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A Novel and Facile One-Pot Solvothermal Synthesis of PEDOT-PSS/ Ni-Mn-Co-O Hybrid as an Advanced Supercapacitor Electrode Material Chengjie Yin, Chunming Yang, Min Jiang, Cuifen Deng, Lishan Yang, Junhua Li, and Dong Qian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11022 • Publication Date (Web): 12 Jan 2016 Downloaded from http://pubs.acs.org on January 13, 2016

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ACS Applied Materials & Interfaces

A Novel and Facile One-Pot Solvothermal Synthesis of PEDOT−PSS/Ni−Mn−Co−O Hybrid as an Advanced Supercapacitor Electrode Material Chengjie Yin,† Chunming Yang,*,† Min Jiang,† Cuifen Deng,† Lishan Yang,† Junhua Li,‡ and Dong Qian,*,‡,§ †

College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081,

PR China ‡

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083,

PR China §

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR

ABSTRACT: In this work, a novel and facile one-pot method has been developed for the synthesis

of

a

hybrid

consisting

of

Ni−Mn−Co

ternary

oxide

and

poly(3,4-

ethylenedioxythiophene)–polystyrene sulfonate (PEDOT−PSS/NMCO) with a hierarchical threedimensional net structure via a solvothermal−coprecipitation coupled with oxidative polymerization route. Apart from the achievement of polymerization, coprecipitation and solvothermal in one pot, the hydroxyl (OH−) ions generated from the oxidative polymerization of

1 Environment ACS Paragon Plus


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organic monomer by neutral KMnO4 solution were skillfully employed as precipitants for metal ions. As compared with the PEDOT−PSS/Ni−Mn binary oxide, PEDOT−PSS/Co−Mn binary oxide and PEDOT−PSS/MnO2, PEDOT−PSS1.5/NMCO exhibits overwhelmingly superior supercapacitive performance, to be specific, a high specific capacitance of 1234.5 F g−1 at a current density of 1 A g−1, a good capacitance retention of 83.7% at a high current density of 5 A g−1 after 1000 cycles, an energy density of 51.9 W h kg−1 at a power density of 275 W kg−1 and an energy density of 21.4 W h kg−1 at an extremely elevated power density of 5500 W kg−1. Noticeably, the energy density and power density of PEDOT−PSS/NMCO are by far higher than those of the existing analogues recently reported. The exceptional performance of PEDOT−PSS/NMCO benefits from its unique mesoporous architecture, which could provide larger reaction surface area, faster ion and electron transfer ability, and good structural stability. The desirable integrated performance enables the multi-component composite to be a promising electrode material for the energy storage applications.

KEYWORDS: Ni–Mn–Co ternary oxide, PEDOT–PSS, 3D net structure, synthesis, supercapacitor

1. INTRODUCTION With the extraordinary growth of the portable electronic devices and hybrid electric vehicles markets, the demands for the high-power and high-energy power resources are rising to unprecedented heights. Electrochemical capacitors, also called supercapacitors, exhibit great promising to meet these demands because of their fast charge and discharge rates, high-power density, and excellent cycling stability.1,2 Supercapacitors have two common types according to


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the different energy storage charge–discharge mechanisms, i.e., electrical double-layer capacitors (EDLCs) and pseudocapacitors.3,4 EDLCs store energy by the electrostatic accumulation of charges in the electrical double-layer near the electrode/electrolyte interface, and the corresponding electrode materials are usually the carbon-based materials (e.g., activated carbon,5 carbon nanotubes6 and graphene7). However, EDLCs cannot afford high specific capacitance and energy density enough to satisfy the requirements of applications in electric vehicles.8 In contrast, pseudocapacitors generally use noble/transition metal oxides (RuO2,9 MnO2,10 CoOx,11 NiO,12 Fe2O3,13 etc.) and conducting polymers (polyaniline,14 polypyrrole,15 poly(3,4ethylenedioxythiophene)16,17 and its derivatives,18,19 etc.) as the electrode materials and store energy via the faradaic reactions of electrode materials with the electrolytes. Obviously, pseudocapacitors can achieve relatively high specific capacitance and energy density through the fast and reversible faradaic redox reactions.20 Although the transition metal oxides can frequently deliver remarkably high specific capacitance, energy density and power density, the poor rate capability and cycling stability arising from their low electrical conductivity and easily damaged structures during the redox processes are the current bottlenecks to their practical applications in pseudocapacitors.21,22 The conducting polymers possess the attractive features of being capable of storing charges throughout their entire bodies and rapid charge–discharge kinetics due to their good conductivity.23 Unfortunately, their relatively low mechanical stability and cycle ability also limit their applications in pseudocapacitors.24 Compositing transition metal oxides with conducting polymers has been proven as an efficient strategy to attenuate the drawbacks of both moieties, benefiting from the reinforced synergic effect between them and complementarity of each other.25−27 Co3O4 has attracted much attention for its extremely high theory specific capacitance (3560 F g−1).28 However, its high cost and



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potential to environmental contamination significantly hinder its extensive applications in pseudocapacitors. To alleviate these problems, Co3O4 was frequently coupled with other ecofriendly and cheaper metal oxides such as NiCo2O4,29 ZnCo2O4,30 MnCo2O4,31 CuCo2O4,32 MgCo2O433 and FeCo2O4.34 Among these metal oxides, NiO and MnO2 are of particular interests. NiCo2O4 is the most studied binary metal oxide electrode material for pseudocapacitors due to its improved electronic conductivity35,36 and high theory specific capacitance of 2584 F g−1 for NiO.37 Recently, MnCo2O4 has been widely investigated owing to the environmental friendliness, low cost, resource abundance and good electrochemical performance of MnO2.38,39 Besides, due to the similar atom radii of Mn, Ni and Co, the introductions of Mn and Ni into Co3O4 can partly replace Co atoms to obtain a Mn−Ni−Co ternary oxide without distinct crystal structure change.40 Despite of the improvements in the supercapacitive performances of combined metal oxides, their conductivities are still too poor to support the rapid electron transport toward high rate capability.41 As aforementioned, the introduction of conducting polymers to metal oxides is one of the most effective approaches. The studies of conducting polymers for supercapacitors mainly focus on polyaniline, polypyrrole, poly(3,4-ethylenedioxythiophene) and its derivatives. Recently, poly(3,4-ethylenedioxythiophene)–polystyrene sulfonate (PEDOT−PSS) has received considerable interests as a promising electrode material for supercapacitors, especially in combining with metal oxides owing to its high conductivity, excellent stability, suitable conjugated backbone, fast redox transition and superb processability.42,43 With PSS doped into PEDOT molecular chains, the solubility of PEDOT can be greatly improved, thereby enhancing its practicability in supercapacitors.44,45 In this study, we, for the first time, demonstrate a facile one-pot method for the synthesis of a hybrid

