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Meso- 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14390 • Publication Date (Web): 23 Nov 2017 Downloaded from http://pubs.acs.org on November 24, 2017

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ACS Applied Materials & Interfaces

Meso- and macropore-dominant nitrogen-doped hierarchically porous carbons for high energy and ultrafast supercapacitors in non-aqueous electrolytes Rong Shao,a,b,# Jin Niu,a,b,# Jingjing Liang,a,b Mengyue Liu,a,b Zhengping Zhang,a,b Meiling Dou,a,b Yaqin Huang,a,* Feng Wang a,b,* a

State Key Laboratory of Chemical Resource Engineering, Laboratory of Electrochemical

Process and Technology for materials, Beijing University of Chemical Technology, Beijing 100029, China b

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing

University of Chemical Technology, Beijing 100029, P. R. China #

These authors contributed equally to this work.

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

ACS Paragon Plus Environment

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energy densities in non-aqueous electrolytes via using a cattle bone-derived porous carbon as electrode material. Due to 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 meso- and macropore-dominant porosity with ultrahigh specific surface area (2203 m2 g-1) of meso- and macropore. 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 Wh kg-1 at 3.13 kW kg-1 in 1.0 M TEABF4/AN, and 75 Wh kg-1 at 3.75 kW kg-1 in neat EMIMBF4), as well as superior cycling performance (over 89 % of the initial capacitance after 10000 cycles at 10 A g-1).

KEYWORDS: biomass, co-activation, hierarchically porous carbons, supercapacitor, nonaqueous electrolytes INTRODUCTION Due to the fossil fuel depletion and the environmental pollution issues, developing the energy storage systems (ESSs) with low cost, high safety, high energy and power is urgently needed. 1-3 Supercapacitors have attracted great attentions due to the 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) for supercapacitors can be determined by = , where C is the capacitance and V is the working voltage.5 The capacitance can be classified into: 1) electrical double layer capacitance (EDLC), which is caused by the electrostatic charges adsorption at the interface of electrolyte and electrode, depending on the specific surface area (SSA) of the electrode that is


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accessible to the electrolyte ions.4,7 2) pseudocapacitance, which is contributed from the surface or near-surface fast reversible faradic processes.6 To enlarge the 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, nonaqueous 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 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,

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In the

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 favourable to the usage of organic and ionic liquid electrolytes. Also, doping nitrogen atoms onto the carbon frameworks not only can increase the pseudocapacitance through enhancing the diffusion of negative charges on the electrode surfaces,16 but also can lead to a clear increase in defect density of carbon 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 only can generate many micropores which are ion-inaccessible combined with a small amount of meso- and macropores. Although meso- and macropore dominant porous carbons can be prepared using templating


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methods, their relatively low SSA would cause low specific capacitances.19 Furthermore, the templating methods are limited by the complicated and high-cost 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 necessary to find an efficient and low-cost method for the direct synthesis of NHPC with large SSA of meso- and macropore that can be used as high performance electrode for supercapacitor in non-aqueous electrolyte. As one kind of abundant biomass wastes, animal bone possesses a high output more than 20 million tons per year in China.22 Animal bone is one kind of natural organic/inorganic composite, which is composed of collagen and hydroxyapatite (HA, Cax(PO4,CO3)y(OH)).23 As 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 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 animal bone.26 Based on our previous work, we prepared a unique NHPC with 3D-interconnected meso- and macroporedominant porosity by co-activation of HA and KOH using cattle bone as precursor (the resultant product was referred to as NHPC-HA/KOH). The resultant NHPC-HA/KOH possessed good electrical conductivity, high defect density and ultrahigh SSA of meso- and macropore. Due to the unique porosity, good electrical conductivity and numerous effective storage sites, the symmetric supercapacitor based on NHPC-HA/KOH electrodes showed integrated high energypower density, and superior long cycle performance in organic electrolyte (1.0 M TEABF4/AN) and ambient-temperature ionic liquid electrolyte (neat EMIMBF4).


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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 pre-carbonized product and KOH was 1:0.3. Subsequently, the mixture was heat-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 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 firstly precarbonized at 400 °C for 3 h (heating rate: 2.5 °C min-1), then the precarbonized powder was washed by 1.0 M HCl and ultrapure water to remove the HA. The dried product was well mixed with KOH. The weight ratio of pre-carbonized product and KOH was 1:0.3 (based on the weight of pre-carbonized 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, and followed by drying in an oven at 80 °C for 24 h. Material Characterization. The morphologies and microstructures of NHPCs were performed on JSM-6701F scanning electron microscopy (SEM) and JSM-2100 high resolution transmission electron microscopy (HR-TEM). Pore structures of NHPCs were characterized by N2 adsorption isotherms using a Micromeritics ASAP 2460. The SSA was determined by BET method, DFT method was used to calculate the pore size distribution (PSD) was. The SSA and pore volume for


