Recent advances in carbon-based materials for solar-driven interfacial photothermal conversion water evaporation: Assemblies, structures, applications, and prospective

Abstract

The shortage of fresh water in the world has brought upon a serious crisis to human health and economic development. Solar-driven interfacial photothermal conversion water evaporation including evaporating seawater, lake water, or river water has been recognized as an environmentally friendly process for obtaining clean water in a low-cost way. However, water transport is restricted by itself by solar energy absorption capacity's limits, especially for finite evaporation rates and insufficient working life. Therefore, it is important to seek photothermal conversion materials that can efficiently absorb solar energy and reasonably design solar-driven interfacial photothermal conversion water evaporation devices. This paper reviews the research progress of carbon-based photothermal conversion materials and the mechanism for solar-driven interfacial photothermal conversion water evaporation, as well as the summary of the design and development of the devices. Based on the research progress and achievements of photothermal conversion materials and devices in the fields of seawater desalination and photothermal electric energy generation in recent years, the challenges and opportunities faced by carbon-based photothermal conversion materials and devices are discussed. The prospect of the practical application of solar-driven interfacial photothermal conversion evaporation technology is foreseen, and theoretical guidance is provided for the further development of this technology.

INTRODUCTION

In the era of carbon peak and carbon neutrality, it is particularly important to develop low-cost, environmentally friendly, and large-scale new energy technologies to replace traditional fossil fuels, which are widely considered to cause the greenhouse effect and frequent climate extremes.^1,2 Solar-driven photothermal conversion (SPC) has attracted extensive attention. SPC refers to converting solar energy into thermal energy and then using the thermal energy to achieve the desired purpose, such as seawater desalination, sewage purification, electric energy generation, photothermal catalysis, and sterilization.^3,4 It is considered an important clean energy utilization technology. Among them, solar-driven interfacial photothermal conversion water evaporation (SIPCWE) has attracted widespread attention because of its ability to treat different water sources (such as seawater and sewage) with solar energy and to obtain fresh water and salt efficiently and economically.^3–6

Freshwater is the basis for the survival and development of human society. At present, the freshwater resources available for direct use by human beings are less than 0.36% of the total water resources.⁷ It has been reported that more than two-thirds of the world's population are facing varying degrees of water scarcity. To solve the global water shortage problem, there are many advanced technologies to obtain fresh water.^7,8 To date, reverse osmosis membrane technology, electrodialysis, and multi-stage flash evaporation technology are the most commonly used technology to obtain pure water from seawater and even sewage.⁷ However, there are still some disadvantages to these technologies, such as high energy consumption, and the difficulty of large-scale equipment and facilities in remote areas. Therefore, it is very important to develop a new technology that can not only reduce energy consumption but also easily and quickly obtain clean water.^6,9

Solar energy is a clean source of energy and can be used as a substitute for traditional energy sources. How to effectively utilize solar energy has become one of the research hotspots and has been favored by many researchers. At present, solar energy conversion and application methods mainly include solar electric-power generation,¹⁰ photothermal catalysis,^10,11 solar cells,^12,13 photothermal conversion,^14,15 and photobiological energy.¹⁶ Among the application methods, photothermal conversion is a solar energy utilization scheme that converts light energy directly into heat energy and performs water evaporation, thus obtaining clean water resources with minimum environmental impact and solving the water shortage problem. However, due to the different absorption of solar energy by different photothermal conversion materials (PCMs) and the serious heat loss, the efficiency of traditional SIPCWE is still low, which limits practical applications.¹⁷

SIPCWE is a complex process, which involves water transport, photothermal conversion, and heat transport. Under 1.0 kW m^–2 light illumination, the water evaporation rate (WER) of the device can be described by m₀ – m = kt, where m₀ and m are, respectively, the mass of water before and after evaporation, t is the irradiation time, and k is the WER. The photothermal conversion efficiency (PCE) η of the device can be calculated using the following equations¹⁸: [Image Omitted. See PDF] [Image Omitted. See PDF]where P₀ is the incident light power, h_v is the gas–liquid phase enthalpy change, Q₁ is the sensible heat per unit mass of water, C is the specific heat capacity (~4.2 J g^–1 ℃^–1) of water, T is the temperature at which the water evaporates, T₁ is the initial temperature of the water, and m is the steady-state evaporation rate (m₁) excluding the evaporation value of the sample under dark conditions (m₀).

In the process of SIPCWE, heat transfer and loss are also important factors affecting evaporation performance. The heat loss during photothermal conversion includes three aspects: heat convection, heat radiation, and heat conduction. The heat radiation loss can be calculated using the Stefan–Boltzmann equation⁴: [Image Omitted. See PDF]where Φ (W) is the radiant heat, A (m²) is the surface or lateral area of the device, σ is the Stefan–Boltzmann constant (5.67 × 10^–8 W m^–2 K^–4), ε is the thermal radiation coefficient of the material, T₁ is the average surface or lateral temperature of the PCM under one solar irradiation, and T₂ is the ambient temperature.

The heat convection losses can be calculated by Newton's formula⁴: [Image Omitted. See PDF]where Q₂ (W) is the convective heat, h is the convective heat transfer coefficient (10 W m^–2 K^–1), and ΔT is the difference between the surface temperature of the PCM and the ambient temperature under one solar irradiation.

The heat conduction loss can be calculated using the following equation⁴: [Image Omitted. See PDF]where Q₃ is the heat conduction loss, m is the mass of water, and ΔT is the temperature difference before and 1 h after one solar irradiation.

In the SIPCWE process, the heat loss will cause an energy exchange between the evaporation device and the environment. The energy exchange between the device and the environment can be calculated by the following equation¹⁹: [Image Omitted. See PDF]where A₁ and A₂ (m²) is the surface area and lateral area of the device, respectively. T₃ is the average lateral temperature. ε is the maximum reflectance of the PCM, ΔT is the temperature difference before and after evaporation under one solar light irradiation.

According to the above equations, it is beneficial to improve the PCE by increasing the surface temperature, reducing heat loss, and obtaining more heat from the environment during water evaporation.

To improve the absorption of solar energy and reduce heat loss, a large number of studies have been carried out to improve the PCE. For example, Neumann²⁰ and Zhao et al.²¹ developed a water evaporation method to heat bulk water. The method involves the dispersion of nanoparticles with good light absorption ability in water. When light irradiates the water, the nanoparticles suspended in the water absorb light and convert it into heat energy. The heat energy transfers from the nanoparticles to the water, increasing the temperature of the water and thus accelerating the production of water vapor (Figure 1A). This method not only ensures the uniformity of temperature in the fluid but also improves thermal conductivity and reduces the energy loss of surface heat. But even so, the heat loss is still very serious, and it is difficult to meet the practical requirements of the photothermal conversion process.

Figure 1. (A) Schematic of nanoparticle-enabled solar-driven steam generation. Reproduced with permission: Copyright 2012, American Chemical Society.20 (B, C) Schematic of the heat loss modes during the SIPCWE process. Reproduced with permission: Copyright 2019, Elsevier6; Copyright 2021, Elsevier.9 (D) Environmental energy-enhanced SIPCWE. Reproduced with permission: Copyright 2018, Elsevier.22 (E) Photothermal conversion mechanism of carbon-based materials. Reproduced with permission: Copyright 2019, Royal Society of Chemistry.23

To further reduce heat loss and improve PCE, the advantages of SIPCWE have been noticed.^24,25 By floating the PCM on the water surface, the heat energy is fixed at the vapor–water interface, thereby avoiding heating the whole bulk water and making most of the heat concentrated at the interface, so as to improve the utilization of heat energy.²⁶ However, it is still impossible to break through the theoretical limit of energy conversion efficiency simply by relying on the interface advantage because there are still energy losses, such as heat convection, heat radiation, and heat conduction between the PCM and the water in the bottom layer and the air in the upper layer (Figure 1B,C). If the temperature of the evaporator can be guaranteed to be lower than the ambient temperature, the PCM can not only convert solar energy into heat energy but also obtain additional energy from the environment, which can offset the heat loss, thus improving the PCE, even beyond the theoretical limit. Therefore, the structural design of the evaporator is important. If the structure of the evaporator evolves from a one-dimensional (1D) and two-dimensional (2D) structure to three-dimensional (3D) structure, it not only can increase the evaporation area of water but also the lateral of the 3D structure device can become cold evaporation surface. The temperature of the lateral surface may be lower than the ambient temperature so that energy can be obtained from the surrounding environment, and the WER and the PCE could be significantly improved.²⁷ Zhu et al.²² demonstrated for the first time that the performance of SIPCWE can be improved by structural design, even beyond the theoretical limit by using ambient heat energy (Figure 1D).

The performance of SIPCWE mainly depends on the PCMs properties, heat insulation materials, and water transmission materials, where PCMs are the core of the solar evaporation unit. There are two ways to maximize the use of solar energy for SIPCWE. One is to design and prepare solar-driven PCMs with ultrawide wavelengths so that they have strong absorption capacity and cover the whole solar spectrum.^4,5,28 In addition, the design and assembly of devices that can make full use of the incident light and reduce heat loss will be the other key to the practical application of SIPCWE technology.^9,22,25,26 By rationally designing PCMs for high-efficiency solar evaporators and water evaporation devices using the concept of interface heating, the PCE can be greatly improved.^27,29

From the perspective of practical application, PCMs should have the following characteristics: (1) high absorption coefficient and wide absorption wavelength; (2) rich sources; (3) low manufacturing cost, and (4) better stability. For thermal insulation materials and water transmission materials, excellent thermal insulation and water transmission performance are equally necessary. With the development of nanotechnology, PCMs, thermal insulation, and water transmission materials have been developed rapidly, especially in PCMs.^23,26

At present, carbon-based materials are very popular among the materials used for SIPCWE because carbon-based materials show strong light absorption capacity, which can better convert light energy to heat energy.^23,30,31 However, in many carbon-based materials, the energy gap between σ and σ* of most single carbon bonds, including C–C, C–H, and C–O, is too large, which is generally equivalent to the light with a wavelength less than 350 nm, so that the electron transfer from σ to σ* cannot be realized under the sunlight irradiation. The π bond is generally weaker than the σ bond, and the electronic strength of the π bond is lower, so the electrons in the π bond can be excited to π* with lower input energy; especially, the conjugated π bond will cause a red shift of absorption spectrum. At the same time, increasing the number of π bonds can reduce the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Therefore, carbon-based materials used for light-to-heat conversion usually contain a large number of conjugated double-bond structures.²³ Similar to the allotrope of graphene, a large number of conjugated π bonds can excite electrons at almost every wavelength of the solar spectrum, resulting in various π–π* transitions, so the material presents a black appearance. When the input light energy matches the electron transition energy in the molecule, the electron absorbs the light and transitions from HOMO to LUMO. The stimulated electrons are relaxed by the electron–phonon coupling, so energy is transferred from the excited electrons to the vibration mode in the entire atomic lattice, resulting in a macroscopic temperature rise of the material (Figure 1E).²³

This paper mainly summarizes the design and development of carbon-based PCMs and devices for SIPCWE, the optimization of solar PCE, the minimization of heat loss, and various approaches to design and provide efficient water vapor transport channels in the SIPCWE process. By summarizing the outstanding challenges and future trends in this field, we aim to stimulate discussions and ideas to accelerate the development of SIPCWE for different practical applications.

CARBON-BASED PCMS

Carbon-based materials are suitable for solar thermal applications because of their strong broad light absorption.^30,32 In addition, carbon-based materials have the merits of high stability and low cost, making them very popular in SIPCWE.³² More importantly, carbon-based materials can be easily made into various structures to enhance light absorption and integrated with various substrates for various solar thermal applications. All these characteristics make them the most promising PCMs. Different carbon-based nanostructures, such as carbon nanotubes (CNTs)-based, graphene-based, activated carbon, and polymer-based materials, have been developed as solar light absorbers for photothermal applications.

Carbon nanotubes-based PCMs

Among many carbon materials, there are a large number of conjugated π bonds in the molecular structure of CNTs and graphene. These conjugated π bonds will cause the red shift of absorption, which is beneficial to improving the PCE.^33,34 Many studies have found that CNTs nanofluids can use solar energy to generate water vapor at low temperatures.³⁵ CNTs first absorb and scatter photons in the upper fluid, and then part of the photons are incident into the lower fluid under sunlight. However, the problem with this process is the recovery and reuse of CNTs. Moreover, the way of heating the fluid leads to considerable heat loss in the bulk water, which limits the improvement of the PCE. To further improve the PCE, reduce the cost, and expand the scale, a series of CNTs-based substrates, such as absorbent papers,³⁶ microporous silica,³⁷ fabric,^38–42 wood,^43,44 sponge,^45–47 hydrogel,^48,49 and aerogels,^50–53 are reported to be used for SIPCWE.

It is still a great challenge to obtain and assemble durable and effective PCMs and devices by using low-cost and simple preparation methods. CNTs film with controllable thickness and pore diameter can be obtained by a series of simple methods such as filtration, spraying, and rotary spraying. To obtain an effective SIPCWE evaporator, Wang et al.³⁷ used vacuum filtration to deposit CNTs on the macroporous SiO₂ substrate. The top membrane of CNTs with hydrophobic properties was used as the PCM layer, and almost all the incident light could be converted into heat. The large pore SiO₂ substrate with the hydrophilic property has well water transfer ability and the function of preventing the heat from diffusing to the bulk water. The double-layer evaporator can self-float on the air–water interface, while the rough surface structure formed by the vacuum filtration process indirectly promotes the light absorption of CNTs film. When the thickness of the CNT film is 2.4 μm, the surface temperature of the film can reach 72°C under the 1 kW m^–2 light irradiation, so the PCE can reach 82%. Zhu et al.³⁸ embedded CNTs on polyacrylonitrile nonwoven fabric through electrostatic spinning technology to form flexible and washable PCM. The light absorption rate of the composite PCM can reach 90.8%, and the WER of seawater can reach 1.44 kg m^–2 h^–1 under 1 kW m^–2 light irradiation. Because of its good washing resistance, the salt precipitated during evaporation could be easily washed off.

