Highly Uniform Anodically Deposited Film of MnO2 Nanoflakes on

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Highly uniform anodically deposited film of MnO2 nanoflakes on carbon fibers for flexible and wearable fiber-shaped supercapacitors Amjid Rafique, Andrea Massa, Marco Fontana, Stefano Bianco, angelica .chiodoni, Candido Fabrizio Pirri, Simelys Hernández, and Andrea Lamberti ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06311 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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ACS Applied Materials & Interfaces

Article type: full paper Highly uniform anodically deposited film of MnO2 nanoflakes on carbon fibers for flexible and wearable fiber-shaped supercapacitors

Amjid Rafiquea*, Andrea Massaa, Marco Fontanaa,b, Stefano Biancoa,b, Angelica Chiodonib Candido F. Pirria,b, Simelys Hernándeza,b and Andrea Lambertia,b a

Politecnico di Torino, Dipartimento di Scienza Applicata e Tecnologia (DISAT), Corso Duca Degli Abruzzi, 24, 10129

Turin, Italy. b

Istituto Italiano di Tecnologia, Center for Sustainable Future Technologies, Corso Trento, 21, 10129 Turin, Italy

*

Corresponding Author: Fax: 011 090 7399; Tel: 011 090 7394; E-mail: [email protected].

Key word: supercapacitors, MnO2, gel polymer electrolyte, flexible, electrodeposition, symmetric Abstract A highly uniform porous film of MnO2 was deposited on carbon fiber by anodic electrodeposition method for the fabrication of high-performance electrodes in wearable supercapacitors (SCs) application. The effects of potentiostatic and galvanostatic electrodeposition and the deposition time were investigated. The morphology, crystalline structure and chemical composition of the obtained fiber-shaped samples were analyzed by field-emission scanning electron microscopy (FESEM), X-ray Diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The charge storage performance of the carbon fibers@MnO2 composite electrode coupled to a gel-like polymeric electrolyte was investigated by cyclic voltammetry and galvanostatic charge-discharge measurements. The specific capacitance of the optimized carbon fiber@MnO2 composite electrodes could reach up to 62 Fg-1 corresponding to 23 mFcm-1 in PVA/NaCl gelpolymer electrolyte, i.e. the highest capacitance value ever reported for fiber-shaped SCs. Finally, the stability and the flexibility of the device were studied and the results indicate exceptional

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capacitance retention and superior stability of the device subjected to bending even at high angles up to 150 0. 1.Introduction The worldwide demand for green and sustainable energy is even more pushing the development of non-toxic and ecological energy harvesting and storage devices characterized by high efficiency, versatility and flexibility. The novel concepts of flexible display, artificial skin, and intelligent glasses1 have increased the interest on ultrathin, flexible, miniaturized and efficient energy storage devices 2,3. Supercapacitors (SC), also known as electrochemical or ultra-capacitors, are the highly interesting candidates for energy storage, thanks to their higher power, faster charge/discharge and longer cyclic life than batteries as well as to their higher energy density than conventional capacitors

4-5

. Thus, SCs bridge the gap between conventional capacitors and

batteries and have been used in many application ranging from portable electronics devices, hybrid electrical vehicles and large industrial scale power and energy management

6-7

. Owing to

these peculiar properties, supercapacitors have attracted considerable attention and aroused widely research 8-10. Concerning the charge storage mechanism, SCs can be categorized into two main groups 11-12, the first is based on the formation of the electric double layer (EDL) while the second on the faradaic reactions, which allow charge transfer at the electrode/electrolyte interface. Several materials have been evaluated as active material for SCs such as activated carbon 13, carbon nanotubes 14, carbon onions 15, carbon quantum dot 16, carbon fibers 4,11,13 and other carbon based materials 1718

, electrically conducting polymers

19-20

and transition metal oxides

21-24

.

