Polyoxometalates‐Modulated Hydrophilic‐Hydrophobic Composite Interfacial Material for Efficient Solar Water Evaporation and Salt Harvesting in High‐Salinity Brine

Solar vapor generation (SVG) represents a promising technique for seawater desalination to alleviate the global water crisis and energy shortage. One of its main bottleneck problems is that the evaporation efficiency and stability are limited by salt crystallization under high‐salinity brines. Herein, we demonstrate that the 3D porous melamine‐foam (MF) wrapped by a type of self‐assembling composite materials based on reduced polyoxometalates (i.e. heteropoly blue, HPB), oleic acid (OA), and polypyrrole (PPy) (labeled with MF@HPB‐PPyn‐OA) can serve as efficient and stable SVG material at high salinity. Structural characterizations of MF@HPB‐PPyn‐OA indicate that both hydrophilic region of HPBs and hydrophobic region of OA co‐exist on the surface of composite materials, optimizing the hydrophilic and hydrophobic interfaces of the SVG materials, and fully exerting its functionality for ultrahigh water‐evaporation and anti‐salt fouling. The optimal MF@HPB‐PPy10‐OA operates continuously and stably for over 100 h in 10 wt% brine. Furthermore, MF@HPB‐PPy10‐OA accomplishes complete salt‐water separation of 10 wt% brine with 3.3 kg m−2 h−1 under 1‐sun irradiation, yielding salt harvesting efficiency of 96.5%, which belongs to the record high of high‐salinity systems reported so far and is close to achieving zero liquid discharge. Moreover, the low cost of MF@HPB‐PPy10‐OA (2.56 $ m−2) suggests its potential application in the practical SVG technique.


Introduction
[18][19][20][21] Since, with the rapid evaporation of water, the concentration of brine in the system gradually reaches saturation and the diffusion rate of salt in water (10 À9 m 2 s À1 ) is far lower than that of water vapor in air (10 À5 m 2 s À1 ), thus leading to the inevitable crystallization, accumulation, and even blockage of salt on the surface of the photothermal material. [22,23]This problem severely hinders the light absorption, water transport, and water vapor escape in the continuous water evaporation process, significantly degrading the performance of SVG and terminating the operation of the device.Thus, the development of photothermal water evaporation materials with both efficient water evaporation performance and self-cleaning/anti-salt fouling properties is a key issue to drive SVG from the laboratory to large-scale practical applications.
To achieve the above aim, the SVG materials should possess three key properties.One is displaying the excellent solar light capture and photothermal conversion capability.The other is possessing an efficient water evaporation pathway.The third one is having suitable hydrophilic-hydrophobic dual interfaces that can improve the anti-salt fouling ability.Recently, a series of reports by Yu et al. have demonstrated that hydrogen bonding or electrostatic interactions between hydrophilic functional groups and water molecules facilitate the evaporation of bulk water as activated water, [10,[24][25][26][27][28][29][30][31] providing a fast and efficient water evaporation pathway (exceeding the theoretical evaporation rate value of 1.47 kg m À2 h À1 ). [32]Such evaporation process requires less energy than the conventional single-molecule water evaporation that requires breaking the hydrogen bonds between all water molecules.][35][36][37][38][39][40] For example, the hydrophobic surface structure designed by Zhu et al. based on natural water lily can effectively inhibit the crystallization of salt on its surface and achieve the complete separation of salt and water.However, the strongly hydrophobic surface weakened the effective contact of water with the photothermal active site and the water activation capacity, which just yielded an evaporation rate of 1.39 kg m À2 h À1 .Therefore, the combination of three properties into one material is obviously a challenging task for exploring efficient, anti-salt fouling, and durable SVG evaporators.
