Vertically Aligned Heteroatom Doped Carbon Nanosheets from

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Vertically Aligned Heteroatom Doped Carbon Nanosheets from Unzipped Self-Doped Carbon Tubes for High Performance Supercapacitor Qi-Ying Lv, Feng Jing, Yuchen Huang, Jian Xiao, Yan Zhang, Fei Xiao, Junwu Xiao, and Shuai Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04674 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 24, 2018

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ACS Sustainable Chemistry & Engineering

Vertically Aligned Heteroatom Doped Carbon Nanosheets from Unzipped Self-Doped Carbon Tubes for High Performance Supercapacitor Qiying Lva‡, Feng Jinga‡, Yuchen Huang,b Jian Xiao a‡, Yan Zhanga, Fei Xiaoa, Junwu Xiaoa, Shuai Wang*a,c a

Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education.

School of Chemistry & Chemical Engineering, Huazhong University of Science and Technology,Wuhan 430074, P. R. China. b

Wuhan Foreign Language School, Hankou Wansongyuan Lu 48, Wuhan 430022, P. R. China.

c

State Key Laboratory of Digital Manufacturing Equipment and Technology, Flexible Electronics

Research Center (FERC), School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R. China. Email: [email protected] ‡

These authors contributed equally to this work. Q. Lv, F. Jing and J. Xiao designed and performed the experiments. Q. Lv designed the experiment and F. Jing carried out the synthesis and conducted the materials characterization. Q. Lv conducted the electrochemical evaluation and wrote the manuscript. J. Xiao drew the illustrations. S. Wang supervised the project and finalized the manuscript. All the authors discussed the results and gave approval to the final version of the manuscript.

KEYWORDS Carbon nanosheets; Aligned; Oxygen, nitrogen co-doped; Unzipped; Supercapacitor

ABSTRACT A common but important problem of carbon-based supercapacitors unresolved is the difficulty of achieving high specific capacitance over a wide voltage window when the electrodes are

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assembled into devices. Adjusting the structure of carbon materials is expected to tune the electrode potential and the surface charge density of the electrode materials, and thus further enhance the energy density of carbon-based supercapacitors. Herein, an efficient surface charge control strategy was developed to remarkably enhance the energy density of porous N, O co-doped vertically aligned carbon nanosheets (VACNs) based solid-state symmetric supercapacitors through unzipping N, O co-doped carbon tubes by transition metal atoms. Originating from the synergetic effect of vertically aligned structure and the doped N, O atoms, the assembled solid-state symmetric supercapacitor based on N, O co-doped VACNs electrodes exhibits excellent electrochemical performance. The synthesis strategy may enlighten the design and fabrication of well-defined carbon-based nanomaterials that have potential applications in energy storage and other area.

Introduction

The fast charging/discharging capability and stable cycling characteristics promote carbon-based supercapacitors (SCs) into various electronics markets.1-2 However, a common but important unresolved problem is that it is difficult to obtain high energy density when carbon-based electrodes are assembled into devices owing to the narrow voltage window and low specific capacitance. It is well known that the specific capacitance of electrode and electrode potential (EP) depend on the surface charge density of the electrode materials, whereas the potential voltage of device is related to the potential variation of respective electrodes.2-4 Thus, tuning the EP and the surface charge density through changing the structure of electrode materials is feasible. Over the last decade, many researchers have devoted to develop synthetic strategies for fabricating carbon-based nanomaterials as well as the manipulation of their morphology, dimension, and crystallinity to maximize the energy density of SCs.5-10 Among reported nanostructures, graphene and graphene-like carbon nanosheets have been received widespread attention owing to their open porous and layered structure.6 The unique nanostructure has a large high surface area, so that it not only provides excellent accessibility to active sites, but also enhances good


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electronic conductivity. However, due to the van der Waals interactions, the individual ultrathin carbon nanosheets tend to stack together and thus lose their inherent characteristic feature, thereby reducing their overall electrochemical performance.10 The carbon-based nanomaterials with vertically aligned structure have unique orientation so that they can expose more sharp edges, larger surface-to-volume ratio, and more open channels between nanosheets,11 which can significantly enhance their electrochemical performances as comparison to the bulk materials. Currently, the vertically aligned graphene (VAGNs) and graphene-like nanosheets are usually achieved by the following techniques: plasma-enhanced chemical vapor deposition (PECVD),12 and chemical synthesis.13-15 Although the PECVD method possesses several advantages, such as lower substrate temperature, higher growth selectivity and better controllability for nanostructure ordering/patterning, the preparation process is rather complicated. Moreover, the morphology and structure of the prepared vertically aligned graphene nanosheets are strongly affected by both the plasma source and power, etching rate, surface temperature, and plasma pre-treatment.16 The chemical synthesis including permanganate treatment,17 metal nanoparticle catalyze,18

