Solar-driven interfacial evaporation technologies for food, energy and water

Introduction

Many people lack secure access to food, energy and water (FEW), especially in rural areas or areas that lack centralized infrastructure and access to efficient distribution networks. In 2022, for instance, 1.3 billion people in rural areas lacked access to safely managed drinking water¹. Technologies are needed that will alleviate FEW insecurities, but these technologies should be cost-effective and community-managed and they need to work off-grid to be effectively implemented.

Solar evaporation is a well-established method of evaporating seawater to obtain salt and fresh water² without external energy inputs. This process can be sped up by floating a solar evaporator on the water’s surface to capture solar energy and localize this energy to evaporate water molecules^3,4. This method, known as solar-driven interfacial evaporation, offers high solar-to-vapour efficiency independent of water volume³ and is adaptable to various applications^5,6,7,8,9. After nearly a decade of research, this technology is approaching its thermodynamic efficiency limit of 92.5%⁵ and produces fresh water at a rate exceeding 80 l m^–2 h^–1 under concentrated sunlight¹⁰, operating stably for over 600 hours¹¹.

Interfacial solar evaporation technologies have potential applications across the FEW nexus (Fig. 1). Potable water can be generated from various water sources (including seawater⁴, brackish water¹² and industrial wastewater¹³) and the resulting water can be used domestically and in food production¹⁴. Wastewater — including domestic sewage¹⁵ and industrial wastewater from textiles¹⁶, thermal power plants¹³ and pharmacy manufacturing¹⁷— can be treated with this technology, reaching zero liquid discharge¹³ (the complete separation of water and solutes). Hot vapour generation with solar evaporation can be used for disinfection and sterilization^18,19. Moreover, interfacial solar evaporation can integrate with other renewable energy technologies to boost evaporation rates, electricity output and system efficiency, enhancing practicality and viability²⁰.

Solar-driven interfacial evaporation technologies for food, energy and water — **Fig. 1: Food, energy and water from interfacial solar evaporation technologies.**

In this Review, we discuss interfacial solar evaporation innovations and design strategies in materials, devices and systems for water, food, energy, resources and wastewater treatment. We assess these technologies within the framework of production capabilities, suitability for low-resource settings, and key challenges for practical implementation. Last, we explore co-generation strategies and synergies with other technologies.

Water production and treatment

Interfacial solar evaporation efficiently separates water and solutes by localizing the solar heat at the liquid–vapour interface, transforming liquid water into vapour while leaving solutes behind⁶ (Fig. 2a). The resulting vapour can be condensed into drinking water or utilized in catalytic processes to produce hydrogen and other valuable chemicals. The solutes can be concentrated and precipitated, facilitating the extraction of valuable minerals like lithium and uranium. This section discusses water-centred applications of solar-driven interfacial evaporation, including clean water production, wastewater treatment and resource recovery from water, and efforts to enhance the performance and practical implementation of these technologies.

Clean water production

Interfacial solar evaporation purifies water by condensing vapour from sources such as seawater, brackish water and industrial wastewater (Fig. 2a). The resulting distilled water consistently meets the World Health Organization’s ion concentration standards for drinking water^3,21,22. This strategy has an estimated cost of US$0.4–2.2 per ton^{23,24,25,26,27}, cheaper than small-scale reverse osmosis (US$5–10 per ton)²⁸ and comparable to large-scale water purification systems (US$0.9–1.4 per ton)²⁹. The decentralized nature of solar evaporation and its potential ability to provide high-quality water at low cost makes it a potential solution for drinking water in rural areas or islands⁸. There are various methods of producing clean water, including tandem evaporation and condensation, solar steam-driven membrane desalination and interfacial solar-driven atmospheric water generation, as discussed below.

Evaporation–condensation purifiers

Water purifiers that use a tandem solar evaporation and condensation strategy operate on the same fundamental principle (Fig. 2a), even when using different system designs. Solar evaporators made from materials with high solar absorptivity, such as carbon-based substances and plasmonic metals, capture and convert solar energy into heat through processes like electron excitation–relaxation^30,31 and the localized surface plasmon resonance effect^4,32. This localized heating at the evaporation interface accelerates the transition of water from liquid to vapour. The generated vapour diffuses towards the condensation surface, driven by the vapour pressure gradient. At the condensation interface, the vapour reaches a supersaturated, high-energy metastable state, leading to nucleation upon perturbation. The nucleated water droplets grow and coalesce, eventually reaching a critical size where they overcome surface tension and slide down under gravity, collecting as condensate water^33,34. Advances in materials and device development have enabled evaporation efficiencies of over 90% under reduced optical concentrations^{35,36,37,38,39}.

Condensation is the limiting factor in water productivity, because the evaporation rate often exceeds the condensation rate, resulting in insufficient vapour recovery⁴⁰. Traditional solar purifiers use a single- or double-sloped transparent cover positioned above the evaporator. The cover allows sunlight to pass through while facilitating condensation when the vapour reaches saturation^41,42 (Fig. 2b). Although easy to install, this structure has two limitations: the polymer or glass materials used for the cover typically have low thermal conductivity (<5 W m^–1 K^–1)^40,43, leading to insufficient heat transfer and hindering condensation; and the condensed droplets exacerbate the thermal barrier and scatter sunlight, resulting in substantial light loss of up to 35%⁴⁴.

To maximize water productivity, the condensation structure can be optimized and condensation efficiency can be enhanced^45,46,47. For example, hydrophilic and/or hydrophobic modifications of the cover have been developed to enhance condensation⁴⁸ (Fig. 2b). Hydrophilic covers promote filmwise condensation, reducing the light scattering caused by droplets. However, the water film formed on the cover has relatively low thermal conductivity (about 0.6 W m^–1 K^–1), which is detrimental to condensation⁴⁹. By contrast, hydrophobic covers encourage dropwise condensation, with the heat transfer coefficient being 5–7 times higher than that of filmwise condensation³⁴. Nevertheless, they suffer from substantial optical loss and potential durability issues.

Inverted-structure solar purifiers have been designed to overcome the limitations and enhance condensation⁴⁰. The inverted structure places the condensation surface below the solar absorber, thereby decoupling the light absorption and condensation (Fig. 2c). As such, the condensation surface does not require high light transmittance⁴⁵. The increased flexibility in material choice allows for the use of materials with high thermal conductivity and hydrophophic properties such as copper⁴⁰ and aluminium (thermal conductivity of >400 W m^–1 K^–1)⁵⁰ to enhance condensation. A 70% solar-to-clean-water efficiency has been achieved in the inverted condensation set-up⁴⁰, compared with 20–50% in traditional set-ups.

Multistage solar water purifiers harvest and reuse the vaporization enthalpy by using multiple hydrophilic water supply layers and condensation layers separated by air gaps^{50,51,52,53,54} (Fig. 2d). This design recycles the latent heat released during condensation in one stage for evaporation in the next, enabling water production efficiency beyond the thermodynamic limit^50,51. Optimizing the structure of a multistage still, such as the number of stages N and the thickness of the air gap t_g, can further improve performance^50,55. For instance, increasing N has produced a sixfold enhancement in water production rate compared with a single-stage device when N > 20 (ref. ⁵⁰). However, water production and investment costs must be balanced when determining the number of stages, because overall efficiency does not increase linearly with N thanks to cumulative heat losses. Additionally, t_g substantially affects water production. A small t_g < 2.5 mm leads to strong heat conduction that decreases efficiency, whereas a large t_g > 2.5 mm increases mass transport resistance, also limiting efficiency⁵⁵. Therefore, quantitative analysis of heat and mass transfer and careful optimization of multistage device design are necessary⁵⁰.

