A self-healing, deformation-resistant MXene double-network hydrogel for stable solar-driven interfacial evaporation

Abstract

Solar-driven interfacial evaporation (SDIE) is an eco-friendly and sustainable technology for desalinating water. Hydrogel-based composite evaporators are effective in resisting salt, but often suffer from reduced shape stability and diminished water transport capability during dehydration and rehydration cycles, which undermines their long-term performance in photo-evaporators. Here, we developed a long-term stable MXene-carrageenan/polyacrylamide (MXene-CA/PAAm) hydrogel evaporator. This device integrates an MXene film for efficient photothermal conversion with a double-crosslinked, high-strength, deformation-resistant, self-healing hydrogel for water supply. A dehydration/rehydration cycling test was developed to evaluate the shape stability and water absorption capacity of hydrogel-based evaporators. The MXene-CA/PAAm hydrogel evaporator operated continuously for 360 hours without performance decrease, achieving an average evaporation rate of 1.78 kg m² h⁻¹ under 1 sun illumination. This study presents a novel approach to creating double-network hydrogel evaporators, enhancing stability and durability, and advancing desalination technology.

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1 Introduction

With rapid global population growth and industrialization, water scarcity has emerged as a critical challenge, prompting the need for efficient seawater/waste-water purification technologies.^1–3 Solar-driven interfacial evaporation (SDIE) has emerged as a promising solution.^4–10 However, SDIE systems are prone to mechanical deformations, which can lead to significant performance degradation or failure.^11–13 Therefore, ensuring the chemical and structural stability of solar evaporators is essential for maintaining their long-term functionality.

Titanium carbide (Ti₃C₂T_x, T = –F, –O, and –OH), a two-dimensional (2D) MXene material, has attracted considerable attention due to its extensive light absorption, excellent photothermal effect, and hydrophilic two-dimensional lamellar-structure facilitating water molecule transport.^14–19 With a photothermal conversion efficiency as high as 99%, MXene membranes demonstrate potential as photothermal conversion materials for solar desalination.²⁰ Hydrogels are functional soft materials characterized by a three-dimensional network structure formed by chemical covalent bonds or physical forces between polymer chains.^21–24 Traditional polymer hydrogels, however, are brittle due to their pure elastic behaviour (lacking energy dissipation mechanisms) and heterogeneous network structures.²⁵ To enhance the mechanical properties of hydrogels, researchers have proposed various synthesis methods such as amphiphilic polymer hydrogels,^26,27 polymer microsphere composite hydrogels,²⁸ nanocomposite hydrogels,²⁹ and double network (DN) hydrogels.³⁰

In recent years, DN hydrogels have attracted attention for their unique network structures and excellent mechanical strength and toughness.^31–33 Indriyati et al. introduces an approach using a hydrogel film based on poly(vinyl alcohol) (PVA) integrated with carbon dots (CDs) to leverage their photothermal effect, which demonstrates remarkable stability, maintaining consistent evaporation rates over several cycles, thus indicating its long-term durability and potential for reuse.³⁴ Zhang et al. synthesized ferric tannate (TA–Fe³⁺)/sodium alginate (SA–Fe³⁺)/polyacrylamide (TFSFP) hydrogels and constructed a solar water evaporator (SWE). Along with a ion cross-linked SA–Fe³⁺ network, a porous hydrogel with a hydrophilic water transportation network maintained continuous water supply to an evaporation interface.³⁵ Although hydrogel-based composite evaporators are effective for SDIE, they often crack and bend during dehydration and rehydration cycles, compromising long-term stability. Moreover, their shape retention and water transport efficiency during photothermal evaporation have yet to be thoroughly investigated.³⁶

