Air-breathing Micro Research Articles

Design and Simulation of Air-Breathing Micro Direct Methanol Fuel Cells with Different Anode Flow Fields.

The design of the anode flow field is critical for yielding better performance of micro direct methanol fuel cells (µDMFCs). In this work, the effect of different flow fields on cell performance was investigated by the simulation method. Compared with grid, parallel and double-serpentine flow fields, a single-serpentine flow field can better improve the mass transfer efficiency of methanol and the emission efficiency of the carbon dioxide by-product. The opening ratio and channel length also have important effects on the cell performance. The cells were manufactured using silicon-based micro-electro-mechanical system (MEMS) technologies and tested to verify the simulation results. The experimental results show that the single-serpentine flow field represents a higher peak power density (16.83 mWcm−2) than other flow fields. Moreover, the results show that an open ratio of 47.3% and a channel length of 63.5 mm are the optimal parameters for the single-serpentine flow field.

Modeling study of an air‐breathing micro direct methanol fuel cell with an extended anode catalyst region

A three-dimensional model was developed for an air-breathing micro direct methanol fuel cell (μDMFC) with an extended anode catalyst region on the cell channels. The model was evaluated against experimental studies for a μDMFC under several anode distribution conditions, and the results showed a close agreement. The model was employed to study if catalysts coated on the fluid-flow channel walls could enhance the power generation performance. Further, the effects of the anode catalyst loading of channel walls on the overall cell and individual electrode performances were examined. The modeling results indicated that the fuel cell with anodes both on the proton-exchange membrane and on channel walls did not show superior performance to the fuel cell with anode catalysts only on the membrane since the overall power generation was mainly limited by the kinetics of the methanol electrode reaction but not the methanol transfer. The modeling results also demonstrated that increasing the anode catalyst loading on channel walls decreased the cathode potential due to an increase in the ohmic loss for the fuel cell with anode catalysts both on the membrane and on channel walls. Reducing channel dimensions decreased the ionic resistance and increased the methanol concentration at the anode and the methanol crossover flux to the cathode.

Development and Characterization of a Micro Redox Fuel Cell

The design approach of catalysed flow field channel walls as an additional surface reaction area for the micro fuel cells was demonstrated in an air-breathing micro iron-oxygen redox fuel cell (μIOFC). The μ IOFC with a graphite channel improved the maximum power density by 280% when compared to the μIOFC with inactive channel walls for the redox couple reaction. In the micro redox anode fuel cell with a graphite channel, the performance is limited by the mass transfer of the redox species but not the anode reaction rate. The proton transfer to the cathode is also a limiting factor determining the μIOFC performance, and the anode layer on the ion exchange membrane should be eliminated to reduce the resistance of the proton transfer.

Fabrication and Characterization of a Thin, Double‐sided Air Breathing Micro Fuel Cell

AbstractAn air‐breathing micro fuel cell (μFC) with laterally fed porous anode electrode is realized and reported. The performance of this microdevice with embedded flow channels and electrodes is characterized in ambient conditions. To realize this device, we used negative epoxy‐based photoresist (SU‐8) as a stamp to transfer a microchannel pattern into a Nafion 1110 membrane by micro hot embossing. A Nafion 212 layer served to seal the device as a blanket top layer. The fabrication process of microchannels and μFC assembly is evaluated with optical and scanning electron microscopy (SEM) step by step. Variations of hydrogen feed rate on performance are investigated. The cell is composed of two individual fuel cells with a shared anode. The characteristics of the cell are further studied by performance and electrochemical impedance spectroscopy (EIS) measurements of the separate constituent cells and compared with data for the complete cell. The maximum power density per superficial (footprint) area is 68.4 mW cm−2 for the shared anode stack. In the present μFC architecture, the external packaging is Nafion polymer, which results in a thin and light energy source with simplicity of manufacturing. The device performance offers a high volumetric and gravimetric energy density for portable applications.