Liu et al.⁴² deposited PANI on carbon fiber by using an electro-polymerization method and obtained CFs/PANI, and then Co²⁺ and Fe³⁺ were adsorbed on the CFs/PANI. The CFs/CNT PCM was prepared by heat treatment of the CFs/PANI-Fe³⁺/Co²⁺ at 500°C for 2 h and at 800°C for 2 h under an N₂ atmosphere with melamine as the carbon source (Figure 2A). During the heat treatment process, Co and Fe nanoparticles were formed and acted as catalysts to promote the growth of CNT. The maximum surface temperature can reach 325°C under 10 kW m^–2 light irradiation (Figure 2B). This is because the light absorption range of CFs/CNTs was expanded to near-infrared light (Figure 2C). Under one solar irradiation, the WER reaches 1.40 ± 0.03 kg cm^–2 h^–1. Chen et al.⁴⁴ used CNTs to modify porous wood to prepare a flexible, recyclable, and efficient PCM. The results of the light absorption test show that the transmittance of the flexible wood film without CNTs coating is 8.6%, the reflectivity is as high as 70%, and the absorption rate is only about 21%. However, the transmittance and reflectivity of CNTs loaded films are very low in the visible and infrared region, resulting in a very high absorption rate (~98%). The absorbance of the two samples is different, which largely depends on their unique structure. For wood films not coated with CNTs, although the hair-like surface can extend the optical path length and increase the scattering, thereby reducing the transmittance; but, the wood surface has a strong light reflection. After coating a layer of CNTs on the surface, due to the good light absorption and the rough hair-like surface of CNTs, the reflectivity of light on the surface is reduced, and the light path of multiple scattering is extended, which helps the CNTs/wood film to have ultrahigh broadband light absorption ability (Figure 2D). The PCE can reach up to 81% under 10 kW m^–2 light irradiation. However, the hydrophobicity of CNTs will bring some obstacles to their processability, so additional hydrophilic treatment is usually needed.

Figure 2. (A) The preparation process of the CFs/CNTs. (B) the corresponding IR thermal images at 120 s under different light irradiation. (C) The absorptivity ability of the CFs/CNTs. Reproduced with permission: Copyright 2016, American Chemical Society.42 (D) Schematic diagram of light absorption of the F-Wood and F-Wood/CNTs films. Reproduced with permission: Copyright 2017, Wiley-VCH.44

Hu et al.⁴⁸ designed and prepared CNT/PVA mixed hydrogels (CPGs) by simply freezing and thawing the mixture of CNT and PVA polymer with the help of glutaraldehyde as the cross-linking agent, in which CNT as the light absorber is well-embedded in the PVA molecular grid (Figure 3A). The PVA hydrogel has a cross-linked network with layered nanostructure. Combined with the superhydrophilicity and capillary effect of PVA porous network, the layered PVA gel can realize efficient water transport from the internal gap to the molecular grid. The PCE of CNTs/PVA hybrid gel is 90.5% and the WER is as high as 2.06 kg m^–2 h^–1. Mu et al.⁵² obtained monolithic carbon aerogels as efficient SIPCWE devices by simply carbonizing microporous conjugated polymer nanotubes. This carbon aerogel is a hierarchical nanopore network structure composed of randomly gathered hollow carbon nanotubes (HCNTs) up to a few microns in length and 100–250 nm in diameter. After treating the aerogel with ammonium persulfate/sulfuric acid solution, it has superhydrophilic wettability and is conducive to the rapid transport of water molecules. Combining the advantages of abundant porosity (92%), open pore structure, low apparent density (57 mg cm⁻³), high specific surface area (826 m² g⁻¹), low thermal conductivity (0.192 W m⁻¹ K⁻¹), and wide light absorption capacity (99%), the PEC can reach 86.8% in a single sunlight exposure. Qin et al.⁵³ used superhydrophilic and ultralight hydroxyapatite (HAP) aerogel as a carrier and loaded a layer of CNTs film on the surface of HAP aerogel to construct a self-floating HAP/CNT SIPCWE. Porous HAP aerogel serves as both a water transport layer and a thermal insulation layer to reduce heat loss and rapid water transport (Figure 3B). Under 1 kW m^–2 light irradiation, the WER reaches 1.34 kg m^–2 h^–1, and the PCE reaches 89.4%, which are much higher than those of pure water evaporation and water evaporation in the presence of HAP aerogel (Figure 3C). At the same time, taking rhodamine B (RhB) as the simulated pollutant, the absorption peak of RhB in the clean water obtained by SIPCWE completely disappeared, indicating that this SIPCWE device is also suitable for obtaining clean water from sewage (Figure 3D).

Figure 3. (A) Preparation process of 3D cross-linked CPG water evaporation device and schematic diagram of water evaporation. Reproduced with permission: Copyright 2019, Wiley-VCH.48 (B) The preparation process of the HAP/CNT aerogel. (C) Comparison of the water PCE (black color) and pure WER (red color) under 1 kW m–2 light irradiation. (D) UV-vis absorption spectra and digital image (inset) of the RhB aqueous solution before and after the SIPCWE. Reproduced with permission: Copyright 2019, Elsevier.53

Graphene-based PCMs

In addition to CNTs, graphite, graphene, and graphene oxide (GO) are 2D flat sheets composed of sp²-bonded carbon atoms and are also widely used in the field of SIPCWE. Graphene has an atomic thickness that ensures its high fluid permeability with energy/cost benefits.^54–58 In addition, through the nanopores of the highly robust graphene layer or the 2D nanochannels between adjacent stacked graphene sheets, there is a good size selection transport potential. In addition to the advantages of CNTs mentioned above, the optical and surface properties (i.e. the hydrophilicity and surface charge) of graphene can be adjusted by adjusting its oxidation degree (such as GO and reduced GO) and doping.⁵⁹

GO or reduced-GO (rGO) is the most important due to its good dispersibility. Therefore, GO and rGO-based materials are the most commonly used PCMs.^58,60–71 Storer et al.⁶¹ reported 3D photothermal aerogels for SIPCWE by combining the rGO nanosheets and cellulose fibers. The cellulose plays a significant role in a supporting framework, which not only reduces the amount of rGO used but also improves the mechanical stability and flexibility of the aerogel. Furthermore, the 3D structure of the device allows the lateral temperature of the evaporator to be lower than the ambient temperature, so additional energy can be obtained from the environment, which exhibits extremely high WER of 2.25 kg m^–2 h^–1 under 1 kW m^–2 light irradiation, and the PCE reaches 88.9%. Zhang et al.⁶² developed a graphene sheet solar steam generator with rich defects by laser irradiation. The defect-rich graphene sheets have excellent light absorption capacity and low thermal conductivity as low as 0.0075 W m^–1 K^–1. During the laser irradiation process, the small-sized GO nanosheets were curled into tubular graphene vortices during cold quenching and assembled into a 3D porous framework. The top surface of rGO exhibits an orderly macroporous structure, which facilitates the rapid release of water vapor during solar steam generation (Figure 4A). In the SIPCWE process, the evaporator with 3D thermal insulation not only inhibits the heat conduction to the bottom water but also well reduces the convective heat loss to the environment caused by airflow, so that the heat is highly concentrated on the absorber surface (Figure 4B). The water is efficiently heated in the hot spots region, and the resulting water vapor is rapidly released through the large pore channels of the rGO, so, the WER can reach 1.78 kg m⁻² h⁻¹ under one solar irradiation. Wang et al.⁶⁴ prepared a novel light evaporation device composed of rGO and polyurethane (PU) for SIPCWE. The covalent cross-linking of rGO nanosheets and PU makes the material have good stability and wide optical absorption. Meanwhile, the thermal insulation performance of PU makes the local temperature of the composite film increase rapidly under light. At 10 kW m^–2 light irradiation, the PCE reaches 81%. Huo et al.⁶⁵ reported the preparation of nitrogen-doped graphene/carbon aerogels (NGCAs) as PCM (Figure 4C). The prepared PCM has low density, high light absorption, low thermal conductivity, and efficient water transport properties, which can ensure water transport and heat localization during the SIPCWE and reduce heat loss. Under one solar light radiation, the PCE reaches 90%, and the WER reaches 1.558 kg m^–2 h^–1, which is 2.83 times higher than that of pure water evaporation. In addition, Zhang et al.⁶⁶ prepared a long-distance vertically aligned GO film (VAGSM) as an efficient PCM for the generation of clean water. They mixed the water suspension of GO with ethanol in a certain proportion, placed the polytetrafluoroethylene (PTFE) mold on the surface of liquid nitrogen for freeze-casting from the bottom to the top, then obtained VAGSM after subsequent freeze-drying and heat treatment. They tentatively suggested that preferential GO adsorption on curved ice crystal surfaces, antifreeze-induced ice growth inhibition, and directional freeze casting lead to the formation of vertically aligned graphene sheets. The VAGSM was prepared using antifreeze-assisted freezing technology so that the film has rich channels for water transport, high light absorption capacity, and excellent light and heat transfer capacity (Figure 4D). Under one solar radiation, the WER is 1.62 kg m^–2 h^–1, and the PCE reaches 86.5%. The WER could reach 6.25 kg m^–2 h^–1 under four solar radiation, and the PCE also increased to 94.2%.

Figure 4. (A) SEM image of rGO. (B) Energy balance and heat loss diagram with a 3D thermal insulating tank during the SIPCWE process. Reproduced with permission: Copyright 2018, Elsevier.62 (C) The stepwise fabrication process of NGCA. Reproduced with permission: Copyright 2019, Elsevier.65 (D) Mechanism illustration and SEM image of water transport channels with VA-GSM for SIPCWE. Reproduced with permission: Copyright 2017, American Chemical Society.66

Although rGO aerogel as a PCM has the characteristics of broadband light absorption ability, high chemical stability, and porous structure, there are still challenges, such as the following: (1) Solar light absorption ability still needs to be strengthened, and the interfacial heat loss still needs to be high. (2) Poor antifouling ability and low antibacterial performance make the aerogel susceptible to microbial adhesion, which affects the performance of the device. Therefore, assembling an evaporator with a wide absorption spectrum, and high WER and PCE, as well as antibacterial properties and stability in different environments, is of great importance in SIPCWE. Zhang et al.⁶⁸ designed an aerogel based on rGO composite polypyrrole (PPy) as a solar desalination evaporator. First, the rGO aerogel was prepared by hydrothermal reaction combined with freeze-drying, and then PPy was prepared by polymerization in the aerogel, thereby obtaining rGO-PPy aerogel. By combining the broadband light absorption capability of rGO and PPy and the antibacterial ability of PPy, the rGO-PPy aerogel exhibits excellent seawater desalination and antibacterial properties. The WER of seawater reaches 1.44 kg m⁻² h⁻¹, and the PCE is higher than 90%. This benefits from the fact that the ultralight rGO-PPy aerogel can float on the liquid surface (Figure 5A), which can limit the absorbed heat to the air–water interface, and it can only heat the water at the interface to complete the evaporation, thereby reducing the heat transfer to the bulk water and improving the WER and PCE. In addition, the evaporator also has excellent antibacterial performance and rapid antibacterial response time. This work provides a new strategy for developing low-cost, high-efficiency self-suspending carbon-based PCM for desalination evaporators.

