Research progress of MXene composite aerogels in the field of electrochemical energy storage
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摘要: MXene材料目前已在电磁屏蔽、传感、污水处理等多个领域具有广泛应用,其优异的电化学性能使得其在储能领域也展现出广阔的应用前景。然而,MXene的自堆叠与易氧化等特性,限制了其进一步发展。将MXene组装成三维(3D)结构复合材料是解决上述问题的有效途径之一。3D多孔结构能为离子传输/存储提供更多通道和活性位点,可有效提高电化学性能。本文主要回顾MXene复合气凝胶的最新研究进展,详细阐述MXene复合气凝胶的制备方法以及其在电池、超级电容器等储能方面的应用。最后,对其发展方向进行了展望。Abstract: MXene materials have been widely used in many fields, such as electromagnetic shielding, sensing, and wastewater treatment. The excellent electrochemical properties of MXene makes it demonstrate broad application prospects in the field of energy storage as well. However, the self-stacking and easy oxidation characteristics of MXene limit its further development. The assembly of MXene into three-dimensional (3D) structural composites is one of the effective way to solve the above problems. The 3D porous structure can provide more channels and active sites for ion transport or storage, which can effectively improve the electrochemical performance. This article reviews the recent researches of MXene composite aerogels. The preparation methods and applications of MXene composite aerogels in energy storage, such as batteries and supercapacitors, are discussed in detail. Finally, the prospects for its future development direction were also presented.
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Keywords:
- MXene /
- aerogel /
- composites /
- preparation processes /
- electrochemical energy storage /
- batteries /
- supercapacitors
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进入21世纪以来,电子技术发展迅速,各类便携式电子设备、电动汽车及目前新兴的智能设备等占据了人们的生活,这些设备使人们对储能设备的能量密度、循环稳定性等提出了更高的需求[1-2]。
MXene是一种新型二维过渡金属碳化物、氮化物、碳/氮化物。MXene是指从母相MAX中选择性去除A层,其中,MAX相是层状三元化合物,通式为Mn+1AXn (n= 1, 2, 3),M代表早期过渡金属,A为主族元素(主要为IIIA或IVA),X表示C和或N。由于蚀刻过程中A层被各类官能团取代,因此MXene表面含有大量的表面末端,如—F、—OH、—Cl等。MXene通式表示为Mn+1XnTx,其中T是表面基团,x是末端数目,为了写得简单明了,Mn+1XnTx一般缩写为Mn+1Xn,如Ti3C2、Ti2C[3-4]。目前已知存在60多种MAX相[5],根据MAX的初步合成方法及新的合成前驱体的发展,确定并预测了200多种稳定相MXene(主要是Ti3C2Tx、Ti2CTx、Nb2CTx、Mo2CTx、Ti4N3Tx、Ta4C3Tx、Cr2TiC2Tx、V2CTx、Zr3C2Tx (Nb0.8Zr0.2)4C3Tx等)[6-7]。MXene的结构灵活性和表面可调节化学的多样性使其成为一种广泛应用的材料。元素“M”的化学多样性使MXene适用于各种应用。例如,V基MXene离子扩散势垒较低,具有良好的储能特性;相比之下,Nb基MXene由于其抗磁行为而表现出磁性相变;而Mo基MXene在电催化和热电领域具有潜力[8]。迄今为止,70%以上的MXene研究都集中在首次发现的Ti3C2Tx MXene上[9]。由此,Ti3C2Tx具有成熟的合成技术,在储能领域应用最广泛[7]。
MXene具有类似石墨烯的碳层结构,表现出良好的导电性;过渡金属层,可表现出类似过渡金属氧化物的性能,赋予MXene良好的储能性能;独特的手风琴层状结构,使层间距加大,有利于插层其他材料,进一步增加材料的密度和体积比容量。优异的电化学性能和力学性能使MXene在储能领域获得广泛关注[10]。然而,与其他二维(2D)材料类似的是,MXene组装过程受到分子间范德华力和氢键[11]的影响,导致片层产生自堆积现象,降低MXene的电子传输能力,造成电化学性能衰减。将2D的层状结构,组装成三维(3D)多孔结构,是一种有效解决MXene自堆积的策略[12],这种3D结构能够提供比一维(1D)和2D结构更大的孔隙率、更高的比表面积和更短的离子传输距离。目前,有多种形成3D MXene结构的方法。Xu及其团队[13-15]利用真空抽滤的方法将2D MXene纳米片与其他材料(如活性炭颗粒、硬炭等)结合,构筑3D导电网络。应用于电化学储能领域时,以MXene为粘结剂和骨架的一体化电极提高了储能器件的电容和倍率性能。此外,发泡法也是构建3D MXene结构的一种途径。通过物理或化学过程将大量气体引入材料中,形成相互渗透或封闭的多孔结构,从而克服范德华力,扩大层间距,达到改善电化学性能的效果[16]。
气凝胶是当前最常用的制备3D结构的方法之一。然而,由于MXene材料横向尺寸小且具有亲水性等,致使其不易形成稳定3D结构。因此,大量研究人员通过将MXene与其他材料结合,制备复合气凝胶以试图解决这一问题。Zhuo等[17]将MXene与纤维素纳米晶体结合制备了一种可压缩、弹性的MXene复合气凝胶;Wang等[18]利用芳纶纤维与MXene集成制备复合气凝胶;Yang等[19]采用明胶辅助MXene形成复合气凝胶。鉴于此,本文主要讨论MXene复合气凝胶的制备方法,并总结其在电化学储能领域的应用研究现状,最后展望了其所面临的挑战和今后发展方向。
1. MXene复合气凝胶的制备
具有3D结构的气凝胶,孔结构丰富,比表面积大,能够负载更多的活性材料,提高储能能力。制备MXene复合气凝胶能有效避免该2D材料的自堆叠,并进一步改善电化学性能。
MXene片层的制备是复合气凝胶合成必不可少的步骤。其他2D材料通常采取各类物理或化学剥离方式,将3D前驱体直接剥离或分层为超薄的单层或少层2D薄片[20]。与此不同的是,MXene在自然界中没有直接的前驱体,通常是从相应的MAX相中派生而来的。由于M—X键(共价键和离子键)和M—A键(金属键)的合成能不同,使得从MAX相选择性去除“A”层成为可能。通过这种方式,可以由其母相MAX合成多层MXene,再通过进一步的分层或剥离,获得单层或少层MXene薄片[21]。采用湿法刻蚀(HCl+LiF)体系从Ti3AlC2 MAX材料中选择性脱除Al是制备Ti3C2Tx MXene薄片最常用的方法[22-23]。
由于MXene纳米片表面存在着相互排斥的亲水性基团(—OH、—O等),使相邻的纳米片难以直接交联,因此常采用与其他材料结合的方法制备3D MXene复合气凝胶。目前广泛使用的制备方法主要包括自组装法、模板法、化学交联法、3D打印、溶胶-凝胶法、离子诱导组装等。
1.1 自组装法
自组装法[12]是基于石墨烯材料制备MXene复合气凝胶的一种常用策略,是指将氧化石墨烯(GO)作为前驱体均匀分散在液相中,随后将GO还原成具有共轭结构的还原氧化石墨烯(rGO),或者直接用rGO作为前驱体,在一定条件下进行自组装形成MXene/石墨烯复合水凝胶,再利用干燥技术转变成复合气凝胶。其具有独特的多孔骨架结构,不仅在很大程度上阻止了MXene纳米片的堆积,而且赋予复合气凝胶高的导电性。以下是最常见的两种自组装方法。
