Ke Lua,
Rongyan Jiangb,
Xiang Gaoa and
Houyi Ma*a
aKey Laboratory for Colloid and Interface Chemistry of State Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China. E-mail: hyma@sdu.edu.cn; Fax: +86-531-88564464; Tel: +86-531-88364959
bSchool of Materials Science and Engineering, Shandong Jianzhu University, Jinan 250101, China
First published on 1st October 2014
This work studied the pseudocapacitive behavior of Fe3O4 and examined approaches to improve its electrochemical activity. Our results show that by optimizing the electrode architectures, we were able to construct ternary Fe3O4/CNTs/PANI composites that delivered excellent electroactivity and cyclic stability because of synergistic effects from each component. In this composite, CNTs were uniformly coated on the surface of Fe3O4 particles and were able to form conductive networks that electrically wired each particle, whereas PANI particles were able to bridge Fe3O4/CNTs composites and further ensure the overall mechanical integrity. Overall, with 8 wt% of PANI the electrodes were able to deliver a high specific capacitance of 260 F g−1 at 0.5 A g−1. The electrodes also had excellent cyclic stability and showed only 6% decay capacity after 1000 cycles.
Among all of the pseudocapacitor materials, Fe3O4 is a particularly attractive material because of its considerably lower cost (compared with, for example, Ni, Co and Ru based materials), low toxicity, high theoretical capacity and relatively low operational potential, which make it suitable for use as an anode material.14–16 However, Fe3O4 has low electrical conductivity and this significantly limits electronic/ionic transfer rate and results in poor capacity under realistic conditions.17 Low electric conductivity is a common problem for metal oxides and many approaches have been proposed to resolve this problem. In particular, an effective approach is to construct binary and/or ternary nanostructured electrodes by integrating metal oxide particles with carbon hosts (such as MnO2 with graphene and carbon nanotubes).1,7,18,19 This is because carbon materials can not only provide effective networks for electrical transport by bridging adjacent metal oxide particles, but also can contribute to the overall capacity through the EDLC mechanism.17,20–23 On the other hand, it has also been discovered that the pseudocapacitance of metal oxides can be improved by adding an appropriate amounts of conducting polymers.24–26 In this regard, polyaniline (PANI) is particularly attractive because of its easy preparation, good environmental stability and relatively high electrochemical activity.19,27,28 However, this material is not stable during charge–discharge processes, and therefore, cannot be used as an active material alone. It should be noted that a combination of PANI with various porous carbon materials and/or transition metal oxides has been shown to be effective for improving its stability, as well as its capacity.28 These previous observations and demonstrations suggest that it is possible to maximize the electrochemical performance of electrode materials through rational selection of functional materials and design of appropriate electrode architectures to enable the synergistic effects of different components.29–31
In this work we aim to study the pseudocapacitive behavior of Fe3O4 and develop effective approaches to maximize its capacity for practical applications. Several combinations of materials, including Fe3O4 alone, Fe3O4/CNTs binary composites and Fe3O4/CNTs/PANI ternary composites, were tested in order to identify the most effective electrode architecture. Overall, we found that electrodes based on the ternary composites delivered the highest specific capacitance, better rate performance and longer cycle life that outperformed both the Fe3O4 particles and the Fe3O4/CNTs composites. We believe the improved electrochemical performance was due to the effective conductive path constructed by CNTs, as well as the synergistic effects of each component in the ternary composites. The network formed by CNTs can act as “conduction bridges” and enhances the electrical conductance between PANI and Fe3O4, as well as providing pathways for charge transfer in the electrode.
The electroactivity of different materials was studied using galvanostatic charge–discharge and cyclic voltammetry (CV) techniques. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 0.05 Hz to 100 kHz under an excitation of a sinusoidal wave of 5 mV at open circuit potential. The impedance data were fitted using ZSimpWin (ver. 3.10).
The specific capacitance (SC) (F g−1), specific energy density (SE) (W h kg−1) and maximum power density (Pmax) (W g−1) of the supercapacitors were calculated using the following equations:
![]() | (1) |
![]() | (2) |
![]() | (3) |
The crystalline properties of the as-synthesized Fe3O4, PANI, Fe3O4/CNTs and Fe3O4/CNTs/P8 composites were studied using XRD, and obtained results are shown in Fig. 3a. The XRD pattern of PANI powder has two broad and weak peaks ranging from 20° to 25°, which can be considered as the characteristic peaks of amorphous PANI.34 The XRD patterns of Fe3O4 and Fe3O4/CNTs both exhibit six characteristic peaks at 2θ = 30.3°, 35.7°, 43.3°, 53.4°, 57.4° and 62.5°, which can be indexed to the (220), (311), (400), (422), (511), and (440) planes of face-centered cubic Fe3O4 (JCPDS card no. 19-0629), respectively.33 On the basis of these results, the as-synthesized Fe3O4 and Fe3O4/CNTs composites had good crystal structure. The characteristic peaks of Fe3O4 and PANI can also be observed in the XRD pattern of the ternary Fe3O4/CNTs/P8 composites.
