Porous Carbon with Willow-Leaf-Shaped Pores for High

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Porous Carbon with Willow Leaf Shaped Pores for High Performance Supercapacitors Yanhong Shi, Lin-Lin Zhang, Tyler B Schon, Huan-Huan Li, Chao-Ying Fan, Xiao-Ying Li, HaiFeng Wang, Xing-Long Wu, Haiming Xie, Haizhu Sun, Dwight S. Seferos, and Jingping Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12776 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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ACS Applied Materials & Interfaces

Porous Carbon with Willow Leaf Shaped Pores for High Performance Supercapacitors Yanhong Shi,†,‡ Linlin Zhang,

†,‖,‡

Tyler B. Schon,§ Huanhuan Li,† Chaoying Fan,†

Xiaoying Li,† Haifeng Wang,† Xinglong Wu,† Haiming Xie,† Haizhu Sun*,†, Dwight S. Seferos*,§, and Jingping Zhang*,†

†College of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, No. 5268 Renmin Street, Changchun 130024, China.

§Department of Chemistry, University of Toronto, 80 St. George, Toronto, ON M5S 3H6, Canada.

To whom correspondence should be addressed. Email: [email protected]; [email protected]; [email protected]; Fax: 86-431-85099667.

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ABSTRACT: A novel kind of biomass-derived high oxygen containing carbon materials doped with nitrogen that have willow leaf shaped pores was synthesized. The obtained carbon materials have an exotic hierarchical pore structure composed of bowl shaped macropores, willow leaf shaped pores, and an abundance of micropores. This unique hierarchical porous structure provides an effective combination of high current densities with high capacitance due to a pseudocapacitive component that is afforded by the introduction of nitrogen and oxygen dopants. Our synthetic optimization allows further improvements in the performance of this hierarchical porous carbon (HPC) material by providing a high degree of control over the graphitization degree, specific surface area, and pore volume. As a result, a large specific surface area (1093 m2 g-1) and pore volume (0.8379 cm3 g-1) are obtained for HPC-650, which affords fast ion transport due to the short ion diffusion pathways. HPC-650 exhibits a high specific capacitance of 312 F g-1 at 1 A g-1, retaining 76.5% of its capacitance at 20 A g-1. Moreover, it delivers an energy density of 50.2 W h kg-1 at a power density of 1.19 kW kg-1, which is sufficient to power a yellow light emitting diode and operate a commercial scientific calculator.

KEYWORDS: Biomass-Derived Carbon, Willow Leaf Shaped Pore, Iron-based Complex, Nitrogen and Oxygen co-Introduction, Extraordinary Supercapacitor Applications


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1. INTRODUCTION

With the increasing use of portable electronics, hybrid electric vehicles and other high power, off-the-grid systems, there is an urgent need to develop new energy storage devices with increased sustainability, high energy density, and excellent rate performance.1-6 Supercapacitors are relatively carbon neutral energy storage devices and have attracted extensive attention due to their high power density, long cycling life and fast charging/discharging processes.7-12 Conductive carbons are an important type of electrode materials for supercapacitors because they possess large specific surface area, high electrical conductivity and excellent electrochemical stability.13-21 However, the relatively low specific capacitance and limited diversity of structural architectures limit their performance in supercapacitors.

To address these issues, it is important to design novel architectures with appropriate pore shapes and sizes. It is known that macropores are favorable for mass transport, mesopores are beneficial for high interfacial contact with the electrolyte, and micropores increase specific surface area and introduce active sites for charge storage.22-24 Therefore, many studies have been devoted to the fabrication of a hierarchical porous structure. For example, Cao et al. prepared hierarchical porous carbon nanosheets with micro-, mesoand macro-pores via simultaneous activation and graphitization of biomass-derived 3 ACS Paragon Plus Environment


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natural silk.25 Yang et al. fabricated three-dimensional beehive-like hierarchical porous carbons by a simple carbonization and KOH activation process.26 A proper pore shape and hierarchical structure are critical to increase the specific surface area, capacitance, and rate performance. However, controllable pore shapes are rarely reported for supercapacitor applications. Therefore, it is crucial to develop a carbon material with a controllable pore shape imparted by a simple, reproducible methodology.

