Layered-MnO2 Nanosheet Grown on Nitrogen-Doped Graphene

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Layered-MnO nanosheet grown on nitrogen-doped-graphene template as a composite cathode for flexible solid-state asymmetric supercapacitor Yongchuan Liu, Xiaofei Miao, Jianhui Fang, Xiangxin Zhang, Sujing Chen, Wei Li, Wendou Feng, Yuanqiang Chen, Wei Wang, and Yining Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10649 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 9, 2016

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ACS Applied Materials & Interfaces

Layered-MnO2 nanosheet grown on nitrogen-doped-graphene template as a composite cathode for flexible solid-state asymmetric supercapacitor Yongchuan Liu, Xiaofei Miao, Jianhui Fang, Xiangxin Zhang, Sujing Chen, Wei Li, Wendou Feng, Yuanqiang Chen, Wei Wang*, Yining Zhang* Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou, Fujian 350002, P. R. China * To whom correspondence should be addressed: [email protected], [email protected] Abstract Flexible solid-state supercapacitor provides a promising energy-storage alternative for the rapid growing flexible and wearable electronic industry. Further improving device energy density and developing a cheap flexible current collector are two major challenges when pushing the technology forward. In this work, we synthesize a nitrogen-doped-graphene/MnO2-nanosheet (NGMn) composite by a simple hydrothermal method. Nitrogen-doped graphene acts as a template to induce the growth of layered δ-MnO2 and improves the electronic conductivity of the composite. The NGMn composite exhibits large specific capacitance of about 305 F·g-1 at a scan rate of 5 mV·s-1. We also create a cheap and highly conductive flexible current collector using Scotch tape. Flexible solid-state asymmetric supercapacitors are fabricated with NGMn cathode, activated carbon anode, and PVA-LiCl gel electrolyte. The device can achieve high operation voltage of 1.8 V and exhibit maximum energy density of 3.5 mWh·cm-3 at the power density of 0.019 W·cm-3. Moreover, it retains more than 90% of its initial capacitance after 1500 cycles. Because of its flexibility, high energy density, and good cycle life, NGMn–based flexible solid state asymmetric supercapacitor has great potential for application in next generation portable and wearable electronics. Keyword flexible solid-state supercapacitor, asymmetric supercapacitor, layered MnO2, graphene, flexible current collector

1. Introduction Flexible and wearable electronics is a rapid growing sector in the consumer electronic industry. After the recent introduction of smartwatch and various personal health-care devices, consumers 1

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are looking forward to more functionalities in next generation devices. The expectation inevitably leads to the demand for high-performance energy storage device. Among different alternatives, Flexible Solid-State SuperCapacitor (FSSSC) recently attracts lots of attention.1-3 It offers several advantages over conventional energy storage devices, such as battery and capacitor. FSSSCs exhibit higher power density and longer cycling life than batteries while possessing higher energy density than conventional capacitors. In addition, FSSSC also shows improved safety comparing to traditional supercapacitors due to the lack of liquid electrolyte.4-9 Still, a few roadblocks need to be removed on the way to its commercialization. For example, further improving FSSSC’s energy density10-11 and searching for a cheap flexible current collector12 presents two major challenges in pushing the technology forward. It is well known that the stored energy in a capacitor depends on its capacitance and voltage. Great efforts have been dedicated to prepare electrode materials with high specific capacitance. The material selection includes carbonaceous materials (activated carbon, carbon nanotubes, graphene, and carbon fibers),13-17 transition metal oxides (RuO2, MnO2, Ni(OH)2, and Fe2O3),18-22 conducting

