A high-performance electrochemical supercapacitor based on a polyaniline/reduced graphene oxide electrode and a copper(II) ion active electrolyte

Abstract

Active electrolyte enhanced supercapacitors (AEESCs) have received increasing attention because of their large specific capacitance and easy fabrication process. The better matching between the active electrolyte and the counter electrode and the slow self-discharge rate are the challenges of this type of supercapacitor. In this paper, a novel AEESC with polyaniline/reduced graphene oxide hydrogel (PANI/RGOHG) as the anode and Cu(II) ions as the cathodic active electrolyte is constructed. Experimental results demonstrate that the electrode potentials of PANI and Cu(II) can match perfectly, thus the device has a wide working voltage range. Because of the large specific capacitance of both PANI and Cu(II), a high average specific capacitance of a single electrode of 1120 F g⁻¹ at 2.6 A g⁻¹ is achieved. Meanwhile, self-discharge is also suppressed because the reduction product of Cu(II) is immobilized on the electrode. These results demonstrate that the performance of AEESCs strongly depends on the choice of a suitable electrode material, and also reveal that Cu(II) is a promising cathodic active electrolyte for AEESCs.

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Introduction

Supercapacitors (SCs) have been attracting significant attention because of their large power density, long cycle life and high reliability.^1–3 According to energy storage mechanisms, supercapacitors can be classified into two types: electric double layer capacitors and pseudo-capacitors.⁴ In particular, pseudo-capacitors have developed rapidly due to their much higher capacitance compared with the former type, because the energy is stored by fast surface redox reactions near the electrode surface. The surface redox reactions can be achieved by coating insoluble electroactive materials onto an electrode, or adding various soluble electroactive molecules or ions into an electrolyte, such as hydroquinone, iodide and methylene blue. The latter types of devices are the so-called active electrolyte enhanced supercapacitors (AEESCs).^5–11 The fabrication of AEESCs is simple, but they possess high specific capacitance. For example, Senthilkumar et al. used hydroquinone as the active electrolyte in polyvinyl-alcohol H₂SO₄ gel electrolyte, and increased the specific capacitance of the device from 425 to 941 F g⁻¹ (average specific capacitance of a single electrode).⁹

Generally speaking, active electrolyte can efficiently increase the specific capacitance of one electrode, depending on its electrode potential. For example, hydroquinone is the reduced species of hydroquinone/p-benzoquinone redox couple, and can be oxidized at ∼0.5 V vs. SCE (saturated calomel electrode); thus it provides high specific capacitance at the anode.⁶ In order to further improve the overall performance of the device, active electrolyte should cooperate with a suitable counter electrode. However, most of the active electrolytes and electroactive electrode materials, such as conducting polymers, RuO₂ and Ni(OH)₂, work at a relatively positive potential. It is difficult to find matching active electrolyte and electrode material, which can yield both a large working voltage and high capacitance.

Herein, we designed an asymmetric AEESC, with a high-performance polyaniline/reduced graphene oxide hydrogel (PANI/RGOHG) as the anode, and CuSO₄ as a redox active electrolyte to increase the cathode capacitance. In our previous work, we have demonstrated that Cu²⁺ is an excellent active electrolyte which works at a low potential of ∼0.1 V vs. SCE.¹² Therefore, it is compatible with PANI, whose typical working potential range is −0.2 to 0.8 V vs. SCE. Since both the PANI/RGOHG anode and Cu²⁺ at the cathode have high specific capacitance, the asymmetric AEESC exhibits high capacitive performance. Besides, because the reduction product of Cu²⁺ is immobilized on the electrode, the fast self-discharge commonly occurring in the AEESC caused by the shuttle effect is also suppressed in our device.¹³

Results and discussion

The PANI/RGOHG composite electrode was fabricated by electrochemical polymerization. A RGOHG slice was used as the matrix for electrochemical polymerization of PANI. After the polymerization, the surface of RGOHG is covered by a thick layer of green PANI, which can be observed by the naked eye. SEM images (Fig. 1) demonstrate the morphology of the as-prepared freeze-dried PANI/RGOHG, which has a phase-separated structure.¹⁷ The RGOHG matrix exhibits a 3D porous network structure (Fig. 1A), which is attributed to the self-assembly of RGO sheets during the reduction of GO.¹⁹ A layer of PANI was found to be coated on the RGOHG matrix. Magnified SEM images (Fig. 1B and C) show that the PANI layer is constructed by coral-like dendritic nanofibers with an average diameter of ∼100 nm. Therefore, the PANI/RGOHG electrode has a layered porous structure. We have reported in our previous work that such a structure could facilitate the electrolyte diffusion, and promote the rate performance of the electrode.¹⁷

Fig. 1 SEM image of the phase-separated PANI/RGOHG composite. (A) RGOHG matrix in the PANI/RGOHG composite. (B) Interface between the PANI layer and the RGOHG matrix. (C) PANI layer in the PANI/RGOHG composite.

