The flow field structures in fuel cells have to meet several requirements, such as homogeneous fluid distribution, product water removal, good current transport, good under-rib convection and high conductivity. Each factor can considerably influence the performance of fuel cells. However, this work, as mentioned previously, is focused on the effects of flow field structure in micro PEMFC.
Yu et al. [1] designed and fabricated the micro flow channels by using MEMS techniques, including photolithography, dry and wet etching, and chemical and physical vapor deposition. The material of wafer base was silicon. This paper’s primary research was to deposit Cu/Au composite layers in different thickness on the top of silicon wafer, including 0.5μmAu, 1.4μm Cu&0.2μm Au, and 1.5μm Cu&0.9μm Au.
The composite layers were primarily to reduce their resistance as the current collectors. The results indicated that the cell performance is improved by increasing the thickness of the composite layer on the silicon wafer.
Meyers and Maynard [2] also fabricated the micro flow channels by using MEMS techniques. The wafer base was silicon. Fig. 1-3 illustrates (a) the bipolar plate design and (b) the monolithic design.
This research was primarily to compare bipolar and monolithic designs.
Bipolar design is similar to the standard cell sandwich commonly used in PEMFC system, and monolithic design is essentially an “unfolded” fuel cell with the anode and cathode on the same substrate. The experimental results were that bipolar design’s performance was better than that of monolithic one, see Fig. 1-4. The research also suggested that many system-level issues, like thermal management, air movement, fuel delivery, humidification control, water management, power load management, and system integration, must be considered in order to achieve higher performance.
Lee and Chuang [3] employed porous silicon as a gas diffusion layer (GDL) to minimize a micro-fuel cell. Pt catalyst was deposited on the surface of the porous silicon by physical vapor deposition (PVD) method to improve the porous silicon conductivity. Porous silicon with Pt catalyst replaced traditional GDL, and the Pt metal, remaining on the rib, was used to form a micro-thermal sensor in single lithographic process.
The researches of [4] and [5] had focused on measuring important data on the effect of cell temperature, fuel temperature, fuel humidity and other key factors on cell performance.
Nguyen and White [6] showed that the potential loss caused by the membrane contributes to a large proportion of the total loss of fuel cell stacks. In order to reduce this loss, reactants at the anode must be humidified. As for the cathode, if air, instead of pure oxygen, is used,
humidification is also needed.
For the studies for micro PEMFC and DMFC, the MEMS technology and Si-based substrate have been widely used. For examples, Kelley et al. [7], Shah et al. [8], Cha et al. [9], and Lu et al. [10] all fabricated flow field structures using Si wafers.
Schmitz et al. [11, 12] successfully designed a planar fuel cell that was primarily made by the print circuit board (PCB) as materials, see Fig.
1-5. The PCB is a new base of bipolar plate which can be used in MEMS technology. They operated the air-breathing fuel cell in the atmosphere, and the power density could reach to 110mW/cm2 in 0.5V.
Schmitz et al. [13] researched the air-breathing fuel cells under the influence of different opening area ratios, which are 33%, 50% and 80%, respectively; see Fig.1-6. They were operated in the room temperature, and it was found that the case of area ratio 80% has the best performance for the water management.
Jeong et al. [14] also proofed the influence on air-breathing PEMFCs by using four kinds of cathode opening area ratio; see Fig.1-7.
At low current density, the power density decrease with increasing the opening ratio. But at the high current density, the level of the power density is in successive order, that is 77%>64%>92%>52%. The opening ratio of 64% will reach the concentration polarization first comparing to that of 92%. And at low current density, the power density will increase with the surrounding humidity, but such trend does not appear at high current density.
Cha et al. [15] covered the investigation range of flow channels from
flow pattern archetype exhibits unique scaling behavior. For most flow pattern archetypes, optimal feature size occurs at an intermediate channel dimension. Extremely small flow channels do not optimize performance despite improved mass transport. Pressure drop loss and flow travel path in the cathode compartment are the major factors to determine the optimal size. The scaling phenomena are explained in conjunction with the details of oxygen distribution in the cathode flow channels and gas diffusion layer.
Jin et al. [16] indicated that the flow-field for reactant distribution is an important design factor that influences the performance of PEMFCs.
They focused on increasing the path-length difference, ∆z, in serpentine flow-fields with the hypothesis that an enhancement of under-rib convection between neighboring channels improved the performance of PEMFCs as illustrated in Figs. 1-8 and 1-9. The resultant maximum path-length difference between neighboring flow-channels is expected to enhance under-rib convection and transport, thereby improving the performance. In addition, the highly-interlaced channel patterns in Multi-Pass Serpentine Flow-Fields (MPSFFs) are expected to improve the uniformity of local conditions, such as reactant and product concentrations, temperature and liquid water saturation in the active cell area.
Jason et al. [17] showed that PEMFC performance is directly related to the flow channel design on bipolar plates. Power gains can be found by varying the type, size, or arrangement of channels. This study presented two new flow channel patterns: a leaf design and a lung design;
see Fig.1-10. These bio-inspired designs combine the advantages of the
existing serpentine and interdigitated patterns with inspiration from patterns found in nature. This research showed promising results for bio-inspired flow patterns with noticeable improvement in pressure loss, overall performance, and peak power density. For the new leaf and lung designs, the optimum operating conditions are found as 65~750C cell temperature, 2-atm backpressure, and 100% relative humidity.