consisting

of

Ni−Mn−Co

ternary

oxide

and


PEDOT−PSS

(denoted

as

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PEDOT−PSS/NMCO) with a hierarchical three-dimensional (3D) net structure via a solvothermal–coprecipitation coupled with oxidative polymerization route. Apart from the achievement of solvothermal, coprecipitation and polymerization in one pot, the hydroxyl (OH−) ions generated from the oxidative polymerization of organic monomer by neutral KMnO4 aqueous solution were skillfully employed as precipitants for metal ions. The designed PEDOT−PSS/NMCO hybrid shows overwhelmingly superior supercapacitive performance in comparison with the PEDOT–PSS/Ni−Mn binary oxide, PEDOT−PSS/Co−Mn binary oxide and PEDOT−PSS/MnO2. Especially, the energy density and power density are by far higher than those of the existing analogues recently reported. As far as we know, no works have been reported on the PEDOT−PSS/NMCO hybrid as the supercapacitor electrode material. 2. EXPERIMENTAL SECTION 2.1. Chemical Reagents and Materials. All of the reagents used in the experiments were of analytical grade and used without further purification. Co(NO3)2·6H2O, Ni(NO3)2·6H2O, ethanol were purchased from Tianjin Fengchuan Chemical Reagent Co., Ltd. KMnO4 was obtained from Hengyang Kaixin Chemical Reagent Co., Ltd. 3,4-ethylenedioxythiophene (EDOT) and polystyrene sulfonate (PSS, MW = 80000, the mass fraction is 25%) were provided by Maklin. The nickel foam (1 × 4 cm2) was cleaned by sequential sonications in 3 M HCl, acetone, distilled water and absolute ethanol for 15 min.



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Figure 1. Schematic illustration of synthesis procedure for PEDOT–PSS/NMCO.

2.2. Synthesis of PEDOT–PSS/NMCO 3D Net Structure Composite. Figure 1 shows the schematic

illustration

of

synthesis

procedure

for

PEDOT−PSS/NMCO.

The

PEDOT−PSS/NMCO hybrid was prepared by a one-pot solvothermal−coprecipitation coupled with oxidative polymerization route. In a typical process, 0.9 g of EDOT monomer and 7.6 g of PSS (the molar ratio of −SO3− in PSS : EDOT = 1.5 : 1) were ultrasonically dispersed in 30 mL of alcohol solution (Vwater : Vethanol = 1 : 1). Then, the obtained suspension was added into 35 mL of KMnO4 aqueous solution (0.67 g of KMnO4 was dissolved in 35 mL of distilled water, the molar ratio of KMnO4 : EDOT = 4 : 6) with the assistance of sonication for 10 min. Meanwhile, 0.813 g of Ni(NO3)2·6H2O and 1.63 g of Co(NO3)2·6H2O were dissolved in 25 mL of distilled water, and the received solution was subsequently added to the above suspension assisted by sonicating for 10 min. The mixture was then transferred into a 150 mL Teflon-lined autoclave for the hydrothermal reaction at 150 °C for 15 h. The black precipitates were collected through filtration and washed with distilled water and absolute ethanol several times, respectively. Finally, the PEDOT−PSS/NMCO hybrid with a mass ratio of PEDOT−PSS : NMCO of 45.5 :


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54.5 was obtained after drying at 60 °C overnight in vacuum. Additionally, in order to investigate the influences of the molar ratios of KMnO4 and −SO3− in PSS to EDOT on the electrochemical properties, PEDOT–PSS/NMCO hybrids with different molar ratios of KMnO4 to EDOT (3 : 6, 4 : 6, 5 : 6 and 6 : 6, labeled as PEDOT–PSS/NM3CO, PEDOT−PSS/NM4CO, PEDOT−PSS/NM5CO and PEDOT−PSS/NM6CO, respectively, the molar ratio of −SO3− in PSS: EDOT = 0.8 : 1) and −SO3− in PSS to EDOT (0.5 : 1, 1 : 1, 1.5 : 1 and 2 : 1, referred as PEDOT−PSS0.5/NMCO,

PEDOT−PSS1/NMCO,

PEDOT−PSS1.5/NMCO

and

PEDOT−PSS2/NMCO, respectively, the molar ratio of KMnO4 : EDOT = 4 : 6) were prepared using

the

similar

procedure.

For

comparison,

PEDOT−PSS/MnO2

(denoted

as

PEDOT−PSS/MO), PEDOT−PSS/Ni−Co binary oxide (denoted as PEDOT−PSS/NMO) and PEDOT−PSS/Co−Mn binary oxide (denoted as PEDOT−PSS/CMO) were also synthesized by the similar procedure through the changes of starting materials ratios. 2.3. Characterizations. Fourier transform infrared (FTIR) analyses were carried out to characterize the bonding properties of samples on a NEXUS 670 FTIR spectrometer with a KBr disk. Crystallite structures of samples were identified on a Japan Rigaku 2550 X-ray powder diffractometer (XRD) with Cu Kα radiation (λ = 1.54056 Å) operating at 40 kV and 250 mA. Brunauer−Emmett−Teller (BET) surface areas and pore volumes of samples were determined by N2 adsorption/desorption technique using a TriStar 3000 system. Scanning electron microscopy (SEM) images of samples were taken by a Hitachi SU8010 ultra-high resolution cold field emission scanning electron microscope. The surface chemical states of samples were investigated utilizing the X-ray photoelectron spectroscopy (XPS) measurement performed on a Thermo ESCALAB 250XI system with a resolution of 0.45−0.6 eV from a monochromatic Al



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anode X-ray source with Kα radiation (1486.6 eV). Conductivity measurements of samples were investigated using a SDY−4 type four-probe conductivity tester. 2.4. Working Electrode Fabrications and Electrochemical Measurements. The working electrode was fabricated by mixing the as-prepared samples, acetylene black and poly(tetrafluoroethylene) with a mass ratio of 80 : 15 : 5 in ethanol to form a slurry. The resulting slurry was compressed on a pretreated nickel gauze using a tablet press machine at 10 MPa, and dried at 60 °C overnight in vacuum. The mass loading of active material in the working electrode was ca. 1.5 mg cm−2. All the electrochemical measurements were conducted in a conventional three-electrode system with the fabricated electrode as the working electrode, Pt foil as the counter electrode and Ag/AgCl as the reference electrode in 6.0 M of KOH aqueous solution on a RST 5200 electrochemical workstation. The electrochemical properties of samples were evaluated by cyclic voltammetry (CV) in the voltage range of −0.1 to 0.6 V (vs. Ag/AgCl) and galvanostatic charge– discharge. The electrode capacitance was measured by the galvanostatic discharge–charge method in the voltage range of −0.2 to 0.35 V, and the corresponding specific capacitance (Cs, F g−1), energy density (E, W h kg−1) and power density (P, W kg−1) can be calculated according to the following equations:46 Cs = I ×∆t/(∆V × m)

(1)

E = 1/7.2 ×Cs ×∆V2

(2)

P = 3600 × E/∆t

(3)


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where I (A) is the discharge current, ∆t (s) is the discharge time, m (g) is the mass of active material and ∆V (V) is the potential window.