Here, we report a MXene-carrageenan/polyacrylamide hydrogel (MXene-CA/PAAm) solar-driven interface evaporator for desalination, employing MXene membranes as the photothermal layer and carrageenan/polyacrylamide (CA/PAAm) double-network (DN) hydrogels with self-healing as the water supply layer. The Ti₃C₂T_x photothermal layer achieves 99% photothermal conversion efficiency, guaranteeing efficient water evaporation. The DN-CA/PAAm hydrogel demonstrates exceptional deformation resilience, crack resistance, and self-healing capabilities, enabling sustained photothermal evaporation. It features strong mechanical properties, with a tensile strain of 718% and compressive strength of 478 kPa, while maintaining its shape and water transport capacity after 10 dehydration/rehydration cycles. The MXene-CA/PAAm hydrogel (MCPH) evaporator demonstrated continuous operation for 360 hours without bending or cracking, achieving an average evaporation rate of 1.78 kg m⁻² h⁻¹ under 1 sun illumination, sustaining high steam generation rates even after prolonged simulated seawater desalination. This work focuses on the structural integrity and water transport stability of hydrogel-based photothermal evaporators, providing a durable and reliable solution for solar-driven interfacial desalination.

2 Experimental section

2.1 Materials

κ-Carrageenan (CA), N,N′-methylenebis(acrylamide) (MBA), lithium fluoride (LiF), and sodium chloride (NaCl) were provided by Shanghai Aladdin Reagent Co., Ltd; acrylamide (AAm), potassium chloride (KCl), and 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO) were provided by Shanghai Yien Chemical Technology Co., Ltd; Ti₃AlC₂ (MAX) powder was provided by Jilin Province Yiyi Technology Co., Ltd; hydrochloric acid (HCl) was provided by Tianjin Fuchen Chemical Reagent Technology Co., Ltd; the polyvinylidene fluoride membrane (PVDF, pore size 0.45 μm) was provided by Tianjin Jinteng Experimental Equipment Co., Ltd; ethanol was provided by Tianjin Fuyu Fine Chemical Co., Ltd.

2.2 Preparation of the photothermal evaporator

2.2.1 Preparation of the MXene photothermal layer. 30 mL of hydrochloric acid was slowly added to 10 mL of deionized water in an ice bath. Next, 1.6 g of LiF was added and stirred at 1000 rpm for 1 min, followed by 1 g of Ti₃AlC₂, which was stirred for another minute. The mixture was then heated in a 45 °C oil bath for 36 hours. After centrifuging with deionized water to achieve neutral pH, the nanosheet-containing supernatant was collected, freeze-dried, and stored. A photothermal layer was formed on a PVDF membrane by vacuum filtration of 10 mg of the prepared MXene monolayer.^37–40

2.2.2 Preparation of κ-carrageenan/PAAm DN hydrogels. The following compounds were added to deionized water and stirred at 90 °C for one hour: CA, KCl, AAm, MBA, and TPO as a UV initiator, maintaining a combined concentration of CA and AAm at 18 wt%. The solution was poured into a mold and cooled at 4 °C for 30 min to form the initial CA network, which was then exposed to UV light for photopolymerization to create CA/PAAm double-network hydrogels, extending the work of Liu et al. on DN-CA/PAAm hydrogels.⁴¹

2.2.3 Assembly of the evaporator. A dual-network hydrogel (MCPH) evaporator was synthesized by first placing a MXene photothermal layer onto a single-network hydrogel, followed by photocrosslinking. This process intertwined the polymer crosslinking chains within the PVDF pores, resulting in a stable, bonded double-network.

2.3 Characterization

Prior to observing the morphology of the hydrogel and MXene film using a scanning electron microscope (SEM, S-4800N, HITACHI), both samples underwent a 72 hour freeze-drying process. The chemical structure of the hydrogel samples was analyzed using Fourier-transform infrared spectroscopy (FT-IR, Nicolet iS20, Thermo Scientific). X-ray photoelectron spectroscopy (XPS, KAlpha, Thermo Scientific) was utilized to examine the chemical binding states of elements C, O, and K in the DN hydrogel. Raman spectroscopy was employed to analyse the hydrogel, utilising a laser micro-Raman spectrometer (Raman, LabRam HR Evolution, HORIBA) with a 633 nm laser. The MXene film was subjected to analysis of its absorbance in the ultraviolet-visible-near infrared spectrum (UV-vis-NIR, PE Lambda 750, PerkinElmer) across a range of wavelengths between 200 and 2500 nm. The crystal structure of the MXene film was analyzed using a SmartLab SE X-ray diffractometer (XRD), manufactured by Rigaku GmbH in Germany.

