Abstract
It has recently been found that there are ideal values for O, OH, and OOH Gibbs adsorption bond strengths to a catalyst that will allow each electron transfer step to have 1.23 V for its reversible potential. In the case of Pt(111), the bond strengths are too high by ~0.9 eV for O and ~0.7 eV for OH and too small by ~0.4 eV for OOH. These discrepancies result in OOH(ads) dissociation to O(ads) + OH(ads) being ~1.2 eV exergonic. The lost Gibbs energy causes the reversible potential for the four-electron reduction on Pt(111) to have a value of ~0.9 V, which is called the effective reversible potential. Volcano plots of activity measured at around 0.9 V depend on the adsorption energies scaling, that is, if one increases they all increase, or if one decreases they all decrease. Volcano plots have been drawn for several transition metal catalysts and platinum and platinum alloy catalysts in the pioneering work of Appleby, Mukerjee, and Adzic and have been seen by many other workers. Although they allow grading the active catalysts in the high overpotential region where reduction current flows, they do not point the direction to better catalysts that will operate at potentials approaching 1.23 V. The search should be for new catalysts which with the right balance of OOH, O, and OH adsorption energies.
Highlights
In the fuel cell literature, great emphasis is given to kinetics in describing the causes of overpotentials
When it is assumed that the strength of OOH bonding to the active site scales with O and OH, current densities measured at an overpotential in the kinetic current range for a series of catalysts obeying the scaling behavior can be graphed as functions of the adsorption bond strength of any one of the intermediates and, with sufficient data, volcano plots emerge
If one is to overcome the overpotential problem, it will be necessary for DG for O, OH, and OOH bonding to the catalyst active site to have particular values which seem incompatible with these alloy catalysts
Summary
In the fuel cell literature, great emphasis is given to kinetics in describing the causes of overpotentials. There is, another dimension to understanding the overpotentials and this may be thought of as a thermodynamic factor, the loss of Gibbs energy for reaction steps during which no transfer of electrons takes place. For the effective reversible potential to increase to the standard value 1.23 V, the OOH adsorption bond strength must instead increase This means catalysts with different adsorption properties must be found if the working potentials oxygen cathodes are to increase beyond those observed with platinum and its alloys that have been studied so far
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