604 675 675

4000 3500 3000 2500 2000 1500 1000 -1

Wavenumbers (cm )

500

Transmittance (a.u.)

606 498

1169 1171

1371 1182

1365

(b)

1171

PEDOT-PSS/NM3CO

1612 1608

3376 3383

PEDOT-PSS/NM4CO

1358

3371

PEDOT-PSS/NM5CO

1618

1618

PEDOT-PSS/NM6CO 3367

(a)

1354

3. RESULTS AND DISCUSSION

Transmittance (a.u.)

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


PEDOT-PSS2/NMCO

PEDOT-PSS1.5/NMCO PEDOT-PSS1/NMCO PEDOT-PSS0.5/NMCO

2000

1500

1000

500

-1

Wavenumbers (cm )

Figure 2. FTIR spectra of PEDOT−PSS/NMCO prepared with different molar ratios of (a) KMnO4 to EDOT (3 : 6, 4 : 6, 5 : 6 and 6 : 6) and (b) –SO3− in PSS to EDOT (0.5 : 1, 1 : 1, 1.5 : 1 and 2 : 1).

3.1. Structures and Morphologies. The FTIR technique was first used to confirm the structure of PEDOT−PSS in the PEDOT−PSS/NMCO hybrids, of which spectra are shown in Figure 2. As seen from Figure 2(a), substantial changes can be observed in IR main peaks with increasing the molar ratios of oxidizing agent KMnO4 to monomer EDOT from 3 : 6 to 6 : 6. In particular, the peak around 1612 cm−1, attributed to the O−H stretching vibration on metal atoms,47 shifts first towards the lower wavenumbers of 1608 cm−1 and then to the higher wavenumbers of 1618 cm−1 with the increase of KMnO4 concentration, confirming the interaction process between metal ions and OH− generated from the oxidative polymerization of



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EDOT by KMnO4. Meanwhile, the peaks at around 1358 and 1171 cm−1, corresponding to the vibration of C−C on thiophene ring and C−O−C vibration mode on ethylenedioxy ring, respectively, shift first towards the higher wavenumbers and then to the lower wavenumbers with increasing the KMnO4 concentration, indicating the formation of conducting PEDOT and the increasing conjugation degree of polymeric chains. The peak at 675 cm−1, ascribed to the deformation vibration of C−S on thiophene ring, moves more than 70 cm−1 towards the lower wavenumbers, certifying the enhancement of oxidation degree of polymer PEDOT.47 The newly emerging peak at ca. 498 cm−1 can be assigned to the M−O vibration absorption.47 Interestingly, almost all the turning points for the above characteristic peaks corresponding to υ (O−H…M), υ (C−C), υ (C−O−C) and δ (C−S) on thiophene ring occur when the molar ratio of KMnO4 to EDOT is 4 : 6, implying that the structure of PEDOT−PSS/NMCO might be experiencing a dramatic change and therefore a performance fluctuation could be anticipated at this point. Additionally, the weak absorption peak at 1037 cm−1, originated from the −SO3− group, suggests that PEDOT is in a doped state by PSS.48 It was also affirmed by the absorption peak strengthen at 1037 cm−1 when varying the molar ratios of −SO3− in PSS to EDOT from 0.5 : 1 to 2 : 1 as displayed in Figure 2(b).


20

30

40

50

60

2 Theta (degree)

70

80

10

20

30

40

50

(440)

(400)

(222)

NiCo2O4

(511)

PEDOT-PSS0.5/NMCO PEDOT-PSS1/NMCO PEDOT-PSS1.5/NMCO PEDOT-PSS2/NMCO

(311)

PEDOT MnO2 (220)

(400)

(222)

(220)

(111) 10

(b)

(111)

2

NiCo2O4

(440)

PEDOT MnO

(511)

PEDOT-PSS/NM6CO PEDPT-PSS/NM5CO PEDOT-PSS/NM4CO PEDOT-PSS/NM3CO

(311)

(a) Relative intensity (a.u.)

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


Relative intensity (a.u.)

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60

70

80

2 Theta (degree)

Figure 3. XRD patterns of PEDOT−PSS/NMCO prepared with different molar ratios of (a) KMnO4 to EDOT (3 : 6, 4 : 6, 5 : 6 and 6 : 6) and (b) –SO3− in PSS to EDOT (0.5 : 1, 1 : 1, 1.5 : 1 and 2 : 1).

Figure 3 presents the XRD patterns of the above PEDOT−PSS/NMCO hybrids for the purpose of identifying the structure of NMCO. As Figure 3 reveals, the diffraction peaks at 2θ of 18.4°, 30.2°, 35.6°, 37.2°, 43.4°, 57.3° and 63.0° can be readily indexed to the (111), (220), (311), (222), (400), (511) and (440) crystal planes of NiCo2O4 (JCPDS No. 73−1702), respectively. The diffraction peaks centered at 12.9°, 18.1°, 28.6°，37.5°, 60.3° and 74.6°, corresponding to αMnO2 crystal planes of (110), (200), (310), (211), (521) and (402) (JCPDS No. 44−0141), respectively, can also be distinguished from Figure 3. It should be noted that the diffraction peaks of the ternary oxide is similar to those of spinel Co3O4.49 This may be due to the fact that the substitutions of Mn and Ni atoms for Co in Co3O4 do not affect its spinel crystal structure significantly because of the rather close covalent radii of Mn (1.17 Ǻ), Ni (1.15 Ǻ) and Co (1.16 Ǻ).



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As can be seen from Figure 3(a), with the increase of the amount of KMnO4, the crystallinity of NMCO becomes higher. This is primarily because OH− ions generated from the oxidative polymerization of organic monomer by the neutral KMnO4 solution serve as the precipitants for metal ions as shown in the following consecutive reactions: 3n EDOT + 2n MnO4− = 3 P(EDOT)n + 2n MnO2 + 2n OH−

(4)

n M2+ + 2n OH− = n M(OH)2 (M = Co2+ and/or Ni2+)

(5)

where n is the degree of polymerization of PEDOT. Therefore, the more OH− ions, the more complete precipitations of metal ions. This means that the amount of KMnO4 actually control the precipitation processes of metal ions. The peaks at 2θ of 8.0° and 13.1°, assigned to the repeat units of PEDOT molecular chains,48 become weak gradually with the increase of KMnO4 concentration, mainly because the intercalation of metal oxides Mn−Ni−Co−O into the polymeric molecular chains could deteriorate the crystallinity order of PEDOT configuration. However, as shown in Figure 3(b), with increasing the amount of dopant PSS, most diffraction peaks of NMCO decrease significantly, suggesting that the crystallinity of NMCO becomes weak. The newly emerging peaks at 13.7°, 25.2° and 26.6°, which can be attributed to the PEDOT molecular chains,50 become strong, indicating the incremental crystallinity of PEDOT. This may be because the macromolecular PSS can not only act as a dopant to induce the configuration transformation of resulting PEDOT molecular chains from intertwined state to expanded state, but also act as a surfactant to induce the directional arrangements of monomer EDOT molecules in the reaction system,51,52 which could simultaneously affect the crystallinity of inorganic metal oxides.


