In-situ spectroelectrochemistry: We constructed a transparent three-electrode microfluidic electrochemical cell according to Wang et al. Electrolyte flowed through the cell at a constant rate of 50 μL/min using an automated syringe pump. The electrolyte was sparged (and later blanketed) with N2 gas for at least 30 mins prior to flowing through the electrochemical cell. The electrochemical cell was mounted on the stage of an inverted optical microscope (Olympus IX73) where a 60× objective UPLANSAPO60x/W) focused light onto a sCMOS camera (Teledyne Prime 95B). An Ivium Compactstat Potentiostat applied a constant potential during each spectral measurement using a Pt wire counter electrode and Ag wire quasi-reference electrode. A Horiba OBB monochromator (1200 gr/mm, 1.2 mm slit) selected light from a Xe lamp source in 2 nm wavelength steps. 𝐴(𝜆) was calculated for each respective image with 𝐼 and 𝐼 0 ROI boundaries for each set of potential-dependent spectral data. Each 𝐴(𝜆) is the average of measurements from 12 to 16 images. The absorbance data was collected using non-sequential applied potentials. A data acquisition system (DATAQ 4108) synchronized the camera, monochromator, and potentiostat signals. The ESI provides further experimental details and imaging procedures.