Transparent supercapacitors of 2 nm ruthenium oxide nanoparticles decorated on a 3D nitrogen-doped graphene aerogel

Abstract

Although ruthenium oxide nanoparticles (RuO₂), graphene, and their composites have been widely used as supercapacitor electrode materials, transparent supercapacitors of these materials have been rarely investigated. In this work, we fabricated high-performance transparent solid-state supercapacitors of 2 nm RuO₂ decorated on a 3D nitrogen-doped reduced graphene oxide aerogel (NGA) with 27–54% transparency. The as-fabricated symmetric supercapacitor of RuO₂/NGA at a finely tuned mass loading of 16.3 μg cm⁻² with a finely tuned transmittance of 34.1% at a wavelength of 550 nm exhibits a maximum areal energy of 0.074 μW h cm⁻² and a maximum areal power of 64 μW cm⁻². The cycling stability of the device can also be maintained at 100% over 2000 cycles. The high transparent supercapacitor in this work may practically be used in many advanced transparent electrical devices.

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1. Introduction

Due to the increasing demand for next-generation energy technologies, transparent and flexible energy storage devices have attracted a lot of interest. They can be used in many advanced applications such as transparent displays, transistors, optical circuits, touch screens, and solar cells.^1,2 Recently, supercapacitors providing high power densities (500–10 000 W h kg⁻¹) and high stability (>100 000 cycles) have been developed with the use of transparent substrates.^3,4 Although many transparent supercapacitor electrodes have been previously studied, transparent full-cell supercapacitor devices with high charge storage capacity are rather difficult to achieve owing to the poor transparent properties and charge storage capacity of the separators and electrode materials being used in the supercapacitor devices. Generally, conventional supercapacitors can be divided into two types based on their charge storage mechanisms including electrical double layer capacitors (EDLCs) and pseudocapacitors (PCs). EDLCs utilize ion adsorption/desorption at the interface between the electrolyte and the carbon-based electrode such as graphene, activated carbon, and carbon nanotubes.^5–7 Meanwhile, PCs store charge via the faradaic redox reaction on the surface of metal oxides or conducting polymers.⁸

Among carbon-based materials, graphene, a two-dimensional carbon monolayer with sp² hybridization, has been recently proposed as an effective active material for transparent supercapacitors^9–13 due to its good electrical conductivity (2 × 10³ S cm⁻¹) and high theoretical surface area (2675 m² g⁻¹).^14,15 However, transparent supercapacitors using graphene sheets still exhibit low charge storage performance (e.g. in-plane CVD graphene (80.7 μF cm⁻²)¹⁴ and laminated ultrathin CVD graphene (4.27 μF cm⁻²)¹⁶) due to the restacking nature of 2D graphene and its low ionic conductivity. Therefore, in this work, a 3D graphene aerogel with nitrogen atom doping (NGA) was used to replace the 2D graphene. The 3D interconnected structure of graphene sheets with their hierarchical porous structure can increase the electron mobility and contact area between the electrode/electrolyte, while nitrogen heteroatoms can generate a pseudocapacitive behaviour and increase the ionic conductivity.¹⁷

To further improve the performance of transparent supercapacitors, ruthenium oxide (RuO₂) nanoparticles (NPs) were also decorated on the structure of the GA because of their high theoretical capacitance (1450 F g⁻¹ for RuO₂ and 1360 F g⁻¹ for RuO₂·0.5H₂O) due to which it can store charge via the following reaction (1).¹⁸

RuO_x(OH)_y + δH⁺ + δe⁻ ↔ RuO_x−δ(OH)_y+δ (1)

