Interfacial Water Flipping and Electrostatic Fields at the Electrode:Electrolyte Interface from operando Nonlinear Optical Spectroscopy

Water's oxygen is the electron source in the industrially important oxygen evolution reaction, but how water interacts with an electrode's active sites remains poorly understood. Much microscopic insight into the Stern layer water structure and the interfacial fields currently comes from atomistic simulations,1-6 with joint theoretical and surface-specific experimental studies just emerging.7-9 The strong absorber problem for water in particular has hampered our molecular understanding of how the water molecules in the Stern layer orient themselves in response to an externally applied potential. Here, we employ operando nonlinear optics with a non-resonant pulse triplet while recording cyclic voltammograms at Ni:NiOx electrodes in contact with pH 13 electrolyte. We quantify the number of net-aligned Stern layer water molecules that point their oxygen atoms towards the electrode in response to the externally applied potential and obtain the total electrostatic field across the electrical double layer by quantifying the total potential. We find that the energy associated with water flipping is parabolic in the fraction of Stern layer water molecules flipped and comparable to the cohesive energy of water and ice, depending on the choice of the Stern layer relative permittivity (2 and 1.33, respectively).