Graphic representation of varying fuel cell technology general operating parameters.
|Click on graphic at left to view, or Click Here to download the Comparison of Fuel Cell Technologies document from the US Department of Energy – Energy Efficiency & Renewable Energy|
Types of Fuel Cells
Over the last half-century, the most promising alternative power source has been the fuel cell. Here are the major types of fuel cells and principles of their operation:
Alkaline Fuel Cells
Proton Exchange Membrane
Direct Methanol Fuel Cells
Microbial Fuel Cells
Zinc Air Fuel Cells
Regenerative Fuel Cells
Protonic Ceramic Fuel Cell
Direct Carbon Fuel Cell
Alkaline Fuel Cell
Long used by NASA on space missions, Alkaline Fuel Cells (AFC) can achieve power generating efficiencies of up to 70 percent. They were used on the Apollo spacecraft to provide both electricity and drinking water. Their operating temperature is 150 to 200 degrees C (about 300 to 400 degrees F). They use an aqueous solution of alkaline potassium hydroxide soaked in a matrix as the electrolyte. This is advantageous because the cathode reaction is faster in the alkaline electrolyte, which means higher performance. Until recently they were too costly for commercial applications, but several companies are examining ways to reduce costs and improve operating flexibility. They typically have a cell output from 300 watts to 5 kW.
Phosphoric Acid Fuel Cell
Phosphoric Acid Fuel Cells (PAFC) are the most mature fuel cell technology and is commercially available today. More than 200 fuel cell systems have been installed all over the world – in hospitals, nursing homes, hotels, office buildings, schools, utility power plants, an airport terminal, 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. Operating temperatures are in the range of 300 to 400 degrees F (150 – 200 degrees C). At lower temperatures, phosphoric acid is a poor ionic conductor, and carbon monoxide (CO) poisoning of the Platinum (Pt) electro-catalyst in the anode becomes severe. The electrolyte is liquid phosphoric acid soaked in a matrix. 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. Disadvantages of PAFCs include: it uses expensive platinum as a catalyst, it generates low current and power comparably to other types of fuel cells, and it generally has a large size and weight.
Molten Carbonate Fuel Cells
Molten Carbonate Fuel Cells (MCFC) use a liquid solution of lithium, sodium and/or potassium carbonates, soaked in a matrix for an electrolyte. They promise high fuel-to-electricity efficiencies, about 60% normally or 85% with cogeneration, and operate at about 1,200 degrees F or 650 degrees C. The high operating temperature is needed to achieve sufficient conductivity of the electrolyte. Because of this high temperature, noble metal catalysts are not required for the cell’s electrochemical oxidation and reduction processes. 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. Carbonate fuel cells for stationary applications have been successfully demonstrated in Japan and Italy. The high operating temperature serves as a big advantage because this implies higher efficiency and the flexibility to use more types of fuels and inexpensive catalysts as the reactions involving breaking of carbon bonds in larger hydrocarbon fuels occur much faster as the temperature is increased. A disadvantage to this, however, is that high temperatures enhance corrosion and the breakdown of cell components.
Solid Oxide Fuel Cells
Solid Oxide Fuel Cells (SOFC) could be used in big, high-power applications including industrial and large-scale central electricity generating stations. Some developers also see SOFC use in motor vehicles and are developing fuel cell auxiliary power units (APUs) with SOFCs. A solid oxide system usually uses a hard ceramic material of solid zirconium oxide and a small amount of ytrria, instead of a liquid electrolyte, allowing operating temperatures to reach 1,800 degrees F or 1000 degrees C. Power generating efficiencies could reach 60% and 85% with cogeneration and cell output is up to 100 kW.
Proton Exchange Membrane Fuel Cells
The Proton Exchange Membrane (PEM) fuel cell (also called the solid polymer electrolyte fuel cell or the polymer electrolyte fuel cell) operates at the lowest temperature range (< 200C), 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.
The proton exchange membrane is a thin plastic sheet that allows hydrogen ions to pass through it. The membrane is coated on both sides with highly dispersed metal alloy particles (mostly platinum) that are active catalysts. The electrolyte used is a solid organic polymer poly-perflourosulfonic acid. The solid electrolyte is an advantage because it reduces corrosion and management problems. Hydrogen is fed to the anode side of the fuel cell where the catalyst encourages the hydrogen atoms to release electrons and become hydrogen ions (protons). The electrons travel in the form of an electric current that can be utilized before it returns to the cathode side of the fuel cell where oxygen has been fed. At the same time, the protons diffuse through the membrane (electrolyte) to the cathode, where the hydrogen atom is recombined and reacted with oxygen to produce water, thus completing the overall process. This type of fuel cell is, however, sensitive to fuel impurities. Cell outputs generally range from 50 to 250 kW.
Direct Methanol Fuel Cells
Direct Methanol Fuel Cells (DMFCs) 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 degrees F or 50 -100 degrees C. This is a relatively low range, making this fuel cell attractive for tiny to mid-sized applications, to power cellular phones and laptops, or portable power sources for soldiers in the field. Higher efficiencies are achieved at higher temperatures.
