In Situ Growth of Free-Standing All Metal Oxide Asymmetric

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In situ Growth of Free-standing All Metal Oxide Asymmetric Supercapacitor Bosi Yin, Zhen-Bo Wang, Siwen Zhang, Chang Liu, Qingqing Ren, and Ke Ke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08037 • Publication Date (Web): 14 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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ACS Applied Materials & Interfaces

In situ Growth of Free-standing All Metal Oxide Asymmetric Supercapacitor Bo-Si Yin 1, Zhen-Bo Wang 1,*, Si-Wen Zhang 1, Chang Liu 1, Qing-Qing Ren, 1 Ke Ke 1* 1

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and

Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No.92 West-Da Zhi Street, Harbin, 150001 China. E-mail: [email protected]; Tel.: +86-451-86417853; Fax: +86-451-86418616 Abstract Metal oxides have attracted renewed interest in applications as energy storage and conversion devices. Here, a new design is reported to acquire an asymmetric supercapacitor assembled by all free-standing metal oxides. The positive electrode is made of 3D NiO open porous nanoribbons network on nickel foam and the negative electrode is composed of SnO2/MnO2 nanoflakes grown on carbon cloth (CC) substrate. The combination of two metal oxide electrodes which replaced the traditional group of carbon materials together with metal oxide has achieved a higher energy density. The self-supported 3D NiO nanoribbons network demonstrates a high specific capacitance and better cycle performance without obvious mechanical deformation despite of undergoing harsh bulk redox reactions. The SnO2/MnO2 nanoflakes as the pseudocapacitive electrode exhibit a wide range of voltage window (-1V~1V),

which

is

conducive

to

electrochemical

energy

storage.

The

(CC/SnO2/MnO2)(−)//(NiO/Ni foam)(+) asymmetric supercapacitor device delivers an energy density of 64.4 Wh kg-1 (at a power density of 250 W kg-1) and two devices in series are applied to light up 24 red LEDs for about 60s. The outstanding electrochemical properties of the device hold great promise for long-life, high-energy and high-power energy storage/conversion applications.

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Keywords: NiO Nanoribbons; SnO2/MnO2 Nanoflakes; Hydrothermal method; Metal oxides; Asymmetric supercapacitor device

1. Introduction The limited availability of fossil fuels together with fast-growing demand of energy will lead to the rapid-growth of high-performance and sustainable electrochemical energy storage devices. Systems for energy storage and conversion include batteries1, fuel cells2, and supercapacitors (SCs)3. Among numerous energy-storage devices, SCs are mostly promising as future self-powered candidates because of many outstanding features4-7. It has been proved that the SCs are able to establish a bond of batteries and conventional capacitors using its merits like good cycle stability, fast charge-discharge characteristics, high power densities and so on8-14. The demand for the next generation devices, such as SCs, need to design more promising nanostructured materials offering the high electrochemical performance as well as the long cycling life15-18. Tin oxide (SnO2), a significant n-type semiconductor is widely used in various energy storage systems including transparent electrodes19, lithium–ion batteries20,21 dye-sensitized solar cells22 and supercapacitors23,24, due to its low toxicity, low cost and excellent electrical properties. There are extensive researches which focus on the synthesis of 0D to 3D nanostructured SnO2 materials. Despite this, the problems like how to improve the capacitance still exist. Therefore, the design of SnO2 in nanoscale zone with better electrical conductivity and larger surface area in order to get excellent performance is seem to be a promising solution. Compared to a bare SnO2 nanostructured electrode, the SnO2@MOx (M=Ni, Co, Mn...) heterostructure composites always show superior electrochemical properties25, so SnO2-based composite electrode is an efficient way to obtain the enhanced electrical conductivity and more efficient active sites with the benefits of synergistic effect. 2


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Vigorous researches utilize SnO2 and their composites as negative electrode, and carbon-based materials, like activated carbons, graphene as positive electrode to assemble a supercapacitor. However, metal oxides (MOs) in general exhibit higher electrochemical energy storage ability arising from faradic redox reactions than carbon-based materials. Unfortunately, the reports which concentrate on fabricating all metal oxide supercapacitor using MOs as both the positive and negative electrode is not too much. There is an imperative need to develop a new class of supercapacitors based on MOs with higher energy densities than normal carbon materials. Herein, we put forward a high-performance (carbon cloth(CC)/SnO2/MnO2)(-)//(NiO/Ni foam)(+) button type alkaline aqueous asymmetric supercapacitor(ASC) device based on self-supported negative electrode and free-standing positive electrode. Although SnO2/MnO2 and NiO electrode material are used widely, respectively, for the first time, the unique design which combined SnO2/MnO2 and NiO together is assembled here and investigated as a novel small size ASC device. As a typical MO, nickel oxide (NiO) possessing high electrical conductivity and high theoretical capacitance (2584 F g-1) is a good choice to replace the positive carbon based electrode26-28. This work provides a fresh perspective on using all metal oxide nanostructures instead of normal carbon/metal oxide hybrid device. To improve the performance of the ASC device, three major strategies have been proposed (Fig. 1). The first effective approach is to enhance the transport kinetics by building a direct mechanical and electrical contact between nanostructure electrode and the current collector3. The second one is to synthesize SnO2 nanostructured negative electrode with desirable architecture and smooth surface, and then an increased specific surface area is obtained by coating MnO2 nanosheet networks on bare SnO2. The reason to choose this route is: I) the capacitive performance of material is associated with its surface area. The Brunauer–Emmett–Teller (BET) surface area of bare SnO2 electrode dramatically increases from 2.6 to 44.5 m2 g−1 after

