Environmental Engineering Reference
In-Depth Information
4.4 APPLICATIONS OF DENSITY FUNCTIONAL THEORY
TO ELECTROCHEMICAL SYSTEMS
Water molecules, water clusters, water layers, or aqueous solution can influence the
kinetics and thermodynamics governing catalytic reactions, even to the extent of com-
pletely altering the preferred reaction path. These mechanistic changes are due to the
ability of water to stabilize charged or highly polarized adsorbed intermediates and
transition-state species, which induces structural, electronic, and energetic variations
in surface adsorption, diffusion, reaction, and desorption processes. We present var-
ious example studies here in which interfacial water significantly influences the reac-
tion energetics at the electrode - aqueous electrolyte interface.
4.4.1 Structure of Water in the Double-Layer Region
The majority of fuel-cell-relevant reactions occur in or near the limits of the double-
layer potential region for the pure electrolyte. In this region, charging of the electrode
results in polarization of the electrode - electrolyte interface with negligible rates of
redox reactions involving the electrolyte. Before examining electrocatalytic reaction
mechanisms under these conditions, we first examine the structure of water at the elec-
trode interface and the effects of potential on this structure. Studies of the structure of
monolayer and multilayer adsorbed water at the metal interface have contributed sub-
stantially to our understanding of the aqueous interface. The adsorption of a single
water molecule to Group VIII and IB metal surfaces is generally quite weak, with
H
2
O bound to atop sites on the metal surface with a tilted configuration and
metal22O bond distances of around 2.122.3
˚
[Michaelides et al., 2003b].
Adsorption energies calculated by DFT for vapor phase H
2
O (low-coverage) bound
to Group VIII and IB metal surfaces are listed in Table 4.1. The vapor phase binding
energies, E
b
(vapor phase), become stronger from right to left across the periodic table,
and especially from the Group IB to VIII metals. At higher coverage, adsorbed water
can form monolayers, bilayers, three-dimensional water clusters, and overlayers in
TABLE 4.1 Metal-Dependent Vapor Phase Binding
Energy for H
2
O Adsorption to Group VIII and IB
(111) Metal Surfaces
a
E
b
(vapor phase) (eV)
Cu(111) 20.24
Ru(0001) 20.38
Rh(111) 20.42
Pd(111) 20.33
Ag(111) 20.18
Pt(111) 20.35
Au(111) 20.13
a
Vapor phase (low-coverage) binding energies from [Michaelides
et al., 2003b].
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