1 Introduction 1 简介

2D transition metal carbides and/or nitrides (MXenes) have attracted increasing attention due to the unique combination of excellent conductivity comparable to metal, remarkable mechanical properties, large aspect ratio and specific surface area, and easy processing in aqueous dispersion derived from the hydrophilic functional groups.^[1] Rational construction of mechanically robust MXene-based macrostructures involving fibers, films, or aerogels to utilize the MXene layers is of vital importance for many functional applications, such as energy storage, catalysis, sensors, microwave absorption, or electromagnetic interference (EMI) shielding.^[2] Particularly, 3D MXene-based aerogels composed of MXene-based frameworks and the porous structure can efficiently avoid the agglomeration of MXene nanosheets, contributing to the preparation of low-density, high-porosity, and large-specific-surface-area MXene-based macrostructures with functionalities.^[3] Therein, many techniques have been proposed to prepare the porous MXene aerogels, including chemical foaming, freeze-drying, or hydrothermal process.^{[2, 4, 5]} For instance, the MXene-based aerogels^[5] with a high EMI shielding effectiveness (SE) were prepared through the freeze-drying method, and Zhang et al.^[5] fabricated the MXene-based porous foams showing excellent EMI shielding performance via the chemical foaming method. However, despite the impressive results, challenges always exist in requiring energy-intensive facilities, such as freeze-driers and autoclaves, or special gases and vacuum/temperature in these typical preparation approaches.^[6] Consequently, both the MXene aerogels’ size and their scalable production were limited, impeding their practical application. Developing a facile, cost-efficient, and scalable preparation approach for fabricating large-area, multifunctional MXene-based aerogels is vital yet remains lacking.
二维过渡金属碳化物和/或氮化物（MXenes）由于具有与金属相当的优异导电性、卓越的机械性能、大长径比和比表面积以及在水分散体中易于加工的独特组合而受到越来越多的关注。组。 ¹合理构建机械坚固的基于 MXene 的宏观结构（涉及纤维、薄膜或气凝胶）以利用 MXene 层对于许多功能应用至关重要，例如能量存储、催化、传感器、微波吸收或电磁干扰 (EMI) 屏蔽。 ²特别是，由 MXene 基骨架和多孔结构组成的 3D MXene 基气凝胶可以有效避免 MXene 纳米片的团聚，有助于制备低密度、高孔隙率和大比表面积的 MXene-基于宏观结构和功能。 ³其中，人们提出了许多制备多孔 MXene 气凝胶的技术，包括化学发泡、冷冻干燥或水热工艺。 ^{2 , 4 , 5}例如，通过冷冻干燥方法制备了具有高EMI屏蔽效能（SE）的MXene基气凝胶⁵ ，Zhang等人。^图5通过化学发泡方法制备了具有优异EMI屏蔽性能的MXene基多孔泡沫。 然而，尽管取得了令人印象深刻的结果，但在这些典型的制备方法中需要能源密集型设施（例如冷冻干燥机和高压灭菌器）或特殊气体和真空/温度方面始终存在挑战。 ⁶因此，MXene 气凝胶的尺寸和可扩展生产都受到限制，阻碍了其实际应用。开发一种简便、经济高效且可扩展的制备方法来制造大面积、多功能 MXene 气凝胶至关重要，但仍然缺乏。

Challenges also exist in constructing mechanically robust, stable, and durable MXene aerogels, which is attributed to the poor interfacial interactions or gelation ability of MXene nanosheets, and the poor oxidation stability of MXenes in the O₂/H₂O environment, resulting in a loss of the electrical conductivity or functionalities of the MXene aerogels.^[7] Compared with reinforcing agents such as carbon nanotube or graphene, requiring more complex operations for dispersing or processing, the polymers as binders or matrices are most exploited to improve the mechanical properties and service stability of MXene aerogels. For instance, hydrophobic polymers such as epoxy or rubber were employed as matrices to prepare the MXene aerogel-embedded composites with good mechanical strength and considerable oxidation stability.^{[1, 8]} In contrast, water-borne polymers could generate strong interactions with the MXenes due to strong hydrogen bonding interactions derived from their numerous hydrophilic functional groups, avoiding severe aggregation of MXene nanosheets.^[9] The interfaces between polymer and MXene can not only improve the mechanical properties due to the efficient stress transfer but also play crucial roles in improving the functionalities of MXene-based aerogels. For example, functional MXene/polyvinyl alcohol (PVA),^[10] MXene/sodium alginate (SA),^[11] or MXene/cellulose^[3] aerogels have been prepared by the freeze-drying approach, wherein the composite aerogels have shown higher compressive strength in comparison to either pure MXene or polymer aerogels. Moreover, the severe mismatch of conductivity in the interfaces can lead to high interfacial polarization under the electric field of incident electromagnetic wave (EMW), promoting the EMWs’ energy loss and thus enhancing EMI shielding performance of MXene-based aerogels.^[12] Nevertheless, the hydrophilicity of the waterborne polymers is harmful in improving the oxidation stability of the MXene, resulting in unsatisfactory MXene-based aerogels. More importantly, the electrical conductivity or functionalities of the MXene-based macrostructures or aerogels are inevitably deteriorated because of the large insulating polymer gaps between the MXene nanosheets.^{[2, 13]} Most MXene-based aerogels have shown an improvement in either mechanical properties or electrical conductivity or functionalities while sacrificing one property over the other. In addition to the commonly used petrochemical polymers, the inherently low mechanical strength of the polymers suggests a huge challenge in preparing lightweight yet robust, sustainable MXene-based aerogels. In short, a facile, large-area, and scalable manufacturing of lightweight, robust, sustainable, and durable MXene-based aerogels with high conductivity and efficient functionalities remains tremendously problematic, especially to find a trade-off in these properties without sacrificing one property at the expense of the other.
构建机械坚固、稳定和耐用的 MXene 气凝胶也存在挑战，这是由于 MXene 纳米片的界面相互作用或凝胶化能力较差，以及 MXene 在 O ₂ /H ₂ O 环境中的氧化稳定性较差，导致MXene 气凝胶的导电性或功能丧失。 ⁷与碳纳米管或石墨烯等增强剂相比，需要更复杂的分散或加工操作，聚合物作为粘合剂或基质最常用于提高 MXene 气凝胶的机械性能和使用稳定性。例如，采用环氧树脂或橡胶等疏水性聚合物作为基质来制备具有良好机械强度和相当大的氧化稳定性的MXene气凝胶嵌入复合材料。 ^{1 , 8}相比之下，水性聚合物由于其众多亲水官能团产生的强氢键相互作用，可以与 MXene 产生强烈的相互作用，从而避免 MXene 纳米片的严重聚集。 ⁹聚合物和 MXene 之间的界面不仅可以通过有效的应力传递来提高机械性能，而且在提高 MXene 基气凝胶的功能方面也发挥着至关重要的作用。 例如，通过冷冻干燥方法制备了功能性MXene/聚乙烯醇（PVA）、 ¹⁰ MXene/海藻酸钠（SA）、 ¹¹或MXene/纤维素³气凝胶，其中复合气凝胶与传统气凝胶相比表现出更高的抗压强度。纯 MXene 或聚合物气凝胶。此外，界面电导率的严重失配会导致入射电磁波（EMW）电场下的高界面极化，促进EMW的能量损失，从而增强MXene基气凝胶的EMI屏蔽性能。 ¹²然而，水性聚合物的亲水性不利于改善 MXene 的氧化稳定性，导致基于 MXene 的气凝胶效果不理想。更重要的是，由于 MXene 纳米片之间存在较大的绝缘聚合物间隙，基于 MXene 的宏观结构或气凝胶的导电性或功能不可避免地会恶化。 ^{2 , 13}大多数基于 MXene 的气凝胶在机械性能、导电性或功能性方面都表现出改进，同时牺牲了一种性能。除了常用的石化聚合物外，聚合物本身机械强度较低，这表明制备轻质但坚固、可持续的 MXene 基气凝胶面临着巨大的挑战。 简而言之，轻质、坚固、可持续和耐用的具有高导电性和高效功能的 MXene 基气凝胶的简便、大面积和可扩展制造仍然是一个巨大的问题，特别是在不牺牲一种特性的情况下找到这些特性的权衡以牺牲对方为代价。

