Polypyrrole Coated Fabric As Electrode Material For Stretchable Supercapacitors

The incorporation of electronics into wearable items has demonstrated significant advances in terms of miniaturisation, functionality and comfort. These e-textiles have found broad applications in continuous personal health monitoring, high performance sportswear, wearable displays and a new class of portable devices. Being an indispensable part of these applications, lightweight, stretchable and wearable power sources including batteries and supercapacitors are strongly demanded. To have good combination with fabric, and to make people comfortable while wearing them, the energy source should be flexible and stretchable, the ideal wearable power sources would be made into breathable textile formats with stretchability (i.e. mechanical resilience) being conformal to the curved surface and sustain its function during the body movement. As one type of power sources, supercapacitors possess the advantages of higher power densities, excellent reversibility and long cycle life. They are gaining increasing interest in the application of light weight, ultrathin energy management devices for wearable electronics. Recently, there has been an emerging interest in flexible or stretchable supercapacitors. However, most research works are about flexible supercapacitors, and stretchable ones have been reported rarely. To the best of the authors’ knowledge, the application of conducting polymer coated fabric in stretchable supercapacitors has not been reported yet.Here we describe a daily-used Nylon Lycra fabric or cotton fabric coated with polypyrrole (PPy) as electrode for stretchable supercapacitors. And different polymerization methods have been investigated to further improve the electrochemical properties of the resultant polymer.(1) Conductive PPy coated cotton fabric has been successfully fabricated via different polymerization methods:in-situ polymerization (CP), chemical vapour deposition (CVD) and interfacial polymerization (IP). And fabrics obtained though different polymerization methods displayed different surface morphology. The surface resistances of CVD and IP fabrics were310Ω/(?) and1200Ω/(?), respectively. CVD fabric showed better electrochemical properties than CP and IP fabrics.For the in-situ polymerizied PPy coated Nylon Lycra fabric, the conductivity of the fabric could be affected by reaction time and dopants. The best reaction time was2hours. When pTS was used as dopant, the surface resistance was highest (32,500Ω/(?)), while the lowest surface resistance of149Ω/(?) could be obtained when Na2NDS was used as dopant with the reaction time of2h at4℃. Compared with the original Nylon Lycra fabric, slightly higher drawing force was shown for PPy coated fabric, which might be attributed to the stiff PPy layer coating. EIS test proved that the sample obtained after2h polymerization showed the best electrochemical properties. The type of electrolyte used in CV test also had an influence on the electrochemical performance of PPy coated fabrics and highest specific capacitance of123.3F g-1was obtained in1M NaCl at a scan rate of lOmV s-1. However, the specific capacitance decreased when the scan rate increased. This might be explained by the entering into/ejecting and diffusion of counter ions being too slow compared to the transfer of electrons in the PPy matrix at high scan rates.(2) PPy coated Nylon Lycra fabric obtained via in-situ chemical polymerization of was investigated as electrode material of stretchable supercapacitor. Such a conductive fabric showed outstanding flexibility and stretchability, and demonstrated strong adhesion between the PPy and the fabric of interest. During stretching at wale direction, the resistance of this PPy coated Nylon Lycra fabric decreased with increasing elongation. After being stretched for1000times, the resistance of this conductive fabric was irreversibly increased. Higher irreversible resistance increase was induced with higher strain applied. Nevertheless, PPy coated Nylon Lycra still sustained its electrochemical properties with less than10%specific capacitance loss after being stretched to100%for1000times. Interestingly, its electrochemical properties could be improved with in-situ strain applied and exhibited a higher specific capacitance. The capacitance increased from69.7and39.4F g-1with0%elongation to101.9and88.2F g-1with60%strain applied at the scan rate of50and100mV s-1, respectively. In a two-electrode system at a current density of1.0A g-1the specific capacitance increased from108.5F g-1with0%strain to117.6,119.6and125.1F g-1with20%,40%and60%elongation, respectively. Also the cycling stability was improved with the applied strain. After500charging/discharging cycles, only12.5%of the initial capacitance was retained for the fabric with no strain applied. The retained capacitance of the fabric increased to45%,53%and55%with20%,40%and60%elongation, respectively. However, we should point out the cycling stability need to be improved for practical applications.