The formation of PANI was confirmed by Raman spectra (Fig. 2). The RGOHG spectrum displays two dominant D band at 1359 cm⁻¹ and G band at 1597 cm⁻¹, which are associated with disordered carbon and the in-plane vibration of the graphitic structure, respectively.²⁰ The PANI/RGOHG spectrum shows an additional band related with PANI. The bands at 1153 and 1583 cm⁻¹ are assigned to the C–H bending of the quinoid ring and the C=C stretching of the benzenoid ring,²¹ the bands around 780 and 820 cm⁻¹ correspond to amine and imine deformation (C–N–C bending).²² The bands at 1326, 1481 and 1250 cm⁻¹ were associated with the vibrations of the semiquinone radical. The C=N stretching vibration of the quinonoid units and the C–N stretching mode of the polaronic units.²³ These spectral data confirm the successful polymerization of PANI.

Fig. 2 Raman spectra of RGOHG and PANI/RGOHG.

The electrochemical performance of PANI/RGOHG was then evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) techniques in a three-electrode system (Fig. 3). As displayed in Fig. 3A, the CV curves of RGOHG are almost rectangular, with weak and wide redox waves around 0.4 V, which can be attributed to the redox of the residual oxygen-containing functional groups on RGO.¹⁹ In the CV curve of PANI/RGOHG, the current density was much higher than that of RGOHG, and the area of the CV curve was also larger. The results show clearly that the introduction of PANI could enhance the capacitive performance of RGOHG. The CV curves of PANI/RGOHG show two pairs of peaks, one pair at −0.1 to 0.3 V is attributed to the transition of PANI between thed leucoemereldine form and the emeraldine salt form,²⁴ and the other pair at 0.3–0.6 V is attributed to the redox of degradation products of PANI.²⁵ The peaks associated with the transition between emeraldine and pernigraniline are not observed because the potential range is not high enough.^26,27Fig. 3B shows the detailed CV behaviors of PANI/RGOHG at different scan rates from 10 to 100 mV s⁻¹. It was found that at a higher scan rate, the anodic peaks right-shifted to high potentials and the cathodic peaks shifted to low potentials, showing the characteristics of a quasi-reversible electrochemical process. The peak current increases linearly with the scan rate, suggesting that the electrode process is not controlled by diffusion. This is essential for high rate performance of the electrode.

Fig. 3 Capacitive performance of the PANI/RGOHG composite. (A) CV curves of PANI/RGOHG and pure RGOHG. (B) CV curves of the PANI/RGOHG composite at different scan rates. (C) GCD curves of the PANI/RGOHG composite at different current densities. (D) Specific capacitance of the PANI/RGOHG composite at discharge current densities.

Fig. 3C depicts the GCD curves of PANI/RGOHG between 0 and 0.8 V. The charge and discharge curves are symmetric, showing the high reversibility of the redox reaction of PANI. It was noted that the specific capacitance decreased slightly with the increase of current density (Fig. 3D). The specific capacitance of PANI/RGOHG can reach 492 F g⁻¹ at 0.6 A g⁻¹, and decreases to 365 F g⁻¹ as the current density increases from 0.6 to 2.27 A g⁻¹. The capacitive retention of 74% indicates a prominent rate capability of the PANI/RGOHG electrode. This good electrochemical performance of PANI/RGOHG is ascribed to its 3D porous structure which is in favor of the transport of electrolyte.²⁸ We then used this PANI/RGOHG composite as the anode to construct an asymmetric supercapacitor. We have reported that Cu(II) is a good active electrolyte, whose electrode potential is about 0.1 V vs. SCE. Therefore, Cu²⁺/Cu can serve as a cathode redox couple to work with the PANI/RGOHG anode, forming an asymmetric supercapacitor with working voltage higher than 0.7 V. In order to increase the specific surface area, RGOHG was used as the porous current collector for Cu²⁺. The configuration of the device was PANI/RGOHG//1 M H₂SO₄ + 0.4 M CuSO₄//RGOHG (Device 1). Another device with the same electrodes but 1 M H₂SO₄ as the inert electrolyte (Device 2), as well as two devices with pure RGOHG as the electrodes, namely, Device 3 (RGOHG//1 M H₂SO₄//RGOHG) and Device 4 (RGOHG//1 M H₂SO₄ + 0.4 M CuSO₄//RGOHG), were fabricated for comparison.

