Synthesis and characterization of sulfonated hafnium oxide nanoparticles for energy storage devices

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• The sulfonated HfO 2 nanoparticles obtained by treating with sulfuric acid (H 2 SO 4 ). • When sulfonation particles size were reduced. • Low Ohmic resistance was obtained in sulfonated HfO 2. • High supercapacitor value exhibited S-HfO 2 (210 Fg −1 ) compare to HfO 2 (126 Fg −1 ). Metal oxide electrode material plays a vital role, particularly in supercapacitor applications and energy storage devices. The present study broadly focuses on the comparative research of the super-capacitive performance of hafnium oxide (HfO 2 ) and sulfonated hafnium oxide (S-HfO 2 ) nanomaterials. The simple precipitation technique has been used to synthesize HfO 2 nanoparticles. Subsequently, sulfuric acid (H 2 SO 4 ) has been used to functionalize the obtained HfO 2 nanoparticles. Afterward, the crystalline phases and orientation of nanoparticles has been analyzed using X-Ray diffraction (XRD), the presence of functional group and possible vibration frequency of the material can be identified by Fourier transform infrared (FT-IR) spectroscopy, crystal structures, and lattice imperfections of the synthesis material were found by High-resolution transmission electron microscope (HR-TEM). Consequently, the nanoparticles' structure, thickness, and elemental composition have been analyzed using the Field-Emission scanning electron microscope (FE-SEM) and Energy Dispersive X-ray spectroscopy (EDS) techniques. To gain the knowledge of supercapacitor behavior of prepared nanomaterial, electrochemical properties, and ion transfer characteristics were investigated by various experiments, namely, the cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD). Moreover, the internal resistances of the material (HfO 2 and S-HfO 2 ) were analyzed using Electrochemical Impedance Spectroscopy (EIS) technique. Both the nanoparticles showed a good improvement in specific capacitance. The supercapacitor values were measured for HfO 2 and S-HfO 2 electrodes, which exhibit a specific capacitance value 126.0 Fg −1 and 210.0 Fg -,1 respectively, at the scan rate of 5 mVs −1 . Hence, it is noted that Sulfonated HfO 2 nanoparticle enhances the specific capacitance and is used as a promising substance for the energy storage device. Besides, this finding benefits the new researchers in developing different kinds of sulfonated metal oxide nanomaterials for energy applications.