Heteroatom doping is an effective way to improve the electrochemical properties of carbon materials for supercapacitors due to the improved wetting of the carbon surface and the introduction of pseudocapacitive charge storage elements.27-31 Sustainable biomass materials, such as chitosan,32 cellulose33-36 and gelatin,37-39 etc. have been frequently used as the carbon source to prepare multi-heteroatom doped carbon materials. For instance, Qiu et al. synthesized two-dimensional porous carbon nanosheets derived from gelatin and dopamine to achieve a specific capacitance of 300 F g-1 at a current density of 0.5 A g-1.40 However, most investigations using biomass-derived carbon materials have low specific capacitance and/or poor rate performance. Moreover, the simultaneous control of heteroatom doping content, graphitization degree, specific surface area and pore volume of the obtained material still remains a challenge.

Herein, we report a novel kind of biomass-derived high oxygen containing carbon materials doped with nitrogen that have willow leaf shaped pores synthesized via a 4 ACS Paragon Plus Environment

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simple carbonization and subsequent KOH activation. The obtained hierarchical porous carbon materials (HPC and HPC-X, X=activation temperature) have an exotic hierarchical pore structure composed of bowl shaped macropores, willow leaf shaped pores, and an abundance of meso- and micropores. The unique hierarchical porous structure promotes an effective combination of high current densities with high capacitance due to a pseudocapacitive element afforded by the introduction of nitrogen and oxygen dopants. Importantly, the unique willow leaf shaped pores can offer not only a larger specific surface area but also facilitate diffusion of the electrolyte. Our synthetic method allows further improvements in the HPC material performance by providing a high degree of control over the graphitization degree, specific surface area, and pore volume. As a result, a large specific surface area (1093 m2 g-1) and pore volume (0.8379 cm3 g-1) are obtained for HPC-650, which affords fast ion transport due to the short ion diffusion pathways. HPC-650 exhibits a high specific capacitance of 312 F g-1 at 1 A g-1, retaining 76.5% of its capacitance at 20 A g-1. Moreover, it delivers an energy density of 50.2 W h kg-1 at a power density of 1.19 kW kg-1, which is sufficient to power a yellow light emitting diode and operate a commercial scientific calculator.



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2. EXPERIMENTAL SECTION

2.1. Materials.

Gelatin, citric acid, anhydrous iron (III) chloride and potassium hydroxide (KOH) were purchased from Aladdin. All other reagents were of analytical grade and were directly used as received without any purification.

2.2. The Preparation of HPC.

3 g gelatin, 1 g citric acid and 4 g anhydrous iron (III) chloride were dissolved in 30 mL deionized (DI) water at 90oC for 4 h. A red brown gel was obtained after drying at 60 o

C in an oven. The obtained gel was subjected to a two-step annealing process: the gel

was calcined at 300oC for 1 h with a heating rate of 3oC min-1 and then at 800oC for 2 h at 5oC min-1 under Ar/H2 atmosphere. The obtained product named HPC was washed with 1 mol L-1 (M) HCl several times and dried under vacuum.

2.3. The Preparation of HPC-X.

200 mg HPC and 600 mg KOH were dissolved in a moderate amount of deionized water (DI) and stirred for 20 minutes. The obtained solution was dried in an oven at 60oC overnight. The mixture was annealed at different calcinations temperature (550oC, 650oC, 750oC) for 2 h with the heating rate of 5oC min-1 under N2 atmosphere. Then the product


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was centrifuged with DI water and dried in an oven. The samples are named according to the calcination temperature i.e. HPC-X (X=550, 650, 750).

