Direct alcohol fuel cells DAFCs

In recent decades, direct alcohol fuel cells (DAFCs) have been extensively studied and considered as possible power sources for portable electronic devices and vehicles in the near future. The application of methanol is limited due to its high volatility and toxicity, although it is relatively easily oxidized to CO2 and protons. So other short chain organic chemicals especially ethanol, ethylene glycol, propanol, and dimethyl... 

C. Lamy, E. M. Belgsir, and J.-M. Leger, Electrocatalytic oxidation of aliphatic alcohols Application to the direct alcohol fuel cell (DAFC), J. Appl. Electrochem. 31, 799-809 (2001). 

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). 

Oxidation of Alcohols in a Direct Alcohol Fuel Cell The electrocatalytic oxidation of an alcohol (methanol, ethanol, etc.) in a direct alcohol fuel cell (DAFC) will avoid the presence of a heavy and bulky reformer, which is particularly convenient for applications to transportation and portable electronics. However, the reaction mechanism of alcohol oxidation is much more complicated, involving multi-electron transfer with many steps and reaction intermediates. As an example, the complete oxidation of methanol to carbon dioxide ... 

In order to improve the fuel utilization in a Direct Alcohol Fuel Cell (DAFC) it is important to investigate the reaction mechanism and to develop active electrocatalysts able to activate each reaction path. The elncidation of the reaction mechanism, thus, needs to combine pnre electrochemical methods (cyclic voltammetry, rotating disc electrodes, etc.) with other physicochemical methods, such as in situ spectroscopic methods (infrared and UV-VIS" reflectance spectroscopy, or mass spectroscopy such as EQCM, DEMS " ), or radiochemical methods to monitor the adsorbed intermediates and on line chromatographic techniques"" to analyze qnantitatively the reaction products and by-products. 

Direct alcohol fuel cells (DAFC) are very attractive as power sources for mobile and portable applications. The alcohol is fed directly into the fuel cell without any previous chemical modification and is oxidized at the anode while oxygen is reduced at the cathode. Methanol has been considered the most promising fuel because it is more efficiently oxidized than other alcohols. Among different electrocatalysts tested in the methanol oxidation, PtRu-based electrocatalysts were the most active [1-3]. In Brazil ethanol is an attractive fuel as it is produced in large quantities from sugar cane and it is much less toxic than methanol. On the other hand, its complete oxidation to CO2 is more difficult than that of methanol due to the difficulty in C-C bond breaking and to the formation of CO-intermediates that poison the platinum anode catalysts. Thus, more active electrocatalysts are essential to enhance the ethanol electrooxidation [3],... 

Ethanol is considered as the ideal fuel for the so-called direct alcohol fuel cells (DAFCs). This is because ethanol has a number of advantages over methanol it can be produced in a sustainable manner, easily stored and transported, and is less toxic or corrosive than methanol. The theoretical mass energy of ethanol is 8.0 kWh kg compared to 6.1 kWh kg" for methanol. The complete oxidation of ethanol releases 12 electrons per molecule its standard electromotive force E° q =1145V, is similar to that of methanol. 

Lamy C, Lima A, LeRhun V, Delime F, Coutanceau C, Leger J-M (2002) Recent advances in the development of Direct Alcohol Fuel Cells (DAFC). J Power Sources 105 283-296... 

Considering all the types of Direct Alcohol Fuel Cells (DAFC) currently in development, that using methanol (DMFC) is closer to massive commercialization. DMFC exhibits higher current and power densities than fuel cells using ethanol, ethylene glycol, etc., mainly due to the difficulty for breaking the C-C bonds of higher alcohols. For this reason this Chapter will be mainly devoted to review the applications of DMFC. 

The dilution of Pd with non-noble metals in a smart catalytic architecture capable of rapidly and stably oxidizing alcohols on anode electrodes would knock down the main barriers to the commercialization of direct alcohol fuel cells (DAFC), especially those fed with primary alcohols, hi er than methanol, and polyalcohols. Indeed, apart from methanol for which there exist platinnm based catalysts capable of prodncing cnrrent densities of several tens of mW cm, the higher alcohols like ethanol and polyalcohols like glycerol are difficnlt to oxidize on platinum or platinum alloyed with either noble or non-noble metals. 

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]. 

One of the main issues in the direct alcohol fuel cells (DAFCs) is that the fuel can easily permeate into the cathode through the proton exchange membrane, which causes dramatic performance loss since the currently used Pt-containing cathode catalysts have no or little methanol tolerance. One of the advantages of Pd-M alloys over Pt in DAFCs is their high methanol and ethanol tolerance in acid. In particular, methanol tolerance was demonstrated for Pd-Fe, Pd-Co, Pd-Cr, Pd-Ni, and Pd-Pt alloys [19, 41, 53, 77-80]. 

In direct alcohol fuels cells (DAFCs), some simple organic molecules such as methanol, ethanol, formic acid, and ethylene glycol are used as alternative fuels. Besides the slow kinetics of ORR in the cathode, the slow alcohol oxidation reaction on Pt is another major contribution to low DAFC performance. 

In this context, hydrogen carriers like alcohols feeding a direct alcohol fuel cell (DAFC) appear advantageous for two main reasons they are liquid (which simplifies the problems of storage and distribution) and their theoretical mass energy density is rather high, close to that of gasoUne (6.1 and 8.0 kWh/kg for methanol and ethanol, respectively [4]). The most studied alcohols are methanol [5], which is the simplest mono-alcohol, and ethanol [6]. 

Direct Alcohol Fuel Cells (DAFCs), Table 1 Thermodynamic data associated with the electrochemical oxidation of some alcohols (under standard conditions) ... 

In order to improve the fuel utilization in a direct alcohol fuel cell (DAFC) it is important... 

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