Harvesting solar energy with a Ni-MOF-based evaporator for efficient solar thermal storage and steam generation

Abstract

Harvesting solar energy for efficient photothermal conversion and steam generation over solar evaporators is particularly significant in the context of comprehensive solar utilization toward solving the global shortage of fresh water. Herein, a flexible Ni-based metal–organic framework composite (NMC) with hierarchical structures is constructed as a photothermal material for solar steam-generation devices. Besides, a phase change material (PCM)-based thermal storage pack is incorporated into the solar evaporator to overcome the solar radiation intermittency. By virtue of the synergistic effect of the promising 3D structure and excellent water transport, the NMC-PCM evaporator possesses an outstanding evaporation performance (2.55 kg m⁻² h⁻¹) under 1 sun irradiation. More strikingly, owing to the enhanced light-to-heat performance as well as large heat storage capacity, the proposed evaporator achieves a high water-evaporation rate of 1.46 kg m⁻² h⁻¹ without sunlight irradiation. This work opens up opportunities to realize excellent energy capture for solar-driven clean water production and solar thermal storage not only under sunlight incidence but also in no light environment.

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1. Introduction

The global population growth and industrialization call for sustainable technologies for water purification.^1–3 Solar-driven interfacial vapor generation (SIVG)-based water treatment offers a promising strategy that enables continuous production of fresh water by utilizing inexhaustible solar thermal energy as the only power input.^4–8 Of particular importance, the facile operation and zero-carbon emission endow solar-driven water treatment with enormous potential specifically in the fields of energy and the environment, such as water desalination,^6,9–12 steam sterilization,^13,14 wastewater treatment,^15–18 electricity generation,^19–24 and fuel production.^25,26 In principle, an ideal interfacial solar evaporator should have high solar to thermal conversion, excellent thermal management structures and tunable water transportation pathways, to increase the water evaporation rate.^27,28 To achieve higher steam yields, one attractive method is to assist water molecules in clusters by optimizing the intrinsic structure of photothermal materials, which can effectively reduce the energy requirement during steam generation.^29,30 Another strategy is to increase the evaporation area through three-dimensional (3D) morphology controlling, which enables the more efficient use of energy by purposefully harvesting heat from the surrounding environment. In this context, enormous efforts have been devoted to exploring solar-driven photothermal materials, including carbon-based materials,^31–33 plasmonic metal nanoparticles,^34,35 semi-conductor materials,^17,36,37 polymers,³⁸ and their hybrids. Specifically, 3D hierarchical architectures have been widely reported due to the reduction of the heat loss and light reflection of photothermal material or the expansion of the evaporation area and light absorption. Inspired by the black scales of Bitis rhinoceros, Chen's group designed a hierarchical MXene nanocoating with broadband light absorption (up to ∼93.2%) and a significant light-to-heat conversion efficiency.³⁹ Du and co-workers reported ultra-black paper-based artificial trees for highly efficient solar steam generation resulting from multiple times of internal reflection of light and thermal radiations between the leaves.⁴⁰ Likewise, Yu's group demonstrated a bionic solar evaporator with a macroporous skeleton and macro-patulous channels to significantly promote abundant water transportation and rapid steam extraction and finally achieve a great steam efficiency.⁴¹ However, those hierarchical architectures involve complicated and energy-consuming processes and remain far from satisfactory in terms of material and manufacturing cost. Lately, triphenylene-based metal–organic frameworks (TP-MOFs) as emerging porous crystalline materials, which possess an open pore framework structure, high surface area and hydrophilicity,^42–47 have been considered to be an ideal candidate for photothermal materials as solar absorbers due to their high intrinsically broad light absorption, thermal stability, and chemical stability (in aqueous and non-aqueous media).⁴⁸ Herein, we designed a flexible TP-involved Ni-MOF^49,50 composite (NMC) with a 3D biomimetic austrocylindropuntia subulata-like structure as a photothermal material via an in situ growth of Ni-CAT-1 on air-laid paper. The flexible air-laid paper made of 55% cellulose and 45% polyester fibers has excellent hydrophilicity and an interconnected porous structure for water supply and vapor escape. Nevertheless, in the case of merely solar energy input, the SIVG rate is theoretically limited to 1.47 kg m⁻² h⁻¹ at 1 sun (1 kW m⁻²).⁵¹ Besides photothermal materials, particular attention should also be paid to solar absorber operation during solar radiation intermittency.

As a promising thermal storage medium, phase change materials (PCMs) can reversibly store and release large amounts of thermal energy during the isothermal phase change, which offers a great opportunity to guarantee continuous operation of various solar heating-related processes.⁵² Hence, we propose a facile way to fabricate high-performance PCM-based solar-thermal packs through impregnating organic paraffin within natural wood pulp sponge (NWPS) for solar-thermal energy storage. By combination of PCM-based solar-thermal packs with TP-involved Ni-MOF porous structures, we rationally designed a NMC-PCM hybrid photothermal evaporator to simultaneously achieve high evaporation rates during solar steam generation as well as excellent thermal storage performance. This presented solar evaporator features a biomimetic hierarchical structure for enhanced light absorption, abundant microchannels with excellent hydrophilicity for water supply and vapor escape, and high-performance solar-thermal heat packs, storing thermal energy within the PCMs to achieve continuous heating for solar steam generation. Under 1 sun irradiation, the NMC-PCM photothermal evaporator demonstrates a water-evaporation rate as high as 2.55 kg m⁻² h⁻¹. More strikingly, the NMC-PCM evaporator maintains a high water-evaporation rate of 1.46 kg m⁻² h⁻¹ without sunlight irradiation. Furthermore, the NMC-PCM photothermal evaporator generates clean water from seawater and wastewater, demonstrating its great potential for water purification.