lithium/liquid

ammonia

intercalation-driven

unwrapping

CNTs,19

and

potassium

vapor-induced splitting.20 Typically, CNTs treated with the permanganate in acid result in a heavily oxidized product that has a similar structure to graphene oxide (GO) nanoribbon. While only partial graphene nanosheets/nanoribbons can be obtained by unzipping CNTs with lithium intercalation and subsequent exfoliation because only the superficial film of carbon nanotubes was exposed to the pre-treatment of the CNTs, thus providing low yield. Similarly, the yield of graphene nanoribbons obtained by the cutting action of catalytic metal nanoparticle is also very low.18 Although many researches have focused on the development of high-performance VAGNs, it remains a challenge to controllably prepare VAGNs composed of porous heteroatoms doped graphene-like nanosheets efficiently in a bulk quantity. Recent researches proposed a template method to construct hierarchically structured transition-metal-oxide (TMO) nanosheets as the template and organic polymer as the carbon source with subsequent heat treatment.9, 21-23 However, randomly interconnected VAGNs were



usually obtained owing to lack of an effective strategy to manipulate the assembly of TMO templates.9, 21-23

It is well known that the novel porous nanostructures is beneficial to shorten the diffusion pathway of

ions and minimize the diffusive resistance of mass transport on electrode/electrolyte interface. In addition, the introduced defects, heteroatom and/or functional groups can also increase more active sites and effectively modulate their electronic and chemical properties.24,

25

Unfortunately, there are still huge

challenges to satisfy all the above-mentioned characteristics of carbonaceous materials simultaneously through existing controllable technologies. Compared with other synthesis methods, the introduction of transition-metal nanoparticles as chemical scissors to unzip carbon nanotubes not only does not involve any aggressive chemical treatment, but also obtains smooth graphitic edges.26 Inspired by this, we propose a controllable and step-wise strategy of unzipping N, O co-doped carbon tubes by using Ni atoms as chemical scissors so as to obtain porous vertically aligned N, O co-doped graphene-like carbon nanosheets (VACNs). Here, we chose ZnO nanoarrays as templates, low-cost and recyclable gelatin and β-cyclodextrin (β-CD) as green mixing carbon precursors, respectively. Gelatin can form a stable hydrogel that is beneficial for coating on the ZnO nanoarrays uniformly. Furthermore, gelatin has multitudinous -OH and -NH groups, which has strong intermolecular interactions with β-CD.27 The β-CD consists of seven glucose units with a hydrophobic cavity and hydrophilic surface, which can function as a special surfactant to help the mixture to form a homogeneous and stable hydrogel. Thus, it is beneficial to the distribution of -NH and -OH groups on the ZnO nanoarray. The as-obtained porous N, O co-doped VACNs possess abundant active site dispersion over hierarchical structure to promote charge/electrolyte transfer, provide high charge capacity, and minimize polarization effects, thus, greatly enhancing the electrochemical performance of the N, O co-doped VACNs-based solid-state SSCs devices. The production of such porous heteroatom self-doped vertically aligned carbon nanosheets may allow VACNs to be integrated on a large scale into energy storage and conversion or nanoelectronics.

Experimental Section


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Preparation of ZnO nanorods (ZNRs) The ZNRs were prepared by electrochemical deposition method as described in literature,28 which was performed in a simple two-electrode system. In brief, a piece of cleaned carbon cloth (1×2 cm2) that was cleaned by sonication in ethanol and deionized water in turn and a pure Zn (99.99% purity) rod were used as the positive and negative electrodes, respectively. The electrodeposition of ZNRs was performed at a current density of 0.8 mA cm-2. During the whole electrodeposition process, the electrochemical cell was put into an oil bath at 80 ℃. The electrolyte Zn(NH3)4(NO3)2 solution was prepared according to previously reported method.28 The ammonium hydroxide (28% NH3 in water, 99.99%) was added dropwise into 0.2 M zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%) aqueous solution until the solution became clear, with a pH value of about 10. Preparation of ZnO nanorods coated with N, O co-doped carbon (ZNR@ N, O co-doped carbon) The carbon cloth grown with ZNRs was immersed into the mixture solution including 4 mg mL-1 gelatin and 2 mg mL-1 β-CD for 0.5 h at 50 ℃. Then, the carbon cloth was taken off and dried up. Subsequently, the sample was annealed at 500 ℃ under Ar (200 sccm) atmosphere with a heating rate of 5 ℃ min-1 for 3 h, forming the ZNRs@ N, O co-doped carbon hierarchical nanoarrays on the carbon cloth. Synthesis of N, O co-doped vertically aligned carbon nanosheets (N, O co-doped VACNs) The details of the preparation of N, O co-doped VACNs are as follows: First, a 20 mM NiCl2·6H2O cholamine solution was prepared and transferred into an autocave. Second, a piece of carbon cloth grown with ZNRs@N, O co-doped carbon was immersed in the solution and aged for 2 h. Third, the autocave was tight sealed, moved to an oven of 120 ℃ and maintained this temperature for up to 2 h. The autocave was then cooled down to room temperature immediately. The carbon cloth was taken out and washed with deionized water several times, and then dried in air. Finally, the sample was heated at 750 ℃ for 1 h under Ar (200 sccm) atmosphere with a heating rate of 5 ℃ min-1, and then annealed for another 10 minutes under a H2: CH4: Ar (10:5:200, v/v/v) atmosphere. The prepared sample was immersed 1 M HCl solution for 24 h to remove the metal, and then soaked in deionized water repeatedly to ensure removal of



