High-performance flexible redox supercapacitors induced by methylene blue with a wide voltage window

Abstract

Flexible redox supercapacitors based on treated carbon cloths were prepared by introducing methylene blue into a gel electrolyte. With the improvement in pseudocapacitance and broadening of the voltage window, the device exhibited a high capacitance of 1738 mF cm⁻² and an energy density of 0.78 mW h cm⁻². In addition, 84.6% capacitance retention was achieved over 8000 cycles.

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To meet the increasing demand for portable electronics such as roll-up displays and wearable devices, high-performance, lightweight, and flexible power sources with long cycle life are attracting considerable attention.^1,2 Based on the above requirements, flexible solid-state supercapacitors (SSCs) have become a focus of research.^3,4 Typically, supercapacitors are classified into electric double-layer capacitors (EDLCs) and pseudocapacitors based on their charge-storage mechanisms.^2,5 EDLCs function based on electrical charge transfer at the surfaces of conducting carbon electrodes including carbon nanotubes (CNTs),^1,3,6 graphene,^7–9 and carbon cloths.^4,10 Compared to EDLCs, pseudocapacitors with fast interfacial faradaic reaction possess higher specific capacitance and energy density.⁵ Nevertheless, typical pseudocapacitive materials such as transition metal oxides (e.g., MnO₂,^11–16 Bi₂O₃,¹⁷ NiCo₂O₄,¹⁸ and RuO₂ (ref. 19)), polyaniline,^20,21 and polypyrrole (PPy)²² are hampered by sluggish reaction kinetics and low conductivity.²³

Recently, a new direction focused on redox additives electrolytes has emerged. Redox additives in the gel electrolyte can promote energy storage by faradaic reactions and they can also enhance the ion conductivity of the electrolyte.²⁴ For example, when Ma et al.²⁵ utilized indigo carmine as a redox additive, the capacitance of the SSC increased to 382 F g⁻¹ from 180 F g⁻¹ for the original device with a voltage window of 1 V. However, the SSC showed a poor cyclic stability of 80.3% after 3000 cycles, severely hindering its application. In addition, the limited voltage window of 1 V also led to a low energy density. According to the equation E = 0.5CV², the energy density (E) can be enhanced by both increasing the specific capacitance (C) and extending the operating cell voltage (V). Hence, it is particularly crucial to obtain SSCs with redox additives that possess high capacitance and can achieve a wide electrochemical stability window.

Methylene blue (MB), a heterocyclic aromatic thiazine dye, can be effectively adsorbed on the surfaces of carbon materials through π–π electron coupling,^26,27 and undergoes an outer-sphere redox reaction so that the redox reaction of MB is irrelevant to the changes in surface chemistry of carbon electrodes.²⁸ Herein, we tuned the porous structure through a simple annealing process to maximize the electric double-layer capacitance and the surface area in contact with redox additives. The surface functional groups can also be optimized to achieve a broader voltage window (1.8 V). The subsequent introduction of MB into the PVA (polyvinyl alcohol)–H₂SO₄ gel led to excellent performance (1738 mF cm⁻²) and cycle life (84.6% capacitance retention over 8000 cycles). The flexibility was maintained, and a high energy density of 0.78 mW h cm⁻² was achieved at a power density of 1.80 mW cm⁻². To highlight the potential as a power source for practical applications, the device flexibility and ability to power red light-emitting diodes (LEDs) were also demonstrated.

Commercial carbon cloths (CC) with graphitic outer layers and amorphous inner layers feature nonporous structures and hydrophobic surfaces (Fig. S1a†).¹⁰ CCs have been annealed at different temperatures according to the method reported for preparing treated CC (TCC_x, where x represents the treatment temperature). As observed from the SEM image in Fig. 1a, TCC₁₀₀₀ maintained the fiber-like structure of the original CC and became more porous. N₂ adsorption–desorption isotherms and pore size distributions were used to quantitatively analyze the prepared electrode materials. Fig. 1b shows the isotherms of TCC₁₀₀₀ and TCC₆₀₀; both curves were classified as type IV isotherms, suggesting the existence of different pore sizes ranging from micro- to mesoscale. TCC₁₀₀₀ presented a higher specific surface area (S_BET) of 772 m² g⁻¹ and a larger pore volume of 0.45 cm³ g⁻¹ compared to TCC₆₀₀ (S_BET of 93 m² g⁻¹ and pore volume of 0.06 cm³ g⁻¹). As shown by the pore size distributions, compared to TCC₆₀₀, TCC₁₀₀₀ had a higher proportion of micropores, which play an important role in enhancing the capacitance of SSCs.^2,10

Fig. 1 (a) SEM image of TCC₁₀₀₀. (b) N₂ adsorption–desorption isotherms and pore size distributions of TCC₁₀₀₀ and TCC₆₀₀.

