Flexible Solid-State Symmetric Supercapacitor Based on (Fe,Cr)2O3

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Flexible solid-state symmetric supercapacitor based on (Fe,Cr)2O3 oxide layer developed on the stainless steel mesh Prashant Ravasaheb Deshmukh, Youngku Sohn, and Weon Gyu Shin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02489 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 2, 2017

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ACS Sustainable Chemistry & Engineering

Flexible solid-state symmetric supercapacitor based on (Fe,Cr)2O3 oxide layer developed on the stainless steel mesh Prashant R. Deshmukha, Youngku Sohnb, Weon Gyu Shina* a

Department of Mechanical Engineering, Chungnam National University, Daejeon, Republic of Korea -34134. b Department of Chemistry, Chungnam National University, Daejeon, Republic of Korea – 34134. *Corresponding Authors: Tel.: +82 42 821 5647; fax: +82 42 822 5642. E-mail address: [email protected] (W. G. Shin).

Abstract The present work describes a simple, easy and scalable thermal oxidation process for the development of polygonal structure of (Fe,Cr)2O3 oxide layer on the stainless steel mesh in an ambient atmosphere. The developed oxide layer shows the maximum areal capacitance of 45.92 mFcm-2 at a scan rate of 5 mVs-1 with an extraordinary capacitance retention of 97% over the 10000 cycles. Furthermore, a flexible solid-state symmetric supercapacitor is assembled with (Fe,Cr)2O3 oxide layer as electrodes using polymer-gel electrolyte, which substantially demonstrate pseudocapacitive nature with the maximum areal capacitance of 16.88 mFcm-2 at a scan rate of 5 mVs-1 with capacitance retention of 90% after 10000 cycles, an energy density (0.57 mWhcm-2) and power density (200 mWcm-2). Furthermore, it demonstrates constant capacitance retention over 500 bending cycles as well as excellent electrochemical durability (90% over 2000 cycles) under bent and twisted conditions. These charming performances suggest the present (Fe,Cr)2O3 oxide layer has a widespread potential application for next generation supercapacitors in high energy density flexible storage systems. Keywords: Stainless steel mesh, Thermal Oxidation, Flexible supercapacitor device.

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Introduction In the era of a fast growing global economy, depletion of coal and oil fuels, concerns of safety environment, highly efficient, clean and green energy storage devices are becoming more imperative. Supercapacitors have attracted significant attention due to its long cycle life and high power density than batteries as well as numerous potential applications in portable electronic devices, consumer electronics, memory back-up systems, public transportation, and so on.1-5 Today recent advances and developments in portable electronic devices (PEDs), foldable, compressible, and stretchable electronics put forward new challenges for the compatible energy storage devices. Such devices require power sources that are flexible, robust, high capacity, excellent conductivity and mechanical properties and hence, flexible solid-state supercapacitors (SSCs) have attracted enormous research attention in these electronic devices.6,7 Also, flexible SSCs have the merits of lightweight, ease of handling, environmental friendliness, small size, low weight, high power density, outstanding cycling stability than batteries, excellent reliability, and ability to accommodate frequent mechanical strain in flexible energy storage applications.7-9 Flexible SSCs are playing significant role in portable electronics due to their flexibility, lightweight and capability to tolerate large twists while retaining normal functions and reliability, which is the key factor for the portable electronic devices.10-13 Considering above facts and several advantages of stainless steel mesh such as lightweight, low-cost and good mechanical strength are utilized in an electrode substrate 2


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for supercapacitor. Recently, iron-based oxides/hydroxide materials, such as Fe2O3, Fe3O4, FeOOH, etc. have received tremendous interests as desired electrode materials for supercapacitors due to their substantial benefits of high theoretical specific capacitances, multiple valence states of iron, natural abundance, environmental friendly, low cost, and non-toxicity. In addition, iron oxides/hydroxides have stable and wide working negative potential window, making it very promising high-performance negative electrodes for asymmetric

supercapacitors

(ASCs),

whereas

most

oxides/hydroxides are employed as positive electrodes.3,

of 14, 15

the

transition

metal

To the best of our

knowledge, until now there have been very few reports about the study of thermally prepared iron based oxide layer on the stainless steel substrate as electrodes for supercapacitor ranging from fundamental understanding to application test (No reports on, only thermal oxidation without any pre-process). For instance, the electrochemical performance of porous sponge like structure of Fe2O3 prepared by anodizing steel in 10 M NaOH shows a capacitance of 18 mFcm−2.16 Iron oxide nanotube and nano-porous oxide layer prepared by electrochemical anodization of pure iron substrate and 302-type stainless steel substrate shows an areal capacitance of 314 and 112 mFcm-2, respectively.17, 18 A nano-porous oxide layer prepared by chemical oxidization of 304type stainless steel shows the capacitance of 75 mFcm-2 at the scan rate of 5 mVs-1 in LiClO4-PC electrolyte.19 Therefore, the main purpose of the present work is a thermal oxidation of lightweight, flexible and inexpensive 304-type stainless steel mesh at various temperature

