Solar-Powered Interfacial Evaporation for Simultaneous Photocatalytic Hydrogen Production and Salinity Gradient Power Generation

Abstract

Solar-driven interfacial evaporation desalination technology offers a feasible solution to the global shortage of freshwater resources. However, previous interfacial evaporation technologies have often only focused on the production of freshwater resources, without fully utilizing the high-energy photons in sunlight and the salinity gradient generated after seawater evaporation. In this work, a solar-driven water–hydrogen–electricity (SWHE) co-production system integrated by solar-driven interfacial evaporation (SIE), interface photocatalytic hydrogen evolution (IPHE), and reverse electrodialysis (RE) was proposed. The aim is to enhance the efficiency of solar energy utilization and achieve simultaneous production of freshwater, hydrogen, and electricity. Under 2-sun irradiation, the SWHE device achieved a water generation rate of 0.77 kg m⁻² h⁻¹, a hydrogen generation rate of 8.57 mmol m⁻² h⁻¹, and a highest power density of 2.9 mW m⁻². Outdoor tests demonstrate that the cumulative water production reached 1.6 kg m⁻² over 6 h, with a total hydrogen yield of 12.22 mmol m⁻² and a highest power density of 0.095 mW m⁻², which validated the environmental adaptability of SWHE system. This novel design strategy is expected to provide a novel form of freshwater resources and energy supply for human society.

1. Introduction

Freshwater resources are a necessary condition for maintaining the stable operation and healthy development of human society [1,2,3]. The rapid increase in population size, climate warmings, and intensification of environmental pollution have intensified the consumption of freshwater resources [4,5,6]. It is predicted that by 2050, the number of urban residents facing water shortages worldwide will rise to 2.373 billion [7]. A total of 97.2% of the water resources on the earth are seawater. Seawater desalination technology is considered as a viable solution for the freshwater resource crisis [8,9,10]. However, previous seawater desalination technologies (e.g., reverse osmosis and multi-stage flash distillation) are typically associated with issues of high energy consumption and environmental pollution [11,12]. Solar energy, as a renewable energy which is abundant and clean, has demonstrated great potential in the field of seawater conversion [13,14], environmental management [15], and energy production [16,17,18]. SIE technology is an advanced technique that takes photothermal materials as the core and converts solar energy into thermal energy by constructing local heat zones to achieve efficient evaporation [14,19].

In recent years, SIE has obtained much research due to its high-efficiency photothermal conversion, zero carbon emissions, and strong scalability [20]. Li et al. [21] typically built an ANF/MoS2@MXene evaporator which reached an evaporation rate of 1.42 kg m⁻² h⁻¹ with 1-sun irradiation and simultaneously realized broadband absorption through MXene and vertically dispersed MoS₂ nanosheets. Han et al. [22] designed a one-dimensional Fe/C structured film for interface evaporation. Its unique one-dimensional evaporation structure reduced the enthalpy of pure water evaporation at 25 °C to 1388 kJ kg⁻¹ and achieved an evaporation rate of 2.6 kg m⁻² h⁻¹ with 1-sun irradiation. Although the materials and structure used in evaporators are constantly being updated, the traditional single-effect evaporation system still faces a bottleneck of single resource output, which means that the potential energy generated as a by-product during the evaporation process has not been fully exploited [23,24,25].

Considering the bottleneck of single output from SIE, researchers have conducted extensive studies on multi-energy cogeneration systems based on SIE to recover the by-product energy such as the residual heat from the light–thermal conversion [17,26], latent heat of steam [27], and ion gradient at the interface [28,29]. Ho et al. [26] connected a Bi₂Te₃-based thermoelectric module to the bottom of an interface evaporator to convert the temperature gradient into electricity, reaching an evaporation rate of 1.36 kg m⁻² h⁻¹ by 1-sun irradiation and a power density of 0.4 W m⁻². Zhou et al. [30] designed a device combining an interfacial evaporator with a Nafion membrane, which recovers the salinity gradient energy generated by seawater evaporation through a cation exchange membrane. With the light intensity of 1-sun irradiation, it achieved a vapor production rate of 1.1 kg m⁻² h⁻¹ and a power density of 1 W m⁻². Huang et al. [31] developed an interface photocatalytic water hydrogen co-production system that consists of a photocatalytic layer and air-water collection materials, achieving a H₂ generation rate of 425.4 μmol·g⁻¹·h⁻¹ while water vapor supply reached 0.121 kg m⁻²·h⁻¹ with the light intensity of 1-sun irradiation. These studies blazed a new trail for the innovation of the SIE system from single freshwater production to water–energy cogeneration.