1.1.1 水热还原组装法
水热还原组装法是将含有GO的混合溶液置于密闭的高压反应釜,经过一定反应时间消除GO含氧官能团,GO片层间相同电荷间斥力作用减弱,片层相互交联,从而形成3D结构的水凝胶。
Zhao等[24]将制备的单层TiVC MXene水溶液与GO水溶液混合,经过水热反应制备了超轻多孔TiVC/rGO双金属气凝胶(TVGA)。如图1(a)中(I)所示,混合溶液在高压反应釜中经过180℃、12 h水热条件构筑3D结构水凝胶,随后冷冻干燥转化为气凝胶。图1(a)中(II)、(III)显示了超轻多孔TVGA气凝胶的光学照片,所制备的电极片具有极好的柔性。气凝胶丰富的孔结构不仅有利于离子的输运,而且易于CO2和电解质的扩散,从而降低传质和荷电阻力。Liu等[25]通过引入N、S等杂原子增强复合材料的电化学性能。采用简便的水热法,将MXene溶液、GO溶液和硫脲以一定比例混合在反应釜中,在120℃下反应6 h后,所得产物用去离子水反复洗涤至pH达到7,最后将水凝胶真空冷冻干燥24 h得到S, N-rGO@MXene复合气凝胶。尽管水热法能够有效辅助形成气凝胶,但高温条件会加速MXene的氧化,破坏其片状结构,降低其电导率。
图 1 (a) 超轻多孔TiVC/还原氧化石墨烯(rGO)双金属气凝胶(TVGA)的合成及Li-CO2电池原理图(I)、TVGA气凝胶光学图(II)和具有良好柔韧性的TVGA阴极片的光学图(III)[24];(b) 制造Ti3C2Tx MXene-石墨烯复合气凝胶(MGA)材料的示意图(I)和在无约束条件下MGA材料的顶部(左)和侧视图的SEM图像(II)[27];(c) Mo-Ti/MXene (Mx)-rGO-纤维素纳米纤维(GN)气凝胶(Mo-Ti/Mx-GN)合成工艺示意图(I)和SEM图像(II)[29];(d) 3D多孔1T MoS2/Ti3C2Tx复合气凝胶 X-Z和X-Y横截面SEM图像[30];(e) 3D打印V2CTx/rGO-碳纳米管(CNT)微栅气凝胶电极的制备过程示意图(I)和30%V2CTx/rGO-CNT微格栅气凝胶的SEM图像(II)[40];(f) 3D打印各种结构的模板[41];(g) Ti3C2Tx/碳纳米纤维(CNF)复合气凝胶制备工艺示意图(I)和Ti3C2Tx/CNF6-2的SEM图像(II)[43];(h) 具有褶皱结构的MXene平台器件(上)和Mg2+-MXene气凝胶的SEM图像(下) [45]Figure 1. (a) Synthesis of ultralight porous TiVC/reduced graphene oxide (rGO) aerogels (TVGA) and schematic diagram of Li-CO2 battery (I), optical photos of block TVGA (II) and TVGA cathode sheet with good flexibility (III)[24]; (b) Schematic illustration of fabricating Ti3C2Tx MXene and graphene aerogel (MGA) material (I), SEM images of top (left) and side views under unconstrained conditions for the MGA material (II)[27]; (c) Schematic diagram illustrating the synthesis process (I) and SEM images (II) of the metallic 1T MoS2 and rich oxygen vacancies TinO2n–1/MXene hierarchical bifunctional catalyst (Mo-Ti/Mx) anchored on a rGO-cellulose nanofiber (GN) host (Mo-Ti/Mx-GN) aerogel[29]; (d) X-Z and X-Y cross-sectional SEM images of 3D highly-porous 1T MoS2/Ti3C2Tx composite aerogel[30]; (e) Schematic diagram of the preparation process of a 3D-printed V2CTx/rGO-carbon nanotube (CNT) microgrid aerogel electrode (I), SEM image of the 30%V2CTx/rGO-CNT microgrid aerogel (II)[40]; (f) 3D printing templates for various structures[41]; (g) Schematic illustration of the fabrication process of Ti3C2Tx/carbon nanofiber (CNF) composite aerogel (I), SEM image of Ti3C2Tx/CNF6-2 (Ⅱ)[43]; (h) MXene platform device with folded structure (top), SEM image of a Mg2+-MXene aerogel (bottom)[45]GO—Graphene oxide; ΔT—Temperature1.1.2 化学还原组装法
化学还原组装法是利用还原剂对GO进行还原,再利用去离子水、乙醇等去除还原剂。L-抗坏血酸是一种广泛使用的绿色还原剂。Shao等[26]在室温下利用L-抗坏血酸还原GO制备MXene/rGO复合气凝胶。将制备好的GO分散体与Ti3C2Tx纳米片分散体混合并超声分散均匀,然后加入L-抗坏血酸,室温静置约3天后,收集MXene/rGO复合水凝胶,随后在乙醇溶剂中透析以去除残留物。最后将复合水凝胶冷冻干燥形成复合气凝胶。Zhou等[27]使用L-抗坏血酸做还原剂的同时利用溶剂热法辅助复合水凝胶的形成。如图1(b)中(I)所示,将GO和MXene分散到去离子水中,超声处理后将L-抗坏血酸加入溶液中,然后在65℃下密封3 h,75℃下密封1 h。溶剂热反应后,清洗复合水凝胶产物以消除未反应的化学物质,最后利用冷冻干燥形成复合气凝胶。图1(b)中(II)显示了Ti3C2Tx MXene-石墨烯复合气凝胶(MGA)在非加压条件下的顶部(左)和横截面(右)的SEM图像。MGA具有平坦的顶部表面,没有可见的孔隙。这种顶部与横截面形状显著减小了电解质与电极之间的接触面积,并抑制了副反应。同时,多孔的横观形貌提供了丰富的空间,为后续优化镀锌提供了条件。除了L-抗坏血酸外,氨水、水合肼等也可作为还原剂还原GO。Wang等[28]利用氨水还原,在高压反应釜中组装MXene/rGO复合材料的3D结构,经过冷冻干燥后制得复合气凝胶。
1.2 模板法
模板法主要指在反应体系中加入模板,根据模板的自身特点来调节制备材料的结构。在气凝胶的制备中加入具有3D骨架结构的材料,依托3D材料的结构来形成MXene复合气凝胶。
冰模板法(又称冷冻铸造)已被广泛应用于气凝胶的制备,它是通过冷冻过程中冰晶的生长来调控所制备产物的3D结构。依据冰晶的生长方式可分为单向冷冻铸造和双向冷冻铸造。Xia等[29]提出了一种金属1T MoS2和富氧空位TinO2n−1/MXene分级双功能催化剂(Mo-Ti/Mx)锚定在rGO-纤维素纳米纤维(GN)主体(Mo-Ti/Mx-GN)上,采用定向冷冻,实现了由长程排列的波浪形多拱形态组成的柔性GN支架的层次结构(图1(c)(I))。这种定向冷冻是通过充入液氮产生温度梯度,由于冰核呈水平方向生长,产生了长程排列的平行冰柱。如图1(c)中(II)所示,这种薄层在完全干燥后可皱缩成波浪形层,为循环时硫/Li2S转化之间的体积膨胀与收缩提供足够的缓冲空间。Bo等[30]同样使用具有丰富氧化还原化学性质和较高理论电容的1T MoS2制备复合气凝胶。所不同的是,采用双向冷冻铸造技术,在水平和垂直温度梯度下,双向生长的冰晶被模板化,冰晶的双向成核与生长扩大了薄片之间的空间(图1(d)),同时组装分散的纳米片成水平和垂直排列的结构。随后,真空干燥除去冰晶,得到Ti3C2Tx连接的1T MoS2复合气凝胶。这种方法形成的多孔排列结构可以为离子的快速传输提供丰富的开放通道,减少离子的扩散路径。
此外,也可通过物理浸渍法保留模板,如Chen等[31]为避免高温炭化,将具有超高电导率和良好电化学活性的MXene涂覆在天然木材细胞壁上,以天然木材切片为3D模板,负载MXene制备复合气凝胶。
1.3 化学交联法