the residue. The N, O co-doped VACNs sample was obtained after dried at 80 ℃. The effective active area and mass of the sample are 1×1.5 cm2 and 0.23 mg/cm2, respectively. To understand the effect of temperature on the formation of N, O co-doped VACNs, materials formed at different temperatures were also prepared. The mass of the active material that prepared at 700 ℃ and 800 ℃ are 0.31 mg/cm2 and 0.08 mg/cm2, respectively. In addition, the sample synthesized by carbonizing the precursor that without Ni2+ and with other cations (Co2+, Fe2+, and Cu2+) that have been reported for unzipping carbon nanotubes as catalysts, were also prepared at the same condition and followed by treated in HCl or FeCl3 solution, respectively. The H2 used here is to rapidly reduce the nickel ions, and then unzipping the N, O co-doped carbon tubes. For comparison, the ZNRs@N, O co-doped carbon absorbed with Ni2+ was also heated under the same condition except the mixture gas without H2. However, the nanosheets were not obtained as expected (Figure S1). Characterization The morphologies, elemental compositions and phase components of the samples were characterized using a field-emission scanning electron microscope (FESEM, Heilios NanoLab G3 UC) equipped with an energy dispersive spectroscopy system (EDS, Team Octane Plus), X-ray diffractometry (XRD, X' Pert PRO, Panalytical B.V., Netherlands), X-ray photoelectron spectroscopy (XPS, VG Multilab 2000), and transmission electron microscopy (TEM, FEI, gG2F20, 200 kV), respectively. Brunauer-Emmett-Teller (BET) specific surface area of the sample was analyzed by N2 adsorpiton/desorption isotherms at 77 K using a Micrometritics ASAP 2020 instructurement, and the pore size distribution was obtained from N2 adsorption isotherm using Barrett-Joyner-Halenda (BJH) method. Raman spectra were recorded on a LabRAM HR800 instrument with Nd: YAG laser source of 532 nm in a macroscopic configuration. Preparation of the gel electrolyte The gel electrolyte was prepared according to our previously reported method29 with slightly modified: For the Li2SO4-PVA gel electrolyte, 4.4 g of Li2SO4 and 4 g of PVA were added into 40 mL of deionized


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water at 95 ℃ with vigorous stirring until the solution became clear. Then, the solution was kept at 95 ℃ without stirring. For the H2SO4-Li2SO4-PVA gel electrolyte, with the former method and the only difference is that the pH of the mixture is adjusted to 2 by dilute H2SO4. Assembly of the devices The solid-state symmetric supercapacitor was assembled with two pieces of self-supported N, O co-doped VACNs electrodes (1×2 cm2) with an electrolyte-soaked separator (filter paper with 8 µm) in between. Prior to the assembling, the N, O co-doped VACNs electrodes were immersed in the Li2SO4-PVA or H2SO4-Li2SO4-PVA gel electrolyte for 5 min, respectively. Then, the two electrodes impregnated with gel electrolyte were assembled into device and kept at room temperature. After the gel electrolyte solidified, the solid-state supercapacitor was prepared. Electrochemical measurements All the electrochemical performances were tested on the Electrochemical Work-station (CHI760E). The electrochemical performances of single electrode were tested in a three-electrode configuration with a platinum gauze counter electrode and saturated calomel reference electrode (SCE) in 1 M Li2SO4 electrolyte. Calculations The specific capacitances of the devices were calculated by the using Equations (1) and (2), respectively: b

C=

∫ I (V )dV a

2 × ∆V × v × A(m)

C=

I × ∆t ∆V × A(m )

(1)

(2)

b

Where,

∫ I (V )dV is the integrated area of the CV curve in one cycle, v refers the scan rate, I is the a

discharge current applied, A is the total area of the device, m represents weight of active material or entire



weight of two electrodes in device, and ∆ V refers to the discharge voltage (excluding voltage drop) within the discharge time ∆ t. The energy density (E) and power density (P) of the devices were calculated by the using Equations (3) and (4), respectively: E =

P=

CdeviceV 2 2

×

1 3600

E × 3600 ∆t

(3)

(4)

Where V refers the applied voltage within discharge time ( ∆ t), and Cdevice is the specific capacitance of the device.

