Alkaline-DAFC

The beneficial effect of catalysis by Pd addition to Pt is ascribed to the facile adsorption of OH on this surface which is further reflected by the high yield of formate as well as low yield of carbonate obtained during the oxidation of methanol on Pt e Pd/C catalyst [64]. An overview of catalysts and membranes for alkaline DAFC is found in Ref. [65]. 

Firstly, Nafion and other perfluorinated sulfonic acid ionomers will be discussed, along with inorganic- and organic-Nafion based composites. Secondly, we will introduce non-fluorinated single and composite membranes, including membranes for high temperature DAFC. Finally we will discuss anion conducting membranes for alkaline DAFC. 

The large-scale spread of DAFCs is closely related to the development of efficient anodic and cathodic materials, characterized by very fast electrochemical kinetics, stability at the high current densities in alkaline environments and modest cost. This objective requires cathodes without noble metals and anodes with very low amounts of noble metals. In order to improve the cheapness and sustainability of the processes described above, the most accepted opinion is the possibility of using solar light by means of the introduction of Ti02, pure or doped, into the electrode material formulation. Figure 4.15 shows a typical laboratory-scale photoelectrocatalytic reactor. 

DMFCs and direct ethanol fuel cells (DEFCs) are based on the proton exchange membrane fuel cell (PEM FC), where hydrogen is replaced by the alcohol, so that both the principles of the PEMFC and the direct alcohol fuel cell (DAFC), in which the alcohol reacts directly at the fuel cell anode without any reforming process, will be discussed in this chapter. Then, because of the low operating temperatures of these fuel cells working in an acidic environment (due to the protonic membrane), the activation of the alcohol oxidation by convenient catalysts (usually containing platinum) is still a severe problem, which will be discussed in the context of electrocatalysis. One way to overcome this problem is to use an alkaline membrane (conducting, e.g., by the hydroxyl anion, OH ), in which medium the kinetics of the electrochemical reactions involved are faster than in an acidic medium, and then to develop the solid alkaline membrane fuel cell (SAMFC). 

The less corrosive nature of an alkaline environment ensures a longer durability of the ADAFC, and the faster kinetics of the ORR allows the use of non-noble, low-cost, metal electrocatalysts. Thus, ADAFC meets a number of potential advantages compared to PEM DAFC, which in turn triggers the resurgence of interest in this kind of fuel cell technology. 

This section is devoted to a brief description of the main comptments of DAFC as an introduction to the most exhaustive analysis in Chaps. 2, 3,4, and 5 for electrocatalysts for methanol, ethanol, and higher alcohols, in Chap. 6 for proton exchange and alkaline membranes, and Chap. 7 for carbonous materials used as catalysts support, gas diffusion layers and bipolar plates. 

The faster kinetics of alcohol oxidation and oxygen reduction reactions in alkaline direct alcohol fuel cells opens up the possibility of using less expensive Pt-free catalysts, as nickel, gold, palladium and their alloys [30]. Thus, the cost of ADAFC could be potentially lower compared to the acid DAFC technology if non-precious metal alloys are used for the alcohol electrooxidation, being the nanoparticulated Ni-Fe-Co alloys developed by Acta (Italy) with the trade name of HYPERMEC a good example. 

The development and application of carbonate as well as anion-exchange PEM electrolytes have significantly renewed interest in the development of alkaline-based DAFC [134]. Many of the reaction intermediates, products and paths discussed above for the catalytic oxidation of alcohols in alkaline media have also been identified or speculated to take part in the electrocatalytic oxidatiOTi of these same alcohols. 

The research and development of nanostructured electrode materials for improved performance of the direct alcohol fuel cells (DAFCs) in alkaline electrolytes has continued to grow. Palladium-based nanocatalysts, in particular, have continued to receive much research attention because of their unique properties in alcohol electrooxidation in alkaline media compared to their platinum-based counterparts [1]. 

Palladium is more abundant in nature and sells at half the current market price of platinum. Unlike Pt, the Pd-based electrocatalysts are more active towards the oxidation of a plethora of substrates in alkaline media. The high activity of Pd in alkaline media is advantageous considering that non-noble metals are sufficiently stable in alkaline for electrochemical applications. Importantly, it is believed that the integration of Pd with non-noble metals (as bimetallic or ternary catalysts) can remarkably reduce the cost of the membrane electrode assemblies (MEAs) and boost the widespread application or commercialization of DAFCs [1]. Palladium has proved to be a better catalyst for alcohol electrooxidation in alkaline electrolytes than Pt [2]. Palladium activity towards the electrooxidation of low-molecular weight alcohols can be enhanced by the presence of a second or third metal, either alloyed or in the oxide form [3]. 

Notable efforts are being carried out to design new catalytic structures for DAFC anodes that do not contain Ft or contain tiny amounts of this rare metal and are able to oxidize primary and secondary alcohols with reasonable fast kinetics and tolerable deactivation. Fd is considered as an attractive replacement for Ft in DAFCs. Fd catalysts, unlike Ft ones, are highly active for the oxidation of a large variety of substrates in alkaline environments wherein non-noble metals are sufficiently stable. The dilution of Fd with non-noble metals in a smart catalytic architecture is expected to increase the efficiency and decrease the cost of the DAFCs. 

Direct Alcohol Fuel Cell (DAFC) and Direct Alcohol Alkaline Fuel Cell (DAAFC)... 

The Portable power supply to electronic equipment from DAFC (direct alcohol fuel cell) and DAAFC (direct alcohol alkaline fuel cell) should have minimum recharging interval of 10-12 hours. Recharging means changing of methanol and ethanol fuel cartridges. Cost targets are (a) stack cost US 150-200/kW, (b) fuel cost US 1-1.5/kg. Fuel Efficiency is 30-40%. 

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