Nanoparticulate magnetite thin films as electrode materials for the fabrication of electrochemical capacitors
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- Pang, S.C., Khoh, W.H. & Chin, S.F. J Mater Sci (2010) 45: 5598. doi:10.1007/s10853-010-4622-1
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Magnetite nanoparticles in stable colloidal suspension were prepared by the co-precipitation method. Nanoparticulate magnetite thin films on supporting stainless steel plates were prepared by drop-coating followed by heat treatment under controlled conditions. The effects of calcination temperature and atmosphere on the microstructure and electrochemical properties of nanoparticulate magnetite thin films were investigated. Nanoparticulate magnetite thin films prepared under optimized conditions exhibited a specific capacitance value of 82 F/g in mild aqueous 1.0 M Na2SO4 solution. Due to their high charge capacity, good cycling reversibility, and stability in a mild aqueous electrolyte, nanoparticulate magnetite thin films appear to be promising electrode materials for the fabrication of electrochemical capacitors.
The ever increasing miniaturization of portable electronic and communication devices such as laptop, cellular phones, and personal digital assistants (PDA) have led to increasing demands for energy-storage systems with high power capability and energy density, high reversibility, and long cycle life. Due to the low power capability of batteries and the low energy density of conventional capacitors, these charge storage systems are unable to meet the demands of modern devices, which require both high power capability and high energy density. Electrochemical capacitors which possess high power capability and moderately high energy density are able to fill in the gap between batteries and conventional capacitors. In addition, the energy-storage mechanisms in electrochemical capacitors are simpler and highly reversible which ensure a very long cycle life of exceeding 100,000 cycles [1, 2].
Recently, various types of transition metal oxides such as manganese oxides and ruthenium oxides have been investigated as electrode materials for the fabrication of electrochemical capacitors. Each of these materials possesses its own limitations and advantages [3, 4]. Nanoparticulate magnetite (Fe3O4) thin film is another potential electrode material which has received considerable attention due to its superior electrochemical properties, environmental friendliness, and low cost. Wu et al. reported that magnetite crystallites electrocoagulated on conductive matrix exhibited specific capacitance values that varied over a large range between 30 and 500 F/g in 1.0 M Na2SO3 aqueous electrolytes [5, 6]. However, studies by Wu et al. focused only on the electrochemical properties of magnetite nanocrystallites without considering the effects of surface morphology and microstructure on their electrochemical properties. Elucidating the microstructure–property relationship for the magnetite electrodes is crucial for enhancing their electrochemical properties by optimizing their surface morphological characteristics and microstructural parameters. In this study, we have focused on the characterization of nanoparticulate magnetite thin films prepared under various synthesis and post-synthesis conditions, and elucidating the effects of microstructure on their electrochemical properties. Besides, the potential utility of nanoparticulate magnetite thin films as electrode materials for the fabrication of electrochemical capacitors was evaluated.
Preparation of magnetite colloidal suspension
The magnetite precipitate formed was separated by magnetic settling, and washed three times with deaerated ultrapure water. The purified magnetite precipitate was then redispersed in 100 mL ultrapure water, and the resulting magnetite colloidal suspension was maintained at a pH above 9 for enhanced stability [9, 10].
Preparation of magnetite thin films and xerogels
Nanoparticulate magnetite thin films were prepared on precleaned stainless steel plates by the drop-coating technique. The relative film thickness was controlled by the magnetite loadings (30 μmol or 120 μmol) which was associated with a fixed volume of magnetite colloidal suspension (0.1–0.4 mL) that covered a known area (1.5 cm × 1.5 cm) of the stainless steel plate. A small amount of methylcellulose (~7.0 × 10−2 wt%) was being added to increase the viscosity of the colloidal suspension in order to improve adherence and prevent cracking of magnetite films formed on the stainless steel plates. The magnetite loading of thin films prepared was determined quantitatively by atomic absorption spectroscopy (AAS).
Xerogels of magnetite were prepared by evaporating a given volume of magnetite colloidal suspension in a desiccator filled with anhydrous silica gels. Magnetite xerogels formed were subsequently calcined in a tube furnace at temperatures up to 500 °C in nitrogen atmosphere for 1 h.
