Electrostatics and Structure at Electrode:Electrolyte Interfaces from Nonlinear Optics

Electrode:electrolyte interfaces are important in many technological applications but are notoriously difficult to probe with surface specificity and without the need for external labels. Here, it is hypothesized that structural and electrostatic information on the Stern and diffuse layers can be provided by measurements of the amplitude and phase of heterodyne-detected second harmonic generation (HD-SHG) signals from electrode:electrolyte interfaces. The project will unveil under what conditions dynamic exchange between these two important components of the electrical double layer (EDL) can occur as they are subjected to external stimuli (Uext, pH, [electrolyte]). The project will also determine the total interfacial potential drop across the electrode:electrolyte interface from the nonlinear optical measurements and employ HD-SHG in a wide-field imaging modality. It is hypothesized that the Stern layer structure and total surface potential drop provided by HD-SHG inform on the non-Coulombic contributions (dipoles, quadrupoles, etc.) to the EDL. Goal 1 is to develop and validate HD-SHG for Au and Ni:NiOx electrodes and to determine Stern layer structure and total surface potential as a function of Uext, pH, and [electrolyte] in their electrochemical stability windows. Goal 2 is to determine the conditions for which the total potential departs from predictions from charge-only mean field theory (e.g. Gouy-Chapman-Stern), and when dipolar and multipolar contributions make up the difference, as is hypothesized. It is also hypothesized that these contributions that are missing in the "standard model" should manifests themselves in the Stern layer water dipole structure encoded in the second-order nonlinear susceptibility provided by HD-SHG. Goal 3 is to develop HD-SHG microscopy as a widefield modality for imaging local deviations from the point of zero charge condition in time and space. The project will also use the oxygen evolution reaction on Au:AuOx and the hydrogen evolution reaction on Ni:NiOx to ask whether second-order nonlinear susceptibility and total interfacial potential hotspots form prior to O2 and H2 production.