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coating MnO2 and the corresponding discharge time of the composite electrode shows a 3-fold increase together with a wider range of voltage window; II) MnO2 is a typical pseudo-capacitive material with high theoretical specific capacitance (1370 F g-1)29–33 which may improve the capacitance ability of the composite electrode. In addition, the interconnected structure of MnO2 is much stable toward cycle performance25. The last improvement plan is using alkaline KOH aqueous electrolyte which has intimate interfacial contact between active materials and electrolyte instead of PVA/KOH gel electrolyte the one being relatively sluggish.

Fig.1 Schematic illustration of the preparation process of the (CC/SnO2/MnO2)(−)//(NiO/Ni foam)(+) ASC device.

2. Results and discussion 2.1 Negative electrode material The scanning electron microscope (SEM) image of the pure SnO2 is shown in Fig. 2a. It is obvious to see that the nanosheets possess clean and smooth surface with the thickness of ~20 nm. The direct contact without any adhesive between SnO2 nanostructure and the CC current collector can avoid the transfer block existed in the electrolyte. Moreover, the 3D SnO2/CC lamellar structures also provide a larger surface area of active material, which is benefit to the further deposition of MnO2 as the supporting material. Through a two-step 4


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hydrothermal growth method, the CC/SnO2/MnO2 heterostructure is obtained (Fig. 2b and the inset). Due to the decomposition of KMnO4 molecules under hydrothermal conditions, numerous tiny primary MnO2 nuclei are produced. Further aggregation of these nuclei based on the surface of SnO2 nanosheets induces the formation of manganese dioxide precursor34. After 1 h of the hydrothermal reaction, the CC/SnO2 surface are is entirely coated uniformly with MnO2 nanoflakes. Such unique architecture of connected MnO2 nanoflakes loaded uniformly on the 3D CC/SnO2 can provide abundant active sites for redox reaction. To examine the crystal phases, the as-prepared products were tested by X-ray diffraction (Fig. 2c). All the diffraction peaks are in agreement with the standard card (no. 41-1445; ao = bo = 4.738 Å, and co = 3.187 Å) and can be well indexed to the tetragonal rutile structure. Here are seven peaks with 2θ values of 26.01, 33.22, 37.21, 51.19, 53.50, 61.02 and 65.16, corresponding to SnO2 crystal planes of (1 1 0), (1 0 1), (2 0 0), (2 1 1), (2 2 0), (3 1 0) and (1 1 2), respectively. The elemental mapping of CC/SnO2/MnO2 heterostructures (Fig. 2d-i) shows the existence of Mn, O and Sn elements and the presence of C (coming from CC), revealing that the SnO2 sheets are homogeneously coated by MnO2 flakes. The red line represents the diffraction peaks of CC/SnO2/MnO2 heterostructures, which is almost the same as that of CC/SnO2 because MnO2 are too weak to discover. In order to further confirm the existence of Mn element, the corresponding EDS spectrum test selected from the yellow area of the inset in Fig. 2b has been performed (Fig. S1, Supporting Information). The loading amount of Mn on the SnO2 nanosheets is 32% (atomic percentage). The microstructures of SnO2/MnO2 compound are further investigated by transmission electron microscopy (TEM). Fig. 2j shows a single SnO2/MnO2 nanofake. The composite exhibits a typical core-shell structure which consists of a SnO2 nanosheet ‘core’(20 nm in thickness) and a MnO2 ‘shell’ (30 nm in thickness). The results that SnO2 nanasheets uniformly covered the MnO2 nanoflakes highly agree with the observations from SEM images (Fig. 2b). The interplanar spacing of 0.23 nm

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in HRTEM image (Fig. 2k) corresponds to the (211) plane of α-MnO2. The fast Fourier transformation (FFT) pattern, which clearly displays regular diffraction spots, is further evidenced the single-crystalline characteristic of MnO2 (the inset of Fig. 2k).

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Fig. 2 (a) The SEM image of the pure SnO2 and the inset is high magnification SEM image of SnO2 nanosheets. (b) The SEM image of the CC/SnO2/MnO2 heterostructure; the yellow area of the inset was tested by EDS. (c) X-ray diffraction pattern of the prepared products. (d-i) The elemental mapping images of CC/SnO2/MnO2 heterostructure. (j) The typical TEM image of a single SnO2/MnO2 nanoflake. (k) HRTEM image and the fast Fourier transformation (FFT) pattern of SnO2/MnO2 heterostructure.