Fig. 4A shows the CV curves of Device 1 and Device 2. The current of the CV curve of Device 1 is larger than that of Device 2, indicating a higher specific capacitance of Device 1. In the CV curve of Device 1, a pair of redox peak at 0.3–0.6 V is observed, making the CV curve of Device 1 quite similar to that of the PANI/RGOHG composite electrode. In fact, on the cathode the following reaction will take place during the charge/discharge process:

Cu²⁺ + 2e⁻ = Cu

Fig. 4 Comparison of the capacitive performance of Device 1 and other devices. (A) CV curves of Device 1 and Device 2. (B) GCD curves of Devices 1, 2, 3, and 4, at a current density of ∼1 A g⁻¹. (C) Specific capacitance of Devices 1, 2, 3, and 4 at different discharge current densities. (D) Capacitance retention of Device 1 after 4000 charge/discharge cycles under 15.6 A g⁻¹. (E) Potential curves of single electrodes of Device 4 during GCD at current densities of 4.6 A g⁻¹ (for the anode) and 6.0 A g⁻¹ (for the cathode). (F) Self-discharge curves of Device 1 and Device 3.

The standard electrode potential of this reaction is 0.1 V vs. SCE.²⁹ Because of the depolarization effect of this reaction, the potential of the cathode was fixed at ∼0.1 V vs. SCE. This made Cu²⁺/Cu@RGOHG behave like a reference electrode, and consequently the CV curve of Device 1 in fact reflects the electrochemical properties of the PANI/RGOHG anode, except for a small potential shift. Thus the peaks at 0.3–0.6 V are attributed to the redox of degradation products of PANI.

The GCD curves at a current density of 1 A g⁻¹ are shown in Fig. 4B. There is a plateau at 0.4–0.5 V vs. SCE on the GCD curve of Device 1, corresponding to the redox peaks on the CV curve. This plateau makes a great contribution to the discharge time. As a result, the specific capacitance of Device 1 calculated from GCD curves are 307.9 F g⁻¹ at 1.3 A g⁻¹ and 230.6 F g⁻¹ at 5.2 A g⁻¹, as shown in Fig. 4C. If these data are converted into an average specific capacitance of a single electrode, high values of 1231.6 F g⁻¹ at 2.6 A g⁻¹ and 922.4 F g⁻¹ at 10.4 A g⁻¹ are obtained. As a comparison, the specific capacitance of Device 2 is only 68 F g⁻¹ at 1.3 A g⁻¹, corresponding to an average specific capacitance of a single electrode of 272 F g⁻¹ at 2.6 A g⁻¹. The specific capacitance was also much higher than those of Device 3 and Device 4. This significant enhancement of specific capacitance of Device 1 comes from a synergetic combination of Cu²⁺ and PANI. Fig. 4D demonstrates that after 4000 charge/discharge cycles, a capacitance retention of 91.4% was obtained in Device 1. Thus, Device 1 has good cyclic stability. In order to further investigate the working mechanism of Device 1, the electrode potentials of the anode and the cathode during the charge/discharge process were measured respectively. As shown in Fig. 4E, during the charge–discharge process, the potential shift of the anode (0.69 V) makes chief contribution to the increase of cell voltage, while the potential of the cathode remains stable (potential change: 0.05 V). This small potential change clearly demonstrates the depolarization effect of Cu²⁺. From the potential curve, the specific capacitance of the cathode was calculated to be 4107 F g⁻¹. Such a high specific capacitance is attributed to the fast kinetics of Cu²⁺/Cu and a large specific surface area of the RGOGH current collector. Since the capacitance of the device is the total capacitance of the parallel-connected anode and cathode, the large specific capacitance of the cathode can significantly increase the capacitance of the device.

For most of the active electrolyte enhanced supercapacitors, the self-discharge (SDC) caused by the shuttle effect of the active electrolyte between the anode and the cathode is a serious problem. Here we evaluated the SDC curve of Device 1. As shown in Fig. 4F, after 6000 s, the voltage of Device 1 dropped from 0.8 V to 0.33 V. The SDC rate was even slower than that of Device 3, which was a double electric layer capacitor without active electrolyte. For Device 3, the self-discharge from 0.8 V to 0.33 V took only 1408 s. It should be noted that the absolute SDC rate depends on the fabrication and the capacitance of the device, but the above comparison clearly reveals that Cu²⁺ does not accelerate the SDC process. In fact, during the charge process, Cu²⁺ is reduced to metal Cu, which is insoluble in the electrolyte and is immobilized on RGOHG. No shuttle of Cu will occur, and consequently the charges stored by Cu²⁺/Cu will not be lost. The slow SDC rate is essential for the practical application of supercapacitors.

