还原氧化石墨烯含量对MXene/还原氧化石墨烯基超级电容器电化学性能的调控机制研究

Tuning Mechanism of Reduced-Graphene Oxide Content on Electrochemical Performance of MXene/Reduced Graphene Oxide-Based Supercapacitors

二维过渡金属碳/氮化物(MXenes)和还原氧化石墨烯(rGO)在超级电容器电极材料的应用方面均展现出非常有潜力的性质。通过超声混合、冷冻干燥和高温碳化成功制备了碳化钛MXene/rGO海绵状复合材料。由该复合材料在有机电解液中组装而成的超级电容器在1 A·g^(-1)的电流密度下循环1×10^(4)圈,其电容保持率可维持在90%~110%,库伦效率100%;通过调节加入的rGO含量,该复合材料在2 mV·s^(-1)的扫描速率下,其最大的比电容值可达到80.0 F·g^(-1)。更重要地,增加rGO含量在导致MXene/rGO复合材料的界面电荷转移电阻上升的同时,也会引起其层间距和比表面积的增大,而这些因素之间相互竞争,共同影响MXene/rGO复合材料的电化学性能:适量的rGO通过增加复合材料的层间距和比表面积来改善其电化学性能,但过量的rGO会降低界面电荷转移速率而导致其电化学性能下降。这种储能机制也受到电解液类别的影响,在粘度较高的PC基电解液中,离子迁移缓慢,界面电荷转移成为限制电化学储能的主导因素。

Excessive use of fossil fuels and emission of greenhouse gases into the environment has become one of the major challenges nowadays.Clean renewable energy and how to use renewable energy safely,conveniently and more efficiently are the key to solve this problem,in which,supercapacitors,as one of the major categories of energy storage devices other than rechargeable batteries,play an important role.Because of its exceptional electrochemical properties,MXenes,the two-dimensional transition metal carbides/nitrides,hold great potential as energy storage materials.In this work,Ti_(3)C_(2)T_(x) MXene was mixed with another two-dimensional material,reduced graphene oxide(rGO),through a simple mixing,freeze drying and heat treatment fabrication process,and formed spongelike composite materials with different rGO ratios.Pristine rGO sponge material exhibited transparent,layered structure.With increasing MXene content,the structure became opaque and showed“long strip”morphology.There was no obvious change of morphology before and after the heat treatment.One thing to note that,there were some white dots observed on the carbonized samples,mainly because of Ti element in MXene oxidized to TiO2.The oxidation was confirmed by transmission electron microscope(TEM)test,X-ray diffraction(XRD)and Raman spectrum results showed the oxidation level was too low to be detected which should not interfere with the following electrochemical measurements.The asprepared composite materials then were assembled into supercapacitors,all with two different organic electrolytes with the same solute,tetraethylammonium tetrafluoroborate(TEABF_(4))but different solvents,acetoni-trile(AN)and propylene carbonate(PC).All supercapacitors exhibited long cycle life(90%to 110%capacitance retention and near-ly 100%coulombic efficiency over 1×10^(4)cycles at a current density of 1 A·g^(-1)).With adding rGO,the specific capacitance increased from 54.2 to 80 F·g^(-1)in 1 mol·L^(-1)TEABF_(4)-AN,while from 29.8 to 34.3 F·g^(-1)in 1 mol·L^(-1)TEABF_(4)-PC,respectively,at a scan rate of2 mV·s^(-1).Moreover,the optimum capacitance was obtained when the ratio of MXene/rGO was 9∶1,despite the fact that the interlayer spacing as well as the surface area/pore volume increased with increasing rGO ratio(pristine Ti_(3)C_(2)T_(x) sponge material had the smallest lattice spacing(d-spacing)of 1.33 nm and lowest specific surface area of 71.9 m^(2)·g^(-1),the composite materials with the 9∶1 MXene/rGO ratio had a d-spacing of 1.43 nm and specific surface area of 212.2 m^(2)·g^(-1)while the composite materials with the 7∶3 MXene/rGO ratio admitted the largest d-spacing of 1.57 nm and and highest specific surface area of 259.9 m^(2)·g^(-1)).Further electrochemical analysis revealed that charge transfer resistance also increased as increasing rGO ratio,meaning that with increasing rGO ratio,the diffusion of electrolytic ions to the electrode/electrolyte interface and the charge transfer on the interface should be retarded.For instance,the charge transfer resistance of the composite in 1 mol·L^(-1)TEABF_(4)(AN)increased from 2.1 to 37Ωwhen the MXene/rGO ratio de-creased from 10∶0 to 7∶3.Based on the above experimental results,the underlying mechanism of how rGO ratio to affect the electro-chemical performance of MXene/rGO composite materials was uncovered,which was the competition result of specific surface area,dspacing together with the ability to transfer charge.In addition,the time constant(τ)related to the diffusion of electrolytic ions was calculated from EIS data at low frequency,which also confirmed MXene/rGO with the composition of 9∶1 showed the lowest value of time constant,indicating the best ionic diffusion and charge transfer performance.There was another interesting phenomenon that for MXene/rGO with the ratio of 7∶3,in 1 mol·L^(-1)TEABF_(4)(PC),after cycling 1×10^(4)cycles at 1 A·g^(-1),its capacitance retention was as high as 211.9%.To explain this interesting phenomenon,it is critical to consider the unique structural changes of our composite with different rGO ratios together with the properties of the electrolyte itself.As known,the viscosity of PC was higher than AN,leading to slower movement of electrolytic ions.For MXene/rGO in the AN-based electrolyte,ion diffusion was fast and complete,while that was slow and incomplete at the beginning of the cycling for MXene/rGO in the PC-based electrolyte,which could also be confirmed by the time constant results,that supercapacitors with AN-based electrolyte admitted lower time constant.As mentioned above,MXene/rGO with the ratio 10∶0 and 9∶1,though with considerable specific surface area,their interlayer spacing were smaller,limiting the diffu-sion of electrolytic ions into the deeper sites within the structure,as the cycling went on,more electrolytic ions gradually reached to the deeper sites,leading to this increase of energy retention.In summary,this work presented the fundamental analysis between elec-trode structures and the electrochemical performance,and the effects of rGO content on the electrochemical performance of MXene/rGO based supercapacitors were systematically studied,that upon changing the rGO ratio,d-spacing,specific surface area and charge transfer resistance changed as well,and the final electrochemical performance was the competition results of them.