MXene‐Modified Aramid Aerogel for Advanced Solar Steam Generation

Developing solar‐driven desalination through interfacial steam generation is crucial to reducing global water shortages. However, traditional solar steam generation systems have faced efficiency, durability, cost, and complexity limitations. To overcome these issues, interfacial solar steam evaporators are fabricated using light‐absorbing materials with low thermal conductivity, high absorption capacity, and sufficient mechanical strength. Herein, an advanced 3D solar evaporator is developed by coating MXene onto the surface of the aramid nanofiber aerogels (MX@ANF aerogels). The MXene coating enhances the ANF aerogels' light absorption and thermal conversion capabilities. Additionally, the hydrophilicity of MXene complements the high porosity of the host aerogels, enhancing continuous water supply by improving the capillary action. Primarily, these MX@ANF aerogels show promising performance at the air–water interface, with an evaporation rate of 1.48 kg m−2 h−1 and steam conversion efficiency of 93.8% under 1 sun irradiation (1 kW m−2). These highlight the effectiveness of the MX@ANF aerogel as a material for solar‐driven desalination. Moreover, using MXene as a photothermal agent in composite materials paves new avenues toward efficient and cost‐effective solutions for addressing water scarcity through solar desalination.


Introduction
Our rapidly growing population and fast-paced industrialization have brought severe issues such as scarcity of freshwater resources and water pollution. [1]According to recent studies, almost 40% of the world's population will have difficulty accessing clean water by 2025. [2]The earth's surface is also estimated to be covered with water on approximately 75%, but only 2.5% is fresh water.DOI: 10.1002/aesr.202300126Developing solar-driven desalination through interfacial steam generation is crucial to reducing global water shortages.However, traditional solar steam generation systems have faced efficiency, durability, cost, and complexity limitations.To overcome these issues, interfacial solar steam evaporators are fabricated using light-absorbing materials with low thermal conductivity, high absorption capacity, and sufficient mechanical strength.Herein, an advanced 3D solar evaporator is developed by coating MXene onto the surface of the aramid nanofiber aerogels (MX@ANF aerogels).The MXene coating enhances the ANF aerogels' light absorption and thermal conversion capabilities.Additionally, the hydrophilicity of MXene complements the high porosity of the host aerogels, enhancing continuous water supply by improving the capillary action.Primarily, these MX@ANF aerogels show promising performance at the air-water interface, with an evaporation rate of 1.48 kg m À2 h À1 and steam conversion efficiency of 93.8% under 1 sun irradiation (1 kW m À2 ).These highlight the effectiveness of the MX@ANF aerogel as a material for solar-driven desalination.Moreover, using MXene as a photothermal agent in composite materials paves new avenues toward efficient and cost-effective solutions for addressing water scarcity through solar desalination.
[18] Finally, 3D interconnected porous structures, including aerogels with vertically aligned vessels, and inherent porous structures, such as loofah fibers, have been examined through capillary action to facilitate water transport in solar steam evaporators. [19,20]any challenges remain to overcome to enhance photothermal conversion efficiency.Achieving stable and efficient performance requires optimizing light-absorbing materials, substrate architecture, thermal management, and water transport channels in solar evaporators, which has remained the researchers' focus.[32][33][34] Last but not least, MXene is hydrophilic inherently because it comprises oxygen-containing functional groups that are hydrophilic, which could promote water penetration into the MXene structure, facilitating water migration within the material. [31]ome previous studies have explored MXenes for solar water evaporation.However, comprehensive analyses of MXene on the surface of 3D architecture for solar steam generations are limited.To best utilize MXene's excellent properties as an interfacial photothermal material, a support material is required, and it needs to meet the following requirements: thermal stability, low cost, hydrophilic and porous structure to support water transport, and higher surface area for water evaporation. [35]Aramid nanofibers (ANFs) are a great candidate as supporting materials for solar steam generation because of their strong mechanical properties and thermal stability under high temperatures.ANFs are usually produced from poly(p-phenylene terephthalamide) (PPTA) fibers via a simple deprotonation process, yielding a dispersion.They can be further processed and assembled as aerogels with robust mechanical properties, high porosity, large specific surface area, and excellent thermal insulation properties. [36,37]Moreover, aramid-based aerogels are highly thermally stable, making them suitable candidates for desalination application compared to other polymer-based porous structures such as melamine or polyurethane foam. [38]However, effectively incorporating MXene in ANF aerogels for solar steam generation is still challenging.ANFs only have good light absorption at lower wavelength (<%480 nm), which means that MXene embedded in the ANFs aerogel is less effective in absorbing light. [39]eanwhile, the effect of introducing MXene on the excellent properties of ANF aerogel is also unknown.Therefore, developing a facile method and a good aerogel structure optimized for photothermal conversion, minimizing heat dissipation, and facilitating water transport are needed to achieve good solar steam efficiency.
