1D nanorod-like porous carbon with simultaneous high energy and large power density as a supercapacitor electrode material

Abstract

A kind of 1D nanorod-like porous carbon PCPAC-2 with a length of about 0.5–2 μm and width of around 100–300 nm was synthesized by pyrolysis of a porous coordination polymer as a carbon source and structure template followed by KOH activation. The PCPAC-2 possesses a high specific BET surface of about 679 m² g⁻¹ and a total pore volume of 1.63 cm³ g⁻¹. When serving as a supercapacitor electrode active material, the highest specific capacitance of 306 F g⁻¹ was obtained at 1 A g⁻¹. The specific capacitance of 205 F g⁻¹ could be preserved at the large current density of 30 A g⁻¹. The high cyclic stability with 96% capacitance retention was obtained after 5000 charge–discharge cycles at 1 A g⁻¹. Moreover, the maximum energy density of 17.6 W h kg⁻¹ could be found at the power density of 272.2 W kg⁻¹, and the energy density of 10 W h kg⁻¹ could be obtained at the high power density of 7530 W kg⁻¹ in 6 M KOH aqueous electrolyte.

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Introduction

As one of the most promising energy conversion/storage devices used for digital communications, hybrid electronic vehicles and so forth, supercapacitors have attracted intense research interest because of their advantages of good power density and long cycle life.^1,2 However, low energy density hinders the application of supercapacitors in heavy equipment and machinery demanding the delivery of much high currents and powers. Hence, enhancing the energy density without sacrificing power density and cyclic stability is still a great challenge for supercapacitors.³ The electrochemical performance of supercapacitors is highly determined by the adopted electrode materials with a rational design of the pore structures and morphologies. Carbon-based materials play an important role as the electrodes of the electrochemical double-layer capacitors (EDLCs), owing to their stable physical and chemical properties, high stability, good electronic conductivity and relatively low cost.^4–8 Up to now, many kinds of carbon materials with variously dimensional morphologies such as zero-dimensional (0D) carbon, one-dimensional (1D) nanofibers, two-dimensional (2D) nanosheets and three-dimensional (3D) porous activated carbons have been fabricated by the different methods for high-performance EDLCs.^9–17 Therefore, designing carbon electrodes with well-defined porous structure and desirable morphology by suitable method is the key to enhance the specific capacity and energy density of supercapacitors.¹⁸ Moreover, the low-dimensional (0-2D) architecture of electrode materials has a significant influence on the electrochemical properties, especially the power density. Because the ion transport time τ could be reduced when the ion transport length I is greatly shortened in the thin dimension based on the equation τ = I²/d, where I is the ion transport length and d is the ion transport coefficient.^19–21 Very recently, porous coordination polymers (PCPs) or metal–ligand coordinated metal-organic frameworks (MOFs), which are porous structures consisting of metal ions coordinated to rigid organic molecules, have been demonstrated as novel carbon sources and structure templates to prepare the porous carbons.²² In this work, 1D nanorod-like porous carbon materials were synthesized by pyrolysis using a kind of Al-based PCP as the carbon source and structure template following with KOH activation. The optimized porous carbon with high conductivity, large specific surface area and hierarchical pore texture exhibits simultaneous high energy and large power density as supercapacitor electrode material in 6 M KOH electrolyte system.

Experimental section

Preparation of the porous carbon

All the chemicals used in the experiments are analytical grade and were used without further purification. In the experiments, Al-based PCP powders as the initial precursor were prepared according to a published literature.²² Firstly, 0.34 mg of Al(NO₃)·9H₂O and 0.11 mg of 1,4-naphthalenedicarboxylic acid were dissolved in 10 mL of deionized water. Then, the mixture was treated by ultrasonic dispersion for 5–10 min under ambient condition. The obtained solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained at 180 °C for 24 h in an electron oven. After that, the autoclave was cooled down naturally to room temperature. The product was collected and washed by centrifugation, followed by vacuum-drying at 60 °C. The obtained pale yellow powders were put into a ceramic boat (1.5 cm × 3.0 cm × 6.0 cm). The furnace was then heated from room temperature to 800 °C with a heating rate of 2 °C min⁻¹ and maintained for 5 h under N₂ flow. After that, the samples were cooled down naturally to room temperature. In order to remove the residual aluminium species, the obtained black powders were immersed in 20 vol% HF under magnetic stirring for 72 h. The black precipitates were collected by centrifugation and washed several times in distilled water. Finally, the black precipitate porous carbon was dried under vacuum conditions for 24 h at 60 °C. The obtained porous carbon was denoted as PCPC.

