Electrocatalytic Properties Toward Oxygen Reduction

The first step in this pathway (A) involves the electrochemical reduction of the inactive, oxidized form of the macrocycle, denoted without loss of generality as M(III)P+, to yield the reduced, electrocatalytically active form ofthe species, M(II)P. 

The latter then reacts with 02 to form the adduct M(II)P02 (a) which, as indicated, can be further reduced chemically by a second M(II)P, present either in solution phase or adsorbed on the electrode surface, (b). Alternatively, M(II)P02 can be reduced by the electrode itself to form [M(II)P]202 (B), which may react with yet another M(II)P species to yield a dimeric dioxygen adduct, [M(II)P]202 (c), dissociate to generate peroxide (d), or be subsequently reduced electrochemically to give water as the final product (C). Systematic studies of the factors that control the stability and reactivity of such adducts can provide much insight into the mechanisms by which macrocycles reduce dioxygen in electrochemical environments [26]. 

The main sections to follow address various aspects of homogeneous (Section 3.2) and heterogeneous (Section 3.3) electrocatalysis, using examples reported in the literature, which, in our view, best illustrate the rich behavioral spectrum this unique type of macrocydic compounds, including dimeric species, display toward oxygen 

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The primaiy emphasis in this review article is to showcase the use of LEISS to examine the outermost layers of Pt-Co alloys in order to correlate interfacial composition with electrocatalytic reactivity towards oxygen reduction. In some instances, it is desirable to compare the properties of the outermost layer with those of the (near-surface) bulk an example is when it becomes imperative to explain the unique stability the alloyed Co under anodic-oxidation potentials. In such cases. X-ray photoelectron spectroscopy and temperature-programmed desorption may be employed since both methods are also able to generate information on the electronic (binding-energy shift measurements by XPS) and thermochemical (adsorption enthalpy determinations by TPD) properties at the sub-surface. However, an in-depth discourse on these and related aspects was not intended to be part of this review article. 

On the basis of the known ability of porphyrins to bind nucleic acids and of the catalytic properties of cobalt porphyrins toward oxygen reduction, a biosensor was prepared for electrocatalytic detection of short noncoding ribonucleic... 

The primary challenge in commercialization of MCFC remains in the proper selection of materials for the cathode. The life expectancy of the electrode structure is aimed toward 40,000 hr for successful commercialization of MCFC. The following cathode properties were recognized as of fundamental importance with respect to the cell performance 1) high electronic conductivity at 650°C (cr > 1 S/cm) 2) low chemical reactivity and solubility in the electrolyte 3) thermodynamic stability at 650°C in carbonate electrolyte at different partial pressures of O2/CO2 mixtures 4) high electrocatalytic activity for the oxygen reduction reaction and 5) suitability for the fabrication of porous electrodes. ... 

ECPs including coordination complexes are also able to show electrocatalytic properties e.g., toward the oxidation of nitric oxide in the case of porphyrin functionalized polypyrroles containing various metallic centres [275], for the oxygen or hydrogen peroxide reduction in the case of cobalt-salen PEDOT [276] or iron-containing polysalen [245], or for oxidation of ascorbic acid in the case of osmium bipyridyl functionalized PPy [277]. 

Electrocatalysts advocated for methanol oxidation at the anode and oxygen reduction at the cathode in DMFCs are required to possess well-controlled structure, dispersion, and compositional homogeneity [46 9]. The electrocatalytic activities of both anode and cathode catalysts are generally dependent on numerous factors such as particle size and particle size distribution [50-54], morphology of the catalyst, catalyst composition and in particular its surface composition [55,56], oxidation state of Pt and second metal, and microstructure of the electrocatalysts [49,57,58]. With the frequently attempted surface manipulation strategies for nanosized electrocatalysts to increase their catalytic efficiencies toward MOR and ORR, rigorous characterization techniques which can provide information about nanoscale properties are critically required. For example, parameters such as particle size and variation in surface composition have strong influence on catalytic efficiency. Further, if the nanoparticles are comprised of two or more metals, both the composition and the actual distribution will... 

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