Design And Zinc Storage Mechanism Of Functionalized Porous Carbon Fibers

Sustainable development strategies and carbon neutrality goals have greatly promoted the research of electrochemical energy storage systems with good security,environmental protection,and low cost.Among various potential energy storage systems,aqueous zinc-ion hybrid capacitors（ZIHCs）composed of porous carbon cathodes and zinc metal anodes have the advantages of high theoretical capacity,non flammable characteristics,low environmental harm and wide resource sources,so they have become the central issues of energy research.Although researchers have struggled mightily in the exploitation of electrode materials and the majorization of electrolytes,there are still two major bottlenecks.First,the Zn anode faces a huge risk of dendrites piercing the separator and causing internal short circuits in the battery,which greatly limits the cycle life of ZIHCs and increases their safety concerns.Second,the limited capacitance level of carbon-based cathodes makes the energy density of ZIHCs not comparable to commercial Li-ion batteries.Therefore,it is urgent to explore suitable methods to improve the cycling stability of Zn anodes and increase the capacitance of carbon-based cathodes to advance the commercialization of ZIHCs.Aim at the above bottleneck matters,this paper takes the carbon nanofibers films as the research object,modifies them by means of structural design and surface modification,and studies the Zn storage mechanism of the modified carbon nanofibers films as the cathode or Zn anode hosts of ZIHCs by various characterization methods and theoretical simulation.The specific research contents are as follows:（1）In Chapter 2,flexible porous carbon nanofibers films were synthesized by electrospinning and KOH activation strategy.The films can be used as both dendrite-free Zn metal anode protection layer and self-supporting flexible cathode of ZIHCs.On the one hand,when it serves as the protective layer of Zn metal anode,the oxygen functional groups on the surface of carbon nanofibers can effectively improve its zincophilicity.Its high conductivity and rich pore structure also promote the uniform distribution of electric field,so as to realize the planar deposition of Zn metal.On the other hand,carbon nanofibers activated by KOH can also be used as flexible capacitive cathode with excellent electrochemical properties.The results of electrochemical analysis and ex-situ characterization show that in addition to the electric double-layer capacitance brought by ultra-high specific surface area,the oxygen functional groups on the surface of carbon nanofibers can also produce a part of pseudocapacitance by chemisorption of Zn²⁺.Therefore,the assembled ZIHCs exhibit high energy/power densities of 112.1 Wh kg^-1/9.92 k W kg^-1 and an ultra long cycle life（69.7%capacity retention after 80000 cycles）.More importantly,the assembled pouch ZIHCs can operate stably even at a bending angle of 180°,indicating its great application potential in wearable electronic products.（2）In chapter 2,it is found that minute quantities of oxygen functional groups on the surface of carbon nanofibers can chemically adsorb Zn²⁺to produce a part of pseudocapacitance.In order to further increase the contribution of pseudocapacitance brought by oxygen functional groups and further understand the energy storage mechanism of such pseudocapacitance,in chapter 3,an oxygen-enriched super hydrophilic flexible porous carbon nanofibers film was constructed through an electrostatic spinning method and nitric acid assisted oxidation technology,and used as the cathode of ZIHCs.By optimizing the oxidation time,it is found that the carbon nanofiber film with an oxidation time of 20 h（OPCNF-20）not only exhibits a near-zero water contact angle and excellent electrical conductivity,but also possesses a hybrid energy storage mechanism of electric double layer capacitance and pseudocapacitance.In addition,the reaction mechanism between Zn²⁺and oxygen-containing functional groups（mainly carbonyl and carboxyl）was also analyzed by a series of ex-situ characterizations and density functional theory calculations.In practical performance tests,the ZIHCs based on OPCNF-20 cathode demonstrated high energy/power densities of 97.7 Wh kg^-1/9.9 k W kg^-1 and ultra-long-term cycle stability（81%capacity retention after 50000 cycles）.More importantly,the ZIHCs can also operate stably under extreme conditions such as high loads(20.1 mg cm^-2),different bending angles（from 0°to 180°）,and quasi-solid electrolytes,demonstrating its potential for practical applications.（3）Chapter 3 has demonstrated the Zn storage mechanism of oxygen functional groups as chemisorption sites,but the excessive introduction of oxygen functional groups will damage the electrical conductivity of carbon nanofibers,thereby affecting the energy transfer of electrodes.To further increase the chemisorption sites of carbon nanofibers,an in-situ exfoliation strategy was reported in Chapter 4 to modulate the chemisorption sites of carbon nanofibers by exfoliating of high pyridine/pyrrole nitrogen doping and carbonyl-functionalized nanosheets.The experimental results and theoretical calculations show that the highly electronegative pyridine/pyrrole nitrogen dopants can not only greatly reduce the binding energy between carbonyl and Zn²⁺by inducing the charge delocalization of carbonyl,but also further promote the adsorption of Zn²⁺by forming N-Zn-O bonds with the carbonyl.Benefiting from the multiple highly active chemisorption sites generated by the synergistic interaction between carbonyl and pyridine/pyrrole nitrogen atoms,ZIHCs based on the obtained carbon nanofiber film cathode exhibit extremely high energy/power densities(98.28 Wh kg^-1and 33.2 k W kg^-1)and ultra-long-term cycling stability(99.2%capacity retention after200000 cycles at 40 A g^-1).This work establishes a general approach to address the bottleneck of insufficient active adsorption sites for carbon-based ZIHCs.（4）In order to take full advantage of the cross-linking ability between oxygen functional groups and Zn²⁺,a progressive Zn deposition mechanism was proposed in Chapter 5 after an in-depth investigation of the formation mechanism of Zn dendrites.This mechanism enables dendrite-free Zn plating behavior by introducing oxygen functional groups and macroporous channels into the carbon nanofibers film（labeled as OPCNF）host.Experimental analysis and theoretical simulation results indicate that the zinc-philic oxygen functional groups act as deposition sites with ultra-low Zn nucleation overpotentials,which can guide the initial nucleation of Zn on the surface of carbon nanofibers.Then,the macroporous channel with higher current density and Zn²⁺concentration will serve as the preferred space for the subsequent growth of Zn.After the surface of the carbon nanofibers is completely occupied by the deposited Zn,the subsequently deposited Zn will progressively fill the inner space of the entire carbon nanofiber film according to the current density of the electrolyte.Therefore,the OPCNF host with a progressive deposition mechanism enables dendrite-free Zn plating/stripping behavior even at a high areal deposition capacity of 15 m Ah cm^-2.Furthermore,symmetric cells with OPCNF-Zn electrodes exhibit a long cycle life of over 1600 h at a current density of 1 m A cm^-2 and a fixed deposition capacity of 1 m Ah cm^-2.In particular,ZIHCs assembled from the OPCNF-Zn anode and the activated porous carbon nanofibers film cathode exhibit excellent energy density,stable cycle life,and extraordinary flexibility.