Core-Shell Composite Fibers for High Performance Flexible

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Core-Shell Composite Fibers for High Performance Flexible Supercapacitor Electrodes Xiaoyan Lu, Chen Shen, Zeyang Zhang, Elizabeth Barrios, and Lei Zhai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12997 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Core-Shell Composite Fibers for High Performance Flexible Supercapacitor Electrodes Xiaoyan Lu†‡, Chen Shen†§,Zeyang Zhang†‡, Elizabeth Barrios†§, Lei Zhai†‡§* †

NanoScience Technology Center, University of Central Florida, Orlando, Florida 32826 USA

§

Department of Chemistry, University of Central Florida, Orlando, Florida 32816 USA

‡

Department of Material Science and Engineering, University of Central Florida, Orlando,

Florida 32816 USA *

Corresponding author’s emai: [email protected]

Keywords. MnO2, polyelectrolyte, polypyrrole, core-shell nanofiber, flexible supercapacitor electrodes

ABSTRACT

Core-shell nanofibers containing polyacrylic acid (PAA) and manganese oxide nanoparticles as the core and polypyrrole (PPy) as the shell were fabricated through electrospining the solution of PAA and manganese ions (PAA/Mn2+). The obtained nanofibers were stabilized by Fe3+ through the interaction between Fe3+ ions and carboxylate groups. Subsequent oxidation of Mn2+ by

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KMnO4 produced uniform MnO2 nanoparticles in the fibers. A PPy shell was created on the fibers by immersing the fibers in a pyrrole solution where Fe3+ ions in the fiber polymerized pyrrole on the fiber surfaces. In the MnO2@PAA/PPy core-shell composite fibers, MnO2 nanoparticles function as high capacity materials while polypyrrole shell prevents the loss of MnO2 during charge-discharge process. Such unique structure makes the composite fibers efficient electrode materials for supercapacitors. The gravimetric specific capacity of the MnO2@PAA/PPy core-shell composite fibers was 564 F/g based on cyclic voltammetry (CV) curves at 10 mV/s and 580 F/g based on galvanostatic charge/discharge (GCD) studies at 5 A/g. The MnO2@PAA/PPy core-shell composite fibers also presents stable cycling performance with 100% capacitance retention after 5000 cycles.

1. Introduction Supercapacitors (SCs) are an emerging technology that fills a crucial gap in today’s rapidly evolving energy storage requirements such as high energy density, high power density, lightweight and flexibility. With desirable properties of high power density, fast chargedischarge rate, and excellent cycle stability,1-5 SCs can be used in a wide range of applications such as hybrid vehicles, portable electronics, pacemakers, etc.6-9 SCs are categorized into electrochemical double layer capacitors (EDLCs) and pseudo-capacitors depending on the charge storage mechanisms. Ion absorption of the electrical double layer at the electrode and electrolyte interface is the mechanism for charge storage in EDLCs which are usually built from carbonbased active materials with high surface area. They have good charge/discharge performance but low specific capacitance (10~200 F/g).2-4, 10-15 Pseudo-capacitors store energy through fast and reversible redox reactions of transition metal oxides and electrical conductive polymers.16-26 Manganese dioxide (MnO2) is a promising material for electrochemical supercapacitors because

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of its high specific capacitance (theoretical value of 1400 F/g), fast charge-discharge process, low cost, and environmentally benign nature.5,

27-28

The MnO2 pseudocapacitive behavior is

attributed to a Mn4+/Mn3+ redox system involving a single-electron transfer.2 In aqueous electrolytes, the general charge/discharge mechanism in MnOO--based supercapacitors can be described by: MnO2 + M+ + e- → MnOOM where M represents hydrated protons (H3O+) and/or alkali cations such as K+, Na+, and Li+.2 Energy extracted from a MnO2 composite electrode strongly depends on crystal and morphological structure of MnO2 as well as the quality of MnO2/current collector and MnO2/electrolyte interfaces. Textural and morphological effects at interface determine the MnO2 capacitance while the material bulk structures affect electrolyte accessibility. 18, 27, 29-31 One-dimensional (1D) MnO2 composite fibers have been manufactured through electrospinning and extensively investigated as supercapacitor electrode materials because they provide large electrolyte/MnO2 interfacial surface area and good control of MnO2 morphology.21, 28, 32-35

