Orbital engineering for enhanced quantum capacitance and ion transport in supercapacitor electrodes: Ni–N Co-Doped V₂C/WS₂ MXene heterostructure
This study presents a comprehensive first-principles density functional theory (DFT) investigation into how single (Ni, N) and synergistic Ni–N co-doping modifies the structural, electronic, and electrochemical properties of V₂C/WS₂ an oxygen-terminated V₂CO₂/WS₂ heterostructure, with the aim of creating mechanically stabilized design principles for next-generation MXene-based devices. An oxygen-terminated ( –O ) surface chemistry was adopted in place of the bare-surface model used in past, since experimentally synthesised MXenes are typically surface-functionalised and –O termination leads to the alkaline/mildly acidic etching conditions. Calculations were performed using the GGA-PBE functional augmented with DFT-D3(BJ) van der Waals corrections and Hubbard U corrections (U eff = 3.5 eV for V-3d; 5.5 eV for Ni-3d) to treat long-range interlayer interactions and correlated d-electron physics, respectively. All doped heterostructure show negative formation energies, confirming thermodynamic feasibility; the Ni–N co-doped system exhibits the most favourable formation energy (− 1.48 eV) and the strongest interfacial adhesion (binding energy: − 2.32 eV), a 25.4% improvement over the pristine system (− 1.85 eV). Electronic structure analysis indicates that Ni–N co-doping raises the density of states at the Fermi level by 5.2-fold relative to the pristine heterostructure, driven by a fully synergistic three-way hybridisation of Ni-3d, N-2p, and V-3d orbitals that cannot be reproduced by either dopant independently. Bader charge analysis confirms a net interfacial charge transfer of + 0.47 e⁻ at the co-doped interface, substantially exceeding single-dopant configurations. Work function calculations identify a reduction of 0.72 eV (from 5.10 eV to 4.38 eV) in the Ni–N co-doped system, facilitating more effective charge injection at the electrode–electrolyte interface. CI-NEB calculations demonstrate that Li⁺ migration barriers decrease from 0.31 eV (pristine) to 0.18 eV (co-doped), a 41.9% reduction in diffusion resistance. The theoretical quantum capacitance of the Ni–N co-doped heterostructure reaches 133 μF cm⁻ 2 (380 F g⁻ 1 ), a 5.3-fold enhancement over the pristine system (25 μF cm⁻ 2 ; 70 F g⁻ 1 ). Phonon dispersion calculations show no imaginary modes for any configuration, and ab initio molecular dynamics ( AIMD ) at 300 K over 5 ps confirms that the structural framework is kept without distortion, together developing dynamical and thermal stability of the O-terminated heterostructure. In-plane elastic-constant calculations further confirm mechanical (Born) stability, with Young's moduli of 136–151 GPa across the doping series. A combined mechanical framework is defined, where Ni–N synergy works through four interconnected pathways: complementary orbital hybridisation, Fermi level stabilisation at a DOS maximum, increased interfacial charge transfer, and higher ion adsorption active sites. These findings offer DFT-guided framework rules for systematic co-doping strategies in MXene-based heterostructure supercapacitor electrodes.