Operando Raman Spectroscopy Reveals Cr-Induced-Phase Reconstruction of NiFe and CoFe Oxyhydroxides for Enhanced Electrocatalytic Water Oxidation

Abstract

Nonprecious NiFe and CoFe oxyhydroxides are among the most active materials for oxygen evolution reaction (OER) in basic media. However, the phase separation in these composites during water oxidation remains a critical issue that often results in degradation of electrochemical performance and debate on the mechanism and the active intermediates. In this study, we show that the introduction of Cr can efficiently transform the crystalline multiphase NiFe and CoFe oxides/ hydroxides into homogeneous amorphous nanodots with sharply reduced nanoparticle size from tens of nanometers to merely 2−3 nm. Serving as an OER catalyst, the ternary NiFeCr and CoFeCr catalysts exhibit a smaller onset potential of ∼1.51 V vs reversible hydrogen electrode (RHE) and a stable OER performance during long-term water electrolysis. The impact of Cr on the NiFe and CoFe catalysts for OER kinetics was systematically investigated by operando electrochemical Raman spectroscopy. It is found that, for the NiFeCr compound, Cr can promote the generation of a more active β-NiOOH phase than that of the NiFe composite during water oxidation. For the CoFe and CoFeCr systems, the introduction of Cr only disturbs the lattice crystallization. However, active CoOOH is spontaneously present on the surface of the composites upon making contact with KOH electrolyte, even without applying a potential. Thus, Co-based catalysts can easily achieve the "ready-to-serve" state for high-performance water oxidation without preactivation.

Modifying redox properties and local bonding of Co3O4 by CeO2 enhances oxygen evolution catalysis in acid

“…Co 3 O 4 /CeO 2 showed a red-shift in the A 1g peak position after the onset of OER at 1.52 V vs. RHE. Red-shifts in Raman signals are commonly observed in OER catalysts (CoO x 63 , 66 , NiOOH 67 , NiFe, and CoFe oxyhydroxides 68 ) at OER operating potentials, and they generally reflect the characteristic vibration for local bonding environment at the outer layer of catalysts with oxidized active site during OER. Thus, the generation of active Co IV species that can participate in a fast and efficient OER process should lead to the observed red-shift of the Raman signals.…”

Engineering the Activity and Stability of MOF‐Nanocomposites for Efficient Water Oxidation

“…The Ni 2p 3/2 can be deconvoluted into four peaks at 855.9, 857.5, 861.8, and 865.3 eV, which are ascribed to Ni 2+ , Ni 3+ , and satellite peak of Ni 2+ . [ 22 ] Similarly, the Fe 2p 3/2 can be deconvoluted into three peaks at 711.3, 714.6, and 719.5 eV, which are assigned to Fe 2+ , Fe 3+ , and satellite peak of Fe 2+ . [ 23 ] The small shoulder peak at 707.3 eV of NiFe‐MOF/G refers to Ni auger.…”

Design and operando/in situ characterization of precious‐metal‐free electrocatalysts for alkaline water splitting

“…With the assistance of operando Raman, Zhao and co‐workers realized real‐time tracking of Cr‐induced structural evolution on the surface of catalysts during OER (Figure 11G,H). 196 In comparison with the NiFe compound, Raman analysis revealed the incorporation of Cr promoted the formation of more active β‐NiOOH phase under the applied potentials, implying the significance of Cr dopants on phase reconstruction. However, for the CoFeCr compound, spontaneous transformation into the active CoOOH species was observed once contact with electrolyte.…”

“…Reproduced with permission: Copyright 2020, American Chemical Society. 196 HER, hydrogen evolution reaction; LDH, layered double hydroxides; OER, oxygen evolution reactions F I G U R E 1 2 HAADF images and EDS mapping of CoS x (A) before OER and (B) after OER for 12.5 minutes. C, Corresponding HR-TEM and SAED patterns analyses for the structural evolution of CoS x .…”

“…Operando Raman spectroscopy measurement of the (G) NiFeCr and (H) NiCoCr under applied potentials. Reproduced with permission: Copyright 2020, American Chemical Society 196 . HER, hydrogen evolution reaction; LDH, layered double hydroxides; OER, oxygen evolution reactions…”