Framework based on number of basis functions complexity measure in investigation of the power characteristics of direct methanol fuel cell

A potential alternative to cell batteries is the air-breathing micro direct methanol fuel cell (μDMFC) because it is environmental friendly, charging-free, possesses high energy density properties and provides easy storage of the fuel. The effective functioning of the complex air-breathing μDMFC system exhibits higher dependence on its operating conditions and the parameters. The main challenge for the experts is to determine its optimum operating conditions. In this context, the mathematical modeling approach based on evolutionary framework of genetic programming (GP) can be applied. However, its successful implementation depends on the complexity chosen in its structural risk minimization (SRM) objective function. In this work, the two measures of complexity based on the standardized number of nodes and the number of basis functions in the splines is chosen. Comparison between the two GP approaches based on these two complexity measures is evaluated on the experimental procedure performed on the μDMFC. The power characteristics considered in this study are power density and open-circuit voltage and the three inputs considered are methanol flow rate, methanol concentration and the cell temperature. The statistical analysis based on cross-validation, error metrics and hypothesis tests is performed to choose the best GP based power characteristics models. Further, 2-D plots for measuring the individual effects and the 3-D plots for the interaction effects of the inputs on the power characteristics is plotted based on the parametric approach. It was found that the methanol concentration influences the power characteristics (power density and OCV) of μDMFC the most followed by cell temperature and methanol flow rate.

The optimization of air-breathing micro direct methanol fuel cell using response surface method

In this paper, the response surface methodology is applied to optimize the operating conditions of μDMFC (micro direct methanol fuel cell). Three operating parameters, the methanol flow rate, the methanol concentration and the cell temperature are considered in this study, and the ranges of them are 0.5–1.5 ml min−1, 0.5–2.0 mol L−1 and 20–60 °C respectively. The individual effects of these operating parameters and the combined effects of multiple operating conditions on air-breathing cell performance are examined. Furthermore, a quadratic model is established to describe the maximum power density and open-circuit voltage as the response value.

Investigations of silicon-based air-breathing micro direct methanol fuel cells with different anode flow fields

Different anode flow fields of air-breathing micro direct methanol fuel cells (μDMFCs) are investigated to improve the cell performances. The single-serpentine flow field can effectively improve the methanol mass transport efficiency and exhibit higher exhaust resultant (CO2) rates than other flow fields such as gird, parallel and double-serpentine. Additionally, the effects of open ratios and channel lengths on the cell performance are evaluated to determine the optimal anode flow field structures. The μDMFCs with different anode flow fields are fabricated using silicon-based micro–electro–mechanical systems (MEMS) technologies and are tested at room temperature. The experimental results show that the single-serpentine flow field exhibits a significantly higher performance than those of other flow fields, demonstrating 16.83mWcm−2 in peak power density and a substantial increase in mass transport coefficients. Moreover, for optimum single-serpentine flow field, it is appropriate for the flow channel dimensions to be in the ratio of 2:3:254 for channel width: ridge width: channel length.

Capillary-based water removal cathode for an air-breathing micro direct methanol fuel cell

This paper presents a capillary-based water removal cathode for an air-breathing micro direct methanol fuel cell (μDMFC). The mechanism of water removal from the cathode is studied and an array of capillaries with hydrophilic surface is designed on the ribs of the cathode structure. Microfabrication techniques, including double-side lithography and ICP, were used to fabricate the anode and cathode plates of the μDMFC on the same silicon wafer simultaneously. The surface of capillary structure was treated by low temperature oxygen plasma to improve the hydrophilicity. One μDMFC with capillary-based water removal cathode and another regular one without were both assembled and characterized. Measured results show that the μDMFC with water removal cathode achieves a power density of 2.35 mW/cm2, 12 % larger than that of the regular one with the value of 2.10 mW/cm2. And the maximum current density of the novel μDMFC is 30 mA/cm2, 20 % larger than that of the regular one, 25 mA/cm2. It is also clearly observed during the μDMFC operation that the water is drawn out from the capillary-based water removal cathode expectantly.