In these studies, MnO2 nanoparticles were produced in carbon nanofiber matrices through

an electronspin/treatment process. The electrospinning of a solution of carbon precursor (e.g. polyvinylpyrrolidone and polyacrylonitrile) and a Mn salt (or MnOx nanoparticles) produced 1D fibers which were converted to carbon/MnO2 composite fibers by a controlled heat treatment. Phase separation of the polymers produced porous structures to control MnO2 morphology and facilitated the accessibility of electrolyte. While these 1D composite fibers have efficient use of MnO2 that leads to high specific capacitance, the charge/discharge mechanism of MnO2 indicates that water soluble MnOO- can diffuse away into solution in the process, imposing a serious stability issue that limits the practical applications for MnO2.36-37

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Here we report the fabrication of freestanding MnO2@PAA/PPy core-shell nanofibers from electrospun poly(acrylic acid) (PAA) fibers loaded with Mn2+ ions (Figure 1). This work is based on our recent discovery that well-defined nanoparticles can be produced from metal ion loaded polyelectrolyte fibers.38

PAA nanofibers with manganese ions, as the core of the

composite fiber, provided a robust and porous nanostructure as a free-standing and flexible supporting scaffold for the production of MnO2 nanoparticles. The Fe3+ ions were used to crosslink PAA fibers to improve their stability, and initiate the polymerization of pyrrole to generate a PPy shell on PAA fibers. The PPy shell inhibited the loss of MnO2 during chargedischarge process. As a result, the core-shell structured fibers exhibited outstanding properties including high specific capacitance, excellent reversible redox reactions and fast chargedischarge ability, making them effective materials for supercapacitor electrodes.

2. Experimental Procedure 2.1 Materials 25% poly(acrylic acid) (PAA, MW = ~250,000 g/mol) solution, was purchased from SigmaAldrich Chemical Co. (USA). Manganese acetate (Mn(CH₃COO)₂), potassium permanganate (KMnO4), ferric (III) chloride (FeCl3), pyrrole, and sodium sulfate (Na2SO4) were purchased from Fisher Scientific (USA). All chemicals were of analytical grade, and they were used as received. All solutions were prepared in deionized water. Samples were synthesized by the following procedures. 2.2 Materials synthesis 2.2.1 Fabrication of Nanofibers

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4 g of 25% PAA (Mw ~240,000, Sigma-Aldrich) solution was mixed with 700 µL 1 M manganese acetate (Mn(CH₃COO)₂) as the precursor solution. The solution was loaded into a plastic syringe equipped with a 16 gauge needle made of stainless steel. The needle was connected to a high-voltage power supply. The solution was pumped continuously by a syringe pump. A piece of aluminum foil as the collector was placed at 27 cm distance of the tip of needle. The electrospinning process was conducted in air at room temperature. The actual voltages ranged between 10 and 12 kV. To obtain continuous nanofibers with consistent thickness, the voltages applied were adjusted during the process whenever necessary. For example, a lower voltage was applied when the concentration and viscosity of the drop at the needle tip increased during the electrspinning. The as–spun nanofibers were simply peered off from the collector as a free-standing fiber mat after electropsinning, and dried at 40 ᵒC under vacuum. 2.2.2 Creating MnO2 Nanoparticles on PAA Nanofibers A solution contained 0.2 M FeCl3 and 0.6 M Mn(CH₃COO)₂ was dropped onto the as-spun nanofibers to fully wet them, and nanofibers were then dried in air. FeCl3 served as the crosslinker of PAA to make fibers insoluble in water. Mn(CH₃COO)₂ was to provide Mn2+ environment to prevent the diffusion of Mn2+ from the nanofibers. After that, 0.01 M KMnO4 solution was brushed onto nanofibers by a small brush with a tip size of 4.1 mm × 2.3 mm where KMnO4 reacted with Mn(CH₃COO)₂ to form MnO2 nanoparticles on nanofibers. As a result, MnO2@PAA nanofibers were formed. The fiber mats were brushed second, third, and fourth time with the 0.01 M KMnO4 solution after the KMnO4 solution from previous brush was completely dried in air to obtain different amount of MnO2 nanoparticles on PAA nanofibers.

