Fabrication of a miniature twin-fuel-cell on silicon wafer

The fabrication and performance evaluation of a miniature twin-fuel-cell on silicon wafers are presented in this paper. The miniature twin-fuel-cell was fabricated in series using two membrane-electrode-assemblies sandwiched between two silicon substrates in which electric current, reactant, and product flow. The novel structure of the miniature twin-fuel-cell is that the electricity interconnect from the cathode of one cell to the anode of another cell is made on the same plane. The interconnect was fabricated by sputtering a layer of copper over a layer of gold on the top of the silicon wafer. Silicon dioxide was deposited on the silicon wafer adjacent to the copper layer to prevent short-circuiting between the twin cells. The feed holes and channels in the silicon wafers were prepared by anisotropic silicon etching from the back and front of the wafer with silicon dioxide acting as intrinsic etch-stop layer. Operating on dry H-2/O-2 at 25 degreesC and atmospheric pressure, the measured peak power density was 190.4 mW/cm(2) at 270 mA/cm(2) for the miniature twin-fuel-cell using a Nafion 112 membrane. Based on the polarization curves of the twin-fuel-cell and the two single cells, the interconnect resistance between the twin cells was calculated to be in the range from 0.0113 Omega (at 10 mA/cm(2)) to 0.0150 Omega (at 300 mA/cm(2)), which is relatively low. (C) 2003 Elsevier Science Ltd. All rights reserved.

Experimental Investigation of Performance of a Miniature Direct Methanol Fuel Cell in Short-Term Microgravity

Experimental study of a liquid fed direct methanol fuel cell has been conducted in different gravity environments. A small single cell with 5 cm x 5 cm active area has single serpentine channel on the graphite cathode polar plate and 11 parallel straight channels on the graphite anode flow bed. Cell voltage and current have been measured and two-phase flow in anode channels has been in situ visually observed. The experimental results indicate that the effect of gravity on power performance of the direct methanol fuel cell is large when the concentration polarization governs fuel cells operation. Gravitational effect becomes larger at higher current density. Increasing methanol feeding molarity is conducive to weaken the influence of gravity on performance of liquid fed direct methanol fuel cells. Increasing feeding flow rate of methanol solution from 6 to 15 ml/min could reduce the size of carbon dioxide bubbles, while the influence of gravity still exist. Transport phenomena inside direct methanol fuel cells in microgravity is also analyzed and discussed.

Two-Phase Flow In Anode Flow Field Of A Small Direct Methanol Fuel Cell In Different Gravities

An in-situ visualization of two-phase flow inside anode flow bed of a small liquid fed direct methanol fuel cells in normal and reduced gravity has been conducted in a drop tower. The anode flow bed consists of 11 parallel straight channels. The length, width and depth of single channel, which had rectangular cross section, are 48.0, 2.5 and 2.0 mm, respectively. The rib width was 2.0 mm. The experimental results indicated that when the fuel cell orientation is vertical, two-phase flow pattern in anode channels can evolve from bubbly flow in normal gravity into slug flow in microgravity. The size of bubbles in the reduced gravity is also bigger. In microgravity, the bubbles rising speed in vertical channels is obviously slower than that in normal gravity. When the fuel cell orientation is horizontal, the slug flow in the reduced gravity has almost the same characteristic with that in normal gravity. It implies that the effect of gravity on two-phase flow is small and the bubbles removal is governed by viscous drag. When the gas slugs or gas columns occupy channels, the performance of liquid fed direct methanol fuel cells is failing rapidly. It infers that in long-term microgravity, flow bed and operating condition should be optimized to avoid concentration polarization of fuel cells.

低甲醇透过直接甲醇燃料电池(Low Methanol Crossover Direct Methanol Fuel Cell)

直接甲醇燃料电池与间接甲醇燃料电池相比，体积更小，重量更轻，因此在一些领域有诱人的应用前景。但是，在它们实际应用之前，必须解决一些具体的技术难题。目前，甲醇从阳极透过到阴极是影响电池性能的主要难题之一，另外，催化剂和电极的制备方法也对电池的性能有重要的影响。本论文的主要目的在于研制低甲醇透过直接甲醇燃料电池并有效地提高电池的性能。为了减小甲醇在Nafion117膜中的透过，提出并研制了铭纳米粒子修饰的Nafion复合膜，该方法包括与[Pd(NH_4)_4]~(2+)离子的离子交换过程和化学还原过程。研究了一种制备高分散性铂基催化剂的方法。另外我们还研究并分析了不同的电池运行参数，例如温度、甲醇浓度等，刘一电池性能和甲醇透过的影响。主要结果如下：1.采用离子交换还原法在Nafionll7膜内部沉积纳米把粒子，制备成高聚物电解质复合膜。研究了镀把前后Nafion膜表面形态、甲醇透过和膜的电导的变化和对直接甲醇燃料电池的性能的影响等。由于把纳米粒子阻碍了甲醇透过，同时，由于它对氢离子的强吸引力，不但不对氢离子的透过产生影响，而且还提高了膜佩狗电导。所以镀把后电解质膜的甲醇透过减少，膜电导增加，无论在低电流密度区还是在高电流密度区，电池性能都有效地提高。2.研究了一种制备高分散性铂基催化剂的新方法一预沉淀还原法。并采用TEM,XRD和电化学等技术来表征催化剂中铂的粒径、晶态结构和催化活性：与传统的化学还原法相比，因为该方法在化学还原过程中反应物与载体的作用力得到增强，所以采用该方法制备的催化剂铂分散性更好、晶态结构更低、粒径更小并且催化活性更好。该方法在直接甲醇燃料一电池中有应用价值。3.研究并分析了不同的电池运行参数，例如温度、甲醇浓度等，对电池性能和甲醇透过的影响。研究发现当电池运行温度增加时，电池性能提高，甲醇透过增加；甲醇浓度增加时，甲醇透过增加，但是，甲醇浓度对电池性能有不同的影响，在低甲醇浓度区，甲醇浓度增加，电池性能提高；在高甲醇浓度区，甲醇浓度增加，电池性能降低；存在一个最佳甲醇浓度，在该甲醇浓度的条件下，电池的性能最高。实验结果为：采用Nafion117膜时，电池的最佳甲醇浓度为2. 0 mol/L，采用镀把Nafion117膜时，电池的最佳甲醇浓度高于4.0 mol/Lo

