Advancing robust all-weather desalination: a critical review of emerging photothermal evaporators and hybrid systems

With the swift expansion of the global economy and the relentless exacerbation of environmental pollution, the critical issue of water scarcity has emerged as an imperative to be addressed (Fig. 1a). Traditional sources such as surface water and groundwater are no longer adequate to supply the ecosystem with sufficient and cost-effective freshwater. Concurrently, the burgeoning global population has led to a significant reduction in per capita freshwater availability (Fig. 1b)¹. Projections indicate that by 2050, the populations and irrigated agricultural lands grappling with water scarcity are poised to escalate by approximately 70.4% and 25.3%, respectively, compared to the figures in 2020 (Fig. 1b)². This stark reality underscores the urgent need for the exploration and implementation of alternative water sources to bolster the freshwater supply^3,4. Seawater, which accounts for over 70% of the Earth’s surface, represents a vast resource, and desalination technology has become a focal point for its potential to mitigate global freshwater shortages. Current desalination methods, including multi-stage flash distillation, low-temperature multi-effect evaporation, and reverse osmosis, are generally characterized by high costs, energy consumption, and inefficiency⁵. In contrast, interfacial solar steam generation (ISSG) technology, as an emerging solution, promises lower energy requirements as well as numerous benefits such as structural simplicity, cost savings, and environmental sustainability, thereby reducing operational expenses and offering an eco-friendly alternative to address the freshwater crisis^6,7,8.

However, due to the day-night cycle, traditional evaporators can only operate intermittently. And when the evaporator encounters overcast weather with low light intensity, its evaporation performance is significantly diminished, by up to 60% at most, owing to a lack of heat. To solve this, a range of photothermal materials, including plasmonic metals, semiconductors, carbon-based, and gel-based materials, have been ingeniously developed to facilitate continuous solar-driven seawater evaporation regardless of weather^9,10. For instance, hydrogel-based photothermal materials can lower evaporation enthalpy, customize water channels, minimize energy needs, and achieve elevated evaporation rates even in low light conditions^11,12. The Marangoni effect in these hydrogel materials also accelerates water flow, thereby enhancing resistance to salt accumulation during prolonged all-weather evaporation. Remarkably, even at a high evaporation rate of 6.0 kg m⁻² under three sun irradiations, no salt accumulation occurs over a 12-h period, maintaining a stable rate. These advantages of hydrogel-based materials substantially enhance the adaptability of evaporators to varying climatic conditions.

In terms of system design, hybrid systems that integrate photothermal evaporators with external heat sources, such as electrothermal converters and phase-change heat storage materials, have demonstrated effectiveness in all-weather solar seawater evaporation^13,14. These systems enable efficient and steady functioning during periods of insufficient light by utilizing the Joule effect for heat generation or releasing stored heat. Furthermore, an innovative hybrid system combining solar seawater evaporation with fog collection has been constructed, exemplified by the micro/nanostructured polyethylene/carbon nanotube foam with interconnected open-cell structure (MN-PCG)¹⁵. This system achieves a daily freshwater output of ~35 kg m⁻², capitalizing on its capacity for rapid water absorption from humid air at night and exceptional solar steam efficiency during the day. Importantly, the acid/alkali tolerance, temperature resistance, and passive/active de-icing capabilities of the MN-PCG foam provide robust adaptability and endurance in outdoor environments for practical applications.

The aforementioned developments notably enhance the all-weather adaptability, evaporation efficiency, and operational stability of photothermal evaporators, facilitating a consistent freshwater supply under diverse environmental conditions. Despite tremendous advances, these studies on all-weather solar seawater evaporation remains relatively scattered. Reviews in this field have primarily focused on daytime evaporation, offering insights into the selection of photothermal materials, the design of macro and micro structures, and current advances^16,17. Moreover, a topical review that summarized the application of phase-change materials in photothermal/phase-change material hybrid systems has laid a foundation for future research¹⁸. However, a comprehensive review about all-weather solar seawater evaporators and hybrid systems is conspicuously absent. This work presents a thorough overview of the architecture, operating principles, and design criteria for all-weather photothermal seawater evaporators. It succinctly categorizes key photothermal materials, elucidates their conversion mechanisms, and highlights recent advances. Furthermore, it discusses the characteristics, working paradigms, and all-weather evaporation performance of different hybrid systems. Our review is vital for researchers wishing to methodically appreciate the underlying knowledge and current research status, giving a comprehensive theoretical framework to guide future studies.

Basic structure, principle, and design criteria of all-weather seawater evaporator

System composition and operating principle

Given the intermittent nature of solar radiation, as well as the limitations during nighttime or cloudy conditions, conventional interface evaporators struggle to achieve stable evaporation. Thus, developing systems capable of maintaining consistent evaporation around the clock is particularly important. All-weather solar seawater evaporators are primarily composed of four key components: the photothermal absorption layer, the evaporation layer, the insulation unit, and the heat supply system (Fig. 2a). Each part plays an indispensable role under varying illumination conditions, collectively ensuring the efficient operation of evaporators. The photothermal absorption layer, the foundation of the system, is responsible for absorbing sunlight and transforming it into thermal energy. This layer employs high-efficiency photothermal conversion materials to maximize solar radiation absorption and convert it into heat for water heating. The evaporation layer, positioned above the photothermal absorption layer, serves as the core energy conversion zone. It utilizes capillary action to draw moisture to the heat-concentrated area, thereby accelerating water evaporation. The insulation unit located at the system’s base primarily functions to maintain the system temperature, prevent heat loss, and ensure the effective flow of vapor to the water collection system. Unlike traditional solar seawater evaporators, the distinctive feature of all-weather solar seawater evaporators is an integrated heat supply system. This system releases stored thermal energy or generates heat via the Joule heating effect during nighttime or low-light conditions, assuring a continuous evaporation process^19,20. Moreover, some all-weather evaporators are equipped with water collection capabilities, allowing for ongoing water collection and purification during nighttime.

The operating principle for all-weather solar seawater evaporators varies throughout the day and at night. During the day or under adequate lighting, water is transported to the photothermal absorption layer by capillary action. The absorption layer then turns light energy into thermal energy, heating the water surface and causing it to evaporate into steam, which eventually flows into the collecting system for water purification or other purposes. Because heat is largely focused at the interface, this evaporation method is more efficient than traditional volumetric evaporation, reducing heat loss. During nighttime or insufficient lighting conditions, some all-weather solar interface evaporators can release stored thermal energy or convert heat through their heat supply system, therefore continually maintaining the evaporation process, while others can gather water via fog harvesting.

Design criteria of efficient evaporators for all-weather seawater desalination

The all-weather seawater evaporation performance of interfacial solar evaporators is closely related to their solar absorption, photothermal conversion, thermal management and heat supply capabilities, and environmental resistance^20,21. To achieve an efficient and consistent evaporation rate throughout the day, a solar seawater evaporator must fulfill the following design criteria.

(1) Effective solar absorption: solar absorption materials must efficiently capture sunlight to fully utilize solar energy.

(2) Efficient photothermal conversion: the absorbed light should be effectively converted into thermal energy rather than other forms of reradiation.

(3) Proper thermal management and heat supply: the heat generated from absorbed light must be efficiently transferred to the evaporation process, with minimized convection and blackbody radiation losses. Nighttime evaporation should be sustained by a constant heat supply from a connected heat storage or heat conversion device.

(4) Robust seawater resistance: seawater, rich in salts and microorganisms, can severely corrode or clog evaporators during a long period of photothermal evaporation. Optimizing material selection and structural design can mitigate issues of corrosion and clogging.

The following sections will look in depth at each of the design criteria for all-weather evaporators to efficiently promote seawater evaporation.

Efficient solar absorption

One of the critical factors in the design of all-weather solar seawater evaporators is efficient solar absorption. To achieve this, the solar absorption layer needs to exhibit high absorption and extremely low transmission and reflectivity across the entire solar spectrum range from 300 to 2500 nm (ASTM G-173, AM 1.5). The ultraviolet (UV) region of the solar spectrum (300–400 nm) comprises ~3% of the power, the visible light range (400–700 nm) accounts for around 45%, and the near-infrared (NIR) region (700–2500 nm) contains about 52% of the power (Fig. 2b). This implies that the material should possess a high absorption rate within the 300 to 2500 nm wavelength range with negligible reflection and transmission, to maximize the capture of solar energy and convert it into thermal energy.