In this paper, a SWHE device with multi-energy collaborative production capabilities was proposed through integrating SIE technology, IPHE technology, and RE technology. SIE absorbs sunlight and transforms it into thermal energy for seawater evaporation. A portion of the water vapor generated during evaporation is split into hydrogen on the surface of a photocatalyst, while the rest condenses on the device walls to form collectable freshwater. The salinity gradient generated by seawater evaporation is recovered by the RE module, which uses ion-selective membranes to generate electricity. This device solely relies on solar energy as its power input source. It is anticipated to supply water and generate electricity for remote regions where there is a shortage of clean water and energy resources.

2. Experiment and Methods

2.1. Design Concept of the SWHE System

The design principle of the SWHE system is illustrated in Figure 1. A photothermal local heating interface was constructed with a polytetrafluoroethylene (PTFE) microporous hydrophobic membrane as the core. Deep gray Pt/TiO₂, which exhibits high light absorption properties, was deposited on the PTFE membrane’s surface to serve as the IPHE module’s photocatalyst and the SIE module’s solar absorber. It is important to emphasize that the low surface energy of the PTFE membrane can effectively prevent the permeation of ions in liquid seawater, thus ensuring the stability of the photocatalyst. By coupling the evaporation interface with the photocatalytic hydrogen evolution interface, the full-spectrum utilization of solar energy can be achieved. Long-wave photons are absorbed to heat seawater and generate water vapor, providing raw materials for the IPHE module. Subsequently, a portion of the water vapor is cracked on the Pt/TiO₂ surface under the action of short-wave high-energy photons, causing the hydrogen evolution reaction to occur, while the unreacted water vapor is collected through the condensation wall. Meanwhile, the successive evaporation of seawater causes a constant accumulation of concentration near the evaporation interface, which is significantly higher than that in the bulk seawater area. This salinity gradient energy caused by evaporation can be recovered through an RE module for electric output. Due to the concentration potential difference, cations in seawater form a directional migration through the cation exchange membrane (Nafion 117) to form a potential difference, and then the electric energy caused by ion migration is output (Ag/AgCl). The specific structure of the device is elaborated in the Supplementary Materials, Figure S1.

2.2. Material Characterization

As shown in Figure 2a, a Pt/TiO₂–PTFE composite membrane was fabricated by loading Pt/TiO₂ photocatalyst onto a PTFE hydrophobic membrane for the integration of SIE and IPHE. Firstly, 10 mg of Pt/TiO₂ powder was mixed with 100 mL deionized water to form a suspension through ultrasonic diffusion. The preparation process of the Pt/TiO₂ photocatalyst is shown in Figure S2. Subsequently, the composite membrane was formed by vacuum filtering the Pt/TiO₂ powder onto the surface of PTFE hydrophobic membrane and then drying it. Figure 2b shows the physical image of the Pt/TiO₂–PTFE composite membrane with a surface color of dark gray. SEM images demonstrate that the Pt/TiO₂ catalyst particles are successfully and consistently bonded to the PTFE membrane’s upper surface’s polyethylene terephthalate support layer (Figure 2c). Meanwhile, the cross-sectional SEM for thickness shows that the thickness of this composite film is only 109.21 μm (Figure S3). The lower surface of the PTFE membrane shows a distinct porous network structure with an average pore diameter of approximately 0.22 μm, which provides a transport channel for water vapor. The static contact angle tests on both sides of the composite membrane were 123.3° and 127.3°, respectively, indicating that the composite membrane has excellent hydrophobic properties (Figures S4 and S5). As shown in Figure 2d, within all solar radiation bands, from 300 to 2500 nm, the composite film’s absorption rate can reach 92.5% (weighted by AM 1.5G) due to the roughness of the Pt/TiO₂ powder and the high blackness. The PTFE surface’s elemental mapping by the EDS further verified that Pt, Ti, and O elements were evenly deposited across the surface. This suggested that the hydrophobic membrane surface had a significant amount of catalysts that could be utilized for photocatalytic reactions (Figure 2e). The XPS confirmed the successful synthesis of Pt/TiO₂ (Figure S6).

