(Invited) Electropolymerized Polyaniline/Manganese Iron Oxide Hybrids with Enhanced Electrochemical Energy Storage and Color Switching Response

Abstract

Polyaniline (PANI) nanocomposites embedded with manganese iron oxide (MnFe2O4) nanoparticles were prepared as thin films by electropolymerizing aniline monomers onto indium tin oxide (ITO) glass slides pre- spin-coated with MnFe2O4 nanoparticles. The UV-visible absorption spectra, FT-IR and SEM results confirmed the formation of the composite films and the chemical interaction between the PANI matrix and MnFe2O4 particles. A coloration efficiency of 206.2 cm2 C-1 was obtained for the PANI/MnFe2O4 nanocomposite film, higher than that of the pristine PANI film, 104.2 cm2 C-1, suggesting a synergistic effect between the MnFe2O4 particles and the PANI matrix. An enhanced areal capacitance as 4.46 mF cm-2 was also achieved in the PANI/MnFe2O4 nanocomposite film compared with that of 3.95 mF cm-2 in the pristine PANI film from the CV at a scan rate of 5 mV s-1. The enhanced capacitance of the composite films are attributed to the pseudocapacitive property of MnFe2O4 and the rougher morphology caused by the embedment of MnFe2O4 particles into the PANI matrix. Finally, the positive roles of decreasing H2SO4 concentration and increasing temperature during a low temperature range were also demonstrated, however relative higher temperatures can destroy the PANI structure and cause the degradation of PANI. Experimental 1.0 mg Mn2FeO4 was dissolved in 10.0 mL ethanol solution under sonication. The MnFe2O4 film was prepared by drop casting about 1.0 mL MnFe2O4 suspension onto the ITO glass and maintained at 2000 rpm for 20 s. The film was dried naturally overnight. The electropolymerization of aniline onto the as-treated ITO glass or formed MnFe2O4 film was performed on an electrochemical working station VersaSTAT 4 potentiostat (Princeton Applied Research). A typical three electrode electrochemical cell was employed, in which a saturated calomel electrode (SCE) served as the reference electrode, a platinum (Pt) wire served as the counter electrode and the MnFe2O4 coated ITO glass or bare ITO glass slide with an effective area of 4.0 cm2 served as the working electrode. A long path length home-made spectroelectrochemical cell was used for optical characterizations. A typical electrochemical polymerization was performed 10 cycles scanned back and forth from 0 to +1.2 V vs. SCE at a scan rate of 50 mV/s in 0.5 M H2SO4aqueous solution containing 0.1 M aniline. Results and Discussion A fewer amount of PANI was deposited on the MnFe2O4 layer due to the increased resistance caused by the introduced MnFe2O4. The relationship between MnFe2O4 and PANI is probably caused by the π-π stacking, electrostatic interactions as well as hydrogen bonding between MnFe2O4 and the -NH group in PANI. The higher coloration efficiency and stable chronocoulometry of MnFe2O4/PANI nanocomposite further confirm the positive role of MnFe2O4 layer. Enhanced capacitance of MnFe2O4/PANI nanocomposite is probably due to the supercapacitive role of MnFe2O4 and resulted rougher morphology. Enhanced capacitances were obtained when increasing temperature in low temperature range and decreasing H2SO4concentration. Conclusion A PANI matrix embedded with MnFe2O4 particles nanocomposite film was successfully prepared by an electrodeposition of PANI monomer onto a MnFe2O4 coated ITO glass. Multi-color electrochromic phenomenon, higher coloration efficiency and faster switching response were obtained due to the major PANI and the inner interactions between the PANI matrix and the MnFe2O4 particles as well as the resulted rougher morphology. The PANI/MnFe2O4 nanocomposite film also exhibits an enhanced areal capacitance compared to that of the pristine PANI film at low scan rates due to the capacitive role of MnFe2O4. A negative role of increasing the H2SO4 concentration and a positive role of increasing temperature on the supercapacitive behaviors of both the pristine PANI and PANI/MnFe2O4 composites films was also demonstrated. Acknowledgments The financial supports from University of Tennessee Knoxville are kindly acknowledged. Figure 1. CV curves of pureMnFe2O4 film, pristine PANI film and PANI/MnFe2O4 nanocomposite film onto ITO glass in 1.0 M H2SO4 at a scan rate of 5 mV/s. Background is the TEM image of PANI/MnFe2O4nanocomposite accompanied with the color changing phenomenon.