Mesopore- and Macropore-Dominant Nitrogen-Doped Hierarchically

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 42797−42805

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Mesopore- and Macropore-Dominant Nitrogen-Doped Hierarchically Porous Carbons for High-Energy and Ultrafast Supercapacitors in Non-Aqueous Electrolytes Rong Shao,†,‡,§ Jin Niu,†,‡,§ Jingjing Liang,†,‡ Mengyue Liu,†,‡ Zhengping Zhang,†,‡ Meiling Dou,†,‡ Yaqin Huang,*,† and Feng Wang*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China ‡ Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: Non-aqueous electrolytes (e.g., organic and ionic liquid electrolytes) can undergo high working voltage to improve the energy densities of supercapacitors. However, the large ion sizes, high viscosities, and low ionic conductivities of organic and ionic liquid electrolytes tend to cause the low specific capacitances, poor rate, and cycling performance of supercapacitors based on conventional micropore-dominant activated carbon electrodes, limiting their practical applications. Herein, we propose an effective strategy to simultaneously obtain high power and energy densities in nonaqueous electrolytes via using a cattle bone-derived porous carbon as an electrode material. Because of the unique co-activation of KOH and hydroxyapatite (HA) within the cattle bone, nitrogen-doped hierarchically porous carbon (referred to as NHPC−HA/KOH) is obtained and possesses a mesopore- and macropore-dominant porosity with an ultrahigh specific surface area (2203 m2 g−1) of meso- and macropores. The NHPC−HA/KOH electrodes exhibit superior performance with specific capacitances of 224 and 240 F g−1 at 5 A g−1 in 1.0 M TEABF4/AN and neat EMIMBF4 electrolyte, respectively. The symmetric supercapacitor using NHPC−HA/KOH electrodes can deliver integrated high energy and power properties (48.6 W h kg−1 at 3.13 kW kg−1 in 1.0 M TEABF4/AN and 75 W h kg−1 at 3.75 kW kg−1 in neat EMIMBF4), as well as superior cycling performance (over 89% of the initial capacitance after 10 000 cycles at 10 A g−1). KEYWORDS: biomass, co-activation, hierarchically porous carbons, supercapacitor, non-aqueous electrolytes

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INTRODUCTION Because of the fossil fuel depletion and the environmental pollution issues, developing the energy storage systems with low cost, high safety, and high energy and power is urgently needed.1−3 Supercapacitors have attracted great attention because of their high power densities and long cycling performance. However, the relatively low energy densities of commercially available supercapacitors seriously limit their further practical applications.4 Generally, the energy density (E) 1 for supercapacitors can be determined by E = 2 CV 2 , where C is the capacitance and V is the working voltage.5 The capacitance can be classified into the following: (1) electrical double-layer capacitance (EDLC), which is caused by the electrostatic charge adsorption at the interface of the electrolyte and electrode, depending on the specific surface area (SSA) of the electrode that is accessible to the electrolyte ions4,7 and (2) pseudocapacitance, which is contributed from the surface or near-surface fast reversible faradic processes.6 To enlarge the © 2017 American Chemical Society

working voltage, one of the most efficient ways is to use the electrolytes with wide potential windows.8 Compared with aqueous electrolytes which only supply operating windows ranging from 1.0 to 2.0 V, non-aqueous electrolytes (e.g., organic and ionic liquid electrolytes) can supply wider operating windows ranging from 2.5 to 4.0 V.8 However, the larger ion sizes, lower ionic conductivities, and higher viscosities of organic and ionic liquid electrolytes cause a poor ion diffusivity in commercially available activated carbons with large SSA but with numerous blind, dead-end, and tortuous micropores, leading to the low power and energy densities for supercapacitors.8−10 Nitrogen-doped hierarchically porous carbons (NHPCs) with numerous defects have shown good capacitive performance as electrodes for supercapacitors in recent years.11,12 In the Received: September 22, 2017 Accepted: November 23, 2017 Published: November 23, 2017 42797

