# Lotus Root Type Nickel Oxide-Carbon Nanofibers: A Hybrid Supercapacitor Electrode Material
## 1 Introduction  
In the realm of electrochemical energy storage and conversion technology, the supercapacitor is one of the renowned systems for its practicality and versatility. Supercapacitors are distinguished from other electrochemical energy storage systems by offering a unique balance of high-power capabilities, exceeding that of batteries, along with enhanced energy density compared to traditional capacitors [1]. Based on the charge storage mechanism, supercapacitors are categorized as electric double-layer capacitors (EDLCs), pseudo-capacitors, and hybrid supercapacitors. EDLCs exhibit high power density and cycle life, which are attributed to the efficient separation of charges through electrostatic attraction within the electrochemical double layer. EDLCs generally employ activated carbon-based nanomaterials with high surface area and electrical conductivity in their electrodes to facilitate the rapid movement of ions and electrons [2]. The nanometer-scale pore size plays a pivotal role in reducing charge separation distances, consequently leading to increased capacitance [3]. Consequently, a common strategy involves the fabrication of nanostructured electrodes to bolster electrode material conductivity [4] while simultaneously reducing diffusion pathways [5], with specific surface area serving as a direct determinant of specific capacitance. In contrast, Pseudo capacitors employ electrode materials consisting of conductive polymers and transition metal oxides to store charge through surface-controlled faradaic reactions taking place at the electrode-electrolyte interface at rates comparable to those of electrochemical double-layer formation in EDLCs. The architecture and composition of electrode materials have asignificant impact on the performance of supercapacitors. Various electrode materials with different dimensional structures such as 0D, 1D, 2D, and 3D have been designed to enhance their performance. Among them, 1D nanomaterials synthesized by the electrospinning method show promising potential due to their easy preparation, high aspect ratio, which facilitates efficient electron and mass transportation, and strong plasticity. The composite materials synthesized using carbon materials, carbide materials, conductive polymers, sulfides, nitrides, metal oxides, etc., show better electrochemical performance due to their high stability, conductivity, and electron storage capacity [6; 7]. In general, transition metal oxides offer higher capacitance than carbon materials and superior charge-discharge cycle stability compared to polymers, despite their lower electronic conductivity [8]. Among transition metal oxides, NiO is a popular choice as a pseudo-capacitor electrode material due to its high theoretical capacitance, large surface area, cycling stability, multi-oxidation states, and reversible redox reactions [7]. However, NiO grapples with challenges such as low electronic conductivity, limited redox active sites, and a low charge/discharge ratio, which inevitably impact capacitance and cycling performance. To address these issues, strategies have been explored, including the nanostructuring of NiO electrodes to enhance the effective surface area, hybridization with highly conductive carbon-based materials, and doping with foreign elements [9]. Various studies have been reported to enhance the capacitance of capacitors by using NiO-based composite materials. For instance, [PERSON] and coworkers achieved 150 F g\({}^{-1}\) capacitance using NiO/CNF with Na\({}_{2}\)SO\({}_{4}\) as electrolyte [10], [PERSON] et al. reported 157.9 F g\({}^{-1}\) with CNF-NiO [11], and 742.2 F g\({}^{-1}\) capacitance was reported by [PERSON] and coworkers [12]. In our study, we have developed a hybrid Nickel oxide-carbon nanofiber (NiO-CNF) hybrid capacitor electrode, notably for its rectangular shape observed in cyclic voltammetry (CV), stable operating voltage, high energy, low cost, self-balancing, and close resemblance to the ideal characteristics of a capacitor [13].