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

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 pure MnFe₂O₄ 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.

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