The most famous example of natural salt-resistant organisms is seaweed. Compared with terrestrial algae, most seaweed cell walls are composed of negatively charged polysaccharides. Due to the chemical potential difference between the cell wall and the seawater, seaweed can maintain the cytosol ion concentration at a relatively low level. Inspired by the salt-resistant mechanism of seaweed cell walls, the evaporator introduced biomimetic negatively charged SPPSU interlayer between the top PANI solar absorber and the bottom PU insulated sponge (Fig. 1). The top PANI layer absorbed broad spectrum sunlight and then converted the solar energy into effective heat [44]. The bottom PU acted as a heat insulation and support layer, and its porous structure can provide an internal skeleton for the SPPSU. Particularly, due to the Donnan effect, the SPPSU interlayer fixed groups (-SO3−) can confine counter-ions (such as Na+) within the interlayer. These confined counter-ions created an energy barrier to alter the salt ions partition equilibrium between the interlayer and bulk water [45]. At the same time, the hydrophilic SPPSU interlayer provides sufficient water for salt ion diffusion [41]. Since the synergistic mechanism of the Donnan effect and salt diffusion, the salt concentration in the interlayer can be kept at a relatively low state.
3.1 Microstructure and chemical composition of the salt-resistant solar evaporator
The synthetic strategy of the PANI-SPPSU@PU was shown in Fig. 2a. Firstly, SPPSU was deposited on the PU sponge (Fig. S3a, Supporting Information). This interlayer surface was relatively dense, and its thickness was about 277 µm (Fig. S3b and S4a, Supporting Information). The SPPSU interlayer was the critical component that served as water transport and salt resistance. The nanostructured PANI layer (with a thickness of about 653 nm, Fig. S4b, Supporting Information) was finally deposited on the SPPSU surface via oxidative self-polymerization. Due to the soft-template mechanism and interface effect, unique nano-heterojunctions of nanoflake-like petals and polyaniline nanospheres covered the surface of PANI (Fig. 2b-d), which were also confirmed in different SPPSU (80% SD-y)@ PU (Fig. S9, Supporting Information). The obtaining of these spheres could be rationalized by a soft template mechanism [46]. During the polymerization, because of the swelling behavior of the SPPSU layer, SPPSU-water cavities spontaneously formed. When aniline molecules diffused into the cavity interior, amine groups from aniline monomers accepted protons from the substrate sulfonic acid groups producing anilinium-sulfonate salts (PPSU ~ SO3−@+H3N ~ ANI) [47]. These aniline-filled cavities served as “micro-reactors” (soft templates) for aniline polymerization, which promoted the formation of polyaniline spheres. As time goes on, the polymerization initially occurred at the hydrophilic -hydrophobic interface (SPPSU–PANI–water), yielding PANI thin sheets. The spontaneously formed hydrogen bonding among PANI thin sheets rendered them to grow together, resulting in the formation of nanoflake-like petals.
XPS analysis was carried out to investigate the chemical composition (Fig. 2e). The PU sponge exhibited only the C 1s and N 1s features. After depositing SPPSU, the new signals of Na 1s, S 2s, and S 2p appeared, owing to the -SO3Na group of SPPSU. After depositing PANI, the new signal of N 1s appeared and the signals of Na 1s, S 2s, and S 2p disappeared attributed to the -NH- and -N = groups, confirming the effective coverage of PANI on the SPPSU interlayer [48].
In addition, the surface chemical state was further characterized by FTIR (Fig. 2f). The characteristic peaks of the polyurethane sponge were stretching vibration of N-H at 3289 cm− 1, -CH2- at 2971 cm− 1, -CH3 at 2867 cm− 1, stretching vibration of C = O at 1720 cm− 1 and the stretching vibration of C-O at 1225 cm− 1 and 1092 cm− 1 [49]. After depositing SPPSU, the symmetric stretching vibrations of -SO2- at 1160 cm− 1 and the asymmetric/symmetric stretching vibrations of the -SO3− groups at 1095 cm− 1 and 1072 cm− 1 could be detected. After depositing PANI, the characteristic peaks of -NH- and -N = were at 1564 cm− 1 and 1486 cm− 1 [48]. And the peak of -SO3− shifted from 1072 to 1062 cm− 1, indicating the interaction between N-H (PANI) and -SO3− (SPPSU) [50]. This confirmed the good interfacial interaction between SPPSU and PANI layer.