3. Results and Discussion

3.1. Evaluation of Basic Performance

To evaluate the water–hydrogen–electricity cogeneration capacity of the system, tests were first conducted in the controlled laboratory environment. Figure 3a shows an experimental test system layout. Figure S7 shows the layout of the actual experimental testing system. The uncertainties of the freshwater production process (U_W), hydrogen production process (U_H), and salinity gradient power generation (U_E) of this system are 5.00%, 5.83%, and 5.00%, respectively. Figure 3b shows the under light intensity of 1-sun irradiation, 1.5-sun irradiation, and 2-sun irradiation, the steady-state temperature of the evaporation interface reached 42 °C, 45 °C, and 50 °C, respectively. Figure 3c shows that the freshwater generation rate of the SWHE reached 0.42 kg·m⁻²·h⁻¹, 0.61 kg·m⁻²·h⁻¹, and 0.77 kg·m⁻²·h⁻¹ under solar intensities of 1-sun irradiation, 1.5-sun irradiation, and 2-sun irradiation, respectively. The primary cause of this is the increase in water vapor partial pressure on the evaporation side due to the rising temperature of seawater; this thus causes the vapor flux to rise. Under the synergistic action of the ultraviolet spectrum and the Pt/TiO₂ catalyst, partial water vapor undergoes the PTFE membrane for cracking to generate H₂, as shown in Figure 3d. After three hours of operation, the hydrogen generation of SWHE reached 11.21 mmol·m⁻², 16.77 mmol·m⁻², and 25.72 mmol·m⁻² under light intensities of 1-sun irradiation, 1.5-sun irradiation, and 2-sun irradiation, respectively. In comparison to 1-sun irradiation of light intensity, the production of hydrogen increased significantly by up to 129% under 2 kW·m⁻² of light intensity. This phenomenon can be ascribed to the photothermal–photocatalytic synergy mechanism. Specifically, the elevation of temperature decreases the activation energy of the reaction. Simultaneously, the augmentation of photon flux density elevates the concentration of electron-hole pairs. This increase effectively surmounts the reaction energy barrier, and the variation in the reaction Gibbs free energy is directly offset by solar energy.

Continuous evaporation leads to varying salt concentrations on either side of the Nafion 117 membrane. Driven by the concentration gradient, Na⁺ migrates through the Nafion 117 membrane towards the side with lower concentration, while Cl⁻ is selectively blocked. The Nernst equation indicates a positive correlation between the ion concentration in solution and voltage output of RE. Under light intensities of 1-sun irradiation, 1.5-sun irradiation, and 2-sun irradiation, the salt content of the brine in the distillation chamber increased from 3.5 wt% to 4.94 wt%, 6.08 wt%, and 7.56 wt%, respectively (Figure S8). This can be primarily attributed to the temperature rise, which enhances the seawater evaporation rate and consequently increases the salinity gradient. Under the synergistic effect of temperature and the salinity gradient, the ion diffusion flux across the Nafion 117 membrane is enhanced, significantly increasing the open-circuit voltage of the system. As shown in Figure 3e, under light intensities of 1-sun, 1.5-sun, and 2-sun irradiation, the open-circuit voltages of the device reached 10.44 mV, 18.45 mV, and 27.9 mV, respectively. A notable acceleration in the voltage increase rate was observed after 60 min, coinciding with the rise and eventual stabilization of the temperature at the evaporation interface. High solar intensity not only increases the concentration gradient between solutions by enhancing the evaporation rate, but also boosts the ion migration efficiency through thermodynamic effects. Under light intensities of 1-sun irradiation, 1.5-sun irradiation, and 2-sun irradiation, the SWHE’s highest output power was 0.28 mW·m⁻², 0.85 mW·m⁻², and 2.9 mW·m⁻², separately (Figure S9). Figure 3f shows the water, hydrogen, and electricity production capacity of the SWHE with various light intensities. Under the irradiation of 2-sun irradiation, the output capacity of the device is the highest, with water generation reaching 1.844 kg·m⁻², hydrogen production reaching 25.72 mmol·m⁻², and the maximum voltage reaching 27.9 mV.

