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University of Texas at Arlington MAE 3183 Measurements II Laboratory Lab Manual Introduction to Fuel Cell Applications Measurements II Lab – 3183University of Texas at Arlington MAE 3183 Measurements II Laboratory Fuel Cell 2/12 Introduction Recent political and environmental concerns have increased our need to find alternative sources of energy and decrease our dependence on fossil fuels. Fuel cells are viewed as one of the most promising solutions. Fuel cells potentially could run our automobiles, our laptops, or even our cell phones. They are currently used primarily as power backup systems for large facilities (such as DFW airport). Fuel cells produce electricity by combining hydrogen and oxygen to make water. While Hydrogen is a very powerful clean fuel, it is not technically a source. Hydrogen does not exist in large quantities naturally; instead it is trapped in other molecules such as water or alcohol. Because of this, hydrogen must be produced by another energy source using either electrolysis, or a reformer. A reformer converts hydrocarbon or alcohol fuels into hydrogen. This process is less efficient but greatly increases fuel cell’s viability. Fuel cells have the ability to operate with as high as 80% efficiency when pure hydrogen is supplied. When a reformer is used to create the hydrogen the total efficiency drops to between 30-40%, which is still an improvement over conventional IC engines. It is still uncertain whether hydrogen fuel cells will be a long term, viable solution only further research and development will make this clear. Theory Hydrogen and oxygen are combined in a fuel cell to produce electrical energy. The main components of a basic PEM (proton exchange membrane) fuel cell that facilitates this process are shown in Figure 1 below. . Figure 1. Example of Fuel Cell The outside layers of a fuel cell are the anode plate, with a positive charge, and the cathode plate, with a negative charge. On the anode side the H2 is forced through the catalyst, which separatesUniversity of Texas at Arlington MAE 3183 Measurements II Laboratory Fuel Cell 3/12 it into H+ ions and electrons e-. The electrons flow through the anode to provide electrical current for a circuit while the protons move through the proton exchange membrane to the cathode. The proton exchange membrane blocks electrons and only allows positively charged ions to flow through it. On the cathode side the O2 undergoes the same process as the hydrogen. The O2 is forced through the catalyst where it is separated into two oxygen atoms with a very strong negative charge. The two negative oxygen atoms attract the two positive hydrogen atoms through the membrane and combine. Two electrons from the circuit are added as well to form water and heat. The list of chemical reactions that take place are as follows Chemistry of a Fuel Cell Anode side: 2H2 => 4H+ + 4e- Cathode side: O2 + 4H+ + 4e- => 2H2O Net reaction: 2H2 + O2 => 2H2O The reverse of this reaction is also possible in what is called electrolysis. What limits the current? The anode reaction releases energy, but not at an unlimited rate. For the energy to be released it must overcome the activation energy. The energy is released when the hydrogen is in contact with the catalyst. There are three methods for increasing the speed of the reaction: the use of catalyst, raising the temperature, and increasing the electrode area. Chemistry of the Hydrogen Fuel Cell For some electrical systems it is easy to define what form of energy is converted to electricity. For example, a wind generator converts the kinetic energy of wind as it moves the blades into electricity. The electrical power and energy output are easily calculated by, IVPower * tIVEnergy ** where V is voltage and I is current. In fuel cells the energy is harder to visualize and not easily calculated. The energy for fuel cells is calculated by using the energy of chemical input and output. In the hydrogen fuel cell the chemical energy comes from H2, O2 and H2O.University of Texas at Arlington MAE 3183 Measurements II Laboratory Fuel Cell 4/12 In this lab the source of H2 and O2 comes from the electrolysis of water. When current is passed through water the energy necessary to dissociate the H2O into H2 and O2 is provided. Chemical energy is not defined simply, but in terms such as enthalpy, Helmholtz function and Gibbs free energy. Gibbs free energy equation is important for fuel cells. It can be defined as the energy to do external work while neglecting work done by changes in pressure and or volume. Chemical energy is really energy storage and acts like potential energy. When defining chemical reactions the zero energy point is normally defined as a pure element in normal state at standard temperature and pressure. The change in Gibbs free energy formation, fG, gives us the energy being released. Gibbs free energy is defined by the difference between the Gibbs free energy of the product and reactants. tsreacfproductsffGGGtan__ For convenience, Gibbs free energy formation is set in per mole form. tsreacfproductsffgggtan__ Looking at the basic hydrogen/oxygen fuel cell. OHOH22212 The Gibbs free energy will become. 222)()()(21OfHfOHffgggg Equation 5 looks simple and straightforward, but the reaction is highly dependent on changes in temperature that can cause the reaction to be either in liquid or gas state. In the hydrogen fuel cell two electrons pass around the external circuit, which produces a water molecule for every 2 molecules of hydrogen consumed. For each mole of hydrogen used, 2N electrons pass around the external circuit. FEvolatageechdoneworkElecticalnumbersAvogadroNelectrononeonechetconsFardayFFNe2*arg_'___argtan_22University of Texas at Arlington MAE 3183 Measurements II Laboratory Fuel Cell 5/12 If the system is assumed as a reversible process then the electrical work done will equal to Gibbs free energy released, fg . Fuel Cell Output and Efficiency In this lab one of the things that the student will determine is how changing the input voltage will affect the efficiency of hydrogen production. The equation that describes this relationship is, volumeH2 voltageV K VIt  where K is a gauge of efficiency for the cell measuring H2/energy produced, I is the current and t is time. This equation can be used to describe


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