Figure 5. (A) Optical images of light RGO/PPy aerogel and agar plates used for incubating Staphylococcus aureus and Escherichia coli under different conditions. Reproduced with permission: Copyright 2022, Tsinghua University Press.68 (B) Scheme of the COF/graphene hydrogel for accelerating SIPCWE. (C) Heavy ion concentration, UV−vis spectra, and photograph of the methyl orange solution before and after purification. Reproduced with permission: Copyright 2022, American Chemical Society.70 (D) Schematic illustration of the fabrication process of CNT−RGO@BC and top surface SEM images of CNT−RGO@BC. (E) The concentration of Na+, Ca2+, Mg2+, and K+ before and after desalination. Reproduced with permission: Copyright 2022, American Chemical Society.71

Using an in situ growth strategy, Thomas' group⁷⁰ developed a dual-region hydrogel (CGH) evaporator by rationally designing and precisely controlling the growth of sulfonic acid-functionalized covalent organic frameworks (COF-SO₃H) on rGO. It consists of a hydrophilic COF-supported rGO (COF@rGO) region and a pure hydrophobic rGO region. The addition of COF enlarges the pores of the hydrogel network to accommodate more water. The hydrophilic –SO₃H/–SO₃^– groups in COF form strong electrostatic interactions and hydrogen bonds with water molecules, making more water molecules bound inside the network. In addition, as a porous material, COF can also adsorb a large number of water molecules. By precisely controlling the distribution of hydrophilic COF-SO₃H on rGO, the composite hydrogel can have size-controllable capillary channels, tunable hydrophilicity and hydrophobicity, and light absorption capacity. Furthermore, the size of the capillary channel and the number of hydrophilic functional groups affect the water content and water state in the hydrogel, which in turn affects the evaporation enthalpy of water in the hydrogel (Figure 5B). Thanks to the advantages of the structure, the WER of the dual-zone hydrogel is as high as 3.69 kg m^–2 h^–1 under one sunlight irradiation, and the PCE is about 92%. More importantly, the hydrogel has a high-water evaporation rate even under weak sunlight. In addition, the hydrogel also demonstrated the ability of efficient solar-driven desalination, removal of heavy metal ions, and sewage purification (Figure 5C). This work not only provides a reference for the preparation of two-domain hybrid materials but also provides a novel and effective method for water state regulation in hydrogels. This method of assembling highly porous materials with adjustable hydrophilic and hydrophobic regions can also be applied in membrane separation and purification technologies. Jin et al.⁷¹ used a facile bioassembly method to design and optimize the hierarchical structure of the film by adding carbon materials in the process of bacterial cellulose (BC) raining and obtained a cellulose-based composite PCM film with a microporous structure. The 3D structure BC as a carrier is combined with 1D CNTs and 2D rGO to form PCM for SIPCWE. By in situ culture of BC, not only can a dense structure be obtained but also the surface of BC contains a lot of hydroxyl groups and many active sites, which is conducive to the load of CNT/rGO. The BC nanofibers, CNTs, and rGO synergistically form a porous network structure, which provides a continuous dual channel for the rapid transport of water molecules and light paths (Figure 5D), thus presenting significant WER and PCE characteristics. Under one sunlight irradiation, the PCE of the device reaches 90.2%, and the WER is 1.85 kg m^–2 h^–1. This superior photothermal performance hybrid film also has self-cleaning and desalination capabilities. The concentration of major metal ions such as Na⁺, Ca²⁺, Mg²⁺, and K⁺ in the purified water obtained after SPC was significantly lower, far below the standards set by WHO and EPA (Figure 5E). In addition, the device can maintain excellent structural and performance stability during operation, thus meeting the requirements of practical applications.

Other carbon-based PCMs

In addition to the above two basic carbon-based PCMs, there are many rich conventional carbon materials, such as carbon black (CB),^72–75 biochar,^76–83 carbon sponge,^84–86 carbon dots (CDs),⁸⁷ black Nylon,⁸⁸ and some kitchen and agricultural residues,^89,90 which can obtain porous carbon through a simple carbonization process. These carbon materials are considered to be the most practical choice for SIPCWE because they are most competitive in terms of cost, abundance, modifiability, stability, and environmental friendliness.

In the process of SIPCWE, high WER cannot be sustained as the solute concentration increases due to the inevitable accumulation of dirt or salt on the PCMs. Xu et al.⁷⁵ designed and assembled a water lily-inspired hierarchical structure (WHS) capable of photothermal water evaporation from high-salinity brine and wastewater containing heavy metal ions. Copper foam with high thermal conductivity and micron-sized pores was selected as the substrate, and the smooth surface of the copper foam was transformed into knife-shaped nanoplates by chemical etching. Then, the surface is coated with Al₂O₃ and decorated with CB nanoparticles to protect the surface and enhance infrared absorption, respectively (Figure 6A). The hydrophobicity of the absorber was enhanced by molecular surface modification. The hydrophobic upper surface not only absorbs sunlight and provides pores for water vapor to escape but also has self-cleaning properties. Perforated polystyrene (PS) with low thermal conductivity was chosen for the stent. The low density of PS allows the device to float on the water surface, while the pores act as water supply and salt/solute drainage channels (Figure 6B). During the entire evaporation process, no WER decrease and no contamination were observed on the PCM until the water and solute were completely separated. With the ability to stabilize high-salinity brines, high rate of evaporation, and effectively separate solutes, this technology is expected to have a direct impact in multiple fields, including wastewater treatment, sea salt production, and metal recovery. Wang et al.⁹¹ prepared ultrablack carbon aerogel (CA) foams with micropores, mesopores, and macropores (Figure 6C). Due to the formation of ice crystals during the pre-freezing process, the carbon nanoparticles are squeezed together to form vertical carbon walls. After drying and removal of ice crystals, certain macropores are retained. Importantly, CAs have many mesopores, which are formed by the accumulation of carbon nanoparticles. These large numbers of mesopores not only are beneficial for thermal insulation but also lead to a decrease in the density of the material, making the material less reflective of light (Figure 6D). In addition, the increased surface electron concentration can accelerate the heat exchange and thermal equilibrium between electrons and phonons, further enhancing the absorption of light and ultimately improving the PCE. Under one solar radiation, the surface temperature of CAs prepared under the optimum conditions can reach 87.6°C, which is 20.8°C higher than the thermal equilibrium temperature of the foam. The WER is as high as 1.29 kg m⁻² h⁻¹. Furthermore, after being activated by CO₂, the thermal electron effect of CAs is further improved and the highest WER reaches  1.37 kg m⁻² h⁻¹, which is about 2.85 times of pure WER, and the PCE is 87.51%. Such CAs with thermal-electron enhancement effect have the advantages of high thermal insulation and stability, and mechanical strength, showing great potential in the production of freshwater from seawater, industrial effluents, and even concentrated acidic or alkaline solutions.

Figure 6. (A) Fabrication processes of the Cu/CB/Al2O3 PCM. (B) Photographs of the WHS. Reproduced with permission: Copyright 2019, American Association for the Advancement of Science.75 (C) Schematic diagram of the preparation of ultrablack CAs. (D) SEM images of the CAs. Reproduced with permission: Copyright 2019, American Chemical Society.91

Fabrication of low-cost and efficient PCM remains a challenge for both industrial and academic research. Zhang et al.⁹² employed ZnCl₂/NaCl eutectic salt (as a catalyst) and a porogen to form “controlled carbonization” of poly(ethylene terephthalate) (PET) at 550°C in a low-cost way, and they easily synthesized graded porous carbon with irregular and interconnected nanoparticle forms (Figure 7A). During the carbonization process, ZnCl₂ catalyzes the dehydration and decarboxylation of PET to form vinyl-terminated segments and aromatic rings, which are then further cyclized and cross-linked to form the carbon material backbone. Meanwhile, ZnCl₂/NaCl eutectic salt serves as a porogen to generate mesopores and macropores (Figure 7B). This layered porous carbon exhibits irregular, interconnected nanoparticle morphology with high specific surface area and abundant oxygen-containing groups. These properties made them have fast water transport, high solar energy absorption efficiency, and low thermal conductivity properties. These characteristics enable the WER to be as high as 1.68 kg m^–2 h^–1 under one solar irradiation and the PCE is 97%. Meanwhile, the hierarchical porous carbon is also suitable for dye-containing wastewater, river water, seawater, and oil/water emulsions. The metal ion removal efficiency of seawater is about 99.9%, and the dye removal rate is >99.9%. This work not only reveals the potential of hierarchical porous carbon for SIPCWE but also sustainably converts low-cost waste polyesters into valuable carbon nanomaterials for energy storage and conversion through a “controllable carbonization” strategy.

Figure 7. (A, B) Schematic illustration and mechanism of controlled carbonization of PET using ZnCl2/NaCl molten salts as catalysts. Reproduced with permission: Copyright 2019, Royal Society of Chemistry.92 (C) The production process of CCY and LPF (D) SEM images of the DCY. Reproduced with permission: Copyright 2022, Wiley-VCH.93

Inspired by the aquatic plant Pistia, Wang et al.⁹³ assembled waste carbon fibers into a stretchable and compressible loop pile fabric (LPF) composed of double helical carbon yarn (DCY) and further assembled into a flexible solar evaporator. In the preparation process, low-modulus cotton fibers were added to provide more entanglement. Then, the fabrication of LPF is completed by weaving, burning, pulling, and other processes (Figure 7C). In this loop pile fabric, the V-shaped pile and loop yarns are distributed on both sides of the base layer. The end of the V-shaped pile was dispersed, which increases the surface area of the evaporator. In addition to the light absorption capacity of the carbon element, the fibers packing pores structure and the unsmooth yarn surface caused by the hairiness could further reduce the light reflection, and the channels between fibers in the DCY allowed for pumping water from the bottom to the top and spreading the water to the basic layer and piles (Figure 7D). The multi-bundle weaving technology facilitates the formation of biomimetic structures, which can reduce heat loss and improves material utilization. With the help of the porous structure of the evaporator and the gradient capillary effect, by adjusting the height of the device, the photothermal layer is in indirect contact with the bulk water, thereby maximizing the utilization efficiency of PCM. The PCE was increased from 52.70% in the original state to 88.7% under one solar irradiation. The maximum WER for 10% brine reaches 1.50 kg m⁻² h⁻¹. With the compressibility and extensibility of the fiber-helix and yarn-helix double-helix evaporator, these excellent mechanical properties can ensure that the steam production performance will not be reduced in long-term use even under extreme conditions, which provides great imagination for its industrial applications.

Plasma-carbon-based composites

To further improve the performance of PCM, researchers have designed two or more light-to-heat composites to optimize its SIPCWE performance. Hybrid materials with synergistic optical properties can achieve higher PCE. Plasma nanoparticles, such as Au,^94–98 Ag,^90–105 and Cu,^106,107 exhibit excellent light absorption and photothermal conversion ability generated by plasmon resonance. When the photon frequency matches with the electron frequency of the metal surface, the resonance photon induces the charge coherent oscillation, which shows the local surface plasmon resonance (LSPR) effect. The LSPR effect includes three phenomena, near-field enhancement, thermos-electron generation, and light-to-heat conversion. When the resonant wavelength of metal nanoparticles is used to irradiate the metal, a plasma exciton-assisted photothermal effect will be produced, which will cause the oscillation of electron gas. Electrons are excited from the occupied state orbit to the unoccupied state orbit, thus forming hot electrons. These hot electrons redistribute the hot electron energy through electron scattering, which can rapidly increase the local surface temperature of metal particles.^17,20 By combining plasma light-to-heat conversion materials with carbon-based materials, due to the LSPR effect, the light absorption ability can be further enhanced, and the surface temperature of the composites can be further improved, which is conducive to the evaporation of water.⁹⁶

Zhou et al.⁹⁶ prepared Au nanorods decorated on GO nanosheets as the PCM for SIPCWE (Figure 8A). The addition of glucose as a ligand and stabilizer is beneficial to the dispersion of Au-GO. The Au-GO hybrid structure shows increased absorption in the broadband reaching the near-infrared region (Figure 8B). Due to the coupling effect between Au nanorods and GO nanosheets, the WER reaches 1.34 kg m^–2 h^–1 under 1 kW m^–2 light irradiation. Compared with a single GO, the PCE is improved by 20% (Figure 8C). Shi et al.⁹⁹ rapidly carbonized melamine forms (MF) to obtain a 3D carbonized-MF (CMF) with good mechanical properties and then polymerized a layer of polydopamine (PDA) with dual functions of reducing and linking the agent on the surface of CMF. When CMF/PDA was immersed in AgNO₃ solution, Ag⁺ was reduced to Ag nanoparticles by reducing groups on the surface of dopamine, thus obtaining Ag-modified CMF. The porous structure not only ensures the transport of water but also scatters the incident light, thus making maximum use of the incident light, and especially, when the porous space is filled with water, the light scattering is further enhanced (Figure 8D). In the SIPCWE process, the side of the 3D device is a cold surface, and the temperature is lower than the ambient temperature, so the device can obtain energy from the surrounding environment (Figure 8E). Compared with the single component evaporator, such as the CMF and MF/Ag evaporator, the Ag-modified CMF evaporator exhibits amazing water evaporation capability under 1 kW m^–2 light irradiation.

Figure 8. (A) The synthesis process of glucose-modified Au-GO nanocomposites. (B) UV–Vis-NIR spectra of different samples. (C) The water mass loss using different materials such as PCM. Reproduced with permission: Copyright 2018, Elsevier.96 (D) Schematic illustration of light scattering by porous structures of the Ag-modified CMF. (E) Lateral IR images of the device under 1.0 kW m–2 illumination for 1 min and 30 min. Reproduced with permission: Copyright 2021, Elsevier.99 (F) Schematic of the plasmon-enhanced SIPCWE process. (G) The absorption spectra of Cu/G, CuNCN, and G. Reproduced with permission: Copyright 2018, Elsevier.107

Xu et al.¹⁰⁷ developed a “popcorn” method for the synthesis of Cu nanodot-embedded N-doped graphene (Cu/G) sea urchins based on spatially constrained pyrolysis of carbodiimide (CuNCN). The Cu nanodots formed in situ are rigidly anchored and form spatial scaffolds in the G matrix. The Cu nanodots convert the energy of incident photons into LSPR oscillations and transfer the plasmonic energy to the interconnected G via direct electron transfer (DET). The interconnected G accelerates photoinduced electron transport and converts excess energy into heat through a thermalization process. The G matrix protects the Cu nanodots from oxidation under solar radiation, thus achieving stable SIPCWE performance. The nitrogen-doped G matrix is hydrophilic, which facilitates water penetration and transport. While the sea urchin-like structure gives the sample self-floating properties, allowing the temperature increase to be localized at the water–air interface rather than uniformly heating the bulk water, resulting in more efficient solar desalination (Figure 8F). Compared with pure G and pristine CuNCN, the Cu/G samples achieve near-full-spectrum solar absorption (99%) over a wider spectral range. Efficient (~82%) and stable desalination was achieved under simulated sunlight (Figure 8G). In practical applications, the device can produce ~5 L m⁻² day⁻¹ fresh water under solar irradiation. The excellent light absorption effect, coupled with the abundance and low cost of the PCM, will enable the large-scale fabrication of other nanophotonic structures and devices.

