Ionization of water in interfacial electric fields: An electrochemical view

► Water ionizes to hydronium H+(H2O)m and hydroxide OH−(H2O)n in fields of ≈0.3 V/Å. ► Ionization fields similar to those at electrode/electrolyte interfaces. ► Dielectric properties, ion cluster size and energetics, and ionization images. ► Ion participation in electrocatalytic reactions: H2 and CO oxidation, O2 reduction. ► Directions for integrated experimental and theoretical research and new techniques.