| Energy source is the foundation to support social progress.With the overusing of traditional non-regenerative energy sources and the increasingly serious global environmental problem,the development of renewable clean energy has become an important strategy for sustainable development.Electrochemical energy storage devices can solve the problem of uninterrupted supply of renewable energy sources such as solar,wind and tidal energies.Studies on the key new technology of large-scale electrochemical energy storage have been widely concerned by researchers all over the world.Carbon foams have advantages such as three-dimensional hierarchical pore structure,large specific surface area,light weight,high conductivity,strong corrosion resistance,low production cost,and so on.And they have bright application prospects in the field of electrode materials for next-generation electrochemical energy storage devices represented by sodium-ion/potassium-ion batteries.Rational pore structure design and heterogeneous atom doping are important ways to improve the structure,composition,as well as electrochemical energy storage performance of carbon foams.However,it is still a great challenge to develop a simple and controllable method to achieve optimal design of carbon foams in these two aspects.There are many precursors for the preparation of carbon foams.Among them,polyacrylonitrile(PAN),which has low raw material cost,natural-contained nitrogen heteroatoms,controllable carbonization degree,and high carbon yield,is one of the most excellent carbon precursors.Carbon dioxide(CO2)is an environmentally friendly,cheap and highly adjustable fluid.CO2 aided batch foaming process can effectively control the pore structure of polymer matrix.However,due to the very close softening temperature and decomposition temperature of PAN materials,as well as no suitable temperature range for foaming,the research of PAN in the field of batch foaming is still in its infancy.Therefore,in this paper,based on PAN as polymer matrix,combined with the characteristics of CO2 aided batch foaming process,carbon foams with controllable pore structure from micron to nano scale were prepared.Moreover,the carbon foams were regulated and improved from the perspectives of open-cell structure design and heteroatom doping for application in electrochemical energy storage.Electrochemical performances and electrochemical mechanisms of the carbon foams as electrode materials were studied.The main research work and results are as follows:(1)An organic solvent assisted non-melting hot pressing strategy was designed for preparing a blocky PAN precursor which can effectively overcome the foaming problem of PAN.A wide foaming temperature window was first proposed for preparing PAN foams.The one-step foaming process of the precursor was studied,and the effect mechanisms of foaming temperature and saturation pressure on cell structure of the foams were revealed.The structural evolution of the foams during carbonization was clarified.The controllable preparation of cell structure of PAN-based carbon foams with cellular micron scale closed-cell structure and good conductivity was achieved.The results show that in a foaming temperature range of 100-150 ? and a saturation pressure range of 10.34-31.03 MPa,the cell density and cell size of PAN foams can be adjusted in ranges of 1.16 × 108-5.93 × 1010 cells cm-3 and 5.34-51.43 ?m,respectively,and the void fraction can be as high as 93.8%.After carbonization,the cellular structure of PAN foams is well preserved,and the cell size shrinkage degrees and density increase rates of different samples are very close.The electrical conductivities of the carbon foams are 92-132 S m-1,and the conductivity decreases with the increase of void fraction.(2)For the application of micron scale porous carbon foams in sodium-ion batteries,a preparation method of carbon foams through foaming and carbonization of polyacrylonitrile/polymethyl methacrylate(PAN/PMMA)dual-phase precursor was proposed,which solves the problem that it is difficult to acquire open-cell structure for PAN-based carbon foams.A series of carbon foams with controllable cellular structures from closed-cell to open-cell were obtained.The influences of precursor composition on the physical and chemical properties of the prepared carbon foams were revealed.Electrochemical performances of the carbon foams used as free-standing anodes for sodium-ion batteries were clarified.The results show that the proposed method can endow carbon foams with interconnected and stable open-cell structure,uniform micropores and mesopores on the carbon walls,and a high level of nitrogen doping.Under coaction of these characteristics,the carbon foams show outstanding sodium-ion storage performances.When the mass ratio of PAN to PMMA is 8 to 2,the fabricated carbon foams exhibit a high reversible specific capacity(281 mA h g-1 after 300 cycles at 50 mA g-1),superior rate performance(67 mA h g-1 at 10 A g-1),and excellent long-term cycling stability(175 mA h g-1 after 3000 cycles at 0.5 A g-1).