Nitrogen-Doped Porous Multi-Nano-Channel Nanocarbons for Use in

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Nitrogen-doped porous multi-nano-channel nanocarbons for use in high performance supercapacitor applications Prakash Ramakrishnan, and Sangaraju Shanmugam ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00289 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 12, 2016

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ACS Sustainable Chemistry & Engineering

Nitrogen-doped porous multi-nano-channel nanocarbons for use in high performance supercapacitor applications Prakash Ramakrishnan Sangaraju Shanmugam* Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), 333, Techno Jungang-daero, Hyeonpung-myeon, Dalseong-gun Daegu, 42988, Republic of Korea

1 Environment ACS Paragon Plus


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ABSTRACT: Herein, we report a simple strategy for the rational design of a three-dimensional carbon material, in situ nitrogen-doped porous multi-nano-channel carbon nanorods (N-MCNR), using immiscible polymers blends. A series of N-MCNR with discrete and well-connected continuous nano-channels ranging from 18 to 75 nm in size was developed. The N-MCNR developed in this work represents the well controllability of nano-pores and nano-channels integrated at the nanoscale level. Three-dimensional N-MCNR nanostructured materials have been recommended as a promising electrode material for use in high-performance supercapacitors (SCs). A prototypical pouch-type symmetric SC was assembled and operated under practical application conditions. The N-MCNR based symmetric SC device fabricated in this study delivered a maximum specific capacitance of 335 Fg-1at 0.25 Ag-1 with corresponding energy density of 11.2 Wh kg-1 , and also exhibited an outstanding long-term cycle stability of 50,000 cycles, with 92.6 % charge retention.

KEYWORDS: Electrospinning, Multi-channel-carbon, Nanocarbon, Nitrogen-doped carbon, Supercapacitor


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INTRODUCTION Supercapacitors (SCs) are electrochemical storage devices that have gained much attention in recent years due to their high power density (>103-105 W kg-1), long life cycles (>105 cycles), rapid charge and discharge times (>5 min), and higher reliability than batteries, albeit with low energy density (up to 10 W h kg-1).1–5 These features suggest that SCs may serve as suitable power sources for applications requiring high charge-discharge rates and limited energy density, such as power and backup applications, consumer electronics, emergency doors and solar panels.6–9 Most SC manufactures use activated carbon as the electrode material. Carboncarbon symmetric SCs are favourable because of their fast charge storage and discharge; moreover, they are inexpensive and exhibit good electrical conductivity and high surface area. The charge storage mechanism of these SCs fundamentally depends on electric double layer (EDL) formation at the electrode/electrolyte interface, thus resulting in low energy density.10–13 Therefore, considerable effort has been devoted to improving the energy density of SCs. In EDLbased SCs with an aqueous electrolyte system, the operating voltage is restricted to the watersplitting potential (~1.2 V); thus, much attention has been devoted to determining how to enhance the specific capacitance (Csp) of the electrode material.14–18 In contrast to carbon materials, pseudocapacitive materials such as metal oxides and conducting polymers use faradic process to achieve high capacitances in excess of 1000 Fg-1, corresponding to an energy density of > 50 Wh Kg-1.19 However, poor cycling stability and low power density hamper the development of pseudocapacitive SC materials

20

. Furthermore, the

strategy of forming surface functional groups on carbon, particularly groups containing nitrogen (N) and oxygen (O), allows for quick faradic reactions. In fact, the strategy has been shown to improve energy storage performance to a small extent.21–26 The polarization of surface functional



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groups would indeed enhance the wettability of porous carbon materials in aqueous media and reduce the necessity of high surface area or high porosity of electrode materials.23,27,28 However, the instabilities of these surface functional groups deteriorate during long-term operation of SCs. Additionally, EDLC devices are generally limited to surface adsorption and surface redox reactions that occur near the inner Helmholtz region, respectively.29 Hence, the utilization of the entire electrode material requires a wide range of pore sizes, such as those associated with micro-, meso- and macro-pores, and carbon wettability toward the polarizing functional groups such as oxygen and nitrogen groups. Herein, we developed highly nitrogen-doped porous multi-nano-channel carbon nanorods (N-MCNR) as the electrode material for carboncarbon based SC device. We developed various multi-nano-channels inside the carbon rods with a high concentration of N-functionalities by simply varying the soft template concentration, using a conventional electrospinning approach and subsequent carbonization process. Because the formation of the nanostructures evolved as a one-step carbonization process, our method does not involve any hazardous chemical leaching process and was very simple, easily controlled and scalable. Thus, our approach highlights the fact that various controlled multi-nano-channel nanocarbons can be designed by an in situ Ndoping method, and tailoring the nano-channel diameter increases the ratio of the surface area to bulk volume, which affects the surface properties of the structures. Furthermore, this rationally designed material could provide enhanced contact between the active material and the electrolyte, allowing for efficient ion transport. Although, several reports have discussed the development of nano-channels using poly(methyl methacrylate) as a soft template, but, the obtained nanochannels were above 50 to 150 nm which supress the decrement in channel numbers in nanocarbons and eventually lower the surface area of nanocarbons.30–35 In our approach,