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ranging from 200 to 800°C in ambient atmosphere by a simple and low cost single step process to achieve an iron based oxide layer. Furthermore, we propose, for the first time to the best of our knowledge, the integration of the iron based oxide layer into the flexible solid-state symmetric supercapacitor with polymer gel electrolyte. Experimental Materials and chemicals 304-type stainless steel mesh purchased from local supplier. Polyvinyl alcohol (PVA), Potassium hydroxide (KOH), purchased from Sigma-Aldrich and used in experimental process. Preparation of Oxide layer (Thermal oxidation of stainless steel mesh) 304-type stainless steel (SS) mesh was used as a starting material for the growth of the nanostructured oxide layer, which can act as an active electrode material. The chemical composition of SS mesh (wt. %) is C (0.07), Si (0.46), Mn (0.78), P (0.029), S (0.006), Ni (8.06), Cr (18.16), Fe (Balance). In prior, SS mesh were cut into 3 cm X 1 cm pieces. Then, the SS mesh pieces were cleaned carefully with ethanol, acetone and DI water in ultrasonic bath for 15 minute each, respectively. The cleaned SS mesh pieces were thermally oxidized at different temperature ranging from 200 to 1000°C for 1 hour using the tube furnace with an ambient air atmosphere. The oxidation of mesh at different temperature was observed as the temperature increased from 200 to 800°C. However, the significant oxide layer was observed at 800°C, after that the mesh becomes mechanically fragile, started to crumble and was easily scrubbed off. In this way, surface texture of SS

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mesh was altered for practical application in supercapacitor. Figure S1 shows the digital photographs of the bare, and thermally oxidized SS mesh at different temperature and its narration is given in the experimental section of Supporting Information. The details of material characterization techniques and electrochemical test have been given in the Supporting Information. Results and discussions The crystallinity of the obtained samples were analyzed by X-ray diffraction. Figure 1 shows the XRD patterns of bare and oxidized SS mesh at different temperature. The patterns are similar to bare SS mesh up to 600°C. However, the color and FE-SEM images of oxidized SS mesh up to 600°C show some changes from the bare SS mesh. This may be due to the fact that developed oxide layer is very thin, which is insufficient to be detected by XRD or existing amorphous oxide layer. The peaks marked by * in the XRD patterns are due to the SS mesh. The SS mesh oxidized at 800°C shows the formation of corundum type hematite iron-chromium oxide on the surface of SS mesh, which is the composite of Fe and Cr oxide i.e. (Fe,Cr)2O3. The obtained peaks at 24.35, 33.38, 35.78, 41.17, and 54.63° corresponds to (012), (104), (311), (113), and (116) Miller indices, respectively. This shows the formation of hematite phase of (Fe,Cr)2O3, (hereafter called as FCO) as per the JCPDS Card number 00-034-0412. In addition, the peaks at 44.46, and 64.90° corresponds to the (110) and (220) planes, which shows the presence of Fe metal with cubic phase formation as per the JCPDS card number 00-0011262.

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(220)

*

(116)

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* -Stainless Steel 00-034-0412 00-001-1262

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Figure 1. XRD patterns of bare and oxidized SS mesh at different temperature.

In order to acquire information on the morphological evolution of SS mesh surface during the thermal oxidation, FE-SEM characterization was conducted. Figure 2(a-d) presents FE-SEM images as well as corresponding EDS spectrum of FCO and Figure S2 shows the FE-SEM images as well as EDS spectra of bare and samples obtained at different thermal oxidation temperature (200, 300, 400, 500, 600, and 700°C) at low and high magnifications. FE-SEM images of bare SS mesh (Figure S2) shows the smooth surface of the mesh. EDS study of bare SS mesh confirms the presence of main constituent elements Fe (48.63 %) and Cr (14.37 %) of bare stainless steel mesh, which 6


