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HISTORY • An engineer, Dr. Francis Thomas Bacon, at Cambridge University in England, wrote the next major chapter in the fuel cell story. In 1932, Bacon resurrected the machine developed by Mond and Langer and implemented a number of modifications to the original design. These included replacing the platinum electrodes with less expensive nickel gauze. He also substituted the sulphuric acid electrolyte for alkali potassium hydroxide, a substance less corrosive to the electrodes. This device, which he named the "Bacon Cell," was in essence the first alkaline fuel cell (AFC). Another 27 years would pass until Bacon could produce a truly workable fuel cell. In 1959, Bacon demonstrated a machine capable of producing 5 kW of power, enough to power a welding machine. • In the early 1960s, aircraft engine manufacturer Pratt & Whitney licensed the Bacon patents for the Alkaline Fuel Cell (AFC). With the goal of reducing the weight and designing a longer-lasting fuel cell than the GE PEM design, Pratt & Whitney improved the original Bacon design. As a result, Pratt & Whitney won a contract from NASA to supply these fuel cells to the Apollo spacecraft. Alkali cells have since been used on most subsequent manned U.S. space missions, including those of the Space Shuttle.
History – Other Source • The AFC was developed in the 1930s by F.T. Bacon and is one of the oldest fuel cell technology types. In 1954 Bacon demonstrated a six cell battery that produced 150W and in 1956 his team built a 40 cell unit based on the same design. The larger unit produced 6kW and was used to power a fork-lift truck, welding equipment and a circular saw. In 1961, Bacon formed Energy Conversion (Ltd) and the company began to develop fuel cells which could be produced commercially. • Whilst development work was progressing on Bacon’s larger unit, two licenses were granted on the technology patents. One of these licenses went to Pratt & Whitney in 1959 and then three year later, in 1962, the company started to develop an AFC power plant for the Apollo space programme. Three AFC units were employed to supply drinking water and power for life support, guidance and communications functions to the shuttle during its two week expedition to the moon and the AFC unit became the first type of fuel cell to be widely used in the U.S space programme.
Technology • All fuel cell systems are based on a central design where two electrodes, a negative anode and a positive cathode, are separated by a solid or liquid electrolyte that carries electrically charged particles between them. • AFC systems are classified as low temperature fuel cells and usually operate between 60 and 900C. As a result of the low operating temperature, it is not necessary to employ a platinum catalyst in the system and instead, a variety of non precious metals can be used to speed up the reactions occurring at the anode and cathode. Nickel is the most commonly used catalyst in AFC units. • The AFC uses an alkaline electrolyte such as potassium hydroxide (usually in a solution of water) in order to operate- the chemical used in Duracell batteries. It's redeeming feature is the fact that it freezes at below -40°C and it also provides a mechanism for cooling and heating the fuel cell. In operation this chemical is controlled at 70°C to give full power. The electrolyte flows through the centre of the fuel cell plates with porous membranes between the electrolyte and the hydrogen (anode) and oxygen (cathode) gas supplies.
The membranes contain the catalysts (platinum & silver) which provides the sites for ion recombination to take place. The result of recombination is water which is formed as vapor in the gas circuits and is condensed out and returned to the KOH header tank to maintain water balance. The process is controlled by maintaining a constant hydrogen pressure and adjusting the volume of air (oxygen) supplied. • The speed of response is determined by service gas pressures. In this system gas pressures are low (1/20 bar) and response time around 10 seconds. The Fuel Cell is consequently used with a battery to supply transient loads. This raises two issues: • Electrical & Gas characteristics of Fuel Cell • Matching batteries to the Fuel Cell • Each fuel cell plate gives 0.93 volts at no load and 0.66 volts at full load. Each plate gives 25 amperes. Modules are matrices of six plates in series and four plates in parallel, each module providing 5.6 volts no load and 4 volts at 100 amperes full load. A 2.5 kW Fuel Cell consists of a single stack of six modules in electrical series.
AFC • The Alkaline Fuel Cell (AFC) is one of the most developed fuel cell technologies and is the cell that flew Man to the Moon. • AFCs consume hydrogen and pure oxygen producing potable water, heat, and electricity. • They are among the most efficient fuel cells, having the potential to reach 70%.
Explanation • The fuel cell produces power through a redox reaction between hydrogen and oxygen. • At Anode, hydrogen is oxidized as below. • This reaction produces water and releases 2 electrons. • These electrons flow through an external circuit and return to cathode, reducing oxygen.
Explanation Cont. • At Cathode, oxygen is reduced as below. • The oxygen molecule combines with the electron and water and gets reduced to hydroxide ions. • The net reaction consumes one oxygen molecule and two hydrogen molecules in the production of two water molecules. Electricity and heat are formed as by-products of this reaction.
