Modulation of the surface humidity with dual convection structure towards highly efficient and stable solar desalination

Modulation of the surface humidity with dual convection structure towards highly efficient and stable solar desalination | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 30 October 2025 V1 Latest version Share on Modulation of the surface humidity with dual convection structure towards highly efficient and stable solar desalination Authors : Yawen Wang , Caicai Li 0000-0002-8480-8829 [email protected] , Weijia Guo , Shunyu Shen , Gaojie Jiao , Dan Sun , Xizheng Liu , Xianyun Peng , Qingfeng Sun 0000-0003-3520-4970 , and Huiqiao Li 0000-0001-8114-2542 Authors Info & Affiliations https://doi.org/10.22541/au.176178443.32158344/v1 179 views 111 downloads Contents Abstract Abstract 1 Introduction 3 Conclusion 4 Experimental Section Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Solar-driven interfacial evaporation has emerged as a promising sustainable technology to address global water scarcity. Notwithstanding significant advancements in photothermal materials, substrates, and their combinations, these strategies to enhance evaporation efficiency seem to have reached a bottleneck for the oversight of the important factor of thermal convection on the performance of the evaporator. Herein, a wood-based evaporator featuring a dual convection structure was developed by creating a central cavity in a wood block and integrating MXene photothermal materials. Specifically, without the need for special materials or complex structural designs, the evaporator achieves an outstanding evaporation rate of 2.16 kg·m⁻2·h-1 with a photothermal conversion efficiency of 118%. Simulation results indicated that the engineered dual-convection structure effectively modulates the heat distribution and reduces the humidity above the evaporation surface, thereby increasing the evaporation rate and the overall photothermal conversion efficiency. Additionally, the device demonstrates excellent salt resistance and long-term operational stability even under extreme pH conditions (e.g., strong acid (pH=2) and strong alkali (pH=12) environments). This study not only provides new insights into the design of efficient, structurally simple solar interfacial evaporators but also expands the potential applications of wood-based materials in solar-driven evaporation systems. Modulation of the surface humidity with dual convection structure towards highly efficient and stable solar desalination Yawen Wang 1 , Caicai Li 1, 2* , Weijia Guo 1 , Shunyu Shen 2 , Gaojie Jiao 1 , Dan Sun 1 , Xizheng Liu 2 , Xianyun Peng 3* , Qingfeng Sun 1* and Huiqiao Li 2, 4* Wang 1 , Prof. C. Li 1, 2 , W. Guo 1 , Prof. G. Jiao 1 , D. Sun 1 , Prof. Q. Sun 1 1 College of Chemistry and Materials Engineering, Zhejiang A&F University, Hangzhou 311300, China. 2 Key Laboratory of Flexible Optoelectronic Materials and Technology (Ministry of Education), School of Optoelectronic Materials & Technology, Jianghan University, Wuhan 430056, China Email: [email protected] Email: [email protected] Prof. X. Peng 3 3 Institute of Zhejiang University-Quzhou, Quzhou, 324000, PR China. Email: [email protected] Shen 2 , Prof. X. Liu 2 , Prof. H. Li 2, 4 2 Key Laboratory of Flexible Optoelectronic Materials and Technology (Ministry of Education), School of Optoelectronic Materials & Technology, Jianghan University, Wuhan 430056, China 4 State Key Laboratory of Material Processing and Die and Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430056, China Email: [email protected] Keywords Wood, MXene, Dual convection structure, Surface humidity modulation, Solar-driven interfacial evaporation Abstract Solar-driven interfacial evaporation has emerged as a promising sustainable technology to address global water scarcity. Notwithstanding significant advancements in photothermal materials, substrates, and their combinations, these strategies to enhance evaporation efficiency seem to have reached a bottleneck for the oversight of the important factor of thermal convection on the performance of the evaporator. Herein, a wood-based evaporator featuring a dual convection structure was developed by creating a central cavity in a wood block and integrating MXene photothermal materials. Specifically, without the need for special materials or complex structural designs, the evaporator achieves an outstanding evaporation rate of 2.16 kg·m⁻ 2 ·h -1 with a photothermal conversion efficiency of 118%. Simulation results indicated that the engineered dual-convection structure effectively modulates the heat distribution and reduces the humidity above the evaporation surface, thereby increasing the evaporation rate and the overall photothermal conversion efficiency. Additionally, the device demonstrates excellent salt resistance and long-term operational stability even under extreme pH conditions (e.g., strong acid (pH=2) and strong alkali (pH=12) environments). This study not only provides new insights into the design of efficient, structurally simple solar interfacial evaporators but also expands the potential applications of wood-based materials in solar-driven evaporation systems. 