Polyaniline based nanofibers for energy storage and conversion devices

Silas K. Simotwo

doi:10.17918/etd-7789

Electrochemical energy devices (EEDs) such as batteries, supercapacitors and fuel cells are promising energy alternative. However, commercialization of these devices is hampered by unsatisfactory performance and high production cost. Performance of the ES&C systems is typically quantified by usable energy density, power delivery and durability. The development of higher energy and power density EEDs, in turn, significantly depends upon the advancement in technology of materials used in these devices. Organic materials such as conducting polymers are excellent candidates for application in EEDs because of their natural abundance and benign environmental effect. (Ref. 1) My dissertation focuses on preparation of polyaniline (conducting polymer) based nanofibers through electrospinning method, and establishing understanding of the property-structure-performance correlation of such nanofibers towards developing high performance electrochemical devices. Polyaniline is attractive for application in electrochemical devices due to its excellent electrical conductivity, high theoretical capacitance (750 F g^-1) (Ref. 2) and tunable synthesis. The electrospinning is a simple and versatile fiber formation technique using a strong electric field to extrude polymer solution, forming ultrathin fibers with diameters in the range of 50-800 nm. (Ref. 3) Supercapacitors are generally characterized by fast charge/discharge kinetics, high power density and long life-cycle performance. (Ref. 4) However, large scale application of supercapacitors is limited by their low energy density (typically <10 W h kg^-1 vs > 100 W h kg^-1 for batteries). In this work I have sought to improve the energy density by: (i) developing self-standing, 3-D polyaniline/polyaniline-carbon nanofiber based electrodes and (ii) investigating electroactivity of the polyaniline electrodes in electrolyte with wider electrochemical voltage window. Electrochemical performance of freestanding high-purity electrospun polyaniline nanofibers was investigated in 0.5 M H₂SO4. The nanofibers showed competitive electrochemical performance, including high gravimetric capacitance (385 F g^-1) and good life cycle performance (~81% capacitance retention over 1000 cycles). The capacitance obtained represents ~40% increase relative to previously reported values for similarly prepared PANI electrodes. (Ref. 5) The promising electrochemical performance stems from its porous three-dimensional nonwoven nanofiber mat morphology. The inter-/intra-fiber porosity and the interconnected nanofiber network facilitates shuttling of charges to the electrode/electrolyte interface. The standard electrode assembly process-utilization of slurries-in most literature is likely to result in a lack of such an open, through-connected pore structure. The voltage window of the acid aqueous electrolytes is thermodynamically limited to 1.0V due to water decomposition. To expand the voltage window, electrochemical performance of polyaniline in neutral aqueous electrolyte (Li2SO4) and protic ionic liquid was investigated. The neutral electrolyte is electrochemically stable than the acid based due to strong solvation energy involved for electrolyte species. We developed an asymmetric supercapacitor (ASC) utilizing Li2SO4 based electrolyte, PANI-carbon cathode and activated carbon nanofibers as the anode. Due to the wide voltage window (1.8V) achieved in such system, a specific energy of 24 W h kg^-1 was obtained with capacitance retention of ~75% after 4500 cycles. In Situ FTIR spectroelectrochemical study confirmed that polyaniline was undergoing redox process in Li2SO4 similar to that observed in acidic media. Redox activity of polyaniline was also investigated in protic ionic liquid with stable voltage window of ~2.7 V using both electrochemical and ex-situ FTIR spectroscopic techniques. Cyclic voltammetry plots (3-electrode set-up) showed that polyaniline was undergoing electrochemical mechanisms similar to those observed prior in acidic media. The promising electrochemical activity of polyaniline in neutral aqueous and protic liquids is crucial for development of advanced polyaniline based supercapacitors with high energy density.

Polyaniline based nanofibers for energy storage and conversion devices

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Polyaniline based nanofibers for energy storage and conversion devices

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