The main problems

limiting the use of these carbonaceous materials in supercapacitors are related to their low energy density storage per cycle (in case of carbon based materials) 25, electrodes stability (in the case of conducting polymers) and relatively high cost when noble-metal-based oxides such as RuO2 are


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used

26

. In recent years, a great interest has emerged on the development of cost-effective

alternative materials

27

. Inexpensive transition metal oxides such as iron oxide and manganese

dioxide have been used as a substitute to the ruthenium oxides 19. Considerable efforts have been made to synthesize composite electrodes made of manganese oxides and carbonaceous materials such as carbon nanotubes 28, carbon nanoforms 29, exfoliated graphite 30, ordered mesoporous carbon 31 and carbon fibers 4,11,13 since manganese oxide-based electrochemical capacitors have a high theoretical capacitance (around 1400 F/g), low-cost, natural abundance and good environmental compatibility.

4,32

. The combination of those two

materials provides a synergic effect able to strongly improve the final device performance 11. Moreover, the approach of using a pseudocapacitive material such as MnO2 with carbon fibers in a composite electrode material together with a gel polymer electrolyte allows to overcome the problems of electrolyte evaporation and bulky size of the devices using an aqueous electrolyte 33. Fiber-shaped SCs have been proposed using both carbonaceous material along with transition metal oxides/Hydroxides and conductive polymers 34-39. For instance, Meng et. al. 34 reported the fabrication of the fibers-shaped supercapacitors in which CNT/PANI nanowires in a gel-polymer electrolyte were used; Lee et al.

35

reported a biscrolled yarn-like supercapacitor by using a

MWNT/PEDOT (poly(3,4-ethylenedioxythiophene) nano-composite yarn on Pt wire as two fiber electrodes; instead, Rafique et al.

36

used ultrafine graphite powder with ZnO to fabricate

composite electrodes. Although several studies about Carbon fibers@MnOx based SCs with aqueous electrolytes, such as LiCl, NaCl, KCl, and Na2SO4 have been reported 4,11,13, to the best of our knowledge, there are not any works in the literature in which carbon fibers coated with manganese dioxides have been used in conjunction with a gel-polymer electrolyte. However, a necessary transition to devices with polymeric or gel electrolytes is obvious from a practical point of view, in order to avoid electrolyte leakage or evaporation during a real application 40.



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In this paper, a simple strategy to fabricate composite electrodes consisting of carbon fiber@MnO2 nanosheets through an anodic electrochemical deposition method is reported. In this composite electrode, the carbon fiber core serves as both structural backbone and current collector for the MnO2 active material. They were assembled in a parallel configuration into a quasi-solid-state supercapacitors, with a gel-electrolyte acting both as separator and ion mediator. The effect of both the electrodeposition current and time on the MnO2 mass loading and corresponding electrochemical performance of the devices were fully investigated, along with the electrolyte composition. The outer MnO2 nanoflakes shell engender a porous structure with a large surface area for energy storage and electrolyte penetration [13]. Thanks to these features, the composite exhibits a high specific capacitance, improved rate capability, and good cycling and bending stability. 2. Experimental section 2.1 Electrode Fabrication Manganese oxide-coated carbon fibers were obtained by electrodeposition. Both galvanostatic and potentiostatic approach were exploited: the first consists in the application of a constant current to the working electrode, leaving the potential free to adapt to the electrodeposition conditions, while in the latter the potential is set as a constant parameter and the current is variable. With the electrodeposition time increasing, potential tends to slightly increase in galvanostatic mode, whereas current decreases in potentiostatic configuration, due to the increase in the deposited layer, which increases the electrical resistance. Figure 1a shows the scheme of the three electrodes configuration (details on the electrode connection are shown in Figure S3). Prior to the deposition, the carbon fibers (Panex® 35, ZOLTEK) were cut (length 3 cm), washed in acetone and deionized water, and laid on microscope glass slides and were used as anode electrode for the electrodeposition of the MnO2. The end of the carbon fibers boubdle was