3.2. Reliability Testing of the Device

To verify the operational reliability of the device in actual complex environments, the performance of the device was systematically evaluated using real seawater as the feed solution. Additionally, a Poisson–Nernst–Planck theory-based numerical simulation model was developed to deeply analyze the physical and chemical processes involved in the device’s operation. The established two-dimensional axisymmetric model is shown in Figure 4a. On both sides, there are two cylindrical nano-solution pools, each measuring Rp in radius and Lp in length. The two solution pools are connected by a cylindrical nano-channel with a radius of Rp and a length of Lp. The nano-channel wall is negatively charged. Details of the boundary conditions (Table S2) and model structural data (Table S3) are provided in the Supplementary Materials.

To investigate the reliability of the SWHE system, the change in seawater salt concentration was calculated based on the system’s water generation rate, and a numerical simulation was conducted to analyze the power output capacity of the RE module. After being exposed to solar intensities of 1-sun irradiation, 1.5-sun irradiation, and 2-sun irradiation for 3 h, the simulated open-circuit voltages were 12 mV, 18.7 mV, and 27.5 mV, respectively. The corresponding experimental values were 10.439 mV, 18.454 mV, and 27.91 mV, with relative errors of 14.9%, 1.33%, and 1.47%, respectively (Figure 4b). Figure 4c shows the trend of the simulated current of the SWHE varying with solar intensity by different solar intensities. The simulation results are essentially in line with the outcomes of the experiment, which shows that ion transport in the Nafion 117 membrane can be adequately described by this model. Furthermore, to investigate the influence of light intensity variation on ion diffusion, this paper calculated the variation in the ion diffusion coefficient along the axial direction of the pore under varying solar intensities. Figure 4d shows that the N⁺ diffusion coefficient grows with the enhancement of light intensity, and this phenomenon is particularly significant on the side closer to the light source. This is explained by the interface evaporator absorbs spectral energy, causing the temperature and concentration of the water body to increase. This promotes the electrophoretic movement of ions from the high-concentration side to the low-concentration side, thereby generating a larger ion current.

Compared with the NaCl solution that simulates seawater, the composition of real seawater is often more complex, which poses challenges to the practical application of the device. This study employed a systematic test with real seawater to evaluate the reliability of the device. Figure 4e shows that with the light intensity of 1 kW m⁻², the device achieved the highest voltage of 14 mV and highest hydrogen generation rate of 11.48 mmol m⁻². Subsequent water quality tests on the desalinated water obtained through condensation and real seawater showed that the concentration of K⁺ dropped from roughly 875 mg L⁻¹ to 1.07 mg L⁻¹, Ca²⁺ from 561 mg L⁻¹ to 6.9 mg L⁻¹, Na⁺ from 9978 mg L⁻¹ to 5.82 mg L⁻¹, and Mg ²⁺ from 1604 mg L⁻¹ to 0.28 mg L⁻¹. These results met the drinking water standards set by the World Health Organization, demonstrating the excellent desalination capability of the SWHE device (Figure 4f).

