DOI:
10.1039/C5RA00447K
(Paper)
RSC Adv., 2015,
5, 39270-39277
Facile solvothermal synthesis of porous ZnFe2O4 microspheres for capacitive pseudocapacitors
Received
9th January 2015
, Accepted 20th April 2015
First published on 20th April 2015
Abstract
A facile and cost-effective solvothermal approach to the fabrication of ZnFe2O4 microspheres composed of nanocrystals has been developed. The morphology and structure of the products were characterized by X-ray powder diffraction, transmission electron microscopy, and field-emission scanning electronic microscopy, and N2-adsorption–desorption. Meanwhile, the magnetic properties of the product were investigated via vibrating sample magnetism. Finally, the electrochemical performance of the obtained ZnFe2O4 microspheres was measured by cyclic voltammetry and galvanostatic charge–discharge techniques. The results show that such structured ZnFe2O4 has a specific capacitance of 131 F g−1 and stable cycling performance with 92% capacitance retention after 1000 cycles, which make it have a potential application as a supercapacitor electrode material.
1 Introduction
The supercapacitor, which has been considered as one of the most promising energy storage devices in practical use, has advantages of high power density, fast charge–discharge, excellent reversibility, as well as long and stable cycle life over other electrochemical energy storage devices.1–5 Based on the charge storage mechanism, supercapacitors can be broadly divided into two categories, electrochemical double layer capacitors (EDLC) and pseudocapacitors.6 Generally, pseudocapacitors, the specific capacitance of which far exceeds that of EDLCs, entail reversible faradic redox reactions on the surface of an electro-active material for charge storage.7–9 RuO2 is one of typical pseudocapacitive materials due to its high pseudocapacitance (∼700 F g−1), high reliability, and excellent reversibility. Nevertheless, Ru is rare on earth and very expensive, which has restricted the commercial application of RuO2 as an active material in pseudocapacitors.10 Therefore, the development of electrode materials with low cost and high capacitance for pseudocapacitors is required.
Recently, attentions have been paid to other transition metal oxides. Such as Fe2O3,11,12 V2O5,13,14 NiO,15,16 Co3O4,17–19 MnO2,20–22 CuO23,24 have been explored as alternatives of RuO2 in fabricating pseudocapacitors. However, most of these materials often suffer from low capacitance and/or poor cycling stability. The development of nanostructured materials provides a promising solution to enhance the capacitive performance because of their high surface area, short electron and ion transport pathways, but the poor intrinsic conductivity of these oxide materials still limits their performance.25,26 In addition, these single-component oxides often suffer from significant capacitance loss after hundreds of charge–discharge cycles.
Compared to the single-component oxides, spinel ferrites (MFe2O4; M = Ca, Mg, Cu, Ni, Zn, Mn) offer richer redox chemistry and combine the contributions from both Fe and M ions.27,28 It has also been demonstrated that binary oxides often exhibit better electrical conductivity and higher electrochemical activity compared to single oxides. There have been several reports on the synthesis and electrochemical evaluation of nanostructured metal ferrite materials. For example, Zhao and coworkers have developed a simple electrospinning followed by direct annealing method for the synthesis of CuFe2O4 hollow nanofibers with pseudocapacitive capacitance.29 Anwar has demonstrated that the EDLC behavior of NiFe2O4 ceramic powders depends on the morphology which can be controlled by the synthesis method adopted.30 Kuo and co-workers compared the capacitance behavior of several metal (including Mn, Fe, Co, and Ni) ferrites. They found that MnFe2O4 exhibits unusually larger capacitances (>100 F g−1) than the others.31 Most recently, our group has synthesized hierarchically porous CuFe2O4 nanospheres through polymer/surfactant-assisted solvothermal method, exhibiting not only high capacitance but also excellent cycling stability.32
In this article, we proposed a facile solvothermal strategy to directly fabricate ZnFe2O4 microspheres consisting of primary nanocrystals. Besides characterizing the product using a series of techniques, the effect of the reaction time on the product morphology was also investigated systematically to reveal the plausible formation mechanism of ZnFe2O4 microspheres. Finally, the pseudocapacitive property of the as-prepared ZnFe2O4 microspheres was presented and discussed in detail.
