Multifunctional Solar Evaporator with Adjustable Island Structure Improves Performance and Salt Discharge Capacity of Desalination

Abstract Interfacial solar steam generation (ISSG) is the main method to get fresh water from seawater or wastewater. The balance between evaporation rate and salt resistance is still a major challenge for ISSG. Herein, a wood aerogel island solar evaporator (WAISE) with tunable surface structure and wettability by synthesizing poly(n‐isopropylacrylamide)‐modified multi‐walled carbon nanotube photothermal layers. Compared to dense surface structure evaporators, interfacial moisture transport, thermal localization, and surface water vapor diffusion of WAISE are greatly promoted, and the evaporation rate of WAISE increased by 87.64%. WAISE allows for record performance of 200 h continuous operation in 20% NaCl solution without salt accumulation. In addition, the photo‐thermal‐electric device is developed based on WAISE with continuous water purification, power generation, and irrigation functions. This work provides a new direction for the development of multifunctional water purification systems.


Preparation of MWCNT-g-PNIPAM
MWCNT was pretreated with aqueous HCl (0.1 M) and sonicated for 3 h at room temperature to remove surface impurities and produce hydroxyl groups on the MWCNT surface.The dried product was immersed in a beaker containing a solution of trichlorovinylsilane (TVS, 1 g) and toluene (49 g) for 12 h at room temperature.
The samples were removed from the solution, washed three times with stirring in toluene (25 mL), and then dried in a vacuum oven for 2 h to obtain TVS modified MWCNT (TVS-MWCNT).TVS-MWCNT was immersed in a mixture consisting of NIPAM (0.375 g), AIBN (0.0082 g) and DMF (45 mL), and the mixture solution was deoxygenated with argon gas and heated in an oil bath at 70°C for 12h.Finally, the powder was washed by centrifugation with a large amount of toluene and subsequently dried under vacuum at 70 °C for 30 min to obtain MWCNT-g-PNIPAM powder.The mechanism of MWCNT-g-PNIPAM synthesis is shown in Supplementary Figure 27.

Preparation of WA
NBW was placed in an aqueous solution of NaClO2 (2 wt%) at 80°C for 24 h to delignify lignin (pH adjusted to 4.7 by acetic acid).The materials were washed in deionized water (80 °C) and dried in a desiccator (80 °C).The dried material was placed in NaOH (8 wt%) aqueous solution for 9 h to remove hemicellulose, and then the material was placed in deionized water and washed thoroughly (Supplementary Figure 28).The cleaned material was frozen in a -40 °C refrigerator for 10 h and then transferred to a freeze dryer for 48 h (at -60 °C) to obtain wood aerogels.

Preparation of ZIF-67 and WAISE
ZIF-67 was prepared by pouring Co(NO3)2•6H2O (4.36 g, 15 mmol) and 2-methylimidazole (4.92 g, 60 mmol) into a solution containing 150 mL of methanol and stirring for 6 h.The ZIF-67 powder was obtained by washing with methanol five times and drying.The mechanism of ZIF-67 synthesis is shown in Supplementary Figure 29a.
Wood aerogel/ZIF-67 was prepared by improving the methods already reported.
Specifically, 15 mmol of Co(NO3)2•6H2O and 60 mmol of 2-Methylimidazole were poured into solution A and solution B containing 150 mL of methanol, respectively.
WA was immersed in solution A and 167 mg of polyvinylpyrrolidone (PVP) was added to enhance the binding of Co 2+ to WA.In order to promote more Co 2+ into the interior of WA, the impregnation was done by vacuum pumping for 30 min, and then put into gas to stand for 30 min at atmospheric pressure, and the process was cycled 2 times.The solution B was slowly poured into A after standing under ambient pressure for 5 h.The above method was used to coordinate the 2-MI organic ligand with Co 2+ .
The mixture was stirred on a thermostatic shaker at 120 rpm for 10 h and then the WA samples were removed and washed with ethanol/water (10/90,v/v) solution.Finally, the ZIF-67@WA composites were obtained by freeze-drying for 24 h.0.2 g of MWCNT-g-PNIPAM powder was evenly dispersed in 2 mL of deionized water solution and sonicated for 10 min to obtain MWCNT-g-PNIPAM aqueous dispersion.The dispersion was evenly coated on the surface of WA/ZIF-67 to obtain WA/ZIF-67@MWCNT-g-PNIPAM intelligent wood-based evaporator (Supplementary Figure 29b).The evaporator samples were frozen in a refrigerator at -40℃ for 4h, and then dried in a freeze dryer for 6h.The robust evaporator was moistened to become flexible and WAISE was obtained by surface grooving (Supplementary Figure 30 and Supplementary Figure 31).