Fig. 3a is the XPS spectra of SnO2/MnO2 nanostructure. The peaks of manganese (Mn 2p) 6


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and tin (Sn 3p, 3d, 4p, 4d) confirm the presence of Mn and Sn elements in the SnO2/MnO2 hybrid. The peaks of Sn 3d5/2 and Sn 3d3/2 located at 487.5 and 495.9 eV have an 8.4 eV peak-to-peak separation (Fig. 3b). The Mn 2p3/2 and Mn 2p1/2 broad peaks around 642.3 and 653.8 eV (Fig. 3c) demonstrate that Mn4+ ions are dominant in the product35. To investigate the capacitive ability of SnO2/MnO2 electrode material, the electrochemical tests were first detected in a three-electrode configuration. Figure 4 a,b shows the cyclic voltammetry (CV) curves of pristine SnO2 and SnO2/MnO2 electrodes at 5-80 mV s-1 (vs. SCE), respectively. The CV curves with rectangular shape of SnO2 indicate an ideal capacitive behavior because of the fast intercalation/de-intercalation of K+ between the electrolyte and active material.

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Fig. 3 The XPS spectra: (a) SnO2/MnO2 nanocomposite; (b) Sn 3d; (c) Mn 2p.

In addition, no obviously change existed in the CV curves shows the good capacitive property (Fig.4a). The CV curves of the SnO2/MnO2 composite present obvious redox peaks which reveal the pseudocapacitance characteristics of MnO2. Compared to bare SnO2, such an enhanced current plateau of SnO2/MnO2 electrode increases the total capacitance coming from the swift and reversible redox reactions (Fig. 4b). The energy storage mechanisms of heterostructure are expressed as follows equations (Eqn.) 25, 34: (MnO2)surface + M+ + e- = (MnOOM)surface

(1)

(SnO2)surface + M+ + e- = (SnO--M+)surface

(2)

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(M+ = Li+, Na+, K+ or H3O+) Both of them are surface processes coming from the adsorption/desorption of alkali cations which is existed in the electrolyte. Both SnO2 and MnO2 can attribute to the entire capacitance. The galvanostatic charge–discharge (GCD) behaviors of both pristine and SnO2/MnO2 nanoflakes network are also presented in Fig. 4c-d. The internal resistance (IR) potential drop in the discharging curve of SnO2/MnO2 is 0.021 V, which is smaller than 0.035 V of the pure electrode, indicating enhanced conductivity, excellent reversibility and high coulombic efficiency of the composite. Importantly, for a hybrid device using aqueous electrolyte, its maximal energy density is limited mainly by the capacitor electrode. The maximal operational voltage always plays a significant role in the maximum energy density36. So, the wider voltage window will bring the incredible increase of the corresponding energy and power densities. After coating MnO2, the working voltage of the electrode increases from (-1 ~ -0.5 V) to (-1 ~ +1 V), which is beneficial to improve the performances of asymmetric device. Electrochemical impedance spectroscopy (EIS) test measured in the frequency between 0.01 Hz and 100 kHz is used to investigate the ion diffusion and charge transfer behavior within the electrode. The Nyquist plots of SnO2 and SnO2/MnO2 are given in Fig. 4e. The real axis intercept in the high frequency region reflects the equivalent series resistance (Rs) which represents the intrinsic resistance of the substrate, ionic resistance in the electrolyte and contact resistance between the current collector and active material37. It is apparent that Rs (0.8 Ω) of SnO2/MnO2 is much smaller than that (2.4Ω) of the pristine SnO2, indicating that the electroactivity and conductivity of the nanostructure on CC is indeed improved after the introduction of MnO2 nanoflakes network. Due to the excellent electrical conductivity of SnO2 nanosheets, there is no distinct semicircle for both the two electrodes which is also suggested a small charge transfer resistance (Rct). The straight line of SnO2/MnO2 composite electrode in the low frequency region is more vertical compared with

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Fig. 4 (a-b) The cyclic voltammetry (CV) curves of pristine SnO2 and SnO2/MnO2 electrodes at 5-80 mV s−1. (c-d) The galvanostatic charge–discharge curves of pristine SnO2 and SnO2/MnO2 nanoflakes. (e) The Nyquist plots of SnO2 and SnO2/MnO2 measured in the same range from 100 kHz to 0.01 Hz. (f) The specific capacitance of SnO2 in the potential range of -1 to -0.5 V and SnO2/MnO2 in the potential range of -1 to 1 V.

