Hydrogen-Oxygen AFCs

In hydrogen-oxygen fuel cells with an alkaline electrolyte, the reactions at the electrodes and the overall current-generating reaction can be formulated as follows  

The electrodes in Bacon s battery measured 370 cm, their thickness was 1.8 mm. At current densities from 200 to 400 mA/cm, the voltage of an individual cell in the battery was 0.90-0.95 V, which is considerably higher than that in phosphoric acid fuel cell (PAFC) and in modern proton exchange membrane fuel cell (PEMFC). A variety of corrosion problems were responsible for the total lifetime of Bacon s battery not reaching more than a few hundred hours. 

In 1962, a research group in the American company Allis-Chalmers started developing a new type of hydrogen-oxygen fuel cell with an alkaline electrolyte. The distinguishing feature of this cell was to use, instead of a fi eely flowing liquid electrolyte (KOH solution or melt, as described above), a quasi-solid electrolyte in the form of potassium hydroxide solution immobilized in an asbestos matrix. Asbestos 

Allis-Chalmers being a manufacturer of agricultural machinery, they installed a battery in an experimental tractor from their product line. It actually was operated under field conditions for some time, and then was demonstrated in many exhibitions and fairs. This was the first use of fuel cells as the power source in an electrically powered vehicle. 

After the successful completion of all Apollo missions, the UTC began developing batteries for a new spacecraft, the Orbiter Space Shuttle. This spacecraft had much larger energy requirements and needed a much more powerful battery. It led to a 

In Bacon s battery, a KOH solution of intermediate concentration (37 to 50%) was the electrolyte. The electrodes were made of porous nickel. This worked without any problems for hydrogen electrodes, as hydrogen is readily oxidized at nickel as a catalytic anode. But problems had to be solved for the oxygen 

A battery weighing about 114 kg was designed for delivering a power of 1.5 kW and sustaining short-term loads of up to 2.2 kW. The lifetime of this improved battery was extended to several thousands of hours, much more than that of Bacon s original battery. 

The Allis-Chalmers battery had a filter-press design. It contained a 5 M KOH solntion in asbestos diaphragms 0.75 mm thick. The working temperature was 65°C, much lower than that in the batteries described above. At a current density of 200 mA/cm and a gas pressure of 1.35 bar, the voltage per cell was 0.75 V, which is somewhat lower fiian in battery types working at higher temperatures. 

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AUcaline fuel cells (AFCs, hydrogen-fuelled cells with an alkaline liquid electrolyte such as KOH(aq)) are the best performing of all known conventional hydrogen-oxygen fuel cells operable at temperatures below 200 C. This is due to the facile kinetics at the cathode and at the anode cheaper non-noble metal catalysts can be used (such as nickel and silver [3,4]), reducing cost. McLean et al. gave comprehensive review of alkaline fuel cell technology [5]. The associated fuel cell reactions both for a traditional AFC and also for an AMFC are ... 

The Alkaline Fuel Cell (AFC) was one of the first modern fuel cells to be developed, beginning in 1960. The application at that time was to provide on-board electric power for the Apollo space vehicle. Desirable attributes of the AFC include its excellent performance on hydrogen (H2) and oxygen (O2) compared to other candidate fuel cells due to its active O2 electrode kinetics and its flexibility to use a wide range of electrocatalysts, an attribute which provides development flexibility. 

Pure hydrogen and oxygen are required to operate an AFC. Reformed H2 or air containing even trace amounts of CO2 will dramatically affect performance and lifetime as described in the following section. 

Anodic hydrogen oxidation and even more cathodic oxygen reduction is kinetically hampered at low temperature, so that anodic hydrogen oxidation in AFCs, PEMFCs, and PAFCs demands catalysts of highest activity, that is, platinum metals and platinum in particular. Also Raney nickel is used in... 

One of the first fuel cell designs was low-temperature alkaline fuel cells (AFCs) used in the U.S. space program. They served to produce both water and electricity on the spacecraft. Some of their disadvantages are that they are subject to carbon monoxide poisoning, are expensive, and have short operating lives. The AFC electrodes are made of porous carbon plates laced with a catalyst. The electrolyte is potassium hydroxide. At the cathode, oxygen forms hydroxide ions, which are recycled back to the anode. At the anode, hydrogen gas combines with the hydroxide ions to produce water vapor and electrons that are forced out of the anode to produce electric current. 

The advantage of AFCs over the other systems lies in the fact that the reduction of oxygen to OH- is much faster than the acidic equivalent of oxygen to H20 due to a better kinetics, which makes the AFC a more efficient system [15]. The hydrogen oxidation reaction in alkaline medium, however, is slower. 

The AFC type was originally created for the Apollo program, after that a modernized version has been developed and is even now in use to provide electrical power for shuttle missions. The electrolyte in this fuel cell is KOH, concentrated (85 wt %) for fuel cells operated at relatively high temperatures, that is, around 250°C, and less concentrated (35-50 wt %) for cells operated at lower temperatures, that is, less than 120°C [6,9,11], In the construction of these fuel cells, the electrolyte is retained in a matrix, typically asbestos, and a wide range of catalysts, for example, Ni, Ag, metal oxides, and noble metals, can be used for both the hydrogen and the oxygen electrodes [8,9],... 

In alkaline conditions, the oxygen reduction reaction is much faster in AFCs while the hydrogen oxidation reaction (HOR) is slower, compared with acidic fuel cells (H2/air and direct methanol fuel cells). 

Although AFCs exhibit the highest electrical efficiency (60%) of all fuel cell systems, their applications are limited to space and military programs, where high costs are permissible and pure hydrogen and pure oxygen can be supplied. 

The Alkaline Fuel Cell, AFC, is the most efficient low-temperature fuel cell presently available with a very high power density, therefore ideally appropriate for mobile applications. The AFC shows a similar performance as the PEFC, but with a much more demanding process control which is complicated because of the requirements of fuel purity (no CO2) and of the corrosive liquid electrolyte. Efficiencies of more than 60 % have been achieved with clean hydrogen and oxygen and noble electrode materials. The AFC was demonstrated to also work with a hydrogen-air system, in a 1 kW stack in Japan and in a 6 kW Russian system with more than 5(XX) h lifetime [34]. PEFC and AFC may play a... 

Apart from exhibiting electrocatalytic activity towards the electrode reactions, the electrocatalysts must be stable within the working cell. For the alkaline fuel cell (AFC) this is relatively easy since many electrocatalytic materials are adequately stable in alkaline solutions. The fact that the AFC is very sensitive to the presence of CO2, either in the fuel stream or in the air stream, has limited its application substantially to those simations where very pure hydrogen and very pure oxygen can be supplied. 

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