Cover matters: enhanced performance of a multistage solar evaporator with tuned optical and thermal cover properties

Abstract

To address global water scarcity and improve off-grid pure water production, this study focuses on enhancing the performance of inverted solar evaporators by optimizing the design of cover materials. By employing a coupled numerical and experimental approach, we systematically investigated the optical and thermal properties of various cover materials, including thin film, transparent silica aerogel, acrylic, and glass, to guide their rational design under different operational conditions. Silica aerogel, with its high solar transmittance and ultra-low thermal conductivity, demonstrated superior evaporation performance in high-stage device designs and retained cost-effectiveness in high-stage and low-solar irradiation (1 sun) scenarios. However, under single-stage design and low-solar irradiation, thin film emerges as a viable and cost-effective alternative. At high solar concentrations, the performance gap between aerogel and other materials narrows, making acrylic a suitable substitute maintaining both efficiency and cost-effectiveness. Experimental results show that the optimized cover thickness for aerogel was 6 mm achieving an evaporation rate of 6.25 kg m⁻² h⁻¹ under 1 sun. These results highlight the critical role of cover material properties in enhancing solar evaporator efficiency, offering valuable insights for the development of high-performance, sustainable water purification systems.

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Broader context

Global water scarcity is a pressing issue that affects billions of people worldwide, necessitating innovative and sustainable solutions for water purification. Traditional water desalination methods are often energy-intensive and costly, making them less accessible for remote or off-grid communities. Interfacial solar evaporation has emerged as a promising technology for producing pure water using abundant solar energy. However, optimizing the efficiency of solar evaporators remains a significant challenge, particularly in balancing thermal insulation and optical transmittance of the cover materials. This study addresses these challenges by systematically investigating and optimizing cover materials for multistage solar evaporators. By employing both numerical modeling and experimental validation, we explored the performance of thin film, glass, acrylic, and silica aerogel covers. Our findings reveal that the superior thermal and optical properties of silica aerogel lead to better performance under certain conditions, acrylic can be a cost-effective and stable alternative, especially at high solar irradiations. This research not only advances the understanding of material properties in solar evaporators but also provides practical solutions for enhancing their performance. By optimizing cover materials, we can improve the efficiency and viability of solar evaporation technologies, contributing to global efforts in addressing water scarcity and promoting environmental sustainability.

Introduction

Solar interfacial evaporation featured with thermal localization offers a promising pathway for off-grid and portable pure water production from various water sources.^1–6 Advancements have been achieved in engineering devices with high evaporation rates.^7–14 However, the purified water collection remains challenging due to (i) less effective condensation compared to significant evaporation rate and (ii) optical transmittance reduction due to condensed water on the front cover. To tackle these challenges, inverted devices have been proposed to condense vapor in a separate dedicated condenser with a large surface area, enhancing condensation performance through an expanded condensing area and preventing droplet-induced optical loss on the cover. Based on this strategy, the single-stage evaporator reached an evaporation rate of 1 kg m⁻² h⁻¹ with a 70% collection efficiency.¹⁵ Further efficiency improvement has been reported by deploying a multistage design to recover released heat from condensation to achieve an evaporation rate of up to 5.18–5.75 kg m⁻² h⁻¹.^11,16–19 However, the front side cover, based on polymer or glass, becomes the most significant contributor to the optical and heat losses of the device, generally accounting for >20% of the total solar input.^17,20 Further device performance enhancements require a front cover with high transmittance in the solar band to maximize optical efficiency, low transmittance in the thermal band to reduce the reradiation loss, and low thermal conductivity to block conduction from the hot absorber to the ambient. Silica aerogel has been reported to perfectly meet these requirements as a cover material to achieve high-performance¹⁸ and high-temperature²¹ solar thermal applications due to its high optical transmittance (>90%) and ultra-low thermal conductivity (∼0.02 W m⁻¹ K⁻¹).^18,21–24 However, for solar interfacial evaporation devices used in purified water production, it remains unclear whether silica aerogel is the optimal choice for the cover material since (i) high quality and large area aerogel requires a complex and costly preparation process (i.e., supercritical drying), (ii) the high stagnation temperature, i.e., >200 °C under 1 sun, enabled by silica aerogel may lead to the destruction of device components, e.g., the hydrophobic porous polytetrafluoroethylene (HBP-PTFE) membrane, and (iii) the mechanical rigid and fragile nature of aerogel presents challenges for operating the device under real on-sun conditions limiting the reliability and lifetime of the device.