Solar absorptance is a measure of a material’s ability to absorb solar radiation, determined by the ratio of total absorbed solar radiation to incident radiation²². For a given incident angle, the total solar absorptance is calculated by weighting the material’s spectral absorbance against the standard solar spectrum (AM 1.5) spectral irradiance distribution and integrating it over the wavelength range where solar radiation reaches the solar absorber surface. The Eq. (1) describes the total solar absorptance of the solar absorber at an incident angle:

Where λ_min and λ_max are 0.3 and 2.5 µm, respectively, and the incident angle θ is measured from the surface normal of the absorber. A(λ) is the wavelength-dependent solar spectral irradiance. R(θ,λ) is the total reflectance at the wavelength λ²³. Hence, all-weather seawater evaporators must have a high total solar absorptivity combined with low transmission and reflectivity in order to maximize solar radiation capture and achieve outstanding evaporation efficiency during the day or under adequate lighting.

Efficient photothermal conversion

Except for solar absorptivity, the subsequent conversion of light energy into thermal energy is equally vital for all-weather solar seawater evaporators. To facilitate comparisons between different materials, photothermal conversion efficiency quantifies the amount of light converted to heat. A direct experimental method for determining photothermal conversion efficiency involves measuring the temperature rise and the generated thermal energy from the incident light source²⁴. Typically, the light source passes through a solution containing the solar-absorbing material, which converts the absorbed light into heat, resulting in an overall temperature increase of the solution. By accounting for heat losses through conduction and other mechanisms, the energy gained by the system (calculated through the temperature increase and specific heat capacity of water) is compared to the absorbed light to estimate the photothermal conversion efficiency. For three-dimensional photothermal evaporators, Gui et al. have proposed the formula for photothermal conversion efficiency, η_PT, as follows:

Where P_heat signifies the power generated in the form of heat, and P_light denotes the optical power. This necessitates that materials effectively convert absorbed solar energy into thermal energy rather than reemitting it as radiation. The frequently reported vaporization efficiency pertains to the light-to-steam generation efficiency. In this context, light is directed onto solar-absorbing materials immersed in water. The mass loss of the water over time is measured, and the energy required to evaporate the water is calculated. Additionally, temperatures of various system components can be measured using thermocouples or infrared (IR) cameras to assess temperature increase and heating uniformity²⁵. The efficiency η is then calculated as follows:

Where m denotes the mass loss rate of water when subjected to illumination, L_v represents the latent heat of vaporization for water, which is conventionally accepted as 2.26 kJ g⁻¹, and Q symbolizes the energy input required to elevate the system’s temperature from an initial state T₁ to a final state T₂. Additionally, P_in is the incident optical power impinging upon the solar absorber. Consequently, the enhancement of photothermal conversion efficiency emerges as a critical imperative for the development of solar seawater evaporators that are effective in all climatic conditions.

Proper thermal management and nighttime heat supply

In the photothermal evaporation process, effectively harnessing thermal energy to achieve water evaporation is directly tied to the evaporator’s efficiency. Typically, the heat generated from photothermal materials is conducted into the water body, while heat convection and radiation affect the surrounding air and environment, leading to partial heat loss (Fig. 2c). Baffou et al. emphasized the necessity of designing an efficient thermal management system to maximize the conversion of absorbed light into heat for interface evaporation. According to the quantitative relationship of thermal convection and radiation losses, the smaller the temperature difference between the evaporation surface and the environment, the lesser the heat loss^7,26. The thermal balance of this process can be expressed by the following formula:

Where A represents the surface area of the absorber that is oriented towards the sun, q_solar is the incident solar flux, and h_fg is the evaporation enthalpy of water. The symbol ε denotes the emittance of the absorber’s surface, (sigma) is the Stefan–Boltzmann constant, h is the convection heat transfer coefficient, and q_water is the heat flux directed towards the underlying seawater, encompassing both conduction and radiation components. The symbol (dot{m}) signifies the rate of steam generation. Furthermore, T and T₀ denote the absorber’s temperature and the ambient temperature, respectively. Consequently, it is of paramount importance for all-weather solar seawater evaporators to meticulously address the challenges associated with heat loss to ensure optimal performance.

Recent studies have concentrated on reducing thermal losses through innovative designs of evaporators and the optimization of water transport layers. Zhou et al. developed a biosafe solar absorber based on sepia ink, specifically designed to optimize water transport regulation²⁷. This avant-garde design effectively limits excess water, thereby minimizing heat loss. Wang et al. introduced a novel concept of confinement capillarity to significantly enhance the efficiency of solar-driven water evaporation²⁸. This is achieved by applying a thin layer of black/hydrophilic nanoparticles on the framework of a standard sponge. The channels formed between these nanoparticles facilitate strong capillary action for water transport. Water is confined and automatically conveyed exclusively within these channels, as opposed to the larger interconnected pores (~200 µm) of the sponge. As a result, without reliance on external devices to precisely manage the water supply, an ultrathin water layer (2–5 µm) naturally forms on both the interior and exterior surfaces of the sponge. These slim water layers expand the evaporation surface by increasing vapor escape paths, simultaneously preventing the overheating of water through solar exposure. Due to the effect of confinement capillarity, the rate of solar steam generation reaches 3.2 kg m⁻² h⁻¹ under one sun illumination. Moreover, to ensure evaporation efficiency and stability during nighttime and under low-light conditions, the design of all-weather interface evaporators must address nighttime heat supply, which is neglected by traditional evaporators. Contemporary methods for nighttime heat supply include two main approaches. One utilizes the Joule effect and other heat conversion methods to sustain evaporation at night. The other employs organic phase-change materials to store heat collected during the day and release it at night.

Robust resistance to salts and impurities

The effectiveness and durability of solar interfacial desalination (SID) systems hinge significantly on the management of brine. When the rate of photothermal concentration surpasses that of brine drainage, salt can accumulate on the photothermal evaporation components (PECs). This accumulation not only obstructs water flow pathways but also impedes light absorption^29,30, thereby diminishing the system’s energy efficiency (Fig. 2d)^31,32. To mitigate these issues, two primary strategies have been developed: salting-out and salt-free approaches.

The salting-out approach entails the complete evaporation of seawater, leading to the formation of solid salt deposits on and within the PECs. This requires subsequent removal methods, such as manual or rolling techniques^33,34,35. Although this method prevents energy loss associated with brine discharge, it necessitates system downtime for salt removal, which reduces overall productivity. Conversely, the salt-free strategy aims to prevent salt saturation by optimizing the PEC’s structure and functionality to control brine transfer or dilution. Several effective techniques have been identified: (1) Ion diffusion control. A thin hydrophobic layer is applied to the hydrophilic surface of the PEC^36,37, facilitating internal diffusion of seawater while preventing lateral permeation. The PEC is also designed with materials or structures capable of local ionization, such as polyelectrolytes³⁸. These materials dissociate corresponding anions and cations, concentrating them within the PEC and creating a Donnan effect that restricts ion permeation from seawater into the PEC. (2) Gravity-assisted cleaning. This technique involves establishing a height differential within the PEC system^39,40. Seawater flows continuously from a higher inflow area, while concentrated brine evaporates and thickens in a lower outflow area. This configuration helps maintain a steady flow and prevents salt buildup. (3) Enhanced convection through structural optimization. By optimizing the PEC’s topology, the density gradient of the brine, which is related to its concentration, can drive more efficient flow through channels. This convection-based approach significantly improves brine transport compared to simple diffusion⁴¹. (4) Self-rotating photothermal evaporator that utilizes a cyclical mechanism of evaporation, salt crystallization, and self-refreshment to ensure continuous evaporation. A spherical self-rotating photothermal evaporator has been designed, with dual evaporation zones—high and low temperature—to improve efficiency⁴². This evaporator is highly sensitive to weight imbalances of less than 15 mg, enabling a swift rotational response to salt accumulation, thereby refreshing the evaporation surface. The dual-zone design optimizes the energy dynamics during solar evaporation, achieving both superior salt resistance and an impressive evaporation rate of 2.6 kg m⁻² h⁻¹. (5) Directional salt crystallization. The generation/recycling of salt during the evaporation process is equally important since salt is valuable. Wang et al. introduced a method for directional salt crystallization by fabricating a 3D evaporator that incorporates hydrophilic nylon threads and scotch tape to anchor alternating sponges^43,44. This innovative design enables selective salt crystallization along the wrapping lines in contact with a superhydrophobic photothermal sponge, with crystallization expanding up and down along the tape. This approach results in abundant salt formation on the entire tape, preventing salt accumulation on the top surface and achieving zero liquid discharge. These innovations provide novel insights into material design and structural engineering for both fundamental and applied research on solar evaporators. Furthermore, unlike the other four strategies, this method of directional salt deposition in 3D solar evaporators not only maintains a high water evaporation rate but also enhances the salt generation rate. Impressively, these evaporators have demonstrated the capability to selectively separate NaCl and LiCl during the solar evaporation process, marking a significant stride in the field of solar-driven salt collection and separation⁴⁴.