DOI: 10.1021/acsami.7b14390 ACS Appl. Mater. Interfaces 2017, 9, 42797−42805

Research Article

ACS Applied Materials & Interfaces

treated at 900 °C for 1 h in an Ar atmosphere (heating rate: 2.5 °C min−1). After the heat treatment, the product was washed with 1.0 M HCl and ultrapure water several times and finally dried in an oven at 80 °C for 24 h. For the preparation of NHPC by HA self-activation (referred to as NHPC−HA), the cattle bone powder was pyrolyzed at 400 °C for 3 h and then at 900 °C for 1 h in an Ar atmosphere (heating rate: 2.5 °C min−1). The obtained product was washed by 1.0 M HCl and ultrapure water, followed by drying in an oven at 80 °C for 24 h. For the preparation of NHPC by KOH chemical-activation (referred to as NHPC−KOH), the cattle bone powder was first precarbonized at 400 °C for 3 h (heating rate: 2.5 °C min−1), and then the precarbonized powder was washed by 1.0 M HCl and ultrapure water to remove HA. The dried product was mixed well with KOH (the weight ratio of the precarbonized product and KOH was 1:0.3 based on the weight of the precarbonized powder) and then pyrolyzed at 900 °C for 1 h (heating rate of 2.5 °C min−1). Subsequently, the product was washed by 1.0 M HCl and ultrapure water, followed by drying in an oven at 80 °C for 24 h. Material Characterization. The morphologies and microstructures of NHPCs were observed by JSM-6701F scanning electron microscopy (SEM) and JSM-2100 high-resolution transmission electron microscopy. Pore structures of NHPCs were characterized by N2 adsorption isotherms using a Micromeritics ASAP 2460 instrument. The SSA was determined by the Brunauer−Emmett− Teller (BET) method, and the density functional theory (DFT) method was used to calculate the pore size distribution (PSD). The SSA and pore volume for micropores were determined by the t-plot method. X-ray diffraction (XRD) was performed on a Rigaku RINT 2200V/PC instrument. X-ray photoelectron spectroscopy (XPS) spectra were collected on an ESCALAB 250 instrument. Raman spectra were collected on a LabRAM HR800 instrument. A Nanometrics HL5550 cryostat based on the four-probe technique was used to test the electrical conductivity. Electrochemical Measurements. The electrochemical measurements were carried out at an ambient temperature in a CR2032 cointype cell. To prepare the working electrode, the active material (80 wt %), acetylene black (10 wt %) and polyvinylidene fluoride (10 wt %), were coated onto nickel foam followed by drying at 120 °C for 12 h, and then the nickel foam was compacted at 10 MPa with a tablet machine. The loading weight of the active material was 1.5−2.0 mg cm−2. TEABF4/AN (1.0 M) was used as the organic electrolyte in the voltage range of 0−2.5 V, and neat EMIMBF4 was used as the ionic liquid electrolyte in the voltage range of 0−3.0 V. The glass-fiber film (Whatman) was used as the separator. Galvanostatic charge−discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) were performed on a CHI660 working station. The EIS measurement was conducted at the open circuit potential with frequency ranging from 100 kHz to 10 mHz. The specific capacitance for a single electrode C (F g−1) is determined based on the equation