Development of a 4-cell air-breathing micro direct methanol fuel cell stack

In this work, we present a 4-cell air-breathing micro direct methanol fuel cell (μDMFC) stack featured with novel n-inlet and n-outlet (NINO) anode flow fields. Compared with the conventional parallel flow field, the NINO flow field with micro structures improves the methanol solution transport efficiency and facilitates the exclusion of CO 2 gas bubbles accumulated in flow channels. The μDMFC stack patterned with novel NINO flow field is fabricated using silicon-based micro electromechanical system (MEMS) technologies. The polydimethylsiloxane (PDMS) distributors are used not only for the stack packaging but also for the uniform distribution of methanol solution through the connected double-side patterns. Experimental results reveal that the NINO flow fields exhibit higher peak power density than that of the conventional flow field. The NINO structures with different patterns have significant influences not only on the momentum transfer but also on the cell performance. The maximum power output of the μDMFC stack can yield about 80 mW at room temperature, which is significative for portable applications.

Design, fabrication and testing of an air-breathing micro direct methanol fuel cell with compound anode flow field

An air-breathing micro direct methanol fuel cell (μDMFC) with a compound anode flow field structure (composed of the parallel flow field and the perforated flow field) is designed, fabricated and tested. To better analyze the effect of the compound anode flow field on the mass transfer of methanol, the compound flow field with different open ratios (ratio of exposure area to total area) and thicknesses of current collectors is modeled and simulated. Micro process technologies are employed to fabricate the end plates and current collectors. The performances of the μDMFC with a compound anode flow field are measured under various operating parameters. Both the modeled and the experimental results show that, comparing the conventional parallel flow field, the compound one can enhance the mass transfer resistance of methanol from the flow field to the anode diffusion layer. The results also indicate that the μDMFC with an anode open ratio of 40% and a thickness of 300 µm has the optimal performance under the 7 M methanol which is three to four times higher than conventional flow fields. Finally, a 2 h stability test of the μDMFC is performed with a methanol concentration of 7 M and a flow velocity of 0.1 ml min−1. The results indicate that the μDMFC can work steadily with high methanol concentration.

A water collecting and recycling structure for silicon-based micro direct methanol fuel cells

An air-breathing micro direct methanol fuel cell (μDMFC) with a water collecting and recycling cathode structure is presented in this paper. The mechanism of water generation in cathode and water collection from the cathode is studied. The capillary channels with hydrophilic surface are designed along the ribs of the air-breathing cathode window to collect excessive water. The collected water is then released via an exhaust pipe. Microfabrication techniques, including double-side lithography and KOH etching, are used to fabricate the anode and cathode plates of the μDMFC on the same silicon wafer simultaneously. The silicon plate surface is treated by low temperature oxygen plasma to improve the hydrophilicity. The μDMFC with the water collecting and recycling structure and the regular one without, both with an active area of 7.8 × 7.8 mm 2, are assembled and characterized. Experimental results show that the maximum power density and current density of the both μDMFCs are more than 10 mW cm −2 and nearly 100 mA cm −2 at room temperature, respectively. After both μDMFCs operate at 0.15 V (about 60 mA cm −2 at room temperature) for 1 h, the outputs of the prototype with water collecting and recycling structure decrease to about 9.7 mW cm −2 and 95 mA cm −2 only, while that of regular one decrease to 9 mW cm −2 and 75 mA cm −2. The water is collected by the exhaust pipe in evidence. As such, it would be possible to recycle the water from cathode to anode.

Development and characterization of a novel air-breathing micro direct methanol fuel cell stack for portable applications

An air-breathing 10-cell micro direct methanol fuel cell (µDMFC) stack with four anode feeding patterns is designed, fabricated and tested. For a better understanding of the operational characteristics of both the single cell and the stack, a two-dimensional numerical model is established and calculated. Employing micro-stamping technology, the current collectors of each single cell are microfabricated on the stainless steel plate with a thickness of 300 µm. The single µDMFC is first tested under various operating parameters. On the basis of the simulation and experimental observation of the single cell performance, the µDMFC stack performance is thoroughly analyzed with different anode feeding patterns. The results indicate that the µDMFC stack with pattern B can ensure the uniform performance of each single cell and generate the highest power output. With pattern B, further experiments are carried out to investigate the influence of the anode flow rate on the stack performance. As a result, the µDMFC stack achieves the best performance with the maximum power density of about 24.75 mW cm−2 at 5.0 ml min−1. Finally, the stack is successfully applied to two electronic devices of different rated power.