polymers

(polyanilines,

polypyrrole,

and

polythiophenes),23-25

and

their

composites.26-28 Among them, MnO2 attracts considerable research interest because of its low cost and high theoretical specific capacitance.29 However, its poor electrical conductivity, low specific surface area, and problematic dissolution in electrolyte hinder its real world application. Combining MnO2 with conductive materials, such as graphene, appears to be an effective approach in solving some of these problems.26-28 Further exploring MnO2 nano-structure in combination with modified graphene could potentially lead to electrode materials with higher specific capacitance. Meanwhile, increasing cell voltage can also improve the energy density of supercapacitors. A supercapacitor’s operating voltage is limited by the stable potential window of its electrolyte (for example, 1.2 V for aqueous based electrolytes). Although organic electrolyte or ionic liquid can increase the operation voltage, they generally lead to lower device power density than aqueous electrolyte.30 An effective way to improve device voltage with aqueous electrolytes is to construct an asymmetric supercapacitor (ASCs), which takes advantage of the electrolyte decomposition over-potential at both electrodes.26, 31-33 In a typical ASC, a battery-like Faradaic electrode serves as the energy source while a capacitive electrode acts as the power source.34 Some efforts have 2


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been made to study flexible solid-state ASCs with different electrode combinations, such as H-TiO2@MnO2//H-TiO2@C,34 VOx//VN,5 and SWCNTs//RuO2.35 More research still needs to be done in order to establish a comprehensive material library for flexible solid-state ASCs. On the other hand, a low-cost and highly conductive flexible current collector is critical for the large scale production of FSSSCs. In a recent review,12 Dubal et al. summarized the current existing conductive substrates, including metal foil, carbon based material, modified porous material, and thin conductive layer coated cable electrode. As a 2D current collector, metal foil exhibits high conductivity and flexibility, but its high mass density limits the specific energy of the final device; carbon based materials, such as carbon cloth34, 36-37 and graphene/carbon-nanotube membrane11,

38-39

show high conductivity, but the high material cost limits their large scale

application; modified porous materials, like conventional paper, sponge, or textile, are inexpensive but suffer from low conductivity; other alternatives like metal-layer coated PET substrates11 may ran into cost and through-put problems during production. Therefore, searching for a low-cost and highly conductive flexible current collector is still an ongoing task. In this work, we design a simple route to synthesize nitrogen-doped-graphene/MnO2-nanosheet (NGMn) composites with excellent electrochemical performance. Nitrogen-doped graphene acts as a template to induce the growth of layered δ-MnO2 nanosheet. NGMn composites exhibit a large specific capacitance of about 305 F·g-1 at a scan rate of 5 mV·s-1. A cheap and highly conductive flexible substrates is created with scotch tape by an extremely simple method. Flexible solid-state ASCs are fabricated based on NGMn cathode and activated carbon anode. The obtained device operates reversibly in the voltage range of 0-1.8 V and exhibits excellent flexibility, good cycling stability, and high energy density. A maximum energy density of 3.5 mWh·cm−3 with a power density of 0.019 W·cm-3 is achieved on our flexible solid-state ASC. These values are higher than most of reported solid state SCs.3, 5, 7, 17, 36-37, 40-41 2. Material and Methods 2.1 Synthesis of NGMn Cathode. All reagents used in this experiment were of analytical grade. Graphene oxide (GO) was synthesized from natural graphite (99.95%, 1.3 µm, Aladdin) by modified Hummers method.42-43 Chemically reduced graphene was prepared by the following method: for example, 0.8 g NaHSO3 was dispersed into 200 mL GO (0.4 mg/ml) solutions under ultrasonic vibration; the mixture was 3