2 Experimental section
2.1 Materials
FeCl3·6H2O, ZnCl2, were purchased from Aladdin Reagent Com. (Shanghai, China). Other chemicals were of analytical grade and provided by Sonopharm Chemical Reagent Company (Shanghai, China). All reagents were used as received without further purification. Double distilled water was employed throughout the experiment.
2.2 Synthesis of ZnFe2O4 microspheres
ZnFe2O4 microspheres were synthesized via a modified solvothermal route. In a typical procedure, 2 mmol ZnCl2, 4 mmol FeCl3·6H2O, and 6 mmol NaAc were dissolved into 30 mL of ethylene glycol by stirring. The mixture was sealed in a Teflon-lined autoclave and maintained at 200 °C for 24 h and then taken out for cooling. After it was cooled down to room temperature, the precipitates were collected using a magnet and rinsed with ethanol and distilled water several times to remove organic residuals and unreacted inorganic moiety. At last, the products were dried under vacuum at 70 °C for 12 h.
2.3 Characterization
X-ray powder diffraction patterns of the samples were conducted on a Bruker D8 diffractometer in reflection mode (Cu Kα radiation). Field emission scanning electron microscopy (FESEM) was performed on Hitachi S4800 electron microscopy. Transmission electron microscopy (TEM) images of the products were taken on a Tecnai-2 electron microscopy. N2 adsorption–desorption was tested on Micromeritics ASAP2020. Magnetic properties of the products were investigated using a vibrating sample magnetometer (VSM, EV7, ADE, USA) with an applied field between −8000 and 8000 Oe at room temperature.
2.4 Electrochemical performance analyses
Before electrochemical measurement, the working electrode should be prepared as following procedure: the as-prepared ZnFe2O4 microspheres, acetylene black, and polyvinylidene difluoride (PVDF) with a weight of ∼20 mg were mixed in a mass ratio of 70
:
15
:
15 and then dispersed in ethanol to form homogeneous slurry. The resulting slurry was coated onto the nickel foam (1 cm × 1 cm) and dried at 80 °C under vacuum for 12 h to fabricate the working electrode. The electrochemical performance analyses were performed on the CHI760e electrochemical working station. All measurements were carried out using a three-electrode system with a working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum wire counter electrode. The electrolyte was a 1 M KOH aqueous solution. CV measurements were performed at various scan rates from 10–100 mV s−1. Galvanostatic charge–discharge tests were carried out between 0 and 0.45 V at various current densities.
3 Results and discussion
3.1 Morphology and structure
Both TEM and HRTEM were employed to observe the morphology, size, as well as microstructure of the as-prepared ZnFe2O4 microspheres. It is obvious from Fig. 1A and B that the microspheres are composed of many nanoparticles having multiple contacts with neighboring nanoparticles. In order to confirm the structure of the as-synthesized ZnFe2O4 microspheres, the sample was examined by HRTEM. The clear lattice fringes, as shown in Fig. 1C, indicate that the product exhibits high crystalline. The lattice fringe spacing of 0.298 nm and 0.27 nm, which were corresponded well to the {022} and {311} lattice planes of cubic ZnFe2O4.
 |
| Fig. 1 TEM (A and B) and HRTEM (C) images of the as-prepared ZnFe2O4 microspheres. | |
Fig. 2 shows the surface morphology of the product investigated by FESEM. As shown in Fig. 2A, the product has a spherical structure with a diameter of 200 nm. By close observation of Fig. 2B, it can be seen that the spheres are assembled by numerous interconnected nanoparticles. A large number of voids and interspaces are present among these nanoparticles. Additionally, energy dispersive X-ray spectroscopy (EDS) results from a single microsphere showed that it was mainly composed of Zn, Fe, and O (Fig. 2C). The ratio of Zn, Fe, and O from EDS spectrum was about 1
:
2
:
4, suggesting that the products had a chemical formula of ZnFe2O4.