FTIR was used to analyze the surface chemistry of the PANI particles, Fe3O4/CNTs and Fe3O4/CNTs/P8 composites. A typical spectrum of PANI is shown in Fig. 3b, and characteristic peaks located at ∼1582 cm−1 and 1150 cm−1 were observed. These peaks can be attributed to the characteristic vibration responses of the NN and C–N bonds in PANI, respectively.34 The spectrum of Fe3O4/CNTs composites is also shown in this figure and the composite showed peaks located at ∼590 and 450 cm−1, which correspond to the Fe–O stretching resulting from the interaction of Fe–O–Fe in Fe3O4. In addition to these two peaks, a peak at 1600 cm−1 was also observed and this peak can be assigned to the COO– stretching band from surface functionalized CNTs. The IR peaks from PANI and Fe3O4/CNTs composites are also present in the spectrum of the ternary Fe3O4/CNTs/P8 composites (marked with a blue line). The inset image in Fig. 3b shows the response of the synthesized Fe3O4/CNTs composites under an applied magnetic field, showing that when a colloidal dispersion containing the composites was placed near a magnet the composites were quickly concentrated near the magnet, further demonstrating the successful formation of Fe3O4 particles.
The electrochemical properties of bare Fe3O4, Fe3O4/CNTs binary composites and Fe3O4/CNTs/PANI ternary composites were studied in 1 M KOH using CV, galvanostatic charge–discharge and EIS techniques. Fig. 4a shows a set of CVs of electrodes prepared with different materials as noted at 5 mV s−1 using a relatively wide potential window of −1.1 to 0 V (vs. SCE). All of the current densities have been normalized based on the mass of electroactive material. The specific currents for CNTs and PANI were noticeably lower than that of the Fe3O4-based electrodes (Fig. 4a, inset), suggesting that both of them have low capacity alone and the pseudocapacitance from Fe3O4 was able to substantially improve the overall capacity. This also shows that all of the Fe3O4-based electrodes have characteristic pseudocapacitive responses related to surface redox-type reactions with considerably higher specific currents. The pseudocapacitance of Fe3O4 in KOH electrolyte may originate from the redox reactions between Fe2+ and Fe3+ with the intercalation of hydroxide ions shown as follows:36,37
FeIIO + 2OH− ↔ FeII(OH)2 + 2e− |
2FeIIO + 2OH− ↔ (FeIIIO)+(OH−)2(FeOIIIO)+ + 2e− |
Furthermore, the CV of the Fe3O4/CNTs electrode shows considerably higher specific current than the electrode made with pure Fe3O4. Such pronounced enhancement suggests that the binary composites may have more accessible sites and higher utilization of Fe3O4 due to better electrical conductivity with the presence of CNTs.38 The CV of the ternary Fe3O4/CNTs/PANI composites shows a pair of broad and reversible redox peaks from the reactions of PANI and has the highest specific current among all of the materials studied, suggesting that a rational combination of these three materials is very effective for improving electrode performance. Fig. 4b presents a set of CVs for the Fe3O4/CNTs/PANI composites with different amounts of PANI at 20 mV s−1. It is also evident that the capacitive currents do not change monotonously with the increase in PANI content, but strongly depend on the composition, suggesting strong synergistic effects between these three compositions. Overall, it was observed that the electrode assembled with 8 wt% PANI had the highest specific capacitance. The behavior of the ternary composites with 8 wt% of PANI under increased scan rates is shown in Fig. 4c. The overall shape of CV curves did not exhibit obvious distortions when the scan rate was increased from 5 to 50 mV s−1. This indicates that this electrode had good reversibility and very rapid current response to the reversal of voltage at each terminal potential.39 On the basis of these results it is evident that the ternary composite based electrodes delivered more ideal supercapacitive behavior.