To address the abovementioned issues, the ambient-pressure-dried (APD) fabrication of large-area, an ultralight yet robust MXene-based aerogels was accomplished within the high-efficiency employment of ultrathin, high-strength, and renewable cellulose nanofiber (CNF) as a high-efficiency cross-linker of MXene nanosheets.^[14] Compared with the commonly employed preparation approaches, the APD approach without any energy-intensive facilities and special gases or vacuum/temperature can achieve eco-friendly, facile, large-area, and scalable preparation. The strong interactions between MXene and CNF contributed to the mechanically robust cell walls, inducing mechanical stability and thus avoiding the structure collapse of aerogels in the APD process. The abundant functional groups of the CNFs are also beneficial for chemical crosslinking of the MXene-based aerogels, significantly improving the hydrophobicity and oxidation stability. The ultrathin (average diameter of merely 1.4 nm), 1D nature of the CNF rendered the minimal gaps between the highly conductive MXene nanosheets, maintaining the high electrical conductivity of the MXene-based aerogels. Consequently, the APD MXene-based aerogels with high porosity showed multifunctionality of high-efficiency dye adsorption, remarkable photothermal and photothermal oil absorption, and EMI shielding performance. The MXene-based aerogels also have controllable wide-ranging densities and MXene contents, contributing to the easy adjustment of the photothermal and EMI shielding performance. Particularly, the thin, robust MXene-based aerogels have reached an EMI SE of 42 to 81 dB at a density of merely 10 to 45 mg cm⁻³ due to the synergistic interactions of the MXene, CNF, and their created micrometer-sized pores. This work thus shows a new avenue for fabricating sustainable, robust, multifunctional APD MXene-based aerogels with great application potential in dye adsorption, oil-water separation, smart heaters, electromagnetic protection, and aerospace.
为了解决上述问题，通过高效利用超薄、高强度、可再生纤维素纳米纤维（CNF），实现了大面积、超轻但坚固的 MXene 基气凝胶的常压干燥（APD）制造。 ）作为 MXene 纳米片的高效交联剂。 ¹⁴与常用的制备方法相比，APD 方法不需要任何能源密集型设施和特殊气体或真空/温度，可以实现环保、简便、大面积和可规模化的制备。 MXene 和 CNF 之间的强烈相互作用有助于形成机械坚固的细胞壁，从而诱导机械稳定性，从而避免 APD 过程中气凝胶的结构崩溃。 CNF丰富的官能团也有利于MXene基气凝胶的化学交联，显着提高疏水性和氧化稳定性。 CNF 的超薄（平均直径仅为 1.4 nm）、一维性质使高导电性 MXene 纳米片之间的间隙最小，从而保持了 MXene 基气凝胶的高导电性。因此，具有高孔隙率的APD MXene基气凝胶表现出高效染料吸附、显着的光热和光热吸油以及EMI屏蔽性能的多功能性。 MXene基气凝胶还具有可控的宽范围密度和MXene含量，有助于轻松调节光热和EMI屏蔽性能。 特别是，由于 MXene、CNF 及其产生的微米级孔的协同相互作用，薄而坚固的 MXene 基气凝胶在密度仅为 10 至 45 mg cm ^-3时已达到 42 至 81 dB 的 EMI SE 。因此，这项工作为制造可持续、坚固、多功能 APD MXene 气凝胶开辟了一条新途径，在染料吸附、油水分离、智能加热器、电磁防护和航空航天等领域具有巨大的应用潜力。

2 Results and Discussion 2 结果与讨论

MXene-based aerogels were prepared with a nanocellulose-assisted APD preparation process (Figure 1a). A stable MXene aqueous dispersion with a Zeta potential of −40 mV was first prepared by etching of Al layer followed by the mechanical delamination of the precursor Ti₃AlC₂ MAX with a compact rock-like structure (Figure S1, Supporting Information). The MXene nanosheet with a hexagonal atomic structure has a lateral size of 2–3 µm (Figure 1b) and a thickness of ≈1.5 nm (Figure 1c). Furthermore, the 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated oxidation and mechanical grinding treatments were employed to prepare the CNF by weakening hydrogen bonds and strengthening electrostatic repulsion between the cellulose chains from cellulose fibers.^{[9, 15]} The mass production of stable CNF aqueous dispersion with a Zeta potential of −60 mV was achieved (Figure S2, Supporting Information), demonstrating the potential for scalable manufacturing of MXene/CNF aerogels. According to the transmission electron microscopy (TEM) and atomic force microscope (AFM) images, the CNFs showed an average diameter of ≈1.4 nm and a large aspect ratio of ≈300 (Figure 1d,e). The similar hydrophilic functional groups of MXene and CNF are of great importance in forming the stable MXene/CNF mixed aqueous dispersion with a high Zeta potential (Figure S3a,b, Supporting Information), which is beneficial for avoiding severe agglomeration of MXene nanosheets in the composites.
采用纳米纤维素辅助的 APD 制备工艺制备了 MXene 基气凝胶（图1a ）。首先通过蚀刻 Al 层，然后对具有致密岩石状结构的前驱体 Ti ₃ AlC ₂ MAX 进行机械分层，制备了 Zeta 电位为 -40 mV 的稳定 MXene 水分散体（图S1 ，支持信息）。具有六方原子结构的 MXene 纳米片的横向尺寸为 2-3 µm（图1b ），厚度约为 1.5 nm（图1c ）。此外，采用2,2,6,6-四甲基哌啶-1-氧基（TEMPO）介导的氧化和机械研磨处理，通过减弱纤维素纤维中纤维素链之间的氢键并增强静电斥力来制备CNF。 ^{9 , 15}实现了 Zeta 电位为 -60 mV 的稳定 CNF 水分散体的大规模生产（图S2 ，支持信息），证明了 MXene/CNF 气凝胶可规模化制造的潜力。根据透射电子显微镜（TEM）和原子力显微镜（AFM）图像，CNF的平均直径约为1.4 nm，长宽比约为300（图1d，e ）。 MXene和CNF相似的亲水官能团对于形成稳定的具有高Zeta电位的MXene/CNF混合水分散体非常重要（图S3a，b ，支持信息），这有利于避免MXene纳米片在纳米片中的严重团聚。复合材料。