The contact angles of the hydrogel and MXene film were determined using a contact angle measuring device (JC2000D1, Powereach). A rotational rheometer (DHR, TA Instruments, USA) was employed to record the storage modulus (G′) and loss modulus (G′′) to evaluate the viscoelasticity of the DN hydrogel. The mechanical properties of the hydrogel were characterized using an electronic universal testing machine (T-30, CAFMAN) equipped with a 100 N load sensor, conducted at room temperature.

2.4 Evaporation assessment

A solar thermal evaporation device was assembled by suspending the MCPH evaporator in simulated seawater to evaluate the hydrogel's performance. A xenon lamp (CEL-PF300-T8) provided illumination, and experiments were carried out using a range of solutions: deionized water, 3.5 wt% simulated seawater, and high-salinity waters (10.0, 15.0, and 20.0 wt%). A precision electronic balance (0.01 g) was used to track real-time mass changes, while an infrared thermal imager (ZC11AFP210W0272) captured surface temperatures and thermal images.

Evaporation rate (v, kg m⁻² h⁻¹) and solar conversion efficiency (η, %) are calculated by eqn (1) and (2) respectively.

where Δm (kg) is the mass of water evaporation in 1 h, S is the area of the cross-section of the evaporator (m²), t (h) is the time of solar irradiation, v (kg m⁻² h⁻¹) is the evaporation rate after subtracting the evaporation rate in the dark, H_LV is the water evaporation enthalpy, C_opt is the irradiation intensity of the incident light source, and P_i (1000 W) is the energy density of the incident light source.⁴²

3 Results and discussion

3.1 Preparation and characterization of the MXene photothermal layer

The MCPH evaporator employs a dual-layer configuration, comprising a MXene membrane as the solar thermal layer and a DN-CA/PAAm hydrogel as the water supply layer. The MXene solar thermal layer was prepared by acid etching of the layered Ti₃AlC₂ (MAX) phase and vacuum-induced self-assembly (Scheme 1). The layered Ti₃AlC₂ precursor (Fig. 1a) was etched selectively with a LiF–HCl mixture to remove aluminum elements (Fig. 1b).^43,44 As shown in Fig. 1c, the X-ray diffraction disappearance of Ti₃AlC₂ at 38.8° (104), and the (002) peak shifted from 9.5° to 6.3°, indicating successful etching of Al.⁴⁵ Subsequently, MXene was dispersed in ethanol and subjected to ultrasound treatment to obtain dispersed sheet-like MXene, as characterized by scanning electron microscopy (SEM) (Fig. 1d). Finally, MXene nanosheets were self-assembled into MXene membranes through vacuum filtration (Fig. 1e and S1†). The Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis confirmed the Ti₃C₂T_x composition of the MXene membrane (Fig. S2†).

Scheme 1 Preparation of the MXene CA/PAAm double-network hydrogel evaporator for stable and efficient photothermal desalination.

Fig. 1 (a) SEM image of Ti₃AlC₂ (b) SEM image of MXene (an accordion-like structure). (c) XRD patterns of MXene and Ti₃AlC₂. (d) SEM image of MXene nanosheets. (e) Cross-section SEM image of the MXene membrane. (f) UV-vis-NIR absorption spectrum of MXene and atmospheric mass 1.5 global (AM 1.5 G) tilted solar spectrum.

The two-dimensional layered structure of the MXene film allows for the unimpeded entry of sunlight, which is then reflected multiple times between the MXene nanosheets.³⁷ This results in the effective absorption of a broad spectrum of sunlight.⁴⁶ Furthermore, the PVDF insulation layer impedes the transfer of heat to the substrate, thereby reducing heat loss. As demonstrated by UV-vis-NIR diffuse reflectance spectroscopy, MXene membranes exhibit a solar energy absorption efficiency exceeding 94% within the wavelength range of 250 to 2500 nm (Fig. 1f). The infrared image (Fig. S3†) shows that the MXene membrane temperature increased from 28.4 °C to 49.1 °C after 30 min of one-sun exposure, demonstrating efficient light absorption.⁴⁷

