Alkaline fuel cells electrode reactions

Alkaline solutions are generally known to lead to better catal5Tic activities than acidic solutions for many relevant electrode reactions. However, owing to the paucity in the development of suitable electrolyte materials, such as alkaline membranes, there has been much less fundamental work in the area of fuel cell catalysis in alkaline media. Nevertheless, there are a few hopeful developments in new alkaline polymer membranes [Varcoe and Slade, 2005] that are currently stirring up interest in smdying fuel cell catalytic reactions in alkalme solution. 

Fuel cells operate in a manner reverse to that of electrolysis, discussed in Chapter 2, combining fuel to make electricity. The basic design consists of two electrodes separated by an electrolyte. The oldest type of fuel cell is the alkaline fuel cell where an alkaline electrolyte like potassium hydroxide is used. The hydrogen enters through the anode compartment and oxygen through the cathode compartment. The hydrogen is ionized by the catalytic activity of the anode material and electrons are released into the external circuit. The protons react with the hydroxyl ions in the electrolyte to form water. The reaction can be written as ... 

Although the Bonnemann method is very interesting by allowing to vary and to control easily the composition and the nanostructure of the catalyst and is adapted to the preparation of real fuel cell electrodes, it displays also some limitations. For example, bismuth-containing colloids could not be prepared with the Bonnemann method, and even in presence of platinum salts. Moreover, the presence of bismuth hinders the reduction of platinum salts [59], However, platinum-bismuth is a good catalyst for ethylene glycol electro-oxidation in alkaline medium [59-62], Moreover, colloid of tin alone could not be obtained, and the reaction was only possible by coreduction in the presence of a platinum salt. Then, other colloidal methods should be developed keeping in mind the necessity of a similar flexibility as that of the Bonnemann method. 

The cathode of the zinc-air cell uses a zinc metal anode and an air electrode similar to that for an alkaline fuel cell. The cell reaction is given in Equation 10.4. The cell is constructed with a pull-off... 

Further research was performed on alkaline fuel-cell concepts (e.g. characterisation of gas diffusion electrodes) as well as on catalytic burners (reaction kinetics of H2/air mixtures). Experimental investigation of dymanic combustion phenomena was performed. Practical tests were carried out on internal combustion engines, including compression ignition engines (Altman el al., 1997 Schucan, 2000)... 

Alkaline fuel cells (AFCs) use an aqueous potassium hydroxide (KOH) solution (around 30%) as electrolyte and have electrode reactions of the form... 

Alkaline fuel cells required very pure hydrogen. That was problematic when hydrogen was produced from common fuels such as natural gas or coal. Any residual C02 in the hydrogen reacts with the liquid alkaline electrolyte, gumming up the electrodes microscopic pores and slowing the overall chemical reactions. 

This low-temperature fuel cell uses H2 and O2 reactants and a highly alkaline aqueous KOH electrolyte. The advantages of this fuel cell are the faster oxygen reduction reaction in the alkaline electrolyte and the possibility of using low-cost, nonprecious metal electrode catalysts, such as Ag-loaded carbon powder. The greatest problem with alkaline fuel cells is that the electrolyte reacts with traces of CO2 to produce insoluble carbonates. 

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. 

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... 

Whereas there is a significant understanding of the phenomena taking place in the interface electrode surface/solution and of the reaction mechanism in the case of ethanol electro-oxidation in acid medium, the ethanol electro-oxidation in alkaline medium has been much less investigated due to some difficulties inherent to the utilization of alkaline fuel cells [74]. 

The anode electrode-catalyst is one of the important components of the alkaline fuel cell as it helps in the electro-oxidation of fuel. It is desirable that the anode electrode-catalyst provides faster reaction kinetics and 100% oxidation of fuels to CO2 and H2O. The most widely used catalyst, without doubt, is platinum. Platinum seems to be the best choice for acidic solutions, but other metallic alloy with platinum or other metals can match its performance in alkaline medium because of the favorable fuel oxidation in alkaline medium. Different anode materials based on Pt (Prabhuram et al. 1998, Moralldn et al. 1995, Tripkivic et al. 1996), Pt-Ru (Wang et al. 2003, Manoharan et al. 2001), Co-W alloys (Shobba et al. 2002), sintered Ag/ PdO (Koscher et al. 2003), spent carbon electrodes impregnated with Fe, Fe" or Ag (Verma 2000), nickel impregnated silicate-1 (Khalil et al. 2005) and nickel dimethylglyoxime complex (Golikand et al. 2005) are some of the catalysts studied for the electro-oxidation of methanol in alkaline medium. 

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