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tied up with a copper tape to improve the electrical contact during the deposition. Manganese oxide deposition on the carbon fibers were carried out by galvanostatic electro-deposition (Biologic potentiostat, VMP-300), applying a constant anodic current of (0.7 mA) in a solution containing 0.1(M) manganese (II) acetate (Mn(CH3COO)2, > (98%), Sigma-Aldrich), as MnO2 precursor, and 0.1(M) of sodium sulfate (Na2SO4, 98%, Sigma-Aldrich), as supporting electrolyte. Different samples were synthesized, by varying the electro-deposition time between 10 and 180 min at room temperature. After the MnO2 electro-deposition on the carbon fibers, the electrodes were washed several times with deionized water and let dry in air and then calcination was done at 300 0 C at ramp (1 0 C/min). The amount of the MnOx loaded on the substrate was evaluated by weighting the carbon fibers before and after electrodeposition using a microbalance (Sartorius CP225D) with capacity and accuracy of 80 and 1x10-5 g respectively. Electrochemical deposition was done using three electrode cell in which Ag/AgCl is used as reference electrode and platinum wire as counter electrode. 2.2 Preparation of Gel-polymer electrolyte The gel-polymer electrolyte was prepared by modifying the procedure described by Haijun et al. 5. PVA + (1M) NaCl gel was prepared by dissolving 2 (grams) of PVA in 10 (ml) of deionized water containing (1 M) NaCl. After 4 hours on hot plate at 80 (oC) with rigorous stirring, the PVA completely dissolves into the solution resulting in a homogeneous and highly viscus electrolytic gel. 2.3 Supercapacitor assembly Supercapacitors were fabricated as schematically shown in the rendering of Figure S4. Two carbon fibers@MnOx electrodes were used as both current collectors and electrodes and the soft polymer is used as electrolyte as well as separator between two electrodes. The carbon fibers@MnOx electrodes were immersed into gel-polymer electrolyte for 20 minutes in order to



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allow a good gel infiltration and then placed onto flexible PET substrate in two parallel electrodes configuration. After partial gel electrolyte solidification, they were covered with Kapton tape as packaging and copper taped ends were connected to crocodile clamping for the electrochemical characterizations. 2.4 Characterizations The morphology of the electrochemically deposited electrodes (carbon fibers@MnO2) was investigated by means of a Zeiss Supra 40 Field Emission Scanning Electron Microscope (FESEM). TEM and STEM analyses were performed on FEI Tecnai F20 ST transmission electron microscope. Concerning sample preparation, parts of the MnO2 layer were detached from the carbon fibers by sonication in ethanol. Then, they were dispersed and subsequently deposited on a holey carbon copper TEM grid. The crystalline structure of the electrode materials was examined using a Panalytical X’Pert Pro Xray diffractometer in Bragg/Brentano configuration with Cu (Kα) as an X-ray source. Raman spectra of the nanostructured films were acquired using a Renishaw InVia micro-Raman spectrometer, with a laser excitation wavelength of 514.5 (nm) and a laser spot size of ~20 (μm). The chemical composition was investigated with a (PHI 5000) Versaprobe scanning X-ray photoelectron spectrometer using monochromatic Al [K-α] X-ray source 1486.6 (eV). The analyzed sample area was (500 (µm) × 500 (µm)). Different pass energy values were employed for survey spectra 187.85 (eV) and for high-resolution acquisitions (23.5 eV) All the spectra were acquired with simultaneous charge compensation. CasaXPS software was used for data analysis. All corelevel peak energies were referenced to C1s peak at 284.8 eV (C-C/C-H bonds)). The capacitive behavior of the MnO2 coated on the carbon fibers was determined by Cyclic voltammetry

measurements

performed

with

a

Metrohm


Autolab

PGSTAT128

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potentiostate/galvanostate. Both kinds of measurements were performed in device configuration (e.g. two symmetric electrodes). 3.Result and discussion The electrochemical deposition (see Figure 1a) has been performed exploring both potentiostatic and galvanostatic approach: the former results with almost no MnO2 coverage, while the latter show better results (see Figure S1). For this reason, galvanostatic method was optimized by finding the best working current and by increasing the deposition time from 10 up to 180 minutes. The obtained materials were firstly characterized by field emission scanning electron microscopy (FESEM) to investigate the evolution of morphology with electrodeposition time. As shown in Figure 1b, the deposition time had some influence on the homogeneity of the MnO2 coverage. Specifically, complete coverage of the fibers cannot be obtained at low deposition times. For example, the 30 min deposition sample exhibits regions were the external surface of the carbon fibers is not covered by MnO2; these uncovered regions can be recognized in lowmagnification FESEM images as they have lower intensity (they are “darker”) than the covered areas. After 120 min deposition, complete coverage of the carbon fibers is obtained, as confirmed by FESEM imaging and by EDX mapping (see Supporting Information). Irregular coverage of the fibers at low deposition times is probably