Experimental

Material

GO was prepared using a modified Hummers method according to the literature.¹⁴ Natural graphite powders, H₂SO₄ (98%), hydrazine monohydrate (85%), aniline (AR) and copper sulfate (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dialysis Membranes (8000–12 000) were purchased from Shanghai Yuanye Bio-Technology Co., Ltd.

Preparation of RGOHG

RGOHG was synthesized by a hydrothermal reaction.^15,16 Firstly, 15 mL of GO aqueous dispersion (2 mg mL⁻¹) was sealed in a 25 mL Teflon-lined autoclave and heated at 180 °C for 12 h. After the autoclave was cooled to room temperature, the black RGOHG monolith in the autoclave was collected and treated by hydrazine monohydrate (50%) at 95 °C for 8 h. Finally, the obtained RGOHG was dialyzed in pure water for 24 h.

Preparation of the PANI/RGOHG composite

The electrochemical polymerization of the PANI/RGOHG composite was carried out in a three-electrode system by electrochemical polymerizing aniline onto the RGOHG electrode.¹⁷ The RGOHG discs act as both the working electrode and the counter electrode, and SCE as the reference electrode. RGOHG discs were prepared by directly compressing the as-prepared wet RGOHG monolith at 0.16 MPa with the final thickness of the RGOHG disc of about 1 mm. The CV technique was used to polymerize aniline in a voltage range between −0.2 and 0.8 V vs. SCE at 10 mV s⁻¹ in the electrolyte containing 0.4 M aniline and 1 M H₂SO₄. After deposition, the electrode was immersed in 1 M H₂SO₄ to remove the aniline monomer.

Fabrication and test of the supercapacitors

A two-electrode device configuration was used in this work, and the PANI/RGOHG composite was chosen as the working electrode, RGOHG discs as the counter electrode, two pieces of Pt foil (1 × 2 cm²) were used as current collectors. A silicone ring was used to seal the device by sandwiching between the Pt foil, with graphene gel, a separator, and the electrolyte solution accommodated within it. The electrolyte solution was 1 M H₂SO₄ + 0.4 M CuSO₄. For comparison, devices with 1 M H₂SO₄ as the electrolyte were also assembled. The electrode process of a single electrode was investigated in a three-electrode cell. To do so, the two-electrode device was immersed into an electrochemical cell filled with the same electrolyte as used in the device. To ensure the connectivity of the electrolyte in the device and the cell, two openings were cut on the silicone ring. The two electrodes of the device were used as working and counter electrodes, and SCE was utilized as the reference electrode.¹²

Before testing the device was activated by cyclic voltammetry cycling from 0 V to 0.8 V for 50 cycles.¹⁸ The specific capacitance of the device was calculated from galvanostatic charge–discharge (GCD) curves according to the following equations:

where J is the mass current density, I is the current applied on the device, t is the discharge time, m is the total mass of two electrodes, V is the highest voltage in the GCD curves, and IR represents the voltage drop at the beginning of the discharge process, caused by the internal resistance of the device. The specific capacitance of the single electrode was calculated from the GCD curves measured in the three-electrode system:where m is the mass of the single electrode and ΔV is the potential change of the electrode during the discharge process; I, t and IR have the same definitions as in eqn (1) and (2).

Characterization

The structure of PANI/RGOHG was characterized using a HITACHI S-4800 scanning electron microscope. All the electrochemical measurements were performed on a CHI 760E electrochemical workstation. Raman spectra were collected from ID Spec ARCTIC confocal Raman Spectroscopy.

Conclusions

We designed and fabricated a novel AEESC, based on the PANI/RGOHG anode and the Cu²⁺ active electrolyte. The Cu²⁺/Cu couple has a relatively low electrode potential of ∼0.1 V vs. SCE on the RGOHG electrode, thus it can match the PANI/RGOHG anode, which works in the voltage range 0–0.8 V vs. SCE. A high specific capacitance of 307.9 F g⁻¹ at 1.3 A g⁻¹ was obtained. Mechanism investigation reveals that, the specific capacitance of Cu²⁺ at the cathode is as high as 4107 F g⁻¹, thus the specific capacitance of the device approaches that of the PANI/RGOHG anode. Furthermore, owing to the immobility of the reduction product of Cu²⁺, our AEESC showed a slow SDC rate. This work demonstrates that Cu²⁺ is a superb cathodic active electrolyte, and by carefully choosing the anode material, high-performance asymmetric AEESCs could be constructed.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21774104).

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