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then kept at 95 °C under vigorous stirring for 3 h;44 after the reaction, the chemically reduced graphene (G) was filtered from the mixture and washed repeatedly with deionized water. The thoroughly washed product was dispersed in DI water under ultrasonic vibration to form a suspension. To synthesize the nitrogen-doped graphene (NG), the graphene suspension was hydrothermally treated with ammonia solution (12.5%~14% v/v%) at 120 °C for 6 h. The NGMn composite was prepared by adding 0.1048 g KMnO4 into 70 mL NG (55 mg) suspension. The homogeneous mixture was transferred to a Teflon-lined autoclave and maintained at 140 °C for 50 min. A magnetic stir bar was put into the autoclave for slow stirring during hydrothermal reaction. After the reaction, the autoclave was naturally cooled to room temperature. The NGMn composite was filtered from the suspension and washed with deionized (DI) water. The final product was dried at 80 °C in air. For comparison, graphene/MnO2-nanosheet (GMn) composite was also prepared with chemically reduced graphene following the same procedure. 2.2 Material Characterization. Microstructures, morphologies, and composition of as-obtained NGMn composite were characterized by scanning electron microscopy (SEM, HITACHI SU8010), transition electron microscopy (TEM, JEOL JEM-2010, 200 kV), X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi), and Raman spectroscopy (Lab RAMHR800, using 532 nm laser as excitation source), respectively. The crystallographic structures were determined by powder X-ray diffraction (XRD, Miniflex600 equipped with Cu Κα radiation). 2.3 Electrochemical Characterization. To make the test electrode, active material, carbon black, and poly(tetrafluoroethylene) are mixed into a paste in a mass ratio of 82:10:8. Then the paste was brushed onto graphite paper current collector and dried at 120 °C. The mass loading of NGMn on the electrode is about 2 mg·cm−2. Cyclic voltammetry (CV) measurements were performed on an electrochemical workstation (Wavedrive10, PINE). Galvanoststic charge/discharge (GCD) measurements were carried out on a Neware battery test system with various charging current. EIS measurements were done on an electrochemical workstation (EC-lab, France) over a frequency range from 105 to 10-2 Hz with 10 mV AC amplitude. The electrochemical characterization of individual electrode was performed in three-electrode cell configuration, with a platinum counter electrode and an SCE reference electrode. The electrochemical properties of the asymmetric supercapacitor were 4


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measured in a two-electrode cell configuration. 2.4 Fabrication of aqueous Asymmetric Supercapacitor (ASC). NGMn cathode and activated carbon (AC) (TF-B520, ShangHai Carbosino Material CO., LTD) anode were assembled into aqueous ASCs. To balance the charge between the electrodes, the mass loading values of anode and cathode were about 3 mg/cm2 and 4 mg/cm2, respectively. LiCl (5 M), LiClO4 (1 M), and Na2SO4 (1 M) aqueous solution were tested to determine the optimal electrolyte. 2.5 Fabrication of Flexible Solid-State Asymmetric Supercapacitor. The flexible substrate was produced simply by pasting a piece of scotch tape over graphite paper (Thermal Conductive Graphite Paper, Langfang Hengtong Sealing Materials Co., Ltd., thickness of 0.33 mm), applying pressure, and then pulling it off. The NGMn and AC electrodes were fabricated with the flexible substrates following the aforementioned method. The Solid-State ASCs was assembled by separating the NGMn and AC electrodes with a thin PVA/LiCl gel membrane, which serves as both separator and electrolyte. PVA/LiCl gel membrane was produced by drying a layer of PVA/LiCl sol on a piece of PET film at 60 °C. The PVA/LiCl sol was prepared by mixing LiCl (2.12 g) and PVA (5.0 g) in 50 mL deionized water and stirred under 85 °C until the solution became clear. Both electrodes were soaked in the PVA/LiCl sol and then assembled into device with a PVA/LiCl gel separator sandwiched in between. The device was kept at 45 °C for 12 h to ensure the complete solidification of PVA/LiCl gel. The thickness of the final device was about 0.45 mm. 3. Theory and Equations: The gravimetric specific capacitance of the individual electrode is calculated from the CV curve based on the following equation:

Cs =

A · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (1) 2 sm∆V

Here, A is the integral area of the closed CV curve, m is the mass of the active material, ∆V is width of the potential widow, s is the potential scan rate. Gravimetric specific capacitance (CG), specific energy (EG) and specific power (PG) of ASCs are calculated from the charge/discharge curve according to the following equation:

CG =

i∆t M ∆V

· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (2)

5



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1 EG = CG ∆V 2 2

· · · · · · · · ·· · · · · · · · · · · · · · · · · · · · (3)