Herein, we have developed a solar evaporator of a composite material by a new combination (MXene/aramid, 2D/1D structure) to make MX@ANF aerogels.We have utilized the light absorption property of MXene and aramid aerogel, which possess excellent mechanical strength, stable chemical properties, lightweight, high porosity, and hydrophilicity, to develop a highly efficient solar evaporator by spray coating of MXene on aramid aerogel surface.
The MX@ANF composite aerogels achieved 93.8% solar steam efficiency under 1 sun illumination and a high evaporation rate of 1.48 kg m À2 h À1 .The simple fabrication approach of MXene and aramid composite aerogels provide an easy, cost-effective, and less complex manufacturing process that can be further scaled up, which opened the door to the future development of solar steam generators based on MXene and aramid fiber (AF).

Preparation of ANF Dispersion, ANF Aerogel, and MX@ANF Aerogel
The preparation of MX@ANF aerogel involves the following steps shown in a schematic (Figure 1, and S1, Supporting Information), including the deconstruction of AFs to ANFs dispersion, fabrication of ANF aerogel, and coating of MXene to form MXene@ANF aerogel.A top-down approach was used to prepare ANF dispersion (i.e., the deconstructions of macroscopic AF into ANF dispersion) by deprotonating the amide bond using potassium hydroxide (KOH)/dimethyl sulfoxide (DMSO) solution and generating the negatively charged polyanions. [40,41]ere, deionized (DI) water was used as a protic solvent to trigger the reprotonation of ANF to form a 3D porous network linked by hydrogen bonds within the ANFs. [42,43]Then, DI water and isopropyl alcohol (IPA) (3:1) were used to completely remove the DMSO from wet ANF gels.Finally, the wet ANF gels were freezedried to obtain ANF aerogels.Due to their nanoscale morphology, high mechanical strength, and impressive chemical and thermal stability, [44] the fabricated ANF aerogels are also lightweight, strong, and thermally stable, making a promising building block to support interfacial photothermal materials.
The morphology of the AF, ANF, and ANF aerogel was investigated using scanning electron microscopy (SEM).AF exhibited a smooth surface (Figure 2a), whereas ANF dispersion showed aggregation of uniform nanofibers (Figure 2b), both of which are quite different from ANF aerogel with 3D entangled networks of nanofibers in a porous architecture (Figure 2c).To analyze the surface hydrophilicity of the ANF aerogel, the water contact angle on the ANF aerogels was measured.We observed the contact angle of %52°, indicating a hydrophilic surface beneficial for water absorption and transport in the porous aerogel network shown in Figure 2d.Moreover, its lightweight nature makes the aerogel stand on the leaves' surface, as shown in the photograph (Figure 2e).This stability ensures that aerogel can remain in the same place and maintain consistent contact with the surface during an extended period of sunlight exposure.
Additionally, because of its low density (calculated 0.0254 g cm À3 ), the ANF aerogel is buoyant and self-floats in water without sinking, as shown in Figure 2f.This buoyancy ensures that aerogel does not need additional support and can be easily retrieved from the water after usage.