2.4. Electrode Preparation.

The working electrode was prepared by mixing 80 wt% electrode materials, 10 wt% acetylene black and 10 wt% polyvinylidene fluoride (PVDF) in N-methyl pyrrolidone (NMP) to a slurry, which was coated on steel mesh. Then the prepared electrode was dried at 60oC for 12 h under vacuum oven.

2.5. Characterization of Materials.

Typical XRD to determine the structure of HPC, HPC-550, HPC-650, HPC-750 was performed on Rigaku P/max 2200VPC with Cu Kα radiation. Raman spectra were collected at room temperature with a JY HR-800 Lab Ram confocal Raman microscope in a backscattering configuration with an excitation wavelength of 633 nm. Nitrogen adsorption-desorption isotherms at 77 K were measured on a micromeritics ASAP 2020 analyzer using vacuum-degassed samples (200 oC for 15 h). The isotherms were used to calculate the specific area by the Brunauer-Emmett-Teller (BET) method and the pore volume and pore size by the Barrett-Joyner-Halenda (BJH) method. Scanning electron microscopy (SEM, XL 30 ESEM-FEG, FEI Company) and transmission electron microscopy (TEM) were conducted to characterize the morphologies of the as-prepared 7 ACS Paragon Plus Environment


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samples. A VGESCALAB MKII spectrometer was used for an X-ray photoelectron spectra (XPS) test using Mg-Kα eacitation (1253.6 eV). Binding energy calibration was based on C 1s at 284.6 eV.

2.6. Electrochemical Characterization.

Electrochemical analyses were carried out in CHI660E electrochemical workstation (Chenhua, Shanghai). The electrochemical performance of individual working electrode samples was conducted in 1 M H2SO4 aqueous electrolyte with a saturated calomel electrode (SCE) as the reference electrode and platinum foil as the counter electrode, respectively. Cyclic voltammetry (CV) tests and galvanostatic charge-discharge (GCD) tests were performed under ambient conditions. Electric impedance spectroscopy (EIS) was performed with an amplitude of 5 mV from 0.01 Hz to 100 kHz. The specific capacitance in this paper was calculated from the galvanostatic charge-discharge (GCD) values according to the equation (1):

C = I∆t / m∆V

(1)

where C (F g-1) is the specific capacitance, I (A) is the discharging current, △V (V) is the potential change within the discharge time △t (s), and m (g) is the mass of the active material in the electrode. The energy density and power density of supercapacitors were calculated in a two-electrode system by the equation (2) and (3): 8 ACS Paragon Plus Environment

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Et =

1 2 Ct (∆V ) 2

(2)

Pt = Et / t

(3)

Where Et (W h kg-1) is the specific energy density, Pt (W kg-1) is the specific power density, Ct (F g-1) is the specific capacitance in two-electrode system, △V (V) is the cell voltage window, and t (h) is the discharge time, respectively.

3. RESULTS AND DISCUSSION

Gelatin was selected as the main carbon source because it is an abundant biomass material with many amino and hydroxyl groups which contribute to the introduction of the resultant material with nitrogen and oxygen atoms (Figure 1). Citric acid was also used due to its large proportion of carboxyl functionalities to further increase the content of oxygen, which can offer more pseudocapacitance.41, 42 Using iron (III) chloride as the pore forming agent leads to stable chemical bonds with gelatin and citric acid form, which facilitates the formation of the willow leaf shaped pores with a large surface area after calcination.



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Figure 1. Schematic illustration of the HPC-X materials.

As an example of the hierarchical porous structure formed in the HPC-X materials, electron microscopy images of HPC-650 show a homogenous distribution of bowl shaped macropores with a diameter of ~500 nm and willow leaf shaped pores with a maximum length of ~150 nm and a maximum width of ~41 nm (Figure 2 and Figure S1, S2). To investigate the process of the pore formation, a sample composed of only gelatin and iron (III) chloride (GC-650), and a sample made of only citric acid and iron (III) chloride (CAC-650) were prepared and examined with electron microscopy (Figure 2c-f). SEM images of GC-650 (Figure 2c) and CAC-650 (Figure 2e) show irregular microstructure devoid of bowl and willow leaf shaped pores. TEM images of GC-650 (Figure 2d) and CAC-650 (Figure 2f) reveal crumpled flake-like structures lacking any apparent porosity. This is consistent with a previously reported carbon material prepared using only gelatin and citric acid that had a layered sedimentary rock structure.43 Therefore, we attribute the