[50][51][52][53][54] Moreover, the oxygen-rich surface structure of HPBs strongly supports their affinity for water, [55,56] exhibiting the potential in the construction of hydrophilic-photothermal evaporation sites.Importantly, the HPBs that hold high-negative charge can be electrostatically self-assembled with cationic polymers to avoid water leaching problems and achieve efficient and stable loading.Further regulation of the surface microenvironment of the functional selfassembled materials is expected to achieve system management and optimization of the surface local hydrophilic-hydrophobic region, offering an effective strategy for constructing hydrophilichydrophobic interfacial SVG evaporators.
Given the above considerations, a new type of photothermal water evaporation materials (abbr.MF@HPB-PPy n -OA) was designed and constructed based on the melamine foam (MF) skeleton, which is covered by a hydrophilic-hydrophobic composite interfacial material via a onestep self-assembly of HPB, oleic acid (OA), and polypyrrole (PPy).On the surface of MF@HPB-PPy n -OA, the hydrophilic regions of HPB and the hydrophobic regions of OA show spatial separation features due to the electrostatic mutual repulsion of HPB and OA molecules.Benefiting from the advantages of the self-assembly strategy and the unique surface structure, the local hydrophilic-hydrophobic interface of this SVG material can be regulated and optimized, thus achieving ultra-high water evaporation and salt collection.In a series of MF@HPB-PPy n -OA, MF@HPB-PPy 10 -OA exhibits the best SVG performance, yielding 3.3 kg m À2 h À1 of evaporation rate and 96.5% of salt harvesting efficiency in the salt-water separation process of high-salinity (10 wt%) brine at 1 solar irradiation, which belongs to the record high of high-salinity systems reported so far and is close to achieving zero liquid discharge.Moreover, MF@HPB-PPy 10 -OA can operate continuously and stably for over 100 h at an ultrahigh evaporation rate of 3.3 kg m À2 h À1 (corresponding to an energy efficiency of 92.1%) in continuous solar desalination of high-salinity brine (10 wt%), and enables real-time antifouling and maintains surface cleanliness.The low-cost, excellent mechanical properties, and processability of MF@HPB-PPy 10 -OA in terms of manufacturing exhibit promising prospects for commercial applications.hydrophilic-hydrophobic composite interfacial materials (denoted as MF@HPB-PPy n -OA, n refers to the amount of Py added, n = 5 μL, 10 μ L, 20 μL).Influenced by the electrostatic repulsion of HPB and OA, the hydrophilic-hydrophobic units formed by them present distinct spatial separation characteristics on the MF@HPB-PPy n -OA surface.As shown in Figure 1b, on the surface region without HPB, the PPy surface is modified by OA, forming a hydrophobic region (left).In contrast, the region containing HPBs exhibits affinity to water owing to the absence of OA modification (right).This spatially separated feature implies that the hydrophilic regions of the HPB might form nano water channels for water transport.On one hand, the exposed hydrophilic regions of HPBs enhance the coupling activation, transport, and convection of bulk water under the action of hydrogen bonding or electrostatic forces, thus enhancing the evaporation efficiency of water.On the other hand, the hydrophobic regions endow the 3D porous MF@HPB-PPy n -OA surface with a powerful anti-salt fouling property.Moreover, salt is also prevented from crystallizing in the hydrophilic regions due to the advantages of the nanoscale dimension of hydrophilic regions and the strong salt-water convection effect between the water channels (It is something like "pits" formed by HPBs with a high salt concentration) and the original microporous channels of MF (with a low salt concentration).The surfacial hydrophilic-hydrophobic dual feature of the porous MF@HPB-PPy n -OA is further modulated by adjusting the loading different amounts of HPBs.Among them, MF@HPB-PPy 10 -OA stands out in terms of its ability in photothermal water evaporation and anti-salt fouling properties; thus, it is selected as an example for elucidation in detail.

