Capacity Enhancement of Low-Cost and Flexible Micro-Supercapacitors By a Redox-Active Electrolyte and Laser-Induced Fibrous Graphene

Abstract

Graphene-based nanomaterials are very promising for wearable and flexible electronics because of their unique electrical and mechanical properties. However, their difficult chemical synthesis has limited wide-spread commercial adoption. In particular, energy storage devices such as micro-supercapacitors require a porous morphology for large specific surface area and patterning such as interdigitated designs, which traditionally requires multiple complex fabrication steps. Laser-induced graphene (LIG) is a recently developed method that solves this problem by simultaneously generating and patterning porous graphene electrodes. Laser irradiation of a polyimide (PI) substrate selectively transforms the PI into graphene. This method shows promise to fabricate miniaturized energy storage devices at low cost and on a flexible substrate. However, the capacitance of LIG micro-supercapacitors needs to be improved by developing novel charge storage mechanisms compatible with LIG electrodes. Typically, energy storage mechanisms are classified as either electrochemical double-layer capacitance (EDLC) or pseudocapacitance. EDLC is enhanced by facile adsorption of electrolyte ions on the electrode material. Therefore, light weight carbon-based materials such as activated carbon, carbon nanotubes, graphene and their different combinations are suitable for EDLC but ultimately their performance is limited by their specific surface area. Pseudocapacitance allows further charge to be stored beyond the limitations imposed by the electrode’s surface area utilizing redox reactions of the electrode material with the electrolyte ions. There has been extensive interest in enhancing pseudocapacitance using various redox reactions between electrode materials such as transition metal oxides, conducting polymers and electrolyte ions. Carbon-based large-scale supercapacitors have been reported with improved performance by adding redox species, which can undergo redox reactions with the carbon-based electrodes, to the electrolyte. Reports on micro-supercapacitors using redox electrolytes have been limited to electrodes fabricated by complex multi-step fabrication processes. Here, we report graphene micro-supercapacitors fabricated by one-step laser scribing with enhanced capacitance utilizing a redox electrolyte. Hydroquinone (HQ) is a promising candidate as a redox additive in the electrolyte owing to its double charge transfer mechanism i.e. loss of 2H+ and 2e- during the charging process, which results in benzoquinone (BQ). Similarly, BQ is reduced to HQ by gaining 2H+ and 2e- during discharging. HQ (0.5 mol/L) was added to the aqueous H2SO4 (1 mol/L) electrolyte, which was deposited onto interdigitated LIG electrodes. The laser parameters were optimized to achieve LIG with a fibrous morphology. Fig.1a shows a scanning electron microscope (SEM) image of the cross-section of the fibrous LIG on PI. The addition of HQ as a redox-active additive enhanced specific capacitance approximately 5 times compared to devices without the HQ additive. The unmodified H2SO4 electrolyte exhibited an areal capacitance of 0.8 mF/cm2. Addition of HQ raised the total capacitance to 3.9 mF/cm2 as calculated from the area under the cyclic voltammetry curve in Fig.1b. Cyclic voltammetry of the device with HQ electrolyte showed very strong peaks due to oxidation and reduction of HQ and BQ as shown in Fig. 1b. A typical cyclic voltammetry graph with rectangular shape due to EDLC was observed in H2SO4 at 10 mV/s scan rate. It remained stable even at a high scan rate of 1000 mV/s as shown in Fig.1c-d. These results demonstrate the conductive nature of the engraved graphene and facile adsorption of the H+ ions. The device was further tested at higher scan rates after the introduction of HQ (10-1000 mV/s as shown in Fig.1e-f. The redox peaks were found very stable at higher scan rates and their peak values move towards extreme potentials at higher scan rates. To further investigate the motion of electrolyte ions within the pores of the fibrous graphene, electrochemical impedance spectroscopy (EIS) was performed with and without HQ in H2SO4 (see Fig.1g-j). A semi circle at low frequencies was observed (see Fig.1j), which indicates only 3 Ω charge-transfer resistance owing to facile motion of the electrolyte ions. In conclusion, we report a facile method to increase the capacitance of micro-supercapacitors with laser-induced graphene electrodes. The method only requires the addition of one redox-active component to the electrolyte. Combined with the one-step fabrication and patterning of graphene by LIG, this method shows promise for low-cost flexible energy storage devices. Figure 1