Hybrid α-Fe2O3@Ni(OH)2 nanosheet composite for high-rate-performance supercapacitor electrode

In this study, we report a facile fabrication of ultrathin two-dimensional (2D) nanosheet hybrid composite, α-Fe2O3 nanosheet@Ni(OH)2 nanosheet, by a two-step hydrothermal method to achieve high specific capacitance and good stability performance at high charging/discharging rates when serving as electrode material of supercapacitors. The α-Fe2O3@Ni(OH)2 hybrid electrode not only has a smooth decrease of the specific capacitance with increasing current density, compared with the sharp decline of single component of Ni(OH)2 electrode, but also presents excellent rate capability with a specific capacitance of 356 F/g at a current density of 16 A/g and excellent cycling stability (a capacity retention of 93.3% after 500 cycles), which are superior to the performances of Ni(OH)2 with a lower specific capacitance of 132 F/g and a lower capacity retention of 81.8% at 16 A/g. The results indicate such hybrid structure would be promising as excellent electrode material for good performances at high current densities in the future.


Results and Discussions
The α -Fe 2 O 3 nanosheet@Ni(OH) 2 nanosheet hybrid was fabricated through a two-step hydrothermal route. First, ultrathin α -Fe 2 O 3 nanosheets were synthesized by a metal ion intervened hydrothermal method with Al 3+ as structure director according to our previous report 26 , then Ni(OH) 2 nanosheets was produced onto the surface of as-prepared α -Fe 2 O 3 with carboxymethylcellulose (CMC) as structure director. For comparison, the same procedure was also applied to prepare Ni(OH) 2 without the addition of α -Fe 2 O 3 . Typical X-ray diffraction (XRD) patterns of Ni(OH) 2 , α -Fe 2 O 3 and α -Fe 2 O 3 @Ni(OH) 2 hybrid are shown in Fig. 1. In Fig. 1a, all of the diffraction peaks can be indexed to hexagonal Ni(OH) 2 (JCPDS No. 03-0177), confirming the formation of single Ni(OH) 2 nanocrystals with high purity. All the diffraction peaks in Fig. 1b can be indexed to hematite α -Fe 2 O 3 (JCPDS No. 33-0664) and no obvious impurity peaks can be observed, indicating the high purity of the product with the addition of Al 3+ . After introducing Ni(OH) 2 onto the surface of α -Fe 2 O 3 , both diffraction peaks of α -Fe 2 O 3 and Ni(OH) 2 emerge as shown in Fig. 1c, suggesting the formation of the composite composed of α -Fe 2 O 3 and Ni(OH) 2 . Figure S1 displays scanning electron microscopy (SEM) images of two single materials. As shown in Figure S1a, the obtained α -Fe 2 O 3 is composed of monodisperse nanosheets with 2D size of 500 nm and thickness of about 2 nm. Figure S1b demonstrates that under the hydrothermal conditions without the addition of α -Fe 2 O 3 , the obtained Ni(OH) 2 crystals have flower-like structures assembled by nanosheets. Figure 2 shows SEM, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of α -Fe 2 O 3 @Ni(OH) 2 nanosheet hybrid. From the SEM images shown in Fig. 2a,b, it is clearly observed that the hybrid presents a sandwich-like structure with wrapped α -Fe 2 O 3 serving as intermediate layer and attached Ni(OH) 2 nanosheet as coating layers. The typical structure can be clearly observed in red rectangle of Fig. 2b. The α -Fe 2 O 3 nanosheets are tightly covered by Ni(OH) 2 nanosheets with a lot of folds. TEM images shown in Fig. 2c,d also show that the center part of the structure is much darker than the edges confirming the sandwich-like structure of the nanosheet hybrid. Central part of the image contains α -Fe 2 O 3 nanosheet covered by Ni(OH) 2 nanosheets. Single wrinkled Ni(OH) 2 nanosheet occupies most parts of edge section. Further evidences can be obtained from HRTEM characterizations.   