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PPy nanowires feature improved conductivity and stability for supercapacitor
Journal Pre-proofs Core/sheath structured ultralong MnOx/PPy nanowires feature improved conductivity and stability for supercapacitor Jiali Fu, Yong Zhang, Hui Zhao, Rijuan Jiang, Renjie Zhang PII: DOI: Reference:
S0021-9797(19)31190-7 https://doi.org/10.1016/j.jcis.2019.10.006 YJCIS 25500
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
4 August 2019 2 October 2019 3 October 2019
Please cite this article as: J. Fu, Y. Zhang, H. Zhao, R. Jiang, R. Zhang, Core/sheath structured ultralong MnOx/PPy nanowires feature improved conductivity and stability for supercapacitor, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.006
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Core/sheath structured ultralong MnOx/PPy nanowires feature improved conductivity and stability for supercapacitor Dedicated to the Memory of Professor Helmuth Möhwald Jiali Fu, [a] Yong Zhang, [a] Hui Zhao, [a] Rijuan Jiang [a] and Renjie Zhang* [a,b,c] [a]
Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education of the P. R.
China, Shandong University, Jinan 250100, P. R. China *E-mail: [email protected] [b] National Engineering Technology Research Center for Colloidal Materials, Shandong University,
Jinan 250100, P. R. China [c]
Key Laboratory of Special Functional Aggregated Materials of the Ministry of Education of the
P.R. China, Shandong University, Jinan 250100, P. R. China
Abstract Increased conductivity of manganese oxide (MnOx) for effectively improved supercapacity is studied in this work by addressing on introduced oxygen vacancies (OVs) besides a porous sheath of conductive polymer (polypyrrole, PPy). The assembly profile of core/sheath structured MnOx/PPy nanowires by in-situ polymerization of PPy under mild condition showing better conductivity, specific capacitance, rate performance and cycling stability than so far reported MnOx based materials. Structural characteristics of the MnOx/PPy nanowires are studied in detail, including high weight percent of MnOx core, underlying PPy layer chemically bonding MnOx core and PPy nanoparticles in outlayer, as well as the simultaneously introduced conductive oxygen 1
vacancies (OVs) in MnOx during formation of PPy sheath. The contribution of assembly profile to the supercapacitor performances is discussed, especially concerning PPy sheath and OVs, essentially yielding improved conductivity between current collector and the MnOx core to assure large energy density and power density.
Keywords: core/sheath structure; MnOx/PPy nanowires; defect engineering; energy storage devices
1. Introduction Good conductivity is essential for fast dynamic electrochemical processes in manganese oxide (MnOx) based supercapacitors. The pseudocapacitor compound MnOx with high theoretical[1] and measured specific capacitance[2] has been extrinsically coated with conducting polypyrrole (PPy)[3, 4] as shell or alternatively intrinsically modified by introducing OVs favourable for conductivity.[5, 6] However, so far there are no deep investigation on using simultaneously the two effective methods to increase conductivity, that is, MnOx with OVs and PPy. So the MnOx-PPy system should be revisited. Moreover, OVs in single component MnO2 have been obtained by hydrogenation at temperature of 650 °C for 3 h.[7] It is scientifically interesting to know whether the OVs can be introduced simply during mild polymerization process of PPy without high temperature and hydrogen, corresponding to practically economic in procedure and energy. Furthermore, supercapacitor performances on β-[8].and γ-MnO2 [9] reported, while it is scientifically unclear on MnOx containing low valences of Mn2+ 2
and Mn3+ together with α-MnO2. Conductivity in supercapacitors involves 2 processes of electron transfer, 1) from current collector to assembly of closely packed MnOx-PPy nanostructures, 2) from the outer shell to the inner core of each MnOx-PPy nanostructure. It has been reported that PPy as sheath can improve the conductivity of materials and improve electrochemical performance.[10-12] Both require a well-conductive PPy rather than MnOx as shell. Porous PPy shell is necessary for supercapacitors involving inherent intercalation/deintercalation of ions in MnOx. That is, the morphology of outer PPy layer should be optimized. Perfectly smooth and physically intact PPy shell by expensive chemical vapor deposition (CVD) on MnO2 [13] should be updated to facilitate inherent ions transfer. Regarding to electrochemical kinetics and stability for potential application, it can be managed to obtain PPy layers consisting of both chemically bonded underlayer of PPy and outlayer of closely packed PPy nanoparticles. In this work, the above issues are addressed by using a simple in-situ polymerization process to assemble PPy sheath on MnO2 nanowire, forming core/sheath structured ultralong MnOx/PPy nanowires. Interesting mesoporous PPy nanoparticles form with the underlayer of PPy chemically bonding MnOx core. The formation process of PPy sheath accompanies introduction of abundant OVs and oxides of low valent Mn2+ and Mn3+. As a result, such core/sheath structured MnOx/PPy nanowires yield so far better supercapacitor performance than reported MnOx-based materials by simultaneously considering conductivity, rate performance, energy density, power density and cycling stability. 3
2. Experimental Section 2.1 Synthesis of MnO2 nanowires KMnO4 (63.2 mg) as oxidant and cetyltrimethylammonium bromide (CTAB) as reductant and surfactant (6.7 mg) were dissolved in H2O (20 mL) at room temperature, respectively. The CTAB solution was poured into KMnO4 solution under stirring for several minutes. Then the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave (100 mL) and maintained at 180 °C for 5 h. The autoclave was cooled to room temperature naturally. The dark precipitation was washed by H2O and ethanol for three times to obtain pure MnO2 nanowires. 2.2 Synthesis of core/sheath structured ultralong MnOx/PPy nanowires MnO2 nanowires (50.0 mg) were dispersed in H2O (20 mL). Sodium ptoluenesulfonate (32.0 mg) and (NH4)2S2O8 (48.0 mg) was dissolved in H2O (20 mL), respectively. The sodium p-toluenesulfonate solution was poured into liquid dispersion of MnO2 nanowires under stirring, which was for coating a uniform PPy sheath on the surface of MnOx.[14] Afterward pyrrole monomer (15 μL) and phytic acid (100 μL 50 wt. % in H2O) was added. After 30 min, (NH4)2S2O8 solution was put into the above solution under stirring. The mixture solution was maintained at 0 - 5 ºC under stirring for 6 h. After polymerization, the black precipitate was centrifuged, rinsed with abundant H2O and ethanol to remove residues and dried in a vacuum oven. 2.3 Electrochemical measurement All electrochemical performances including cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were studied on a CHI660E electrochemical 4