尽管部分材料能够依靠分子间的范德华力组装成3D气凝胶结构,但仍存在仅仅依靠分子间作用力组装的片层结构不够理想的情况。为此,通常选择加入交联剂辅助形成MXene复合气凝胶,如乙二胺[32-34]、乙二醇[35]、海藻酸钠[36]、聚乙烯醇[37]等,使得材料间产生高分子交联结构,增强气凝胶的稳定性和力学性能。除此之外,Pei等[38]以三聚氰胺甲醛树脂为交联剂,通过氢键和静电作用,将Ti3C2Tx MXene和三聚氰胺甲醛树脂集成3D互连多孔网络结构。Yang等[39]以L-半胱氨酸为交联剂,L-抗坏血酸为还原剂,通过MXene和GO纳米片表面丰富的—OH与L-半胱氨酸的—NH2和—SH反应,使MXene和rGO纳米片相互交联,形成3D大孔结构。
1.4 3D打印
3D打印近些年发展迅速,通过使用数字技术分层打印可以形成特定形状。这项技术应用于3D气凝胶材料的构建,具有可自定义结构与尺寸、制备迅速等优点。Wang等[40]选用具有较高的电化学活性和良好的离子导电性的V2CTx MXene材料,采用3D打印技术成功制备了V2CTx/rGO-碳纳米管(CNT)微网格气凝胶,并将其作为钠金属阳极的基体材料,如图1(e)中(I)所示。图1(e)中(II)的SEM图像显示了分层多孔3D打印结构,不仅保持了电极结构的稳定性,而且提供了较大的比表面积,增强了Na+电子传输动力。另外,3D打印技术可实现材料结构自行设计调控[41],展现出高度的结构设计自由度(图1(f))。
1.5 溶胶-凝胶法
溶胶-凝胶法(Sol-Gel法,简称S-G法)是以无机物或金属醇盐作前驱体,在液相中将这些原料均匀混合,并进行水解、缩合化学反应,在溶液中形成稳定的透明溶胶体系,溶胶经陈化、胶粒间缓慢聚合,形成3D空间网络结构凝胶。凝胶经过干燥、烧结固化制备出分子乃至纳米亚结构的材料。
Zhou等[42]设计了一种独特的MXene/SiO2杂化气凝胶多孔材料。首先将正硅酸四乙酯(TEOS)加入MXene水分散液中,通过水解和聚合反应可以将TEOS转化为SiO2。SiO2表面的羟基官能团可以与Ti3C2Tx MXene的含氧官能团形成氢键,进一步形成MXene/SiO2杂化水凝胶,最后通过真空冷冻干燥制备该气凝胶。所制备的多孔材料表现出由Ti3C2Tx MXene和SiO2组装的3D互连网络结构。Liu等[43]以聚丙烯腈(PAN)纳米纤维作为交联和插层构件制备了具有“层状结构”支撑3D微观结构的Ti3C2Tx/碳纳米纤维(CNF)复合气凝胶。利用两种材料之间的强界面相互作用,实现了2D Ti3C2Tx薄片与1D PAN纳米纤维的3D组装,如图1(g)中(I)所示,可有效地控制少量层状Ti3C2Tx薄片的再堆叠。制备的Ti3C2Tx/CNF复合气凝胶具有高度有序的微孔隙结构,CNF贯穿于层状MXene薄片之间,使复合气凝胶具有良好的结构稳定性和柔韧性,微观结构中的孔隙为电子传输和离子迁移提供了方便的通道(图1(g)(II))。
1.6 离子诱导组装
离子诱导的材料组装主要分为两种[44]:(1) 阳离子通过改变溶液的离子强度或者中和纳米材料表面的电荷,使纳米材料间的吸引力大于排斥力进而引发纳米材料的组装;(2) 二价或者多价离子通过与纳米材料表面的配体形成化学键,作为连接纳米材料之间的桥梁,使纳米材料发生组装。Ding等[45]设计了一个具有大比表面积的褶皱结构的Ti3C2Tx MXene平台(图1(h)(上)),以促进Mg2+诱导组装。利用气凝胶蜂窝状的孔隙(图1(h)(下))及Mg2+与MXene纳米片的—OH形成强络合,减少MXene自堆叠,使大面积的Mg2+-MXene气凝胶在没有聚合物粘结剂的情况下形成。同时,Mg2+的嵌入在很大程度上保留了MXene纳米片的导电特性。
2. MXene复合气凝胶在电化学储能领域的应用
2.1 碱金属离子电池
Ti3C2Tx在碱金属离子电池,如Li+、Na+、K+电池的电化学存储方面具有很大的潜力。MXene由于具有—OH、—O、—F官能团和高电导率,在锂离子电池中具有广阔的应用前景。MXene复合气凝胶丰富的孔结构能够促进锂离子的传输,提高锂离子电池的容量。Butt等[46]通过增强两组分之间的静电相互作用,将Nb2C MXene片分散到GO片上形成多孔结构,从而促进离子的运动,为离子提供插层位。这种杂化多孔Nb2C-rGO气凝胶作为锂离子电池的阳极,在0.05 A·g−1下,具有357 mA·h·g−1的容量,即使在1 A·g−1下循环1000次后也表现出极好的稳定性。
钠离子储能系统以其低成本和丰富的资源成为下一代储能的候选技术[47]。Song等[48]利用氨基丙基三乙氧基硅烷、Mn2+、Fe2+、Zn2+和Co2+作为界面介体,在室温下实现GO辅助组装Ti3C2Tx MXene复合气凝胶(MGA)。在此基础上结合硫改性工艺制备硫改性MGA (SMGA),作为钠离子储存的自支撑阳极时,显示出良好的电化学性能。在12.3 mg·cm−2的超高负载质量以及0.1 A·g−1的电流密度下,获得了1.26 mA·h·cm−2的面积容量。得益于室温条件下的组装,该Ti3C2Tx/rGO整体复合气凝胶具有良好的结构稳定性和多孔性(图2(a)),不会引起Ti3C2Tx的明显氧化降解。图2(b)显示了SMGA电极在不同扫速的CV曲线。进一步组装的混合钠离子电容器能很好地为发光二极管(LED)阵列和电子温度计供电(图2(c))。
图 2 (a) 氧化石墨烯(GO)辅助组装Ti3C2Tx MXene复合气凝胶经硫改性(SMGA)后的SEM图像[48];(b) SMGA电极在0.1~3 mV·s−1不同扫描速下的CV曲线[48];(c) 组装活性炭(AC)//SMGA SiC供电LED阵列和电子温度计的数码照片[48];(d) 锑单原子和量子点(~5 nm)共修饰Ti3C2Tx MXene气凝胶(Sb SQ@MA)的SEM图像[50];(e) Sb SQ@MA与钾离子电池(PIBs)中其他报道的Sb基阳极的倍率性能比较[50];(f) Sb SQ@MA长期循环性能[50]Figure 2. (a) SEM image of graphene oxide (GO)-assisted assembly of Ti3C2Tx MXene aerogel after sulfur modified (SMGA)[48]; (b) CV curves of the SMGA electrode at different scan rates from 0.1-3 mV·s−1[48]; (c) Digital photos of LED arrays and electronic thermometer powered by the assembled activated carbon (AC)//SMGA SiC[48]; (d) SEM image of antimony single atoms and quantum dots (~5 nm) codecorated Ti3C2Tx MXene-based aerogels (Sb SQ@MA)[50]; (e) Comparison of the rate performance of the Sb SQ@MA with other reported Sb-based anodes in potassium-ion batteries (PIBs)[50]; (f) Long-term cycling performance of the Sb SQ@MA[50]NC—N-doped three-dimensional porous carbon由于锂资源成本的昂贵与稀缺[49],而钾离子电池具有丰富的天然钾储量、高的离子电导率以及与锂相似的化学性质,引起广泛关注。但钾离子电池的大半径使钾化过程体积膨胀大,动力学迟缓。Guo等[50]制备了用于高性能钾离子电池(PIBs)的锑单原子和量子点(~5 nm)共修饰Ti3C2Tx MXene气凝胶(Sb SQ@MA)(图2(d))。发现原子分散的Sb可以改变Sb/Ti3C2Tx复合材料的电子结构,改善电荷转移动力学,提高异质界面的储钾能力。采用GO辅助的3D多孔Ti3C2Tx MXene复合气凝胶具有丰富的表面官能团和缺陷,为复合材料提供了丰富的锚定位点,同时克服了MXene的自堆叠,增强了复合材料的结构稳定性和高效的电子传递能力。高载量的Sb (~60.3wt%)和短的离子传输途径是理想的钾储库。这些特性协同提高了PIBs中Sb SQ@MA电极的性能,且优于大多数先前报道的Sb基纳米复合材料(图2(e))。该电极在0.1 A·g−1下表现出521 mA·h·g−1的可逆容量,在1 A·g−1循环1000次后仍保持94%的高容量(图2(f))。
2.2 锂硫电池
锂硫电池具有理论能量密度高、生产成本低等优点,被认为是一种很有前途的新一代储能系统[51-53]。然而,严重的多硫化物穿梭效应、导电性差及硫阴极的剧烈体积变化等问题阻碍了锂硫电池的商业化[54]。为了解决这些问题,近年来许多研究集中在具有丰富表面官能团和优异机械柔性的MXene纳米材料。