Porous carbon/RuO₂ nanocomposites have been widely studied as active materials in supercapacitors due to the combination of high pseudocapacitance from RuO₂ and electrochemical double layer capacitance from carbon materials.¹⁹ For example, F. Z. Amir, et al. reported that the specific capacitance (SC) of RuO₂/reduced graphene oxide composites synthesized by an in situ sol–gel deposition method is 500 F g⁻¹ at a current density of 1 A g⁻¹ (ref. 20) and the SC of RuO₂/holey reduced graphene oxide (HRGO) is 418 F g⁻¹ at a current density of 1 A g⁻¹.²¹ R. B. Rakhi and M. L. Lekshmi et al. reported that the SC of RuO₂/carbon nanocoil/reduced graphene oxide composites is 725 F g⁻¹ at a scan rate of 20 mV s⁻¹.²² Y. Zhang and S.-J. Park showed that the RuO₂/charcoal-derived activated carbon exhibited a SC of 510 F g⁻¹ at a current density of 1 A g⁻¹.²³ S. Kong et al. electrodeposited RuO₂ on graphene/carbon nanotubes achieving a SC of 480.3 F g⁻¹ at a current density of 0.53 mA cm⁻².²⁴ J. Zhang et al. showed that the SC of RuO₂ decorated on the C₃N₄@rGO aerogel is 704.3 F g⁻¹ at a current density of 0.5 A g⁻¹.²⁵ Although there are a few reports on using RuO₂ composites as an active material in transparent supercapacitors, the effect of loading contents has not yet been investigated.^26–28

In this work, we fabricated a new solid-state supercapacitor of 2 nm ruthenium oxide (RuO₂) nanoparticle decorated NGA with 27–54% transparency for which a polymer gel of PVA/H₂SO₄ was used as the solid-state electrolyte and separator. The as-fabricated symmetric supercapacitor using RuO₂/NGA at a mass loading of 16.3 μg cm⁻² exhibits the highest electrochemical performance (1.57 mF cm⁻² at 0.5 A g⁻¹) with a finely tuned transmittance of 34.1% at a wavelength of 550 nm. The cycling stability of the as-fabricated device can also be maintained at 100% over 2000 cycles.

2. Experimental

2.1 Chemicals and materials

Graphite powder (<20 μm, Aldrich), sodium nitrate (NaNO₃, 99% Ajax Finechem), potassium permanganate (KMnO₄, 99%, UNIVAR), sulphuric acid (H₂SO₄, 98%, QRec), nitric acid (HNO₃, 65%, QRec), hydrogen peroxide (H₂O₂, 30%, Chem Supply), hydrochloric acid (HCl, 37%, Qrec), hydrazine (N₂H₂, 99%, LOBA Chemie), ruthenium(III) chloride trihydrate (RuCl₃·3H₂O, Aldrich), 1-methyl-2-pyrrolidone (NMP, 99.5%, QRec), and poly(vinyl alcohol) (PVA, M_w = 85 000–124 000, 99+% hydrolysed, Aldrich) were of analytical reagent grade and used without further purification. Deionized water from a Millipore system (DI water, 15 MΩ cm) was used in all experiments.

2.2 Synthesis of graphene oxide

Graphene oxide (GO) was prepared using a modified Hummer's method. Briefly, 5.0 g graphite powder and 7.5 g NaNO₃ were mixed in 500 ml conc. sulphuric acid in an ice-bath keeping the temperature below 20 °C. 40 g KMnO₄ was slowly added into the mixture and stirred for 24 h at ambient temperature. Then, 500 ml DI water and 150 ml H₂O₂ were subsequently added to the mixture and stirred for 24 h. The mixture was then centrifuged with DI water several times until neutral. Finally, the GO powder was obtained by drying at 60 °C for 72 h.²⁹

2.3 Synthesis of the nitrogen-doped reduced graphene oxide aerogel (NGA)

The N-rGO aerogel was synthesized by a hydrothermal method using hydrazine hydrate as a reducing agent. Firstly, 300 mg of GO was sonicated in 150 ml of DI water for 2 h. Then, 0.5 M N₂H₂ was added to the GO solution. The solution was then transferred into a Teflon-lined stainless-steel autoclave and heated at 80 °C for 72 h. The obtained product was then washed with DI water several times and freeze dried for 72 h.³⁰

