This journal is © The Royal Society of Chemistry 2021 J. Mater. Chem. C Cite this: DOI: 10.1039/d0tc06085b Copper-doped a-MnO 2 nano-sphere: metamaterial for enhanced supercapacitor and microwave shielding applications† Dheeraj Mondal, a Biplab Kumar Paul, a Debopriya Bhattacharya, b Debopriyo Ghoshal, b Somen Biswas, ac Kaustuv Das a and Sukhen Das * a a-MnO 2 nanoparticles with increasing copper-doping concentration have been synthesized through a modified hydrothermal technique. Doping-induced microstructural defects inside the host material lead to a giant dielectric constant 1.6  10 6 with moderate tangent loss at 20 Hz frequency. In the microwave frequency region, synthesized nanoparticles showed a dual loss mechanism which includes valuable dielectric and magnetic loss of the order of B0.45. Consequently, this dual loss mechanism leads to strong electromagnetic interference (EMI) shielding effectiveness (SE) of B38 dB at 14 GHz and 49 dB at 17.5 GHz for 15 wt% of Cu doped a-MnO 2 thin layer of thickness B600 mm. This result reveals 499.999% EMI SE against hazardous electromagnetic waves in the microwave/GHz frequency region. Additionally, cyclic voltammetry and galvanostatic charge–discharge measurements in the potential range of 0.4 V to +0.2 V demonstrate an enhanced capacitance value of 334.2 F g 1 at a current density of 0.5 A g 1 with 96% charge retentivity up to the 2000th cycle. Finally, a solid-state supercapacitor device was fabricated which could light up 2 red LEDs for 10 s. 1. Introduction With the rhythms of technological fascination, the practice of using portable electronic gadgets is increasing day by day. These types of electronics appliances are becoming an integral part of our daily life, making us more comfortable, time-saving, and hassle-free. In such a way, they are being used to fulfill our daily needs even in medical demands. To make devices active and operative, it is essential to attach a charge storing device such as a battery or supercapacitor to power up devices. Based on the recent investigation, it is evident that the supercapacitor is more effective and stronger than conventional batteries 1–4 in the field of power electronics. Most experimental studies in last few decades reveal that the fabrication of supercapacitors with high power and energy density with long cycle life is the central focused area among scientists. 5 To date, several attempts have already been made to fabricate supercapacitors with high energy density, sustainable cycle life, and an extensive potential window of operation. 5,6 To achieve excellencies in supercapacitor devices, carbon-based materials (activated carbon, carbon nanotubes, (CNT) and reduced graphene oxide (rGO)), 7–9 con- ducting polymers (polypyrrole, polyaniline, PEDOT–PSS, and polythiophene), 10,11 transition metal oxides (nickel oxide (NiO), cobalt oxide (Co 3 O 4 )) and even manganese dioxide (MnO 2 ) have been recognized as suitable electrode materials for high charge storage supercapacitor applications. Therefore, various combi- nations and composition of carbon-based metal oxide and conducting polymer composite-based electrodes, gradually have become necessary for producing high capacitive performance and good cyclability. Zhang et al. showed that the specific capacitance performance of pure MnO 2 was nearly 258.7 F g 1 at the KMnO 4 /MnCl 2 molar ratio of 3 : 2. 12 The performance was modified utilizing graphene micro–nano spheres inside the hollow-porous MnO 2 structure. 13 Furthermore activated carbon, carbon nanofiber, and carbon nanotube were incorporated into the MnO 2 matrix and a specific capacitance of 375, 238, and 349 F g 1 respectively, was reported. 14 To modify the performance, bamboo charcoal was impregnated into the MnO 2 matrix in different weight percentages and results revealed that 3 wt% bamboo charcoal incorporated MnO 2 nano- composite with a maximum capacitive performance of nearly 307 F g 1 at 100 mA g 1 . 15 Recently, they investigated the working electrode performance of NiMoO 4 -multi-walled carbon nanotube composites and found better specific capacitance of a Department of Physics, Jadavpur University, Kolkata 700 032, India. E-mail: sukhendasju@gmail.com b Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711 103, India c Department of Physics, Bangabasi College, Kolkata 700009, India † Electronic supplementary information (ESI) available: To support the performance of our handmade device, a video (duration_10 s) of lighting up two commercial LEDs by our device is provided. See DOI: 10.1039/d0tc06085b Received 29th December 2020, Accepted 11th March 2021 DOI: 10.1039/d0tc06085b rsc.li/materials-c Journal of Materials Chemistry C PAPER Published on 16 March 2021. Downloaded by Jadavpur University on 4/5/2021 9:57:16 AM. View Article Online View Journal