1 Introduction The growing global freshwater scarcity crisis has intensified the demand for efficient and sustainable desalination technologies. [1–4] Recently, solar-driven interfacial evaporation has emerged as a highly promising solution, drawing considerable attention for its ability to produce fresh water with high evaporation efficiency at an affordable cost while circumventing the need for fossil energy consumption and intricate installations. [5,6] Recently, timber has provided a platform for significant advances in wood-based solar evaporators for its inherent thermal insulation properties and the layered pore structure, which facilitate rapid moisture transport ( Figure 1 a). However, significant challenges remain. Although carbonization can enhance the photothermal conversion performance to some extent, and the unique advantages of wood, namely its oriented and continuous vertical microchannels and pitted structure, are conducive to water transport, the significant heat loss of such evaporators, limits the further improvement of their evaporation performance (Figure 1b). [7–9] To enhance the evaporation performance, a direct and effective approach is to load photothermal materials onto the wood substrate surface. [10,11] For instance, two-dimensional MXene materials, owing to their unique surface plasmon resonance effect and broadband light-absorbing capability exhibits exceptional photothermal conversion efficiency. [12,13] Simultaneously, the low thermal conductivity of wood helps reduce heat exchange between the photothermal layer and the water, thereby optimizing thermal management and improving evaporation efficiency. Despite significant progress in enhancing the performance of wood-based evaporators, these endeavors that focus solely on advancements in photothermal materials or substrates seem to have reached a bottleneck and achieved further breakthroughs in photothermal performance remain a formidable challenge. [14,15] This limitation arises because, during the evaporation process, thermal convection must be carefully considered alongside light absorption and water transport. In fact, in the photothermal evaporation process, generated steam tends to accumulate at the interface between the photothermal layer and the surrounding air, [16–19] which not only hampers heat transfer at the interface but also increases the humidity at the evaporator surface, thereby reducing the evaporation rate and photothermal conversion efficiency. [20,21] Although the temperature field on the evaporation surface creates a single convection flow above the interface to promote gas escape, this convection forms slowly. This results in higher humidity at the interface edges, obstructing heat transfer and lowering evaporation rates (Figure 1c). [22] Nonetheless, the influence of thermal convection during the evaporation process on the evaporation performance has not yet received sufficient attention and the underlying mechanism remains unclear. [23,24] Therefore, regulating the thermal convection above the evaporation surface to decrease the surface humidity and thus enhance the evaporation performance of the evaporator is a meaningful task that has been overlooked in current studies. [25–28] Herein, a solar interfacial evaporator with a dual convection structure was constructed by removing the central part of a wood block and integrating with the photothermal material MXene (Figure 1d). Specifically, under a standard solar intensity (1 kW m⁻²), the evaporator achieved an evaporation rate of 2.16 kg·m⁻ 2 ·h⁻¹ with a photothermal conversion efficiency as high as 118%. Further, the theoretical simulations indicate that compared with traditional single-convection interfacial evaporators, the dual convection structure lowers humidity at the evaporation surface, reduces the escape resistance of water vapor, promotes the release of free gas, and thereby substantially improving the evaporation efficiency (Figure 1e). In outdoor simulated seawater desalination experiments, the evaporator maintained a salt-free surface after 8 hours of continuous operation, demonstrating excellent salt resistance. Moreover, the evaporator exhibited stable evaporation performance even under extreme pH conditions. Concurrently, we also verified the effectiveness of removing the center for evaporators of different sizes. This work not only provides innovative insights for designing high-efficiency solar evaporators but also expands the application prospects of biomass materials in sustainable energy fields. Figure 1. a) Biomimetic design of MXene-RCBW. b) Interfacial evaporation performance of carbonized wood. c) Solar interfacial evaporator with a single-convection structure. d) The interfacial evaporator with a dual-convection structure. e) Working mechanism of the developed MXene-RCBW evaporator. 