CO selective oxidation in a microchannel reactor for PEM fuel cell

It is indispensable to remove CO at the level of less than 50ppm in H-2-rich feed gas for the proton exchange membrane (PEM) fuel cells. In this paper, catalyst with high activity and selectivity, and a microchannel reactor for CO preferential oxidation (PROX) have been developed. The results indicated that potassium on supported Rh metal catalysts had a promoting effect in the CO selective catalytic oxidation under H-2-rich stream, and microchannel reactor has an excellent ability to use in on-board hydrogen generation system. CO conversion keeps at high levels even at a very high GHSV as 500 000 h(-1), so, miniaturization of hydrogen generation system can be achieved by using the microchannel reactor. (C) 2004 Elsevier B.V. All rights reserved.

Preparation and characterization of PtSn/C anode electrocatalysts for direct ethanol fuel cell

Highly active PtSn/C catalyst was prepared by a polyol method. The catalyst was reduced in H-2/Ar atmosphere at 600 degreesC for 2 h in order to obtain different metallic phase. TEM images show uniform dispersion of spherical metal nanoparticles with average diameters of 1.8 and 3.9 nm for the as-prepared and treated catalysts, respectively. UV-vis spectrophotometry is employed to monitor the preparation process and the results indicate that Pt-Sn complex formed once the precursors of Pt and Sn were mixed together. The structure properties of the samples were characterized using X-ray diffraction. The results show that after reduction, the catalyst tends to form PtSn alloy. TPR experiment results show that Sn exists in multivalent state in the as-prepared sample while only zero-valence Sn was detected in the treated sample, while it could not be excluded that the multivalent tin existed in the treated sample. Cyclic voltammetry (CV) technique and single direct ethanol fuel cell (DEFC) tests indicate that the as-prepared catalyst possesses superior catalytic activity for ethanol oxidation to the treated sample. The results suggest that Pt and multivalent Sn are the active species for ethanol oxidation. (C) 2004 Elsevier B.V. All rights reserved.

Performance improvement in direct methanol fuel cell cathode using high mesoporous area catalyst support

Black Pearls 2000 (designated as BP- 2000) and Vulcan XC-72 (designated as XC-72) carbon blacks were chosen as supports to prepare 40 wt % (the targeted value) Pt/C catalysts by a modified polyol process. The carbon blacks were characterized by N-2 adsorption and Fourier tranform infrared spectroscopy. The prepared catalysts were characterized by inductively coupled plasma atomic emission spectroscopy, transmission electron microscopy, scanning electron microscopy (SEM), in situ cyclic voltammetry, and current-voltage curves. On BP- 2000, Pt nanoparticles were larger in size and more unevenly distributed than on XC-72. It was observed by SEM that the corresponding catalyst layer on BP- 2000 was thicker than that of XC-72 based catalyst at almost the identical catalyst loading. And the BP- 2000 supported catalyst gave a better single cell performance at high current densities. These results suggest that the performance improvement is due to the enhanced oxygen diffusion and water removal capability when BP- 2000 is used as cathode catalyst support. (C) 2004 The Electrochemical Society.

Novel synthesis of highly active Pt/C cathode electrocatalyst for direct methanol fuel cell

A 40 wt% Pt/C cathode electrocatalyst with controlled Pt particle size of similar to 2.9 nm showing better performance than commercial catalyst for direct methanol fuel cell was prepared by a polyol process with water but without using stabilizing agent.

Composite electrode for unitized regenerative proton exchange membrane fuel cell with improved cycle life

To improve the cycle life of unitized regenerative fuel cells (URFCs), an electrode with a composite structure has been developed. The cycle life and polarization curves for both fuel cell and electrolysis modes of URFC operation were investigated. The cycle life of URFCs was improved considerably and the performance was fairly constant during 25 cycles, which illustrates that the composite electrode is effective in sustaining the cyclic performance of URFCs. It shows the URFCs with such an electrode structure are promising for practical applications. (C) 2004 The Electrochemical Society.