(3) Electrochemical polymerization was used to obtain PPy coated cotton fabric with better electrochemical performance. The cotton fabric displayed a surface resistance of105Ω/(?) after being sputter coated with gold and electrochemically coated with PPy with50%strain applied. For the Au coated cotton fabric, the normalized electrical resistance increased rapidly with strain at the initial stage of stretching with violent vibration. Then the resistance was decreased and gradually stabilized. The electrical resistance of PPy coated cotton fabric showed a much smaller initial increase with strain after which the resistance was gradually stabilized. They both exhibited excellent stretch ability and sustained their conductivity even at140%elongation. During cycling test, the normalized resistance initially increased and then decreased with elongation. When the strain was released, the resistance was also increased first and then decreased. After being stretched to50%for1000times, the resistance of this conductive fabric was irreversibly increased to1645Ω/(?).CV results showed good capacitive behaviour of this PPy coated cotton fabric even at high scan rates. It delivered a specific capacitance of254.9,216.7,196.8,166.4and144.8F g-1at a scan rate of10,50,100,200and300mV s-1, respectively. Those values were much higher than that of the fabric obtained via chemical polymerization. While30%strain was applied to the fabric, its specific capacitance slightly increased to256.3,225.0,203.4,175.0and149.8F g-1at a scan rate of10,50,100,200and300mV s-1, respectively. The cycling stability of the PPy coated fabric obtained via electrochemical deposition was also better than that of the chemically polymerized fabric,51%of the initial capacitance was remained after500CV cycles at a scan rate of100mV s-1. And it was further improved while the fabric was tested with30%strain applied. However, after3000cycles of CV, the specific capacitance still decreased severely, only13%of the initial capacitance was retained.Due to the different stretchability of Nylon Lycra fabric and cotton fabric, their electrochemical properties also exhibited different variation trends with strain applied. For the Nylon Lycra fabric, its electrochemical properties could be improved with in-situ strain applied (0%-60%) and exhibited a higher specific capacitance. As to the cotton fabric, its specific kept nearly unchanged with or with30%strain applied. This could provide a reference for the supercapacitor electrode material choosing according to the different needs for various occasions.(4) Copolymerization was introduced to improve the performance of the resultant polymer. The copolymer of pyrrole and3-(4-tertbutylphenyl)thiophene (TPT) was synthesized via electropolymerization in acetonitrile with ClO4-as dopant. SEM, FTIR and elemental analysis results showed that the copolymer included both Py and TPT units while the amount of TPT unit was much smaller than Py unit. The resultant homopolymers and copolymer were assembled into supercapacitors to investigate their electrochemical performances. Though the homopolymer PTPT displayed very poor electrochemical properties, an introduction of small amounts of TPT units leads to superior electrochemical properties in comparison with the homopolymer PPy or PTPT. It might be ascribed to the introduction of the PTPT units into PPy improved its available surface area. Copolymer delivered the highest capacitance of291and203F g-1at a scan rate of5and500mV s-1, in comparison with216and166F g"1for PPy,26and6F g-1for PTPT, respectively. In charge/discharge tests, the copolymer electrode exhibited a capacitance of279F g-1at0.5A g-1, much higher than that of PPy (227F g-1) and PTPT (45F g-1). The copolymer electrode also showed an improved cycling stability. After1000charge/discharge cycles at a current density of5A g-1only a9%decrease of capacitance was observed, while PTPT and PPy electrodes lost60%and16%of their initial capacitance, respectively. The cycling stability has also been tested via CV at a scan rate of100mV s-1. After10000CV cycles,67%of the initial capacitance was remained for the copolymer supercapacitor, while the remained values were48%and40%for supercapacitors based on PPy and PTPT, respectively.