A Wire-Shaped Supercapacitor in Micrometer Size Based on Fe3O4 Nanosheet Arrays on Fe Wire

One-dimensional (1D, wire- and fiber-shaped) supercapacitors have recently attracted interest due to their roll-up, micrometer size and potential applications in portable or wearable electronics. Herein, a 1D wire-shaped electrode was developed based on Fe3O4 nanosheet arrays connected on the Fe wire, which was prepared via oxidation of Fe wire in 0.1 M KCl solution (pH 3) with O2-rich environment under 70 °C. The obtained Fe3O4 nanosheet arrays displayed a high specific capacitance (20.8 mF cm−1 at 10 mV s−1) and long cycling lifespan (91.7% retention after 2500 cycles). The excellent performance may attribute to the connected nanosheet structure with abundant open spaces and the intimate contact between the Fe3O4 and iron substrate. In addition, a wire-shaped asymmetric supercapacitor was fabricated and had excellent capacitive properties with a high energy density (9 µWh cm−2) at power density of 532.7 µW cm−2 and remarkable long-term cycling performance (99% capacitance retention after 2000 cycles). Considering low cost and earth-abundant electrode material, as well as outstanding electrochemical properties, the assembled supercapacitor will possess enormous potential for practical applications in portable electronic device. Electronic supplementary material The online version of this article (doi:10.1007/s40820-017-0147-3) contains supplementary material, which is available to authorized users.

Abstract One-dimensional (1D, wire-and fiber-shaped) supercapacitors have recently attracted interest due to their roll-up, micrometer size and potential applications in portable or wearable electronics. Herein, a 1D wireshaped electrode was developed based on Fe 3 O 4 nanosheet arrays connected on the Fe wire, which was prepared via oxidation of Fe wire in 0.1 M KCl solution (pH 3) with O 2 -rich environment under 70°C. The obtained Fe 3 O 4 nanosheet arrays displayed a high specific capacitance (20.8 mF cm -1 at 10 mV s -1 ) and long cycling lifespan (91.7% retention after 2500 cycles). The excellent performance may attribute to the connected nanosheet structure with abundant open spaces and the intimate contact between the Fe 3 O 4 and iron substrate. In addition, a wire-shaped asymmetric supercapacitor was fabricated and had excellent capacitive properties with a high energy density (9 lWh cm -2 ) at power density of 532.7 lW cm -2 and remarkable long-term cycling performance (99% capacitance retention after 2000 cycles).
Considering low cost and earth-abundant electrode material, as well as outstanding electrochemical properties, the assembled supercapacitor will possess enormous potential for practical applications in portable electronic device.

Introduction
Nowadays, the increasing demand for portable electronic devices in modern industry requires compatible flexible, lightweight and even wearable miniature energy storage system [1][2][3]. Therefore, due to the inherent characteristics of roll-up and micrometer size, one-dimensional (1D) wireshaped and fiber-shaped supercapacitors (SCs) are being identified as one of the most promising miniature energy storage systems for these portable electronic devices [1]. Compared with the typical two-dimensional (2D) sandwich-structured SCs [4][5][6][7], 1D SCs possess many versatile advantages such as smaller size and higher bendability and also can be converted into many other conceivable model or even woven into textile for unique electronic devices in practical applications [8][9][10]. Recently, high-performance wire-or fiber-shaped SCs have been extensively explored based on carbon/CNT (carbon nanotube) fibers [9,11,12], Cu wire and Ti wire [13,14]. However, the complicated synthesized procedure and relatively high cost, as well as low energy density values, hamper their wide applications. Note that iron-based materials have received hugely interest and have been widely used as electrode material for SCs [15][16][17][18]. In particular, among the ordinary electrode materials (nickel, cobalt, manganese, iron and molybdenum), iron is of higher abundance and lower price. In addition, iron oxides have received growing attention due to their suitable negative working window for aqueous supercapacitors [15,19,20]. Thus, developing efficient iron-based material for SCs should be highly economically desirable. So far, various iron-based materials, including Fe 2 O 3 and Fe 3 O 4 , exhibit a charming electrochemical performance for SCs [15,16,18,[21][22][23][24][25][26]. For instance, the hollow and porous Fe 2 O 3 , which was derived from industrial mill scale, delivers a high capacitance value of 346 F g -1 with outstanding cycling property (88% retention after 5000 cycles) [21]. In addition, Yang and co-authors [27], for the first time, synthesized Fe 3 O 4 nanoparticles, which showed good capacitive property, including high specific capacitance (207.7 F g -1 ), prominent rate capability and superior cycling stability (100% capacitance retention after 2000 cycles). Nevertheless, to the best of our knowledge, a simple and effective strategy for the preparation of iron-based material remains a great challenge.
Here  CF@MnO 2 cathodes were soaked in the hot gel electrolyte (50-60°C) for 10 min to allow the electrolyte diffuse into their porous structures and then were carefully entangled with each other. The assembled device was further heated at 35°C for 12 h to remove excess water in the electrolyte.
The specific capacitance is about 3.0 cm, which was calculated based on the length of the total device. The calculation process is shown in Support Information in detail.