3.2 Photothermal performance
The solar absorption capability of the PANI-SPPSU@PU was investigated by UV-Vis-NIR Spectrometer (Fig. 3a-b). The PANI-SPPSU@PU enabled wide-range and high-efficient solar absorption. Due to the wide-spectrum absorption of the PANI, the PANI-SPPSU@PU enabled wide-range and high-efficient solar absorption. In addition, the PANI nano-heterojunction on the surface induced light refraction and reduced light reflection further improving the light absorption efficiency.
Except for the competitive solar absorption capacity, the PANI-SPPSU@PU also showed a pronounced photothermal response nature (Fig. 3c). After 180 s irradiation, the surface temperature of the PANI-SPPSU@PU (dry) quickly increased to 86.1 ℃ under 1 sun irradiation. Meanwhile, only a slight temperature increase appeared on the surfaces of the PU sponge (33.8 ℃) and the SPPSU@PU (40.1 ℃). Besides, during solar evaporation tests, the surface temperature of the PANI-SPPSU@PU (wet) reached 42.3 ℃ after 30 min under 1 sun irradiation, further confirming the pronounced photothermal response nature of PANI-SPPSU@PU (Fig. S5, Supporting Information). PANI-SPPSU(80%-7)PU exhibited a more favorable solar absorption capacity and photothermal response nature (Fig. S6, Supporting Information). It indicated that SPPSU with a high sulfonation degree promoted the oxidative self-polymerization of PANI.
3.3 Water and ion management in the SPPSU layer
According to the water contact angle test, the deposited SPPSU layer greatly enhanced the hydrophilicity SPPSU@PU (Fig. 3d). It was due to the extensive -SO3− group in SPPSU, which tended to bond with water molecules. While the hydrophilicity of PANI-SPPSU@PU decreased to some extent. Because PANI covered the hydrophilic sites (-SO3−), the groups of PANI (-NH-, -N=) could only weakly bond to water molecules.
To better reveal the role of the SPPSU interlayer, the hydrophilicity and the ion content of SPPSU was also tested. The high degree sulfonation of SPPSU indicated higher hydrophilicity (Fig. 3e), thus improving the water bonding capability. The high water binding capability not only facilitated water transportation but also reduced the required energy for water evaporation (Evaporation enthalpy test, Supporting Information) [2]. PANI-SPPSU@PU achieved a remarkable solar-to-vapor efficiency of 83.59% under 1 sun irradiation [51]. Furthermore, the ion content of SPPSU at 20% SD, 40% SD, 60% SD, and 80% SD were 0.97, 1.56, 2.13, and 2.66 mmol·g− 1, respectively (Fig. 3f). The converted charge densities were 7.8 wt.%, 12.3 wt.%, 17.0 wt.%, and 21.3 wt.%. As the charge density of the formed SPPSU layer was also increased, a higher Donnan energy barrier was formed.
3.4 Interfacial Solar-Powered Evaporation Performance.
Real-time mass change of PANI-SPPSU@PU with different charge densities was investigated by self-made experimental equipment (Fig. 4a-b). The evaporation rate increased with the degree of sulfonation. We attributed the trend to the following two reasons: (1) With the charge densities increased, the salt-resistant performance of the PANI-SPPSU@PU evaporator was improved. When the evaporator with 20% SD was used for solar evaporation tests, salt crystallization appeared on the surface only after 1.5 hours of illumination (Fig. S7, Supporting Information). As the SD increased, the salt crystallization disappeared. (2) Thanks to the desirable hydrophilicity of high sulfonation SPPSU, sufficient water transportation enabled a timely supply for water evaporation (Fig. 3e-f and Fig. S8, Supporting Information) [52–54]. It is well documented that the PANI-SPPSU@PU evaporators with hydrophilic and charged interlayers were more likely to achieve a balance between water evaporation and transportation.