hierarchical porosity, meso- and macropores with relatively large pore sizes can accelerate the ion transport and diffusion to enhance the charge storage under high current loads,13−15 which would be favorable to the usage of organic and ionic liquid electrolytes. Also, doping nitrogen atoms onto the carbon frameworks can not only increase the pseudocapacitance through enhancing the diffusion of negative charges on the electrode surfaces,16 but can also lead to a clear increase in defect density of carbons to increase the EDLC.17 In this regard, many works have been carried out to design and synthesize novel NHPC electrodes to improve the performance of supercapacitors. Generally, the hierarchical porosity of carbon materials can be introduced using templating methods and activation methods.18−21 However, the activation methods can only generate many micropores which are ion-inaccessible combined with a small amount of meso- and macropores. Although mesopore- and macropore-dominant porous carbons can be prepared using templating methods, their relatively low SSA would cause low specific capacitances.19 Furthermore, the templating methods are limited by the complicated and highcost templated synthesis processes, poor dispersion of the templates in the precursors, and harsh experimental conditions to remove templates.18 Moreover, for nitrogen doping, the current methods are mainly dependent on the post-synthesis treatments.4 Therefore, it is still a great necessity to find an efficient and low-cost method for the direct synthesis of NHPC with a large SSA of meso- and macropores that can be used as a high-performance electrode for supercapacitors in non-aqueous electrolytes. As one kind of abundant biomass wastes, an animal bone possesses a high output more than 20 million tons per year in China.22 The animal bone is one kind of a natural organic/ inorganic composite, which is composed of collagen and hydroxyapatite [HA, Cax(PO4,CO3)y(OH)].23 As an organic component, collagen is rich in carbon and nitrogen elements, which can be used as the carbon and nitrogen sources to form nitrogen-doped carbons. As an inorganic component, HA crystals disperse homogeneously within the cattle bone as the natural templates.24,25 In our previous work, we reported that HA could be thermally decomposed to generate H2O and CO2 during the pyrolysis process, resulting in an in situ physical activation of the animal bone.26 On the basis of our previous work, we prepared a unique NHPC with 3D-interconnected mesopore- and macropore-dominant porosity by co-activation of HA and KOH using a cattle bone as a precursor (the resultant product was referred to as NHPC−HA/KOH). The resultant NHPC−HA/KOH possessed good electrical conductivity, high defect density, and an ultrahigh SSA of mesoand macropores. Because of the unique porosity, good electrical conductivity, and numerous effective storage sites, the symmetric supercapacitor based on NHPC−HA/KOH electrodes showed integrated high energy−power density and superior long cycle performance in an organic electrolyte (1.0 M TEABF4/AN) and an ambient-temperature ionic liquid electrolyte (neat EMIMBF4).

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C=

2I Δt mΔV

(1)

where Δt is the discharge time (s), I is the discharge current (A), m is the active material mass (g) on one working electrode, and ΔV is the potential change (V) excluding the IR drop. The energy density E (W h kg−1) and power density P (W kg−1) are determined by the following equations27,28

EXPERIMENTAL METHODS

Sample Preparation. For the preparation of NHPC−HA/KOH, the cattle bone powder was precarbonized under an Ar atmosphere at 400 °C for 3 h (heating rate: 2.5 °C min−1). Then, the product was well-gridded with KOH. The weight ratio of the precarbonized product and KOH was 1:0.3. Subsequently, the mixture was heat-

E=

1 C ΔV 2 2 × 4 × 3.6

(2)

P=

3600E Δt

(3)

The complex form of capacitance C(ω) is dependent on the real part of the capacitance C′(ω) and the imaginary part of the capacitance C″(ω), which is defined as follows29

C(ω) = C′(ω) − jC″(ω) 42798

(4) DOI: 10.1021/acsami.7b14390 ACS Appl. Mater. Interfaces 2017, 9, 42797−42805

Research Article


Figure 1. SEM images of (a1,a2) NHPC−HA, (b1,b2) NHPC−KOH, and (c1,c2) NHPC−HA/KOH; TEM images of (a3) NHPC−HA, (b3) NHPC−KOH, and (c3) NHPC−HA/KOH.

Figure 2. (a) N2 adsorption−desorption isotherms, (b) PSD plots, (c) XRD patterns of NHPCs. (d) Fitting of the Raman spectra for NHPCs using five Gaussian peaks. (e) Content of integrated fitting peaks for NHPCs. (f) XPS survey spectra and high-resolution (g) C 1s and (h) N 1s spectra of NHPCs.

C′(ω) =

− Z″(ω) ω |Z(ω)|2

C″(ω) =

Z′(ω) ω |Z(ω)|2

frequency ω is determined by ω = 2πf. τ0 is the relaxation time constant which is calculated by τ0 = 1/f 0.

