| With the rapid development of portable electronic devices, the current lithium-ion battery technology can not meet the increasing requirement of energy density for the protable electronic products. As a new generation of portable power sources,μDMFC (Micro Direct Methanol Fuel Cell) has a great development potential, because of its high theoretical energy density and the convenient fuel storage and refilling, which has became a focus of attention in both of industry and academia. However, there still are several technical barriers for the development ofμDMFC, such as the low integrated level and power density, the high cost of bipolar plate and its immature preparation process. Fortunately, The development of MEMS (Micro-Electro-Mechanical Systems) supplies a good opportunity to solve the technical problems mentioned above. In this paper, we mainly applied MEMS techniques to fabricate the bipolar plates, and studied their properties in the passiveμDMFC.For the fabrication of stainless steel bipolar plates, an analytical model, with three parameters of the stencil width, the etching depth and the etching time, was firstly built to study the etching rate variation in the wet chemical etching of stainless steel. Comparison of the experimental data, the model can correctly predict the relationship between the etching depth and the etching time, when the stencil widths were between 50μm to 500μm, and the value of the coefficient of determination can reach above 0.94. The study found that the variation ofφ(the ratio of the area being etched to the area of the stencil opening) was the root causing the etching rate variation. The model can quantitatively explain some experimental results, especially for the reason of the right move of CLW (Critical Line width) and the variation of etching rate.Subsequently, the mechanism that the micro structure array of the woven mesh can improve the performance ofμDMFC was studied. A resistance network model was built to explore the current traces in a stainless steel woven wire mesh. The results showed that the currents mainly flow from Rfp (the contact resistance between a stainless steel fiber and a bipolar plate) to Rbulk (the bulk resistance of a stainless steel fiber) and then to Rfc (the contact resistance between a stainless steel fiber and a piece of carbon paper).Rff (the contact resistance between two neighbor stainless steel fibers) is very large, almost no current flowing through them. The measurement results also showed that the mesh with the fiber diameter of 130μm and the fiber pitch of 450μm had the lowest resistance. When the pressure density was 1 MPa, the power consumption of Rfc and Rfp were 90.1% and 9.8% of the total power consumption by the mesh. Therefore, how to decrease Rfc and Rfp is the key point to decrease the power consumption by the mesh.Based on the properties and geometric parameters of woven mesh, two kinds of stainless steel bipolar plates, respectively with the structures of micro channel arrays and micro pillar arrays, were fabricated by using several lithography and wet chemical etching processes. The quality and the performance of the fabricated bipolar plate were also studied. The result showed that the bipolar plates fabricated by using wet chemical etching do not have the process defects such as flying edge, burr, processing deterioration layer and heat affected zone. The micro channel arrays can improve the mass transfer at anode and increase the peak power density by 17～35%. The micro pillar arrays can decrease the contact resistance, make the area of GDL (Gas Diffusion Layer) exposed to fuel more than 95% and increase the peak power density by 77%. The micro pillar array can break the interrelationship constraint between the contact resistance and the mass transfer ability ofμDMFC bipolar plate design.To make theμDMFC stack more compact and easy connectivity, a passiveμDMFC stack, which was based on the principle of the self-circulation, was developed to achieve the interconnection and mass transportation by using bipolar plate structure. For the requirements of the self-circulation flow field design, ABS (Acrylonitrile Butadiene Styrene), a kind of polymer with 1.05 g·cm-3 density, was formed as the substrate material of the bipolar plates by using UV-LIGA (Ultraviolet-Lithographie, Galvanoformung, Abformung). Then, a 5μm thick nickel layer was electroplated on the ABS bipolar plates in a low cost way. After that, the surface was modified to ultra hydrophilic by using nano self-assembly. Then, a single fuel cell with the designed bipolar plates was assembled and studied. The results showed that the self-pumping rate of the single fuel cell in 5 hours was 0.1-0.15 mL·h-1. The fuel cell can work under different orientations, moreover, the bubble could discharge even when the channel was placed upside down.By improving the processes of polymer bipolar plates and the design of self-circulation single fuel cell, aμDMFC stack was fabricated, assembled and tested. The size of the polymer bipolar plate for the stack was only 10 mm×10 mm×2.4 mm, which integrated a self-circulation anode flow field, a self-breathing cathode flow field, two fuel reservoirs and a fuel pipe. Especially, our smallestμDMFC, which can be expanded to multi-unit serial connected, was only 0.5 mL in volume,0.59 g in weight,7.5 W·L-1 in volumne power density and 6.36 W·kg-1 in weight power density. |
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