Oxygen reduction reaction alcohol oxidation

Recent intensive research efforts have led to the development of less expensive and more abundant electrocatalysts for fuel cells. This book aims to summarize recent advances of electrocatalysis in oxygen reduction and alcohol oxidation, with a particular focus on low- and non-Pt electrocatalysts. The book is divided into two parts containing 24 chapters total. All the chapters were written by leading experts in their fields from Asia, Europe, North America, South America, and Africa. The first part contains six chapters and focuses on the electro-oxidation reactions of small organic fuels. The subsequent eighteen chapters cover the oxygen reduction reactions on low- and non- Pt catalysts. 

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. 

For a given electrochemical system, the increase of the voltage efficiency is directly related to the decrease of the overpotentials of the oxygen reduction reaction, t]c, and alcohol oxidation reaction, T]a, which needs to enhance the activity of the catalysts at low potentials and low temperature, whereas the increase of the faradic efficiency is related to the ability of the catalyst to oxidize completely or not the fuel into carbon dioxide, i.e. it is related to the selectivity of the catalyst. Indeed, in the case of ethanol, taken as an example, acetic acid and acetaldehyde are formed at the anode [10], which corresponds to a number of electrons involved of 4 and 2, respectively, against 12 for the complete oxidation of ethanol to carbon dioxide. The enhancement of both these efficiencies is a challenge in electrocatalysis. 

A facile method for the stereospecific labeling of carbon atoms adjacent to an oxygenated position is the reductive opening of oxides. The stereospecificity of this reaction is due to virtually exclusive diaxial opening of steroidal oxides when treated with lithium aluminum hydride or deuteride. The resulting /ra/w-diaxial labeled alcohols are of high stereochemical and isotopic purity, with the latter property depending almost solely on the quality of the metal deuteride used. (For the preparation of m-labeled alcohols, see section V-D.)... 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

The product of this metabolic sequence, pyruvate, is a metabolite of caitral importance. Its fate depends upon the conditions within a cell and upon the type of cell. When oxygen is plentiful pyruvate is usually converted to acetyl-coenzyme A, but under anaerobic conditions it may be reduced by NADH + H+ to the alcohol lactic acid (Fig. 10-3, step h). This reduction exactly balances the previous oxidation step, that is, the oxidation of glycer-aldehyde 3-phosphate to 3-phospho-glycerate (steps a and b). With a balanced sequence of an oxidation reaction, followed by a reduction reaction, glucose can be converted to lactate in the absence of oxygen, a fermentation process. The lactic acid fermentation occurs not only in certain bacteria but also in our own muscles under conditions of extremely vigorous exercise. It also occurs continuously in some tissues, e.g., the transparent lens and cornea of the eye. 

One of the main problems of low-temperature (20°C-80°C) PEMFCs is the relatively low kinetics rate of the electrochemical reactions involved, e.g., the oxygen reduction at the cathode and fuel (hydrogen from a reformate gas, or alcohols) oxidation at the anode. Another problem, particularly crucial for the electric vehicle, is the relatively low-working temperature (70°C-80°C), which prevents efficient exhaust of the excess heat generated by the power fuel cell. 

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