Fabrication and characterization of PANI nanoarrays solar evaporator. Figure 1a demonstrates the schematic of the PANI nanoarrays solar evaporator. The PANI nanoarrays were grown in situ on a PES macroporous membrane with a sponge-like pore structure (Supplementary Fig. 1) that serves as a substrate. The polymerization process formed nano-fibers by rapidly polymerizing aniline monomers via ammonium persulfate51. After being treated with oxidative polymerization in a dilute solution for 12 hours, the surface and internal pores of the PES substrate were coated with vertically aligned PANI nanofibers with 30–50 nm in diameter and 50–250 nm in length (Fig. 1b-d). The resulting PANI nanoarrays-coated PES membrane is hydrophilic, with an instantaneous water contact angle of 11° (inset in Fig. 1b). The pore size distribution of the PES substrate before and after the growth of PANI nanoarrays was measured using the bubble pressure method. As demonstrated in Fig. 1e, the average pore diameter of the PES substrate reduces from 345 nm to 225 nm due to the growth of PANI nanoarrays on its inner surface.
A stable and efficient water supply is a crucial component of the solar evaporator. During the steady interface evaporation process, water molecules removed by evaporation are replaced by water flow toward the air-water interface. The cohesive force between water molecules is crucial in maintaining a continuous water supply, generating tension between the molecules45,46. The water surface tension that forms in micrometer-sized pores or channels can generate a capillary pressure that is the primary driving force for water transport in a solar evaporator. The capillary pressure (Pc) can be estimated using the Young-Laplace equation, Pc= 4γcosθ/d, where γ is the surface tension of water, θ is the water contact angle on the membrane surface, and d is the pore diameter. According to the Young-Laplace equation, pores or channels present in both PES substrate and PANI nanoarrays solar evaporator can produce capillary pressures of 7.6 and 12.3 bar in the liquid phase, respectively (Fig. 1f). This result proves that smaller pore size and a hydrophilic surface can increase capillary pressure.
Efficiently absorbing broad-spectrum sunlight and converting it into heat are crucial steps in solar-driven evaporation. The PANI evaporator presents a dark surface resulting from its exceptional sunlight absorption capacity, particularly in the visible range, reaching as high as 96% (Fig. 1g). The PANI nanoarrays efficiently trap incident light and promote its multiple reflectance until absorption, contributing to the high absorption capacity52. To evaluate the light-to-heat conversion performance, the surface temperature of the PANI nanoarrays evaporator was monitored under 3 sun illumination using an infrared thermal imager (Fig. 1h). Within 2 minutes of lighting, the surface temperature of the wet PANI nanoarrays evaporator increases rapidly from 22.0 to 49.2°C. After 5 minutes, the surface temperature reaches a maximum of 54.0°C and remains stable. The excellent light-to-heat conversion performance can be attributed to the superior light-harvesting efficiency of PANI nanofiber arrays. Water evaporation rates were measured at 3.13 kg m− 2 h− 1 under 3 sun illumination by recording the weight loss (Fig. 1i). This remarkable performance can be attributed to the excellent photothermal conversion efficiency and efficient water supply of the PANI nanoarrays evaporator.