In a freezing process of the aqueous dispersion, the water molecules connected to each other by hydrogen bonds in a wide range, and their volume increased to form loose crystals. Then the MXene and CNF are squeezed into the gaps of the randomly grown ice crystals, leading to the formation of a continuous MXene/CNF hybrid phase under the action of hydrogen bonding.^[16] Subsequently, the frozen samples were immersed in ethanol. Since the freezing point of ethanol is lower than that of water, the ice crystals would rapidly dissolve in ethanol.^[17] After ethanol exchange, since the CNF and MXene were insoluble in ethanol, the MXene/CNF cell walls would become thinner and the hydrogen bonding force was enhanced, improving the stability of the whole structure. Thanks to the low surface tension of ethanol, the MXene/CNF framework did not collapse in the APD process while the hydrogen bonding force was further enhanced, resulting in a stable, robust MXene/CNF aerogel. Herein, the large aspect ratio and exceptional mechanical properties of both CNFs and MXene and the strong hydrogen bonding interactions between the CNF and MXene made for achieving robust MXene-based cell walls, maintaining the microstructural stability to prevent the collapse of aerogels in the APD process. In contrast, if the frozen MXene/CNF hydrogels were dissolved at room temperature, the high surface tension of water would destruct the structure of the MXene/CNF walls, eventually turning into a collapsed aerogel or even an MXene/CNF film (Figure S3c, Supporting Information). This process was different from the commonly employed freeze-casting approach,^{[4, 18]} where a high vacuum environment is essential for sublimating the ice crystals to prepare the aerogels. After the chemical crosslinking treatment of the MXene/CNF aerogels with poly ((phenyl isocyanate)-co-formaldehyde) (PMDI) (named C-MXene/CNF), large-area MXene-based aerogels with dimensions of ≈21 cm were prepared (Figure 1f, Figure S3d,e, Supporting Information). This aerogel, containing 30 wt.% CNF, was manufactured with excellent mechanical strength even at a density of 18 mg cm⁻³ such that it could sustain a load weighing >4000 times heavier (Figure 1f, inset).
在水分散体的冷冻过程中，水分子在大范围内通过氢键相互连接，其体积增大，形成松散的晶体。然后MXene和CNF被挤压到随机生长的冰晶的间隙中，在氢键的作用下形成连续的MXene/CNF杂化相。 ¹⁶随后，将冷冻样品浸入乙醇中。由于乙醇的凝固点比水低，冰晶会迅速溶解在乙醇中。 ¹⁷乙醇交换后，由于CNF和MXene不溶于乙醇，MXene/CNF细胞壁会变薄，氢键力增强，提高整个结构的稳定性。由于乙醇的低表面张力，MXene/CNF框架在APD过程中不会崩溃，同时氢键力进一步增强，从而形成稳定、坚固的MXene/CNF气凝胶。在此，CNF 和 MXene 的大长径比和优异的机械性能以及 CNF 和 MXene 之间强大的氢键相互作用，有助于实现坚固的 MXene 基细胞壁，保持微观结构稳定性，防止 APD 过程中气凝胶的塌陷。相反，如果冷冻的MXene/CNF水凝胶在室温下溶解，水的高表面张力会破坏MXene/CNF壁的结构，最终变成塌陷的气凝胶甚至MXene/CNF薄膜（图S3c ，支持信息）。 该过程不同于常用的冷冻铸造方法^{4 , 18} ，其中高真空环境对于升华冰晶以制备气凝胶至关重要。用聚（异氰酸苯酯）-共聚甲醛（PMDI）（命名为C-MXene/CNF）对MXene/CNF气凝胶进行化学交联处理后，制备了尺寸约21 cm的大面积MXene基气凝胶。 （图1f ，图S3d，e ，支持信息）。这种含有 30 wt.% CNF 的气凝胶即使在 18 mg cm ^-3的密度下也具有优异的机械强度，因此它可以承受重量 >4000 倍的负载（图1f ，插图）。

Scanning electron microscopy (SEM) image clearly shows the 3D porous structure (Figure 1e) of the MXene-based aerogels at a density of 18 mg cm⁻³. Randomly isotropic pores with an average diameter of 100 µm and interconnected, robust MXene/CNF hybrid cell walls were observed (Figure 1f,g). The CNF content could be further increased, resulting in the preparation of APD robust MXene-based aerogels embedded with wide-ranging CNF contents (Figure S4a–c, Supporting Information) of 30 wt.% to 100 wt.% (pure CNF aerogel). In contrast, too less CNF, such as 15 wt.% CNF, resulted in the remarkable volume shrinkage of the MXene-based aerogels in the APD process. The MXene hydrogels without the CNFs were directly dispersed in the ethanol and thus collapsed in the same process. Therefore, the vital role the CNF played in achieving APD MXene-based aerogels was ascertained. Moreover, the CNFs could adhere well to MXene nanosheets because of the strong hydrogen bonding interactions, inducing intercalation between the MXene nanosheets in the hybrid cell walls of aerogels. This contributed to the formation of biomimetic cell walls showing the nacre-like “brick and mortar” microstructure (Figure 1h,i), which was beneficial for enhancing the mechanical strength of the MXene-based aerogels even at low densities.
扫描电子显微镜（SEM）图像清楚地显示了密度为18 mg cm ^-3的MXene基气凝胶的3D多孔结构（图1e ）。观察到平均直径为 100 µm 的随机各向同性孔和互连、坚固的 MXene/CNF 混合细胞壁（图1f、g ）。 CNF 含量可以进一步增加，从而制备出嵌入了 30 wt.% 至 100 wt.%（纯 CNF 气凝胶）的广泛 CNF 含量（图S4a-c ，支持信息）的 APD 稳健的 MXene 基气凝胶。 。相反，太少的 CNF，例如 15 wt.% CNF，会导致 APD 过程中 MXene 基气凝胶的显着体积收缩。不含 CNF 的 MXene 水凝胶直接分散在乙醇中，因此在同一过程中塌陷。因此，确定了 CNF 在实现 APD MXene 气凝胶中发挥的重要作用。此外，由于强氢键相互作用，CNF 可以很好地粘附到 MXene 纳米片上，从而诱导气凝胶混合细胞壁中 MXene 纳米片之间的插入。这有助于仿生细胞壁的形成，显示出珍珠质状的“砖和砂浆”微观结构（图1h，i ），这有利于增强基于MXene的气凝胶的机械强度，即使在低密度下也是如此。

Abundant hydroxy groups of CNFs were instrumental in the chemical crosslinking with PMDI as well as introducing hydrophobic functional groups, enhancing the hydrophobicity and stability in the O₂/H₂O environment.^[19] Fourier-transform infrared spectroscopy (FTIR) analysis ascertained the reaction between the CNF and PMDI (Figure 2a). After chemical crosslinking, a new peak at 1680 cm⁻¹ appears, suggesting the apparency of the CONH resulting from the crosslinking of NCO groups of PMDI and OH groups of CNFs. The CNF-assisted crosslinking introduced the hydrophobic backbone of PMDI, contributing to the change of water contact angles from 0 to 120° (Figure 2b). Combined with the generated covalent interactions, the excellent stability and water resistance of the Mxene-based aerogels are accomplished. After an ultrasound treatment in water for 20 min, obviously, the APD MXene-based aerogels were quite stable, showing the potential for practical applications (Figure 2c). In addition to the numerous functional groups, the high porosity of the MXene-based aerogels was beneficial for the high-efficiency organic dye adsorption, e.g., removal efficiency of 100% of methyl blue (MB) stock was shown (Figure 2d, Figure S5, Supporting Information), extending the functionalities of the APD MXene-based aerogels.
CNFs丰富的羟基有助于与PMDI的化学交联以及引入疏水性官能团，增强其在O ₂ /H ₂ O环境中的疏水性和稳定性。 ¹⁹傅里叶变换红外光谱 (FTIR) 分析确定了 CNF 和 PMDI 之间的反应（图2a ）。化学交联后，在1680 cm ^-1处出现一个新峰，表明PMDI的NCO基团和CNF的OH基团交联产生CONH 。 CNF辅助交联引入了PMDI的疏水主链，有助于水接触角从0°变化到120°（图2b ）。结合产生的共价相互作用，Mxene 基气凝胶具有优异的稳定性和耐水性。在水中超声处理 20 分钟后，显然，APD MXene 基气凝胶相当稳定，显示出实际应用的潜力（图2c ）。除了众多的官能团之外，MXene基气凝胶的高孔隙率有利于高效有机染料吸附，例如，甲基蓝（MB）库存的去除效率为100％（图2d ，图S5） ，支持信息），扩展了 APD MXene 基气凝胶的功能。