Results and Discussion

Experimentally, the ZnO nanoarray with 4 – 6 µm in length was used as the starting template. The fabrication process for the N, O co-doped VACNs is schematically illustrated in Figure 1a. The carbon fabric grown with ZnO nanoarray was prepared (Step ℃) and were then coated with a layer of gelatin (G) and cyclodextrin (CD) to yield the ZnO@G-CD composite. After carbonization, the ZnO@G-CD was converted into ZnO@N,O-co-doped-carbon (Step ℃). Magnification scanning electron microscopy (SEM) images show that the hexagonal ZnO nanorods (Figure 1b and f) become rougher and porous when coated with N, O-co-doped-carbon (Figure 1c and g). Additionally, the ZnO@N, O-co-doped-carbon nanoarray was well maintained. Then, the ZnO@N,O-co-doped-carbon nanoarray was used as a carrier for Ni ions (Step ℃), followed by a reduction with flowing CH4/H2/Ar at appropriate temperature (Step ℃). The product was quickly cooling down to room temperature. In order to remove the residual ZnO and Ni nanoparticles, the product was immersed in 1 M HCl solution for 24 h. The final product was washed repeatedly with deionized water and dried (Step ℃). Interestingly, the surface of the ZnO@N, O-co-doped-carbon becomes smooth with the adsorption of Ni ions (Figure 1d and h). By


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thermal treating the ZnO@N, O-co-doped-carbon adsorbed with Ni2+ under a mixed gas stream, part of the ZnO nanorods can be removed,30 and the leaving carbon tubes adsorbed with Ni2+ were unzipped. Although the Ni atom cannot be observed in SEM image (Figure S2a), and the X-ray diffraction (XRD) pattern of the sample also does not indicate the peak of Ni (Figure S3) for the small amount of adsorption, energy dispersive spectroscopy (EDS) demonstrates the presence and content of Ni (Figure S2b). The final sample, having been treated with HCl, was revealed that almost all of the tubes were unzipped along the axis (Figure 1e and i). The curling and roughness of the nanosheets edges can minimize its surface energy.31 To reveal the effect of nickel species on the formation of N, O co-doped VACNs, the sample prepared by carbonizing the precursor that without Ni2+ and with other cations (Co2+, Fe2+, and Cu2+) that have been reported for unzipping carbon nanotubes as catalysts, were also prepared at the same condition and followed by treated in HCl or FeCl3 solution, respectively. As shown in Figure S4, the N, O co-doped carbon tubes are well maintained without nickel species. However, it is surprising to be found that the carbon tubes almost also maintained by carbonizing the precursor that introducing Co2+, Fe2+, and Cu2+ ions. Especially, the ZnO@N, O co-doped carbon nanorods are fully covered by iron compounds nanowalls when introducing iron ions and then some incomplete tubes are obtained in the subsequent annealing treatment and acid etching (Figure S5a-f). To further uncover the effect of temperature on the formation of N, O co-doped VACNs, materials formed at different temperatures were also studied (Figure S6a-b). The N, O co-doped VACNs prepared at 700 ℃, almost complete tubes were obtained. However, the N, O co-doped VACNs prepared at 800 ℃, most all of these nanosheets were adhered to the substrate. This is probably due to the ZnO nanorods were reduced to Zn and then volatilized at high temperature.32 As a result, the N, O co-doped carbon layer, which lost of the support, collapsed. The details of the unzipped carbon nanosheet can be observed more clearly with transmission electron microscopy (TEM). The porous N, O co-doped carbon was uniformly coated on ZnO nanorod (Figure S7), which facilitates the adsorption of metal ions. After annealed in flowing CH4/H2/Ar at appropriate



temperature and the subsequent metal removal, the N, O co-doped carbon tubes were opened. Figure 2a-b reveals that the unzipped carbon tubes are laminar and contain obvious boundary that comes from the edge of ZnO nanorod. In addition, the width of the unzipped carbon is approximately 609 nm, which is close to the circumference of ZnO nanorod. A high-resolution TEM image shows that the sample is compose of amorphous carbon and a small amount of crystalline carbon (Figure 2c). Typical elemental mapping images demonstrate the presence of C, N and O elements which are homogeneously distributed on carbon nanosheets (Figure 2d-f). X-ray diffraction (XRD) pattern of the sample (Figure 2h) exhibits the characteristic (002) and (100)/(101) diffraction peaks at 2θ values of 25.5° and 43.2°, respectively, which is similar to the diffraction pattern of N-doped graphene.33 Raman spectroscopy was used to assess the degree of graphitization of the sample, as shown in Figure 2i, there are two distinct bands centered at 1354 and 1589 cm-1, assigned to the D and G bands of carbon, respectively, suggesting that the precursor has been successfully transformed into graphitic carbon atoms.34 The band at 1354 cm-1 (D-band) mainly reflects the structural defects in the graphitic structure. And the peak at 1589 cm-1 is due to the distorted structures caused by the incorporation of five-membered rings or hereroatoms in the graphene hexagonal network, indicating the occurrence of doping of heteroatoms into the carbon matrix, thereby imparting electrochemical activity.35,

36

The intensity ratio of the G to D bands (denoted as IG/ID) is directly

proportional to the degree of graphitization.37 As shown in Figure 2i, the IG/ID value of the sample is 0.59, suggesting that the sample has a great degree of graphitization. Meanwhile, the broad band appearing in the 2500 - 3500 cm-1 region can be assigned to a combination of D + D and D + G bands,34 further indicating the microcrystalline graphite in the sample.38 Nitrogen adsorption-desorption isotherms suggests the corresponding pore size of N, O co-doped VACNs is mainly falls in the range of 1 to 10 nm (Figure 2j), indicating the sample has a micro-/meso-porous structure. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) of N, O co-doped VACNs (280.1 m2 g-1) is much higher than the pristine carbon cloth (14.1 m2 g-1). All the results show that the sample has the potential of increased rate capability.


