Alkaline fuel cell

The alkaline fuel cell (AFC), also known as the Bacon fuel cell after its British inventor, is one of the most developed fuel cell technologies and is the cell that flew Man to the Moon. NASA has used alkaline fuel cells since the mid-1960s, in Apollo-series missions and on the 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

The fuel cell produces power through a redox reaction between hydrogen and oxygen. At the anode, hydrogen is oxidized according to the reaction:

producing water and releasing two electrons. The electrons flow through an external circuit and return to the cathode, reducing oxygen in the reaction:

producing 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.

Electrolyte

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 (CO₂) so the fuel cell is easily "poisoned" through the conversion of KOH to potassium carbonate (K₂CO₃). Because of this, alkaline fuel cells typically operate on pure oxygen, or at least purified air. Because the generation and storage requirements of oxygen make pure-oxygen AFCs relatively 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 K₂CO₃, 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.

Basic Designs

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.

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 . 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 K₂CO₃, however, some published reports indicate thousands of hours of operation on air. These designs have used both platinum and non-noble metal catalysts, trading off volumetric and specific efficiencies with cost.

The EloFlux design, with its transverse flow of electrolyte, has the advantage of low-cost construction and replacable electrolyte, but so far has only been demonstrated using oxygen.

Further variations on the alkaline fuel cell include the metal hydride fuel cell and the direct borohydride fuel cell.

Commercial Prospects

AFCs are, however, the cheapest of fuel cells to manufacture. 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.

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.

The world's first Fuel Cell Ship (HYDRA) was realised by Christian Machens and used an AFC system with 6.5 kW net output.

Another very interesting recent development (though not necessartily for high power applications) is the solid-state alkaline fuel cell, utilising anion-exchange membranes rather than a liquid. This work is pioneered at the University of Surrey in the United Kingdom. See details at http://mypages.surrey.ac.uk/chs1jv/.

See also

• Hydrazine
• Hydrogen technologies