due to the actual current density provided to the

carbon fiber, which was considerably lower than the set current density. The latter, in fact, was estimated from the macroscopic area of the carbon fiber array, whereas the real fiber surface was far higher. This resulted in lower electrodeposition rates which required longer times for the complete coverage of the fibers, if compared to other substrates, such as Ti foil 41. The morphology of the nanostructured MnO2 layer was investigated by high-resolution imaging and it did not appear to be influenced by the deposition time. Figure 1 (c and d ) are representative images of the morphology of the MnO2 coverage, which is composed of extremely



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thin nanosheets (thickness < 10 nm, see STEM and TEM characterization in figure 1 (e-f), as expected for this particular electrodeposition process, as previously reported in the literature 42-43. Some insights on the structural properties of the material were gathered with X-Ray Diffraction and Raman measurements. XRD (Figure 2a) shows the presence of an amorphous material, with only a very weak and broad signal at 2θ ~ 37° in agreement with results obtained by electron diffraction, as reported in the Supporting Information. Raman characterization (Figure 2b) shows features from both manganese oxide in the 200 - 800 cm-1 region and from carbon fibers in the 1200 - 1700 cm-1 region.

Figure 1. 3D scheme of the experimental set up and MnO2 electrodeposition on carbon fibers (a). FESEM images showing that higher deposition times lead to better coverage of the carbon fibers with MnO2 layer (b). FESEM image of the MnO2 layer, completely covering the carbon fiber (c). High-magnification FESEM image showing the morphology of the electrodeposited MnO2 nanostructures (d). STEM HAADF image of the MnO2 nanosheets (e). Bright-field TEM images of two different regions (f).


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In general, it is well known that manganese oxides in their various forms show Raman signals related with the motion of the oxygen atoms within the MnO6 octahedral units in MnOx 43. In particular, here presented material shows a strong phonon band around 650 cm-1 (related with A1g spectroscopic species with Mn−O symmetric vibraVons) and a weak phonon band in the region 200 - 500 cm-1 range with not well-defined resonances (a merge of Mn-O symmetric vibrations and Mn-O bending vibrations). These features are compatible with the formation of an amorphous material, in-line with XRD findings. At higher wavenumbers, the typical D and G peaks from carbon fibers are revealed at ~1350 cm-1 and ~1580 cm-1, respectively. XPS characterization was adopted to determine the amorphous manganese oxide composition, which is obtained after the electrodeposition process. This technique allows for the identification of manganese oxides, since there are features of the photoelectron spectrum that are sensitive to the oxidation state of Mn. Specifically, the two most important regions of the XPS spectrum for the identification of manganese oxide phases are Mn2p and Mn3s. Concerning the Mn2p region, a multiplet structure is expected to arise for Mn(II), Mn(III) and Mn(IV) due to the presence of unpaired d electrons

44

. The particular multiplet structure depends on the oxidation state and it

can be used for phase identification. Moreover, further information can be obtained from shakeup features: for example, MnO exhibits a clear shake-up component at the higher binding energy side of the 2p3/2 peak 45.



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Figure 2. (a) XRD spectrum from carbon fibers@MnO2 sample (JCPDS references for α-MnO2: 44-0141); (b) Raman spectrum from carbon fibers@MnO2 sample. XPS high-resolution scans of Mn 2p (c) and Mn 3s (d) regions of the photoelectron spectrum for the carbon fibers@MnO2 sample.

In the case of the carbon fibers@MnO2 samples, the peak structure of the Mn2p region (Figure 2c) is indicative of MnO2: the presence of multiplet splitting components, the absence of a shake-up satellite near the 2p3/2 peak and the overall shape of the convoluted spectrum agree with previous results published in the literature 46-47. In accordance with the previously cited papers, the peak at lower binding energy is not considered as a multiplet splitting component but it is attributed to Mn3+, since this peak does not emerge from theoretical calculations of multiplet structure of core 2p levels for Mn