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are analogous with stainless steel alloys material. This indicates that there is only SS material, without any significant surface oxide layer on the mesh. Here, low magnification images show the high uniformity and homogeneity of the samples, while high magnification images are useful for understanding the effect of the oxidation temperature on the surface of SS mesh. Thermal oxidation at 200 and 300°C does not show significant change in the surface of SS mesh, which remained as like surface of bare mesh. The smooth surface of substrate turned into short uniform wrinkles/particles at 400°C thermal oxidation. At this thermal oxidation, one can see the roughness is appearing on the surface. Subsequent, thermal oxidation at 500 and 600°C, surface shows the irregular shape of particles grown in larger size with apparent noteworthy roughness on the surface. These irregular particles turned into the polygonal, hexagonal plate like structures with various size at 800°C thermal oxidation. The size and shape of polygonal, hexagonal plate/particles varied from few nanometer to several nanometers. The developed oxide layer is relatively thicker as compared to other oxidation temperature. EDS spectrum shows the presence of Fe, Cr and O as main elements in the surface oxide layer at 800°C (Figure 2 (c & d)), which further support to the formation of (Fe,Cr) oxide layer in the XRD pattern. Cr rich hexagonal, polygonal like structure, while Fe present in a surface layer is identified from the elemental mapping of the oxide layer along with O elements (Figure 2e). In addition, EDS analysis at various places (Figure S4) shows the Cr rich hexagonal, polygonal like structure and the surface oxide layer constitute of Fe rich element.

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Figure 2. (a, b) FE-SEM images; (c, d) EDS spectrum; and (e) elemental mapping of thermally oxidized SS mesh at 800°C (FCO). We performed X-ray photoelectron spectroscopy (XPS) measurement to study the composition and oxidation state of elements on the selective samples as bare, oxidized mesh at 400 and 800°C. Figure 3a shows the XPS wide scan survey spectra collected from surface of the samples, which confirm the presence of Fe, Cr and O element. Figure 3b presents the Fe 2p core level XPS spectra of the bare surface, oxidized at 400 and 800°C SS mesh. The Fe 2p3/2 and Fe 2p1/2 peaks of bare, oxidized at 400 and 800°C SS 9



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mesh are centered at the binding energies of 711.30, 711.30, 711.28 and 724.60, 724.50, 724.45 eV, respectively, which are the typical values for Fe3+ species present in the surface oxide layer. A satellite peak of the main Fe 2p3/2 peak located around at 720.2 eV further specifies +3 oxidation state of Fe.2, 20 There is no obvious difference between these spectra, indicating the similar oxidation state of Fe in the samples. However, from the Fe 2p core level spectra, it can be observed that the intensity of Fe 2p3/2 and Fe 2p1/2 peak is higher in substrate oxidized at 400°C, which shows the more accumulation of the Fe species on the substrate surface. This is analogous with the brown color photo and FESEM image of the SS mesh oxidized at 400°C. Figure 3c shows the Cr 2p core level XPS spectra of the surface layer of bare, oxidized at 400, and 800°C SS mesh. The Cr 2p3/2 and Cr 2p1/2 peaks of bare and oxidized at 800°C substrates are centered at 576.72, 576.86 and 586.70, 586.50 eV, respectively, which are typical values for the Cr3+ species present in the oxide layer. The Cr 2p peak in substrate oxidized at 800°C has well resolved Cr 2p3/2 and Cr 2p1/2 peaks at 576.86 and 586.50 eV, respectively, with an energy separation of 9.6 eV indicating the principal presence of Cr.21, 22 The surviving Cr 2p peak in the substrate oxidized at 400°C does not exist, due to the surface mainly constituent of Fe3+ species. The intensity of Cr 2p peak in the substrate oxidized at 800°C is higher indicating the oxide layer contain major Cr3+ species. This can be analogous to the substrate color of oxidized SS mesh at 800°C, XRD and elemental mapping discussed in the previous sections. The substrate color changed from brown at 400°C to brownish black at 800°C. This is in support with observed result in the XPS analysis. The O1s

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spectrum of bare, oxidized at 400 and 800°C SS mesh oxide layer (Figure 3d) exhibits strong peaks at 530.49, 529.90 and 530.40 eV, respectively due to the lattice O2- species present in the oxide layer. The intensity of O1s peaks is higher in the substrate oxidized at 400 and 8000C indicating the formation of oxide layer on the surface of substrates.

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(e)

100

Atomic Concentration (%)

(f)

160k

O Fe Cr Ni

Region II

Region I

80 120k

O Fe Cr Ni

60 40

80k Region I

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20 0

Intnesity

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0k

0

5

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Sputter Time (min)

25

0

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Figure 3. (a) XPS wide scan spectra; high-resolution core-level spectra of (b) Fe 2p; (c) Cr 2p and (d) and O 1s of bare and developed oxide layer at 400 and 800°C of SS mesh; AES depth profile showing the element content in the surface oxide layer of FCO ((e) atomic concentration and (f) intensity).