Fig. 2f shows perfect aligned crystal lattice planes from outer edge, corresponding to {100} planes of Ni(OH) 2, whose interplanar spacing is 0.27 nm. The HRTEM images presents the changes of 2D lattice fringe from inside to outside, directly proving the structure characteristic of hybrid. Figure 3 displays the scheme of the whole synthesis process. After the formation of α -Fe 2 O 3 nanosheets, they were used as templates with CMC as the directing agent. The functionalized side groups on molecular chain of CMC are easy to combine with Ni 2+ by electrostatic force to form polymer-inorganic composite, which can work as a template to ensure the anisotropic growth of the inorganic precursor and result in the formation of inorganic nanosheets 27 . The functionalized side groups of CMC can also enable the polymer-inorganic composite to be adsorbed onto the surface of α -Fe 2 O 3 nanosheets due to the interaction between the functional groups (such as carboxyl and hydroxyl groups) and Fe 3+ . Under hydrothermal conditions, Ni(OH) 2 nanosheets formed coating on the surface of α -Fe 2 O 3 . IR spectra of α -Fe 2 O 3 , Ni(OH) 2 , and Fe 2 O 3 @Ni(OH) 2 are displayed in Figure S2 with the wavenumber ranging from 4000 to 300 cm −1 . The sharp absorption peaks of around 3640 cm −1 and 533 cm −1 of both Ni(OH) 2 and α -Fe 2 O 3 @Ni(OH) 2 are related to stretching vibration and lattice vibration of hydroxyl respectively. Low wavenumber peaks around 534 cm −1 of α -Fe 2 O 3 , Ni(OH) 2 and α -Fe 2 O 3 @Ni(OH) 2 attribute to the lattice vibration of Ni-O, Fe-O, and nearly equal value indicates possible interaction between two compounds. Besides, the IR spectra also contain stretching (3404 cm −1 ) and bending vibration (1610 cm −1 ) of adsorbed water. No obvious extra peaks are observed in α -Fe 2 O 3 @Ni(OH) 2 , indicating no emergence of newly functional group, which illustrates possibly simple attachment of Ni(OH) 2 onto the surface of α -Fe 2 O 3 nanosheet.
The acquisition of the α -Fe 2 O 3 @Ni(OH) 2 hybrid gives us opportunity to investigate the electrochemical performance of such novel nanosheet composites. In general, cyclic voltammetry (CV) is used to research the capacitive behavior and reversibility of an electrode material 5 . The electrochemical tests were carried out in a three-electrode system with a Pt wire counter electrode and an Ag/AgCl reference. Figure 4a,b show the CVs of the single Ni(OH) 2 nanosheets and the α -Fe 2 O 3 @Ni(OH) 2 nanosheet hybrid, which were conducted at the scan rates of 5, 10, 20, 50 and 100 mV/s with potential windows ranging from 0.2 to 0.6 V. Compared to the Ni(OH) 2 electrode, although the CV peaks of α -Fe 2 O 3 @Ni(OH) 2 hybrid electrode broaden, the two strong redox peaks still exist, which reveals the pseudocapacitance characteristics. The quasi symmetric characteristic of the redox peaks and cycles in the first few loops indicates the excellent reversibility of the Ni(OH) 2 and hybrid electrodes. Figure 4c shows chronopotentiometry (CP) curves of the hybrid electrode within a potential window of 0.2-0.6 V at different current densities from 2 to 16 A/g. From the CP curves, it can be observed that the discharge time is  longer than charge time at low current density, indicating that some undesirable reduction reaction may happen during the electrochemical process. It is also clearly observed from the CP curves that the each discharge curve contains two section: a rapid potential descent process and a slow potential decay process. The former represents a low internal resistance and the latter indicates the capacitive character of the electrode. Figure 4d shows the 10 charge-discharge cycles at a current density of 4 A/g, which indicates the superior reversible characteristics of hybrid electrode.