44

. The detailed description of the peak fitting procedure and line-shape


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parameters can be found in the supporting information. The assignment of MnO2 phase gains further confirmation by the analysis of the Mn3s region (Figure 2d). In this case, multiplet splitting is responsible for the presence of two main peaks, whose binding energy separation can be used for phase identification. For the analyzed sample, a peak separation of about 4.8 eV is compatible with the MnO2 phase, according to previous reports. 48-49 The wearable devices were assembled in a two-parallel wire configuration employing a polymeric electrolyte acting both as separator and ion mediator (a photograph of the assembled device is shown in figure 3a). The electrochemical performance of the carbon fiber@MnO2 composite electrodes was investigated by using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements, while all the energy storage parameter can be evaluated by the equations reported in the supporting information. As shown in Figure 3 (b and c), the deposition time strongly influences the capacitive performance of the device, with a marked increase of the current in the CVs and consequent rise of the specific capacitance, respectively. The CV curves look like ideal rectangular shape and smooth, while the charge - discharge cycles shown in Figure 3c exhibit a good triangular profile. Even if the MnO2 works as pseudocapacitive materials, there are no redox peaks observed in the CVs. This finding is in-line with many other studies, since MnO2 pseudocapacitance is due to fast and reversible faradaic reactions, which involve the electrochemical interconversion between Mn4+ and Mn3+ in the solid, and corresponding insertion of cations from the electrolyte 50.



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Figure 3. 3D image of the assembled device (a), electrochemical measurements were performed in 2 electrode configuration and CVs recorded at 5mV/s on electrode prepared at different times: 10, 30, 60, 120 and 180 minutes’ (b), galvanostatic charge-discharge measurements at different current of the 2 hours’ deposition device (c), plot of the specific capacitance vs scan rate for the different devices (d,e) in F/g and (mF/cm) respectively. comparison of mass loading and specific capacitance vs deposition time (f)

The highest specific capacitance at 5 mV/s was calculated (using the mass loading of the both electrodes according to equation S1 & S2) to be 62 Fg-1 as gravimetric capacitance, and 23 mFcm-1 as length capacitance for the two hours’ deposition electrodes, which is almost 4 order of magnitude higher than the pristine carbon fibers (0.02 Fg-1and 0.06 mFcm-1, respectively) as can be seen in comparison in the figure 3(d,e). This result is also much higher than other recently reported composite electrodes such as RGO-MnO2/ RGO (22 Fg-1) 33 or CNT@MnO2 / CNT (12.5 Fg1

) in PVA based gel-polymer electrolytes

34

. In order to underline the effect of the MnO2

deposition, Figure 3(f) shows the trend of the capacitance vs the active material loading on the carbon fibers. The capacitance was also calculated by using the equation (S7) 51 from the charge


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and discharge measurements. The specific capacitance is 62.5 F/g at current density of 200 mA/g, which is in-line with the results obtained from the cyclic voltammetry. It is generally believed that the charge transfer resistance, ohmic resistance (electrode and electrolyte) and mass transfer resistance can affect the capacitive performance of the electrodes 52- 53

. By exploiting nanostructured materials, some of the above-mentioned factors may be

negligible because they provide both high specific surface area and fast redox reaction for charge transfer. Thanks to their micro/nano features, the carbon fibers@MnO2 composite electrodes structure not only facilitate the cations fast diffusion between electrolyte and electrode but they are also beneficial to overcome the poor electrical conductivity of the MnO2, both of which would enhance the electrochemical performance of supercapacitors

50

. The specific capacitance

performance for the 180-min deposition of MnO2 is lower than the 120-min deposition time, since the increase in the mass loading correspond to an increase of MnO2 thickness on carbon fiber (carbon fibers and MnO2 ratio shown table 1) and probably only a layer of MnO2 was charged and discharged during supercapacitor testing, resulting in a decrease in the energy storage behavior 51. This shows that

Table 1 carbon fiber to MnO2 mass loading ratio Time of deposition(min) 10 30 60 120 180

Mass of MnO2 deposited(mg/cm) 0.2 0.6 1.1 2.56 2.99

Ratio (fiber: MnOx) 1:0.011 1:0.033 1:0.061 1:0.14 1:0.16



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capacitance of the MnO2 is strongly dependent on the amount and the thickness of the material involved in the fabrication of the electrodes 30. Long-term cyclic stability is the one of the most important factors for supercapacitor devices. Cyclic stability of the device was demonstrated in a voltage window of 1 V for 10000 cycles at 50 mV/s and specific capacitance retention was evaluated and shown in Figure 4(a,b). We found that the device retained above 90 % specific capacitance after 10000 cycles. Cyclic voltammograms were also plotted to show the degradation after different cycles and we found a slight change in voltammetry. This shows that the gel-polymers electrolytes PVA+NaCl avoid loss of specific capacitance by preventing the evaporation of the electrolyte 53.