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X-ray photoelectron spectroscopy (XPS) is usually used to characterize the elemental composition, chemical state and electronic state of the elements existed in a material. As shown in Figure 3a, a survey scan indicated that the N, O co-doped VACNs is composed of C, O, N, and a very small amount of Ni elements, which confirms the doping of the carbon nanosheet. On the basis of the XPS analyses, the elemental composition of the sample is listed in Table S1 in the supporting information. The N and O contents in the sample is estimated to be around 9.47 and 4.89 at.%, respectively. The predominant asymmetric C 1s peak shown in Figure 3b implies the existence of C-C, C-N, and O=C-O bonds in the graphitic network. The high-resolution C1s peak at 285.0 eV is corresponding to the graphite-like sp2 C, indicating that most of the C atoms in the sample are arranged in a conjugated honeycomb lattice.39 The small peaks at 286.0 and 288.8 eV reflect different bonding structure of the C-N and C-O bonds, respectively.40 The high-resolution N 1s peak in Figure 3c could be deconvoluted into three peaks: the peak at 398.8 eV is attributed to pyridine-like structure (N-6) that the N atoms are doped at the edge site of carbon plane,22, 41-42 the weak peak located at 400.3 eV is attributed to the nitrogen atoms in pyrrolic moieties (N-5), and the peak centered at 401.4 eV is associated with N atoms doped at the center of carbon planes (N-Q).39, 43-46 Among these types, the N-6 and N-5 species are the dominant N-containing functional groups in the sample. Moreover, the amount of pyridinic N (39.0%) is much higher than that of pyrrolic N (21.8%), which is agreement with reported in literature that the pyridinic N is more stable than pyrrolic N at high temperature.47, 48 The presence of the pyridinic N and pyrrolic N in the sample will not only can improve the wettability of VACNs in electrolytes and thus increase electrolyte accessible surface area, but also can enhance the capacitance performance of the electrode due to their pseudocapacitive effect.45, 46 In addition, due to the higher electronegativity (3.5) and smaller atomic diameter (70 pm) of nitrogen than carbon (3.0, 77 pm), the graphitic N will enhance the conductivity of VACNs and thus facilitate electron transfer.46,

49-51

All of the doped atoms may contribute to an enhancement of the

interfacial capacitance by redox reaction.43, 52-53 In the O 1s spectrum (Figure 3d), three peaks located at 531.4 eV, 532.5 eV, and 533.5 eV represent the carbonyl groups (C=O), phenol-type groups



(C-OH)/ether-type groups (C-O-C) and chemicsorbed O and/or adsorbed water, respectively, which can enhance the wettability and contribute to pseudocapacitance.54, 55 The XPS analysis further supports the results of XRD, Raman spectra, and TEM. Compared with reported methods, the synthetic requirements for the N, O co-doped VACNs in this work could easily be achieved. Moreover, besides the mild reaction conditions, the synthetic process of N, O co-doped VACNs can be widely extended due to the advantages of the initial ZnO template, such as low cost, controllability of synthesis, and recyclable utilization of Zn2+. Importantly, the length and diameters of ZnO nanorods can be controlled by regulating the deposition time and the concentration of ZnO precursor. Thus, it is expected that the size of the N, O co-doped VACNs is controllable according to the ZnO templates. The Ni-catalyzed nanomaterial cutting process may be attributed to the following: N bonded to the carbon atoms in a pyridinic way certainly induces voids in the graphitic layers, which could be the reason of more turns during the cutting process. The Ni ions supported on N, O co-doped carbon tubes have multiple anchoring points for the coordination ability of N and O atoms. Thus, their mobility is much lower compared to nanoparticles with single point contact on carbon support. The result is well agreed with previous reports that the incorporated nitrogen in the graphitic lattice and might considerably affect the cutting action along a straight line.18, 56 Electrochemical performance Due to its unique porous structure, N, O co-doped VACNs has the potential to be used as electrode material for supercapacitors. We first evaluated the capacitive potential range of N, O co-doped VACNs electrodes with cyclic voltammetry (CV) with a neutral aqueous solution of 1 M Li2SO4 electrolyte in a conventional three-electrode system. For comparison, the capacitance of the N, O co-doped carbon nanomaterial electrodes prepared at different temperature were intensively studied. As expected, the N, O co-doped carbon nanomaterial electrodes prepared at 750 ℃ and 800 ℃ (denoted as sample-750 and sample-800, respectively) show more rectangular CV curves in both positive and negative potential


