Oxygen Isotope Labeling Experiments Reveal Different Reaction Sites for the Oxygen Evolution Reaction on Nickel and Nickel Iron Oxides

Abstract

Nickel iron oxide is considered ab enchmark nonprecious catalyst for the oxygen evolution reaction (OER). However,t he nature of the active site in nickel iron oxide is heavily debated. Here we report direct spectroscopic evidence for the different active sites in Fe-free and Fe-containing Ni oxides.Ultrathin layered double hydroxides (LDHs) were used as defined samples of metal oxide catalysts,a nd 18 O-labeling experiments in combination with in situ Raman spectroscopy were employed to probe the role of lattice oxygen as well as an active oxygen species,NiOO À ,inthe catalysts.Our data show that lattice oxygen is involved in the OER for Ni and NiCo LDHs,but not for NiFeand NiCoFeLDHs.Moreover,NiOO À is ap recursor to oxygen for Ni and NiCo LDHs,b ut not for NiFeand NiCoFeLDHs.These data indicate that bulk Ni sites in Ni and NiCo oxides are active and evolve oxygen via aN iOO À precursor.F ei ncorporation not only dramatically increases the activity,but also changes the nature of the active sites.

Spectroscopic and Electrokinetic Evidence for a Bifunctional Mechanism of the Oxygen Evolution Reaction

“…These two bands correspond to the Ni-O bending and stretching vibrations of NiOOH, respectively. 22,[32][33][34][35] Their appearance indicated the presence of NiOOH at these potentials, again consistent with previous XAS data. 19 At the three chosen spots on the surface, the relative intensities of the 480 cm -1 and 560 cm -1 bands (IB/IS) and the half-widths of the two bands vary, indicating different local environments around the Ni-O bonds.…”

“…For FeOOH-NiOOH at OCP to about 1.3 V, the peak of FeOOH remained at the same position whereas the peaks of Ni II -O, Ni II -OH appeared to be shifted, but the shift could not be quantified due to an overlap of peaks. (Figure 3a, left) For NiFe LDH same as reported previously, [32][33] the peaks of Ni II -O, Ni II -OH at OCP to about 1.35 V red-shifted by about 22 cm -1 , indicating the exchange of lattice 16 O with 18 O of the electrolyte (Figure 3a, right). Upon formation of NiOOH, and more obviously at 1.55 V, the Ni III -O bands were observed at around 455 and 535 cm -1 for both FeOOH-NiOOH and NiFe LDH, red-shifted by about 22 cm -1 relative to those of 16 O-labeled samples.…”

“…22,32,35 Accordingly, the amount of Fe dopant in the NiOOH at the three spots follows the order of: III > I > II. A broad band in the frequency range of 900 to 1150 cm -1 , previously attributed to Ni-OO -, [32][33][34]36 was observed from 1.375 V ( Figure S13a). The Operando Raman spectra collected of NiFe LDH from OCP to 1.5 V ( Figure S13b) show no spectral features corresponding to γ-FeOOH.…”

“…30,35,37 The two Raman bands of Ni III -O (from NiOOH) began to grow from 1.375 V and the growth was completed at around 1.45 V ( Figure S13b). The broad band in higher frequency range of 900 to 1150 cm -1 , due to Ni-OO -, [32][33][34]36 appeared from about 1.4 V.( Figure S13b). We compare directly the representative Raman spectra of FeOOH-NiOOH and NiFe LDH at OCP and 1.5 V ( Figure S15).…”

Anchoring Co3O4 nanoparticles on MXene for efficient electrocatalytic oxygen evolution

“…Electrochemical water splitting represents a convenient means to produce oxygen via oxygen evolution reaction (OER), which involves a complex fourelectron process (2H 2 O ? O 2 + 4H + + 4e À ) and the correlations between different elementary reactions often limit the reaction rate [2]. Currently, Ru/Ir-based oxide is commonly proposed as the effective electrocatalysts for OER performance but their practical applications are restricted by the low abundance and high costs [3].…”