Observing Surface Metal Sites on Nickel Anodes Exposed to Trace-Level Iron Electrolyte with <i>in situ</i> and <i>operando</i> Resonantly Enhanced Second Harmonic Generation

Trace-level iron incorporation into nickel anodes is emerging as a promising route for lowering the overpotential and increasing the efficiency of the oxygen evolution reaction (OER), but the identification of the active iron species at the interface has been elusive. Here, we employ a spectroscopic autocorrelator to identify, using surface-specific in situ second harmonic generation (SHG) spectroscopy, a resonance band at 565 nm with ca. 25 nm bandwidth that appears during the anodic polarization of nickel nanolayers in pH 14 solution containing trace amounts of iron at well-characterized concentrations. The resonant signals are consistent with the presence of high-valence iron-oxo species. Scan rate-dependent SHG measurements (up to 200 mV s–1) demonstrate the technique is feasible under operando conditions. We envision that the development of surface-specific SHG spectroscopy to access electronic transitions associated with electrocatalytically active sites reported herein is of interest for other doped transition metals as well as mixed-metal anodes.