To investigate the influences of CNF on the mechanical robustness of the MXene-based aerogels, a compression test was conducted (Figure 2e). By introducing the CNF “mortars” to enhance the interfacial interactions between MXene “nanobricks”, the compressive strength and modulus of the MXene-based aerogels were enhanced significantly. Since the pure MXene aerogels could not be fabricated in the APD approach, the freeze-dried MXene aerogels (FD-MXene) were prepared for contrast. By adding 30 wt.% CNFs, the compressive modulus of the APD MXene-based aerogel reached 298 kPa, which was ≈155% higher than that of the FD-MXene (≈18 kPa) (Figure 2f). With the further addition of CNF to 40 wt.%, the highest compressive strength and modulus were achieved for the MXene-based aerogels. The modulus was up to 780 kPa, remarkably higher than that of the FD-MXene.
为了研究 CNF 对 MXene 基气凝胶机械鲁棒性的影响，进行了压缩测试（图2e ）。通过引入CNF“砂浆”来增强MXene“纳米砖”之间的界面相互作用，MXene基气凝胶的压缩强度和模量显着增强。由于纯 MXene 气凝胶无法通过 APD 方法制造，因此制备了冻干 MXene 气凝胶（FD-MXene）作为对比。通过添加 30 wt.% CNF，APD MXene 基气凝胶的压缩模量达到 298 kPa，比 FD-MXene (≈18 kPa) 高约 155%（图2f ）。随着 CNF 的进一步添加至 40 wt.%，MXene 基气凝胶获得了最高的压缩强度和模量。模量高达780 kPa，明显高于FD-MXene。

The 1D, ultrathin nature of CNFs was beneficial for minimizing the interlayer gaps between the MXene nanosheets to maintain the excellent electrical conductivity of hybrid cell walls. The X-ray diffraction (XRD) patterns of the MXene-based aerogels were employed to evaluate the gaps for the hybrid aerogels (Figure 2g). A slight downshift of the (002) characteristic peak of MXene with increasing CNF content corresponded to a slightly broadened interlayer spacing between MXene nanosheets for the MXene-based aerogels. For instance, compared with an interlayer gap of ≈1.52 nm for FD-MXene aerogels, the MXene/CNF hybrid aerogels containing 30 and 80 wt.% CNFs showed gaps of 1.56 and 1.64 nm, respectively. This gap value agreed with the data observed from the TEM image of MXene-based hybrid cell walls containing 30 wt.% CNF (Figure 1i). The high conductivities were thus promising for MXene-based aerogels because of the ultrathin insulating gap between MXene nanosheets. In contrast, the freeze-dried MXene/PVA aerogels were prepared and showed a more remarkable decrease in conductivity with increasing polymer contents (Figure 2h). For instance, MXene/PVA aerogels show conductivities of 67 and 0.02 S m⁻¹ respectively at polymer content of 30 wt.% and 80 wt.%, which were lower than those of MXene/CNF aerogels at the same polymer content, respectively. Particularly, at a polymer content of 80 wt.%, the APD MXene/CNF aerogels have a conductivity of 1.8 S m⁻¹, two orders of magnitude higher than that of the MXene/PVA aerogels, showing the vital role of CNF played in achieving high conductivity of the MXene-based aerogels.
CNF 的一维超薄性质有利于最大限度地减少 MXene 纳米片之间的层间隙，以保持混合细胞壁的优异导电性。采用 MXene 基气凝胶的 X 射线衍射 (XRD) 图案来评估混合气凝胶的间隙（图2g ）。随着 CNF 含量的增加，MXene 的 (002) 特征峰略有下移，这对应于 MXene 基气凝胶的 MXene 纳米片之间的层间距略有扩大。例如，与 FD-MXene 气凝胶约 1.52 nm 的层间间隙相比，含有 30 和 80 wt.% CNF 的 MXene/CNF 混合气凝胶的间隙分别为 1.56 和 1.64 nm。该间隙值与从含有 30 wt.% CNF 的 MXene 基混合细胞壁的 TEM 图像中观察到的数据一致（图1i ）。由于 MXene 纳米片之间的超薄绝缘间隙，MXene 基气凝胶的高电导率很有前景。相比之下，制备的冻干 MXene/PVA 气凝胶随着聚合物含量的增加显示出更显着的电导率下降（图2h ）。例如，当聚合物含量为30 wt.%和80 wt.%时，MXene/PVA气凝胶的电导率分别为67和0.02 S m ^-1 ，分别低于相同聚合物含量下MXene/CNF气凝胶的电导率。特别是，当聚合物含量为80 wt.%时，APD MXene/CNF气凝胶的电导率为1.8 S m ^-1 ，比MXene/PVA气凝胶高两个数量级，这表明CNF在实现了 MXene 基气凝胶的高导电性。

The EMI SE in the X-band frequency range (8.2–12.4 GHz) of the APD MXene-based aerogels with various CNF loadings was obtained (Figure 3a). At a thickness of 2 mm and a density of 18 mg cm⁻³, the MXene-based aerogels with 30 wt.% CNF have the highest EMI SE values of 64 dB (Table S1, Supporting Information). Even at a CNF content of 80 wt.%, the EMI SE values were nearly close to the commercial SE value of 20 dB, corresponding to a 99% attenuation of incident EMW (Table S2, Supporting Information). To better ascertain the important influences of the introduced 1D, ultrathin CNFs on EMI SE of the MXene-based aerogels, the average X-band SE values of freeze-dried MXene/PVA aerogels and APD MXene/CNF aerogels was compared at the same density and thickness (Figure 3b, Figure S6, Supporting Information). Clearly, the MXene/CNF aerogels display higher EMI SE than the MXene/PVA aerogels at the same polymer contents, which is consistent with the behavior in electrical conductivity of the MXene-based aerogels. For example, at a polymer content of 30 wt.%, the EMI SE values of MXene/CNF and MXene/PVA aerogels reached 64 and 19 dB, respectively. It was ascribed to that the ultrathin, 1D CNFs could minimize the gaps between the MXene nanosheets, fully utilizing the conductivity and EMI shielding properties of MXenes. Therefore, 1D, ultrathin CNFs showed great superiority in fabricating the lightweight yet robust MXene-based aerogels without remarkably compromising the conductivity and EMI SE, demonstrating the potential for EMI shielding applications.
获得了具有不同 CNF 负载量的 APD MXene 气凝胶在 X 波段频率范围（8.2–12.4 GHz）内的 EMI SE（图3a ）。在厚度为2 mm、密度为18 mg cm ^-3时，具有30 wt.% CNF的MXene基气凝胶具有64 dB的最高EMI SE值（表S1 ，支持信息）。即使 CNF 含量为 80 wt.%，EMI SE 值也几乎接近 20 dB 的商业 SE 值，相当于入射 EMW 衰减 99%（表S2 ，支持信息）。为了更好地确定引入的一维超薄 CNF 对 MXene 基气凝胶 EMI SE 的重要影响，在相同密度下比较了冻干 MXene/PVA 气凝胶和 APD MXene/CNF 气凝胶的平均 X 波段 SE 值和厚度（图3b ，图S6 ，支持信息）。显然，在相同聚合物含量下，MXene/CNF 气凝胶比 MXene/PVA 气凝胶表现出更高的 EMI SE，这与 MXene 基气凝胶的电导率行为一致。例如，当聚合物含量为30 wt.%时，MXene/CNF和MXene/PVA气凝胶的EMI SE值分别达到64和19 dB。人们认为，超薄的一维 CNF 可以最大限度地减少 MXene 纳米片之间的间隙，充分利用 MXene 的导电性和 EMI 屏蔽特性。因此，一维超薄 CNF 在制造轻质而坚固的 MXene 气凝胶方面表现出巨大的优越性，且不会显着影响电导率和 EMI SE，展示了 EMI 屏蔽应用的潜力。