Steam-driven membrane desalination and beyond

Solar steam-driven membrane desalination uses high-temperature high-pressure steam generated by interfacial solar heating to push salty water through a reverse osmosis or nanofiltration membrane to produce clean water¹⁰ (Fig. 2e). Membrane desalination involves separating ions and water molecules using a selective membrane and requires much less energy than a single-stage solar evaporation–condensation purifier — the theoretical energy consumption for seawater desalination is 5.76 kJ kg^–1, nearly three orders of magnitude lower than for the single-stage evaporation-condensation purifier, which is constrained by the vaporization enthalpy of water (about 2,400 kJ kg^–1)⁵⁶. Owing to the relatively low separation energy requirement, these membrane-desalination-based devices have reached rates of up to 81 l m² h^–1 under 12-sun illumination¹⁰. This rate is higher than for single-still evaporation–condensation purifiers, which have a theoretical water production rate of 1.47 l m² h^–1 under one-sun illumination⁵.

Thermal-responsive shrinking can drive water through a membrane by utilizing materials that undergo substantial shape or size changes in response to temperature variations, such as solar heating. This mechanism has been demonstrated in a solar-driven water purifier that combines a graphene nanofiltration membrane and poly(nisopropylacrylamide) (poly-NIPAM) with switchable hydrophilicity⁵⁷. In this system, the purifier absorbs a large amount of water from a polluted water source. Upon photothermal heating, the absorbed water is released via the thermal-responsive hydrophilicity switching in poly-NIPAM (from hydrophilic to hydrophobic) while the outer graphene membrane effectively retains ions and molecules with a high rejection rate (>99%)⁵⁷.

Integrating interfacial solar heating and atmospheric water harvesting allows direct generation of water from atmospheric moisture, further broadening the potential applications of both techniques^58,59. This combination was demonstrated in an interfacial solar-driven atmospheric water generator that combined interfacial solar evaporators with an atmospheric water-harvesting sorbent based on ionic liquid⁵⁸ (Fig. 2f). The system operates through a simultaneous adsorption–desorption process, and under outdoor conditions had a water production rate of 2.8 l m^–2 per day in outdoor conditions⁵⁸. In principle, this system can provide fresh water flexibly in areas that have an acute need for clean water but do not have a surface source of liquid water (like the ocean)⁶⁰. However, improvements are needed, given that its water yield is typically lower than that of liquid-water-based processes^61,62.

Application and the next generation

Clean-water production is an established application of interfacial solar evaporation with production rates of up to 81 l m^–2 h^–1 (ref. ¹⁰) under concentrated sunlight (Fig. 2g and Supplementary Table 1). Several simple and portable evaporation–condensation solar-powered water purifiers have advanced beyond the laboratory and are now commercially available (Fig. 2h). Once deployed, they float on water, absorb sunlight and convert it into heat, producing water vapour that condenses into purified water that is collected in a water bag attached to the device. These stills can reportedly produce approximately 2 l of water per day.

Advanced multistage^{50,51,52,53,54} and solar-membrane water purifiers¹⁰ have been evaluated at a scaled performance of 1 m² and are capable of producing enough drinking water for an individual (3 l per person per day) and even meeting the basic clean-water needs for living (50 l per person per day). For example, a 10-stage device is expected to generate 36–60 l of clean water per day during the sunny summer months⁴⁵. Solar steam-driven membrane desalination can produce over 600 l per day under 8–10 hours of 12-sun irradiation¹⁰. However, these systems are not yet commercially available. Scalability remains a challenge — they are effective at the laboratory scale (typically under 1 m²), but adapting them for larger communities or industrial use requires substantial technological refinement and infrastructure.

Although real-world application demonstrates the usefulness of this technology in clean-water production, further development and engineering are needed to address challenges such as the intermittent production caused by diurnal cycles, scale, stability and cost. The next generation of prototypes should aim to produce several litres of water per day to meet human daily needs. This scalability is feasible owing to the cost-effective and industrially mass-produced materials used in solar evaporation. Intermittent water production could be addressed through combining solar energy technologies like photovoltaic (PV) cells⁶³ and phase-change materials⁶⁴ to store energy during the day and use it at night for water purification, enabling continuous, all-day solar-powered water production.

Improving condenser performance is key to overall system performance, because the condenser typically contributes the most mass to the system. Indeed, the weight of the condenser is more than 40 times that of evaporator or sorbent⁶⁵, greatly reducing the water yield per unit mass of the device. Strategies such as nanostructuring the surface of the condenser^33,66 and implementing passive radiative cooling^67,68,69 are expected to enhance the performance of the condenser. Nanostructuring creates micro- and nanoscale textures that promote the rapid coalescence of water vapour to liquid³³. Passive radiative cooling lowers the surface temperature by emitting thermal radiation to the cold sky, creating a temperature gradient that accelerates condensation.

Active solar water generators, powered by electricity from solar energy, deliver higher water productivity than passive systems⁷⁰. PV-driven reverse osmosis, a typical active system, converts solar energy into electricity via PV cells to drive the reserve osmosis membrane, producing clean water at a rate of 12–15 l m^–2 h^–1. These rates are an order of magnitude higher than those of most passive systems⁷¹. However, PV-driven reverse osmosis systems are heavier and larger owing to the additional PV modules.

Finally, most solar water generators are tested under stable indoor conditions with standard 1-sun illumination or in ideal outdoor conditions, such as sunny summer days or at noon⁹. These conditions are not realistic in terms of real-world operation, which has fluctuating solar illumination and substantial temperature variations. Therefore, the productivity and stability of upcoming solar water generators should be monitored and maintained over several months in the field. Factors such as water quality, especially organic matter content, must be tracked, as volatile organic compounds can evaporate with water, contaminating the distillate and compromising drinking water safety. Additionally, investment and maintenance costs must be thoroughly evaluated to assess the technology’s readiness for long-term field operation.

Wastewater treatment

Interfacial solar evaporation is being tested in the treatment of various types of wastewater, such as industrial wastewater, brines and domestic sewage^21,72 (Fig. 3a). In this process, liquid water is continuously evaporated until only solids remain, achieving zero liquid discharge⁷³. This approach mitigates the ecological threat of discharging highly concentrated brine and allows for the reuse of evaporated water by coupling a condenser, with the remaining solutes used for resource extraction.

However, given that wastewater is a complex mixture that can contain high levels of salts, bacteria, oil and organic compounds, it can cause severe fouling on the solar evaporator. This fouling can block water flow and light absorption, leading to evaporator malfunction^74,75,76,77. Therefore, developing evaporators with excellent operational stability, including resistance to salt, biofouling and organic contamination, is crucial for the successful application of interfacial solar evaporation in the treatment of wastewater.