2. Materials and methods

2.1. Materials

Lead nitrate [Pb(NO₃)₂], chromium chloride hexahydrate (CrCl₃·6H₂O), cadmium nitrate tetrahydrate [Cd(NO₃)₂·4H₂O], copper(II) sulfate pentahydrate (CuSO₄·5H₂O), iron(III) chloride hexahydrate (FeCl₃·6H₂O), zinc sulfate heptahydrate (ZnSO₄·7H₂O), nickel(II) acetate tetrahydrate [Ni(CH₃COO)₂·4H₂O] (NAT), methyl orange (MO), methylene blue (MB), rhodamine B, and organic paraffin were provided by Sinopharm Chemical Reagent Co., Ltd., China. Lithium chloride hydrate was provided by Shanghai Bide Medical Technology Co., Ltd. Air-laid paper was purchased from Suzhou Tainuo Purification Co., Ltd., China. 2,3,6,7,10,11-Hexahydroxytriphenylene (HHTP) was obtained from Shanghai Xianding Biotechnology Co., Ltd., China. Cobalt(II) acetate tetrahydrate [Co(CH₃COO)₂·4H₂O] was provided by Shanghai Aladdin Biochemical Technology Co., Ltd., China. Pearl cotton was obtained from Ningbo Youku Pinsheng Trading Co., Ltd., China. Aluminum foil was provided by Wuhan Aizhijia Industrial Co., Ltd., China. Natural wood pulp sponge was purchased from Dengfeng Jinguan Trading Co., Ltd., China. All materials and chemicals were used as received without further purification.

2.2. Fabrication of Ni-based metal–organic framework composites (NMCs)

The biomimetic hierarchical nanostructure NMCs were prepared by in situ growth of Ni-MOF on air-laid paper. First, a piece of air-laid paper was cut into a cross shape for easy placement on the PCM column and drops of nickel acetate solution. 10 mL nickel acetate solution (10 mg mL⁻¹) is added dropwise to the air-laid paper until it is saturated with air-laid paper. The paper was then placed in a polypropylene ziplock bag and 10 mL of HHTP solution (5 mg mL⁻¹) was added. The polypropylene ziplock bag was sealed and placed horizontally in an oven at 85 °C for 12 h. The reaction mixture was naturally cooled to room temperature and the NMC was obtained after washing with deionized water for three times.

2.3. Fabrication of PCM composites

In a typical experiment, a natural wood pulp sponge (NWPS) was cut into cuboids of different sizes (2.5 cm × 2.5 cm × 1 cm, 2.5 cm × 2.5 cm × 2.5 cm, and 2.5 cm × 2.5 cm × 5 cm). The perimeter and bottom of the cuboids were encapsulated with pearl cotton for heat insulation. 100 g organic paraffin was placed in an oven at 70 °C until completely melt and was then impregnated into the 3D interworking framework of NWPS. After cooling to room temperature and encapsulating the upper surface with aluminum foil, the PCM composites were obtained for further use.

2.4. Assembly of evaporators

The 3D evaporator consists of a piece of NWPS filled with organic paraffin for support as a PCM composite, a watch-glass filled with water, and a piece of air-laid paper with Ni-based metal–organic framework composites as the photothermal material (Fig. S1†). The length and width of the NWPS are 2.5 cm. The PCM composites with heights of 1 cm, 2.5 cm and 5 cm were named PCM-1, PCM-2 and PCM-3, respectively. Blank 1, blank 2 and blank 3 represent evaporators with a height of 1, 2.5 and 5 cm combined with a NWPS column without paraffin filling and an air-laid paper, respectively. Blank-PCM-1, 2 and 3 represent evaporators with a height of 1, 2.5 and 5 cm combined with a PCM composite and an air-laid paper, respectively. Similarly, NMC-PCM-1, 2 and 3 represent evaporators with a height of 1, 2.5 and 5 cm combined with the PCM composite and NMC, respectively. The thickness of the NMC photothermal material varies from ∼371 μm (1 layer) to ∼1110 μm (3 layers) and then ∼1850 μm (5 layers), which are denoted as NMC-PCM-2, NMC-3-PCM-2 and NMC-5-PCM-2, respectively. The body and bottom of all NWPS columns are wrapped in pearl cotton as well as the top is covered with aluminum foil.

2.5. Characterization

X-ray diffraction (XRD) patterns were recorded on an XRD (Bruker D8 phaser) using Cu Kα radiation. Structure and morphology characterization was conducted with a transmission electron microscope (TEM, FEI Tecnai F20) and field emission scanning electron microscope (FESEM, Hitachi SU8010). Atomic force microscopy (AFM, tapping mode) measurements were carried out on an SPM 9700 microscope, examining film areas of 5 μm × 5 μm. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific K-Alpha. The absorption, reflection and transmission spectra were characterized using an ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometer (Shimadzu Corp., Tokyo, Japan, UV-3600 equipped with an integrating sphere). A Labram HR 800 spectrometer and INVENIO R spectrometer were applied to record Raman spectra and Fourier transform infrared spectroscopy (FTIR) spectra, respectively. The heat flow signals of pure water and water-bearing NMC were characterized with a differential scanning calorimeter (DSC, DSC2500). Thermogravimetric (TG) results were obtained from a PerkinElmer instrument (heating from 30 to 600 °C with a rate of 5 °C min⁻¹). The hydrophilicity of the NMC and air-laid paper were analyzed using a surface tension-contact angle meter (Dataphysics-OCA20). N₂ adsorption–desorption measurement was conducted on a Micromeritics ASAP2420 apparatus at 77 K. The ion concentrations of real seawater before purification and the collected water after purification were measured with an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP7200).

