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| [[Image:alkalinecell.svg|thumb|270px|Diagram of an Alkaline Fuel Cell. 1: Hydrogen 2:Electron flow 3:Load 4:Oxygen 5:Cathode 6:Electrolyte 7:Anode 8:Water 9:Hydroxyl Ions]]
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| The alkaline fuel cell ('''AFC'''), also known as the [[Francis Thomas Bacon|Bacon]] fuel cell after its British inventor, is one of the most developed [[fuel cell]] technologies. [[NASA]] has used alkaline fuel cells since the mid-1960s, in [[Project Apollo|Apollo]]-series missions and on the [[Space Shuttle program|Space Shuttle]]. 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%.
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| ==Chemistry==
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| The fuel cell produces power through a [[redox reaction]] between [[hydrogen]] and oxygen. At the [[anode]], hydrogen is oxidized according to the reaction:
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| <math>\mathrm{2H}_2 + \mathrm{4OH}^- \longrightarrow \mathrm{4H}_2\mathrm{O} + \mathrm{4e}^-</math>
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| producing water and releasing two electrons. The electrons flow through an external circuit and return to the [[cathode]], reducing oxygen in the reaction:
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| <math>\mathrm{O}_2 + \mathrm{2H}_2\mathrm{O} + \mathrm{4e}^- \longrightarrow \mathrm{4OH}^-</math>
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| producing [[hydroxide]] ions. The net reaction consumes one oxygen atom and two hydrogen atoms in the production of one water molecule. Electricity and heat are formed as by-products of this reaction.
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| ==Electrolyte==
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| The two electrodes are separated by a porous matrix saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH). Aqueous alkaline solutions do not reject carbon dioxide (CO<sub>2</sub>) so the fuel cell can become "poisoned" through the conversion of KOH to potassium carbonate (K<sub>2</sub>CO<sub>3</sub>). Because of this, alkaline fuel cells typically operate on pure oxygen, or at least purified [[air]] and would incorporate a 'scrubber' into the design to clean out as much of the carbon dioxide as is possible. Because the generation and storage requirements of oxygen make pure-oxygen AFCs expensive, there are few companies engaged in active development of the technology. There is, however, some debate in the research community over whether the poisoning is permanent or reversible. The main mechanisms of poisoning are blocking of the pores in the cathode with K<sub>2</sub>CO<sub>3</sub>, which is not reversible, and reduction in the ionic conductivity of the electrolyte, which may be reversible by returning the KOH to its original concentration. An alternate method involves simply replacing the KOH which returns the cell back to its original output.<!-- Need references, here; see articles by K. Kordesch and S. Gamburzev, possibly also J. Appleby -->
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| ==Basic designs==
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| Because of this poisoning effect, two main variants of AFCs exist: static electrolyte and flowing electrolyte. Static, or immobilized, electrolyte cells of the type used in the Apollo space craft and the Space Shuttle typically use an asbestos separator saturated in potassium hydroxide. Water production is managed by evaporation out the anode, as pictured above, which produces pure water that may be reclaimed for other uses. These fuel cells typically use platinum catalysts to achieve maximum volumetric and specific efficiencies.
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| Flowing electrolyte designs use a more open matrix that allows the electrolyte to flow either between the electrodes (parallel to the electrodes) or through the electrodes in a transverse direction (the ASK-type or EloFlux fuel cell). In parallel-flow electrolyte designs, the water produced is retained in the electrolyte, and old electrolyte may be exchanged for fresh, in a manner analogous to an oil change in a car <!-- reference needed - see Karl Kordesch's papers -->. In the case of "parallel flow" designs, greater space is required between electrodes to enable this flow, and this translates into an increase in cell resistance, decreasing power output compared to immobilized electrolyte designs. A further challenge for the technology is that it is not clear how severe is the problem of permanent blocking of the cathode by K<sub>2</sub>CO<sub>3</sub>, however, some published reports indicate thousands of hours of operation on air. <!-- Source: a recent article by Ovonic Fuel Cell in Fuel Cell Today and probably past articles by Gamburzev --> These designs have used both platinum and non-noble metal catalysts, resulting in increased volumetric and specific efficiencies and increased cost. <!-- references need; see again Kordesch, Gamburzev, Appleby and others -->
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| The EloFlux design, with its transverse flow of electrolyte, has the advantage of low-cost construction and replaceable electrolyte, but so far has only been demonstrated using oxygen. <!-- need to confirm this with a literature search; see material published by Gaskatel: Dr. August Winsel, Dr. Hans-Joachim Kohnke, and Joachim Helmke -->
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| Further variations on the alkaline fuel cell include the [[metal hydride fuel cell]] and the [[direct borohydride fuel cell]].
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| ==Commercial prospects==
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| AFCs are the cheapest of fuel cells to manufacture. The catalyst required for the electrodes can be any of a number of different chemicals that are inexpensive compared to those required for other types of fuel cells.
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| The commercial prospects for AFCs lie largely with the recently developed bi-polar plate version of this technology, considerably superior in performance to earlier mono-plate versions.
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| The world's first Fuel Cell Ship [[Hydra (ship)|HYDRA]] used an AFC system with 5 kW net output.
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| Another recent development is the solid-state alkaline fuel cell, utilizing [[alkali anion exchange membrane]]s rather than a liquid. This resolves the problem of poisoning and allows the development of alkaline fuel cells capable of running on safer hydrogen-rich carriers such as liquid urea solutions or metal amine complexes.
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| ==See also==
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| {{Portal|Sustainable development|Energy}}
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| *[[Gas diffusion electrode]]
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| *[[Glossary of fuel cell terms]]
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| *[[Hydrazine]]
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| *[[Hydrogen technologies]]
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| ==External links==
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| * [http://www.cenergie.com Cenergie Corporation plc]
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| * [http://www.independentpower.biz/ Independent Power]
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| * [http://www.intensys.com/ Intensys]
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| * [http://www.afcenergy.com Technological milestone]
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| '''Developers'''
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| * [http://www.cygnusatratusgroup.com The Cygnus Atratus Group]
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| * [http://www.afcenergy.com AFC Energy]
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| {{Fuel cells}}
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| [[Category:Fuel cells]]
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| [[ja:燃料電池#アルカリ電解質形燃料電池 (AFC)]]
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