Characterization of magnetite thin films and xerogels
The surface morphology of magnetite thin films was observed using a field emission scanning electron microscope (FESEM, LEO 1525). The specific surface area and pore size distribution of magnetite xerogels calcined at various temperatures were determined by the BET nitrogen adsorption/desorption method (Micromeritics ASAP 2010). The electrochemical properties of magnetite thin films were characterized by cyclic voltammetry (CV) using a standard three-electrode cell configuration. A saturated calomel electrode (SCE) fitted with a Vycor bridge, and a platinum foil (~2 cm2) were used as the reference electrode and the counter electrode, respectively. In all cyclic voltammetric experiments, a geometric area of 0.1257 cm2 of each thin film sample was being exposed to 1.0 M Na2SO4 aqueous electrolyte. Cyclic voltammograms (CV) were generated by scanning within the potential range of 0.0–1.0 V (vs. SCE) at a scan rate of 50 mV/s.
Results and discussion
Theoretically, the charge capacities of nanoparticulate magnetite thin films should correlate positively to their specific surface areas since high surface areas should enhance accessibility of electrolyte ions to active sites of electrode materials. However, the charge capacity and capacitive behavior of magnetite thin films were greatly influenced by their microstructural parameters such as mean pore size and pore size distribution. Since the size of hydrated ions in an aqueous electrolyte is within the range of 6–7.6 Å, the minimum effective pore size should be greater than 15 Å. In general, a pore size within the range of 30–50 Å is required to maximize the capacitance of an electric double-layer capacitor . In this study, substantially higher charge capacity was obtained from magnetite thin film calcined at temperature 300 °C with the highest mean pore radius of 14.5 Å as compared with those of films without heat treatment (6.5 Å), calcined at 100 °C (6.5 Å) or 500 °C (5.7 Å). The higher charge capacity of films calcined at 300 °C could be due to their considerably larger mean pore radius, which in turn, enabled electrolyte ions to penetrate deeply into active sites within the bulk of oxide matrices, and hence led to enhanced utilization of active materials.
Wu et al. reported that the specific capacitances of mixed electrode containing magnetite:carbon black in the ratio of 9:1 were 27.0, 5.3, and 7.6 F/g-Fe3O4 in 1 M aqueous electrolytes of Na2SO3 and Na2SO4, and in saturated Na3PO4, respectively, over the scan potential window of −1.2 V to +1.0 V vs. Ag/AgCl. However, the charge capacities of magnetite electrodes in Na2SO3 aqueous electrolyte were greatly affected by the dispersed magnetite crystallites on the conductive matrix, with their specific capacitances varied substantially between ~30 F/g and 510 F/g. Such different capacitive behaviors were attributed to the limited conductivity of pure magnetite which could be enhanced by adding optimum amount of conductive additive such as carbon black to form composite or mixed electrode. Indeed, pure magnetite electrode was reported to exhibit a specific capacitance of less than 0.1 F/g-Fe3O4 in 1 M Na2SO3, but it increased to about 30 F/g-Fe3O4 with an addition of 10 wt% of carbon black. The specific capacitance of coprecipitated electrode of 3 wt% of magnetite loading was reported to be as high as ~510 F/g-Fe3O4 in 1 M Na2SO3 aqueous electrolyte [5, 6].
In contrast to the preceding findings by Wu et al. and Wang et al., the specific capacitances of nanoparticulate magnetite thin films (~100% pure magnetite) prepared under various synthesis conditions in this study were observed to vary between 35 and 82 F/g-Fe3O4 in 1.0 M sodium sulfate aqueous electrolyte. The highest specific capacitance of 82 F/g-Fe3O4 in 1.0 M sodium sulfate was obtained for optimized magnetite thin film of magnetite loading of 0.4 μmol/cm2 and heat treated at 300 °C in nitrogen. These specific capacitance values of pure nanoparticulate magnetite thin films in sodium sulfate aqueous electrolyte were substantially higher than those of pure magnetite and magnetite/carbon black (9:1) composite electrodes in various aqueous electrolytes including sodium sulfite as previously reported. These observed different capacitive behaviors could be mainly attributed to microstructural effects (as shown in Figs. 2, 3) associated with the nanoparticulate and highly porous nature of magnetite thin films prepared in the present study.
Our study has demonstrated that the calcination temperature could have substantial effect on the overall microstructure and charge capacities of nanoparticulate magnetite thin films. Enhanced electrochemical properties were observed for magnetite thin films that had been calcined at 300 °C in nitrogen, which could be attributed to optimized microstructure of films, and consequently, enhanced accessibility of electrolyte ions to electroactive sites within the oxide matrices. Given their ease of preparation, low cost, and low toxicity, nanoparticulate magnetite thin films appeared to be a promising electrode material for the fabrication of electrochemical capacitors. Further optimization and enhancement of electrochemical properties is envisaged through better microstructural control of these nanoparticulate magnetite thin films.
This work was funded by Malaysian Ministry of Science Technology and Innovation (MOSTI) through the IRPA Grant No. 03-02-09-1019 EA001.