SnO2 sample, which not only demonstrates that the diffusion of ions is essentially modified after coating MnO2 from solution to the SnO2 nanosheets but also implies a better capacitive performance because of a faster ion diffusion rate38. According to the CV curves, the specific capacitance can be evaluated for pristine and composite, respectively, as shown in Fig. 4f. The specific capacitances of SnO2/MnO2 composite are 782.2, 610, 323.4, 273.4, 211.5 F g-1 at 5, 9



10, 30, 50, and 80 mV s-1. But the specific capacitances of SnO2 electrode are only 291, 254.3, 151.4, 134.6, 133.8 F g-1 at the same scan rates (based on the Eqn. S(1) in supporting information). Besides the tests of SnO2, the shell MnO2 is also been studied. The SEM image and CV curves of MnO2 are shown Fig. S2a and 2b (Supporting Information). The contribution of capacitances from MnO2 and carbon cloth are also presented (Fig. S2c and 2d). According to the N2 adsorption isotherm, the Brunauer-Emmett-Teller specific surface areas of before and after coating MnO2 are calculated to be 2.6 and 44.5 m2 g−1 (Fig. 5a,b). From the narrow hysteresis loop at the low relative pressure and no significant delay in the capillary evaporation, it can be inferred that the SnO2/MnO2 nanostructure is quite open. The

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0.012 0.010 0.008 0.006 0.004 SnO2

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Fig. 5 (a-b) Typical nitrogen adsorption-desorption isotherm of SnO2 and SnO2/MnO2. (c-d) The average pore sizes of SnO2 and SnO2/MnO2.

average pore sizes are shown in Fig. 5c and d. The high surface area with more active sites and moderate pore size (2-5 nm) of SnO2/MnO2 composites provide more opportunities for 10


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efficient transport tunnel of the ions and electrons and lead to enhance electrochemical properties39. Cycle stability is a significant consideration in practical applications of materials. Fig. S3 (Supporting Information) is the cycle performance of SnO2/MnO2 at scan rate of 1.5 mV s-1 for 4000 repetitive cycles. The SnO2/MnO2 composite electrode exhibits an ideal long-term stability with a capacitive retention of 94.5%, which is superior to that of bare SnO2 (91.5%). Such a good performance of SnO2/MnO2 composite electrode could result from its unique structural feature. During the cycle process, the interconnected MnO2 networks can alleviate the structural damage caused by volume expansion and lead to its enhancement of stability. The SnO2/MnO2 electrode with excellent stability, low resistance and unique structure make it a qualified material candidate for energy conversion and storage applications.

2.2 Positive electrode materials The SEM image of the NiO-coated Ni foam through the hydrothermal process is shown in Fig. 6a. With immersing in Ni(NO3)2 solution and high temperature treatment, a layer of nanoribbons is found deposited uniformly on the Ni foam. As is known to all, the kinetic behaviors of the electrode materials are depended on the transportation of electrons and ions in the active material. This NiO film is made up of many interconnected nanoribbons, forming a highly efficiently exposed framework structure which allows easy access of electrolyte to every nanoribbon. Besides, NiO directly grown on the Ni foam without any block of a binder is also facilitating the charge transport and ion diffusion when using as an electrode. Upon closer examination (the inset image in Fig. 6a, these smooth NiO nanoribbons with the thickness of 20 nm form into a uniform structure. From the cross-sectional view of the nanoribbon networks (Fig. 6b), it can be seen that the length of uniform NiO nanoribbons is about 12 µm. TEM and X-ray diffraction were used to further study crystal structure of the as-obtained NiO nanoribbons. Fig. 6c displays the TEM image of a single NiO nanoribbon

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peeled from the substrate after ultrasonic processing. As seen that NiO nanoribbon shows a unique belt-like shape with transparent feature, indicating its ultrathin nature with a width of

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*

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Fig. 6 (a) The SEM image of the NiO-coated Ni foam from the hydrothermal process and the inset image is the closer examination. (b) The crosssectional view of the nanoribbon networks. (c) The TEM image of a single NiO nanoribbon peeled from the substrate by ultrasonication and the inset is the SAED pattern of NiO nanoribbons. (d) High resolution TEM (HRTEM) image of NiO taken from the edge of single nanoribbon. (e-h) The elemental mapping performed on NiO networks. (i) The XRD pattern of NiO.

300 nm. Furthermore, the SAED pattern with regular diffraction spots reveals the single crystalline characteristics and high crystallinity of NiO nanoribbons (Fig. 6c). The clear lattice fringe of 0.209 nm shown in the high resolution TEM (HRTEM) image corresponds to the (2 0 0) crystal plane of cubic NiO (Fig. 6d). The elemental mapping performed on a

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representative composite (Fig. 6e-h) reveals the co-existence and homogenous distribution of O and Ni located on the whole Ni framework. The XRD results shown in Fig. 6i can be well indexed to the standard powder diffraction pattern of bunsenite (JCPDS card No.47-1049). Except for the signals originating from Ni substrate, other peaks located at angles of 37.210, 43.610, 62.910 accord with (1 1 1), (2 0 0) and (2 2 0) planes of cubic NiO, respectively. Due to the excellent crystallinity which can decrease the charge recombination, the porous NiO network with enhanced electron conductivity provides a good electrochemical performance. The nature of NiO was confirmed by EDS test (green area in Fig. S4, Supporting Information). The signals of O and Ni can easily be recognized, which confirms the existence of nickel oxide. The loading amounts of Ni (including Ni substrate) and O are 85.6% and 14.4% (atomic percentage), respectively.

