Electrolytes for AFC

As the electrode reactions involve OBT ions, the electrolyte is usually taken to be an alkaline hydroxide solution. For the electrode reactions to proceed without interruption, the hydroxyl ions should be produced and migrated by electrolyte towards the anode. The commonly used electrolyte for AFC is potassium hydroxide (KOH) or sodium hydroxide (NaOH). They are potential candidates due to their low cost, easy availabUily, least corrosive nature and high solubility. However, it should be noted that if different electrolytes are used, the catalysis, electrodes as well as operating conditions also get changed. Although other fuel cells have reduced the importance of AFC in the recent era, AFC are of technological importance due to their low systems cost and absence of bipolar plates. 

The electrodes and electrolyte for AFC are relatively cheap. Secondly, the bipolar plates are expensive, which contributes to the cost of fuel cells. 

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Alkaline fuel cells (AFCs) were the first type of fuel cell to be widely used in space exploration applications-for example, in NASA s Apollo and space shuttle flights. Figure 1.8 shows a schematic of an AFC stmcture. AFCs use H2 and 02 as fuel and oxidant, respectively. The electrolyte is a concentrated KOH solution absorbed into an asbestos matrix. The temperature for AFCs ranges from 100-250°C and the efficiency can be > 60%. OH ions are transported through the electrolyte from cathode to anode. The reactions are as follows ... 

The optimization of interfaces for low temperature fuel cells has always depended on the availability of specific materials to control the porosity and wetting behavioiu" of the respective pores by the electrolyte. For example, one of the breakthroughs in the development of AFCs was the work of Bacon at a time when the chemically stable hydrophobizing agent, PTFE, was not yet available [4]. Bacon managed interface control with a dual-layer, dualporosity electrode, made out of Ni powders of different grain size. The later availability of PTFE opened up new possibilities for improved GDE designs. 

To develop a highly efficient AFC, an electrode stracture with more than one pore system or with a bimodal pore distribution is required. The liquid and gaseous reactants must reach the electrolyte that has direct contact with the anode and cathode catalysts. For AFCs, different electrode types have been used. The main types of electrodes are the PTFE-bonded GDE and the DSK. 

Different catalysts are used for AFCs. For the hydrogen oxidation reaction, carbon-supported platinum and platinum-palladium catalysts (e.g., noble metal catalysts) are suitable. However, one of the advantages of the AFC compared with acid electrolyte fuel cells, including the phosphoric acid fuel cell (PAFC) and the... 

Alkaline fuel cells (AFC) use concentrated (85 %) KOH as the electrolyte for high temperature operation (250°C) and less concentrated (35-50 %) for lower temperature operation (<120 °C). The problem of slow reaction rate is overcome by using highly porous electrodes, with a platinum catalyst, and by... 

AFC, schematically shown in Figure 8.1, is using 30%-40% KOH(aq) as an electrolyte and operating at temperatures up to around 90°C. The electrolyte is selected to optimize its properties such as conductivity and concentration. Temperature is elevated for improving the electrochemical kinetics and reducing the electrolyte resistivity. AFC does not require using Pt for electrodes, so cheaper material (e.g., Ni) can be used. 

As the name indicates, the electrolyte is an alkaline solution for AFC (OH ion moving across the electrolyte). There are three types of AFCs mobile electrolyte, static electrolyte and dissolved fuel. The first two are being used in the space program. 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

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