In this study, to better understand the impact of the optical and thermal properties of the front cover on the device performance, we discuss the design of the front cover of a multistage solar evaporator (MSE) device assisted with a coupled numerical and experimental method. Four cover materials, i.e., thin film, glass, acrylic, and aerogel, with distinct optical and thermal properties are explored and compared under various solar irradiations and cover thicknesses to guide the choice and design of the front cover. In particular, we searched for the design and operation spaces in which the silica aerogel cover can be replaced by a thin film (n = 1, 1 sun) or acrylic (n = 10, 3 suns) cover, featured with low cost and long lifetime, without sacrificing the device's performance. Additionally, we optimized the thickness of the cover for the MSE device under various materials with different optical and thermal properties, device configurations, and environmental conditions offering design guidelines for high performance solar evaporators.

Results and discussion

Fig. 1(a) shows the schematics of the MSE device under solar irradiation with arrows indicating the optical and heat transfer processes. The cover is located at the front of the device, allowing for solar irradiation to pass through and dissipating heat to the ambient by reradiation and convection. The amount of heat dissipated depends on the temperature of the upper surface of the cover. Therefore, a small thermal conductivity is required to minimize heat losses from the front in addition to high solar band transmittance. Note that the reradiation from the absorber has less chance to pass through the front cover for commonly used cover materials and, hence, is neglected in this study. The absorber under the cover is an HBP-PTFE membrane loaded with nanocarbon powder (details in ESI†), which mainly absorbs sunlight to raise the temperature as well as prevents seawater from being in contact with the absorbers to avoid salt precipitation. Absorbers conduct (q_cond) and reradiate (q_rad,abs-cov) heat to the cover, which is further transferred to the ambient through the cover. The first stage comprised the absorber, the evaporation layer, the PTFE layer, the condensation layer, and the sealing layer (dash orange box marked Stage 1 in Fig. 1(a)). The other single-stage components are similar to those of the first stage but without the absorber (indicated by the dashed orange box marked as Stage n). The condensation heat released from the former stage provides the evaporation heat to the next stage. To facilitate the condensation, we designed a passive cooling layer to pump the seawater to the back of the device. The fresh water was passively pumped to a reservoir by cellulose tissue.

Fig. 1 The impact of the transmittance and thermal conductivity of the cover on the performance of the MSE device. (a) Schematics of the MSE device with detailed energy distributions including solar radiation (q_in), reflection loss (q_ref), convection loss (q_conv), conduction loss (q_cond), radiation loss by the absorber to the cover (q_rad,abs-cov), the cover to ambient (q_rad,cov-amb), useful energy (q_u), and latent heat (q_lat). The key components consist of a transparent cover, absorber layer, evaporation layer, PTFE layer, condensation layer, sealing layer, and cooling layer. Effects of transmittance and thermal conductivity on normalized evaporation efficiency (η_evap,norm) with different stages (n) and different solar irradiation conditions: (b) n = 1, 1 sun, (c) n = 10, 1 sun, and (d) n = 10, 3 suns.

We developed a lumped parameter model based on an equivalent thermal network to investigate the effects of optical and thermal properties on the evaporation efficiency (see ESI† for the model details). The cover materials were selected to cover a wide range of both thermal and optical properties, i.e., transmittance 71–96% and thermal conductivity 0.011–1 W m⁻¹ K⁻¹ for the three selected materials in this study (see Table S1, ESI†). Note that the absorber temperature (T_abs) were also predicted (see Fig. S4, ESI†). The cases with T_abs over 85 °C are identified as device failures due to the loss of the function of the HBP-PTFE membrane at high temperatures (see Fig. S22 and S23, ESI†). In principle, the higher cover solar band transmittance and lower thermal conductivity lead to a higher normalized evaporation efficiency (η_evap,norm = η_evap/η_ref) of the single-stage (n = 1) MSE device under 1 sun (Fig. 1(b)). Interestingly, the difference of the η_evap,norm at n = 1 under 1 sun was insignificant, which was below 0.5, indicating that the use of aerogel leads to marginal benefits. The acrylic and glass covers may be an appropriate choice considering the mechanical stability, cost, and large-scale fabrication advantages over the aerogel. As the number of MSE device stages increases, the latent heat released by condensation can be utilized multiple times, and the η_evap,norm increased for all cover materials (see Fig. 1(c)). Hence, with a higher n, the difference in η_evap,norm among different cover materials was enlarged, i.e., η_evap,norm increased from 3 to 6 with n increased from 1 to 10, which justified the choose of aerogel. The enlarged difference in η_evap,norm was due to the glass and acrylic cover result higher q_loss,conv and q_loss,rad as a results of higher T_abs (see Fig. S5, ESI†), which emphasizes the importance of low thermal conductivity. In addition, at high n, e.g., n = 10 in Fig. 1(c), the requirement for optical transmittance of the aerogel can be relaxed. The η_evap,norm reached up to 5 even when the optical transmittance is only 78%, which was higher than that achieved with acrylic and glass, both having transmittance higher than 95%.

