Electrochemical oxygen reduction, kinetic catalysts

The functions of porous electrodes in fuel cells are 1) to provide a surface site for gas ionization or de-ionization reactions, 2) to provide a pathway for gases and ions to reach the catalyst surface, 3) to conduct water away from the interface once these are formed, and 4) to allow current flow. A membrane electrode assembly (MEA) forms the core of a fuel cell and the key electrochemical reactions take place in the MEA. MEA performance is severely affected by electrode composition, structure, and geometry, and especially by cathode structure and composition, due to poor oxygen reduction kinetics and transport liniitations of the reactants in the cathode catalyst layer. 

The kinetics and mechanism of electrochemical oxygen reduction at different catalysts has been widely studied from the late 1950s onward, as can be seen in the monographs of Hoare (1968), Breiter (1969), and Kinoshita (1992). 

During the 1990s, van Veen and collaborators mainly studied the electrochemical kinetics of oxygen reduction. Their results are presented in Sect. 3. These mechanistic studies were, however, always based on the model in which the C0-N4 or Fe-N4 moieties of the respective macrocycles were retained intact at all pyrolysis temperatures. Their last contribution to the molecular structure of the catalytic site was a study in 2002 of catalysts obtained by adsorption of iron tetramethoxyphenyl porphyrin chloride (ClFeTMPP) on Vulcan XC-72, heat treated between 325 and 800°C in inert atmosphere, and characterized by EXAFS and Mossbauer spectroscopy, as well as by cyclic voltammetry". The loading of these catalysts was 7 wt% chelate ( 0.5 wt% Fe). 

Ramaswamy N, Allen RJ, Mukerjee S (2011) Electrochemical kinetics and X-ray absorption spectroscopic investigations of oxygen reduction on chalcogen-modified ruthenium catalysts in alkaline media. J Phys Chem C 115(25) 12650-12664. 

The RDE or RRDE in an electrochemical cell (Fig. 15.2) is widely used to study ORR kinetics on M-N4/C complexes in electrolyte solutions. Co and Fe macrocycles have been studied extensively as catalysts for oxygen reduction [27, 49-93]. Co-phthalocyanine (CoPc) and Co-tetrasufonated phthalocyanine (CoTsPc) adsorbed on carbon surfaces have been fotmd to catalyze 2e reductimi of O2 to form H2O2 in both alkaline and acid solutions, while Fe-phthalocyanine (FePc) and Fe-tetrasufonated phthalocyanine (FeTsPc) catalyze the overall 4e reduction in alkaline solutions [83-86]. hi an acid solution, certain face-to-face Co-porphyrins, which can form a dioxygen bridge between two Co-centers on graphite surface, have been shown to catalyze 4e reduction [58-60]. 

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. 

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