Results and Discussion
Scanning electron microscope (SEM) and transmission electron microscope (TEM) show the morphology of MF@HPB-PPy 10 -OA.As shown in Figure 2a and Figure S1, Supporting Information, the HPB, PPy, and OA are clearly self-assembled on the smooth skeleton surface of MF (Figure S1a,b, Supporting Information), forming a HPB-PPy 10 -OA self-assembled coating.Meanwhile, the resulting MF@HPB-PPy 10 -OA preserves the original 3D porous structure of MF.TEM further reveals that HPB-PPy 10 -OA is stacked by irregular lamellar structures (Figure S3b,c, Supporting Information).In the HR-TEM image (Figure 2b), we observe that the HPB clusters are uniformly dispersed in the amorphous PPy lamellar structures.The uniformity distribution of C, N, O, Mo and P elemental components and the completeness of HPB-PPy 10 -OA coating coverage are evidenced by TEM-EDX and SEM-EDX mapping analysis (Figure 2c), respectively.Notably, the crosssectional SEM images (Figure S1e,h,k, Supporting Information) display that the self-assembled layers of MF@HPB-PPy 5 -OA, MF@HPB-PPy 10 -OA and MF@HPB-PPy 20 -OA became progressively thicker with increasing the proportion of Py substrate in the reaction system.At the same time, the loading of HPB is gradually diminished (Figure S4, Supporting Information), implying that the hydrophilic-hydrophobic properties of the surface are controllable.The corresponding water contact angle measurements verify this conclusion (Figure S1f,i,l, Supporting Information).A control sample MF@PPy 10 -OA without HPB is firstly prepared (see Synthesis 2.1 section for details), and the water contact angle is about 132.0 AE 3°, exhibiting a strong hydrophobicity as demonstrated in Figure 2d.When the hydrophilic HPB is introduced, the water contact angle on the surface of the material gradually decreases with the increase of HPB loading, and the corresponding hydrophilicity is obviously enhanced (Figure S4, Supporting Information).Among them, the optimal MF@HPB-PPy 10 -OA possesses a water contact angle of about 106.0 AE 2°(Figure 2e), corresponding to a HPB loading of about 31.6 wt% (calculated as a percentage of Mo weight).
In such materials, OA has a crucial influence on the hydrophobic management of their surfaces.The OA-free MF@HPB-PPy 10 displays a strong hydrophilicity with a contact angle of 78.0 AE 2°(Figure 2f).These results demonstrate the significant contribution of HPB and OA in balancing and optimizing the hydrophilic-hydrophobic interfaces of the composite material.In addition, OA also serves the function of maintaining the mechanical properties of the material.The compressive stress-strain measurements (Figure 2g) demonstrate the excellent mechanical properties of MF@HPB-PPy 10 -OA.After 50 cycles of compressive stress-strain measurements (Figure 2g, top, inset), it still maintains excellent and reversible elastic deformation.Moreover, MF@HPB-PPy 10 -OA also exhibits good bendability, twistability, and tailoring properties.In contrast, the elastic deformation of the OA-free MF@HPB-PPy 10 fails to recover after the compressive stress-strain measurements (Figure 2g, bottom, inset).During the compressive stressstrain measurements, the HPB-PPy 10 coating on MF@HPB-PPy 10 is prone to peeling.To further confirm the composition of material, its Fourier transform infrared spectra (FTIR), X-ray powder diffraction (XRD), and XPS are investigated.As shown in the FTIR (Figure S5b, Supporting Information), the characteristic absorption peaks of HPB (including ν P-O : 1053.7 cm À1 , ν Mo-O : 954.3 cm À1 , ν Mo-O-Mo : 869.5 and 770.8 cm À1 ) and PPy (including ν C=C : 1533.2 cm À1 and ν C-N : 1147.1 cm À1 ) and OA (ν C=O : 1703.2 cm À1 , ν CH3 : 2922.4 cm À1 and ν CH2 : 2850.7 cm À1 ) all are exhibited on MF@HPB-PPy 10 -OA, evidencing that the successful assembly of HPB, PPy, and OA on MF.A survey scan of MF@HPB-PPy 10 -OA confirms the presence of these elements (Figure S6, Supporting Information), which is consistent with the results of EDX mapping (Figure 2c and Figure S3c, Supporting Information).HR-XPS of the corresponding Mo-3d indicates that the Mo3d 3/2 and Mo3d 5/2 at 235.3 eV and 232.2 eV are split into two sets of peaks 235.4/232.4eV and 234.4/231.3eV, respectively, which are referred to as Mo 6+ and Mo 5+ (Figure 2h). [57,58]This reduced state of Mo confirms the presence of HPB that is favorable for solar absorption and photothermal conversion.

