Overpotentials carbons

Oscillometry 527 as analytical tool, 528 titrations (H.F.), 527 Osmium tetroxide catalyst 381 Ostwald s dilution law 31 Ovens electric, 97 microwave, 97 Overpotential 506 Overvoltage see Overpotential Oxalates, D. of as calcium carbonate via oxalate, (g) 484... 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

At the electrode surface there is competition among many reduction reactions, the rates of which depend on iQ and overpotential q for each process. Both /0 and q depend on the concentration of the electroactive materials (and on the catalytic properties of the carbon surface). However, the chemical composition of the SEI is also influenced by the solubility of the reduction products. As a result, the voltage at... 

Oscillatory reactions carbon monoxide oxidation, 388 electrochemical promotion of, 389 Overpotential activation, 124 anodic, 122 cathodic, 122 cell, 123... 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

The above brief analysis underlines that the porous structure of the carbon substrate and the presence of an ionomer impose limitations on the application of porous and thin-layer RDEs to studies of the size effect. Unless measurements are carried out at very low currents, corrections for mass transport and ohmic limitations within the CL [Gloaguen et ah, 1998 Antoine et ah, 1998] must be performed, otherwise evaluation of kinetic parameters may be erroneous. This is relevant for the ORR, and even more so for the much faster HOR, especially if the measurements are performed at high overpotentials and with relatively thick CLs. Impurities, which are often present in technical carbons, must also be considered, given the high purity requirements in electrocatalytic measurements in aqueous electrolytes at room temperature and for samples with small surface area. 

CO electro-oxidation exhibits a strong particle size dependence on both carbon-and titania-supported Au catalysts a strong deactivation of the reaction is observed for particle sizes below about 3 nm. In the case of the titania supports, however, a distinct activation of the reaction is also evident. This manifests itself in a strong decrease in the overpotential for the reaction, and an increase in activity as the particle size decreases in the range 8-3 nm. The result is a maximum in the catalytic activity with particle size. 

Fig. 23. (a) Experimental IR-free overpotentials in MCFC-based separator. Cell performance 0.25% C02 Feed. All curves calculated [32] (b) C02 production scheme using molten carbonate fuel cell stack. 

It is clear from the calculated limiting-current curves in Fig. 3a that the plateau of the copper deposition reaction at a moderate limiting-current level like 50 mA cm 2 is narrowed drastically by the surface overpotential. On the other hand, the surface overpotential is small for reduction of ferri-cyanide ion at a nickel or platinum electrode (Fig. 3b). At noble-metal electrodes in well-supported solutions, the exchange current density appears to be well above 0.5 A/cm2 (Tla, S20b, D6b, A3e). At various types of carbon, the exchange current density is appreciably smaller (Tla, S17a, S17b). 

Since noncatalyzed carbon dioxide reduction shows a large overpotential and potentials far more negative than -2.0 V versus SCE... 

Carbon electrodes modified by polymeric films of [Ru(bpy)(CO)2] appear to be efficient molecular cathodes for selective reduction of C02 into CO (rj >95%) especially in pure aqueous electrolyte, at a moderate overpotential (—1.2V vs. Ag AgCl).93 Strongly adherent thin films of [Ru(bpy)(CO)2]ra can also be easily prepared from the electroreduction of monobipyridyl mono-or binuclear complexes of Ru containing two leaving groups per Ru, such as [Ru(bpy)(CO)2Cl2], [Ru(bpy)(CO)2(MeCN)2]2+, [Ru(bpy)(CO)2(MeCN)]22+, and [Ru(bpy)(CO)2Cl]2.94"97... 

From gas phase measurements CO is known to prefer top sites on all three low index faces, with the CO molecule perpendicular to the surface and bonded through the carbon end of the molecule except at high coverages (27). It is likely that HCOOH and COOH are adsorbed in a similar way. It is not likely that they could "enter the "troughs , which seems to be possible for anions. For Pt(100) on the other hand, upon sweep reversal and gradual oxide reduction, anions are immediately adsorbed on that "flat" surface. They block adsorption of HCOOH. Adsorption of anions decreases as potential becomes more negative. The oxidation of HCOOH commences and the rate increases as at more negative potentials, i.e. at lower overpotential. A competition between anions and HCOOH adsorption explains this apparently anomalous behaviour. The explanation of the "anomalous behaviour of the Pt(110) surface can be also found in the data for stepped surface vicinal to the (100) and (110) orientations. 

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