Operating Conditions • Alkaline fuel cells use an electrolyte that is an aqueous (water-based) solution of potassium hydroxide (KOH) retained in a porous stabilized matrix and can use a variety of non-precious metals as a catalyst at the anode and cathode. • The concentration of KOH can be varied with the fuel cell operating temperature, which ranges from 65°C to 220°C (149°F to 428°F). • However, newer AFC designs operate at lower temperatures of roughly 23°C to 70°C (74°F to 158°F).
Operating Conditions Cont. • The charge carrier for an AFC is the hydroxyl ion (OH-) that migrates from the cathode to the anode where they react with hydrogen to produce water and electrons. • Water formed at the anode migrates back to the cathode to regenerate hydroxyl ions.
Advantages of AFC • AFCs are the cheapest fuel cells to manufacture. (Reason: The catalyst required for the electrodes can be any of a number of different chemicals that are relatively inexpensive compared to those required for other types of fuel cells). • AFCs' has the highest performance among all types of fuel cells. (Reason: The rate at which chemical reactions take place in the cell is very fast). • Low material costs – plastics, carbon, base metals and metal oxides; no platinum. • Long life span – 2000-plus hours currently. • Superior electrochemical conversion efficiency to other fuel cells and the internal combustion engine. • Quick start, even in sub-freezing temperatures down to minus 40 degrees C. • Simpler heat and water management when compared to other fuel cell technologies. • Like other fuel cells, it is odorless and quiet for enclosed applications
Limitations of AFC • AFCs are very sensitive to CO2 that may be present in the fuel or air. • The CO2 reacts with the electrolyte, poisoning it rapidly, and severely degrading the fuel cell performance. • Therefore, AFCs are limited to closed environments, such as space and undersea vehicles, and must be run on pure hydrogen and oxygen. • Furthermore, molecules such as CO, H2O and CH4, which are harmless or even work as fuels to other fuel cells, are poisons to an AFC. • AFC stacks have been shown to maintain sufficiently stable operation for more than 8,000 operating hours. To be economically viable in large-scale utility applications, these fuel cells need to reach operating times exceeding 40,000 hours.
Issues • AFCs are not being considered for automobile applications. Their sensitivity to poisoning, which requires use of pure or cleansed hydrogen and oxygen, is an insurmountable obstacle at the present time. (Note: NASA has made the decision to shift to Proton-exchange fuel cells for the next generation of Space Shuttles). • Conversely, AFCs operate at relatively low temperatures and are among the most efficient fuel cells, characteristics that would enable a quick starting power source and high fuel efficiency, respectively.
AFC Applications Deployed • Used by Space Program. • Developed by NASA to power the Gemini Missions and Subsequent Shuttle Operations.
Present Applications • Fuel Cell Taxi & Boat http://www.infotools.hfpeurope.org/energyinfos__e/fuelcells/main06.html • Generator and Golf Car http://www.astris.ca/PR/PR50.php
Future Developments - ALKANLINE FUEL CELL – TECHNOLOGY FOR THE 21st CENTURY • Alkaline cells, just like alkaline based batteries, are reliable performers that can be built inexpensively from down-to-earth materials – carbon, plastic, base metals; they use cheap electrolyte, start instantly and perform even in deep subzero temperatures. They do not depend on expensive platinum catalyst. And one of the alkaline cell problems often quoted by their detractors – their limited tolerance to carbon dioxide – has been solved by engineers many times over. • The complete article is in a PDF format and its uploaded in directory “MURU” under the name “AFC-Future.PDF”.
Activities - 1 • What is the electrolyte used in AFC? • NaOH • KOH • LiOH • Ca(OH)2
Activities - 2 • How much operating hours should any fuel cell posses to be economically viable in large-scale utility applications? • > 8000 Hours • Between 8000 Hours & 40,000 Hours • < 40,000 Hours • > 40,000 Hours
Activities - 3 • What makes AFCs cheaper to manufacture? • Due to inexpensive cathode. • Due to inexpensive anode. • Due to lesser working temperature conditions. • Due to inexpensive electrolyte.
Activities - 4 • Which fuel cell is being replaced for AFCs in the next generation space shuttles? • Phosphoric Acid Fuel Cell. • Solid Oxide Fuel Cell. • Proton Exchange Fuel Cell. • Molten Carbonate Fuel Cell.
Fuel Cell Resources • http://www.fctec.com/fctec_history.asp • http://www.rmi.org/sitepages/pid537.php • http://www.rmi.org/sitepages/pid556.php • http://www.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_parts.html • http://www.kettering.edu/~altfuel/links.htm
Resources for Pictures • http://www.kettering.edu/~altfuel/fcpicts.htm • http://www.ectechnic.co.uk/pictures.html
