Hygroscopic holey graphene aerogel fibers enable highly efficient moisture capture, heat allocation and microwave absorption

Aerogel fibers have been recognized as the rising star in the fields of thermal insulation and wearable textiles. Yet, the lack of functionalization in aerogel fibers limits their applications. Herein, we report hygroscopic holey graphene aerogel fibers (LiCl@HGAFs) with integrated functionalities of highly efficient moisture capture, heat allocation, and microwave absorption. LiCl@HGAFs realize the water sorption capacity over 4.15 g g−1, due to the high surface area and high water uptake kinetics. Moreover, the sorbent can be regenerated through both photo-thermal and electro-thermal approaches. Along with the water sorption and desorption, LiCl@HGAFs experience an efficient heat transfer process, with a heat storage capacity of 6.93 kJ g−1. The coefficient of performance in the heating and cooling mode can reach 1.72 and 0.70, respectively. Notably, with the entrapped water, LiCl@HGAFs exhibit broad microwave absorption with a bandwidth of 9.69 GHz, good impedance matching, and a high attenuation constant of 585. In light of these findings, the multifunctional LiCl@HGAFs open an avenue for applications in water harvest, heat allocation, and microwave absorption. This strategy also suggests the possibility to functionalize aerogel fibers towards even broader applications.

This work reports hygroscopic holey graphene aerogel fibers for highly efficient moisture capture, heat allocation and microwave sorption. A series of experiments were conducted to support the conclusions and claims. However, there is lack of novelty and there are still a few questions to be resolved: 1. For moisture sorption, sorption capacity defined as weight of sorbed water/weight of sorbent was adopted to depict the water sorption capacity. Compared with other sorbents, the author claimed that the holey graphene aerogel have the highest sorption capacity. However, the excellent sorption capacity of the fiber is mainly due to its low density, which is not available in real applications. I suggest that the author supplement the following experiments: various moisture sorbents with the same weight or volume are put into a closed space with the same initial relative humidity; after a period of time, to evaluate which sorbent reduces the relative humidity most. 2. For microwave sorption, this work show that the fiber possess good microwave sorption own to the entrapped water. Therefore, an experiment about the sorption stability of microwave should be supplemented in that the water will be heated after microwave sorption, which will inevitably lead to the decrease of sorption performance. 1

Reply to reviewers
For Reviewer #1: We appreciate the reviewer's positive evaluation of our manuscript: "This paper reports fabrication and performance of hygroscopic holey graphene aerogel fibers which can be applied for moisture capture, heat allocation, and microwave absorption. This theme is interesting and it is worthy to be studied. The paper has clearly presented the detailed fabrication process and performance data of LiCl@HGAFs. This paper should be published." We thank the reviewer very much for the constructive comments and suggestions to improve our manuscript greatly. We have addressed all the issues in a point-by-point manner and all the changes to the original manuscript are highlighted in blue.

Revision:
The following content has been added in the manuscript on page 13:

Response:
We thank the reviewer for the comments on the improvement of the figures. In the heating mode, the sorbents LiCl@HGAFs capture water molecules and release adsorption heat for indoor heating ( Figure   R3, left). In the cooling mode, the evaporation of working fluid (water), driven by the water sorption of LiCl@HGAFs, enables indoor cooling by removing the heat of evaporation ( Figure R3, right). The coefficient of performance (COP) is defined as useful energy output divided by the required energy as input, which is described as COP H and COP C for the heating mode and the cooling mode, respectively. ( (4) 7 Figure R2. Working principle of the heating mode and cooling mode. In the heating mode, the sorbents LiCl@HGAFs capture water molecules and release adsorption heat (Q ads ) to the house for indoor heating. As the adsorbent will become saturated with water, regeneration is required. Energy is taken up at a relatively high temperature (Q regen ) to desorb the water, which is subsequently condensed, releasing heat at an intermediate temperature (Q con ) to the house. Both of the released Q ads and Q con contribute to indoor heating. In the cooling mode, heat is taken up from the house by the evaporation of the working fluid (Q eva ), driven by the water sorption of the sorbents LiCl@HGAFs. Therefore, one can operate such a switchable sorption cycle as a heat pump to produce heating energy using Q con and Q ads , or to produce cooling energy by using Q eva . mode. In the heating mode, the sorbents LiCl@HGAFs capture water molecules and release adsorption heat (Q ads ) to the house for indoor heating. As the adsorbent will become saturated with water, regeneration is required. Energy is taken up at a 9 relatively high temperature (Q regen ) to desorb the water, which is subsequently condensed, releasing heat at an intermediate temperature (Q con ) to the house. Both of the released Q ads and Q con contribute to indoor heating. In the cooling mode, heat is taken up from the house by the evaporation of the working fluid (Q eva ), driven by the water sorption of the sorbents LiCl@HGAFs. Therefore, one can operate such a switchable sorption cycle as a heat pump to produce heating using Q con and Q ads , or to produce cooling by using Q eva .