Total EMI SE (SE_T) of conductive composites involves shielding by reflection (SE_R) and absorption (SE_A), and multiple reflections, mainly corresponding to the mobile charge carriers, electric dipoles, and reflections at interior surfaces/interfaces, respectively (Figure 3c).^[20] In addition to reducing the weight, micrometer-sized pores of the aerogels can induce abundant multiple reflections, extending the propagation paths of incident EMWs and increasing the interactions between the cell walls and EMWs.^[21] Combined with the high EMW loss capability of the MXene-based cell walls, derived from sufficient charge carriers and heterogenous MXene-CNF interfaces, the high SE_A can be obtained. Also, the SE_A decreased with increasing CNF content or decreasing MXene content. SE_R values slightly decreased with increasing CNF content due to the slightly decreased conductivity of the MXene-based aerogels. Therefore, the SE_T, the sum of SE_A and SE_R, decreased with increasing CNF content. Nevertheless, it is attractive that the EMI SE values of the APD MXene/CNF aerogels with a small fraction of CNFs were higher than those of the FD-MXene aerogels at the same thickness and density. This was attributed to that a small quantity of 1D, ultrathin CNF slightly affected the conductivity and charge carrier amount of the hybrid cell walls of MXene-based aerogels, maintaining the SE_R values. Moreover, the CNF-induced numerous MXene/CNF interfaces with a large mismatch of conductivity led to a high interfacial polarization under the electric field of incident EMWs for the MXene-based aerogels. Furthermore, in comparison to the FD-MXene aerogels showing numerous holey cell walls (Figure S7, Supporting Information), the APD MXene/CNF aerogels had more intact cell walls with good integrity, which was beneficial for the interactions between the cell walls and incident EMWs. Thus, the SE_A of the MXene/CNF aerogels was higher than that of the FD-MXene aerogels, leading to the higher SE_T value of the APD MXene/CNF with a small fraction of CNF. In a word, the intercalation of 1D, ultrathin CNFs can not only contribute to the formation of nacre-like “brick-and-mortar” biomimetic microstructure for significantly improved mechanical properties but also maintain or even promote the excellent EMI shielding performance of the APD MXene-based aerogels.
导电复合材料的总 EMI SE (SET _T ) 涉及反射屏蔽 (SE _R ) 和吸收屏蔽 (SE _A ) 以及多重反射，主要分别对应于移动电荷载流子、电偶极子和内表面/界面处的反射 (图3c )。 ²⁰除了减轻重量之外，气凝胶的微米级孔隙还可以引起丰富的多次反射，延长入射电磁波的传播路径，并增加细胞壁和电磁波之间的相互作用。 ²¹结合 MXene 基细胞壁的高 EMW 损失能力（源自充足的电荷载流子和异质 MXene-CNF 界面），可以获得高 SE _A。此外，SE _A随着 CNF 含量的增加或 MXene 含量的减少而降低。由于 MXene 气凝胶的电导率略有下降，SE _R值随着 CNF 含量的增加而略有下降。因此，SE _T （SE _A和 SE _R之和）随着 CNF 含量的增加而降低。然而，引人注目的是，在相同厚度和密度下，含有少量 CNF 的 APD MXene/CNF 气凝胶的 EMI SE 值高于 FD-MXene 气凝胶。这是由于少量的一维超薄 CNF 轻微影响了 MXene 基气凝胶杂化细胞壁的电导率和载流子数量，保持了 SE _R值。 此外，CNF 诱导的大量 MXene/CNF 界面具有较大的电导率失配，导致 MXene 基气凝胶在入射电磁波电场下出现高界面极化。此外，与显示出大量多孔细胞壁的 FD-MXene 气凝胶相比（图S7 ，支持信息），APD MXene/CNF 气凝胶具有更完整的细胞壁，完整性良好，这有利于细胞壁和事件​​之间的相互作用。电磁波。因此，MXene/CNF 气凝胶的 SE _A高于 FD-MXene 气凝胶，导致含有少量 CNF 的 APD MXene/CNF 具有较高的 SE _T值。总之，一维超薄CNF的嵌入不仅有助于形成珍珠质“砖块和砂浆”仿生微观结构，显着提高机械性能，而且可以保持甚至促进APD优异的EMI屏蔽性能MXene 基气凝胶。

One more superiority of our APD preparation approach is that the density/porosity of MXene-based aerogels can be effortlessly controlled by tuning the water fraction in the precursor dispersion (Figure 3d). Higher water fraction led to higher porosity and lower density of the 3D aerogels (Table S3, Supporting Information). The aerogels containing 30 wt.% CNF show a density down to 10 mg cm⁻³ and porosity up to 99.6% (Figure S8a, Supporting Information). The pore size of such ultralight MXene-based aerogels was a bit larger than that of aerogels with a higher density, e.g., average pore size was 130 and 100 µm for the MXene-based aerogels at densities of 10 and 18 mg cm⁻³, respectively (Figure S8b, Supporting Information). Nevertheless, the intact cell walls of the APD aerogels in the ultralow-density range (≤ 10 mg cm⁻³) were maintained, and thus the EMI SE was still high enough. For instance, the 2 mm-thick MXene-based aerogels showed SE_T values of 41 dB at a density of merely 10 mg cm⁻³. Higher density of the aerogels also increased the EMI SE, e.g., the SE values reached >80 dB for the APD MXene-based aerogels at a density of 45 mg cm⁻³, further demonstrating the wide-ranging controllability of the EMI shielding performance.
我们的 APD 制备方法的另一个优点是，可以通过调节前驱体分散体中的水含量来轻松控制基于 MXene 的气凝胶的密度/孔隙率（图3d ）。较高的水含量导致 3D 气凝胶的孔隙率较高且密度较低（表S3 ，支持信息）。含有30 wt.% CNF的气凝胶显示出密度低至10 mg cm ^-3和孔隙率高达99.6%（图S8a ，支持信息）。这种超轻MXene基气凝胶的孔径比具有较高密度的气凝胶的孔径稍大，例如，密度为10和18mg cm ^-3时，MXene基气凝胶的平均孔径为130和100μm，分别（图S8b ，支持信息）。尽管如此，超低密度范围（≤10 mg cm ^-3 ）的APD气凝胶仍保持完整的细胞壁，因此EMI SE仍然足够高。例如，2毫米厚的MXene基气凝胶在密度仅为10 mg cm ^-3时表现出41 dB的SE _T值。气凝胶的较高密度也增加了EMI SE，例如，APD MXene基气凝胶在密度为45 mg cm ^-3时SE值达到>80 dB，进一步证明了EMI屏蔽性能的广泛可控性。