Liu等[55-57]试图利用2, 2, 6, 6-四甲基哌啶氮氧化物(TEMPO)氧化纤维素纳米纤维(TEMPO-oxidized cellulose nanofibers,T-CNF)解决MXene材料堆积的问题。首先,将1D T-CNF和2D rGO纳米片作为“钢筋”和“水泥”,构建了一个坚韧的骨架,单层MXene均匀地包裹在骨架上,形成了具有高离子与电子导电性的MXene包覆的3D复合气凝胶(MCG)。复合材料中MXene、rGO和T-CNF的质量比6∶4∶4,即标记为MCG-6。如图3(a)所示,MCG-6复合气凝胶中单个孔隙的直径约为5 μm,有利于电解质离子的快速传输,增加有限区域内的硫负载量,缓解硫的体积膨胀。该复合气凝胶无需任何粘合剂即可用作自支撑电极,这种3D结构为硫的体积膨胀提供了空间和可支撑的韧性框架(图3(b))。具有单层MXene涂层的T-CNF/rGO杂化气凝胶作为锂硫电池的阴极材料在0.1 C下表现出1470 mA·h·g−1的高放电容量(图3(c))和在5 C下744 mA·h·g−1的倍率容量。接着,又采用冷冻干燥和碳化法将Ti3C2Tx MXene和T-CNF复合,形成MXene/T-CNF 3D复合气凝胶结构,在0.1 C的电流密度下具有1119.6 mA·h·g−1极佳的可逆放电比容量,200次循环后容量保持率为99.4%。最后,为了进一步探究不同电荷状态(带正电荷或负电荷)的纤维素纳米纤维(CNF)和分层MXene (d-MXene)复合材料对锂硫电池电化学性能的影响,Liu等[57]采用冰模板定向组装法将两种不同类型的纤维素纳米纤维(阳离子纤维素纳米纤维(C-CNF)和T-CNF)与d-MXene复合,分别制备了d-MXene/C-CNF (MC)和d-MXene/T-CNF (MT)复合气凝胶,MXene在整个混合溶液中的百分比分别为60wt%、40wt%时,即标记为MC-6、MT-4。结果表明,d-MXene与C-CNF静电相互作用更强,结合更牢固,有效抑制了多硫化物的穿梭效应,MC-6复合气凝胶具有规则的孔结构和孔分布(图3(d)),作为锂硫电池阴极具有超高容量,0.1 C时容量为1573.7 mA·h·g−1 (图3(e))和优异的长期循环性能(200次循环后容量留存率为96.3%);d-MXene与T-CNF在水溶液中具有更好的分散特性,制备的复合气凝胶具有更好的多孔结构(图3(f)),有利于锂离子的快速扩散,MT-4复合气凝胶阴极具有优异的倍率性能,在2 C电流密度下容量为848.1 mA·h·g−1(图3(g))。
图 3 (a) MXene涂层三维气凝胶(MCG-6)复合气凝胶在不同放大倍数的SEM图像[55];(b) MCG-6复合气凝胶电极提高锂硫电池动力学反应速率的示意图[55];(c) MCG-6电极在各种速率下的恒流充放电曲线[55];(d) MXene/C-CNF复合气凝胶(MC-6)的SEM图像[57];(e) MC-6复合气凝胶电极在不同速率下的恒流充放电曲线[57];(f) MXene/T-CNF复合气凝胶(MT-4)的SEM图像[57];(g) MT-4复合气凝胶电极在不同速率下的恒流充放电曲线[57];(h) 层状(PA) MXene-CNT-50复合气凝胶的SEM图:(I) 俯视图(插图是PA MXene/CNT-50整体的照片);(II) 侧视图; (III) 高放大率图[54]Figure 3. (a) SEM images of MXene-coated three-dimensional aerogel (MCG-6) composite aerogel under different magnifications[55]; (b) Illustration of MCG-6 composite aerogel electrode enhancing the kinetic reaction rate of lithium-sulfur battery [55]; (c) Galvanostatic charge and discharge curves of MCG-6 electrodes at various rates[55]; (d) SEM image of MXene/C-CNF composite aerogel (MC-6)[57]; (e) Galvanostatic charge and discharge curves of MC-6 composite aerogel electrode at various rates[57]; (f) SEM image of MXene/T-CNF composite aerogel (MT-4)[57]; (g) Galvanostatic charge and discharge curves of MT-4 composite aerogel electrode at various rates[57]; (h) SEM images of the parallel-aligned (PA)-MXene/CNT-50 composite aerogel: (I) Top-view (Inset is a photo of the PA-MXene/CNT-50 monolith); (II) Side-view; (III) High magnification[54]T-CNF—TEMPO-oxidized cellulose nanofibers; C-CNF—Cation-CNF; TEMPO—2, 2, 6, 6-tetramethylpiperidoxylZhang等[54]采用单向冷冻干燥法制备了Ti3C2Tx MXene/CNT三明治结构的层状复合气凝胶(PA-MXene/CNT),MXene 的重量百分比为50wt%时,即标记为PA-MXene/CNT-50。PA-MXene/CNT-50三明治的片层(图3(h))形成多重物理屏障,结合MXene的化学捕获和催化活性,有效地抑制了高硫负载下的多硫化锂(LiPS)穿梭,更重要的是,显著提高了无微孔和介孔的三维宿主的LiPS约束能力。组装好的锂硫电池在硫负载为7 mg·cm−2时,可提供712 mA·h·g−1的高容量。同时,具有优越的循环稳定性,在0.5 C下循环800次后,每次循环的容量衰减仅为0.025%。即使在10 mg·cm−2的硫负载下,300次循环后也可获得高于6 mA·h·cm−2的面积容量。
2.3 超级电容器
超级电容器具有快速充放电速率、高功率密度和优异的循环稳定性等优点,在储能领域,特别是高功率输出情况下具有潜在的应用前景。超级电容器的电化学性质在很大程度上取决于电极材料。在现有电极材料中,2D材料具有高比表面积、特殊的电子性能和丰富的电化学活性位点,因其双层或赝电容储能机制,被广泛用于超级电容器和其他储能设备中[58]。MXene材料作为超级电容器的电极具有良好的电容和循环稳定性,通过材料组成和结构的设计,可以进一步提高MXene基电极的性能[59]。
Zheng等[32]采用水热和湿化学两步法,将MXene、NiCO2-层状双氢氧化物(Layered double hydroxides,LDHs)和rGO相结合,制备了MXene复合气凝胶(NiCO2-LDHs@MXene/rGO)。以该复合气凝胶为阴极,MXene/rGO为阳极的典型混合超级电容器(HSC)器件,在700 W·kg−1的功率密度下提供了65.3 W·h·kg−1的优越能量密度,并在5 A·g−1下10000次循环后保持92.8%的容量保持率,展现了优异的稳定性和极好的能量及功率密度。如图4(a)所示,将两个HSC器件串联,可以为温度计供电超过8 min。
图 4 (a) 两个混合超级电容器(HSC)器件串联,为温度计供电超过8 min[32];(b) (I)自愈机制示意图和光学图;(II) 自愈后的聚氨酯(PU)展示:弯曲状态(左上),平坦状态(左下),支撑500 g质量(右),矩形表示伤口/愈合的位置;(III) MXene/rGO 复合气凝胶的SEM图像[60];(c) (I) MLSG-6复合气凝胶的SEM图像;(II) 显示LSG//MLSG-6不对称装置与其他最新超级电容器相比的面积能量和功率密度的Ragone图;(III) 由一个LSG//MLSG-6不对称超级电容器供电的小灯泡[61]Figure 4. (a) Two hybrid supercapacitor (HSC) devices connected in series to power a thermometer more than 8 min[32]; (b) (I) Schematic diagrams and optical images of the self-healable mechanism; (II) Demonstration of after self-healing polyurethane (PU): Under bending state (top left), under flat state (bottom left), supporting a 500 g mass (right) (Rectangles indicate the wound/healing positions); (III) SEM