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accountability of channel size, nano-channel numbers, and continues nano-channels were controlled for the advancement in multi-nano channel nanocarbons for energy applications. Because of its well-controlled nano-architecture, the N-MCNR material provides a high power rate (up to 35 Ag-1), high power density (12.9 kW kg-1), and long-term cycle stability (50,000 cycles). Notably, the post-mortem analysis of cycled SC electrodes was conducted by X-ray photo-electron spectroscopy (XPS) to determine the carbon functionalities’ potential for long term use. Moreover, the nanocarbons developed using this approach could also be generalized to other energy storage applications. EXPERIMENTAL Fabrication of porous multi-nano-channel nitrogen doped nano-carbons. Polyacrylonitrile, PAN (Sigma-Aldrich, Mw= 150,000 g mol-1) of 0.8 g (8 wt%) and Polystyrene, PS (Sigma-Aldrich, Mw= 150,000 g mol-1) of 0.4g (4 wt%) were blended together in 10 mL of N,N-dimethylformamide (DMF) and stirred at 80oC. The solution was allowed to stir until a clear homogeneous solution was observed. The prepared solution was fed into 10 mL syringe of simple electrospinning setup and allowed to electrospun for 10 h. The following electrospinning parameters were followed: the flow rate of 1.0 ml h-1, the collector drum to the syringe needle was set at 10 cm, a collector drum rotating speed of 300 rpm and a high voltage power supply of 13 kV under humidity < 30% RH at

15-20 oC. The obtained product were

subject to stabilization at 250 oC for 2 h at a slow heating rate of 2oC min -1 under air atmosphere and followed by carbonization at 800 oC for 1h at a rapid heating rate of 5 oC min-1 under argon atmosphere. The product yield of ~25% was obtained after carbonization. The obtained product was ground using mortar and pestle. Thereby prepared nitrogen doped porous multi-nano-



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channel carbon nanorods and labelled as N-MCNR1. In order to optimize the morphology conditions, the above electrospinning and carbonization procedures were repeated with varied PS amount 0.5, 0.6 and 0.7 g, in the preparation of precursor solution thus, obtained nitrogen doped porous multi-nano-channel carbon nanorods were denoted as N-MCNR2, N-MCNR3 and NMCNR4. The digital images of polyacrylonitrile (PAN) and polystyrene (PS) at different concentration shows no phase separation observation for PAN and PS polymer solution after mixing, but they were immiscible together (Figure S1). For comparison, carbon nanostructure without multi-nano-channel pores was fabricated using similar synthesis protocol as followed for N-MCNR material but without the soft template, PS polymer, inclusion and the obtained product was labelled as carbon nano rod, CNR. Materials characterization. The morphologies of the samples were characterized using field emission scanning electron microscope (S4800 FE-SEM) and high resolution transmission electron microscope (HF 3600 HR-TEM). The high resolution X-ray diffraction (HR-XRD) and Raman spectroscopy (with source wavelength of 514 nm) were used to understand the graphitic phase of carbon nature. The elemental analyzer (EA, Vario MICRI cube) used for quantitative analysis of heteroatom functionalities. The X-ray photo-electron spectroscopy (XPS, Thermo scientific ESCALAB 250Xi) performed to unveil the chemical nature of heteroatom species. The Bruner– Emmett–Teller (BET, Micromeritics ASAP 2020) were used to understand the surface area, and micropore texture information such pore size distribution, PSD (using non-local density functional theory, PSDNL-DFT; Barrett-Joyner-Halenda, PSDBJH) and micro texture information (using t-plot method, Harkins and Jura formula). All the samples were degassed at 180 oC overnight in prior to analysis.