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window than the N, O co-doped carbon nanomaterial electrode prepared at 700 ℃ (denoted as sample-700) (Figure S8a-b). This is due to the sample-750 and sample-800 not only guarantee sufficient electrochemically active sites but also shorten the diffusion and migration of the electrolyte ion for their nanosheets nanostructure, thus improving electrochemical kinetics during galvanostatic charge/discharge (GCD) processes. However, the sample-750 electrode exhibits higher specific capacitances (negative potential window, 295 F/g; positive potential window, 283 F/g) than those of the sample-800 (negative potential window, 191 F/g; positive potential window, 167 F/g) because it provides more electrochemically active sites for its vertically aligned nanostructure (Figure S8c). And the specific capacitances of the sample-750 electrode are almost double than those of the sample-700. The results are well agreement with the mentioned-above speculation. To proceed further, a conventional SSC based on the N, O co-doped sample-750 that with VACNs nanostrucutre electrodes was assembled with PVA-Li2SO4 gel electrolyte and the electrochemical performances were investigated. The electrochemical impedance spectroscopy (EIS) shows that the SSC features almost vertical curve in the low and middle frequency region, suggesting the dominance of electric double layer effect. The equivalent series resistance (ESR) obtained from the intercept of the Nyquist plot on the real axis was ~ 0.95 Ohm (expand view insert in Figure 4a), showing a good interface contact between solid-state electrolyte and electrode as well as excellent electric conductivity of electrode materials due to the N, O co-doped porous vertically aligned structure. Bode plot in Figure 4b shows a phase angle of 83.7° at 0.01 Hz for the SSC. Additionally, the capacitor response frequency f0 for the SSC at a phase angle of -45° was 10 Hz. The corresponding relaxation time constant τ0 (τ0 = 1/f0) was calculated to be 100 ms, indicating that the device possesses pure capacitive behavior and most of its stored energy is accessible at frequencies below 10 Hz. The typical rectangular EDLC-like CV curves at scan rates from 5 to 5000 mV/s in a wide potential window were obtained (Figure 4c). Besides, the CV rectangular shape is well maintained in the charge and discharge processes even the scan rate increasing to 20,000 mV/s (Figure S9a), indicating a



substantially high rate performance of the SSC. All GCD curves (Figure 4d) show symmetric features with a fairly linear slope, further indicating ideal capacitive behavior. High specific capacitances of 378 mF/cm3 (34 F/g) based on the volume of device (total mass of two electrodes) (Figudre 4e) and 104 F/g based on the mass of active materials (Figure S10) can be achieved at a current density of 0.2 mA/cm2, respectively, which is consistent with the results that calculated from CV curves (Figure S9b-c). What’s more, over a wide range of current densities, the SSC device continued to provide a well-behaving GCD curve and high capacitance. And even if the current density increased to 50 mA/cm2, a high specific capacitance of 258 mF/cm3 (71 F/g and 23 F/g based on the total mass of the active materials and the total mass of two electrodes, respectively) based on the volume of device can be obtained. Generally, energy and power densities are two key parameters for evaluating the entire device. As shown in Figure 4f, the SSC exhibits a maximum volumetric energy density of 0.2 mW·h/cm3 and power density of 793.7 mW/cm3, respectively. In addition, the gravimetric energy densities of the SSC (Figure S11) are as high as 57.5 W·h/kg and 18.0 W·h/kg based on the total mass of active materials and the entire mass of two electrodes, respectively. It is well known that the excellent wettability of carbon materials can be obtained when carbon materials were doped with nitrogen/oxygen species, especially with high nitrogen content doping into graphitic lattice. Besides, the high nitrogen doping level, particularly pyridinic N and pyrrolic N, provides the electrode material with large numbers of active sites, which boost the pseudocapacitive effect greatly by redox reactions.57 To further verify this viewpoint, the performance of the N, O co-doped VACNs was also investigated in 0.5 M H2SO4 with two electrodes. Unlike the typical EDLCs electrode, the curves of the N, O co-doped VACNs electrode maybe deconvoluted into two parts: (i) a nearly rectangular EDLC-like curve; and (ii) a couple of symmetric Faradaic charging/discharging peaks that located at -0.08 to 0.23 V, which indicates the contribution of pseudocapaitance by the surface reaction of N and O functional groups. Importantly, the corresponding shape and symmetry features could be maintained when the scan rate increased from 10 to 5,000 mV/s (Figure S12). This indicates that both EDLC-like and


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redox reactions have fast charging-discharging kinetics. Remarkably, compared to the SSC device based on based on N, O co-doped VACNs electrodes assembled with PVA-Li2SO4 gel electrolyte, the gravimetric and volumetric specific capacitance of the device greatly enhanced in 0.5 M H2SO4 electrolyte. Therefore, in order to investigate the potential application, a solid-state symmetric device based on N, O co-doped VACNs was also assembled with PVA-Li2SO4-H2SO4 gel electrolyte (recorded as H-SSC). The H-SSC shows an ideal capacitive behavior with rectangular CV curve, even at the potential window up to 2.0 V (Figure 5a). It is well known that the energy density is proportional to the square of working potential, and the expanded working potential can significantly enhance the energy density.58 Thus, the potential window of 2.0 V was chosen to further investigate the electrochemical performances of the H-SSC device. The CV curves of the H-SSC were measured at scan rates from 5 to 5,000 mV/s between 0 and 2.0 V. CV curves in Figure S13a exhibits rectangular shapes with weak redox peaks that induced by N, O species at low scan rate. Moreover, the CV curves retain a relatively rectangular shape with increasing scan rates, even at a high scan rate of 5000 mV/s (Figure 5b), suggesting an ideal fast charge/discharge property. Consistent with the CV results, all GCD curves at various current densities (Figure 5c) show symmetric features with little plateaus, indicating that the solid-state H-SSC device has a good electrochemical capacitive characteristic and excellent reversible redox reactions. Moreover, the cycling curve is still symmetrical even at a current density as high as 100 mA/cm2, indicating very high rate stability. Figure 5d shows that specific capacitance achieved 1579 mF/cm3 (122 F/g and 501 F/g based on the entire mass of two electrodes and the total mass of the active materials, respectively. Figure S14) at a current density of 0.2 mA/cm2, which is much higher than that of SSC with PVA-Li2SO4 gel electrolyte. Importantly, the H-SSC device not only continues to provide good GCD behavior over a wide range of current densities but also exhibits high capacitance, achieving 842 mF/cm3 at 10 mA/cm2, over 50% retention of the value measured at 0.2 mA/cm2. Remarkably, the specific capacitance of the H-SSC device still retain 488