2.6. Solar evaporation performance test

For a solar vapor generation test under laboratory conditions, a solar simulator (CEL-HXF300, Education Au-light Co., Beijing, China) with an AM 1.5 G filter was used as the only light source. The solar illumination was calibrated to 1 sun (1 kW m⁻²) with an optical power meter (CEL-NP2000-2, Beijing Education Au-light Co., Ltd.). The light intensity was always calibrated before tests. The evaporator was loaded onto a computer-controlled electronic balance with 0.001 g resolution (Mettler Toledo, ME204). The real-time mass change during evaporation was regularly measured and recorded in each test group. An IR camera (FLIR E4 Pro, America) was applied to record the temperature distribution and the variation of the surface temperature of samples during the evaporation process. A thermocouple (KEYSIGHT 34972A LXI Data Acquisition/Switch Unit, America) was applied to measure and record temperature changes at various locations on the evaporator. Note that, in our evaporation measurements, evaporation in the dark was included in the measured vapor mass.

2.7. COMSOL multiphysics simulations

To gain insight into the intensifying solar-driven heat accumulation over a top interfacial layer of NMC-PCM-3 solar evaporator, and efficient thermal management of PCM composites followed by the well-known heat storage module. The steady-state heat transfer simulations of NMC-PCM-3 filled with fluid were assessed using the following equations:

Q = (ρC_p)_eff + ρC_Pu·∇T + ∇·q (i)

q = −k_eff∇T (ii)

(ρC_P)_eff = θ_Pρ_PC_p,P + (1 − θ_P)ρC_p (iii)

k_eff = θ_Pk_P + (1 − θ_P)k (vi)

where Q represents the heat flux (W m⁻³), ρ is the water density (1000 kg m⁻³), C_p denotes the water heat capacity (4200 J kg⁻¹ K⁻¹), and (ρC_p)_eff shows the effective volume heat capacity under constant pressure. The Darcy velocity (u) depicts the volume of fluid flow rate per unit cross-sectional area (m s⁻¹). The fluid velocity inside the micro-channel of the NMC was considered as u_L = u/θ_p, where θ_p represents the open porous of the evaporator, ∇T is the temperature variation (K m⁻¹), and q shows the heat flux vector (W m⁻²).

3. Results and discussion

3.1. Characterization of the NMC microstructures

Inspired by the unique texture of Austrocylindropuntia subulata, we created a biomimetic Ni-based metal–organic framework composite (NMC) via in situ growth of Ni-CAT MOF on air-laid paper. The fabrication process of the NMC by a one-step hydrothermal method is schematically illustrated in Fig. 1A. We employed scanning electron microscopy (SEM) to study the morphology of the obtained NMC. From the low magnification images in Fig. S2 and S3,† it can be seen that multiple flat microfibers are intertwined and overlap each other for the NMC and air-laid paper, resulting in the formation of abundant microchannels. Unlike the smooth surface of the air-laid paper, the NMC exhibits a biomimetic hierarchical structure comprising uniform nanorods with a diameter of 60–80 nm tightly anchoring on the microfibers (Fig. 1B). The crystal structure of the NMC was further examined with a transmission electron microscope (TEM). The TEM image in Fig. S4† displays clearly a lattice fringe of 1.1 nm. The energy-dispersive spectroscopy (EDS) mapping verified uniform and continuous distributions of carbon (C), oxygen (O), and nickel (Ni) for the synthesized NMC (Fig. S5†). Power X-ray diffraction (XRD) was used to study the phase structure of the Ni-MOF powder. The XRD pattern of the synthesized Ni-MOF powder matched well with that of the Ni-CAT MOF (Fig. S6†).^50,51 The space filling drawings of the single-crystal structure of the Ni-CAT MOF are shown in Fig. 1C. X-ray photoelectron spectroscopy (XPS) spectrum was then collected to verify the three main characteristic peaks appearing at the binding energy of 284.35, 531.68 and 855.66 eV, which belong to the C 1s, O 1s and Ni 2p peaks of NMC, respectively (Fig. S7†). The above features are in agreement with that of the Ni-CAT MOF reported previously.⁵⁰ The chemical structure of the NMC was further analyzed by Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy.^50,53,54 The FTIR spectrum shows a couple of peaks at 1236 and 1708 cm⁻¹, which correspond to the C–O and C=O stretching vibrations of carboxyl from HHTP (Fig. S8†). The peaks locating at 1326 and 1460 cm⁻¹ are assigned to the C–C stretching vibration of the benzene ring from HHTP. A group of peaks at 1095, 3281, 840 and 716 cm⁻¹ are attributed to the C–H in-plane deformation, C–H stretching vibration, C–H out-of-plane deformation vibration, and ring deformation vibration of the benzene ring from HHTP, respectively. Moreover, the band centered at 443 cm⁻¹ is assigned to the Ni–O stretching vibration, confirming that nickel ions in the hydrothermal process have combined with the organic ligands (HHTP) via coordination bonds. In the Raman spectrum of NMC, the bands at 1610, 1445, 1285 and 1143 cm⁻¹ correspond to the C–H in-plane bending mode of the benzene ring, and the peaks at 3067 and 856 cm⁻¹ belong to the C–H out-of-plane bending mode of the benzene ring (Fig. S9†). Additionally, the Raman band at 473 cm⁻¹ is associated with the tensile vibration of Ni–O. All the results coincide with the FTIR analysis. To evaluate the surface roughness of the NMC, atomic force microscopy (AFM) surface scans were performed in tapping mode (Fig. 1D and S10†). The calculated Ra surface roughness of the NMC is 136.57 nm. The hydrophilicity of the photothermal material is a key criterion to evaluate the speed of water transfer in evaporators. The contact angle of water on air-laid paper and the NMC is ca. 30.25° and 22.8° at 0.02 s, respectively, followed by a rapid drop to 0° at 0.04 s (Fig. 1E). The instantaneous wettability experiments of the air-laid paper and NMC are presented in Movies S1 and S2,† which verified that the air-laid paper and NMC were super hydrophilic. We then analyzed the solar light absorption performance of the air-laid paper and NMC using an ultraviolet-visible-near-infrared (UV-vis-NIR) spectrophotometer across the spectrum of 250–2500 nm. As shown in Fig. 1F, the biomimetic coating of Ni-MOF substantially improves the optical absorption of air-laid paper from 35.1% to 97.9%. In comparison, the NMC exhibits extremely less optical reflectance (2.12%) and transmittance (0.02%) than air-laid paper in the 250–2500 nm region (Fig. S11†), which demonstrates that the NMC possesses outstanding light absorption capability with a remarkable overlap with the entire solar spectrum. Unlike the air-laid paper surface that indiscreetly reflects light, the biomimetic hierarchical structure of the NMC can efficiently capture incoming light, which is probably due to its nanowhisker texture that could induce light scattering and absorption, eventually leading to an improved light-trapping behavior and laying the foundation for interfacial solar heating (Fig. 1G). As shown in Table S1,† the specific surface area of the NMC was 5.316 m² g⁻¹, which is 483.3 times larger than that of air-laid paper (0.011 m² g⁻¹). Additionally, the Thermogravimetric (TG) analysis of the NMC in nitrogen proves the excellent thermal stability up to 168 °C, with a mass loss of ca. 1.0%. Specifically, when the temperature reaches 300 °C, the mass loss is only 6.2% (Fig. S12†). Additionally, different types of patterns can be obtained by cutting the NMC, confirming the satisfactory flexibility of the NMC (Fig. S13†).