workstation at room temperature. For three-electrode system, 1.0 M aqueous Na2SO4, Ni foam loading core/sheath structured ultralong MnOx/PPy nanowires, Pt plate and Ag/AgCl electrode was used as electrolyte, working electrode, counter electrode and reference electrode, respectively. The working electrode was prepared as follows: mixing active materials, ketjen black and polyvinylidene fluoride (PVDF) in N-methyl2-pyrrolidinone (NMP) with the weight ratio of 50:40:10. The above mixture slurry was coated on the surface of Ni foam evenly and dried at 60 °C in a vacuum oven overnight. The mass loading of active material on the working electrode was about 0.7 mg cm-2. Then the electrode was pressed under 10 MPa and immersed in 1.0 M Na2SO4 aqueous overnight. Electrochemical impedance spectroscopy (EIS) was carried out on a Zahner IM6 in a range of 10-2 - 105 Hz with 5 mV amplitude at open circuit potential. The specific capacitance (C, F g-1) of samples was calculated according to Eq. 12: I × Δt
C=m
(1)
× ΔV
CMnOx =
C - CPPyωPPy
(2)
ωMnOx
where I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the weight of active material on one electrode in three-electrode system, ΔV (V) is the voltage window after voltage drop (IR drop), CMnOx (F g-1) is the specific capacitance of pure MnOx in the MnOx/PPy nanowires, CPPy (F g-1) is the specific capacitance of pure PPy, ωMnOx and ωppy are the weight ratio of MnOx and PPy in the MnOx/PPy nanowires, respectively. The assembled asymmetric supercapacitor (ASC) using core/sheath structured 5
ultralong MnOx/PPy nanowires as positive electrode and activated carbon (AC) as negative electrode was measured in a two-electrode configuration. The polyvinyl alcohol/Na2SO4 (PVA/ Na2SO4) gel electrolyte with safety, stability and good flexibility[1] was made by dissolving 4.0 g PVA in 30 mL H2O at 85 °C (water bath). After the solution became transparent, 10 mL 0.4 g mL-1 Na2SO4 solution was put into the PVA solution under vigorous stirring until the mixture appeared clear. The weight ratio of MnOx/PPy and AC in electrodes for optimal electrochemical performance is calculated to be 1:4.7 according to charge balance by Eq. 3: m+ m-
C - × ΔV -
= C+
(3)
× ΔV -
where m+ and m- (g) are the weight of active materials on positive electrode and negative electrode; C+ and C- (F g-1) are the specific capacitance of active material on positive electrode and negative electrode; and ΔV+ and ΔV- (V) are the voltage of positive electrode and negative electrode, respectively. The specific capacitance (C, F g-1), energy density (E, W h kg-1) and power density (P, W kg-1) of samples were calculated according to Eq. 4-6: I × Δt
C=m 1
E=2× P=
(4)
× ΔV C × ΔV2
(5)
3.6
3600E
(6)
Δt
where I (A) is the discharge current, m (g) is the total weight of active material on two electrodes, ΔV (V) is the voltage window after IR drop, Δt (s) is the discharge time. For the leakage current test, the device was first charged to 1.0 V at 2 mA and then the potential was kept at 1.0 V for 2 h while acquiring the current data. For the self6
discharge test, the device was first charged to 1.0 V at 2 mA and kept at 1.0 V for 15 min, and then measured the open circuit potential over time. 3. Results and Discussion
Scheme 1 Schematic of the formation of MnOx/PPy nanowires.
Core/sheath structured ultralong MnOx/PPy nanowires assembled by in-situ polymerization of pyrrole on MnO2 (Scheme 1) are about 30 nm in diameter and several tens of micrometers in length (Fig. 1a-b and Fig. S1a-c). The elemental mapping images (Fig. S1e–h) show that C, N, Mn and O elements coexist and distribute uniformly in MnOx/PPy nanowires. The aspect ratio of length to diameter is as large as ca. 1000. The PPy sheath is about 4 nm in thickness on the MnOx core of about 20 nm in diameter (Fig. 1b). The FTIR spectrum of PPy (Fig. S2a) exhibits characteristic vibration peaks of PPy, such as the C-N and C=C stretching vibration at 1468 and 1550 cm-1, the C-H in-plane vibration at 1309 cm-1 and the ring deformation at 900 cm-1.[15, 16] The characteristic Raman peaks of PPy also prove the existence of PPy again (Fig. S2b), such as the peak at 924 cm-1 attributed to the quinoid polaronic structure, 1047 cm-1 attributed to the C-H in-plane deformation, 1385 and 1584 cm-1 attributed to ring stretching mode of the polymer backbone and the π-conjugation structure, respectively.[16] The conductive PPy sheath with large aspect ratio would essentially 7
assure fast electron transfer between current collector and MnOx core. The content of MnOx in the nanowires is 71.4 wt. % (Fig. S2c), higher than that of reported MnOxbased materials, [2, 17] which would produce large specific capacitance and energy density.
Fig. 1 (a) SEM image, (b) TEM images (inset in b: magnified TEM image) and (c) HRTEM image of MnOx/PPy nanowires.
OVs with improved conductivity for MnOx in MnOx/PPy nanowires are simply introduced during polymerization process of the PPy sheath, as verified by a subpeak at 531.3 eV in O 1s XPS spectra (Fig. 2a), corresponding to the adsorbed oxygen species on the OVs.[18] The other two deconvoluted subpeaks correspond to the O atom in (Mn)-O-(Mn) at 529.5 eV, and the O atom in (Mn)-O-(H) at 532.3 eV.[19] In contrast, the O 1s XPS spectra of the MnO2 nanowires show only the O atoms in (Mn)O-(Mn) and (Mn)-O-(H). The multivalent Mn causes the surrounding O atoms to accumulate more charges,[7] resulting in higher binding energy of O atom in (Mn)-O(H) of MnOx/PPy. The content of OVs in MnOx/PPy nanowires is calculated to be 41.8% by integrating peak areas, indicating abundant OVs. The OVs are also demonstrated by 8
a symmetric electron spin signal at g=2.006 in electron spin resonance (ESR) spectrum (Fig. 2c) only for the MnOx/PPy nanowires and not for MnO2 nanowires.