Preparation of activated porous carbon materials

The as-synthesized porous carbon PCPC was further activated with KOH, which performed by heating the mixture of the carbon and KOH (the mass ratios of KOH and carbon precursor are 1 : 1, 2 : 1 and 4 : 1). The mixture was placed in a tube furnace and then heated to the activation temperature of 700 °C with a heating rate of 3 °C min⁻¹ under Ar flow. After that, the mixture was removed and washed several times with 20 wt% HCl at room temperature and then ultrasonic cleaning for 5 min. The suspension was collected by suction filtration, and washed with distilled water and ethanol for several times. At last, the resultant products were dried at 100 °C for 12 h. The samples after activation with KOH at 700 °C were labelled as PCPAC-X, where X indicates the mass ratio of KOH and carbon precursor.

Physicochemical characterization

Scanning electronic microscopy (SEM) images and the energy dispersive X-ray spectroscopy (EDS) data were obtained on the JSM-7500F (5 kV) instrument. Surface morphology of the produced carbon nanorods were examined by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEOL-2100F, JEOL Corporation, Japan) at 200 kV. The crystal structure of the as-prepared samples was determined by the X-ray diffraction (XRD, Shimadzu XRD-6000) using Cu-Kα as a radiation source. Raman spectroscopy analysis was performed on the confocal microprobe Raman system (Lab RAM HR800) with a laser wavelength of 514 nm. N₂ sorption analyses were obtained on the Quantachrome instruments (USA) equipped with an automated surface area at 77 K using the Brunauer–Emmett–Teller (BET) calculation for the surface area.

Electrochemical measurements

The working electrode was prepared by mixing 80 wt% porous carbon, 10 wt% carbon black (Super-P) and 10 wt% polytetrafluoroethylene poly(vinylidenefluoride) (PVDF) binder in N-methylpyrrolidone (NMP) solvent. The slurry of the mixture was painted between two pieces of the nickel foam and pressed under a pressure of 500 kg cm⁻², and then dried at 80 °C overnight in vacuum oven. All electrochemical characterizations were carried out on the CHI660c electrochemical workstation (Shanghai Chenhua Instruments Co.) at room temperature. The three-electrode cyclic voltammetry (CV) curves were tested with Pt wire as the counter electrode and Ag/AgCl as the reference electrode in 6 M KOH. A total of 2025 stainless steel coin cell was assembled with two symmetrical electrodes separated by a porous polypropylene membrane (Nihon Kaiheiki Ind. Co.) in 6 M KOH aqueous electrolyte. In the two-electrode devices, the specific capacitance of one electrode was calculated based on the equation C = 2IΔt/(mΔV), where I is the discharge current (A), Δt is the discharge time (s), m is the active material weight of one electrode (g), and ΔV is the discharge voltage (V). The energy density was calculated by using the formula E = CΔV²/(8 × 3.6), and the power density was calculated from the formula P = E/t.

