Electrocatalytic activity catalyst utilization

Modified electrodes containing cyclam derivatives have been prepared. The approach utilizing cyclam incorporated in Nafion film on a carbon electrode shows that the catalytic efficiency of the system is much lower than observed when the catalyst is adsorbed on the mercury. With electrodes prepared following the Langmuir Blodgett technique, only the electrode materials that allow the orientation of the monolayer so that the tail points to the substrate were found to be electrocatalytically active.165... 

Noble metals applied as electrocatalysts for the oxygen reduction have been largely utilized because of their high electrocatalytic activity and stability. Investigations are concentrated on platinum, palladium, silver and gold. The application of noble metal catalysts is limited by two fundamental disadvantages high cost and low availability. Thus, it is important to construct cathodes with small amounts of the noble metal which are obtained, for example, by dispersed platinum on an appropriate support. 

So far, we have focused on the formal description of current generation in the catalyst layer and discussed major effects of structure and composition on exchange current density and catalyst utilization. In the remainder of this chapter, we will explore in detail, how electrocatalytic activity interferes with other processes at the catalyst surface (e.g. surface diffusion) and transport in the bulk phases. The key measure of catalyst layer performance is the current density that could be extracted from a cell for a given cell potential. This links the spatially varying concentrations and reaction rates with the global performance, rated in terms of power density and fuel cell efficiency. 

In addition to improvements in the intrinsic electrocatalytic activity, research has been carried out to modify ( design , possibly) the catalyst surface in order to enhance the catalyst utilization efficiency and ultimately the fuel oxidation superficial current density. The latter can be expressed in terms of catalyst layer physico-chemical properties as [218] ... 

As discussed above, f is a static materials property. It is the product of the specific electrocatalytic activity of the catalyst surface times statistical factors that arise at all scales due to the random morphology and distribution of the catalyst in the composite CL, as considered in Equation 8.2. The reaction penetration depth is a steady state property, which is mainly determined by the nonlinear coupling between transport of oxygen and protons and exchange current density. Together, both parameters, f and, determine the overall effectiveness of catalyst utilization. 

The statistical surface area utilization factor Fstat has been considered under different conditions, specifically in catalyst powders and in MEAs of operational PEFCs. The electrocatalytically active surface area in the catalyst powder can be obtained from the charge under the H-adsorption or CO-stripping waves measured by... 

Higher Pt loading causes a transition of the Pt/C surface from predominantly hydrophobic to predominantly hydrophilic. The change in surface wettability of Pt/C triggers a structural inversion in the ionomer film. This inversion of the orientation of the ionomer film has implications for catalyst utilization, density distribution and conductivity of protons, electrocatalytic activity, and water balance in catalyst layers. 

The definition of the electrochemical Thiele modulus [Eq. (9b)J characterizing the degree of electrocatalyst utilization is a prerequisite for properly tailoring the micromorphology of porous electrocatalytic electrode coatings and fuel cell electrodes, as it allows matching of the coating or catalyst particle dimension to the catalytic activity of the material ... 

Hydrogen Oxidation. Transition metal carbides have shown activity in electrocatalytic oxidation of hydrogen, alcohols, CO, formic acid etc. These chemicals are potential fuels of low temperature fuel cells, which utilize Pt-based electrocatalysts. The Pt catalysts are expensive and easily poisoned and represent one of the most serious barrier for practical applications of such fuel cells. Thus it is natural that many research groups have investigated the feasibility to replace them with transition metal carbides. 

The heme-containing cytochrome P450 catalysts that hydroxylate CH bonds and are active as epoxidation catalysts are known to be monooxygenases. They utilize molecular oxygen as a source of two electrons to catalyze oxygen insertion into a CH bond. The overall electrocatalytic cycle for the oxidation reaction is shown in Scheme... 

Multicopper oxidases, such as laccase, also activate oxygen at a site containing a three-plus-one arrangement of four Cu atoms, and are known to catalyze the four-electron reduction of >2 Electrocatalytic reduction of O2 has been performed to probe the catalytic reactivity of synthetic CcO model complexes, and some copper complexes have also been utilized as electro-catalysts. In a homogeneous system, a copper complex [(L)Cu°f (L = tris(2-pyridylmethyl)amine) can efficiently catalyze the four-electron reduction of O2 by one-electron reductants, such as ferrocene derivatives, in the presence of HCIO4 in acetone, as shown in Scheme 4.10. The initial electron transfer from Fc to [(LjCu ] with O2 affords the two-electron reduction of... 

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