2 Results and Discussion 2.1 Construction and Characterization of MXene-RCBW evaporator Inspired by the transpiration process of tree, a wood-based solar-driven interfacial evaporator with a dual convection structure using MXene as photothermal material was developed. Ti 3 C 2 T x MXene nanosheets were obtained by etching the MAX-Ti 3 AlC 2 phase ( Figure 2 a). As shown in Figure S1a (Supporting Information), the X-ray diffraction (XRD) pattern of Ti 3 AlC 2 exhibits a typical characteristic peak at 38.97°, which is absent in the MXene sample, indicating the effective removal of the Al layer. Additionally, a significant red shift from 9.53° to 6.53° in the XRD peak is indicative of an increase in layer spacing and successful etching of the MXene. The transmission electron microscopy (TEM) image (Figure S1b, Supporting Information) reveals clearly defined lattice stripes with a spacing of 1.09 nm, confirming the successful fabrication of MXene. The RCBW was prepared by selectively removing a portion of the center of a basswood block using a milling machine (Figure 2b and Figure S2, Supporting Information). Then, the MXene-RCBW evaporator was fabricated by immersing and stirring the RCBW substrate in an MXene solution (Figure 2b and Figure S3, Supporting Information). Fourier transform infrared spectroscopy (FT-IR) analysis (Figure 2c) revealed a characteristic peak at 3155 cm⁻¹ for RCBW, corresponding to the stretching vibration of the -OH group. After incorporating MXene, the hydroxyl peak in MXene-RCBW shifted to a higher wavenumber (rightward shift), indicating the formation of hydrogen bonds between the hydroxyl groups of RCBW and MXene. Additionally, the peak at 1034 cm⁻¹, corresponding to the C-O-C bond, significantly shifted compared to 1047 cm -1 . This shift is likely due to the coating of MXene on the cellulose/lignin structure, altering the original vibrational modes of these components, which suggests that MXene has successfully integrated into the wood matrix. The morphology and structure of the prepared RCBW and MXene-RCBW evaporators were examined using scanning electron microscopy (SEM) (Figure 2d,e). Figure 2d and Figure S4 (Supporting Information) reveal that basswood is rich in pore channels in both the transverse and longitudinal cross-sections. These pore channels facilitate water transport to the evaporator surface via capillary action and enhance fluid flow, promoting the upward and downward migration of ions and preventing salt accumulation on the evaporator surface. [29] After loading MXene, the wood retained its original pore structure, with MXene distributed at various locations within the wood. The energy dispersive spectroscopy (EDS) mapping (Figure 2f and Figure S5, Supporting Information) demonstrated the distribution of C, O, F, and Ti elements on the evaporator surface. The presence of C and O can be attributed to both RCBW and MXene, while the elements F and Ti originate solely from MXene. The consistent distribution of these elements throughout the region confirms the homogeneous dispersion of MXene on the porous RCBW structure. Figure 2. a) Fabrication process of the MXene. b) Fabrication process of the MXene-RCBW evaporator. c) FT-IR spectrum of RCBW and MXene-RCBW. d) SEM image of RCBW in radial section. e) SEM image of MXene-RCBW in radial section. f) EDS mapping of MXene-RCBW. 2.2 Light Absorption, Hydrophilicity, Photothermal Conversion and Evaporation Performance of MXene-RCBW evaporator Light absorption capacity and the photothermal conversion efficiency are pivotal factors influencing the evaporation performance of solar-driven interface evaporator. [30,31] Therefore, the light absorption and photothermal conversion performance of the MXene-RCBW evaporator were evaluated. As shown in Figure 3 a, compared to RCBW, MXene-RCBW exhibited excellent sunlight absorption capacity across the wavelength range of 300~2500 nm with a stable absorption rate of over 85%, and even reached more than 95% in the UV-visible light range. The outstanding light absorption is attributed to the strong absorption properties of MXene. Meanwhile, the MXene coating on the surface of the evaporator also minimizes light reflection and transmission, effectively trapping light within the porous structure, and thereby enhancing the light absorption. [32,33] The excellent light absorption and photothermal conversion ability of MXene enable the materials to generate a large amount of heat under sunlight. The inherent thermal insulation properties of wood coupled with its intricate network of vertical pores serve to confine heat to the evaporator surface, thereby enhancing the photothermal conversion efficiency. [34,35] The hydrophilicity of the photothermal material and efficient water transport during evaporation are crucial for solar-driven interfacial evaporation. [36,37] As demonstrated in Figure 3b, water droplets cannot be