Fig. 1 (A) Scheme illustrating the fabrication process of the NMC. (B) SEM image of the NMC. (C) Space filling drawings of the single-crystal structure of the Ni-CAT MOF along the c axis. Color code: C, blue; O, red; Ni, cyan. (D) AFM scans of a 5 μm × 5 μm area of the NMC with a color bar displaying the film heights. (E) Time-dependent water contact angle measurements capturing the wetting process of a water droplet on the air-laid paper (upper) and NMC (bottom). (F) Solar spectral irradiance (AM 1.5 G) (right axis) and optical absorption (left axis) spectra of the wet air-laid paper and NMC in the wavelength range of 250–2500 nm. (G) Schematic illustration of the biomimetic Ni-CAT nanocoating with broadband light absorption and enhanced light-to-heat performance.

3.2. Solar vapor generation test

Homemade solar vapor generation devices were designed to systematically study the steam generation performance, as illustrated in Fig. 2A. We placed the NMC (as a light-to-heat conversion layer) on top of the PCM supporting column (as a thermal pack). While increasing the evaporation area, the side area NMC provides continuous upward transport of water molecules due to its super-hydrophilicity. The device was located in the water and acted as an integrated solar-thermal evaporator. We evaluated the performance of solar vapor generation by measuring the water evaporation rate. The steam generation process is shown in Fig. S1.† In the dark, the evaporation rates of pure water, blank-PCM-2 and NMC-PCM-2 were 0.08, 0.37 and 0.45 kg m⁻² h⁻¹, respectively (Fig. 2B). The solar thermal driven evaporation curves of time-dependent water mass change under 1 sun illumination are shown in Fig. 2C. The evaporation rate of pure water, blank-PCM-2 and NMC-PCM-2 increased to 0.21, 1.32 and 2.47 kg m⁻² h⁻¹, respectively, demonstrating the excellent photothermal conversion and rapid water evaporation performance of NMC-PCM-2. Under the vertical irradiation of 1 sun, the evaporation rates of blank-1, blank-2, blank-3 and blank-PCM-2 were 1.57, 1.62, 1.65 and 1.32 kg m⁻² h⁻¹, respectively (Fig. 2D). The water evaporation rate of the blank group increased slightly with the height of the evaporator; however blank-PCM-2 containing organic paraffin had a lower evaporation rate than blank-2, since organic paraffin absorbed some of the heat from the photothermal interface during evaporation. Notably, we also measured the evaporation rate of each evaporator within 30 min after the simulated sunlight lamp was turned off. The evaporation rates of blank-1, blank-2, blank-3 and blank-PCM-2 were 0.94, 0.98, 1.04 and 1.16 kg m⁻² h⁻¹, respectively (Fig. 2E). Interestingly, after the simulated light was turned off, the evaporation rate of the non-organic paraffin-containing evaporator decreased dramatically, while the evaporation rate of the evaporator containing organic paraffin is up to 1.16 kg m⁻² h⁻¹, further demonstrating that the PCM continues to produce water even after the sunlight is interrupted. We then investigated the effect of the evaporator height and light intensity on the rate of evaporation. The evaporation rates of NMC-PCM-1, NMC-PCM-2 and NMC-PCM-3 under one sun were 2.32, 2.47 and 2.55 kg m⁻² h⁻¹, respectively (Fig. 2F), verifying that the evaporation rate of NMC-PCMs also increased slightly with the height of the evaporator. When the simulated light source was turned off, the evaporation rates of NMC-PCM-1, NMC-PCM-2 and NMC-PCM-3 were 1.12, 1.20 and 1.16 kg m⁻² h⁻¹, respectively (Fig. 2G). Moreover, the evaporation rate of NMC-PCM-1/2/3 was 4.22, 4.40, and 4.47 kg m⁻² h⁻¹ and 1.22, 1.28, and 1.3 kg m⁻² h⁻¹ before and after turning off the simulated light at 2 kW m⁻², respectively (Fig. 2H and I), which demonstrated that the evaporation rate of NMC-PCM elevated significantly when the simulated light intensity increased. Additionally, when the light intensity was increased to 3 kW m⁻², the evaporation rate of NMC-PCM-1/2/3 rose to 5.07, 5.16, and 5.33 kg m⁻² h⁻¹ and 1.32, 1.38, and 1.42 kg m⁻² h⁻¹ before and after turning off the simulated light, respectively (Fig. S14†). The above results indicated that the water evaporation rate can be observably enhanced by the height of NMC-PCM and increased light intensity. More strikingly, the evaporation rate of NMC-PCM-3 was 1.42 kg m⁻² h⁻¹ after the simulated light was turned off, which was 9.5 times larger than that of pure water (0.15 kg m⁻² h⁻¹), attributed to the excellent light-to-heat conversion performance of the NMC and the good thermal storage capacity of the PCM composite. The residual heat of the NMC and the thermal energy stored by the PCM composite can continue to provide heat for evaporation after the lamp source was turned off. As shown in Fig. S15a and b,† we studied the thickness effect of the NMC by varying the layers of the NMC photothermal material from 1 to 5 layers (about 370–1850 μm) and tested their photothermal performances. The evaporation rates of NMC-3-PCM-2 and NMC-5-PCM-2 decreased to 2.10 and 1.70 kg m⁻² h⁻¹, respectively. After turning off the simulated light at 1 kW m⁻², the evaporation rate of NMC-3-PCM-2 and NMC-5-PCM-2 was 0.97 and 0.81 kg m⁻² h⁻¹, respectively. The result shows that the evaporation rate decreases significantly with the increase of the NMC thickness. In addition, the top surface, middle and bottom temperatures of NMC-3-PCM-2 and NMC-3-PCM-2 were lower than those of NMC-PCM-2 at 1 kW m⁻², further illustrating a deteriorated photothermal performance (Fig. S15c and d†), which might be ascribed to the great hydrophilic properties of the NMC and capillary effect. When the thickness of the NMC photothermal material increases, more water localized in the NMC material could lead to a heat loss during evaporation, which results in a reduced photothermal conversion. When one layer of the NMC material (∼370 μm thickness) is used, the upwardly transported water is able to absorb the interfacial heat in time, creating a dynamic equilibrium of rapid evaporation for optimal light-to-heat conversion performance.