Salt resistance

Salt resistance is essential for solar evaporators, because salt buildup is a common fouling issue in wastewater treatment^9,78,79. When the rate of water evaporation exceeds the rate at which salt ions return to the bulk water, the local salt concentration increases rapidly, resulting in salt deposition on the evaporator. Strategies to enhance salt resistance can be grouped into three main approaches: reducing local salt concentration^80,81,82,83, directing salt precipitation⁸⁴ and implementing contactless evaporation²².

A Janus evaporator with dual hydrophilicity can reduce the salt concentration on its top surface, thereby preventing salt fouling^80,85,86,87 (Fig. 3b). In this design, the upper surface is hydrophobic, acting as a protective barrier to prevent salt ions from reaching the evaporator’s top surface. The lower surface is hydrophilic, facilitating the back diffusion of salt ions into the bulk water and reducing the salt concentration within the evaporator. This approach is simple and feasible, using a common hydrophobic coating — fluorinated alkyl silane — which can be easily brushed onto the evaporator’s surface, and it maintained a stable water yield of 1.46 l m^–2 h^–1 for 15 days using seawater from the Yellow Sea⁸⁵. However, ensuring the long-term stability of the hydrophobic and hydrophilic surfaces is a challenge. Prolonged exposure to sunlight and oxidative substances in water potentially degrades these functional surfaces, diminishing the performance of the evaporator^88,89.

The Donnan effect can also be used to reduce salt concentration in solar evaporators⁸³ (Fig. 3c). This approach uses a polyelectrolyte hydrogel evaporator with anions (such as COO⁻) fixed on its surface and cations (such as Na⁺) within to maintain electric neutrality. The confined Na⁺ ions establish a Donnan distribution equilibrium at the boundary between the evaporator and the surrounding brine, which reduces salt ion diffusion into the evaporator and minimizes local salt concentration. This approach supports stable evaporation of 1.3 l m^–2 h^–1 for 11 days with a 15 wt% NaCl solution⁸³. However, the effectiveness of Donnan-effect-based salt resistance is reduced in high-salinity water with salinity greater than 150 g l^–1 owing to the charge screening effect⁸³.

Structural design can also be used to reduce salt concentration within the evaporator. For example, incorporating macroscopic water transport channels into the evaporator improves water convection, facilitating salt ions flow back to the bulk water and preventing salt buildup^81,90 (Fig. 3d). However, this enhanced water convection also increases heat loss to the surrounding water, reducing evaporation efficiency to approximately 57% under 1-sun illumination. Therefore, several materials and structure designs have been developed to balance thermal localization and salt rejection^91,92, such as enhancing conductive heat recovery⁹³ and optimizing transport channel diameter⁹⁴. Specifically, selecting an appropriate channel size induces natural convection, which speeds up salt rejection and minimizes heat loss, achieving an efficiency of 91%⁹⁴.

Directing salt crystallization to specific areas of the evaporator and regularly removing it can enhance operational stability while maintaining high photothermal evaporation rates^11,13. These salt crystallizers feature elaborately designed structures and shapes, such as disc¹¹, cone⁸⁴, cup^74,95 and particle shapes^13,76 (Fig. 3e–h). The disc evaporator¹¹ uses a one-dimensional water uptake thread at the centre, creating a radial concentration gradient that directs salt crystallization to the edges (Fig. 3e). The disc design enables continuous, stable steam generation and salt harvesting for over 600 h with a 3.5 wt% NaCl solution¹¹. Similarly, 3D cone evaporators form a water film with varying thickness and temperature gradients, causing salt crystallization at the apex, where the water film is thinner and hotter⁸⁴ (Fig. 3f). This design achieves an evaporation rate of 2.63 l m^–2 h^–1 and an energy efficiency of over 96% with a 25 wt% NaCl solution⁸⁴. Cup evaporators physically separate the water evaporation and the light absorption surface⁹⁵ (Fig. 3g). The bottom and inner walls absorb solar energy and then transfer the heat to the outer wall, where evaporation and salt crystallization occur, achieving an evaporation rate of 2.42 l m^–2 h^–1 with 21.6 wt% brine⁹⁵.

Manual^74,95 and automatic^13,42 methods can be used to remove salt crystals. For example, a dynamic and self-cleaning salt crystallizer has been developed as an automatic method¹³ (Fig. 3h). In this approach, a thin water film at the top (red spots in Fig. 3h) promotes preferential salt crystallization. As the salt crystals grow and disrupt the mechanical balance, the spherical crystallizer rotates, self-cleans and restarts the process. The surface tension causes these spherical crystallizer to act collectively, because the rotation of one crystallizer can induce correlative rotations of its nearby ones, leading to simultaneous self-cleaning of the whole system¹³. Of the crystallizer shapes discussed, spherical salt crystallizers are well suited for practical applications thanks to their self-cleaning ability and scalability. Over 100 m² of crystallizers have been constructed and used to treat wastewater from coal-fired power plants, accelerating the concentration process¹³ (Fig. 3i).

Contactless evaporation is the most stable and effective salt-resistant strategy^94,96, because it spatially separates the solar evaporator from the water (Fig. 3j). This approach requires an evaporator with asymmetric optical properties: the top surface needs high visible absorption and low infrared emission, whereas the bottom surface requires high infrared emission⁹⁶. Such an evaporator can efficiently convert sunlight into thermal radiation, which is then emitted towards the wastewater and absorbed within a very thin layer (<100 μm) beneath the water–vapour interface, forming an interfacial evaporation structure²². This method offers long-term stability for high-salinity wastewater treatment and complete separation of solution and solute. However, owing to its non-contact nature, the photothermal conversion efficiency of contactless evaporation is limited to a maximum of 51%⁹⁴, much lower than traditional contact-mode solar interfacial evaporation (>90%)⁶. Therefore, further improvements are needed to optimize heat and mass transport to improve solar utilization efficiency to make the contactless evaporation commercially viable. Higher efficiency would increase water yield and lower costs, making the process more competitive and sustainable for large-scale applications like desalination and wastewater treatment, where contact-mode systems face salt-fouling challenges.

Biofouling and organic contamination resistance

Solar evaporators are vulnerable to biofouling and organic contamination. For example, microorganisms and bacteria can adhere to the evaporator, forming biofilms that hinder water transport and light absorption, compromising operational stability⁹⁷. A key strategy to combat biofouling is integrating antibacterial materials into the evaporator (Fig. 3k). Common antibacterial materials include inorganic nanomaterials like metallic nanoparticles^98,99, reduced graphene oxide^77,100, MXene^101,102, and organic polymers such as chitosan¹⁰³ and polyethyleneimine¹⁰⁴. These materials act by disrupting cell walls¹⁰¹, denaturing bacterial proteins⁹⁸ and/or generating reactive oxygen species¹⁰³. Evaporators decorated with these antibacterial materials have demonstrated nearly 100% effectiveness against various bacteria, including Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus^103,105.

