Synthesis Of Ultra-thin Carbon Nanostructure By Coupling Chemical Activation With Hard Template And Acting As Cathode Materials For Zinc Ion Hybrid Capacitors

The ever-increasing demand for renewable energy sources has led to the consideration of developing high safety,high stability,low cost and green electrochemical energy storage systems.As an emerging energy storage device,aqueous zinc-ion hybrid capacitors（ZHCs）have gained widespread attention due to their low cost,excellent cycling stability,satisfactory energy and power output characteristics.However,it remains challenging to design unique and stable carbon nanostructures for the construction of ZHCs with high capacity,excellent rate performance,and ultra-long cycle life.In order to achieve the above objectives,we have been working on the design of a simple,efficient and green synthetic route for the preparation of porous carbon cathodes with excellent overall performance for ZHCs from both nanostructure modulation and atomic doping.We investigated the role of heteroatoms in promoting the storage of zinc ions by theoretical calculations.Some testing and characterization tools,such as XPS,XRD,SEM etc.were used to investigate the mechanism of energy storage in ZHCs.（1）3D honeycomb hierarchically porous carbon cathodes with high specific area was obtained by one-step pyrolysis using glucose as the carbon precursor,micron-sized commercial magnesium oxide as the hard template and potassium bicarbonate as the activator.The nanostructure modulation was achieved by the synergistic effect of template and activator.The obtained hierarchically porous carbon has a high surface area of 2265 m² g^-1 and prominent mesopores.As a cathode for ZHCs,a specific capacity of 147m Ah g^-1 was obtained at a current density of 0.2 A g^-1,and a capacity of 71 m Ah g^-1 was obtained even at a high current density of 50 A g^-1,showing an excellent rate performance.Furthermore,the ZHCs based on the hierarchically porous carbon cathode has 88%initial capacity retention after 20,000 cycles,which fully confirms the good stability of this honeycomb structure.（2）The C@MgO core-shell structure was formed by one-step pyrolysis using polyvinylpyrrolidone as both carbon and nitrogen sources,micron-sized commercial Mg O as hard template,and potassium bicarbonate as activator.Subsequently,hollow structured hierarchically porous carbon with high specific area,ultrathin carbon layer,and heteroatom doping was obtained after etching off the template with acid,and the nanostructure modulation and heteroatom doping were successfully achieved by a one-step method.The ZHCs assembled with this hollow carbon structure as the cathode are able to obtain a high capacities of 136.2-66.7 m Ah g^-1 in the current density range of 0.1-50 A g^-1.Furthermore,it exhibited a cycle life of up to 24,000 cycles,which fully demonstrated the excellent comprehensive performance of this ultrathin hollow carbon structure.Finally,SEM and XRD were used to make a preliminary investigation of the energy storage mechanism of ZHCs.（3）Inspired by the above two works,we used hexamethyltetramine with high nitrogen content as the carbon and nitrogen source,magnesium chloride as the complexing agent and structure inducer to firstly prepare nitrogen-rich magnesium-based complex precursors by complexation reaction in aqueous solution,and then obtained oxygen-rich boron and nitrogen co-doped porous carbon nanosheets by one-step pyrolysis with the assistance of boric acid.The heteroatom content of the obtained porous carbon nanosheets is close to 29%（15.26%N,10.18%O,3.21%B）.In this work,the formation mechanism of such boron and nitrogen co-doped nanosheets was investigated in detail,and the effects of different doping conditions on the structure and morphology of the products were analyzed.This ZHCs based on boron and nitrogen co-doped nanosheet cathodes are able to obtain a specific capacity of 130.7-47.2 m Ah g^-1 in the current density range of 0.1-50 A g^-1.Finally,the role of heteroatoms in promoting Zn ion storage was investigated by theoretical calculations,and the energy storage mechanism of ZHCs based on boron and nitrogen co-doped carbon cathodes was studied in detail by XRD and XPS.