image of MXene/rGO composite aerogel[60]; (c) (I) SEM images of MLSG-6 composite aerogel; (II) Ragone plot displaying areal energy and power densities of LSG//MLSG-6 asymmetric device in comparison to the other state-of-the-art supercapacitors; (III) A small bulb powered by one LSG//MLSG-6 asymmetric supercapacitor[61]PANI—Polyaniline; ESCNF—Electrospinning carbon nanofiber; PEDOT—Poly(3, 4-ethylenedioxythiophene); PSS—Polystyrene sulfonate; MLSG—Lignosulfonate (LS) modified-MXene (Ti3C2Tx)-reduced graphene oxide; LSG—LS-functionalized reduced graphene oxide; CDC—Carbide-derived carbons为解决3D电极在实际应用中容易受到机械变形破坏的问题,Yue等[60]采用自愈合聚氨酯(PU)作为外壳,制备了一种由Ti3C2TxMXene/rGO复合气凝胶电极组成的自愈合3D超级电容器(MSC)。图4(b)中(I)显示了器件的自愈机制,通过切割界面的氢键作用实现。尽管自愈器件表面存在愈合疤痕,但仍具有较好力学性能(图4(b)(II))。如图4(b)中(III)所示,该气凝胶结合了rGO的大比表面积和MXene的高电导率,不仅可以防止MXene层状结构的自堆叠,而且可以在一定程度上避免MXene的不良氧化。基于3D MXene/rGO复合气凝胶的MSC在1 mV·s−1的扫描速率下提供34.6 mF·cm−2的大面积比电容及在15000次循环中电容保持率高达91%的出色的循环性能。3D MSC具有良好的自愈能力,第5次愈合后比电容保持率为81.7%。这种自愈合三维MSC的制备,为设计和制造下一代长寿命多功能电子器件提供了一种新策略。
同样,为了解决MXene片层自堆叠对电容器性能的影响,Ma等[61]注意到木质素磺酸盐(LS)的p-π共轭结构赋予α、β碳较强的化学反应活性和局部正电位,能够修饰MXene表面,避免重堆问题。由此合成了LS修饰的MXene/rGO 3D多孔复合气凝胶(MLSG),MXene和GO的质量比为6∶4时候,即标记为 MLSG-6。如图4(c)中(I)所示,MLSG-6复合气凝胶互连的多孔网络结构,与纯MXene相比,MLSG-6复合气凝胶在5.1 mg·cm−2的高质量负载下具有更好的电化学性能。作为超级电容器电极,MLSG-6复合气凝胶在2 mV·s−1扫速时具有386 F·g−1和1967 mF·cm−2的高比电容,在100 mV·s−1扫速时具有241 F·g−1的优异倍率性能。此外,利用LS在正电位下的氧化还原赝电容特性,将3D多孔LS官能化还原GO气凝胶(LSG气凝胶)与MLSG复合气凝胶相匹配,构建了电位范围为1.45 V的全赝电容非对称超级电容器。在功率密度为4900 μW·cm−2的情况下,非对称超级电容器的能量密度为142 μW·h·cm−2,10000次充放电循环后电容保持率为96.3%,超过了许多导电聚合物和贵金属氧化物衍生的超级电容器(图4(c)(II))。图4(c)(III)显示该非对称超级电容器可成功为小灯泡供电。不同MXene复合气凝胶超级电容器的电化学性能对比结果如表1所示[62-74, 36]。
表 1 MXene复合气凝胶超级电容器电化学性能Table 1. Electrochemical properties of supercapacitors assembled by MXene composite aerogelsMaterials Key assembly method Electrochemical performance Ref. Ti3C2Tx/cellulose
nanocrystalWaterborne polyurethane (WPU) crosslinking Cycling stability: 86.7%, 4000 cycles; Energy density:
38.5 μW·h·cm−2 at power density of 1375 μW·cm−2[62] Ti3C2Tx Urea-assisted hydrothermal process for nitrogen doping Cycling stability: 85%, 5000 cycles; Energy density: 21.7 W·h·kg−1
at a power density of 600 W·kg−1[63] Ti3C2Tx/rGO Ascorbic acid reduction Specific capacitance: ~65 F·g−1 at 0.5 A·g−1; Cycling stability: 94.5%,
10000 cycles; Energy density: 3.81 W·h·kg−1 at a power density of 163 W·kg−1[64] Ti3C2Tx/rGO/MnO2 Ascorbic acid reduction Cycling stability: 90.5%, 5000 cycles; Energy density: 50.1 W·h·kg−1 at a power density of 2.1 kW·kg−1 [65] Ti3C2Tx/CoS Melamine foam loading Specific capacitance: 41.5 F·g−1 at 0.2 A·g−1; Cycling stability: 80.39%, 5000 cycles; Energy density: 10.66 W·h·kg−1 at a power density of 135.96 W·kg−1 [66] Ti3C2Tx/rGO/Co3O4 Hydrazine hydrate
reductionSpecific capacitance: 345 F·g−1 at 1 A·g−1; Cycling stability: 85%,
10000 cycles; Energy density: 8.25 W·h·kg−1 at a power density of 159.94 W·kg−1[67] Ti3C2/rGO Zn foil reduction Specific capacitance: 41 mF·cm−2, 1 mA·cm−2; Cycling stability: Almost no capacitance decay, 1000 cycles; Energy density:
~2.1 μW·h·cm−2 at a power density of ~301.2 μW·cm−2[68] MXene/zeolitic imida-
zolate framework-8
(ZIF-8)Metal-organic frameworks deposition Specific capacitance: 1176 F·g−1 at 0.5 A·g−1; Cycling stability: 90.88%, 10000 cycles; Energy density: 57.84 W·h·kg−1 at a power density of 628 W·kg−1 [69] Ti3C2Tx/rGO Ascorbic acid reduction Cycling stability: 80%, 4000 cycles; Energy density: 7.5 W·h·kg−1
at a power density of 500 W·kg−1[70] Ti3C2Tx/rGO/Fe3O4 Ethylenediamine (EDA) crosslinking Specific capacitance: 365 mF·cm−2, 1 mA·cm−2; Cycling stability: 84.5%, 30000 cycles; Energy density: 130 μW·h·cm−2 at a power density of 802 μW·cm−2 [71] Ti3C2Tx/rGO/NiCo2O4 Ascorbic acid reduction Specific capacitance: 87 F·g−1 at 1 A·g−1; Energy density: 40.5 W·h·kg−1 at the power density of 1125.1 W·kg−1 [72] Ti3C2Tx/CNT Ice template method Specific capacitance: 410.7 mF·cm−2, 0.8 mA·cm−2; Cycling