completely absorbed by RCBW within 10 s. In contrast, MXene-RCBW can completely absorb the water droplets within 0.5 s (Figure 3c). The excellent hydrophilicity of MXene-RCBW can be attributed to the presence of rich hydrophilic hydroxyl and carboxyl groups. Furthermore, the multi-channel structure of RCBW can also facilitate water adsorption and enhance the water transport capacity of the evaporator. Subsequently, the evaporation performance of the evaporator was examined. Surface temperatures of MXene-BW and MXene-RCBW were measured using an infrared thermal imager under one sun illumination and the results are shown in Figure 3d,e. When subjected to equivalent light intensity, both MXene-BW and MXene-RCBW exhibited faster temperature increases compared to BW (Figure S6, Supporting Information). Specifically, MXene-RCBW reached 40.3 °C within 10 minutes. The finding suggests that the photothermal material exhibits good photothermal conversion performance, and the localized heating effect induced by the surface plasmon resonance of MXene also contributes to this effective photothermal conversion performance. Furthermore, during solar evaporation under a single sun, the temperature distribution of MXene-BW in a stable evaporation state decreases gradually from the center to the edge. In contrast, the temperature distribution of MXene-RCBW increases first and then decreases from the inner edge center to the outer edge (Figure 3f). This temperature trend resulted in a reduction in energy loss and an enhancement in evaporation efficiency, thereby optimizing the performance of the evaporator. The evaporation rate of the MXene-RCBW evaporator is influenced by various factors, and the real-time mass changes were measured using a computer-connected electronic balance with an accuracy of 0.1 mg (Figure S7, Supporting Information). Figure S8 (Supporting Information) demonstrates the mass change of water for different evaporators under 1 sun illumination. The evaporation rates for the BW, MXene-BW, and MXene-RCBW evaporators were 0.95, 1.75, and 2.16 kg·m −2 ·h −1 , respectively (Figure 3g). And the photothermal conversion efficiencies of these evaporators were 50%, 99% and 118%, respectively (Figure 3g). The superior performance of MXene-BW compared to BW can be attributed to the abundant active groups and the high specific surface area of MXene, which enable efficient absorption of sunlight and conversion into heat, thereby enhancing the thermal localization effect on the evaporator surface. [38,39] Consequently, the light-to-heat conversion ability is significantly strengthened, leading to a higher evaporation rate. Specifically, the evaporation rate and photothermal conversion efficiency of the solar interfacial water evaporator in this study are significantly higher than those of most evaporators reported to date (Figure 3h). [40–49] Figure 3. a) Absorption spectra of RCBW and MXene-RCBW. b), c) Wettability of the RCBW and MXene-RCBW. d), e) Infrared thermogram of MXene-BW and MXene-RCBW. f) IR image and the temperature distribution along the dash line (from middle to edge). g) Evaporation rates and efficiencies of BW, MXene-BW and MXene-RCBW under 1 sun irradiation. h) Comparison of evaporation rates and solar-steam conversion efficiencies of different wood-based evaporators. To investigate the impact of the central cavity size on the evaporation performance of the evaporator, we varied the size of the central cavity in the wood block (2×3 cm) and measured the resulting changes in evaporation rates and overall efficiency. The samples with 1.5, 2 and 2.5 cm 2 of center area removed were labelled as L1.5, L2 and L2.5, respectively (Figure S9, Supporting Information). As demonstrated in Figure S10 and S11 (Supporting Information), the highest evaporation rate was observed for L2 in comparison to the other two samples. Although there was no statistically significant difference in evaporation performance between L2 and L2.5, L2 consistently outperformed L2.5 in seven consecutive tests (Figure S12, Supporting Information). In summary, the L2 evaporator exhibited superior performance in comparison with the L1.5 and L2.5 evaporators, with an augmentation in evaporation rate of 19% and a reduction in solar thermal material utilization of 6.7%. Furthermore, the surface temperatures of all three evaporators demonstrate a tendency to initially increase and subsequently decrease from the center to the edges, thereby facilitating to minimize heat loss and enhance the efficiency of solar heat conversion (Figure S13 and S14, Supporting Information). The evaporation rates of MXene-RCBW under different light intensities were also studied. As shown in Figure S15 and S16 (Supporting Information), the evaporation rates of MXene-RCBW under 0.5, 1 and 2 sun illumination were 1.01, 2.16 and 4.04 kg·m −2 ·h −1 , respectively. The experimental results demonstrate that the evaporation rate increases with the rise of light intensity. The data show a 27% increase in evaporation efficiency when the light intensity increases from 0.5 suns to 2 suns (Figure S17, Supporting Information). 