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. 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%.
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 atom and two hydrogen atoms in the production of each water molecule. Electricity and heat are formed as by-products of this reaction.
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 can become "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 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 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. An alternate method involves simply replacing the KOH which returns the cell back to its original output.
When carbon dioxide reacts with the electrolyte carbonates are formed. The carbonates could precipitate on the pores of electrodes that eventually block them. It has been found that AFCs operating at higher temperature does not show a reduction in performance while around room temperature, significant drop in performance has been shown. The carbonate poisoning at ambient temperature is thought to be a result of the low solubility of K2CO3 around room temperature, which leads to precipitation of K2CO3 that blocks the electrode pores. Also, these precipitants gradually decrease the hydrophobicity of the electrode backing layer leading to structural degradation and electrode flooding.
On the other hand, the charge-carrying hydroxyl ions in the electrolyte can react with carbon dioxide from organic fuel oxidation (i.e. methanol, formic acid) and/or air to form carbonate species.
Carbonate formation depletes hydroxyl ions from the electrolyte, which reduces electrolyte conductivity and consequently cell performance. As well as these bulk effects, the effect on water management due to a change in vapor pressure and/or a change in electrolyte volume can be detrimental as well .
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, resulting in increased volumetric and specific efficiencies and increased cost.
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.
The electrodes consist of a double layer structure: a an active electrocatalyst layer and a hydrophobic layer. The active layer consists of an organic mixture which is ground and then rolled at room temperature to form a crosslink self-supporting sheet. The hydrophobic structure prevents the electrolyte from leaking into the reactant gas flow channels and ensures diffusion of the gases to the reaction site. The structure is made by rolling porous organic layer to crosslink the layer and form a self-supporting sheet. The two layers are then pressed onto a conducting metal mesh. Sintering complete the process eventually.
Further variations on the alkaline fuel cell include the metal hydride fuel cell and the direct borohydride fuel cell.
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.
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 used an AFC system with 5 kW net output.
Another recent development is the solid-state alkaline fuel cell, utilizing alkali anion exchange membranes 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.