The thickness of the MXene-based aerogels was also controlled effortlessly, allowing for a wide-ranging adjustment of the EMI SE. The EMI SE of the MXene-based aerogels with 30 wt.% CNF increased with increasing thickness, e.g., EMI SE values reached >90 dB, at a thickness of 6 mm and a density of 18 mg cm⁻³ (Figure 3e). Even at a thickness of merely 1.0 mm, the EMI SE was also close to 50 dB, corresponding to transmission of EMWs less than 0.001% (Table S2, Supporting Information). The MXene-based aerogels showed higher SE values than other porous composites ever reported at similar thicknesses, e.g., 5 mm-thick silver nanowires/PI, 2 mm-thick CNT/PI, 2.3 mm-thick CNT/PU, 2.3 mm-thick graphene/PE aerogels showed SE values up to 13, 41, 51, and 13 dB, respectively. Generally, in the GHz band, the higher frequency can result in a lower transmission of the EM waves through the shields.^[22] The average SE values of the MXene-based aerogels in X-band (8.2–12.4 GHz), Ku-band (12.4–18 GHz), K-band (18–26.5 GHz), and Ka-band (26.5–40 GHz) can reach 64.7, 66.9, 70.1, and 86.7 dB, respectively, demonstrating the excellent EMI shielding performance in an ultra-broadband frequency range (Figure 3f). The high yet controllable EMI SE values in such an ultra-broadband frequency range showed the great promise of APD MXene-based aerogels for EMI shielding applications.
基于 MXene 的气凝胶的厚度也可以轻松控制，从而可以对 EMI SE 进行大范围的调整。具有30wt.％CNF的基于MXene的气凝胶的EMI SE随着厚度的增加而增加，例如，在6mm的厚度和18mg cm ^-3的密度下，EMI SE值达到％3E90dB（图3e ）。即使厚度仅为 1.0 mm，EMI SE 也接近 50 dB，相当于电磁波传输率低于 0.001%（表S2 ，支持信息）。基于 MXene 的气凝胶显示出比相似厚度下报道的其他多孔复合材料更高的 SE 值，例如 5 毫米厚的银纳米线/PI、2 毫米厚的 CNT/PI、2.3 毫米厚的 CNT/PU、2.3 毫米厚石墨烯/PE气凝胶的SE值分别高达13、41、51和13 dB。一般来说，在 GHz 频段，频率越高，穿过屏蔽层的电磁波传输就越少。 ²² MXene基气凝胶在X波段（8.2-12.4 GHz）、Ku波段（12.4-18 GHz）、K波段（18-26.5 GHz）和Ka波段（26.5-40 GHz）的平均SE值GHz）可分别达到 64.7、66.9、70.1 和 86.7 dB，展示了出色的 EMI 屏蔽性能超宽带频率范围（图3f ）。在如此超宽带频率范围内的高且可控的 EMI SE 值显示了基于 APD MXene 的气凝胶在 EMI 屏蔽应用中的巨大前景。

CNF-assisted chemical crosslinking of the MXene-based aerogels contributed to the hydrophobicity and water resistance, which was beneficial for the oxidation stability in O₂/H₂O environment. The FD-MXene aerogels showed remarkably decreased SE after they were stored in a 95% relative humidity environment and a temperature of 70 °C for 2 days, and the aerogels have ignorable EMI shielding capability after 6 days in the same environment. In contrast, APD MXene-based aerogels maintained a high EMI SE of >60 dB after they were stored for 6 days in the same condition (Figure 3g). The significantly increased oxygen content of the FD-MXene aerogels compared to APD MXene aerogels after being stored in this condition for 6 days further implied the improved oxidation stability of the APD MXene (Figure S9, Supporting Information). Although the MXene-based aerogels were stored for 15 days in such harsh conditions, the EMI shielding can still be efficient. EMI SE values remain stable even after the MXene-based aerogels were immersed in the commonly employed solvents such as water or acetone for 15 days at room temperature (Figure 3h), showing the stability of MXene-based aerogels against various solvents. Besides, optical topography pictures also confirm this merit (inset in Figure 3h). These efficiently proved the long durability and service stability of the APD MXene-based aerogels.
CNF辅助化学交联使MXene基气凝胶具有疏水性和耐水性，有利于O ₂ /H ₂ O环境中的氧化稳定性。 FD-MXene气凝胶在95%相对湿度和70℃的环境中储存2天后，SE显着降低，并且在相同环境中储存6天后，气凝胶具有可忽略的EMI屏蔽能力。相比之下，基于 APD MXene 的气凝胶在相同条件下储存 6 天后仍保持 >60 dB 的高 EMI SE（图3g ）。在这种条件下储存 6 天后，与 APD MXene 气凝胶相比，FD-MXene 气凝胶的氧含量显着增加，进一步表明 APD MXene 的氧化稳定性得到改善（图S9 ，支持信息）。尽管基于 MXene 的气凝胶在如此恶劣的条件下储存了 15 天，但 EMI 屏蔽仍然有效。即使将 MXene 基气凝胶在室温下浸入水或丙酮等常用溶剂中 15 天，EMI SE 值仍保持稳定（图3h ），显示了 MXene 基气凝胶对各种溶剂的稳定性。此外，光学形貌图片也证实了这一优点（图3h中的插图）。这些有效地证明了APD MXene基气凝胶的长期耐用性和使用稳定性。