The η_evap,norm differences among three cover materials was narrowed at higher solar irradiation (n = 10 and 3 suns, see in Fig. 1(d)). This can be related to a more significant reduction in q_loss,conv and q_loss,rad in acrylic and glass compared with the aerogel (see Fig. S5a, ESI†) when increasing from 1 sun to 3 suns, resulting from much higher T_abs increase of the aerogel case (see Fig. S5b, ESI†). It is interesting to note that η_evap,norm of acrylic covers can be higher than 5 (maximum 5.2), which were also close to the best efficiency values, ∼6, for aerogel covers. This gives the potential to replace aerogel with acrylic as a cover. Moreover, the device failure may happen for the aerogel covers (see Fig. 1(d)) due to overheating, i.e., temperature > 85 °C, limiting the design space. Hence, for a small number of stages without solar concentration, the solar evaporation efficiency was less sensitive to the cover materials. In contrast, at large stage numbers, the aerogel covers outperformed other materials due to their low thermal conductivity. The differences in η_evap,norm will reduce at higher solar concentrations, with aerogel, in general, showing better performance. However, the overheating of the HBP-PTFE membrane that can reduce the design space of the aerogel has to be carefully considered.

Idealized cover materials require a 100% solar band transmittance and a close to 0 thermal conductivity (dash line in Fig. 2(a)). In practice, it is a challenge to meet these requirements yet keep low cost and high mechanical stability. Fig. 2(a) shows the direct hemispherical transmittance of glass, acrylic, and aerogel samples with a thickness of 10 mm in the 0.25 to 20 μm band, i.e., solar band plus thermal band. The transmittance was calculated based on the weighted average of the transmittance over the solar band, noted as solar-weighted transmittance since sunlight has different magnitudes of energy in different wavelength bands. The solar-weighted transmittance of aerogel (90%) is slightly lower than that of glass (93%), but higher than that of acrylic (88%). In the thermal band, glass, acrylic, and aerogel all have low transmittance, which means suppressed thermal radiation from the absorber to the ambient. Note that aerogel shows an emission peak at 4 μm and has a slightly higher thermal-weighted transmittance (1.08%, 0.05% for glass, and 0.01% for acrylic). Additionally, blackbody spectra corresponding to T_abs of the MSE device under 1 sun and 3 suns were plotted (see Fig. S12, ESI†). The peaks of the blackbody spectra at 50 °C and 80 °C are located at approximately 8.9 μm and 8.3 μm, respectively, which deviate from the aerogel's transmission peak (∼4 μm). This allows us to conclude that the aerogel's peak near 4 μm has almost no effect on its thermal insulation performance. Fig. 2(b) shows the transmittance of different covers as a function of thickness using the Beer–Lambert law. Note that the thermal and optical properties of the cover materials were determined by the chemical components (i.e., elements, molecular weights, etc.), and microstructures (i.e., packing, particle and pore sizes, etc.). Hence, we chose 3 materials (i.e., glass, acrylic, aerogel) to cover a wide range of both thermal and optical properties. As expected, the optical transmittance generally decreases with increasing thickness for all cover materials. The glass shows the highest transmittance, followed by aerogel and acrylic. The transmittance differences were increased with increasing thickness. For example, the transmittance difference was 6% at 10 mm and increased to 25% at 180 mm. Although a thinner cover favors optical transmittance, the heat conduction from the absorber may result in large heat losses from the front. Hence, a low thermal conductivity can further reduce heat losses while maintaining high optical transmittance. Silica aerogel shows the lowest thermal conductivity, i.e., 0.02 W m⁻¹ K⁻¹, which is 6 times lower than acrylic and 32.5 times lower than glass (see Fig. 2(c)). Transmittance and thermal conductivity properties of cover materials led to distinct stagnant temperatures (Fig. 2(d)) with 177 °C for the aerogel, 78 °C for the acrylic, and 69 °C for the glass. As expected, the aerogel shows the highest stagnation temperature, indicating excellent optical and thermal performance.