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The remaining FeCl3 and unreacted KMnO4 and Mn(CH₃COO)₂ on the dry nanofibers were washed away by de-ionized water. 2.2.3 Depositing Polypyrrole Shell on the Surface of MnO2@PAA Nanofibers The MnO2@PAA nanofiber mat was immersed into a mixed solution of 2 mL pyrrole and 10 mL ethanol for 10 min for polymerization. After washed with ethanol and water, the polypyrrole coated nanofiber was dried at 40 ᵒC under vacuum. The free-standing nanofiber mat was pasted onto a piece of graphite electrode (1 cm x 3 cm) by silver paint as the working electrode for three-electrode electrochemical workstation. The loading mass was around 0.2 mg for each electrode. 2.3 Characterization The morphologies were characterized by scanning electron microscopy (SEM, ZEISS ULTRA 55) and transmission electron microscopy (TEM, JEOL 1011). The size and distribution of MnO2 nanoparticles was determined by high resolution transmission electron microscopy (HRTEM, TECNAI F30) equipped with an energy-dispersive X-ray spectroscopy (EDS). The chemical compositions of nanofibers were analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5400). X-Ray diffraction (XRD) spectra were obtained using a PANalytical Empyrean Xray diffractometer. To compare the electrochemical performance of different electrode materials, a three-electrode system consisting a working electrode, a platinum counter electrode and a saturated calomel electrode (SCE) as reference electrode was used. The representative cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD) were performed on this three-electrode configuration in 1 M Na2SO4 solution at room temperature. CV was carried out at different scan rates of 2, 10, 20, 50, and 100

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mV s−1. GCD was measured at 0.5, 5, 10, 25, and 50 A/g. EIS measurements were performed in a frequency range from 1 Hz to 1000 kHz. To measure the electrical conductivity, PAA/Mn2+, MnO2@PAA, MnO2@PAA/PPy fiber mats were cut into 1 cm × 1 cm mats (the typical thickness of mats was 1.5 µm.). Silver pastes were drawn on two opposite sides of fiber mats to get sheet resistance. The electrical conductivity was calculated by equation 2.1:

σ=

(2.1)

where σ is the electrical conductivity, is the length of the piece of material, ℎ crosssectional area of the specimen, and is the electrical resistance of the specimen. The specific capacitances (Csp, F/g) are calculated from CV study according to equation 2.2:

=

)

− )

(2.2)

where I(V) is the charge–discharge current of the CV loop; is the scan rate, m is the mass of the active materials (g), which is MnO2 in this study; and V1, V2 are the switching potential in the CV curve.39 The specific capacitance (Csp, F/g) of the electrode can also be calculated from the constant current discharging curve of GCD experiment using equation 2.3:

=

× ×

(2.3)

where I is the constant current, ∆t is the discharge time, ∆V is the discharge potential range and m is the mass of the active materials (MnO2 in this study).

3. Results and Discussion

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3.1 Preparation of supercapacitor electrodes

Figure 1. The scheme of the fabrication of the MnO2@PAA/PPy core-shell nanofibers from PAA electrospun fibers loaded with Mn2+ ions. Figure 1 illustrates the fabrication procedure of MnO2@PAA/PPy core-shell nanofibers. The advantage of using electrospun poly(acrylic acid) (PAA) fibers loaded with Mn2+ ions to fabricate MnO2@PAA/PPy core-shell nanofibers as supercapacitor electrodes are following. Small and well dispersed MnO2 nanoparticles can be produced in PAA matrix upon oxidation due to the interaction between carboxylate groups and Mn2+ ions. PAA nanofibers not only provides the support of MnO2 nanoparticles but also functions as an electrolyte media to facilitate cation diffusion during the charge-discharge process. Such structure increases the contact area between MnO2 and electrolyte, and significantly improves the utilization efficiency of MnO2. In addition, the loaded Fe3+ ions stabilize PAA fibers and function as an oxidant of pyrrole polymerization to produce a polypyrrole (PPy) shell on PAA fibers. The PPy shell can improve the electrical conductivity as a conductive polymer, increase the capacitance of the electrode as a pseudo capacitive material22, and hinder the diffusion of MnOO- to improve the cyclic stability of the electrode.

