Structural Design And Regulation Of Carbon Based Electrode Materials For Zinc-ion Hybrid Capacitors

Aqueous zinc-ion hybrid capacitors（ZICs）have attracted tremendous attention due to the unique properties of Zn metal,such as high theoretical capacity(820 m Ah g^-1;5855 m Ah cm^-3),relative low redox potential（-0.76 V vs.SHE）and high stability in water.However,the imbalanced kinetics between cathodes and anodes,the poor compatibility between cathode and electrolyte and the growth of Zn dendrites on the Zn anode severely limit the development of ZICs.In this thesis,an ion modulation layer is introduced on the surface of the zinc anode,which improves the slow energy storage dynamics of the zinc anode,and achieves the double improvement of the rate performance and power density of the zinc ion hybrid capacitors.The ion transport and chemisorption capacity of the carbon cathodes were enhanced by optimizing their porous structure and introducing reasonable heteroatom dopings.The electrochemical conversion mechanism of the modified Zn anodes and the effect of heteroatom dopings on the surface of the carbon-based cathodes were revealed.The structure-activity relationship between the composition and structure of the carbon-based cathode materials and the electrochemical performance was investigated.The main results are summarized as follows.（1）The construction of ion modulation layer for the modification of Zn anode.ZIF-8crystals were grown on the surface of pre-oxidized carbon fiber cloth via the hydrothermal process.The as-prepared carbon fiber cloth grown with ZIF-8（CC@ZIF-8）was used as an ion modulation layer to modify the zinc anode.Based on the ZIF-8 induced adsorption of zinc ions and the 3D continuous conductive network of carbon cloth,zinc ions can be reduced to zinc nanocrystal nuclei and distributed on the surface of carbon cloth during the charging and discharging process,which can accelerate the diffusion of zinc ions and increase the deposition rate of zinc ions on the anode side.Density functional theory（DFT）calculations confirm that ZIF-8 can induce the growth of Zn deposits in a parallel direction to avoid the formation of Zn dendrites.The homogenous distribution of concentration and electrical field on the modified Zn anode was revealed by dual-field and electroplating simulations.Compared with ZICs using bare Zn anode,the ZICs with CC@ZIF-8 modified Zn anode exhibit higher power density of16.8 k W kg^-1,demonstrating that the improved zinc-ion diffusion and plating rate on the Zn anode is the key factor for increasing the power output of ZICs.（2）Fabrication of interconnected carbon nanocages with low surface oxygen content via chemical forming,in-situ activation and hydrogen reduction.Polyvinylpyrrolidone with great dispersibility and high melting point was used as a carbon source,and potassium nitrate was used as a foaming agent to prepare three-dimensional carbon frameworks（3DCF）with low surface oxygen content and connected nanocages via the chemical foaming,in-situ activation and surface hydrogen reduction for deoxygenation（DO）.The interconnected structures inhibit the surface contacted resistance and the low surface oxygen improves the high-voltage capability.The three-dimensional carbon frameworks after deoxygenation（3DCF-DO）was used as electrode for a 4 V ionic liquid-based symmetric supercapacitor,which exhibits a high energy density of 34 Wh kg^-1 at an ultra-high power density of 150 k W kg^-1.The 3DCF-DO based ZICs with CC@ZIF-8 modified Zn anode shows a high specific capacitance of 372 F g^-1 at 0.5 A g^-1 and a great rate performance of 233 F g^-1 at 20 A g^-1.（3）Fabrication of N,P,O tri-doped nanocages via electrostatic-driven phase transfer strategy.Based on the oriented arrangement of zwitterionic surfactant phosphatidylcholine（PC）with charged hydrophilic groups at the interface of oil/water,the phase transfer strategy driven by the electrostatic interaction was used to achieve the precisely controlled coating of PC on metal organic framework（MOF）nanoparticles in the oil-in-water emulsion.N,P,O tri-doped carbon nanocages were fabricated after annealing.According to the DFT calculations,it is demonstrated that the P-O sites and N vacancies on carbon-based cathodes can enhance the chemical zinc adsorption to provide the stable and reversible pseudo-capacitance.As a result,the assembled ZIC with CC@ZIF-8 modified Zn anode delivers a high energy density of 43Wh kg^-1 at an ultrahigh power density of 137.9 k W kg^-1.（4）Fabrication of MOF-based carbon composites via bidirectional electrostatic self-assembly-assisted strategy.Through the large dipole moments between the covalently bonded negative and positive charges in the hydrophilic group of the zwitterionic surfactant dodecyl dimethyl betaine,the homogeneous electrostatic interaction at the interface between surface-charged substrates and metal ions in solution can be established,resulting in the heterogeneous nucleation of metal ions on the surface of substrates.Based on the bidirectional electrostatic self-assembly,the in-situ growth of single-layer nanoscale MOFs on a range of substrates can be achieved.After annealing,the ultrahigh N doped（12.4 at.%）carbon composites with hierarchical porosity(V_meso/V_micro=2.4)and dual electrical conductive networks were fabricated.The ZICs with the carbon composites as cathode and CC@ZIF-8 modified Zn foil as anode exhibit 90.1%specific capacitance retention after the ultra-long cycling of 230,000 cycles at500 m V s^-1.