Fig. 2 (a) O 1s, (b) Mn 2p XPS spectra and (c) ESR spectra of MnOx nanowires and MnOx/PPy nanowires.
Due to the formation of OVs, Mn2+ and Mn3+ form based on electrostatic equilibrium.[20] Mn2+ in Mn-(O), Mn3+ in Mn-(N), Mn3+ in Mn-(O) and Mn4+ in Mn(O) at the ratio of 15.7, 22.8, 20.2 and 41.3 at.% among total Mn atoms are shown by the deconvoluted four peaks at 640.6, 641.8, 642.7 and 643.3 eV, respectively, from the Mn 2p XPS spectra of MnOx/PPy nanowires[21, 22]. As comparison, the MnO2 nanowires without PPy only show Mn4+ peak in the Mn 2p XPS spectra (Fig. 2b). Mn4+ in MnOx/PPy nanowires exists as α-MnO2, as revealed by the XRD peak at 2θ = 18.1° (Fig. S2d) corresponding to the interplanar (200) lattice of α-MnO2 with an interplanar spacing of 0.49 nm, the same as that shown in HRTEM (Fig. 1c). The presence of αMnO2 is also demonstrated by the (200) and (310) lattices of α-MnO2 revealed by diffraction spots in SAED image of MnOx/PPy nanowires (Fig. S1i). Mn2+ and Mn3+ should exist in amorphous oxides since no related XRD peaks appear. Between MnOx and contacted PPy, about 20.2 at.% interfacial Mn atoms form Mn-N chemical bond, 9
as indicated by above Mn 2p XPS spectra and also by the N 1s XPS spectra of MnOx/PPy nanowires, the latter can be deconvoluted into four subpeaks at 397.8, 399.4, 399.8 and 401.8 eV, corresponding to N atoms in =N-, (Mn)-N, -N-(H) and -N+ (Fig. S3a), respectively.[23] It is worth noting that the interfacial Mn-N chemical bond would not only maintain structural stability of MnOx/PPy nanowires but also promote fast electron transfer between MnOx core and PPy sheath. Left ca. 80 at.% interfacial Mn atoms allow access of ions to MnOx during charge/discharge process. So these advantages would yield good cycling stability, conductivity and rate performance of MnOx/PPy nanowires in supercapacitors.
Fig. 3 (a) Nyquist plots (inset in a: an equivalent circuit) of MnO2 and MnOx/PPy nanowires. (b) Plot of specific capacitances against different current densities of MnO2, PPy and MnOx/PPy nanowires. (c) Cycling stability tests of MnO2, PPy and MnOx/PPy nanowires.
The outlayer of PPy sheath consists of characteristic nanoparticles (Fig. 1a) during in-situ polymerization of pyrrole, superior to smooth intact layer of PPy by CVD.[13] PPy nanoparticles increase specific surface area of MnOx/PPy nanowires to 80.5 m2 g1,
much higher than that of MnO2 nanowires before coating (37.7 m2 g-1) (Fig. S3b-c). 10
PPy nanoparticles also form mesopores including interparticle mesopores of PPy as well as mesopores between PPy sheath and MnOx core, as indicated by a hysteresis loop starting at around a relative pressure (p p0-1) of 0.4-1.0.[24] The ratio of mesopores and macropores to total pores is 60.4 and 39.6 %, respectively. Large specific surface would sufficiently expose PPy to electrolyte, while rich mesopores would shorten ionic transport path and diffusion time to accelerate electrolyte transport during charge/discharge process, both essential for high electrochemical performance.[25] Last but not least, the outlayer of PPy nanoparticles would improve cycling stability due to 1) inherent intercalation/de-intercalation and the compositional change of MnOx accompanied by volumetric change[5] and 2) protection against direct exposure and easy dissolution in neutral or alkaline aqueous electrolyte.[26] Due to the conductive PPy layer and OVs, MnOx/PPy nanowires show improved conductivity with smaller series resistance (Rs) of 1.26 Ω than that of MnO2 (2.05 Ω), implying much faster electron transfer between current collector and MnOx core in MnOx/PPy nanowires. Also benefiting from large specific surface area and mesopores, MnOx/PPy nanowires show much smaller charge transfer resistance (Rct) of 0.64 Ω than that of MnO2 (5.13 Ω) (Fig. 3a). Both would greatly accelerate access of electrolyte and transport rate of electrons and ions, and consequently speeding up electrochemical dynamics. As a result, the MnOx/PPy nanowires show a larger specific capacitance and ion transport rate, as indicated by significantly larger CV curve area (1.09 C) than that of MnO2 nanowires (0.52 C) at a scan rate of 50 mV s−1 (Fig. S4a-c). For Ni foam, the lower current response than that of MnO2 and MnOx/PPy implies that the capacitance 11
of Ni foam can be ruled out. Fast electrochemical dynamics enables MnOx/PPy nanowires to show excellent pseudocapacity by quasi-rectangular shape of CV curves at different scan rates.[27] Good rate performance of MnOx/PPy nanowires is shown by the well-kept quasi-rectangular CV curves (Fig. S4b) as scan rates increase.[28] There are no obvious redox peaks appeared in the CV curves, implying that the capacitive performance is based on Faradaic redox processes in MnOx/PPy nanowires as expressed with fast intercalation/de-intercalation of Na+ (Eq. 7) in MnO2 and doping/de-doping of SO42− (Eq. 8) in PPy:[3] MnO2 + Na+ + e- ⇌ MnOONa
(7)
(PPy+)2SO42- + 2e- ⇌ 2PPy + SO42-
(8)
The specific capacitance of MnOx/PPy nanowires is as large as 1091.4 F g-1 at 1 A g-1, 2 times larger than that of MnO2 nanowires (480.1 F g-1) (Fig. 3b) and reported MnOx-based materials, such as TiN@MnO2[29], hollow MnO2 nanofibers[30] and MnO2/CNT[31]. Moreover, good rate capability of MnOx/PPy nanowires is proved by a large specific capacitance of 890.1 F g-1 with a retention ratio of 81.6% even when the current density is increased to 20 A g-1. The specific capacitance of PPy is 449.8 F g-1 (Fig. 3b and Fig. S5), so the specific capacitance based on the weight of MnOx at 1 A g-1 is calculated to be 1348.4 F gMnOx-1, higher than reported values of MnOx-based materials (Table S1). Due to the protective PPy sheath in the MnOx/PPy nanowires, corresponding cycling stability, one important factor affecting the application of supercapacitors, is significantly increased. First, excellent electrochemical reversibility as well as fast 12