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Fig. 7 (a-d) The X-ray photoelectron spectroscopy of nanoribbon networks, which clearly shows the presence of Ni and O in the NiO. 13



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Fig. 7a depicts the X-ray photoelectron spectroscopy of nanoribbon networks, which further confirms the existence of O and Ni elements in the NiO. The signal of C 1s located at 284.8 eV originates from the carbon sample holder (Fig. 7b). The O 1s band can be deconvoluted into two peaks (seen in Fig. 7c). The low energy peak located at 529.1 eV is attributed to the formation of O−Ni bond and the higher binding energy of 533.1 eV can be ascribed to the Ni−OH bond40. In Fig. 7d, the signals at 834.9 and 871.8 eV are belonging to the Ni 2p3/2 and Ni 2p1/2 which also agree with the previous report of NiO41. No signals of other elements exist which is also a good match with the results of EDS analysis. The as-obtained NiO nanoribbon as a working electrode was directly tested in a three-electrode configuration using 1 mol L-1 KOH aqueous electrolyte to study its electrochemical behaviors. Fig. 8a presents galvanostatic charging-discharging plots of NiO at current densities of 0.5, 1.0, 2.5, 4.0, 5.0 and 10 A g-1. Its charging-discharging curves show a platform at variety current densities due to the redox reactions corresponding to the peaks shown in CV curves at various scanning rates (Fig. 8b). The redox couple can be explained by the conversion between NiO and NiOOH as follows41. NiO + OH-→NiOOH + e-

(3)

Clearly, the nonlinear charge-discharge curves and redox peaks in CV curves further verify its faradic behaviors and the redox reaction appeared between NiO and NiOOH. The capacitance of the NiO electrode according to the CV curves at the potential range between 0 and 0.4 V is shown in Fig. 8c. Its corresponding specific capacitance is 2479.2, 1355, 1059.4, 596.4, 447 and 367.5 F g-1 at scan rates of 2, 5, 8, 20, 30 and 50 mV s-1, exhibiting a high rate capability. The decrease in capacitance may be result from the presence of incompletely redox transitions and less contribution on capacitance of active material at higher scan rates. Fig. 8d shows the capacities of NiO nanoribbons network at different rates. For the first 100 cycles at scan rate 14


Page 15 of 28

of 2 mV s-1, the NiO electrode shows a stable capacitance of 2479.2 F g-1. Following the next 500 cycles, NiO always presents a stable capacitance although the scan rates changed sequentially. 0.5

0.08

0.5 A g-1

(a)

-1

0.06

2.5 A g-1

0.04

1.0 A g

0.4

(b)

0.02

5.0 A g-1 0.3

10.0 A g-1

0.2

Current (A)

Potential (V)

4.0 A g-1

0.00 2 mV s-1

-0.02

5 mV s-1

-0.04

8 mV s-1 10 mV s-1

-0.06

0.1

20 mV s-1 30 mV s-1

-0.08

0 3000

50 mV s-1

-0.10

0.0

0.0

500 1000 1500 2000 2500 3000 3500 4000 4500 5000

0.1

0.2

0.3

0.4

Potential (V)

Time (s)

(c)

2800

(d)

2.0 mV s-1

2500

2.0 mV s-1

2400 Specific capacitance (F g-1)

Specific capacitance (F g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000 1500 1000 500

2479.2 F g-1

1600

5.0 mV s-1 8.0 mV s-1

1200 800

20.0 mV s-1 30.0 mV s-1 50.0 mV s-1

400 0 0

5

10

15

20

25 30

35

40

45 50

55

2472.5 F g-1

2000

0

60

0

Scan rate (mV s-1)

100

200

300

400

500

600

700

800

Cycles

Fig. 8 (a) The galvanostatic charging-discharging plots of NiO. (b) The CV curves of NiO at different scanning rates. (c) The capacitance of the NiO electrode elucidated in the potential range of 0 to 0.4 V at different scan rates. (d) The capacitances of NiO nanoribbons network at different rates.

