ORIGINAL PAPER Influence of thiourea concentration on the CuS nanostructures and identification of the most suited electrolyte for high energy density supercapacitor Nandhini Sonai Muthu 1 & Shobana Devi Samikannu 1 & Muralidharan Gopalan 1 Received: 18 January 2019 /Revised: 15 March 2019 /Accepted: 5 April 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The energy density of a supercapacitor is largely reliant on functional parameters of electrode material and electrolyte. To improve the energy density of the CuS asymmetric device, optimization of sulfur concentration (thiourea) in the precursor and identification of the most suited electrolyte have been attempted. The changes in thiourea concentration greatly affect the physical and electro- chemical features of CuS. The highest specific capacitance of 298 F g -1 at 2 A g -1 was obtained for copper sulfide nanoparticles prepared with 1:2 ratio of copper acetate and thiourea (C3). It exhibits excellent cycling stability in 2 M KOH electrolyte. In addition, to evaluate the most suited electrolyte, electrochemical studies were performed with different electrolytes (H 2 SO 4 , Na 2 SO 4 , KOH and LiClO 4 in propylene carbonate). Based on the electrochemical results, it was found that an outstanding performance has originated from H 2 SO 4 electrolyte (773 F g -1 at 2 A g -1 ). The C3 electrode exhibits no perceptible degradation in capacity even after 4000 charge-discharge cycles in acidic electrolyte. Further, for real-life applications, an asymmetric device was fabricated using C3 as a cathode and PVA/ H 2 SO 4 as electrolyte. The device attained a highest energy density of 21 W h kg -1 at a power density of 310 W kg -1 . Furthermore, lighting up of red and yellow LEDs is demonstrated using the fabricated asymmetric device. The efficient device performances concluded that C3 is a potential cathode material for future supercapacitor applications. Keywords Copper sulfide . Hydrothermal method . Various electrolytes . Cyclic voltammetry . Specific capacitance Introduction The exploration of new kind of nanomaterials is crucial to develop nanostructures with properties such as size reduction and confinement at low dimensions, and enhanced interfacial reaction kinetics towards applications such as supercapacitors and batteries [1]. Nanostructured materials reveal outstanding optical, chemical, mechanical, and electrochemical properties when their morphologies change [2]. Though number of ma- terials such as transition metal oxides, polymers, carbon nanomaterials, and their composites have been examined and exploited towards energy storage applications, another important class of materials such as nanostructured metal sul- fides remains as a less explored material. These are new cat- egory materials that could be possibly exploited in the area of pseudocapacitors that involve the Faradic redox reactions of the metal [3]. The quantum size effects and rich crystal chem- istry of the metal sulfides lend them useful in various techno- logical applications such as sensors, fuel cells, batteries, light- emitting diodes, and electrochemical devices [4]. The metal sulfides, which usually exist as mineral in nature, are available abundantly and they are cheaper. The structural and chemical properties of sulfide are remarkably varied from correspond- ing oxides [5, 6]. The fact that the electronegativity of sulfur (2.5) is lower than oxygen (3.5) signifies bonding between metal-sulfur is more covalent than the metal-oxide bond [7]. In metal sulfides, metal is trigonal-prismatically coordinated with sulfur atoms through the formation of S–S bonds so as to produce stable sulfide structures unlike oxides [ 8 ]. Furthermore, the average polarizability of sulfur (2.9 × 10 -24 cm 3 ) is also higher than oxygen (0.802 × 10 -24 cm 3 ) that consequences advanced structural and electrochemical prop- erties than parent oxide material [9]. * Muralidharan Gopalan muraligru@gmail.com 1 Department of Physics, The Gandhigram Rural Institute (Deemed to be University), Dindigul, Tamil Nadu 624 302, India Ionics https://doi.org/10.1007/s11581-019-03002-8