3.2 Preparation and characterization of the DN-CA/PAAm hydrogel

The CA/PAAm dual-network hydrogel was prepared by a sol–gel process and light-initiated polymerization. The CA, AAm, MBA, and TPO were dissolved, homogenized, then subjected to heating and cooling. UV irradiation then induced the polymerization of PAAm, forming a second chemically crosslinked network (SN-PAAm). This process produced DN hydrogels (DN-CA/PAAm) with interpenetrating CA and PAAm networks (Scheme 1). Photographs comparing hydrogel samples with single and double networks are shown in Fig. S4†. SEM analysis (Fig. S5†) characterizes the network structure of SN-CA and SN-PAAm. Fig. 2a shows a porous structure of DN-CA/PAAm, indicating that DN-CA/PAAm substrates can effectively absorb and transport water to the MXene layer.

Fig. 2 (a) Cross-sectional SEM image of the DN hydrogel. (b) FTIR spectra and (c) XPS spectra of SN-CA, SN-PAAm and DN-CA/PAAm hydrogels. (d) Raman spectrum of the DN-CA/PAAm hydrogel. (e) WCA of the DN-CA/PAAm hydrogel and MXene membrane. (f) Saturated water content of DN-CA/PAAm hydrogels in DI water and 3.5 wt% NaCl solution.

Fig. 2b illustrates the FTIR spectra of SN-CA, SN-PAAm and DN-CA/PAAm hydrogels. The peaks of the SN-CA hydrogel are located at 3411, 2921, 1069, and 1647 cm⁻¹, attributed to the stretching vibrations of O–H, C–H, C–O and –COO– stretching vibrations, respectively. The symmetric stretching vibration of the O–H group of CA at 3411 cm⁻¹ was observed in the DN-CA/PAAm hydrogel, yet no new peaks emerged, indicating that hydrogen bonding was established between the O–H group of the SN-CA chain and the N–H group of the SN-PAAm chain. The FTIR results substantiate the formation of an interpenetrating double network comprising CA and PAAm. The surface elemental composition and chemical bonding states of the hydrogels were evaluated by XPS. As illustrated in Fig. 2c, the XPS spectrum exhibited distinctive peaks for C 1s, N 1s, O 1s, and K 2p_3/2. The intensity of the peaks around 290 eV is greater in DN-CA/PAAm hydrogels than in SN-CA and SN-PAAm hydrogels, which provides preliminary evidence of the successful synthesis. In addition, the oxygen-containing groups promote the water-absorption of the hydrogels (Fig. S6†).

Fig. 2d shows the Raman spectra of the DN-CA/PAAm hydrogel, with peaks at 3215–3316 cm⁻¹ and 3417–3520 cm⁻¹ corresponding to strong (FW) and weak (IW) hydrogen bonds, respectively (Fig. S7a and b†).⁴⁸ The degree of DN-CA/PAAm hydrogen bonding in hydrogel samples can be quantified by measuring the peak intensity ratio of I₃₂₃₀/I₃₄₂₀. The reduction in I₃₂₃₀/I₃₄₂₀ in DN hydrogels relative to SN-PAAm hydrogels indicates that DN-CA/PAAm may disrupt the robust hydrogen bonds between water molecules. It can be postulated that the hydration of K⁺ may promote the conversion of free water to bound water, thus counteracting the freezing effect.⁴⁸ The increase in I₃₂₃₀/I₃₄₂₀ in DN hydrogels in comparison to SN-CA hydrogels indicates that the incorporation of PAAm may augment the quantity of IW within the hydrogels, thereby providing a greater supply of activated water for evaporation. Consequently, the effective water delivery network of the DN-CA/PAAm hydrogel can reliably meet the evaporated bulk water delivery.

Additionally, the water contact angle (WCA) of the DN hydrogel was 10.2°, while that of the MXene membrane was 75.1° (Fig. 2e). The smaller WCA indicates that the DN-CA/PAAm hydrogel exhibits excellent hydrophilic properties. Given that the WCA of the top MXene photothermal layer was larger than that of the bottom DN-CA/PAAm hydrogel, and considering the porous structure of the hydrogel (Fig. 2a), the CPDH evaporator was able to efficiently direct the water flow from the bulk water stream to the evaporator surface. Furthermore, the saturated water content of the DN-CA/PAAm hydrogel was evaluated (Fig. 2f), which demonstrated a notable capacity for water absorption in simulated seawater relative to the SN hydrogels (Fig. S7c and d†). Consequently, the MCPH evaporator for simulated seawater desalination exhibits the capacity to absorb water rapidly, which facilitates efficient photothermal evaporation and ensures timely water replenishment.