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formation of the novel willow leaf shaped pores to the synergetic interaction of the gelatin, citric acid and iron (III) chloride. Moreover, in the transparent sol system, the citric acid formed a clathrate with iron (III) chloride. Increasing the temperature and pH value by the addition of gelatin led to release of iron (III) from the clathrate. After calcination, some of the iron (Ⅲ) turns into FeCx to form willow leaf iron composites that are then etched by acid to form the willow leaf shaped pores. Additionally, some of the iron (Ⅲ) converted to iron (0) to form bowl shaped pores when etched by acid which is confirmed by the XRD pattern (Figure S3-6). Elemental mapping reveals that iron is present in the sample surface upon carbonization (Figure S4) and the inside (Figure S6), which exhibits a homogeneous metal introduction. After acid washing and removal of the iron and iron compounds, willow leaf shaped and bowl shaped pores form. Activation with potassium hydroxide further etches the carbon material, leading to the formation of micropores. This unique morphology yields HPC and HPC-X materials with large specific surface area and an abundance of pore channels, which are beneficial for facile ion transport and short ion diffusion pathways.



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Figure 2. SEM images of (a) HPC-650, (c) GC-650 and (e) CAC-650; TEM images of (b) HPC-650, (d) GC-650 and (f) CAC-650, indicating the novel willow leaf shaped pores are derived from the synergetic interaction of the gelatin, citric acid and ferric iron.

The content of the heteroatom doping, the graphitization degree, the specific surface area and pore volume of the HPC-X materials were further optimized by varying the activation temperature. SEM and TEM images of the porous carbon at different calcination temperatures (Figure S7) confirm the retention of internal willow leaf shaped pores and bowl shaped macropores on the surface of the HPC-X materials. The atomic compositions of the HPC materials were measured by elemental analysis (Table 1). The results show that HPC-650 possesses relatively high content for both nitrogen (7.03 wt%) and oxygen (14.03 wt%). SEM imaging and elemental mapping of C, N and O in


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HPC-650 show that the nitrogen and oxygen elemental distribution is homogenous (Figure S8). The homogeneous distribution and high percentage of nitrogen dopants of high oxygen-containing carbon materials provides a favorable pseudocapacitive environment for charge storage and excellent electrochemical performance.41, 42

Table 1. Elemental analysis of the as-obtained HPC-X materials Sample

C (wt.%)

N (wt.%)

H(wt.%)

O (wt.%)

HPC

78.583

5.778

1.236

14.403

HPC-550

76.172

5.976

1.435

16.417

HPC-650

77.461

7.030

1.478

14.031

HPC-750

83.826

1.784

0.641

13.749

XPS measurements were performed to further analyze the chemical environments of the heteroatoms. The characteristic peaks of C, N, O (Figure 3a) are located at ~284, ~400 and ~533 eV, respectively. The N 1s XPS spectrum of HPC-650 (Figure 3b) can be deconvolved into three individual peaks centered at 397.9, 399.7 and 400.8 eV, corresponding to pyridinic, pyrrolic and quaternary nitrogen environments, respectively. Pyridinic and pyrrolic nitrogen dopants in carbon materials contribute mostly to pseudocapacitance. Moreover, quaternary nitrogen atom dopants facilitate electron transfer, promote high conductivity, and increase wettability of carbon materials.41, 42 The C 1s XPS spectrum (Figure 3c) can be deconvoluted into five individual peaks, which correspond to sp2 C-C (284.4 eV), C-O-H (285.2 eV), C-O-C (286.4 eV), C-N (288.6 eV) and O-C=O (289.1 eV) environments. The C-N (288.6 eV) also reflects quaternary 13 ACS Paragon Plus Environment


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nitrogen (graphitic N, 400.8 eV) environments in N 1s spectrum. The O 1s spectrum can be resolved into three peaks at 530.9, 532.1 and 533.3 eV (Figure 3d) which correspond to C-O-C, C-O-H, and C=O environments, respectively. These results confirm the successful synthesis of nitrogen doped and high oxygen containing porous carbon materials.