Moreover, organic pollutants in seawater, such as oils and organic compounds, can form oil films or deposits on the evaporator surface during the evaporation process, further impeding thermal conduction and evaporation efficiency. These organics may also chemically react with evaporator materials, leading to material degradation. Additionally, microorganisms in seawater, including bacteria and algae, can proliferate on the evaporator surface, forming biofilms. These biofilms not only obstruct thermal conduction but can also cause clogging and corrosion of the evaporator. Therefore, an ideal all-weather seawater evaporator should incorporate design considerations to mitigate or eliminate the adverse effects of the complex components in seawater, thereby enhancing evaporation efficiency and extending the equipment’s lifespan.

Recent developments in photothermal materials for seawater evaporation

The design and development of broadband, highly efficient solar-absorbing photothermal conversion materials lie at the heart of ISSG technology. To date, various materials have been employed for photothermal seawater evaporation. Depending on the mechanisms of photothermal conversion, different materials with efficient photothermal conversion have been developed, such as plasmonic metals, semiconductors, and carbon-based materials. However, due to the limitations of the aforementioned materials in photothermal evaporation efficiency and desalination, combining carbon-based materials or transition metals with 3D aerogel/hydrogel to construct composites offers significant advantages. These advantages include low thermal conductivity, smooth transport channels, and self-floating capabilities^45,46,47, resulting in notable improvements in light absorption, photothermal conversion, and desalination. This chapter provides a comprehensive review of the research progress in photothermal conversion materials for seawater evaporation, through a comparative analysis of various materials’ photothermal conversion principles, influencing factors, and conversion efficiencies.

Plasmonic metals

Plasmonic metals are metallic materials with negative permittivity, enabling interaction with light through surface plasmon resonance (SPR). In SPR, photons transfer energy to free electrons, known as surface plasmons, at the material’s interface⁴⁸. In localized surface plasmon resonance (LSPR), these excited electrons oscillate coherently with the incident electromagnetic field^49,50. They then relax through processes such as electron-electron scattering and energy transfer from the electron gas to the metal lattice via electron–phonon coupling, leading to phonon-phonon interactions. This relaxation process converts the kinetic energy of the excited electrons into thermal energy, rapidly heating the surrounding environment of the metal surface (Fig. 3a). This “green” heat has potential applications in various fields, including the evaporation of seawater for desalination.

A significant challenge in using plasmonic metal particles as solar absorbers is their limited absorption range, typically confined to one or a few LSPR wavelengths. To address this, researchers have developed advanced fabrication techniques and plasmonic structures. For example, Hu’s group created a novel plasmonic material by decorating metal nanoparticles of Au, Pd, and Ag onto a 3D mesoporous matrix of natural wood⁵¹. This plasmonic wood achieved high light absorption of ~99% across a broad wavelength range from 200 to 2500 nm, due to the wood’s microchannels and the plasmonic effect of the nanoparticles, resulting in a solar conversion efficiency of about 85% under 10 kW m⁻². Zielinski et al. demonstrated a technique for fabricating bimetallic Ag/Au hollow mesoporous plasmonic nanoshells, which exhibited superior solar vapor generation⁵². These nanoshells generate vapor bubbles from both their interior and exterior when exposed to light, accelerating vapor nucleation. This makes them promising for seawater desalination through rapid, increased saltwater evaporation, achieving a conversion efficiency of 69% under 11 kW m⁻². Besides, Zhou et al. developed Al nanoparticles for plasmon-enhanced solar desalination using a self-assembly approach⁵³. The structure included an anodized alumina membrane (AAM) with closely packed aluminum nanoparticles. This design facilitated strong plasmon hybridization, enabling over 96% solar absorption across the spectrum. The setup allowed the plasmonic material to float on water, promoting steam generation and condensation in a chamber, with the AAM providing an efficient pathway for water supply and steam flow. These examples illustrate the diverse approaches and innovative techniques being developed to enhance the efficiency and broaden the absorption range of plasmonic metals for solar desalination applications.

Inorganic semiconductors

In the realm of semiconducting materials, optical absorption exhibits a pronounced dependence on the wavelength, particularly in the vicinity of the bandgap energy. Upon illumination, semiconducting materials give rise to electron–hole pairs that possess energy commensurate with the bandgap. These excited electrons ultimately revert to lower energy states, releasing energy either through radiative relaxation as photons or via non-radiative relaxation as phonons (heat)⁵⁴. The latter process involves the transfer of energy to impurities, defects, or surface dangling bonds within the material. When energy is dissipated in the form of phonons, it induces localized heating of the lattice, thereby establishing a temperature gradient that is contingent upon the material’s optical absorption and its bulk and surface recombination properties. The photothermal effect, as depicted in Fig. 3b, is a manifestation of this temperature gradient within the material, arising from the diffusion and recombination of optically excited carriers.

Semiconductor photothermal nanomaterials, such as metal oxides and chalcogenides, have demonstrated remarkable potential due to their facile synthesis, low cost, invulnerability to photobleaching and photodegradation, and fine-tunable absorption capacity. At present, black TiO₂^55,56,57, Ti₂O₃⁵⁸, MoO₃⁵⁹, and Fe₃O₄⁶⁰ have been developed as light-to-heat converters. Especially for black TiO₂, since the first discovery in 2011⁶¹, various fabrication strategies have been proposed to improve its photothermal properties. The two primary techniques of bandgap engineering are the introduction of surface disorder and the formation of oxygen vacancies. Surface disorder can disrupt lattice periodicity and modify the edges of the conduction and valence bands, resulting in a narrowing of the band gap. The created oxygen vacancies can serve as traps for reducing the recombination of photogenerated charge carriers and thus improve the photothermal performance. Notably, the bandgap of black TiO₂ was reported in 2017 to be further narrowed by incorporating Ti³⁺ ions⁶². Ti₂O₃ nanoparticles have been synthesized, exhibiting exceptional light absorption capabilities and a remarkable photothermal conversion efficiency of 92%. Furthermore, Ti–O Magnéli phase oxides, such as Ti₄O₇, stand out as promising photothermal materials due to their cost-effectiveness, high chemical stability, superior thermal conductivity, extensive solar spectrum absorption ranging from 200 to 2500 nm, and efficient photothermal conversion^63,64,65. In comparison to other black titania materials, Ti₄O₇ boasts high electrical conductivity and exhibits a pronounced electrothermal effect under the application of a mere low voltage. In addition, various metal chalcogenide semiconductors, including copper sulfides (Cu_2-xS) and Cu₁₂Sb₄S₁₃, have garnered significant interest in photothermal applications⁶⁶. Zhang et al. reported high-quality Cu₇S₄ nanocrystals (NCs) with different morphologies for light-induced water evaporation⁶⁷. At one sun irradiation, water evaporation efficiencies of 60.5, 51.6, 53.1, and 47.8% were attained for monodisperse disk-like, monodisperse spherical, polydisperse disk-like, and polydisperse spherical Cu₇S₄ NCs, respectively. Monodisperse disk-shaped NCs performed best due to their strong LSPR effect and high electron (hole) carriers.

Carbon-based materials

The majority of single carbon bonds in carbon-based photothermal materials, such as C–C, C–H, and C–O, have large energy gaps between σ and σ*, corresponding to solar spectrum wavelengths less than 350 nm. As a result, the σ to σ* transition is impossible under solar irradiation. On the other hand, pi (π) bonds are usually weaker than σ-bonds due to less strongly bonded electrons; these electrons can be stimulated from the π to the π* orbital with less energy input^68,69. Furthermore, conjugated π bonds can cause a red shift in the absorption spectrum. As the number of π bonds increases, the energy gap between the highest-occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) narrows. In graphene-like allotropes, the enormous number of conjugated π bonds allows the excitation of electrons by nearly every wavelength of solar light irradiation, coupled with numerous π–π* transitions, resulting in black materials. When a material is irradiated with light energy that corresponds to a probable electronic transition inside the molecule, the electron that absorbs the light is promoted from the ground state (HOMO) to a higher energy orbital (LUMO), as seen in Fig. 3c. The excited electrons relax by electron–phonon coupling, and so the light energy absorbed is transferred from the excited electrons to vibrational modes across the atomic lattices, leading to a macroscopic increase in the material’s temperature⁷⁰.