4. Outdoor Performance Evaluation

To verify the operational capability of the SWHE equipment in real outdoor environments, on 8 April 2025, from 9:00 to 15:00, an outdoor experiment was carried out on the rooftop of the Wuhan Institute of Technology laboratory building. Figure 5a shows the layout of the outdoor experimental setup, where a hygrograph and a solar intensity meter are used to detect the real-time variation in environmental temperature and solar intensity over time. The setup collects hydrogen gas in the chamber every 30 min using a micro-injector and delivers it to a gas chromatograph for detection. The voltage output of the device is monitored in real time by a KDMS-2450 source meter.

Figure 5b shows that the average daily light intensity was 803 W·m⁻² while the average ambient temperature was 31.36 °C. The SWHE is exposed to sunlight, allowing water vapor to pass through the Pt/TiO₂–PTFE composite membrane and condense on the wall surface, thereby transforming into water droplets to be collected (Figure 5c). Figure 5d shows the variation in hydrogen generation and voltage during the operation of the SWHE device. Eventually, 12.22 mmol m⁻² of hydrogen was collected. In addition, a 14.56 mV open-circuit voltage as well as a 0.095 mW·m⁻² highest power density were attained (Figure 5e). It is worth noting that the hydrogen production rate fluctuates with the variation in light intensity, as shown in Figure 5f. At the peak solar intensity period of 12:30 (0.97 kW m⁻²), the hydrogen evolution rate reached a peak of 3.9 mmol·m⁻²·h⁻¹, indicating that the fluctuation of light intensity directly affects the carrier separation efficiency of the photocatalytic active sites. After 6 h of outdoor operation, SWHE achieved the water production of 1.6 kg·m⁻², hydrogen generation of 12.22 mmol·m⁻², and a maximum output power of 95.5 µW m⁻² (Figure 5g). The outdoor experimental results fully demonstrate that the SWHE device can stably produce freshwater, hydrogen, and electricity in an environment with fluctuating natural solar intensity and temperature changes.

In summary, this system not only produces freshwater through interface evaporation, but also simultaneously generates hydrogen energy and electrical energy by utilizing the high-energy photons in sunlight and the salinity gradient generated during evaporation. Thus, it achieves efficient utilization of solar energy. A detailed comparison of this SWHE multi-energy combined system with other systems can be found in Supplementary Materials Table S4.

5. Conclusions

In summary, this study successfully developed and demonstrated a hybrid solar-driven water–hydrogen–electricity co-production system that integrates interfacial evaporation, photocatalytic hydrogen evolution, and reverse electrodialysis. The key conclusions are as follows: (i) Unlike the previous SIE systems, the SWHE system effectively utilizes the additional salinity gradient generated during the interface evaporation process and the high-energy photons in sunlight, significantly improving the efficiency of solar energy utilization and achieving additional hydrogen and electrical energy outputs. The analysis of the experimental results shows that with the light intensity of 2-sun irradiation, SWHE can achieve a freshwater generation rate of 0.77 kg·m⁻² h⁻¹, a hydrogen generation rate of 25.72 mmol·m⁻², and a highest power density of 2.9 mW·m⁻². (ii) Experimental results align well with numerical simulations based on the Poisson–Nernst–Planck model, confirming the reliability of the proposed mechanism and system design. (iii) Moreover, SWHE can still maintain stable operation in the outdoor real environment (the average light intensity is 803 W m⁻²), achieving a freshwater generation rate of 0.27 kg m⁻² h⁻¹, a hydrogen production rate of 3.9 mmol m⁻² h⁻¹, and a highest power density of 0.095 mW m⁻².

Although this study demonstrates the successful co-production of freshwater, hydrogen, and electricity, certain limitations in the SWHE system remain and warrant further investigation: (i) The hydrogen production efficiency of the device needs to be further improved. In the future, higher-efficiency and low-cost photocatalytic materials can be adopted to increase the hydrogen output of the device. (ii) The system performance needs to be optimized. By introducing low-grade waste heat or improving the structure of the device, it can adapt to low light intensity and fluctuating natural environments, enabling the device to operate. Overall, the proposal of this system provides a new idea for efficient solar energy utilization in the interface evaporation process and helps to build a sustainable new energy production system.