stability: 91.2%, 5000 cycles[73] Polypyrrole (PPy)@poly-
vinyl alcohol (PVA)/
bacterial cellulose (BC)/Ti3C2TxIce template method Specific capacitance: 3948 mF·cm−2, 0.47 mA·cm−2; Cycling stability: 120%, 10000 cycles; Energy density: 178 μW·h·cm−2
at a power density of 951 μW·cm−2[74] Ti3C2Tx/sodium
alginate (SA)Ice template method Specific capacitance: 284.5 F·g−1, 2 mV·s−1; Cycling stability:
Almost no capacitance decay, 20000 cycles[36] 2.4 其他
除了上述碱金属离子电池,MXene还可用于构造其他金属离子电池电极材料。同时,鉴于目前对于环境问题的日益重视,开发具有环保理念的MXene储能器件也尤为重要。Zhou等[27]利用溶剂热和冷冻干燥法制备了MGA复合气凝胶(图1(b)),该材料作为Zn封装的三维主体,所组装的柔性电池在2 C电流密度下,经过不同程度折叠后,仍可稳定提供110 mA·h·g−1的初始容量和90.3%的容量保持率(图5(a)、图5(b))。
Zhao等[24]制备了基于TiVC-石墨烯气凝胶的电催化阴极(图1(a)),组装的Li-CO2电池具有27880 mA·h·g−1的优异放电容量,2.77 V稳定的放电平台,过电位为1.5 V。除此之外,MXene复合气凝胶还能作为一种高效催化剂用于金属空气电池。Faraji等[75]制备MXene、氮掺杂石墨烯复合气凝胶(MXene/NGA),具有丰富的富氮催化中心、较高的碳石墨化度和合适的比表面积,是氧化还原反应的良好催化剂。基于MXene/NGA催化剂组装的Zn-Air电池在功率密度、充放电电位和耐久性方面超过了20wt%Pt/C和IrO2电池。
3. 结论及展望
MXene材料目前已成为新兴的热门材料,在储能等诸多领域具有广阔的发展空间和应用前景。尽管MXene复合气凝胶在电化学储能领域的应用已经取得一些进展,但其自身存在的易堆叠、氧化等问题仍然会影响材料性能。为进一步扩大MXene的应用,未来可以从以下几个方面开展研究:
(1) 对MXene制备合成工艺优化,降低对人体与环境的危害。对前驱体Ti3C2Tx刻蚀使用的HF,对人体与环境具有危害,目前为止只有少数研究采取了无氟合成。因此,还需探索绿色高效、对环境友好、无污染的合成工艺;
(2) 探索更多高效合成单层或少层MXene纳米片的方法,并提升纳米片的产率。MXene日益广泛用于储能器件,且具有良好的电化学性能。这些储能器件如广泛使用,势必需要大规模产业化的技术要求。因此,应探索具有高效、简洁、低成本等条件的大规模制备方法;
(3) MXene复合气凝胶的孔隙结构目前主要采用冷冻干燥技术对冰晶升华制备,对于孔结构以及大小的调控,也将是今后重要的研究方向之一。
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图 1 (a) 超轻多孔TiVC/还原氧化石墨烯(rGO)双金属气凝胶(TVGA)的合成及Li-CO2电池原理图(I)、TVGA气凝胶光学图(II)和具有良好柔韧性的TVGA阴极片的光学图(III)[24];(b) 制造Ti3C2Tx MXene-石墨烯复合气凝胶(MGA)材料的示意图(I)和在无约束条件下MGA材料的顶部(左)和侧视图的SEM图像(II)[27];(c) Mo-Ti/MXene (Mx)-rGO-纤维素纳米纤维(GN)气凝胶(Mo-Ti/Mx-GN)合成工艺示意图(I)和SEM图像(II)[29];(d) 3D多孔1T MoS2/Ti3C2Tx复合气凝胶 X-Z和X-Y横截面SEM图像[30];(e) 3D打印V2CTx/rGO-碳纳米管(CNT)微栅气凝胶电极的制备过程示意图(I)和30%V2CTx/rGO-CNT微格栅气凝胶的SEM图像(II)[40];(f) 3D打印各种结构的模板[41];(g) Ti3C2Tx/碳纳米纤维(CNF)复合气凝胶制备工艺示意图(I)和Ti3C2Tx/CNF6-2的SEM图像(II)[43];(h) 具有褶皱结构的MXene平台器件(上)和Mg2+-MXene气凝胶的SEM图像(下) [45]
Figure 1. (a) Synthesis of ultralight porous TiVC/reduced graphene oxide (rGO) aerogels (TVGA) and schematic diagram of Li-CO2 battery (I), optical photos of block TVGA (II) and TVGA cathode sheet with good flexibility (III)[24]; (b) Schematic illustration of fabricating Ti3C2Tx MXene and graphene aerogel (MGA) material (I), SEM images of top (left) and side views under unconstrained conditions for the MGA material (II)[27]; (c) Schematic diagram illustrating the synthesis process (I) and SEM images (II) of the metallic 1T MoS2 and rich oxygen vacancies TinO2n–1/MXene hierarchical bifunctional catalyst (Mo-Ti/Mx) anchored on a rGO-cellulose nanofiber (GN) host (Mo-Ti/Mx-GN) aerogel[29]; (d) X-Z and X-Y cross-sectional SEM images of 3D highly-porous 1T MoS2/Ti3C2Tx composite aerogel[30]; (e) Schematic diagram of the preparation process of a 3D-printed V2CTx/rGO-carbon nanotube (CNT) microgrid aerogel electrode (I), SEM image of the 30%V2CTx/rGO-CNT microgrid aerogel (II)[40]; (f) 3D printing templates for various structures[41]; (g) Schematic illustration of the fabrication process of Ti3C2Tx/carbon nanofiber (CNF) composite aerogel (I), SEM image of Ti3C2Tx/CNF6-2 (Ⅱ)[43]; (h) MXene platform device with folded structure (top), SEM image of a Mg2+-MXene aerogel (bottom)[45]
GO—Graphene oxide; ΔT—Temperature
图 2 (a) 氧化石墨烯(GO)辅助组装Ti3C2Tx MXene复合气凝胶经硫改性(SMGA)后的SEM图像[48];(b) SMGA电极在0.1~3 mV·s−1不同扫描速下的CV曲线[48];(c) 组装活性炭(AC)//SMGA SiC供电LED阵列和电子温度计的数码照片[48];(d) 锑单原子和量子点(~5 nm)共修饰Ti3C2Tx MXene气凝胶(Sb SQ@MA)的SEM图像[50];(e) Sb SQ@MA与钾离子电池(PIBs)中其他报道的Sb基阳极的倍率性能比较[50];(f) Sb SQ@MA长期循环性能[50]
