Activated Carbon Fibers Based On Pore Structure Design For As Anode Electrode Of Potassium-ion Capacitors

Alkali metal ion capacitors combine the advantages of rechargeable batteries and electrochemical capacitors,achieving a balance between energy density and power density,which has attracted great attention.Among them,potassium ion capacitors have become potential substitutes for lithium/sodium ion capacitors due to their rich potassium resources,rapid transport kinetics of K⁺in electrolytes,and low standard electrode potential.However,the specific capacity of the anode electrode of a potassium ion capacitor is relatively low,which cannot match the high specific capacity characteristics of the cathode electrode of the battery type.This is the main factor leading to the low specific energy density of the potassium ion capacitor.Capacitive anode electrodes store energy through reversible anionic adsorption/desorption on the surface,so obtaining electrode materials with large specific surface area and suitable pore structure is the key to achieving high specific capacity anode electrodes.In addition,heteroatom doping can change the bonding environment,electronegativity,and charge density of carbon atoms,thereby changing the pseudocapacitance behavior of carbon atoms.It is also an effective method to improve specific capacity.However,the anion storage mechanism related to pore structure and heteroatom doping is still unclear,which is not conducive to improving the performance of potassium ion capacitors.Therefore,in this thesis,the effects of pore structure and oxygen content on the anionic storage behavior of activated carbon fiber cathode were studied from two aspects:pore structure design and chemical structure design.The specific work is as follows:1.Starting with pore structure design,activated carbon fibers with different pore structures were obtained by placing carbon fibers in different concentrations of KOH solution for activation treatment.Research shows that increasing the concentration of KOH solution can significantly increase the specific surface area and enrich the pore structure of carbon fibers.Activated carbon fibers obtained under different activation conditions are used as anode electrode materials for potassium ion batteries.Through multiplication tests,it is found that large specific surfaces and pore sizes suitable for accommodating anions can significantly improve specific capacity.Impedance tests and constant current intermittent titration tests at different temperatures have also verified that large specific surface areas and suitable pore structures improve the storage kinetics of anions and promote the adsorption and desorption of anions by carbon anode electrodes.2.Starting from the chemical structure design,the prepared activated carbon fibers（ACNFs-O）were subjected to hydrogen reduction treatment to obtain oxygen free doped activated carbon fibers（ACNFs-H）.The effects of oxygen doping in activated carbon fibers on anionic storage performance were compared and studied.According to the magnification tests under different current densities,oxygen doping can significantly improve the specific capacity.The anode electrode of ACNFs-O has a high reversible capacity of 88.4 m Ah g^-1 at 0.1 A g^-1,and a reversible capacity of 57.3 m Ah g^-1 at 6.0 A g^-1,which is much higher than ACNFs-H.Ex-situ FT-IR and XPS characterization,as well as density functional theory calculations,have demonstrated that redox active carbonyl（C=O）groups adjust the electrochemical properties of carbon,providing a pseudocapacitive contribution to improving anion storage.3.Considering the advantages of a dual-carbon type potassium ion capacitor,a dual-carbon type potassium ion capacitor CNFs||ACNFs-O was assembled using activated carbon fibers as the cathode electrode of the potassium ion capacitor,and ACNFs-O with high oxygen content and rich porosity as the anode electrode.The assembled capacitor can operate under a high voltage window of 0-4 V.After optimizing the anode and cathode electrode mass ratio,CNFs|ACNFs-O provides 125.4Wh kg^-1 energy density at 193 W kg^-1 power density,which is50%higher than ACNFs-H anode electrode performance.CNFs||ACNFs-O also has good cyclic stability,with a capacity retention rate of 80.3%after 4000 cycles at a high current density of 4A g^-1.