Auger electron spectroscopy (AES) in conjunction with ion sputtering has proven to be a wide spread depth-profiling tool in the past decades. AES combined with sputtering was used to obtain elemental depth profiles of the oxide layer on the surface of thermally oxidized mesh at different temperature. AES depth profiling was performed on layers grown on the surface of SS mesh oxidized at different temperature. Only the main and major constituent elements Fe, Cr, Ni and O are considered for depth profiling. Atomic concentration and intensity as a function of sputtering time drawn from the AES data of samples are depicted in the Figure 3 (e & f) and Figure S5. There are two regions observed in the depth profiles, which can be divided as surface oxide layer and metallic SS mesh. These two region significantly observed in the higher thermal

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oxidation of mesh showing the sufficient growth of oxide layer. However, at lower thermal oxidation (>600°C), we can see the existence of only metallic SS mesh. Yet, the oxide layer at 400°C is not significantly observed, while it started appearing around 600°C. This is well confirmed from the atomic concentration and intensity (Figures S5) and further supported to the XRD results up to 600°C. At 800°C thermal oxidation, we observed the substantial growth of oxide layer on the surface of SS mesh. Moreover, the surface oxide layer is enriched by Cr element than the Fe and Ni. In the metallic region, the concentration of Fe, Cr and Ni are approximately consistent with concentration of elements in the bare substrate observed from the EDAX analysis. Considering the sputtering rate and time, the observed average thickness of surface oxide layer is approximately 400 nm for SS mesh thermally oxidized at 800°C (FCO). The electrochemical performance of the as grown oxide samples was evaluated using cyclic voltammetry (CV), charge-discharge (CD) and electrochemical impedance spectroscopy (EIS) techniques in a three-electrode system with 1 M KOH electrolyte. (Supporting Information, Figures S6, S7 & S8). Figure 4a shows the CV measurements of FCO electrode at different scan rates ranging from 5 to 100 mVs-1. CV curves exhibit typical pseudocapacitive behavior at different scan rates, as an indicative of good charge propagation at the electrode surface. The current response in the CV curves increases approximately proportional with increasing scan rates, suggesting that the rates of electronic and ionic transport are not limiting at scan rates as high as 100 mVs-1. Moreover, the cathodic peak and anodic peak shifted to negative and positive potential,

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respectively as the scan rate increases, which exhibits the good reversible redox reaction. For instance, at 100 mVs-1, the CV curve has an obvious pair of redox peaks approximately at -0.50 and -0.70 V that indicates good electrochemical performance attributed to the change in valence state of Fe, corresponding to conversion between Fe2+ ↔ Fe3+. During the cathodic scan, at more negative potential, Fe3+ is reduced to Fe2+, meanwhile, Fe2+ is oxidized to Fe3+ when scanned to the positive direction, consistent with the pair of redox peak observed in the CV.23, 24 Such type of oxidation-reduction peaks are observed in the CV curves of iron oxide decorated carbon and Fe2O3-graphene composite in the 2 M KOH electrolyte. The observed redox peaks due to the faradic reaction between Fe3+ and Fe2+ through the electrolyte that gives the pseudocapacitance. Moreover, it can be specified that the intercalation/deintercalation of K+ within the interlayer, the tunnels and holes in the crystal structure would be the main cause for the contribution to the pseudocapacitance.25-27 Wu et al.27 reported the Li+ intercalation into the holes existed in the surface of hematite α-Fe2O3 nanostructures. A slight shift in reduction peak (which is expected at ~-0.94 V in Fe3O4 electrode) to lower negative voltage can be attributed the Cr rich surface oxide layer. Xie et al.28 observed such type of shift in redox peaks due to mass fraction difference in the electrode materials. The observed maximum areal capacitance is 45.92 mFcm-2 at a scan rate of 5 mVs-1. The values of areal capacitances are 25.36, 14.43, 8.80, 7.37 and 6.75 at scan rates of 10, 20, 50, 75 and 100 mVs-1, respectively. The observed areal capacitance are higher than reported areal capacitance of iron based and other metal oxide/carbide electrodes.15, 23, 29-

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33

For example; Fe2O3 NWs (~25 mFcm-2),15 Fe3O4 NRAs (~40 mFcm-2) and Fe3O4-

MoO2 thin film (~45 mFcm-2),23 Fe2O3 NTs (29.4 mFcm-2),29 Ni-NiO core-shell inverse opal (~9.2 mFcm-2),30 α-Fe2O3 NRs (9.2 mFcm-2),31 SiC NWs (240 µFcm-2 at 100 mVs1 32

