Self‐Cleaning Integrative Aerogel for Stable Solar‐Assisted Desalination

Abstract The solar‐assisted desalination generator (SADG) shows great potential for solving water scarcity problems. However, salt precipitation and accumulation is still a challenge for SADG, which slows down solar steam generation performance of evaporator during operation. Here, a facile integrative evaporator featuring stable and high evaporation performance breaks this bottleneck. By using a rational design in which amorphous carbon particles are evenly composited within the porous chitosan aerogel, the evaporator not only integrates excellent light absorption, heat management, and water transportation abilities but also endows a large vapor escape space. Upon desalination, salt concentration ingredients between carbon particles and chitosan membranes can be quickly balanced by water transport in interconnected chitosan chains, and thus salt precipiation on the evaporator surface would be avoided. Compared to other salt‐rejection evaporators, the integrative evaporator can operate in 3.5 wt% brine for 60 days without salt precipiation and exhibits a stable evaporation rate (1.70 kg m−2 h−1), indicating its potential for practical applications in seawater desalination and the harvest of clean drinking water.

alpha anode emitter as the excitation source. The absorptance spectrum of each sample was transformed from its diffuse reflectance spectral and transmittance spectral, which measured from 2500 to 250 nm via a UV-Vis-NIR spectrometer (V-570, JASCO, Japan) equipped with an integrating sphere. The infrared reflectance spectral of materials were measured by a FTIR spectrometer (Nicolet IS10, Thermo Fisher, USA) connecting an IntegratIR TM mid-infrared integrating sphere with Mercury-Cadmium-Telluride (MCT) detector (PIKE). The macropore parameter of PPCA y was measured by mercury porosimetry (AutoPore IV 9500, Micrometritics, USA). Thermal conductivity of PPCA y was measured by thermal conductivity instrument (TCi, C-therm, Canada). Zeta potentials of materials were investigated by laser particle analyzer (MS2000, Malvern, UK).

Solar steam generation experiment:
The whole process of the experiment was conducted at an ambient temperature of 25 ± 2 °C and a relative humidity of 48 ± 2%. The pretreatment PPCA y sample was loaded on homemade MTS to conduct experiment. A xenon lamp (CEL-S500/350, ZJJY, Beijing) with an AM1.5 optical filter was used as the light source. A piece of Fresnel lens with 20 cm focal length was applied to enhance incident light. During SSG experiment, light intensity of 1 kW m -2 was calibrated by an optical power densitometer (843-R, Newport, USA) with a thermopile sensor (919P-010-16, Newport, USA). The mass change of sample was recording by a high-precision electric balance (ATX224, Shimadzu, Japan) for 60 min at constant condition. The surface temperature distribution of PPCA y was captured by an infrared camera (E60, FLIR, USA). The temperature of vapor was measured by a thermal sensor probe (BD-PT100-3022A).
In long-term solar-assisted desalination experiments, brine of different concentrations (3.5, 7, 10, 13, 17 and 20 2 Pb were selected as model heavy metal ion (Mn 2+ , Fe 3+ , Ni 2+ , Cu 2+ , Pb 2+ ) contamination to evaluate solar-assisted water treatment performance. The concentration of the heavy metal in the production fresh water was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 8000, PerkinElmer, Waltham, MA, USA). Moreover, NaCl, MgCl 2 , KCl, and CaCl 2 were selected for simulated desalination experiment, the production water was measured by ICP-AES for evaluating water quality.
The outdoor experiment was performed through a prototype made of polymethyl methacrylate (PMMA) box with double inclined planes, and polyvinyl chloride tube as the water chute for collecting the condensed water. An expanded polystyrene foam with 2×2 array of PPCA 5 with an effective area of 28 cm 2 . The simulated seawater containing 3.5 wt% of NaCl was placed into the bottom of the prototype. The outdoor experiment was carried out under natural sunlight with a solar flux of ~0.56 kw m -2 .

Absorptivity calculation:
According to Kirchhoff's law, the absorptivity was calculated by Eq. (1): Where R and T are the reflectivity and transmissivity, respectively. The absorptivity values of different samples were calculated based on Eq. (2), resulting from the solar energy dependent on the wavelength in the solar spectrum. [1] Where, P sun (λ) is the normal solar irradiance defined by the ISO standard 9845-1 (1992) for air mass (AM)1.5. The obtained absorptivity values of samples are shown in Tale S3.
Evaporation rate calculation: The evaporation rate is a key factor to evaluate the photothermal conversion performance of SSG. The evaporation rate ν is given by Where ṁ is the mass flux of loss water under SSG, which can be represented as the slope of the mass loss curve, obtained by linear fitting in the steady state. A proj is the projection area of sample.
Moreover, the mass flux of loss water generated by the sample consists of natural evaporation(ṁ nature ) and light-driven evaporation(ṁ light ). Which is given by Photothermal conversion efficiency calculation: Based on the unique of system, the photothermal conversion efficiency η 1 can be represented as follow: Specifically Where c opt is the optical concentration, I is the nominal direct solar illumination (1 kW m −2 ).
C p × ∆T is sensible heat. C p is the specific heat capacity of water (4.18 J g −1 K −1 ). ΔT represents the difference between the vapor temperature and the ambient temperature. ∆H vap is latent heat, which depends on Eq. (6). Where T vap represents vapor temperature. A, B, C, D, E, τ, T c are standard constant. [2][3] The photothermal conversion efficiency of each sample is shown in Table S4.
Emissivity calculation: The thermal emissivity refers to the proportion of blackbody radiation absorbed by the sample to the total black body radiation, which is dependent on the nature and surface state of the materials. The emissivity ε of the material can be obtained by measuring its infrared reflectance spectrum and calculating, the formula is as follows: [1] Where P B (λ) is the spectral radiance of blackbody at a temperature T. According to Plank's law: Where Specifically, the SSG experiment was conducted in lab environment. The temperature was adjusted at 25 ± 2 °C, and the illumination intensity was controlled at 1 kW m -2 by light source.
The surface reflection happens on the surface of aerogel, P ref is given by Where P light = l kW m -2 .
The heat radiation (P ra, vap ) between system and vapor is calculated by the Stefan-Boltzmann Where σ=5.67×10 -8 W m -2 k -4 denotes the Stefan-Boltzmann constant. T ab is the temperature of absorber. [1] The heat convection (P cv, vap ) between system and vapor is defined by Newton's law of Where h = 5 W m -2 k -1 denotes the convective heat transfer coefficient of the planar system. [1] The heat conduction mainly consists of heat transferring from system to water and EPE foam. The heat conduction from system to water (P cd, water ) is given by where m 1 is the mass of the remaining bulk water, and ΔT 1 is the temperature change of bulk water. [1] The heat conduction from system to EPE (P cd, EPE ) is given by Where κ is the thermal conductivity of EPE foam. A transfer is the heat transfer area of the absorber towards EPE foam, and is the temperature gradient of the absorber towards EPE foam. [1] The thermal utilization η 2 of each sample with MTS device is given by The thermal utilization and every step of heat loss of sample are listed in Table S6. And the SSG energy flow and resistance network diagram of PPCA 5 was shown in Figure S10. Figure S1.           Tables   Table S1. The Zeta potential of each component.