2.4 Synthesis of RuO₂/NGA nanocomposites

RuO₂/NGA nanocomposites were synthesized by chemical oxidation of RuCl₃ on the NGA. Firstly, 0.14 g of the N-rGO aerogel powder was mixed with 0.14 g RuCl₃·3H₂O (1 : 1 by weight) in 10 ml of DI water by sonication for 1 h. Then, 73.5 ml of H₂O₂ solution (13.5 vol%) was slowly added to the mixed solution under magnetic stirring. The final solution was placed in a static water bath at 90 ± 5 °C for 2 h³¹ and then it was cooled down to room temperature. Finally, the solution was filtered with DI water and dried at 60 °C (Fig. 1).

Fig. 1 Schematic illustration for the preparation of RuO₂/NGA and a transparent solid-state supercapacitor with a PVA/H₂SO₄ gel electrolyte.

2.5 Morphological and structural characterization

The morphology of the as-prepared materials was characterized by field emission scanning electron microscopy (FE-SEM, JEOL, JSM-7610F) and transmission electron microscopy (TEM, FEI Tecnai G2 20). The crystallographic structure was characterized by X-ray diffraction (XRD, Bruker D8 ADVANCE using Cu-Kα radiation, λ = 1.5418 Å). The specific surface area was measured by nitrogen adsorption/desorption with the BET method (3Flex, Micromeritics). The surface composition was analyzed by X-ray photoelectron spectroscopy (XPS, JEOL, JPS-9010MC). The chemical structure was investigated by Raman spectroscopy (Bruker, an excitation wavelength of 532 nm) and Fourier transform infrared spectroscopy (ATR-FTIR, PerkinElmer). The transmission spectra were recorded by UV-vis spectroscopy (PerkinElmer, Lambda 1050). The thermal gravimetric analysis technique (TGA, STA PT1600, Linseis) was used to measure the amount of RuO₂ in the composite. The thickness of the material on FTO was measured with an optical microscope (SteREO Discovery V12, Zeiss).

2.6 Fabrication of transparent supercapacitors and electrochemical evaluation

RuO₂/NGA nanocomposites were dispersed in NMP at different concentrations by sonication for 1 h. The homogeneous solution was spin-coated on an FTO glass substrate with an active area of 4 cm² at 1000 rpm for 25 s. Then, the electrode was dried at 60 °C for 24 h. Note that the mass loadings of the RuO₂/NGA nanocomposites are 12.5, 16.3, and 22.5 μg cm⁻² at the concentrations of 2.5, 5.0, and 10.0 mg ml⁻¹, respectively. Polyvinyl alcohol in sulfuric acid (PVA/H₂SO₄) was used as the gel electrolyte for which it was prepared by stirring 1 g of PVA in 10 ml of 1 M H₂SO₄ at 80 °C. The gel electrolyte was spin-coated on the electrode at 2000 rpm for 25 s. Then, the two electrodes were assembled into a symmetric transparent solid-state supercapacitor (see Fig. 1). For electrochemical evaluation, cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed using a Metrohm AUTOLAB potentiostat (PGSTAT 302N). Note that all calculation details can be found in the ESI.†

3. Results and discussion

3.1 Physicochemical characterization

The morphologies of the NGA and RuO₂/NGA nanocomposites synthesized from the oxidation of ruthenium chloride (RuCl₃) with H₂O₂ on NGA sheets were investigated by FE-SEM and TEM. Fig. 2a shows the FE-SEM image of the NGA which exhibits a 3D interconnected structure from graphene nanosheets. After combining with RuO₂, the small particles of RuO₂ were decorated on the graphene layers without destroying the graphene aerogel frameworks (Fig. 2b). The TEM images of the NGA in Fig. 2c and e also show the transparent thin layer of graphene sheets with many wrinkles. Meanwhile for the RuO₂/NGA nanocomposites in Fig. 2d and f, it can be clearly observed that tiny RuO₂ NPs with a particle size of ca. 2 nm were homogeneously decorated on the surface of the NGA corresponding to the FE-SEM results. The HRTEM image of the RuO₂/NGA nanocomposites and their particle size distribution are shown in Fig. S3.† It can be clearly seen that the particle size of RuO₂ in the RuO₂/NGA nanocomposites is in the range of 1.5–2.0 nm. The STEM image, EDX elemental mapping data, and EDX spectrum of RuO₂/NGA are shown in Fig. S4.† The cross-sectional optical micrographs are shown in Fig. S5.† The thicknesses of the NGA and RuO₂/NGA at the mass loadings of 12.5, 16.3, and 22.5 μg cm⁻² are ca. 10.8, 6.5, 8.7, and 16.8 μm, respectively.

