Graphene-Graphite Polyurethane Composite Based High-Energy Density Flexible Supercapacitors (original) (raw)

based energy storage technologies have been explored meet the demand in these applications. [7,12-17] Currently, the electrochemical based energy storage is largely based on Li ions batteries (LIBs), sodium ion batteries, or zinc-air batteries. [13-15,18-21] In particular, LiBs offer high energy density (≈500 W h kg −1) and benefit from well-developed manufacturing processes. [14,18,19] But majority of them are not flexible and their weight, low power density, long charging time (1-2 h), heat generation, [22] environmental concerns, [18,19,22,23] etc. limit their use in applications such as wearable systems. These limitations have already caught the attention of the research community , as evident from various works on flexible/stretchable batteries [12,13,24,25] and supercapacitors (SCs) with long cycling stability. [26-28] In particular, the SCs offer excellent energy and power densities with low-cost of fabrication. [26-31] Further they offer rapid charging (minutes vs hours in LiBs), long life cycle, do not generate heat [26,30] and are generally environment friendly. [28,29] With flexible and stretchable form factors, SCs can also conform to curved surfaces. [28] Among various types of SCs, the electrochemical double layer capaci-tors (EDLCs) [28,29] based on carbon materials are the most promising [28-32] because of their long lifetime (more than 10 6 charging/discharging cycles), low environmental impact, ease of maintenance, and flexible form factors. [26,27,30] The performance of SCs is mainly governed by their structure , surface morphology, electrolytes, and the electrochemical and electrical properties of active electrodes. [26,30,33-35] For this reason, the choice of electrode materials and the electrolyte are critical. Since the energy storage (areal energy density, E A = C A V 2 /2) depends on the potential window (V) and the specific capacitance (e.g., areal capacitance C A), researchers have focused on the ways to improve these values. [35-38] For example, a variety of carbon-based structures (e.g., graphene foam, [35] reduced graphene oxide (rGO), [36] etc.) have been explored for EDLC fabrication. The choice of electrolyte is also important to increase the V and hence the energy density. [37] Table S1 in the Supporting Information provides a comparison of C A , and V for EDLCs developed with various active carbon-based materials. The low values of C A (<10 mF cm −2) reported in the majority of the SCs can be attributed to the lack of elec-troactive surface per unit area needed to store the charge at the electrode-electrolyte interface. [33,38] The electrodes with multi-layer structures have been explored to overcome such issues Energy autonomy is critical for wearable and portable systems and to this end storage devices with high-energy density are needed. This work presents high-energy density flexible supercapacitors (SCs), showing three times the energy density than similar type of SCs reported in the literature. The graphene-graphite polyurethane (GPU) composite based SCs have maximum energy and power densities of 10.22 µWh cm −2 and 11.15 mW cm −2 , respectively, at a current density of 10 mA cm −2 and operating voltage of 2.25 V (considering the IR drop). The significant gain in the performance of SCs is due to excellent electroactive surface per unit area (surface roughness 97.6 nm) of GPU composite and high electrical conductivity (0.318 S cm −1). The fabricated SCs show stable response for more than 15 000 charging/ discharging cycles at current densities of 10 mA cm −2 and operating voltage of 2.5 V (without considering the IR drop). The developed SCs are tested as energy storage devices for wide applications, namely: a) solar-powered energy-packs to operate 84 light-emitting diodes (LEDs) for more than a minute and to drive the actuators of a prosthetic limb; b) powering high-torque motors; and c) wristband for wearable sensors. Supercapacitors