current-voltage response[19] is indicated by triangular and substantially symmetrical GCD curves of MnOx/PPy nanowires at different current densities with a small IR drop of only 0.01 V at 2 A g-1 (Fig. S4d-f). As comparison, the IR drop values of MnO2 nanowires (0.02 V) and reported CNTs/MnO2 (0.03 V) and Co-MnO2 (0.08 V)[25] are much bigger. Then the specific capacitance retention percentage of MnOx/PPy electrodes is measured to be as high as 97.4% of their initial values after 10000 GCD cycles at 10 A g-1 (Fig. 3c), much higher than that of MnO2 (67.0%), PPy (50.9%) and reported composites such as N-doped carbon nanotubes/MnO2,[17] MnO2/polyaniline,[1] carbon nanotube/MnO2/reduced graphene oxide[32] and MnO2/C/Ag.[33] The morphology and crystal phase also show good stability without obvious changes after 10000 cycles (Fig. S6a-b). The cycling stability is further proved by Nyquist plots before and after 10000 cycles. The plots are similar without obvious changes in Rs and Rct from 1.26 and 0.64 to 1.29 and 0.83 Ω after 10000 cycles, respectively (Fig. S6c). In order to practically illustrate the excellent electrochemical performance of core/sheath structured ultralong MnOx/PPy nanowires, MnOx/PPy//AC ASC is assembled with MnOx/PPy as positive electrode, AC as negative electrode and PVA/Na2SO4 gel as electrolyte (Fig. 4a). The stable working voltage window of the MnOx/PPy nanowires and AC is 0 - 1.0 V and -1.0 - 0 V, respectively (Fig. S7 and S8a). The MnOx/PPy//AC ASC works steadily at 2.2 V (Fig. S8b-c), capable of lighting a red light-emitting diode (LED) with only one ASC (Fig. 4b). The CV curves show a quasirectangular shape, indicating fast current-voltage response and near-ideal capacitance 13
characteristics (Fig. S8d). In 0-2.2 V, the GCD curves at 1-20 A g-1 show symmetrical shapes and excellent electrochemical reversibility (Fig. S8e). The specific capacitance is as high as 214.2 F g-1 at 1 A g-1. Even at 20 A g-1, the capacitance is still up to 171.8 F g-1 with capacitance retention of 80.2%, implying excellent rate capability (Fig. 4b). The MnOx/PPy//AC ASC exhibits excellent cycling stability with a capacitance retention of 98.1% after 10000 cycles (Fig. S8f).
Fig. 4 (a) Schematic of the charge/discharge process of assembled MnOx/PPy//AC ASC (b) Plot of specific capacitances against different current density (inset in b: photograph of the assembled ASC lighting a red LED), (c) Leakage current curve of MnOx/PPy//AC ASC charged at 2 mA to 1.0 V and kept at 1.0 V for 2 h. (d) Selfdischarge curve of MnOx/PPy//AC ASC after charged at 1.0 V for 15 min. (e) Ragone plots of the MnOx/PPy//AC and reported MnOx-based ASCs.
For practical application, it is important to know the leakage current and selfdischarge characteristics of ASC. For the MnOx/PPy//AC ASC, the leakage current dropped significantly at the beginning and then quickly stabilized at 9.8 μA (Fig. 4c). 14
The self-discharge test further shows the time courses of the open circuit potential. The ASC undergoes rapid self-discharge process at the beginning, and then the selfdischarge process is quite slow after several hours. Finally, the device shows a stable output voltage of 0.56 V after 24 h (Fig. 4d). The maximum energy density of MnOx/PPy//AC ASC reaches a large value of 144.0 W h kg-1 at a power density of 1100 W kg-1. Even at a high power density of 22000 W kg-1, the energy density can still be as large as 116.4 W h kg-1 (Fig. 4e). The specific capacitance, energy density and power density of MnOx/PPy//AC ASC are larger than that of reported ASCs of MnOx-based materials (Table S2). 4. Conclusion In summary, the assembled core/sheath structured ultralong MnOx/PPy nanowires prove to be promising electrode materials in future energy storage devices, based on better supercapacitor performances such as conductivity, specific capacitance, energy density, power density and cycling stability than reported MnOx-based materials. The conductive PPy sheath with underlying PPy layer chemically bonding MnOx, abundant OVs in MnOx formed during mild in-situ polymerization of PPy and the interfacial MnO bonds are of great importance to speed up electron transfer between current collector and MnOx core (Fig. 4a). The large specific surface due to the PPy nanoparticles in outlayer of PPy sheath, the large aspect ratio of the MnOx/PPy nanowires, as well as mesopores in MnOx/PPy nanowires speed up diffusion and access of electrolytes and transport of Na+ ions to and from MnOx which are not chemically bonded by PPy during intercalation/de-intercalation. The sheath of PPy with both Mn-N chemical bonds and 15
PPy nanoparticles stabilizes the MnOx core considering volumetric change and dissolution inhibition of MnOx, consequently assuring excellent cycling performance of MnOx/PPy nanowires. This work optimizing assembly profile of simply synthesized conductive PPy sheath with PPy nanoparticles rather than physically intact PPy layer accompanying with mildly introduced abundant OVs in MnOx for high energy density and power density energy devices is of great importance as a breakthrough in controllable assembly of functional nanomaterials.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21872086) and Natural Science Foundation of Shandong Province, China (No. ZR2019MB011). We thank Prof. Houyi Ma for providing the impedance analyzer. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/xxx References [1] N. Liu, Y.L. Su, Z.Q. Wang, Z. Wang, J.S. Xia, Y. Chen, Z.G. Zhao, Q.W. Li, F.X. Geng, ACS Nano 11 (2017) 7879. [2] J.P. Li, Z.H. Ren, S.G. Wang, Y.Q. Ren, Y.J. Qiu, J. Yu, ACS Sustainable Chem Eng 4 (2016) 3641. [3] W.D. He, C.G. Wang, F.W. Zhuge, X.L. Deng, X.J. Xu, T.Y. Zhai, Nano Energy 35 (2017) 242. 16
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Graphical abstract
Core/sheath structured ultralong MnOx/PPy nanowires feature improved conductivity and stability for supercapacitor Jiali Fu, Yong Zhang, Hui Zhao, Rijuan Jiang and Renjie Zhang*
20
Conflict of Interest Form Declarations of interest: none
21
S0021-9797(19)31190-7 https://doi.org/10.1016/j.jcis.2019.10.006 YJCIS 25500
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
4 August 2019 2 October 2019 3 October 2019
Please cite this article as: J. Fu, Y. Zhang, H. Zhao, R. Jiang, R. Zhang, Core/sheath structured ultralong MnOx/PPy nanowires feature improved conductivity and stability for supercapacitor, Journal of Colloid and Interface Science (2019), doi: https://doi.org/10.1016/j.jcis.2019.10.006
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© 2019 Published by Elsevier Inc.
