Aerogel fiber is the rising star for the applications of thermal insulation and wearable textiles, with a high specific surface area, high porosity, and low density. The porous structure of aerogel fiber can be easily combined with other functional components, providing unique advantages in realizing material functionalization. However, current functional aerogel fibers are single functional materials, therefore, designing and preparing multifunctional integrated intelligent fibers are highly desired. Here, by introducing the hygroscopic LiCl into the perforated graphene aerogel fiber, a hygroscopic graphene aerogel smart fiber (LiCl@HGAFs) was obtained, realizing the integration of moisture harvest, adsorption-based heating and cooling, and broadband microwave absorption.
Fig. 1 | Fabrication strategy and application of LiCl@HGAFs. a Schematic illustration of the fabrication of hygroscopic holey graphene aerogel fibers (LiCl@HGAFs). HGAFs were obtained by wet spinning, HI reduction, and supercritical drying. LiCl was introduced by simple impregnation. b Schematic illustrations on moisture capture, heat allocation, and microwave absorption, respectively.
The material synthesis has been illustrated in Fig. 1. The holey graphene oxide (HGO) was prepared by etching graphene oxide (GO) in H2O2 at 100 oC, followed by washing and centrifuging. Then, the LiCl@HGAFs were fabricated by wet spun, reduction, supercritical drying (Sc-drying), and filling with LiCl in sequence. Water molecules can be captured by LiCl@HGAFs and quickly released based on graphene's electro-thermal or photo-thermal effect. Efficient heat allocation was realized since heat is reversibly generated and released along with the adsorption and desorption of water. With water saturated in the interconnected porous structure, LiCl@HGAFs showed outstanding microwave adsorption performance.
Fig. 2 | High efficient moisture capture by LiCl@HGAFs. a Schematic illustration of the moisture sorption process. In this process, LiCl reacts with water first to form LiCl·H2O, and then deliquesce to form LiCl solution, and the captured moisture exists in the form of liquid water (blue-shaded region). b Water uptake of LiCl@GAFs and LiCl@HGAFs at 25 oC and 90% RH in 30 min. c Water uptake of LiCl@HGAFs with different salt contents at 25 oC and 90 % RH. d Water uptake of LiCl@HGAFs under the relative humidity of 30%, 60%, and 90%. e Comparison of water sorption capacity with reported hygroscopic materials. f Temperature response of HGAFs and LiCl@HGAFs under one-sun irradiation. g Temperature response of LiCl@HGAFs under various voltages. h Cycling stability of the sorption-desorption process of LiCl@HGAFs under photo-thermal or electro-thermal conditions.
Etching the graphene sheets enhanced the kinetics of the moisture capture of the fibers. The captured water of LiCl@GAFs and LiCl@HGAFs were 1.37 g g-1 and 1.81 g g-1 at 30 min, respectively, increasing by 32.1%. And LiCl@HGAFs have a high moisture capture capacity in a wide humidity range. Under the condition of 90% RH, the mass of captured water can reach 4.14g g-1 in 6 h without leakage. Even under the condition of 30% RH, it can still reach 0.66g g-1. The high capture capacity is attributed to the super hygroscopicity of LiCl and the excellent confinement HGAFs. In addition, LiCl@HGAFs have good photo responsiveness and electrical responsiveness. Under one solar irradiation, the temperature of the fibers can increase from 22℃ to 46℃ in 44 s, and the regeneration degree can reach 83.4% under photothermal conditions. The fibers surface temperature can reach 131℃ under 12 V, making the fiber regeneration completely. Further cyclic tests show that the adsorption capacity of LiCl@HGAFs does not degrade during the ten sorption-desorption cycles.
When LiCl@HGAFs is used in adsorption heating system, heat storage density and performance coefficient COPH are the main parameters. According to the Clausius-Clapeyron equation, when the relative partial pressure of water vapor is 0.1, the heat storage density of fiber is 0.19 kWh kg-1, which is higher than the requirement of the U.S. Department of Energy (DOE) (0.071 kWh kg-1). It has the advantage of high energy density and a performance coefficient of 1.73, higher than silica gel sorbents (1.65), suitable for thermal storage applications. The moisture capture process of the fibers will promote the evaporation of the working fluid (water) to absorb heat from the environment and achieve the purpose of refrigeration. Performance coefficient COPc and unit mass refrigeration parameter SCP is the primary evaluation indexes for the adsorption refrigeration system. The performance coefficient of the fiber can reach COPc = 0.7 at 373 K driving temperature. In addition, the unit refrigeration parameter of the fiber can get SCP=297 W kg-1 due to its excellent adsorption and desorption kinetics, which is superior to commercial sorbents and show great potential for practical applications.
Fig. 3 | Heat allocation of LiCl@HGAFs. a Working principle of LiCl@HGAF in an ATH device. b Water sorption isotherms of LiCl@HGAFs-7 at 293 K, 303 K, 313 K, and 323 K. c Isosteric enthalpy of adsorption for water at LiCl@HGAFs-7 (black) and the corresponding heat storage capacity (red). d Comparison of energy density among our LiCl@HGAF sorbent and other sorbents. e Working capacity as a function of driving temperature for heating conditions. f The COP values for cooling and heating at different driving temperatures.
Water is ideal for designing broadband absorbers because of its dispersion and high loss at microwave frequencies. Therefore, LiCl@HGAFs have better microwave absorption performance after moisture absorption. As shown in Fig. 4, after hygroscopic, the effective absorption bandwidth of the material is 8.31 GHz-18 GHz at a thickness of 2.5mm, and the lowest reflection loss RL= -27.9 dB at a frequency of 17.3 GHz.
Fig. 4 | Microwave absorption characterization. Reflection loss (RL) value of a HGAFs, b LiCl@HGAFs, and c LiCl@HGAFs-H2O. Calculated |Zin/Z0| values of d HGAFs, e LiCl@HGAFs, and f LiCl@HGAFs-H2O at different thicknesses. g ε′−ε″ plot of LiCl@HGAFs-H2O. h The |Zin/Z0| value of HGAFs, LiCl@HGAFs, and LiCl@HGAFs-H2O with a thickness of 2.5 mm. i The integrated effective absorption bandwidth (EAB) at the scope of 1-18 GHz.
This work provides new inspiration for the multi-functionalization of aerogel fibers. All the material functionalities are centered on the hygroscopicity of the fiber. And this strategy is not be limited to the combination of aerogel and hygroscopic salt. It may also suggest the possibility to functionalize aerogel fibers with other choices of functional materials. It is envisioned that our results will also spur future efforts for the development of advanced adsorbents, dehumidifiers, sorption-based heat transfer systems, adsorption-driven refrigeration, and beyond.