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Electrochemical measurements. The electrochemical measurements, using biologic bi-potentiostat VSP-modular 2 channels, was performed in both three and two electrode system cell containing 1M H2SO4 aqueous solution. The platinum wire and Ag/AgCl were used as the counter and reference electrode, respectively. The active material, N-MCNR, of 2.5 mg was dispersed in 150 µl of isopropyl alcohol and 2 µl Nafion (5%) using an ultrasonic bath for 30 min and a constant mass loading of ~0.5 mg, on glassy carbon working electrode (0.0760 cm2), was kept constant for all the samples. Prior to all the electrochemical studies, the electrolyte solution was de-aerated in N2 for 30 min. The specific capacitance was calculated from cyclic voltammetry (CV) and galvanostatic charge/dis-charge (GCD) studies using the following equation :36,37 For CV studies:

Csp =

(1)

For GCD studies:

Csp=

)

(

(2)

Where, Csp (F g-1) is specific capacitance; I (mA) the discharge current; m (g) mass of the active material; (mVs-1) is the scan rate; dV/dt is the slope of the discharge curve (V s-1).



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Fabrication and evaluation of supercapacitor devices. Based on preliminary three electrode test, only the best performed material (N-MCNR2) was opted to construct the laminated symmetric SCs device, in which both the negative and positive electrode materials were same. Both the electrodes were separated by Whatman glass fiber filter paper (70 mm diameter). The working electrode slurry was prepared by 80% of active material, N-MCNR and 20% of binder, polyvinylidene difluoride with N-methylpyrrolidone (NMP) as a solvent and dried overnight at 80 oC. The prepared slurry was coated on stainless steel (SS) current collector foil (20 µm thickness). The mass loading of ~4.5 mg of active material was achieved in this process. The electrode mass measurement was carried out in high precision weighing balance accuracy up to four decimal points. Notably, no conductive filler was used in electrode preparation. However, for better contact between SS foil and active material, conductive adhesive paste made of acetylene black of 20 µm thickness was coated as follows in industrial standard. The total cell capacitance (Ccell), specific gravimetric energy density (E.D, W h kg-1) and power density (P.D, W kg-1) were calculated according to the following equation (3), (4) and (5), respectively: 36,38,39

Ccell = 4

E. D=

)

(

Ccell (V-Vdrop)2

P. D = ∆

(3)

(4)

(5)

Where, Factor 4 is used to compensate the mass of a single electrode and capacitance of both electrodes; M(g) mass of the both electrodes (~4.5 mg), V and Vdrop is the maximum applied


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potential (1V) and IR drop potential. ∆T (h) is the potential decline during the discharge with excluding IR drop. RESULTS AND DISCUSSION Physicochemical characterizations. N-MCNR samples were synthesized by a simple two-step process, conventional electrospinning process followed by a carbonization step, as illustrated in Figure 1.

Figure 1. Pictorial representation of synthesis and formation mechanism of N-MCNR samples, respectively. The FE-SEM images of all the carbonized N-MCNR samples confirm the formation of multi-nano-channel porous fibrous structures (Figure 2(a-h) and Figure S2(a-d)). The average diameters of N-MCNR1, N-MCNR2, N-MCNR3 and N-MCNR4 were observed to be 415, 430, 650 and 670 nm, respectively, as shown in Figure S2(e-h). Interestingly, all the N-MCNR samples possessed cross-sectional pores with different pore diameters. To understand the extension of the cross-sectional pores in the N-MCNR samples, HR-TEM analysis was performed; the resulting images are were shown in Figure 3(a-h).



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Figure 2. FESEM images of N-MCNR samples. (a, b) N-MCNR1; (c, d) N-MCNR2; (e, f) NMCNR3; (g, h) N-MCNR4. The HR-TEM images of N-MCNR1 (Figure 3(a, b) and Figure S3a) shows discrete nano-channels with diameters of 20-22 nm. The images of N-MCNR2 in Figure 3(c, d) and Figure S3b; N-MCNR3 in Figure 3(e-f) and Figure S3c; N-MCNR4 in Figure 3(g-h) and Figure S3d display continuous nano-channels with diameters mostly within the ranges of 18-22, 40-45 and 55-75 nm, respectively. It was also noted that the number of nano-channels observed for N-MCNR2 (> ~6) was greater than that observed for other samples; this discrepancy was attributed to the fact that increasing the concentration of PS decreases the number of nanochannel numbers and widens the nano-channel diameter. All the N-MCNR samples showed similar length range from 3 to 9 µm, which reasonably account to the same grinding condition of


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carbonized N-MCNR membranes. In addition, CNR sample fabricated without using soft template (PS) displayed rod like morphology without multi-nano-channel pores, as shown in Figure S4 (a, b).