To enhance the ANF aerogel's light absorption and photothermal capability, we spray-coated MXene on the surface to prepare MX@ANF aerogel.MXene dispersions used for spray-coating were treated with mild sonication (3 min in bath sonication) to achieve a moderate and uniform sheet size.Four different concentrations of MXene (3, 5, 10, and 15 mg mL À1 ) were spray-coated onto the top surface of ANF aerogel, which is denoted as MX3, MX5, MX10, and MX15@ANF, respectively (Figure 3a-d).The morphology of all MX@ANF aerogels was investigated using SEM to examine whether the highly porous structure of the ANF aerogel remains intact after MXene modification at different concentrations (Figure 3e-h).It can be seen clearly that MXene nanosheets were evenly distributed on the surface of the aerogel.Some pores remained open when low concentrations of MXene dispersions were applied, while the coverage increased with the increase in MXene concentration.It is worth noting that higher MXene loading potentially provides a higher solar absorbance, but it also causes the blocking of the pores of the ANF aerogels.Thus, the appropriate balance between absorbance and porosity must be maintained to optimize solar steam generation.At the same time, the cross-sectional SEM for MX5@ANF aerogel exhibits the hierarchical 3D network channels that facilitate the water uptake during solar steam generation, as shown in Figure S3, Supporting Information.
To confirm the loading of MXene on the ANF surface, X-ray diffraction (XRD) measurements were performed on pure MXene, ANF aerogel, and MX@ANF aerogel (Figure 3i).The characteristic peak of MXene at %6.5°corresponds to (002) planes in its crystal structure, while ANF aerogel shows distinct peaks at the diffracting angle at 20.5°, 22.7°, and 28.7°which can be assigned to the (110), (200), and (004) lattice planes of PPTA crystal structure, respectively. [45]The XRD patterns of MX@ANF show both the characteristic peaks from MXene and ANF, confirming the attachment of MXene on the surface of ANF aerogel.
Moreover, all MX@ANF aerogels showed excellent hydrophilicity, confirmed by water contact angle measurement.Typically, water droplets were absorbed into the MXene-coated aerogel surface in a few seconds.A droplet of water spreads out and gets absorbed by MX5@ANF within 6 s (Figure 3j).This confirms that MX@ANF aerogels provide a high porosity and excellent hydrophilicity to facilitate fast water transport and heat insulation to enhance solar steam generation.
To study the porosity of ANF aerogel and the impact of MXene on the porosity of ANF aerogel, nitrogen adsorption-desorption isotherm was conducted, which shows a type II isotherm with an H3-like hysteresis loop for the ANF aerogel over the whole pressure range (Figure 3k).This suggests the presence of narrow pores within the material.After MXene coating, the nitrogen adsorption-desorption isotherm exhibits a similar type II isotherm with an H3-like hysteresis loop for MX@ANF aerogel.This type of isotherm is typically observed in materials with a hierarchical pore structure, with both micropores and mesopores, which are irregular and open with good connectivity with different shapes.The presence of hierarchical narrow pores in the ANF aerogel promotes capillary action, allowing the ANF aerogel to transport and distribute water within its structure effectively. [46]This structure is ideal for water transport and provides space, which is beneficial for uptake and evaporation during solar steam generation, as shown in Figure 3k.In addition, a high porosity enables the material to have a large surface area and provide efficient thermal insulation.The insulation properties of the aerogel help reduce heat loss from the system, allowing the surface of the evaporator to be rapidly heated.This, in turn, facilitates the conversion of water on the surface into vapor, contributing to the efficient generation of solar steam.
To understand the thermal characteristics, AF, ANF aerogel, and MX5@ANF aerogel were subjected to thermogravimetric analysis (TGA).It was observed that both AF and ANF aerogels are thermally stable up to %508 °C, indicative of outstanding heat stability shown in Figure 3l.At the same time, no significant change in thermal decomposition was observed up to %526 °C in MX5@ANF aerogel.This indicates MX5@ANF aerogel is more stable than individual AF and ANF aerogel.Therefore, this good thermal stability makes them suitable candidates to serve as a substrate for photothermal evaporators often operated under elevated temperatures.To evaluate the mechanical properties of the ANF aerogel, it was compressed at a compression rate of 5 mm min À1 with a strain of 30% maximum, as shown in Figure 3. a-d) Top-view as-prepared MX3@ANF, MX5@ANF MX10@ANF, and MX15@ANF aerogel; e-h) SEM images of MX3@ANF, MX5@ANF MX10@ANF, and MX15@ANF aerogel; i) XRD patterns of pure MXene, ANF, and MX5@ANF aerogel; j) contact angle of MX5@ANF aerogel; k) nitrogen adsorption/desorption isotherm of ANF aerogel and MX5@ANF aerogel; l) TGA curves of AF, ANF aerogel, and MX5@ANF aerogel; and m) cyclic compressive fatigue tests with strain at 30%.