Results and discussion

The morphologies and microstructures of the PCPC and PCPAC-X samples were analyzed by the SEM (Fig. 1a–d) and (HR)TEM (Fig. 1e–g). The obtained PCPC precursor has a 1D nanorod-like morphology (Fig. 1a) with smooth surface (Fig. 1b). The lengths are about 0.5–2 μm and the widths are around 100–300 nm. The KOH activation may hardly change the appearance (Fig. 1c) but destroy the microstructure of the sample leading to the rough surface with more large cracks/voids in the structure of PCPAC-X, e.g., PCPAC-2 (Fig. 1d), which could provide a favourable path for the electrolyte ions to transport and/or penetrate. The short rod sectional dimension may reduce the charge carriers transport time thus enhancing the power density. Moreover, the TEM images (Fig. 1e and f) clearly show the cracks/voids in the structure of PCPAC-2. The HRTEM image (Fig. 1g) further demonstrates that there are many nanopores randomly disturbed in the structure of PCPAC-2. The EDS analyses indicated that the carbon contents of PCPC and PCPAC-2 are about 98 wt%.

Fig. 1 SEM image of (a and b) PCPC precursor as well as SEM (c and d), TEM (e and f) and HRTEM (g) images of the PCPAC-2 sample.

The degree of carbonization and graphitization of the PCPAC-X samples closely related to their conductivities²³ were determined by XRD. The XRD patterns (Fig. 2a) show a broad diffraction peak at 2θ = 23° ascribing to the (002) reflection of the graphitic-type lattice (hexagonal, space group P6₃/mmc (no. 194), and JCPDS card no. 65-6212), which indicates the well-developed graphitization of the PCPAC-X samples.¹⁷ The weak peak at about 43° ascribed to a superposition of the (100) and (101) reflections, i.e., (10) reflection, indicates there is a certain extent interlayer condensation for the PCPAC-X samples. The intensities of (002) and (10) reflections decreased a little for the PCPAC-X samples with the increase of KOH and carbon ratio. In the process of activation, the PCPC precursor may actively react with KOH based on the chemical reactions: 6KOH + 2C → 2K + 3H₂ + 2K₂CO₃, K₂CO₃ → K₂O + CO₂, CO₂ + C → 2CO, K₂CO₃ + 2C → 2K + 3CO, and C + K₂O → 2K + CO. Thus, many cracks/voids could form under the KOH treatment, which may reduce the ordering of the carbon arrangement. However, the high activation temperature may enhance the ordering of the carbon arrangement.

Fig. 2 (a) XRD patterns and (b) Raman spectra of the PCPC and PCPAC-X (X = 1, 2 and 4) samples; and (c) N₂ adsorption–desorption isotherms of the PCPAC-2 sample with insert of the pore size distribution.

Raman spectroscope analysis was employed to further investigate the ordering degree of carbon atoms in the PCPAC-X samples (Fig. 2b). All the samples showed a broad disorder-induced D-band (∼1340 cm⁻¹) and in-plane vibrational G-band (∼1590 cm⁻¹). The former peak is assigned to the breathing mode of κ-point phonons with A_1g symmetry and is related to the disordered carbon or defective graphitic structures.²⁴ The latter peak is a characteristic feature of graphite layers, corresponding to E_2g phonon vibrations of the sp²-bonded carbon atoms.²⁵ The intensity ratio (I_G/I_D) may illustrate the degree of disordering or defective graphitic structure of the carbon materials.²⁶ The I_G/I_D ratio of the PCPC, PCPAC-1, and PCPAC-2 increases from 0.71, 0.77, to 0.83 (Table 1), indicating the decrease of the degree of disordering or defective graphitic structure with the rising ratio of KOH/PCPC. The I_G/I_D value decreases to 0.79 when the mass ratio of KOH/PCPC is 4, where the largest amount of KOH could intensively destroy the long rang ordered structure of carbon material leading to the most defective graphitic structure. So, the PCPAC-2 has the most suitable ordering degree of carbon atoms with the highest value of I_G/I_D ratio (0.83). The value is about twice that of the commercial activated carbon (0.52), presenting the good electrical conductivity of PCPAC-2.¹¹