The DN-CA/PAAm hydrogels display remarkable deformability, tensile strength, and mechanical resilience, attributable to their soft and elastic double-network.⁴⁹ A comparison of the stress–strain curves of hydrogels with different monomer ratios revealed that the hydrogel with CA/PAAm mass ratio of 1 : 8 exhibited the highest tensile and compressive strengths, which align with the requirements of a high-strength hydrogel evaporator (Fig. S8†). Accordingly, the 1 : 8 mass ratio hydrogel was selected for the photothermal evaporation experiment. The stress–strain curves of DN-CA/PAAm hydrogels in the tensile state are illustrated in Fig. 3a. The stress and strain at break for the DN-CA/PAAm hydrogel were 132.6 kPa and 718.4%, respectively, whereas those for SN-CA hydrogels were 5.1 kPa and 16.4%, and for SN-PAAm hydrogels were 8.0 kPa and 270.5%. The mechanical properties of DN hydrogels showed a significant improvement over the single network hydrogels. Fig. 3b shows that the stress of the DN-CA/PAAm hydrogel reached 478.04 kPa at 80.31% strain, which is four times higher than that of the SN-CA hydrogel and 9.6 times higher than that of the SN-PAAm hydrogel.

Fig. 3 (a) Tensile strength and (b) compressive strength of SN-CA, SN-PAAm and DN-CA/PAAm hydrogels. (c) Rheological properties of the DN-CA/PAAm hydrogel. (d) Image illustrating the self-healing behavior of DN-CA/PAAm hydrogels. (e) Image of DN-CA/PAAm hydrogels supporting a 50 g weight after self-healing. (f) Tensile strength of DN-CA/PAAm after self-healing for 10, 20, 30, and 40 min, respectively.

Rheological analysis of the DN-CA/PAAm hydrogel (Fig. 3c) shows that the storage modulus (G′) exceeds the loss modulus (G′′) at ambient temperature. G′ remains consistently higher than G′′ across the entire frequency range, indicating dominant elasticity over viscosity and suggesting substantial cross-linking within the system. The higher G′ values correlate with stronger mechanical properties, which result from the synergistic interaction between the two networks.⁵⁰

The self-healing ability of DN-CA/PAAm hydrogels was investigated. As shown in Fig. 3d, the gel strips were cut into five segments and pressed end-to-end for 12 hours, and then the gels showed self-healing behavior. The healed DN-CA/PAAm hydrogels were able to withstand a load of 50 g (Fig. 3e). Fig. 3f shows the tensile strength increased with healing time, reaching a maximum at 30 minutes. After this point, further healing led to increased gel deformation, but no significant increase in strength. These results highlight the excellent self-healing properties of DN-CA/PAAm hydrogels.⁴¹

Hydrogels are commonly used as water transport layers in photothermal evaporators, where they absorb and transport water while resisting salinity.³⁷ However, repeated cycles of water absorption and loss can lead to physical degradation and reduced water transport capacity, ultimately causing evaporator failure.²¹ We designed a dehydration–rehydration cycling test to assess the shape retention and water transport efficiency of hydrogel-based evaporators. The hydrogels were dried at 60 °C for 12 hours, then immersed in water for 12 hours to swell (Fig. 4a). This dehydration/rehydration cycle was repeated multiple times.

Fig. 4 (a) Schematic illustration of the dehydration/rehydration cycling test. (b) Images of SN-CA, SN-PAAm, and DN-CA/PAAm hydrogels after dehydration/rehydration cycles, (c) mass and (d) relative water content of SN-CA, SN-PAAm and DN-CA/PAAm hydrogels. (e) Water absorption capability of DN-CA/PAAm hydrogels during 10 dehydration/rehydration cycles.

