Chemical Energy - Fuel Cells - Essay Example

Fuel Cells In science, a fuel cell is defined as a device that can convert chemical energy using oxidizing agent oroxygen into electricity. Fuel cells require a continuous supply of oxygen and air for them to operate. 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. There are different fuel cells that are grouped according to the electrolyte they use. They include fuel cell, phosphoric acid fuel cell, high-temperature fuel cells and hydrogen-oxygen fuel cell. The chemical reactions in fuel cell occur at the interfaces of the three different segments resulting in consumption of fuel, water or carbon dioxide is 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 ions can pass through them but electrons cannot. The free electrons creates the electric current while the ions travel through the electrolyte to the cathode. Upon reaching the cathode, the electrons and the ions are reunited, and they react with a third chemical preferably oxygen to form water and carbon dioxide. The most common fuel used in fuel cells is hydrogen. The anode is usually made up of very fine platinum powder. The cathode catalyst turns the ions into waste chemicals like water or carbon dioxide, and it is usually made up of nickel but it can also be a nonmaterial-based catalyst (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. The various types of fuel cells mentioned earlier are discussed independently below. Proton exchange membrane fuel cells In this type of fuel cell, it’s 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 produced react with oxidants turning them to what are commonly referred to as multi-facilitated proton membranes. The protons conduct through the membrane to the cathode while the electrons travel through an external circuit because the membrane is electrically insulating. 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 is passed to the cathode using phosphoric acid. These cells usually work at very high temperatures of about 150 to 200 degrees Celsius. The high temperatures cause a heat and energy loss if the heat is not removed and properly used. This high heat can be turned into a useful source of energy for air conditioners and other thermal energy consuming systems (Onovwiona and Ugursal 389). This property makes the phosphoric acid fuel cell more useful and improves its efficiency to about 80%. The anode electrode in this cell uses a platinum catalyst that fastens the conversion of hydrogen into free ions, and electrons are produced (Scherer 34). The electrons travel from the anode to the cathode through an external electrical circuit since phosphoric acid is a non-conducting electrolyte. This cell, however, has one major disadvantage 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 about800-1000 0C 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 this arrangement, the cathode acts as the positive electrode and the anode as the negative electrode (Onovwiona and Ugursal 388). Oxygen gas enters the cell through the cathode where it absorbs electrons to form oxygen ions. The ions then travel through 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 as 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 coking’. 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 Pennsylvania. Another problem with the high-temperature cells is the slow startup time. Hydrogen-oxygen fuel cell 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 or sodium hydroxide (Onovwiona and Ugursal 384). It fills the space between the electrodes. Hydrogen gas and oxygen gas are bubbled through the porous electrodes, and the overall cell reaction involves a combination of hydrogen and oxygen gases to form water. This cell operates at a temperature range of between 343K and 413K. Molten carbonate fuel cell 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 (Onovwiona and Ugursal 384). The chemical reactions for the carbon-oxygen fuel cell are as shown below: Anode reaction: CO32− + H2 → H2O + CO2 + 2e− Cathode Reaction: CO2 + ½O2 + 2e− → CO32− Overall Cell Reaction: H2 + ½O2 → 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 electrolyte. 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 coking’ 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 this field. Work Cited H.I. Onovwiona and V.I. Ugursal. Residential cogeneration systems: review of the current technology. Renewable and Sustainable Energy Reviews, 10(5): page 389 – 431, 2006. Scherer, Günther G, S A. Gürsel, and Marc J. M. Abadie. Fuel Cells. Berlin: Springer, 2008. Print. Barbir, Frano. Fuel Cells. Amsterdam: Elsevier, 2006. Print. Read More