Core/sheath structured ultralong MnOx/PPy nanowires feature improved conductivity and stability for supercapacitor Dedicated to the Memory of Professor Helmuth Möhwald Jiali Fu, [a] Yong Zhang, [a] Hui Zhao, [a] Rijuan Jiang [a] and Renjie Zhang* [a,b,c] [a]
Key Laboratory of Colloid and Interface Chemistry of the Ministry of Education of the P. R.
China, Shandong University, Jinan 250100, P. R. China *E-mail: [email protected] [b] National Engineering Technology Research Center for Colloidal Materials, Shandong University,
Jinan 250100, P. R. China [c]
Key Laboratory of Special Functional Aggregated Materials of the Ministry of Education of the
P.R. China, Shandong University, Jinan 250100, P. R. China
Abstract Increased conductivity of manganese oxide (MnOx) for effectively improved supercapacity is studied in this work by addressing on introduced oxygen vacancies (OVs) besides a porous sheath of conductive polymer (polypyrrole, PPy). The assembly profile of core/sheath structured MnOx/PPy nanowires by in-situ polymerization of PPy under mild condition showing better conductivity, specific capacitance, rate performance and cycling stability than so far reported MnOx based materials. Structural characteristics of the MnOx/PPy nanowires are studied in detail, including high weight percent of MnOx core, underlying PPy layer chemically bonding MnOx core and PPy nanoparticles in outlayer, as well as the simultaneously introduced conductive oxygen 1
vacancies (OVs) in MnOx during formation of PPy sheath. The contribution of assembly profile to the supercapacitor performances is discussed, especially concerning PPy sheath and OVs, essentially yielding improved conductivity between current collector and the MnOx core to assure large energy density and power density.
Keywords: core/sheath structure; MnOx/PPy nanowires; defect engineering; energy storage devices
1. Introduction Good conductivity is essential for fast dynamic electrochemical processes in manganese oxide (MnOx) based supercapacitors. The pseudocapacitor compound MnOx with high theoretical[1] and measured specific capacitance[2] has been extrinsically coated with conducting polypyrrole (PPy)[3, 4] as shell or alternatively intrinsically modified by introducing OVs favourable for conductivity.[5, 6] However, so far there are no deep investigation on using simultaneously the two effective methods to increase conductivity, that is, MnOx with OVs and PPy. So the MnOx-PPy system should be revisited. Moreover, OVs in single component MnO2 have been obtained by hydrogenation at temperature of 650 °C for 3 h.[7] It is scientifically interesting to know whether the OVs can be introduced simply during mild polymerization process of PPy without high temperature and hydrogen, corresponding to practically economic in procedure and energy. Furthermore, supercapacitor performances on β-[8].and γ-MnO2 [9] reported, while it is scientifically unclear on MnOx containing low valences of Mn2+ 2
and Mn3+ together with α-MnO2. Conductivity in supercapacitors involves 2 processes of electron transfer, 1) from current collector to assembly of closely packed MnOx-PPy nanostructures, 2) from the outer shell to the inner core of each MnOx-PPy nanostructure. It has been reported that PPy as sheath can improve the conductivity of materials and improve electrochemical performance.[10-12] Both require a well-conductive PPy rather than MnOx as shell. Porous PPy shell is necessary for supercapacitors involving inherent intercalation/deintercalation of ions in MnOx. That is, the morphology of outer PPy layer should be optimized. Perfectly smooth and physically intact PPy shell by expensive chemical vapor deposition (CVD) on MnO2 [13] should be updated to facilitate inherent ions transfer. Regarding to electrochemical kinetics and stability for potential application, it can be managed to obtain PPy layers consisting of both chemically bonded underlayer of PPy and outlayer of closely packed PPy nanoparticles. In this work, the above issues are addressed by using a simple in-situ polymerization process to assemble PPy sheath on MnO2 nanowire, forming core/sheath structured ultralong MnOx/PPy nanowires. Interesting mesoporous PPy nanoparticles form with the underlayer of PPy chemically bonding MnOx core. The formation process of PPy sheath accompanies introduction of abundant OVs and oxides of low valent Mn2+ and Mn3+. As a result, such core/sheath structured MnOx/PPy nanowires yield so far better supercapacitor performance than reported MnOx-based materials by simultaneously considering conductivity, rate performance, energy density, power density and cycling stability. 3
2. Experimental Section 2.1 Synthesis of MnO2 nanowires KMnO4 (63.2 mg) as oxidant and cetyltrimethylammonium bromide (CTAB) as reductant and surfactant (6.7 mg) were dissolved in H2O (20 mL) at room temperature, respectively. The CTAB solution was poured into KMnO4 solution under stirring for several minutes. Then the homogeneous solution was transferred into a Teflon-lined stainless steel autoclave (100 mL) and maintained at 180 °C for 5 h. The autoclave was cooled to room temperature naturally. The dark precipitation was washed by H2O and ethanol for three times to obtain pure MnO2 nanowires. 2.2 Synthesis of core/sheath structured ultralong MnOx/PPy nanowires MnO2 nanowires (50.0 mg) were dispersed in H2O (20 mL). Sodium ptoluenesulfonate (32.0 mg) and (NH4)2S2O8 (48.0 mg) was dissolved in H2O (20 mL), respectively. The sodium p-toluenesulfonate solution was poured into liquid dispersion of MnO2 nanowires under stirring, which was for coating a uniform PPy sheath on the surface of MnOx.[14] Afterward pyrrole monomer (15 μL) and phytic acid (100 μL 50 wt. % in H2O) was added. After 30 min, (NH4)2S2O8 solution was put into the above solution under stirring. The mixture solution was maintained at 0 - 5 ºC under stirring for 6 h. After polymerization, the black precipitate was centrifuged, rinsed with abundant H2O and ethanol to remove residues and dried in a vacuum oven. 2.3 Electrochemical measurement All electrochemical performances including cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) were studied on a CHI660E electrochemical 4
