Three-Dimensional Carbon Nitride Nanowire Scaffold for Flexible Supercapacitors
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Herein, a 3D composite electrode supported by g-C3N4 nanowire framework as scaffold and poly(3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT: PSS) as conducting polymer is reported for flexible solid-state electrochemical capacitors. Compared to pure PEDOT: PSS, the composite electrodes have a greatly increased specific surface and showed good electrochemical performance. A specific capacitance of 202 F g−1 is achieved, and 83.5% of initial capacitance maintained after 5000 cycles. The device based on the 3D g-C3N4/PEDOT: PSS electrode also exhibits good performance in capacitance, flexibility, and cycling stability.
KeywordsElectrochemical supercapacitor 3D g-C3N4 PEDOT:PSS Flexible device
Electrochemical double-layer capacitors
Electrochemical impedance spectroscopy
Equivalent series resistance
Field emission scanning electron microscopy
Fourier transform infrared spectroscopy
Graphitic carbon nitride
Transition metal oxides
PSS: (3,4-ethylenedioxythiophene): poly(4-styrenesulfonate)
Transmission electron microscopy
X-ray photoelectron spectroscopy
X-ray diffraction patterns
Wearable energy storage devices, especially flexible supercapacitors, are getting extra attention due to their higher cycling stability and power density [1, 2, 3, 4]. As for material systems of supercapacitor electrodes, recent researches mainly focus on three principle types: carbon-based high surface area materials (activated carbon, graphene, carbon fibers, and so on), transition metal oxides (MOs), and conducting polymers (CPs) [5, 6, 7, 8]. The storage mechanism of the first type is electrochemical double-layer capacitors (EDLCs) while the others are pseudocapacitors [9, 10, 11]. Compared to EDLCs, the pseudocapacitors with Faradaic charge storage mechanism show higher specific capacitance, which become an essential part of high-performance supercapacitors. MOs possess high theoretical capacities. However, low conductivity, toxicity, poor stability, and high cost restrict the application of MOs. In contrast, CPs overcoming these problems are suffering the constraint of relatively low mechanical and cycle ability. What is more, the low specific surface is one of the most disadvantages which impede the application of CPs in flexible energy story device.
So far, each of the materials mentioned above has strengths and weaknesses, and none of them is ideal. In order to enhance the performance of devices, compositing materials and optimizing structure are both effective strategies. As for flexible supercapacitors, the composite of 3D EDLC materials and MO (or CPs) pseudocapacitance materials, which keep high electrochemical performance (capacitance, stability) along with well mechanical performance (flexible, light), becomes one of the most suitable choices [12, 13, 14]. Although carbon-based materials acted as EDLC materials get some satisfying results, new candidates with competitive performance, low cost, easy fabrication, and eco-friendly properties are still drawing researchers’ attention.
Graphitic carbon nitride (g-C3N4), a two-dimensional graphene derivative, has been explored due to its interesting electronic feature, low cost, and high environmental-friendly features [15, 16]. In recent years, the application field of g-C3N4 is mainly focused on photocatalysis [17, 18, 19, 20, 21, 22]. Few investigations on the application of supercapacitor for g-C3N4 got competitive results. Its energy storage potentials are far from fully developed since the molecular structure advantage is not totally explored. The most commonly used microstructure of g-C3N4 was a 2D structure, while 3D g-C3N4 structure was rarely reported [23, 24, 25, 26, 27]. On the other hand, (3,4-ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT: PSS) as a kind of CP is extensively utilized in ES electrode. PEDOT: PSS has high conductivity and relatively much higher chemical and mechanical stability which are basic requirements for wearable energy storage devices. In order to improve its capacitance, enlarging its active surface area is the most direct and effective strategy.
Herein, a 3D g-C3N4/PEDOT: PSS composite material has been developed where g-C3N4 nanowire (GCNW) acts as a 3D skeleton structure supporting PEDOT: PSS. The composite materials achieve a specific capacitance of 202 F g−1, meanwhile exhibiting an excellent electrochemical performance in the form of all-solid-state flexible supercapacitor. The as-prepared device possessed excellent flexibility and stability. Moreover, the effect of g-C3N4 ratio on the structure and electrochemical properties had been studied in detail.