Fig. 2 FE-SEM and TEM images of the NGA (a, c, and e) and 2 nm RuO₂/NGA nanocomposites (b, d, and f).

The XRD patterns of the as-prepared materials are shown in Fig. 3a. The XRD patterns show a broad peak at around 23–25° and a weak peak at around 44.5° corresponding to the (002) and (100) planes of graphene,³² respectively. The (002) plane peak of RuO₂/NGA is shifted from 24.3° to 24.6°. The d_spacing values of the NGA and RuO₂/NGA are 3.66 and 3.62 Å, respectively. These d_spacing values are higher than that of graphite (3.35 Å (ref. 33)) indicating the presence of some functional groups on the graphene sheet.³⁴ However, the diffraction peak of RuO₂ nanoparticles cannot be seen due to their small particle size and amorphous structure.^20,35Fig. 3b shows the nitrogen adsorption–desorption isotherm of both the NGA and RuO₂/NGA nanocomposites exhibiting a type-IV isotherm with an H3-hysteresis loop indicating their mesoporous structure.³⁶ The BET specific surface area of the NGA and RuO₂/NGA is 133 m² g⁻¹ and 92 m² g⁻¹, respectively. Note that the specific surface area of RuO₂/NGA is lower than that of the pristine NGA because of the decoration of RuO₂ nanoparticles on the NGA surface resulting in the loss of the specific surface area. The FTIR spectra of the NGA and RuO₂/NGA are shown in Fig. 3c. The peaks at 2981 and 2885 cm⁻¹ can be attributed to the asymmetric stretching and symmetric vibrations of CH₂, respectively,³⁷ whilst the peaks at 1559, 1204, and 1054 cm⁻¹ correspond to the C=C,³⁸ C–O vibration of C–OH,³⁹ and C–O–C in the epoxy groups,⁴⁰ respectively. Raman spectroscopy was also carried out to study the defect characteristics of the as-prepared materials, and the results are shown in Fig. 3d. These results show that the major vibration peaks consist of D and G bands. The D band at 1350 cm⁻¹ represents the defects, edges, and disordered carbon while the G band at 1579 cm⁻¹ represents the in-plane vibration of sp² carbon atoms. The peaks at around 2692 and 2938 cm⁻¹ are the 2D band and the combination mode of G and D bands,⁴¹ respectively. The characteristic peak of RuO₂ NPs is observed at around 620 cm⁻¹ (ref. 24) (see the inset image of Fig. 3d). In addition, an intensity ratio of D and G bands (I_D : I_G) of the NGA is 1.12, which is lower than that of the RuO₂/NGA (I_D/I_G = 1.21). This is because the chemical reduction of RuO₂ can introduce more lattice defects on the structure of the NGA.⁴² In addition, the amorphous content can also be calculated by the deconvolution of the amorphous band at around 1520 cm⁻¹ as shown in Fig. S1.†⁴³ The amorphous carbon contents of the NGA and RuO₂/NGA are 23 and 21%, respectively. To determine the RuO₂ content in the composite, TGA was carried out, and the results are shown in Fig. S6.† The results show a slight weight loss at a temperature lower than 200 °C due to the removal of physically adsorbed water. The weight loss at 200–400 °C is due to the removal of the water molecules of hydrous RuO₂, and the large weight loss at >400 °C is due to the removal of the NGA. After 500 °C, the remaining weight remains stable indicating that the RuO₂ content in the RuO₂/NGA composite is about 20%.