Fig. 5a shows a set of typical charge–discharge curves of Fe3O4, Fe3O4/CNTs, and Fe3O4/CNTs/P8 obtained with a potential range of −1.0 to 0 V at 0.5 A g−1. The behavior of pure CNTs and PANI are also included for comparison (Fig. 5a, inset). The charge–discharge characteristics of Fe3O4/CNTs/P8, Fe3O4/CNTs and Fe3O4 were similar, but the time required for fully charging or discharging the ternary composite was noticeably longer. The specific capacitance of different combinations of materials was determined using eqn (1) and the results are summarized in Fig. 5d. In good agreement with the CV tests, it was found that the ternary Fe3O4/CNTs/P8 composites delivered the highest specific capacitance of 260 F g−1, whereas the capacitances of Fe3O4/CNTs and Fe3O4 electrodes were 208 and 128 F g−1, considerably lower, respectively. Furthermore, we also studied ternary composites with different amounts of PANI in order to identify optimal compositions (Fig. 5b); the specific capacitances of Fe3O4/CNTs/P6, Fe3O4/CNTs/P8 and Fe3O4/CNTs/P10 in 1 M KOH at 1 A g−1 are 156 F g−1, 160 F g−1 and 138 F g−1, respectively. The specific capacitance of the ternary composites also strongly depends on the content of PANI (i.e. wt%) and the best performance was obtained with a loading of 8 wt%, which corresponds well with the CV results (Fig. 4b). It is interesting to note that the composite with 10 wt% PANI had an even lower SC than the composite without PANI. This decrease in activity might be because of the aggregation of PANI particles when their content is too high, which affects the overall interconnection between Fe3O4, CNTs and PANI and decreases the synergistic effect.
The rate performance of different electrode materials was also studied. Fig. 5c shows the galvanostatic charge–discharge profiles of the Fe3O4/CNTs/P8 electrode at increasing current densities from 0.5 to 5 A g−1. The specific capacitance was calculated and compared with Fe3O4/CNTs binary composites and pure Fe3O4 in Fig. 5d. The ternary composite had the highest specific capacitance and outperformed both of the other two materials under all of the current densities studied, and was able to deliver a high specific capacitance of 122 F g−1 at 5.0 A g−1. Recently, Fe-based electrode materials have been demonstrated to display excellent capacitive properties as promising high-performance materials in supercapacitors. When compared to the electrochemical performances previously reported for Fe-based materials, the specific capacitances obtained are comparable and directly reflect the effectiveness of the active materials (see Table 1). Nonetheless, we would like to mention that the scope of this work is to identify approaches to improve the performance of iron oxides and to demonstrate synergistic effects; we think our approach for the determination of specific capacitance should be effective.
Materials | SC | Cell type | Electrolyte | Potential window | Processing of electrode | Ref. |
---|---|---|---|---|---|---|
Fe3O4/CNFs | 135 F g−1 at 0.42 A g−1 | 3-ED | 1 M Na2SO4 | −0.9–0.1 V | Mixed with PVDF (5%), and pressed onto Ni foam | 7 |
Fe3O4 nanoparticles | 37.9 F g−1 at 0.5 mA cm−2 | 2-ED | 6 M KOH | 0–1.2 V | Mixed with AB (10%) and PTFE (5%), and pressed onto Ni grid | 14 |
Fe3O4/CNTs | 110.5 F g−1 at 0.5 A g−1 | 3-ED | 1 M KOH | −1.0–0 V | Mixed with AB (10%) and PVDF (10%), and pressed onto Ni foam | 17 |
Graphene/Fe2O3/polyaniline | 638 F g−1 at 1 mV s−1 | 3-ED | 1 M KOH | −1–0.1 V | Mixed with AB (10%) and PTFE (5%), and pressed onto Ni foam | 24 |
Fe3O4/reduced graphene oxide | 160.5 F g−1 at 0.5 A g−1 | 3-ED | 6 M KOH | −1.1–0 V | Mixed with AB (10%) and PTFE (10%), and pressed onto Ni foam | 35 |
Fe3O4 films | 118.2 F g−1 at 6 mA | 3-ED | 1 M Na2SO3 | −1.2–0.2 V | Deposited onto stainless steel foil | 40 |
Fe3O4–GNS | 88 F g−1 at 0.25 A g−1 | 2-ED | 1 M H2SO4 | 0–1 V | Mixed with KB (10%) and TAB (10%), and pressed onto stainless steel mesh | 41 |
a-Fe2O3 nanotubes/reduced graphene oxide | 69 F g−1 at 10 A g−1 | 3-ED | 1 M Na2SO4 | −1–0 V | Mixed with super P carbon (15%) and Kynar 2801 (15%), and pressed onto etched-copper foil | 42 |
Fe3O4@SnO2 core–shell nanorod film | 2.7 mF cm−2 at 1.0 mA cm−2 | 3-ED | 1 M Na2SO3 | −0.7–−0.2 V | Nanorod films were directly used as the working electrode | 43 |
CNT/Fe3O4 | 129.3 F g−1 at 2.5 mA cm−2 | 3-ED | 6 M KOH | −1–0.1 V | Mixed with AB (15%) and PTFE (5%), and pressed onto Ni foam | 44 |
