Proton exchange membrane DEFC

As described before in this chapter, conventional DEFCs can be divided into two types as a function of fhe employed membrane, namely proton exchange membrane DEFCs (PEM-DEFCs) and anion exchange membrane DEFCs (AEM-DEFCs), used in acidic and alkaline medium, respectively. As previously reported, Pt-based catalysts undergo rapid poisoning of the catalytic sites, which compromises cell performance. On the other hand, the kinetics of both ethanol oxidation (OER) and the oxygen reduction reaction (ORR) in alkaline medium are much faster than the corresponding kinetics in acidic medium, which substantially improves cell performance. The main limitation to the cell performance in AEM-DEFCs is the physical and chemical stability of the AEM [71]. Another problem encountered with the AEM is that its ionic conductivity is about one order of magnitude lower than that of Nafion membranes. 

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). 

After rehearsing the working principles and presenting the different kinds of fuel cells, the proton exchange membrane fuel cell (PEMFC), which can operate from ambient temperature to 70-80 °C, and the direct ethanol fuel cell (DEFC), which has to work at higher temperatures (up to 120-150 °C) to improve its electric performance, will be particularly discussed. Finally, the solid alkaline membrane fuel cell (SAMFC) will be presented in more detail, including the electrochemical reactions involved. 

The DEFC transforms directly the Gibbs energy of combustion of ethanol into electricity, without a fuel processor. This greatly simplifies the system, reducing its volume and cost [22, 23]. The important development of DEFCs is due to the use of a proton exchange membrane as electrolyte, instead of a liquid add electrolyte, as done previously. 

Direct Alcohol Fuel Cells (DAFCs), Fig. 1 Schematic diagram of a DEFC based on a proton exchange membrane... 

As an example, the working principle of a DEFC is illustrated in Fig. 9.1. The electrochemical cell consists of two electronic conducive electrodes, an anode and a cathode separated by an ionic conductive solid electrolyte (a proton exchange membrane generally of Nation type). At the anode the electro-oxidation of alcohol takes place as follows ... 

Direct alcohol fuel cells (DAFCs) are a type of proton exchange membrane fuel cell in which alcohol is directly used as fuel. The majority of the studies about such type of fuel cells have been conducted by using methanol as fuel, leading to the so-called Direct Methanol Fuel Cells (DMFCs). However, the use of ethanol, rather than methanol, as fuel is attracting a great deal of attention. This is because ethanol is, or can be, obtained from renewable sources such as sugarcane in which case, the energy obtained by Direct Ethanol Fuel Cells (DEFCs) could result in net zero CO2 emissions. Furthermore, ethanol is less toxic and less... 

The principle of PEM-DEFC (proton exchange membrane direct ethanol fuel cell) operation is illustrated in Eigure 15.1. The anode consists of an ethanol solution, while the cathode is composed by humidified air or oxygen, so that good conductivity is maintained in the PEM. 

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