Fig. 3 (a) XRD patterns, (b) nitrogen adsorption–desorption isotherms, (c) FTIR spectra, and (d) Raman spectra of the NGA and RuO₂/NGA.

To investigate the chemical composition and structure of the NGA and RuO₂/NGA, XPS was carried out, and the results are shown in Fig. 4. The wide-scan XPS spectrum of RuO₂/NGA in Fig. 4a confirms the existence of RuO₂ NPs and nitrogen-containing functional groups of the NGA. In the narrow-scan XPS spectrum of Ru 3p, the RuO₂/NGA exhibits the binding energy of Ru 3p_3/2 at 463.2 eV and Ru 3p_1/2 at 485.5 eV which can be assigned to RuO₂.⁴⁴ The C 1s XPS spectrum of the NGA in Fig. 4c can be deconvoluted into three peaks at 284.7, 285.6, and 286.7 eV which can be attributed to C=C, C–N, and C–O, respectively,⁴⁵ whilst, the C 1s peak of the RuO₂/NGA nanocomposites in Fig. 4d can be deconvoluted into five peaks at 281.8, 284.7, 285.6, 286.7, and 288.5 eV which can be attributed to Ru 3d_3/2, C=C, C–N, C–O, and O–C=O,⁴⁵ respectively. Note that the O–C=O functional group occurs on the surface of RuO₂/NGA due to the further oxidation of the NGA in the presence of hydrogen peroxide. The N/C and O/C ratios of the NGA are 0.15 and 0.21 while the N/C, O/C and Ru/C ratios of RuO₂/NGA are 0.13, 0.28 and 0.15, respectively.

Fig. 4 (a) Wide-scan XPS spectra of the NGA and RuO₂/NGA, (b) Ru 3p XPS spectra of the NGA and RuO₂/NGA, (c) C 1s XPS spectra of the NGA, and (d) C 1s XPS spectra of RuO₂/NGA.

3.2 Electrochemical evaluation of transparent solid-state supercapacitors

CV was used to measure the electrochemical performance of the as-fabricated supercapacitors. Fig. 5a shows the CVs of the NGA and RuO₂/NGA at different mass loadings on FTO glass electrodes and at a scan rate of 25 mV s⁻¹ in the cell voltage range of 0–1.2 V. The CVs of the RuO₂/NGA devices show no obvious redox peak of RuO₂ due to the fast reversible redox reaction of tiny RuO₂ NPs.²¹ The area under the CV of RuO₂/NGA at a mass loading of 16.3 μg cm⁻² is larger than those of other samples, indicating its better charge storage performance. The CVs of RuO₂/NGA (16.3 μg cm⁻²) at various scan rates including 25, 50, 75, and 100 mV s⁻¹ are also shown in Fig. 5b. The specific and areal capacitances of RuO₂/NGA are higher than those of the pristine NGA due to the combination of pseudocapacitance from RuO₂ and electrochemical double layer capacitance from the NGA.³⁵ Among the RuO₂/NGA transparent solid-state supercapacitors, the RuO₂/NGA at a mass loading of 16.3 μg cm⁻² exhibits the highest specific capacitances of 79.6, 79.2, 75.9, and 72.3 F g⁻¹ and areal capacitances of 1.30, 1.29, 1.23, and 1.18 mF cm⁻² at a scan rate of 25, 50, 75, and 100 mV s⁻¹, respectively.

Fig. 5 (a) CV curves of the NGA and RuO₂/NGA at a scan rate of 25 mV s⁻¹, (b) CV curves of RuO₂/NGA at a mass loading of 16.3 μg cm⁻² at different scan rates, and (c) specific and (d) areal capacitances of the NGA and RuO₂/NGA as a function of scan rates.