EG · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (4) ∆t

PG =

, where i is the charge/discharge current, ∆t is the time of discharging, M is the total mass of two electrodes and ∆V is the device operating voltage. Volumetric specific capacitance (CV), energy density (EV) and power density (PV) of solid-state flexible ASCs are calculated from the charge/discharge curves based on the following equation:

CV =

i∆t · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (5) V ∆V

1 EV = CV ∆V 2 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (6) 2

PV =

EV ∆t

· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · (7)

, where i is the charge/discharge current, ∆t is time of discharging, V is the volume of the whole flexible solid-state device (including current collector and solid electrolyte) and ∆V is the device operating voltage. For asymmetric supercapacitors, the charge balance (q+=q-) between cathode and anode is required. Typically, the charge balance is determined based on the electrochemically properties of both electrodes under 10 mV·s-1 scan rate. The mass balancing is calculated from Equation (9) based on the specific capacitance (Cs) and potential range (∆V):

q = m × Cs × ∆V m m

+

=

−

· · · · · · · · · · · · · · · · · · · · · · · · · · (8)

Cs − × ∆V− · · · · · · · · · ·· · · · · · · · · · · · · · · · · (9) Cs + × ∆V+

, where m+ is the mass of active materials in cathode, m- is the active-material mass of anode. Because the specific capacitance generally changes with the mass loading of active materials and cannot be treated as a constant, we adopt a more comprehensive graphic method to determine the mass balancing in this work. (See details in Results and Discussion) 4. Results and Discussion Previous research has shown the electrochemical properties of MnO2 depend on its crystallographic nature and layered birnessite type δ-MnO2 is a promising MnO2 phase for energy 6


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storage application.45 Therefore, we use Nitrogen-doped Graphene (NG) in our synthesis for two reasons: first, to improve the electrical conductivity of the composite; second and more importantly, to serve as the template to induce the growth of layered birnessite type δ-MnO2. Compared with the graphene oxide template in a reported method of MnO2 synthesis,46 NG template is a stronger reducing agent and hence should react with KMnO4 more readily and facilitate the growth of MnO2. The short reaction time (50 min) and constant stirring during reaction both suppress the grain growth and therefore enable the formation of MnO2 nano-structure. The reaction between KMnO4 and NG can be expressed as follows:

4MnO-4 +3C+H 2 O → 4MnO 2 +CO32- +2HCO3After the hydrothermal reaction, MnO2 nanosheets were grown homogeneously on NG. Figure 1 shows the typical morphology and microstructure of the NGMn composite. Low-magnification SEM image (Figure 1a) shows its curly and interconnected structure, with MnO2 grown on graphene surface. High-magnification SEM image (Figure 1b) clearly reveals the corrugated nanosheet morphology, which can be easily accessed by the electrolyte ions. Meanwhile, the interconnected NG can effectively improve the conductivity of the composite. Figure 1c shows the X-ray diffraction (XRD) patterns of the NGMn composites produced by different reaction time (50 min and 120 min). In the 50-min sample, a broad hump around 2θ of 25° indicates the disordered stacking of graphene sheets. No obvious diffraction can be observed, likely because of the poor crystallinity and small grain size of MnO2 resulted from stirring and short reaction time. Extending the reaction time to 120 min reveals the formation of birnessite-type δ-MnO2 (JCPDS No.42-1317).47-48 Energy dispersive X-ray spectroscopy (EDS) show C, Mn, O, and N are the major elements in the composite (Figure S1). The presence of N indicates the successfully doping of graphene sheets. K can be detected likely because of its intercalation into δ-MnO2. The Al signal comes from the Al sample holder. Figure S2 shows a SEM image of NGMn composite and the corresponding EDS elemental mapping, which reveals the uniform distribution of MnO2 over nitrogen doped graphene.

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Figure 1. (a) Low- and (b) high-magnification SEM images of the NGMn composites. (c) X-ray diffraction (XRD) patterns of the NGMn composites produced by different reaction times (50 min and 120 min).