2.3 Simulation of MXene-RCBW evaporator To gain further insights into the reasons for the enhanced performance of the evaporator after the central part was removed, simulations were carried out using COMSOL. The modelling process is detailed in the supporting material. The evaporators are designated as follows: MXene-BW (without center removal) and MXene-RCBW (with center removal). As demonstrated in Figure 4 a,b, surface temperature distributions of these evaporators reveal significant differences. For MXene-BW, a high-temperature zone forms centrally on the evaporating surface, leading to a large temperature gradient and substantial heat loss. This configuration also decreases the effective evaporation area. In contrast, MXene-RCBW achieves a more uniform surface temperature distribution over a 10-minute period, increasing the effective evaporation area and reducing heat loss. This enhances thermal management and evaporation performance. Furthermore, air convection above the evaporator during evaporation was also simulated. MXene-BW forms a single convection centrally after 4 seconds (Figure 4c). In this instance, the surrounding vapor could not be efficiently removed, resulting in elevated local humidity and thereby a reduced evaporation rate. In contrast, MXene-RCBW exhibits two convections (Figure 4d), facilitating the removal of vapor and enhancing evaporation efficiency. This dual convection reduces overall humidity and accelerates evaporation. Humidity distribution simulations above the evaporators further confirm the significant effect of dual convection on evaporation performance. While MXene-BW shows uneven humidity distribution with localized high humidity (Figure 4e), MXene-RCBW’s dual convection leads to a more uniform humidity distribution (Figure 4f). Numerical simulations indicate that the performance of MXene-RCBW outperforms MXene-BW was due to the effective introduction of dual convection on the evaporation surface. This ingenious design effectively decreases the humidity above the evaporation surface, resulting in an increased evaporation rate and efficiency. Figure 4. a), b) Simulated temperature distributions of the evaporator after solar irradiation for the MXene-BW and MXene-RCBW. c), d) Simulations of convection patterns above the corresponding evaporating region. e), f) Simulations of the corresponding humidity. 2.4 Application of MXene-RCBW Evaporator In the application of seawater desalination, the presence of a salt deposition layer has significant consequences. The salt deposition layer not only amplifies the reflection of sunlight and reduces the rate of water transmission but also diminishes the evaporation rate and considerably impacts the service life of the evaporator. [50,51] Therefore, the salt resistance of the evaporator is a crucial factor for ensuring long-term, efficient, and stable operation of evaporator. To investigate the salt resistance of the evaporator, the evaporation rates of MXene-RCBW at different salt concentrations were examined. As shown in Figure 5 a,b, the evaporation rates of MXene-RCBW in pure water, 3.5 and 10wt% brine were 2.16, 2.11 and 2 kg·m −2 ·h −1 , respectively. It is noteworthy that even in high concentrations of saltwater, the photothermal conversion efficiency remains stable at more than 100% (Figure S18, Supporting Information), thereby affirming the evaporator’s capacity to sustain a steady evaporation rate of 2 kg·m −2 ·h −1 even in high-salinity brine environments. To further investigate the durability of the evaporator, MXene-BW and MXene-RCBW were placed in 3.5 wt% brine and evaporated continuously for eight hours under one sunlight. As shown in Figure S19 (Supporting Information), the average evaporation rate of MXene-BW after 8 cycles is 1.72 kg·m −2 ·h −1 , producing 13.33 kg·m −2 of fresh water. In contrast, the average evaporation rate of the MXene-RCBW was 2.05 kg·m −2 ·h −1 after 8 cycles, producing 16.42 kg·m −2 of fresh water (Figure 5c). The comparison shows that removing part of the central area of the evaporation surface is an effective strategy, which not only improves the evaporation rate, but also enhances the stability and salt resistance of the evaporator. Even after 8 cycles, no salt accumulation was observed on the evaporation surface, and photothermal materials remained intact (Figure S20 and S21, Supporting Information). The good salt resistance of the evaporator can be attributed to the vertical porous channel structure inside BW. This structure not only serves as a support but also promotes the continuous delivery of water to the evaporator surface through capillary action. Simultaneously, the anisotropic structure of BW enhances ion exchange capacity between the channels. The liquid on the evaporator surface establishes a concentration gradient due to efficient