Sodium hydroxide (NaOH) and urea were obtained from Beijing Chemical Corp. PEDOT: PSS solution (1.0 wt.% in H2O, high-conductivity grade) was purchased from Sigma-Aldrich Co. None of the above products have been further purified.
Synthesis of g-C3N4
This preparation used urea as the precursor. Ten grams of urea was heated to 550 °C (10 °C min−1) and kept for 2 h in a muffle furnace, producing the yellow powder.
Three-Dimensional Fabrication of the GCNW
Briefly, 500 mg CN power was mixed with 20 ml of aqueous NaOH and stirred at 60 °C for 12 h. The sealed flasks were ultrasonic cleaned for 2 h. The suspension was dialyzed to remove the excess NaOH. The final pure g-C3N4 nanowire aerogel was obtained through freeze-drying.
Three-Dimensional Preparation of GCNW/PEDOT: PSS Composite Material
The composite materials were prepared with different mass ratios of g-C3N4 nanowire hydrogels (6 mg ml−1) to PEDOT: PSS, namely 10%, 20%, 50%, and 80% GCNW/PEDOT: PSS. The homogeneous solution had been gotten after 12 h of stirring. Finally, the product was obtained using the freeze-drying process. The pure PEDOT: PSS thin film was prepared by filtration method for comparison.
The morphologies and structures of samples were characterized by field emission scanning microscopy (FESEM, 7610, JEOL), transmission electron microscopy (TEM, Tecnai F20), and D-MAX II A X-ray diffractometer (XRD). Fourier transform infrared spectroscopy (FTIR) was carried out using Nicolet-6700 (Thermofisher). X-ray photoelectron spectroscopy (XPS) measurements were tested with ESCALABMK II X-ray photoelectron spectrometer.
Electrochemical performance was carried out using a CHI 660E electrochemical workstation. In the three-electrode configuration, the platinum foil and saturated calomel (SCE) electrodes were used as counter and reference electrodes. The working electrodes were prepared by pressing the composite on a carbon cloth with a loading amount 1 mg cm−2. The electrolyte was 1 M H2SO4. Cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) curves were tested at the potential range of 0 V to 1 V. The electrochemical impedance spectroscopy (EIS) measurements were recorded under an open circuit potential in the frequency range of 1–105 Hz with a modulating amplitude of 5 mV.
For the two-electrode devices, 2 mg of active material was loaded on the carbon cloth as working electrodes. Then, a small amount of H2SO4/PVA hydrogel was dripped on the non-woven fabric (NKK-MPF30AC-100) as a separator. Finally, the separator was placed between two working electrodes to assemble a symmetrical capacitor. Electrochemical testing of two electrodes was carried out in a CHI 660E electrochemical workstation.
Results and Discussion
In summary, for the first time, 3D GCNW/PEDOT: PSS composite materials have been prepared and applied as an electrode of flexible supercapacitor successfully. Due to the improvement of the active surface, the capacitance of the composite reached 202 F g−1 in the three-electrode system and 78 F g−1 in the symmetric device at the scan rate of 5 mV s−1, resulting in a high energy density of 6.66 Wh Kg−1. The 3D structure was of great significance to enhance electrochemical performance. The as-prepared device also exhibited excellent flexible and stable performance in the bending cycle test. Taking into account the cost and preparation convenience, the results obtained herein open new prospects for 3D g-C3N4/CP composite as an efficient electrode material in flexible energy storage device and commercial applications.
The authors thank Dr. Liying Wang form the Key Laboratory of Advanced Structural Materials, Ministry of Education, and Department of Materials Science and Engineering, Changchun University of Technology for the TEM characterizations.
This work was supported by the National Nature Science Foundation of China (Grant No. 61604017, 61574021), Open Subject Fund by Key Laboratory of Materials Modification by Laser Ion and Electron Beams Ministry of Education (KF1703).
Availability of data and materials
The datasets used or analysed during the current study are available from the corresponding author on reasonable request.
WL and XZ conceived the idea. ZT carried out the experiments. ZT, AM, and LD took part in the experiments and the discussion of the results. XZ and ZT drafted the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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