Alkaline fuel cell

The alkaline fuel cell (AFC), also known as the Bacon fuel cell) after its British inventor, Francis Thomas Bacon, 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. ( Alkaline fuel cells 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%.) The alkaline fuel cell (AFC), also known as the Bacon fuel cell) after its British inventor, Francis Thomas Bacon, 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. ( Alkaline fuel cells 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%.) The fuel cell produces power through a redox reaction between hydrogen and oxygen. At the anode, hydrogen is oxidized according to the reaction: H 2 + 2 O H − ⟶ 2 H 2 O + 2 e − {displaystyle mathrm {H} _{2}+mathrm {2OH} ^{-}longrightarrow mathrm {2H} _{2}mathrm {O} +mathrm {2e} ^{-}} producing water and releasing electrons. The electrons flow through an external circuit and return to the cathode, reducing oxygen in the reaction: O 2 + 2 H 2 O + 4 e − ⟶ 4 O H − {displaystyle mathrm {O} _{2}+mathrm {2H} _{2}mathrm {O} +mathrm {4e} ^{-}longrightarrow mathrm {4OH} ^{-}} 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. 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 do not show a reduction in performance, whereas at around room temperature, a 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. C O 2 + 2 K O H ⟶ K 2 C O 3 + H 2 O {displaystyle mathrm {CO} _{2}+mathrm {2KOH} longrightarrow mathrm {K} _{2}mathrm {CO} _{3}+mathrm {H} _{2}mathrm {O} } 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) or air to form carbonate species. 2 O H − + C O 2 ⟶ C O 3 2 − + H 2 O {displaystyle mathrm {2OH} ^{-}+mathrm {CO} _{2}longrightarrow mathrm {CO} _{3}^{2-}+mathrm {H} _{2}mathrm {O} } 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 .