X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface chemistries of the samples. Table S1† presents the atomic percentages of C and O as well as the C/O atomic ratio. After thermal treatment, the oxygen content increased slightly from 2.74 at% in CC to 4.86 at% in TCC₁₀₀₀. Various C and O components were further identified by fitting the C 1s and O 1s fine-scan spectra. As shown in Table S2 and Fig. S7,† the amounts of C–C and C=C along with the intensity of graphitic shake-up satellites were significantly increased. Meanwhile, as C–O related species were easily broken with increasing temperature and generated C=O bonds, the abundance of C–O was reduced, while the percentage of C(O)O increased. In addition, the C–F₃ disappeared, implying that the hydrophobic coating on the surface of CC was thoroughly damaged after annealing, thereby enhancing the wettability. The C 1s peak-fitting results indicated that the annealing process can improve the electronic conductivity of the final products, favoring the restoration of the sp² conjugated carbon lattice. Moreover, after carbonization at 1000 °C, the majority of unstable oxygen-containing groups were eliminated. According to the sequence of thermal stability for oxygen-containing groups on the carbon surface [carbonyl/quinone (CO) > lactone (CO₂) > phenol (CO) > anhydride (CO and CO₂) > carboxylic acid (CO₂)], the functional groups related to C–O and O–H existed in the form of more thermally stable isolated ethers and phenols, respectively, and some unstable C=O was either reduced by thermal treatment or transformed into more thermally stable carbonyls. As a result, the hydroxyl and carboxyl groups at edge carbon sites were electroactive and thus supplied pseudocapacitance. Additionally, oxygen-containing groups benefit the overpotential of hydrogen evolution, and the negative potential can extend from −0.2 to −0.8 V. Thus the operating voltage window can extend from −0.2–1 V to −0.8–1 V. Introducing oxygen-containing groups to carbon can broaden the operated voltage by suppressing hydrogen evolution (Fig. S2†).^29–32

TCC₁₀₀₀ was then used as the working electrode to explore the electrochemical stability window in 1 M H₂SO₄ with the addition of 0.01 M MB. As is shown in Fig. S8,† an operating window between −0.8 and 1 V vs. SCE enabled the reversible redox reaction of MB. The capacitance of liquid-state supercapacitors clearly increase in the presence of MB (Fig. 2a). As illustrated by Fig. 2b, each MB molecule can be reversibly reduced and oxidized, and store three protons and two electrons in acidic media, thus leading to enhanced pseudocapacitance. The cyclic voltammetry (CV) curves at different rates and galvanostatic charge–discharge (GCD) tests were measured in 1 M H₂SO₄ with 0.05 M MB (Fig. S9†). In addition, the calculated areal capacitance is plotted as a function of current density in Fig. 2c. The porous, binder-free and three-dimensional electrode provided a conductive pathway for efficient charge transport and facilitated fast ion diffusion along with facile electrolyte permeation, consequently resulting in good rate capability after the addition of MB similar to the original rate capability which derived from EDLC. The areal capacitance of TCC₁₀₀₀ was enhanced from 2250 to 3925 mF cm⁻² at 15 mA cm⁻². Thus, the capacity enhancement of TCC₁₀₀₀ after the addition of MB was 1675 mF cm⁻², much larger than the enhancement observed for TCC₆₀₀ (216 mF cm⁻²; Fig. 2d). This demonstrates that the high surface area enhanced the electric double-layer capacitance and facilitated the adsorption of additives from the electrolyte (Fig. S4†), thereby greatly improving the pseudocapacitance.

Fig. 2 (a) CV curves of the electrolyte with and without 0.05 M MB (scan rate = 5 mV s⁻¹). (b) Schematic of the redox process of MB. (c) Plots of areal capacitance vs. current density. (d) Bar graph showing the performances of TCC₆₀₀ and TCC₁₀₀₀.

With the aim of exploring bendable and lightweight energy sources, SSCs are a promising replacement for liquid-state supercapacitors on account of their safety and applicability.^25,33 Herein, to assemble SSCs, filter paper was placed as a separator between two pieces of TCC₁₀₀₀ coated with gel electrolyte in parallel. To investigate the effect of MB concentration, gel electrolytes were prepared with various MB concentrations (0, 0.01, 0.05, and 0.10 M). The working voltages of all the above SSCs were greater than 1.8 V, higher than the SSCS with other additives listed in Table S4.† Fig. S10a† shows the areal capacitances of electrolytes with different concentrations of MB calculated from the discharge times obtained in GCD tests at 2 to 10 mA cm⁻² (Fig. S11†). Significantly, the areal capacitance varied greatly with the concentration (lower than 0.05 M) as a result of the fast faradaic reaction of MB, while it had negligible change at 0.1 M MB concentration, possibly because of electrostatic interaction between ions.^34,35 The CV curves of the SSCs with MB clearly showed much larger areas compared to the curve of the original SSC without MB (Fig. 3a), further demonstrating the role of MB in delivering pseudocapacitance. The MB concentration of 0.05 M was determined to be optimal as the addition of more MB to the electrolyte did not further enhance the performance. SSCs with 0.05 M MB were then tested at various scan rates from 2 to 10 mV s⁻¹ (Fig. S10b†). Clearly, a combination of both electric double-layer capacitance and pseudocapacitance was observed. The charge and discharge times of the SSCs with 0.05 MB were almost equal (Fig. S11c†), implying good electrochemical reversibility. The nonlinear profile further demonstrated the contribution of pseudocapacitance from MB, in agreement with the CV results. Meanwhile, 1738 mF cm⁻² at 2 mA cm⁻² (262 F g⁻¹ and 19.3 F cm⁻³) can be achieved in PVA–H₂SO₄ with 0.05 M MB; these values are comparable to or even better than previously reported values.^3,8,33,36,37 Similar to the phenomenon in liquid electrolyte above, the rate capability was maintained after adding 0.05 M MB to the system, indicating that effective redox reactions can happen in solid electrolytes.

