SHORT COMMUNICATION Nano-dimensional iron tungstate for super high energy density symmetric supercapacitor with redox electrolyte Sagar Jadhav 1,2,3 & Pratiksha D. Donolikar 2,3 & Nilesh R. Chodankar 3,4 & Tukaram D. Dongale 2,3 & Deepak P. Dubal 3,5 & Deepak R. Patil 1,3 Received: 18 August 2019 /Revised: 18 August 2019 /Accepted: 11 October 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract In present work, we have developed 2.0 V symmetric supercapacitor with rationally prepared iron tungstate (FeWO 4 ) nanopar- ticles as electrodes and redox-active electrolyte. It is revealed that the electrochemical performances of FeWO 4 -system were significantly improved due to the addition of potassium iodide (KI) redox additive in conventional KOH electrolyte in terms of the specific capacitance and energy density. Notably, FeWO 4 -based symmetric cell with KI-additive shown two-fold enhance- ment in specific energy (113 Wh/kg) compared with the cell with pristine KOH electrolyte (41.62 Wh/kg). Such an excellent enhancement is attributed to the improvement in the stability of existing KOH electrolyte by KI which influences the strength of OH bond in aqueous media and prevents the breakdown of electrolyte without adversely affecting the redox behavior and on contrary supporting the interactions at the higher potential to produce better results. Keywords Symmetric supercapacitors . Redox species . High energy . Voltage Introduction The electrochemical energy storage systems become more im- portant for controlling air pollution by reducing the consump- tion of harmful fossil fuel energy resources. The batteries and supercapacitors (SCs) are vital parts of the use of sustainable energy and energy storage to tackle the energy crisis and global warming issues. Especially, SCs are shown a promising view owing to their high power densities and fast charge/discharge rates along with excellent life cycles; however, the energy den- sity of SCs (being ˂ 10 Wh/kg) is still lagging behind than that of batteries, which has become a major concern to employ them as a replacement for batteries [1]. Hence, it is necessary to find some advanced materials having the ability to store more charges than the conventional electrode materials. In addition to this, developing the rational and eco-friendly synthesis route is essentially required for the modern energy storage system. In literature, the energy density of the SCs has been en- hanced by assembling the asymmetric SCs with different pos- itive and negative electrodes. Typically, the carbonaceous ma- terials like activated carbon (AC), graphene, or carbon nano- tubes (CNTs) [2–4], with higher conductivity and surface area, are well studied as negative electrodes. On the other hand, the redox-active transition metal oxides (TMOs) including RuO 2 , TiO 2 , (Ni, Mn)Co 2 O 4 , MnO 2 , NiO, NiMoO 4 ,V 2 O 5 , BiVO 4 , Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10008-019-04427-x) contains supplementary material, which is available to authorized users. * Deepak P. Dubal dubaldeepak2@gmail.com * Deepak R. Patil deepphy24@gmail.com 1 Centre for Materials for Electronics Technology, Ministry of Electronics and Information Technology (MeitY), Pune 411008, India 2 Computational Electronics and Nanoscience Research Laboratory, School of Nanoscience and Biotechnology, Shivaji University, Kolhapur 416004, India 3 Department of Physics and Astronomy, Center for Novel State of Complex Materials Research, Seoul National University, Seoul 151-747, Korea 4 Department of Energy & Materials Engineering, Dongguk University, Seoul 100-715, Republic of Korea 5 School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4001, Australia Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-019-04427-x