Solar Vapor Generation under 1 Sun
Ultraviolet-visible near-infrared (UV-Vis-NIR) measurements (Figure 3a) display excellent solar absorption properties of MF@HPB-PPy n -OA.The MF@PPy 10 -OA has good light absorption in the spectral range of 200-2500 nm because of the PPy coverage on MF.The absorption of MF@HPB-PPy 10 is visibly strengthened in the Vis-NIR region when the HPB component with excellent light absorption properties in Vis-NIR is introduced.Based on the advantages of the 3D porous structure of MF@HPB-PPy 10 -OA and the stability and homogeneity of the HPB-PPy 10 -OA coating, MF@HPB-PPy 10 -OA demonstrates nearly complete light absorption (95.3%, weighted under AM 1.5G), presenting excellent photothermal conversion potential.In Figure 3b, the surface temperature of MF@HPB-PPy 10 -OA in the dry state rapidly increases to 89.5 °C and remains stable within 3 min under 1 sun.The photothermal conversion capacity of MF@HPB-PPy 10 -OA is obviously higher than that of MF, MF@PPy 10 -OA and MF@HPB-PPy 10 , which is attributed to the superiority of HPB composition (Figure S7, Supporting Information) and the unique surface structure.When MF@HPB-PPy 10 -OA is in the wetted state, the temperature steady-state value of surface is slightly lower than that of MF@PPy 10 -OA and MF@HPB-PPy 10 (Figure 3c).Specifically, the absorbed sunlight is applied to the photothermal water evaporation; thus, the lower temperature steadystate value implies stronger evaporation efficiency.The photothermal water evaporation experiments further confirm the above results.Before executing the water evaporation, MF@HPB-PPy n -OA improves the evaporation area and the internal temperature of structure by tailoring the edges into an orthogonal prismatic table structure (Figures S8  and S9c, Supporting Information).As displayed in Figure 3d and Figure S9d, Supporting Information, taking the bulk water as control group, the overall mass change with MF@HPB-PPy n -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. [59]][62][63][64][65] Evaporation rates of 1.9 and 2.9 kg m À2 h À1 were obtained for the evaporation systems driven by the hydrophobic MF@PPy 10 -OA and hydrophilic MF@HPB-PPy 10 , respectively (Figure 3d).Compared with MF@PPy 10 -OA, the significant increase in evaporation rate of MF@HPB-PPy 10 proves the positive contribution of hydrophilic HPB to the overall evaporation performance.][62][63][64][65] This is attributed to the ability of the HPB-PPy 10 -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-PPy 10 -OA allow more hydrophilic evaporation sites to be exposed compared to HPB-PPy 10 (Figure 2b and Figure S3a, Supporting Information).The excellent evaporative property of MF@HPB-PPy 10 -OA exceeding the theoretical limit value (1.47 kg m À2 h À1 ) should be contributed to the active intermediate state of water molecules in the hydrophilic region.The active intermediate water that is a state intermediate between bound and free water is generally referred to as activated water. [10,31,38]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 highnegative charge. [55,56]As shown by thermogravimetric analysis (TG) (Figure S11, Supporting Information), 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 °C in the corresponding DSC curve (while the endothermic peak of bulk water is around 100 °C).The computational analysis indicates that the evolution of crystalline water in pure POM (850.0J g À1 ) requires only low energy compared to bulk water (2327.5J g À1 , see Figure S12, Supporting Information, for details), confirming the active water molecule intermediate state in the POM structure.In the DSC analysis of Figure S12, Supporting Information, water in MF@HPB-PPy 