Li+/Mg2+ separation performance of PA membrane. A membrane with high mono-/di-valent ion separation performance is highly required to extract lithium from brine effectively. State-of-the-art mono-/di-valent ion separation membranes are based on a thin film composite (TFC) design, which deposits a polyamide (PA) selective layer, formed by interfacial polymerization between piperazine (PIP) and trimesoyl chloride (TMC), on top of a porous substrate. We have previously reported that deposition of a thin layer of single-walled carbon nanotubes (SWCNTs) film on macroporous PES membrane greatly improves the quality of the obtained PA layer thanks to the high porosity, smoothness, and narrowly distributed pores of the SWCNT film19,24,53. Following our previous reports, PA membranes were fabricated on the surface of SWCNTs film via interfacial polymerization (Fig. 3a). Firstly, a thin SWCNT film was prepared through vacuum-filtrating a certain amount of SWCNTs dispersion onto a macropous PES membrane with pore size of ~ 0.2 µm. The surface SEM image of SWCNTs film shows that a network porous structure is formed from interconnected carbon nanotubes with a high porosity (Supplementary Fig. 2a). Then a typical interfacial polymerization process, in which PIP and TMC react at water–hexane interface to form the PA layer, was induced on the surface of SWCNT film. A smooth layer of PA was formed after the interfacial polymerization process using 2 mg mL− 1 TMC and 10 mg mL− 1 PIP solutions (Supplementary Fig. 2b). The cross-sectional SEM image shows a bilayer structure with a thickness of 125 ± 5 nm (Fig. 3b).
The crosslinking degree of the resultant PA membrane reflects the efficiency of the interfacial reaction and determines the effective pore size. To study the effect of pore size on Li+/Mg2+ separation, the crosslinking degree of the PA layer was tuned by changing the TMC concentrations of 1, 2, to 4 mg mL− 1 during the interfacial polymerization process. The obtained membrane was denoted as PA-X, X representing the TMC concentration used in the interfacial polymerization process. The chemical composition of obtained PA layers was analyzed by X-ray photoelectron spectroscopy (XPS) (Supplementary Fig. 3). In the C1s XPS spectra, four peaks are detected, which are ascribed to O = C–O (centered at 288.5 eV), O = C–N (centered at 287.7 eV), C–N (centered at 286.0 eV), and C–C (centered at 284.8 eV) in the PA layer, further confirming the formation of the polyamide structure. The O = C–N group is derived from the reaction between the acyl chloride in TMC monomers and secondary amine in PIP monomers. The O = C-O group can be attributed to the carboxyl acid group formed by the hydrolysis of the unreacted acylchloride group. With the TMC concentration increasing from 1 to 2 and 4 mg mL− 1, the O = C–N content gradually increases from 6.35–6.48%, and 7.26%. The O = C–O content decreases from 2.85–2.45% and 1.95% (Supplementary Table 1). The crosslinking degrees of PA layers were calculated based on the O/N elemental ratio from the XPS spectra. As shown in Fig. 3c, the crosslinking degrees of PA-1, PA-2, and PA-4 are 70.6%, 74.1%, and 78.3%, respectively. These results indicate that the increase in the TMC concentration could improve the crosslinking degree of the PA layer during the PIP-TMC interfacial polymerization process.
The molecular weight cut-off (MWCO) and pore size distribution of the PA membranes were calculated using neutral organic molecules as probes (detailed information see Supplementary Methods). As shown in Fig. 3d, the MWCO of the PA membrane decreases from 422 Da to 309 Da by increasing the TMC concentration from 1 to 4 mg mL− 1. According to the function fitting of the corresponding pore model, the mean pore size is 0.63, 0.61, and 0.57 nm for PA-1, PA-2, and PA-4, respectively (Fig. 3e). This result indicates that the effective pore size of the PA layer decreases with the increase of the crosslinking degree. The performance of the PA membrane was first evaluated by experimenting with a cross-flow filtration with transmembrane pressure of 4.0 bar. The rejections to MgCl2 (1.0 g L− 1) are 95.1%, 96.1%, and 96.9% for PA-1, PA-2, and PA-4, respectively. The rejection of LiCl (1.0 g L− 1) slightly increases from 59.1–61.3% with the increase of crosslinking degree. These results are reasonable when compared to the hydrated ion diameter and hydration energy of Li+ and Mg2+. The hydrated Mg2+ diameter is 0.86 nm, larger than most of pores in the PA membrane. As Mg2+ enters PA membrane, the energy required to strip water from the hydration shell of Mg2+ is 437.4 kcal mol− 1 54. In contrast, the hydrated Li+ diameter is 0.76 nm, whereas the energy required to strip water from the hydration shell of Li+ is 113.5 kcal mol− 1. Therefore, the PA membrane demonstrated a higher rejection rate to Mg2+ than Li+.