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As mentioned previously, the morphology of MnO2 determines its electrochemical properties. To produce small and uniform MnO2 nanoparticles dispersed in the fiber structure, the precursor manganese acetate (Mn(CH3COO)2) was added to PAA solution before electrospinning, and converted to MnO2 through an oxidation reaction by KMnO4. The rational of preloading manganese acetate into PAA solutions is based on our previous observation that loading metal ions into polyelectrolyte fibers generates metal ion concentration gradient and nanoparticle aggregates upon subsequent reaction (Figure S1).40 While the interaction between Mn2+ and carboxylate group on PAA is bidentate, the Mn2+ interaction with the second ligand is much weaker than its interaction with the first one. Therefore, Mn2+ does not affect the viscosity of the PAA solution and electrospinning process. As shown in Figure 2a and Figure S2, PAA/Mn2+ nanofibers have an average diameter of 200 ~ 250 nm and similar morphology to pure PAA nanofiber (Figure 2b).

Figure 2. SEM images of PAA/Mn2+ (a) and PAA fibers (b). On the other hand, the weak Mn2+ interactions with PAA make the fiber soluble in water. To stabilize the PAA nanofiber, the as-prepared fiber mat was wetted by a FeCl3 solution, utilizing the strong interactions between PAA and ferric ion.38 It was observed that the nanofibers were dissolved or fused together when exposed to the FeCl3 solutions with a concentration of Fe3+ lower than 0.2 M. In contrast, the PAA nanofiber mat shrank and became stiff and fragile when exposed to the FeCl3 solutions with a concentration of Fe3+ higher than 0.5 M. PAA fibers are

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undergoing two processes when they are exposed to a ferric solution, dissolving of PAA and crosslinking of PAA by ferric ions. In the solutions of low ferric concentration, not enough Fe3+ ions are available to crosslink PAA and prevent the fiber from dissolving. While at higher concentration, too many Fe3+ ions bonded with carboxylate groups and made the fiber brittle and fragile. To keep the morphology and softness of nanofibers, 0.2 M FeCl3 solution was used to stabilize PAA nanofiber (Figure 3a). After crosslinked with Fe3+, PAA nanofiber mats were stable in water and the nanostructure was maintained after being immersed in water for 5 days (Figure S5). Our previous study shows that the nanostructure of the fiber mat was kept constant after 15 days when immersed in PBS solution after being corsslinked with Fe3+ ions.38 a

b

3rd$step

1µm

1µm c

d

2µm

1µm

Figure 3. SEM images of PAA/Mn2+ fibers crosslinked by 0.2 M FeCl3 (Inset is a higher resolution SEM image of the fiber) (a), the crosslinked fibers after 18-hour immersion in water (b), PAA@MnO2 (c) and PAA@MnO2/PPy (d) fibers after 18-month immersion in water.

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MnO2 nanoparticles were produced by the oxidation of Mn(CH3COO)2 by KMnO4. The amount of MnO2 in PAA fiber can be tuned by multiple oxidation by KMnO4. With one-time oxidation, the fiber mat was still flexible and no obvious size change was observed compared with PAA/Mn2+ fibers (Figure 4a). However, the color of the fiber mat changed from light orange to brown after the reaction, indicating the formation of MnO2 nanoparticles on the fibers. According to TEM, the size of the MnO2 nanoparticles for one-time KMnO4 oxidation is smaller than 5 nm (Figure S3). The formation of small MnO2 nanoparticles is due to the interaction between Mn2+ ions and carboxylates in polyelectrolyte fibers. Our previous study38 revealed that the formation of nanostructures on nanofibers was affected by two factors, the diffusion of the embedded ions to the surface and the reaction rate of the metal ions with other chemicals. Since the diffusion of Mn2+ in PAA nanofiber is slow due to the interaction between Mn2+ and carboxylate group, the fast oxidation of Mn2+ to MnO2 by KMnO4 leads to small MnO2 nanoparticles. Multiple KMnO4 oxidation made fiber mat darker brown and more fragile, suggesting the production of more MnO2 nanoparticles upon each oxidation (Figure 4b-d). The TEM image of PAA@MnO2 fibers after fourth oxidation also shows the formation of larger amount of MnO2 nanoparticles on the fibers (Figure S4). The weight % and atom % of Mn on PAA@MnO2 fibers surface after first oxidation and fourth oxidation was examined by electron diffraction spectroscopy (EDS) in SEM, showing that the values increased from 26.04% and 9.31% to 39.63% and 18.10%, respectively (Table S1.).