When the rate goes back, the capacitance (2472.5 F g-1) without any obvious change compared with the initial capacitance shows that NiO open porous structure has an excellent rate performance. Typical Nyquist plots of NiO nanoribbons after the 1st, 300st and 6000th cycles are presented in Fig. 9. Its Rs is smaller after 300 cycles (0.9 Ω at first and decrease to 0.3Ω), then increases to 2.3Ω after 6000 cycles. The reduction of Rs at the first 300st cycle may attribute to the more efficient use of active sites after electrochemical activation. In the high frequency region, the NiO electrode with an invisible semicircle before cycle demonstrates a very low Rct. It is further confirmed that the open porous structure of NiO 15



nanoribbon networks with the increased contact area between the electrode and electrolyte is

-Z＂(Ω)

10

5 After 1st cycle After 300th cycle After 6000th cycle

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Z＇ (Ω)

Fig. 9 Typical Nyquist plots of NiO electrode after the 1st, 300th and 6000th cycles.

good for ion transportation and charge transfer. In addition, the increasing of the Warburg resistance (Rw) after 6000 cycles may be attributed to the slight peeling of NiO nanoribbons to prevent the efficient diffusion of ions during the working process. Long-term cycle stability is an important element for evaluating the electrode in real applications. (b)

(a)

1µm

5µm

100µm 3000

(c)

2500

100

(d)

2500

85.6%

80

2000 2122.2 F g

-1

60

1500

40

1000

20

500

Specific capacitance (F g-1) Specific capacitance (F g -1 )

120

Capacitance retention(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 28

2000

1500 90% 1000 86% 82%

500

80% 0 0

2000

4000

6000

8000

0 10000

0 -1

2 mV s

5 mV s

-1

-1

8 mV s

20 mV s-1

75% 30 mV s-1

62% 50 mV s-1

Cycles

Fig.10 (a-b) The SEM images after 6000 cycles. (c) The capacitance retention of NiO electrode. (d) The electrochemical rates of NiO electrode after 6000 cycles.

The SEM images after 6000 cycles are shown in Fig. 10a and b. The general structure of 16


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the nanoribbons still remains the connection with each other and no obvious structural deformation is observed after 6000 cycles. As a result of the electrochemical activation commonly happened in electrochemical processes42, its capacitance retention is slightly over 100% during the first 300 cycles which also matches with the results of EIS test. Then it decreases gradually but still retains 85.6% after 6000 cycles (Fig. 10c). The electrochemical rate tests of the homemade electrode have also been carried out again after cycles (Fig. 10d). Its capacitance retention is still 62% of the initial capacitance at the high scan rate of 50 mV s-1, demonstrating outstanding high-rate capability after long term cycle performance.

2.3 Asymmetric supercapacitor device To further demonstrate the feasibility of CC/SnO2/MnO2 composite and NiO/Ni foam electrode for real application, an ASC device was assembled and tested (inset of Fig. 11a). Fig. S5 (Supporting Information) shows the CV profiles of the SnO2/MnO2 electrode (Fig. S5a, Supporting Information), NiO electrode (Figure S5b, Supporting Information) and the supercapacitor system (Fig. S5c, Supporting Information) at 50 mV s−1. In contrast to symmetric SCs (such as AC//AC system) with rectangular CV profiles, our (CC/SnO2/MnO2) (−)//(NiO/Ni

foam)(+) device has an asymmetric CV profile, indicating introduction of a

faradaic reaction into the system. Prior to assembling the device, the charge between the CC/SnO2/MnO2 negative and the NiO/Ni foam positive needs to be optimized, and the optimal mass ratio between these two electrodes was calculated to be 1:2.8, on the basis of the Eqn. S(2) and (3) in supporting information. Fig. 11b presents the CV curves of the ASC device at various scanning rates. Taking full advantage of the different potential windows of CC/SnO2/MnO2 and NiO/Ni foam, the stable voltage window of the fabricated ASCs can be enlarged to 1.5 V. The redox peaks in CV curves obtained at different scan rates between 10 and 200 mV s-1 reveal that the

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

assembled ASC device shows a pseudocapacitance capacitance at the working voltage window. Additionally, when the scan rate increases, the shape of the CV curve remains the same, indicating the desirable fast charge/discharge nature for the power device. The specific capacitances of the ASC device in the potential range of 0 to 1.5 V at different scan rates are shown in Fig. S6a (Supporting Information) and the CV curves collected at 100 mV s-1 in different voltage windows for the device is presented in Fig. S6b (Supporting Information). This device demonstrates good performance behaviors under different voltage windows, revealing a good stability. GCD curves of the ASC at different current densities are given in Fig. 11c. Good symmetrical profiles of the charge-discharge curves can further confirm the perfect electrochemical behaviour of the as-fabricated ASC in the potential range. But it still can be seen that the discharge curves only remain relatively linear at high currents, while a sloping plateau can be found at low currents which is originating from the battery characteristics of SnO2 under low currents without obvious polarization. EIS testing was performed and Nyquist plots of the devices with different electrolytes (pure KOH aqueous and PVA/KOH gel) are illustrated in Fig. 11d (inset is the enlarged view). Evidently, the Rs value for the pure KOH aqueous device (1.7 Ω) is smaller than that of the PVA/KOH gel device (5.9 Ω), suggesting a small resistance of KOH aqueous electrochemical system with an excellent ionic conduction. Moreover, the slope of the straight line in the low frequency region for quasi-solid state device is much lower than that of the aqueous one, demonstrating a much larger ion diffusion resistance. It has been proved that the charge transfer and ion diffusion in the quasi-solid state should be more sluggish together with the loose interfacial contact, which definitely has an influence on the electrochemical kinetics at high rates. These results are also in coincidence with the previous reports18, 42. As one of the most important parameters for a real energy storage device, the capacitance retention ratio of the fabricated ASC device is still maintained at about 75% after 6000 times charge/discharge cycles at a