Fig. 2 (A) Scheme illustrating the solar steam generation. (B) Time-dependent mass change under dark conditions. (C) Time-dependent mass change under one sun illumination. (D) Time-dependent mass change of different evaporators under one sun illumination. (E) Evaporation rate of the control group after turning off the light. (F) Time-dependent mass change of NMC-PCM-1/2/3 under one sun illumination and (G) evaporation rate after turning off the light source. (H) Time-dependent mass change of NMC-PCM-1/2/3 under two sun illumination and (I) evaporation rate of after turning off the light.

To further confirm the photo-thermal conversion performance of the evaporator, we investigated the temperature variation of the top surface, middle and bottom of NMC-PCM-1/2/3 under different light intensities. As shown in Fig. 3A–C, when NMC-PCM-3 was placed directly under the solar simulator, the temperature of the top surface under 1, 2, and 3 kW m⁻² has promptly increased to 31.6, 41.4 and 51.2 °C, respectively. Simultaneously, the middle and bottom temperatures of NMC-PCM evaporators exhibited a remarkable increase within a short duration and remained stable under simulated solar irradiation (Fig. S16 and S17†). To further verify heat localization performance, typical IR images displayed the temperature distribution of NMC-PCM-3 for different periods under 1 sun illumination (Fig. 3D). The top temperature of the evaporator was 20.3, 22.8, 27.0, 28.5, 30.8, 31.0, and 31.5 °C at 10, 20, 60, 120, 300, 800, and 1400 s, respectively. To further clarify and understand heat transfer in NMC-PCM, COMSOL Multiphysics was employed to simulate the actual temperature distribution under one-sun radiation using a finite element method. Initially, a simulated 3D model with an identical dimension to that of the NMC-PCM-3 evaporator was constructed. The simulation of the surface temperature distribution of the NMC-PCM-3 solar evaporator is shown in Fig. 3E, where the surface temperature could reach about 31.5 °C quickly, which agreed with our experimental phenomenon (typical IR images, Fig. 3D), further confirming effective interfacial heat confinement occurring in our solar vapor generation systems. Due to capillary forces, a constant flow of water was transferred upwards from the lower sides of NMC-PCM-3 to the top surface for evaporation, so that the temperature at the center of the upper surface was higher than that at the edges. Meanwhile, it was found that the lower the height of NMC-PCM (i.e., the lower the content of organic paraffin), the greater the temperature difference between the middle and bottom temperatures of NMC-PCM and the top surface temperature, which was due to the higher temperature at the location closer to the top surface (Fig. 3A–C and S16, S17†). According to the thermal analysis of the energy loss in the entire solar evaporation process, the heat radiation, heat convection and heat convection losses were 2.53%, 2.57% and 3.18%, respectively (Calculation S1–S4 and Fig. S18†).

Fig. 3 Temperature response profiles of wet state NMC-PCM-3 when the solar illumination turns on and off under different irradiations of (A) 1, (B) 2, and (C) 3 sun illumination, respectively. (D) IR thermal images of NMC-PCM-3 during 1 sun illumination. (E) Simulation of the top temperature distributions of NMC-PCM-3 under 1 sun irradiation.