(3)For the application of micron scale open-cell carbon foams in potassium-ion batteries,a phosphorylation modification strategy of PAN was designed.The strategy improves the preparation method of PAN/PMMA dual-phase precursor-based carbon foams,and overcomes the challenge of increasing the phosphorus content in carbon structure.A series of phosphorus/nitrogen co-doped micron scale open-cell carbon foams were obtained.The effect mechanisms of the strategy on the physical and chemical properties of the prepared carbon foams were studied.The electrochemical performances of the carbon foams used as free-standing anodes for potassium-ion batteries were clarified.The influences of phosphorus functional groups on potassium-ion storage were revealed.The results show that when the concentration of phosphoric acid is 55 wt%,the proposed phosphorylation modification strategy endows carbon foams with high phosphorus and nitrogen contents of 11.93 wt%and 6.97 wt%,respectively.Benefiting from the synergistic effect of three-dimensional open-cell structure and high contents of phosphorus/nitrogen,the carbon foams deliver an excellent reversible specific capacity(396 mA h g-1 at 0.1 A g-1 after 300 cycles)with high initial Coulombic efficiency(63.6%),a great rate performance(168 mA h g-1 at 5 A g-1)and an ultra-long cycling stability(218 mA h g-1 at 1 A g-1 after 3000 cycles).It is revealed that in phosphorus/nitrogen co-doped carbon materials,P-C bonds devote more to enhancing potassium-ion storage via adsorption and improving electronic conductivity of carbon,while P-O bonds contribute more to enlarging interlayer distance of carbon and reducing ion diffusion barrier.(4)A preparation method of carbon foams through two-step foaming and carbonization of a thin-film PAN precursor was proposed.The foaming process parameter window of PAN-based nano scale pore structure carbon foams was first revealed.The influences of foaming temperature and saturation pressure on pore structure of the carbon foams were studied.The formation mechanism of nano scale pore structure was elucidated,and the effect mechanisms of carbonization temperature on structure and composition of the carbon foams were revealed.The controllable preparation of PAN-based nano scale porous carbon foams was realized.The results show that under a saturation temperature of 40? and a foaming temperature of 80?:when the saturation pressure is below 31.03 MPa,carbon foams present interconnected nanoporous scaffold structure,which is under control of CO2 spinodal decomposition foaming mechanism;when the saturation pressure is 31.03 MPa,carbon foams show worm-like structure due to high solubility of CO2.With the increase of carbonization temperature from 600? to 900?,although the pore morphology of carbon foams changes inconspicuous,the ordering degree of carbon layer structure,the graphitization degree,the specific surface area and total pore volume increase,while the content of heteroatoms decreases.(5)For the application of nano scale porous carbon foams in potassium-ion batteries,carbonization temperature was optimized,and PAN-based carbon foams with nanoporous scaffold structure as anode materials for potassium-ion batteries were prepared.The differences between carbon foams with nanoporous scaffold structure and unfoamed carbon materials in structure,composition and electrochemical performance were clarified.The influence mechanisms of nanoporous scaffold structure on potassium-ion storage were revealed.The results show that the PAN-based nano scale porous carbon foams obtained by carbonization at 750? own moderate graphitization degrees,pecific surface area,pore volume,and heteroatom doping level.When used as anode materials for potassium-ion batteries,the carbon foams deliver a high reversible specific capacity(325 mA h g-1 at 0.1 A g-1 after 300 cycles),an excellent rate performance(118 mA h g-1 at 10 A g-1)and an ultra-stable cyclability(184 mA h g-1 at 1 A g-1 after 5000 cycles).The high electrochemical performances of the carbon foams can be ascribed to the synergic effect from interconnected micro-meso-macroporous structure and nitrogen/oxygen co-doping. |
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