10 -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 S14, Supporting Information) 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: where, ṁ is the water net evaporation rate (ṁ light -ṁ dark kg m À2 h À1 ), h is the evaporation enthalpy (J g À1 ) of the water in MF@HPB-PPy 10 -OA, C opt is the optical concentration on the evaporator surface, P 0 is the solar radiation power (1 kW m À2 ).Thus, the energy conversion efficiency of MF@HPB-PPy 10 -OA reaches 94.9% under 1 solar illumination (Figure 3e), obviously higher than that of MF@PPy 10 -OA (76.0%) and MF@HPB-PPy 10 (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-PPy 10 -OA yields a satisfactory evaporation rate of 5.6 kg m À2 h À1 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-PPy 10 -OA is equally efficient in high-salinity brine (10 wt%), and even accomplishes complete salt-water separation.As shown in Figure 4a,b, an inexpensive filter paper diffusion layer (with a dimensional radius cm larger than that of MF@HPB-PPy 10 -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-PPy 10 -OA.In the complete salt-water separation, the average water evaporation rate of MF@PPy 10 -OA is maintained at 1.8 kg m À2 h À1 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@PPy 10 -OA surface, salt starts to crystallize in the diffusion layer instead of at the surface during up to 32 h of continuous operation (Figure S15a, Supporting Information).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@PPy 10 -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-PPy 10 drives solar salt-water separation, the average rate is improved.But the salt tolerance is visibly reduced compared to that of MF@PPy 10 -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 the hydrophilic surface.It is satisfactory that MF@HPB-PPy 10 -OA exhibits excellent photothermal water evaporation and salt-water separation capabilities.Under the identical conditions, its evaporation rate and salt harvesting efficiency are up to 3.3 kg m À2 h À1 and 96.5% (Figure 4c and Figure S16, Supporting Information), respectively, which belongs to the record high of high-salinity or local salt crystallization systems reported so far and is close to achieving zero liquid discharge.Simultaneously, the MF@HPB-PPy 10 -OA is quite stable and maintains the high levels of evaporation rate and salt harvesting efficiency even after 5 consecutive cycles of salt-water separation demonstrated in Figure 4e, Figures S17 and S18, Supporting Information.Continuous operation for up to 100 h in 10 wt% brine that reveals stable evaporation efficiency and the ability to self-clean in real time further confirms the outstanding of MF@HPB-PPy 10 -OA (Figure 4f).
Such outstanding water evaporation, salt-water separation performance, and continuous operation capability for high-salinity brine derive primarily from the contribution of the unique structure and the Figure 2. Characterization of the as-prepared MF@HPB-PPy 10 -OA.a) Scanning electron microscope (SEM) image of MF@HPB-PPy 10 -OA evaporator at low magnification.Inset image is high-resolution SEM image of the cross-section of MF@HPB-PPy 10 -OA skeleton, showing a uniform wrapping of the HPB-PPy 10 -OA coating on the MF.b) High-resolution transmission electron microscope (HR-TEM) image of HPB-PPy 10 -OA nanosheets.c) Energy dispersive X-ray (EDX) elemental mapping results of MF@HPB-PPy 10 -OA.d-f) Water contact angle behavior of MF@PPy 10 -OA, MF@HPB-PPy 10 -OA, and MF@HPB-PPy 10 .g) Cyclic compressive stress-strain curves of MF@HPB-PPy 10 -OA (at a set strain of 75%) and the digital photographs (inset).h) High-resolution x-ray photoelectron spectroscopy analysis of Mo-3d on HPB-PPy 10 -OA.