workstation at room temperature. For three-electrode system, 1.0 M aqueous Na2SO4, Ni foam loading core/sheath structured ultralong MnOx/PPy nanowires, Pt plate and Ag/AgCl electrode was used as electrolyte, working electrode, counter electrode and reference electrode, respectively. The working electrode was prepared as follows: mixing active materials, ketjen black and polyvinylidene fluoride (PVDF) in N-methyl2-pyrrolidinone (NMP) with the weight ratio of 50:40:10. The above mixture slurry was coated on the surface of Ni foam evenly and dried at 60 °C in a vacuum oven overnight. The mass loading of active material on the working electrode was about 0.7 mg cm-2. Then the electrode was pressed under 10 MPa and immersed in 1.0 M Na2SO4 aqueous overnight. Electrochemical impedance spectroscopy (EIS) was carried out on a Zahner IM6 in a range of 10-2 - 105 Hz with 5 mV amplitude at open circuit potential. The specific capacitance (C, F g-1) of samples was calculated according to Eq. 12: I × Δt
C=m
(1)
× ΔV
CMnOx =
C - CPPyωPPy
(2)
ωMnOx
where I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the weight of active material on one electrode in three-electrode system, ΔV (V) is the voltage window after voltage drop (IR drop), CMnOx (F g-1) is the specific capacitance of pure MnOx in the MnOx/PPy nanowires, CPPy (F g-1) is the specific capacitance of pure PPy, ωMnOx and ωppy are the weight ratio of MnOx and PPy in the MnOx/PPy nanowires, respectively. The assembled asymmetric supercapacitor (ASC) using core/sheath structured 5
ultralong MnOx/PPy nanowires as positive electrode and activated carbon (AC) as negative electrode was measured in a two-electrode configuration. The polyvinyl alcohol/Na2SO4 (PVA/ Na2SO4) gel electrolyte with safety, stability and good flexibility[1] was made by dissolving 4.0 g PVA in 30 mL H2O at 85 °C (water bath). After the solution became transparent, 10 mL 0.4 g mL-1 Na2SO4 solution was put into the PVA solution under vigorous stirring until the mixture appeared clear. The weight ratio of MnOx/PPy and AC in electrodes for optimal electrochemical performance is calculated to be 1:4.7 according to charge balance by Eq. 3: m+ m-
C - × ΔV -
= C+
(3)
× ΔV -
where m+ and m- (g) are the weight of active materials on positive electrode and negative electrode; C+ and C- (F g-1) are the specific capacitance of active material on positive electrode and negative electrode; and ΔV+ and ΔV- (V) are the voltage of positive electrode and negative electrode, respectively. The specific capacitance (C, F g-1), energy density (E, W h kg-1) and power density (P, W kg-1) of samples were calculated according to Eq. 4-6: I × Δt
C=m 1
E=2× P=
(4)
× ΔV C × ΔV2
(5)
3.6
3600E
(6)
Δt
where I (A) is the discharge current, m (g) is the total weight of active material on two electrodes, ΔV (V) is the voltage window after IR drop, Δt (s) is the discharge time. For the leakage current test, the device was first charged to 1.0 V at 2 mA and then the potential was kept at 1.0 V for 2 h while acquiring the current data. For the self6
discharge test, the device was first charged to 1.0 V at 2 mA and kept at 1.0 V for 15 min, and then measured the open circuit potential over time. 3. Results and Discussion
Scheme 1 Schematic of the formation of MnOx/PPy nanowires.
Core/sheath structured ultralong MnOx/PPy nanowires assembled by in-situ polymerization of pyrrole on MnO2 (Scheme 1) are about 30 nm in diameter and several tens of micrometers in length (Fig. 1a-b and Fig. S1a-c). The elemental mapping images (Fig. S1e–h) show that C, N, Mn and O elements coexist and distribute uniformly in MnOx/PPy nanowires. The aspect ratio of length to diameter is as large as ca. 1000. The PPy sheath is about 4 nm in thickness on the MnOx core of about 20 nm in diameter (Fig. 1b). The FTIR spectrum of PPy (Fig. S2a) exhibits characteristic vibration peaks of PPy, such as the C-N and C=C stretching vibration at 1468 and 1550 cm-1, the C-H in-plane vibration at 1309 cm-1 and the ring deformation at 900 cm-1.[15, 16] The characteristic Raman peaks of PPy also prove the existence of PPy again (Fig. S2b), such as the peak at 924 cm-1 attributed to the quinoid polaronic structure, 1047 cm-1 attributed to the C-H in-plane deformation, 1385 and 1584 cm-1 attributed to ring stretching mode of the polymer backbone and the π-conjugation structure, respectively.[16] The conductive PPy sheath with large aspect ratio would essentially 7
assure fast electron transfer between current collector and MnOx core. The content of MnOx in the nanowires is 71.4 wt. % (Fig. S2c), higher than that of reported MnOxbased materials, [2, 17] which would produce large specific capacitance and energy density.
Fig. 1 (a) SEM image, (b) TEM images (inset in b: magnified TEM image) and (c) HRTEM image of MnOx/PPy nanowires.
OVs with improved conductivity for MnOx in MnOx/PPy nanowires are simply introduced during polymerization process of the PPy sheath, as verified by a subpeak at 531.3 eV in O 1s XPS spectra (Fig. 2a), corresponding to the adsorbed oxygen species on the OVs.[18] The other two deconvoluted subpeaks correspond to the O atom in (Mn)-O-(Mn) at 529.5 eV, and the O atom in (Mn)-O-(H) at 532.3 eV.[19] In contrast, the O 1s XPS spectra of the MnO2 nanowires show only the O atoms in (Mn)O-(Mn) and (Mn)-O-(H). The multivalent Mn causes the surrounding O atoms to accumulate more charges,[7] resulting in higher binding energy of O atom in (Mn)-O(H) of MnOx/PPy. The content of OVs in MnOx/PPy nanowires is calculated to be 41.8% by integrating peak areas, indicating abundant OVs. The OVs are also demonstrated by 8
a symmetric electron spin signal at g=2.006 in electron spin resonance (ESR) spectrum (Fig. 2c) only for the MnOx/PPy nanowires and not for MnO2 nanowires.
Fig. 2 (a) O 1s, (b) Mn 2p XPS spectra and (c) ESR spectra of MnOx nanowires and MnOx/PPy nanowires.