Semiconductor-carbon-based composites

For most semiconductors, bandgap energy is a key factor that affects sunlight absorption ability. When semiconductor materials are excited by light with an energy similar to or greater than their band gap, electron-hole pairs are generated, and the electron transitions into the high-energy conduction band (CB). When the excited electrons finally return from the CB to the valence band (VB), the energy is released by radiative relaxation in the form of photons or nonradiative relaxation to the impurities/defects or surface dangling bonds of the semiconductor. When the energy is released by a phonon, it can cause the local heating of the lattice to establish the temperature distribution, thus realizing the conversion of light energy to heat energy.^17,23 The composite of semiconductors with carbon-based materials can also enhance the absorption of sunlight, so as to improve the PCE.^108–126 Compared with plasma-carbon-based composites, semiconductor materials can be used for the degradation of pollutants, so that photothermal and photocatalysis can be carried out at the same time.^117,118 Noureen et al.¹¹⁸ combined BiVO₄ and rGO to prepare composite hydrogel for enhanced SIPCWE and decontamination of polluted water. BiVO₄/rGO hydrogel floats on the water surface, and the WER increases to 1.6 kg m^–2 h^–1 under 1 kW m^–2 light irradiation. In addition, due to the excellent photocatalytic activity of BiVO₄, the hydrogel can simultaneously decompose toxic organic dyes in source water during SIPCWE.

The water transport of the device is affected by the capillary force, which naturally causes the liquid to rise to wet the capillary wall, and the capillary force is closely related to the internal pore size of the capillary and the surface tension of the liquid. Therefore, without considering the interface PCE, the water transport speed of the device significantly affects the water evaporation efficiency to a certain extent. Sun et al.¹¹⁹ assembled a high-yield and low-cost solar steam generator using low-cost natural corn stover as the matrix and multi-walled CNs and TiO₂ as the PCMs. The superhydrophilic properties of the corn stover surface are mainly due to the small pore size of the dispersed vascular bundles, which generate large capillary force. This capillary force can break the constraints of gravity and accelerate the upward movement of water molecules in the capillary tube. At the same time, due to the large pore size of the porous basic tissue, the cell wall is thinner, which is more conducive to the storage and transportation of water molecules. It has a porous cavity and nanoscale biofilm, forming a molecular filter screen, which can effectively separate organic impurities and fresh water, especially sea salt and other macromolecular substances. Therefore, the evaporator has the properties of multi-layer self-cleaning of sea salt and a large seawater storage capacity. Under sunlight irradiation, the WER of the evaporator is up to 2.48 kg m^–2 h^–1. This study not only provides the possibility to find for corn stalk as a low-cost, expandable, and efficient evaporative heat exchanger device for high-efficient desalination but also provides innovative inspiration for reducing the greenhouse effect caused by corn stalk combustion, and promotes the efficient utilization of biomass stover.

Gong et al.¹²² developed a new structure composed of G and MoO_3−x coated porous nickel (Ni-G-MoO_3−x) as a PCM by combining simple chemical vapor deposition (CVD) with the hydrothermal method. The existence of oxygen vacancies (OVs) in MoO_3−x improves the PCE of materials (Figure 9A). The PCM has excellent light absorption (96%), outstanding WER (1.50 kg m^–2 h^–1), and high PCE (95%) under 1 kW m^–2 light irradiation (Figure 9B). Ren et al.¹²⁴ grew TiO₂@TiN hyperbranched nanowires on carbonized wood (CW) to construct a highly efficient SIPCWE device. The existence of hyperbranched TiO₂@TiN can enhance light absorption by increasing the light capture area and contact area with water for heat transfer. The TiO₂@TiN-CW possesses an excellent WER of 1.52 kg m^–2 h^–1 and a remarkable PCE of 94.01% under 1 kW m^–2 light irradiation. Guo et al.¹²⁵ assembled a new system for SIPCWE by growing superhydrophilic PEGylated-MoS₂ nanoparticles on a highly air-permeable cotton cloth (CC) as a light absorption layer and polyethylene foam as a support layer and heat insulation layer. Due to the good air permeability of CC and hydrophilicity of PEGylated-MoS₂, efficient solar steam generation of 80.5–90 ± 3.5% is achieved at optical densities of 1–5 kW m^–2. The WER of PEGylated MoS₂-CC maintains a stable value after long-term light exposure (4 h) and 32 cycles of testing.

Figure 9. (A) Absorption of the Ni, Ni-G, Ni-MoO3−x, Ni-G-MoO3−x. (B) WER and PCE correspond using different PCMs. Reproduced with permission: Copyright 2021, Elsevier.122 (C) Preparation process of 1T/2H-MoS2/A-CFC. (D) Absorption properties of the prepared samples. (E) The light and thermal management of 1T/2H-MoS2/A-CFC. Reproduced with permission: Copyright 2022, American Chemical Society.126

Yang et al.¹²⁶ assembled 1T/2H-MoS₂ nanosheets on activated carbon fiber cloth (A-CFC) by a simple hydrothermal method and fabricated a 3D artificial transpiration device (ATD) based on the plant transpiration process (Figure 9C). This ATD combines a 3D conical absorber with a 1D water loop to increase evaporation parameters such as area and rate, thereby reducing operating temperature and heat loss. The combination of A-CFC and 1T/2H phase MoS₂ leads to high light absorption (~97.5%) (Figure 9D), excellent mechanical stability, a large evaporation area, and easy vapor escape. Moreover, the 3D-ATDs can take light from multiple angles just like plants, making effective use of light energy (Figure 9E). In addition, MOS₂/CFRP 3D hollow cones can recover reflected and radiated energy to realize efficient and continuous SIPCWE. An excellent WER of 1.61 kg m^–2 h^–1 and an optimal PCE of 97% were achieved under one sun irradiation. This nature-inspired 3D 1T/2H-MoS₂/A-CFC design is expected to facilitate large-scale applications in seawater purification and desalination.

In a word, semiconductor-carbon-based light-to-heat conversion composites can combine photothermal and photocatalysis to degrade pollutants while evaporating water, realize the synergy between them, and provide a new idea for single photocatalytic degradation.

Polymer-based composites

Polymer materials, typically conjugated polymers, have been used as PCMs for a long time because of their high optical absorption coefficient, low cost, lightweight, and easy chemical treatment.⁴ Both polymer PCMs and carbon-based PCMs generate heat by absorbing sunlight and causing lattice vibration.¹²⁷ Common polymer PCMs mainly include polypyrrole (PPy),^18,128–131 polyaniline (PAN),^132,133 polydopamine (PDA),^134–136  and so on.

Li et al.¹²⁸ fabricated a stretchable, efficient, and durable bilayer polymer foam for efficient and stable SIPCWE. It utilizes the double-layer structure to endow different layers with different functions, among which the PPy coating is used for light absorption and water evaporation, and the bottom pre-pressed MF with excellent mechanical properties and good thermal insulation confines the energy to the evaporation surface (Figure 10A,B). The pre-compressed MF still retains the network structure, and the PPy coating makes the skeleton surface of the MF form a rough surface and has a large pore size, which is conducive to water transportation and steam release, so as to achieve efficient water evaporation (Figure 10C). Under one sunlight irradiation, the average WER can reach 1.574 kg m^–2 h^–1, and the PCE is as high as 90.4% (Figure 10D). Benefiting from the MF superelastic and strong pre-compressed skeleton, the double-layer foam exhibits remarkable structural stability and evaporation performance under a range of harsh conditions. This low-cost, large-scale, and durable SIPCWE device is expected to produce fresh water efficiently and stably under natural light conditions, making it particularly attractive in remote areas where fresh water supply is lacking. In addition, this approach may inspire the possibility of using other PCM (PAN, PDA, GO, CNTs, etc.) and other polyporous materials (polyurethane foams, graphene aerogels, cellulose aerogels, etc.) to create double-layer solar steam generation devices for solar desalination.

Figure 10. (A, B) Schematic illustrations for the preparation and SIPCWE mechanism of the bilayer polymer foam. (C) SEM images of the bilayer form, pristine MF, pre-pressed MF, and upper PPy layer. (D) The mass change of water by using different PCMs and without PCM under 1 sun illumination. Reproduced with permission: Copyright 2019, Elsevier.128 (E) Schematic illustration of the PPy–wood PCM for the SIPCWE. (F) Absorption of PPy–wood at different incident light angles. (G) Illustration of the salt accumulation and dissolution process. Reproduced with permission: Copyright 2019, Royal Society of Chemistry.131

Using a simple “immersion polymerization” process, Huang et al.¹³¹ assembled a 3D porous water evaporation device by in situ polymerizations of pyrrole monomers into the porous wood matrix (Figure 10E). PPy has good light absorption performance, and the 3D porous wood matrix can further enhance light absorption on the basis of light harvesting and multiple scattering. In addition, the hole and the improved cavity roughness at the top of the 3D device increase light collection and multiple dispersion, allowing PPy-wood to maintain more than 93% light absorption at different angles (Figure 10F). The hydrophilicity of PPy-wood and the numerous arrayed microchannels ensure a constant water supply to the interface, and because of the low thermal conductivity of wood, PPy-wood can locate the converted heat on the device surface. Based on these advantages, PPy-wood can achieve a high WER of 1.33 kg m^–2 h^–1 and a PCE as high as 83% under one sun. In addition, since the device is composed of different layered channel structures and pits (1–2 mm), the salt concentration gradients are formed between the big channels and the small channels because of their different hydraulic conductivities in the SIPCWE process. The salinity gradients drive interchannel salt exchange through the well-maintained pits, resulting in the dilution of salt in the small channels, which establishes a good balance between the salt accumulation and dissolution process. During the evaporation process, steam continuously and rapidly escapes from the water–air interface, and the salt concentration on the top surface of the PPy-wood increases, followed by the gradual formation of salt crystals on the surface of the evaporator and in the channel. After turning off the lamp, the accumulated salt crystals gradually dissolve back into the water, which accumulates in the channel and diffuses into the body water (Figure 10G). Benefiting from the low cost and simple manufacturing process, PPy-wood with these advantages is one of the most suitable candidates for SIPCWE.

Li et al.¹³³ fabricated a reconfigurable Ti₃C₂T_x MXene/GO/PANI (MGP) hybrids with variable shapes and patternable surfaces by PANI-assisted assembly of GO and MXene for efficient SIPCWE and wastewater purification. Figure 11A shows the process of preparing plastic MGP hybrid by PANI-assisted electrostatic self-assembly at room temperature. Due to similar zeta potentials and good hydrophilicity, GO and MXene can be easily mixed to form homogeneous suspensions (Step I). In the absence of an initiator, aniline spontaneously polymerizes and generates short-chain PANI macromolecules on the surface of MXene sheets. MXene and GO sheets can be embedded into each other to form a GO/MXene/GO/MXene layered structure (Step II). In the presence of short-chain PANI, GO and MXene sheets can aggregate to form plastic hybrids on the basis of π–π conjugation, hydrogen bonding, and electrostatic interactions. After being shaped through a cylindrical mold, a vertically oriented structure is produced by placing the MGP hybrid on a Cu disk immersed in liquid nitrogen for directional freezing (Step III), during which many icicles are grown vertically to expel the GO and MXene sheets. Finally, a black and plastic MGP hybrid was obtained after thawing at ambient temperature. Water is distributed in three different states in these plastic MGP hybrids. During the SIPCWE process, MGP has abundant oxygen-containing groups, and the interaction of these oxygen-containing groups with water can effectively reduce the enthalpy of water (Figure 11B). The variable shape, patternable surface, and reusability of the plastic MGP hybrids are attributed to the strong interaction of PANI with GO and MXene (Figure 11C). Thanks to the excellent PCE of hydrophilic GO and MXene, the variable shape and patternable surface of MGP, and the reduced enthalpy of vaporization of water, the average WER with planar and concave pyramidal surfaces are as high as 2.89 and 3.30 kg m^–2 h^–1, respectively, under one solar irradiation. Because of the Marangoni effect, the temperature field and water tension field on the concave pyramid structure are distributed in a gradient (Figure 11D), which results in a larger water flow on the evaporation surface of the concave pyramid. The rapid flow of water reduces the viscosity of the internal water and activates the free water around the surface, thus increasing the WER. The excellent performance, patternable surface, remodelability and reusability, and scalable manufacturing process of plastic MGP make it promising for efficient solar-driven desalination and wastewater purification.

Figure 11. (A) The preparation process of MGP plastic hybrids for SIPCWE. (B) The distribution of the three states of water inside the MGP and water transport routes. (C) Schematic of different surface patterning models. (D) Schematic illustrations of heat flow and water flow promoted by the Marangoni effect. Reproduced with permission: Copyright 2021, Wiley-VCH.133

SOLAR-DRIVEN WATER EVAPORATION DEVICES

PCMs are the core of SIPCWE devices, but it is also very important to design a reasonable structure for the device. With the continuous development of SIPCWE technology, it is found that the development of SIPCWE system can improve the PCE when the heat is limited to the vapor–liquid interface rather than the bulk water.^17,26,27 In this part, the main structures of solar water evaporation devices are summarized. It can be divided into three types: one is a single-layer structure, which is mainly the PCM film or sheet and provides a channel for water molecules; the other is a double-layer structure, which consists of two parts: the PCM layer and the heat insulation layer. The main difference from the single-layer structure is the separation of the PCM layer (water molecular channel layer) and the heat insulation layer. The last one is the overall 3D structure, which is the most complex but can effectively reduce the heat loss of the device, and can continuously transport water to the top of the PCM layer, so as to achieve efficient SIPCWE.