3.3. Solar-thermal storage performance

Due to the excellent photothermal conversion performance of the NMC, it can effectively convert solar radiant energy to thermal energy under simulated solar irradiation such that the solar-thermal storage component of the evaporator can absorb solar thermal energy from the heat locating layer. As presented in Fig. 4A–C, the temperature of NMC-PCM ascends obviously with the increase of light intensity from 1 to 3 sun (Fig. 4). Interestingly, the top surface temperature of NMC-PCM-1, 2 and 3 is greatly close under different irradiations at 1, 2 and 3 sun, respectively, suggesting that the height of NMC-PCM has no significant effect on the surface temperature. We then further investigated the middle and bottom temperatures of NMC-PCM-1, 2 and 3 after 1 h of vertical light irradiation with the simulated sunlight source turned off (Fig. 4D–I). It can be concluded that the higher the height of NMC-PCM, the slower its middle temperature decreased. Furthermore, the bottom temperature of NMC-PCM-1, 2 and 3 after turning off the simulated sunlight source for 1 h of light exposure under different light intensities is shown in Fig. 4G–I. When the organic paraffin content in NMC-PCM was a fixed value, the stronger the light intensity, the more solar-thermal heat was stored in the PCM composite, and the higher its bottom temperature in the absence of light. Evidently, the higher the height of NMC-PCM, the more slowly its bottom temperature decreased in the absence of light. As a control, we monitored the temperature variation of the top surface, middle and bottom of the evaporator with or without the phase change material at 1 kW m⁻². As presented in Fig. 5A and B, the temperature is plotted as a function of the irradiation time. Upon light irradiation, the top surface temperature of NMC-NWPS-2 and NMC-NWPS-PCM-2 quickly reached 55.1 and 46.6 °C, respectively. However, the middle and bottom temperatures of NMC-NWPS-PCM-2 were higher than those of NMC-NWPS-2. Expectedly, the temperature in various regions rapidly decreased over NMC-NWPS-2, while the temperature over NMC-NWPS-PCM-2 showed a slow downward trend after turning off the light. The above results demonstrated that the solar-thermal energy stored in the organic paraffin in NMC-NWPS-PCM-2 was gradually released, which enabled the solar-driven steam device to continue producing water to overcome solar radiation intermittency. Additionally, typical IR images of NMC-NWPS-PCM for different periods under 1 sun illumination are displayed in Fig. 5C. The top temperature of NMC-NWPS-PCM was 19.8, 32.0, 38.2, 42.0 and 45.6 °C at 0, 120, 500, 800 and 1400 s, respectively. With the light source turned off at 3600 s, the top temperature of NMC-NWPS-PCM was 32.5, 27.2, 23.8 and 22.0 °C at 3800, 4200, 4600 and 5000 s, respectively. To gain a better theoretical insight into the improved solar-thermal storage performance, the simulation of the surface temperature distribution of NMC-NWPS-PCM is shown in Fig. 5D, which indicated that the simulation result was consistent with the observation from infrared thermal imaging experimental results. The investigation above presents that the NMC could achieve effective photothermal conversion and that the PCM composites could effectively transfer and store solar heat downwards, ensuring continuous thermal energy supply, which provided an attractive method for continuous water evaporation in an individualized mode in the absence of solar illumination.

Fig. 4 (A–C) Time-dependent temperature variation at the top evaporation surface of wet state NMC-PCM-3 when the solar illumination turns on and off under different irradiations of (A) 1, (B) 2, and (C) 3 sun illumination, respectively. (D–F) Time-dependent temperature variation in the middle of wet state NMC-PCM-3 when the solar illumination turns off under different irradiations of (D) 1, (E) 2, and (F) 3 sun illumination, respectively. (G–I) Time-dependent temperature variation at the bottom of wet state NMC-PCM-3 when the solar illumination turns off under different irradiations of (G) 1, (H) 2, and (I) 3 sun illumination, respectively.

Fig. 5 Temperature response profiles of the dry state (A) NMC-NWPS-2 and (B) NMC-NWPS-PCM-2 when the solar illumination turns on and off under 1 sun illumination. (C) IR thermal images and (D) corresponding simulated top temperature distributions of the dry state NMC-PCM-2 when the solar illumination turns on and off under 1 sun illumination.

