Design And Regulation Of Carbonaceous Cathode Materials And Their Energy Storage Mechanism For Aqueous Zinc Ion Capacitors

Zinc-ion capacitors（ZICs）,as an emerging hybrid device that inherit the advantageous characteristics of high energy of ZIBs and high power of supercapacitors,have attracted extensive attention.However,the mismatch of kinetics and capacity between Zn anode and capacitive-type cathode,worse Zn²⁺storage capability,poor rate performance,the low utilization of active sites and unsatisfactory cycle life are still the Achilles’heel of this technology.This thesis focuses on the problems and challenges of ZICs,and based on the design and preparation of carbonaceous cathode materials,which significantly improve the electrochemical performance of the materials by the regulation of specific surface area（SSA）,pore structure,defects and heteroatoms of material.Secondly,the energy storage mechanism of the materials in electrochemical reactions.Systematic ex-situ characterization analysis coupled with in-situ electrochemical quartz crystal microbalance and Raman spectra measurements testify that the energy storage mechanism of the materials,including the coupling mechanism of electric double-layer capacitance and redox reaction of oxygen functional groups,the dual ion adsorption and reversible chemical adsorption.Finally,it is proposed to achieve a good match between material pore size and carrier by eliminating the microporous domain-limiting effect of the material,to improve the active site utilization of the material,and to identify a co-adsorption mechanism of Zn²⁺,H⁺and SO₄^2-with reversible chemisorption to synergistically enhance the Zn²⁺storage capability.This thesis addresses the problems and challenges faced by aqueous ZICs,and successfully constructs high-performance aqueous ZICs energy storage devices by investigating the design of positive materials,the regulation of material structure and composition,the kinetic matching of positive and negative materials,and the energy storage mechanism.The details are as follows:1.The carbon cathode material cannot effectively match the high theoretical capacity of the Zn cathode,resulting in worse electrochemical performance of ZICs.Herein,we report the fabrication of oxygen functionalized hierarchical porous carbon（HPC）materials by direct annealing of potassium citrate and used for ZICs.The material structure and oxygen functional groups can be efficiently regulated by adjusting the pyrolysis temperature.The porous carbon material affords more Zn²⁺adsorption/desorption sites by the high SSA and the suitable pore structure promotes the ions fast transport.Additionally,oxygen functional groups,especially the hydroxyl group of HPC-600 can effectively boost the chemical adsorption of Zn²⁺.Therefore,HPC-600-based ZIC demonstrates prominent electrochemical properties,with specific capacity as high as 169.4 mAhg^-1 at current density of 0.1 A g^-1 and impressive specific energy of 125.1 Whkg^-1.Even at a high specific current of 20 A g^-1,the HPC-600 can still achieve high specific capacity and power density of 97.6 mAhg^-1 and 16.1 k W kg^-1,respectively.Moreover,the corresponding quasi-solid ZICs also exhibit satisfactory rate properties and high cyclic stability.Furthermore,the ex-situ mechanism investigation deciphering that the outstanding electrochemical characteristics are stem from the synergically contribution of the electric double-layer capacitance（EDLC）of porous carbon and pseudocapacitances of oxygen functional groups reversible transformation.2.A good match in capacity and kinetics between carbon cathode materials and zinc cathodes is essential for the construction of high energy/power densities ZICs.Herein,the tetra-alkali metal pyromellitic acid salts（PMA4M,M=Li,Na,K,Rb,and Cs）were used as precursors to prepare porous carbons via a carbonization/self-activation procedure.The optimized rubidium activated porous carbon（Rb PC）demonstrates affluent defect-rich graphitic tissue,high SSA of 1527.7 m² g^-1,hierarchical porous structure and high oxygen doping of 6.02 at%.Benefiting from the advantageous features,the Rb PC-based ZICs delivered excellent electrochemical performance with high energy density(178.2 Whkg^-1)and power density(72.3 k W kg^-1).Meanwhile,the Rb PC-based quasi-solid-state ZIC holds particular promise for serving as energy storage components for wearable electronics.Systematic ex-situ spectral investigations in combination with in-situ electrochemical quartz crystal microbalance experiments certify that the remarkable electrochemical capability is ascribed to the synergistic effect of dual-ion adsorption and reversible chemical adsorption of Rb PC.The numerous lattice defects providing a multitude of sites for the adsorption of Zn²⁺and CF₃SO₃^-,which exhibit outstanding capacitive kinetics.Moreover,the strong interaction between oxygen functional groups and Zn ion resulting in affluent redox-active pseudocapacitance further boost the ZICs electrochemical capability.3.Acknowledgedly,[Zn（H₂O）₆]²⁺,as the main solvation structure with a large ion size of 0.86 nm,is identified as the primary charge carriers for ZICs during charge/discharge processes.Due to the micropore confinement effect（the mismatch between pore size and Zn hydrate ion）,the large ion size of[Zn（H₂O）₆]²⁺is difficult to access the deep micropores of the microporous-based carbon materials,resulting in a severe loss of space utilization and poor Zn²⁺storage capability.Herein,we provide new insights to boost the Zn²⁺storage capability of activated nitrogen-doped hierarchical porous carbon materials（ANHPC-x）by effectively eliminating micropore confinement effect and synchronously enhancing the utilization of active sites.The optimized ANHPC-2 displays a SSA as high as 3553.1 m² g^-1,suitable pore size,sufficient active sites,and abundant oxygen functional groups,thus favorable for promoting the chemical adsorption and accelerating the kinetics of Zn²⁺storage.Importantly,the more matchable pore size can be favorable to improve the accommodation of[Zn（H₂O）₆]²⁺charge carriers,leading to excellent capability of Zn²⁺storage.Consequently,the as-fabricated ZIC with ANHPC-2 cathode delivers a specific capacity as high as 199.1 mAhg^-1,high energy density(155.2 Whkg^-1),high power density(41.4 k W kg^-1),and excellent cycling stability（99.1%capacity retention over65000 cycles）.Systematic in situ electrochemical quartz crystal microbalance（EQCM）and Raman spectra measurements testify that the excellent electrochemical properties are attributed to the synergistic effect of the Zn²⁺,H⁺,and SO₄^2-co-adsorption mechanism and reversible chemical adsorption.More encouragingly,the quasi-solid-state ZIC demonstrates superior rate property,favorable mechanical stability,and ultralong lifespan up to 100 000 cycles with 98.8%capacity retention.