Single-layer structures

When the nanoparticles with plasma efficiency are dispersed in bulk water for heating, although the thermal conductivity is improved, a lot of heat is lost, resulting in low efficiency of light-to-heat conversion.^20,21 With the development of research, it has been found that the interface solar evaporation system can improve the PCE. The PCM is placed at the water/air interface instead of dispersed in the bulk water, and the heat is only absorbed by the surface water, so the PCE can be improved. In recent years, researchers have begun to design the structure of evaporation devices, such as self-assembled thin films,¹³⁷ or unused substrates such as foam nickel and copper,^75,108 stainless steel sheet,⁸⁹ titanium sheet,¹⁰⁹ silicon carbide,¹³⁸ and so on. By means of spraying, in situ growth, and other methods, the PCMs and substrates are well composite, and they are integrated into a single-layer structure for SIPCWE.^36,72,113

Ren et al.⁵⁴ designed and synthesized graphene porous foams using foamed nickel as substrate by a simple one-step plasma-enhanced CVD (PECVD) growth method. Compared with ordinary graphene foams, the porous structure significantly enhanced the broadband absorption of sunlight and significantly improved the surface temperature. Ito et al.⁵³ prepared N-doped graphene sheets as SIPCWE devices and adjusted the pore size of the graphene sheets through the temperature. Low CVD temperatures at 800°C produce an average pore size of 100–300 nm, while CVD at 950°C results in coarsened pores of 1–2 μm. The original water wettability of the graphene sheets is changed after N doping, making it hydrophilic and able to quickly disperse water. In addition, it also shows a lower thermal conductivity, which reduces heat loss during water evaporation. The subsequent water evaporation test on it showed that the sample obtained at 950°C shows the largest WER of 1.50 kg m^–2 h^–1. Therefore, the reasonable design of the pore diameter of the material is helpful for the transportation of water in the process of water evaporation, so as to improve the WER and PCE. In addition to using some metal sheets or metal mesh as the matrix, many researchers also focus on flexible and portable materials as the matrix, such as absorbent paper and carbon cloth. Liu et al.⁷² used dust-free paper as a substrate and coated the prepared carbon particles on the dust-free paper. Utilizing the small pores of the dust-free paper to transport water from bottom to top, the thermal conductivity of the dust-free paper is small so that it can reduce heat loss, which is conducive to improving the PCE.

Wang et al.¹⁰⁹ heat-treated kitchen residues such as cherry, grape, apple, and other fruit residues by hydrothermal treatment and supplemented by carbonization. The carbon materials were grown on the upper surface of the porous titanium sheet as PCM, and a single-layer water evaporation device was assembled (Figure 12A). Under 1.5 solar irradiation, the WER is 1.42 kgm^–2 h⁻¹, and the PCE is 59.43%. Zhou et al.¹³⁷ designed a liquid-liquid phase conversion process and combined gas foaming process for rapid and continuous treatment of uniform hollow carbon spheres (HCS). The obtained HCS has a particle size of millimeters and a hierarchical structure with mutually permeable, open carbon shells and huge external voids, so it allows molecules to be quickly transported through and out of the hollow structure. Because of its lightweight and hydrophobicity, when it is placed in water, HCS always floats on the water surface and forms a photothermal conversion film at the air–water interface (Figure 12B). Combined with their chemical inertia and light absorption characteristics, these floating HCS can convert sunlight energy into heat energy, resulting in an increase in the surface temperature of the water and a significant increase in the WER. To determine the evaporation capacity of water, a certain amount of HCS-1(7 wt% CNTs) was evenly distributed on the surface of 3.5% saline water in a beaker. Under the illumination intensity of 1 kW m^–2, the surface temperature quickly increased to 50.4°C, which was 11.8°C higher than the bulk water temperature without HCS, and the WER reached 1.24 kg m^–2 h^–1. More importantly, because HCS has an inert chemical structure and mechanical robustness, it can be easily collected and washed repeatedly with deionized water, so as to realize the reusability of light-to-heat materials.

Figure 12. (A) Schematic diagram of the preparation of carbon-loaded porous TiO2 form. Reproduced with permission: Copyright 2019, Elsevier.109 (B) Schematic diagram of the experiment of enhancing water evaporation of self-floating HCSs through interface heating. Reproduced with permission: Copyright 2016, Wiley-VCH.137

Although the single-layer structure of light-to-heat conversion film makes the light-to-heat conversion occur at the water–air interface, which is conducive to the rapid escape of water vapor, the reflection of light on the film surface reduces the utilization rate of the incident light. What's more, the heat is very easy to diffuse to the bulk water, resulting in the reduction of the surface temperature, which is unfavorable to the evaporation of water. Therefore, the structure of the solar water evaporation device needs to be optimized to obtain excellent water evaporation performance.

Double-layer structures

When a single-layer thin film is used as an SIPCWE device, some materials have a good pore structure that can transmit water molecules to the PCM layer, but the thermal conductivity is relatively high or it cannot float in water. To achieve the purpose of high-efficiency SIPCWE and further reduce heat loss, people began to combine low thermal conductivity materials with PCMs to form double-layered water evaporation devices. Compared with the single-layer water evaporation device, the double-layer device composed of a light-to-heat layer and thermal insulation layer shows excellent WER and PCE, and has turned into a widely used device type.^{41,81,115,139} Polyurethane (PU), PS, and other foamed plastics as excellent commercial insulation materials^39,64,94 are widely used in the research of SIPCWE. They not only enable evaporation devices to float on the water surface but also have good thermal insulation functions and reduce the transfer of thermal energy to bulk water.

Kou et al.³⁹ bleached and dyed cotton with CNT ink as the PCM layer, and PS foam was used as an adiabatic layer to form a double-layer solar device (Figure 13A). The composite material with an absorption rate of 95.7% is in the range of 250–2500 nm. At 1 kW m^–2 light irradiation, the evaporation rate of seawater reaches 1.59 kg m^–2 h^–1, and the salt deposited on the surface is easy to be washed away. Wang et al.⁴¹ fabricated a new MXene/CNT/cotton fabric through layer-by-layer assembly, which was used as a solar steam generator for textile wastewater purification. The device includes MXene/CNT/cotton fabric as the solar absorber and steam evaporator, a PS foam as a float and heat insulation layer, and a bunch of cotton fiber as a water channel depending on the capillary effect. Because of the single water supply channel and PS foam insulation layer, the loss of heat conduction to the bottom water is effectively restrained. Under the illumination intensity of 1 kW m^–2, the high WER (1.35 kg m^–2 h^–1) and textile wastewater (1.16–1.27 kg m^–2 h^–1) are obtained. Xu et al.¹³⁹ reported a flexible carbon cloth nanocomposite with a biomimetic geranium petal-like surface for SIPCWE; the device has excellent hydrophilicity and an interconnected porous structure to ensure timely water supply and effective in-situ steam diffusion. Combining 2D water channel and insulating PS foam, the absorbance of the device is increased to 95.31%,  the WER can reach 1.484 kg m^–2 h^–1 under 1 kW m^–2 light intensity, and the corresponding PCE is 93%. Liu et al.¹⁴⁰ used natural soil polymer and carbonized corn straw to obtain biomass porous carbon (BMC). After pretreatment and molding, soil polymer was compounded with BMC to obtain a soil polymer biomass mesoporous carbon composite (GBMCC) device (Figure 13B). Under one and three solar irradiation, the WER of GBMCC is 1.58 and 2.71 kg m^–2 h^–1, and the PCE is 84.95% and 67.6%, respectively. The WER reached a record of 7.55 kg m^–2 h^–1 under one sun radiation at a wind speed of 3 m s^–1. This is mainly due to the extremely low thermal conductivity (0.15–0.48 W m^–1 K^–1) of the soil polymer as a supporting part, so the heat scattering is reduced. BMC not only has strong absorption of light but also has a surface area as high as 467 m² g^–1, which is conducive to water transport.

Figure 13. (A) Schematic diagram of flexible water evaporation device for CNT-dyed cotton fabric. Reproduced with permission: Copyright 2019, Elsevier.39 (B) Schematic of the mass and heat transportation of GBMCC device. Reproduced with permission: Copyright 2018, Wiley-VCH.140 (C) Schematics and flowchart of the solar desalination devices. Reproduced with permission: Copyright 2016, National Academy of Sciences.141

Jiang et al.¹⁴² prepared bacterial nanocellulose foam as a heat insulation layer and rGO as a light absorption layer to prepare a double-layer solar evaporator. Cellulose foam with rich pores has low thermal conductivity and good hydrophilicity, ensuring sufficient moisture transport so that efficient SIPCWE can be achieved. Under the simulated light of 10 kW m^–2, the PCE is about 83%. Li et al.¹⁴¹ made a folded GO film as a solar absorbing layer. The water dispersions of GO were filtered through a porous mixed cellulose membrane filter. After drying, the films were formed and then combined with the PS foam coated with hydrophilic cellulose. PS is used as the heat insulation layer and cellulose is used as the transmission channel of water, thus forming the 2D SIPCWE device (Figure 13C). The absorption efficiency of the film to solar energy is higher than 94%, and the PEC of the device can achieve 80% and four-times desalination rate under one sunlight irradiation. Remarkably, in this device, water is transported through a limited 2D channel, which can meet the water supply and prevent heat loss at the same time. The GO as PCM does not directly contact the bulk water but is separated by a thin layer of cellulose as the thermal insulator with a thermal conductivity of only 0.04 W m^–1 K^–1, so the heat loss is suppressed. The whole structure can float on the surface of water naturally; only the cellulose is in direct contact with the bulk water at the bottom, while the water supply is supplied to the top surface by the cellulose through the capillary force. Different from the direct water supply, heat dissipation through water will be minimized due to the reduction of the water supply dimension. Therefore, the use of a 2D water supply channel can achieve efficient water supply and inhibit heat dissipation at the same time. This device still has the function of solar desalination without an additional insulation layer.

In a word, in the double-layer-structure SIPCWE device, the rapid development of thermal insulation materials with good water delivery channels, and low thermal conductivity can effectively improve the PCE and accelerate the WER.

3D structures

The heat transfer process of solar steam generation includes three energy flows: solar energy input, steam output, and heat exchange with the environment. In recent years, improving light absorption and reducing heat loss to the environment have been key to improving PCE.^22,143 Compared with the single-layer structure, the double-layer structure could be used to reduce heat loss and improve PCE, but it still inevitably produces light reflection and heat radiation loss. This is because the temperature of the PCM layer is higher than the ambient temperature, so there will always be heat loss to the environment.²² In this case, a part of the input solar energy is converted into internal steam energy, and the other part is lost to the environment through heat exchange, so the PCE is lower than the theoretical value. However, if the temperature of the evaporation device is lower than the ambient temperature, the evaporation device can obtain energy from the environment, and the PCE can be improved, even exceeding the theoretical limit (Figure 1D).²² To further improve the PCE, researchers have begun to explore more effective light-to-heat conversion devices. Since 3D-structured evaporation devices greatly reduce heat loss and have the potential to gain additional energy from the environment, various 3D-structured devices were designed and assembled.^{18,29,51–53,56,60,69,70,74,77,78,83,123,132,144–155}

Li et al.²² carefully designed a cylindrical steam generator with a height of 10 cm and a diameter of 5.7 cm. It was proved for the first time that this evaporator can use environmental energy to improve the PCE of the SIPCWE device. As shown in Figure 14A, the cylindrical material is composed of a cotton core and plant cellulose used for papermaking, which has a sufficient water supply channel, high light absorption capacity, and a large evaporation area. The low-cost cotton core with good water supply capacity is used as a 1D water supply path to restrain the heat conduction loss of bottom bulk water. CB nanoparticles were chosen as the PCM to absorb sunlight. The outer hydrophilic cellulose coating has a continuous layered pore structure, which provides a continuous channel for water vapor to escape and air to penetrate, thus increasing the effective evaporation area. During the water evaporation process, the top surface of the PCM absorbs most of the incident solar energy, so that the surface temperature is higher than the ambient temperature, and the energy of the PCM surface is lost to the environment. However, the side of the PCM cannot absorb too much solar energy, and the temperature of the side surface is lower than the ambient temperature due to evaporative cooling, so energy can be obtained from the environment through convection and radiant heat transfer. By minimizing the energy loss of the top surface and maximizing the obtained energy of the sides, the steam output exceeds the theoretical limit. The relationship between the WER and the top and side temperature of the absorber under different light intensities is proved. The infrared image of the steam generator was used to determine the surface temperature of the device under 25, 50, 100, and 120 mW cm^–2, respectively (Figure 14B–E). As shown in Figure 14F, the ambient temperature was maintained at 24°C during the measurement. The average top temperature is 19.6°C, 24.7°C, 26.6°C, and 31.6°C, and the side temperature of the evaporator is, respectively, 18.5°C, 20.3°C, 20.4°C, and 25.4°C under different light intensity. Obviously, the top temperature always exceeds the side temperature. With the increase in light intensity, most of the solar energy is absorbed by the top surface, and the increase of the top surface temperature is greater than that of the side temperature, so more energy can be obtained from the environment. The finding provides a new way to improve the WER and PEC.