Figure 3. HR-TEM images of N-MCNR samples. (a, b) N-MCNR1; (c, d) N-MCNR2; (e, f) N-MCNR3; (g, h) N-MCNR4. Remarkably, morphology analyses suggest that the fibre diameter and multi-nanochannels could be tailored in all the samples by simply varying the precursor condition. The mechanism of nano-channel formation in the N-MCNR was attributed to the immiscibility of the polymer blends, resulting from the phase separation between PS and PAN, likely due to the mixing entropy phenomenon. Once sufficient phase separation was thermodynamically set between the polymers PAN and PS, one polymer was dispersed into the matrix of the other.40 Thus, during the carbonization process, the complete decomposition of the thermoplastic nature of PS occurs, which confirmed by thermo gravimetric analysis (TGA) analysis (Figure S5a), and hence internal pores can be created; simultaneously, the thermosetting nature of the organic backbone of PAN was retained, which results in a mostly carbon composition. Moreover, the formation of nano-channels inside the carbon rod was likely due to H-bonding (physically)



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between) the two immiscible polymers (PAN and PS), as shown in Figure 1, thus generates separate phase region, which later form nanotube during carbonization process. The results of Xray diffraction (XRD) and Raman spectroscopy characterization reveal the existence of disordered carbon frameworks and a low degree of graphitization (Figure S5 b, c). The N2 adsorption-desorption isotherms of all N-MCNR samples were shown in Figure S6a, and the results obtained were tabulated in Table S1. The isotherm plots exhibit typical type I and IV behaviour for all samples, indicating the co-existence of micro- and meso-pore structures. The specific surface areas for the N-MCNR1, N-MCNR2, N-MCNR3 and N-MCNR4 samples were determined to be 419, 840, 642 and 577 m2 g-1, respectively. Although, the NMCNR1 and N-MCNR2 material exhibit similar diameter, however, N-MCNR1 sample possess voids or discrete channels inside the carbon nanostructure as observed in FE-SEM image (Figure. S3a) and HR-TEM image (Figure 3b), which were not accessible for N2 adsorption and desorption analysis, as shown in Figure S6a. On the other hand, N-MCNR2 sample possess continuous channel inside the carbon nanostructure, which were accessible for N2 adsorption and desorption analysis, as shown in Figure S3b and Figure 3d. Hence, the poor surface area of NMCNR1 accounts mostly from the exterior volume. Information regarding PSDBJH indicates the presence of a broad range of meso-pores in all the samples (Figure 4a). Specifically, the meso-pore size distribution was observed to shift to larger sizes from sample N-MCNR1 to N-MCNR4 owing to the effect of the soft template concentration. In particular, samples N-MCNR1 and N-MCNR2 exhibited narrow meso-pores measuring 3.42 and 3.79 nm, respectively, and wide meso-pores (attributed to nano-channels) measuring 18.51 and 21.12 nm, respectively, whereas samples N-MCNR3 and N-MCNR4 samples solely exhibited wide pores spanning the meso- to macro-pore size ranges (10 to 70 nm ).


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The above mentioned results were in good agreement with the results of HR-TEM analysis for the observation of nano-channels. Interestingly, N-MCNR1 and N-MCNR2 sample displayed narrow mesopores compared to the other samples which could be attributed to the existence of with low phase separation possess small (~3.8 nm) and large (~18 - 22 nm) voids in mesopore range. In the case of N-MCNF3 and N-MCNF4, phase separation occurs in large voids (30 to 75 nm) due to high phase separation domains between polymers before carbonization. The mesopores observed were substantially larger than the electrolyte ions that are favourable for ion pathway and charge storage, which results in high capacitance.41 As the BJH studies were limited to examining pores measuring 1.5 nm, to obtain micro-pore textural information, so the T-plot and NL-DFT methods were carried out for all samples, and the obtained results were tabulated in Table S1. The proportions of the pore area occupied by micro-pores in samples N-MCNR1, NMCNR2, N-MCNR3 and N-MCNR4 were determined to be 77, 75, 77 and 80%, respectively. The PSDNL-DFT analysis (Figure S6b) suggests behaviour dominated by micro-pores with diameters of 0.58, 0.64, 059 and 0.71 nm for N-MCNR1, N-MCNR2, N-MCNR3 and NMCNR4, respectively. Such sub-nanometre pores, > 1 nm, would favour nano-confinement of electrolyte ions and facilitate to minimize the relaxation time constant (τo), thereby enhancing both the energy and power characteristics of SCs.41,42 Hence, among the different N-MCNR samples fabricated in the study, those with the highest surface area and substantial numbers of sub-nanometre micro- and meso-pores would be the most suitable for high-energy, high-power SC applications. Quantitative elemental analyses were carried out for all N-MCNR samples, and the obtained C, N and H compositions were observed to be similar (Table S1). In particular, the N