Figure 3. (a) XPS spectra of HPC-550, HPC-600, HPC-650, HPC-750, and (b) N 1s, (c) C 1s, (d) O 1s of HPC-650, confirming the successful introduction of HPC-X materials with N and O.

In addition to tuning the content of heteroatoms, the graphitization degree of the HPC materials can also be adjusted by varying the temperature. HRTEM images (Figure S9) show varying amounts of graphitized carbon between samples subjected to different 14 ACS Paragon Plus Environment

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calcination temperatures. The degree of graphitization was further investigated by XRD and Raman spectroscopy. The two peaks located at approximately 24.5o and 43.9o in the XRD pattern of HPC-X materials (Figure S10a) correspond to the (002) and (100) planes of graphitic carbon. The broad peak shape reflects the partly defective nature and their amorphous structure.44 The sharp peak at 26.5o corresponds to the (300) in HPC-550 (JCPDS No.50-0927) and indicates a higher degree of crystallinity compared to the samples activated at higher temperatures (Figure S11).45 HPC-600, as a comparison, displays decreased peak intensity which demonstrates that the degree of crystallinity decreases with increasing the temperature. In the Raman spectra, the increasing ID/IG values from 1.03 to 1.10 going from HPC-550 to HPC-750 (Figure S10b), with the D band at 1336 cm-1 and the G band at 1589 cm-1, show that HPC-550 is more graphitic than HPC-600, HPC-650 and HPC-750, which is consistent with the XRD results. These results show that in addition to tuning the heteroatom content, temperature can be used to achieve varying degrees of graphitization and crystallinity in the porous carbon materials.

Another important factor that was investigated is how to adjust the pore size distribution and the pore volume. Nitrogen adsorption-desorption measurements of all HPC-X materials (Figure 4a) show a type I/IV isotherm with an H3 hysteresis loop. The resultant

isotherms

are

different

from

one

of

traditional

activated

carbon

adsorption-desorptions, due to the co-existence of micro-, meso-, and macro-pores in the 15 ACS Paragon Plus Environment


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HPC-X materials. The specific surface area of HPC-X materials was calculated by the BET method and the pore size distribution of the HPC-X materials was calculated using the BJH method (Figure 4b and Table S1). HPC-650 has a large surface area of 1093 m2 g-1, which offers ample electrode/electrolyte interfacial contact with sufficient active sites for a high capacitance and a high rate performance. With increasing calcination temperature, all of samples show an increasing pore volume of 0.3383 cm3 g-1 for HPC, 0.5778 cm3 g-1 for HPC-550, 0.8379 cm3 g-1 for HPC-650, and 1.189 cm3 g-1 for HPC-750. The ratio of micropores is 90.7% for HPC, 35.2% for HPC-550, 30.6% for HPC-650 and 36.2% for HPC-750, respectively. The specific surface area gradually increases from 935.5 to 1926 m2 g-1 with the total pore volume from 0.3383 to 1.189 cm3 g-1, which shows that an increase of the pore volume favours a high specific surface area. Moreover, the relatively small specific surface area (849.5 m2 g-1) and micropore volume (0.2066 cm3 g-1) of HPC-550 is probably because part of the micropores become mesopores during the KOH activation. The mesopore distribution of HPC-X materials contains the willow leaf shaped pores and the common mesopores derived from the KOH activation process. According to the results of BJH simulation (Figure S12), the distribution of pores at ~40 nm results from willow leaf shaped pores. We can clearly observe that the ratio of willow leaf shaped pores increase from HPC, HPC-750, HPC-550 and HPC-650. Namely, the ratio is the highest for HPC-650. Considering that this trend is in accordance with the results of electrochemical performance of the HPC 16 ACS Paragon Plus Environment

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materials, we can conclude that willow leaf shaped pores have an important influence on the performance of the resulted materials. It is expected that the synergy between the hierarchical porous structure and the high heteroatom content of the HPC-X materials will be beneficial for improving the electrochemical performance of supercapacitor electrode materials.

