Strong Polyaniline Conductive Polymer Hydrogels:Design And Application In Flexible Supercapacitors

Mechanically flexible supercapacitors(SCs)are regarded as a promising power units for flexible electronic devices due to high power density,modest energy density,fast charge-discharge capability,good cyclic stability and excellent mechanical flexibility.The key point of fabricating flexible SCs is the design and development of flexible SCs electrodes.Currently,the traditional fabricated approaches of flexible supercapacitor electrodes are that the electroactive materials was transferred or covered to non-active flexible substrates.However,those fabricated methods are complicated and high cost.The non-active substrates occupy a significant amount of weight and volume in the final supercapacitor devices,which is not favorable for flexible electronic devices.Conductive polymer-based hydrogels(CPHs)with a 3D network structure can sustain a large mechanical deformation by themselves,which can be as electrode materials applied to flexible supercapacitors.However,most of current CPHs are mechanically weak,and the electrochemical and mechanical stability of current CPHs also needs to be improved.This dissertation mainly focuses on the design and fabrication of strong and robust polyaniline-polyvinyl alcohol conductive polymer hydrogels and their application in flexible supercapacitors.This dissertation provides new ideas and methods on how to design CPHs with outstanding mechanical and electrochemical properities and plays an important significance on the application of CPHs in flexible supercapacitors.The main contents of this dissertation are summarized briefly as follows:(1)The supramolecular self-assembly of rigid polyaniline and soft polyvinyl alcohol at molecular level through dynamic boronate bond as cross-links yields the polyaniline-poly(vinyl alcohol)hydrogel(PPH)with outstanding mechanical and electrochemical properties.The experiment results show that PPH has a remarkable tensile strength of 5.3 MPa and electrochemical specific capacitance of 928 F/g at a current density of 0.5 A/g.The mechanical properties,electrochemical properties and stabilities of PPH are much higher than that of previously reported CPHs.The flexible all-solid-state supercapacitor based on PPH provides a large specific capacitance of 153 F/g at a current density of 0.25 A/g.The robustness of the PPH-based supercapacitor is demonstrated by the 100%capacitance retention after 1000 mechanical folding cycles and the 90%capacitance retention after 1000 galvanostatic charge-discharge cycles.The high activity and robustness enable the PPH-based supercapacitor as a promising power device for flexible electronics.(2)We report that a simple physical process(freeze-thaw cycles,FT cycles)to deal with PPH.This method can reform the microstructure of conductive polymer hydrogels from clustered nanoparticles to interconnected nanosheets,leading to enhanced mechanical and electrochemical properties.The polyaniline-poly(vinyl alcohol)hydrogel after five freeze-thaw cycles(PPH-5)showed remarkable tensile strength of 16.3 MPa and an elongation at break of 407%,and high electrochemical specific capacitance of 1053 F/g at current density of 0.5 A/g.The flexible supercapacitor based on PPH-5 provided a large specific capacitance of 210 F/g at a current density of 0.25 A/g and a high energy density of 18.7 Wh/kg.The outstanding electrochemical stability of this device was demonstrated by 100%capacitance retention after 1000 galvanostatic charge-discharge cycles or after 1000 mechanical folding cycles.This demonstrates the merit of FT cycles for enhancing the performance of functional hydrogels.(3)Based on previous work,we try to imporve and optimize the PPH system.A strong,highly conductive and stretchable polyaniline-poly(vinyl alcohol)hydrogel is reported via adjusting the relative content of the polyaniline polymer and poly(vinyl alcohol)polymer,which exhibits a high tensile stress of 2.8 MPa,an elongation at break of 136%,and a high conductivity of 2.1 S/cm.This hydrogel film can be directly used as an all-in-one stretchable electrode without needing any extra current collector or elastic substrate,which can be repeatedly stretched to 50%elongation without damaging its electrochemical properties.An all-polymer stretchable supercapacitor(SSCs)based on this hydrogel film is fabricated,which exhibits a specific capacitance of 516 F/g,an areal capacitance of 4528 mF/cm2 and a maximum volume energy density of 0.32 mWh/cm3 at a current density of 1 mA/cm2.This SSCs also show 95%capacitance retention at 50%elongation state,and more than 90%capacitance retention after 300 dynamic stretching and releasing cycles.The design method using the hydrogel films as stretchable electrodes with all-in-one structure is innovation and novelty.These outstanding properties enable this stretchable supercapacitors as a promising power device for stretchable electronic.