To study the capacitance difference of Ni(OH) 2 and hybrid electrodes, the CPs of Ni(OH) 2 and α -Fe 2 O 3 nanosheets were also carried out, which are displayed in Figure S3. The specific capacitance can be calculated from CPs according to the equation: C = (I d × Δ T)/(Δ V × m), where C is the specific capacitance (F/g), I d is discharge current(A), Δ T is discharge time (s), Δ V is the potential change (V), and m represents the mass of the active material within the electrode (g). Figure 5a shows the obvious specific capacitance difference of three electrodes at a current density of 2, 4, 8, 12 and 16 A/g, respectively, from which it can be clearly observed that α -Fe 2 O 3 @ Ni(OH) 2 hybrid electrode has a smooth decrease of specific capacitance with increasing current density, compared with the sharp decline of single component of Ni(OH) 2 electrode. The hybrid electrode exhibits higher specific capacitance than Ni(OH) 2 electrode at high current densities, despite of lower capacitance than Ni(OH) 2 at low current densities. As known, Ni(OH) 2 is a promising pseudocapacitive material due to its high specific capacity but α -Fe 2 O 3 is not an ideal material for supercapacitor electrode. At low current densities, the addition of α -Fe 2 O 3 would no doubt sacrifice some capacity because of the low capacitance of α -Fe 2 O 3 . But in the other hand the better conductivity of α -Fe 2 O 3 nanosheets would make the hybrid stand up the impact of large-current, which may attribute to the enhanced conductivity to support fast electron transport required by high rates 4 . EIS tests of α -Fe 2 O 3 , Ni(OH) 2 , and α -Fe 2 O 3 @Ni(OH) 2 were carried out and the curves are shown in Figure S4a. From the semicircles in the EIS curves, it can be seen that the hybridation of α -Fe 2 O 3 with Ni(OH) 2 results in the decrease of charge-transfer resistance (Rct) comparing to that of single Ni(OH) 2 , indicating the more facile charge transfer ability of α -Fe 2 O 3 @Ni(OH) 2 nanosheet hybrids, which may enhance the charge-discharge performance at high current densities. This is an exciting phenomenon that the rapid charging/discharging performance of Ni(OH) 2 can be greatly improved by combining with the ultrathin ferric oxide. It is important for supercapacitor electrodes to keep capacitance extremely with low capacitance loss at long term circulations 28 . Therefore, cycling-life tests of the hybrid and the single component of Ni(OH) 2 electrodes were carried out to discuss the behavior of capacitance decay. Figure 5b shows the cycling test of at a current density of 4 A/g and 16 A/g for the hybrid electrode and 16 A/g for the Ni(OH) 2 electrode. The hybrid electrode presents good cycle stability with a extremely high capacity retention of 96.0% at a current density of 4 A/g and 93.3% at 16A/g after 500 cycles, confirming the good stability of hybrid electrode at both low rate and high rate. However, single component of Ni(OH) 2 electrode displays a poor performance with a lower capacity retention of 81.8% at a current density of 16 A/g, which illustrates again that α -Fe 2 O 3 @ Ni(OH) 2 nanosheet hybrid has an excellent performance of fast charging/discharging. At the same time, we also conducted the cycle performances of α -Fe 2 O 3 @Ni(OH) 2 and Ni(OH) 2 for 2000 cycles to research the long-term stability. As shown in Figure S4b after 2000 cycles, the hybrid electrode still has a higher specific capacitance than Ni(OH) 2 electrode with good stability. The superior electrochemical performance of hybrid electrodes can be attributed to the synergistic effects of α -Fe 2 O 3 and Ni(OH) 2 . α -Fe 2 O 3 nanosheets not only provide abundant active sites for increased capacitance but also help Ni(OH) 2 nanosheets stabilize nanostructure at high current densities. In this case, the superiority of 2D composite materials is reflected and the hybrid has potential to be developed as an outstanding supercapacitor electrode material. It is also highly expected to explore the supercapacitor devices in the future with α -Fe 2 O 3 @Ni(OH) 2 nanosheets for high-performance supercapacitors with fast charging/discharging ability.