Figure 2. (a) TEM images of the NGMn composite; (b) zoom-in TEM image of the blue dashed line area in (a); (c) high resolution TEM image of the NGMn composite, showing MnO2 lattices; (d) selected area electron diffraction (SAED) pattern of the NGMn composite.

TEM images further confirm the deposition of layered-structure MnO2 nanosheet on NG surface (Figure 2a, Figure S3a, Figure S3b). Figure 2b is the high-resolution TEM (HRTEM) 8


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image collected at the blue dash-line area in Figure 2a, showing a thin MnO2 layer on NG sheet. Various domains and grain boundaries can be observed, indicating the polycrystalline nature of MnO2 nanosheets. The measured lattice spacing of the MnO2 nanosheet from Figure 2c is 0.24 nm, which corresponds to the (11-1) plane of birnessite-type δ-MnO2. The selected area electron diffraction (SAED) pattern (Figure 2d) contains three faint continuous diffraction rings, showing the polycrystalline nature of the composite. These diffraction rings can be indexed as (001), (110), (11-1), and (31-2) plane according to JCPDF card No.42-1317.

Figure 3. XPS spectrum: survey (a), Mn 2p (b), O 1s (c) and N 1s (d) of NGMn.

The X-ray photoelectron spectroscopy (XPS) survey spectrum (Figure 3a) confirms the presence of C, Mn, O, and N on the surface of NGMn composite. In the high resolution Mn 2p spectrum (Figure 3b), two peaks centered at 654.1 eV and 642.3 eV can be attributed to the binding energy of Mn 2p1/2 and Mn 2p3/2, respectively. The spin-energy separation of 11.8 eV indicates the oxidation state of Mn is +4. The O1s spectrum (Figure 3c) of NGMn can be fitted into three peaks at binding energies of 529.9, 531.8, and 533.5 eV, which corresponds to the O-Mn, O-C, and O-H bond, respectively. The N1s spectrum in Figure 3d indicated the presence of three different types of nitrogen in NGMn, namely pyridinic-N (399.0 eV), pyrrolic-N (400.0 eV), and 9



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graphitic-N (401.1 eV). The total nitrogen atomic concentration is about 1.75 %. Raman spectrum of the NGMn composite is shown in Figure S4, the existence of D band (1352 cm−1) and G band (1595 cm−1) reveals the presence of graphene. Meanwhile, a weak peak centered at 561 cm-1 could be attributed to the stretching vibration of MnO6 groups.

Figure 4. (a) CV curves of NGMn electrode at different scan rates in 5 M LiCl electrolyte. (b) CV curves of NGMn electrode in three electrolytes at a scan rate of 10 mV·s-1. (c) Comparison of specific capacitance of NGMn electrode in three electrolyte solutions at different scan rates. (d) Galvanostatic charge/discharge curves of NGMn symmetric supercapacitor under various charging current densities in 5 M LiCl aqueous electrolyte.

We characterized the electrochemical properties of NGMn electrode in a three-electrode cell configuration with a Pt counter electrode and an SCE reference electrode. The cyclic voltammetry (CV) curves of NGMn at different scan rates were first measured in three different aqueous electrolytes, including 5 M LiCl (Figure 4a), 1 M LiClO4 (Figure S5a), and 1 M Na2SO4 electrolytes (Figure S5b). All CV curves possess a symmetric rectangular shape at applied scan rates. Among the three tested electrolytes, NGMn exhibits the most ideal capacitive behavior in LiCl electrolyte at all scan rates. The 10 mV·s-1 CV curves collected in the three electrolytes are compared in Figure 4b. The redox peaks of NGMn in LiCl electrolyte are nearly reversible and the current density is also higher than in the other electrolytes. Hence, we decide to use LiCl 10


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electrolyte for all further electrochemical characterizations. Specific capacitance (Cs) values of NGMn electrode at different scan rates in LiCl electrolyte are shown in Figure 4c. NGMn electrode yields the highest specific capacitance of 248 F·g-1 at the scan rate of 5 mV·s −1, which converts to a value of 305 F·g-1 for NGMn based on the mass loading ratio. When the scan rate increases from 5 to 100 mV·s

