## 1 Introduction  
Renewable energy sources for the storage and conversion of energy are under extensive development to reduce dependency on fossil fuels and thereby control the worsening environmental pollution [1, 2]. Along with batteries, fuel cells' rechargeable supercapacitors have started gaining wide attention as a clean energy source for storing and converting energy. The combined features of the supercapacitors, such as superior power density, fast charging capability, long cycle stability, high specific capacitance, environmental safety, and low cost, make them extensively propitious in backup power supplies, emergency lighting, electric vehicles, and portable electronic devices [3, 4, 5]. In supercapacitors, the conversion and storage of energy take place through electrochemical reactions. The method of charge storage execution splits supercapacitors into electric double-layer capacitors (EDLC) and pseudocapacitors (faradaic supercapacitors) [6]. Charge storage in EDLC is based on double-layer ion adsorption normally associated with carbon-based electrode materials. In pseudocapacitors, charge storage takes place through reversible faradaic reactions at the surface of the electrode or inner surface [7, 8]. Pseudocapacitive electrodes of various materials, including transition metal oxides (TMOs), hydroxides, sulfides, conductingpolymers, and carbonaceous materials, have been prominently employed as a cathode in energy storage devices [9; 10]. Their remarkable physical and chemical properties enriched electronic conductivity, and decent thermal and mechanical stability favor transition metal sulfides in overriding the electrochemical performance of the other materials [10; 11]. Binary/ternary metal sulfides are the most favored materials over single-component sulfides in transition metal sulfides. Here, the synergistic effect between two metal cations results in the procurement of large redox reactions, which ultimately enhances the overall electrochemical activities compared to single-metal-component-based sulfides [11; 12; 13]. Recent research on supercapacitors implies that alongside a selection of superior material, the in situ development of hierarchical nanostructures of that material on a conductive substrate gives a fascinating electrochemical performance. Directly grown hierarchical nanostructures have various benefits, such as a large surface area, binder-free tight physical contact between the conductive substrate and as-grown nanostructure, and good electrical contact without any conducting additives, which constitute attractive electrochemical properties even in case of high mass-loading [14; 15]. In recent years, hierarchical nanostructures of transition metal sulfides have been extensively used for various applications such as catalysis, photocatalytic, biomedical, and sensor technologies, and energy storage and conversion devices [11; 16; 17; 18; 19]. Various synthesis methods have been adopted with which to develop these materials with a tuned morphology. Hydrothermal (water-assisted) or solvothermal (solvent-based) methods are commonly used synthetic approaches with which to procure types of morphological tuning of the various materials to make them useful for tremendous applications [20]. In the past, many researchers adopted a two-step hydrothermal method to grow the different nanostructures of sulfide-based materials. In the first step, hydroxides of the metal cations were grown at a specific temperature, and sulfurization was conducted in the second step using the sulfur source and using water as a solvent. The two-step approach is a prolonged technique that involves multiple processing steps, requiring high energy consumption and a high cost to procure the final product. The fabrication of numerous sulfide-based nanostructures with a cost-effective, one-step hydrothermal/solvothermal method is very rare.