Organic pollutants that are found in wastewater and affect evaporator performance include non-volatile organic compounds, such as oils, and volatile organic compounds (VOCs). High-viscosity oil contaminants in wastewater, for example, can clog pores and obstruct water flow in evaporators¹⁰⁶. An effective way to prevent oil fouling is by creating superoleophobicity on the evaporator surface^77,107 (Fig. 3l). Cassie contact induced by nanostructures can enhance surface oleophobicity, achieving a contact angle of 143° with soybean oil underwater⁷⁷. This angle allows oil droplets to retain their spherical shape, thereby preventing them from sticking to and clogging the evaporator.

VOCs can evaporate together with water and pose health risks^108,109,110, so physical and chemicals approaches are being developed for their removal from wastewater with solar evaporation¹⁰⁹. Merging photocatalysis with interfacial solar evaporation is the most common chemical VOC removal approach (Fig. 3m). This method uses sunlight to excite photocatalysts to generate strong oxidizing •OH radicals that degrade VOCs into CO₂ and other nonhazardous products^{111,112,113,114}. For example, a reduced graphene oxide/polypyrrole aerogel has achieved an evaporation rate of 2.08 l m^–2 h^–1 and 94.8% VOC removal at 20 parts per million (ppm)¹¹⁵. However, photocatalysis becomes less effective at higher VOCs concentrations (<20 ppm), limiting its use to low-concentration wastewater¹¹⁶.

Physical methods, such as molecular sieving¹¹⁷ (Fig. 3n) and semipermeable membrane filtration^77,110,118 (Fig. 3o) have been proved effective in removing higher concentrations of VOCs in condensate water during solar evaporation. Both methods allow water to pass through while blocking VOCs, but they rely on different mechanisms. Molecular sieving is based on the size-exclusion effect, whereas semipermeable membranes exploit different affinities of the membranes for water and VOCs. These physical methods effectively treat high-concentration VOCs (up to several hundred parts per million) with a retention rate of 99% at 400 ppm (ref. ¹¹⁷). A potential issue with physical VOCs removal methods is that they do not degrade the pollutants, which could lead to secondary pollution. In this regard, developing physicochemical strategies that combine the advantages of both physical and chemical approaches is essential to achieve the efficient degradation of high-concentration VOCs.

Application and the need for rigorous long-term testing

For use in the real world, with complex contaminants in wastewater, solar evaporators designed for wastewater treatment need to be resistant to fouling from many sources. Some progress has been made in this area, such as the development of a multi-effect anti-fouling evaporator that resists salt, bacteria, oil and VOCs. This evaporator features a graphene/alginate hydrogel evaporator with a dense internal structure and bio-inspired surface engineering⁷⁷. Ion rejection (>99.3%) is achieved by creating a high osmotic pressure difference between the graphene/alginate hydrogel and contaminated surface water. The underwater anti-oil-fouling function is achieved through superoleophobicity (contact angle >140°) based on hydrophilic micro–nanostructure-induced Cassie contact. VOCs are removed (>99.5%) through the differential transport abilities of water and contaminant molecules in graphene/alginate hydrogel. Antibiotic properties are conferred by the exposed reduced graphene oxide nanosheets. The multiple anti-fouling mechanisms suggests that this graphene/alginate hydrogel can process wastewater with complex components into safe drinking water.

An important prerequisite for the commercialization of interfacial solar-driven wastewater treatment is a thorough assessment of stability over several months in real wastewater. Technology validations on simulated wastewater are acceptable, provided that these solutions mimic the physical, chemical and biological constituents of real wastewater. However, many studies primarily use sodium chloride solutions, which do not adequately represent real wastewater complexities. Difficult-to-remove salts, such as calcium sulfate, barium sulfate and silica, can form challenging scales; excluding these from simulations can lead to misleading results⁹⁵.

Monitoring the performance and stability of wastewater evaporators in the field for several months is crucial to prove the technology’s long-term viability. Extended field testing allows evaluation under real environmental conditions, with variations in temperature, humidity and pollutant levels that can strongly affect performance. Additionally, long-term monitoring provides insights into system lifespan and maintenance needs. However, most reported field tests last less than 1 month. A minimum field test period of 6–12 months is recommended to assess performance stability, accounting for changes in weather and wastewater composition.

Resources from water

Resource extraction from water through interfacial solar evaporation is emerging as a way to meet the growing demand for resources such as critical metals^119,120 and fuels^{121,122,123,124,125}. For instance, valuable metals like lithium and uranium can be extracted from brines during evaporation and catalytic techniques like photocatalysis and electrolysis can be integrated with solar-driven interfacial evaporation to produce chemical fuels like hydrogen. This section describes resource recovery and generation using interfacial solar evaporation.

Critical metals

Interfacial solar evaporation can enable the selective extraction of critical metals (like lithium and uranium) from aqueous sources (such as seawater, salt lakes, and geothermal aquifers)¹¹⁹. However, the salinity of seawater and brines (35 to 400 g l^–1) and the trace level of target resources in aqueous solutions (sometimes as low as parts per billion¹²⁶) mean that these technologies must be salt-resistant and highly selective.

In one approach, interfacial solar evaporation has been combined with membrane technology to use the large, passively generated capillary pressures during interfacial solar evaporation to drive the membrane separation process, thereby eliminating the need for mechanical pumps¹¹⁹. For example, a solar transpiration-powered lithium extraction and storage (STLES) device comprising three functional layers has been developed for sustainable lithium recovery from brines¹¹⁹ (Fig. 4a). The top layer is a solar transpirational evaporator, acting as a green high-pressure (around 18 bar) and high-permeance (1.8 l m⁻² h⁻¹) pump. In the middle is a lithium storage layer, which conducts pressure and water between the evaporator and membrane and stores the extracted lithium salts. The bottom layer is a Li⁺-selective membrane, which allows Li⁺ to pass through and block other coexisting ions, like Ca²⁺ and Mg²⁺. The fabricated STLES operates through three main steps. First, solar transpiration creates a high capillary pressure within the evaporator. Next, this transpiration pressure is transmitted to the membrane, resulting in an influx of lithium from the brine to the lithium-storage layer. Finally, water circulation transports the extracted lithium to the reservoir and regenerates the device. The modular configuration allows scalable lithium extraction by combining multiple STLES units into an extended platform (Fig. 4a). For proof of concept, a 50 cm × 30 cm STLES platform was fabricated and successfully operated using solar energy to extract lithium from brines.

**Fig. 4: Resource extraction via solar-driven interfacial evaporation.**

The evaporation-driven membrane filtration strategy is versatile and can be adapted for various applications by replacing the membrane¹²⁷. For instance, capillary-powered desalination water bottles have been developed by combining solar evaporation with a reverse osmosis membrane^128,129. These bottles, with a 9.4 cm diameter and a 10 cm wide annular fin, are capable of producing approximately 1 l of fresh water daily from seawater, thus facilitating water production in coastal regions.

Another strategy for extracting critical metals from water sources combines interfacial solar evaporation with selective adsorption^130,131 (Fig. 4b). Interfacial solar evaporation offers two key benefits. First, it accelerates ion adsorption through enhanced diffusion and mass transport¹³²; second, it boosts ion adsorption capacity because ion adsorption is typically endothermic, and increasing temperature shifts the equilibrium towards greater adsorption capacity, according to Le Chatelier’s principle¹³³. This approach has been implemented in a Li⁺ sieve-integrated solar evaporator that was used for lithium recovery from a hypersaline salt lake¹³¹. Notably, both lithium capture capacity and adsorption kinetics are doubled thanks to the elevated temperature from the photothermal effect, which raises the temperature by 32 °C. Moreover, the produced fresh water is recycled for elution and Li⁺ sieve regeneration, achieving near-zero water and carbon footprint.

