  The design of better heterogeneous catalysts for applications such as fuel
cells and electrolyzers requires a mechanistic understanding of
electrocatalytic reactions and the dependence of their activity on operating
conditions such as pH. A satisfactory explanation for the unexpected pH
dependence of electrochemical properties of platinum surfaces has so far
remained elusive, with previous explanations resorting to complex co-adsorption
of multiple species and resulting in limited predictive power. This knowledge
gap suggests that the fundamental properties of these catalysts are not yet
understood, limiting systematic improvement. Here, we analyze the change in
charge and free energies upon adsorption using density-functional theory (DFT)
to establish that water adsorbs on platinum step edges across a wide voltage
range, including the double-layer region, with a loss of approximately 0.2
electrons upon adsorption. We show how this as-yet unreported change in net
surface charge due to this water explains the anomalous pH variations of the
hydrogen underpotential deposition (Hupd) and the potentials of zero total
charge (PZTC) observed in published experimental data. This partial oxidation
of water is not limited to platinum metal step edges, and we report the charge
of the water on metal step edges of commonly used catalytic metals, including
copper, silver, iridium, and palladium, illustrating that this partial
oxidation of water broadly influences the reactivity of metal electrodes.
