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Vaporized Methanol Fuel Cells Essay

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In 1839 Sir William Grove invented the fuel cell. He produced electricity by combining hydrogen and oxygen at electrodes that were separated by an aqueous liquid acid electrolyte. [1] Subsequently, throughout the 19th and 20th centuries, a variety of researchers investigated fuel cell reactions for the conversion of chemical energy into electrical energy. [2] The first practical application of fuel cells occurred during the Apollo space program from 1960 to 1965. [1] Those fuel cells provided both drinking water and electricity.

In 1970 Kordesch built a car that operated using the combination of an alkaline fuel cell and a lead acid battery.

Recently environmental pollution issues have been the driving force for the development of fuel cells. [3] Fuel Cells – Definition, Composition, and Operating Principles A fuel cell is a device that generates electricity and produces heat from an electrochemical reaction using suitable catalysts. The most common fuel used in fuel cells is hydrogen. It is used in combination with oxygen to produce electricity based on the following overall chemical reaction [4]:

H2 + 1/2 O2 > H2O In general, a fuel cell assembly can be divided into five layers as shown in figure 1.

On each side of the five layers there are flow channels that are usually machined into solid graphite plates. The anode side of the fuel cell includes the anode-backing layer (anode diffusion layer) and the anode catalyst layer. The cathode side includes the cathode backing layer (cathode diffusion layer) and the cathode catalyst layer. The electrolyte layer separates the anode side from the cathode side of the fuel cell.

If the electrolyte is a membrane, the five layers are called a Membrane Electrode Assembly (MEA) [5]. Hydrogen or a hydrocarbon-containing fuel such as methanol is supplied to the anode side and electrochemically oxidized. Oxygen or air is supplied to the cathode side and electrochemically reduced. The anode reaction that occurs at the anode side (anode catalyst layer) produces protons and electrons. [6] Electrons flow through (a) the electronically conducting anode material, to (b) the external circuit that contains an electrical load, to (c) the cathode side.

The protons travel through a specific electrolyte (such as a membrane), to (d) meet the electrons and oxygen at the cathode side, where the cathode reaction occurs [1]. Each of the fuel cell layers has individual characteristics. The backing layers in the anode and the cathode have several functions. They are the thickest of the layers and they provide the mechanical strength for the MEA that includes the other layers of the fuel cell. They are made from either carbon paper or carbon cloth composed of fibers that create a porous structure.

The channels containing the flowing reactants are interspersed with “lands”, or regions of the backing layers that are not exposed to reactants. The porous structures of the backing layers permit the reactants to diffuse in a direction perpendicular to the catalyst layers and thereby provide a more even concentration of reactants at the interface between the catalyst layer and the backing layer [4]. The carbon backing layers also provide both good electronic conductivity for the electrons that participate in the electrochemical reactions and good heat conductivity for the heat released when the electrochemical reactions that occur.

Often the backing layers are made hydrophobic by using Teflon, in order to avoid flooding that comes from water produced by the reaction. The need for hydrophobicity at the cathode is generally greater than that at the anode, as most of product water is produced at the cathode [7]. The next layer is the anode or cathode catalyst layer that is a thin porous solid. It is often composed of a precious metal (Pt, Ru) on a carbon support. The carbon support provides a large surface area on which the precious metal can be dispersed. This permits a large surface area of the precious metal to contact the reactants.

The last layer that separates the two sides is an electrolyte. The type of electrolyte is usually the characteristic that distinguishes the different fuel cell types. The purpose of the electrolyte is to transport ionic species (eg. H+) from one side to the other [1]. Advantages of Fuel Cell Compared to Combustion In principle fuel cells processes can have substantially greater energy efficiencies than combustion processes. During combustion a fuel is burned in the presence of an oxygen-containing gas (usually air) to produce heat (eg. burning gasoline inside the internal combustion engine of a car) [2].

The durability of materials at high temperatures limits the theoretical energy efficiencies of combustion processes to approximately 60%. The energy efficiencies obtained in practice are normally in the 30-40 % range. In fuel cells, the fuel reacts electrochemically to produce electricity [8]. Fuel cells are not required to operate at temperatures as high as those in combustion processes. In contrast to combustion processes, the theoretical energy efficiencies of fuel cells can be above 90%. One of the goals of current fuel cell research is to demonstrate fuel cell energy efficiencies that exceed those of combustion processes [8].

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