  We find that ion creation/destruction dominates the behavior of
electrochemical reaction barriers, through grand-canonical electronic structure
calculations of proton-deposition on transition metal surfaces. We show that
barriers respond to potential in a nonlinear manner and trace this to the
continuous degree of electron transfer as an ion is created or destroyed. This
explains both Marcus-like curvature and Hammond-like shifts. Across materials,
we find the barrier energy to be driven primarily by the charge presented on
the surface, which in turn is dictated by the native work function, a
fundamentally different driving force than non-electrochemical systems.