Revision
Comment 3: "On the right side of Eq. S5, it misses the sensible heat items."

Reply:
We have carefully checked our calculation, and in the original manuscript, we considered the sensible heat term during the calculation but missed the writing when typing the eq. S5. Therefore, we have added the sensible heat items in the revised manuscript.

Revision:
is the energy taken up by evaporation, is the energy released by the condenser, is the energy gained during the adsorption process, and is the energy required by regeneration, they can be calculated by 2-4 (S3) Where is evaporation enthalpy and is the condensation enthalpy. and are temperatures of evaporator and condenser, respectively.
In the above equation, is the molar mass of working fluid (water), is the adsorption enthalpy, and are the maximal and minimum working capacity at sorption condition and desorption condition.
T des is the desorption temperature. T IC is the terminal temperature of the sorbent after Isosteric cooling. and are the specific heat capacity of sorbent (Supplementary Figure 29) and working fluid. is the density of working fluid. is the energy released during adsorption of the sorbent, which can be calculated by: ∫ Comment 4: "In Eq. S6, T con and T cond are not correct. They should be T abs ."

11
Reply: We appreciate the reviewer for raising this point in Eq. S6. We have replaced T con and T cond with T abs in Eq. S6. During the calculation, the values of T abs and T con are the same, so the calculation result is not affected.
Revision: Based on the reviewer's suggestion, we have revised Eq. S6 as follows: Here, and refers to the desorption temperature of sorbent and the temperature of absorption.
Comment 5: "Q sorption is redundant and Eq. S7 is not correct."

Reply:
The original Eq. S7 has been included in the revised Eq. S5 and therefore we delete the original Eq. S7.

Revision:
The item Q sorption mentioned in Eq. S7 has been deleted. The order of new equations has been updated.    HAGFs is maintained at a relatively stable level up to 10 cycles, both demonstrating rapid cycling capability of water capture and water release ( Fig. 2h, Supplementary Fig.22).  Transactions, Vol. 101, No. 2, pp. 348-357, 1995." Reply: We appreciate the reviewer for this suggestion. It is essential to study the adsorption equilibrium model of adsorbents for their 21 performance in a heat allocation system. We have fitted the water uptake data of sorption isotherm with both the Freundlich and S-B-K models at moderate relative pressure from 0.15 to 0.7. And we have added the fitting results in the Supplementary Information.