3.4. Water evaporation enthalpy and mechanism

The water evaporation in the dark process under ambient conditions was tracked to investigate the different water vaporization behaviors of bulk pure water and the NMC. As confirmed in Fig. 2B, the water evaporation rate in the dark of NMC-PCM-2 (0.45 kg m⁻² h⁻¹) was much higher than for pure water (0.08 kg m⁻² h⁻¹), suggesting that NMC-PCM-2 was capable of activating water and enhancing the vaporization of water effectively. We assumed that the rapid vaporization of water from NMC-PCM-2 arises from a lower energy consumption of water evaporation (i.e., vaporization enthalpy). To prove this, the water vaporization enthalpy under dark conditions was evaluated by spontaneous evaporation (Calculation S5, ESI†). The water loss of pure water, air-laid paper, and the NMC under dark conditions for 1 h was 273, 335 and 402 mg, respectively (Fig. 6A). The water molecules confined in the NMC exhibited a lower vaporization enthalpy (1653 J g⁻¹ for NMC-PCM-2) than that of blank (1984 J g⁻¹) and bulk pure water (2434 J g⁻¹). To further prove the change of water vaporization enthalpy, the heat flow signals of pure water and over the NMC characterized by DSC were investigated (Fig. 6B). Notably, the heat flow signal peak of the NMC was smaller than that of pure water. The above analysis result revealed that the NMC could effectively reduce the water evaporation enthalpy and improve the evaporation rate of the solar evaporator for practical solar-driven vapor generation applications. Based on the results above, the solar-steaming mechanism of the NMC was proposed (Fig. 6C). Impressively, the NMC features an interconnected and biomimetic hierarchical structure with broadband light absorption and excellent hydrophilicity for water supply, vapor escape and enhanced light capture, as well as significant reduction in the enthalpy of water evaporation to promote vaporization. Meanwhile, the hydrophilic groups of the NMC (e.g., –OH) formed hydrogen bonds with water molecules. According to the difference of intermolecular hydrogen bonding, there were two types of water molecules in the solar steam generation system of the NMC, including intermediate water (IW) and free water (FW). We analyzed the O–H stretching region in the Raman spectrum to reveal the water state in the NMC (Fig. S19a†). The –OH peaks of the adsorbed water molecules in the NMC could be classified into two types: the in-phase and out-of-phase –OH stretching vibrations of FW with two typical hydrogen bonds at 3233 and 3401 cm⁻¹, respectively. And the symmetric and asymmetric –OH stretching modes equipped with weak hydrogen bonds located at 3514 and 3630 cm⁻¹ indicated the presence of intermediate water. The IW formed hydrogen bonds with the bonded water, which diminished hydrogen bonds among intermediate water molecules and significantly reduced the energy supply for the liquid-to-vapor conversion process of water. Under such a circumstance, we assumed that some of the water molecules in the NMC evaporate in the form of water clusters and performed a model solar-driven evaporation experiment utilizing LiCl solution since Li⁺ was easily carried by water clusters during solar evaporation. The Li⁺ concentration in the condensed water using the NMC is 88.8 ppm, which was 40 times higher than that of pure water without evaporators (Fig. S19b†), demonstrating that the water vapor escaping from the NMC was in the form of water clusters.

Fig. 6 (A) Measured water evaporation mass change under dark conditions and the calculated equivalent water evaporation enthalpy values. (B) DSC curves of confined water in the NMC compared to bulk water. (C) Scheme illustrating the mechanism of solar steam generation of the NMC.

3.5. Applications in water treatment

In addition to rapid water evaporation, NMC-PCM can be applied as a functional solar evaporator to purify various types of waste water. To demonstrate the water purification performance of the NMC-PCM-based fresh water production system, the typical ion concentrations of real hot spring, snow water, lake water, pond water, seawater, and rain water before and after purification (Fig. S20–S24†) were measured with an inductively coupled plasma optical emission spectrometer (ICP-OES). The results showed that all ion concentrations in various water samples significantly decreased after solar-driven evaporation over NMC-PCM-3. Specifically, the concentrations of Ca²⁺, Na⁺, Mg²⁺ and K⁺ in actual hot spring water decreased from 410.41, 13.34, 310.55 and 5.67 to 0.06, 0.07, 0.20 and 0.04 ppm, respectively (Fig. 7A). The concentrations of Na⁺, Ca²⁺, Mg²⁺, K⁺ and B³⁺ in snow water were as low as 0.38, 0.11, 0.20, 0.03 and 0.02 ppm after solar-driving steaming, respectively (Fig. 7B). The ion concentrations of Na⁺, Ca²⁺, Mg²⁺, K⁺ and B³⁺ decreased from 27.75, 90.42, 81.21, 2.68 and 2.44 in lake water to 0.11, 0.28, 0.08, 0.01 and 0.01 ppm, respectively (Fig. 7C). The typical ion concentrations also greatly reduced for pond and rain water after purification. Fig. 7D shows that the ion concentrations of Na⁺, Ca²⁺, Mg²⁺, K⁺ and B³⁺ in actual seawater greatly decreased from the original concentrations of 21654.15, 1107.95, 9266.75, 812.60 and 20.13 to 0.70, 0.23, 1.18, and 0.02 ppm, respectively, which are much lower than the drinking water standards defined by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA). The corresponding ion rejection of hot spring water, snow water, lake water, pond water, rain water and seawater undergoing solar thermal-driven water purification was nearly 100% (Fig. S25†). The results above demonstrated that the NMC-PCM solar evaporator had high performance in wastewater purification and desalination. Additionally, we studied the treatment of dye-containing wastewater over the NMC-PCM solar evaporator by using methyl orange (MO), methylene blue (MB), and rhodamine B (RB) as model contaminants. As presented in Fig. 7E, the color of the MO, MB and RB solutions changed from orange, blue and pink to clear accompanied by the disappearance of absorption peaks of the solutions after solar evaporation from UV-vis-NIR spectra, indicating effective dye removal properties due to the solar-driven evaporation process. Moreover, we investigated the purification ability of NMC-PCM toward heavy metal-contaminated water. As illustrated in Fig. 7F, after the interfacial solar-thermal evaporation, the concentrations of Cu²⁺, Fe³⁺, Co²⁺, Pb²⁺, Zn²⁺, Cd²⁺ and Cr³⁺ decreased from 1000 ppm to 0.04, 0.08, 0.02, 0.10, 0.25, 0.15 and 0.02 ppm, respectively. The overall ion rejection of 99.9% suggested the high-efficiency purification properties of NMC-PCM for wastewater treatment (Fig. S26†). Especially, the vegetation compatibility of the condensed water was studied through a flexible solar desalination–cultivation system for wheat growing with soil (Fig. S27†). As displayed in Fig. S28,† the wheat irrigated by desalinated seawater through the solar desalination–cultivation system grew well in real time, demonstrating that the purified seawater was compatible with crop growth and could definitely satisfy the water demand for wheat growth. The stability of the NMC-PCM evaporator was evaluated through a water evaporation cycle test. The water evaporation performance of NMC-PCM-3 was stable over 15 cycles (1 h simulated solar illumination for each cycle and after the light is turned off.) of the interfacial solar-thermal evaporation process in the actual seawater (Fig. 7G and S29†). Similarly, the water evaporation performance of NMC-PCM-3 was investigated using water samples from different sources (i.e., hot spring water, snow water, lake water, pond water, rain water and multi-heavy metal ions solution), and NaCl solutions with different concentrations, demonstrating the excellent cycling stability and water treatment properties of the NMC-PCM-3 evaporator (Fig. 7H and S30†). Notably, the water vaporization rate achieved by NMC-PCM-3 at 1 sun illumination was obviously higher than that of other recently reported evaporators (Fig. 7I, S31 and Tables S2, S3†). In particular, the NMC-PCM-3 achieved a record evaporation rate of 1.42 kg m⁻² h⁻¹ after turning off the simulated sunlight source, and the evaporation rate after turning off the light source was rarely gaining attention. All these results demonstrated that the integrated solar-thermal evaporation system based on NMC-PCM exhibited remarkable potential for the practical purification of various water samples, offering an attractive design method for the next generation of photothermal materials.