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(5)

RESULTS AND DISCUSSION The morphologies of all the NHPC samples were characterized by SEM and TEM analysis. As shown in Figure 1a1, the fiber bundle-like structure was maintained on NHPC−HA after direct carbonization and self-activation of the cattle bone, which

(6)

where Z′(ω) is the real component and Z″(ω) is the imaginary component of the complex impedance, respectively. The angular 42799


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ACS Applied Materials & Interfaces Table 1. Porous Structure Parameters and XPS Results of NHPCs % of total N 1s

a

% of total C 1s

sample

SBETa (m2 g−1)

Smeso/macrob (m2 g−1)

Vtc (cm3 g−1)

Vmeso/macrod (cm3 g−1)

C%

O%

N%

N-6

N-5

N-Q

CC

C−C

C−N, C−O

CO

NHPC−HA NHPC−KOH NHPC−HA/KOH

2111 3236 2476

801 1027 2203

1.38 1.93 3.14

0.74 0.96 3.00

83.2 84.2 88.0

13.0 12.5 9.3

2.9 2.5 2.3

6.5 13.4 7.3

13.2 30.5 36.2

80.3 56.2 56.5

12.6 11.5 10.2

45.8 50.0 48.5

12.6 12.1 11.4

29.0 26.4 29.9

Total specific surface area. bThe specific surface area of meso- and macropore. cTotal pore volume. dThe pore volume of meso- and macropore.

was derived from the collagen fibril bundle in the bone.26 In addition, a coarse surface with some mesopores could be observed from Figure 1a2,a3. Moreover, when HA was first removed and KOH was used as the activator, the fiber bundlelike structure could not be maintained, leading to the irregular bulk structure of NHPC−KOH (Figure 1b1). The highresolution SEM image (Figure 1b2) and TEM image (Figure 1b3) showed that the individual KOH activation generated numerous mesopores with a small pore size on the surface of NHPC−KOH. As HA and KOH co-existed, highly interconnected mesopores and macropores were observed in the fiber bundle-like structure of NHPC−HA/KOH, which was resulted from the co-activation of HA and KOH inside and outside of the cattle bone powder, respectively (Figures 1c1−c3 and S1). It should be noted that the pores formed by HAinduced activation at high temperatures might supply many channels for the permeation of KOH into the cattle bone powder to enhance the activation inside, resulting in the highly interconnected porosity of NHPC−HA/KOH. To further investigate the pore structure of the samples, we employed the N2 adsorption−desorption measurements and DFT calculations for the corresponding PSDs. As shown in Figure 2a, all the samples exhibited the combination of type I and IV adsorption−desorption isotherms with hysteresis loops at high relative pressure, indicating the existence of micro- and mesopores.30 It should be noted that the hysteresis loops of NHPC−HA and NHPC−KOH were much smaller than that of NHPC−HA/KOH. Furthermore, an obvious uptake of the hysteresis loop for NHPC−HA/KOH existed at the high relative pressure above 0.7, indicating that the presence of the macroporous structure in NHPC−HA/KOH. The PSD plots confirmed the existence of hierarchically porous structures for all the NHPC samples (Figure 2b). NHPC−HA and NHPC− KOH possessed numerous micro- and mesopores with a pore size below 8 nm, whereas NHPC−HA/KOH exhibited micro-, meso-, and macropores with a wide PSD ranging from 0.5 to 200 nm. These results were in accordance with the SEM and TEM characterizations. The pore parameters for all the samples were calculated, and they are shown in Table 1. All the samples possessed a large SSA more than 2000 m2 g−1. Among them, NHPC−HA possessed the lowest mesopore SSA (801 m2 g−1, accounting for 38% of the total) and mesopore volume (0.74 cm3 g−1, accounting for 54% of the total). NHPC−KOH possessed the largest SSA of 3236 m2 g−1, which was almost contributed from micropores, showing almost the same porous structure with that of NHPC−HA. The mesopore SSA (1027 m2 g−1) accounted for 31% of the total, whereas the mesopore volume (0.96 cm3 g−1) accounted for 50% of the total. Different from NHPC−HA and NHPC−KOH, NHPC−HA/KOH possessed an ultrahigh meso- and macropore SSA (2203 m2 g−1), which was rarely reported in the previous works using template and activation methods (Tables S1 and S2).31−33 The meso- and macropore volume was as high as 3.00 cm3 g−1 and

