Reaction barriers dictate the rates of elementary reactions, and therefore
are crucial to understanding electrochemical kinetics. Since these reactions
tend to occur at heterogeneous surfaces, principles from catalysis can be
expected to apply. Here, we use electronically grand-canonical calculations on
a model system of the proton-deposition reaction on a range of metal surfaces
to explore the extent to which these principles hold. First, we show that while
reaction barriers are functions of potential, unlike endstates they tend to
exhibit a nonlinear dependence, and this nonlinearity can be traced to
differences in the electron transfer at the reaction barrier. We show that
along a reaction path, this same electron-transfer difference forces the
barrier to move earlier for downhill reactions, enforcing and explaining the
Hammond-Leffler postulate for electrochemical reactions, as well as explaining
the curvature in the Marcus-like relations. We further examine trends in
barrier energies, at equivalent driving forces, for this reaction across
metals. We find that the barrier energy correlates weakly to the hydrogen
binding energy and the d-band center, but instead correlates strongly with the
charge presented at the metal surface -- which is a direct consequence of the
native work function of the material. This suggests that the energetics of the
barrier are driven more strongly by the electrostatic, rather than the
covalent, nature of the metal-adsorbate interaction, and suggests
electrochemical barriers may have an independent driving force from
electrochemical adsorbates