Fig. 7 Water purification performance of NMC-PCM-3. Evaluation of evaporation performance with primary ions in (A) hot spring water, (B) melted snow, (C) lake water, pond water and rain water before and after solar distillation. (D) Evaluation of the desalination performance with primary ions in a seawater sample (from the South China Sea, China) before and after solar distillation. The green and pink lines refer to the WHO and EPA standards for drinkable water. (E) The capability of NMC-PCM-3 to remove water-soluble organic dyes. (F) The overall heavy metal removal performance of NMC-PCM-3 before and after solar distillation. (G) Evaporation rate cycle performance of NMC-PCM-3 under one sun illumination. (H) Evaporation rates of NMC-PCM-3 using various water sources. (I) Solar steam generation rate of various photothermal materials under 1 sun illumination.

4. Conclusions

In summary, we have demonstrated a NMC-PCM evaporator that could accelerate the solar driven vaporization process, store solar-thermal energy, and achieve a high water evaporation rate. Water molecules are confined as clusters and transported automatically within the nanochannels of the NMC, thereby reducing the water vaporization enthalpy. The NMC-PCM evaporator exhibits desirable properties for solar steam generation, including enhanced optical absorption, high light-to-heat performance, favorable steam escape channels and remarkable solar-thermal storage. As a result, the solar water evaporation rate of the NMC-PCM evaporator is determined to be as high as 2.55 kg m⁻² h⁻¹ under 1 sun illumination, with a record rate of 1.46 kg m⁻² h⁻¹ with the light source switched off, which is highly desirable for efficiently capturing solar energy and using it for sustainable solar evaporation. Notably, after solar thermal-driven water purification, high-quality clean water could be produced from various water samples. Such an integrated solar thermal energy storage-steam generator presented here not only provides an attractive design method to construct solar evaporators for continuous water evaporation, but also achieves the overall comprehensive solar utilization. This environmentally friendly strategy offers great potential for practical water treatment applications in various resource-constrained areas to alleviate the global water scarcity and energy issues.

Author contributions

Y. Xu: conceptualization, methodology, investigation, writing – original draft preparation, and writing – review & editing; Z. Guo: conceptualization; X. Wang: supervision; Z. Wang: methodology; M. Irshad and N. Arshad: data curation; J. Gong, H. Liu and G. Li: funding acquisition, project administration and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Key R&D Program of China (2022YFE0117000) and National Natural Science Foundation of China (22005109 and 52373099). The authors acknowledge all the funding for supporting this work. We also acknowledge the support of the Analytical and Testing Center of the Huazhong University of Science and Technology for the XRD, TEM, and SEM measurements.

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Footnote

† Electronic supplementary information (ESI) available: The analysis of heat transfer processes and water evaporation enthalpy; digital photographs and SEM images of the air-laid paper and NMC; XRD patterns of the standard peaks for Ni-CAT-1 MOF crystals and the Ni-CAT MOF; XPS survey, HRTEM images, EDX elemental mapping results, and FTIR and Raman spectra of the NMC; AFM scans of the NMC; transmittance and reflectance spectra of the wet/dry state air-laid paper and NMC; TGA of the NMC from 30 to 600 °C in a N₂ atmosphere; schematic of various patterning models; digital photographs of the NMC-PCM evaporator for efficient solar vapor generation; time-dependent mass change of NMC-PCM-1/2/3 under 3 sun illumination and its water evaporation rate after turning off the light source; temperature response profiles of NMC-PCM-1/2 when the solar illumination turns on and off under different irradiation; energy balance and heat loss diagram of the NMC-PCM-3 evaporator; temperature response profiles of dry state NMC-NWPS-2 and NMC-NWPS-PCM-2; IR thermal images and corresponding simulated top temperature distributions of dry state NMC-PCM-2; Raman spectrum of the wet-state NMC; concentration of Li⁺ in the condensed water of NCF and without evaporators; photographs of the different water samples and the condensed water; the ion rejection of different water samples after purification; the experimental set-up for the desalination–cultivation system; digital photographs of wheat seeds grown at different times using desalinated seawater; evaporation rates of NMC-PCM-3 in NaCl solution with different concentrations; the specific surface area of air-laid paper and the NMC; the comparison of the water evaporation rate using various solar evaporators under one sun illumination. See DOI: https://doi.org/10.1039/d4ta00203b

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