Figure 3m
. The stress-strain curves suggested that the aerogel exhibits an elastic zone when the strain is below %10%, and when the strain was increased further, it was likely that the internal pores started to be compressed and become dense.The aerogel could recover to its original shape after applying a strain of 30%.The aerogel showed a stable mechanical property after the first five compress-release cycles, and the maximum stress at 15 cycles (91 kPa) remains almost consistent with that in the initial cycle (94 kPa).These results indicate that ANF aerogel has excellent mechanical stability as a substrate for supporting solar evaporation.On the other hand, as shown by SEM analysis (Figure 3e-h), only a thin layer of MXene is located on the top surface of the ANF aerogel after spray coating.Therefore, the MXene layer does not significantly alter the ANF aerogel's mechanical strength and compression behavior.

Solar Photothermal Conversion Performance of ANF Aerogels and MX@ANF Aerogels
We first characterized and compared the light absorbance across a broad-spectrum range (200-2000 nm) covering the UV to NIR region.The light absorbance spectra of ANF, MX3@ANF, MX5@ANF, MX10@ANF, and MX15@ANF aerogels are shown in Figure 4a.The ANF aerogel generally indicates a low absorbance across the vis-NIR spectrum, but exhibits a high absorbance of green and blue light (<%500 nm).After modification of ANF aerogel using MXene, the light absorbance has significantly increased in the visible and NIR regions, while the absorbance in <500 nm remains high.It is worth noting that the high absorption in the NIR region is attributed to the MXene coating, which suggests that the MXene@ANF aerogel as a photothermal evaporator can utilize NIR light compared to some of the other photothermal evaporators reported that can use only the visible spectrum. [1,13,47,48]The excellent light absorption properties of MXene combined with the interconnected porous structure create multiple areas for scattering and reflection of the incident light, increasing the optical path and the overall absorption within the aerogels. [49]ifferent MX@ANF aerogels with additional MXene loading exhibited other absorption characteristics.At lower MXene concentrations, it is less effective at absorbing light.At higher concentrations, MXene interacts with light more effectively, thereby increasing the overall light absorption, but this is at the cost of lower light absorbance at the blue/green spectrum (<500 nm) and blockage of the pores (Figure 3h) and potentially hindering water evaporation on the aerogel surface.These results indicate that an optimal MXene concentration is needed to balance the improved light absorption and water evaporation to achieve an optimized solar steam generation performance.
Evaporation rates and solar steam efficiencies of the prepared MX@ANF aerogels were evaluated under 1 sun (1 kW m À2 ) irradiation (Figure 4b).Using an IR camera, each sample's upper and bulk water temperature was recorded, as shown in Figures 4c,f.Additionally, continuous evaporation of water was investigated during the given period.All the evaporators showed a linear decrease in water mass with time, as shown in Figure 4b.The steeper slope of the line indicates a higher evaporation rate and vice versa.The surface temperature of ANF aerogel could reach 34.0 °C from the initial value of 28.6 °C for 60 min, as shown in Figure 4c, while the evaporator MX5@ANF increased to 37.6 °C from 29.3 °C due to solar thermal conversion capabilities of MXene, as shown in Figure 4f.Additionally, as shown in Figure S5, Supporting Information, the ANF aerogel and MX5@ANF aerogel showed low thermal conductivity (K ) due to their porous structure, making them insulators by which heat is entrapped, retained, and produced by MXene photothermal conversion within the aerogel structure, which is still lower than commonly used foams. [50]Also, the aerogels assist in minimizing the heat loss to the surroundings and maximizing the temperature on the surface of the aerogel, which enhances the overall photothermal performance of the composite synergistically.The change in surface temperature of MX15@ANF aerogel was found to be the highest among all the aerogels, while the evaporation rate and efficiency were found to be highest in the case of MX5@ANF owing to reaching an optimal level of concentration in achieving the highest solar steam generation performance (Figure 4d,e).The reason is that with increasing MXene loading, more and more surface pores on the aerogel are blocked (as confirmed by the SEM analysis), hindering water evaporation.The MX5@ANF still showed the highest evaporation rate and efficiency owing to its optimal pore opening and MXene loading level, resulting in the highest solar steam generation performance.Compared with the evaporation rate of pure water (0.36 kg m À2 h À1 ), the evaporation rates of the ANF, MX3@ANF, MX5@ANF, MX10@ANF, and MX15@ANF aerogels were remarkably enhanced to 1.12, 1.32, 1.48, 1.42, and 1.44 kg m À2 h À1 , respectively, with a calculated solar steam efficiency at 70%, 84%, 93.8%, 90%, and 90%, respectively (Figure 4e).