Due to the formation of OVs, Mn2+ and Mn3+ form based on electrostatic equilibrium.[20] Mn2+ in Mn-(O), Mn3+ in Mn-(N), Mn3+ in Mn-(O) and Mn4+ in Mn(O) at the ratio of 15.7, 22.8, 20.2 and 41.3 at.% among total Mn atoms are shown by the deconvoluted four peaks at 640.6, 641.8, 642.7 and 643.3 eV, respectively, from the Mn 2p XPS spectra of MnOx/PPy nanowires[21, 22]. As comparison, the MnO2 nanowires without PPy only show Mn4+ peak in the Mn 2p XPS spectra (Fig. 2b). Mn4+ in MnOx/PPy nanowires exists as α-MnO2, as revealed by the XRD peak at 2θ = 18.1° (Fig. S2d) corresponding to the interplanar (200) lattice of α-MnO2 with an interplanar spacing of 0.49 nm, the same as that shown in HRTEM (Fig. 1c). The presence of αMnO2 is also demonstrated by the (200) and (310) lattices of α-MnO2 revealed by diffraction spots in SAED image of MnOx/PPy nanowires (Fig. S1i). Mn2+ and Mn3+ should exist in amorphous oxides since no related XRD peaks appear. Between MnOx and contacted PPy, about 20.2 at.% interfacial Mn atoms form Mn-N chemical bond, 9
as indicated by above Mn 2p XPS spectra and also by the N 1s XPS spectra of MnOx/PPy nanowires, the latter can be deconvoluted into four subpeaks at 397.8, 399.4, 399.8 and 401.8 eV, corresponding to N atoms in =N-, (Mn)-N, -N-(H) and -N+ (Fig. S3a), respectively.[23] It is worth noting that the interfacial Mn-N chemical bond would not only maintain structural stability of MnOx/PPy nanowires but also promote fast electron transfer between MnOx core and PPy sheath. Left ca. 80 at.% interfacial Mn atoms allow access of ions to MnOx during charge/discharge process. So these advantages would yield good cycling stability, conductivity and rate performance of MnOx/PPy nanowires in supercapacitors.
Fig. 3 (a) Nyquist plots (inset in a: an equivalent circuit) of MnO2 and MnOx/PPy nanowires. (b) Plot of specific capacitances against different current densities of MnO2, PPy and MnOx/PPy nanowires. (c) Cycling stability tests of MnO2, PPy and MnOx/PPy nanowires.
The outlayer of PPy sheath consists of characteristic nanoparticles (Fig. 1a) during in-situ polymerization of pyrrole, superior to smooth intact layer of PPy by CVD.[13] PPy nanoparticles increase specific surface area of MnOx/PPy nanowires to 80.5 m2 g1,
much higher than that of MnO2 nanowires before coating (37.7 m2 g-1) (Fig. S3b-c). 10
PPy nanoparticles also form mesopores including interparticle mesopores of PPy as well as mesopores between PPy sheath and MnOx core, as indicated by a hysteresis loop starting at around a relative pressure (p p0-1) of 0.4-1.0.[24] The ratio of mesopores and macropores to total pores is 60.4 and 39.6 %, respectively. Large specific surface would sufficiently expose PPy to electrolyte, while rich mesopores would shorten ionic transport path and diffusion time to accelerate electrolyte transport during charge/discharge process, both essential for high electrochemical performance.[25] Last but not least, the outlayer of PPy nanoparticles would improve cycling stability due to 1) inherent intercalation/de-intercalation and the compositional change of MnOx accompanied by volumetric change[5] and 2) protection against direct exposure and easy dissolution in neutral or alkaline aqueous electrolyte.[26] Due to the conductive PPy layer and OVs, MnOx/PPy nanowires show improved conductivity with smaller series resistance (Rs) of 1.26 Ω than that of MnO2 (2.05 Ω), implying much faster electron transfer between current collector and MnOx core in MnOx/PPy nanowires. Also benefiting from large specific surface area and mesopores, MnOx/PPy nanowires show much smaller charge transfer resistance (Rct) of 0.64 Ω than that of MnO2 (5.13 Ω) (Fig. 3a). Both would greatly accelerate access of electrolyte and transport rate of electrons and ions, and consequently speeding up electrochemical dynamics. As a result, the MnOx/PPy nanowires show a larger specific capacitance and ion transport rate, as indicated by significantly larger CV curve area (1.09 C) than that of MnO2 nanowires (0.52 C) at a scan rate of 50 mV s−1 (Fig. S4a-c). For Ni foam, the lower current response than that of MnO2 and MnOx/PPy implies that the capacitance 11
of Ni foam can be ruled out. Fast electrochemical dynamics enables MnOx/PPy nanowires to show excellent pseudocapacity by quasi-rectangular shape of CV curves at different scan rates.[27] Good rate performance of MnOx/PPy nanowires is shown by the well-kept quasi-rectangular CV curves (Fig. S4b) as scan rates increase.[28] There are no obvious redox peaks appeared in the CV curves, implying that the capacitive performance is based on Faradaic redox processes in MnOx/PPy nanowires as expressed with fast intercalation/de-intercalation of Na+ (Eq. 7) in MnO2 and doping/de-doping of SO42− (Eq. 8) in PPy:[3] MnO2 + Na+ + e- ⇌ MnOONa
(7)
(PPy+)2SO42- + 2e- ⇌ 2PPy + SO42-
(8)
The specific capacitance of MnOx/PPy nanowires is as large as 1091.4 F g-1 at 1 A g-1, 2 times larger than that of MnO2 nanowires (480.1 F g-1) (Fig. 3b) and reported MnOx-based materials, such as TiN@MnO2[29], hollow MnO2 nanofibers[30] and MnO2/CNT[31]. Moreover, good rate capability of MnOx/PPy nanowires is proved by a large specific capacitance of 890.1 F g-1 with a retention ratio of 81.6% even when the current density is increased to 20 A g-1. The specific capacitance of PPy is 449.8 F g-1 (Fig. 3b and Fig. S5), so the specific capacitance based on the weight of MnOx at 1 A g-1 is calculated to be 1348.4 F gMnOx-1, higher than reported values of MnOx-based materials (Table S1). Due to the protective PPy sheath in the MnOx/PPy nanowires, corresponding cycling stability, one important factor affecting the application of supercapacitors, is significantly increased. First, excellent electrochemical reversibility as well as fast 12