Figure 4. (a) Nitrogen adsorption-desorption isotherm and (b) pore size distribution curves of HPC, HPC-550，HPC-650，HPC-750.

Cyclic voltammetry (CV) curves of all samples in a three-electrode system (Figure 5a) present a similar rectangular shape with small anodic and cathodic peaks at 0.41 and 0.36 V respectively in a potential window of 0.0 to 0.9 V. The redox peaks are the most pronounced in the HPC-550 material, which has the most oxygen content. This suggests that the oxygen functionalities mainly contribute to the Faradaic processes occurring. HPC-650 has a higher current density than the other samples, giving rise to an improved specific capacitance because of high nitrogen dopant concentration. The high specific 17 ACS Paragon Plus Environment


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surface and pore volume offer sufficient active sites in favour of a high specific capacitance. The high nitrogen concentration likely leads to an improved wettability of the carbon material, allowing a greater amount of interfacial contact between the electrode material and the electrolyte. This explains why the HPC-750 material, with the highest surface area, a high oxygen content, but low nitrogen content, has a relatively low capacitance. These results further demonstrate the improvement of electrochemical property is beneficial from the co-contribution of N and O doping and the increase of specific surface area and pore volume. The CV curves of HPC-650 between 1 and 100 mV s-1 (Figure 5b) retain its quasi-symmetric rectangular shape, indicating that it has an excellent rate capability which is attributed to the unique hierarchical porous structure yielding fast ion transport and short ion diffusion pathways. The galvanostatic charge/discharge (GCD) curves of HPC and HPC-X materials at a current density of 1 A g-1 (Figure 5c) show nearly symmetric triangles, demonstrating the reversible electrochemical performance in the charging-discharging process. HPC-650 has the highest specific capacitance out of all the samples, attaining 312.3 F g-1 at a current density of 1 A g-1 compared to HPC-550 (246.8 F g-1) and HPC-750 (235.9 F g-1). The GCD curves of HPC-650 from 1 to 20 A g-1 (Figure 5d and Figure S14b) show a good rate performance with almost no IR drop even at a high current density of 100 A g-1.


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Figure 5. Electrochemical performance of the HPC and HPC-X materials tested in a three-electrode system with 1 M H2SO4 as the aqueous electrolyte: (a) CV curves of HPC, HPC-550, HPC-650 and HPC-750 at the scan rate of 10 mV s-1; (b) CV curves of HPC-650 at different scan rates from 1 to 100 mV s-1; (c) GCD curves of HPC, HPC-550, HPC-650 and HPC-750 at the current density of 1 A g-1; (d) GCD curves of HPC-650 at different current densities from 1 to 20 A g-1; (e) the specific capacitance at different current densities for HPC, HPC-550, HPC-650, HPC-750; (f) Comparison of Nyquist



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plots of HPC, HPC-550, HPC-650, HPC-750. The inset image shows the magnified data in the high frequency region.