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contents of N-MCNR1, N-MCNR2, N-MCNR3 and N-MCNR4 were observed to be nearly equal to one another (9.56, 9.55, 10.18, and 9.7 wt%, respectively).

Figure 4. BJH pore size (PBJH) distribution of all the samples; (b) High resolution XPS N1s spectra of N-MCNR2; Schematic representation of Pyridinic-N (c) and Pyrrolic-N (d); (e) Schematic model of nitrogen- and oxygen functionalities on carbon network based on XPS studies. To understand the chemical bonding nature of nitrogen and oxygen species over the carbon framework, XPS analysis was carried out exclusively for a high-surface-area sample (NMCNR2). The XPS signals show three major peaks related to the C1s, N1s and O1s spectra, and quantitative analysis shows that N-MCNR2 was composed of 85.27 wt% C, 8.81 wt% of N, and 5.85 wt% O. The deconvoluted C1s and O1s spectra were shown in Figure S7 a, b. Figure 4b shows the deconvoluted peaks of the N1s spectrum; the peak values were tabulated in Tables S2


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and S3. The N1s spectrum was deconvoluted into four major peaks, and their corresponding quantitative details were tabulated in Table S4. The N1 peak at a B.E. of 398.0 eV corresponds to pyridinic N groups bonded to two carbon atoms at the edge of the sp2 carbon framework, and the N3 peak at a B.E. of 400.5 eV was attributed to pyrrolic N groups bonded to five-membered sp2 carbons.23,43 As illustrated in Figure 4c, pyridinic-N has one lone pair of electrons that was not involved in the aromatic π carbon framework, and it can interact easily with protons or oxygen molecules and hence behaves as a basic site.44 In contrast, as shown in Figure 4d, the pyrrolic-N group contributes a lone pair of p-electrons to the aromatic π carbon framework and thus restricts the basic nature of the corresponding site for protonation. However, the sharing of lone-pair electrons with the aromatic ring generates a partial negative charge due to resonance, and thus, the interaction of protons or hydronium ions could be possible in the carbon framework to destabilize the resonance in the carbon network. The role of these nitrogen functionalities in enhancing capacitance has been theorized by Frackowiak et al. using density functional theory and by Yu et al. using the Hartree-Fock method and Koopmans theory.45,46 The peak positions of N2 and N4 at 399.8 and 401.3 eV suggest that the peaks were associated with pyrrolidonic and graphitic N species, respectively. The presence of graphitic N (16.3%) could potentially reduce the energy gap differences between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the carbon framework,46 thus resulting in better conductivity. To verify this reasoning, the electrical conductivity was measured using a fourprobe technique, and the conductivity of N-MCNR2 was observed to be 3.5 S cm-1. Furthermore, the presence of N functionalities in N-MCNR2 were highly favourable for the formation of acidbase conjugates, thus encouraging the use of an acid electrolyte rather than an alkaline or neutral electrolyte.47–49 In addition, the presence of appreciable oxygen surface functionalities plays an



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important role in determining electrolyte wettability. Figure 4e shows the schematic representation of all possible functional groups in the carbon framework based on the results of XPS analysis. Electrochemical evaluation of N-MCNR sample. Preliminary electrochemical performance measurements were first conducted using a three-electrode system to understand the effects of morphology on charge storage ability for all the N-MCNR samples. The CV curves for all the samples performed at 2 mVs-1 were shown in Figure 5a.

Figure 5. Three-electrode system electrochemical performance: All the N-MCNR samples in 1 M H2SO4: (a) CV curves at a 2 mV s-1 scan rate, (b) Effect of scan rates on Csp of all N-MCNR samples, (c) GCD profiles at a 0.25 A g-1 current density, and (d) Effect of current densities on Csp of all N-MCNR samples.