Figure 14. (A) Photograph of the PCM. Infrared images of the device under (B) 25, (C) 50, (D) 100, and (E) 120 mW cm–2 illumination. (F) Temperatures of the top and side surfaces under different light intensities. Reproduced with permission: Copyright 2018, Elsevier.22

Wu et al.¹⁹ designed a new type of light-to-heat reservoir, which is composed of an absorbent cotton core encapsulated by an rGO-based aerogel as PCM. Due to the excellent water storage property of the absorbent cotton core, it can absorb water 6.5 times its own weight and can maintain solar water evaporation for one day without an external water supply. Under 1 kW m^–2 irradiation, the top surface of the water reservoir (5.2 cm in diameter and 10 cm in length) absorbs sunlight and makes the temperature (29.2°C) higher than the ambient temperature (25°C), while the side temperature is always stable at 19.4°C, which is lower than the ambient temperature. This temperature deficiency leads to the side surface being able to obtain net energy from the environment. By calculating the energy between the top surface, side surface, and the environment in the water evaporation process, it is found that the energy lost from the top surface to the environment is 0.138 W, while the energy obtained from the environment is 1.448 W. Therefore, the net increased energy is 1.31 W, which greatly promotes water evaporation, making the WER reach 3.3 and 3.4 kg m^–2 h^–1, and the corresponding PCE is 133–139%. Li et al.¹⁴⁴ constructed an integrated evaporator with a concave structure by using 3D printing technology. The overall structure is composed of CNT/GO layer, GO/nanocellulose (NFC) layer, and GO/NFC wall successively, which has high PCE performance under one sunlight. The CNT/GO layer has high-efficiency broadband solar absorption and good PCE performance. The thin and porous CNT/GO layer contributes to the escape of water vapor and the localization of heat. The porous and hydrophilic GO/NFC wall as the support can effectively absorb water from the bottom to the adjacent porous CNT/GO layer due to capillary action, forming a continuous bottom-up water delivery channel. The air surrounded by the GO/NFC layer can play an effective role in heat insulation, so as to reduce the heat loss to a large extent and make the device have efficient SIPCWE performance. The 3D-printed porous evaporator with low intrinsic thermal conductivity makes it possible to localize the heat, thus effectively reducing the heat dissipation of bulk water. This integrated structure design using 3D printing technology provides a new way for solar energy collection and efficient water evaporation.

In addition to using PCMs to wrap the water absorption layer and assemble the solar evaporator, using the structural characteristics of biomass materials to build water evaporation devices has also attracted great attention.¹⁴⁶ Bian et al.⁷⁸ obtained a 3D structure device with excellent SIPCWE performance through simple carbonization of bamboo (Figure 15A). Through simple carbonization treatment, bamboo can maintain good mechanical properties and hydrophilicity. This is because the multiple micropores inside the bamboo are connected to each other, and water molecules can be quickly transferred from bulk water to the light-to-heat conversion region through diffusion and capillary forces. More importantly, carbonized bamboo reduced the enthalpy of evaporation and heat radiation loss during evaporation. Under solar irradiation, the WER was as high as 3.13 kg m^–2 h^–1. This high WER benefits from its unique 3D structure. After carbonization, the inner wall of bamboo can reuse the scattered light energy from the 3D bottom and the heat loss through radiation, while the outer layer can obtain additional energy from the warm environment (Figure 15B). This bamboo-based high-performance, low-cost, self-cleaning, durable, and expandable 3D steam generation device has attractive application prospects in seawater desalination and industrial and domestic wastewater reduction. Xu et al.¹⁴⁷ first used carbonated mushrooms as the SIPCWE device. The PCE of natural mushrooms and carbonized mushrooms was 62% and 78%, respectively. The results showed that the unique natural structure of mushrooms, such as umbrella-shaped black pili, porous environment, and small cross-sectional fiber stalk, promoted the production of water vapor. These characteristics can not only produce efficient light absorption, rapid water supply, and steam escape but also inhibit heat loss. Sun et al.¹⁴⁸ used a carbonized sunflower head with a 3D structure as an effective SIPCWE generator (Figure 15C). Benefiting from the numerous 3D cavities densely distributed on the top surface, the energy loss through diffuse reflection and thermal radiation can be effectively re-absorbed, realizing an enlarged water/air interface through which steam escapes. The WER and PCE are 1.51 kg m^–2 h^–1 and 100.4%, respectively, which exceed the theoretical limit. Subsequently, carbonized cattail and corncob (Figure 15D) have also been proven to be able to use in environmentally enhanced SIPCWE devices.^149,150

Figure 15. (A, B) schematic of the carbonized bamboo-based 3D solar vapor generation device and the heat behavior in the device. Reproduced with permission: Copyright 2019, Wiley-VCH.78 (C) Schematic of a carbonized sunflower head-based device and the schematic of the reflection of light in the cavity. Reproduced with permission: Copyright 2019, American Chemical Society.148 (D) Schematic of C-corncob-based water evaporation generator: (1) corn ears; (2) N-corncob; (3) device; (4) cross-section of C-corncob; (5) aligned porous sheet on the side surface of C-corncob; (6) carbon microfiber arrays on the side surface of C-corncob. Reproduced with permission: Copyright 2021, Elsevier.150

In addition to obtaining energy from the environment to enhance SIPCWE performance, it is proved that extracting energy from the environment and water can further obtain at the same time. In the process of SIPCWE, the top surface of 3D PCM is used as solar evaporation surface (SES) to dissipate energy in the environment, while the side surface is used as cold evaporation surface (CES) to obtain energy from the environment. If the area of CSE can be increased to make the side temperature lower than both the ambient temperature and the water temperature, the side can simultaneously extract energy from the environment and water. Based on this theory, Xu et al.¹⁵¹ designed 3D water evaporators with different heights to achieve this purpose (Figure 16A,B). The 3D water evaporator is composed of rGO-C as PCM and cylindrical cotton core as side surface, and the temperature changes of the cylindrical cotton core at different heights from the water surface (0, 1, 2, 3, 4, 5, and 6 cm) are compared. Under the 1 kW m^–2 irradiation, the temperature of top SES (T_SES) rises and promotes evaporation. At the same time, as a result of the cooling effect of water evaporation, the side that did not receive light experienced cold evaporation, and its surface temperature (T_CES) was lower than the ambient temperature (Figure 16C,D). Through two hours of water evaporation test, it is found that the temperature of water in 0, 1, and 2 cm heights increase by 0.5°C, 0.4°C, and 0.2°C, respectively, and the corresponding conduction losses are 0.262, 0.209, and 0.105 W (Figure 16E). These results indicated that when the height of the CES is low, the temperature of the SES can be transferred down to the water to increase the temperature of the water, and the increased temperature of water decreases with the increase of the height of the CES. When the height of the CES increases to 3–4 cm, the water temperature basically remains unchanged. When it continues to increase to 5–6 cm, the water temperature decreased by 0.2°C and 0.3°C, respectively, indicating that the device extracts energy from the water. When the CES area between SES and body water exceeds 50.3 cm² (i.e., the height of the evaporator > 4 cm), CES can eliminate all conducted energy losses of SES but actually absorb energy from the bulk water to improve the SIPCWE performance. With the increase in height, the WERs of 0–6 cm evaporators are 1.28, 1.49, 1.66, 2.03, 2.22, 2.53, and 2.95 kg m^–2 h^–1, respectively. To further explore how much energy the device can extract from water, they increased the height of CES to 7, 8, 9, and 10 cm. When the height is 7 cm, the water temperature decreases by 0.3°C as when the height is 6 cm. When it further increases to 8, 9, and 10 cm, the water temperature decreases by 0.4°C, reaching the limit value (0.209 W) of extracting energy from water, and the WER of the 7–10 cm evaporator is 3.20, 3.46, 3.70 and 3.95 kg m^–2 h^–1, respectively. These results show that it is feasible to enhance the SIPCWE performance by making the device obtain energy from the surroundings and water through a reasonable design.

Figure 16. (A, B) Schematic diagram of heat conduction between SES, CES, and a large amount of water of cylinder evaporator with small and large CES. (C) Infrared image of the top surface of 0–6 cm high cylindrical evaporator under 1.0 sunlight irradiation in initial and steady state. (D) Time-dependent temperature curve of a large amount of water during 2-h solar evaporation using a 0–6 cm high cylindrical evaporator. (E) The temperature change of a large amount of water after solar evaporation for 2 h using a 0–6 cm cylindrical evaporator. Reproduced with permission: Copyright 2020, Elsevier.151

Not all 3D devices can obtain energy from the environment to improve the PCE. In recent research, some researchers use the unique pore structure of biomass (such as wood and sugarcane) or sponge as a heat insulation layer and water transport layer and deposit PCM in situ or carbonize themselves to form 3D water evaporation devices.^{43,44,80–83,152–154} Zhu et al.¹⁵³ put one end of a wood on a metal surface heated to 500°C for carbonization treatment. The thickness of carbonization can be controlled by carbonization time. After carbonization, it becomes a 3D SIPCWE device. This tree-inspired design provides unique advantages for rapid water transport and evaporation in mesopores. The carbonized layer on the surface gives the material a high absorptivity (≈99%), and the channel thermal conductivity after carbonization is low, so the heat loss can be effectively avoided. Xue et al.¹⁵⁴ demonstrated that wood can be an ideal absorbent after a very simple flame treatment. The absorbance of flame-treated wood (F-wood) can reach up to 99%. In addition, due to the self-floating ability, inherently low thermal conductivity (0.33 W m^–1 K^–1) and naturally arranged microchannels for transporting water to the device surface, combined with strong heat localization ability, it achieves high WER of 1.05 kg m^–2 h^–1 and PCE of 72% under 1 kW m^–2. Fan et al.¹⁵⁵ designed a solar energy utilization device with a light reflection layer to improve light absorption. As an efficient SIPCWE and salt-resistant evaporator, the rGO-MF sponge (as PCM layer) and Al foil (as the reflective layer) assembled evaporator has high broadband light absorption (6.5% higher than evaporators without reflective layer), excellent thermal insulation (0.0148 W m^–1 K^–1 in dry condition), and continuous water permeability. In addition, the MF sponge-based evaporator with 3D network structure showed stable salt resistance even in 20 wt% brine. The evaporator with 3D macroporous network structure shows excellent water transport performance and excellent thermal management; it realizes an excellent WER of 1.498 kg m^–2 h^–1, and the PCE is 87.5% under 1 kW m^–2 light irradiation.

APPLICATIONS OF PHOTOTHERMAL CONVERSION TECHNOLOGY

With the rapid development of the SIPCWE system, the application of solar energy heat transfer is more and more extensive. This section focuses on the applications of photothermal conversion technology in seawater desalination and wastewater purification, electric energy generation, and photothermally enhanced photocatalytic pollutant degradation.^156–171

Seawater desalination and wastewater purification

In recent years, with the development of PCMs and the progress of devices' structural design, the development of SIPCWE systems has attracted attention. In desalination, PCE and salt tolerance are regarded as two mutually restrictive measures. Therefore, it is still challenging to fabricate solar evaporators with high PCE and excellent salt tolerance function. In the process of seawater desalination, with the continuous evaporation of water, the concentration of salt in the solution increases; when it reaches the saturation state, salt particles will precipitate on the surface of PCM. On the one hand, it will reduce the absorption of sunlight. On the other hand, salt particles will block the water transmission channel and steam escape channel, thus reducing the WER and PCE of the device.^156,172 To solve this problem, it is necessary to prepare a water evaporation device with a well salt resistance function. According to different salt evolution mechanisms, SIPCWE devices can be divided into three categories. One is to design a water transmission channel with a certain aperture, which can dilute the salt in the channel when evaporating high-concentration seawater, so as not to precipitate salt on the surface of PCMs.^{156–158,172} The second is to design water evaporation devices to precipitate salt particles outside the devices.^159–161 The last is to use the hydrophilic and hydrophobic properties of Jauns structure, with the hydrophilic end as the light absorption layer and the hydrophobic end as the salt resistance layer, thus ensuring the long-term usability of the devices.^162,164

Inspired by nature, Hu et al.¹⁷² reported a self-regenerative device with excellent antifouling performance using a reasonably designed artificial channel array on natural wood substrate. In the SIPCWE process, due to its different hydraulic conductivity, a salt concentration gradient is formed between the millimeter-sized drilled channel (low salt concentration) and the miniature wood channel (high salt concentration). The concentration gradient allows spontaneous interchannel salt exchange through 1–2 μm pits, resulting in salt dilution in the micro wood channel. Therefore, the drilling channel with high hydraulic conductivity acts as a salt-blocking channel, which can quickly exchange salt with bulk solution, so as to realize the real-time self-regeneration of the evaporator (Figure 17A,B). Hu et al.¹⁵⁶ also prepared a salt-resistant bimodal porous evaporator by using surface-carbonized balsa wood to achieve efficient and stable seawater desalination. The surface carbonized layer is about 2.5 mm thick, which can effectively absorb incident light and convert solar energy into heat energy for the desalination of brine. The natural light wood layer below can quickly pump brine upward to the carbonized layer by capillary action, ensuring continuous upward pumping of brine, and all microchannels of bimodal wood are filled with artificial brine during seawater desalination. Many micro pits (~1–2 μm) and nanopores can be found on the cell wall of the microchannel and many aligned parenchyma cells are arranged radially outward through the device and regularly penetrate these microchannels. These pits, nanopores, and ray parenchyma cells are closely connected with adjacent microchannels, and facilitate the lateral transport, diffusion, and convection of brine in the process of solar desalination. The unique bimodal porous microstructure plays a key role in long-term effective desalination. Specifically, rapid capillary suction of large container channels and water diffusion and convection between microchannels in hydrophilic wood evaporators bring about rapidly replenishing salt water evaporation on the surface to avoid salt accumulation, thereby ensuring rapid and continuous generation of freshwater. In high salinity (15 wt%) salt water, the rapid WER of 6.4 kg m^–2 h^–1 is obtained under 6 kW m^–2 irradiation, which has excellent stability and durability. In addition, the wood evaporator can effectively remove excess salt while maintaining a fast and stable evaporation rate.