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mF/cm3 even the current density is increased to 100 mA/cm2, indicating outstanding rate capability. The N, O co-doped VACNs-based solid-state H-SSC device also exhibits good cycling stability with a high capacitance retention over 100% after 10000 charging/discharging cycles at 10 mA/cm2 (Figure 5e). A Ragone plot (Figure 5f) for the N, O co-doped VACNs-based solid-state H-SSC device depicts that the specific energy density is about 0.9 mW·h/cm3 at power density of 2.7 mW/cm3. Moreover, it still maintains 0.3 mW·h/cm3 even at an ultrahigh power density of 1625.1 mW/cm3. The high volumetric energy density not only substantially outweights the SSC with PVA-Li2SO4 gel electrolyte, but also higher than a commercial supercapacitor (5.5 V, 100 mF, 0.55 mW·h/cm3), the recently reported carbon-based supercapacitors,2,

24, 59

and hybrid supercapacitors.29,

60-63

Additionally, the gravimetric

energy and power densities based on the total mass of active materials are as high as 139 W·h/kg and 258 kW/kg and can still be up to 46 W·h/kg and 85 kW/kg based on the entire mass of two electrodes of the solid-state H-SSC device, respectivley (Figure S15). Due to the high energy density, two solid-state H-SSC devices connected in series can light a red light-emitting diode (LED) with a minimum operating voltage of 2 V (Figure 5e, inset). EIS was tested to evaluate the electronic and ionic transport of N, O co-doped VACNs-based solid-state H-SSC device. As shown in Figure 5g, the EIS shape nearly parallel to the Y-axis at low- and mid-frequency regions indicates an ideal capacitive behavior of the H-SSC device. And the H-SSC device has almost the same ESR (0.93 ohm) with SSC device with PVA-Li2SO4 gel electrolyte (inset in Figure 5g), better than that of most reported carbon-based supercapacitors.24 This may be attributed to the better wettability of N, O co-doped VACNs material, which lowers the interface resistance. Specifically, the frequency (of -45°) when the resistance and reactance have equal magnitudes is 7 Hz for the H-SSC, giving a relaxation time constant (τ0 = 1/f0) of 143 ms (Figure 5h), which is lower than that of OMFLC-N symmetric electrochemical cell (2.1 s).43 In particular, the ESR and the corresponding time constant τ0 actually reduced after 10,000 cycles life because the porous nanostructure of the electrode materials increased the fast transmission of ion/electron between the electrode materials and electrolyte, as


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illustrated in Figure 6, which strongly confirms that the solid-state H-SSC device has good stability and great potential to deliver high power and energy densities. The excellent electrochemical performance of the H-SSC device based on N, O co-doped VACNs material can be ascribed to its N, O co-doped unique 3D nanostructures. The N, O co-doped VACNs not only guarantees sufficient electrochemically active sites but also shorten the diffusion and migration of the electrolyte ion for their vertically aligned structure, improving electrochemical kinetics and enhancing structural stability during charge/discharge processes. Furthermore, the mesopores in the nanosheets also provide fast transportation channels for electrolyte ions. More importantly, the N and O atoms in the N, O co-doped VACNs can improve the wettability of VACNs in electrolytes and thus promote the electrolyte transfer efficiency at the solid-liquid interface.45, 46 In addition, the electronic-rich N-Q can enhance the conductivity of VACNs by modifying the band gap and thus facilitate electron transfer.46,

49-51

Additionally, the presence of the pyrridinic N, pyrrolic N, and O also can enhance the capacitance performance for their great pseudocapacitive effect, which is well agreed with previous proposed redox mechanism.43, 64-65 Except that, the introduced heteroatoms may passivate the electrode surface, thereby enhancing the activation overpotential of water electrolysis.3 Thus, the electrode potential is expanded. Conclusions In summary, the energy density of porous N, O co-doped VACNs-based SSCs can be significantly boosted via maximizing its operating voltage by tuning the surface charge density of N, O co-doped VACNs electrode materials through introducing transition-metal atoms as chemical scissor to cut self-doped carbon tubes without any aggressive chemical treatment. The operating voltage of the VACNs-based SSCs was expanded from 1.2 V to 2 V. More importantly, the optimized VACNs-based SSCs yielded an extraordinary energy density, long cycling stability, and good capacity retention at high rates. Furthermore, this study demonstrates the potential of porous N, O co-doped vertically aligned carbon nanosheets derived from unzipping carbon tubes by nickel atoms as impressive candidates for electrochemical energy storage applications.



ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Corresponding Author *To whom correspondence should be addressed. E-mail: Shuai Wang (S. Wang) [email protected] Present Addresses a

Key laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education.

School of Chemistry & Chemical Engineering, Huazhong University of Science and Technology,Wuhan 430074, P. R. China. b

Wuhan Foreign Language School, Hankou Wansongyuan Lu 48, Wuhan 430022, P. R. China.

c

State Key Laboratory of Digital Manufacturing Equipment and Technology, Flexible Electronics

Research Center (FERC), School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P.R. China. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Funding Sources This work is supported by the National Natural Science Foundation (Project No. 51173055, 51572094) and China Postdoctoral Science Foundation Funded Project (Project No. 2015M572135, 2017T100547). Notes The authors declare that they have no competing financial interest.


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FIGURES


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Figure 1. a) Schematic illustration of the fabrication of N, O co-doped VACNs on the carbon cloth substrate. (1), (2), (3), and (4) are the ZnO nanorods grown on carbon fiber, ZnO nanorods coated with N, O co-doped carbon, ZnO@N, O co-doped carbon absorbed with Ni2+, and N, O co-doped VACNs, respectively. SEM images of ZNs (b, f), ZNs@N, O co-doped carbon (c, g), ZNs@N, O co-doped carbon adsorbed with Ni ions (d, h), and the N, O co-doped VACNs (e, i) on the carbon cloth, respectively. In (i), arrows guide the cutting procedure along the axis of nanorods with Ni atom. The scale bars in (b, c, d, e) and (f, g, h, i) are 10 µm and 200 nm, respectively.

29 ACS Paragon Plus Environment


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Figure 2. a-b) Low-magnification TEM image of the unzipped N, O co-doped carbon nanosheets. c) HRTEM image of the unzipped N, O co-doped carbon nanosheet. d-f) TEM elemental mapping of C, N and O in N, O co-doped carbon nanosheet, respectively. h) X-ray diffraction patterns and i) Raman spectra of the N, O co-doped VACNs. j) Pore size distribution curves of N, O co-doped VACNs; insert is the nitrogen adsorption-desorption isotherms at 77 K of the N, O co-doped VACNs and carbon cloth, respectively.


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Figure 3. a) XPS survey spectrum of the obtained N, O co-doped VACNs. b-d) XPS spectra of C 1s (b), N 1s (c), and O 1s (d), respectively.



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Figure 4. Electrochemical performance of solid-state SSC device based on N, O co-doped VACNs electrodes was assembled with PVA-Li2SO4 gel electrolyte. a) Impedance spectra of the SSC device. Inset: an enlarged view of the impedance spectrum in the high frequency region. b) The phase angle versus the frequency of the solid-state SSC device. c) CV curves for the SSC device at different scan rates. d) GCD curves of the SSC device collected at different current densities. e) Volumetric specific capacitance of the SSC device calculated from GCD curves as a function of current densities. f) The volumetric energy and power densities of the solid-state SSC device.


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Figure 5. Electrochemical performance of solid-state SSC device based on N, O co-doped VACNs electrodes was assembled with PVA-Li2SO4-H2SO4 gel electrolyte (recorded as H-SSC). a) CV and GCD curves for the H-SSC device from 1.0 to 2.0 V at 100 mV/s. b) CV curves for the H-SSC device at different scan rates. c) GCD curves of the H-SSC device collected at different current densities. The inset shows an enlarged view of the GCD curves collected at 10, 20, 50, and 100 mA/cm2, respectively. d) Volumetric specific capacitance of the H-SSC device calculated from GCD curves as a function of current densities. e) Cycling performance of the H-SSC device at a current density of 10 mA/cm2. The inset is the schematic illustration of transport pathways for the electrons/ions in the N, O co-doped VACNs and the optical photographs of the fabricated solid-state H-SSC. f) The volumetric energy and power densities of the solid-state H-SSC device. g) Impedance spectra of the H-SSC device before and after cycle stability test. The inset shows an enlarged view of the impedance spectra in the high frequency region. h) The phase angle versus the frequency of the H-SSC device.



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Figure 6. Schematic illustration of the electrolyte travel through the N, O co-doped VACNs electrode, with the arrows highlighting the ion transport pathway.


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Table of Contents, Graphic, and Synopsis An efficient surface charge control strategy was developed to remarkably enhance the energy density of solid-state symmetric supercapacitors based on porous N, O co-doped vertically aligned carbon nanosheets through unzipping self-doped carbon tubes by transition metal atoms.


Vertically Aligned Heteroatom Doped Carbon Nanosheets from

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