Facile preparation of flexible binder-free graphene electrodes for high-performance supercapacitors

Abstract

A facile two-step strategy to prepare flexible graphene electrodes has been developed for supercapacitors using thermal reduction of graphene oxide (GO) and thermally reduced graphene oxide (TRGO) composite films. The tunable porous structure of the GO/TRGO film provided channels to release the high pressure generated by CO₂ gas. The graphene electrode obtained from reduced-GO/TRGO (1 : 1 in mass ratio) film showed great flexibility and high film density (0.52 g cm^-3). Using the EMI-BF₄ electrolyte with a working voltage of 3.7 V, the as-fabricated free-standing reduced-GO/TRGO (1 : 1) film achieved a great gravimetric capacitance of 180 F g^-1 (delivering a gravimetric energy density of 85.6 W h kg^-1), a volumetric capacitance of 94 F cm^-3 (delivering a volumetric energy density of 44.7 W h L^-1), and a 92% retention after 10 000 charge/discharge cycles. In addition, the solid state flexible supercapacitor with the free-standing reduced-GO/TRGO (1 : 1) film as the electrodes and the EMI-BF₄/poly (vinylidene fluoride hexafluopropylene) (PVDF-HFP) gel as the electrolyte also demonstrated a high gravimetric capacitance of 146 F g^-1 with excellent mechanical flexibility, bending stability, and electrochemical stability. The strategy developed in this study provides great potentials for the synthesis of flexible graphene electrodes for supercapacitors.

Fig. 1. Schematic illustration of synthesis process of flexible free-standing reduced-GO/TRGO film.

Fig. 2. Cross-sectional SEM images of (a) GO film; (b) GO/TRGO (1 : 1) film; (c) reduced-GO/TRGO (1 : 1) film, and (d) bent reduced-GO/TRGO (1 : 1) film with radius of curvature of 1.28 mm.

Fig. 3. TEM images of GO/TRGO composite film and reduced-GO/TRGO film. (a) Low magnification TEM image showing an overview of GO/TRGO nanosheets. (b) HRTEM image of GO/TRGO nanosheets marked in (a), with d-spacing of 0.23 nm corresponding to (100) of graphite. The crystalline domain is due to TRGO and the amorphous region pertains to GO. (c) Low magnification TEM image showing an overview of reduced-GO/TRGO film. (d) HRTEM image of reduced-GO/TRGO film showing clear crystallinity in graphene after rapid thermal reduction.

Fig. 4. XRD patterns of GO, GO/TRGO, and reduced-GO/TRGO films.

Fig. 5. Raman spectra of GO, GO/TRGO, and reduced-GO/TRGO films.

Fig. 6. (a) XPS survey spectra of GO, GO/TRGO, and reduced-GO/TRGO films. (b) XPS high-resolution C 1s spectra of GO, GO/TRGO, and reduced-GO/TRGO films.

Fig. 7. (a) Nitrogen adsorption/desorption isotherms and (b) pore size distribution of GO, GO/TRGO, and reduced-GO/TRGO films.

Fig. 8. (a) CV curves at slow voltage sweep rate of 10 and 100 mV s⁻¹. (b) Charge and discharge curves at constant current density of 0.2 A g⁻¹. (c) Gravimetric and volumetric specific capacitance at various current densities from 0.2 to 10 A g⁻¹. (d) Ragone plots of supercapacitor with free-standing reduced-GO/TRGO (1 : 1) film and comparison with devices using other 2D materials reported in literature. (e) Nyquist plots of supercapacitors with free-standing reduced-GO/TRGO (1 : 1) film and filtered TRGO film electrodes, respectively. (f) Cycling stability of supercapacitor with free-standing reduced-GO/TRGO (1 : 1) film.

Fig. 9. (a) Cyclic voltammetry curves and (b) charge and discharge curves of flexible solid state supercapacitor at different bending angles. (c) Gravimetric specific capacitance of flexible solid state supercapacitor at various current densities from 0.1 to 1.0 A g⁻¹.