Conclusions
In summary, α -Fe 2 O 3 nanosheet@Ni(OH) 2 nanosheet, a hybrid of two-dimensional ultrathin nanomaterial was synthesized and developed as electrode of supercapacitor through a simple and cost-effective approach. In such composites, α -Fe 2 O 3 nanosheets serves as core and Ni(OH) 2 nanosheets as shell, and this distinctive design effectively shortens the ion diffusion path and provides abundant active sites, meanwhile stabilizes the structure. The hybrid electrode presents a more smooth specific capacitance change than that of Ni(OH) 2 electrode with the increase of current density. The electrode based on the hybrid composites shows excellent electrochemical performances at high current densities, including the increase of specific capacitance and enhancement of stability. The specific capacitance of α -Fe 2 O 3 nanosheet@Ni(OH) 2 nanosheet reached 356 F/g, and had a capacity retention of 93.3% at a current density of 16 A/g after 500 cycles, which was superior to the performance of Ni(OH) 2 with a specific capacitance of 132 F/g and a capacity retention of 81.8%. The results illustrate that the hybrid electrode has a more excellent fast charging/discharging performance at high current densities than single component of Ni(OH) 2 . It is worthy to expect that the fabricated α -Fe 2 O 3 nanosheet@Ni(OH) 2 nanosheet composite architectures would be applied in high-performance supercapacitors with fast charging/discharging ability and high energy/power densities in the future.

Methods
Synthesis of α-Fe 2 O 3 @Ni(OH) 2 nanosheet hybrid and Ni(OH) 2 nanosheets. α -Fe 2 O 3 @Ni(OH) 2 nanosheet hybrid material was prepared via a simple two step process. First, α -Fe 2 O 3 nanosheets were synthesized by applying metal ions Al 3+ as structure-directing agents, as described in our previous work 26 . Then α -Fe 2 O 3 naonosheets were added to a Teflon-lined autoclave (40 mL) containing nickel acetate and carboxymethylcellulose aqueous solution under magnetic stirring. After 30 min of stirring, appropriate amount of ammonia solution (25%, analytically pure) was added to the autoclave for further 20 min of stirring. Then the mixture was sealed, transferred to oven, and kept at 80 °C for 12 h. The red precipitate was obtained by centrifugation (10 000 rpm, 1 min) and washed with deionized water and ethanol for 3 times, and dried in air. The synthesis of Ni(OH) 2 nanosheet was same as α -Fe 2 O 3 @Ni(OH) 2 hybrid except the addition of α -Fe 2 O 3 nanosheets.
Characterizations. Scanning electron microscopy (SEM) was performed on Hitachi S-4800 at 10 kV.
Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained by using a JEOL JEM-2100 transmission electron microscope operating at 200 kV. Powder X-ray diffraction (XRD) patterns were collected by using a Bruker D8 ADVANCE diffractometer with CuKα radiation (λ = 1.5418 Å).
Electrochemical test. The working electrode was obtained by mixing the electroactive material, acetylene black, and polymer binder (polyvinylidene difluoride, PVDF) in a mass ratio of 75:15:10 with solvent (N-methylpyrrolidone, NMP). Then the homogeneous slurry was coated on the pre-treated nickel foam as the working electrode after stirring for one night, and dried at 50 °C for 12 h in a vacuum oven. Finally, the Ni foam with active materials was pressed with mass loading about 1.0 mg·cm −2 under 10 MPa for 40 seconds. Electrochemical measurements including cyclic voltammogram (CV) and chronopotentiometry (CP) were operated using a three-electrode system with a CHI 660 d electrochemical workstation in an electrolyte aqueous KOH electrolyte (1.0 M). A Pt wire was used as the counter electrode and a Ag/AgCl electrode was served as the reference electrode. In details, CV experiments were performed at various scan rates of 5, 10, 20, 50, and 100 mV/s. CP charge/discharge curves were obtained at various current densities of 2, 4, 8, 12 and 16A/g to evaluate the specific capacitance. A potential window in the range from 0.2 to 0.6 V was used during all measurements.