Figure 2. (a) SEM image of graphene oxide (GO)-assisted assembly of Ti3C2Tx MXene aerogel after sulfur modified (SMGA)[48]; (b) CV curves of the SMGA electrode at different scan rates from 0.1-3 mV·s−1[48]; (c) Digital photos of LED arrays and electronic thermometer powered by the assembled activated carbon (AC)//SMGA SiC[48]; (d) SEM image of antimony single atoms and quantum dots (~5 nm) codecorated Ti3C2Tx MXene-based aerogels (Sb SQ@MA)[50]; (e) Comparison of the rate performance of the Sb SQ@MA with other reported Sb-based anodes in potassium-ion batteries (PIBs)[50]; (f) Long-term cycling performance of the Sb SQ@MA[50]
NC—N-doped three-dimensional porous carbon
图 3 (a) MXene涂层三维气凝胶(MCG-6)复合气凝胶在不同放大倍数的SEM图像[55];(b) MCG-6复合气凝胶电极提高锂硫电池动力学反应速率的示意图[55];(c) MCG-6电极在各种速率下的恒流充放电曲线[55];(d) MXene/C-CNF复合气凝胶(MC-6)的SEM图像[57];(e) MC-6复合气凝胶电极在不同速率下的恒流充放电曲线[57];(f) MXene/T-CNF复合气凝胶(MT-4)的SEM图像[57];(g) MT-4复合气凝胶电极在不同速率下的恒流充放电曲线[57];(h) 层状(PA) MXene-CNT-50复合气凝胶的SEM图:(I) 俯视图(插图是PA MXene/CNT-50整体的照片);(II) 侧视图; (III) 高放大率图[54]
Figure 3. (a) SEM images of MXene-coated three-dimensional aerogel (MCG-6) composite aerogel under different magnifications[55]; (b) Illustration of MCG-6 composite aerogel electrode enhancing the kinetic reaction rate of lithium-sulfur battery [55]; (c) Galvanostatic charge and discharge curves of MCG-6 electrodes at various rates[55]; (d) SEM image of MXene/C-CNF composite aerogel (MC-6)[57]; (e) Galvanostatic charge and discharge curves of MC-6 composite aerogel electrode at various rates[57]; (f) SEM image of MXene/T-CNF composite aerogel (MT-4)[57]; (g) Galvanostatic charge and discharge curves of MT-4 composite aerogel electrode at various rates[57]; (h) SEM images of the parallel-aligned (PA)-MXene/CNT-50 composite aerogel: (I) Top-view (Inset is a photo of the PA-MXene/CNT-50 monolith); (II) Side-view; (III) High magnification[54]
T-CNF—TEMPO-oxidized cellulose nanofibers; C-CNF—Cation-CNF; TEMPO—2, 2, 6, 6-tetramethylpiperidoxyl
图 4 (a) 两个混合超级电容器(HSC)器件串联,为温度计供电超过8 min[32];(b) (I)自愈机制示意图和光学图;(II) 自愈后的聚氨酯(PU)展示:弯曲状态(左上),平坦状态(左下),支撑500 g质量(右),矩形表示伤口/愈合的位置;(III) MXene/rGO 复合气凝胶的SEM图像[60];(c) (I) MLSG-6复合气凝胶的SEM图像;(II) 显示LSG//MLSG-6不对称装置与其他最新超级电容器相比的面积能量和功率密度的Ragone图;(III) 由一个LSG//MLSG-6不对称超级电容器供电的小灯泡[61]
Figure 4. (a) Two hybrid supercapacitor (HSC) devices connected in series to power a thermometer more than 8 min[32]; (b) (I) Schematic diagrams and optical images of the self-healable mechanism; (II) Demonstration of after self-healing polyurethane (PU): Under bending state (top left), under flat state (bottom left), supporting a 500 g mass (right) (Rectangles indicate the wound/healing positions); (III) SEM image of MXene/rGO composite aerogel[60]; (c) (I) SEM images of MLSG-6 composite aerogel; (II) Ragone plot displaying areal energy and power densities of LSG//MLSG-6 asymmetric device in comparison to the other state-of-the-art supercapacitors; (III) A small bulb powered by one LSG//MLSG-6 asymmetric supercapacitor[61]
PANI—Polyaniline; ESCNF—Electrospinning carbon nanofiber; PEDOT—Poly(3, 4-ethylenedioxythiophene); PSS—Polystyrene sulfonate; MLSG—Lignosulfonate (LS) modified-MXene (Ti3C2Tx)-reduced graphene oxide; LSG—LS-functionalized reduced graphene oxide; CDC—Carbide-derived carbons
图 5 (a) LiMn2O4 (LMO)//Ti3C2Tx MXene-石墨烯复合气凝胶(MGA)@Zn软包电池在不同折叠状态下的循环性能[27];(b) 分别在10、30、50次循环时软包电池的容量曲线[27]
Figure 5. (a) Cycling performance of the LiMn2O4 (LMO)//Ti3C2Tx MXene and graphene aerogel (MGA)@Zn pouch cells at various folding times[27]; (b) Capacity profiles of the pouch cells at 10, 30, 50 cycles, respectively[27]
表 1 MXene复合气凝胶超级电容器电化学性能
Table 1 Electrochemical properties of supercapacitors assembled by MXene composite aerogels
Materials Key assembly method Electrochemical performance Ref. Ti3C2Tx/cellulose
nanocrystalWaterborne polyurethane (WPU) crosslinking Cycling stability: 86.7%, 4000 cycles; Energy density:
38.5 μW·h·cm−2 at power density of 1375 μW·cm−2[62] Ti3C2Tx Urea-assisted hydrothermal process for nitrogen doping Cycling stability: 85%, 5000 cycles; Energy density: 21.7 W·h·kg−1