), and graphene-MnO2 paper (17.9 mFcm-2).33 Therefore, the present electrode material

could be alternative for carbon (low specific capacitance due to charge storage mechanism) in the development of asymmetric supercapacitor. Furthermore, coupling with the high conducting and stable polymer such as PEDOT, polyaniline, and polypyrrole, performance of this electrode can be sufficiently improved for the development of asymmetric devices instead of carbon based asymmetric devices. Figure 4b shows the charge-discharge curves of FCO electrode at different current densities, which shows the consistent result with CV curves. Importantly, no obvious iR drop observed at the beginning of the discharge curve, which suggest a low internal resistance and good electrochemical reversibility. The values of areal capacitance calculated from charge-discharge curves are 7, 6.6, 4.8, and 4.2 mFcm-2 at constant current densities of 0.5, 1, 2, and 3 mAcm-2, respectively. The values of areal capacitance decreases at higher scan rate/current densities due to insufficient period to utilize the whole surface area of the electrode for surface redox reactions. Areal capacitance of FCO electrode is higher and comparable with values reported in the literature34-36 such as, Fe3O4 NRs and Fe3O4-SnO2 core-shell hybrid nano thin film (5.156 and 7.013 mFcm-2 at 0.20 mAcm-2, respectively),34 ZnO, AZnO and HZnO NWs (0.27, 0.46, and 1.38 mFcm-2

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at 0.05 mAcm-2, respectively),35 graphene-polyaniline bilayer film (2.09 mFcm-2 at 3 Acm-3).36

Figure 4. (a) CV curves at different scan rates; (b) CD curves at different current densities; (c) Capacitance retention over 10000 CV cycles (Inset is the CV curves ); and (d) Nyquist plot before and after 10000 CV cycles of FCO electrode (Inset is enlarged view at high frequency). Figure 4c shows the capacitance retention of FCO electrode over 10000 CV cycles and remarkably a very high electrochemical stability was observed with

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capacitance retention of 97% of the initial capacitance. Electrochemical stability of FCO electrode tested over successive 10000 CV cycles at a scan rate of 100 mVs-1 (Inset of Figure 4c). CV curves shows the almost mirror shape, even after 10000 CV cycling. Undoubtedly, the redox processes were found to be extraordinarily reversible as the electrode showed negligible loss in capacitance. This may be due to stable oxidationreduction peaks, which does not shows any significant changes in the surface morphology or crystalline structure during the extended CV cycles. No significant changes in FE-SEM images (Figure S3) and transparent electrolyte solution without any degradation of the electrode even after the 10000 cycles further supports the high stability of oxide layer. On the other hand, Qu et al.37 reported some structural changes in the FeOOH electrode during the CV cycles. These results demonstrate that thermal oxidization of SS mesh is a suitable method to produce high-surface-area electrodes for supercapacitor with excellent cycling lifetime. The results are higher than the reported values of iron-based oxide and other electrodes (Supporting Information, Table S1).2, 29, 31, 34, 38-42

Thus, the superior electrochemical performance of thin FCO oxide layer

developed on stainless steel mesh is attributed to an integrated structure of the electrode consisting a current collector, active electrode material, porous structure and high surface area. These unique characteristics of the electrode offers rapid ion and electron transfer, minimum diffusion distance as well as a large effective interfacial area between the active material and electrolyte for faradaic reactions. Figure 4d shows the Nyquist plot of FCO before and after 10000 CV cycles. It shows the almost similar semicircle at high

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frequency (before and after), while the more inclination of the straight line towards the real axis in low frequency is observed after the 10000 cycles. This may be one of the reason to decrease in small amount of capacitance retention. Inset is the enlarge view of Nyquist plot at high frequency region. The energy densities of FCO electrode calculated from charge-discharge curves are 0.97, 0.91, 0.66, and 0.58 mWhcm-2 that corresponds to the power densities of 250, 500, 1000 and 1500 mWcm-2, respectively. The observed energy density is higher than the energy density of Co3O4 (4.06 µWhcm-2) and rGOCo3O4 (7.24 µWhcm-2) composite aerogel electrodes.43 Compared with three-electrode cell, measurement by means of two electrode cell is more suitable for estimating the operation of SCs because they simulate the physical configuration, interior voltages, and charging transference in present SC uses, consequently providing the best estimation electrode materials. Therefore, to test the viability of FCO for flexible devices, a flexible solid-state symmetric (SSCs) supercapacitor FCO//FCO (FCO-SSCs) device was assembled with FCO electrodes as an anode and cathode using PVA–KOH gel electrolyte (See Supporting Information). Figure 5 shows the schematic of SSCs device fabrication from FCO electrodes. Two FCO electrode (area = 1 cm2) was dipped in PVA-KOH gel electrolyte. Then, FCO electrode with PVA-KOH gel electrolyte were pressed together to make a flexible SSCs. Then this device was wrapped in Teflon tape. Polymer gel acts as both electrolyte ions and porous separator as well as provides the mechanical integrity to the assembly. In all, approximate weight of FCO-SSCs was 260.57 mg. 18


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Figure 5. Schematic of flexible solid-state symmetric (SSCs) device fabrication from FCO electrodes and actual device photo.