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The slopes of the CV curves suggest partial pseudocapacitance behaviour, especially in the cathodic region, which accounts for the faradic reaction between the protic ion (H3O+) and the surface functionalities (pyridinic N and pyrrolic N) in the N-MCNR, as indicated by the XPS N1s spectra. The faradic reactions responsible for the pseudocapacitance were shown below: 44,50

(6)

(7)

It can be observed that the N-MCNR2 sample exhibited a high specific capacitance of 456 Fg-1 at a scan rate of 2 mVs-1 in 1 M H2SO4, whereas samples N-MCNR1, N-MCNR3 and N-MCNR4 samples showed Csp values of 248, 352 and 323 Fg-1, respectively. Notably, in Figure 5a, the redox peak for N-MCNR2 at 0.2-0.4V was more prominent than N-MCNR1, NMCNR3, and N-MCNR4 samples due to the enhancement of nitrogen functionalities redox reaction (according to equation 6 and 7), which was facilitated by high surface area nature of NMCNF2. Also, due to the effect of carbon morphologies, N-MCNR1, N-MCNR2, N-MCNR3 and N-MCNR4 showed different rate capabilities of 66, 80, 60 and 58%, respectively. Thus NMCNR2 with higher rate capability indicates good power performance, as shown in Figure 5b. Additionally, GCD measurements were performed to validate the CV results, as shown in Figure 5c and S8(e-h). N-MCNR2 showed a high Csp value of 461 Fg-1 at a current density of 0.25 Ag-1,



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whereas N-MCNR1, N-MCNR3 and N-MCNR4 exhibited Csp values 255, 370 and 312.2 Fg-1, respectively. Symmetric SC device. Based on preliminary studies, N-MCNR2 electrode material in 1 M H2SO4 was desirable for high-performance SC applications. Hence, the N-MCNR2 symmetric pouch SC cell in 1 M H2SO4 was constructed and labelled as N-MCNR2-Cell. The CV curve (Figure 6a) of N-MCNR2-Cell displayed a quasi-rectangular shape indicating the existence of both faradic and non-faradic mechanisms for charge storage, as explained in equation (6, 7). Furthermore, the curve shows a maximum Csp value of 332 Fg-1 at 2 mVs-1 and the corresponding volumetric capacitance was 61.2 Fcm-3 based on the cell volume (2.5 cm of length x 2.5 cm of height x 40 µm active material thickness). To understand the ability of ions to adsorb onto the electrode surface, the CV scan rate was increased to 100 mV s-1; the obtained CV curves were observed to retain the original shape, suggesting high reversibility. The Csp value was plotted as a function of the scan rate in Figure 6b, demonstrating good rate capability at high scan rates. The GCD profiles at different current densities (0.25 to 5 Ag-1) obtained for N-MCNR2-Cell exhibits near symmetric charge- and discharge profiles (Figure 6c), and also shows a maximum Csp of 335 Fg-1 at a 0.25 Ag-1 current density which was higher than that reported for N-doped carbon materials (Table S4). To understand the rate capability the GCD studies were extended to a maximum current density of 35 Ag-1 as shown in Figure S9a. The GCD profiles retained nearly symmetric triangular shapes up to a current density of 20 Ag-1. As the current density was increased to 35 Ag-1, the N-MCNR2-Cell could preserve a perfect triangular shape, suggesting a high rate


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capability. To gain a better understanding of the cell’s behaviour, ESR was calculated (Figure S9b); the ESR values calculated by correlating the linear slope linear slope between the IR drop voltages at zero discharge time w.r.t different discharge current densities for N-MCNR2-Cell. It was worth to mention, for practical applications, the coulombic efficiency was highly warranted and critical as well. The coulombic efficiency was almost 98 % in all current densities as the GCD profiles were isosceles triangle in shape. The rate capability of 70 %

Figure 6. Symmetric pouch N-MCNR2-cell: (a) CV curves at different scan rates and (b) effect of different scan rates on Csp; (c) GCD profiles at different current densities and (d) effect of various current densities on Csp. The Csp values obtained from GCD profiles were IR drop corrected. was achieved up to moderate 25 Ag-1 and further decrement at higher current densities. The decrement at high current densities was ascribed to the existence of high micro-pore content