Fig. 6a shows the galvanostatic charge–discharge (GCD) curves of the NGA and RuO₂/NGA supercapacitors at a current density of 0.5 A g⁻¹. These curves show a symmetrical triangular shape with a small iR drop indicating their good charge storage behaviour. The GCD curve of RuO₂/NGA at a mass loading of 16.3 μg cm⁻² exhibits the highest discharge time, which is in good agreement with the CV results. The GCD curves of RuO₂/NGA at a mass loading of 16.3 μg cm⁻² at various applied current densities of 0.5–10 A g⁻¹ are shown in Fig. 5b. The RuO₂/NGA nanocomposites at a mass loading of 16.3 μg cm⁻² exhibit the highest specific (96.4, 93.5, 72.7, 67.2, and 63.4 F g⁻¹) and areal capacitances (1.57, 1.52, 1.35, 1.18, 1.09, and 1.03 mF cm⁻²) in accordance with the CV results. Note that the areal capacitance of the RuO₂/NGA nanocomposites at a mass loading of 22.5 μg cm⁻² is higher than that at a mass loading of 12.5 μg cm⁻² at low applied current densities. This is because increasing the mass loading (or thickness) of the active material can increase the active surface area of the electrode which facilitates the storage of a large number of ions. However, a higher mass loading will create a longer diffusion pathway for electron transport^46,47 reducing the charge storage performance. Therefore, the specific and areal capacitances at a mass loading of 22.5 μg cm⁻² are lower than those at a mass loading of 16.3 μg cm⁻².

Fig. 6 (a) GCD curves of the NGA and RuO₂/NGA at a current density of 0.5 A g⁻¹, (b) GCD curves of RuO₂/NGA at a mass loading of 16.3 μg cm⁻² at different current densities, and (c) specific and (d) areal capacitances of the NGA and RuO₂/NGA as a function of current densities.

The optical transmittance spectra of all transparent solid-state devices are shown in Fig. 7. Note that a symmetric supercapacitor of bare FTO only provides a transmittance of 70% at 550 nm. The RuO₂/NGA devices with the mass loadings of 12.5, 16.3, and 22.5 μg cm⁻² exhibit the C_sp and C_ae values of 69.9 F g⁻¹ (0.87 mF cm⁻²), 96.4 F g⁻¹ (1.57 mF cm⁻²), and 58.4 F g⁻¹ (1.31 mF cm⁻²) at a current density of 0.5 A g⁻¹ with transmittances of 48.1%, 34.1%, and 27.0% at 550 nm, respectively. Note that these results are higher than those of the NGA supercapacitor (40.8 F g⁻¹ and 0.77 mF cm⁻²) which exhibits a transmittance of 53.6% at 550 nm.

Fig. 7 (a) Optical transmittance spectra and (b) specific and areal capacitances as a function of % transmittance@550 nm of the NGA and RuO₂/NGA transparent solid-state supercapacitors.

Electrochemical impedance spectroscopy (EIS) of the symmetric supercapacitors using the RuO₂/NGA nanocomposites on the FTO glass electrode using PVA–1 M H₂SO₄ gel electrolyte was further carried out in the frequency range of 10 mHz to 100 kHz at an excitation amplitude of 5 mV, and the results are shown in Fig. 8a. The slope in the low-frequency region approaching the Y-axis indicates the ideal capacitive behaviour and low diffusion resistance of the ions in the electrolyte. The RuO₂/NGA at a mass loading of 16.3 μg cm⁻² exhibits a nearly vertical line as compared to other devices, whilst the X-intercept of the semicircle in the high-frequency region represents the equivalent series resistance (ESR) related to the resistances of electrode materials, electrolyte, and their contact.⁴⁸ The ESR values of the NGA and RuO₂/NGA at the mass loadings of 12.5, 16.3, and 22.5 μg cm⁻² are 23.09, 22.41, 16.22, and 51.22 Ω, respectively. Note that the charge transfer resistance (R_ct) or the diameter of the semicircle of all RuO₂/NGA devices is smaller than that of the bare NGA indicating that the electrochemical reactions on the interface of RuO₂/NGA electrode/electrolyte are more facile.²⁸ In addition, the equivalent circuit model of RuO₂/NGA at a mass loading of 16.3 μg cm⁻² is also shown in Fig. 8b with an ESR of 16.3 Ω and R_ct of 190 Ω. The relaxation time (τ₀) which is the minimum time required for the discharge process can also be calculated by using the complex power⁴⁹ as shown in Fig. 8c. The τ₀ of the RuO₂/NGA transparent supercapacitor with PVA–1 M H₂SO₄ gel electrolyte is 0.30 s indicating the high-power capability of the device, whilst the bare NGA transparent supercapacitor exhibits a τ₀ of 0.59 s as seen in Fig. S2†. For the cycling stability test, the RuO₂/NGA transparent supercapacitor at a mass loading of 16.3 μg cm⁻² was tested at an applied current density of 1 A g⁻¹. It exhibits a remarkable capacitance retention of 100% after being tested for 2000 cycles.