−1

, NGMn shows superior rate performance by retaining 65.6% of

its capacitance. Figure 4d is galvanostatic charge/discharge (GCD) curves of NGMn symmetric supercapacitor in 5 M LiCl aqueous electrolyte at different current densities. The GCD curves are slightly distorted from the ideal triangle shape, because of the pseudocapacitive contribution from MnO2. Based on equation 2, NGMn electrode achieved specific capacitance value of about 217.8 F·g−1 and 150.8 F·g−1 at the current density of 0.25 A·g-1 and 2 A·g-1, respectively. The excellent electrochemical properties of NGMn composite can be attributed to the synergistic effect between the nitrogen doped graphene and layered-MnO2. To show the electrochemical enhancement, we also characterize the electrochemical properties of chemically reduced graphene (G), nitrogen doped graphene (NG), and graphene/MnO2-nanosheet (GMn). The results are shown in Figure S6. The NGMn electrode shows rectangular CV curves while the CV curves of GMn deviate from the ideal rectangular shape (Figure S6d). Moreover, the NGMn electrode exhibits superior rate performance comparing to GMn electrode (Figure S6e and Table S1). Both observations indicate the decrease of electrochemical impedance by nitrogen doping. On the other hand, the increase of specific capacitance by pseudo-capacitive MnO2 is more significant on NG (77.1% increase under 5 mV·s-1) than G (50.4% increase under 5 mV·s-1). This proves NG serves as a more effective template for growing δ-MnO2 nanosheet.

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Figure 5. (a) CV curves of AC electrode at different scan rates in a potential window of -0.9 to 0.1 V in 5 M LiCl electrolytes; (b) Specific capacitance calculated for NGMn and AC electrodes based on different mass loading values at the scan rate of 10 mV·s-1; (c) Electric charge value calculated for NGMn and AC electrodes based on different mass loading values under 10 mV·s−1 scan rate; (d) CV curves collected for NGMn and AC electrodes at a scan rate of 10 mV·s−1.

Since NGMn composite demonstrates high specific capacitance and stable voltage windows between -0.1 and 0.9 V (vs. SCE), it could be a good cathode candidate in an ASC. To fabricate ASCs, commercial AC is chosen as anode material. CV curves of AC electrode at different scan rates between -0.9 V and 0.1 V (vs. SCE) are shown in Figure 5a. AC electrode exhibits ideal capacitive behavior with a nearly rectangular shape in the measured potential window. During ACS fabrication, it is important to balance the stored charge (q+ = q-) between cathode and anode. Conventionally, the mass loading of active material in cathode and anode is determined by Equation (9). However, as show in Figure 5b, Cs decreases significantly with increasing mass loading. The phenomena has also been pointed out in the literature.49-50 Therefore, using equation [9] with a single unchanged Cs to determine the mass loading value of both electrodes may be problematic. Here, we present a more comprehensive approach. We calculated electric charge 12