Hydrogen generation from solid NaBH 4 with catalytic solution for planar air-breathing proton exchange membrane fuel cells

In the pursuit of the development of alternative mobile power sources with high energy densities, this study elucidated a new hydrogen generation approach from solid NaBH 4 using a new catalyst, sodium hydrogen carbonate (NaHCO 3), which was placed in a small and compact cartridge. A planar air-breathing PEMFC system fitted with the cartridge has been investigated for testing hydrogen generation from NaBH 4 and NaHCO 3. NaHCO 3 allowed the hydrogen cartridge to control hydrogen generation and to improve the power density, fuel efficiency, energy efficiency, and cell response. The cell performance of solid NaBH 4 air-breathing PEMFC system strongly depended on the operating conditions: the feeding rates and concentrations of catalytic solutions for NaBH 4 hydrolysis. In various concentrations (5 - 12 wt %) of NaHCO 3 aqueous solutions, 10 wt % NaHCO 3 aqueous solution exhibited the highest maximum power density of 128 mW cm −2 at 0.7 V, which was estimated to be a Faradic efficiency of 78.4% and an energy efficiency of 46.3%. The data illustrated that NaHCO 3 was an effective catalyst for hydrogen generation with the solid NaBH 4, which is considered as a hydrogen carrier for air-breathing micro PEMFCs operated without auxiliary hydrogen controller or devices.

An air-breathing micro direct methanol fuel cell stack employing a single shared anode using silicon microfabrication technologies

This paper presents a silicon-based air-breathing micro direct methanol fuel cell (μDMFC) stack with a shared anode plate and two air-breathing cathode plates. Three kinds of anode plates featured by different methanol transport methods are designed and simulated. Microfabrication technologies, including double-side lithography and bulk-micromachining, are used to fabricate both anode and cathode silicon plates on the same wafer simultaneously. Three μDMFC stacks with different kinds of anodes are assembled, and characterized with a single cell together. Simulation and experimental results show that the μDMFC stack with fuel transport in a shared model has the best performance, and this stack achieves a power of 2.52 mW which is almost double that of a single cell of 1.28 mW.

Development of Micro Direct Methanol Fuel Cell Systems and Their Possible Applications

To meet the demands for high power microelectronic devices, research efforts at SIMIT have involved the optimization of Si based flow fields using the MEMS technology, the development of key components for the air-breathing micro direct methanol fuel cells (µDMFCs), the assembly of µDMFC stacks and the integration of micro energy systems. We found that the microstructure of both the anodic and cathodic flow fields has a significant influence on the performance of the µDMFCs. By adjusting the microstructure of the flow field plates, it is possible to obtain the better performance of a µDMFC with Si based flow field plates than that using traditional flow field plates. Recent research results including the fabrication of the membrane electrode assembly with improved performance and the water management under the air-breathing mode will be presented. Integrated µDMFC systems and their possible applications will be discussed.

Development of a Silicon Based Air-Breathing Micro Direct Methanol Fuel Cell

As one part of the efforts to break the bottleneck of power sources in development of integrated micro systems, a silicon based air-breathing micro direct methanol fuel cell (μDMFC) was developed in this work. By using micro-machining technologies compatible with that in processing of the other MEMS devices, the anode and cathode micro flow-field plates had been successfully fabricated on a pair of 2-inch silicon wafers. The silicon μDMFC was evaluated under ambient conditions using aqueous methanol solution with different concentrations. Results show that open circuit potential (OCP) of the μDMFC was above to 0.6 V, and by using 3mol/L methanol, the peak current density and power density of the silicon μDMFC could reach 28mA/cm2 and 8mW/cm2, respectively.

Design, fabrication and testing of a silicon-based air-breathing micro direct methanol fuel cell

A silicon-based air-breathing micro direct methanol fuel cell (μDMFC) with an active area of 13 mm × 11 mm has been developed for portable application. The prototype features unique cathode structures fabricated by low-cost MEMS technologies and a reliable assembly method by using PDMS and holders. I–V performance curves with different anode flow rates, methanol concentrations and operating temperatures have been obtained. The results show that this μDMFC has generated a maximum power density of 2.31 mW cm−2 using 1 M methanol solution at room temperature, which is comparable with our previous active μDMFCs. It has also been demonstrated that temperature is a critical experimental determinant of μDMFC performance. So thermal management may play a very important role in future μDMFC performance.