accounted for 96% of the total, indicating a mesopore- and macropore-dominant porosity of NHPC−HA/KOH. Figure 2c showed the XRD patterns for all the samples. All the patterns showed broad diffraction (002) peaks and inconspicuous (100) peaks centered at 23° and 43°, indicating that all the samples were primarily in amorphous structures with small regions of crystallinity.17 It is worth noting that the (002) peaks for NHPC−KOH and NHPC−HA/KOH were broader and weaker than that of NHPC−HA, which could be attributed to the fewer stacked graphene layers resulted from the high defect density.26 The defects of NHPC samples were further determined by Raman spectroscopy. The Raman spectra for these three samples were fitted by five peaks including I peak (∼1220 cm−1), D peak (∼1350 cm−1), D″ peak (∼1490 cm−1), G peak (∼1580 cm−1), and D′ peak (∼1620 cm−1) (Figure 2d).26,34 The G peak is caused by the sp2-hybridized graphitic carbons, and the D peak represents the sp3 defects on the graphene layers. The D′ and D″ peaks arise from the lattice disorder and graphene layer stacking faults, and I peak is attributed to the doped nitrogen.35 The relative content of the five peaks for NHPC samples was calculated from the integrated peak area. As shown in Figure 2e, the content of G peaks for all the NHPC samples was less than 20%, indicating an existence of numerous defects. Moreover, NHPC−KOH and NHPC−HA/KOH possessed higher D peak content and lower (D′ + D″) content than NHPC−HA, indicating more intrinsic defects and fewer stacked graphene layer defects. This result corresponded to the XRD characterizations. Apart from intrinsic defects, all the NHPC samples also possessed heteroatom-induced defects. The content of I peaks for all the NHPC samples was more than 20%. The numerous intrinsic defects and heteroatom-induced defects would cause the coarse surface of NHPC samples, which were believed to be able to supply additional EDLCs.36 The surface heteroatoms were characterized by XPS measurements. As shown in Figure 2f, carbon, nitrogen and oxygen species were detected for all the samples. The quantitative analysis for each element was shown in Table 1. All the NHPC samples possessed a similar content of surface N atoms. Among them, NHPC−HA/KOH possessed the relatively low content of surface O atoms. The C 1s spectra (Figure 2g) of all the NHPC samples could be deconvoluted into four peaks, which were centered at 284.6, 285.2, 286.7, and 288.5 eV, that can be assigned to CC, C−C, C−N or C−O, and CO, respectively.37 As shown in Table 1, most of the surface carbon atoms were in the form of sp3-hybridized and heteroatom-bonded carbon atoms. The high-resolution N 1s spectra (Figure 2h) showed that the N atoms exhibited three forms of pyridine nitrogen (N-6, 398.4 eV), pyrrolic nitrogen (N-5, 399.9 eV), and quaternary nitrogen (N-Q, 401.0 eV).37 It is reported that N-6 and N-5 might provide additional chargestorage sites, whereas N-Q could enhance the electron transfer.38,39 The electrical conductivities of NHPC−HA, 42800


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Figure 3. Electrochemical performance of NHPC electrodes in the 1.0 M TEABF4/AN electrolyte. (a) CV curves of NHPC electrodes at the scan rate of 200 mV s−1. (b) GCD curves of NHPC electrodes at the current density of 50 A g−1. (c) CV curves of NHPC−HA/KOH at different scan rates. (d) GCD curves of NHPC−HA/KOH at different current densities. (e) Specific capacitances of NHPC electrodes calculated from GCD curves at different current densities. (f) Cycling stability of NHPC−HA/KOH at the current density of 10 A g−1 over 10 000 cycles.

