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

While water's oxygen is the electron source in the industrially important oxygen evolution reaction, the strong absorber problem clouds our view of how the Stern layer water molecules orient themselves in response to applied potentials. Here, we report nonlinear optical measurements on nickel electrodes held at pH 13 indicating a disorder-to-order transition in the Stern layer water molecules prior to the onset of Faradaic current. A full water monolater (1.1 x 10^15 cm-2) aligns with oxygen atoms pointing towards the electrode at +0.8 V and the associated work is 80 kJ mol-1. Our experiments identify water flipping energetics as a target for understanding overpotentials, advance molecular electrochemistry, provide benchmarks for electrical double layer models, and serve as a diagnostic tool for understanding electrocatalysis.