Generally, reaching higher SE values of EMI shields at a minimum material consumption is challenging for high-performance EMI shields. To make a better comparison, SE divided by thickness (SE/d) was concluded for the shields with various densities (Figure 4a). Here, even at an ultralow density range of 10 mg cm⁻³, the APD MXene-based aerogels still showed an excellent EMI SE, far surpassing the commercial SE values. Moreover, the EMI SE was widely modulated by controlling the density, thickness, or CNF/MXene ratios. The SSE/d values, defined as the SE divided by the thickness and density of shields to evaluate the lightweight, EMI shielding architectures, were calculated up to 26 500 dB cm² g⁻¹, which was comparable to that of the best EMI shields including the films or aerogels ever reported (Table S4, Supporting Information). More importantly, both excellent SE and SSE/d values were accomplished for the APD MXene-based aerogels, significantly outperforming the shields including carbon-based, metal-based, and other MXene-based architectures (Figure 4b). For instance, at a remarkable SE of 81 dB, the SSE/d values can reach 9039 dB cm² g⁻¹, showcasing the superior EMI shielding performance compared with most shields with similar SE values (Table S4, Supporting Information). As compared with other EMI shields, the exceptional EMI shielding performance of our APD aerogels was attributed to the high-efficiency utilization of 1D, ultrathin CNF for constructing the ultralight, biomimetic MXene-based aerogels (Figure 4c). Briefly, the CNF efficiently maintained the high conduction loss of incident EMWs while improving the interfacial polarization loss capability resulting from the introduced abundant MXene-CNF interfaces. Moreover, abundant charge carriers of MXene nanosheets as well as the functional groups led to the high polarization loss capability of MXene-based aerogels. Combined with the multiple reflections of incident EMWs derived from abundant micrometer-sized pores and MXene-based layered microstructure, the excellent EMI shielding performance of the MXene-based aerogels was accomplished. In summary, a radar plot was employed to show the advantages of our APD preparation approach for the MXene-based aerogels, showcasing not only the sustainable, renewable, environmental-friendly process but also superior superiorities in both energy and time consumption compared with other typical preparation approaches for the aerogel-based shields, such as CNT, graphene, silver nanowire or other MXene-based aerogels (Figure 4d). In addition, our APD fabrication approach for MXene-based aerogels showed not only an environment-friendly process but also superior advantages in both time and energy consumption compared to typical fabrication methods for MXene-based aerogels (Table S5, Supporting Information). Therefore, our APD MXene-based aerogels are promising for large-scale production in practical applications.
一般来说，以最少的材料消耗达到更高的 EMI 屏蔽 SE 值对于高性能 EMI 屏蔽来说是一项挑战。为了更好地进行比较，对不同密度的屏蔽体得出了 SE 除以厚度 (SE/ d ) 的结论（图4a ）。在这里，即使在10 mg cm ^-3的超低密度范围内，APD MXene基气凝胶仍然表现出优异的EMI SE，远远超过商业SE值。此外，通过控制密度、厚度或 CNF/MXene 比率来广泛调节 EMI SE。 SSE/ d值（定义为 SE 除以屏蔽层的厚度和密度，用于评估轻量级 EMI 屏蔽架构）的计算结果高达 26 500 dB cm ² g ⁻¹ ，与最佳 EMI 屏蔽层相当包括曾经报道过的薄膜或气凝胶（表S4 ，支持信息）。更重要的是，APD MXene 基气凝胶实现了优异的 SE 和 SSE/ d值，显着优于包括碳基、金属基和其他 MXene 基架构在内的屏蔽（图4b ）。例如，在高达 81 dB 的 SE 下，SSE/ d值可以达到 9039 dB cm ² g ^-1 ，与大多数具有类似 SE 值的屏蔽相比，展现出卓越的 EMI 屏蔽性能（表S4 ，支持信息）。与其他 EMI 屏蔽相比，我们的 APD 气凝胶卓越的 EMI 屏蔽性能归因于高效利用一维超薄 CNF 来构建超轻、仿生 MXene 基气凝胶（图4c ）。 简而言之，CNF 有效地保持了入射 EMW 的高传导损耗，同时由于引入了丰富的 MXene-CNF 界面而提高了界面极化损耗能力。此外，MXene纳米片丰富的载流子以及官能团导致MXene基气凝胶具有高偏振损耗能力。结合丰富的微米级孔隙和 MXene 基层状微结构产生的入射电磁波的多次反射，实现了 MXene 基气凝胶优异的 EMI 屏蔽性能。总之，采用雷达图展示了我们的 APD 制备 MXene 基气凝胶方法的优势，不仅展示了可持续、可再生、环境友好的过程，而且与其他典型气凝胶相比，在能源和时间消耗方面也具有优越性。基于气凝胶的屏蔽的制备方法，例如CNT、石墨烯、银纳米线或其他基于MXene的气凝胶（图4d ）。此外，与基于 MXene 的气凝胶的典型制造方法相比，我们的基于 MXene 的气凝胶的 APD 制造方法不仅显示出一种环境友好的工艺，而且在时间和能耗方面也具有卓越的优势（表S5 ，支持信息）。因此，我们的APD MXene基气凝胶有望在实际应用中大规模生产。

In addition to the hydrophobicity, water resistance, and EMI shielding, developing more functionalities is also crucial for APD MXene-based aerogels. Benefiting from the local surface plasmon resonance (LSPR) characteristics,^[23] the MXene-based aerogels possessed outstanding photothermal conversion performance. Under the irradiation of xenon lamp simulating sunlight, the temperature of the aerogels with various MXene contents exhibited a dramatically increasing trend (Figure 5a). With increased MXene content from 10 wt.% to 70 wt.%, the equilibrium temperature lifted linearly at the light power density of 200 mW cm⁻² and reached 77 to 100°C, respectively. In the lack of MXene, the equilibrium temperature of pure CNF aerogels was merely 57°C, showcasing the positive influences of MXene on photothermal conversion performance. Moreover, the equilibrium temperature of the ambient-dried aerogels could be controlled by adjusting MXene content. The enhanced light power density from 25 to 200 mW cm⁻² also contributed to the equilibrium temperature of MXene/CNF aerogels from 45 to 100 °C, respectively (Figure 5b). Even after the loop was executed ten times for a long working time of 1200 s, the equilibrium temperature of MXene/CNF aerogel was repeatable and stable (Figure 5c). To further reveal the stability of photothermal conversion, the equilibrium temperature during high and low power density circulation was demonstrated in Figure 5d. It was obvious that the APD MXene-based aerogels still maintained the regular ascending and descending temperature cycles, exhibiting excellent responsiveness and cycling stability. Through the infrared thermal images, the temperature distribution on the surface of aerogel was uniform (Figure 5e), further showing the reliable photothermal conversion. Based on the high photothermal performance, a demonstration of a high-performance photothermal therapy device was well displayed (Figure 5f). When the aerogel was placed on the human skin, thanks to the rapid photothermal response of aerogels, the obvious temperature rising could achieve effective local photothermal therapy without skin burns. The superior hydrophobicity and photothermal conversion also contributed to the efficient deicing capability of aerogel samples. In Figure 5g, without light irradiation, a piece of ice placed on the aerogel barely melted within 90 s. Obviously, after exposure under light illumination, the ice rapidly melted into water and then flowed down the aerogel. Benefiting from the hydrophobicity, the melted water cannot infiltrate into the aerogel, which could avoid structural damage to the aerogel. In summary, the controllable, high-performance, and stable photothermal conversion performance was achieved, contributing to the exploration of multifunctional applications for APD MXene-based aerogels.
除了疏水性、防水性和 EMI 屏蔽之外，开发更多功能对于 APD MXene 气凝胶也至关重要。受益于局域表面等离子体共振（LSPR）特性， ²³ MXene基气凝胶具有出色的光热转换性能。在模拟太阳光的氙灯照射下，不同MXene含量的气凝胶的温度呈现出急剧升高的趋势（图5a ）。随着MXene含量从10 wt.%增加到70 wt.%，平衡温度在200 mW cm ^-2的光功率密度下线性升高并分别达到77至100°C。在缺乏 MXene 的情况下，纯 CNF 气凝胶的平衡温度仅为 57°C，显示了 MXene 对光热转换性能的积极影响。此外，常温干燥气凝胶的平衡温度可以通过调节 MXene 含量来控制。光功率密度从25 mW cm -2 提高到200 mW cm ^-2也有助于MXene/CNF气凝胶的平衡温度从45°C提高到100°C（图5b ）。即使在 1200 秒的长时间工作时间内执行十次循环后，MXene/CNF 气凝胶的平衡温度也是可重复且稳定的（图5c ）。为了进一步揭示光热转换的稳定性，高低功率密度循环期间的平衡温度如图5d所示。很明显，APD MXene基气凝胶仍然保持规律的升降温度循环，表现出优异的响应性和循环稳定性。 通过红外热图像，气凝胶表面的温度分布均匀（图5e ），进一步显示了可靠的光热转换。基于高光热性能，很好地展示了高性能光热治疗装置的演示（图5f ）。当气凝胶放置在人体皮肤上时，由于气凝胶的快速光热响应，明显的温度升高可以实现有效的局部光热治疗，而不灼伤皮肤。优异的疏水性和光热转换也有助于气凝胶样品的高效除冰能力。在图5g中，在没有光照射的情况下，放置在气凝胶上的一块冰在90秒内几乎没有融化。显然，在光照下，冰迅速融化成水，然后顺着气凝胶流下来。得益于疏水性，融化的水无法渗透到气凝胶中，从而避免了气凝胶的结构损坏。综上所述，实现了可控、高性能、稳定的光热转换性能，有助于探索APD MXene基气凝胶的多功能应用。