Graphene-based photothermal materials have sparked significant attention in solar desalination⁷¹. Li et al. created porous cellulose coated in polystyrene foam with a graphene oxide (GO) film on the top surface⁷². The cellulose created a two-dimensional (2D) water channel, while the GO served as a solar absorber. The photothermal material proved effective in suppressing thermal losses, allowing for a four-fold reduction in salinity under the illumination of one sun. Recently, Liu et al. developed a new bilayered wood-GO structure for solar evaporation⁷³. The GO layer formed on the microporous wood had a high photothermal conversion efficiency, with the wide optical absorption of GO resulting in a fast temperature increase at the liquid surface. The wood, on the other hand, acted as a thermal insulator, confining the photothermal heat and allowing for effective water transfer from the bulk to the photothermally active region. The composite has a solar thermal efficiency of around 83% at a light power density of 12 kW m⁻², making it a promising photothermal material for cost-effective, resource-limited desalination. In an endeavor to design cost-effective and easily manufacturable evaporators with enhanced stability, Wang and colleagues engineered a polyurethane (PU)-based evaporator that utilized crosslinked reduced graphene oxide (rGO) nanosheets as solar absorbent materials⁷⁴. The rGO/PU foam achieved an evaporation efficiency of 81% under solar irradiation at 10 kW m⁻², highlighting its significant potential for practical application. Furthermore, biomaterials and natural products, as eco-friendly and sustainable carbon sources, possess unique structural features that align with the requirements for efficient photothermal applications. Through hybridization, carbonization, or flame treatment, these materials can attain remarkable photothermal conversion efficiency⁷⁵. Specifically, materials such as biofoam/rGO, carbon nanotube (CNT)/rGO, and GO/wood have been developed to achieve broadband absorption, photothermal conversion, and thermal localization, further expanding the horizons of photothermal materials research and application.

Other carbon-based photothermal materials, such as carbon black nanoparticles, have also been identified for this use. Xu et al. used electrospinning to create Janus absorbers for effective solar desalination⁷⁶. Their developed material had two layers: a lower hydrophilic polyacrylonitrile (PAN) layer for a water pathway and an upper hydrophobic carbon black nanoparticles/polymethylmethacrylate (PMMA) layer for light absorption and salt resistance. An evaporation rate of 1.3 kg m⁻² h⁻¹ was recorded, with an energy conversion efficiency of 72% under 1 kW m⁻². Salt rejections with 3.5 wt% salt water was also examined, with Na⁺, Mg²⁺, Ca²⁺, K⁺, and B³⁺ salt concentrations dropping to below 1.4 mg L⁻¹, substantially lower than that obtained through RO.

Conjugated polymers

Conjugated polymers, characterized by a continuous p-conjugated carbon atom backbone, such as polydopamine (PDA), polyaniline (PANI), and polypyrrole (PPy), have risen as promising candidates for solar-absorbing materials, outpacing conventional inorganic photothermal conversion materials in terms of fabrication ease and light absorption proficiency^77,78,79. Yu et al. crafted a hydrophilic carbon cloth embedded with PPy nanoparticles (termed CC-PPy arrays) for solar-driven evaporation applications⁸⁰. They observed the formation of micro-meniscuses and microdroplets on the surface, which significantly lowered the system’s evaporation enthalpy. This approach achieved an evaporation rate of 2.16 kg m⁻² h⁻¹, with the system maintaining stable evaporation performance for 72 h in 10.0 wt% NaCl solution. Moreover, certain polymers have garnered significant interest due to their biodegradability. Li’s research team developed an evaporator based on PDA-filled cellulose aerogel (PDA-CA) for solar-driven evaporation, which exhibited an evaporation rate of 1.36 kg m⁻² h⁻¹ under one sun illumination, attaining a solar energy utilization efficiency of 86%⁸¹. Additionally, the PDA-CA aerogel effectively adsorbed organic dye pollutants, highlighting its versatile application potential.

MXenes

Since the initial synthesis of Ti₃C₂ in 2011, MXenes have swiftly risen to prominence in the field of two-dimensional nanomaterials⁸². Characterized as carbides and nitrides of transition metals, MXenes adhere to the chemical formula M_n+1X_nT_x, where M denotes an early transition metal (such as Sc, Ti, V, Cr), X signifies carbon or nitrogen, and T represents surface terminations (such as OH, O, F, Cl). These materials are synthesized through the selective etching of their ternary MAX phase precursors, where A indicates elements from the IIIA or IVA groups. Due to the inclusion of transition metals, MXenes possess a free charge carrier density of approximately 10²² cm⁻³, with metallic conductivities reported to reach up to 20,000 S cm⁻¹. Their exceptional conductivity and layered architecture endow MXenes with excellent electromagnetic interference shielding properties. Beyond merely reflecting electromagnetic waves on their surfaces, residual waves undergo multiple internal reflections within MXene layers, leading to increased absorption and attenuation. Moreover, the high density of free carriers in MXenes enhances LSPR, with plasmonic characteristics tunable via modifications in the X and M site structures and surface terminations. MXenes with diverse compositions can achieve tunable plasmonic resonances within the visible and NIR spectra, and interband transitions may elicit significant absorption in the ultraviolet region⁸³. These distinctive features position MXenes as promising candidates for applications in light harvesting and thermal energy generation.

Yet, excessive reflection from flat surfaces poses challenges to solar conversion efficiency. To mitigate this, incorporating microporous structures within MXenes has been suggested as a strategic approach to extend the optical path for improved light scattering and absorption. For example, Zhao et al. converted 2D MXenes into 3D configurations by adhering MXene nanosheets (3.4 mg cm⁻³) to melamine foam using polyvinyl alcohol (PVA) as a binder⁸⁴. This resulting 3D structure captures approximately 95% of light in the 350 to 1500 nm range, achieving an 88.7% photothermal conversion efficiency under solar illumination. However, high MXene concentrations incur substantial costs and constrain manufacturing and practical deployment. In efforts to address these issues, Li et al. engineered bioinspired two-dimensional wrinkled MXenes for efficient solar steam generation (SSG)⁸⁵. This methodology entailed spraying low-concentration (0.32 mg cm⁻²) 2D fragmented MXenes onto a thermoresponsive polystyrene (PS) substrate. Subsequent air drying and heating above the PS glass transition temperature (100 °C) induced shrinkage to 50% of the initial length and 25% of the area, forming isotropic wrinkled nanostructures. Upon initial deformation, flat MXene nanocoatings transform into wrinkled structures, facilitating intense scattering and multiple reflections of incident light, thereby achieving 84.9 to 86.9% broadband ultraviolet-visible-NIR light absorption. Further deformations elevated light absorption to 93.2%. Under one sunlight exposure, the SSG system reached a water evaporation rate of 1.33 kg m⁻² h⁻¹.

Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs)

MOFs, renowned for their lattice structures, exceptional crystallinity, and high porosity, present substantial potential for a myriad of functional applications. However, their utilization as photothermal agents in solar desalination has been constrained, predominantly due to their limited solar absorption and insufficient stability in aqueous media⁸⁶. Zhu et al. have delineated a series of radical MOFs, synthesized from one-dimensional montmorillonite chains of free radical tetrathiafulvalene tetrabenzoate trimers and hexanuclear rare earth clusters⁸⁷. These MOFs exhibit remarkable stability across non-aqueous and aqueous solutions, as well as in acidic and alkaline conditions (pH 1–12). Their photothermal conversion efficiency under solar irradiation is exceptional, attributed to efficient intramolecular charge transfer and low thermal conductivity, coupled with their notable stability. These attributes suggest a significant potential for solar desalination applications. In a parallel advancement, Zhang et al. reported the fabrication of a MOF-based solar thermal water evaporator with a hierarchical structure, achieved by the in situ growth of Cu-CAT-1 MOF crystals on a copper mesh⁸⁸. The MOF-coated Cu mesh demonstrates enhanced evaporation efficiency under solar radiation. The synergistic catalytic and adsorption properties of these MOF-based materials are poised to garner increased interest in multifunctional solar-driven water purification applications.

Besides, COFs have emerged as a novel class of crystalline porous polymeric semiconductor materials, ideally suited for the design of photothermal materials due to their highly ordered structures, open porous channels, and tunable functionalities⁸⁹. Lu and colleagues successfully designed and synthesized sea-urchin-shaped photothermal COFs (UCOF-366-OH) to facilitate efficient solar-driven interfacial water evaporation⁹⁰. By meticulously controlling the nucleation and growth of COFs, they achieved a transition to a three-dimensional structure, thereby effectively managing their nanostructural morphology. The synthesized UCOF-366-OH exhibits remarkable crystallinity, a large specific surface area, superior photothermal conversion efficiency, and strong hydrophilicity. This material, when processed into photothermal composite films, enhances localized solar heating and optimizes interfacial water transport, achieving an impressive evaporation rate of 2.51 kg m⁻² h⁻¹ and a solar-steam conversion efficiency of 97.3%. This research paves the way for the multiscale structural design of COF-based photothermal materials, underscoring their innovative potential in solar water evaporation applications.

