“A brief synopsis of fuel cells”
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INTRODUCTION Fuel cells produce electricity from an electrochemical reaction betweenreactants such as oxygen and hydrogen, although other fuels besides hydrogen can be used. The reaction produces water and heat as byproducts. Some benefits of fuel cells are that they are much more efficient than the internal combustion engine; they produce more usable energy, and they don’t produce pollution (as our current IC engines do). The typical polymer electrolyte membrane fuel cell, orPEMFC, contains two electrodes: one positively charged, called the anode, and one negatively charged, called the cathode. The anode and cathode are made of an electrically conductive carbon paper or carbon cloth-backing layer, coated with a catalyst layer. Between them is an electrolyte membrane, which is the heart of the fuel cell; it conducts protons from the anode to the cathode. To understandhow it works, we need to get down to the molecular level. When the hydrogen gas enters the anode, it comes into contact with the catalyst, which splits the gas into positive ions (hydrogen protons), and electrons. The electrons traveling to the cathode via an external circuit create the electrical current which runs the vehicle, or whatever device the fuel cell is running. The protons can travelthrough the membrane to the cathode. At the same time, oxygen is being fed to the cathode, where a catalyst layer creates oxygen ions. When the hydrogen protons arrive at the cathode side, they bond with these oxygen ions, creating water and heat as the byproduct of the electrochemical reaction. Since a single fuel cell isn’t enough to power most devices, fuel cell manufacturers stack them togetherin a series, which is why they are called fuel cell stacks. The greater the number of fuel cells in the stack, the higher the voltage. The greater the area of the electrodes, the greater the current. Voltage times current is total power output. Figure 1 shows a schematic of a single fuel cell.
Figure 1. Illustration of a single cell fuel cell
PARTS OF A FUEL CELL The Polymer ElectrolyteMembrane The standard electrolyte material currently being used in PEM fuel cells is a Teflon-based polymer membrane produced by DuPont for space applications in the 1960s. The DuPont electrolytes have the generic brand name Nafion®, and the specific type used most often is number 117. The Nafion® membranes exhibit exceptionally high chemical and thermal stability. They are stable against chemicalattack in strong bases, strong oxidizing and reducing acids, and chlorine, hydrogen and oxygen at temperatures up to 125 °C. The proton-conducting membrane usually consists of a polytetrafluoroethylene, or PTFE-based polymer backbone, to which sulfonic acid groups are attached (this ia a negatively charged group, that “carries” the hydrogen protons through the membanre). The chemical formula forNafion® 117 is:
Figure 2. Chemical structure of a Nafion® membrane The proton-conducting membrane works well for fuel cells because the hydrogen protons jump from SO3 site to SO3 site throughout the material, and emerge on the other side of the membrane. However, the membrane must remain hydrated in order to conduct the protons, which limits the operating temperature of PEM fuel cells to underthe boiling point of water, and makes water management a key issue in PEMFC development. The Electrodes The electrodes are usually made of a porous mixture of carbon-supported platinum and a carbonbased backing layer. In order to catalyze the reactions effectively, the catalyst particles must have contact with both the carbon-based backing layer and the electrolyte (Nafion®) membrane. Furthermore,...