at a power density of 600 W·kg−1[63] Ti3C2Tx/rGO Ascorbic acid reduction Specific capacitance: ~65 F·g−1 at 0.5 A·g−1; Cycling stability: 94.5%,
10000 cycles; Energy density: 3.81 W·h·kg−1 at a power density of 163 W·kg−1[64] Ti3C2Tx/rGO/MnO2 Ascorbic acid reduction Cycling stability: 90.5%, 5000 cycles; Energy density: 50.1 W·h·kg−1 at a power density of 2.1 kW·kg−1 [65] Ti3C2Tx/CoS Melamine foam loading Specific capacitance: 41.5 F·g−1 at 0.2 A·g−1; Cycling stability: 80.39%, 5000 cycles; Energy density: 10.66 W·h·kg−1 at a power density of 135.96 W·kg−1 [66] Ti3C2Tx/rGO/Co3O4 Hydrazine hydrate
reductionSpecific capacitance: 345 F·g−1 at 1 A·g−1; Cycling stability: 85%,
10000 cycles; Energy density: 8.25 W·h·kg−1 at a power density of 159.94 W·kg−1[67] Ti3C2/rGO Zn foil reduction Specific capacitance: 41 mF·cm−2, 1 mA·cm−2; Cycling stability: Almost no capacitance decay, 1000 cycles; Energy density:
~2.1 μW·h·cm−2 at a power density of ~301.2 μW·cm−2[68] MXene/zeolitic imida-
zolate framework-8
(ZIF-8)Metal-organic frameworks deposition Specific capacitance: 1176 F·g−1 at 0.5 A·g−1; Cycling stability: 90.88%, 10000 cycles; Energy density: 57.84 W·h·kg−1 at a power density of 628 W·kg−1 [69] Ti3C2Tx/rGO Ascorbic acid reduction Cycling stability: 80%, 4000 cycles; Energy density: 7.5 W·h·kg−1
at a power density of 500 W·kg−1[70] Ti3C2Tx/rGO/Fe3O4 Ethylenediamine (EDA) crosslinking Specific capacitance: 365 mF·cm−2, 1 mA·cm−2; Cycling stability: 84.5%, 30000 cycles; Energy density: 130 μW·h·cm−2 at a power density of 802 μW·cm−2 [71] Ti3C2Tx/rGO/NiCo2O4 Ascorbic acid reduction Specific capacitance: 87 F·g−1 at 1 A·g−1; Energy density: 40.5 W·h·kg−1 at the power density of 1125.1 W·kg−1 [72] Ti3C2Tx/CNT Ice template method Specific capacitance: 410.7 mF·cm−2, 0.8 mA·cm−2; Cycling
stability: 91.2%, 5000 cycles[73] Polypyrrole (PPy)@poly-
vinyl alcohol (PVA)/
bacterial cellulose (BC)/Ti3C2TxIce template method Specific capacitance: 3948 mF·cm−2, 0.47 mA·cm−2; Cycling stability: 120%, 10000 cycles; Energy density: 178 μW·h·cm−2
at a power density of 951 μW·cm−2[74] Ti3C2Tx/sodium
alginate (SA)Ice template method Specific capacitance: 284.5 F·g−1, 2 mV·s−1; Cycling stability:
Almost no capacitance decay, 20000 cycles[36] -
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期刊类型引用(2)
1. 范玮. 储能技术研究进展及经济性分析. 煤质技术. 2024(03): 21-29+38 . 百度学术
2. 任露. 石墨烯复合气凝胶在矿井水污染治理的实践研究. 内蒙古石油化工. 2024(08): 14-17 . 百度学术
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目的
随着智能化时代的到来,各类电子产品大量涌现。人们迫切需要具有较高容量和良好稳定性的储能器件,为电子设备供电。MXene作为一种新兴的二维(2D)过渡金属碳化物、氮化物、碳/氮化物,具有优异的电化学性能和力学性能,在储能领域的应用十分广泛。受分子间范德华力和氢键的影响,MXene片层易产生自堆积,降低其电子传输能力,造成电化学性能衰减。将2D片状结构组装成三维(3D)结构,可显著改善MXene的自堆积现象,进一步提升MXene的电化学性能。气凝胶是当前最为常用的制备3D结构材料的方法之一。然而,由于MXene材料横向尺寸小且具有亲水性等,致使其不易形成稳定3D结构。因此,通常将MXene与其他材料结合,制备复合气凝胶以试图解决这一问题。本文讨论了不同MXene复合气凝胶的制备方法,并总结其在电池、超级电容器等电化学储能领域的应用研究现状,最后对其所面临的挑战和今后发展方向进行展望。
方法通过对近些年国内外MXene复合气凝胶材料文献的归纳整理,总结了MXene复合气凝胶材料的制备方法,分析了不同制备方法的优缺点。基于MXene复合气凝胶良好的性能与结构,概述了该复合材料在碱金属离子电池、锂硫电池、超级电容器等电化学储能领域的应用。
结果从现有研究中可以看出,目前广泛使用的制备MXene复合气凝胶的方法主要包括自组装法、模板法、化学交联法、3D打印、溶胶-凝胶法、离子诱导组装等。(1)自组装法。主要是基于石墨烯材料的一种组装方法,利用氧化石墨烯(GO)经还原后亲疏水性能的转变,制备MXene复合水凝胶,随后冷冻干燥形成复合气凝胶。自组装法包括水热还原组装法和化学还原组装法。由于MXene易氧化的特性,高温水热还原条件会加速MXene的氧化,破坏其片状结构,降低其电导率。化学还原组装利用还原剂还原GO,能避免高温对MXene性能的影响。(2)模板法。包括冰模板法和保留模板的物理浸渍法等。根据模板的自身特点可调节MXene复合气凝胶材料的结构。(3)化学交联法。在反应体系中加入交联剂,增强复合气凝胶的稳定性和力学性能。(4)3D打印。利用可自定义结构与尺寸的优点,能快速制备各种结构的MXene复合气凝胶材料。(5)溶胶-凝胶法。含MXene原料经由反应形成溶胶,进而形成3D空间网络结构凝胶,凝胶经过干燥等技术转变为复合气凝胶材料。(6)离子诱导组装。利用离子表面电荷产生吸引力或与纳米材料表面配体形成化学键,可使MXene与其他材料结合,形成稳定3D结构。得益于良好的电化学性能和稳定结构,MXene复合气凝胶材料在电化学储能领域应用广泛。应用于碱金属离子电池例如锂离子电池、钠离子电池、钾离子电池等方面时,MXene复合气凝胶材料丰富的孔隙能够促进离子传输,所制备的电极材料具有高容量和良好的稳定性。作为锂硫电池的电极材料,MXene复合气凝胶的稳定结构能抑制多硫化锂穿梭,缓解硫的体积膨胀。MXene复合气凝胶材料作为超级电容器的电极具有良好的电容和循环稳定性,通过材料组成和结构的设计,可以进一步提高MXene基电极的性能。除此之外,MXene复合气凝胶还可用于金属空气电池、电催化等领域。
结论MXene独特的结构和优异的电化学性能,使得其在电化学储能领域已取得一些进展。但仍然存在诸多问题需要深入研究,如:对MXene的合成工艺优化,使用无毒、无污染试剂,降低对人体和环境的危害;为提升MXene产率,需探索大规模制备方法;MXene复合气凝胶的孔隙结构和大小调控,也是今后研究的重要方向。