Polyphenol-based coordination compounds

Polyphenols, characterized by di- or tri-hydroxyphenyl structures, are ubiquitous in natural plants and marine organisms, encompassing a diverse range of compounds. Notable polyphenols include tannic acid (TA), gallic acid (GA), tea polyphenols, laccase, and plant flavonoids. These polyphenols exhibit exceptional hydrophilicity, adhesion, secondary reactivity, and photothermal properties, offering significant benefits for the development of eco-friendly, high-performance photothermal interfacial evaporation system (PIES) materials⁹¹. The domain of polyphenol-based PIES has recently experienced a significant upsurge in research, emerging as a vibrant and burgeoning area of focus. In this context, a novel composite coating, tannic acid@aminopropyltriethoxysilane@Fe³⁺ (TA@APTES@Fe³⁺), has been engineered for solar steam generation, exhibiting multifunctional performance and universal applicability⁹². The TA@APTES@Fe³⁺ coating demonstrates remarkable stability and adheres tenaciously to a variety of substrate surfaces, irrespective of their shape, including cotton, filter paper, wood, polyurethane sponge, and even chemically inert and highly hydrophobic polyvinylidene fluoride membranes. It withstands rigorous rinsing treatments (3000 r/min for 96 h), cyclic frost-thaw tests (−18 to 30 °C, 90 cycles), and extreme pH conditions, outperforming many other reported coatings such as carbon black, which succumb under similar stress. To exemplify the concept, poplar wood coated with TA@APTES@Fe³⁺ has achieved a water evaporation rate of approximately 1.8 kg m⁻² h⁻¹ under one sun illumination, setting a record among wood-based photothermal materials. In recent developments, a plethora of innovative designs and technologies leveraging polyphenols like GA and tea polyphenols have been brought to fruition. For instance, Li and his team utilized a TA-centered supramolecular adhesive to fabricate a novel regenerated lignocellulose-modified membrane (LCPT@CF), which achieved an evaporation rate of 1.39 kg m⁻² h⁻¹ and an evaporation efficiency of 84% under one sunlight exposure⁹³. Similarly, Wang and colleagues employed GA@APTES as an intermediary layer to firmly bind polypyrrole to a cotton fabric substrate, resulting in an evaporation rate of 1.54 kg m⁻² h⁻¹ under comparable conditions⁹⁴. These milestones underscore the unique properties of polyphenols and their competitive edge in the construction of photothermal evaporators, positioning them as formidable contenders to traditional photothermal materials and highlighting their potential to revolutionize the field of solar-driven water evaporation technologies.

Gel-based hybrid materials

Aerogel-based photothermal materials

In recent years, aerogels have received a lot of attention in the development of ISSG systems. Aerogels are not only ultra-light materials with extraordinarily high porosity (90 to 99%), but they also have a rough macroscopic surface and a complex porous micro-nano structure. This leads in extremely low thermal conductivity, which keeps heat at the water vapor contact and reduces thermal losses. Their micro-nano porous structure effectively separates internal air and vapor into small units, reducing macroscopic convective heat exchange while significantly increasing solar energy absorption.

Graphene, as a typical two-dimensional nanomaterial, has a unique dimensional morphology and exceptional broadband absorption capabilities. Its composites that combine the benefits of graphene and aerogels were employed for efficient solar evaporation. Yu’s group prepared a rGO aerogel as a solar evaporator⁹⁵, reaching a water evaporation rate of 3.39 kg m⁻² h⁻¹ under the solar irradiation of 1 kW m⁻². Similarly, an N-doped rGO composite aerogel exhibits a water evaporation rate of 2.53 kg m⁻² h⁻¹ under the same conditions⁴⁶. Besides, Zhou and associates strengthened ionic interactions and inhibited surface salt accumulation to produce ultrahigh water flux and efficient separation by adjusting graphene interlayer spacing via photonic control⁹⁶.

Other two-dimensional nanomaterials, such as MoS₂ and MXene, were also combined with aerogels for enhanced solar evaporation. Wang et al. developed a three-dimensional MoS₂-based aerogel that demonstrated over 95% sunlight absorption across the entire solar spectrum, an evaporation efficiency of 88.0% under one sun, and an ultrahigh evaporation efficiency exceeding 90% under 1.5 to 3.0 kW m⁻² solar irradiation, displaying remarkable desalination performance⁹⁷. Moreover, a Janus composite aerogel (JCM) was created using cellulose nanofibers (CNFs) as the principal framework and Ti₃C₂T_x MXenes as the photothermal filler⁹⁸. Under 1 kW m⁻² solar irradiation, this Janus structure converts photothermal energy with 88.2% efficiency and evaporates water at a rate of 2.287 kg m⁻² h⁻¹. Recently, Zhao and his team constructed crosslinked MXene aerogels (CMA) as an all-weather photothermal evaporation system capable of constantly producing steam by alternating photothermal and electrothermal conversion during the day and night⁹⁹. The combined impact of 0.5 kW m⁻² solar irradiation and 2.5 V power supply voltage resulted in a water evaporation rate of 1.624 kg m⁻² h⁻¹, much higher than the rate of 1.337 kg m⁻² h⁻¹ attained with solar irradiation alone.

Hydrogel-based photothermal materials

Hydrogels are emerging materials synthesized through physical or chemical crosslinking of monomers or polymers¹⁰⁰. They have an internal three-dimensional crosslinked polymer network that contains a large amount of water. These hydrogels can be categorized as solitary networks, semi-interpenetrating networks, and interpenetrating networks^101,102,103. In practical applications, photothermal evaporation systems based on hydrogels accomplish effective water evaporation via two major strategies: (i) energy utilization strategy, which reduces evaporation enthalpy and customizes water channels, lowering the energy demand of the evaporation process; (ii) Marangoni effect, which modifies surface morphology, increases water flow rates beneath the evaporation surface, and thus improves photothermal conversion efficiency, water evaporation rates, and anti-salt accumulation (Fig. 3d).

In 2018, Yu’s group proposed employing composite hydrogels with hierarchical nanostructures to accomplish fast water evaporation and saltwater separation under one solar irradiation¹⁰⁴. Similarly, combining polystyrene sulfonate with PVA can result in interpenetrating polymer network gels (IPNG)¹⁰⁵. The polystyrene sulfonate network in IPNG converts more than half of the water in the hydrogel into intermediate water via electrostatic and hydrogen bonding interactions, significantly lowering the equivalent evaporation enthalpy and raising the water evaporation rate. IPNG-based solar evaporators use activated carbon as a light absorber and can achieve a water evaporation rate of 3.9 kg m⁻² h⁻¹ under one sun illumination, with an evaporation efficiency of roughly 92% and outstanding desalination performance. Besides, Yang et al. created a hybrid hydrogel for solar evaporation by embedding Ti₃C₂T_x MXene and rGO nanosheets in a polymer network of polyvinyl alcohol and chitosan (CS)¹⁰⁶. The tailored surface structure of the Ti₃C₂T_x MXene/rGO hydrogel evaporator reduces evaporation enthalpy and induces the Marangoni effect at the evaporation surface, which accelerates water flow. Under 1 kW m⁻² solar irradiation, the photothermal conversion efficiency approaches 91%, with a water evaporation rate of 3.62 kg m⁻² h⁻¹, more than double the theoretical limit of traditional 2D photothermal evaporators.

In addition, the porous structure of hydrogels was demonstrated to promote rapid internal water movement while avoiding salt accumulation on the surface. Zheng’s group utilized CS and cuttlefish ink (CI) powder to form a porous hydrogel structure that functions as a highly efficient and stable salt-rejecting solar evaporator¹⁰⁷. Under 1 kW m⁻² solar irradiation, the water evaporation rate was 4.1 kg m⁻² h⁻¹. They also used marine biological materials to create a hydrogel solar evaporator with vertical microchannels via ice templating and freeze-drying methods. The evaporator achieved continuous and long-term desalination under one sun irradiation¹⁰⁸, with a water evaporation rate of up to 5.15 kg m⁻² h⁻¹ and a solar desalination rate of 4.0 kg m⁻² h⁻¹.

Suitable choice of substrate material and structure

In solar-driven evaporation systems, the selection of substrates for supporting photothermal materials is of paramount importance, requiring a balance between mechanical strength and a porous architecture. The mechanical strength ensures the durability of the substrate against degradation over extended periods, while the porosity facilitates optimal sunlight absorption and efficient water management. A range of substrates have been explored, including cellulose foam, PU, PS foam, anodic aluminum oxide (AAO), air-laid paper, natural wood, and cotton¹⁰⁹. These materials are characterized by their low thermal conductivity and high porosity, which provide nanoscopic channels for water transport.

