Aspects of Electrochemical Oxygen Reduction

An alternative reaction path involving Fe would be possible after (3.4). This path is described by the following reaction steps  

Precursor Carbon support Heat-treatment temperature (°C) (mV/ decade) Electrolyte References 

2 CoNPc and 2.3 CoNPc Printex XE2 and Black Pearls 500 42 and 40 0.25 MH2SO4 [51] 

ClFeTMPP Black Pearls 800 60-68 0.1 MH2SO4, Ha04,H3P04 [68] 

Tafel slopes which are usually found for O2 reduction on Fe- and Co-based catalysts and on Pt. A list of Tafel slopes experimentally found for these catalysts is given in Table 3.4. 

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Kinetic and Mechanistic Aspects of Electrochemical Oxygen Reduction... 

A widespread interest for the electrochemical oxygen reduction reaction (ORR) has two aspects. The reaction attracts considerable attention from fundamental point of view, as well as it is the most important reaction for application in electrochemical energy conversion devices. It has been in the focus of theoretical considerations as four-electron reaction, very sensitive to the electrode surface structural and electronic properties. It may include a number of elementary reactions, involving electron transfer steps and chemical steps that can form various parallel-consecutive pathways [1-3]. 

The third aspect to consider is the electrochemical stability of the material used. For the oxygen reduction reaction, the electrode potential is highly anodic and at this potential, most metals dissolve actively in acid media or form passive oxide films that will Inhibit this reaction. The oxide forming metals can form non-conducting or semi-conducting oxide films of variable thickness. In alkaline solutions, the range of metals that can be used is broader and can include non-precious or semi-precious metals (Ni, Ag). 

Respiration, electrochemical aspects — The function of the enzymes in the mitochondrial respiratory chain is to transform the energy from the redox reactions into an electrochemical proton gradient across the hydrophobic barrier of a coupling membrane. Cytochrome oxidase (EC 1.9.3.1, PDB 20CC) is the terminal electron acceptor of the mitochondrial respiratory chain. Its main function is to catalyze the reaction of oxygen reduction to water using electrons from ferrocytochrome c 4H+ + O2 + 4e 2H2O. Cytochrome oxidases can trans-... 

In spite of the considerable effort expended in trying to unravel the fundamental aspects of the O2 electroreduction reaction, many details about the mechanism are not fully understood. The electrochemical reduction of oxygen is a multielectron reaction that occurs via two main pathways one involving the transfer of two electrons to give peroxide, and the so-called direct four-electron pathway to give water. The latter involves the rupture of the 0-0 bond. The nature of the electrode strongly influences the preferred pathway. Most electrode materials catalyze the reaction via two electrons to give peroxide Peroxide pathway in acid... 

Hoare JP (1979) Some aspects of the reduction of oxygen at a platimmi-oxygen alloy diaphragm. J Electrochem Soc 126 1502-1504... 

The reaction has been subject of numerous experimental and theoretical studies mechanistic aspects based on theoretical considerations are described and summarized in Section 3.3.2. The detailed explanation of the reaction mechanism serves as the basis for the forthcoming discussion of the appropriate choice of catalysts and requirements due to the different intermediates. Experiments were either performed in the gas phase or in an electrochemical environment, whereas only the electrochemical experiments can be considered to bear a realistic resemblance of the fuel cell environment. Contrary to PEMFCs, where the oxygen reduction reaction is the rate-limiting step, in DMFCs the slow kinetics of the MOR represent the hmiting factor. 

Hoare, J. (1979). Some Aspects of the Reduction of Oxygen at a Platinum-oxygen Alloy Diaphragm, J. Electrochem. Soc., 126, pp. 1502-1504. 

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