Similarly, various sorbents have been anchored to porous solar evaporators for selective metal recovery^130,134. However, a potential limitation of this approach is stability — the sorbent particles are loosely attached to the evaporator and can fall off during operation¹³⁵. An elegant solution is to directly functionalize the evaporator backbone. For instance, lithium-specific DNAzyme can be tethered to the polyacrylamide network via copolymerization, forming a stable DNA hydrogel for uranium extraction from natural seawater¹²⁰.

Solar-driven interfacial evaporation is one of the few technologies that can completely dehydrate hypersaline solutions. By exploiting differences in solubility (different salts crystallize under distinct conditions or spatial locations), this process can selectively separate valuable metals from brines. For instance, string-based evaporators have been developed for harvesting lithium from saline water based on spatially separated crystallization¹³⁶ (Fig. 4c). The string-based evaporators are made of porous, twisted cellulose fibres featuring a hydrophilic core and a hydrophobic exterior. When their ends are immersed in saltwater, the porous structure can raise water via the capillary action and allows for fast water evaporation on the side surfaces¹³⁷. As water evaporates, salts with higher concentrations and lower solubilities, like sodium chloride, crystallize at lower heights of the fibre evaporator. By contrast, salts with lower concentrations and higher solubilities, like lithium chloride, move further upward and precipitate near the top (Fig. 4c). This spatially separated crystallization enables the collection of lithium and sodium individually, eliminating the need for additional chemicals. Another notable advantage of string-based evaporators is their scalability and adaptability. For example, a prototype array consisting of 100 string-based evaporators has been successfully demonstrated (Fig. 4d). However, further investigation is required to examine the effects of scaling up a treatment system that incorporates a larger number of strings. Potential drawbacks could include reduced vapour diffusion¹³⁷.

Hydrogen

Merging interfacial solar evaporation with photocatalysis is being developed for the direct production of hydrogen from seawater^{121,122,123,124,125}. The interfacial photothermal–photocatalysis approach offers three key benefits: the water vapour generated is inherently purified, eliminating the need for large-scale capital desalination plants and complex corrosion-resistant electrodes; the configuration of the liquid–vapour interface facilitates the rapid removal of evolved hydrogen (H₂) and oxygen (O₂) gases, promoting the catalysis reaction towards the desired product; and vapor-phase photocatalysis demands less energy for water splitting due to the lower standard Gibbs free energy of formation of gaseous water compared with liquid ((Delta {G}_{{text{H}}_{2}text{O}(text{g})}) = –228.6 kJ mol^–1 versus (Delta {G}_{{text{H}}_{2}text{O}(text{l})}) = –237.2 kJ mol^–1)¹³⁸.

One interfacial photothermal–photocatalytic approach uses a biphase system, which is constructed by spin-coating photocatalytic cobalt oxide (CoO) nanoparticles onto carbonized wood slices¹²³ (Fig. 4e). Under sunlight, the charred wood generates abundant water vapour via photothermal transpiration, which is then split into H₂ by CoO. This photothermal–photocatalytic biphase system kinetically lowers the hydrogen gas’s transport resistance by nearly two orders of magnitude and thermodynamically reduces the interface barrier in the adsorption process of gas-phase water molecules to photocatalysts. As a result, the CoO–wood biphase system achieved a hydrogen production rate of up to 220.74 μmol h^–1 cm^–2, which is 17 times higher than that of CoO nanoparticles alone¹³⁹.

Following the same principle as the biphase systems, hybrid photothermal–photocatalyst sheets have been developed for concurrent water purification and hydrogen generation from seawater¹²¹. The hydrophobically treated sheets can float on water, allowing for complete separation of the photocatalysts from the solution underneath. This separation prevents fouling of the photocatalysts, which would otherwise compromise their performance over time, thereby enhancing their stability and effectiveness. As a result, this design confers an operational stability exceeding 154 h in seawater and other aqueous waste streams.

Interfacial evaporation can also be integrated with PV electrolysis to generate hydrogen¹⁴⁰. A prototype has been developed comprising a transparent cover, a PV cell and an electrolyser. The PV cells convert solar energy into electricity to power the electrolyser, which then uses this electricity for water splitting, producing H₂ and O₂. Importantly, the photothermal heating of the electrodes raises the local catalytic temperature, enhances convection above the catalyst surface for faster gas release, and enriches OH⁻ near the catalyst surfaces, all of which boost catalytic performance¹⁴⁰.

Scale and multi-product recovery are needed

Sustainable resource extraction through interfacial solar evaporation is interlinked with the concept of circular economy. Processing methods that allow for the simultaneous or sequential recovery of multiple products are needed to reduce the impact of resource extraction. For example, existing solar evaporitic technologies for lithium extraction from brines primarily focus on extracting lithium while discarding other valuable materials like the magnesium, potassium, calcium, sodium and boron present in the brines. To ensure sustainability and improve economic feasibility, future technologies should be designed for the joint recovery of two or more of these products.

Huge volumes of brine are processed at the industrial scale. For example, producing 20,000 tons of lithium carbonate requires the processing of approximately 7,668,254 m³ of a 700 ppm Li⁺ brine¹²⁶. Interfacial solar evaporation techniques for resource recovery will need to scale appropriately to be applicable in the real world. Therefore, experiments conducted under elevated solar irradiations or within small-scale closed chambers are useful for research purposes, but are only a starting point, and larger demonstrations are needed.

Food production

Interfacial solar evaporation produces water that is suitable for agriculture, driving efforts to integrate interfacial solar desalination with agriculture. Interfacial solar desalination–agriculture systems could be particularly useful in coastal areas where fresh water and fertile soils are scarce. Coastal farming often relies on costly and energy-intensive desalination methods^141,142, but solar evaporation systems passively desalinate water, making it more cost-effective^143,144,145. Solar evaporation systems can wash saline or contaminated soils, thereby expanding arable land¹⁴⁶. Finally, some of these systems can operate on the ocean, which reduces land use and is useful in space-constrained areas¹⁴. This section discusses interfacial solar evaporation applications in direct food production and soil remediation to increase land available for safe food production.

Farming on ocean via desalination

The Farming On Ocean via Desalination (FOOD) strategy integrates interfacial solar-driven seawater desalination with agriculture¹⁴. A FOOD system can have two separate chambers connected by a pipe to transport fresh water from the desalination chamber to the plant-growth chamber (the separate FOOD system in Fig. 5a). Desalination and agriculture can be independently optimized in this system, and strategies to enhance fresh water production could be implemented into the desalination unit. For instance, optical concentration of sunlight has been used to reduce the spatial requirements for solar evaporation (Fig. 5a), leaving more area available for agriculture¹⁴⁷. Multistage distillation¹⁴⁸, environment-enhanced evaporation¹⁴⁶ and Marangoni-driven salt rejection¹⁴⁹ (the arch evaporator in Fig. 5a) have also been used to improve the water production and long-term stability of the FOOD system.

