DFT-Guided Operando Raman Characterization of Ni-Based Phases Relevant to Electrochemical Systems

TL;DR

This study combines DFT calculations with operando Raman spectroscopy and TEM to characterize Ni-based phases relevant to electrochemical oxygen evolution, providing insights into their stability and vibrational properties.

Contribution

It introduces an integrated approach using idealized DFT models to interpret complex experimental spectra of Ni oxides and hydroxides in electrochemical systems.

Findings

Cubic NiO is stable and matches experimental Raman modes.

Hexagonal NiO is metastable, supported by TEM.

Ni(OH)2 polymorphs are vibrationally stable semiconductors.

Abstract

We present a phase-resolved investigation of Ni-based oxides and hydroxides relevant to the oxygen evolution reaction (OER), combining ground-state DFT+U calculations with operando and in situ Raman spectroscopy, supported by high-resolution TEM. Five crystalline phases-cubic and hexagonal NiO, monoclinic and trigonal Ni(OH)2, and NiOOH-are systematically characterized in terms of their vibrational and electronic structure. Although the DFT models are idealized (0 K, defect-free, no solvation), they serve as clean, phase-specific references for interpreting complex experimental spectra. Cubic NiO is confirmed to be dynamically and electronically stable, consistent with dominant Raman modes observed experimentally. Despite dynamic instabilities in phonon dispersions, hexagonal NiO is structurally verified via TEM, suggesting substrate- or defect-stabilized metastability. Ni(OH)2 polymorphs are both vibrationally stable semiconductors, with the trigonal phase exhibiting stronger spin polarization. NiOOH exhibits spin-polarized electronic states across the Brillouin zone, consistent with its asymmetric band structure under ferromagnetic ordering. Independently, phonon calculations reveal soft modes near the Gamma-point, indicating dynamic instability under idealized conditions, yet operando Raman spectra align closely with calculated zone-center modes. However, introducing 0.03 Angstrom symmetry-breaking displacements relaxes the NiOOH lattice off its saddle point, removing imaginary phonon modes and stabilizing the phase. This integrated framework demonstrates how idealized DFT can reveal intrinsic fingerprints that anchor the interpretation of vibrational and electronic responses in catalytically active, dynamically evolving Ni-based materials.