To further evaluate the electrochemical rate performance of the HPC-X materials, the specific capacitance was calculated at different current densities (Figure 5e) and different scan rates (Figure S15). HPC-650 retains a specific capacitance of 239.0 F g-1 at a high current density of 20 A g-1, having a high capacitance retention of 76.5% compared with that measured at 1 A g-1. The ultra-high electrochemical performances of HPC-650 are attributed to the synergetic contribution of willow leaf and bowl shaped pores, the pseudocapacitance provided by the high content of oxygen, and the improved wettability of the carbon surface due to the high nitrogen content. HPC-750, with a largest surface area and pore volume, has a lower capacitance than HPC-550 and HPC-650 due to the ultralow nitrogen content. The electrochemical behavior of the materials was further explored by electrochemical impedance spectroscopy (EIS) measurements (Figure 5f). The intercept of the curves at the real component of the impedance gives the series resistance of the electrochemical system (Rs), which is similar for all samples (HPC: ~0.86 Ω; HPC-550: ~0.85Ω; HPC-650: ~0.84 Ω; HPC-750: ~0.85 Ω).46 In the high frequency range (inset in Figure 5f), the diameter of the semicircle provides the charge transfer resistance, which is comparatively lower for HPC-650 sample, showing that this sample has optimal pore sizes and heteroatom doping content 20 ACS Paragon Plus Environment

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for facile charge transfer. The low frequency region is almost a perfect vertical line for the HPC-X samples, indicating that these samples behave as ideal capacitors due to the fast ion diffusion, which is favorable for rapid charging-discharging.

To further demonstrate the practical application of the HPC-650 electrodes in supercapacitors, a symmetric supercapacitor was assembled with a potential window of 1.2 V. The CV curves exhibit a pseudo-rectangular shape with scan rates ranging from 1 to 100 mV s-1 (Figure 6a), showing an ideal supercapacitor behavior. The HPC-650//HPC-650 symmetric device shows a wide reversible peak at 0.2 V with a slightly distorted rectangular CV curves and the highest capacitance, demonstrating the good pseudocapacitive properties afforded by the nitrogen and oxygen heteroatoms. The HPC-650 supercapacitor has a specific capacitance of 347.4 F g-1 at 1 mV s-1 with 55.2% retention of capacitance upon increasing the scan rate to 300 mV s-1 (Figure S17). The GCD curves of HPC-650 from 1 to 20 A g-1 show symmetric triangular shapes and nearly linear slopes, showing the excellent coulombic efficiency and ideality of the devices (Figure 6b and Figure S18). The specific capacitance at 1 A g-1 is 251.8 F g-1 and 164.4 Fg-1 at 100 A g-1, demonstrating good rate performance of the symmetric supercapacitor (Figure 6c). From the Ragone plot (Figure 6d), the HPC-650 supercapacitor reaches an energy density of 50.22 W h kg-1 at a power density of 1.19 kW kg-1 and retains 27.38 W h kg-1 at 109 kW kg-1. The symmetric supercapacitor in this work exhibits higher energy 21 ACS Paragon Plus Environment


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density and power density compared to previously reported devices (Figure 6d), including PCNS-G-4//PCNS-G-4 (10.3 Wh kg-1 at 18.4 kW kg-1),40 N-CNFs /Ni(OH)2//N-CNFs (51 Wh kg-1 at 0.90 W kg-1),47 HPNCT-800//HPNCT-800 (24 Wh kg-1 at 7 kW kg-1),48 PNPEA-KOH-800// PNPEA-KOH-800 (20.84 Wh kg-1 at 19.27 kW kg-1),49 and HPC-3//HPC-3 (8.3 Wh kg-1 at 5.48 kW kg-1).50

Figure 6. Electrochemical performance of HPC-650 carried out in a two-electrode system with 1 M H2SO4 as the aqueous electrolyte: (a) CV curves of HPC-650 at different scan rates from 1 to 100 mV s-1; (b) GCD curves of HPC-650 at different current densities from 1 to 20 A g-1; (c) the specific capacitance at different current


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densities for HPC-650; (d) Ragone plot of the two-electrode supercapacitor assembled by the active material HPC-650.

