Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est Robust electrochemical performance of polypyrrole (PPy) and polyindole (PIn) based hybrid electrode materials for supercapacitor application: A review Ram Bilash Choudhary ⁎ , Sarfaraz Ansari, Bela Purty Nanostructured Composite Materials Laboratory, Department of Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India ARTICLE INFO Keywords: Polypyrrole Polyindole Symmetric supercapacitor Surface morphology Specific capacitance Electropolymerization ABSTRACT One of the basic intentions of science and technology is the evolution of novel and more efficient devices to make human life more easy and comfortable. A supercapacitor is one such device that can be utilized as a com- plementary and to some extent a replacement of electrochemical battery to store electrical energy. The per- formance of a supercapacitor very much depends on the electrode material used and hence a variety of materials are employed to form electrodes of supercapacitors in order to improve its performance. In this review, we have discussed the preparation and properties of polypyrrole (PPy) and polyindole (PIn) based electrode material for supercapacitor applications. PPy shows a high degree of flexibility which makes it markedly suitable and ap- plicable for making flexible supercapacitors. PIn exhibit slow hydrolytic degradation and hence long charge- discharge time. Thus, PIn can be utilized to form supercapacitors having the ability to store energy for a longer period of time. Nonetheless, their properties can be enhanced by using binary and ternary composites of PPy and PIn with different carbon-based and metal-based materials. This review briefly discusses some of these com- posites and devices formed by them. 1. Introduction Energy has been one of the most important and fundamental needs of human from ancient times. It is both the cause and effect of the development of a society. Coal, petroleum, wood and natural gas are some conventional energy resources. Unfortunately, these conventional energy resources are limited and depleting very rapidly with the growth of industrialization and urbanization. Further, they are unfriendly and incompatible with the environment [1]. On the other hand, renewable energy resources are environment friendly, easily accessible and abundant in nature [2]. Therefore renewable and sustainable energy resources, like hydropower energy, wind energy, solar energy, geo- thermal energy, marine energy, etc. are rapidly getting its importance [3]. But these energy sources are sporadic and cannot be utilized con- tinuously for a long period of time. For instance, we cannot use solar energy at night [4]. For the continuous use of renewable energy, there is a dire need of energy storage devices like batteries, capacitors and supercapacitors to store a huge amount of energy for later times. Re- chargeable battery is the most widely used energy storage device which stores energy via charge transfer in-between the electrodes through redox reaction [5]. Although batteries can be utilized as a constant power source for a convenient period of time, there are a number of shortcomings such as low power density, relatively poor life span, spark hazard, and adverse effect on the environment, etc. [6]. Whereas, su- percapacitor, also called ultracapacitor or power capacitor, is an en- vironment friendly energy storage device. This is because of the non- toxicity of materials generally used as the electrode and the electrolyte in a supercapacitor [7]. Moreover, supercapacitors can be prepared by bio-wastes such as dead leaves, coconut shells, eggshells, etc. In this way supercapacitor also provides an effective and sustainable method for recycling of these bio-wastes and hence helps in minimizing en- vironmental pollution. It delivers relatively larger specific power (SP) than that of batteries and relatively larger specific energy (SE) than that of ordinary capacitors. Hence, it is supposed to bridge the gap between capacitors and batteries [8]. Further, it is more energy efficient and longer life than that of rechargeable batteries. Due to these advantages supercapacitors have attracted huge attention in the various fields of research and are considered as a future replacement of batteries. A brief comparison among capacitor, supercapacitor and battery are shown in Table 1 [9,10]. In the year 1957, H. I. Becker invented a low voltage capacitor by using porous carbon electrodes [11,12]. This was the first energy https://doi.org/10.1016/j.est.2020.101302 Received 10 October 2019; Received in revised form 23 January 2020; Accepted 16 February 2020 ⁎ Corresponding author. E-mail address: rbcism@gmail.com (R.B. Choudhary). Journal of Energy Storage 29 (2020) 101302 2352-152X/ © 2020 Elsevier Ltd. All rights reserved. T