In the realm of 2D substrates, thin films and membranes with moderate thicknesses have demonstrated remarkable performance. Qu and colleagues have enhanced polytetrafluoroethylene and cellulose membranes with photothermal materials to fabricate composite films with exceptional light absorption capabilities¹¹⁰. Electrospinning techniques have been employed to create nanofiber films, promoting bilayer designs and validating the stability and efficacy of Janus absorbers⁷⁶. Paper, with its inherent porosity and large surface area, has attracted significant research interest^111,112. Wang et al. have exploited the foldable nature of porous cellulose filter paper to develop an origami structure coated with GO as the photothermal agent, enabling an in-depth investigation of the origami structure’s impact on evaporation¹¹³. Fabric, due to its flexibility and porosity, is considered ideal for integrating photothermal materials¹¹⁴. An indirect contact evaporation technique has been proposed, suspending hydrophilic photothermal fabrics in air with both ends immersed in seawater³⁹. In this setup, PANI serves as a model photothermal material, leveraging its cost-effectiveness and broad light absorption capabilities to achieve a rapid evaporation rate of 1.94 kg m⁻² h⁻¹ while collecting runoff seawater.

In nature, many biomass materials possess the requisite strength and porosity, with oxygen-containing functional groups providing affinity sites for water adsorption, thereby facilitating water transport to solar absorbers¹¹⁵. Wood-based materials are particularly prominent in this context. Hu’s research demonstrates flexible wood membranes modified with carbon nanotubes, which exhibit superior mechanical strength and cost-effectiveness, achieving an evaporation rate of 11.22 kg m⁻² h⁻¹ under 10 kW m⁻² solar irradiation and a solar thermal efficiency of 81%¹¹⁶. Controllable methods have been explored beyond the uncontrollable microstructures of biomass, with 3D foam structures emerging as prevalent substrates. A variety of methods and materials exist for foam fabrication, with cellulose being a key raw material due to its broad applicability¹¹⁷. Researchers have developed bilayer composite biomass foams comprising BNC and rGO, achieving high solar thermal performance through thermal localization¹¹⁸. Similarly, bilayer aerogel evaporators formed from cellulose nanofibers and carbon exhibit compressible, efficient solar steam generation¹¹⁹. Additionally, some waste biomass has been repurposed into foam matrices for further utilization, showcasing the versatility and sustainability of these materials in solar-driven evaporation systems¹²⁰.

Design strategies of photothermal hybrid systems for all-weather seawater evaporation

The development of all-weather solar seawater evaporation systems is a critical solution to the global freshwater scarcity issue. However, the intermittent nature of solar energy limits the capacity of solar-driven interface evaporation technologies to meet continuous freshwater supply demands, creating a substantial barrier to the growth of ISSG systems. Researchers have lately tackled this challenge by developing hybrid systems that integrate photothermal conversion with additional heat supply technologies including phase-change heat storage and electrothermal conversion. These hybrid systems have shown to be suitable for continuous seawater evaporation under low light and harsh weather conditions.

An ideal all-weather interface solar evaporation system should embody several core features: first, efficient photothermal conversion materials to ensure maximum collection and conversion of solar energy; second, excellent hydrophilicity to facilitate rapid water transport to the evaporation surface; third, a well-designed thermal management structure to optimize thermal energy utilization; and finally, extra heat supply devices capable of releasing or transforming heat at night or in low light conditions to allow ongoing evaporation. Currently, there are two major types of systems suitable for all-weather solar-driven seawater evaporation: one based on thermal evaporation for continuous seawater desalination, and the other combining photothermal evaporation with fog collection systems, both of which provide a comprehensive solution for constant freshwater acquisition. This article will review the designs of these two system categories.

Photothermal/additional heat supply hybrid systems for all-weather seawater evaporation

Photothermal/Joule heat hybrid systems

In the field of all-weather solar seawater evaporation, the Joule effect is commonly used for nighttime evaporation. This effect enables hybrid systems to transform electrical energy into thermal energy, allowing the photothermal layer to absorb external heat and maintain constant evaporation efficiency at night. Currently, the architectures of all-weather photothermal/Joule heat hybrid systems are categorized into two distinct configurations: the external Joule heat supply mode and the built-in Joule heat source mode.

1. (1)

   External Joule heat coupled photothermal systems

   To combine the photo-electrothermal effect into the evaporation system, Qu et al. have engineered a graphene foil-supported porous graphene sponge architecture (PGS/GF) for use as an evaporator (Fig. 4a)¹²¹. The porous graphene sponge (PGS) functions as an absorber layer, capturing solar energy and concentrating heat to produce water vapor, while also providing efficient channels for water supply and vapor release. Concurrently, the graphene foil (GF) operates as an electrothermal conversion layer, offering an additional heat source that significantly amplifies the rate of steam generation. In this integrated photo-electrothermal PGS/GF system, the GF is powered electrically by a solar cell, thereby supplying additional heat energy to the evaporation system. This distinctive graphene-based evaporator achieves a substantial water production rate of 2.01–2.61 kg m⁻² h⁻¹ under solar illumination of 1 kW m⁻², even in the absence of system optimization. Further enhancement in the production rate is feasible through structural modulation of the graphene sponge and by increasing the electric power input from the applied solar cells.

   

   a Scheme of photo-electrothermal effect on the all-graphene hybrid architecture of PGS/GF¹²¹. b The evaporation rates of P/ET-SG when drawing seawater in the simulated different light environments of the day and night. c Evaporation rate comparison of P/ET-SG with relevant literature¹³. Mass changes of 3.5 wt% NaCl solution over time under d different conditions and e 1 sun + 1–5 V¹²⁶. f Atomic structure of MGM sandwich membranes. g The evaporation rate of MGM is driven by photothermal and Joule heating. h The ion concentration of the seawater, and condensed water before and after desalination¹²⁷.

   Full size image

   In addition, Sun’s group proposed a fabric-based photo-electrothermal steam generator (P/ET-SG) with adjustable surface temperature and evaporation rate¹³. The P/ET-SG consisted of a photothermal layer made of microfiber non-woven fabric (MNF) wrapped in a PPy thin layer, as well as an electrothermal layer made of PP fabric coated with nano-silver and silicone resin. As depicted in Fig. 4b, the system achieved an evaporation rate of 1.86 kg m⁻² h⁻¹ under 0.5 sun illumination with a 1 V input voltage, effectively emulating the light conditions typically encountered in the early morning and late afternoon hours (7:00–10:00, 16:00–19:00). An even higher evaporation rate of 2.52 kg m⁻² h⁻¹ was realized under 1 sun illumination with a 1 V voltage, mirroring the bright midday hours between 10:00–16:00. Post-sunset, the evaporation rate commendably sustained at 1.05 kg m⁻² h⁻¹ with a mere 1 V input voltage from 19:00 to 7:00. The evaporation rate of P/ET-SG notably surpassed the performance of the majority of solar steam evaporators reported in the literature (Fig. 4c)^122,123,124.

2. (2)

   Photothermal systems with built-in Joule heat source

Incorporating electrothermal layers into photothermal evaporation materials has become a pivotal research focus, with Qiu et al. developing a novel solar-electric dual-driven water purification evaporator that integrates an electrically heated mesh within a microporous hydrogel composed of CNTs and polyacrylamide (PAAm)¹²⁵. This system capitalizes on the synergistic effects of photothermal and Joule heating, achieving an impressive evaporation rate of 16.35 kg m⁻² h⁻¹ under one sun irradiation with a 3 A current input. Notably, even under dark conditions, the evaporator maintains a robust performance, with an evaporation rate of 6.45 kg m⁻² h⁻¹, outperforming existing photothermal-driven systems.

Furthermore, functional carbon materials, such as graphene and carbon fabric, which possess excellent photothermal and electrothermal conversion properties, are being utilized as photo-electrothermal materials for all-weather seawater evaporation. Li et al. developed a novel Janus graphene@silica sponge (p-GS)-based all-weather evaporator that synergistically harnesses photothermal and electrothermal effects¹²⁶. The evaporation rate of a 3.5 wt% NaCl solution saw a notable surge, escalating to 2.34 kg m⁻² h⁻¹ under the conditions of 1 sun + 1 V, a stark contrast to the rate achieved with 1 sun illumination alone (Fig. 4d). Concurrently, the top surface temperature of the sponge increased rapidly to 40.3 °C within a mere 2 min under 1 sun + 1 V, a rate significantly expedited compared to that under sole solar irradiation. Remarkably, under the more intensified conditions of 1 sun + 5 V, the evaporation rate peaked at an impressive 6.53 kg m⁻² h⁻¹ (Fig. 4e). Moreover, even under the subdued indoor light conditions of approximately 0.079 sun at 18 °C, the evaporation rate impressively sustained at 1.5 kg m⁻² h⁻¹ with a 5 V solar cell supplement, outperforming all previously reported salt-resistant solar-driven interfacial evaporators. Pandit et al. focused on both evaporation efficiency and salt resistance of the evaporator, developing a novel thiol-functionalized carbon fabric (t-CF) solar evaporator for all-weather seawater desalination¹⁹. During the day, the evaporator attained one of the highest documented evaporation rates of 11.2 kg m⁻² h⁻¹ with a 3 V input voltage. During seawater desalination experiments, t-CF maintained a constant evaporation rate of 11.57 kg m⁻² h⁻¹ while efficiently rejecting salt. Furthermore, t-CF demonstrated outstanding performance at low voltage circumstances (1–3 V), making it feasible for all-weather solar seawater desalination. Zhang et al. created a unique method by integrating a sandwich-like MXene-graphene oxide-MXene (MGM) hybrid material with a commercial PVDF membrane (MGM@PVDF) (Fig. 4f)¹²⁷. This system demonstrated an impressive evaporation efficiency of 92.5% over a 10-h period in a 10 wt% NaCl solution, with no salt accumulation. Additionally, the system can harness the Joule heating effect to facilitate water evaporation under dark or cloudy conditions. The maximum evaporation rate of this system, driven by sunlight (0.5 sun) and electricity (36 V), can reach 10.5 kg m⁻² h⁻¹ (Fig. 4g). In light of the practical application potential of MGM-based evaporators, desalination experiments utilizing real seawater were conducted. The concentration of cations (Na⁺, K⁺, Mg²⁺, Fe³⁺, etc.) in the collected condensed water was found to be in compliance with the World Health Organization (WHO) drinkable water standard (Fig. 4h), underscoring the effectiveness of the system in producing potable water from seawater. The comparative summary of photo-electrothermal desalination technologies is shown in Table 1.