photothermal conversion, gradually reaching saturation through effective evaporation. At this stage, the dissolved salt ions in the saturated solution gradually flow back into the brine along the water transport channels in MXene-RCBW evaporator. This significant improvement can be attributed to the superior photothermal materials, ingenious structural design, and efficient upward water transport. The above results verify the reliability and practicability of the MXene-RCBW evaporator used in seawater desalination. To evaluate the practical application of the MXene-RCBW evaporator, its seawater desalination performance was tested under outdoor natural light conditions (Figure 5d). The MXene-RCBW evaporator was placed in an outdoor environment and subjected to test from 9 am to 5 pm. The ambient temperature, evaporator surface temperature, sunlight intensity, and mass loss were recorded throughout the experiment. As shown in Figure 5e, the evaporator surface temperature was significantly higher than the ambient temperature, increasing with the light intensity and reaching a peak of 51.3 °C. In Figure 5f, light intensity fluctuations can be observed. The light intensity gradually increases from 51 mW·m -2 at 9 am to a maximum of 91 mW·m -2 between 12:00 and 14:00. Over the 8-hour period from 9:00 to 17:00, the MXene-RCBW evaporator collected 20.03 kg·m −2 of fresh water (Figure 5f). These results demonstrate the evaporator’s ability to efficiently convert solar energy into thermal energy and drive the evaporation process under natural sunlight. To further assess the practicality of the evaporator, it was placed outdoors to simulate real desalination applications with fresh water collected using a condensation device (Figure S22, Supporting Information). The concentrations of Ca 2+ , K + , Mg 2+ , and Na + were analyzed before and after seawater purification using inductively coupled plasma-optical emission spectrometry (ICP-OES). As shown in Figure 5g, the concentration of metal ions decreased significantly after purification, meeting the standard set by the World Health Organization (WHO) and the US Environmental Protection Agency (EPA) for the salinity content of drinking water. The result demonstrates that the purification system employing the MXene-RCBW evaporator can produce clean fresh water from seawater through the photothermal interface evaporation technology, which provides a new and effective solution to address the pressing freshwater scarcity. Furthermore, the evaporation performance of evaporators under extreme conditions was investigated to simulate domestic and industrial wastewater discharged into seawater. As shown in Figure 5h, the evaporation rates of MXene-RCBW were 2.13 kg·m −2 ·h −1 under strong acid (pH=2) and 2.03 kg·m −2 ·h −1 under strong base (pH=12). These results show that the evaporator can maintain stable evaporation performance and achieve effective desalination even under challenging seawater conditions. This highlights the robustness and versatility of the MXene-RCBW evaporator in various environmental conditions. Figure 5. a) The mass loss of different brines. b) The evaporation rates of different brines. c) Mass change under eight hours evaporation of the MXene-RCBW. d) Digital image of the outdoor evaporation device. e) The change of ambient temperature and surface temperature of MXene-RCBW evaporator. f) Changes in sunlight intensity and water evaporation quality. g) Changes in metal ion concentration before and after seawater purification. h) Mass change under strong acids and bases. 2.5 The Universality of Strategies and Considerations for Large-scale Application of Evaporators To validate the effectiveness of center removal in enhancing the performance of evaporators with different surface areas, the evaporative performance for evaporators of different areas with central cavity removed was tested ( Figure 6 a and Figure S23, Supporting Information). The findings of the research indicate that the evaporation performance of L1, L2 and L4 has been augmented by 38%, 23% and 28% in comparison with the control samples devoid of center removal (Figure 6b). Concurrently, the surface temperatures of evaporators with center removal were consistently lower than those without center removal (Figure S24 and S25, Supporting Information), exhibiting an increasing then decreasing trend from the center towards the edge (Figure 6c,d). The above results demonstrate that for evaporators with different surface areas, removal of the central part could greatly enhance the evaporation performance. This phenomenon occurs because the elimination of the core results in a reduction of temperatures near the center of the evaporative surface, which will minimize the energy losses and ultimately improve the solar thermal conversion efficiency. Moreover, the formed dual convection structure with the central part removal can reduce surface humidity, thereby accelerating the evaporation. These findings