First-Principles Simulation of the Active Sites and Reaction Environment in Electrocatalysis - Fuel Cell Catalysis: A Surface Science Approach - page 122

concentration), however, it demonstrates that the electrolyte structure a few water
layers from the adsorbate will affect the energetics associated with electron transfer,
and cautious generalization could be made to other electrochemical reactions. In
fact, this concept is not too far from that of OHP electron transfer reactions, such as
the Fe 2 þ /Fe 3 þ redox event that occurs a few ˚ngstr¨ ms away from the electrode
[Smith and Halley, 1994].
Subsequent diffusion of the proton through a Grotthius mechanism to form OOH
was found to be an activated process. The barrier was calculated as a function of poten-
tial using the climbing nudges elastic band method [Henkelman and Jonsson, 2000;
Henkelman et al., 2000; Mills et al., 1995] and added to the reaction energy of step
(4.15) (i.e., referenced to the initial state with the proton and electron in bulk
reference states) to give the overall barrier for the reduction of O 2 . The potential-
dependent reaction barrier and the transition state structure at 1.0 V/NHE are illus-
trated in Fig. 4.15. The calculated barrier at 0.8 V/NHE of 0.34 eV agrees well
with that measured by Damjanovic and Sepa (0.26 eV) [Damjanovic and Sepa,
1990]. Although further work examining the dependence of this barrier on the
water/electrolyte structure is planned, this initial agreement with experiment illus-
trates the ability of this method to examine ion transport and electron transfer at the
aqueous, electrified interface.
4.5 SUMMARY AND CONCLUSIONS
The chemistry at the electrified aqueous/metal interface is quite fascinating, as its
structure, properties, and dynamics can significantly influence reaction energetics,
dictate the kinetics that control catalytic selectivity, and open up novel reaction
pathways and mechanisms.
The presence of water on a metal substrate acts to stabilize both charged and
polarized surface intermediates, thus favoring heterolytic bond activation processes.
Water enhances reactions that result in charged or polarized transition states through
classic charge stabilization. More interestingly, there are a number of reactions
where water can act directly as a co-catalyst in facilitating proton transfer. This is
seen in the heterolytic activation of water, CO oxidation, and methanol dehydrogena-
tion. In a similar manner, water can help to “catalyze” diffusion by providing proton
shuttling pathways.
The structure of water that forms in the double layer is controlled by an optimal bal-
ance between the interactions of adsorbed water and the metal surface and hydrogen
bonding between co-adsorbed water molecules at the interface, as well as adsorbed
water and water in solution. The interfacial water structure undergoes significant
changes as a result of changes in the potential. Higher potentials show stronger
affinities for the oxygen, lead to the migration of OH x intermediates to higher-fold
coordination sites, and ultimately aid in O22H bond activation.
Changes in the potential can significantly polarize the water structure at the
aqueous/metal interface and thus alter the overall reaction energies. This is particu-
larly true for systems in which there is a significant change in the dipole between