Significantly enhanced energy density of magnetite/polypyrrole nanocomposite capacitors at high rates by low magnetic fields
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One great challenge existing in electrochemical capacitors (ECs) is to achieve high energy densities at high rates. Currently, most research efforts are focused on development of new electrode materials or modification of the microstructure of traditional electrode materials. Herein, we propose a new strategy for significant enhancement of the energy density of ECs at high rates, in which an external magnetic field is exerted. Results indicate that exertion of a magnetic field can increase the energy density of nanocomposite capacitors significantly. In particular, the energy densities of the magnetite/polypyrrole nanocomposite capacitors containing different content magnetite nanoparticles achieve an increase of more than 10 times at a high current density of 10 A/g, compared to the counterparts without a magnetic field. The possible mechanism is that the magnetic field induces electrolyte ion movement enhancement and charge transfer resistance reduction, which remarkably cause the increase of capacitance and energy density. This work provides an innovative strategy to significantly enhance the rate capabilities of current ECs by a simple physical process rather than chemical process.
KeywordsElectrochemical capacitors Energy density High rates Magnetic field
Sustainable energy reservoirs such as wind, solar` and tides are in urgent demand due to the increasing energy crisis arising from the limited reserves of fossil fuels [1, 2, 3, 4, 5]. Electrochemical energy storage (EES) systems including batteries, electrochemical capacitors (ECs), and fuel cells are critically needed to store these transient and unstable energies and release them in a stable manner [6, 7, 8, 9, 10]. As a key member of EES, ECs have attracted tremendous attention due to their advantages of high power density, exceptional long cycling life, and reliability [11, 12, 13, 14].
The electrode materials, which is a key component of ECs , generally fall into three categories: (1) porous carbons with high specific surface area (e.g., metal–organic frameworks-derived nanoporous carbon [16, 17], carbide-derived carbon , carbon nanotubes , graphene ), (2) intrinsically conductive polymers (ICPs, e.g., polyaniline [21, 22, 23], polypyrrole [24, 25], poly(DNTD) [26, 27]), and (3) transition metal oxides/hydroxides (e.g., RuO2 , MnO2 , MoS2 , Ni(OH)2 , Co(OH)2 ). Generally, porous carbons serve as electrode materials of electric double-layer capacitors (EDLCs) to store energy via an electrostatic charge accumulation , while the ICPs and transition metal oxides/hydroxides serve as electrode materials of pseudocapacitors to store energy via redox reactions . Actually, ICPs and their composites have received ever increasing attention because of their low cost, environmental friendliness, good redox reversibility, and high pseudocapacitance values [35, 36].
One great challenge that ECs faced with is their low energy density compared to batteries (e.g., less than 10 Wh/kg for commercialized ECs vs. 180 Wh/kg for lithium-ion batteries (LIB)), especially at high rates [37, 38]. This challenge originates from the slow transportation of counter ions, which cannot rapidly compensate those consumed in the oxidation-reduction process at higher scan rate or current densities. In addition, the utilization efficiency of electrode materials will be severely compromised at higher rates, and only relatively large pores can be entered by counter ions . To solve the issue, great efforts have been dedicated to fabricating nanostructured electrode materials with larger specific surface area  or shorter diffusion path . Unfortunately, there exist still some problems, like the increased undesirable reactions at the electrode/electrolyte interface due to the high specific surface area, and reduced electrical conductivities compared to those of their bulk counterparts due to the shortened electron mean free path arising predominantly from the surface scattering in the nanomaterials . More recently, Juliette  et al. found that using low external magnetic field can produce high-rate LIB graphite anodes with an out-of-plane aligned architecture. The magnetically responsive graphite flakes coated by Fe3O4 could align perpendicularly to the current collector paths by the magnetic field in the electrode fabrication, and therefore reduce the diffusion path and expose preferential insertion/extraction sites for Li+. Motivated by the observation above, herein, we propose a new magnetic field exertion strategy to enhance the energy density of nanocomposite ECs at high rates. In particular, by exerting a low magnetic field on the capacitors, the energy densities of magnetite/polypyrrole (Fe3O4/PPy) nanocomposite capacitors containing 10.0, 20.0, and 40.0 wt% Fe3O4 nanoparticles gain an increase of more than 10 times at a high current density of 10 A/g, compared to the counterparts without a magnetic field.