Figure 4. Electrochemical performance of NHPC electrodes in a neat EMIMBF4 electrolyte. (a) CV curves of NHPC electrodes at the scan rate of 200 mV s−1. (b) GCD curves of NHPC electrodes at the current density of 20 A g−1. (c) CV curves of NHPC−HA/KOH at different scan rates. (d) GCD curves of NHPC−HA/KOH at different current densities. (e) Specific capacitances of NHPC electrodes calculated from GCD curves of NHPC−HA, NHPC−KOH, and NHPC−HA/KOH at different current densities. (f) Cycling stability of NHPC−HA/KOH at the current density of 10 A g−1 over 10 000 cycles.

and NHPC−HA/KOH electrodes at a scan rate of 200 mV s−1. All the NHPC electrodes showed near-rectangular shapes, and no apparent pseudocapacitance peaks were observed, suggesting the predominated EDLC characters. The CV loop of the NHPC−HA/KOH electrode exhibited the largest area among all the NHPC electrodes, implying its highest specific capacitance. The GCD curves for all the NHPC electrodes at the current density of 50 A g−1 were shown in Figure 3b. NHPC−HA/KOH possessed the longest discharge time, indicating its best capability among all the NHPC electrodes. The CV curves of NHPC−HA, NHPC−KOH, and NHPC− HA/KOH electrodes at different scan rates were shown in

NHPC−KOH, and NHPC−HA/KOH were measured to be 782, 134, and 1144 S m−1, respectively, which were higher than the commercial activated carbon.40 Particularly, NHPC−HA/ KOH possessed the highest electrical conductivity because of its fiber bundle-like structure and its lowest O content among the three samples. The electrical conductivity of NHPC−HA/ KOH was much higher than those of porous carbons which were previously reported (Tables S1 and S2). To obtain high energy densities of the symmetric supercapacitors, 1.0 M TEABF4/AN was first used as the electrolyte, which could supply a high working voltage window of 2.5 V. Figure 3a shows the CV curves of NHPC−HA, NHPC−KOH, 42801


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capacitance retention of 89.2% at 10 A g−1 after 10 000 cycles (Figure 4f). It has been calculated that the porous electrodes with pore sizes around 0.7 nm could show a maximum surface area normalized double-layer capacitance in TEABF4/AN and ionic liquid electrolytes.43−46 However, the theoretical best pore size was calculated for ideal spherical or slit pores. In fact, micropores for activated carbons were primarily irregular, tortuous, and dead-end, which could not efficiently store the electrolyte ions although the pore size matched the ion size.47 Different from micropores, the interconnected meso- and macropores of the NHPC−HA/KOH electrode with a much larger pore size were ion-accessible. Therefore, the capacitances of NHPC electrodes were not positively correlated with the micropore SSA but were correlated with the meso- and macropore SSA (Figure S5). Although meso- and macropores might not exhibit the maximum surface area normalized double-layer capacitance, the large efficient SSA supplied by meso- and macropores still led to the largest specific capacitance of the NHPC−HA/KOH electrode among all the NHPC electrodes. EIS measurements were conducted to investigate the reason why the NHPC−HA/KOH electrode possessed the best capacitive performance among all the NHPC electrodes. Figure 5a,b show the Nyquist plots for the NHPC electrodes in

