Phosphoric Acid Fuel Cell - Essay Example

Fuel Cells Chemistry Fuel cells Introduction In science, a fuel cell is defined as a device that can convert chemical energy using oxidizing agents. Hydrogen acts as the basic fuel, although fuel cells require oxygen as well. The electricity produced through a chemical reaction does not involve combustion. Fuel cells require a continuous supply of oxygen and air for them to operate. In addition, for electricity to be produced, carbonate fuel cells produce hydrogen directly from a source of fuel. All fuel cells contain an anode, the positive electrode, a cathode-the negative electrode and an electrolyte-a liquid that allows charges to move between the two sides of the fuel cell. The electrolyte has to be in liquid form to reduce hydrogen molecule leakage via the container’s walls. There are different fuel cells that are grouped according to the electrolyte they use. Hey include phosphoric acid fuel cells, high-temperature fuel cells, and hydrogen-oxygen fuel cells. The chemical reactions in a fuel cell occur at the interfaces of the three different segments resulting in consumption of fuel, water, or carbon dioxide in created and an electric current that can be used to power vehicles is created (Scherer 33). The anode reaction involves the oxidation of the fuel by a catalyst. All electrolytes are designed such that charged icons can pass through them but electrons cannot. The free electrons create the electric current while the ions travel through the electrolyte to the cathode. Upon reaching the cathode, the electrons and ions are reunited and they react with a third chemical preferably oxygen to form water or create hydrogen peroxide. The anode is usually made up of very fine platinum powder. The cathode catalyst turns the ions into waste chemicals like water and carbon dioxide, and it is usually made up of nickel but it can also be a non-platinum material (Barbir 12). Most fuel cells produce a voltage from 0.6V to 0.7V, but this voltage decreases as current increases due to activation loss, ohmic loss, and mass transport loss. These are voltage loss indicators. The various types of fuel cells mentioned earlier are discusses independently below. The fuel cell is designed such that a proton-conducting polymer membrane separates the anode and the cathode. On the anode electrode, hydrogen diffuses to the anode catalyst, and it later dissociates into protons and electrons. The protons will react with oxygen molecules at the cathode side, infusing via the polymer electrolyte membrane as well as the electrons coming through the outer circuit to make water molecules. The protons are conducted through the membrane to the cathode while the electrons travel through an external circuit since the covering is insulated electrically. The reaction on the cathode electrode involves oxygen molecules and both the electrons and the protons (Scherer 35). Phosphoric acid fuel cell In this type of cell, hydrogen ions are passes to the cathode using phosphoric acid. These cells usually work at moderate temperatures. The relatively high temperatures cause a heat and energy loss if the heat is not removed and properly used. The heat can be turned into a useful source of energy for air conditioners and other thermal energy consuming systems (Onovwiona ans Ugursal 389). This property makes the phosphoric acid fuel cell more useful and improves the efficiency to about 80%. The anode electrode in this cell uses a platinum catalyst that accelerates conversion of hydrogen into free ions and electrons are produced (Scherer 34). The electrons travel from the anode to the cathode cell through an external electrical circuit since phosphoric acid is a non-conducting electrolyte. This cell, however, has one major disadvantage, and that is the use of an acidic electrolyte. Phosphoric acid is highly corrosive and leads to oxidation of the components which are in contact with it. High temperature fuel cells These are unique cells that operate at very high temperatures of about 800-1000C and can be run by using several fuels including natural gas. In these cells, the negatively charged ions are the ones that travel from the cathode to the anode, unlike the other cells. In definition, a cathode is that negatively charged electrode that attracts positive charges, where the current flows out, while an anode is the source of an electron acceptor or a terminal in which current flows from outside. In this arrangement, the cathode acts as the positive electrode and the anode as the negative electrode (Onovwlona and Urgusal 388). Oxygen gas enters the cell through the cathode where it absorbs electrons to form oxygen ions. The ions then travel trough the electrolyte to the anode to react with the hydrogen gas. This reaction at the anode produces electricity and water as by-products. The chemical equations for a high temperature cell are shown below: Anode reaction: 2H2+2O2=2H2O+4e- Cathode reaction: O2+4e=2O2 Overall reaction: 2H2+O2=2H2O These fuel cells face some challenges resulting from the high operating temperatures. One of the challenges is that there is “carbon cooking.” This is the buildup of carbon dust on the anode which slows down the internal reforming process. This problem is, however being addressed by researchers in the University of Pennsylvia. Another problem with these high-temperature cells is the slow startup time. Hydrogen-oxygen fuel cells This is a simple type of a cell which uses two porous carbon electrodes impregnated with a suitable catalyst such as platinum. The electrolyte in these cells is a concentrated solution of potassium hydroxide and sodium hydroxide (Onovwiona ans Urgusal 384). It fills the space between the electrodes. Hydrogen gas and oxygen gas are simmered using the porous electrodes, and the overall cell reaction involves a combination of hydrogen and oxygen gases to from water. This cell operates at a temperature range of between 343k and 413. Carbon-oxygen fuel cells These cells operate at relatively high temperatures of about 6500C and use lithium potassium carbonate as an electrolyte. Hydrogen gas reacts with carbonate ions in the anode, producing water, carbon dioxide, electrons, and small amounts of other small chemicals. The electrons travel via an external circuit creating electricity and go back to the cathode. The chemical reactions for the carbon-oxygen fuel cell are as shown below: Anode reaction: CO32-+H20+CO2+2e- Cathode reaction: CO2+1/2O2+2e=CO32- Overall Cell reaction: H2+1/2O2=H2O The major setback to the technology of carbonate cells is their short lifespan caused by corrosion of the anode and the cathode by the carbonate. They also have slow start-up times. These cells nevertheless have numerous advantages over other cells in that they are resistant to impurities; they are not prone to “carbon cooking” and have relatively high efficiencies. In conclusion, the field of fuel cells is very broad and still more research is needed to figure out every aspect of it. Read More