Morphology and Structure Characterization
The morphology and structure of the samples were characterized using a field-emission scanning electron microscopic (FESEM, Model JSM-7600F), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (JEOL JEM-20100). Powder X-ray diffraction (XRD) patterns of the samples were recorded with a Bruker D8 Advance powder X-ray diffractometer with Cu Ka (k = 0.15406 nm) radiation. Raman spectra were recorded on a RENISHAW in via instrument with an Ar laser source of 488 nm in a macroscopic configuration. X-ray photoelectron spectroscopic (XPS) measurements were taken using a PHI X-tool instrument (Ulvac-Phi).

Electrochemical Measurements
The electrochemical performances were measured on an electrochemical workstation (CHI 660e, CH Instruments Inc., Shanghai) using a three-electrode mode in 3.0 M LiCl aqueous solution. The as-prepared Fe@Fe 3 O 4 or CF@MnO 2 , a platinum electrode and a saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. Cyclic voltammetry (CV) tests were done between -0.65 and -1.15 V for Fe@Fe 3 O 4 electrode, 0 and 1.0 V for CF@MnO 2 (vs. SCE) at different scan rates, respectively. The electrochemical impedance spectroscopy (EIS) measurements were taken in the frequency range from 0.01 Hz to 100 kHz.

Fe@Fe 3 O 4 Negative Electrode Materials
The SEM images of the Fe@Fe 3 O 4 -30 are shown in Fig. 1. In Fig. 1a [29][30][31]. No additional peaks of other phases have been detected, indicating high purity and good crystallinity of the obtained Fe 3 O 4 . In addition, Raman spectra of the Fe 3 O 4 nanosheets are shown in Fig. S5. The fundamental Raman scattering peaks were observed at 540 and 670 cm -1 , corresponding to the T 2g and A 1g vibration modes, respectively [32][33][34]. The T 2g is attributed to asymmetric stretch of Fe and O, and the A 1g is attributed to symmetric stretch of oxygen atoms along Fe-O bonds.
The XPS was further employed to investigate the composition and valence states of the Fe 3 O 4 gently scratched from the Fe wire. The full XPS spectrum of the Fe 3 O 4 reveals the presence of Fe and O elements along with a small quantity of C element (Fig. 3b). Moreover, the Fe spectrum is depicted in Fig. 3c, and two dominant peaks located at 710.5 and 723.8 eV are in good accordance with Fe 2p 3/2 and Fe 2p 1/2 spin orbit peaks accompanied by their satellite peaks between 717.2 and 731.2 eV, respectively, which are again consistent with the standard Fe 3 O 4 XPS spectrum [22,35,36]. Furthermore, the O 1s spectrum could be deconvoluted into two peaks at 530.3 and 531.8 eV, which results from the oxygen bonds of Fe-O and H-O, as shown in Fig. 3d. The electrochemical properties of as-prepared samples were studied by CV in a typical three-electrode system in 3.0 M LiCl electrolyte. The morphologies, XRD and Fe 3 O 4 content of Fe@Fe 3 O 4 oxidized in different time are shown in Fig. S4, S6 and Table S1. One can see that the nanosheet array structure breaks up under longer oxidation time of 40 min (Fig. S4), and the capacitive performances are reduced due to the poor electron transportation. In addition, it is easy to see that with the increase in reaction time from 0 to 30 min, the content of Fe 3 O 4 is increased, whereas the content of Fe 3 O 4 is decreased when the reaction time is over 40 min. The reason may be that the as-formed Fe 3 O 4 is easy to fall out from Fe substrate, as shown in Fig. S7.
As expected, the  Fig. 4b and quasi-rectangular shape is inherited even at a very high scan rate of 200 mV s -1 , indicating excellent fast electron-transfer characteristics. This was further supported by the low resistance value R ct of 1.2 X (Fig. S8). The quasi-rectangular CV shape without any redox peaks indicates a double-layer capacitive behavior [24,27]. Figure 4c summarizes the specific capacitance from CV tests with different scan rates. The high specific capacitance of 20.8 mF cm -1 is obtained at the scan rate of 10 mV s -1 . To further evaluate the electrochemical properties of the as-prepared Fe@Fe 3 O 4 -30 electrode, galvanostatic chargedischarge (GCD) tests were performed. The GCD curves (Fig. 4d) at different current (0.5-2.4 mA) display a nearly triangular shape, implying a good electrochemical reversibility. The specific capacitance of the Fe@Fe 3 O 4 -30 electrode can also be calculated from the GCD curves (Fig. 4e) and is 12, 8.0, 6.6, 5.8, 4.5, and 4.2 mF cm -1 at 0.6, 0.9, 1.2, 1.5, 2.1, and 2.4 mA, respectively. With the increasing current, the specific capacitance decreases which is similar to the foregoing CV results. In addition, prominent long-term stability is a most important characteristic for state-of-the-art electrode material. The cycling property of the Fe@Fe 3 O 4 -30 electrode was tested by continuous GCD curves in Fig. 4f. As expected, the  Fig. 1 and S4, the connected nanosheet architecture of Fe@Fe 3 O 4 -30 was evidently observed. The as-formed connected nanosheet structure leads to abundant open spaces, which can provide more active surface sites for effective penetration of the electrolyte and accordingly enhance capacitive property. Thus, we think that the enhanced property results from good conductivity of Fe wires, intimate contact between Fe wire and Fe 3 O 4 , and the unique nanosheet architecture.

