硅酸盐学报

利用相变材料储存太阳能在能源利用领域极具发展前景，为此，本工作利用低成本埃洛石制备了一种三维孔隙结构的埃洛石气凝胶，并用于封装十四酸，并以碳纤维为导热增强剂，制得3种不同碳纤维含量的埃洛石气凝胶复合相变材料。对其进行了组分与微观形貌、储热性能、导热性能，光热转换性能以及水蒸发性能的综合测试。结果表明：复合材料中的相变材料负载率高达84%，碳纤维的加入将复合材料的导热系数提高了158%；最佳样品的潜热可达140.65 J/g；在1个太阳光强度下照射下，吸热储能3 h后，将其用于夜间的太阳能界面蒸发器，夜间水蒸发速率相较于纯水提高了4.21倍，并探讨其水蒸发性能的提升机理。为太阳能的高效利用效率和海水淡化效果提升提供了参考依据。

Introduction For a challenge of global water scarcity, the development of efficient and sustainable freshwater production technologies is urgently needed. Solar-driven interfacial water evaporation(SIE) technology has attracted recent attention due to its eco-friendly and low-energy-consumption characteristics. However, the inherent intermittency of solar energy availability, both temporally and spatially, prevents SIE systems to maintain a high efficiency during nighttime, severely limiting further performance improvements. The introduction of phase change materials(i.e., PCMs) is generally considered as an effective approach to address the intermittency of solar water evaporation systems and enhance evaporation efficiency. Nevertheless, leakage issues associated with PCMs during their phase transition process significantly restrict their application scope. To overcome this problem, researchers are dedicated to developing porous mineral-based supporting materials. However, the performance of mineral carriers encapsulating PCMs in some existing studies still requires a further enhancement. Furthermore, the problem of low thermal conductivity in composite PCMs also hinders their application progress in SIE systems. Halloysite-based aerogels represent a highly promising supporting material capable of preparing composite PCMs with high loading rates, good stability, and large thermal energy storage density. Carbon fiber can be incorporated at different ratios to enhance the thermal conductivity of the composite PCMs. This study was to investigate the thermal properties, photothermal conversion efficiency, and water evaporation rate within an SIE system for the composite PCMs, and analyze the enhancement mechanisms responsible for the improved water evaporation performance. Methods Dopamine was firstly used to coat halloysite to obtain dopamine-coated halloysite powder(HNTP), and then 1 g of the prepared HNTP powder was added into 40 mL of deionized water. After stirring uniformly, the mixture was ultrasonicated for 30 min. Afterwards, 1 g of sodium alginate(SA) was added to the resulting suspension under continuous stirring until the SA was completely dissolved. The mixture was evenly divided into four portions and transferred into molds. The samples were snap-frozen in a freezer until completely solidified and then freeze-dried(lyophilized) for 72 h. The aerogels were then washed with anhydrous ethanol and dried in an oven, ultimately yielding the halloysite-based aerogel HA. Halloysite-based aerogels with different carbon fiber contents were prepared via replacing HNTs proportionally with carbon fiber(CF). These aerogels were named as HCA 4:1 and HCA 3:2 based on a mass ratio of halloysite to carbon fiber. The composite phase change materials(PCMs) were fabricated by a vacuum impregnation method. Myristic acid(MA) was placed in an oven and heated beyond its phase transition temperature until it completely melted into its liquid state. The prepared aerogels(i.e., HA, HCA 4:1, HCA 3:2) were then submerged in a liquid MA. Under vacuum conditions, the MA gradually infiltrated into the pores of the aerogels. After complete adsorption, the composite phase change materials(i.e., MA@HA, MA@HCA 4:1, and MA@HCA 3:2) were obtained. Results and discussion The SEM images of the halloysite aerogel(HA) clearly reveal its abundant porous structure. HNTs are uniformly distributed within the three-dimensional network framework. This highly porous structure provides ample space for encapsulating the phase change material. The results of leakage tests confirm a high phase change material loading rate of up to 84% within the composite. A significant supercooling reduction also occurs. The latent heat of the sample MA@HCA 3:2 measured is 140.65 J/g. The XRD patterns and FTIR spectra show no emergence of new peaks, indicating a superior compatibility between the MA and the aerogel without chemical reactions. This ensures the core thermal energy storage functionality of the MA remains unaffected. To enhance the thermal conductivity of MA@HA, carbon fiber(CF) is introduced as a thermal conductivity enhancer. The results measured by a thermal conductivity analyzer demonstrate that compared to MA@HA, the thermal conductivity of the composites with the incorporation of carbon fiber is increased by 158%. This significant improvement is attributed to a high intrinsic thermal conductivity of carbon fiber, which forms a continuous and oriented thermally conductive network within the composite, providing highly efficient pathways for heat transfer. The composite PCMs can efficiently store thermal energy through sunlight absorption during the day and release this stored heat to drive water evaporation during the night due to their superior photothermal conversion and thermal conductivity properties. This effectively mitigates the bottleneck of conventional SIE technology, which relies solely on daytime solar energy for high-efficiency water production. Based on this principle, a novel SIE evaporator incorporating the halloysite aerogel composite PCMs can be designed. The results show that under one sun illumination after 3-h energy storage, the water evaporation rate of the MA@HCA 3:2 sample is 4.21 times greater than that of pure water. The mechanisms behind this enhanced water evaporation performance are investigated. This work provides valuable insights and a reference basis for enhancing the efficiency of solar energy utilization and improving the effectiveness of seawater desalination. Conclusions This study fabricated a highly porous aerogel using halloysite for encapsulating myristic acid(MA). Carbon fiber was incorporated as a thermal conductivity enhancer to modify the composite phase change materials(PCMs). The resulting composite PCMs exhibited a high loading rate, significantly reduced supercooling, and markedly improved photothermal conversion and thermal conductivity properties. The photothermal conversion model designed based on this strategy ensured that more of the heat stored within the PCMs could be utilized for water evaporation. This enabled an efficient operation under various conditions, including nighttime and low-light environments, achieving all-weather high-efficiency evaporation in the solar-driven interfacial evaporator. These results could demonstrate promising applications for enhancing solar energy utilization efficiency and seawater desalination performance.