Tuning Hierarchical Ferric Nanostructures-Decorated Diatomite for Supercapacitors

FeOOH nanosheets on porous diatomite have been successfully prepared by a facile two-step hydrothermal approach for supercapacitors, and then α-Fe2O3 and γ-Fe2O3 nanostructures are obtained via calcination under different atmospheres and temperatures. The morphologies and structures of all the samples are investigated in detail to make the hierarchical architecture clear. Besides, systemic tests are carried out in 1 M Na2SO4 electrolyte to characterize the electrochemical properties of these materials. Among the iron-related composite electrodes, diatomite@FeOOH owns the highest specific capacitance (157.9 F g−1 at a current density of 0.5 A g−1) and best cycling performance (98.95% retention after 1000 cycles), which is considered to be a potential material for high-performance supercapacitors. Furthermore, the synthesizing strategy can be extended to the preparation of other metallic oxide-derived functional materials towards energy storage and conversion. Electronic supplementary material The online version of this article (10.1186/s11671-018-2822-9) contains supplementary material, which is available to authorized users.


Background
Up to now, major challenges for supercapacitor technologies include low-energy density and high production cost. Some research efforts have been devoted to improving its disadvantages [1]. Some transition metal oxides or hydroxides, such as MnO 2 [2][3][4], FeOOH [5], NiO [6], and CuO [7], are regarded as potential candidates for active electrode materials. Among these transition metal oxides, ferric oxides/hydroxides have drawn considerable attention due to their nature abundance, variable oxidation states and environmental friendliness [8][9][10]. Besides, ferric oxides/hydroxides have been considered as especially desirable electrode materials for supercapacitors because its structure (like tunnel-type FeOOH) can accelerate ion transport. However, ferric oxides/hydroxides still own two major obstacles (small surface area and low electro-conductivity). Nanostructures can change the obstacles and provide enormous advantages in energy storage system, which are considered to be high charge-discharge rates by accelerating high specific surface areas, fast redox reactions, and short diffusion paths for electrons and ion [11]. Nevertheless, researches indicated that ferric oxide nanostructures had a tendency to get aggregate and transform into large particles causing a severe loss of specific surface area, which has a seriously terrible effect on electrochemical properties [12]. Therefore, the primary issue currently is to find a simple and feasible way to effectively disperse the nanostructures of ferric oxides, such as fabricating ferric metal oxides on the surface of porous templates.
As an important natural porous nanomaterial, diatomite is an attractive porous template on account of its high porosity, low-volume density, stable chemical property, and large specific area [13][14][15]. Diatomite template can increase the low surface area and avoid aggregation of nanostructures. Until now, despite that the characteristics of their structures are obvious and promising, ferric oxides/hydroxides-based diatomite composites to form a hierarchically porous structure have rarely yet to be studied in supercapacitors.
In this paper, we report successful fabrication of ferric oxides/hydroxides on diatomite as supercapacitor electrode materials by an effective two-step hydrothermal approach. We aimed to use diatomite as a template to distribute evenly ferric oxide/hydroxide nanostructures on the diatomite surface, which solves the problem on how to effectively disperse its nanostructures and greatly improve the surface area and electrochemical properties of ferric oxides/hydroxides, as shown in Additional file 1: SI-5. Remarkably, with diatomite and ferric oxides/hydroxides' synergistic effect, the final composite, diatomite@FeOOH, indicated promising electrochemical properties in supercapacitors.

Materials Synthesis
All the chemical reagents were of analytical purity and used without any further purification. Synthetic processes are as follows ( Fig. 1; more details in Additional file 1: SI-1).
Initially, the natural diatomite was purified via a simple oil bath method by the following procedures. Then, the MnO 2 -decorated diatomite were prepared by a hydrothermal method. Typically, the KMnO 4 solution (30 mL, 0.05 M) was mixed with the purified diatomite (30 mg). Afterwards, the mixture was transferred into a Teflon-lined autoclave (50 mL) which was heat-treated at 160°C for 24 h. The as-prepared diatomite was centrifuged, washed by distilled water and then dried at 60°C. After that, MnO 2 -decorated diatomite was obtained.
In addition, a solution of FeSO 4 ·7H 2 O (0.01 M, 30 mL) was applied towards totally transformation of the pretreated diatomite@MnO 2 (30 mg) at 120°C for 2 h. In the end, the different crystal forms (α-Fe 2 O 3 and γ-Fe 2 O 3 ) of ferric oxide-decorated diatomite were prepared by calcining at 350°C for 2 h under O 2 atmosphere and 500°C for 2 h under N 2 atmosphere, respectively.

Characterization
Focused ion beam scanning electron microscopy (Zeiss Auriga FIB/SEM) was employed for observing the morphologies. And the phase analysis and structure were established by powder X-ray diffraction (XRD; D/ max 2500, Cu Kα).