18


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current density of 1.0 A g-1, which indicates a remarkable cycle stability (Fig. 11e). The excellent cycle stability of the high-performance (CC/SnO2/MnO2)

(−)//(NiO/Ni

foam)(+)

alkaline aqueous ASC device has considerably exceeded those previously reported asymmetric supercapacitors, such as a 36.4% loss for the Ni(OH)2//active carbon (AC)43, and a 33% loss for the Ni(OH)2/CNT//AC44. On the basis of Eqn. S(4) and S(5) in supporting information, a Ragone plot of the device depicting the relationship between energy density and power density is obtained (Fig. 11a). The photo of the device is shown in the inset. The ASC hybrid device achieves a high energy density up to 64.4 Wh kg-1 at an average power density of 250 W kg-1 and a high energy density of 35.8 Wh kg-1 at a power density of 1875 Wh kg-1, again confirming the excellent rate performance of our fabricated (CC/SnO2/MnO2)(−)//(NiO/Ni foam)(+) button type ASC device. As compared to similar systems reported previously, our device exhibits higher energy density than those of similar component devices reported by other groups, for example, SnO2/CC//rGO/CC

asymmetric

supercapacitor

(22.8

Wh

kg-1,

850

W

kg-1)45,

SiC-N-MnO2//active carbon (30.6 Wh kg-1, 113.9 W kg-1)46, GHCS/MnO2//GHCS (15.0 Wh kg-1, 1000 W kg-1)47, graphene/MnO2//CAN (23.0 Wh kg-1, 4000.0 W kg-1)48, CNTs@NCS@MnO2//AC (27.3 Wh kg-1, 4500.0 W kg-1)49, NiO//graphene (21.8 Wh kg-1, 250 W kg-1)50. To demonstrate the potential applications, two devices in series are applied to light up 24 red LEDs after charging at 1.0 A g-1 for about 60s, confirming the large energy density of the (CC/SnO2/MnO2)(−)//(NiO/Ni foam)(+) device which is expected to be a highly promising candidate for high-rate, high-energy, and long-life storage systems.

19



10

2

(a) -1

Energy density (Wh kg )

Our work

SiC-N-MnO2//AC, Ref.49 CNTs@NCS@MnO2 //AC, Ref.52 SnO2/CC//rGO/CC, Ref.37 NiO//graphene, Ref.53

graphene/MnO2//CAN, Ref.51

GHCS/MnO2 //GHCS, Ref.50

10

1

1

10

10

2

10

3

10

4

-1

Power density (W kg ) 1.6

(b)

0.10

0.33 A g-1 0.66 A g-1 1 A g-1 1.66 A g-1 2.5 A g-1

1.2 Potential (V)

Current (A)

(c)

1.4

0.05

0.00 10 mV s-1

20 mV s-1

-0.05

30 mV s-1 50 mV s-1

-0.10

1.0 0.8 0.6 0.4

100 mV s-1

0.2

200 mV s-1

-0.15 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0

1.6

0

1000

Potential (V)

5000

120

(d) Capacity retention(%)

3000

2000

2000

Time (s)

(e)

100

4000

-Z''(Ω)

KOH

50

PVA/KOH

40

30

-Z''(Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

20

1000 10

75% 80 60 40 20

0 0

20

40

Z'(Ω)

0 0

200

400

600

800

0

1000 1200 1400 1600

0

Z'(Ω)

1000

2000

3000

4000

5000

6000

Cycles

Fig.11 (a) Ragone plot of the assembled (CC/SnO2/MnO2)(-)//(NiO/Ni foam)(+) in comparison with other similar reported in the literatures. (b) The CV curves of the ASC device at different scanning rates. (c) Galvanostatic charge-discharge curves of the ASC at various current densities. (d) Nyquist plots of the devices with different electrolytes (pure KOH aqueous and PVA/KOH gel). (e) The capacity retention ratio of the fabricated ASC cell.

3. Conclusion In summary, the design and synthesis of a novel alkaline asymmetric supercapacitor device was proposed here. This device exhibits a high energy density (64.4 Wh kg-1) with high power density (250 W kg-1), good cycle stability and high environmental suitability. Having an efficient contact with CC, binder-free SnO2 nanosheets coated by MnO2 nanoflakes were used as negative electrode. Open porous self-supported NiO nanoribbons network with high capacitance was used as positive electrode. Both the two electrode contributed to the 20


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high performances of ASCs. Our work provides a facile way in exploring the application possibility of developing new types of all metal oxide-based energy storage and conversion devices with high-power density, high-energy density and long-life.

