EPR investigation of point defects in HfB₂ and their roles in supercapacitor device performances

Boron-based materials have various attractive properties and gained increased attention in recent years as promising materials for energy storage applications. Despite vast literature on structural and mechanical properties of transition metal diborides, hafnium diboride (HfB2) in particular, research that addresses the use of HfB2 as an electrode for supercapacitor devices is lacking. Herein, we report both the synthesis and characterization of HfB2 and its electrochemical performance as the electrode for all-in-one symmetric and asymmetric supercapacitor devices. HfB2 powders were synthesized by mechanical activation assisted carbothermal reduction of hafnium oxide and boron oxide precursors. To improve the electrochemical energy storage performance of the electrodes, point defects (either Hf or B vacancies/interstitials) were formed in HfB2 through annealing at different temperatures (1450 and 1650 °C) under a flowing Ar atmosphere. The origin of point defects and their localization on the surface in HfB2 were identified using electron paramagnetic resonance (EPR) spectroscopy and discussed both from chemical and materials point-of-view. The defective HfB2 electrode exhibited higher performance than that of the non-defective one with specific energy and power densities of 0.144 W h kg−1 and 33.3 W kg−1; specific charge–discharge capacities of 0.32 and 0.31 mA h g−1; and 115.5%, 106.2%, and 84.1% retention of the initial capacitances, respectively. The relation between the defect content and the improved supercapacitor performances was explained by employing several structural (x-ray diffractometer and x-ray fluorescence), electronic (EPR), and electrochemical (potentiostatic electrochemical impedance spectroscopy, cyclic voltammetry, galvanostatic cycling with potential limitation) characterization tools.