Figures S2a1,a2 and 3c, respectively. NHPC−HA/KOH electrode maintained a quasi-rectangular shape even at 500 mV s−1, implying its good rate performance. As shown in Figure 3d, the isosceles triangular GCD curves of the NHPC− HA/KOH electrode at different current densities indicated the reversible EDLC behavior and high charge−discharge efficiency. It should be noted that the NHPC−HA/KOH electrode showed a small IR drop even at the high current density of 150 A g−1 (Figure S4a), indicating its superior rate capability. On the basis of the results of GCD curves (Figures S2b1,b2, and 3d), the specific capacitances of NHPC−HA, NHPC−KOH, and NHPC−HA/KOH electrodes were calculated according to eq 1. As shown in Figure 3e, all the NHPC electrodes showed high specific capacitances because of their hierarchical porosity and high defect density. NHPC−HA and NHPC−KOH electrodes exhibited the specific capacitances of 181 and 185 F g−1, respectively, which were slightly higher than or comparable to the performance of previously reported hierarchically porous carbons (Table S1). For NHPC−HA/ KOH electrodes, however, an extremely high specific capacitance of 224 F g−1 was achieved at 5 A g−1 and maintained 156 F g−1 even at the high current density of 150 A g−1. To the best of our knowledge, only a few works have reported on such high specific capacitance for a porous carbon electrode in a TEABF4/AN electrolyte. Apart from ultrahigh specific capacitance and good rate capability, the NHPC−HA/ KOH-based symmetric supercapacitor also possessed a remarkable cycling stability (96.2% retention of the initial capacitance after 10 000 cycles at 10 A g−1, Figure 3f). We further tested the capacitive performance in symmetric supercapacitors using NHPC electrodes in an ionic liquid electrolyte (neat EMIMBF4) at an ambient temperature. Ionic liquids possess high safety, good thermal stability, and high operating voltage windows above 3 V.9 Normally, as the ionic liquids were used as the ambient-temperature electrolytes for supercapacitors based on the activated carbon electrodes, their high viscosities, large ion sizes, and low ionic conductivities would cause the high equivalent series resistance (ESR) and low power densities for supercapacitors.41,42 As shown in Figure 4a,b, NHPC−HA/KOH electrodes still exhibited the largest CV loop area at 200 mV g−1 and possessed the longest discharge time at 20 A g−1 in the neat EMIMBF4 electrolyte, indicating its best capacitive performance among all the NHPC electrodes. Furthermore, the CV curves of NHPC−HA/KOH electrodes maintained a quasi-rectangular shape even at the high scan rates (Figure 4c), indicating its good rate capability. The GCD curves of NHPC−HA, NHPC−KOH, and NHPC− HA/KOH electrodes are shown in Figures S3b1,b2 and 4d. NHPC−HA/KOH electrodes exhibited the smallest IR drop among all the NHPC electrodes (Figure S4b), implying the lowest internal resistance due to their open porosity and good electrical conductivity. The calculated specific capacitances of NHPC electrodes were shown in Figure 4e. Compared with the specific capacitances in organic electrolytes, NHPC−HA and NHPC−KOH electrodes could only deliver 136 and 160 F g−1 at 5 A g−1 in ionic liquid electrolytes, respectively. However, the NHPC−HA/KOH electrode could still show a high specific capacitance of 240 F g−1 at 5 A g−1. A capacitance of 140 F g−1 was still maintained even at the current density of 100 A g−1. The capacitive performance of the NHPC−HA/KOH electrode was superior to those of the reported porous carbon electrodes (Table S2). Moreover, the NHPC−HA/KOH electrode also showed a superior long cycling stability with a

Figure 5. Nyquist plots of NHPC electrodes in (a) 1.0 M TEABF4/ AN electrolyte and (b) neat EMIMBF4 electrolyte; the insets show an enlarged view of the high-frequency and middle-frequency regions. The normalized imaginary part capacitances vs frequency plots of NHPC electrodes in (c) 1.0 M TEABF4/AN electrolyte and (d) neat EMIMBF4 electrolyte.

organic electrolytes and ionic liquid electrolytes, respectively. In the low frequency regions, all of the electrodes showed nearly vertical lines because of the predominated EDLC chargestorage mechanism.48 In the high- and middle-frequency regions (the insets of Figure 5a,b), the intercepts for the curves represent the ESRs. The NHPC−HA/KOH electrode possessed the lowest ESR because of its good electrical conductivity and low contact resistance.49 More importantly, the interconnected meso- and macropores of the NHPC−HA/ 42802