**Fig. 5: Food production via solar-driven interfacial evaporation.**

The installation of FOOD systems with separate desalination and agricultural chambers is straightforward, but the system has two fundamental drawbacks¹⁴. First, the physically separated operational units can use a substantial amount of land (even with optical concentrators), which can be problematic in land-constrained areas. Second, the overall efficieny of solar energy utilization is low because photosynthesis relies solely on visible light, which is only 51% of solar energy^150,151. To overcome these limitations, vertical double-layer FOOD systems integrate solar desalination and plant cultivation on the same land¹⁵² (Fig. 5b). This design has two chambers, with agriculture in the upper chamber and fresh water production through interfacial solar desalination in the lower chamber. A water-transportation belt connects the two chambers and continuously supplies the desalinated water from the lower desalination chamber to the agricultural chamber. The vertical FOOD system uses space efficiently but lacks solar efficiency, because the upper agricultural chamber limits the sunlight reaching the lower desalination chamber.

Solar spectral management FOOD systems address space and energy utilization limitations by dividing solar energy into two components¹⁴. Shorter wavelengths (such as ultraviolet and visible light, about 300–800 nm) are used for plant growth and longer wavelengths (in the near-infrared, about 800–2,500 nm) are used for solar desalination (Fig. 5c). A key advantage of the solar spectral management FOOD system is that sunlight and fresh water are adjustable for different plants and growth stages. For instance, during the germination stage, seeds require abundant water but sunlight is unnecessary. A black dome, which absorbs the entire spectrum of sunlight (ultraviolet, visible and infrared), is installed and maximizes water production while completely blocking sunlight (Fig. 5d). During the plant growth stage, plants require both sunlight and water. Therefore, the black dome is replaced with a semi-transparent dome made of a selective photonic material that absorbs near-infrared light while allowing ultraviolet and visible light to pass through. This configuration enables ultraviolet and visible light to reach the plants for photosynthesis, while near-infrared light is absorbed by the outer cover for desalination (Fig. 5e). Consequently, this strategy optimizes both land and solar energy utilization efficiency.

Coupling other energy extraction technologies (such as salinity-gradient energy¹⁵³, tidal energy¹⁵⁴, thermoelectric energy¹⁵⁵, and wave energy¹⁵⁶) with the FOOD system can generate electricity in addition to food and fresh water, enhancing overall solar energy utilization. For example, a trinity FOOD system combining solar desalination, power generation and crop irrigation simultaneously produces fresh water, electric power and crop cultivation media using solar energy¹⁵³ (Fig. 5f). Solar desalination increases the salinity of seawater from around 3.50 wt% to around 5.19 wt%, providing a continuous supply of high-salinity seawater. Energy is extracted from the salinity gradient between high-salinity seawater and surface water using reverse electrodialysis, generating a voltage of approximately 220 mV and a maximum power density of 0.25 W m^–2. The fresh water produced from the solar desalination process (9.28 l m^–2 per day) is centrally collected and supplied to the wheat cultivation section, which requires 6.0 l m^–2 per day.

The floating cultivation farm based on interfacial solar evaporation leverages ocean and sunlight for agriculture, providing food in coastal regions. Crops such as wheat¹⁵³ and vegetables such as lettuce¹⁴⁸ and peas¹⁴ have been successfully cultivated via FOOD. To date, solar desalination–cultivation farms, as an emerging technology, have been mainly confined to laboratory-scale studies. Scaling these systems will require substantial engineering advances. The low condensation efficiency of passive devices poses challenges for improving water productivity, highlighting the need for enhanced condensation designs. Long-term practical studies are essential to evaluate the potential for bioaccumulation and the durability of devices in extreme weather events, such as cyclones.

To fully harness the potential of FOOD systems, further development and testing are required to produce food and achieve material circularity. Saline-tolerant, profitable species such as white-leg shrimp, Indian prawn, tilapia fish and milk fish could be farmed in the desalination pool. Nutrients and critical minerals can be extracted from the concentrated salts left from desalination. Moreover, agricultural waste, such as rice straw and corn cobs^157,158,159, can be repurposed to construct solar evaporators, reducing material costs and minimizing waste generation. Solar pyrolysis can drive this process, converting by-products into eco-friendly, high-performance photothermal materials¹⁶⁰. This circular approach enhances both sustainability and the economic viability of FOOD systems.

Soil remediation

Solar-driven interfacial evaporation can remediate contaminated soils through washing¹⁴⁶, biodegradable soil enrichment¹⁶¹ and artificial phytoremediation^{162,163,164,165,166,167,168}. The remediated soils can then be used for food production, after careful safety testing.

Soil washing with fresh water produced by interfacial solar desalination leaches contaminants such as excess salts, heavy metals and pesticides from the soil. Its exclusive use of free solar energy and applicability to various water resources (including seawater) makes it particularly well suited for coastal areas, where interfacial solar desalination can provide a steady supply of fresh water from seawater for both saline soil remediation and agricultural irrigation¹⁴⁶. A typical set-up for interfacial solar fresh water leaching includes a seawater tank, an interfacial solar desalination chamber and a saline soil chamber (Fig. 5g). Sunlight transmits through the transparent cover and is absorbed and converted into heat by a photothermal evaporator. This heat facilitates seawater evaporation and the resulting vapour condenses into fresh water that drips down into the soil. Field tests demonstrate that this method removes hazardous contaminants at a rate three times faster than solar distillation, based on natural evaporation, completing the process in 16 days instead of 50 days¹⁴⁶.

Soil quality deteriorates due to nutrient and microbial loss during soil washing, which is problematic when the treated soil is intended for agriculture¹⁶⁹. Biodegradable solar evaporators (such as polysaccharide-based photothermal aerogel beads) are designed to mitigate these issues. They are initially used in wastewater remediation, where they adsorb nutrients from wastewater¹⁶¹. Once saturated, the beads are added to soil, where they slowly degrade and release the nutrients (such as phosphate, nitrate and potassium) (Fig. 5h).

Artificial phytoremediation absorbs, concentrates and removes contaminants (such as heavy metals) from soil^163,164 by mimicking plant-based phytoremediation. Solar evaporators can be placed directly on contaminated soil to extract heavy-metal ions through transpiration¹⁶³. Interfacial solar heating enhances evaporation to facilitate the migration of heavy metals from the soil into the evaporator along with water (Fig. 5i). As water evaporates, contaminants are concentrated and retained by the evaporators. Artificial phytoremediation is more efficient than plant-based phytoremediation and has several advantages¹⁶⁵: effectiveness with high contamination levels; rapid contaminant absorption; and high contaminant-absorption capacity. In a trial addressing lead (Pb) contamination in soil, an artificial phytoremediation system used an evaporator equipped with Pb-binding agents, and was compared with a traditional plant-based phytoremediation using ryegrass¹⁶³. After 2 weeks, the bioavailable Pb fractions in the soil decreased by 40% with artificial phytoremediation, compared with a 20% reduction with plant-based remediation.