Table 1 The intensity ratio (I_G/I_D) of the PCPC and PCPAC-X samples

Sample	IG/ID
PCPC	0.71
PCPAC-1	0.77
PCPAC-2	0.83
PCPAC-4	0.79

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N₂ adsorption–desorption isotherm at 77 K was used to estimate the porous texture of the PCPAC-2 sample. Fig. 2c shows that the PCPAC-2 exhibits a distinct hysteresis loop at the pressure range of 0.45–1.0 P/P₀ which can be classified as a type-IV with type-H3 hysteresis loop, indicative of porous structures. The specific BET surface of the PCPAC-2 sample is about 679 m² g⁻¹ and the total pore volume is 1.63 cm³ g⁻¹. The high surface area may provide the abundant electrochemical charge accumulation at the electrode/electrolyte interface to store energy. The pore size distribution calculated using the NLDFT model demonstrates that the PCPAC-2 possesses hierarchical pore structure, which consists of a large fraction of mesopores (80.5%) with a wide pore diameter range from 3 to 50 nm (inset of Fig. 2c). In addition, the small micropore with the pore size of about 0.96 nm may be observed from the NLDFT pore size distribution. The hierarchical pore structure is advantageous for enhancing the electrochemical capacitor application since the micro/mesopores could greatly contribute the high specific surface area providing more active sites for electrochemical reactions and the mesopores may permit rapid electrolyte ions transport enhancing the power density.

Cyclic voltammetry (CV) of the PCPAC-X samples were determined at a scan rate of 50 mV s⁻¹ in the voltage range from −1.0 to 0.1 V in 6 M KOH aqueous electrolyte (Fig. 3a). The porous PCPAC-X electrodes showed the typical electrical double layer capacitive behaviour with rectangular shape. The specific capacitance values calculated from the CV curves under different scan rates from 2 to 500 mV s⁻¹ are shown in Fig. 3b. It is clear that the specific capacitances decrease along with the increase of the scan rate. The PCPAC-2 electrode exhibits the best capacitance response at the same sweep rates among all the PCPAC-X samples. The specific capacitance value of the PCPAC-2 electrode can reach 537, 348, 298, 286, 273, 255 and 236 F g⁻¹ at the san rate of 2, 10, 20, 50, 100, 200 and 500 mV s⁻¹, respectively.

Fig. 3 (a) CV curves of the PCPC and PCPAC-X (X = 1, 2 and 4) electrodes at a scan rate of 50 mV s⁻¹; (b) dependence of the capacitances for the PCPC and PCPAC-X (X = 1, 2 and 4) electrodes on the scan rate from 2 to 500 mV s⁻¹; (c) the galvanostatic charge/discharge curves at different current densities for the PCPAC-2 supercapacitor; (d) dependence of the capacitances for the PCPAC-2 electrode at the current density from 1 to 30 A g⁻¹; (e) the long-term cyclic stability of the PCPAC-2 supercapacitor at the current density of 1 A g⁻¹ over 5000 cycles with the insert of the cycling stability at progressively varied current densities; (f) the Nyquist plots for the PCPC and PCPAC-X electrodes in the frequency range from 0.01 Hz to 0.1 MHz with the inserts of the enlarged Nyquist plots and the equivalent circuit; and (g) the Ragone plot of PCPAC-2 comparing with the reported values for the porous carbon materials.

The galvanostatic charge–discharge curves of the optimized PCPAC-2 material as supercapacitor electrode were investigated in 6 M KOH aqueous electrolyte in two-electrode coin cell assembly. Fig. 3c shows highly linear and symmetric triangular-type shapes with a little galvanostatic discharge decrease caused by the inner resistance throughout the current range of 1–30 A g⁻¹, indicating a dominant electric double-layer capacitor formation at the interface of electrode and electrolyte. The discharge curves show only a small voltage drop (i.e., IR drop) of 0.01 V at 2 A g⁻¹, implying the device possesses a low equivalent series resistance (ESR). The calculated specific capacitance from the discharge curves of PCPAC-2 is about 306, 289, 274, 266, 262, 243 and 205 F g⁻¹ at the current density of 1, 2, 5, 8, 10, 15 and 30 A g⁻¹, respectively (Fig. 3d). With the increase of the current density, the specific capacitance value decreases because of the limited diffusion of the active ions on the electrode surface in the case of fast charging–discharging processes.