The crosslinked APD MXene-based aerogels possessed high porosity and hydrophobic/lipophilic properties, which could be employed as superior absorbers for oil absorption and oil/water separation.^[24] Upon the thin oil with low viscosity, the crosslinked aerogel exhibited excellent absorption capability (Figure 6a), e.g., when the engine oil, dyed with red before, dripped down onto the aerogel, the oil was completely adsorbed by the aerogels within 2 s. Moreover, the MXene aerogels can absorb oil dozens of times its weight (Figure S10a, Supporting Information), which shows excellent oil absorption capacity, outperforming that of other aerogels.^[25] Furthermore, after leaving the aerogels filled with adsorbed oil for a week, the weight of the aerogel samples was slightly decreased (Figure S10b, Supporting Information), showcasing the superior oil retention capability of the APD aerogels. Due to the hydrophobic/lipophilic properties, the aerogel could be also utilized to adsorb the oil that floated on the water surface for achieving efficient oil/water separation (Figure 6b). In terms of viscous oil, such as crude oil, lubricating grease, or other heavy organic solvents, the high viscosity hindered the absorption rate. Based on the previous research the viscosity of oil decreased obviously with the temperature increasing, which could improve the fluidity of oil. Precisely, the excellent photothermal performance of the APD MXene-based aerogels could lead to a significant decrease in the viscosity of the oil by heating the viscous oil,^[26] promoting the absorption of the thick oil. For instance, without the solar irradiation, the viscous oil on the aerogel was hardly adsorbed for a long time (Figure 6c), while exposed to the light illumination (Figure 6d) the oil rapidly penetrated the aerogels and was completely adsorbed within 150 s. Therefore, the high-efficiency absorption to either thin or thick oil was achieved, showing the application potential in oil pollution for our MXene-based aerogels.
交联的APD MXene基气凝胶具有高孔隙率和疏水/亲油特性，可用作吸油和油/水分离的优异吸收剂。 ²⁴对于低粘度的稀油，交联气凝胶表现出优异的吸收能力（图6a ），例如，当之前染成红色的发动机油滴到气凝胶上时，油在 2 s 内完全被气凝胶吸附。 。此外，MXene气凝胶可以吸收数十倍于其重量的油（图S10a ，支持信息），表现出优异的吸油能力，优于其他气凝胶。 ²⁵此外，将充满吸附油的气凝胶放置一周后，气凝胶样品的重量略有下降（图S10b ，支持信息），显示了 APD 气凝胶卓越的保油能力。由于气凝胶的疏水/亲油特性，还可以利用气凝胶吸附漂浮在水面上的油，以实现有效的油/水分离（图6b ）。对于粘性油，如原油、润滑油脂或其他重有机溶剂，高粘度阻碍了吸收速率。根据以往的研究，随着温度的升高，油的粘度明显降低，这可以改善油的流动性。准确地说，APD MXene基气凝胶优异的光热性能可以通过加热粘稠油来显着降低油的粘度， ²⁶促进稠油的吸收。 例如，在没有太阳照射的情况下，气凝胶上的粘稠油在很长一段时间内几乎不被吸附（图6c ），而暴露在光照下（图6d ），油迅速渗透气凝胶并在150秒内被完全吸附。因此，实现了对稀油或稠油的高效吸收，显示了我们的 MXene 基气凝胶在石油污染方面的应用潜力。

3 Conclusion 3 结论

We have reported an energy-efficient ambient-pressure-dried large-area, multifunctional MXene aerogels associated with ultralight yet mechanically robust nature through high-efficiency employment of the 1D, ultrathin, high-strength, renewable CNFs. The CNF rendered the minimal gaps between MXene nanosheets, maintaining the excellent electrical conductivity of MXene-based aerogels. Meanwhile, the strong interactions of the MXene and CNF contributed to the mechanical strength of cell walls and aerogels, efficiently maintaining the structure stability of MXene-based aerogels in the APD process. The abundant hydrophilic functional groups of the CNFs were also beneficial for chemical crosslinking of the MXene-based aerogels, significantly improving the hydrophobicity, water resistance, and oxidation stability. The aerogels showcased a wide range of controllable densities/porosities, high conductivity, remarkable compression strength, and multifunctionality, involving designed hydrophobicity, EMI shielding, photothermal conversion, and oil-water separation. Wide-ranging CNF contents were achieved, contributing to the easy modulation of the EMI shielding and photothermal performance. MXene-based aerogels reached an EMI SE of 42 to 81 dB at a density of merely 10 to 45 mg cm⁻³, contributing to the superior SSE/d values comparable to those of the best EMI shields ever reported, due to the synergies of the MXene, CNF, and abundant, micrometer-sized pores. In addition to the high-efficiency adsorption to thin oil, high photothermal performance rendered the high-efficiency oil-water separation to viscous crude oil. This work presents a wide range of possibilities for fabricating high-cost-performance, multifunctional MXene-based aerogels with great promises in applications involving electromagnetic blockers and absorbers, smart heaters, dye adsorbers, oil-water separators, and even defense equipment.
我们报道了一种节能常压干燥的大面积、多功能 MXene 气凝胶，通过高效利用一维、超薄、高强度、可再生 CNF，具有超轻但机械坚固的特性。 CNF 使 MXene 纳米片之间的间隙最小，保持了 MXene 基气凝胶的优异导电性。同时，MXene和CNF的强相互作用有助于提高细胞壁和气凝胶的机械强度，有效维持APD过程中MXene基气凝胶的结构稳定性。 CNF丰富的亲水官能团也有利于MXene基气凝胶的化学交联，显着提高疏水性、耐水性和氧化稳定性。该气凝胶具有多种可控密度/孔隙、高导电性、卓越的压缩强度和多功能性，包括设计的疏水性、EMI屏蔽、光热转换和油水分离。实现了广泛的 CNF 含量，有助于轻松调节 EMI 屏蔽和光热性能。基于 MXene 的气凝胶在密度仅为 10 至 45 mg cm ^-3时就达到了 42 至 81 dB 的 EMI SE，由于以下物质的协同作用，其优异的 SSE/ d值可与迄今为止报道的最佳 EMI 屏蔽相媲美。 MXene、CNF 和丰富的微米级孔隙。除了对稀油的高效吸附外，高光热性能还使得对粘稠原油的高效油水分离成为可能。 这项工作为制造高性价比、多功能 MXene 气凝胶提供了广泛的可能性，在电磁屏蔽器和吸收器、智能加热器、染料吸附器、油水分离器甚至国防设备等应用领域具有广阔的前景。

4 Experimental Section 4 实验部分

The detailed experimental process is available in the Supporting Information.
详细的实验过程可在支持信息中找到。

Conflict of Interest 利益冲突

The authors declare no conflict of interest.
作者声明不存在利益冲突。