Energy

Energy can be harvested from water evaporation through techniques such as thermoelectric, pyroelectric, salinity gradient and hydrovoltaic power generation (Fig. 6a). Interfacial solar evaporation can also be integrated with solar PV cells to enhance the performance of both technologies, allowing for joint production of clean water and electricity. Interfacial water–electricity co-generation is off-grid and decentralized, so it could provide an energy source for remote and rural areas that lack access to grid electricity. This section discusses methods of generating energy from interfacial solar evaporation and possible combinations with other energy-generation technologies.

**Fig. 6: Electricity–water co-generation through solar-driven interfacial evaporation.**

Salinity-gradient energy generation

Rapid water evaporation creates a salinity gradient between the high-salinity evaporation interface and the surrounding low-salinity solution. In interfacial solar evaporation systems, the energy density of the salinity gradient power can be higher than that of river–sea mixing and can be used for electricity generation^170,171,172. For example, mixing evaporation-induced high-salinity brine (6 mol l^–1) with seawater (0.6 mol l^–1) yields about 12 kJ l^–1, whereas mixing seawater (0.6 mol l^–1) with river water (0.01 mol l^–1) yields only 2 kJ l^–1 (ref. ¹⁵³). One system for solar evaporation and salinity power generation has an ion-selective membrane between the evaporator and seawater¹⁷⁰ (Fig. 6b). Under solar irradiation, water evaporates quickly and creates a concentration difference between the solution beneath the evaporator and the bulk seawater. This gradient drives Na⁺ ions to diffuse through the membrane, generating 1 W m^–2 of power and evaporating water at a rate of 1.15 l m^–2 h^–1.

Another approach uses a membrane to interface evaporated high-salinity brine with untreated low-salinity brine¹⁷¹ (Fig. 6c). An ionization electronegativity hydrogel is both the solar evaporator for daytime evaporation and the selective ion membrane for nighttime electricity generation. The system therefore produces either fresh water or electricity 24 hours of the day. Integrating evaporation and ion-selective functions into a single material reduces component complexity, potentially lowering operational costs. However, the need to switch modes between day (water generation) and night (energy generation) presents challenges in material design, mechanical durability and thermal management, requiring careful optimization for long-term performance.

Two considerations are key to advancing salinity-gradient energy generation. First, the current power output of about 1 W m^–2 is far below the theoretical maximum of 12.5 W m^–2 (ref. ¹⁷⁰). Optimizing the ion-selective membrane’s properties, such as pore size, surface functionalization¹⁷³ and selectivity, is expected to further enhance the output. Second, the high cost and complex fabrication of ion exchange membranes are a major barrier to widespread adoption. Ionized wood membranes could be more cost-effective for this application¹⁷⁴. A wooden membrane (1.2 m × 1 m × 1 mm) costs about US$10, compared with US$350 for commercial membranes¹⁷⁵. However, the relatively large thickness of wood membranes could constrain the power and energy density of wood-based salinity energy generators, as performance typically scales inversely with membrane thickness¹⁷⁴.

Thermal-gradient energy generation

Thermal energy from interfacial solar evaporation can be extracted and converted to electricity through thermoelectric^176,177,178, thermoelectrochemical^179,180, and pyroelectric^41,181,182 methods. Thermoelectric methods generate electricity by using a temperature difference across a thermoelectric module, which operates via the Seebeck effect¹⁸³. In this set-up, one side of the thermoelectric module (Bi₂Te₃) is connected to the heated part of a solar evaporation system, such as hot vapour¹⁷⁷ or the evaporator¹⁷⁶, while the other side is kept in contact with a cooler area, like ambient air or surrounding water (Fig. 6d). This temperature gradient across the module creates a voltage, which can be used to power external devices or stored in batteries. This approach has generated power outputs of 292.9 W m^–2 under 30-sun irradiation and 63 mW m^–2 under 1-sun irradiation¹⁷⁷.

Thermoelectrochemical approaches exploit the temperature-dependent electrochemical redox potentials to generate energy¹⁸⁴ (Fig. 6e). A typical thermoelectrochemical cell has two identical electrodes, a redox couple electrolyte, and an external connection. Applying a thermal gradient induces oxidation at the anode and reduction at the cathode, with reduced species diffusing back to the anode to sustain continuous reaction and current flow¹⁸⁵. For example, these cells have been applied to harvest the latent heat released from vapour condensation, with a maximum power output of 6.94 mW m^–2 under natural solar irradiation¹⁷⁹. Compared with thermoelectric methods, thermoelectrochemical cells are more cost-effective but typically have lower power densities.

Unlike thermoelectric and thermoelectrochemical devices, pyroelectric devices do not require a spatial temperature gradient. Instead, they convert temporal temperature fluctuations into electricity¹⁸⁶ (Fig. 6f). This conversion is achieved by integrating a composite film of tungsten-doped vanadium dioxide (W-doped VO₂) and polyvinylidene fluoride (PVDF) into the interfacial solar evaporation system¹⁸². Upon sunlight exposure, the W-doped VO₂ undergoes a phase transition that alters its solar-to-heat conversion and generates temperature oscillations. These temperature fluctuations activate the PVDF layer, producing continuous electrical output. The system exhibited self-adaptive temperature oscillations up to 7 °C, with a maximum electrical power density of 104 μW m^–2 (ref. ¹⁸²).

Solar-driven interfacial evaporation has an abundant dynamic flow of water, vapour and ions. Therefore, evaporation-induced kinetic energy can be converted into electrical energy by utilizing streaming potential — the voltage difference created by fluid movement through a porous medium. For instance, the evaporation-induced directional flow of seawater through ionic channels in a porous evaporator can generate a stable potential of 117.8 mV (ref. ¹⁸⁷). The evaporation of pure water vapour from nanostructured carbon surfaces can produce voltages of up to 1 V, sustained for over 300 hours¹⁸⁸.

Water–electricity co-generation

Coupling PV with interfacial solar evaporation can improve the PV panel’s performance, enhance the overall utilization efficiency of solar irradiance, and simultaneously produce electricity and clean water^189,190 (Fig. 6g). The set-up comprises three components that work synergistically: a PV cell at the top to generate electricity, a thermal interconnecting layer in the middle to transfer the thermalization heat from the PV cell to the water purifier, and an interfacial solar water purifier at the bottom to generate clean water¹⁸⁹. The solar panels can be up to 11 °C cooler by evaporation from the purifier below, enabling an increase in electricity generation of up to 7.9% relative to the PV system without the purifier¹⁸⁹. The heat from the PV cells is transferred to the water purifier to purify water at a rate of 0.80 l m^–2 h^–1 under natural sunlight. The tandem system also maximizes photon utilization. The top PV cells use above-bandgap photons while the bottom purifier uses below-bandgap photons, representing an overall solar energy utilization efficiency of 74.6%¹⁸⁹. The water-production rate can be further increased through a PV-multistage distillation device, in which the latent heat released during vapour condensation is recycled^53,54. A three-stage version of this system has a water production rate of 1.64 l m^–2 h^–1 (ref. ⁵⁴), while a five-stage system produces 2.35–2.45 l m^–2 h^–1 of fresh water from seawater (ref. ⁵³).