 |
| Fig. 2 FESEM images (A and B) and EDS spectrum (C) of the obtained ZnFe2O4 microspheres. | |
The phase composition of the as-synthesized ZnFe2O4 microspheres was determined by XRD. The XRD diffraction pattern of the as-prepared ZnFe2O4 microspheres was displayed in Fig. 3. It can be clearly seen that almost all the diffraction peaks match well with the standard pattern of the spinel-type of ZnFe2O4 (JCPDS 89-4926), which is identified as face-centered-cubic (FCC) franklinite ZnFe2O4.33 Seven characteristic peaks at 2θ values of 30.06°, 35.42°, 37.04°, 43.06°, 53.44°, 56.96°, and 62.56° were observed. These peaks can be ascribed to (220), (311), (222), (400), (422), (511), and (440) crystal planes of spinel ZnFe2O4, respectively. Meanwhile, the XRD peaks were very intense and sharp, suggesting that the ZnFe2O4 sample was well crystalline.
 |
| Fig. 3 XRD diffraction pattern of the as-prepared ZnFe2O4 microspheres. | |
It is well known that the magnetic properties of metal ferrite nanostructures have received growing attention owing to their potentially application in magnetic resonance imaging (MRI), magnetically induced hyperthermia, separation and purification, and drug delivery.34–36 Thus, the magnetic behavior of the as-synthesized hollow ZnFe2O4 microspheres was further investigated here. Fig. 4 shows magnetization curves of ZnFe2O4 at the full scale measured at room temperature at a magnetic field of H = 8000 Oe. As depicted in Fig. 4, the values of saturation magnetization (Ms) of the ZnFe2O4 microspheres were measured to be 37.6 emu g−1, and the coercivity values Hc is too small to be read. The inset of Fig. 4 presented very small hysteresis loops in the enlarged manner, indicating the ferromagnetic behavior for these samples.
 |
| Fig. 4 Magnetization hysteresis of ZnFe2O4 samples and the partial magnified curve (inset). | |
The as-prepared ZnFe2O4 product exhibits porous nature, which has been examined by N2 adsorption–desorption technique. Fig. 5A shows the BET isotherm of the ZnFe2O4 sample. According to the IUPAC classification, the isotherm belongs to type IV with hysteresis loop. This type of hysteresis loop is generally related to textural slit-like pores, but recently also observed extensive voids or hollow particles with mesoporous walls. As a result, the hysteresis loop in our case can be attributed to the hollow chambers and mesoporous walls assembled by the nanoparticles. The BET surface area of the product can be calculated to be 37.6 m2 g−1. The pore size distribution was calculated from Barrett–Joyner–Halenda (BJH) model, which is shown in Fig. 5B. It is observed that the maxima peak centered at 10 nm. The average pore diameter was 5 nm. These results confirm the highly porous nature of ZnFe2O4 sample. Thus, it is concluded that the porous nature is the cause for enhancing ion diffusion and the easy access of electrolyte ions into the electroactive surface.
 |
| Fig. 5 N2 adsorption–desorption isotherm (A) and the pore size distribution curve (B) of the as-prepared ZnFe2O4 microspheres. | |
3.2 Formation mechanism
The evolution of the ZnFe2O4 microspheres prepared at different hydrothermal time is clearly shown with TEM images in Fig. 6. As can be seen from the pictures, there has been a strong correlation between the time and the resulting product. For the material obtained at the initial stage (the first 6 h), there have been numerous agglomerated nanowhiskers of ZnFe2O4, as shown in Fig. 6A. As the reaction time processes, the nanowhiskers changed into particle-like morphology on hydrothermal treatment for 12 h, as displayed in Fig. 6B. Fig. 6C and D shows the FESEM images of the products obtained by further increasing the hydrothermal reaction time to 18 h and 24 h, respectively. It can be seen that large amount of ZnFe2O4 microspheres were formed and each ZnFe2O4 microsphere was composed of many ZnFe2O4 nanocrystals.
