First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 123

the reactants and product states. The change in dipole leads to significant differences in
the reactants' and products' responses to changes in the applied potential or electric
field. The changes in applied potential for these systems are rather nonlinear, as the
interfacial capacitance plays an important role. For reactions in which the reactant
and product states have similar dipoles, however, the polarization of both states is simi-
lar and therefore tends to cancel out. The potential effects for these systems can then
safely be decoupled from the reaction energies. The reaction energies for one-electron
oxidation or reduction processes are then simply linear with respect to the potential, as
capacitance differences do not alter relative energies.
The common framework for describing adsorption in the gas phase can break down
when modeling adsorption at an aqueous/metal interface. While there is a gain in
energy as solute molecules approach the surface, the water molecules at the surface
must be displaced from the surface in order for the solute to adsorb. The displacement
will depend upon the potential as well as upon both enthalpic and entropic consider-
ations. We have shown here that water can only displace O 2 from the Pt(111) surface
at higher potentials that tend to be outside the typical operating ranges of a PEMFC
cathode. This analysis only describes the overall equilibrium behavior. Water may
begin to compete at lower potentials if the system is kinetically limited. Sulfate
anions in solution, on the other hand, have been shown to displace oxygen and
water from the surface at potentials greater than 0.9 V, and thus act to inhibit the
kinetics of different surface reactions.
The elementary reaction energies and thermodynamics for methanol dehydrogena-
tion have been shown to be significantly influenced by electrode potential. The
oxidation pathways become much more favorable at higher potentials. The relative
barriers of O22HtoC22H bond activation decrease with increasing potential,
which decreases the overall selectivity to CO and CO 2 and increases the yield of
formaldehyde. This is consistent with experimental studies. The oxidation of CO
intermediates appears to occur via adsorbed hydroxyl intermediates. The hydroxyl
intermediates are more weakly held to the surface than atomic oxygen, and thus
have significantly lower barriers for the oxidation of CO.
Lastly, we have shown that transport of ions across the double layer is facilitated by
water via proton transfer and that the barrier for the reduction of O 2 is controlled by
electron transfer that occurs as the proton moves close to the adsorbed O 2 to form a
reactive center. Electron transfer appears to occur before the actual formation of the
OO22H bond.
REFERENCES
Anderson AB. 1981. Reactions and structures of water on clean and oxygen covered Pt(111) and
Fe(100). Surf Sci 105: 159 - 176.
Anderson AB. 1990. The influence of electrochemical potential on chemistry at electrode
surfaces modeled by MO theory. J Electroanal Chem 280: 37 - 48.
Anderson AB, Albu TV. 1999. Ab initio determination of reversible potentials and activation
energies for outer-sphere oxygen reduction to water and the reverse oxidation reaction.
J Am Chem Soc 121: 11855 - 11863.