We report a facile route for the synthesis of hyperbranched polyamido-graphenes (HBP(A-G)) as non-metallic high capacity electrodes for charge storage in supercapacitors. HBP(A-G) were synthesized by Michael addition of polyamines, ethylene diamine (EDA), diethylene triamine (DETA), triethylene tetraamine (TETA), to acrylamide grafted graphene oxide, followed by reduction in situ. A 3D network of stiff graphene sheets connected by flexible polyamine chain was formed. Hyperbranching increased d-spacing and BET surface area of graphene stacks, which enhanced accessibility of sites for ion storage, and prevented restacking. HBP(A-G) electrodes displayed electric double layer capacitance behaviour with excellent specific capacitance. The charge storage capacity of the electrodes increased with an increase in the amine chain length. Specific capacitance of electrodes containing HBP(TETA-G) was found to be 269 F g-1 in a symmetric two electrode electrochemical cell, using 1 M H2SO4 electrolyte at a current density of 1 A g-1 for voltage range of 0- 1 V. An increase in the length of amine chains improved ion mobility, lowered equivalent distributed resistance and time constant of the electrodes. The time constant of HBP(TETA-G) capacitor was only 538 ms. Hydrophilic amine linkages on graphene backbone provided stability against electrode volume changes during charging and discharging and gave 89 % capacity retention over 10,000 cycles at 10 A g-1 current density for HBP(TETA-G) electrodes. HBP(A-G) electrodes demonstrated superior performance as electrodes for supercapacitors and has shown potential for use in other electrochemical applications. Graphical abstract Symmetric supercapacitor fabricated using hyperbranched poly(amidographene)s, synthesized via novel route by Michael addition of polyamines to acrylamide-grafted-graphene oxide followed by in situ reduction to graphene, gave high specific capacitance with good energy density.

Introduction

The recovery and storage of electrical energy is one of the most important challenges facing our society that is progressing in the direction of harnessing renewable energy, use of hybrid vehicles, etc. Batteries and electrochemical capacitors are most common energy storage technologies [1]. The cutting-edge battery technology is not adequate for powering these applications, mainly due to slow charge/discharge time, short life and high cost. Electrochemical capacitors, also known as supercapacitors (SC) or ultracapacitors, combine the benefits of both, batteries and capacitors. Supercapacitors have much higher energy capacity than conventional capacitors, with similar rapid charge/discharge characteristics. Electric double layer capacitors (EDLC) are one type of SC containing high surface area material on which the charges accumulate. The other type of capacitor, known as pseudocapacitor, consists of conducting materials or those that undergo redox transformations [2]. Since pseudocapacitive materials have several times higher capacity, hybrid capacitators combing capacitive carbon based electrode with conductive or redox active materials have been in focus recently [3–۷]. Graphene composed of sp2 hybridized (2D) carbon atoms has driven great interest in the field due to unique properties such as high surface area, excellent thermal and mechanical stability, high conductivity, etc., which makes it an ideal material for EDLC [8,9]. One of the disadvantages of graphene is, its tendency to stack thereby decreasing the accessible surface area and the performance [10]. Studies on derivatives of graphene with conducting polymers such as polyethylenedioxythiophene [11,12], polyaniline [13,14], polypyrrole [15–۱۷] are reported with the aim of decreasing the stacking tendencies.