Nature Inspired MXene-Decorated 3D Honeycomb-Fabric Architectures Toward Efficient Water Desalination and Salt Harvesting

Highlights The 3D honeycomb-like fabric decorated with MXene is woven as solar evaporator. The honeycomb structure enables light-trapping and recycling of convective and radiative heat. The 3D honeycomb-fabric evaporator possesses high solar efficiency up to 93.5% under 1 sun irradiation and excellent salt harvesting ability. Abstract Solar steam generation technology has emerged as a promising approach for seawater desalination, wastewater purification, etc. However, simultaneously achieving superior light absorption, thermal management, and salt harvesting in an evaporator remains challenging. Here, inspired by nature, a 3D honeycomb-like fabric decorated with hydrophilic Ti3C2Tx (MXene) is innovatively designed and successfully woven as solar evaporator. The honeycomb structure with periodically concave arrays creates the maximum level of light-trapping by multiple scattering and omnidirectional light absorption, synergistically cooperating with light absorbance of MXene. The minimum thermal loss is available by constructing the localized photothermal generation, contributed by a thermal-insulating barrier connected with 1D water path, and the concave structure of efficiently recycling convective and radiative heat loss. The evaporator demonstrates high solar efficiency of up to 93.5% and evaporation rate of 1.62 kg m−2 h−1 under one sun irradiation. Moreover, assisted by a 1D water path in the center, the salt solution transporting in the evaporator generates a radial concentration gradient from the center to the edge so that the salt is crystallized at the edge even in 21% brine, enabling the complete separation of water/solute and efficient salt harvesting. This research provides a large-scale manufacturing route of high-performance solar steam generator. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00748-7.


Note S1 Measurement of the effective porosity of the 3D honeycomb fabric
Under the dry state, the fabric was weighed with an analytical balance. After soaking in water for one hour to reach a fully wetted state, it was weighed again. The effective porosity of the fabric can be calculated according to the Eq. (S1).
Where m1 and m2 are the weight of the fabric before and after wetting (mg), respectively, and ρ1 and ρ2 are the densities of cellulose (1.5 g cm −2 ) and water (1 g cm −2 ), respectively. In order to minimize the experimental error, the fabric was measured three times and the average data was used.

Note S2 Thermal conductivity measurements of dry/wet 3D honeycomb fabric
The thermal conductivity of the dry/wet MXene/3D honeycomb fabric was measured by sandwiching the material between two 1 mm thick glass slides [S1]. The "sandwich" was placed between a heat source (JF-956, China) and a cold source (ice water bath). The temperature distribution along the cross section of the sandwich structure was monitored using an IR camera. The thermal conductivity is calculated based on Fourier's law (Eq. 2).
Since the thermal conductivity (K) of the glass slide (1.05 W m −1 K −1 ) is known, ∆T is the temperature difference, and ∆X is the distance difference, the heat flux q' per unit area was calculated accordingly. Assuming that the sample and the slide have the same heat flux, the thermal conductivity of the fabric was calculated.

Note S3 Calculation of the theoretical overall heat loss and efficiency
Theoretical overall heat loss and efficiency were calculated to explore the functionality of the MXene modified honeycomb fabric evaporator. Using the Stefan-Boltzmann equation, Newton's law of cooling and heat flux for a large amount of water, a detailed heat loss analysis was performed as shown in Eqs.
(3) and (4) [S4-S7]: Here, ε is the emissivity (i.e., 0.98), σ is the Stefan-Boltzmann constant (i.e.,5.67 × 10 −8 W m −2 K −4 ), h is the convective heat transfer coefficient of about 5 W m −2 K −1 , C is the specific heat capacity of water (i.e., 4.18 J C −1 g −1 ), and m (i.e.,30 g) is the weight of water. T2 (41 °C) is the mean surface temperature evaporator steady state temperature for about 1 hour and T1 is the environment temperature surrounding the sample. According to previous reports [S5, S6], the heat gathered on the solar absorber is exchanged with the surrounding solar steam in a small area rather than dissipated into the environment. In particular, since that the top surface of the absorber is surrounded by water and hot steam, the temperature of the environment adjacent to the top of the absorber is close to that of the absorber and steam (Fig. S19). In this work, a thermocouple was used to measure the vapor temperature top of the sample, and T1 was measured to be about 39.0 °C (about 7 mm above the center of the device). ∆T/t is the change in total water temperature during the test (i.e., 0.3 °C within t seconds (3600 s)), and A is the area of water transfer. According to the equation (S3 and S4), the power of heat loss is recalculated to be approximately 32.8 W m −2 . Most absorbed solar energy is still used to evaporate the water sheet on top of the absorber surface rather than being lost through these channels. The theoretical efficiency of solar energy utilization is 96.72%.

Note S4 Calculation of solar evaporation efficiency
The solar evaporation efficiency of samples under the solar simulator, which is defined as the Eq. (5) [S2]: Here, m is the evaporation rate, hLV is the total enthalpy including sensible heat and the liquid-vapor phase change, Copt denotes the optical concentration, and qi is the solar radiation. It should be noted that m should be the difference between the total evaporation rate and the natural volatilization rate without light irradiation. The natural volatilization rate of MXene/3D honeycomb fabric and MXene/2D plain fabric were measured to be 0.22 and 0.20 kg m −2 h −1 , respectively. Therefore, the light induced evaporation rates of MXene decorated honeycomb fabric under 1 sunlight was 1.62 minus 0.22 kg m −2 h −1 , while the light induced evaporation rates of MXene modified plain fabric was 1.39 minus 0.20 kg m −2 h −1 .
The variation of hLV at different temperatures and its relationship can be described by an approximate Eq. (S6) [S3]: where α = 2500.304, β = −2.2521025, γ = −0.021465847, δ = 3.1750136×10 −4 , ε = −2.8607959 ×10 −5 are constants and T is the temperature (°C). Thus, the hLV of MXene/2D plain fabric with the surface temperature of 45 °C is 2394.19 kJ kg −1 , and the hLV of MXene/3D honeycomb fabric with the surface temperature of 41 °C is 2403.78 kJ kg −1 . In the wet state, evaporation on the large surface area of the honeycomb fabric is more intense than on the liquid/vapour interface of a plain fabric, requiring more evaporative water energy and thus lowering the surface temperature. Thus, the evaporation efficiency of MXene/3D honeycomb fabric in one sunlight irradaition is 93.5% while MXene/2D plain fabric is 79.5%.

Note S5 Calculation method of 3D honeycomb fabric evaporator performance
It is worth mentioned that the area for the calculation of water evaporation rate is the light-receiving area, rather than the actual evaporation area. We assume that the concave structure of the single honeycomb is similar to a quadrangular pyramid (Fig. S18). The bottom area of the quadrangular pyramid corresponds to that exposed to sunlight for the fabric. The bottom rectangle is about 10 mm in length and 8 mm in width, and the light-receiving area is calculated by Eq. (S7): The height of the quadrangular pyramid is 8 mm. The area of the quadrangular pyramid is calculated by Eq. (S8): Hence, we can calculate that the area of actual evaporation is about 2.06 times that of receiving light. Figures  Fig. S1 a Schematic illustration of preparation process of MXene. b Aqueous solution of MXene nanosheets   The image of the water evaporation device. The size of the spot is controlled through an aperture to ensure that it equals to the size of the absorber. The evaporative mass is measured by a balance with high precision. The data is then transferred to a computer to evaluate the evaporation rate and the solar thermal conversion efficiency     Table   Table S1 The solar-thermal performance and structure properties of materials under one sun illumination