Fuel Cells Prof Park UTI 111 Essex County College What Is A Fuel Cell In principle a fuel cell operates like a battery Unlike a battery a fuel cell does not run down or require recharging It will produce energy in the form of electricity and heat as long as fuel is supplied A fuel cell consists of two electrodes sandwiched around an electrolyte Oxygen passes over one electrode and hydrogen over the other generating electricity water and heat What Is A Fuel Cell What Is A Fuel Cell Hydrogen fuel is fed into the anode of the fuel cell Oxygen or air enters the fuel cell through the cathode Encouraged by a catalyst the hydrogen atom splits into a proton and an electron which take different paths to the cathode The proton passes through the electrolyte The electrons create a separate current that can be utilized before they return to the cathode to be reunited with the hydrogen and oxygen in a molecule of water What Is A Fuel Cell A fuel cell system which includes a fuel reformer can utilize the hydrogen from any hydrocarbon fuel from natural gas to methanol and even gasoline Since the fuel cell relies on chemistry and not combustion emissions from this type of a system would still be much smaller than emissions from the cleanest fuel combustion processes Types of Fuel Cells 1 Phosphoric Acid 2 Proton Exchange Membrane 3 Molten Carbonate 4 Solid Oxide 5 Alkaline 6 Direct Methanol 7 Regenerative 8 Zinc Air 9 Protonic Ceramic 10 Microbial Fuel Cell Phosphoric Acid fuel cell PAFC Phosphoric acid fuel cells are commercially available today Hundreds of fuel cell systems have been installed in 19 nations in hospitals nursing homes hotels office buildings schools utility power plants landfills and waste water treatment plants PAFCs generate electricity at more than 40 efficiency and nearly 85 of the steam this fuel cell produces is used for cogeneration this compares to about 35 for the utility power grid in the United States Phosphoric acid fuel cells use liquid phosphoric acid as the electrolyte and operate at about 450 F One of the main advantages to this type of fuel cell besides the nearly 85 cogeneration efficiency is that it can use impure hydrogen as fuel PAFCs can tolerate a CO concentration of about 1 5 percent which broadens the choice of fuels they can use If gasoline is used the sulfur must be removed Proton Exchange Membrane fuel cell PEM These fuel cells operate at relatively low temperatures about 175 F have high power density can vary their output quickly to meet shifts in power demand and are suited for applications such as in automobiles where quick startup is required According to the U S Department of Energy DOE they are the primary candidates for light duty vehicles for buildings and potentially for much smaller applications such as replacements for rechargeable batteries This type of fuel cell is sensitive to fuel impurities Cell outputs generally range from 50 watts to 75 kW Molten Carbonate fuel cell MCFC Molten carbonate fuel cells use an electrolyte composed of a molten carbonate salt mixture suspended in a porous chemically inert matrix and operate at high temperatures approximatelly 1 200 F They require carbon dioxide and oxygen to be delivered to the cathode To date MCFCs have been operated on hydrogen carbon monoxide natural gas propane landfill gas marine diesel and simulated coal gasification products 10 kW to 2 MW MCFCs have been tested on a variety of fuels and are primarily targeted to electric utility applications Solid Oxide fuel cell SOFC Solid oxide fuel cells use a hard non porous ceramic compound as the electrolyte and operate at very high temperatures around 1800 F One type of SOFC uses an array of meter long tubes and other variations include a compressed disc that resembles the top of a soup can Tubular SOFC designs are closer to commercialization and are being produced by several companies around the world SOFCs are suitable for stationary applications as well as for auxiliary power units APUs used in vehicles to power electronics Alkaline fuel cell AFC Long used by NASA on space missions alkaline fuel cells can achieve power generating efficiencies of up to 70 percent They were used on the Apollo spacecraft to provide both electricity and drinking water Alkaline fuel cells use potassium hydroxide as the electrolyte and operate at 160 F However they are very susceptible to carbon contamination so require pure hydrogen and oxygen Direct Methanol fuel cell DMFC These cells are similar to the PEM cells in that they both use a polymer membrane as the electrolyte However in the DMFC the anode catalyst itself draws the hydrogen from the liquid methanol eliminating the need for a fuel reformer Efficiencies of about 40 are expected with this type of fuel cell which would typically operate at a temperature between 120 190 F This is a relatively low range making this fuel cell attractive for tiny to mid sized applications to power cellular phones and laptops Higher efficiencies are achieved at higher temperatures Companies are also working on DMFC prototypes to be used by the military for powering electronic equipment in the field Regenerative fuel cell Regenerative fuel cells are attractive as a closed loop form of power generation Water is separated into hydrogen and oxygen by a solar powered electrolyzer The hydrogen and oxygen are fed into the fuel cell which generates electricity heat and water The water is then recirculated back to the solarpowered electrolyzer and the process begins again These types of fuel cells are currently being researched by NASA and others worldwide Zinc Air fuel cell ZAFC In a typical zinc air fuel cell there is a gas diffusion electrode GDE a zinc anode separated by electrolyte and some form of mechanical separators The GDE is a permeable membrane that allows atmospheric oxygen to pass through After the oxygen has converted into hydroxyl ions and water the hydroxyl ions will travel through an electrolyte and reaches the zinc anode Here it reacts with the zinc and forms zinc oxide This process creates an electrical potential when a set of ZAFC cells are connected the combined electrical potential of these cells can be used as a source of electric power This electrochemical process is very similar to that of a PEM fuel cell but the refueling is very different and shares characteristics with batteries Zinc Air fuel cell ZAFC ZAFCs contain a zinc fuel tank and a zinc refrigerator that automatically and silently regenerates