Electrochemical Performance of the WSSC
The WSSC was assembled by using the Fe@Fe 3 O 4 -30 as negative electrode and CF@MnO 2 as positive electrode (Scheme 1). The gel state PVA-LiCl solution was used as the solid electrolyte. Figure S11 shows the SEM images of as-assembled WSSC. The length and diameter of the WSSC are about 3 cm and 0.5 mm, respectively. Figure 5a displays the CV curves of the assembled WSSC collected in different potential windows, indicating that the potential window of the assembled WSSC can reach up to 2.0 V. Moreover, the CV tests at different scan rates were carried out within the potential window of 0-2.0 V, as shown in Fig. 5b. The voltammetric feature of the assembled WSSC remains almost unchanged with the increasing scan rate from 10 to 500 mV s -1 , suggesting fast electron-transfer kinetics. Figure 5c gives the GCD curves of the WSSC at different currents. The corresponding specific capacitances calculated according to the GCD curves are summarized in Fig. 5d. One can see that the WSSC exhibits a length specific capacitance of 5 mF cm -1 and an area specific capacitance of 16 mF cm -2 at the current of 0.5 mA. The delivered specific capacitances are also much higher than that of reported WSSC (Table 1). It is well known that the energy density (E) and power density (P) of a supercapacitor could be calculated according to Eq. S3 and Eq. S4, respectively. Therefore, this WSSC will also deliver a superior energy density and power density which are plotted on the Ragone diagram in Fig. 5e. Impressively, a maximum energy density of 9 lWh cm -2 at power density of 532.7 lW cm -2 is achieved at a working voltage of 2.0 V. Meanwhile, the large energy density of the assembled WSSC is superior to previously reported WSSCs systems such as MWCNT//MWCNT/ MnO 2 , NPG wire/MnO 2 //CNTs/carbon paper (Table 1). Furthermore, as shown in the inset of Fig. 5e, a single WSSC device could light a commercial red-light-emittingdiode (1.5 V) for 2 min, implying its practical application. c GCD curves of the device at different current. d Specific capacitance of the device as a function of current. e Ragone plots of the device calculated from GCD curves. The inset is a photograph of a red LED turned on by a wire-shaped all-solid-state asymmetric supercapacitor device. f Cycling stability of the device at a current density of 3.0 mA. The inset shows the last 10 charge/discharge profile More importantly, the WSSC device also reveals a good cycling stability and 100% of capacitance is retained over 2000 cycles (Fig. 5f).

Conclusion
In summary, Fe 3 O 4 -connected nanosheet arrays growing on the surface of the Fe wire substrate have been successfully synthesized by directly oxidization of Fe wire. Benefiting from the connected nanosheet structure and the intimate contact between the Fe 3 O 4 and Fe substrate, the obtained Fe@Fe 3 O 4 exhibits excellent capacitive behavior with a length specific capacitance of 12 mF cm -1 at 0.6 mA. What is more, the as-assembled asymmetrical WSSC device also presents a high energy density (9 lWh cm -2 ) at power density of 532.7 lW cm -2 and remarkable long-term cycling performance (100% capacitance retention after 2000 cycles), which will possess enormous potential for practical applications in portable electronic devices. C L : length specific capacitance; C A : area specific capacitance; E and P are the energy and power energy