Fig. 2 Characterization of optical and thermal properties of different cover materials. (a) Transmittance spectra of the glass, acrylic, and aerogel in the solar and thermal bands. The blue dotted line is the transmittance of an ideal cover in the solar and thermal bands. (b) Solar-weighted transmittance as a function of cover thickness for glass, acrylic, and aerogel. (c) Thermal conductivity of the glass, acrylic, and aerogel. (d) Temperature response of the absorber and cover surface of the devices under 1 sun for stagnation temperature measurement.

To further validate our findings, we tested MSE devices utilizing 3 different cover materials under various solar irradiation (see Fig. 3). Note that the thin film (i.e., polyethylene film), a common sealing material, was used as a control in this study. As shown in Fig. S17 (ESI†), the thin film has little effect on the heat convection and radiation losses compared to the uncovered case based on our simulation results. Fig. 3(a) shows the η_evap at n = 1 and 1 sun for different cover conditions. The aerogel case showed the highest η_evap of 88% which was ∼10% higher than other cases. Note that this efficiency enhancement is minor, i.e., only ∼0.1 kg m⁻² h⁻¹, which indicates that aerogel is not necessary at low stage numbers and low solar irradiation. Note that η_evap for acrylic, glass, and thin film also showed negligible differences within 10%. The insignificant differences can be attributed to the fact that the benefit from the low thermal conductivity of aerogel can be partially counteracted by higher q_loss,opt. Meanwhile, the thin film configuration provides a cost-effective (see Fig. S31a, ESI†) alternative with minimal performance reduction. However, the aerogel cover is suitable for the MSE device when operated at n = 10, 1 sun. As seen in Fig. 3(b), the aerogel case has an η_evap of 414%, which is 96% higher than the following-up case, the acrylic case, and 164% higher than the worst case, the thin film case. Meanwhile, the aerogel case also shows the lowest cost in the economic analysis due to significantly higher performance, as shown in Fig. S31b (ESI†). In line with the model prediction, at a higher solar irradiation of 3 suns, the efficiency differences decrease as seen in Fig. 3(c). The best aerogel case showed an η_evap of 425% (simulation value) which is only 102% (164% at 1 sun) higher than the worst thin film case. Note that efficiency values for aerogel at n = 10 and 3 suns were simulation values because a device failure was detected during the experiment due to overheating. The T_abs exceeded 85 °C after 32 mins’ testing (see Fig. S22 and S23 for details, ESI†). Acrylic cover is a preferred choice at n = 10, 3 suns, considering only a 9% efficiency reduction compared to the aerogel case (equivalent to 0.45 kg m⁻² h⁻¹ out of 5 kg m⁻² h⁻¹), as shown in Fig. 3(c). This suggests that at high n and high solar irradiation, acrylic is a well-suited substitute for aerogel to simultaneously achieve high efficiency as well as material and device stability. Meanwhile, the economic analysis results indicate that the acrylic case is more cost-effective than the aerogel case at n = 10 under 3 suns (see Fig. S31c, ESI†).

Fig. 3 The evaporation efficiency (η_evap), evaporation rate (r_evap), and energy breakdown of MSE devices with different types of cover materials (thin film, glass, acrylic, and aerogel) at (a) n = 1, 1 sun, (b) n = 10, 1 sun, and (c) n = 10, 3 suns. The energy breakdown is defined as the ratio of target energy to total solar energy. The total solar energy includes evaporation (q_evap), optical loss (q_loss,opt), convection loss (q_loss,conv), and radiation loss (q_loss,rad). The dashed and solid lines are the simulated values and the corresponding experimental values, respectively.