Energy Environ.Mater.2024, 7, e12647 hydrophilic-hydrophobic engineering of MF@HPB-PPy 10 -OA surface.For one, the nano water channels and the photothermal active site HPB in the hydrophilic regions guarantee the excellent water transport and high-efficiency water evaporation performance at the interface.On the other hand, the OA molecular region imparts excellent real-time selfcleaning ability to the whole porous 3D structure by significantly improving the hydrophobicity of the surface, effectively suppressing the salt crystallization on surface (Figure 4c Figure S19, Supporting Information).Besides, according to the Hagen-Poiseuille law, the hydraulic conductivity of micropores is ∼ 10 12 times higher than that from nano channels.The salt concentration in the nanoscale water channels of hydrophilic regions is effectively diluted by the rapid saltwater convection with the original macroporous water transport channels, resembling a "drilling effect", [66] which boosts the anti-salt fouling performance.Benefit from the self-assembly of HPB, OA, and PPy, MF@HPB-PPy 10 -OA is characterized by excellent photothermal stability in high-salinity salt-water separation.This may be attributed to the unique function of excited state electron transfer between HPB and PPy, which substantially inhibits the photobleaching of PPy and the photooxidation of HPB.It is worth mentioning that the above-mentioned brine convection effect is reinforced in the modified evaporation system by an innovative technology, i.e. the diffusion layer size effect.The slightly larger size of the hydrophilic filter paper diffusion layer accelerates the brine exchange between it and the 3D porous MF@HPB-PPy 10 -OA, altering the radial concentration gradient of salt in the evaporation system and allowing the salt to preferentially crystallize at the periphery of the farthest diffusion layer (See Figure 4b, Figure S20, Supporting Information and Discussion section for details).The appreciable convective effect ensures the rapid diffusion and transport of brine in the MF@HPB-PPy 10 -OA and suppresses the crystalline precipitation of salt on the surface to some extent. [18,60,67,68]

Laboratory and Outdoor Solar Desalination Performance
To evaluate water purification effect of MF@HPB-PPy 10 -OA in a practical application state, a practical photothermal water evporation system consisting of MF@HPB-PPy 10 -OA with dimensions of 30 × 30 × 0.5 cm under natural sunlight is meticulously monitored at the athletic field of Northeast Normal University (NENU: longitude: 125.3 E, latitude: 43.9 N) on July 11, 2022 (Figure 5a-c).As shown in Figure 5d, the system collects up to 1.8 L of purified water in 12 h, even in a remote area with low sunlight intensity (daily average intensity: about 0.61 kw m À2 ), which is sufficient to meet the average daily drinking water demand of an adult.Impedance measurements are conducted by multimeter to evaluate the electrolyte concentration and purity of the purified water.In Figure 5e-h, the water purification effect is outstanding and the water quality is markedly superior to the drinking water supplied by daily water purifier and well above that of seawater.After desalination, the salinities of water in samples (3.5 and 10 wt% salinity) are all below the WHO and EPA (World Health Organization and US Environmental Protection Agency) drinking water quality standards by 1-2 orders of magnitude.The major ions, such as Na + , Mg 2+ , K + , and Ca 2+ , are decreased in concentration by 3-4 orders of magnitude compared to seawater (Figure S21, Supporting Information).This system can also be applied to the purification of industrial wastewater containing heavy the surface temperature rise of MF (reference), MF@PPy 10 -OA, MF@HPB-PPy 10 , MF@HPB-PPy 10 -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@PPy 10 -OA, MF@HPB-PPy 10 , MF@HPB-PPy 10 -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@PPy 10 -OA, MF@HPB-PPy 10 , MF@HPB-PPy 10 -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-PPy 10 -OA under 0-2 sun (0-2 kW m À2 ) radiation.