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Figure 6. Ragone plots of NHPC−HA/KOH-based symmetric supercapacitors in comparison with state-of-the-art works in the electrolytes of (a) 1.0 M TEABF4/AN and (b) neat EMIMBF4. (c) Photograph of a small fan powered by one NHPC−HA/KOH-based symmetric supercapacitor in the electrolyte of 1.0 M TEABF4/AN. (d) Photograph of 33 LEDs powered by one NHPC−HA/KOH-based symmetric supercapacitor in the electrolyte of neat EMIMBF4.

capacitive performance, the assembled NHPC−HA/KOH electrode-based symmetric supercapacitors in the 1.0 M TEABF4/AN electrolyte could easily drive a small fan (Figure 6c) and successfully power 33 LEDs (Figure 6d), showing the promising practical applications.

KOH electrode effectively decrease the ESR resulted from the desolvation process for the organic electrolyte and high viscosity for the ion liquid electrolyte.26,40 Moreover, the NHPC−HA/KOH electrode exhibited the smallest semicircle in the middle-frequency region among all the NHPC electrodes, demonstrating its fastest charge transfer because of its open porosity and good electrical conductivity.49 The relaxation time constant τ0 for these three electrodes were further determined. Figure 5c,d shows the evolution of the normalized imaginary [C″(ω)] capacitances of the electrodes versus frequency in organic electrolytes and ionic liquid electrolytes, respectively. The relaxation time constants (τ0) for NHPC−HA, NHPC−KOH, and NHPC−HA/KOH electrodes were calculated to be 1.78, 1.47, and 0.38 s in organic electrolytes. It should be noted that the relaxation time constant for NHPC−HA/KOH in organic electrolytes was even better than the previously reported activated carbons in aqueous electrolytes (0.7−3.3 s).29 In addition, the NHPC− HA/KOH electrode also possessed the shortest τ0 (4.65 s) in the ionic liquid electrolyte, confirming that the interconnected meso- and macropore porosity and good electrical conductivity were beneficial for the superior capacitive performance of the NHPC−HA/KOH electrode. The Ragone plots of NHPC−HA/KOH electrode-based symmetric supercapacitors in organic and ionic liquid electrolytes are shown in Figure 6a,b, respectively. In the organic system, the supercapacitor based on NHPC−HA/KOH electrodes could deliver a high energy density of 48.6 W h kg−1 at the power density of 3.12 kW kg−1 and maintain 27.1 W h kg−1 even at the power density of 84.0 kW kg−1. In the ionic liquid electrolyte, the symmetric supercapacitor could exhibit a remarkable energy density of 75 W h kg−1 at 3.75 kW kg−1, and a high energy density of 31.3 W h kg−1 was maintained even at the power density of 63.5 kW kg−1. The capacitive performance of the NHPC−HA/KOH electrode-based symmetric supercapacitors are superior to the state-of-the-art works in recent years (Figure 6a,b and Tables S1, S2).50−66 Because of the good

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CONCLUSION In general, cattle bone-derived NHPC was successfully synthesized through the co-activation of HA and KOH. The resultant product possessed high defect density, good electrical conductivity, and numerous highly interconnected meso- and macropores with a high SSA. Because of these favorable features, the symmetric supercapacitors based on this carbon electrode showed outstanding capacitive performance with ultrahigh specific capacitances, high energy-power density, and superior long cycling performance in non-aqueous electrolytes. This work provides a facile and low-cost method to prepare the mesopore- and macropore-dominant hierarchically porous carbons and opens a new way to simultaneously increase energy and power densities for supercapacitors in non-aqueous electrolytes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14390. Other characterizations along with additional supporting data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.H.). *E-mail: [email protected] (F.W.). ORCID

Feng Wang: 0000-0002-7901-3693 42803


Research Article

ACS Applied Materials & Interfaces Author Contributions

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R.S. and J.N. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Funds of China (51432003). REFERENCES

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Mesopore- and Macropore-Dominant Nitrogen-Doped Hierarchically

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