Fe3O4/graphene sheet | 368 F g−1 at 1 A g−1 | 3-ED | 1 M KOH | −1–0 V | Mixed with AB (10%) and PTFE (10%), and pressed onto Ni foam | 45 |
N-doped graphene/Fe3O4 | 212 F g−1 at 1 A g−1 | 3-ED | 1 M H2SO4 | −0.2–0.8 V | Mixed with AB (10%) and PVDF (10%), and pressed onto graphite plate | 46 |
Fe3O4/CNTs/P8 | 122 F g−1 at 5 A g−1 | 3-ED | 1 M KOH | −1–0 V | Mixed with AB (15%) and PVDF (5%), and pressed onto Ni mesh | This work |
EIS is an effective tool to analyze the internal resistance and the electron transfer rate at the electrode–electrolyte interface. Fig. 6 shows typical Nyquist plots for electrodes made with the three materials as noted; all of the plots showed a straight line at low frequencies and a semi-circle at high frequencies. Theoretically, the Nyquist plot for an ideal capacitor connecting its series with a resistor is a straight line parallel to the vertical axis. The straight lines for the spectra shown in Fig. 6; however, all had some degrees of deviation depending on the active materials, suggesting that the electrodes did not have ideal capacitive behavior. The capacitive loop in the high-frequency range is attributed to the relaxation process of the double-layer capacitance (Cdl) and the charge-transfer resistance (Rct) due to Faradaic reactions. Overall, the impedance plots may be analyzed with the equivalent circuit shown in the inset of Fig. 6. In this circuit, Rs represents internal resistance, Q stands for a constant phase element to represent the Cdl, W stands for the Warburg impedance related to the transport process of electrochemical active species and CL is the limit capacitance. The circuit element values were determined by fitting the impedance spectra and were listed in Table 2. The addition of CNTs and PANI is helpful in improving the capacitive behavior of Fe3O4. On the basis of EIS measurements, a calculation of the maximum power densities of Fe3O4/CNTs/P8, Fe3O4/CNTs, and Fe3O4 based on eqn (3) gives 6.2 W g−1, 6.0 W g−1 and 5.3 W g−1, respectively. Thus, the ternary composites exhibited the largest SC values and the improved electrochemical performance of the composite electrode could be because of the good conductivity of the carbon nanotubes and the synergistic effects between each component.
![]() | ||
Fig. 6 Nyquist plots for various electrodes. The inset shows the equivalent circuit used to fit the impedance spectra. |
Rs (Ω cm2) | Rct (Ω cm2) | W (Ω−1 s1/2 cm−2) | Q | CL (F cm−2) | ||
---|---|---|---|---|---|---|
Y (Ω−1 sn cm−2) | n | |||||
Fe3O4 | 0.32 | 0.78 | 0.050 | 0.00030 | 0.78 | 0.0076 |
Fe3O4/CNTs | 0.23 | 0.40 | 0.095 | 0.00015 | 0.89 | 0.0096 |
Fe3O4/CNTs/P8 | 0.25 | 0.42 | 0.098 | 0.000062 | 0.99 | 0.011 |
The cyclic stability of electrodes prepared with Fe3O4/CNTs/P8, Fe3O4/CNTs and Fe3O4 was evaluated by a 1000 cycle galvanostatic charge–discharge test conducted at 5 A g−1. The changes in specific capacitance as a function of cycle number are summarized and compared in Fig. 7a. The electrode with Fe3O4/CNTs/P8 composites had excellent stability and its capacity only decayed by ∼6% after being cycled for 1000 times. In contrast, the capacity decays for the Fe3O4/CNTs and Fe3O4 electrodes were considerably higher and were 9% and 17%, respectively.
On the basis of the results discussed above, it is evident that electrodes based on the ternary Fe3O4/CNTs/P8 composites have the best performance metrics for practical applications. The excellent capacitive behavior of the ternary composites could be attributed to the synergistic effects between each component when constructed properly. Such effects could be better explained by the illustration shown in Fig. 8 in which interactions between each component are shown. In this composite, Fe3O4 is the main electrochemically active material and its redox process provides most of the capacity. CNTs could act as conductive bridges to connect individual Fe3O4 particles and improve the overall electrical conductivity.38,47 PANI particles can alleviate the volume changes of the active materials during redox reactions and prevent the agglomeration/detachment of Fe3O4 particles.24,48 Working together, the synergistic effect between Fe3O4, CNTs and PANI enables the ternary Fe3O4/CNTs/P8 composites with greatly improved electrochemical performance.
This journal is © The Royal Society of Chemistry 2014 |