Energy Environ.Mater.2024, 7, e12647 Figure 4. a) Schematic illustration of the improved solar salt-water separation system, which consists of a rthogonal prismatic table structure MF@HPB-PPy 10 -OA, a filter paper diffusion layer, a polystyrene foam insulation layer, and a cotton swab brine transport line.The radius of the filter paper diffusion layer is 0.2 cm larger than that of the MF@HPB-PPy 10 -OA evaporator; b) salt-water convection including the evaporator interior and the whole system is enhanced by combining the unique hydrophilic nano-channels of the 3D evaporator surface and the original macroporous structure as well as the size effect of the diffusion layer, achieving improved anti-salt fouling performance; c) water evaporation rate and salt harvesting efficiency of the evaporators in saltwater separation for high-salinity sample (10 wt%).Insets correspond to digital photographs of the respective salt-water separation final results; d) photothermal water evaporation performance of the evaporator at wide range of salinities compared to recent literature reports.Circles and triangles refer to the reports of salt local crystallization and PPy-based evaporators, respectively; MF@HPB-PPy 10 -OA executes e) cycles of salt-water separation and f) up to 100 h photothermal water evaporation measurements for high-salinity brine samples.and c) internal construction view.d) Real-time monitoring of SVG data from 5:00 to 17:00.Purity measurements by using a multimeter: e) world sea, f) purified water, g) drinking water (from the water purifier supply system of NENU), and h) ultrapure water.i) Salinity of two simulated seawater samples (3.5 and 10.0 wt%) before and after purification based on MF@HPB-PPy 10 -OA.
Energy Environ.Mater.2024, 7, e12647 metals or dyes (Figure S22a,b, Supporting Information), where the concentration of pollutants in the purified water is also reduced greatly and meets the discharge standards.These outstanding purification results demonstrate the great potential of MF@HPB-PPy 10 -OA for practical photothermal seawater desalination and wastewater purification.Furthermore, the advantages of MF@HPB-PPy 10 -OA in terms of mechanical properties, applicability, and fabrication process (simple, low-cost, and batch fabrication) allow for facile scale-up in commercialization (2.7 USD m À2 , see Section 2.2 synthesis for details).And the photothermal water evaporation system can also be further improved and optimized, such as designing suitable condensers to improve water collection efficiency and further promote photothermal water evaporation. [69]

Conclusion
In conclusion, a new type of functional photothermal materials (MF@HPB-PPy n -OA) with controllable interfacial hydrophilicity-hydrophobicity are successfully designed and obtained by the self-assembly of HPB, OA, and PPy on the 3D porous MF surface for high-efficiency water evaporation and complete salt-water separation of high-salinity samples.By virtue of the unique spatial separation structure of the hydrophilic region HPB and hydrophobic region OA, the hydrophilichydrophobic interface of the 3D porous SVG materials is managed for the first time to balance the contradiction between efficient water evaporation and salt crystallization.Meanwhile, the strong brine convection effect formed between the nano-water channels on the hydrophilic region and the microporous channels of MF@HPB-PPy n -OA further enhances the anti-salt fouling performance.Among them, MF@HPB-PPy 10 -OA can operate continuously and stably for over 100 h at ultrahigh evaporation rate of 3.3 kg m À2 h À1 under 1 sun while surface free of salt crystallization for desalination of high-salinity brine (10 wt%).More notably, MF@HPB-PPy 10 -OA accomplishes the complete saltwater separation of 10 wt% brine, yielding up to 3.3 kg m À2 h À1 and 96.5% evaporation rate and salt harvesting efficiency, which realizes a record-high for high salinity or local salt crystallization systems.Apart from the high-efficiency performance, MF@HPB-PPy 10 -OA also holds the advantages of simple process, low-cost, and batch manufacturing, indicating broad applicability in the fields of solar humidification, salt extraction from seawater, and wastewater purification.This study not only demonstrates the vital role of interfacial hydrophilic-hydrophobic engineering for high-efficiency, salt-resistant, and sustainable operation but also advances the development of novel HPB-based SVG functional materials for seawater desalination and zero liquid discharge.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.

2. 1 .