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which resist to electrosorb the ions. Also, for comparison, the symmetric SC device for CNR electrode material was designed and evaluated the electrochemical performance. CNR SC device shows a maximum Csp of 154 Fg-1 at 2 mVs-1, and 151 Fg-1 at 0.25 Ag-1 (Figure S10 a, b). Such low charge storage capability was due to morphology texture of CNR electrode material having the specific surface area of 484.1 m2 g-1. To further understand the electrochemical performance characteristics from a quantitative standpoint, such as ionic diffusion through the porous carbon network, and the charge transfer resistance (R1) of N functionalities, potentiostatic electrochemical impedance spectroscopy (EIS) measurements were carried out for both cells. The impedance measurements were carried out with an open-circuit voltage over the frequency range of 200 kHz to 10 mHz with a perturbation potential amplitude of 10 mV AC. The Nyquist plot for N-MCNR2-Cell shows a semicircle in the high-frequency region; furthermore, the plot shows a steep slope at intermediate frequencies and a straight line in the low-frequency region, as shown in Figure 7a. The semi-circle region was attributed to blocking behaviour due to faradic resistance from N-functional groups, as described in equation (6, 7), and the steep region with a slope of 45° observed at intermediate frequencies represents the combination of the resistive and capacitive behaviours of ions penetrating the nano-pores of the electrode. Finally, the straight line observed at low frequency represents the dominance of the capacitance due to the formation of an EDL at the surfaces of nano-pores. Thus, the Nyquist plot was fitted using the equivalent circuit model, as shown in Figure 7d. Rs and ESR generally describe the resistance of the electrolyte solution combined with the internal resistance of the electrode. The ESR and R1 values obtained for the N-MCNR2-Cell as 0.75 and 1.01 Ω, respectively, which suggest that the electrode material exhibits prolonged lifetime performance


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and supports high redox reaction rates at the electrode-electrolyte interface, respectively. We believe that the facilitation of electronic conduction in the N-MCNR2 carbon framework due to N-doped atoms and the excellent contact area of the electrode-electrolyte interface yield low ESR and R1 values. The constant phase element (CPE) represents the EDL capacitance at the interfaces between solids and electrolyte solutions due to the separation of ionic charges. The Warburg resistance (W) of this device represents the frequency-dependent diffusion of ions into the nano-porous electrode material.

Figure 7. EIS studies of N-MCNR2-Cell: (a) Nyquist impedance plots and inset shows the magnified view at high frequency; (b) complex blode plot; (c) frequency response of capacitance imaginary part to determine relaxation time constant; (d) equivalent circuit used to fit for obtain Nyquist plots. Moreover, in general, the power dissipated can be related to the frequency response of any device, as shown Figure 7b. The N-MCNR2-Cell shows average power dissipation at intermediate frequencies as the phase angle approaches 0°. In addition N-MCNR2-Cell exhibits the phase angle of 81°, which was very close to the ideal capacitor behaviour (90°), as shown in Figure 7b Furthermore, the dielectric relaxation time constant (τo = 1 / 2 π f) was calculated to determine the response time of the device using a complex Bode plot.17 The local maxima was



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determined to be 10 s (1.61 Hz), as shown in Figure 7c. It was evident that the lower τo value of N-MCNR2-Cell which features a small voltage perturbation, supports a higher rate of kinetic diffusion between the electrode and electrolyte interface. To further evaluate the applicability of N-MCNR2-Cell, Ragone plot was created from the results of the GCD experiments, as shown in Figure 8a. The N-MCNR2-Cell reached an E.D of 11.2 Wh kg-1 at a P.D of 118.7 W kg-1 (340 s) and retained an E.D of 4.0 Wh kg-1 at a maximum P.D of 12.9 kW kg-1(1.4s). Thus, N-MCNR2-Cell device can be used exclusively for high-power-rate applications; the obtained E.D was also higher or comparable to that of the other N-doped carbon materials listed in Table S4. Such performance was mainly attributed to the role of N functionalities and the textural properties of the multi-nano-channel carbon rods. The superior performance obtained can be attributed to the following factors: (1) Continuous nano-channels provided a suitable conduction pathway and localized or confinement of hydronium and sulfate ions near the electrode surface. (2) The presence of both pseudocapacitive N-pyridinic and N-pyrrolic functionalities promoted the electron transfer between the electrode and electrolyte interface. (3) The existence of micro- and meso-pores provided additional room for confining the electrolyte ions thus enabling high polarization of the carbon surface. The key advantage of SCs over other energy storage devices was their long-term cycling stability. The N-MCNR2-cell was tested for up to 50,000 charge-discharge cycles at a current density of 2 Ag-1, as shown in Figure 8b. After 50,000 cycles, 92.6% of the initial capacitance was retained, indicating excellent cycling stability. To understand the modest capacity retention loss, EIS studies were carried out for both cycled cells. The results indicate that increased Rs and


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R1 values of 1.22 and 6.05 Ω, respectively, which can be attributed to the change in the pH of the electrolyte and trivial oxidation of carbon in the acidic environment after prolonged cycling (Figure S11).