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Figure

4.

SEM

images

of

(a)

PEDOT−PSS/MO,

(b)

PEDOT−PSS/NMO,

(c)

PEDOT−PSS/CMO, (d–g) PEDOT−PSS/NMCO with different molar ratios of KMnO4 to EDOT



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(3 : 6, 4 : 6, 5 : 6 and 6 : 6), and (h–k) PEDOT−PSS/NMCO with different molar ratios of –SO3− in PSS to EDOT (0.5 : 1, 1 : 1, 1.5 : 1 and 2 : 1).

The morphologies of all the prepared hybrids were observed by SEM as shown in Figure 4. From Figure 4(a), it can be found that PEDOT−PSS/MO is mainly composed of micron rods in approximately 50−150 nm diameter mixed with few nanoparticles and octahedral crystals. In many cases, MnO2 prepared by KMnO4 via a hydrothermal method exhibits rod-like morphology.53,54 With the respective introductions of Ni−O and Co−O to PEDOT−PSS/MO, both of the hybrids comprise irregular blocks accompanied by few micron rods as shown in Figure 4(b) and (c). As for the PEDOT−PSS/NMCO hybrids, with increasing the amounts of KMnO4 in their preparations, the morphologies change from irregular blocks or micron rods to leaf-like nanosheets, and finally to regular microspheres of 200–300 nm as indicated in Figure 4(d−g). In particular, the PSS doping amounts have pronounced effects on the morphologies and backbones

of

PEDOT−PSS/NMCO

hybrids.

As

seen

from

Figure

4(h−k),

the

PEDOT−PSS1.5/NMCO hybrid displays a 3D porous net architecture, which is expected to provide a large electroactive surface area for redox reactions and thereby facilitate the easy ion access of the electrode/electrolyte interface and capacitance increment when applied as a supercapacitor electrode material. The significant influence of doping amounts on the morphologies of PEDOT−PSS/NMCO hybrids could stem from the dual functions of PSS as a dopant and a molecule-directing agent in the preparations of PEDOT−PSS/NMCO hybrids as mentioned above. The detailed mechanism for the formations of different morphologies of PEDOT−PSS/NMCO hybrids with changing the starting materials ratios during their preparations is still ambiguous due to the fact that the PEDOT−PSS/NMCO preparations involve


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too many reactions (polymerization, coprecipitation and solvothermal) and starting materials, which deserves further exploration in the future work.

0.0008

(a)

200

-1

150

-1

dV/dD (cm g nm )

250

(b)

50.1 nm

0.0006

0.0004

3

3

-1

Quantity absorbed ( cm g STP)

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


100

Desorption

50

Absorption

0 0.0

0.2

0.4

0.6

0.8

0.0002

0.0000

1.0

0

Relative pressure (P/Po)

30

60

90

120

150

180

Pore diameter (nm)

Figure 5. (a) N2 adsorption/desorption isotherms and (b) pore size distribution curve of the PEDOT−PSS1.5/NMCO.

N2 adsorption/desorption technique was further used to evidence the microstructure of PEDOT−PSS1.5/NMCO. The hysteresis loop in the N2 adsorption/desorption isotherms of PEDOT−PSS1.5/NMCO

(Figure

5(a))

suggests

the

existence

of

mesoporous

in

PEDOT−PSS1.5/NMCO, and the mean pore diameter of 50.1 nm is obtained from its pore size distribution curve (Figure 5(b)). The BET specific surface area and pore volume of PEDOT−PSS1.5/NMCO are determined to be 64.8 m2 g−1 and 0.7 cm3 g−1 from the desorption branch of the N2 adsorption/desorption isotherms using the Barett–Joyner–Halenda method. As we expected above, the BET specific surface area and pore volume of PEDOT– PSS1.5/NMCO are far larger than those of PEDOT−PSS/NMO (49.4 m2 g−1 and 0.41 cm3 g−1), PEDOT−PSS/CMO (47.3 m2 g−1 and 0.38 cm3 g−1) and PEDOT−PSS/MO (39.1 m2 g−1 and 0.29 cm3 g−1).



(a) O1s Ni2p

Intensity (a.u.)

Co2p

C1s Mn2p

S2p

1200

1000

800

600

400

200

0

Binding energy (eV)

(b)

(c) Intensity (a.u.)

Intensity (a.u.)

C1s

295

290

285

280

Ni 2p Ni 2p3/2

Ni 2p1/2 Sat

890

(d)

Co 2p

Sat

800

790

780

Mn 2p3/2

660

O 1s

535

530

Binding energy (eV)

Mn 2p

650

640

Binding energy (eV)

(g)

S 2p

Intensity (a.u.) 540

850

Mn 2p1/2

Binding energy (eV)

(f)

860

(e) Intensity (a.u.)

Intensity (a.u.)

Co 2p3/2

810

870

Binding energy (eV)

Co 2p1/2

Sat

Sat

880

Binding energy (eV)

Intensity (a.u.)

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 42

525

S 2p3/2 S 2p1/2

172

168

164

160

Binding energy (eV)

Figure 6. XPS spectra of (a) survey scan, (b) C 1s, (c) Ni 2p, (d) Co 2p, (e) Mn 2p, (f) O 1s and (g) S 2p regions of PEDOT−PSS1.5/NMCO.


Page 17 of 42

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


In order to gain insights into the surface elemental composition and oxidation states of PEDOT−PSS1.5/NMCO, XPS characterization was performed, and the corresponding results are depicted in Figure 6. The survey scan spectrum (Figure 6(a)) indicates the presence of Ni, Co, Mn, S, O as well as C, agreeing with the elemental composition of PEDOT–PSS1.5/NMCO. In the C1s region (Figure 6(b)), the peak at 284.8 eV is corresponding to the C−C bond. The Ni 2p region (Figure 6(c)) consists of two characteristic spin−orbit doublets of Ni2+ (at 856.2 eV) and Ni3+ (at 873.7 eV), and two shakeup satellites (denoted as Sat) at 861.8 and 879.7 eV. The Co 2p spectrum (Figure 6(d)) also comprises two characteristic spin−orbit doublets of Co2+ (at 781.4 eV) and Co3+ (at 797.2 eV), and two shakeup satellites at 787.5 and 804.3 eV. The coexistence of Co2+, Co3+, Ni2+ and Ni3+ coincides well with the XPS results in previous reports for the Ni and Co composite oxides.55−57 In the Mn 2p region (Figure 6(e)), the spin−orbit doublets for Mn 2p3/2 and Mn 2p1/2 are located at 642.6 and 654.5 eV, respectively, which can be ascribed to Mn4+.58 The peak at 531.7 eV in the O 1s region (Figure 6(f)) corroborates the existence of composite oxides for Co, Ni and Mn.52,55 In the S 2p region (Figure 6(g)), two spin−orbit doublets of PEDOT for S 2p3/2 (at 164.1 eV) and S 2p1/2 (at 165.1 eV) can be detected.59 The peak at a higher binding energy of 168.3 eV would be attributed to the S species resulted from PEDOT+SO3− formed by the incorporation of counter ion SO3− into PEDOT chains, affirming the successful doping of PEDOT by PSS.60,61 3.2. Electrochemical Properties.



