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

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.