To further optimize the evaporation performance of the evaporator, real-time mass change of PANI-SPPSU@PU with different deposit densities was also investigated (tuned by the mass of coating SPPSU/DMSO solution). It was found that the evaporation rate of the PANI-SPPSU (80%SD-7g)@PU was the best (Fig. 4c-d). For the evaporator with lower deposit density (PANI-SPPSU (80%SD-3g)@PU and PANI-SPPSU (80%SD-5g)@PU, the SPPSU layer was not dense enough to generate effective capillarity channels for water transportation (Fig. S9, Supporting Information). While, for the PANI-SPPSU (80%SD-9g)@PU and PANI-SPPSU (80%SD-11g)@PU, the SPPSU layer became too dense, resulting in higher mass transfer resistance and lower evaporation area (Fig. S9, Supporting Information). Therefore, the PANI-SPPSU (80%SD-7g)@PU with the best performance was further tested.
We investigated the different PU evaporation systems by recording the real-time mass change under simulated sunlight (10 wt.% NaCl 25℃ 40% RH 1Sun). Figure 4e shows the real-time mass change curves of the PU, SPPSU@PU, and PANI-SPPSU@PU. The PANI-SPPSU@PU achieved a mass loss of 2.56 kg/m2 after 90 min solar irradiation, which was 2.4 times that of the PU. As shown in Fig. 4f, the evaporation rates of the PU, SPPSU@PU and PANI-SPPSU@PU were 0.73, 0.86, and 1.80 kg/m2 h, respectively, which were 1.49 1.76 and 3.67 times that of the bulk seawater (0.49 kg/m2 h). The excellent evaporation rate of PANI-SPPSU@PU was mainly due to: (1) The PANI layer with nano-heterojunction (nanospheres and nanoflake-like petals) had high-efficiency light absorption and photothermal conversion capability; (2) The SPPSU interlayer with many -SO3− provided sufficient water channels for the solar desalination.
3.5 Salt-resistance performance
Desalination experiments were carried out at different salt concentrations to reveal the salt-resistance performance of PANI-SPPSU@PU (Fig. 5a-b). Towards 5 wt.% NaCl solution, the evaporation rate was the highest at ~ 2.06 kg/m2 h under 1 sun irradiation. When the salt concentration increased to 10 wt.%, the evaporation rate decreased to 1.80 kg/m2 h. While the evaporation rates of 10 wt.% 15 wt.% 20 wt.%, and 25 wt.% NaCl solutions were almost the same. No salt crystals were observed during the evaporation process. The evaporation rate and salt-resistance ability reported here exceed most reported for other sponge materials (Fig. 5e, Supporting information Table 1). It can be found that when the salt concentration was higher than 10 wt.%, the evaporation rate changed slightly, even for the ultra-high-concentration NaCl solution (almost saturated seawater). This phenomenon was not consistent with the common rule for desalination, and could not be explained by the unilateral salt diffusion effect. According to the previous discussion, we believed that the negatively charged interlayer played a key role in the management of salt transportation. Based on that, we proposed a mechanism for synergistic salt resistance:
On the one hand, the hydrophilic SPPSU with massive water/ion channels was conducive to salt diffusion back into the bulk water; On the other hand, the SPPSU layer formed an energy barrier that restrict the migration of salt ions into the interface layer (Fig. 6a-b): The negatively charged SPPSU rich in -SO3− groups confined Na+ within the channel of the interlayer, which created a high chemical potential between the interlayer and bulk water [55]. It is worth noting that the Donnan effect may play a critical role at high salt concentrations, which regulated the interface salt concentration within a certain range. Thus, the evaporation rate remained at a high level even for saturated seawater.
For further evaluation of long-time salt resistance and performance durability, PANI-SPPSU@PU was also tested in 10 wt.% NaCl solution for 10 hours (Fig. 5d). At the beginning, the water management and photothermal conversion did not reach a dynamic equilibrium, so the evaporation rate increased gradually. After reaching dynamic equilibrium, the evaporation rate is maintained at a high level (1.91 kg/m2 h), which is with the state-of-the-art results from the reported sponge salt-resistant solar evaporator [56–58]. No salt crystals were observed during the 10 h continuous test, indicating the great salt-resistant ability of PANI-SPPSU@PU. These results illustrated that the PANI-SPPSU@PU with the negatively charged interlayer demonstrates outstanding long-time salt resistance and performance durability.
For expanding the solar desalination application of PANI-SPPSU@PU, the concentrations of various main ions (Na+ Mg2+ Ca2+ K+) in simulated seawater were characterized by ICP-OES. After solar desalination, the ion concentration in the collected water is reduced greatly, which meets the requirement of the World Health Organization (WHO) and the United States Environmental Protection Agency (EPA) standard (Fig. 5c).