current-voltage response[19] is indicated by triangular and substantially symmetrical GCD curves of MnOx/PPy nanowires at different current densities with a small IR drop of only 0.01 V at 2 A g-1 (Fig. S4d-f). As comparison, the IR drop values of MnO2 nanowires (0.02 V) and reported CNTs/MnO2 (0.03 V) and Co-MnO2 (0.08 V)[25] are much bigger. Then the specific capacitance retention percentage of MnOx/PPy electrodes is measured to be as high as 97.4% of their initial values after 10000 GCD cycles at 10 A g-1 (Fig. 3c), much higher than that of MnO2 (67.0%), PPy (50.9%) and reported composites such as N-doped carbon nanotubes/MnO2,[17] MnO2/polyaniline,[1] carbon nanotube/MnO2/reduced graphene oxide[32] and MnO2/C/Ag.[33] The morphology and crystal phase also show good stability without obvious changes after 10000 cycles (Fig. S6a-b). The cycling stability is further proved by Nyquist plots before and after 10000 cycles. The plots are similar without obvious changes in Rs and Rct from 1.26 and 0.64 to 1.29 and 0.83 Ω after 10000 cycles, respectively (Fig. S6c). In order to practically illustrate the excellent electrochemical performance of core/sheath structured ultralong MnOx/PPy nanowires, MnOx/PPy//AC ASC is assembled with MnOx/PPy as positive electrode, AC as negative electrode and PVA/Na2SO4 gel as electrolyte (Fig. 4a). The stable working voltage window of the MnOx/PPy nanowires and AC is 0 - 1.0 V and -1.0 - 0 V, respectively (Fig. S7 and S8a). The MnOx/PPy//AC ASC works steadily at 2.2 V (Fig. S8b-c), capable of lighting a red light-emitting diode (LED) with only one ASC (Fig. 4b). The CV curves show a quasirectangular shape, indicating fast current-voltage response and near-ideal capacitance 13
characteristics (Fig. S8d). In 0-2.2 V, the GCD curves at 1-20 A g-1 show symmetrical shapes and excellent electrochemical reversibility (Fig. S8e). The specific capacitance is as high as 214.2 F g-1 at 1 A g-1. Even at 20 A g-1, the capacitance is still up to 171.8 F g-1 with capacitance retention of 80.2%, implying excellent rate capability (Fig. 4b). The MnOx/PPy//AC ASC exhibits excellent cycling stability with a capacitance retention of 98.1% after 10000 cycles (Fig. S8f).
Fig. 4 (a) Schematic of the charge/discharge process of assembled MnOx/PPy//AC ASC (b) Plot of specific capacitances against different current density (inset in b: photograph of the assembled ASC lighting a red LED), (c) Leakage current curve of MnOx/PPy//AC ASC charged at 2 mA to 1.0 V and kept at 1.0 V for 2 h. (d) Selfdischarge curve of MnOx/PPy//AC ASC after charged at 1.0 V for 15 min. (e) Ragone plots of the MnOx/PPy//AC and reported MnOx-based ASCs.
For practical application, it is important to know the leakage current and selfdischarge characteristics of ASC. For the MnOx/PPy//AC ASC, the leakage current dropped significantly at the beginning and then quickly stabilized at 9.8 μA (Fig. 4c). 14
The self-discharge test further shows the time courses of the open circuit potential. The ASC undergoes rapid self-discharge process at the beginning, and then the selfdischarge process is quite slow after several hours. Finally, the device shows a stable output voltage of 0.56 V after 24 h (Fig. 4d). The maximum energy density of MnOx/PPy//AC ASC reaches a large value of 144.0 W h kg-1 at a power density of 1100 W kg-1. Even at a high power density of 22000 W kg-1, the energy density can still be as large as 116.4 W h kg-1 (Fig. 4e). The specific capacitance, energy density and power density of MnOx/PPy//AC ASC are larger than that of reported ASCs of MnOx-based materials (Table S2). 4. Conclusion In summary, the assembled core/sheath structured ultralong MnOx/PPy nanowires prove to be promising electrode materials in future energy storage devices, based on better supercapacitor performances such as conductivity, specific capacitance, energy density, power density and cycling stability than reported MnOx-based materials. The conductive PPy sheath with underlying PPy layer chemically bonding MnOx, abundant OVs in MnOx formed during mild in-situ polymerization of PPy and the interfacial MnO bonds are of great importance to speed up electron transfer between current collector and MnOx core (Fig. 4a). The large specific surface due to the PPy nanoparticles in outlayer of PPy sheath, the large aspect ratio of the MnOx/PPy nanowires, as well as mesopores in MnOx/PPy nanowires speed up diffusion and access of electrolytes and transport of Na+ ions to and from MnOx which are not chemically bonded by PPy during intercalation/de-intercalation. The sheath of PPy with both Mn-N chemical bonds and 15
PPy nanoparticles stabilizes the MnOx core considering volumetric change and dissolution inhibition of MnOx, consequently assuring excellent cycling performance of MnOx/PPy nanowires. This work optimizing assembly profile of simply synthesized conductive PPy sheath with PPy nanoparticles rather than physically intact PPy layer accompanying with mildly introduced abundant OVs in MnOx for high energy density and power density energy devices is of great importance as a breakthrough in controllable assembly of functional nanomaterials.
Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21872086) and Natural Science Foundation of Shandong Province, China (No. ZR2019MB011). We thank Prof. Houyi Ma for providing the impedance analyzer. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/xxx References [1] N. Liu, Y.L. Su, Z.Q. Wang, Z. Wang, J.S. Xia, Y. Chen, Z.G. Zhao, Q.W. Li, F.X. Geng, ACS Nano 11 (2017) 7879. [2] J.P. Li, Z.H. Ren, S.G. Wang, Y.Q. Ren, Y.J. Qiu, J. Yu, ACS Sustainable Chem Eng 4 (2016) 3641. [3] W.D. He, C.G. Wang, F.W. Zhuge, X.L. Deng, X.J. Xu, T.Y. Zhai, Nano Energy 35 (2017) 242. 16
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Graphical abstract
Core/sheath structured ultralong MnOx/PPy nanowires feature improved conductivity and stability for supercapacitor Jiali Fu, Yong Zhang, Hui Zhao, Rijuan Jiang and Renjie Zhang*
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Conflict of Interest Form Declarations of interest: none
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