 |
| Fig. 6 TEM images of the products prepared at different hydrothermal times: 6 h (A), 12 h (B), 18 h (C) and 24 h (D). | |
Besides TEM images, XRD was also applied to provide evidence for the formation mechanism of ZnFe2O4 microspheres. As displayed in Fig. 7a and b, no shark peaks were observed for the products obtained at early stage (2 h and 4 h). The inset of the Fig. 7 shows the enlarged curves of (a) and (b). Only several broad peaks were checked from products obtained at 4 h. These wide peaks can be attributed to amorphous precursors, which implied that no crystalline ZnFe2O4 phase was fabricated. When the reaction time was prolonged to 6 h, Fig. 7c shows that the product is mainly composed of ZnFe2O4 phase, since some sharp peaks were observed and all these peaks were in agreement with the spinel-type of ZnFe2O4. As the reaction processed (12, 18, and 24 h), no other phases were fabricated (Fig. 7d–f). It should be noted that only the particle-shaped ZnFe2O4 was obtained if the reaction time is less than 12 h, while ZnFe2O4 microspheres assembled by numerous nanocrystals were obtained if the reaction time is prolonged to 18 h or 24 h.
 |
| Fig. 7 XRD patterns of the products collected at different reaction time. (a) 2, (b) 4, (c) 6, (d) 12, (e) 18, (f) 24 h. The inset is the magnified curves of a and b. | |
As discussed above, the formation mechanism of the ZnFe2O4 microspheres can be schematic illustrated as Fig. 8. Firstly, the solid precursor FeCl3·6H2O dissolved in ethylene glycol, and the hydrolysis of Fe3+ and Zn2+ occurs, under hydrothermal condition in the presence of NaAc, to form ZnFe2O4 whiskers. Secondly, due to the Ostwald ripening, the ZnFe2O4 whiskers were transformed into nanocrystals under high temperature. Finally, with the elongation of synthesis time, the ZnFe2O4 nanocrystals grow larger and assembled to porous nanospheres by self-assemble. As for the driving force for oriented aggregation of nanocrystals, it can be generally attributed to the fact that the ZnFe2O4 nanocrystals with small sizes are tend to aggregate through both the attachment among the primary nanoparticles as well as the rotation of these primary nanoparticles to reduce the high surface energy. The similar phenomena were also observed by others in other systems.37,38
 |
| Fig. 8 Schematic illustration for the formation mechanism of ZnFe2O4 microspheres. | |
3.3 Electrochemical performance
3D hierarchically porous nanostructures are promising candidates for a series of applications such as adsorbent, catalyst, chemical sensor, lithium ion battery, and pseudocapacitor, due to the void space as well as large specific area endowed by their unique structures, which are much favored for the active specie diffusion and mass transport in materials during reaction process, thus leading to the enhanced performances.39–41 The electrochemical performance of ZnFe2O4 microspheres is evaluated as electrodes for pseudocapacitors. Fig. 9A shows the CV curves of an electrode manufactured of the as-synthesized ZnFe2O4 microspheres measured at various scan rates. The curves reveal a pair of redox couples which suggest that the measured capacitance mainly results from a pseudocapacitive capacitance caused by reversible electrochemical reactions related to Zn(or Fe)–O/Zn(or Fe)–O–OH, unlike that of the EDLC which has a CV curve of a rectangular figure. As the scanning rate increases, the oxidation and reduction peaks shifts to higher and lower potentials, respectively. Furthermore, the oxidation peak becomes less obvious while the reduction peak becomes more obvious. In addition, the reduction peak current value show linear relationship to the square root of the scanning rate (Fig. 9B), revealing that the electrochemical reaction occurring on the electrode surface is a diffusion-controlled process.
 |
| Fig. 9 (A) CV curves of ZnFe2O4-based electrodes in 1 M KOH electrolyte at various scanning rate ranging from 10 to 100 mV s−1. (B) The relationship curve of the reduction peak current value versus the square root of scanning rate. | |
The typical discharge curves of the as-prepared porous ZnFe2O4 microspheres at different current densities were shown in Fig. 10A. It can be seen that all curves are nonlinear regardless of the applied current density, indicating the pseudocapacitance behavior of the ZnFe2O4 sample is influenced by the electrochemical adsorption–desorption or quasi-reversible redox reactions at the electrode–electrolyte interface.42,43 The specific capacitance values of ZnFe2O4 sample have been calculated using the following equation:
where
i (A) is the applied charge or discharge current,
m (g) is the mass of the active electrode material, and Δ
t (s) and Δ
V (V) is discharge time and the potential drop. The specific capacitance values of the ZnFe
2O
4 sample at different current densities were calculated as shown in
Fig. 10B. It is observed that the decrease in specific capacitance values with increasing current density (in the range of 0.1–1.0 A g
−1) is very slightly. The highest specific capacitance value can be reached at 131 F g
−1 when the current density is 0.1 A g
−1. Even when the current density increases to 1.0 A g
−1, the specific capacitance value still retains 124 F g
−1, which is much larger than that of ZnFe
2O
4 powder (36.7 F g
−1) or ZnFe
2O
4 replica with biological hierarchical structure (92.6 F g
−1).