Fig. 8 (a) Nyquist plots of the NGA and RuO₂/NGA at different mass loadings, (b) Nyquist plot with the equivalent circuit model, (c) complex power analysis, and (d) cycling performance at a current density of 1 A g⁻¹ of the as-fabricated RuO₂/NGA supercapacitor at a mass loading of 16.3 μg cm⁻².

To evaluate the performance of the as-fabricated RuO₂/NGA transparent solid-state supercapacitor, the Ragone plot was constructed which compares the RuO₂/NGA supercapacitor with other transparent/flexible supercapacitors in other previous reports in terms of areal energy and areal power as shown in Fig. 9. The areal energy and areal power of the transparent supercapacitor based on RuO₂/NGA at a mass loading of 16.3 μg cm⁻² are higher than those of the NGA (Fig. 9). The areal energy of the RuO₂/NGA device is 0.074 μW h cm⁻² at an areal power of 4.75 μW cm⁻² and 0.022 μW h cm⁻², and at an areal power of 64 μW cm⁻², whilst the areal energy of the GA device is 0.03 μW h cm⁻² at an areal power of 5.2 μW cm⁻² and 0.002 μW h cm⁻², and at an areal power of 28 μW cm⁻². These performances are higher than those of other reports based on transparent/flexible devices using PEDOT:PSS/RuO₂ (0.018 μW h cm⁻² at 1 μW cm⁻²), reduced multilayer graphene oxide (0.015 μW h cm⁻² at 9 μW cm⁻²), CVD graphene (0.003 μW h cm⁻² at 2 μW cm⁻²), graphene Qdots (0.001 μW h cm⁻² at 0.06 μW cm⁻²), and convex carbon nanocups (0.00005 μW h cm⁻² at 0.02 μW cm⁻²).²⁶ Note that the performance of our transparent supercapacitors in terms of areal capacitance was also compared with those of other previous reports (see the ESI†).

Fig. 9 Ragone plot showing the comparison of the NGA and RuO₂/NGA devices with other transparent/flexible supercapacitors reported by C. Zhang, et al.²⁶

4. Conclusions

Tiny 2 nm RuO₂ nanoparticles were synthesized by an oxidation reaction of RuCl₃ with H₂O₂ and were decorated on the surface of the NGA. The as-synthesized RuO₂/NGA nanocomposites were spin-coated on the FTO substrate and used as the electrode of solid-state transparent supercapacitors. The polymer gel of PVA–H₂SO₄ was used as the solid-state electrolyte and separator. The as-fabricated symmetric supercapacitor using RuO₂/NGA at a finely tuned mass loading of 16.3 μg cm⁻² exhibits a maximum areal energy of 0.074 μW h cm⁻² and a maximum power density of 64 μW cm⁻² with a cycling stability of 100% over 2000 cycles. The supercapacitor with high transparency and charge storage performance in this work may practically be used in many advanced transparent electrical devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Vidyasirimedhi Institute of Science and Technology and Thailand Research Fund (RSA6180031). Support from the Frontier Research Centre at VISTEC is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8se00177d

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