Figure 17. (A) The self-regenerating solar evaporator with channel-array design (left) and (right) multidirectional mass transfer in the evaporator. (B) Photographs of the evaporator with integrated structure and channel array. Reproduced with permission: Copyright 2019, Wiley-VCH.172 (C) Schematic diagram of the water transport and salt exchange in the carbonized loofah device. Reproduced with permission: Copyright 2020, Royal Society of Chemistry.157

Liu et al.¹⁵⁷ prepared an SIPCWE device based on a loofah that is sustainable, efficient, and easy to use, and has a double-layer structure. The top carbonized layer is used as the PCM layer, which has a wide range of light absorption and high light capture capabilities. At the bottom layer, due to the natural hydrophilicity, graded macropores, and microchannels of loofah fiber, enough water is pumped to the local heating interface. In the process of seawater desalination, the continuous evaporation of water on the upper surface will inevitably lead to the increase of salt concentration on the local heating surface. Due to the limited water in the microchannel, the salt concentration in the microchannel will be much higher than that in the macropores in the sponge, resulting in a horizontal salt concentration gradient. The salt concentration gradient leads to a salt exchange between the microchannel and the 3D porous structure of the sponge. At the same time, in the vertical direction, the high-salinity water in the upper layer spontaneously exchanges with the low-salinity water in the lower layer to dilute the salt concentration in the upper layer (Figure 17C). In addition, loofah sponge has excellent hydrophilic properties, absorbs water quickly, replenishes the brine evaporated on the heating surface, and avoids salt accumulation. The inherent porous structure and microchannel of the fiber spontaneously solve the problem of salt accumulation.

The following is an introduction to the second category of devices. Song et al.¹⁵⁹ fabricated a novel type of 3D evaporator with asymmetric scale gradient grooves and microcavity arrays by 3D printing technology, in which CNTs are selected as PCM, and sodium citrate particles are used as surface distribution hole generators, which has particularly excellent water absorption and show obvious temperature gradient under illumination. Because the device has asymmetric scale gradient grooves and microcavity arrays, when it contacts with water, the water can quickly diffuse to the top of the device. During desalination, when the concentration is close to the critical concentration of a saturated solution (~26.4 wt%, 25°C), the salt will crystallize on the evaporator. The location of salt crystallization or aggregation is spatially located at the apex of the 3D evaporator. Because the position-dependent evaporation of water occurs along the liquid film, the salinity gradient is further generated along the sidewall of the bionic 3D evaporator, and the salt concentration of the top liquid film is higher than that of the bottom liquid film. In the process of continuous water evaporation, compared with the bottom, the top liquid film is easier to reach the critical crystallization concentration; that is, the higher the position on the bionic 3D evaporator, the easier it is to crystallize NaCl. For such high salinity, NaCl also crystallizes on the sidewall of the 3D evaporator surface. Significantly, the salt that stays on the 3D evaporator can be easily washed away, so as to achieve salt resistance. The astonishing WER of 2.63 kg m^–2 h^–1 and PCE of >96% under 1 kW m^–2 solar illumination and high salinity (25 wt% NaCl) are achieved.

Li et al.¹⁶⁰ constructed a migration crystallization device (MCD) with superhydrophilic carbonized green algae (SH-CGA) as PCM and demonstrated a strategy to build a salt-rejection solar steam generation system, and the surface crystallization is prevented by adding cotton thread at the edge of SH-CGA, so as to achieve salt resistance. The vessel impregnated with SH-CGA is located in the center of the MCD. Three cotton threads are used in MCD. One end of the cotton thread is inserted into normal saline, and another end is exposed outside the container by 1–2 cm the container. The function of the cotton thread is to transfer the crystals to the second container (crystal layer). The third layer is an isolation layer, which is arranged to prevent the crystallization layer from polluting the collection layer of distilled water. The outermost layer of the device is set to collect distilled water. After evaporation for 15 days, the area and thickness of the crystals attached gradually increased. A total of 24.26 g of NaCl crystalline products were collected, while there was no crystallization on the surface of SH-CGA. In the evaporation process, the capillary flow from the center of the droplet to the outside brings the suspended particles to the edge; suspended particles are highly concentrated along the edge, and finally, they are deposited in an annular manner. The adhesion of NaCl solution to the glass container, SH-CGA, and cotton threads is greater than the cohesion. Because it has the strongest adhesion to cotton threads, NaCl solution is immersed into cotton threads first. With the evaporation of moisture, crystal nuclei are formed on cotton threads. After the crystal nucleus is formed, solute particles accumulate on the surface of the crystal nucleus in turn, so that the crystal nucleus grows continuously and forms crystals. Finally, the salt particles are led out of the device to achieve salt resistance.

For the third type, Xu et al.¹⁶² demonstrated that stable and effective solar desalination can be achieved by using a flexible Janus absorber. The unique structure of Janus was utilized to separate the functions of vapor generation, solar energy absorption, and water pumping into separate layers (Figure 18A). The upper hydrophobic CB nanoparticle-coated polymethylmethacrylate (PMMA) layer is used as PCM, and the lower hydrophilic polyacrylonitrile (PAN) layer is used for water pumping. Therefore, in the process of seawater desalination, salt can only be deposited in the hydrophilic PAN layer and dissolved rapidly due to continuous pumping. Under 1 kW m^–2 solar illumination, the WER of the Janus absorber can reach 1.31 kg m^–2 h^–1, and the PCE is 72%. Four representative NaCl solutions (0.8, 3.5, 4, and 20 wt%) were used to validate the Janus absorber solar desalination test. The results showed that after SIPCWE, the Na⁺ concentration in the collected water was significantly reduced by at least 4 orders of magnitude, far below the salinity level defined by the WHO and EPA (Figure 18B). Jiao et al.¹⁶⁴ further proved that the Janus absorber had good salt tolerance. They prepared a self-floating Janus sponge composed of hydrophobic CB coating and hydrophilic porous thermoplastic polyurethane-carbon nanotube (TPC) nanofiber substrate (TPC@CB) by simple electrospinning and gas template expansion. The Janus sponge has a hierarchical structure consisting of a hydrophobic CB coating and a hydrophilic TPC sponge with a macroporous structure, where the TPC sponge is formed by interconnecting thin nanofiber layers. The Janus TPC@CB sponge exhibits high PCE due to a unique three-layer functional structure: the upper superhydrophobic solar absorption coating, the middle ultrathin thermal positioning layer, and the lower honeycomb thermal insulation layer. During the solar desalination process, the asymmetric wettability and porous structure of the Janus sponge enable the device to float naturally on the water surface, and the hydrophobic CB layer is upward to achieve efficient light absorption and photothermal conversion. Thermal localization of finite thickness, such as fiber layer thickness, can be achieved through a unique porous structure consisting of continuously interconnected hydrophilic nanofibers. Therefore, the heat loss of bulk water and salt crystallization can be effectively suppressed in the Janus TPC@CB evaporator, and high-efficiency solar steam generation can be achieved even at salinity as high as 25 wt% for the treatment of simulated saturated wastewater (Figure 18C). This work provides a promising approach for desalination and wastewater treatment.

Figure 18. (A) Schematic diagram of Janus CB@PAAM/PAN solar steam generation and seawater desalination. (B) The desalination results of the four simulated seawater samples. Reproduced with permission: Copyright 2018, Wiley-VCH.162 (C) The working mechanism of Janus TPC@CB sponge SIPCWE device. Reproduced with permission: Copyright 2022, American Chemical Society.164

The above three types of SIPCWE devices show excellent performance in the process of seawater desalination, which provides a new practical way for seawater desalination. With the development of desalting seawater by SIPCWE, some new structures have been proposed. Inspired by natural insulation materials with large pore structures, such as penguin feathers and arctic bear hair, Zhao et al.¹⁶⁶ prepared a 3D extended truss sponge with a layered micropore skeleton by using a melamine sponge (MS) as the matrix. Compared with thermalized MS (TMS) without macroporous skeletons, thermalized ammonium-dihydrogen-phosphate-modified melamine sponge (TAMS) exhibits lower thermal conductivity due to 3D interconnected hollow skeletons. When used as solar absorbers for SIPCWE, TAMS have the potential to inhibit thermal convection and promote salt tolerance. Meanwhile, the 3D layered isotropic truss structure can induce multiple light reflections to achieve omnidirectional light absorption, and the bimodal pores promote ion diffusion to inhibit salt deposition.

Liang et al.¹⁶⁷ designed and assembled a layered Co-CNS/M foam using MXene nanosheets and cobalt-based metal-organic frameworks (Co-MOFs) as raw materials. It consists of a 3D MXene microporous framework and vertically aligned MXene nanosheets decorated with vertical arrays of MOF-derived 2D carbon nanosheets embedded with cobalt nanoparticles. This array has the following five characteristics. (1) The vertical array of 2D carbon nanoplates derived from MOFs can suppress the incident light reflection through multiple light scattering and reflection, resulting in the extension of the absorption wavelength over a wide spectral range. (2) The photothermal mechanism combines the thermal vibration of molecules and localized heating of plasma to ensure high PCE. (3) The amorphous carbon nanoplate array combined with the porous skeleton structure reduces the thermal conductivity of the mixed foam. (4) The vertically aligned porous structure with intrinsic hydrophilic cell walls allows water to rapidly transport solar absorbers. (5) The surface of the carbon nanoplate array MXene nanosheets significantly improves the chemical stability of the MXene-based framework under harsh conditions (Figure 19A). Using PS foam with boreholes as the thermal insulation layer not only further restrains heat loss but also improves the salt repellency of the device (Figure 19B). This excellent salt resistance is mainly attributed to the PS foam beneath the Co-CNS/M foam. Hydrophilic nonwovens connecting channels in the hydrophobic PS layer can advect and diffuse concentrated salts back into the water.

Figure 19. (A) Schematic of the SIPCWE system using Co-CNS/M foam as PCM. (B) The different ion concentrations in an actual seawater sample before and after desalination. Reproduced with permission: Copyright 2020, Wiley-VCH.167 (C) Schematic diagram for the synthesis of Fe3O4@G. (D) Schematic diagram depicting the dynamic movement of the conic array (CA) assembly under a variable magnetic field and real pictures of the CA assembly. (E) The concept of dynamic evaporation enabled by reconfigurable Fe3O4@G assembly. The nanoparticles with different color depth show their relative positions. Reproduced under terms of the CC-BY license. Copyright 2022, The Authors, published by Springer Nature.168

Qu et al.¹⁶⁸ proposed the concept of dynamic SIPCWE and realized a magnetic response evaporator with a reconfigurable structure through a controllable and reversible Fe₃O₄@GO nanoparticle self-assembly process, which solved the problem of water vapor diffusion obstruction in the evaporation process by using the synchronous dynamic reconstruction of magnetic nanoparticles at micro and macro levels. This deformable and dynamically assembled system can be deformed and dynamically assembled with changing magnetic fields at both macroscopic and microscopic scales. Therefore, the internal water transport and external vapor diffusion of the material can be significantly improved and enhanced, and the WER is 23% higher than that of a static evaporation system. As shown in Figure 19C, the GO sheet was wrapped on the surface of the Fe₃O₄ nanoparticles by the electrostatic mutual attraction between the GO and the Fe₃O₄ nanoparticles, and then reduced to construct graphene-coated Fe₃O₄ nanoparticles. Under the action of an external magnetic field, nanoparticles can spontaneously aggregate to form conical array structures. When the magnetic field is removed, the structure can be quickly disassembled into dispersed nanoparticles, and can be moved, deformed, and reassembled under the control of the external magnetic field (Figure 19D). The coating of graphene changes the surface behavior of nanoparticles. On the one hand, it inhibits the irreversible aggregation between particles and ensures the disassembly ability of the assembly. On the other hand, the microstructure of the assembly is changed, and a rich multi-stage pore structure is introduced into the assembly, which enhances the rate of water transport. The dynamic evaporation system was constructed by using the motion behavior of the reconfigurable magnetic response assembly under varying the external magnetic field. The cone-shaped structure can still ensure high absorbance when the macroscopic deformation angle is changed. The macroscopic deformation can not only increase air convection to the surface air disturbance but also enhance the water vapor diffusion process by exposing the evaporating surface to a low-humidity environment. In addition, at the same time as the macroscopic deformation, the nanoparticles inside the assembly are also undergoing synchronous and rapid reorganization. The microscopic motion of the nanoparticles simultaneously promotes the material and heat transport process inside the cone-shaped structure, enhances the upward supply of water and salt-up process of shipping, and plays an efficient water and salt-resistant effect. If not reconfiguration, water and salt ions can only migrate in one direction, while microscopic reconfiguration establishes an internal cycle of water, salt ions, and heat (Figure 19E). This proof-of-concept work demonstrates that dynamic structural reassembly is a promising prospect and direction for the development of high-performance SIPCWE systems.

