Investigation Of New Materials And Systems For High-performance Supercapacitors

Supercapacitors were considered as one of the promising energy storage and conversion devices for their high power density,long cyclic life,high charge-discharge rate and environmentally friendiness.However,compared with batteries,their low energy density cannot meet the increasing need of energy.Therefore,developping supercapacitors with high energy density is necessary for their widely application.The energy density is proportional to the capacitance and the square of the cell voltage.Therefore,this paper focuses on increasing the energy density of supercapacitors by improving electrode capacitance and widening the cell voltage,investigating the phenomenon of asymmetric behavior of positive and negative electrodes and its underlying mechanism,and studing the approaches and strategies to improve the performance from electrode materials,the balance of the two electrodes and redox couples.The main contents can be summarized as follows:(1)Asymmetric behavior of positive and negative electrodes in carbon//carbon supercapacitors and its underlying mechanism.The asymmetric behavior in carbon//carbon supercapacitors restricts operating potential window of the device and consequently its energy density.However,the underlying mechanism is still unclear.Cyclic voltammetry and galvanostatic charge-discharge were employed to investigate the asymmetric behavior of activated carbon in TEABF4/PC and LiPF6/PC electrolyte with different concentrations.It was found that the shape of CV curves,the stable potential windows and the capacitance of positive and negative electrodes were asymmetric.With the help of molecular dynamics simulation,it found that sizes of solvated ions,diffusion coefficient and desolvation energy were the main factors.This can provide scientific basis and theoretical guidance on how to improve the energy density of carbon//carbon supercapacitors.(2)Synthesis of multi-heteroatom self-doped porous carbon and study of its electrochemical properties.The limited capacitance of carbon materials is still not sufficient for widely application.Doping is an effective strategy to enhance the capacitance.Animal precursors contain amounts of heteroatoms and can be derived into self-doping activated carbon.Therefore,swim bladders were selected as the carbon precursor.Multi-heteroatom activated carbon with high surface area was synthesized by carbonization and KOH activation and employed as electrode materials in supercapacitors.The effect of activation temperature on the materials structure and electrochemical performance was studied.A surface area of 3068 m2 g-1 and N,O,S self-doped activated carbon was obtained.The high capacitance come from the electrical double layers capacitance of the high surface area and the pseudocapacitance of high doping.The long cyclic stability is attributed to the doping elements from the precursor.(3)The battery capacitor with unipolar redox couple:battery capacitor can make full use of two materials with different potential window to winden the cell voltage.But the activated carbon based negative electrode has a smaller capacitance than the positive electrode.It would limit the cell capacitance and the energy density.So we introduce the redox-active molecules into the negative electrolyte to enhance the capacitance and balance the capacitance of positive and negative.The effect of concentration of redox electrolyte and the weight of electrode materials on capacitance was studies.The battery capacitor employed activated carbon in 150 mM 4-OH-TEMPO + 1 M KOH electrolyte as negative and Ni(OH)2/Ni in 1 M KOH as positive.The device exhibited a wide voltage window and high energy density.This work can give a new strategy to balance the capacity of positive and negative electrode.(4)The hybrid energy storage system based on bipolar redox-ative coulpes:Based on the work above,redox-active couples are expaned their application from one electrode to two electrodes.TEMPO was added into positive and negative electrolyte for that it can be oxidized and reduced.The electrochemical reaction kinetics of TEMPO on carbon electrode and the effect of concentration of redox electrolyte on capacity were investigated.With 10 mM TEMPO in the electrolyte,the capacity of positive and negative was 1.1 and 1.5 times more than that without the TEMPO.The full cell exhibited an excellent electrochemical performance with an energy density of 84 Wh kg-1 and the powder density of 13 kW kg-1.This flexible,versatile,convenient method has a promising application.