Question

Please Write a overview on the Alkaline fuel cells that includes chemical reaction with there advantages and disadvantages and also conditions of its operation.

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Answer 1

Alkaline fuel cells (AFCs) have been used by NASA during space missions; they can achieve power generation efficiencies of up to 70%. The operating temperature of these batteries varies between 150 and 200°C. An aqueous alkali potassium hydroxide solution embedded in a matrix serves as an electrolyte. This is an advantageous configuration because the reaction at the cathode is fast in an alkaline electrolyte, which allows a better performance. Several manufacturers are examining ways to reduce the cost and improve the operational flexibility of these fuel cells. Alkaline fuel cells typically have powers of 300 watts to 5 kW.

A fuel cell is an electrochemical cell that uses a spontaneous redox reaction to produce current that can do work. The net reaction is exothermic. Combining the 2 half cell potentials for the electrochemical reaction gives a positive cell potential.

H2 + 1/2 O2 H2O	Gf = -229 kJ/mol
H2 2 H+ + 2 e-	E0= 0.00 V
O2 + 4 e- + 4 H+ 2 H2O	E0= 1.23 V

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 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.

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.

On the other hand, the charge-carrying hydroxide ions in the electrolyte can react with carbon dioxide from organic fuel oxidation (i.e. methanol, formic acid) or air to form carbonate species.

Carbonate formation depletes hydroxide 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 .

Alkaline fuel cells have as electrolyte an aqueous solution of potassium hydroxide. Usually, this solution has a concentration of around 30%. It is necessary to insert the hydrogen gas at the anode and to insert the oxygen gas at the cathode, as shown in Fig.

Alkaline fuel cells (AFC) are the type developed by Bacon and refined for and used in the space program. This type of requires pure gas inputs, both hydrogen and oxygen. In space, these pure elements are already available and used for propulsion, so the adaptation of there use in the fuel cell was natural. Any impurities in the fuel, such as carbon dioxide or monoxide, will react to form a solid carbonate. This solid carbonate interferes with the chemical reactions in the cell and reduces efficiency and power production. An additional benefit of the AFC is the pure water produced, which on the manned spacecraft was put to good use as drinking water.

The electrolyte of the alkaline fuel cell is potassium hydroxide (KOH) dissolved in water. The electrodes are platinum, an expensive material that also acts as the catalyst for the reaction. The porous catalysts were developed by Bacon in the late 1930s along with the use of pressurized gasses to keep the electrolyte from flooding the electrodes. Figure shows the operation of an AFC. Note the pure water being produced at the anode.

Alkaline fuel cells have the advantage of allowing the use of non-precious catalysts based on nickel for the anode and activated carbon for the cathode.

They may also have the following advantages

operation at atmospheric pressure and at low temperature;

low costs of the electrolyte and catalysts;

response time and fast start time;

high electrical efficiency;

operates at low temperature (below 0°C).

The main disadvantage posed by this type of battery is their sensitivity to carbon dioxide (CO2) which, remember, is present in the air. This sensitivity implies a thorough purification of hydrogen (total elimination of CO2) when it is obtained by reforming a hydrocarbon fuel. For this reason, the AFC battery has been abandoned for transport applications. Another reason for this is due to the liquid and corrosive state of the electrolyte which, subject to the transport constraints (vibrations, accelerations, etc.), can leak from the battery.