Revision:
The following sentences were added on pages 15-16 of the manuscript: As a multifunctional hygroscopic material, in addition to obtaining water from the air, it can also be used as a thermal energy storage material along with an excellent water sorption property…. The isotherm curves obtained at four different temperatures (293 K, 303 K, 313 K, and 323 K) are nearly linear at moderate pressures from 0.15-0.7 and can therefore be described by the Freundlich model and S-B-K model (Supplementary  As for the novelty, we highlight the following five innovative aspects of this work: (1) For the first time, the hygroscopic holey graphene aerogel fibers with high sorption capacity, fast moisture capture rate, and superior recyclability are prepared for water capture from the air in this manuscript. This is the first water harvesting material based on flexible aerogel fibers. The hygroscopic holey graphene aerogel fibers exhibit an excellent moisture absorption capacity over 4.15 g g -1 at 25 °C and 90% relative humidity, fast moisture capture rate of 1.81 g g -1 in 30 min, and the moisture absorption capacity retains 95.5% of the initial capacity after 10 cycles.
(2) Compared with graphene aerogel fibers, the etched nanopores of holey graphene aerogel fibers provide abundant water transport 25 pathways, which is conducive to improving the mass transfer characteristics of materials.
(3) Different from other moisture sorption materials, the hygroscopic holey graphene aerogel fibers can be regenerated using both photothermal and electrothermal methods with little energy input. The macroscopic one-dimensional conductive material can be easily applied voltage to achieve electrothermal regeneration. Combined with its great adsorption capacity and fast kinetics, it is possible to carry out multiple water harvesting cycles at night.
(4) In this work, we also probe the potential use of hygroscopic holey graphene aerogel fibers in adsorptive heat transfer devices. With water as the working fluid, adsorptive heat transfer and adsorptive clean water production can be integrated to produce cooling and heating energy. Comment 1: "For moisture sorption, sorption capacity defined as weight of sorbed water/weight of sorbent was adopted to depict the water sorption capacity. Compared with other sorbents, the author claimed that the holey graphene aerogel have the highest sorption capacity. However, the excellent sorption capacity of the fiber is mainly due to its low density, which is not available in real applications. I suggest that the author supplement the following experiments: various moisture sorbents with the same weight or volume are put into a closed space with the same initial relative humidity; after a period of time, to evaluate which sorbent reduces the relative humidity most."

Reply:
We thank the reviewer for this comment. According to the suggestions, the results of the dehumidification experiment have been provided. At room temperature, a series of 2 g and packing volume of 5 cm 3 dehumidifying materials were respectively placed in a sealed chamber with a size of 60*50*50 cm and an initial relative humidity of 90%, and the relative humidity in the chamber was detected after 6 hours. For 27 the dehumidification test of moisture sorption materials with the same mass ( Supplementary Fig. 19a), LiCl@HGAFs outperforms most of the compared sorbents and are slightly weaker than LiCl. For the dehumidification test of moisture sorption materials with the same volume ( Supplementary Fig. 19b LiCl@HGAFs, on the other hand, not only can be desorbed by photothermal energy in the daytime, but also can be electrically desorbed at night, which means it can perform more adsorption-desorption cycles over a whole day. Therefore, the hygroscopic holey graphene aerogels in our work would be of great benefit for water harvest and the multifunctional materials are greatly desirable in practical applications.

Revision:
The following content has been added in the manuscript on page 12: Combining the various pore structure and high porosity of the aerogel matrix with the strong moisture sorption of LiCl, LiCl@HGAFs 28 show excellent moisture sorption capacity... Furthermore, dehumidification tests of a series of moisture sorption materials were conducted in a closed chamber with the same initial relative humidity ( Supplementary Fig.19). For the dehumidification performance of the sorbents with the same mass ( Supplementary Fig. 19a), LiCl@HGAFs outperforms most of the compared sorbents and is slightly weaker than LiCl. For the sorbents with the same volume ( Supplementary Fig. 19b), LiCl@HGAFs shows moderate hygroscopic performance compared with other sorption materials.
Comment 2: "For microwave sorption, this work show that the fiber possess good microwave sorption own to the entrapped water. Therefore, an experiment about the sorption stability of microwave should be supplemented in that the water will be heated after microwave sorption, which will inevitably lead to the decrease of sorption performance."

Reply:
We appreciate the reviewer for the comment on the sorption stability of the microwave. During the irradiation of microwave, the water content entrapped in the fibers should be a concern under a high electromagnetic power because of the loss of water. However, under a mild or low electromagnetic power, the fibers can well retain the water since the LiCl@HGAF-H 2 O sample is sealed in the paraffin during the testing with the network analyzer, and therefore show good stability for microwave sorption. Furthermore, we are more concerned with the adjustable microwave absorption properties of materials. Once the traditional microwave absorption materials are prepared, their microwave sorption properties cannot be modulated. In our work, the microwave 30 absorption properties can be tuned by adjusting the water sorption content and tuning the power of the external stimuli (solar energy or electric field). Based on the reviewer's suggestion, we have supplemented the experiment about the sorption stability of microwaves.
We placed the LiCl@HGAF-H 2 O fibers in the testing chamber and set the test power to +10 dBm (10 mW) to keep the materials in the electromagnetic environment all the time. After that, this test was conducted every 2 h to obtain the changes in the microwave absorption properties of the materials.