Surface‐Enhanced Raman Spectroscopic Evidence of Key Intermediate Species and Role of NiFe Dual‐Catalytic Center in Water Oxidation

Abstract

NiFe‐based electrocatalysts have attracted great interests due to the low price and high activity in oxygen evolution reaction (OER). However, the complex reaction mechanism of NiFe‐catalyzed OER has not been fully explored yet. Detection of intermediate species can bridge the gap between OER performances and catalyst component/structure properties. Here, we performed label‐free surface‐enhanced Raman spectroscopic (SERS) monitoring of interfacial OER process on Ni3FeOx nanoparticles (NPs) in alkaline medium. By using bifunctional Au@Ni3FeOx core‐satellite superstructures as Raman signal enhancer, we found direct spectroscopic evidence of intermediate O‐O− species. According to the SERS results, Fe atoms are the catalytic sites for the initial OH− to O‐O− oxidation. The O‐O− species adsorbed across neighboring Fe and Ni sites experiences further oxidation caused by electron transfer to NiIII and eventually forms O2 product.

Probing the Oxygen Reduction Reaction Intermediates and Dynamic Active Site Structures of Molecular and Pyrolyzed Fe–N–C Electrocatalysts by In Situ Raman Spectroscopy

“…No chemical shift associated with the H/D isotope substitution was observed, confirming the absence of H in the adsorbed oxygen-containing intermediates. In fact, the broad Raman band between 1000 and 1200 cm –1 has also been observed on other oxygen reduction/evolution electrocatalysts [such as Ni­(Fe) oxyhydroxides, CoPc, and polycrystalline Au] and was commonly assigned to O–O stretching vibration of superoxo species (O 2 – ). − Following these studies and our own results, we assign the observed Raman signal between 1100 and 1200 cm –1 to the superoxo species adsorbed at the Fe active site. The splitting of the broad peak was ascribed to the coupling between the O–O stretching vibration mode of adsorbed O 2 – and the Fe–N ring vibration modes, , indicating a high vibration flexibility of the FePc molecule.…”

“… In contrast, the 1133 cm –1 Raman peak shows a blue shift to 1166 cm –1 , which is in principle impossible since substitution of H in the intermediates by the heavier element D should induce a slower interatomic vibration and hence a red Raman shift. Following the general assignment of the broad Raman band between 1000 and 1200 cm –1 to the O–O stretching vibration of O 2 – observed on various oxygen reduction/evolution electrocatalyst surfaces, − we attribute the 1133 cm –1 Raman band to the O 2 – adsorbed on C–N active sites. The blue shift after H/D substitution could be rationalized by altered adsorption of O 2 – by D 2 O in the electrolyte through hydrogen bonding, according to a similar phenomenon observed on Fe-based oxyhemoglobins previously .…”

A polymeric hydrogel electrocatalyst for direct water oxidation

“…4d–f ). In particular, the characteristic vibration of the superoxide intermediate (*O–O) has been mostly detected by in operando Raman spectroscopy to provide fingerprint information for OER 36 – 38 . As such, we first recorded the Raman spectra by stepwise varying the anodic potential in O 2 -saturated 1 M 16 O-KOH solution.…”

Immobilizing Low‐Cost Metal Nitrides in Electrochemically Reconstructed Platinum Group Metal (PGM)‐Free Oxy‐(Hydroxides) Surface for Exceptional OER Kinetics in Anion Exchange Membrane Water Electrolysis

“…[37] Increasing intensity of NiOOH peaks at higher overpotentials reveals constantly generated NiOOH during OER, indicating a chemically stable, electrically conductive catalyst, [17] which contributes to the higher activity of the V-NiFeOOH/Ni 3 N catalyst during OER. Furthermore, broad peaks ≈480 and 560 cm −1 have appeared as we applied potentials higher than 1.35 V, which are attributed to the, for example, bending and A 1g stretching vibration mode of the Ni-O in the NiOOH phase, [38][39][40] respectively. Interestingly, we could not obtain any Raman signals after 1.47 V due to severe bubbles generated from the working electrode, further suggesting ultra-high fast kinetic of the V-NiFeOOH/ Ni 3 N catalyst.…”