strongly demonstrate the practicality and universality of the strategy of removing the center to enhance convection, thereby regulating the humidity of the evaporative surface to improve the performance of evaporators. In the aforementioned study, it was established that elimination of the central part to enhance the convection has the potential to improve the evaporator performance. Consequently, a more profound comprehension of solar interface evaporators influenced by surface convection would facilitate the determination of the number of pores removed from the surface, a pivotal factor in evaluating evaporative performance within a specified evaporator area. Therefore, evaporators with different numbers of pores, designated as P1, P2, and P3 respectively, were fabricated and their interfacial evaporation performance were tested (Figure 6e and Figure S26, Supporting Information). The results suggest that under solar radiation evaporation conditions, as illustrated in Figures 6f,g, the evaporation rates of P1, P2 and P3 were 2.39, 2.74 and 2.65 kg·m -2 ·h -1 , respectively. It is evident that P2 exhibited superior evaporation performance in comparison to the other two. In the context of single-sun illumination conditions, the surface temperatures of P1, P2 and P3 were measured using an infrared thermal imager, with the results presented in Figure S27 (Supporting Information). In conditions of identical light intensity, the temperature of P2 rises fastest, reaching 41.7 °C within a period of 10 minutes. Consequently, P2 demonstrates superior evaporation rates and solar thermal conversion performance, indicating that although removing the center can enhance convection, thereby improving the performance of the evaporator, the efficiency of the evaporator does not keep increasing with the number of holes. The phenomenon is due to the fact that the presence of multiple holes enhances convection while also causing radiative heat loss. Moreover, as the number of holes increases, it hinders the timely and continuous supply of water to the interstitial areas. Consequently, local drying of the evaporator occurs, leading to a reduction in the evaporation rate. Furthermore, it is evident that the P2 evaporator with two holes removed is equivalent to two small evaporators in series (Figure 6i and Figure S28, Supporting Information), and the same applies to P3 (Figure 6i and Figure S30, Supporting Information). Consequently, a separate evaluation was conducted on the performance of monolithic and tandem evaporators. The findings reveal that, as demonstrated in Figure 6h, the evaporation rate of a single P2 evaporator is significantly higher than that of the combined evaporator. As demonstrated in Figure S29 (Supporting Information), the surface temperature of the single P2 evaporator rises more rapidly than that of the combined P2 evaporator, reaching 41.7 °C within 10 minutes of sunlight exposure. Conversely, both the evaporation rate and the temperature rise of the single P3 evaporator are lower than those of the combined evaporator (Figure S31, Supporting Information). These results further demonstrate that a higher number of holes does not necessarily mean better performance. The strategy of removing the central portion of the evaporator surface can achieve significantly higher evaporation performance with less material. However, the performance of the evaporator is not absolutely positively correlated with the number of pores. In practical applications, higher evaporation performance can be further obtained by connecting multiple center-removed evaporators in series. These studies provide important guidance for the large-scale application of evaporators, as raw material cost and evaporation rate are two of the most critical factors in large-scale applications. Figure 6. a) Schematic diagrams of L1, L2 and L4. b) Evaporation rates of L1, L2 and L4 under 1 sun irradiation. c), d) IR image and the temperature distribution along the dash line (from middle to edge). e) Schematic diagrams of P1, P2, and P3. f) Mass change curves of P1, P2 and P3 under 1 sun irradiation. g) Evaporation rates and evaporated water of P1, P2 and P3. h) Evaporation rates of P2 and P3. i) The following schematic diagram illustrates the P2 and P3 components, both in isolation and in combination. 