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Operating Characteristics of Proton-Exchange-Membrane (PEM) Fuel Cells 398 1.1. Fuel Cell Description The basic physical structure or building block of a fuel cell consists of an electrolyte layer in contact with a porous anode and cathode on either side. A schematic representation of a fuel cell with the reactant/product gases and the ion conduction flow directions through the cell is shown in Fig. 1. Figure 1: Schematic of an individual fuel cell In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment and an oxidant (i.e., oxygen from air) is fed continuously to the cathode (positive electrode) compartment; the electrochemical reactions take place at the electrodes to produce an electric current. A fuel cell, although having components and characteristics similar to those of a typical battery, differs in several respects. The battery is an energy storage device. The maximum energy available is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the chemical reactants are consumed (i.e., discharged). In a secondary battery, the reactants are regenerated by recharging, which involves putting energy into the battery from an external source. The fuel cell, on the other hand, is an energy conversion device that theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes (EG&G Services, 2000). 2.0. Fabrication of Al/xxxx/Al and Al/xxxx/Cu Fuel cells (/xxx/=-C/Pt-PEM-C/Pt-) The search for alternative electrodes apart from Pt is aiming to reduce the cost of fabrication of the cell. Carbon paper impregnated with platinum powder (C/Pt) was chosen as the ideal candidate. This C/Pt is really brittle and cannot support external connections; therefore, aluminum mesh conductors were used for the preliminary fabrication to form an Al/xxx/Al fuel cell. For long term use of the cell, the oxygen side will form aluminum oxide and thus increase the resistance of the electrode. The oxygen side was therefore changed to a copper conductor, forming a Al/xxx/Cu fuel cell, as copper may not react directly with oxygen at the usual working temperature.
399 R. Y. Tamakloe, K. Singh and Clovis A. Linkous 2.1. Preparation of PEM film The PEM film was prepared for catalyst application by dipping the membrane in six different heated solutions in glass beakers. The solutions were all held between 80ºC and 85ºC by immersing the beakers in an ultrasonic washer containing heated water. The bath is heated by means of an immersion heater. Each beaker held the PEM for 30 minutes in sequence. The sequence of beakers used was set up as follows: Beaker 1 –> 150 ml of distilled water to hydrate the membrane and dissolve surface contaminants. Beaker 2 -> 150 ml of 3% hydrogen peroxide solution to remove organic contaminants from the PEM surface. Beaker 3 -> 150 ml of sulfuric acid (new battery electrolyte) to remove metal ion contaminants from the PEM surface, and sulfonate the PEM surface. Beaker 4 -> 150 ml of distilled water to rinse sulfuric acid from the surface and hydrate the PEM. Beaker 5 -> 150 ml distilled water to repeat rinse. Beaker 6 -> 150 ml distilled water repeat rinse. A mercury thermometer was used for checking the beaker bath temperature. From time to time, more water had to be added to the bath surrounding the beakers, due to evaporation. The ultrasonic action quickens the washing process. After the PEM disk was dipped in each of the six hot solution beakers for 30 minutes, it was then air-dried in a clean place (Pyle W., et al, 2003). 2.2. Hot-Pressing the Sandwich Together Two sheets, 3 cm by 5.5 cm were cut from the platinum-carbon impregnated sheet to serve as electrodes. A membrane of 5 cm by 7 cm was also cut. A hot press was made using a workshop vice and two metallic containers, each 200 cm 3 capable of holding the immersion heater, shown below. Since water is the heating medium the highest temperature attained was 100ºC. First, the surfaces of the heating containers in contact with the catalyst were coated with graphite from a 2B pencil. The three layers, catalyst-PEM-catalyst, of the sandwich were then set in between the two heating containers. The layers were carefully aligned so that the smaller catalyst disks were centered above and below the larger PEM disk. At this time the heaters were turned off and the plates let cool to room temperature. Next, the two temperature controllers were initiated and the sandwich was taken up to 100ºC for one hour. Once the heating plates and the sandwich reached 100ºC, pressure was applied by tightening the vice. After this time the heater was turned off and the plates and sandwich cooled to room temperature. The heating containers were opened, and the finished fuel cell sandwich was removed using the tweezers. The PEM sandwich was placed between two aluminum meshes to enable external electrical connection, thus forming the Al/xxx/Al cell. The five-layered sandwich was then placed in between machined transparent Acrylic blocks. The indenture retains the passing fuel for enough time to allow for absorption or consumption. An Al/xxx/Cu was also fabricated and tested. 3.0. Results The measured values of current and voltages for Al/xxx/Al and Al/xxx/Cu Cells are presented in Table 1 and Table 2 respectively.
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