煤基泡沫炭制备及其电化学性能影响研究进展

Research progress of preparation of coal-based carbon foam and factors affecting its electrochemical performance

泡沫炭具有比表面积大、孔隙率高、结构可控等特点,使其在超级电容器电极材料领域具有广阔的应用前景,而以煤、煤液化残渣、煤液化沥青、煤焦油沥青等煤系物为原料制备得到的煤基泡沫炭具有较高的导电性且制备工艺简单、原料来源广泛及产品价格低廉,因而可将煤基泡沫炭作为电极材料应用于超级电容器。基于不同的前驱体材料制备方法也有所区别、制备而得的泡沫炭性能及其应用场景也有差异,需对煤基泡沫炭制备及其电化学性能影响研究进展进行汇总分析。简介煤基泡沫炭的结构,剖析煤基泡沫炭制备方法中的分离焦化法、混合前驱法、高压渗氮法、限制膨胀法、超临界发泡法、自发泡法、模板法,阐述炭电极材料的比表面积、孔径分布、表面化学基团、石墨化程度对电化学性能的影响机制,为合理调控各影响因素之间关系及得到最佳电化学性能提供一定的参考。研究表明:较高的有效比表面积有利于煤基泡沫炭的比电容和能量密度,但其超高比表面积主要由微孔贡献,孔径较小的微孔会导致电解质离子和溶剂化离子无法进入孔内且在微孔孔口堆积,影响双电层的有效形成;理想炭电极材料具有三维分层多孔结构,相互连通的微—介—大孔有助于离子扩散和电子转移,发达的孔隙结构有助于提高倍率性能、功率密度和循环寿命;导电性取决于石墨化程度,炭化温度越高则石墨化程度越高、导电性越强,但过高的炭化温度会造成孔泡完整度下降,破坏泡沫炭三维分层孔结构,且使材料表面杂原子、官能团析出,降低材料润湿性及法拉第赝电容,造成比电容大幅下降;杂原子掺杂有助于提高材料润湿性与总比电容、贡献赝电容、调控结构和提升材料循环稳定性及电容保持率,但同时对石墨化程度、比表面积等也会产生不利影响。炭电极材料的比表面积、孔径分布、表面化学基团、石墨化程度4个因素相互影响与制约,找到合理的平衡点可有效调控其比电容、导电性、循环稳定性、倍率性能、能量密度、功率密度等,但在实际研究中难以兼顾炭电极材料比表面积、孔径分布等4个因素,因此需采用其他方法以实现煤基泡沫炭电化学性能的优化,未来需通过模板复制、物理化学活化、杂原子掺杂、二元/三元复合等手段以实现泡沫炭结构的设计和电化学性能的调控。

Carbon foam has broad application in the field of supercapacitor electrode materials due to its large specific surface area,high porosity and controllable structure.The carbon foam prepared from coal,coal liquefaction residue,coal liquefaction pitch,coal tar pitch and other coal-based materials has high conductivity,simple preparation process,wide source and low price of raw materials,which can be used as electrode materials for supercapacitors.Different precursors have different ways,different the the properties and application are,therefore,summarizing and analyzing the preparation and factors affecting electrochemical properties of coal-based carbon foam is necessary.The structure of coal-based carbon foam is briefly introduced and the methods of separation coking,mixed precursor,high pressure nitriding,limited expansion,supercritical foaming,spontaneous foaming and template are analyzed.The influence mechanism of specific surface area,pore size distribution,surface chemical group and degree of graphitization on electrochemical performance is expounded,which provides a reference for reasonably regulating the relationship between various factors and electrochemical performance.The results show that higher effective specific surface area is beneficial to the specific capacitance and energy density,but the ultra-high specific surface area is mainly contributed by micropores,which will cause the electrolyte ions and solvation ions inaccessible and accumulate at the micropore orifice,affecting formation of the electric double layer.The ideal carbon electrode materials have three-dimensional layered porous structure,the interconnected micro-meso-macropores will benefit to ion diffusion and electron transfer,and the advanced pore structure will profit to improve rate performance,power density and capacity retention.The conductivity depends on the degree of graphitization,the higher the carbonization temperature,the higher graphitization degree is,and the stronger the conductivity will be.However,high temperature will cause the decrease of structural integrity,destroying the three-dimensional layered porous structure,and the heteroatoms and functional groups of the materials will be separated out,reducing the wettability and Faraday pseudocapacitance of the materials,resulting in a significant decrease in specific capacitance.Heteroatom doping helps to improve the wettability and total specific capacitance of the material,which will contribute pseudocapacitance,regulate the structure and improve the cycle stability and capacitance retention rate of the materials,but it also has adverse effect on the degree of graphitization and specific surface area.The four factors of specific surface area,pore size distribution,surface chemical groups and degree of graphitization interact and restrict each other,there is a optimal experimental condition,which may effectively regulate its specific capacitance,conductivity,cycle stability,rate performance,energy density,power density,but it is difficult to take into account all of them in practical research simultaneously.Therefore,other methods are necessary in order to optimize the electrochemical performance.With template replication,physical and chemical activation,heteroatom doping,binary/ternary composite and other measures to design the structure of carbon foam and optimize its electrochemical performance will be a research highlights.