Contents lists available at ScienceDirect Nano Energy journal homepage: www.elsevier.com/locate/nanoen Full paper A novel design of hybrid transparent electrodes for high performance and ultra-flexible bifunctional electrochromic-supercapacitors Riski Titian Ginting a,b,1 , Manoj Mayaji Ovhal a,1 , Jae-Wook Kang a, ⁎ a Department of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju 54896, Republic of Korea b Department of Electrical Engineering, Universitas Prima Indonesia, Medan 20118, Indonesia ARTICLE INFO Keywords: Bifunctional electrochromic-supercapacitor Silver-nanowire Hybrid flexible transparent electrode Solution-processed Polyelectrolyte Nickel hydroxide ABSTRACT In this study, a simple and cost-effective method of fabricating hybrid transparent conductive electrodes (TCEs) based on embedded silver nanowires (Ag NWs)/PEDOT: PSS was developed with the addition of low-tem- perature synthesis of Ni(OH) 2 and polyethylenimine ethoxylated (PEIE) composites as a novel interlayer. The hybrid TCEs with a Ni(OH) 2 -PEIE interlayer exhibit remarkable volumetric capacitance of 443 F cm -3 with transparency of 86%, which is one of the highest values reported to date in the transparent supercapacitor. The fabricated bifunctional solid-state electrochromic-supercapacitor device with a transparency of 80% shows stable cyclic stability up to 10,000 charge/discharge cycles, extremely high coloration efficiency of 517 cm 2 C -1 at 633 nm, and a fast switching speed (< 0.6 s). The noted improvement is mainly caused by the Ni(OH) 2 -PEIE interlayer influence the pore density of PEDOT: PSS which provides high surface area, thus resulting in efficient charge transfer pathways and fast ion diffusion. Moreover, a capacitance retention of 90% is achieved even after 8000 bending cycles at a bending radius of 1 mm and 15 times of crumpling is tolerated without noticeable degradation, implying excellent mechanical robustness and flexibility. The results present the significant po- tential of transparent hybrid electrodes for efficient energy storage and electrochromicity with stable trans- mittance changes, even during fast charge/discharge processes, demonstrating their potential as smart wearable energy storage devices. 1. Introduction Flexible transparent conductive electrodes (TCEs) have attracted considerable attention for use in solar cells, neural networks, light emissions, touch screens, and energy storage [1–4]. Among these ap- plications, the supercapacitor based on flexible TCEs is a potential candidate for wearable energy storage devices owing to its fast charge/ discharge rates, long cycling life, light weight, high power density, and compatible integration with other power sources [5–7]. Recent studies of TCEs have not only focused on improvement of electrochemical properties, but have also further extended the application to bifunc- tional electrochromic and supercapacitors devices as intelligent dis- plays and smart windows [8,9]. Moreover, many efforts have been made to fabricate transparent and flexible conductive electrodes for electrochromic supercapacitors (EC-SCs). Nevertheless, it remains challenging to achieve both high coloration efficiency, a fast switching time, and high capacitance on flexible substrates due to the require- ment of a low temperature process [10]. Two main issues hinder the development of flexible transparent EC-SCs. Firstly, flexible TCEs—- namely, conducting polymers, carbon nanotubes, and graphene as both current collectors and active materials—usually exhibit low con- ductivity, low capacitive behavior, and poor optical transmittance [4,11], while indium tin oxide (ITO) has mechanical brittleness [12]. Secondly, even though most conductive flexible TCEs, such as metal nanowire networks, show good conductivity and optical transmittance, the electrode itself demonstrates relatively poor electrochemical beha- vior [13]. Therefore, an alternative method is needed to combine both transparent metal electrodes and pseudocapacitive materials as flexible- based hybrid electrodes [14]. There have been many attempts to fabricate metal nanowire net- works as electrodes for transparent SC applications, such as nickel [14] and gold networks [15], silver grids [16], and silver nanowires (Ag NWs) [10]. Among them, Ag NWs are promising because they are printable, scalable, cost effective, and exhibit superior flexibility [17,18]. Despite many studies examining the use of Ag NWs as trans- parent electrodes for optoelectronics devices, their applications for https://doi.org/10.1016/j.nanoen.2018.09.016 Received 21 May 2018; Received in revised form 24 August 2018; Accepted 9 September 2018 ⁎ Corresponding author. 1 R.T. Ginting and M.M. Ovhal made equal contribution to this paper. E-mail address: jwkang@jbnu.ac.kr (J.-W. Kang). Nano Energy 53 (2018) 650–657 Available online 13 September 2018 2211-2855/ © 2018 Published by Elsevier Ltd. T