44 This is an indication of the superior rate behavior of the as-prepared ZnFe
2O
4 microspheres. This is owing to the fine microstructure of the ZnFe
2O
4 microsphere, which is in fact consisting of many primary small ZnFe
2O
4 nanocrystals, resulting in numerous nanochannels. The porous morphology acts as perfect “OH
− ion-buffering reservoir” and facilitates enhanced contact, diffusion as well as penetration of OH
− ions for faster electrode kinetics and maximum reversible redox processes for charge storage.
 |
| Fig. 10 (A) Discharge voltage profiles of ZnFe2O4 microspheres at various current densities ranging from 0.1 to 1.0 A g−1. (B) The specific capacitance as a function of current density. | |
As higher life-cycle stability is significantly crucial for the practical application of an electrode material in electrochemical capacitor, the variation of the specific capacitance of the as-prepared ZnFe2O4 microspheres over 1000 cycles is displayed in Fig. 11. The specific capacitance increases slightly with the cycle number during the first 150 cycles, and thereafter the capacitance starts to decrease. After 1000 cycles, the capacitance maintains about 120 F g−1, which is about 92% of the first cycle. This indicates that the galvanostatic charge–discharge processes do not seem to generate significant microstructure or property changes of the ZnFe2O4 microsphere, as expected for pseudocapacitance reactions. The long-term life cycle stability suggests that the as-prepared ZnFe2O4 is a promising candidate to supercapacitor electrode materials.
 |
| Fig. 11 Cycle life of the ZnFe2O4 electrode at 0.1 A g−1 in 1 M KOH electrolyte. | |
Generally, electrochemical impedance spectroscopy (EIS) is applied to gain insight of the resistive and capacitive elements associated with the electrode. Fig. 12 shows Nyquist plot for ZnFe2O4 microsphere in the frequency range of 0.01 Hz and 1 × 105 Hz. Nyquist plot is a plot of the imaginary component (Z′′) of the impedance against the real component (Z′). The impedance spectra include a sloped line in the low-frequency and one semicircle in the high frequency. The interfacial charge transfer resistance (RCT) and double layer capacitance CDL are parallely connected to represent the semicircle in the high frequency region. RCT is measured by the intercept of the semicircle with the real axis (Z′). The RCT value for ZnFe2O4 was determined to be 66.58 Ω. In addition, the spectra inclined at an angle to the real axis, suggesting that a resistance is associated with CL. This resistance is termed as leakage resistance (RL) and connected parallel to CL. The transition from high frequency semicircle to low-frequency tail is represented by Warburg element.
 |
| Fig. 12 Nyquist plot of ZnFe2O4 electrode. | |
4 Conclusions
In summary, ZnFe2O4 microspheres have been successfully synthesized through a facile and cost-effective solvothermal approach without using any template. The microstructure characterization reveals that each ZnFe2O4 microsphere is assembled by numerous ZnFe2O4 nanoparticles with average size less than 20 nm. Owing to such particular structure, the resultant ZnFe2O4 microsphere exhibits high specific surface. Finally, the as-prepared hollow porous ZnFe2O4 microspheres were employed to be an electrode material for construction of pseudocapacitor. Owing to the combination the high surface accessibility of porous materials and the contributions from both Fe and Zn ions, the as-prepared ZnFe2O4 microsphere demonstrated excellent electrochemical properties in terms of cycling life and specific capacitance. It is expected that this method developed here may provide a novel pathway to develop advanced nanomaterials for applications like supercapacitors, lithium ion batteries, and environmental treatment and so on.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant no. 21403091, 51473070, and 51202091), the Natural Science Foundation of Jiangsu Province, China (Grant no. BK20130486), and a Project Funded by Jiangsu University for Senior Intellectuals (Grant no. 12JDG093).
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