Furthermore, the solar evaporators were tested in real seawater (collected from Waterfront, Eastern Beach Reserve, Geelong, Victoria 3220, Australia).The mass changes as a function of irradiation time, and the evaporation rates of seawater, ANF, and MX5@ANF aerogels were 0.22, 1.18, and 1.56 kg m À2 h À1 , respectively, as shown in Figure 5a.Furthermore, to evaluate the stability and durability of MX5@ANF aerogel, the solar evaporation test was repeated for ten cycles using seawater, which demonstrated a linear relationship and a steady evaporation rate of %1.42 kg m À2 h À1 in all the evaporation tests with no significant change in evaporation rate over 10 h (Figure 5b).Moreover, we did not observe any MXene flakes getting out from the surface of MX5@ANF aerogel, which shows that MXene and ANF aerogel have firmly interacted because of their rough and porous structure of aerogel by making the presence of hydrogen bonding between them. [51]The stability of the MXene coating is also reflected by the long cycling experiment, which inferred that the solar evaporation performance remained stable during ten cycles.
These results indicate MX5@ANF aerogel maintained its photothermal characteristics, which provide stability to support reliable evaporation and are promising for integration in larger solar steam generation systems.To demonstrate the desalination capability, MX@ANF aerogels (in this case, MX5@ANF aerogel) were assembled and tested in a solar steam evaporator system.To make a simulator solar steam evaporation system, a beaker was placed on top of the sample submerged in seawater, serving as a water vapor condensation collector, as shown in Figure 5c(i).After exposure to 1 sun for 60 min, the water droplets were observed on the upper part of the condensation beaker and collected for further analysis (Figure 5c(ii)).The resistance of seawater and purified collected water was compared with a multimeter using two electrodes at a constant distance to assess the purification of collected purified water (Figure 5d).The resistance value of saline water was measured to be 133.8KΩ (Figure 5d(i)), which represents the electrical resistance of seawater, while the resistance of evaporated (purified) water was found to be 1.789 MΩ (Figure 5d(ii)).This significant increase in the resistance of purified water suggests the reduction in salts and minerals in the purified water.These results demonstrate that higher-quality desalinated water can be derived from MX@ANF aerogel solar steam evaporators.
Moreover, the results suggest that the MX@ANF aerogel can be used for effective solar steam generation.MXene nanosheets on the aerogel surface act as photothermal layers in the composite material and increase the structural light absorption capabilities, allowing it to utilize a broader range of wavelengths.Simultaneously, ANF aerogel serves as a water transport channel within composite structures, facilitating water transport to the evaporator's surface through an interconnected porous network.As a result of its hydrophilic properties, MXene further enhances the capillary action of the aramid aerogel, enhancing its ability to transport water.The overall low density of the MX@ANF aerogel enables it to self-float on the water, facilitating water evaporation and ensuring ease of maintenance.

Conclusion
In conclusion, this work demonstrated the successful steam generation using a composite of ANFs and MXene.The synergistic effect of the porous structure of ANF aerogel and the photothermal properties of MXene enables solar energy harvesting across a broad spectrum of solar light.The significant porosity allows for efficient water absorption and exceptional solar steam evaporation, achieving solar steam efficiency of 93.8% under 1 sun irradiation.Its porous structure, stability, and efficient solar energy utilization make it an attractive candidate for finding practical solutions in many areas, such as clean water.
