Role of defects in enhancing the supercapacitive performance of HfB2–HfC composite

Abstract

Supercapacitors play a pivotal role in next-generation energy storage, bridging the gap between high energy and high-power density systems. Transition metal diborides (TMB2s), particularly hafnium diboride (HfB2), combine metallic conductivity, chemical robustness, and mechanical integrity; however, their defect-governed electrochemical mechanisms remain underexplored. Here, we report a defect-engineered boride–carbide composite (HfB2–HfC) that couples intrinsic defect modulation with efficient charge transport. Pure HfB2, HfC, and HfB2–HfC composites are synthesized via mechanical alloying and systematically characterized. Electron paramagnetic resonance (EPR) spectroscopy reveals a distinct g-factor shift from ≈2.0049 (HfB2) to ≈2.002 (HfB2–10HfC), suggesting interfacial defect passivation and enhanced spin homogeneity. This spectroscopic signature is consistent with improved charge-transfer kinetics and reduced impedance, verified through cyclic voltammetry and impedance spectroscopy in a symmetric two-electrode configuration. The optimized HfB2–10HfC composite delivers a specific capacitance of 214 mF g−1, an energy density of 0.107 Wh kg−1, and a power density of 1133.7 W kg−1, while maintaining excellent coulombic stability. The performance enhancement arises from the synergistic interfacial coupling between HfB2 and conductive HfC domains, which facilitates defect-mediated electronic uniformity and efficient ion transport. This study establishes HfB2–10HfC as a model defect-engineered ceramic electrode and demonstrates a fundamental pathway for integrating EPR-verified electronic homogenization into high-performance boride–carbide supercapacitor architectures.