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Figure 4. SEM images of PAA/MnO2 fibers produced through one (a), two (b), three (c), and four (d) times oxidation of KMnO4. Insets are the digital pictures of fibers showing darker color with more times oxidation. The composition of PAA/Mn2+ fibers stabilized by Fe3+ and PAA@MnO2 fibers was also investigated using X-ray photoelectron spectroscopy (XPS). XPS results show that the as-spun PAA/MnAc2 fibers have 1.4% atomic percentage of Mn (Figure S6). Additionally, the high resolution XPS spectrum (Figure 5a) of PAA/Mn2+ fibers was used to investigate the chemical state of Mn in PAA/Mn2+ fibers. Characteristic XPS peaks of Mn 2P1/2 (652.8eV), Mn 2P3/2 (641.2eV), and the satellite peak (645.5eV) indicate the existence of Mn2+ in as-spun fibers.41-44 The successful loading of FeCl3 and additional Mn2+ by exposing as-spun fibers to the FeCl3 and Mn(CH₃COO)₂ mixed solution is verified by XPS. 5.7% of Mn and 2.7% of Fe in atomic percentage clearly show the existence of Fe and Mn ions in fibers (Figure S7). Similar to the asspun fibers, signature peaks of Mn 2P1/2 (652.8eV), Mn 2P3/2 (641.1eV), and the satellite peak

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(645.2eV) in Figure 5b indicate Mn2+ in fibers before KMnO4 oxidation. The XPS survey results of fibers after KMnO4 oxidation show higher Mn atomic percentage compared to fibers before KMnO4 oxidation (Figure S8). The atomic percentage of Mn increased with increased times of KMnO4 treatment. Additionally, the atomic percentage of Mn after 3 and 4 times of oxidation is almost identical, which indicates a saturated Mn loading after 3 times of KMnO4 treatment. The chemical state of Mn after oxidation was examined by high resolution XPS, and the results are shown in Figure 5c. The XPS spectra of samples after different times of KMnO4 oxidation are almost identical. They all have Mn 2P1/2 peak at around 654eV and Mn 2P3/2 peak at around 642eV. These two characteristic peaks indicate the formation of MnO2 among all samples after KMnO4 oxidation.23, 43-45 In the XPS investigation of Mn 3s peak, Mn 3s duplets at 83.1 eV and 87.8 eV are observed. The energy separation between these two peaks is 4.7 eV (Figure S9). The position of duplets and the separation between them verifies the formation of MnO2 nanocrystals in the composite fibers. The X-ray diffraction spectra of the composite fibers clearly show the signature XRD peaks of α-MnO2 (Figure S10). The lattice constants were calculated to be a = 9.375(5) Å and c = 2.908(2) Å, which are in good agreement with the standard values (JCPDS 44–0141). Furthermore, the atomic percentage of Mn after two or more times of KMnO4 brushing is higher than the theoretical value (Table S2). If 5.7 % of Mn2+ in atomic percentage was within fibers before KMnO4 brushing and 3Mn2+ + 2Mn7+ = 5Mn4+ was the only reaction happened, the highest Mn atomic percentage could reach is 8.6%. The high Mn atomic percentage (i.e. > 20%) is probably due to the reaction between KMnO4 and acrylic acid residues in PAA to produce MnO2.