Supporting Information Supplementary material (experimental section, characterization and electrochemical performance measurements ， the growth mechanism for SnO2, the corresponding EDS spectrum test, the SEM image and electrochemical performance of MnO2, cycle stability of SnO2 and SnO2/MnO2, the EDS analysis of NiO, the comparative CV curves of SnO2/MnO2 anode, NiO cathode and (CC/SnO2/MnO2)(-)//(NiO/Ni foam)(+) supercapacitor, the capacitance of the supercapacitor and the CV curves versus different potential windows）is available.

Acknowledgments This research is financially supported by the National Natural Science Foundation of China (Grant No. 21273058 and 21673064), China postdoctoral science foundation (Grant No. 2012M520731) and Heilongjiang postdoctoral financial assistance (LBH-Z12089)

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MnO2 Nanowires as High Performance Supercapacitor Electrodes and Photocatalysts with Enhanced Photocatalytic Activities. CrystEngComm. 2014, 16, 9999-10005. 40. Singh, A. K.; Sarkar, D.; Khan, G. G.; Mandal, K. Hydrogenated NiO Nanoblock Architecture for High Performance Pseudocapacitor. ACS Appl. Mater. Interfaces. 2014, 6, 4684-4692. 41. Alammar, T.; Shekhah, O.; Wohlgemuthb, J.; Mudring, A. V. Ultrasound-assisted Synthesis of Mesoporous β-Ni (OH)2 and NiO Nano-Sheets Using Ionic Liquids. J. Mater. Chem. 2012, 22, 18252-18260. 42. Li, H. L.; He, Y.; Pavlinek, V.; Cheng, Q. L.; Sah, P.; Li, C. Z. MnO2 Nanoflake/Polyaniline Nanorod Hybrid Nanostructures on Graphene Paper for High-performance Flexible Supercapacitor Electrodes. J. Mater. Chem. A 2015, 3, 17165-17171. 43. Lang, J. W.; Kong, L. B.; Liu, M.; Luo, Y. C.; Kang, L. Asymmetric Supercapacitors Based on Stabilized α-Ni(OH)2 and Activated Carbon. J. Solid State Electrochem. 2010, 14, 1533-1539. 44. Tang, Z.; Tang, C. H.; Gong, H. A High Energy Density Asymmetric Supercapacitor from Nano-architectured Ni(OH)2/Carbon Nanotube Electrodes. Adv. Funct. Mater. 2012, 22 1272-1278. 45. Zhang, Y. D.; Hu, Z. G.; Liang, Y. R.; Yang, Y. Y.; An, N.; Li, Z. M.; Wu, H. Y. Growth of 3D SnO2 Nanosheets on Carbon Cloth as a Binder-free Electrode for Supercapacitors. J. Mater. Chem. A 2015, 3, 15057-15067. 46. Kim, M.; Kim, J. Development of High Power and Energy Density Microsphere Silicon Carbide-MnO2 Nanoneedles and Thermally Oxidized Activated Carbon Asymmetric Electrochemical Supercapacitors. Phys. Chem. Chem. Phys. 2014, 16, 11323-11336. 47. Lei, Z.; Zhang, J.; Zhao, X. Ultrathin MnO2 Nanofibers Grown on Graphitic Carbon

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Spheres as High-performance Asymmetric Supercapacitor Electrodes. J. Mater. Chem. 2012, 22, 153-160. 48. Fan, Z.; Yan, J.; Wei, T.; Zhi, L.; Ning, G.; Li, T.; Wei, F. Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density. Adv. Funct. Mater. 2011, 21, 2366-2375. 49. Li, L.; Li, R.; Gai, S. L.; Gao, P.; He, F.; Zhang, M. L.; Chen, Y. J.; Yang, P. P. Hierarchical Porous CNTs@NCS@MnO2 Composites: Rational Design and High Asymmetric Supercapacitor Performance. J. Mater. Chem. A 2015, 3, 15642-15649. 50. Dam, D. T.; Wang, X.; Lee, J. M. Graphene/NiO Nanowires: Controllable One-pot Synthesis and Enhanced Pseudocapacitive Behavior. ACS Appl. Mater. Interfaces 2014, 6, 8246-8256.

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In situ Growth of Free-standing All Metal Oxide Asymmetric Supercapacitor Bo-Si Yin 1, Zhen-Bo Wang 1,*, Si-Wen Zhang 1, Chang Liu 1, Qing-Qing Ren, 1 Ke Ke1* 1

School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No.92 West-Da Zhi Street, Harbin, 150001 China.

The device achieves a high energy density up to 64.4 Wh kg-1 at an average power density of 250 W kg-1 and a high energy density of 35.8 Wh kg-1 can be retained at a power density of 1875 Wh kg-1, confirming the excellent rate performance of our fabricated (CC/SnO2/MnO2)(−)//(NiO/Ni foam)(+) ASC hybrid device.


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In Situ Growth of Free-Standing All Metal Oxide Asymmetric

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