Figure 4 voltammograms at different cycles (a), capacitance retention plot after 10000 cycles(b), voltammogram at 90 degrees bending of 20 cycles(c), current Vs voltage at different bending angles (d), capacitance retention at different angles (e), Ragone plot comparison with other composite electrodes in gel polymer electrolytes (f).


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The cyclic voltammetry’s were also recorded at different angles in the range 30 to 180 degrees to check the bending stability of the devices (photo attached inset of figure 4(e). Voltammograms were plotted as shown in the Figure 4 (c,d) to shown stability at 90o and also comparison at different angles( other angles voltammograms were shown in supporting information Fig.S6). We found that the capacitance retention is almost 100 % as shown in the Figure 4(e) which is consistent with previous literature reports

51

, hence demonstrating the suitability of the here

presented device to be integrated into textiles. The specific power density and energy density values for the best performing device were evaluated through the equations S4 and S6 to be 530 W/kg and 1.087 Wh/kg, respectively. Ragone plot was constructed to compare the composite electrodes performance in gel-polymer electrolyte with other reported composite electrodes as shown in Fig.4(f). We found that carbon fibers@MnO2 composite electrodes gave higher energy density and power density, when compared with other carbon-based composite electrodes in gelpolymer electrolytes as shown in table 2

Table 2 comparison of PVA based fiber shaped supercapacitors Sr#

Electrodes

electrolyte

Cs (F/g)

E. density (µWh/cm) .17

P. density (µW/cm) 100

Reference

40

Cs (mF/cm) 27.1

1

rGO

PVA/H2SO4

2

CNT fiber

PVA/H2SO4

-

.006

-

-

55

3

CNT fiber

PVA/H3PO4

20

.027

-

-

56

4

OMC/CNT yarn

PVA/H2SO4

-

1.91

.085

1.55

57

5

MnO2/rGO fibers

PVA/H2SO4

36

0.143

-

-

58

6

MWCT/carbon fibers

PVA/H2SO4

6

6.3

0.7

13.7

59

7

MnO2/carbon fibers (Current study)

PVA/NaCl

63

24

1.089

129.65

This work

54

4.conclusion In summary, novel carbon fibers@MnO2 composite electrode made by galvanostatic electrodeposition of MnO2 nanoflakes film on carbon fibers were fabricated and fully ACS Paragon Plus Environment


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characterized from the chemico-physical points of view. Flexible symmetric supercapacitors were assembled in parallel configuration exploiting a gel-polymer electrolyte. Galvanostatic chargedischarge and cyclic voltammetry measurements show excellent electrochemical performance as compare to the present literature. The dependence of the electrodeposition time on the energy storage behavior was investigated and maximum values of gravimetric capacitance and length capacitance of 62 Fg-1 and 23 mFcm-1, respectively, were obtained for two hours of MnO2 deposition. Finally, cycling and bending stability was analyzed and the obtain results showed that these fiber-shaped supercapacitors possess suitable mechanical and electrochemical performance as wearable power source for textile electronic devices. This novel approach allows the desirable large-area fabrication and provides a unique combination of high MnO2 loading, binder-free feature, excellent electrical conductivity, open macro-/mesoporous structure, high-accessibility for the electrolyte, use of low-cost and no-critical raw materials for the EU community as well as optimum stability over the time.

Associated content 1. Supporting Information available 1. Additional FESEM and EDX characterization 2. Additional structural characterization of MnO2 structures 3. Additional XPS characterization 4. Electrodeposition additional comments 5. Device assembling 6. Supercapacitor parameter evaluation 7. Effect of temperature

8. Bending stability 9. Reference

References


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graphical abstract


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Highly Uniform Anodically Deposited Film of MnO2 Nanoflakes on

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