Fig. 4b shows that the SN-CA hydrogel deformed after one cycle, and the SN-PAAm hydrogel deformed after three cycles and failed to recover its shape. In contrast, the DN hydrogel withstood three cycles without significant deformation or cracking, demonstrating superior stability. After three cycles, the DN-CA/PAAm hydrogel retained 82% of its initial mass, indicating sustained water absorption and transport (Fig. 4c). In comparison, the mass of SN-CA and SN-PAAm hydrogels decreased to 72% and 45%, respectively, reflecting a significant loss in water absorption capacity. In order to visualize the change of the water content of the hydrogels, we define a new concept called relative water content, which is obtained by normalizing the water content (Fig. S9†). The relative water content of SN-CA and SN-PAAm hydrogels decreased to 56% and 44%, respectively, while the DN-CA/PAAm hydrogel maintained 80% of its original water content (Fig. 4d). Similar results were observed in simulated seawater (Fig. S10†), where SN hydrogels failed after three cycles, whereas DN-CA/PAAm hydrogels preserved both structure and water content.

We extended the dehydration/rehydration tests to 10 cycles for the DN-CA/PAAm hydrogel. Notably, after 10 cycles, the hydrogel maintained its shape and retained 66% of its water absorption capacity, demonstrating exceptional structural stability and water absorption ability (Fig. 4e and S11†), even in both DI water and simulated seawater, highlighting its potential for practical applications.

3.3 Photothermal evaporation performance of DN hydrogel evaporators

The photothermal evaporation of the MCPH evaporator was evaluated under simulated sunlight. The device is illustrated in Fig. S12†, and the evaporation rate was determined by monitoring the mass change of water feed. Fig. 5a shows that the surface temperature of the MCPH evaporator increased rapidly, reaching 54.2 °C after 30 min, as monitored by infrared imaging (Fig. S13†). Fig. S14† illustrates the process of changing the surface temperature of the MXene-CA hydrogel (MCH), MXene-PAAm hydrogel (MPH), and MCPH evaporators.

Fig. 5 (a) Infrared thermal image of the MCPH evaporator under 1 sun for 0–60 min. (b) Mass change and (c) evaporation rate of water for the MCPH evaporator under 1 sun in 0, 3.5, 10, 15 and 20 wt% NaCl conditions. (d) Mass change of MCH, MPH and MCPH hydrogel evaporators in simulated seawater. (e) Mass change and (f) evaporation rate for the MCPH evaporator under 1, 2 and 3 suns in simulated seawater.

The MCPH evaporator can facilitate long-term, continuous desalination of simulated seawater, exhibiting consistent evaporation performance across varying salinity levels (0–20 wt%). Fig. 5b and c illustrate the time-dependent mass and evaporation rates of the MCPH evaporator in salt solutions of varying concentrations over a 60 min period under 1 sun. The evaporation rate of the MCPH evaporator was as high as 1.81 kg m⁻² h⁻¹ in deionized water, 1.76 kg m⁻² h⁻¹ in simulated seawater (3.5 wt%), and not less than 1.25 kg m⁻² h⁻¹ in highly concentrated salt solutions (10–20 wt%), and photothermal evaporation efficiency of up to 91.2% under 1 sun. Compared to the MCH and MPH evaporators, the MCPH showed a significantly higher evaporation rate during the course of the experiment (Fig. 5d).

The MCPH evaporator demonstrated effective evaporation in simulated seawater (3.5 wt%) under varying light intensities (Fig. 5e and f). The evaporation rates were 2.20 kg m⁻² h⁻¹ and 2.70 kg m⁻² h⁻¹ under 2 and 3 suns, respectively, with corresponding photothermal conversion efficiencies of 72.3% and 79.7%. Moreover, the light-to-heat evaporation efficiency of the MCPH evaporator in 3.5 wt% saltwater is superior to some of the previous studies (Table S1†).