2.1 Fabrication of Fe3O4/PPy nanocomposites
Fe3O4/PPy nanocomposites were synthesized using a facile surface initiated polymerization method as reported before . Briefly, magnetite nanoparticles with a size of 12 nm (Nanjing Emperor Nano Material Co., Ltd., 99.5%) were dispersed in 100-mL deionized water containing 15 mmol p-toluene sulfonic acid (C7H8O3S, Sigma-Aldrich, ≥ 98.5%) and 9 mmol ammonium persulfate ((NH4)2S2O8, Sigma-Aldrich, 98%) under sonication and mechanical stirring for 1 h in an ice-water bath. Then, 18 mmol pyrrole monomers (C4H5N, Sigma-Aldrich, ≥ 98%) in 25 mL deionized water were mixed with the above magnetite nanoparticle suspension, and the mixture was mechanically and ultrasonically stirred for another one and a half hours for a complete polymerization of the monomers. The product was filtered and rinsed with around 250 mL deionized water. The precipitant was further washed with 1.0 M p-toluene sulfonic acid. The final nanocomposites were dried at 60 °C overnight in a traditional oven. The pure PPy and its magnetite nanocomposites with an initial loading of 10.0, 20.0, 40.0 wt% magnetite nanoparticles were synthesized and denoted as PPy, M-10.0, M-20.0, and M-40.0, respectively.
2.2 Characterizations of Fe3O4/PPy nanocomposites
Bruker Inc. Vector 22 coupled with an ATR accessory was used to obtain the Fourier transform infrared (FT-IR) spectra of the Fe3O4/PPy nanocomposites over the range of 500 to 2200 cm−1 at a resolution of 4 cm−1. Thermogravimetric analysis (TGA) was operated on a TGA Q-500 and heated from 25 to 700 °C at a heating rate of 10 °C min−1 under an air-flow rate of 60 mL min−1. The scanning electron microscope (SEM) of the samples coated with carbon was performed on a JEOL JSM-6510LV system. The high-resolution transmission electron microscope (HRTEM) was performed on the Hitachi H9000NAR. The electrical resistivity was measured by a standard four-probe method.
2.3 Preparation of Fe3O4/PPy nanocomposite electrodes
About 1-mg Fe3O4/PPy nanocomposites with different loadings of magnetite nanoparticles were weighed using UMX2 ultra-microbalance and pressed uniformly onto a PELCO Tabs™ double-coated carbon conductive tape (6 mm OD), which was adhered to a carbon paper substrate. Each sample was weighed for five times to obtain an average value within a deviation of ± 3%.
2.4 Electrochemical property measurements
3 Results and discussion
3.1 Structure and morphology characterization
3.2 Cyclic voltammetry
3.3 Galvanostatic charge-discharge measurement
3.4 Electrochemical impedance spectroscopy
3.5 Mechanism of magnetic field-enhanced electrochemical performance
Based on the EIS analysis, the mechanism of the magnetic field-enhanced electrochemical performance is proposed and shown in Scheme 1b–d. In the condition without a magnetic field, the charged electrolyte ions moved randomly. The ions moving parallel to the electrical field get accelerated by the electrostatic force, F E , and diffuse faster toward the electrode materials to participate in the redox reactions. For those moving perpendicular to the electrical field, it may take longer for their moving direction to be changed via collisions with other ions or may never reach the electrode material, as illustrated in Scheme 1b. Upon applying a magnetic field, the movement of the former is not affected. However, the dynamic motion of the latter can be altered by the Lorentz force in the magnetic field, and then get accelerated under a combined effect of the electrostatic force and the Lorentz force (magnetohydrodynamics effect, MHD), as shown in Scheme 1c. The faster-moving ions are capable of getting accessible to the electrode surfaces much more efficiently to compensate the ions consumed by the redox reactions in the electrode materials, as shown in Scheme 1d, which leads to an enhanced ion movement and reduces R ct , and therefore gives rise to an increased capacitance, and ultimately larger energy densities (Eq. S7). Since the combination of the electrostatic and the Lorentz forces increases with increasing the current density, it can be inferred that the magnetic field is much more effective in enhancing the energy densities at higher rates, and therefore improving the rate capabilities of ECs.
In summary, we have demonstrated that the magnetic field can significantly enhance the energy densities of magnetite/polypyrrole nanocomposite electrodes at high rates. The combined force under the electric and magnetic fields increased the specific capacitance, and therefore attributed to enlarged energy densities through the manipulation of the dynamic motion of the electrolyte ions and the reduction of the charge transfer resistance. The magnetic field increased the energy density by a factor of 10.5, 11.1, and 19.3, at a high current density of 10 A/g for the magnetite/polypyrrole nanocomposite electrodes at a magnetite particle loading of 10.0, 20.0, and 40.0 wt%, respectively. This simple approach provides an innovative alternative to significantly enhancing the energy density of ECs at high rates without compromising their power densities and is envisioned to influence other storage units such as LIBs and fuel cells as well.
This work is financially supported by National Science Foundation–Nanomanufacturing (Grant Number: CMMI-13–14486); Nanoscale Interdisciplinary Research Team and Materials Processing and Manufacturing (Grant Number: CMMI 10–30755); Tianjin Natural Science Foundation (Grant Number: 1600030041). D. C. is thankful for National Science Fund for Distinguished Young Scholars (No. 21625601).
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