Electrochemical Measurements
Electrochemical study on the materials in a threeelectrode system: All the electrochemical properties of the as-obtained diatomite composites were characterized though a conventional three-electrode equipment filled with 1 M Na 2 SO 4 electrolyte. Before the measurements, the working electrodes were formed with mixing active material (diatomite@MnO 2 , diatomite@FeOOH, diatomite@α-Fe 2 O 3 , and diatomite@γ-Fe 2 O 3 ), acetylene black, and polyvinylidene fluoride (PVDF) at a weight ratio of 7:2:1 in N-methyl-2-pyrrolidone (NMP). The slurry was coated on pieces of foamed nickel foam (1 × 1 cm 2 ), which was heated to evaporate the dissolvent (120°C for 12 h). About 2 mg electrode material was loaded on the nickel foam. The electrochemical performances and capacitance values of the composites electrodes were characterized with cyclic voltammetry (CV), galvanostatic charging/discharging (CC) methods, and electrochemical impedance spectroscopy (EIS).
The specific capacitance (C m ) is calculated by the following equation: where I is the discharging current, △t is the discharging Fig. 1 Preparative route of ferric oxides/hydroxides-based diatomites time, △V is the potential window during discharging, and m is the weight of active materials. To investigate the electrochemical properties of the four samples, a three-electrode system was carried out in 1 M Na 2 SO 4 aqueous electrolyte. The differences in morphologies and structures of these four samples can lead to diverse electrochemical performances. Diatomite served as a substrate contributes to efficient transport of ions owing to its porous structures.

Results and Discussion
As shown in Additional file 1: SI-3(a, b), the CV and CC curves of diatomite@MnO 2 electrode are quasi-rectangular and nearly symmetrical triangular, respectively. There are no distinct redox peaks, which deviates from the ideal rectangle manifesting faradic pseudocapacitive nature of the electrode. As revealed in Additional file 1: SI-3(c, d), diatomite@FeOOH has the better capacitive properties than other two samples (diatomite@α/γ-Fe 2 O 3 ). The specific capacitance of diatomite@FeOOH electrodes is about 157.9 F g −1 at a current density of 0.5 A g −1 , demonstrating that the highly porous structure can transfer more ions into its surface and can promote more redox faradic reactions. Agreeing with the SEM results, the distances of α-Fe 2 O 3 nanosheets are so large that the surface of active material make less use of the cations, while γ-Fe 2 O 3 can provide the smallest specific area for ions among the three ferric oxide samples. Therefore, the distance of the nanosheets of the samples is very important. Besides, as showed in Table 1, the diatomite@FeOOH electrode in this work has a higher specific capacitance among these ferric oxide/ hydroxide-based electrodes compared with previous work.
Such being the case, systematic tests are carried out to better investigate the electrochemical properties of diatomite@FeOOH electrode. Figure 3a shows typical CV curves of FeOOH sample in potential range from − 1 to 0 V at different scan rates. Galvanostatic charge-discharge curves of diatomite@FeOOH electrode at different current densities are presented in Fig. 3b. The shape of CV and CC curves of diatomite@FeOOH electrode demonstrates the pseudocapacitance characteristics of diatomite@FeOOH. Figure 3c further illustrates the relationship between specific and current density. The cycling ability of diatomite@FeOOH electrode was subjected to a long-cycle test for consecutive 1000 cycles (Fig. 3d), and capacity retention after 1000 cycles is about 98.95%. The CC curves of the last 10 cycles suggest no major structure variation during the charge-discharge processes. Additionally, the Nyquist plots for the FeOOH sample electrode (Additional file 1: SI-4) contain a semicircle in the high-frequency boundary and a straight line in the low-frequency range. The internal resistance (R s ) of the electrode is about 3.0 Ω and 3.5 before and after 1000 cycles without much variation, while the charge-transfer resistance (R ct ) is about 1.2 and 4.0 Ω before and after 1000 cycles. These findings could be responsible for the good electrochemical properties of the diatomite@FeOOH electrode.

Conclusions
In summary, we prepare ferric oxides-decorated diatomite combined with a subsequent replacement process by a facile and effective hydrothermal approach. These ferric oxides/hydroxides own finely controlled morphologies and nanosheet structures. Diatomite@FeOOH material exhibits promising electrochemical properties, which is superior to the other ferric oxide materials. The specific capacitance of diatomite@FeOOH is 157.9 F g −1 at a current density of 0.5 A g −1 , and its cycle performance is good (98.95% retention after 1000 cycles). Actually, the hierarchical and porous diatomite@FeOOH could be a promising active material for supercapacitors. Furthermore, such synthesizing strategy can be extended to the preparation of other metallic oxide-derived functional materials towards energy storage and conversion.

Additional File
Additional file 1: Experimental section. Figure S1. The SEM images of the samples in low magnification. Figure S2. (a) CV and (b) CC curves of the diatomite@MnO 2 in 1 M Na 2 SO 4 ; (c) CV and (d) CC curves of three kinds of ferric oxides-decorated diatomite. Figure S3.