Photothermal/heat storage hybrid systems

The core of this system integrates ISSG technology with phase-change thermal storage technology to maximize the use of solar-converted heat. The system is principally composed of photothermal and organic phase-change components. Photothermal materials transform solar energy into heat, which not only covers daytime seawater evaporation demands but also stores excess heat in organic phase-change materials for release at night, assuring continuous evaporation. Organic phase-change materials, encompassing substances such as polyethylene glycol, paraffin, lauric acid, and myristic acid, are widely acclaimed for their cost-effectiveness, eco-friendliness, high heat storage density, superior thermal reliability, and the facile adjustability of their phase-change temperatures¹²⁸. These attributes render them extensively applicable. However, the inherent limitations of these materials, including issues of liquid leakage, low thermal conductivity, poor electrical conductivity, and weak light absorption capabilities, impede the rate of energy storage and release, thereby constraining their practical applications. In response to these challenges, researchers have developed high thermal and electrical conductivity multifunctional supporting materials to encapsulate organic phase-change materials, resulting in the creation of composite phase-change materials¹²⁹. The synergistic coupling of phase-change materials with these functional supporting materials facilitates a variety of energy conversion applications, notably including photothermal conversion. Niu et al. offered a concept and application for an all-weather evaporator²⁰. This system consists of layers that are stacked sequentially: the first layer is a hydrophilic membrane, followed by a hydrogel layer, and finally a second hydrophilic membrane. The first layer’s surface is coated with photothermal nanoparticles, the hydrogel layer comprises microencapsulated phase-change materials, and the second hydrophilic membrane is composed of fiber fabric or fiber fabric with photothermal nanoparticles attached. This unique design leverages the synergistic effects of hydrogels, microencapsulated phase-change materials, and photothermal nanoparticles. Under 1 kW m⁻² irradiation, the evaporator achieved an evaporation rate of 2.67 kg m⁻² h⁻¹ with an efficiency of 89.5%. In the dark, the heat released by the phase-change layer supported an evaporation rate of 0.43 kg m⁻² h⁻¹, which is 3.6 times that of pure water, successfully alleviating the heavy reliance of water evaporation technology on sunlight. Besides, Muhammad et al. created worm-like SrCoO₃@PPy nanocomposites on hydrophilic PU foam and used paraffin blocks with mortise and tenon joints for all-weather solar evaporation¹³⁰. This design allowed for convective water transfer through the paraffin blocks and mortise and tenon joints, while a waste heat recovery device greatly minimized heat loss. The solar evaporator exhibited an excellent evaporation rate of 2.13 kg m⁻² h⁻¹ and a solar-steam conversion efficiency of up to 93% under one sun irradiation. At night, the heat released by organic phase-change materials kept the evaporator running smoothly and steadily. This solar evaporator performed well in all weather conditions, generating up to 14.96 kg m⁻² of water in 8 h of continuous operation.

Furthermore, the anti-salt capacity is critical in hybrid systems. Geng et al. developed a novel solar-driven interface evaporator that combines polypyrrole-impregnated nylon thread (PNT) as the photothermal layer and octadecane/polypyrrole nanotube aerogel composites as the photothermal energy storage device to generate high-efficiency all-weather solar steam (Fig. 5a)¹³¹. Simultaneously, benefiting from the powerful self-circulating water pumping system of PNT, the evaporator achieved a high solar-to-vapor efficiency of more than 85.3% even in high-concentration saline (20%), as well as continuous evaporation in 3.5% brine for 6 h without performance degradation and salt precipitation. In the outdoor experiment, clean water was collected at a high rate of 9.64 kg m⁻² day⁻¹ using the Cppy-O evaporator (Fig. 5b). The comparison of photothermal-phase-change materials is summarized in Table 2.

Photothermal/ambient heat hybrid systems

In 2018, the pioneering concept of leveraging ambient energy to enhance interfacial solar steam generation was introduced¹³². Li and collaborators, through precise structural innovation, effectively utilized ambient energy to augment the efficiency of these devices, achieving performance beyond theoretical efficiency limits (assuming 100% solar-to-steam energy conversion) under various light intensities. Recently, the development of novel materials with heightened surface area-to-volume ratios has been a significant advancement. Structures on a macroscale with expansive surface areas, such as cylinders, triangles, and vascular bundles, have been demonstrated to amplify evaporative cooling, enabling evaporator surface temperatures to descend below ambient levels¹³³. This phenomenon facilitates increased heat absorption from the surroundings, thereby improving steam generation efficiency under solar irradiation. Drawing inspiration from natural heat extraction mechanisms, researchers have engineered evaporator structures with comprehensive environmental contact, showing promise for substantial improvements in energy extraction capabilities and enabling steam generation even in the absence of active solar irradiation¹³⁴. Wang et al. crafted a unique “white” wood evaporator, featuring a pyramidal architecture designed to facilitate efficient water evaporation even under dark conditions (0 kW m⁻²) by adeptly harnessing ambient thermal energy¹³⁴. The microvessels of wood ensure rapid water transport, while the meticulously engineered pyramidal surface architecture promotes effective evaporative cooling (Fig. 5c, d), thereby extracting energy from the surrounding environment. This approach enables swift water evaporation independent of solar heat input. Their experiments have demonstrated a remarkable vapor generation rate of up to 2.15 kg m⁻² h⁻¹ under dark conditions, surpassing the theoretical limit of conventional solar thermal evaporators operating under 1 sun (1 kW m⁻²) illumination by a factor of 1.4 (Fig. 5e). In a 24-h continuous evaporation test, the evaporator exhibited a daily vapor generation rate of up to 50.8 kg m⁻² day⁻¹ on cloudy days and an impressive 60.7 kg m⁻² day⁻¹ on sunny days (Fig. 5f). These findings introduce a groundbreaking avenue for the development of 24-h full-time water evaporators.

Photothermal/fog collection hybrid systems for all-weather water production

Fog is easily formed in the environment at night or in low-light conditions, providing a natural water source. Solar seawater evaporation and fog harvesting are two independent strategies for creating freshwater. Combining the two processes described above results in a highly efficient hybrid system for all-weather water production. The system consists of three major components: photothermal conversion materials, adsorptive materials used to collect fog at night, and moisture storage devices. During the day, photothermal conversion materials capture and convert solar energy into heat, which is subsequently utilized to evaporate water. At night or in low-light circumstances, the adsorptive materials constantly gather moisture from the environment, ensuring that the evaporation process continues. Finally, freshwater is stored in water storage containers. The identification and optimization of appropriate adsorbent materials are essential for atmospheric water harvesting (AWH), as they critically affect the system’s efficiency in extracting moisture from the air. An ideal adsorbent must exhibit a substantial water absorption capacity, rapid adsorption-desorption kinetics, efficient desorption at low energy levels, and robust cyclic stability¹³⁵. Noteworthy advancements have been made with various adsorbent types, including MOFs, COFs, hygroscopic salt-based composites, and polymer hydrogels¹³⁶. Despite sharing a fundamental adsorption mechanism, the distinct characteristics of these materials necessitate careful consideration in their design and implementation to enhance their overall performance¹³⁷. In the case of MOFs and COFs, their commendable water uptake capabilities at low relative humidity (RH) coupled with rapid adsorption kinetics render them highly effective. Polymer hydrogels, on the other hand, demonstrate superior water absorption at high RH, coupled with the advantage of facile energetic regeneration. Hygroscopic salts and their composite materials exhibit comprehensive water uptake across the entire RH spectrum, making them valuable contenders in the pursuit of efficient AWH systems.