Before fabricating the symmetric supercapacitor, we optimized positive working potential window of FCO electrode as shown in Figure S9 (See Supporting Information). The negative and positive working potential windows of FCO electrode in 1 M KOH electrolyte at a scan rate of 100 mVs-1 are shown in Figure S10. According to the CV study for a single electrode, we performed CV of FCO-SSCs in different potential windows from 0.6 to 1.6 V (Figure S11). The fabricated FCO-SSCs shows a typical capacitive behavior with nearly rectangular CV curves until 1V, while a pseudocapacitive behavior was observed at higher potential up to 1.6 V. Moreover, the current response of 19



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CV curves increases with increase in potential window, giving a stable maximum operating potential window of 1.6 V. On the other side, CV curves maintain a good pseudocapacitive behavior within this voltage range. The observed 1.6 V working potential window may be due to the positive and negative working potential of FCO electrode as well as polymer based gel electrolyte that can be used to attain extended potential window in supercapacitor devices compared to aqueous electrolyte.44 Xia et al.45 reported symmetric RuO2/RuO2 supercapacitor operating in 1.6 V. Owusu et al.46 obtained the optimal potential window of 1.7 V for the low–crystalline iron oxide hydroxide nanoparticles based supercapacitor. Further, CV curves of the FCO-SSCs at different scan rates in the potential window of 1.6 V are shown in Figure 6a. The observed redox peaks as well as increase in current density with respect to the scan rate demonstrated the typical reversible electrochemical behavior of FCO-SSCs. The areal capacitance of FCO-SSCs calculated from the CV curves in the different potential windows at a scan rate of 50 mVs-1 are 0.42, 0.64, 0.96, 1.31, 1.80, and 3.79 mFcm-2 for the 0.6, 0.8, 1, 1.2, 1.4 and 1.6 V, respectively. FCO-SSCs exhibit the areal capacitance of 16.88, 11.45, 5.43, 3.79, 3.46 and 3.05 mFcm-2 at a scan rate of 5, 10, 20, 50, 75 and 100 mVs-1, respectively in the potential window of 1.6 V.

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-1

0.4

-2

0.5 mAcm -2 0.6 mAcm -2 0.7 mAcm -2 0.8 mAcm -2 0.9 mAcm -2 1 mAcm

(b)

1.6

Potential (V)

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(a)

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0.2 o

20

Ref.55

Cycle Number

0

16

Ref.54 (d) Present Work

Ref.57

0

10

10

-0.05

0

8

10

0.00

20

-2

1.4 V

Energy Density (mWhcm )

0.15

60

1.2 V

4

Time (sec)

100 2000 100 2000 100 2000 100 20 100 2000

0.20

80

Current (mA)


1V

0.8 V

100

Current (mA)

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


(f) 90%

100

60 40 20

80

Bending

Normal 60 40 20 0

100

200

300

400

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0.0

0.4

0.8

1.2

1.6

0

Potential (V)

1000

2000

3000

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Page 22 of 39

Figure 6. (a) CV curves at different scan rates; (b) CD curves at different current densities; (c) Capacitance retention at a scan rate of 100 mVs-1 in different potential windows over 10000 CV cycles. Inset is the 1st and 2000th CV cycle in each potential window; (d) CV curves collected at a san rate of 50 mVs−1 and digital photographs (inset) of the FCO-SSCs under different bending conditions; (d) Ragone plot of the FCO-SSCs as a function of areal energy density vs. areal power density; (e) Capacitance retention under bent and twisted conditions. Inset shows the capacitance retention after 500 bending cycles and (g) a simple application to light commercial light-emitting diodes (LEDs). The observed areal capacitance for FCO-SSCs is higher than the previous works published recently.47-55 For instance; graphene structures (0.29 mFcm-2),47 carbon paper on a MnO2-modified nanoporous gold wire (12 mFcm–2),48