Further performance improvement can be achieved by tuning the thickness of the cover to balance the optical loss and the conductive loss from the absorber. Fig. 4(a)–(c) shows the effects of cover thickness and T_abs on the absorber efficiency (η_abs = q_u/q_in) for different cover materials. A high T_abs generally leads to a low η_abs for a given cover thickness for glass (Fig. 4(a)) and acrylic (Fig. 4(b)) covers. However, the aerogel cover is not sensitive to T_abs up to ∼180 °C due to low thermal conductivity. This also enables the aerogel to be utilized in high temperature applications, especially for superheated steam generation in which temperature requirements are usually >100 °C. Taking η_abs ≥ 40% as an example, we observed an enlarged area from glass to acrylic and further to the aerogel. The glass cover can only achieve 73 °C, while 107 °C for the acrylic cover and 180 °C for the aerogel cover (see the red lines in Fig. 4(a)–(c)).

Fig. 4 Optimization of cover thickness for MSE devices with different cover materials. Absorber efficiency (η_abs) as a function of thickness and T_abs for (a) glass, (b) acrylic, and (c) aerogel under 1 sun. The solid white and red lines are for T_abs = 100 °C and η_abs = 40%, respectively. The f_evap as a function of (d) glass, (e) acrylic, and (f) aerogel thickness and solar irradiation at the MSE device (n = 10). The f_evap is defined as the ratio of the evaporation efficiency of the MSE device with a cover to that without a cover (thin film case), i.e. f_evap = η_evap,cov/η_evap,thin. Different types of white contours represent different f_evap-values. Two regions are defined: f_evap > 1, enhanced region; f_evap < 1, suppressed region. The highest f_evap is marked with a blue dot. The experimental validation cases are marked with a red dot (10 mm thick cover) and orange dot (70 mm thick cover), respectively.

The MSE device was further optimized with various solar irradiations and cover thicknesses. The relative efficiency enhancement, f_evap, compared to the thin film cover cases. Two regions, i.e., the enhanced region and the suppressed region, were identified for all cover materials. The boundary of the two regions is marked with f_evap = 1 (solid white lines in Fig. 4(d)–(f)). The suppression efficiency was majorly due to the dominate optical loss at a high cover thickness. We also observed an optimal cover thickness for a given solar irradiation. Taking 1 sun as an example, the optimal cover thickness was 20 mm for glass, 11 mm for acrylic, and 6 mm for aerogel (blue dots). To validate our predictions, we chose cover thicknesses of 10 mm (red dots, in the enhanced region) and 70 mm (orange dots, in suppressed region). Our experimental results agree well with the predictions with a maximum difference in f_evap of only 8.7%. In principle, the optimal thickness of the cover material will vary with the cover's optical and thermal properties, device configurations (dimension of each layer, number of stages, etc.), and environmental conditions (solar radiation, ambient temperature, wind speed, etc.). For instance, as solar irradiation increases from 1 to 4 suns, the optimal thickness of the same cover material decreases accordingly, with glass at 9 mm, acrylic at 6 mm, and aerogel at 4 mm (see Fig. 4(d)–(f)). The optimal thickness will also change with design configurations. For instance, when the stage reduced from 10 to 1, the optimal thickness for glass, acrylic, and aerogel will be 1 mm, 5 mm, and 4 mm (see Fig. S9, ESI†).

We further demonstrate optimized MSE devices with n = 10 under 1 sun using different cover materials with optimized thickness, i.e., 20 mm thick glass, 10 mm thick acrylic, and 6 mm thick aerogel. Fig. 5(a) shows the photograph of the experimental setup. The mass change with operation time under 1 sun is shown in Fig. 5(b), with a fresh water collection rate of 4.24–6.25 kg m⁻² h⁻¹ and a measured evaporation efficiency in a range of 283–417%. Under 1 sun, the MSE device can be continuously operated in simulated seawater for more than 8 h with the evaporation rate of ∼5.9 kg m⁻² h⁻¹ and the evaporation efficiency of ∼395%. At the same time, the concentration of major salt ions Na⁺, Mg⁺, Ca⁺, and K⁺ in the simulated seawater decreased from the 15 642.32, 2680.19, 2180.05, and 1011.54 mg L⁻¹ before desalination to 1.32, 0.07, 1.09, and 0.47 mg L⁻¹ after desalination, which meets the drinking water standards set by the World Health Organization (WHO). We compared the evaporation efficiency for the various evaporation device demonstrations with different cover materials and device stage configurations in Fig. 5(e). Different cover materials are identified with different filled colors. In general, higher stage numbers lead to higher efficiency due to multiple times of condensation heat recovery. Note that for 10-stage MSE devices, the aerogel and acrylic covered devices shows high efficiencies, e.g., >300% in general. Our devices, marked with stars, outperform the best-reported cases as a result of further optimized cover designs, i.e., optimized material choice and corresponding thickness. The devices optimized in this study with aerogel and acrylic covers showed an absolute efficiency increase of 31.7% and 10.8% compared to the best existing value, 385%, by SJTU/MIT with the aerogel cover (without optimization in cover thickness).¹⁸ For single-stage designs, our devices with 10 mm thick covers with different cover materials also show efficiencies in the high-performing region with efficiencies close to 90%, which also overlap with various reported data points indicating that cover designs are less important when heat recovery is not considered.