Application and viability

The demonstration of four types of water–electricity co-generation technique highlights the viability of solar-driven interfacial evaporation technologies for decentralized electricity generation (Fig. 6h and Supplementary Table 2). Hybrid systems that combine interfacial solar evaporation with thermal, salinity or hydrovoltaic energy-harvesting techniques are lightweight and cost-effective, and off-grid solutions can generate 1–10 W m^–2 of electricity. These technologies are suitable for small-scale, distributed, low-energy applications such as Internet of Things devices and sensors. For example, the evaporation-induced hydrovoltaic device is sensitive to ambient wind speed, which can be utilized to construct wind-speed sensors¹⁹¹.

Solar PV–desalination hybrid systems are better suited for larger-scale implementation, as these systems have reached up to 204 W m^–2 for electricity¹⁸⁹ and 2.5 l m^–2 h^–1 for water⁵³ in separate systems. A roof-sized PV-desalination hybrid system can produce the daily drinking water and energy needs of a household, defined here as a family of four with average consumption of 50–60 l of drinking water per day and 10–12 kWh of electricity.

Power generation through interfacial solar evaporation is technologically immature, with opportunity for material and system development. For example, we could envision a dual-function strategy that combines solar-driven interfacial evaporation with hydrovoltaic technology¹⁹² to achieve simultaneous resource recovery and power generation. In this dual-function system, solar evaporation utilizes solar energy to evaporate water, concentrating valuable minerals and salts for easier extraction, while hydrovoltaic technology converts the kinetic energy of water movement into electrical energy.

Comprehensive assessment and optimization of evaporative power generation technologies are needed to maximize their efficiency and address potential challenges. For instance, although pairing interfacial evaporation with solar cells results in higher efficiency for both PV and desalination, there are still uncertainties regarding the technology’s technical and economic aspects¹⁹³. The potential biofouling of panels by microbial biofilms could pose a greater problem over water than on land and could decrease PV output¹⁹⁴.

Summary and future perspectives

Solar-driven interfacial evaporation technologies use solar energy to treat or desalinate water, extract resources, aid food production and/or produce power. They can function off-grid, making them potentially suited to addressing FEW demands in rural areas with limited infrastructure. Advances in materials, devices and systems have improved the efficiency, productivity and stability of the systems. However, in most cases, FEW resources are produced individually, reducing energy and space efficiency while increasing costs. Co-generation strategies that can produce FEW resources simultaneously within a unified system need to be prioritized to maximize energy efficiency and deliver collective benefits, such as lower cost, better space efficiency, and improved availability and stability (Box 1).

Because different solar technologies rely on different parts of the solar spectrum, efficient solar energy management is key to maximizing energy utilization and engineering interfacial solar evaporation systems for co-generation. Solar radiation spans approximately 200 to 2,400 nm, with 47% of its total energy in the visible (380–780 nm), 51% in the infrared (780–2,500 nm), and the remaining 2% in the ultraviolet parts of the spectrum (Box 1). Interfacial solar evaporation can harness almost the entire solar spectrum, but PV converts sunlight into electricity primarily within the visible (380–780 nm) and near-infrared (780–1,100 nm) ranges (Box 1). Photosynthesis operates in much narrower bands, specifically 400–500 nm and 600–700 nm (refs. ^150,151). Engineering co-generation systems that independently harness different parts of the spectrum, working synergistically, would enable usage of nearly the entire spectrum.

Serial transduction is also important to boosting overall output, as it captures energy intermediates (Box 1). For instance, the kinetic and internal energy of hot vapour, often discarded as waste, can be recovered by recycling the heat released during condensation⁵⁰. Pairing the photothermal evaporator with radiative cooling to create thermal gradients can generate electricity through thermoelectric systems.

From these principles, we propose strategies for FEW co-generation with high energy efficiency and outputs, including syngas–water co-generation, heat–water co-generation and electricity–water co-generation with cooling, heating, water or biochar (Box 1). Developing these systems will require integrating advanced materials, optimizing heat and mass transfer, precisely controlling energy flows and synchronizing various techniques. Ultimately, it is essential to monitor the long-term productivity and stability of these systems in real-world conditions. Moreover, we strongly recommend comprehensive life-cycle assessments to evaluate environmental impacts before implementation to guide application and more efficient and sustainable use. Finally, advances in hardware design and mass production will be crucial to reducing costs and making these technologies widely accessible, ensuring global access to FEW solutions.

Box 1 Food, energy and water co-generation

Different solar technologies rely on different parts of the solar spectrum (panel a of figure), so efficient solar energy management and serial transduction (panel b of figure) are important for co-generation. The following strategies could potentially be used to integrate interfacial solar evaporation with co-generation of electricity, heating, cooling, food and water, all assuming approximately 1,000 W m^–2 of solar radiation¹⁹⁵. Detailed calculations, system assumptions and underlying considerations are given in the Supplementary Information.

Electricity, water and heat

Photovoltaic (PV) panels, interfacial solar water purification and heat recovery technologies could be integrated to use solar radiation (about 1,000 W m^–2) (panel c of figure). PV panels convert 15–20% of the solar radiation into electricity (150–200 W m^–2) and the remaining solar energy (around 800 W m^–2) is absorbed by the interfacial solar water purifier, producing about 1.2 l m^–2 h^–1 of fresh water. The heat released during condensation (about 700 W m^–2) is captured by a heat-recovery system that reuses it for space heating.

Electricity, water and cooling

PV panels, interfacial solar water purification, and adsorption refrigeration technologies could be integrated (panel d of figure). Solar energy is used as in the electricity–water–heat systems, but the latent heat released during condensation powers an adsorption refrigeration system, providing cooling energy (about 100 W m^–2).

Electricity, food and water

PV panels, photosynthesis and interfacial multistage water purifiers could be integrated (panel e of figure). PV panels use 15–20% of the incoming solar energy for electricity generation (150 W m^–2) and photosynthesis converts around 10% of the solar energy in the 400–700 nm range (about 100 W m^–2) into biomass or food. The remaining solar energy (about 700 W m^–2) powers the interfacial multistage water purifier, yielding approximately 4.0 l m^–2 h^–1 of fresh water.

Water, electricity and biochar

Interfacial multistage desalination could be integrated with microbial fuel cells (MFCs) and biochar production (panel f of figure). The interfacial multistage desalination unit captures about 100% of solar energy to produce fresh water at approximately 5.0 l m^–2 h^–2. The brine or organic waste generated is fed into MFCs, which generate around 10 W m^–2 of electricity while treating the waste. The by-products from MFCs are converted into biochar.

Syngas and water

Solar-driven biomass gasification could be integrated with interfacial solar desalination (panel g of figure). Solar radiation drives biomass gasification to syngas via solar-driven thermochemical and catalytic processes. The waste heat powers a thermal desalination unit, generating about 1.0 l m^–2 h^–1 of fresh water. The syngas can be used for combined heat and power applications.

Water and heat

Concentrating solar power could be combined with solar-steam-driven membrane desalination and thermal storage systems (panel h of figure). The concentrating solar power system converts almost 100% of the solar radiation into high-temperature heat, which is used to produce high-pressure steam to drive a membrane desalination process, yielding approximately 80 l m^–2 h^–1 of fresh water. Waste heat (about 200 W m^–²) is stored in thermal storage materials (such as molten salts), enabling continuous power and water production.