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value for NGMn and AC electrodes based on different mass loading values and plot the results in Figure 5c. From Figure 5c, the optimal mass loading of the NGMn and AC electrode are determined to be 3 mg·cm-2 and 4 mg·cm-2 respectively, based on the Cs values at the scan rate of 10 mV·s-1 in their corresponding potential windows. Figure 5d shows CV curves of NGMn and AC electrodes at a scan rate of 10 mV·s−1 with optimized mass loading. When AC is adopted as anode against NGMn cathode, the operating voltage of the cell could extend to 1.8 V (Figure 5d). We measure the CV curves of an NGMn//AC ASC under different operation voltages in 5 M LiCl aqueous solution at a scan rate of 10 mV·s−1 (Figure 6a). The ACS exhibits ideal capacitive characteristic with rectangular CV curve, even when the device operates to 1.8 V. The Cs values of the ACS measured within different potential windows under 1 A·g-1 current density are shown in Figure S7. The Cs value increases from 46.1 to 66.7 F·g-1 with the increasing operation voltage from 1 to 1.8 V (Figure 6b). CV curves of the ASC measured at different scan rates between 0 and 1.8 V are plotted in Figure 6c. The CV curves possess rectangular shapes, even under a high scan rate of 100 mV·s-1, revealing the ideal capacitive behavior and fast charge/discharge property of the ASC. GCD curves at various current densities (Figure 6d) are quite symmetric with little IR drop, indicating its low internal resistance and excellent electrochemical reversibility. Gravimetric specific capacitance (CG) was calculated from the GCD curve using the Equation (2). The CG of an ACS reaches 71.9 F·g −1 at the current density of 0.25 A·g-1. According to the Equation (3) and (4), the maximum specific energy (EG) of the ACS is 31.67 Wh·kg-1 with a specific power (PG) of 222.5 W·kg-1 and the EG remains 20.85 Wh·kg-1 under a high PG of 1614.6 W·kg-1. The excellent performance of NGMn//AC ASCs are attributed to the high Cs and low internal resistance of both NGMn composite and AC.

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Figure 6. (a) CV curves of an optimized NGMn//AC asymmetric supercapacitor measured at different potential windows in 5 M LiCl aqueous solution at a scan rate of 10 mV·s−1. (b) Specific capacitance of the asymmetric supercapacitor measured with different potential windows at a current density of 1 A·g-1. (c) CV curves of NGMn//AC asymmetric supercapacitor measured at different scan rates between 0 and 1.8 V. (d) Galvanostatic charge/discharge curves of NGMn//AC supercapacitor at different current densities from 0 to 1.8 V.

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Figure 7. (a) Optical photographs of the flexible current collector and the fabricated solid-state supercapacitor device. The left panel shows the good electrical conductivity and flexibility of the current collector made from Scotch tape. The upper right panel presents the appearance of the devices and the bottom right panel demonstrates its flexibility. (b) CV curves of the fabricated solid-state supercapacitor measured at different scan rates between 0 and 1.8 V. (c) CV curves collected at a scan rate of 10 mV·s−1 for the flexible device under various bending conditions. (d) Galvanostatic charge/discharge curves of the flexible solid-state supercapacitor at different current densities between 0 and 1.8 V. (e) EIS results of ASC devices. Inset shows an enlarged portion of the plot near the origin.

As we discussed in the introduction, existing flexible current collectors suffer from various problems, such as heavy weight, low conductivity, or high cost. Here, inspired by the mechanical exfoliation of graphite, we prepare a flexible current collector by simply pulling a piece of scotch 15



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tape from graphite paper, as shown in Figure 7a. This current collector shows great flexibility and high electrical conductivity. The resistance is 14.2 Ω across about 30 cm in distance. Flexible all-solid-state ASCs are fabricated with NGMn cathode and AC anode on our flexible current collector. PVA/LiCl gel was utilized as both separator and electrolyte. The fabricated solid-state ASCs are lightweight and flexible (Figure 7a right panel). The CV curves of the Flexible Solid-State SuperCapacitor (FSSSC) are shown in Figure 7b, demonstrating the near ideal capacitive characteristics of the device. In Figure 7c, the CV curves measured at various bending angles almost overlap with each other, showing the robust electrochemical properties of the device upon flexing. A video of a red LED powered by an FSSSC is provided in the supporting information to demonstrate the flexing stability of the device. Figure 7d shows GCD curves of the FSSSC measured between 0 and 1.8 V under different current densities. The triangular-shaped GCD curves again demonstrate its capacitive characteristics. The FSSSC reaches a high volumetric specific capacitance (CV) of 8.15 F·cm-3 under current density of 1 mA·cm−2. The result of EIS measurement on FSSSC is shown in Figure 7e. The straight line in the low frequency region reveals the ideal capacitive behavior of the device. The arc in the low frequency area has a small radius, indicating low charge transport resistance. The equivalent series resistance (ESR) of the electrode is about 4 Ω by extrapolating the straight line to the Z’ axis, which proves the high electrical conductivity of both electrodes and the high ionic conductivity of the PVA/LiCl solid electrolyte.