MnO2@SiNWs (13.38

mFcm−2),49 CNC & CNT-CNC (0.6 & 1.0 mFcm-2, respectively),50 OLC (0.9 mFcm-2),51 CNT yarn coated with MnO2 (3.7 mFcm-2 at 2 nAcm-2),52 pen ink (19.5 mFcm-2),53 VOPO4/graphene (8.360 mFcm-2),54 metal-like cellulose paper (1.35 mFcm-2).55 22


Page 23 of 39 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


Charge−discharge performance of electrode is crucial in the performance estimation of the active material under real operational circumstances. Figure S12 shows the charge-discharge curves of FCO-SSCs at 0.5 mAcm-2 in the different potential windows from 0.6 to 1.6 V. Charge-discharge curves exhibits the similar electrochemical behavior as like CV curves showing a unique behavior of FCO-SSCs in the all operational potential window. The charging and discharging time increased as the potential window increased from 0.6 to 1.6 V. Through the charge−discharge processes, the curve of charging step is not quite symmetrical with its related discharging step, showing a small decline in internal resistance (IR drop) and indicating pseudocapacitive behavior. Figures 6b shows the charge−discharge curves of FCO-SSCs at different current densities (0.5, 0.6, 0.8, 0.9 and 1 mAcm−2) in the 1.6 potential window. Whole curves exhibit symmetrical characters of the charging and discharging counterparts, suggesting the perfect pseudocapacitive character of rapid charge/discharge processes. Furthermore, based on the discharge times, the areal capacitances of the overall device is calculated. The maximum areal capacitance for the FCO-SSCs device is 1.62 at a current density of 0.5 mAcm−2. Further, it shows decrease in areal capacitance as 1.50, 1.44, 1.29, and 1.18 mFcm-2 with increase in current density as 0.6, 0.7, 0.8, 0.9 and 1 mAcm-2, respectively. Energy density and power density are two important parameters to characterize the performance of any SCs devices. FCO-SSCs has the maximum energy density of 0.57 mWhcm-2, which is achieved at a power density of 200 mWcm-2. Several energy densities

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Page 24 of 39

can be reached with corresponding power densities; 0.53 mWhcm-2 at 240 mWcm-2, 0.51 mWhcm-2 at 280 mWcm-2, 0.46 mWhcm-2 at 320 mWcm-2, and 0.42 mWhcm-2 at 400 mWcm-2. Ragone plot drawn from the areal energy density and power density of FCOSSCs compared with previous report is shown in Figure 6d, which validate the overall performance of FCO-SSCs. Remarkably, this FCO-SSCs exhibits the better values than symmetric and asymmetric supercapacitor reported previously (Supporting Information, Energy Density and Power Density). The energy density of the supercapacitor can be increased further, instead of increasing the capacitance, by using the organic or ionic electrolytes that have the maximum operating potential window (2-3 V) than the aqueous electrolyte. The long cycle performances are important parameter for electrochemical supercapacitors. FCO- SSCs cycle life tested through repeating CV cycles at a scan rate of 100 mVs−1 over 2000 CV cycles in each potential window from 0 to 0.6-1.6 V. Capacitance retention of FCO-SSCs is shown in Figure 6c. To understand cycle performance conveniently, the 1st and 2000th CV cycle in each potential windows are shown in the inset of Figure 6c. CV curves of FCO-SSCs are nearly symmetric in all potential windows, and the curves after 2000 cycles are almost overlay, indicating good reversibility with 90% capacitance retention of the initial capacitance after the 10000 cycles. This excellent capacitance retention might be due to the solid electrolyte, minimizing the degradation of (Fe,Cr)2O3 as well as integrated characteristic of oxide layer, which showed excellent 97% capacitance retention in three-electrode system. The

24


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observed excellent capacitance retention is higher than the values of iron based and other supercapacitor devices reported in the literature (Supporting Information, Table S2). This is the best cycling performance ever reported for a flexible solid-state symmetric supercapacitors based on thermally oxidized mesh. Thus, all in one integrated thin FCO oxide layer electrode developed on flexible conductive stainless steel mesh not only results in large surface area, fast charge transport pathways, easy diffusion of the electrolyte and small contact resistance but also produce binder-free electrode for flexible devices. Figure S13 shows the Nyquist plots of FCO-SSCs before and after 10000 CV cycles, which consists of semi-circle at high frequency and an inclined line at low frequency indicating an ideal capacitive behavior with excellent charge transfer and diffusion. The semicircle intercept on the real axis (Z′) at high frequency shows the charge transfer resistance, which includes the inherent electrode resistance, electrolyte resistance, and contact resistance. EIS shows increase in ESR from 23 to 60 Ω after 10000 CV cycles, which may be one of the reason for decrease in capacitance retention. This increase in resistance attributed to the degradation of the electrolyte and an active material.56 A very high equivalent series and charge transfer resistance in the higher frequency region due to the conductivity of electrode and ion migration in the PVA-KOH gel electrolyte. CV curves of as-assembled flexible FCO-SSCs under different bending conditions (0 to 180°) shows the negligible electrochemical performance (Figure 6e). Figure 6f

