Electrocatalyst thermodynamics

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

The potency of an electrocatalyst is usually defined in terms of the potential required to carry out a specific process at a prescribed rate, or current, per unit area of electrode. Platinum, for example, promotes hydrogen evolution and hydrogen oxidation in aqueous electrolytes, at very high rates at potentials very close to the thermodynamic redox potential for the reaction H+(aq) I e - - ViH2(g), that is, small overpotentials, T. Hence, it is a far more potent electrocatalyst than, for example, Hg or carbon, for which the onset for either reaction occurs at potentials far removed from that predicted value, that is, large T. ... 

Often analytes are irreversibly (slowly) oxidized or reduced at an electrode, that is, require a substantial overpotential to be applied beyond the thermodynamic redox potential (E°) for electrolysis to occur. This problem of slow electron transfer kinetics has spawned much research in the development of electrocatalysts, which may be covalently attached to the electrode, chemisorbed, or trapped in a polymer layer. The basis of electrocatalytic CMEs is illustrated in Figure 15.5. Red is the analyte in the reduced form, which is irreversibly oxidized, and Ox is its oxidized form. The redox mediator is electrochemically reversible and is oxidized at a lower... 

Recently it was proposed that PEMLC electrocatalysts may also be prepared by water-in-oil microemulsions. These are optically transparent, isotropic, and thermodynamically stable dispersions of two nonmiscible liquids. The method of particle preparation consists of mixing two microemulsions carrying appropriate reactants (metal salt + reducing agent), to obtain the desired particles. The reaction takes place during the collision of water droplets, and the size of the particles is controlled by the size of the droplets. Readers are referred to the early work of Boutonnet et al. [149], the review paper of Capek [150] and refs. [128,151], and 152 for fuel cell apphcations. The carbonyl route has the ability to control the stoichiometry between bimetallic nanoparticles, but also the particle size. The reader is referred to review papers for more details [106,107]. Other methods, including sonochemical and radiation-chemical, have been used successfully for the preparation of fuel cell catalysts (see, e.g., review articles 100 and 153). 

Interestingly, when an aqueous solution of the electrocatalyst [Cp Ir(bpy)(Cl)][Cl] is irradiated with visible light, H2 evolves near the thermodynamic potential. No activity is observed in the dark. This peculiar mechanism depends on the fact that a single complex is both the active light absorber and the active electrocatalyst. By optimising the electronic structure of the complex and reaction conditions, higher rates of H2 evolution could be achieved even at milder applied potentials. ... 

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