Spinel NiFe₂O₄ nanoparticles decorated 2D Ti₃C₂ MXene sheets for efficient water splitting: Experiments and theories

Design and demonstration of cost-effective, robust, and earth-abundant electrocatalysts for efficient water splitting have attracted a great deal of interest. Herein, we have decorated NiFe₂O₄ nanoparticles on the emerging novel two-dimensional (2D) Ti₃C₂ (MXene) sheets in order to achieve better electrocatalytic performance for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The synthesized NiFe₂O₄/Ti₃C₂ composite showed extraordinary kinetic metrics for electrocatalytic OER, revealing a low overpotential of 266 mV at a current density of 10 mA/cm², and a small Tafel slope of 73.6 mV/dec. For HER, the composite exhibited an overpotential of 173 mV at 10 mA/cm² with a small Tafel slope of 112.2 mV/dec. The high electrocatalytic performance of NiFe₂O₄/Ti₃C₂ composite is believed to be originated from a well-constructed nanoparticle-sheet interface, synergistic effect, and the high metallic conductivity of Ti₃C₂ MXene sheets. These experimental results are further supported by the state-of-the-art density functional theory (DFT) simulations. The study providing information about the structure, electronic properties, bonding, and interaction mechanism between Ti₃C₂ (MXene) and NiFe₂O₄. Moreover, offering the values of the theoretical overpotential of Ti₃C₂ (MXene), NiFe₂O₄, and the NiFe₂O₄/Ti₃C₂ composite for both OER and HER activities. Interestingly, the theoretical overpotential follows the qualitative trend of NiFe₂O₄/Ti₃C₂ < NiFe₂O₄ < Ti₃C₂ MXene for OER and NiFe₂O₄/Ti₃C₂ < Ti₃C₂ MXene < NiFe₂O₄ for HER, agreeing with the experimental observations. There is charge transfer from NiFe₂O₄ to MXene leading to enhancement in electronic states near Fermi level which may be due to interactions between C 2p orbital of Ti₃C₂ MXene and 3d orbital of Ni and Fe. Therefore, this work provides new insights for designing new efficient non-noble metal-based electrocatalysts in the future.