinforced with 6-9 wt% glass or carbon fibers. When compression molded into a solid, flat preform, the wet-lay material exhibits excellent bulk (in-plane) conductivity (> 250 S/cm). The material also exhibits tensile and flexural strengths of 57.5 and 95.8 MPa, respectively, higher than other polymer composite material being considered for bipolar plate production. However, formability and through-plane conductivity needs improvement. The laminate bipolar plates developed at Virginia Tech are made using wet-lay material in the core and a thermoplastic/graphite mixture on the surfaces. The wet-lay material provides mechanical integrity, while a powder form of PVDF or PPS and graphite mixture added to the surfaces to improve through-plane conductivity and formability. The manufacturing scheme for the production of laminate bipolar plates is based on the pre-consolidation of the wet-lay material, which establishes a solid, flat surface for the continuous addition of laminate powder. Because the laminate powder only requires heating, radiation heating is used in the process design to pre-heat the preform prior to compression molding. The heated preform passes underneath a press, where forming of channels takes place along with cooling of the bipolar plate. It is estimated that the entire process can take one minute to produce a bipolar plate. The cost of manufacturing a bipolar plate is estimated to be $8/kW, below the goal of $10/kW. The annual production is determined to be 250,000, with over 500,000 possible depending on certain design factors. / Ph. D.

Polymer Electrolyte Membrane Fuel Cell (PEMFC) systems are promising technology for contributing to meet the deficiency of world`s clean and sustainable energy requirements in the near future. Metallic bipolar plate (BPP) as one of the most significant components of PEMFC device accounts for the largest part of the fuel cell`s stack. Corrosion for metallic bipolar plates is a critical issue, which influences the performance and durability of PEMFC. Corrosion causes adverse impacts on the PEMFC`s performance jeopardizing commercialization. This research is aimed at determining the corrosion resistance of metallic BPPs, particularly stainless steels, used in PEMFC from different aspects. Material selection, coating selection, manufacturing process development and cost considerations need to be addressed in terms of the corrosion behavior to justify the use of stainless steels as a BPP material in PEMFC and to make them commercially feasible in industrial applications. In this study, Ti, Ni, SS304, SS316L, and SS 430 blanks, and BPPs comprised of SS304 and SS316L were examined in terms of the corrosion behavior. SS316L plates were coated to investigate the effect of coatings on the corrosion resistance performance. Stamping and hydroforming as manufacturing processes, and three different coatings (TiN, CrN, ZrN) applied via the Physical Vapor Deposition (PVD) method in three different thicknesses were selected to observe the effects of manufacturing processes, coating types and coating thicknesses on the corrosion resistance of BPP, respectively. Uncoated-coated blank and formed BPP were subjected to two different corrosion tests: potentiostatic and potentiodynamic. Some of the substantial results: 1- Manufacturing processes have an adverse impact on the corrosion resistance. 2- Hydroformed plates have slightly higher corrosion resistance than stamped samples. 3- BPPs with higher channel size showed better corrosion resistance. 4- Since none of the uncoated samples meet the 2015 target of the U.S. Department of Energy, surface coating is required. 5- ZrN and CrN coated BPPs exhibited higher corrosion resistance meeting DOE target while TiN coated samples had the lowest corrosion resistance. Higher coating thicknesses improved the corrosion resistance of the BPPs. 6- Process sequence between coating and manufacturing is not significant for hydroforming case (ZrN and CrN) and stamping case (CrN) in terms of the corrosion resistance. In other words, coating the BPP`s substrate material before manufacturing process does not always decrease the corrosion resistance of the BPPs.

In this research the effects of the fabrication processes of MEA on the output power of a DMFC stack are studied by changing hot-pressing conditions including pressure, temperature and time. Additionally, the effects of the various treatments of the MEAs on the output voltage and power are also studied after the hot-pressing process of MEA is finished. In the first experimental study the catalyst of cathode is 4.0mg/cm unsupported HP Pt black, Anode is 4.0mg/cm 80% HP Pt-Ru Alloy (1: 1), membrane is Nafion 117, and bipolar plates is heterogeneous carbon fiber bipolar plate developed by our fuel cell laboratory. The MEA for single cell includes the area of membrane 3*3 cm2 the active area of electrode 1.5*1.5 cm2. Under the hot-pressing conditions 120 oC, 100 bar and 90s, the maximum power density can reach a value of 18 mW/cm2 at the conditions of methanol concentration 3 M, air-breathing, and room temperature After several experiments, we observed that performances of MEAs decayed with time. So we designed a series of experiments to inspect the various possible reasons and try to solve this problem. The cylindrical DMFC is one of the most important developments in our lab. However, the MEAs made for plate-type DMFC do not fit the cylindrical DMFC stack properly. The electrodes easily pealed off from the membrane and the contact resistance increases after certain periods. So the hot-pressing device had been redesigned to fit the cylindrical DMFC stack. After that the total power of the 6-cell stack with total active area 15 cm2 can reach a value 135 mW. If the performance of each cell of the 6-cell stack is uniform, we expect that the total power of this stack can reach a higher value 195 mW, which can be applied to some portable electronic products.

Membrane electrode assembly (MEA) is the foundation of the single cell as well as the core of the fuel cell when generating electricity. Its work efficiency is the key factor for single cell performance. This study aims to understand the variation between the conventional method and the decal method during the MEA process. By observing the microstructure morphology of electrode and the performance of single cell, as well as analyzing internal resistance and its stabilization, the advantages and disadvantages of MEA in the two methods is analyzed. The decal condition is 135¢XC, 15 kg/cm , 2.5 min at a high temperature (50¢XC 3M methanol), in air-breathing under atmosphere system. The maximum power density is approximately 22.5 mW/cm which is very close to the result of conventional method. The decal method is better than the conventional method particularly in regards to the high current density performance. It shows that there is an efficient influence of the decal method on the methanol mass transfer and it also improves its polarization and enlarges the current. If the single cell is operated in the high temperature, the fuel mass transfer can be advanced in the decal method and its performance can be raised. However, in the manufacturing process, more time has to be spent when producing the MEA. This experiment can be used as a reference on the single cell operation environment and manufacturing time for future studies.

Fuel cells are electrochemical energy conversion devices that convert chemical energy to electrical energy efficiently. Bipolar plates form an integral part of a fuel cell and their high manufacturing cost and low production rate have hindered the commercialization of fuel cells. Bipolar plates require high electrical conductivity, strength, chemical resistance and thermal conductivity. This thesis presents efforts to manufacture bipolar plates which meet these requirements using compression or injection molding. Compression or injection molding processes allow cost-effective, large-scale manufacturing of bipolar plates. A variety of material systems for the fabrication of bipolar plates are processed, molded and characterized. / text

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