The long term charge–discharge property of the PCPAC-2 electrode was investigated at a current density of 1 A g⁻¹ (Fig. 3e). The supercapacitor retains about 96% of the initial specific capacitance after 5000 cycles of continuous charging–discharging processes, demonstrating its excellent stability. Moreover, the rate performance tests cycled at progressively increasing current densities are shown in the insert of Fig. 3e. When the current density turns back to the initial 1 A g⁻¹ for another 800 cycles, the capacitance almost recovers the initial value. These results demonstrate a good long-term cyclic stability of the PCPAC-2 supercapacitor.

Electrochemical impedance spectroscopy was employed to understand the ionic/electronic transports in the PCPAC-2 electrode (Fig. 3f). The intercept on the real axis of the Nyquist plots at high-frequency region is associated to the resistance of the electrolyte (R_s). The semicircular pattern at middle-frequency is ascribed to the charge transfer resistance (R_ct). The slope line at the low frequency region is due to the Warburg impedance (W₀) of ion diffusion and CPE is related to the constant phase element.²⁵ The impedance data are stimulated by an equivalent circuit and shown in the inset of Fig. 3f. The R_s of 0.5 Ω shows a low resistance of the electrolyte. The R_ct of 0.2 Ω indicates that the porous structures can efficiently facilitate the charge transfer between electrode and electrolyte. The W₀ of 0.9 Ω suggests that the PCPAC-2 electrode has very small resistance with good ion response. The PCPAC-2 electrode possessing of the lower R_ct and W₀ than that of the other PCPC, PCPAC-1 and PCPAC-4 electrodes (Table 2) is due to the suitable hierarchical pore textures and high ordering of carbon atoms in the structure.

Table 2 The R_s, R_ct and W₀ of the PCPC and PCPAC-X samples in 6 M KOH

Sample	Rs^a [Ω]	Rct^b [Ω]	W0^c [Ω]
a Rs is the internal resistance.b Rct is the charge transfer resistance.c W0 is the Warburg impedance.
PCPC	0.7	0.5	1.6
PCPAC-1	0.4	0.3	1.1
PCPAC-2	0.5	0.2	0.9
PCPAC-4	0.6	0.1	1.0

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The Ragone plot of the PCPAC-2 is shown in Fig. 3g. At a low power density of 272.2 W kg⁻¹, the energy density obtained for the PCPAC-2 electrode is 17.6 W h kg⁻¹. When the power density increases to 7530 W kg⁻¹, the energy density may arrive at 10 W h kg⁻¹ for the PCPAC-2 electrode. Apparently, the PCPAC-2 material exhibits the simultaneous high energy-power density superior to many typical porous carbon materials.^9,27–33

Conclusions

1D nanorod-like hierarchically nanoporous carbon material PCPAC-2 has been successfully synthesized by pyrolysis of the porous coordination polymer as the carbon source and structure template following with KOH activation. The PCPAC-2 can exhibit long cyclic stability and simultaneous high energy and large power density in the safe and cheap 6 M KOH aqueous system. The unique 1D nanorod-like morphology with the nanosized short sectional dimension reducing the charge carriers transport time could enhance the power density. The hierarchical nanopores with high specific surface area and a large fraction of mesopores are helpful for the high performance supercapacitor utilization since the high specific surface area may provide abundant active sites for electrochemical reactions enhancing the energy density and the mesopores could permit rapid electrolyte ions transport enhancing the power density. The high ordering degree of carbon atoms in the structure of PCPAC-2 may bring about the high conductivity, thus contributing the high power out obviously. The promising results obtained in this study open up new avenues for the construct of advanced porous carbon materials, which could potentially be used in future energy storage/conversion technologies.

Acknowledgements

This work is supported by National Basic Research Programs of China (973 Program, No. 2014CB931800), Chinese National Science Foundation (No. 21571010 and U0734002) and Chinese Aeronautic Project (No. 2013ZF51069).

Notes and references

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