Giant Magnetocapacitance in Magnetic Polypyrrole/Magnetite Nanocomposites under Low Magnetic Field

Electrochemical capacitors have attracted significant attention for their promising potential applications ranging from portable electronic devices to hybrid electrical vehicles and large industrial equipments. Most of the research efforts have been confined within the electrode materials to improve the energy density or power density till now. Here we first report a unique giant magnetocapacitance (GMC, huge capacitance change upon applying a magnetic field) phenomenon in a polymer nanocomposite under a small magnetic field of 1000 G (0.1 T). The capacitance has been increased by 714.0% for the polypyrrole (PPy) nanocomposites with a magnetite (Fe₃O₄) nanoparticle loading of 40.0 wt%, and the corresponding energy density and power density have been increased by a factor of 19.3 and 1.5, respectively, at a current density of 10 A/g. Two aspects have been proposed to interpret the observed GMC phenomenon. The first is the magnetohydrodynamics (MHD) describing an enhanced movement of the charged electrolyte ions that renders more ions accessible to the electrode materials under both the electric and magnetic fields. The other is the external magnetic field induced electron transfer in the electrode material, where the electrons in the magnetite nanoparticles were transferred to the PPy matrix instead of taking part in the redox reactions, as confirmed by the cyclic voltammetry analysis. Significantly enhanced ion diffusion coefficient and reduced charge transfer resistance have been obtained from electrochemical impedance spectroscopy measurement study and are inferred to be responsible for the superior performances witnessed in the electrode materials under the magnetic field. This observed unique GMC phenomenon provides an innovative alternative approach to substantially enlarge the electrochemical energy storage capacities of the current supercapacitors by just employing a small external magnetic field on the magnetic nanocomposite electrodes.

Experimental

Magnetite/PPy nanocomposites were synthesized using a facile surface initiated polymerization method as reported before.¹ The pure PPy and magnetite/PPy nanocomposites with an initial loading of 10.0, 20.0, 40.0 and 60.0 wt% magnetite nanoparticles were synthesized and denoted as PPy, M-10.0, M-20.0, M-40.0, respectively. About 1 mg magnetite/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. Electrochemical properties of the nanocomposites were investigated by CV, GCD, and EIS performed on an electrochemical working station VersaSTAT 4 potentiostat (Princeton Applied Research) using a three electrode setup.

Results and Discussion

The charge-discharge curves of the PPy/magnetite nanocomposites at a current density of 10.0 A/g is shown in Figure 1. The nanocomposites with 10.0 wt% magnetite nanoparticles display the longest discharge time, indicating the largest specific capacitance. When further increasing the magnetite loading, a reduced capacitance was observed and is attributed to the impedance of efficient electron transportation within nanocomposites due to the poor conductivity of the magnetite nanoparticles.¹ The magnetic field has a remarkable effect on the nanocomposites. The specific capacitance is calculated to increase by the capacitance is increased by 126.8, 474.9 and 714.0% for the composites with a magnetite nanoparticle loading of 10.0, 20.0 and 40.0 wt%, respectively, at a current density of 10 A/g. The enhanced movement of the charged electrolyte ions under both electric and magnetic field effects,² Figure 2, is inferred to be responsible for the improved performances. The faster-moving ions are capable of getting accessible to the electrode surfaces much more efficiently for compensating the ions consumed by the redox reactions.

Conclusion

PPy/magnetite nanocomposites have been investigated for electrochemical capacitor applications under the magnetic field. Both the pure PPy and the PPy/magnetite composites displayed far superior supercapacitive performances. This study provides an innovative alternative approach to substantially enlarge the electrochemical energy storage capacities of the current supercapacitors by just employing a small external magnetic field on the magnetic nanocomposite electrodes.

Acknowledgments

The financial supports from Seeded Research Enhanced Grant (REG) is kindly acknowledged.

Figure 1 Charge-discharge cures of PPy/Magnetite nanocomposites (A) without and (A') with magnetic field at 10 A/g.

Figure 2 (a) The movement of charged electrolyte ions under normal condition (without the magnetic field), (b) changed ion movement by MHD, and (c) enhanced ion diffusion by MHD.

References

(1) J. Guo, Z. Guo, J. Phys. Chem. C, 117, 10191(2013).

(2) R. Tacken, L. Janssen, J. Appl. Chem. 25, 1 (1995)