CHEMISTRY: Fuels cells are electrochemical devices which combine hydrogen and oxygen to produce heat, electricity and water. Fuel cells convert a fuel's chemical energy directly to electrical energy with high efficiency. They are made up of an anode, electrolyte, cathode and a catalyst. Pressurized hydrogen gas is feed into the anode side of the fuel cell where the catalyst in the form of platinum splits it into hydrogen ions and electrons. The electrons are then put through a circuit from anode to cathode producing electricity. Oxygen gases are then feed through the cathode side of the fuel cell where the platinum catalyst breaks it up into oxygen atoms. These oxygen atoms then react with the hydrogen ions to form water. In principle, a fuel cell differs from primary and secondary galvanic cells in several ways. The major distinction is that the fuel cells store neither their reactants nor the products. Their action is to supply electricity at a constant rate as reactants are fed into them and products are removed. The fuel cell relies on a basic oxidation/reduction reaction, as with a battery, but the reaction takes place on the fuel rather than the electrodes. An ion-conducting membrane separates the two, allowing the reaction to take place without affecting the electrodes. In this example, the only waste product is water vapor and/or liquid water. The most successful fuel cell so far developed has been based on the combination of hydrogen and oxygen. The net reaction is the same as that for the combustion of hydrogen. 2H2(g) + O2(g) 2H2O(l) ( Add diagram of 4.16 here ) Figure 4.16 shows one form of hydrogen-oxygen furl cell. Hydrogen gas is supplied to the anode chamber and oxygen to the cathode chamber. The gases diffuse through the electrodes which are porous metals such as platinum or nickel and act as a catalyst. The gasses react with the electrolyte which may be acidic or alkaline. For an acidic electrolyte, (for eg Polymer Electrolyte Membrane fuel cells, electrolytes contains sulfuric acid) (the reactions are given by the following equations: Anode: H2(g) 2H(aq) + 2e– Cathode: O2(g) + 4H (aq) + 4e– 2H2O(l) If the electrolyte is alkaline (NaOH), the reactions are: Anode: H2(g) + 2OH-(aq) 2H2O(l) + 2e– Cathode O2(g) + 2H2O(l) + 4e- 4OH-(aq) In both types of fuel cell the overall equation is the same: 2H2(g) + O2(g) 2H2O(l) All electrochemical reactions consist of two separate reactions: an oxidation half-reaction occurring at the anode and a reduction half-reaction occurring at the cathode. One of the more common types of fuel cell is the Proton exchange membrane fuel cell or the Polymer Electrolyte Membrane (PEM) fuel cell. This is the type of fuel cell that will end up powering cars, buses and maybe even your house. The PEM fuel cell consists of an electrolyte membrane sandwiched between an anode (negative electrode) and a cathode (positive electrode).The PEM is a thin, solid, organic compound, typically the consistency of plastic wrap and about as thick as 2 to 7 sheets of paper. This membrane functions as an electrolyte: a substance that conducts charged ions (in this case protons), but does not conduct electrons. This allows the solution to conduct electricity. This membrane must be kept moist to conduct particles through it. The anode is the electrode at which oxidation (loss of electrons) takes place. In a fuel cell, the anode is electrically negative. The anode is composed of platinum particles uniformly supported on carbon particles. The platinum acts as a catalyst, increasing the rate of the oxidation process. The anode is porous so that hydrogen can pass through it. The cathode is the electrode at which reduction (gaining of electrons) takes place. In a fuel cell, the cathode is electrically positive. The cathode is composed of platinum particles uniformly supported on carbon particles. The platinum acts as a catalyst, increasing the rate of the reduction process. The cathode is porous so that oxygen can pass through it. The electrolyte is fluorocarbon polymer with sulfonic acid functional groups; the membrane provides a conducting path for the hydrogen ions. Figure 1. The parts of a PEM fuel cell Flow plates in the proton exchange membrane fuel cell serve several important functions: 1. they channel hydrogen and oxygen to the electrodes, 2. they channel water and heat away from the fuel cell, and 3. they conduct electrons from the anode to the electrical circuit and from the circuit back to the cathode. Hydrogen fuel (H2) is channeled to the anode, where the catalyst separates the hydrogen's negatively charged electrons from the positively charged protons. The membrane allows the positively charged protons to pass through to the cathode, but not the negatively charged electrons. The negatively charged electrons must flow around the membrane through an external circuit. This flow of electrons forms an electrical current. At the cathode, the negatively charged electrons and positively charged hydrogen ions (protons) combine with oxygen to form water (H20) and heat. The amount of power produced by a fuel cell depends on several factors, including fuel cell type, cell size, temperature at which it operates, and pressure at which the gases are supplied to the cell. A single fuel cell produces less than 1.16 volts - barely enough electricity for even the smallest applications. To increase the amount of electricity generated, individual fuel cells are combined in series, into a fuel cell "stack." A typical fuel cell stack may consist of hundreds of fuel cells. In this type of fuel cell (PEM), the membrane must be hydrated, requiring water to be evaporated at precisely the same rate that it is produced. If water is evaporated too quickly, the membrane dries, resistance across it increases, and eventually it will crack, creating a gas "short circuit" where hydrogen and oxygen combine directly, generating heat that will damage the fuel cell. If the water is evaporated too slowly, the electrodes will flood, preventing the reactants from reaching the catalyst and stopping the reaction. Methods to manage water in cells are being developed by fuel cell companies and academic research labs. The same temperature in a fuel must be maintained throughout the cell in order to prevent destruction of the cell through thermal loading.
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