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Figure 8. (a) Galvanostatic charge/discharge curves of single SC and two SCs connected in series; (b) Photograph of a blue LED powered by the two supercapacitors in series; (c) Cycling stability test of the solid-state ASC device measured at a current density of 5 mA·cm-2; (d) Ragone plots for ASCs reported in the literature and our work.

Figure 8a shows the GCD curves of a single FSSSC and a pair of FSSSCs connected in series. The connected pair can operate reversibly between 0 and 3.6 V. Figure 8b is the photograph of a blue LED (2.5 V voltage requirement) powered by two supercapacitors connected in series. GCD curves of the solid-state supercapacitor operating between different potential ranges under a current density of 4 mA·cm-2 are shown in Figure S8a. The FSSSC exhibits columbic efficiency higher than 88% when operating under various potential windows (Figure S8b). However, it can be found from Figure S8 that the columbic efficiency of the FSSSC decreases from 97.5% to 88% with the increasing of operating voltage from 1 V to 1.8 V, because of the irreversible electrode reaction and polarization of the electrode. To further evaluate the cycle stability of the solid-state device, GCD measurements were carried out at a current density of 5 mA·cm-2. According to Figure 8c, the FSSSC kept 90.5% of its initial capacitance after 1500 cycles, demonstrating good cycling stability. Figure 8d shows the Ragone plots for the entire solid-state ASC device. The energy density of our solid-state ASC was higher than most of reported solid state SCs. A 17



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remarkable energy density of 3.5 mWh·cm−3 was achieved with a power density of 0.019 W·cm-3 with an operating voltage of 1.8 V. In addition, when the power density reaches 0.18 W·cm-3, the energy density can still maintain at 2.02 mWh·cm-3. The results indicate that NGMn//AC flexible solid-state asymmetric supercapacitor has great potential for use in portable and wearable energy-storage systems.

5. Conclusion In summary, we design a strategy for synthesizing nitrogen-doped-graphene/MnO2-nanosheet composite with excellent electrochemical performance. We use nitrogen-doped graphene as template to induce the growth of layered-structure birnessite-type δ-MnO2. NGMn composite exhibits a large specific capacitance of about 305 F·g-1 at a scan rate of 5 mV·s-1. An FSSSC was fabricated with NGMn-composite cathode and AC anode. A novel graphic approach, taking into account of the specific capacitance change with active mass loading, is adopted to determine the cathode-anode charge balance in ASCs. By pulling Scotch tape from graphite paper, we make a cheap and highly conducive flexible current collector for the FSSSC fabrication. The FSSSC can operate reversibly to 1.8 V and retain more than 90% of its initial capacitance after 1500 cycles. Additionally, the device exhibits a maximum energy density of 3.5 mWh·cm-3 at the power density of 0.019 W·cm-3. Because of its flexibility, high energy density, and good cycle life, NGMn//AC FSSSC shows great potential for application in next generation portable and wearable electronics.

Supporting Information. EDS spectrum, SEM/EDS elemental mapping, TEM images, and Raman spectrum of the NGMn composite; CV curves of NGMn electrode at different scan rates in LiClO4 and Na2SO4 electrolytes; Electrochemical characterization of NG, GMn, and G electrodes; GCD curves of the NGMn–based asymmetric supercapacitor at different operation voltages; GCD curves of the NGMn//AC flexible solid-state supercapacitor and the coulombic efficiency under different operation voltages at current density of 4 mA/cm2.

Notes The authors declare no competing financial interest.

Acknowledgments We thank the financial support from Fujian Provincial Department of Science and Technology (Grant No.2014H2008 and No. 2015H0052). 18


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TOC Figure

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Layered-MnO2 Nanosheet Grown on Nitrogen-Doped Graphene

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