(a)

(b) 0.3

PEDOT-PSS/NM4CO

0.2

Current (A)

Current (A)

PEDOT-PSS/NM4CO PEDOT-PSS/NM5CO 0.2 PEDOT-PSS/NM3CO PEDOT-PSS/NM6CO 0.1

0.0

0.1 0.0

-1

10 mV s -1 20 mVs -1 50 mV s -1 100 mV s -1 150 mV s -1 200 mV s

-0.1 -0.2

-0.1 -0.2 -0.1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

-0.3 -0.2 -0.1 0.0

0.7

Potential (V) vs Ag/AgCl left PEDOT-PSS/NM6CO PEDOT-PSS/NM5CO PEDOT-PSS/NM3CO right PEDOT-PSS/NM4CO

0.3 0.2

0.1

(d) 0.4

0.1 0.0 -0.1

0.3

0.4

0.5

0.6

0.7

PEDOT-PSS/NM4CO

-1

1 A.g -1 2 A.g -1 5 A.g -1 10 A.g -1 15 A.g -1 20 A.g

right

0.3 0.2

left

0.1 0.0

-0.1

-0.2

-0.2

0

100

200

300

400

500

0

100

200

Time (s)

800 600 400 200

0

5

10

15

20

(f) -1

PEDOT-PSS/NM3CO PEDOT-PSS/NM4CO PEDOT-PSS/NM5CO PEDOT-PSS/NM6CO

1000

(a)

500

600

600

400

200

0

0

200

Current density (A g )

7.

400

PEDOT-PSS/NM3CO PEDOT-PSS/NM4CO PEDOT-PSS/NM5CO PEDOT-PSS/NM6CO

800

-1

Figure

300

Time (s) Specific capacitance (F g )

(e)

0

0.2

Potential (V) vs Ag/AgCl



(c) 0.4

-1 Specific capacitance (F g )

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 18 of 42

CV

400

600

800

1000

Times (s)

curves

of

PEDOT−PSS/NM3CO,

PEDOT−PSS/NM4CO,

PEDOT−PSS/NM5CO and PEDOT−PSS/NM6CO electrodes at a scan rate of 20 mV s−1. (b) CV curves of PEDOT−PSS/NM4CO at different scan rates from 10 to 200 mV s−1. (c) Galvanostatic discharge curves of PEDOT−PSS/NM3CO, PEDOT−PSS/NM4CO, PEDOT−PSS/NM5CO and PEDOT−PSS/NM6CO at a current density of 1 A g−1. (d) Galvanostatic discharge curves of PEDOT−PSS/NM4CO PEDOT−PSS/NM3CO,

at

different

current

densities.

PEDOT−PSS/NM4CO,

(e)

Specific

capacitances

PEDOT−PSS/NM5CO

of and

PEDOT−PSS/NM6CO derived from the discharge curves at different current densities of 1, 2, 5, 10, 15 and 20 A g−1. (f) Capacitance cycling performances of PEDOT−PSS/NM3CO,


Page 19 of 42

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


PEDOT−PSS/NM4CO, PEDOT−PSS/NM5CO and PEDOT−PSS/NM6CO at a current density of 5 A g−1.

3.2.1. Effect of the Molar Ratios of KMnO4 to EDOT on Electrochemical Properties of PEDOT− − PSS/NMCO. The supercapacitive behaviors of PEDOT−PSS/NMCO with different molar ratios of KMnO4 to EDOT were investigated by CV, galvanostatic charge–discharge and capacitance cycling tests in a three-electrode configuration with 6 M of KOH aqueous solution as the electrolyte, of which results are shown in Figure 7. Figure 7(a) gives the CV curves of PEDOT−PSS/NM3CO,

PEDOT−PSS/NM4CO,

PEDOT−PSS/NM5CO

and

PEDOT−PSS/NM6CO electrodes at a scan rate of 20 mV s−1. The appearance of the redox peaks in the CV curves verifies the pseudocapacitive behavior of PEDOT−PSS/NMCO resulting from the

faradaic

redox

reactions.

Among

these

four

PEDOT−PSS/NMCO

hybrids,

PEDOT−PSS/NM4CO has the largest CV curve area, demonstrating the largest specific capacitance

of

PEDOT−PSS/NM4CO.

Figure

7(b)

presents

the

CV

curves

of

PEDOT−PSS/NM4CO at different scan rates ranging from 10 to 200 mV s−1. With increasing the scan rate the peak shapes change little, implying a reversible and high-rate faradaic pseudocapacitive behavior of PEDOT−PSS/NM4CO. Meanwhile, a negative shift of the reduction peak and a positive shift of the oxidation peak can be observed, which is mainly owing to the resistance of the electrode.62 Figure 7(c) exhibits the galvanostatic discharge profiles of PEDOT−PSS/NM3CO, PEDOT−PSS/NM4CO, PEDOT−PSS/NM5CO and PEDOT−PSS/NM6CO electrodes at a current density of 1 A g−1. From Figure 7(c), palpable discharge voltage plateaus exist, further



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 42

confirming the pseudocapacitive behavior of PEDOT−PSS/NMCO. According to the equation (1), the specific capacitance of PEDOT−PSS/NM4CO is calculated to be 921.8 F g−1, followed by PEDOT−PSS/NM3CO (685.5 F g−1), PEDOT−PSS/NM5CO (594.5 F g−1) and PEDOT−PSS/NM6CO (536.4 F g−1). To further highlight the superior supercapacitive performance of PEDOT−PSS/NM4CO, its galvanostatic discharge profiles at different current densities of 1, 2, 5, 10, 15 and 20 A g−1 were collected as shown in Figure 7(d). The corresponding specific capacitances of PEDOT−PSS/NM4CO are 921.8，767.3，654.5，581.8， 545.5 and 509.1 F g−1 at the different current densities of 1, 2, 5, 10, 15 and 20 A g−1, respectively, calculated according to the equation (1). For comparison, the specific capacitances of

