Enhanced Hydrogen Peroxide Production by Nickel Re-Dispersion in Oxidized Ni@TiN Catalysts

The oxygen reduction reaction (ORR) is a fundamental electrochemical process that underpins numerous energy conversion technologies, including fuel cells, metal-air batteries, and hydrogen peroxide (H₂O₂) production. Despite its significance, achieving both high activity and selectivity in ORR remains a major challenge with conventional catalysts. Single atom catalysts (SACs), characterized by their atomically dispersed active sites, have demonstrated exceptional promise in addressing these challenges by enabling precise control over reaction pathways and enhancing catalytic efficiency.
This study investigated the structural transformations and H₂O₂ selectivity of Ni@TiN catalysts subjected to different oxidative treatments (O₂, H₂O, and CO₂). After oxidation at 500°C, the re-dispersion behavior of Ni nanoparticles was observed through scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and X-ray absorption spectroscopy (XAS). To elucidate the surface structure of Ni@TiN, molecular dynamics (MD) simulations utilizing machine learning potential were performed.
Computational results revealed distinct surface transformations based on the type of oxidant used. The O₂-treated Ni@TiN surface transformed into a TiO₂-like structure resembling anatase-phase TiO₂, consistent with X-ray diffraction (XRD) analysis. As oxidation peaks were detected through X-ray photoelectron spectroscopy (XPS) even on the TiN surface without oxidation treatment, simulations for CO₂ and H₂O treatments were conducted in the presence of O₂ to accurately mimic realistic oxidative environments. After CO₂ and O₂ treatment, partial oxidation of the TiN surface without complete structural reconstruction was observed. Additionally, H₂O and O₂ treatment led to the adsorption of OH groups on the surface. These observations were further supported by density functional theory (DFT) calculations to provide detailed insights into the reaction energetics associated with ORR on these reconstructed surfaces.
Since MD simulations confirmed that the re-dispersion of Ni nanoparticles occurred irrespective of the oxidant used, Ni was modeled as a single atom for DFT calculations. For the O₂-treated catalyst, the surface was modeled with an anatase-phase TiO₂ layer. For the CO₂-treated surface, Ni was positioned on a partially oxidized TiN substrate. Finally, for the H₂O-treated surface, OH groups were adsorbed on the TiN surface with a Ni single atom. For H₂O₂ production, DFT calculations confirmed that the O₂- and CO₂-treated surfaces exhibit unfavorable pathways with positive reaction energies. In contrast, the H₂O-treated surface demonstrated enhanced selectivity toward H₂O₂ formation, with a thermodynamically favorable pathway characterized by negative reaction energy. These results were consistent with the H₂O₂ production test, which exclusively detected H₂O₂ on the H₂O-treated Ni@TiN catalyst.
This work underscores the importance of tailoring surface properties and oxidative treatments to enhance SAC performance for 2e^- ORR. The synergy between computational simulations and experimental validation provides a comprehensive understanding of the catalytic mechanisms, offering practical strategies for the design of highly selective and efficient catalysts. These findings pave the way for sustainable and energy-efficient processes, and contribute valuable insights into the broader field of single atom catalysis.