Figure 3. (a) UV-Vis-NIR spectra of MF@PPy10-OA, MF@HPB-PPy10, MF@HPB-PPy10-OA. Inset image is the air mass 1.5 global (AM 1.5 G) solar spectrum; (b) The surface temperature rise of MF (reference), MF@PPy10-OA, MF@HPB-PPy10, MF@HPB-PPy10-OA relative to irradiation time under 1 sun. Inset: infrared image displaying the temperature distribution after 180 s of irradiation time; (c) Plot displaying the surface temperature of bulk water (reference) and MF@PPy10-OA, MF@HPB-PPy10, MF@HPB-PPy10-OA under 1 sun relative to irradiation time. Inset: infrared image displaying the temperature distribution after 600 s of irradiation time; (d) Mass change of MF@PPy10-OA, MF@HPB-PPy10, MF@HPB-PPy10-OA under 1 sun illumination, with pure water as the control. And the corresponding (e) solar water evaporation rate and energy efficiency; (f) Mass change over time with MF@HPB-PPy10-OA under 0 to 2 sun (0 to 2 kW m-2) radiation.
Figure 3d and Figure S8d, taking the bulk water as control group, the overall mass change with MF@HPB-PPyn-OA is recorded under 1 solar irradiation. As in some previous studies, a light cut-off device with an aperture was placed between the lamp and the evaporator to ensure the same size of light irradiation and evaporator area.[51] The general agreement of the bulk water evaporation rate value (0.43 kg m-2h-1) under this condition with the reported literature values supports the reliability of this test system.[9,22,52-57] Evaporation rates of 1.9 kg m-2 h-1 and 2.9 kg m-2 h-1 were obtained for the evaporation systems driven by the hydrophobic MF@PPy10-OA and hydrophilic MF@HPB-PPy10, respectively (Figure 3d). Compared with MF@PPy10-OA, the significant increase in evaporation rate of MF@HPB-PPy10 proves the positive contribution of hydrophilic HPB to the overall evaporation performance. When the interfacial hydrophilic-hydrophobicity of the materials is further modulated, MF@HPB-PPy10-OA presents optimal performance with 3.4 kg m-2 h-1 evaporation rate (Figure 3d and Figure S8e), which ranks among the highest levels reported in recent years (Figure S8f).[9,17,22,30,52-57] This is attributed to the ability of the HPB-PPy10-OA self-assembled coating to wrap the MF in a more stable and uniform manner, thus presenting excellent light absorption and photothermal response. On the other hand, the small and uniformly dispersed HPB clusters of the self-assembled HPB-PPy10-OA allow more hydrophilic evaporation sites to be exposed compared to HPB-PPy10 (Figure 2b and Figure S2a). The excellent evaporative property of MF@HPB-PPy10-OA exceeding the theoretical limit value (1.47 kg m-2 h-1) should be contributed to the active intermediate state of water molecules in the hydrophilic region. The active intermediate water which is a state intermediate between bound and free water is generally referred to as activated water. [22,23,30] With its subtle interactions between hydrophilic sites and adjacent water molecules, the vaporization process of activated water consumes less energy than that of bulk water. It is well known that POM has good affinity for water molecules by virtue of its oxygen-rich surface structure and high negative charge.[47,48] As shown by thermogravimetric analysis (TG) (Figure S10), the water molecules in the crystal structure of PMA evolve gradually in three stages. It is noteworthy that in the first weight loss step, the free crystalline water evolves with increasing temperature and endothermic peak 1 emerges at a low temperature of 71.9 oC in the corresponding DSC curve (while the endothermic peak of bulk water is around 100oC). The computational analysis indicates that the evolution of crystalline water in pure POM (850.0 J g-1) requires only low energy compared to bulk water (2327.5 J g-1, see Figure S11 in Supporting Information for details), confirming the active water molecule intermediate state in the POM structure. In the DSC analysis of Figure S11, water in MF@HPB-PPy10-OA similarly proceeds with low energy consumption, indicating the activation process of water on the hydrophilic region of HPB. The equivalent dark evaporation measurements (Figure S13) results basically coincide with the DSC test results, which supports this low-energy requirement evaporation process. The energy efficiency (η) of the SVG in this system can be calculated using the following equation:
Ƞ = ṁh/CoptP0
where, ṁ is the water net evaporation rate (ṁlight- ṁdark kg m-2 h-1), h is the evaporation enthalpy (J g-1) of the water in MF@HPB-PPy10-OA, Copt is the optical concentration on the evaporator surface, P0 is the solar radiation power (1 kW m-2). Thus, the energy conversion efficiency of MF@HPB-PPy10-OA reaches 94.9% under 1 solar illumination (Figure 3e), obviously higher than that of MF@PPy10-OA (76.0%) and MF@HPB-PPy10(88.2%). Meanwhile, the evaporation system is found to have only a low heat loss in the energy loss assessment (see Section 4.4 in Supporting Information for details). In addition, 3D MF@HPB-PPy10-OA yields a satisfactory evaporation rate of 5.6 kg m-2 h-1 with 2 solar irradiations by virtue of its unique structure (Figure 3f).
Salt -water separation in highly concentrated brine(10 wt%)
It is worthwhile to mention that MF@HPB-PPy10-OA is equally efficient in high salinity brine (10 wt%), and even accomplishes complete salt-water separation. As shown in Figure 4a and Figure 4b, an inexpensive filter paper diffusion layer (with a dimensional radius 0.2 cm larger than that of MF@HPB-PPy10-OA) is introduced on the basis of the conventional photothermal water evaporation system (including a light absorber layer, insulation layer and delivery line) to reinforce the transport of brine in MF@HPB-PPy10-OA. In the complete salt-water separation, the average water evaporation rate of MF@PPy10-OA is maintained at 1.8 kg m-2 h-1 in the optimized evaporation system, which is only slightly lower than that of bulk water (Figure 4c). Owing to the strong hydrophobicity of the MF@PPy10-OA surface, salt starts to crystallize in the diffusion layer instead of at the surface during up to 32 h of continuous operation (Figure S14a). Eventually, a salt harvesting efficiency of about 88.7% is obtained around the diffusion layer. Unfortunately, after two cycles of salt-water separation, the MF@PPy10-OA becomes significantly lighter in color and has a significantly lower photothermal water evaporation efficiency, which is assigned to the photobleaching effect of PPy. When MF@HPB-PPy10 drives solar salt-water separation, the average rate is improved. But the salt tolerance is visibly reduced compared to that of MF@PPy10-OA (Figure 4c). Due to the continuous evaporation of water, the brine concentration in the system gradually increases and approaches saturation, inevitably leading to the gradual precipitation of salt on