PEDOT−PSS/NM3CO,

PEDOT−PSS/NM4CO,

PEDOT−PSS/NM5CO

and

PEDOT−PSS/NM6CO at the different current densities of 1, 2, 5, 10, 15 and 20 A g−1 are merged in Figure 7(e). As displayed in Figure 7(e), all of the specific capacitances of PEDOT−PSS/NM4CO

at

different

current

densities

are

the

highest

among

these

PEDOT−PSS/NMCO hybrids. Meanwhile, the cycle stabilities of these PEDOT−PSS/NMCO electrodes were examined at a current density of 5 A g−1, as exhibited in Figure 7(f). From Figure 7(f), it can be seen that the specific capacitance retentions of PEDOT−PSS/NM3CO, PEDOT−PSS/NM4CO, PEDOT−PSS/NM5CO and PEDOT−PSS/NM6CO are 65.7%, 76.3%, 65.9% and 66.4% after 1000 cycles, respectively, suggesting that PEDOT−PSS/NM4CO possesses the best durability. As indicated by the above tests, PEDOT−PSS/NM4CO presents the best supercapacitive performance, which exactly coincides with the above anticipation in the FTIR analyses for PEDOT−PSS/NMCO. During the preparation of PEDOT−PSS/NMCO, KMnO4 served as the oxidant for the polymerization of organic monomer, and concurrently the OH− ions generated from the oxidative polymerization by KMnO4 were used as precipitants for


Page 21 of 42

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


metal ions. Therefore, the amount of KMnO4 has pronounced effects on both the degree of polymerization of organic monomer and the formation and growth of inorganic metal oxides. When the quantity of KMnO4 is less, the complete polymerization of organic monomer cannot achieve, leading to the incomplete formation of the organic conductive interpenetrating network. On the other hand, the less OH− ions generated from the oxidative polymerization by KMnO4 make the metal ions be not completely precipitated. On the contrary, when the quantity of KMnO4 is too much, the overoxidation of organic monomer occurs, giving rise to the formation of rigid, intractable and poorly conductive overoxidation polymers. Moreover, too much OH− ions will accelerate the precipitations of metal ions, leading to the agglomeration of metal oxide particles and their growth outside the polymer skeleton. All of the above situations will inevitably deteriorate the electrochemical properties of PEDOT−PSS/NMCO. In addition, the electrical

conductivities

of

PEDOT−PSS/NM3CO,

PEDOT−PSS/NM4CO,

PEDOT−PSS/NM5CO and PEDOT−PSS/NM6CO are measured to be 90, 119, 84 and 32 S cm−1, respectively. The highest electrical conductivity of PEDOT−PSS/NM4CO can accelerate the electron transfer rate and enhance the electron storage property, propitious to the enhancement of specific capacitance.



(b) 0.3

PEDOT-PSS1.5/NMCO PEDOT-PSS2/NMCO 0.20 PEDOT-PSS0.5/NMCO PEDOT-PSS1/NMCO 0.15 0.10 0.05 0.00

-0.10

0.0

-1

10 mV s -1 20 mV s -1 50 mV s -1 100mV s -1 150mV s -1 200mV s

-0.1

-0.3

-0.2 -0.1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

-0.1

0.7

0.0

Potential (V) vs Ag/AgCl left PEDOT-PSS2/NMCO PEDOT-PSS0.5/NMCO PEDOT-PSS1/NMCO right PEDOT-PSS1.5/NMCO

0.3 0.2 0.1 0.0 -0.1

0.1

0.2

0.3

0.4

0.5

0.6


(d)0.4 Potential (V) vs Ag/AgCl

(c) 0.4 Potential (V) vs Ag/AgCl

0.1

-0.2

-0.05

-1

PEDOT-PSS1.5/NMCO

1 Ag -1 2 Ag -1 5 Ag -1 10 A g -1 15 A g -1 20 A g

right

0.3 left

0.2 0.1 0.0

-0.1

-0.2 0

100

200

300

400

500

600

-1

Specific capacitance (F g )

1000 800 600 400 200 0

5

10

15

20 -1

8.

(a)

CV

100

200

300

400

500

600

700

Time (s) 800

600

400 PEDOT-PSS0.5/NMCO PEDOT-PSS1/NMCO PEDOT-PSS1.5/NMCO PEDOT-PSS2/NMCO

200

0

0

200

400

600

800

1000

Cycles


Figure

0

(f)

PEDOT-PSS0.5/NMCO PEDOT-PSS1/NMCO PEDOT-PSS1.5/NMCO PEDOT-PSS2/NMCO

1200

0

-0.2

700

Times (s)

(e)1400 -1

PEDOT-PSS1.5/NMCO

0.2

Current (A)

Current (A)

(a) 0.25


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 22 of 42

curves

of

PEDOT−PSS0.5/NMCO,

PEDOT−PSS1/NMCO,

PEDOT−PSS1.5/NMCO and PEDOT−PSS2/NMCO electrodes at a scan rate of 20 mV s−1. (b) CV curves of PEDOT−PSS1.5/NMCO at different scan rates from 10 to 200 mV s−1. (c) Galvanostatic

discharge

curves

of


PEDOT−PSS1/NMCO,

PEDOT−PSS1.5/NMCO and PEDOT−PSS2/NMCO at a current density of 1 A g−1. (d) Galvanostatic discharge curves of PEDOT−PSS1.5/NMCO at different current densities. (e) Specific

capacitances

of



PEDOT−PSS1/NMCO,

Page 23 of 42

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


PEDOT−PSS1.5/NMCO and PEDOT−PSS2/NMCO derived from the discharge curves at different current densities of 1, 2, 5, 10, 15 and 20 A g−1. (f) Capacitance cycling performances of


PEDOT−PSS1/NMCO,

PEDOT−PSS1.5/NMCO

and

PEDOT−PSS2/NMCO at a current density of 5 A g−1.

3.2.2. Effect of the Doping Amounts of PSS on Electrochemical Properties of PEDOT−PSS/NMCO. Similarly, the above electrochemical characterization techniques were also used to study the effect of doping amounts of PSS on electrochemical properties of PEDOT−PSS/NMCO, and the corresponding curves are illustrated in Figure 8. The CV curves of PEDOT−PSS0.5/NMCO,

PEDOT−PSS1/NMCO,

PEDOT−PSS1.5/NMCO

and

PEDOT−PSS2/NMCO electrodes at a scan rate of 20 mV s−1 in the 6 M KOH solution (Figure 8(a)) shows that the potential differences of PEDOT−PSS0.5/NMCO, PEDOT−PSS1/NMCO, PEDOT−PSS1.5/NMCO and PEDOT−PSS2/NMCO are 0.20, 0.15, 0.16 and 0.20 V, respectively. The lowest potential difference for PEDOT−PSS1.5/NMCO manifests its lowest internal resistance among these four hybrids.63 Besides, with the increase of the molar ratio of −SO3− in PSS to EDOT, the area of CV first increases and then decreases. The maximum is obtained

for

PEDOT−PSS1.5/NMCO.