Fabrication of EGIS
An aqueous dispersion of PTMs (MWCNT-g-PNIPAM) at 0.1 g/ML were dispensed and coated on the surface of a commercial thermoelectric device (TED), and the photothermal material-thermoelectric device (PTM-TED) was obtained by baking under a simulated light source.The PTM-TED was fixed to the foam and a absorbent paper was attached to the water underneath it (the absorbent paper is used to absorb water as a cold side).The WAISE was fixed on the other side of the foam on the basis of the above to obtain evaporation-generation units.The water collection device was designed and prepared, and the evaporation-generation units was placed in it to obtain EGIS.

Preparation of cellulose aerogel
Cellulose aerogels were prepared with reference to the literature [S1].

Solar steam generation measurement
The prepared evaporation material (20×12×10 mm 3 ) was fixed in the foam for evaporation experiments.The experiments were performed under constant temperature and humidity laboratory conditions (24 °C, RH=60%).The sunlight was simulated using a xenon lamp light source (Beijing CEL-HXF300-T3, AM 1.5G).The solar flux was measured with a light power meter (CEL-NP2000-2A).The change in mass of water was measured using a balance with an accuracy of 0.1 mg.

Characterize
The scanning electron microscopy (SEM) images and Energy-dispersive X-ray spectroscopy (EDS) mapping images of the wood aerogels before and after modification were observed using field-emission scanning electron microscopy (Hitachi, Regulus 8100, Japan).Transmission Electron Microscope (TEM) images were measured by a High-resolution TEM (JEM-2100 UHR, Japan).Atomic Force Microscopy (AFM) images were captured by AFM (Dimesion Edge, Germany).The chemical composition of materials investigates by X-ray photoelectron spectroscopy (ESCALAB 250Xi, America).The Contact Angle (CA) Tester (DSA30, US) was used to analyze material wettability.The functional groups were texted by infrared spectrometer (FTIR) (VERTEX 80V, Germany).The main ion concentration in water measured by Inductively Coupled Plasma-Optical Emission Spectrometer (ICP-OES).
Voltage and current data were tested with an electrochemical workstation (CHI760E, China) and a multimeter.
C is the specific heat capacity of water (4.2 kJ -1 K -1 ), and ΔT is the value of the temperature difference at the absorbing surface in one hour.

Supplementary Notes 3: Simulation calculation steps of MDS
In this paper, all molecular dynamics simulations were performed using Gromacs software for kinetic calculations, visualized using VMD, and UFF force fields were selected for kinetic calculations to explain the intermolecular interactions.In addition, the water molecule density number were determined using the Forcite module in the Materials Studio 2018 program of BIOAccelrys.
(a) System of WA/water and WA/ZIF-67/water: The modeling part uses Packmol software, and a 27 Å × 27 Å periodic simulation pool was established to simulate the evaporation process.Firstly, the wood molecules (A cellulose chain consisting of five monomers was used) are spread and fixed on the bottom surface of the simulated pool, and then a free water layer (500 water molecules, with a density of 1 g/cm 3 ) is added to the wood surface.To prevent molecular overlap, a vacuum layer of 2 Å is added between water and wood.This model is used as the initial model for wood/water, with ZIF67 added above the free water layer as the initial model for wood/ZIF/water.The model image is shown in the support information.
After minimizing the energy of the model, molecular dynamics simulations were run.Using the temperature control method of Nose, the temperature is controlled at 316 K, and the cutoff radius is selected as 12.5 Å.Under the NVT ensemble, perform a dynamics simulation with a step length of 1 fs, and totaling time is 500 ps, to ensure that the dynamic results are balanced.Use code to calculate the relationship between the amount of hydrogen bonds and water molecules evaporated in the system and time.
(b) System of water and MWCNT-g-PNIPAM/water: The modeling part adopts packmol, considering the periodicity of carbon nanotubes, and establishes a cross-section of 25 Å × 23.7 Å simulation pool is used to simulate the evaporation process.Fix the CNT with PNIPAM branches (polymerization degree is 3), and then pack the free water layer (500 water molecules with a density of 1g/cm 3 ) near the CNT.This model serves as the initial model for CNT/water.
Additionally, establish a cross-section of 25 Å × 23.7 Å simulation pool of 23.7 Å is placed with a free water layer as a control.All simulation pools have 200 Å heights to simulate the evaporation environment.