The practical application of a symmetric HPC-650 supercapacitor was demonstrated by connecting two as-fabricated symmetric supercapacitors in series and using it to power a yellow LED (inset of Figure 6d). In further demonstrations, we used three supercapacitors in series to power a scientific calculator (Video S1). HPC-650 also possesses excellent cycling stability (Figure 7) with almost no capacitance loss over 10000 cycles in a three-electrode system. The excellent electrochemical performance benefits from the co-contribution of the willow leaf and bowl shaped pores of HPC-650 with abundant micropores and the nitrogen and oxygen dopants that contribute to a high pseudocapacitance. The high performance and the simple one-pot synthesis process make HPC and HPC-X one of the most promising materials for supercapacitor applications.

Figure 7. Cycling stability at a current density of 5 A g-1 for HPC-650.



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4. CONCLUSIONS

In summary, we have synthesized a novel willow leaf shaped porous, high oxygen-containing carbon material doped with nitrogen atom in one-pot synthetic route. This novel hierarchical porous structure emphasizes the co-contribution of bowl shaped macropores, willow leaf shaped pores and abundant mesopores and micropores to the excellent electrochemical performance. The uniform nitrogen doping of the HPC materials with high content oxygen atoms provides high capacitance via a Faradaic process. Furthermore, successful control of the graphitization degree, the specific surface area and pore volume provides a route to obtain optimal material performance. The as-obtained HPC-650 shows a specific capacitance of 312.3 F g-1 at a current density of 1 A g-1 and with a small decrease in capacitance of 23.5% when increasing the rate 20-fold to 20 A g-1, showing its excellent electrochemical performance. As a symmetric supercapacitor, HPC-650 delivers an energy density of 50.22 W h kg-1 at a power density of 1.19 kW kg-1. The incredible cycling stability was also demonstrated, having almost no loss in capacitance at 5 A g-1 over 10000 cycles. This work not only provides an effective method to design an exotic and novel willow leaf shaped porous carbon material co-introduced with nitrogen and oxygen atoms, but also shows how one can effectively optimize the obtained structure by successfully tuning the content of heteroatom dopants, the graphitization degree, the specific surface area and pore volume. All of these 24 ACS Paragon Plus Environment

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excellent properties make these materials superior electrodes, showing promise in energy storage applications.

ASSOCIATED CONTENT

Supporting Information. SEM images and TEM images of HPC-550, HPC-650 and HPC-750, SEM image and corresponding elemental mapping images, XRD pattern of HPC before acid cleaning, SEM and TEM images and Fe elemental mapping image of the HPC before acid washing, EDX pattern of the HPC before acid washing, HRTEM images of HPC-550, HPC-600, HPC-650, HPC-750, XRD pattern and Raman spectra of HPC-550, HPC-600, HPC-650 and HPC-750, the specific capacitance at different scan rates for GC-650, CAC-650 and HPC-650 in a three-electrode system, CV and GCD images of HPC-650 in a three-electrode system, specific capacitance at different scan rates for HPC, HPC-550, HPC-650 and HPC-750 in a three-electrode system, CV and GCD images of HPC-650 in a two-electrode system, Coulombic efficiency at different scan rates for HPC-650 in three-electrode and two-electrode systems, specific capacitance at different scan rates for HPC-650 in a two-electrode system, cycling stability at a current density of 5 A g-1 for HPC-550 and HPC-750, textual parameters of HPC, HPC-550, HPC-650, and HPC-750, Video S1, this material is available free of charge via the internet at http://pubs.acs.org.



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AUTHOR INFORMATION Corresponding Author * (H.S.) E-mail: [email protected]. (Haizhu Sun)

* (D.S.) E-mail: [email protected]. (Dwight S. Seferos)

* (J.Z.) E-mail: [email protected]. (Jingping Zhang) Present Addresses ||

The address of author, Linlin Zhang, is different from the one given in the affiliation

line, and the present address is included here: Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education) College of Chemistry, Nankai University, Tianjin 300071, China.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the NSFC (21574018 and 51433003), Jilin Provincial Education Department (543), and Jilin Provincial Key Laboratory of Advanced Energy Materials (Northeast Normal University), and Changbai Mountain Scholar Project by Education Department of the Jilin Province.



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Porous Carbon with Willow-Leaf-Shaped Pores for High

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