Fig. 3 (a) CV curves of PVA–H₂SO₄ with and without 0.05 M MB (scan rate = 5 mV s⁻¹). (b) Plots of areal capacitance vs. current density for SSCs. (c) Nyquist plots of PVA–H₂SO₄ without and with 0.05 M MB. (d) Ragone plots.

Nyquist plots of PVA–H₂SO₄ with and without 0.05 M MB for frequency varying from 0.01 to 100 kHz are shown in Fig. 3c. Each curve consists of a semicircle and a straight line. The equivalent series resistance (ESR) was 11.40 Ω for PVA–H₂SO₄ with 0.05 M MB, smaller than that of PVA–H₂SO₄ without MB (18.0 Ω). This can be attributed to the improvement in ion conductivity by the addition of MB to the electrolyte.^25,37 The straight line corresponding to Warburg resistance revealed the fast transport of ions and electrons between the electrolyte and electrodes of both kinds of SSCs. Energy density and power density are known to be two crucial factors for practical applications. The Ragone plots of the gel electrolyte with 0.05 M MB and without MB derived from the discharge curves are shown in Fig. 3d. PVA–H₂SO₄ with 0.05 M MB delivered a maximum areal energy density (E_A) of 0.78 mW h cm⁻² at an areal power density (P_A) of 1.80 mW cm⁻², corresponding to a volumetric energy density (E_V) of 8.69 mW h cm⁻³ and a volumetric power density (P_V) of 20 mW cm⁻³. This E_A is five times higher than that of a TCC SSC (E_A = 0.128 mW h cm⁻² at 2 mA cm⁻²)¹⁰ and comparable to those of recently reported SSCs, including a CNT–PPy–HQ fiber–SSC (E_V = 3.6 mW h cm⁻³)³⁷ and ZnO–C–MnO₂ SSC (E_V = 0.04 mW h cm⁻³ at P_V = 20 mW cm⁻³).³⁸

Furthermore, cyclic life is an important indicator for the practical application of SSCs. As shown in Fig. 4a, a capacitance decay of only 15.4% was observed after 8000 cycles at 20 mA cm⁻² for the SSC with MB. This performance was better than that observed for the SSC without MB (Fig. S13†), and the coulombic efficiency simultaneously remained above 95%, demonstrating remarkable stability and reversibility comparable to those of recently reported SSCs with redox additives (Table S4†). The electrochemical impedance spectra (Fig. 4b) suggested that the ESR of the device with 0.05 M MB increased with cycling because of the evaporation of water from the solid-state electrolyte. Moreover, to evaluate the flexibility, the electrochemical performance of the SSC with MB was tested in a flat state and different bending states using CV tests at 10 mV s⁻¹. Fig. 4c and d indicate that the electrochemical performance of the SSC with MB was not affected by bending. The curves in Fig. 4d remained the same after bending the device 1000 times, indicating that the gel electrolyte endowed the fabricated SSC with excellent flexibility and mechanical stability. Significantly, the 1 × 1 cm² SSCs were able to power 40 red LEDs after charging for more than 90 s (Movie S1†).

Fig. 4 (a) Cyclic life of PVA–H₂SO₄ with 0.05 M MB. (b) Nyquist plot of PVA–H₂SO₄ with 0.05 M MB before and after cycling. (c) CV curves with different bending states (scan rate = 10 mV s⁻¹). (d) CV curves before and after bending 1000 times.

Conclusions

In summary, high-performance flexible SSCs based on activated carbon cloths were successfully achieved by introducing MB into PVA–H₂SO₄ gel electrolyte. The introduction of MB both provided pseudocapacitance via fast faradic reactions and enhanced the ion conductivity of the electrolyte. Doping the treated carbon cloths with O can extend the operating voltage window to 1.8 V by suppressing the hydrogen evolution reaction, resulting in a high energy density of 0.78 mW h cm⁻² at a power density of 1.80 mW cm⁻². In addition, the as-assembled SSCs manifested various favorable features including enhanced capacitance (1738 mF cm⁻²), outstanding cyclic stability (15.4% decay after 8000 cycles) and excellent mechanical flexibility. This novel strategy provided a facile and economic approach to obtain devices with high energy storage capacity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Key R&D Program of China (2016YFA0202900), the National Natural Science Foundation of China (21622308, 91534114, 21376208), the Key Program Supported by the Natural Science Foundation of Zhejiang Province, China (LZ18B060002), and the Fundamental Research Funds for the Central Universities are greatly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00492c

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