Figure

8(b)

shows

the

CV

curves

of

PEDOT−PSS1.5/NMCO at different scan rates ranging from 10 to 200 mV s−1. From Figure 8(b), it can be seen that the peak shapes keep almost the same with the increase of scan rate, also evidencing

a

reversible

and

high-rate

faradaic

pseudocapacitive

PEDOT−PSS1.5/NMCO.


behavior

of


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 24 of 42

The galvanostatic discharge profiles of PEDOT−PSS0.5/NMCO, PEDOT−PSS1/NMCO, PEDOT−PSS1.5/NMCO and PEDOT−PSS2/NMCO electrodes at a current density of 1 A g−1 are depicted in Figure 8(c). The specific capacitances, calculated from Figure 8(c) according to the equation (1), decrease in the order of PEDOT−PSS1.5/NMCO (1234.5 F g−1), PEDOT−PSS1/NMCO

(1032.7

F

g−1),

PEDOT−PSS0.5/NMCO

(869.1

F

g−1)

and

PEDOT−PSS2/NMCO (865.5 F g−1). Similarly, we found that PEDOT–PSS1.5/NMCO owned the highest electrical conductivity of 160 S cm−1, followed by PEDOT–PSS2/NMCO (151 S cm−1), PEDOT–PSS1/NMCO (120 S cm−1) and PEDOT–PSS0.5/NMCO (59 S cm−1). As mentioned above, the high electrical conductivity is beneficial to the enhancement of specific capacitance. On the other hand, due to the big molecular weight of PSS, the proportion of metal oxides will decrease with increasing the PSS doping amount. As a result of a low theoretical specific capacitance of PEDOT, the specific capacitance of hybrid will decline. Moreover, the sulfuric groups in excessive PSS will coordinate with metal ions to form micelles, which will hinder the growth of metal oxides on PEDOT and the formation of inorganic oxide crystals. Therefore, there exists an inflection point for the specific capacitance with changing the PSS doping amount. This may be also the reason why PEDOT−PSS2/NMCO which has the second highest electrical conductivity delivers the lowest specific capacitance. The specific capacitances for PEDOT−PSS1.5/NMCO calculated from its galvanostatic discharge profiles at different current densities (Figure 8(d)) are 1234.5，905.5，718.2，600.0，545.5 and 509.1 F g−1 at 1, 2, 5, 10, 15 and 20 A g−1, respectively. The specific capacitances of PEDOT−PSS0.5/NMCO, PEDOT−PSS1/NMCO, PEDOT−PSS1.5/NMCO and PEDOT−PSS2/NMCO at the different current densities of 1, 2, 5, 10, 15 and 20 A g−1 are drawn together in Figure 8(e). As illustrated in Figure 8(e), the specific capacitances of PEDOT−PSS1.5/NMCO at different current densities


Page 25 of 42

are higher than those of other PEDOT−PSS/NMCO hybrids, especially at low current densities. The cycle stabilities of these PEDOT−PSS/NMCO electrodes were also investigated at a current density of 5 A g−1, of which results are presented in Figure 8(f). As expected, PEDOT−PSS1.5/NMCO has the highest capacitance retention of 83.7%, succeeded by PEDOT−PSS2/NMCO (77.5%), PEDOT−PSS1/NMCO (76.3%) and PEDOT−PSS0.5/NMCO (65.0%). The reason is that the polymer chain structure plays an important role in the cycle stability of PEDOT−PSS/NMCO. The doping of PSS into PEDOT endows PEDOT with plentiful branched chain, resulting in the enhancements of rigidity and structure stability of PEDOT, which can efficaciously alleviate the volume changes of metal oxides during cycling. However, as mentioned above, the excessive PSS doping amount will impede the growth and formation of metal oxides on PEDOT, leading to the cycle stability of PEDOT−PSS/NMCO fading. (a)0.3


Current (A)

(b) 0.4

PEDOT-PSS/CMO PEDOT-PSS/NMCO PEDOT-PSS/NMO PEDOT-PSS/MO

0.2

0.1

0.0

left

0.3 0.2

PEDOT-PSS/MO PEDOT-PSS/CMO PEDOT-PSS/NMO PEDOT-PSS/NMCO

right

0.1 0.0

-0.1

-0.1

-0.2

-0.2 -0.1 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0

0.7

100

200

1200 1000 800 600 400 200 0

0

5

10

15

20 -1


400

500

600

700

(d)900 -1

-1

PEDOT-PSS/MO PEDOT-PSS/NMO PEDOT-PSS/CMO PEDOT-PSS/NMCO


(c)1400

300

Time (s)



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


800 700 600

PEDOT-PSS/MO PEDOT-PSS/NMO PEDOT-PSS/CMO PEDOT-PSS/NMCO

500 400 300 200 100 0

0

200

400

600

800

Cycles


1000


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 26 of 42

Figure 9. (a) CV curves of PEDOT−PSS1.5/NMCO, PEDOT−PSS/NMO, PEDOT−PSS/CMO and PEDOT−PSS/MO electrodes at a scan rate of 20 mV s−1. (b) Galvanostatic discharge curves of PEDOT−PSS1.5/NMCO, PEDOT−PSS/NMO, PEDOT−PSS/CMO and PEDOT−PSS/MO electrodes at a current density of 1 A g−1. (c) Specific capacitances of PEDOT−PSS1.5/NMCO, PEDOT−PSS/NMO, PEDOT−PSS/CMO and PEDOT−PSS/MO derived from the discharge curves at different current densities of 1, 2, 5, 10, 15 and 20 A g−1. (d) Capacitance cycling performances of PEDOT−PSS1.5/NMCO, PEDOT−PSS/NMO, PEDOT−PSS/CMO and PEDOT−PSS/MO at a current density of 5 A g−1.

3.2.3. Effect of the Metal Oxide Compositions of Hybrids on Their Electrochemical Properties. The PEDOT−PSS/MO, PEDOT−PSS/NMO and PEDOT−PSS/CMO hybrids with different metal oxide compositions were prepared for further comparison, and their supercapacitive performances together with PEDOT−PSS1.5/NMCO are shown in Figure 9. From their CV curves at a scan rate of 20 mV s−1 (Figure 9(a)), two pairs of redox peaks can be observed

for


PEDOT−PSS/CMO,

PEDOT−PSS/NMO

and

PEDOT−PSS/MO. Interestingly, although four kinds of active centers for the redox reactions from Co2+/Co3+, Co3+/Co4+, Mn3+/Mn4+ and Ni2+/Ni3+ are present in PEDOT−PSS1.5/NMCO, there exist only two pairs of redox peaks.64 This may be ascribed to the fact that the similar redox potentials of MnO2, Co3O4 and NiO, and the surface modification of NMCO by the conductive polymer can induce the appreciable broadening of redox peaks.65 The largest integrated area of CV curve for PEDOT−PSS1.5/NMCO under the same testing conditions indicates the highest specific capacitance among these hybrids. This is also validated by the galvanostatic discharge profiles

of


PEDOT−PSS/NMO,


PEDOT−PSS/CMO

and

Page 27 of 42

PEDOT−PSS/MO electrodes at a current density of 1 A g−1 (Figure 9(b)), from which their respective specific capacitances are calculated to be 1234.5, 507.3, 458.2 and 221.8 F g−1 in terms of the equation (1). Furthermore, all the specific capacitances of PEDOT−PSS1.5/NMCO at the different current densities of 1, 2, 5, 10, 15 and 20 A g−1 are the highest among these hybrids

displayed

in

Figure

9(c).