Preparation and Structural Characterization of 3D MF@HPB-PPy 10 -OA The preparation scheme of MF@HPB-PPy n -OA is shown in Figure 1a.First, a mixed ethanol solution containing H 3 PMo 12 O 40 ÁnH 2 O (POM), pyrrole (Py), and OA is poured on commercially available the melamine foam (MF) that serves as a carrier for self-assembly.In this process, POM is reduced to HPB by triggering the oxidative polymerization of Py to PPy.Then, HPB and OA (acting as electron donors) undergo, respectively, electrostatic self-assembly with PPy (serving as electron acceptor) on the surface of MF to form a series of Sihang Cheng received his Ph.D. degree from Northeast Normal University (NENU) in 2023 with the supervision of Prof. Yangguang Li.He is now a lecturer in College of Chemical and Materials Engineering in Bohai University.His research focuses on the design and synthesis of polyoxometalate (POM)-based self-assembled functional materials and inorganic nanostructures for energy and environmental catalysis.Cuimei Liu received Ph.D. degree from Northeast Normal University (NENU) in 2023 with the supervision of Prof. Chungang Wang.Her main research topics include the rational design and synthesis of novel inorganic nanomaterials for photothermal antibacterial and anticancer applications.Yingqi Li received his Ph.D. degree from Jilin University in 2018, and then joined Jilin Jianzhu University as a lecturer in School of Materials Science and Engineering.He is now a lecturer in Key Laboratory of Polyoxometalate and Reticular Material Chemistry of Ministry of Education and Faculty of Chemistry in Northeast Normal University.His major research topics include rational design of novel nanomaterials for micro/nanosized energy storage devices.Huaqiao Tan received Ph.D. degree from NENU in 2012 with the supervision of Prof. Enbo Wang.Later, he worked as a "Hong Kong Scholars" (2019) with Prof. Wing-Kei Ho in the Education University of Hong Kong.He has been a full professor at NENU since 2020.His main research interest is concentrated on the design and synthesis of inorganic nanostructures, polyoxometalates for energy and environmental catalysis.Yonghui Wang obtained her B.S., M.S., and Ph.D. degrees at Northeast Normal University (NENU) in China from 1993 to 2003.She then started her postdoc career at Bielefeld University and Ulm University in Germany from 2004 to 2006.She started her own research at NENU in China in 2007.Since then her research effort has been focusing on Polyoxometalate (POM) chemistry.Energy Environ.Mater.2024, 7, e12647

Yangguang
Li obtained his B.S., M.S. and Ph.D. degrees at Northeast Normal University (NENU) in China from 1993 to 2003.He then started his postdoc career at CNRS-CRPP in France and Karsluhe University in Germany from 2004 to 2006.He started his own research at NENU in China in 2007.His group's research is Polyoxometalate (POM) chemistry, especially concentrated on the photo-and electro-functional materials based on POM clusters towards the application of new energy area.Energy Environ.Mater.2024, 7, e12647

Figure 1 .
Figure 1.a) Schematic view of the preparation of 3D MF@HPB-PPy n -OA by facile one-step selfassembly of H 3 PMo 12 O 40 ÁnH 2 O (POM), pyrrole (Py), and oleic acid (OA) on melamine foam (MF); b) on the surface coating of MF@HPB-PPy n -OA, the hydrophobic regions (left) is mainly occupied by OA molecules, and the hydrophilic regions (right) is mainly composed of HPBs.

Figure 5 .
Figure 5. Solar-powered water purification equipment based on MF@HPB-PPy 10 -OA in realistic outdoor environment.a) Lateral view, b) top view (photograph credit: Chun Chang, NENU),and c) internal construction view.d) Real-time monitoring of SVG data from 5:00 to 17:00.Purity measurements by using a multimeter: e) world sea, f) purified water, g) drinking water (from the water purifier supply system of NENU), and h) ultrapure water.i) Salinity of two simulated seawater samples (3.5 and 10.0 wt%) before and after purification based on MF@HPB-PPy 10 -OA.