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Figure 5. (a) The deconvoluted high resolution XPS spectrum of as-spun PAA/MnAc2 nanofibers. (b) The deconvoluted high resolution XPS spectrum of PAA/MnAc2 fibers stabilized by Fe3+. (c) The high resolution XPS spectra of fibers after different times of KMnO4 oxidation (1-4 represents the oxidation times.) To prevent the degradation of MnO2 through the diffusion of MnOO- during charge/discharge process, a positively charged conductive polymer layer was deposited as a shell on the surface of PAA nanofiber, which also can improve the conductivity of the electrode. Poly(3,4-ethylenedioxythiophene) (PEDOT), polyaniline (PANI) and polypyrrole (PPy) are the most common conductive polymers used in supercapacitors.10,

15-16, 18, 22, 24-26, 29-30, 46-51

Polypyrrole was chosen as the shell material in this study because of its low cost and neutral polymerization condition.16, 18, 22, 50-51 In our studies, the PAA@MnO2 fibers were immersed in pyrrole solutions for one minute where the Fe3+ ions in PAA fibers polymerized pyrrole and produced 50 nm PPy shells on PAA fibers (Figure 6). Compared with the reported approach of producing PPy shell using oxidants in solutions,52-53 our method produced a uniform PPy shell on the fiber surface since the polymerization was controlled by the diffusion of Fe3+ ions in PAA fibers. Longer time polymerization would produce a thick PPy shell and granule that would significantly reduce the porosity and surface area of the fiber mat, and reduce the accessibility of electrolyte, imposing negative effect on the electrochemical performance (Figure S11). Based on

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the mass ratio and atomic percentage of each element obtained from the EDS in HRTEM, the weight percentage of MnO2 in the electrode material is 14.43% and 13.19% for one-time KMnO4 oxidation on PAA@MnO2 and PAA@MnO2/PPy, respectively. This was taken as the active material mass for electrochemistry study, because the EDS on HRTEM reveals the composition of the whole fiber.54 The as prepared PAA@MnO2 and PAA@MnO2/PPy fiber mats were stable and the fibrous structure was kept even after 18-month immersion in water (Figure 3c and 3d). Electrical conductivities of PAA/Mn2+, PAA@MnO2, and PAA@MnO2/PPy fibers are 3.22×10-4 S/m, 2.61×10-4 S/m, and 1.19×10-3 S/m, respectively. The data shows that PPy shell increases the electrical conductivity of PAA/MnO2 fibers.

Figure 6. (a) An SEM image of PAA@MnO2/PPy fiber produced by one minute polymerization. The inset shows that the fibers are flexible. (b) A HRTEM image of the fiber showing about 50 nm thick PPy shell and small MnO2 particles inside the fiber. 3.2 Electrochemical characterization To study the electrochemical performance of the as-prepared free-standing PAA@MnO2/PPy fibers, cyclic voltammetry (CV) measurements at different scan rates were performed by a threeelectrode configuration in 1 M Na2SO4 aqueous solution. Figure 7b shows that the CV curves of

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PAA@MnO2/PPy fibers, which is more rectangular than that of PAA@MnO2 fibers (Figure 7a), indicating PAA@MnO2/PPy fibers have lower charge transfer resistance, leading to improved capacitive behavior. The higher current density in PAA@MnO2/PPy fibers compare to other composite fibers indicates more efficient MnO2 utilization and electron transportation as PPy is used as a protecting shell. The CV measurement of PAA@MnO2/PPy at different scan rates ranging from 2 to 100 mV/s (Figure 7b) shows rectangular consistency at different scan rates, suggesting the outstanding capacitive behavior of PAA@MnO2/PPy. This superior performance is attributed to several facts: (1) small MnO2 nanoparticles significantly increased the number of electrochemically active sites for the redox reaction, consequently increase the MnO2 utilization efficiency; (2) the porous structure of PAA fibers, although cannot contribute to electron conductivity as other carbon materials,55 improves electrolyte penetration, and promote the ion diffusion within the electrode materials; (3) PAA-Fe3+ crosslinking network provides effective pathways for charge transport;56 (4) PPy, as the conductive shell, also enormously improved the electron transfer at the interface.