Fig. 5 Measured performance of MSE devices with optimized cover thickness. (a) The photograph of the MSE device with a 6 mm thick aerogel cover. (b) The mass change of the MSE device with different cover materials and optimized thickness (glass: 20 mm, acrylic: 10 mm, and aerogel: 6 mm) at n = 10, 1 sun. (c) The mass change and the corresponding evaporation rate of the MSE device with simulated seawater. (d) The ion concentrations measured before and after desalination. (e) Comparison of the evaporation efficiency with previous reports. Typical literature data points are marked with color-filled circles. Different colors represent different cover materials. The details for each date point are summarized in Table S5 (ESI†). Our results are marked with color-filled stars. The black circle contours are for unconcentrated devices, and the gray contours are for concentrated devices. Note that we only compare our results with devices capable of collecting water, rather than those designed solely for evaporation.

Conclusions

This study highlights the critical role of cover materials in optimizing the performance of multistage solar evaporators, through a combined numerical and experimental approach. The transparent silica aerogel was found to exhibit superior evaporation performance due to its ultra-low thermal conductivity and high solar band transmittance. Specifically, the aerogel demonstrated high evaporation efficiency under high-stage design. However, its performance advantage was reduced in single-stage designs and under low-solar irradiation (1 sun), where the evaporation rate was only marginally higher than that of other materials by less than 0.1 kg m⁻² h⁻¹. This led to the thin film case being a cost-effective alternative with minimal performance reduction. Acrylic also emerged as a promising alternative to aerogel, especially when mechanical stability, cost, and scalability are crucial. With high-stage design under high-solar irradiation, the performance gap between aerogel and acrylic narrows, making acrylic a viable substitute that maintains both efficiency and cost-effectiveness. Experimental results further emphasized the impact of optimizing cover thickness, with the optimal thicknesses being 20 mm for glass, 11 mm for acrylic, and 6 mm for aerogel at 1 sun. Notably, a 6 mm thick aerogel cover achieved an evaporation rate of 6.25 kg m⁻² h⁻¹ under 1 sun, showcasing its outstanding thermal properties, which make it particularly suitable for high-temperature applications such as superheated steam generation.

In summary, this study provides valuable insights into the design and optimization of cover materials for multistage solar evaporators, emphasizing the balance between thermal insulation, optical transmittance, and practical considerations such as cost and stability. These findings contribute to the development of high-performance, sustainable water purification systems suitable for addressing global water scarcity.

Data availability

The data supporting this article have been included as part of the ESI.† Additional data related to this paper may be requested from the corresponding author Meng Lin at linm@sustech.edu.cn. Some data are not publicly available due to privacy restrictions but can be accessed upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The Shenzhen Science and Technology Innovation Commission under grant no. GJHZ20210705141808026 and KCXST20221021111207017, Shenzhen Key Laboratory of Intelligent Robotics and Flexible Manufacturing Systems under grant no. ZDSYS20220527171403009, Guangdong Basic and Applied Basic Research Foundation under grant no. 2023A1515011595, Guangdong grant under grant no. 2021QN02L562, and Guangdong Major Project of Basic Research under grant no. 2023B0303000002 are acknowledged. The National Natural Science Foundation of China under Grant 52376191 is acknowledged. The SUSTech High Level of Special Funds under grant no. G03034K001 is also acknowledged. The SEM data were obtained using equipment maintained by Southern University of Science and Technology Core Research Facilities. The computation in this work is supported by Center for Computational Science and Engineering at Southern University of Science and Technology.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ee02710h

‡ These authors contributed equally to this work.

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