Background

The demand for power sources with superior performance has increased due to the rapid growth of the portable electronics market. Moreover, technology progresses over time have enabled developments in electronics to move us into a microelectronics age. When devices make smaller, new functionalities are added. However, power consumption rises alarmingly.

Therefore, the power sources must produce adequate power output while at the same time maintaining criteria such as a very small volume and lightweight packaging. Primary and secondary batteries have been the energy storage solution for these devices. However, batteries are a chemical process, but they do not last long enough. Until recently, one innovative way is to utilize the hydrogen and feed it to a fuel cell, producing electricity.

Kundu et al. [1] presented fuel cells promise to provide higher power density and longer durability than batteries (Table 1-1). In fuel cell, electricity and water are produced from hydrogen and oxygen in an electrochemical reaction. Besides, fuel cells are quiet, efficient and convert energy electrochemically rather than mechanically. Therefore, fuel cells are widely regarded as the most promising energy storage devices for mobiles, laptops, and personal digital assistants (PDA) in the 21^st century due to their properties of high energy density, low noise, and low pollution.

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into electrons through a direct fuel cell system. In direct fuel cell system, the main studies focused on DMFC, but DMFC has still issues due to a low rate of oxidation and a high crossover rate. The PEMFC is highly attractive for both portable and stationary application due to its high operating efficiency and environmental friendliness. This gives the PEMFCs great flexibility of a wide range of applications, Fig 1-1. PEMFCs have been proposed as battery replacements. Ball and Wietschl [2] presentedapplications of PEMFC in portable power sources need to carry enough hydrogen fuel. However, the hydrogen storage problem is still difficult to overcome. To solve this technical difficulty, one possible solution is to employ a reformer. Indirect energy conversion systems is to first reform methanol, ethanol, gasoline followed by feeding the reformate gas into miniature PEM fuel cell. The reformers often have characteristic dimensions, such as channel gaps, which are on the micro-scale (typically<1000μm) or meso-scale (1000μm to a few centimeters) and will be referred to in this article as micro-reformers. These features are significantly smaller than many conventional reformers, and they can significantly enhance mass and heat transfer rates.

Therefore, micro-reformers are being developed for using with the miniature PEMFCs, to overcome the high risk of carrying a large quantity of hydrogen.

In reforming systems, the electronic energy system is generated using concentrated hydrogen produced by reforming from a fuel such as methanol. Holladay et al. [3] showed methanol is an attractive fuel because of its low reforming temperatures, good miscibility with water and low content of sulfur compounds. From the technological point of view, methanol clearly has distinct advantages as a fuel for fuel cell applications. First, methanol is liquid at atmospheric conditions and has a high hydrogen-to-carbon ratio relative to gasoline. Secondly, it can be reformed to hydrogen at much lower temperatures (200-300℃), and is more efficient as compared to gasoline and methane (700-800℃). Finally, methanol is an environmentally friendly fuel, as it is readily biodegradable in air, soil and water. Therefore, methanol clearly

has distinct advantages as a fuel for fuel cell applications due to its higher hydrogen-to-carbon ratio, low reforming temperature and greater environmental friendliness.

Holladay et al. [4] presented the majority of reformers currently being developed are designed to produce hydrogen from methanol. Three basic reforming technologies are steam reforming, partial oxidation, and autothermal reforming. Table 1-2 summarizes the advantages and challenges of each of these processes. Endothermic methanol steam reforming requires an external heat source. Partial oxidation is an alternative to steam reforming, where the reaction heat is provided by the partial combustion of the methanol with oxygen. The autothermal reforming process is a thermally neutral hybrid of steam reforming and partial oxidation. The partial oxidation and autothermal processes do not require an external heat source, but an expensive and complex oxygen separation unit is needed. Because of the steam reforming produces higher yields of hydrogen than autothermal reforming and partial oxidation of hydrocarbon fuels. The requirement of an external heat source can be addressed through the advanced heat and mass transfer provided by combustors. Hence, steam reforming is generally the preferred process for hydrogen production. A portable hydrogen production unit based on methanol steam reforming would be simpler and less costly than other alternatives.

The plate reformers have better performance than cylindrical reformers due to better heat and mass transfer is presented by Kolb et al. [5]. The plate methanol steam reformers are regarded as being micro-structured coated wall reformers, when patterning channels or similar fluid paths with a size below 1 mm. The advantages of micro-structured reformers enhanced heat and mass transfer are observed. Micro-structured reformers are much more suitable for

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multi-channel reformers work under laminar flow conditions demonstrating low pressure drop compared to randomly packed beds, and allow easy thermal integration of the processes involved. From the stated above, the plate methanol steam micro-reformer with small PEMFC has become a potential candidate for portable electronic products in the near future.

Fig. 1-2 shows a photograph of the micro-reformer and micro fuel cell, while Fig 1-3 shows the schematic of a system that consists of a methanol steam reformer and a fuel cell.

First, methanol is fed with water and is heated by the vaporizer. The methanol is reformed by the reforming catalyst to generate hydrogen in the steam reformer. To supply heat to the steam reformer, part of methanol be fed to the combustor that generates sufficient amount of heat to sustain the steam reforming of methanol. For PEM fuel cells, the carbon monoxide levels need to be below 10 ppm. Therefore, a final polishing step (preferential oxidizer (PrOx) reactor) is used. Fig. 1-4 shows the photograph of the plate methanol steam micro-reformer including the etched glass wafers, the cross-section of the reformer, and the complete micro-reformer. Fig. 1-5 presents a simplified cross-sectional diagram of the plate methanol steam micro-reformer. The micro-reformer is composed of four units with vaporizers, catalytic combustor, and a CO remover. The functions are separated into two reaction systems.

One reaction system is the hydrogen production system in which the methanol aqueous solution is fed and then vaporized at vaporizer 1, it is reformed to H2 with CO2 and a trace of CO at the methanol reformer; and finally the CO is preferentially oxidized at the CO remover.

The other reaction system, the catalytic combustion system, is used to supply heat to the hydrogen production system. In this system, the methanol is vaporized at vaporizer 2 and then burned at the catalytic combustor. Fig. 1-6 shows a schematic of these two reaction systems.

Consequently, safety issues, storage problems, and size or portability considerations make pure hydrogen feeding relatively difficult for electronic equipment applications. The combination of a methanol reformer with a PEMFC overcomes the high risk involved in

carrying a large quantity of hydrogen, and is thus a promising choice for miniaturized portable electronic systems, and could soon become the new choice for portable fuel cell applications.