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micropore were determined by t-plot method. X-ray diffraction (XRD) was performed on Rigaku RINT 2200V/PC. X-ray photoelectron spectroscopy (XPS) spectra were collected on ESCALAB 250. Raman spectra were collected on LabRam HR800. Nanometrics HL5550 cryostat based on four-probe technique was used to test the electrical conductivity. Electrochemical measurements. The electrochemical measurements were carried out at ambient temperature in CR2032 coin-type cell. To prepare the working electrode, active material (80 wt%), acetylene black (10 wt%) and PVDF (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 active material was 1.5-2.0 mg cm-2. 1.0 M TEABF4/AN 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, respectively. The glass-fibre 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 (F g-1) is determined based on the equation: ∆

= ∆

(1)

where ∆ is the discharge time (s), is the discharge current (A), is the active material mass (g) on one working electrode, and ∆ is the potential change (V) excluding the IR drop. The energy density E (Wh kg-1) and power density P (W kg-1) are determined by the following equations:27, 28

= ××. ∆ =

(2)

(3)

∆


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The complex form of capacitance (ω) is dependent on the real part of the capacitance ′(ω) and the imaginary part of the capacitance "(ω), which is defined as follows:29 (ω) = (ω) − (ω)

(4)

!! (ω)

(ω) = ω| (ω) =

(5)

(ω)|# ! (ω)

(6)

ω| (ω)|#

where $ (ω) is the real imaginary component and $ʺ(ω) is the imaginary component of the complex impedance, respectively. the angular frequency ω is determined by & = 2(). τ0 is the relaxation time constant which is calculated by * = 1/) . 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 of NHPC-HA after direct carbonization and self-activation of cattle bone, which 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 and a3. Moreover, when HA was firstly removed and KOH was used as the activator, the fiber bundle-like structure could not be maintained, leading to the irregular bulk structure of NHPC-KOH (Figure 1b1). The high-resolution SEM image (Figure 1b2) and TEM image (Figure 1b3) showed that the individual KOH activation generated numerous mesopores with 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 (Figure 1c1-c3, Figure S1). It should be noted that the pores formed by HA-induced activation at high temperature, might supply many channels for the


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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 adsorptiondesorption measurements and density functional theory (DFT) calculations for the corresponding pore size distributions. 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 was existed at the high relative pressure above 0.7, indicating that the presence of macroporous structure in NHPC-HA/KOH. The pore size distribution (PSD) plots confirmed the existence of hierarchically porous structures for all the NHPC samples (Figure 2b). NHPC-HA and NHPC-KOH possessed numerous microand mesopores with pore size below 8 nm, while 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 then were shown in Table 1. All the samples possessed 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, while 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


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works using template and activation methods (Table 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 meso- 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 centred 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 fault, and I peak is attributed to the doped nitrogen.35 The relative content of the five peaks for NHPC samples was calculated from 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 the 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


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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 similar content of surface N atoms. Among them, NHPC-HA/KOH possessed the relatively low content of surface O atoms. The C1s spectra (Figure 2g) of all the NHPC samples could be deconvoluted into four peaks, which were centred 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 existed 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 charge-storage sites, while N-Q could enhance the electron transfer.38, 39 The electrical conductivities of NHPC-HA, 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 due to 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 (Table S1 and S2). To obtain high energy densities of the symmetric supercapacitors, 1.0 M TEABF4/AN was firstly 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 and NHPC-HA/KOH electrodes at the 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 NHPC-HA/KOH electrode exhibited the largest area among all the


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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 Figure S2a1, a2 and Figure 3c, respectively. NHPC-HA/KOH electrode kept 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 NHPC-HA/KOH electrode at different current densities indicated the reversible EDLC behavior and high charge-discharge efficiency. It should be noted that NHPC-HA/KOH electrode showed small IR drop even at the high current density of 150 A g-1 (Figure S4a), indicating its superior rate capability. Based on the results of GCD curves (Figure S2b1, b2 and Figure 3d), the specific capacitances of NHPCHA, NHPC-KOH and NHPC-HA/KOH electrodes were calculated according to Equation 1. As shown in Figure 3e, all the NHPC electrodes showed high specific capacitances due to 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 little 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 been reported on such high specific capacitance for porous carbon electrode in 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 10000 cycles at 10 A g-1, Figure 3f).


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We further tested the capacitive performance in symmetric supercapacitors using NHPC electrodes in an ionic liquid electrolyte (neat EMIMBF4) at 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,

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As shown in Figure 4a and b, NHPC-HA/KOH

electrode 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 electrode maintained quasi-rectangular 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 were shown in Figure S3b1, b2 and Figure 4d. NHPC-HA/KOH electrode exhibited the smallest IR drop among all the NHPC electrodes (Figure S4b), implying the lowest internal resistance due to its 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 electrolyte, NHPC-HA and NHPC-KOH electrodes could only deliver 136 and 160 F g-1 at 5 A g-1 in ionic liquid electrolyte, respectively. However, 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 NHPC-HA/KOH electrode was superior to those of the reported porous carbon electrodes (Table S2). Moreover, NHPC-HA/KOH electrode also showed a superior long cycling stability with a capacitance retention of 89.2 % at 10 A g-1 after 10000 cycles (Figure 4f).