3 Conclusion In summary, inspired by the transport systems of wood, a solar-driven interface evaporator with a dual convection structure for desalination was developed. Experimental data and numerical simulations demonstrated that removal of the central part of the evaporation surface creates a dual convection structure, reducing the humidity above the evaporation surface, and thereby increasing the evaporation rate. At the same time, this design decreases the temperature near the center area, resulting in less energy loss and higher photothermal conversion efficiency. Benefiting from its efficient thermal management of MXene, high-flux water transport via the hierarchical porous structures of the wood substrate and the ingenious structure, the MXene-RCBW evaporator exhibited a high evaporation rate of 2.16 kg·m⁻²·h⁻¹ and a solar thermal conversion efficiency of 118% under a single solar irradiation. Furthermore, the natural microchannels within the timber form a macropore network that effectively prevents salt supersaturation at the evaporation interface, thereby ensuring operational stability. Thus, the evaporator maintained consistent performance even under harsh conditions. Simultaneously, it is important to note that the strategy of removing the central part of the evaporation surface is applicable to evaporators of various sizes. The findings of this study underscore the significance of humidity modulation in evaporators and offer significant insights for the design and preparation of efficient solar desalination evaporators. 4 Experimental Section Materials: The MAX (Ti 3 AlC 2 ) powder (98%) was purchased from Forsman. LiF (99.9%) was purchased from Aladdin. Hydrochloric acid (HCl, 12 M), sodium hydroxide and ethanol (95%) were purchased from Sinopharm. Basswood was purchased from Langlang Rongcheng Wood Industry Co., Ltd. Preparation of Wood Substrate with Removed Center: A part of the central area (2×1×2 cm) of the wood block (3×2×2 cm) was removed by a milling machine and the obtained wood was marked as remove the center of the basswood, i.e. RCBW. Preparation of MXene: 3.0 g LiF was dissolved in 50 mL HCl solution and stirred at room temperature. After stirring for 10 min, 2.5 g of Ti 3 AlC 2 was gradually added to the above mixture and stirred at 40 °C for 24 h. The etched solution was washed several times with deionized water by centrifugation until the pH of the supernatant reached 6. The clay-like sediments were then collected, dispersed in deionized water (150 mL), and sonicated for 1 h, followed by centrifugation at 3500 rpm for 1 h. Finally, the obtained supernatant was Ti 3 C 2 T x (MXene). Preparation of the MXene-RCBW photothermal evaporator: 30 mg of MXene was dispersed in 50 mL of deionized water and sonicated for 2 h. The RCBW substrate was then immersed in the MXene solution and stirred for 24 hours to obtain the MXene-RCBW photothermal evaporator. Characterizations: The morphology of MXene was examined using transmission electron microscopy (TEM, JEM-2100, JEOL), and the surface morphology of the BW and MXene-RCBW was studied by scanning electron microscopy (SEM, JSM IT500HR). The phase structure and composition of MXene were analyzed via X-ray diffraction patterns (XRD, Bruker D8 Advance) using Cu Kα radiation (λ = 0.15406 nm). Fourier transform infrared (FT-IR) spectroscopy (Thermo Scientific Nicolet iS50) was employed to analyze the functional groups of the RCBW and MXene-RCBW. The water contact angle of MXene-RCBW was measured using an optical contact angle meter (JC2000D3R). The absorption spectra of MXene-RCBW were recorded using an ultraviolet-visible-near-infrared spectroscopy (UV-3600IPLUYS, 220C) equipped with an integrating sphere. The concentration of metal ion was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES, Avio 200, PE). Solar-driven interfacial evaporation experiment: Solar radiation was simulated using a solar simulator (CEL-HXF300, Xenon lamp) equipped with an AM1.5G filter. The optical density of sunlight exposure was determined using an auxiliary detector (CEL-NP2000, optical power meter, China), and the density on the sample was adjusted by controlling the power and optical distance of the solar simulator. 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Keywords dual convection structure mxene solar-driven interfacial evaporation surface humidity modulation wood Authors Affiliations Yawen Wang Zhejiang A&F University View all articles by this author Caicai Li 0000-0002-8480-8829 [email protected] Zhejiang A&F University View all articles by this author Weijia Guo Zhejiang A&F University View all articles by this author Shunyu Shen Jianghan University View all articles by this author Gaojie Jiao Zhejiang A&F University View all articles by this author Dan Sun Zhejiang A&F University View all articles by this author Xizheng Liu Jianghan University View all articles by this author Xianyun Peng Zhejiang University View all articles by this author Qingfeng Sun 0000-0003-3520-4970 Zhejiang A and F University View all articles by this author Huiqiao Li 0000-0001-8114-2542 Huazhong University of Science and Technology View all articles by this author Metrics & Citations Metrics Article Usage 179 views 111 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Yawen Wang, Caicai Li, Weijia Guo, et al. Modulation of the surface humidity with dual convection structure towards highly efficient and stable solar desalination. Authorea . 30 October 2025. DOI: https://doi.org/10.22541/au.176178443.32158344/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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