a

b

c

Figure 7. CV measurements of (a) PAA@MnO2, (b) PAA@MnO2/PPy at different scan rates and (c) PAA/Fe3+, PAA/Fe3+/PPy, PAA@MnO2 and PAA@MnO2/PPy at a scan rate of 10 mV/s. In order to study the contribution of Fe3+ and PPy to the specific capacitance, CV 16


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measurements of PAA/Fe3+ and PAA/Fe3+/PPy without MnO2 were carried out. The results reveal that the contribution of Fe3+ and PPy is not significant (Figure 7c). The specific capacitance of PAA/Fe3+ and PAA/Fe3+/PPy was less than 5% and 10% of the capacitance obtained from PAA@MnO2/PPy (Table S3). The gravimetric specific capacitance of PAA@MnO2/PPy fibers reaches up to 564 F/g at a scan rate of 10 mV/s which is about twice of that of PAA@MnO2 fibers (288 F/g). The specific capacitance calculation is based on the substation of the capacitance which is attributed to Fe3+ and PPy from the total capacitance. PAA@MnO2/PPy fibers showed a capacitance about twice of PAA@MnO2 fibers at different scan rates. The loading mass of MnO2 can be controlled by varying the KMnO4 oxidation time. However, higher loading mass of MnO2 reduced the gravimetric specific capacitance. For example, the specific capacitance of the sample with 2-time oxidation by KMnO4 was 320 F/g, and the one with 3-time loading was only 203 F/g with 0.2 mg loading mass for both electrodes. According to the fiber morphology, multiple oxidation by KMnO4 increased the size of MnO2 particle, herein reduced its utilization and lowered the conductivity of the electrode.

a

b

Current density (A/g)

Cs(F/g)

Current density (A/g)

Cs(F/g)

5

367.5

5

580

10

311.8

10

470

25

257.8

25

406.2

50

210

50

372.5

Figure 8. Galvanostatic charge/discharge (GCD) measurements of PAA@MnO2 (a) and PAA@MnO2/PPy (b). The insets are specific capacitance at different current density.

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To study the charge storage capacity of different composite fibers, we performed galvanostatic charge/discharge (GCD) measurements from 0 V to 0.8 V at current density ranging from 5 A/g to 50 A/g, as shown in Figure 8. The charging and discharging curves were symmetric, indicating that the electrochemical behavior of the composite fibers was reversible. While PAA@MnO2/PPy electrodes have high specific capacitance (692 F/g) at 0.5 A/g, the charge-discharge curve is asymmetric. It is believed that the long discharging time at 0.5 A/g led to water splitting during charge-discharge process and caused the asymmetric charging and discharging curves.57-58 At low current density, the charge-discharge process is slow, which gives it enough time for water to be electrolyzed, while at high current density, the process is much faster and the contribution of water splitting is insignificant. Therefore, the specific capacitance obtained at 5A/g reflects more realistic performance. In our study, PAA@MnO2/PPy fibers have higher specific capacitance than PAA@MnO2 fibers due to the PPy shell. The specific capacitance of PAA@MnO2/PPy fibers is higher than most of the reported systems with MnO2 on carbon nanofibers.20,

28, 59-60

A MnO2 on carbon nanofibers and a MnO2/PPy on carbon

nanofibers have been reported to have high specific capacitance (e.g. 855 F/g 34 and 700 F/g 22), the performance reduced to 87 % after 5000 cycles and 91% after 2000 cycle, respectively. In comparison, the specific capacitance of PAA@MnO2/PPy in our study remains 100% of its initial value after 5000 cycles at a scan rate of 100 mV/s (Figure 9). The increase of the capacitance of PAA@MnO2/PPy before 1000 cycles results from the improved usage of MnO2, the wetting of the electrode surface by the electrolyte and the swelling of the fiber during the charge/discharge process.61 In the beginning of the charge/discharge process, MnOO- diffuses from the core and deposits as MnO2 at the PAA/PPy interface, leading to an improved utilization of MnO2. At the same time, PPy shell prevents the MnOO- from diffusing into the electrolyte

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solution. The capacitance value stops increasing when the diffusion of MnOO- species reaches the equilibrium. In contrast, the step-like decrease of capacitance of PAA@MnO2 in Figure 9 was attributed to the loss of MnO2 during the charge/discharge process.19 MnOO- species diffuse from the fiber to the electrolyte solution without the protection of PPy shell, leading to decreased capacitance.