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It has been calculated that the porous electrodes with pore sizes around 0.7 nm could show 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 NHPCHA/KOH electrode with 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 macropore might not exhibit maximum surface area normalized double-layer capacitance, the large efficient SSA supplied by meso- and macropores still leaded to the largest specific capacitance of NHPCHA/KOH electrode among all the NHPC electrodes. EIS measurements were conducted to investigate the reason why NHPC-HA/KOH electrode possessed the best capacitive performance among all the NHPC electrodes. Figure 5a and Figure 5b show the Nyquist plots for the NHPC electrodes in organic electrolyte and ionic liquid electrolyte, respectively. In the low frequency regions, all of the electrodes showed nearly vertical lines due to the predominated EDLC charge-storage mechanism.48 In the high and middle frequency regions (the insets of Figure 5a and Figure 5b), the intercepts for the curves represent the ESRs. NHPC-HA/KOH electrode possessed the lowest ESR due to its good electrical conductivity and low contact resistance.49 More importantly, the interconnected mesoand macropores of NHPC-HA/KOH electrode effectively decrease the ESR resulted from the desolvation process for organic electrolyte and high viscosity for ion liquid electrolyte.26,

40

Moreover, NHPC-HA/KOH electrode exhibited the smallest semicircle in middle frequency


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region among all the NHPC electrodes, demonstrating its fastest charge transfer due to its open porosity and good electrical conductivity.49 The relaxation time constant τ0 for these three electrodes were further determined. Figure 5c and Figure 5d show the evolution of the normalized imaginary [C′′(ω)] capacitances of the electrodes vs. frequency in organic electrolyte and ionic liquid electrolytes, respectively. The relaxation time constant (τ0) for NHPC-HA, NHPC-KOH and NHPC-HA/KOH electrodes were calculated to be 1.78 s, 1.47 s and 0.38 s in organic electrolyte. It should be noted that the relaxation time constant for NHPC-HA/KOH in organic electrolyte was even better than the previously reported activated carbons in aqueous electrolytes (0.7-3.3 s).29 In addition, NHPC-HA/KOH electrode also possessed the shortest τ0 (4.65 s) in ionic liquid electrolyte, confirming that the interconnected meso- and macropore porosity and good electrical conductivity were beneficial for the superior capacitive performance of 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 and Figure 6b, respectively. In the organic system, the supercapacitor based on NHPC-HA/KOH electrodes could deliver a high energy density of 48.6 Wh kg-1 at the power density of 3.12 kW kg-1, and maintain 27.1 Wh 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 Wh kg-1 at 3.75 kW kg-1 and a high energy density of 31.3 Wh 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, 6b and Table S1, S2).50-66 Due to the good capacitive performance, the assembled NHPC-HA/KOH electrode-based symmetric supercapacitors in 1.0 M TEABF4/AN electrolyte could easily drive a small fan


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(Figure 6c) and successfully power 33 LEDs (Figure 6d), showing the promising practical applications. CONCLUSION In general, cattle bone-derived nitrogen-doped hierarchically porous carbon 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 high SSA. Due to 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 meso- and macropore-dominant hierarchically porous carbons, and opens a new way to simultaneously increase energy and power densities for supercapacitors in non-aqueous electrolytes. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Other characterizations along with additional supporting data (PDF) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by National Natural Science Funds of China (51432003). REFERENCES (1)

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


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Figure 2. (a) N2 adsorption-desorption isotherms, (b) pore size distribution 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, (h) N 1s spectra of NHPCs.


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Figure 3. Electrochemical performance of NHPC electrodes in 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 of at different current densities. (f) Cycling stability of NHPC-HA/KOH at the current density of 10 A g-1 over 10000 cycles.


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Figure 4. Electrochemical performance of NHPC electrodes in neat EMI MBF4 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 NHPCHA/KOH at different current densities. (f) Cycling stability of NHPC-HA/KOH at the current density of 10 A g-1 over 10000 cycles.


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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 middlefrequency 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.


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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 electrolyte 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 NHPCHA/KOH based symmetric supercapacitor in the electrolyte of neat EMIMBF4.


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Table 1. Porous structure parameters and XPS results of NHPCs. Vt c

SBET a

Smeso/macrob

(m2 g-1)

(m2 g-1)

(cm3 g-1)

(cm3 g-1)

NHPC-HA

2111

801

1.38

0.74

NHPC-KOH

3236

1027

1.93

NHPCHA/KOH

2476

2203

3.14

Sample

a

% of total N1s

Vmeso/macrod

O %

N %

83.2

13.0

0.96

84.2

3.00

88.0

C%

% of total C1s

N-6

N-5

NQ

C=C

C-C

C-N,CO

C=O

2.9

6.5

13.2

80.3

12.6

45.8

12.6

29.0

12.5

2.5

13.4

30.5

56.2

11.5

50.0

12.1

26.4

9.3

2.3

7.3

36.2

56.5

10.2

48.5

11.4

29.9

Total specific surface area; b The specific surface area of meso- and macropore; c Total pore

volume; d The pore volume of meso- and macropore.


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