Preparation of ANF Dispersion: An ANF dispersion was prepared using a top-down approach by deconstructing macroscopic AFs into nanoscale fibers.AFs were first washed in ethanol and then dried at room temperature.ANF dispersion (%2 wt%) was prepared by adding 2 g of AFs and 3 g of KOH in 100 mL of DMSO under magnetic stirring at 55 °C for a week. [44][54] Then, a solution of 1.6 g LiF in 20 mL of 9 M HCl was prepared.After that, 1 g of Ti 3 AlC 2 MAX powder was slowly added, and the etching was continued continuously for %28 h.The product was washed using a centrifuge with argon-purged DI water until the pH reached above 6.To obtain a stable MXene dispersion, sonication was performed for 3 min, and centrifugation was carried out for 15 min at 1500 rpm to remove bulk particles and unetched MAX.To prepare the MXene dispersions for further processing, the dispersions were diluted with DI water to specific concentrations. [52]reparation of ANF Aerogel and MX@ANF Aerogel: ANF dispersion (%20 mg mL À1 , 3 mL) was placed into sample bottles (diameter = 30 mm) and frozen for 7 h at %4 °C.About 15 mL of (DI) water of the same temperature was slowly poured on top of frozen samples.The solvent in each vial was replaced twice a day for 7 days with fresh DI water to eliminate DMSO from the ANF gel.The gel's color changed from dark red to yellow, showing that PPTA chains in aramid are moderately reprotonated by water.The formed aramid gels were exchanged using a 3:1 solution of DI water and isopropyl alcohol twice a day to remove the DMSO completely.The ANF aerogel was obtained after freeze-drying for 30 h.Further, MXene nanosheets were assembled on the ANF aerogels using a spray coater to prepare MX@ANF aerogel and dried at room temperature overnight.
Characterization: SEM (Zeiss Supra 55VP) was performed to study the morphology of prepared aerogels at different stages with an acceleration voltage of 5 kV.Small sample pieces were mounted on an aluminum stub and carbon-coated before SEM observation.The optical properties of pure and MXene-modified ANF aerogels were measured using an UV-vis-NIR spectrophotometer (Carry 5000) in the 200-2500 nm spectral range at room temperature.ANF aerogel thermostability was studied using TGA.The ramp temperature for TGA samples was set to 10 °C min À1 under N 2 .The wettability of the aerogels was characterized using the Theta flow tensiometer using a 10 μL water droplet for 10 s.The thermal conductivity of samples was analyzed using the laser lash method (NETZSCH, LFA 457) and calculated using (K = α.C p .ρ), where ρ is the density of aerogel, C p is the specific heat capacity, and α represents the thermal diffusivity.The mechanical properties of ANF aerogel were tested on an Instron 30 KN Tensile tester at a strain of 30% in compressive mode at a rate of 5 mm min À1 .ANF aerogels' porosity was determined by measuring nitrogen adsorption-desorption isotherm on Quantachrome Autosorb.
Solar Steam Generation and Measurements: A simulated solar source (Xenon lamp, Newport Model: 67005) generated interfacial steam under 1 sun irradiation (1 kW m À2 ) using an electronic balance to monitor mass changes.An IR camera (FLIR) was used to record variations in the surface temperature of ANF and MX@ANF aerogels.A beaker containing DI water or seawater was used to measure the rate and efficiency of evaporation.Blocking the extra light from the light source is achieved using polystyrene foam with a hole that matches the size of the solar evaporator.
The water evaporation efficiency (η) was calculated according to the following equation: where h LV is the phase-change enthalpy of water from liquid to vapor, C opt is the optical concentration, and P 0 is the solar illumination.The evaporation rate of water without irradiation was deducted to reduce the effects of natural evaporation.While evaporation rate (v) can be calculated as where m is the mass loss of water, S is the surface area of photothermal material under solar illumination, and t is the illumination time.

Figure 2 .
Figure 2. SEM images of a) AF, b) ANF dispersion, c) ANF aerogel, d) the contact angle of aerogel, e) ANF stands on leaves, and f ) self-floating of as-prepared aerogel in water.

Figure 5 .
Figure 5. Performance of ANF aerogel and MX@ANF aerogel for seawater desalination.a) The mass change of real seawater as a function of irradiation time of ANF aerogel and MX5@ANF aerogel.b) Evaporation cyclic stability of MX5@ANF aerogel.c) Images of vapor condensation and water collection after 1 h of 1 sun irradiation.d) Evaluation of saline water and purified water by a multimeter with a constant distance between electrodes.