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Page 26 of 39

shows the electrochemical stability of the FCO-SSCs under bent (180°) and twisted conditions performed at a scan rate of 100 mVs-1 over 2000 CV cycles. The bent or twisted FCO-SSCs device is able to retain the 90% of its capacitance indicating the excellent durability performance. Furthermore, inset of Figure 6f shows the capacitance retention of FCO-SSCs over the 500 bending cycles at a scan rate of 100 mVs-1. It displays unceasing capacitance retention even after 500 bending cycles presenting an excellent flexibility. The promising electrochemical stability and durability of the flexible device under bent and twisted conditions can be ascribed to the excellent mechanical strength, high flexibility, structural integrity and good capacitive performance. This result designates thermally oxidized mesh has large potential and attractive electrochemical and mechanical assets for real-world application in flexible energy-storage devices, such as folded devices and portable electronics. Furthermore, we demonstrates the practical application by assembling two and three FCO-SSCs in series to light up red (2.0 V), yellow (2.0 V) and blue (3.2 V) LED, respectively as shown in Figure 6g.

Conclusions The present work demonstrates that the thermal oxidization is an effective way of nanostructuring lightweight flexible stainless-steel mesh in order to change its surface texture making a suitable high surface area electrode for supercapacitors. Electrochemical investigation shows the pseudocapacitive nature of the (Fe,Cr)2O3 oxide layer. Auxiliary, a high stability (97% capacitance retention) after long cycles suggest the remarkable

26


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reversible redox processes of (Fe,Cr)2O3 oxide layer. We visualize that this simple preparative approach could be easily extended to construct a wide diversity of oxide layer on flexible substrate with superior electrochemical performance for energy storage devices. Furthermore, we demonstrates the design of a low-cost and lightweight high performance solid-state symmetric supercapacitor based on (Fe,Cr)2O3 oxide layer electrodes. This device operates in the potential window of 1.6 V, with an areal capacitance of 16.88 mFcm-2, and a high capacitance retention of 90% over 10000 cycles. Quite attractively, a stable electrochemical performance of a solid-state flexible supercapacitor perceived under numerous bending environments (up to 180°). Besides, it exhibits not only excellent electrochemical durability (90% over 2000 cycles) under bent and twisted conditions but also constant capacitance retention over 500 bending cycles. The present excellent electrochemical properties of polygonal structure of thermally prepared oxide layer could be considered as potential electrode materials for next generation flexible solid-state symmetric as well as asymmetric supercapacitors device (since it shows excellent electrochemical behavior in negative window) for flexible, energy storage device applications.

Acknowledgment This research was supported by the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP: Ministry of Science, ICT and Future Planning) (NRF-2016R1D1A1B03933910).

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Supporting Information Digital

photographs

of

bare

and

thermally

oxidized

mesh,

material

characterizations, electrochemical measurement techniques, calculation formulae for areal capacitance, energy density, power density, FE-SEM images and EDS spectra of bare and thermally oxidized mesh, AES depth profile study, electrochemical study of oxidized mesh, PVA-KOH gel electrolyte, electrochemical study of FCO-SSCs device, energy density and power density, comparison of capacitance retention of FCO electrode and FCO-SSCs device References (1)

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10.1038/ncomms3431. (55) Ko, Y.; Kwon, M.; Bae, W. K.; Lee, B.; Lee, S. W.; Cho, J. Flexible supercapacitor electrodes based on real metal-like cellulose papers, Nat. Commun. 2017, 8, 536, DOI: 10.1038/s41467-017-00550-3. (56) Yang, P.; Ding, Y.; Lin, Z.; Chen, Z.; Li, Y.; Qiang, P.; Ebrahimi, M.; Mai, W.;

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Wong, C. P.; Wang, Z. L. Low-cost high-performance solid-state asymmetric supercapacitors based on MnO2 nanowires and Fe2O3 nanotubes, Nano Lett. 2014, 14, 731−736, DOI: 10.1021/nl404008e.

Graphical Abstract

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Synopsis Low cost, lightweight and flexible stainless steel mesh surface has been engineered using simple thermal oxidation process for the development of supercapacitor as a green energy source.

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Flexible Solid-State Symmetric Supercapacitor Based on (Fe,Cr)2O3

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