Solar-driven lithium extraction. After evaluating the basic properties of PANI nanoarrays solar evaporator and PA membrane, we proceeded the investigation of the separation performance of solar-driven Li+/Mg2+ separation. A solar-driven Li+/Mg2+ separation device was designed and constructed, which incorporates a PANI nanoarrays solar evaporator as the top photothermal layer and a PA-4 membrane as the selective barrier for Li+/Mg2+ separation (Fig. 4a). To evaluate Li+/Mg2+ separation performance during interfacial evaporation, we employed a mixed solution containing 0.45 g L− 1 of LiCl and 0.90 g L− 1 of MgCl2 (Li+ and Mg2+ have similar molality of ~ 10 mM). When the PA membrane is introduced beneath the PANI nanoarrays solar evaporator, there is a slight decrease in the rate of water evaporation to 2.95 kg m− 2 h− 1 under 3 sun irradiation (Supplementary Fig. 4). After observing 120 minutes of stable evaporation, white salt powders accumulate on the surface of the PANI nanoarrays solar evaporator (Fig. 4b), indicating that capillary pressure enables water and ions permeation through the PA membrane. The transmembrane rate of Li+ is found to be 0.91 mol m− 2 h− 1, which is 13 times faster than that of Mg2+ in the ion separation membrane-based solar evaporator (Fig. 4c). This indicates that the ultrathin PA membrane maintains its selective transport properties, even under capillary pressure-driven separation process.
To investigate the effect of the Mg2+/Li+ ratio on solar-driven lithium extraction, the LiCl concentration was kept constant at 0.45 g L− 1 and the MgCl2 concentration gradually increased from 0.45 to 4.50 g L− 1. Under 3 sun irradiation, the water evaporation rate remains around 2.9-3.0 kg m− 2 h− 1. However, it is observed that the LiCl crystallization rate increases from 37.3 to 61.8 g m− 2 h− 1 with the increase of Mg2+/Li+ ratio. This phenomenon can be explained by the Donnan equilibrium, where the increase in the Mg2+/Li+ ratio causes a proportional rise in Cl− concentration. With a higher amount of Cl− in the solution, more Cl− can permeate through the PA membrane. To maintain equilibrium on both sides of the membrane, Li+ with smaller hydrated ion diameter and lower hydration energy also trans through the PA membrane, increasing the LiCl crystallization rate.
Figure 5b demonstrates the proportion of LiCl in solution and in salt powder collected on the PANI nanoarrays solar evaporator. During the evaporation process of an MgCl2/LiCl mixed solution with mass ratio of 1: 1, the proportion of LiCl increases from 50.0% in the feed solution to 94.2% in the salt powder. In the case of an MgCl2/LiCl mixed solution with a mass ratio of 10:1, the proportion of LiCl is enriched from 9.1% in the feed solution to 52.4% in the salt powder, indicating a high lithium extraction efficiency.
To further examine the effect of salt concentrations on solar-driven lithium extraction, the total concentration of MgCl2 and LiCl was increased from 10 to 80 g L− 1 while maintaining a MgCl2/LiCl mass ratio of 1:1. Under 3 sun irradiation, the water evaporation rate remains around 3.0 kg m− 2 h− 1, while the LiCl crystallization rate increases from 41.2 to 186.2 g m− 2 h− 1 (Fig. 5c). Notably, the PA membrane keeps a stable Mg2+/Li+ separation performance, with the LiCl/MgCl2 separation factor remaining above 16.2 (Fig. 5d). The proportion of LiCl in the salt powder collected from the surface of PANI nanoarrays solar evaporator is in the range of 94.2 to 95.1%, indicating that the device can effectively extract lithium ions from Mg2+/Li+ mixtures even in high concentration and high Mg2+/Li+ ratio.