In practice, Xie and colleagues presented an MN-PCG with linked open pores¹⁵. The SEM image and 3D surface profilers reveal that the carbon nanotubes and the graphite oxide@carbon nanotubes on the MN-PCG foam surface exhibit commendable geometric uniformity (Fig. 6a, b). At night, the 3D micro/nanostructure on the foam surface provides abundant nucleation sites for tiny water droplets effectively capturing moisture from the humid air. This results in an average fog-collection rate of 2.65 kg m⁻² h⁻¹ and a cumulative water production of 31.77 kg m⁻² after a 12-h period (Fig. 6c, d). During daylight hours, solar intensity and environmental temperature peak, reaching a maximum of 0.6 kW m⁻² at 12:30 and 43.5 °C between 15:30 and 16:30 (Fig. 6e). Leveraging its exceptional photothermal properties and ample steam escape channels, the system achieves an optimal water production rate of 0.60 kg m⁻² h⁻¹ during the interval of 12:30–13:30, culminating in a total generation of 3.50 kg m⁻² after 12 h of continuous solar exposure (Fig. 6f). Consequently, an impressive daily yield of ~35 kg m⁻² is achieved through the synergistic effect of fog collection and solar-driven evaporation. Furthermore, the foam’s robust superhydrophobicity, resistance to acids and alkalies, thermal stability, and passive/active de-icing capabilities ensure its longevity and reliability in real-world outdoor settings.

Additionally, Shi et al., drawing inspiration from natural structures, have applied 3D micro-topologies to the surface of a PVA/PPy hydrogel membrane to enhance fog collection efficiency, facilitate droplet movement, and encourage drainage¹³⁸. Specifically, they engineered arrays of 3D micro-trees, each composed of self-similar branching micro-cones that mimic the spine structure of cactus stems. The fog collection rate of this device was ascertained to be 5.0 g cm⁻² h⁻¹, as shown in Fig. 6g. To quantify the impact of conical geometries on water droplet transport and fog collection rate, they fabricated and tested analogous PVA/PPy gel membranes featuring equally spaced, geometrically identical surface micro-topologies of cones, cylinders, and flat surfaces. Figure 6h discloses that the micro-tree array boasts a 34% higher fog collection rate compared to a flat surface, with the cone array being 17% more efficient and the cylinder array 29% less effective, after normalization by total surface area. Utilizing a homemade rooftop water harvesting system, this hydrogel membrane is capable of producing fresh water at a daily yield of approximately 34 L m⁻² in outdoor tests, underscoring its potential to alleviate global water scarcity. The comparison of photothermal-atmospheric water harvesting systems is presented in Table 3.

Practical applications of photothermal hybrid systems in all-weather seawater desalination

Advancements in seawater desalination technology are increasingly oriented toward enhancing efficiency and ensuring continuous operation. Environmental factors such as temperature, humidity, solar intensity, and wind speed exert a significant influence on the efficiency and continuity of desalination processes¹³⁹. Solar-driven desalination technologies excel under ample sunlight but face diminished efficiency during overcast conditions or at night. To surmount these challenges, numerous globally successful all-weather desalination projects have emerged, demonstrating their adaptability and technological benefits across diverse environmental conditions.

Zheng’s group has introduced an innovative solar-powered freshwater ecological membrane, crafted from buoyant transparent materials designed to float on the ocean’s surface¹⁴⁰. This avant-garde design incorporates multiple cavities with specific structural configurations to concentrate sunlight, thereby augmenting photothermal conversion efficiency and freshwater yield. Moreover, it ingeniously provides a space for crop cultivation. The system boasts an evaporation rate of 0.1951 kg m⁻² h⁻¹ and a photothermal conversion efficiency of 16.5%, paving the way for floating agricultural practices. In a concurrent development, Qu et al. have proposed a proof-of-concept self-sufficient system that leverages a graphene framework with a highly ordered vertical column array for efficient solar-driven water purification¹⁴¹. Under natural light conditions at ~15 °C, this system achieves an average water production rate of 0.8 kg m⁻² h⁻¹. Although the evaporation rate is comparatively lower, the potential for clean water generation is substantial, with future improvements anticipated through enhanced insulation and sealing mechanisms. The exploration of practical applications for desalinating high-salinity seawater has also gained momentum. Chen and colleagues have employed a 3D hemispherical steam generator in outdoor settings, successfully reducing seawater total dissolved solids (TDS) from 49,400 mg L⁻¹ to 251 mg L⁻¹ and salinity from 32.01 to 0.07%, thereby surpassing WHO standards¹⁴². Similarly, Li’s investigation into a cellulose-based solar evaporator has demonstrated a reduction in TDS from 48,900to 21 mg L⁻¹, and salinity from 32.00 to 0.010%, exceeding potable water and WHO criteria¹⁴³. Chen’s team has presented a model solar distiller, featuring a black cellulose fabric and a polystyrene foam structure, designed to effectively reject salts³¹. This system achieves a rooftop freshwater output of 2.81 and 2.5 L m⁻² day⁻¹ from ocean tests, sufficient to meet personal drinking requirements. With material costs at a mere $3 m⁻² and no reliance on energy infrastructure, this system presents an economical solution for providing clean water to off-grid communities. Furthermore, drawing inspiration from fog collection and photothermal hybrid systems, Zhang’s group has designed a conical array structure evaporator (mMF-JA)¹⁴⁴. Over an 8-day period devoid of rainfall, this system has demonstrated a water evaporation rate of 13.27–22.83 kg m⁻² during daylight hours and a fog collection rate of 2.86–7.00 kg m⁻² at night. These findings suggest that intense light exposure and high-humidity conditions substantially enhance the water collection performance of the mMF-JA. By effectively condensing and collecting evaporated water vapor and atmospheric fog, the mMF-JA is capable of generating at least 19 liters of pure water per cubic meter per day, fulfilling the daily water requirements of eight adults. These groundbreaking developments underscore a significant stride in the economic feasibility and round-the-clock operational capabilities of seawater desalination technology, heralding a new era of sustainable water resource management.

Conclusion and perspectives

This review examines the development status of all-weather solar seawater evaporators. Initially, it provides an overview of the system composition and operating principles of all-weather evaporators, as well as the design criteria for high-performance evaporators, such as broad solar absorption, effective photothermal conversion, rational thermal management, nighttime heat supply, and strong environmental resistance. It then goes over several common photothermal conversion materials for seawater desalination, with a focus on those utilized in all-weather operations. Finally, it gives a detailed introduction to the developed photothermal hybrid systems for all-weather seawater evaporation, including photothermal/external heat supply hybrid systems and photothermal/fog collecting hybrid systems. Their working principles, the evaporation efficiency across the day-night cycle, and the practical applications were thoroughly reviewed.

Looking ahead, as solar technology progresses and costs fall, all-weather solar seawater evaporation technologies have the potential to be a viable option for desalination plants. However, the existing all-weather interfacial evaporation systems continue to encounter a number of challenges when confronted with real climatic conditions and large-scale applications, including:

(1) Harsh environmental conditions such as high temperatures, high humidity, and salt fog can negatively impact system stability and efficiency. Addressing these adaptation challenges is critical to improving system practicality. This may be accomplished by optimizing material structure and composition to increase performance in high-temperature and high-humidity environments, using self-cleaning designs to prevent salt accumulation, and utilizing magnetic materials to improve cleaning and maintenance efficiency.

(2) Exploration of innovative designs for all-weather solar seawater evaporators will be a key part of future efforts. Thermal management, nighttime heat supply, water flow regulation, and salt resistance are critical considerations in evaporator design. A desired evaporator should be capable of performing several functions in a single unit. For example, it is necessary to improve energy collection and utilization for all-weather seawater evaporation, optimize water flow to ensure a balanced water supply and evaporation, and minimize salt accumulation on evaporation surfaces.

(3) Despite significant progress in all-weather solar seawater evaporation technologies on the laboratory scale, practical integration and scale applications continue to encounter problems with reduced system efficiency and high costs. However, system scaling-up does not imply directly applying larger evaporation surfaces, since it has been established that increasing the size of the evaporation surface reduces evaporation rate. As a result, a preferable option for building large-scale interfacial solar evaporation systems is to make small evaporators as units and then combine them in appropriate patterns to form an interconnected system. Furthermore, adopting low-cost materials and streamlining the manufacturing process will significantly reduce the overall system cost.

In short, we believe that much work remains to be done in the field of all-weather solar seawater evaporation, but efforts should be refocused on more applied research aimed at enabling the eventual commercial adaptation of this technology, or more fundamental research that may lead to new scientific breakthroughs, to further advance this field.