Improved Capacitance of Electropolymerized Aniline Using Magnetic Fields

Abstract

With the rise in intermittent energy production methods and portable electronics, energy storage devices must continue to improve. Supercapacitors are promising energy storage devices that are known for their rapid charging and discharging, but poor energy density. Experimentally, one can improve the energy density by improving the operating cell voltage and/or improving the overall capacitance, which have traditionally been achieved using difficult, complicated, or expensive syntheses involving additional chemicals or many steps. In this work, we demonstrate a method to improve the capacitance of electropolymerized polyaniline (PANI, a conductive polymer common in supercapacitor applications) with zero additional energy input or chemical additives: the use of a permanent magnet. Using a pulsed-potential polymerization method, we show that the inclusion of a 530 mT magnetic field, placed directly under the surface of the working electrode during electropolymerization, can result in a PANI film with a capacitance of 190.6 mF; compare this to the same polymerization performed in the absence of a magnetic field, which has a significantly lower capacitance of 109.7 mF. Electrochemical impedance spectroscopy indicates that PANIs formed in the presence of magnetic fields demonstrate improved capacitor behavior, as well as lower internal resistance, when compared to PANIs formed in the absence of magnetic fields. To probe the performance and stability of PANI films synthesized in the presence and absence of magnetic fields, galvanostatic charge–discharge was completed for symmetric capacitor configurations. Interestingly, the PANI films formed in the presence of 530 mT magnetic fields maintained their capacitance for over 75,000 cycles, whereas the PANI films formed in the absence of magnet fields suffered serious capacitance losses after only 29,000 cycles. Furthermore, it is shown that performing the polymerization in magnetic fields results in a higher-capacitance polymer film than what is achieved using other methods of forced convection (i.e., mechanical stirring) and outperforms the expected capacitance (based on yield) by 13%, suggesting an influence beyond the magnetohydrodynamic effect.