The device was further exploited to process a simulated salt-lake brine containing 268.0 g L− 1 NaCl, 5.1 g L− 1 LiCl, 66.1 g L− 1 MgCl2, and 9.2 g L− 1 CaCl2 (referring to the compositions of the Uyuni salar brine). In the simulated salt-lake brine, total salt concentration is as high as 348.4 g L− 1 and Mg2+/Li+ mass ratio is up to 19.8. Under 3 sun irradiation, the water evaporation is around 2.2 kg m− 2 h− 1, and salt crystallization rate is 480 g m− 2 h− 1. The salt powder can be directly collected from the surface of the PANI nanoarrays solar evaporator. The composition was analyzed by inductively coupled plasma atomic emission spectroscopy. Compared with the composition of simulated salt-lake brine, NaCl proportion in the solid salt powder increases from 76.9–96.3%, LiCl proportion increases from 1.5–2.3% (Supplementary Fig. 5). Correspondingly, MgCl2 proportion decreases from 19–0.5% and CaCl2 proportion decreases from 2.7–0.9%. These results indicate that monovalent salts can be enriched by the synergistic effect of membrane-based ion separation and solar-driven evaporation. To determine the salt rejection, we assume that the salt powder is dissolved in the evaporated water. The rejection to MgCl2 is as high as 98.3%, and the rejection to CaCl2 is 78.5% (Fig. 5e and Supplementary Table 2). The slightly lower rejection to CaCl2 than that of MgCl2 is ascribed to the smaller hydrated diameter of Ca2+ than Mg2+. In contrast, the rejection to NaCl and LiCl is 21.7% and 2.0%, respectively, indicating that monovalent salt ions, like Na+ and K+, can pass through the PA membrane, while most of the Mg2+ and Ca2+ ions are rejected.
The ratio of magnesium and lithium in brines is a crucial factor in determining the process of lithium extraction. When the Mg2+/Li+ mass ratio is lower, it becomes easier to extract lithium from salt-lake brines. The mass ratio of Mg2+/Li+ in the salt powders collected on the surface of the PANI nanoarrays solar evaporator is 0.3. This is a significant decrease compared to the simulated salt-lake brine, with a reduction of 66 times (Fig. 5f). At a low Mg2+/Li+ ratio, after adding NaOH and sodium oxalate to remove residual small amounts of Mg2+ and Ca2+, lithium can be readily precipitated out in the form of Li2CO3 by adding Na2CO3. The sediment was separated by centrifugation, washed using deionized water, and then dried in a vacuum oven. The collected white powder (Fig. 5g inset) was characterized by powder X-ray diffraction spectroscopy, whereby the XRD pattern fits well with the standard pattern of Li2CO3 (PDF#22-1141) without any impurity signals being detected. Further quantitative elemental analysis shows that the purity of Li2CO3 is around 99% which meets the requirements of battery-grade Li2CO3 purity.
In summary, drawing inspiration from the selective water/ion uptake and salt secretion processes in mangroves, we reported the synergistic design of ion separation membrane and solar-driven interfacial evaporator, which effectively extracts solid LiCl from a mixed salt solution, achieving a purity level of up to 94%. Notably, our ion separation membrane-based solar evaporator demonstrates the ability to directly treat simulated salt-lake brine, even at concentrations as high as 348.4 g L− 1, resulting in a remarkable 66-fold reduction in the Mg2+/Li+ ratio (from 19.8 to 0.3). At a low Mg2+/Li+ ratio, battery-grade Li2CO3 powder can be precipitated out through a simple precipitation process. By combining ion separation with solar-driven interfacial evaporation, our research introduces a new and promising approach for lithium extraction utilizing renewable energy sources. Hence, it is expected that this approach will lead to the development of a promising process to secure the lithium supply for future energy uses.