PEDOT−PSS1.5/NMCO

are

much

higher

as

Notably, than

the

those

specific of

capacitances

PEDOT−PSS/NMO

of and

PEDOT−PSS/CMO, demonstrating a reinforced synergic effect between cobalt oxide and nickel oxide. To further evidence the synergic effects among cobalt oxide, nickel oxide and manganese oxide, the XRD patterns of PEDOT-PSS/MO and PEDOT−PSS1.5/NMCO was compared (Figure S1, Supporting Information). Interestingly, as shown in Figure S1, α-MnO2，δ-MnO2 and γ-MnO2 coexist in PEDOT-PSS/MO, but only α-MnO2 can be detected in PEDOT−PSS1.5/NMCO. This means that the presences of Ni2+ and Co2+ ions favor the formation of α-MnO2, which possesses higher specific capacitance than δ-MnO2 and γ-MnO2.54 Not

surprisingly,

the

specific

capacitance

retentions

of


PEDOT−PSS/NMO, PEDOT−PSS/CMO and PEDOT−PSS/MO after 1000 cycles at 5 A g−1 are 83.7%, 70.6%, 73.6% and 63.9%, respectively, as illustrated in Figure 9(d).

5 4 3 2 1 0 10

20

30

40

50 -1

Energy density (W h kg )

(b) 10 -1

-1

PEDOT-PSS/NMCO PEDOT-PSS/NMO PEDOT-PSS/CMO PEDOT-PSS/MO

Power density (kW kg )

(a) 6 Power density (kW kg )

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


this work PEDOT-MnO 2 (Ref. 66) CSS/PEDOT/MnO 2 ( Ref. 67)

8

FRGO-MnCoNiO 2 (Ref. 68)

6

NiCo 2O4@NiO (Ref. 21)

4 2 0 0

10

20

30

40

50 -1

Energy density (W h kg )



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 28 of 42

Figure 10. (a) Ragone plots of energy densities and power densities for the PEDOT– PSS1.5/NMCO, PEDOT–PSS/NMO, PEDOT–PSS/CMO and PEDOT–PSS/MO electrodes and (b) comparison of Ragone plots for PEDOT–PSS1.5/NMCO and the existing analogues recently reported.

The

values

of

the

energy

density

and

power

density

deliveries

for

the

PEDOT−PSS1.5/NMCO, PEDOT−PSS/NMO, PEDOT−PSS/CMO and PEDOT−PSS/MO electrodes are calculated by means of equations (2) and (3), and displayed with Ragone plots as illustrated

in

Figure

10(a).

As

seen

in

Figure

10(a),

the

energy

densities

of

PEDOT−PSS1.5/NMCO, PEDOT−PSS/NMO, PEDOT−PSS/CMO and PEDOT−PSS/MO are 51.9, 17.3, 17.0 and 9.0 W h kg−1 at a power density of 275 W kg−1, respectively. Even at a high power density of 5500 W kg−1, the PEDOT−PSS1.5/NMCO electrode still affords an energy density as high as 21.4 W h kg−1. As compared with the existing analogues recently reported (Figure 10(b)), the designed PEDOT−PSS1.5/NMCO shows superior properties in terms of energy density and power density over NiCo2O4@NiO (31.5 W h kg−1 at 215.2 W kg−1),21 PEDOT/MnO2 (12.6 W h kg−1 at 210 W kg−1 and 1.3 W h kg−1 at 8.5 W kg−1),66 commercial supercapacitor separator (CSS)/Graphite/PEDOT/MnO2 (31.1 W h kg−1 at 90 W kg−1 and 1 W h kg−1 at 4500 W kg−1)67 and poly(diallyldimethylammonium chloride)-functionalized graphene nanosheets (FRGO)/MnCoNiO2 (16.4 W h kg−1 at 150 W kg−1 and 9.4 W h kg−1 at 2700 W kg−1).68 4. CONCLUSIONS


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In summary, we have successfully demonstrated a novel and facile method to synthesize the PEDOT−PSS/NMCO

hybrid

with

a

hierarchical

3D

net

structure

via

a

solvothermal−coprecipitation coupled with oxidative polymerization route, which was used as the supercapacitor electrode material for the first time as far as we know. The results show that the PEDOT−PSS/NMCO hybrid offers overwhelmingly superior supercapacitive performance in comparison with PEDOT−PSS/MO, PEDOT−PSS/NMO and PEDOT−PSS/CMO. Specifically, the PEDOT−PSS1.5/NMCO hybrid affords a high specific capacitance of 1234.5 F g−1 at a current density of 1 A g−1, a good capacitance retention of 83.7% at a high current density of 5 A g−1 after 1000 cycles, an energy density of 51.9 W h kg−1 at a power density of 275 W kg−1 and an energy density of 21.4 W h kg−1 at an extremely elevated power density of 5500 W kg−1. The unique mesoporous architecture and synergy effects between components contribute to the exceptional supercapacitive performance of PEDOT−PSS/NMCO. We believe that this fascinating PEDOT−PSS/NMCO and other similar hybrids hold great potential as an advanced supercapacitor electrode material.

ASSOCIATED CONTENT Supporting Information XRD pattern comparison of PEDOT-PSS/MO and PEDOT−PSS1.5/NMCO. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author



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* E-mail: [email protected]. Telephone: +86-731-88872213. Fax: +86-731-88872213 (C.Y.). * E-mail: [email protected]. Telephone: +86-731-88879616. Fax: +86-731-88879616 (D.Q.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Supported by National Natural Science Foundation of China (No. 21171174 and 21505035), the opening subject of State Key Laboratory of Powder Metallurgy, the Open-end Fund for the Valuable and Precision Instruments of Central South University, and the Key Scientific Research Fund of Hunan Provincial Science and Technology ( No. 2011GK2014). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (No. 21171174 and 21505035), the opening subject of State Key Laboratory of Powder Metallurgy, the Open-end Fund for the Valuable and Precision Instruments of Central South University, and the Key Scientific Research Fund of Hunan Provincial Science and Technology (No. 2011GK2014). ABBREVIATIONS 30 Environment ACS Paragon Plus

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PEDOT−PSS/NMCO,

poly(3,4-ethylenedioxythiophene)–polystyrene

sulfonate/Ni−Mn−Co

ternary oxide; EDLCs, electrical double-layer capacitors; 3D, three-dimensional; EDOT, 3,4ethylenedioxythiophene;

PEDOT−PSS/MO,

PEDOT−PSS/MnO2;

PEDOT−PSS/NMO,

PEDOT−PSS/Ni−Mn binary oxide; PEDOT−PSS/CMO, PEDOT−PSS/Co−Mn binary oxide; FTIR,

Fourier

transform

infrared;

XRD,

X-ray

powder

diffractometer;

BET,

Brunauer−Emmett−Teller; XPS, X-ray photoelectron spectroscopy; CV, cyclic voltammetry; Sat, satellites; CSS, commercial supercapacitor separator; FRGO, functionalized graphene nanosheets.

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Table of Contents Graphic



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A Novel and Facile One-Pot Solvothermal ... - ACS Publications

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