Figure 8. (a) Ragone plot of gravimetric energy density (E.D) Vs. Power density (P.D); the IR drop correction was considered for E.D and P.D evaluation. (b) Long term cycle stability test performed for N-MCNR2-Cell at a current density of 2 A g-1 Post-mortem studies. Post-mortem studies were carried out for the cycled device. For this purpose, the NMCNR2-Cell device were dismantled and the electrodes were dried at 50°C for 12 h. Morphological analysis of the cycled electrodes yielded the following results, after 50,000 cycles, the cycled N-MCNR2-Cell electrode retained its multi-nano-channel carbon structure, and the surface roughness was smoothed, which can be attributed to the oxidation of the carbon material (Figure S12a, b). Furthermore, XPS analysis revealed the reason for the long-term cycle stability and the modest decrease in performance of N-MCNR2-Cell. For comparison, a fresh electrode was also subjected to XPS analysis. The XPS elemental analysis (Table S5) shows that the



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oxygen contents of N-MCNR2 was increased to 27.6 wt%, which simultaneously suppressed the relative quantitative nitrogen and carbon contents, as suggested by oxygen content of the fresh electrode; additionally, the increased oxygen content was due to carbon oxidation. The binding energy values and extracted parameters were tabulated in Table S6-S8. The C1s and O1s spectra (Figure S13a and c) obtained after cycle testing showed no significant changes in the functionalities contained in the cell structures; indeed, the cycled cell retained the same functional groups as the fresh electrode. The N1s spectra for the fresh and cycled electrodes followed the same trend (Figure S13b). The above-described results indicate that N-MCNR2 was a promising electrode material for advanced carboncarbon based SC systems. Other favourable synergistic effects provided by the material were listed below: (i)

The high surface area (840 m2 g-1) without activation provides an appropriate electrode/electrolyte interface for facilitating fast charge transfer.

(ii)

The multi-nano-channels offer complete accessibility to H3O+/SO42- species for pseudocapacitive N-functionalities pathways, as described in Figure 9. Thus, easy transport of electrolyte ions facilitates the use of the material in high-energy and high-power-storage applications.


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Figure 9. Pictorial representation of Cell-1 electro-sorption and redox reaction of hydronium ions (H3O+) in N-MCNR2 electrode. CONCLUSIONS We have successfully fabricated a novel porous multi-nano-channel nanocarbon material by in situ N doping. The material exhibits highly favourable morphological texture and pseudocapacitive N-functionalities for SC applications. The preliminary electrochemical optimization studies with different N-doped multi-nano-channel nanocarbon (N-MCNR) materials could suggest the best electrode material was exploited in symmetric pouch cell device. The N-MCNR2-cell with the maximum Csp of 335 Fg-1 which corresponds to the maximum E.D of 11.4 Wh kg-1 and could able to achieve a maximum P.D of 12.9 kW kg-1 at a high current density of 35 Ag-1. In addition, the SC device delivered excellent rate capabilities up to 35 Ag-1, and coulombic efficiency of 98 % in all current densities indicating good macro-, meso- and micro-pore connectivity and a conductive carbon framework. Notably, Cell-1 showed outstanding long-term cycle stability over 50,000 cycles, with 92.6% capacity retention, suggesting the devices’ viability for commercial applications. Moreover, the novel multi-nano-



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channel nanocarbon material fabricated in this study could open new opportunities for applications in H2 and CO2 adsorption and metal-free electro-catalysts for oxygen and CO2 reduction reactions. ASSOCIATED CONTENT Supporting Information. FE-SEM images, CV and GCD profiles data, Post-mortem cycled cell FE-SEM and XPS analysis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the DGIST R&D Program (16-RS-04) of the Ministry of Education, Science and Technology of Korea and also partly by the DGIST MIREBraiN program.


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For Table of Contents Use Only Nitrogen doped porous multi-nano-channel nanocarbons for use in high performance supercapacitor applications Prakash Ramakrishnan Sangaraju Shanmugam*

Novel nitrogen doped porous multi-nano-channel nanorod supercapacitor device delivers energy density of 11.4 Wh kg-1 and excellent 50,000 cycle stability.


Nitrogen-Doped Porous Multi-Nano-Channel Nanocarbons for Use in

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