Electrochemical Polymerization Of Conducting Polymer Based High Energy Density Supercapacitor Electrodes And Devices

Since it was published the first time in a US patent in 1957,till nowadays applied to power public transit in many countries and areas around the world,supercapacitors have developed drastically during the last few decades.As the new energy vehicles and intelligent wearable devices become more and more popular,supercapacitors will find widespread applications in the future.As a type of electronic devices bridging the energy and power gaps between electrolytic capacitors and batteries,supercapacitors can be used to harvest power from regenerative braking systems and release power to help hybrid buses accelerate,to provide cranking power and voltage stabilization in start/stop systems,backup and peak power for key automotive applications or to be used in blade pitch systems and to help increase reliability and stability to the energy grid.In a word,supercapacitors have become an important and indispensable power solution in many fields.However,with the increasing demand for energy storage and conversion devices,the relatively low energy density that supercapacitors store has become a central issue of hindering their development as practical energy storage devices.Now the commercial supercapacitor devices mainly adopt carbon materials as the electrodes.Their energy density can hardly exceed 10 Wh kg-1,which is much lower than that of batteries.Among all the available choices for supercapacitors electrode materials,conducting polymers are considered to be an attractive option as they have higher charge density over carbonous materials and lower cost over the relatively expensive metal oxides.Conducting polymers also have plastic properties and are therefore easily processing and flexible,particularly as thin films.However,they usually suffer from relatively low power density and cyclic instability.And the energy stored is strongly dependent on doping level,which is typically below one dopant per polymer unit: about 0.3-0.5.Too high doping levels will cause the collapse of polymer structure,thus impeding the further enhancement of energy density.Bearing all these in mind,aiming for the fabrication of high energy density supercapacitor devices,we applied several approaches intending to overcome these obstacles.1、 In chapter 2,we introduce inherently rich micropores into the structure of conducting polymers to construct 3D network configuration.The films are unique in that they are porous and possess extended π conjugation.By virtue of these features,we developed the conjugated microporous electrodeposited films as versatile platforms for supercapacitor electrode materials.The preliminary tests demonstrated high specific capacitance value as 142 F g-1 for pZn-mTCPP and 246 F g-1 for pBThCz,which were among the peak values reported on pure conducting polymer based electrodes.The supercapacitor demonstrated robust stability at high current densities benefited from the highly-crosslinked network skeleton and rich inherent pores that promote high-rate electron transfer.pBThCz electrode remains 93% of its maximum capacitance even at a high charge/discharge current density of 100 A g-1.Apart from the traditional conducting polymers like PANI,PPy,and PTh,supercapacitors now have new electrode materials choices of these novel conjugated microporous films.2、In chapter 3,we designed two donor-acceptor(D-A)monomers with different linkage modes as the precursors of bipolar doping electrode materials.Polymers fabricated from these D-A type monomers can be p-doped and n-doped.We investigated the influences of structure on the electropolymerization behaviors,doping processes,and supercapacitive performance.Supercapacitor porotypes based on electrodes pCzAQCz and pAQ3 Cz achieved maximum operating voltages of 2.4 V and 2 V,respectively.Wide voltages bring huge elevation on the device energy density and power density.The pAQ3 Cz device has a maximum energy density of 19.5 Wh kg-1(at power density of 1.2 kW kg-1),maximum power density of 12.2 kW kg-1(at energy density of 16.4 Wh kg-1);The pCzAQCz device has a maximum energy density of 29.1 Wh kg-1(at power density of 1.3 kW kg-1),maximum power density of 11.8 k W kg-1(at energy density of 23.3 Wh kg-1).These values are much higher than that of the commercial supercapacitors,proving the superiority of bipolar doping electrode materials.Meantime,we discussed the charge trapping effect which severely impedes the cycling stability of type III supercapacitors.The results show that the charge trapping effect happening during the n-doping process is the main cause of electrode and device instability.The polymer structure has a vital effect on the degree of charge trapping effect.Because of the extended conjugated structure and the loose surface structure,pAQ3 Cz shows the effectively reduced charge trapping at n-doping process,and exhibits excellent cyclic stability.Moreover,the decay caused by charge trapping effect may be recovered partially or almost totally by several cyclic voltammetry scans,thereby extending the lifetime of supercapacitor devices to some degree.3、In chapter 4,we first explored the application of two redox active mediators in organic electrolytes,i.e.,ferrocene(Fc)and 4-oxo-2,2,6,6-tetramethylpiperidinooxy(4-oxo TEMPO),both of which exhibited fast and reversible redox processes.On this basis,we fabricated flexible and all-solid-state poly(3,4-ethylenedioxythiophene)(PEDOT)supercapacitors.Benifited from the wide voltages endowed by organic electrolytes and the additional capacitances brought by redox mediators,PEDOT supercapacitor exhibited a widened working voltage of 1.5 V,an extraordinary capacitance of 363 F g-1 and high energy density of 27.4 Wh kg-1.Thus,it is proved that doping redox mediators in the electrolyte is a simple,universal and efficient approach to enhance the capacitance of supercapacitors.The redox mediators reported here,are the first time being applied in supercapacitors,especially in the gel state.In addition to providing additional faradaic capacitances,they also exhibit synergistic interaction with PEDOT and improve the cycling stability of supercapacitors.In summary,aiming at fabricating high energy density,flexible supercapacitors devices,we introduced inherent pores or functional redox moiety in the backbone of conducting polymers to work as electrode materials and fabricated gel electrolytes by adding redox mediators to provide additional capacitance.The modification of electrode materials or electrolytes effectively enhanced the capacitance of electrodes and the energy density,power density of devices.Besides,we carefully investigated the relationship between electrode properties and their supercapacitive performance.So,this paper will play a positive role in both broadening our knowledge of the energy storage mechanism of conducting polymers and extending available materials systems for supercapacitor application.