Graphene–Graphite Polyurethane Composite Based High‐Energy Density Flexible Supercapacitors

Abstract 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.

As compared to GS/GPUSC with separator wrapped around the active electrodes (GS/GPUSC -1), the single separator GS/GPUSC shows a significant change in the value resistance and reactance. Instead of straight line in Nyquist plot in low frequency range ( Figure 4) of wrapped separator in GS/GPUSC, the single separator-based device has a semicircle arc in the low frequency range, Figure S2i. It potentially due to the ion adsorption rather than diffusion in low frequency range. The configuration of GS/GPUSC have strong influence on the value of capacitance. It was observed that, in low frequency ionic resistance have strong influence and hence phase angle reach to zero. Moreover, the maximum value of phase angle observed for single layered separator GS/GPUSC is -37° and is very low as compared to wrapped separator GS/GPUSC (-77°). This shows that electrochemical and capacitive performance of the device is poor. respectively. (f) Self-charging of the GS/GPUSC (g) GCD curve at fixed potential window of GS/GPUSC based on NaOH electrolyte (h) GCD curve for saturation potential of GS/GPUSC based on NaOH electrolyte.

Comparison of Charging-Discharging analysis of SCs
In the case of GSSC, the charging-discharging occurred at very low current densities (in the range of 2-8 µA cm -2 ), as shown in Figure S3d for V sat . Figure S3d and S3e shows the GCD measurements carried out in GSSC and GPUSC to obtain the V sat . It is worth noticing that even though the GSSC and GPUSC can reach V sat between 2 -3V, using high current densities, both SCs discharge at a faster rate than GS/GPUSC. As a result, the applicability of GSSC and GPUSC will be limited. The poor operation at high current density and the quick discharging of these devices can be overcome by using the double-layered structure as in the case of GS/GPUSCs.

Role of Electrolyte on SCs Performance
From EIS and CV analysis, we have observed that the SC cell configuration with separator also plays a crucial role towards the enhancement of the electrochemical performance of the GS/GPUSCs. In addition, to evaluate the influence of electrolyte on the performance of the SCs, we have carried out GCD measurements for GS/GPUSC with NaOH electrolyte and compared with results obtained by using H 3 PO 4 electrolyte ( Figure S3g and S5h and comparison under different current densities in Table S2). We noted that, as compared to H 3 PO 4 , the NaOH shows faster discharging times, which could be due to low ionic concentration for EDL formation. The H + ions in H 3 PO 4 can diffuse more efficient into GPU matrix than Na + ions from NaOH electrolyte.

Operational efficiency of SCs
The operational efficiency or performance of the SCs, is determined thorough the areal capacitance (C A ), energy (E A ) and power (P A ) densities, the equations (1) to (3).
The specific capacitance (C A ) of the SC at different current densities were obtained from the following equation 11 Where I is the discharge current Δt is the discharge time of the electrochemical capacitor, ΔV is the potential window (here we consider 0 to 1V and 0 to V sat ) and A is the area of active material.
The energy and power density of the fabricated GSC were calculated by using The specific capacitance (C A ) of the SC can be calculated with IR voltage drop by using the equation Where IR drop is the IR drop voltage at high current densities. The energy and power density of the fabricated GSC were calculated by using

Supplementary Movie Notes
Supplementary Movie-1 Stable and reliable operation of flexible supercapacitors is crucial for several applications. In this regard, we have demonstrated the applicability of fabricated GS/GPUSC in cyclic bending with a radius of 24 mm as an example.

Supplementary Movie-2
The proposed supercapacitors and its performance in solar charging demonstrate its potential application for deployment in portable and remote area usage. We demonstrate this through integrating supercapacitor with solar cell and powered to 84 LEDs, which is modelled as a letter of BEST and UoG.

Supplementary Movie-3
The operating voltage of ~2.5 V allows the SCs power general-purpose electronics and hence makes them suitable for a wide range of applications. As a demonstrations 3 supercapacitors connected to a single motor (normally used in electric toy cars) has a speed of 2300 RPM at 4.5VDC in 70mA without any load. The SCs were charged by using 120 mA current at 2.5V.

Supplementary Movie-4
The performance of the SCs (9 SCs connected) for three motors operation is shown in this movie.

Supplementary Movie-5
Energy autonomy is currently a major challenge in autonomous robotics and prosthetics. Together with solar cells, the SCs offer an attractive solution for future energy autonomous robotics and prosthetics. In this movie, the index and thumb fingers are actuated through PWM to perform grab and release action.