To evaluate the stability of the MCPH evaporator during continuous evaporation, hydrogel evaporators were subjected to 360 hour photothermal tests (Fig. 6 and S15†). As shown in Fig. 6a, the MPH evaporator underwent significant deformation, leading to a reduction in the light-absorbing surface area, while the MCH and MCPH evaporators maintained their structural integrity. Images taken at 72 hour intervals provide clear visual evidence of the sustained functionality and stability of the MCH and MCPH evaporators throughout the test. To quantify the results, we tracked changes in the curving angle of the evaporator to assess gel deformation (Fig. S16†), and monitored changes in mass to evaluate the gel layer's water absorption capacity. As illustrated in Fig. 6b, the MPH evaporator exhibits the poorest structural stability. Furthermore, the mass of the MPH evaporator increases with the duration of the evaporation process, which may be attributed to salt clogging (Fig. S17†). Fig. 6c demonstrates that the MCPH evaporator exhibited long-term stability, with an average evaporation rate of 1.78 kg m⁻² h⁻¹, outperforming both the MPH and MCH evaporators. The MPH evaporator suffered from structural bending, which reduced photothermal efficiency. To further assess stability, the SN-CA hydrogels were immersed in 60 °C water for 120 min, with no mass loss observed (Fig. S18†).

Fig. 6 (a) Photographs of MCH, MPH and MCPH evaporators after long-term evaporation in simulated seawater, (b) changes in curving angle and M/M₀ of MCH, MPH and MCPH evaporators during evaporation, and (c) evaporation rates of MCH, MPH and MCPH in simulated seawater under 1 sun for 360 h.

Additional seven-day evaporation tests were performed using the MCPH evaporator in seawater from the Bohai Sea. Fig. 7a shows that the average evaporation rate from the MCPH evaporator was 1.53 kg m⁻² h⁻¹ under 1 sun (1000 W) for 8 hours per day. Fig. 7b illustrates the outdoor evaporation system. Furthermore, 10 hour outdoor evaporation experiments under natural sunlight were conducted (Fig. 7c), and the average evaporation rate was up to 0.88 kg m⁻² h⁻¹. Under real sunlight, the photothermal evaporation rate is lower than under simulated sunlight in the laboratory, due to the difference in light angle and the temperature of surroundings. These results highlight the continuous photothermal performance and robust mechanical properties of the MCPH evaporator, demonstrating its potential for sustained desalination over extended periods.

Fig. 7 (a) Evaporation rates and humidity of the MCPH evaporator in Bohai Sea seawater over 8 hours under 1 sun per day (b) photograph of an outdoor evaporation unit. (c) 10 hour outdoor evaporation rate under natural sunlight for the MCPH evaporator in Bohai Sea seawater. (d) Concentration of four major ions before and after desalination.

The MCPH evaporator demonstrates exceptional desalination performance in the Bohai Sea seawater, with Na⁺ concentration reduced by four orders of magnitude, well below the World Health Organization's drinking water standard (Fig. 7d). Additionally, concentrations of K⁺, Ca²⁺, and Mg²⁺ were also reduced by four orders of magnitude after photothermal evaporation. These results indicate that the MCPH evaporator effectively desalts ions of seawater, meeting drinking water quality standards.

4 Conclusions

In conclusion, we developed a highly stable MXene-carrageenan/polyacrylamide hydrogel (MCPH) evaporator. The MCPH evaporator achieved 91.2% photothermal evaporation efficiency. The CA/PAAm dual-network hydrogel demonstrated good mechanical properties, self-healing ability, and retained its shape and water adsorption ability through 10 dehydration/rehydration cycles. The MCPH evaporator operated continuously for 360 hours without efficiency decrease, with an average evaporation rate of 1.78 kg m⁻² h⁻¹ under 1 sun illumination. This work presents an effective approach for durable and stable solar-driven interfacial desalination.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Ruiqi Zhao: investigation, data curation, formal analysis, validation, visualization, writing – original draft. Xushuai Chen: investigation, software. Panpan Zhang: visualization. Xi Chen: conceptualization, writing – review & editing. Chunjia Luo: conceptualization, methodology. Pengfei Zhang: resources. Min Chao: methodology, resources. Luke Yan: funding acquisition, project administration, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Innovation Capability Support Program of Shaanxi (2023-CX-TD-43), Key Research and Development Program of Shaanxi (2023-YBSF-498 and 2024GX-YBXM-412), and Fundamental Research Funds for the Central Universities, CHD (300102313208, 300102314105, and 300102314401).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta08803d

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