Figure 9. Cycling performance of PAA@MnO2 and PAA@MnO2/PPy at a scan rate of 100 mV/s.

Figure 10. SEM images of PAA@MnO2 (a) and PAA@MnO2/PPy (b) after 5000 cycles at a scan rate of 100 mV/s. The morphology and structure of PAA@MnO2 fibers and PAA@MnO2/PPy fibers after cycle was determined by SEM (Figure 10). The PAA@MnO2 fiber structure has been 19



completely disintegrated after long-term cycle, while PAA@MnO2/PPy fiber structure was still preserved. According to EDS in SEM, the atomic percent of Mn was 21% and 2.6% for PAA@MnO2 fiber and PAA@MnO2/PPy fiber, respectively. The preserved morphology and the superior electrochemical performance of PAA@MnO2/PPy fibers suggest that the PPy shell greatly improved the cycling stability of MnO2 nanoparticles by preventing MnOO- from immigrating out of the fibers during the charge-discharge process. As shown in Figure 9, PAA@MnO2/PPy fibers maintained full capacitance while PAA@MnO2 fibers retain 77% of their initial value after 5000 cycles. The loss of MnO2 was also reflected by the color change of electrolyte solutions after 5000 cycles, where the electrolyte solution for PAA@MnO2 fibers became yellow, while that for PAA@MnO2/PPy remained colorless. In addition, the Nyquist plots of PAA@MnO2 fibers and PAA@MnO2/PPy fibers before the cycle test (Figure S12) show that the solution resistance of PAA@MnO2 fibers and PAA@MnO2/PPy fibers is 3.42 Ω and 1.48 Ω, respectively, indicating that PPy significantly enhanced the conductivity of the electrode material.

4. Conclusion In summary, PAA@MnO2/PPy core-shell freestanding and flexible fiber mats were fabricated from electrospun PAA fibers loaded with Mn2+ ions. The fibers were stabilized by Fe3+ ions which were subsequently used as the oxidants to polymerize pyrrole. Small and uniform MnO2 nanoparticles were produced in PAA fibers through the oxidation of KMnO4. The obtained flexible nanostructured mats were further investigated as supercapacitor electrodes, showing remarkable enhancements in specific capacitance, charge/discharge ability, as well as cyclic stability. The reported method provides a facile and effective approach to produce flexible supercapacitor electrodes.

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*Corresponding author: Tel: +1 (407)882-2847, Fax: +1 (407)882-2819, E-mail address: lzhai@ ucf.edu Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Funding Sources The financial support from National Science Foundation (1462859) and Garmor, Inc. is greatly appreciated. ACKNOWLEDGEMENTS The authors are grateful to the support from National Science Foundation (CMMI- and Materials Characterization Facility (MCF) at University of Central Florida. ACCOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM image of a MnO2@PAA nanofiber produced by a post-loading approach. TEM images of PAA/Mn2+ fiber. TEM images of PAA/MnO2 fibers after one-time and four-time KMnO4 oxidation. Summary of the elemental composition from XPS. XPS spectrum of as-spun fibers, fibers before KMnO4 oxidation, and after one, two, three, four times of KMnO4 oxidation. XPS spectrum of Mn 3s in the composite fibers. XRD of composite fibers after KMnO4 oxidation. SEM image of PAA/MnO2/PPy fibers produced by five minutes and ten minutes polymerization, and EIS of PAA@MnO2/PPy. 21



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Core-shell nanofibers of polyacrylic acid (PAA) and manganese oxide nanoparticles as the core and polypyrrole (PPy) as the shell were fabricated from electrospun fibers containing PAA and manganese ions. 102x55mm (219 x 219 DPI)


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Core-Shell Composite Fibers for High Performance Flexible

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