Preparation Of Oxygen/Sulfur-doped Biomass-based Porous Carbon And Electrochemical Energy Storage Research

Supercapacitors（SCs）with ultra-high power density,fast charge/discharge,and long cycle life have received continuous attention.However,due to the poor stability of electrode materials and low boiling point of electrolytes at high temperatures,SCs have been studied mainly at room temperature,and less attention has been paid to high-temperature SCs（working temperature≥60°C）.Most of the energy storage devices in daily life are required to have high-temperature operation capability.Moreover,the energy density of most SCs is as low as 5-10 Wh kg^-1,which severely limits their wide-scale application.As the core component of SCs,the electrode material is closely related to its performance index.Therefore,it is significant to develop high-temperature stable,and high-performance electrode materials for SCs.In addition,metal ion（Li,Na,K）hybrid capacitors combine the high energy density of batteries with the high power density and extended cycle life of SCs,which are expected to be widely used in the field of large-scale energy storage.Benefiting from abundant potassium resources,low cost,and low K⁺reduction potential,potassium ion hybrid capacitors（PIHCs）have attracted great attention.However,most of the anode materials show volume expansion and slow kinetics when K⁺（1.38（?））is embedded/detached,which leads to problems such as poor multiplicative performance and cycle life.In addition,the low reversible capacity of the anode material and the imbalance of the two electrode kinetics are also critical factors limiting the development of PIHCs.Considering economic and technical factors,biomass-based porous carbon is the most prospective electrode material for the construction of SCs and PIHCs.In this paper,a series of high-performance porous carbon electrode materials for SCs and PIHCs are prepared through the effective combination of pore structure modulation engineering and heteroatom doping strategy,and the main results are as follows:（1）Using the jujube pits of Xinjiang jujube as the carbon source,a multi-step strategy of hydrothermal pretreatment and subsequent low-temperature carbonization and high-temperature activation was used to prepare oxygen-doped hierarchical porous carbon electrode materials（HJPC-4）which has a high specific surface area(2771 m² g^-1),extremely large active ion-accessible pore volume(V_{0.5-3 nm}=1.067 cm³ g^-1,V_{0.76-3 nm}=1.025 cm³g^-1)and multi-scale pores（combined microporous,supermesoporous and macroporous）At room temperature,SCs based on both 6 M KOH and EMIMBF₄ electrolytes constructed demonstrated ultrahigh charge storage capacity(6 M KOH:1 A g^-1,316 F g^-1;EMIMBF₄:1 A g^-1,204 F g^-1),excellent rate performance(6 M KOH:100 A g^-1,231 F g^-1;EMIMBF₄:30 A g^-1,154 F g^-1)and long cycle life(6 M KOH:60,000 cycles at 10 A g^-1 with 92.11%capacity retention;EMIMBF₄:10,000 cycles at 5 A g^-1 with 80%capacity retention).The systematic electrochemical and physical characterization revealed the mechanism of the pore structure and the contribution of heteroatoms to charge storage.Finally,high-temperature SCs based on EMIMBF₄ electrolyte were successfully constructed with superior electrochemical performance and ultimate energy density/power density up to 97.12 Wh kg^-1/70.7k W kg^-1.（2）High sulfur-doped hierarchical hollow wrinkled carbon spheres（EGS₃PC）were prepared by using green tea extract epigallocatechin gallate（EGCG）and glyoxal as raw material,tetraethyl orthosilicate as in situ silicon source,and the ammonia-catalyzed polymerization to form Si O₂ as the core precursor,followed by carbonization,Na OH solution etching,and high-temperature sulfur doping steps.Owing to the reasonable sulfur doping,sulfur configuration,and hollow hierarchical pore structure,as a K/Na half-cell anode,it demonstrated superior K/Na storage capacity(K half-cell:0.1 A g^-1,476 m Ah g^-1;Na half-cell:0.1 A g^-1,601 m Ah g^-1),outstanding rate performance(K half-cell:5 A g^-1,192 m Ah g^-1;Na half-cell:10 A g^-1,189 m Ah g^-1),and high cycling stability(K half-cell:2 A g^-1 cycle 1000 cycles,still retained 264 m Ah g^-1specific capacity;Na half-cell:5A g^-1cycle 4000 cycles,still retained 231 m Ah g^-1specific capacity).Systematic research proves that sulfur doping can significantly increase the reversible specific capacity,initial Coulomb efficiency,and capacitive energy storage percentage of the carbon-based anode,and the mixed capacitive-diffusive energy storage mechanism with dominant capacitive behavior can achieve faster K⁺/Na⁺storage kinetics.In addition,the performance index of HJPC-4 as the cathode of PIHCs was evaluated.Finally,the PIHCs constructed with EGS₃PC as the anode and HJPC-4 as the cathode can offer 121.5 Wh kg^-1/24 k W kg^-1ultimate energy/power density with no capacity degradation at 2 A g^-1 for 10,000 cycles,which exhibited favorable potential for practical applications.