Methanol is used because of its high power density, safety, low cost, ease of handling and distribution, and high electrochemical activity. Using a liquid fuel instead of a gaseous fuel also simplifies the system design. In a DMFC, methanol solutions in water are fed into the anode as fuel. This allows for a substantial system simplification relative to reformate-based fuel cells and a higher energy density than that presently available with hydrogen-based systems. However, at present DMFCs require much higher platinum loadings than either hydrogen or reformate-based systems.
Microbial Fuel Cells
Microbial fuel cells (MFCs) are devices that use bacteria as the catalysts to oxidize organic and inorganic matter and generate current. Electrons produced by the bacteria from these substrates are transferred to the anode (negative terminal) and flow to the cathode (positive terminal) linked by a conductive material containing a resistor, or operated under a load. In most MFCs the electrons that reach the cathode combine with protons that diffuse from the anode through a separator and oxygen provided from air; the resulting product is water. Microbially catalyzed electron liberation at the anode and subsequent electron consumption at the cathode, when both processes are sustainable, are the defining characteristics of an MFC.
Zinc-Air Fuel Cells
A breakthrough in rechargeable fuel cells, the Zinc-Air Fuel Cell (ZAFC) derives its energy from a solid state fuel, which provides a comparatively inexhaustible power source. The density of the metal fuel source combined with its electrochemical properties and availability make this variety of fuel cell inexpensive, environmentally friendly and highly versatile. The simple structure of the zinc-air cell can be adapted to almost any size for use in most any application. 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. The chief advantage zinc-air technology has over other battery technologies is its high specific energy, which is a key factor that determines the running duration of a battery relative to its weight. When ZAFCs are used to power EVs, they have proven to deliver longer driving distances between refuels than any other EV batteries of similar weight. Moreover, due to the abundance of zinc on earth, the material costs for ZAFCs and zinc-air batteries are low. Hence, zinc-air technology has a potential wide range of applications, ranging from EVs, consumer electronics to military.
Regenerative Fuel Cells
Still a very young member of the fuel cell family, regenerative fuel cells would be attractive as a closed-loop form of power generation. Water is separated into hydrogen and oxygen by a solar-powered electrolyser. The hydrogen and oxygen are fed into the fuel cell which generates electricity, heat and water. The water is then recirculated back to the solar-powered electrolyser and the process begins again. These types of fuel cells are currently being researched by NASA and others worldwide.
Protonic Ceramic Fuel Cell
This new type of fuel cell, the Protonic Ceramic Fuel Cell (PCFC), is based on a ceramic electrolyte material that exhibits high protonic conductivity at elevated temperatures. PCFCs share the thermal and kinetic advantages of high temperature operation at 700 degrees Celsius with molten carbonate and solid oxide fuel cells, while exhibiting all of the intrinsic benefits of proton conduction in polymer electrolyte and phosphoric acid fuel cells (PAFCs). The high operating temperature is necessary to achieve very high electrical fuel efficiency with hydrocarbon fuels. PCFCs can operate at high temperatures and electrochemically oxidize fossil fuels directly to the anode. This eliminates the intermediate step of producing hydrogen through the costly reforming process. Gaseous molecules of the hydrocarbon fuel are absorbed on the surface of the anode in the presence of water vapor, and hydrogen atoms are efficiently stripped off to be absorbed into the electrolyte, with carbon dioxide as the primary reaction product. Additionally, PCFCs have a solid electrolyte so the membrane cannot dry out as with PEM fuel cells, or liquid can’t leak out as with PAFCs.
Direct Carbon Fuel Cell
The Direct Carbon Fuel Cell (DCFC) technology was developed at Lawrence Livermore National Laboratory (LLNL), and converts the chemical energy in coal, or anything else that contains carbon, directly into electricity by electrochemically oxidizing the carbon, without the need for gasification. It is reported that efficiencies are double that of burning coal in a coal-fired power plant. The byproduct is carbon dioxide — but it is emitted in such a pure form that it’s easy to contain. The source of solid carbon-rich fuels can be any type of hydrocarbon, including coal, lignite, natural gas, petroleum, petroleum, coke, and biomass, and is readily available and abundant. DCFC technology doesn’t require expensive sulfur-sensitive catalysts, so it effectively utilizes fuels with high sulfur content, and does not require any hydrogen to generate electricity. Because it is carbon, and not hydrogen, that fuels this cell, hydrogen is released as a byproduct of the cell reaction and could potentially be captured for use in a separate hydrogen-powered fuel cell. According to a LLNL newsletter, the technology uses aggregates of extremely fine carbon particles, from 10 to 1,000 nanometers in diameter, distributed in a mixture of molten lithium, sodium, or potassium carbonate at 750-850°C. Total cell efficiencies are projected to be 70-80%, with power generation in the 1 kW/m2 range, sufficient for practical applications. The carbon fuel particles can be produced through pyrolysis of hydrocarbons, a thermal decomposition method well-known as the source of carbon black for tires, ink, and other applications in manufacturing industries. While the concept has been successfully demonstrated with a 3 W cell, this technology is still in the experimental phase of development. Because this is a high-temperature cell, it would be best suited for stationary applications, particularly in combination with CHP utilizing the waste heat energy.