In addition to desalination, SIPCWE devices can also be used for the treatment of heavy metal pollution and wastewater purification. When the solar evaporator is used in high salinity brine, the hydration of ions increases the energy required for water evaporation, thereby reducing the SIPCWE performance, and is not conducive to the long-term use of the device. In view of this problem, Yu et al.¹⁶⁹ reported polyzwitterionic hydrogels (PZHs) that can be used for solar seawater desalination, in which activated carbon particles are used as PCM, poly(ethylene glycol)diacylate wasused as a cross-linking agent, and N,N,N′,N′-tetramethyl-ethylenedianine was used as a functional group. Since the main chain of the zwitterionic polymer contains both positive and negative functional groups, salt ions in brine can combine with counterions on the polymer chain in the salt solution of appropriate concentration, which will expand the molecular chain to a certain extent and improve the ability of the polymer to combine with water. In addition, its morphology will also change significantly in high-concentration brine, and the multi-size distribution network structure can effectively improve the ability of liquid conduction (Figure 20A). These changes enable the PZHs evaporator to exhibit better performance in brine, reaching a WER of 4.14 kg m^–2 h^–1 in 10% brine, which is about 20% higher than that in pure water. The actual test of the water purification performance of the PCM shows that the concentrations of Na⁺, Mg²⁺, K⁺, Ca²⁺, and B³⁺ ions in seawater are reduced by 2 to 4 orders of magnitude after treatment (Figure 20B). At the same time, the evaporator can also effectively purify wastewater containing heavy metal ions, so that the concentrations of Cu²⁺, Ni²⁺, Zn²⁺, Co²⁺, and Cr⁶⁺ are significantly reduced (Figure 20C), indicating that the material has a good application in removing heavy metal ions in wastewater through SIPCWE process. Li et al.¹⁷⁰ reported a 3D solar evaporator with high WER and heavy metal ion removal function. The device is composed of MoS₂/C microspheres and assembled on 3D PU sponge functionalized by polyelectrolyte. When the 3D porous sponge is processed into a 3D spined structure, the WER reaches to 1.95 kg m⁻² h⁻¹ under one sun radiation. At the same time, it can purify metal ions, sterilize and disinfect, and reduce the alkalinity and hardness of river water. In particular, the strong adsorption of MoS₂ on mercury reduced the mercury content in water from 200 to 1 ppb, which reached the strict standard set by EPA.

Figure 20. (A) Schematics of mechanisms about enhanced solar vapor generation performance of PZH. (B, C) Evaluation of the desalination performance with primary ions in a seawater sample and purification performance with heavy metal ions in a wastewater sample before and after SIPCWE. Reproduced with permission: Copyright 2022, Wiley-VCH.169

Photothermal conversion electric energy generation

The resource gap can be effectively filled by SPC technology coupling water purification and electric energy generation to simultaneously generate pure water and electricity. From the perspective of energy conversion, the SPC process is also an effective way to obtain electric energy.^{8,45,98,139,173–179} It converts solar energy into heat energy and stores it in the form of hot steam or water. However, there is a huge waste of energy between the solar energy input and the final pure water. Reasonable utilization of energy loss in the SIPCWE process will bring more opportunities to solve water and energy shortage problems.

Zhu et al.⁸ developed a kind of integrated device for water evaporation and electricity generation driven by solar energy with carbonized MF sponge as an absorption layer and PS as an insulation layer. This kind of elastic porous sponge can limit the water content in the heating area, which can improve the PCE. Meanwhile, the waste heat can be used for electric generation, and the maximum output power can reach 240.7 μW m^–2. Ho et al.⁴⁵ reported a compressible and easily reconstituted all-organic 3D sponge. It has broadband light absorption, heat insulation, and shaping capabilities, which can achieve efficient SIPCWE and electricity generation (Figure 21A). The surface temperature of the 3D sponge is higher than that of the bulk water due to localized heating. This low-grade solar thermal is considered a promising energy source for converting waste energy into electricity. Low-level heat can be harvested through the Seebeck effect induced by the static temperature difference between the solar absorber and the bulk water. Moreover, using the thermoelectric module as an insulator not only can reduce heat loss and improve the PCE but also simultaneously collect heat and convert it into electricity. In the process of SIPCWE and electric energy generation, the lower the temperature of the bulk water, the greater the temperature difference, and the higher the thermoelectric potential (Figure 21B). This synergistic coupling of SIPCWE and solar electrical energy technologies improves the overall comprehensive utilization of solar energy.

Figure 21. (A, B) Schematic and the all-organic 3D sponge device photograph, the WER, and open circuit voltage at different flow water temperatures. Reproduced with permission: Copyright 2019, Wiley-VCH.45 (C, D) The fabrication process of PSS@CNT/rGO and the liquid-flow-induced streaming potential in the film microchannel. Reproduced with permission: Copyright 2022, American Chemical Society.174 (E) Schematic of the storage and recycling of interfacial solar steam enthalpy for simultaneous generation of clean water and electricity by using the PSS@CNT/rGO device. (F) Maximum output power of the PSS@CNT/rGO thermoelectric device under different solar irradiations. Reproduced with permission: Copyright 2018, Elsevier.177

Yuan et al.¹⁷⁴ constructed PSS@CNT/rGO lamellar films by self-assembling 1D polystyrene sulfonate (PSS)@CNT and 2D matrix rGO at the molecular scale through a multistage assembly strategy (Figure 21C). This film has a controllable surface microstructure and adjustable interlayer spacing. The porous network system composed of nanochannels can provide efficient water supply and steam transfer capacity, and enhance thermal localized and insulation properties. At the same time, the interaction between the water flow and the PCM interface was also used to generate electrical energy. When the fluid flows in the micro/nanochannel, the interface between the electrolyte and the channel wall creates a double layer, and the capillary force and evaporation accelerate the fluid flow (Figure 21D). With the help of fluid flow, the movement of charges can be promoted, resulting in a maximum open-circuit voltage of 0.46 V at both ends of the PSS@CNT/rGO layered film, which successfully realizes the multi-efficiency utilization of energy. Li et al.¹⁷⁷ developed a solar evaporation thermoelectric integrated interface system that used CB nanoparticles as PCM and PS foam as an insulation layer, which uses solar energy as the only input energy, and generates clean water and electric energy at the same time. Compared with the traditional large amount of latent heat wasted in the SIPCWE process, the integrated system stores and recovers the steam enthalpy as thermal energy, and converts the steam enthalpy into electrical energy through a thermoelectric module (Figure 21E). The output of a thermoelectric module is highly dependent on the temperature difference between its two sides. Once the light source is turned on, the open-circuit voltage and short-circuit current first increase with the chamber temperature until a steady state of around 100°C is reached. With the increase of solar power illuminance, more steam will be generated, and more latent heat of condensation will be transferred to the thermoelectric module, thereby further increasing the open circuit voltage and short circuit current. At 30 kW m^–2, the open circuit voltage and short circuit current reach 3.87 V and 0.55 A, respectively (Figure 21F). The generated electricity can drive a fan (1 W) and 28 LEDs (total power 1.5 W) to run continuously. These results demonstrate that the multi-level assembly strategy is a facile and effective means to develop high-efficiency photothermal conversion films for efficient SIPCWE and electric energy generation, providing a broad prospect for alleviating energy and environmental crises.

During the simultaneous generation of pure water and electricity process through SPC technology, the precipitation and accumulation of salt crystals will not only reduce the water purification efficiency but also affect the working process of the power generation module, thus inhibiting the performance improvement of the energy-intensive cogeneration system. To solve this problem, Mori et al.¹⁷⁸ prepared starch–polyacrylamide (S-PAM) hydrogels modified by soluble starch containing a large number of hydroxyl groups with PAM as the basic skeleton. The improved interaction between polymer chains and water molecules can significantly improve the robustness and water absorption speed of the gel. The unique 3D porous structure can provide a wide storage area for moisture and avoid the influence of junction salt on solar energy absorption rate. Based on this evaporator, a cogeneration model of thermoelectricity and water purification with high compatibility and high energy utilization is further designed. The thermoelectric generator (TEG) was placed in the middle of the generator to generate electricity (Figure 22A). The hot end of the thermoelectric sheet is covered by a TiNO_x film with a high absorption rate and low emissivity. As the input end of the main energy, the evaporator is placed at the cold end of the thermoelectric sheet, where the waste heat can be used to generate steam. Since the S-PAM gel evaporator can store sufficient water inside, it can ensure the continuous water purification capacity of the cogenerator. During evaporation, the latent heat of phase transition is used to adjust the thermal behavior and reduce the cold side temperature of the TEG, thus improving the thermoelectric performance. The model focuses on micro-energy acquisition to improve the overall energy utilization rate, optimizes the heat transfer channel, increases the utilization of cold-end waste heat, and utilizes the latent heat of evaporation to improve the electrical output performance of the model. Under the simulated illumination of 1 kW m⁻², the voltage density of the photovoltaic cell (PV) is higher than that of the waste heat generator due to the direct energy conversion process. Moreover, with the lights off, the thermoelectric performance is still good using an 80°C heat source that simulates residual heat at night. In contrast, PV cells almost fail (~0 V m^–2) (Figure 22B,C). The water supply method of this model adopts the spray type, which eliminates the conduction heat loss from the evaporator to the bulk water, resulting in a higher electrical output gain. Under the continuous 8 h irradiation of natural sunlight, the outdoor cogeneration equipment has obtained a water collection rate of 0.92 kg m^–2 and a maximum output voltage of 3.96 V. The output power can be used without any auxiliary equipment. The LED lamps can be directly driven under the conditions, indicating that the generated electric energy has the expected use prospects.

Figure 22. (A) The diagram of the proposed S-PAM/TiNOx hybirds cogenerator for electricity generation and water evaporation. (B) The electricity performance comparison of PV cells and thermoelectricity when light is on and off. (C) The synchronous curves of the output voltage and two side temperatures of TEG spraying 10°C water under 1 kW m–2 illumination. Reproduced with permission: Copyright 2022, Royal Society of Chemistry.178 (D) Schematic diagram of water evaporation and electricity generation cogeneration system by using DDPA-PDN as PCM. (E) IR images of thermoelectric device surface and water under different light intensities. (F) Thermoelectric conversion capability under different light intensities. Reproduced with permission: Copyright 2021, Wiley-VCH.179

Organic small-molecule PCMs have good structure tunability and strong resistance to photobleaching, which, thus, are also promising solar energy absorbers. Cui et al.¹⁷⁹ constructed a donor–acceptor organic small molecule solar energy absorber 2,17-Bis(diphenylamino)dibenzo[a,c]naphtho[2,3-h] phenazine-8,13-dione (DDPA-PDN). Using cellulose paper as the carrier, the SIPCWE is carried out, and at the same time, the thermoelectric device is used as the carrier to make full use of the waste heat to generate electricity (Figure 22D). When the DDPA-PDN cellulose paper absorbs sunlight, the upper part of the thermoelectric device has a higher temperature, which forms a temperature difference with the lower static water as a power generation core, which can convert waste heat into electricity. Under 1 kW m⁻² solar irradiation, the temperature difference (ΔT) of DDPA-PDN cellulose paper is significantly higher than that of blank cellulose paper, and the temperature difference also increases gradually with the increase of light intensity (Figure 22E). Temperature difference causes the thermoelectric device to produce current, which increases with the increase of temperature difference. The voltage of the cogeneration unit can reach 43 mV under 1 kW m⁻² solar irradiation (Figure 22F). This study demonstrates the application of organic small molecules as PCM in water evaporation and electric energy generation, thus providing valuable prospects for their application in solar energy harvesting.

Photothermally enhanced photocatalytic pollutant degradation

In the past decades, photocatalytic technology has been considered a green and promising technology for environmental remediation.^180,181 Under sunlight irradiation, the excited electrons and holes of the photocatalyst can completely decompose organic pollutants into CO₂ and H₂O, and so on. The photocatalytic performance depends on the redox ability of electrons, holes, and active species such as •O₂^– and •OH. Because of their strong redox ability, these active species can completely degrade pollutants.¹⁸² In recent years, it has been found that PCMs can be embedded into the photocatalytic reaction through the photothermal effect, thus significantly improving the catalytic efficiency.^{113,116–118,183–185}

Xue et al.¹¹³ reported carbon dots (CQs) decorated black TiO₂ nanotube arrays@Ti foam (CDs/black TNA@Ti) as an efficient PCM for SIPCWE and photocatalytic pollutant degradation. The results display that the PCM has good solar absorption capacity, wettability, and hierarchical porous structure. In the SIPCWE experiment, the WER and PCE reached 1.762 kg m⁻² h⁻¹ and 55.3%, respectively. The photocatalytic performance of CDs/Black TNA@Ti is also higher than those other samples without hydrogenation and/or CQs decoration, and the degradation rate of RhB reaches 0.8694 h⁻¹ under visible light irradiation. Noureen et al.¹¹⁷ prepared multifunctional Ag₃PO₄-rGO nanocomposite-coated textiles for SIPCWE, photocatalysis, and disinfection (Figure 23A). The Ag₃PO₄-rGO nanocomposite was coated on a cotton textile substrate that could float on the water surface to absorb sunlight and convert it into heat, thereby increasing the surface temperature and promoting SIPCWE. The composite-coated textile can reach a high WER of 1.31 kg m^–2 h^–1 under sunlight. Moreover, the textile can simultaneously decompose organic dyes in water and disinfect pathogenic microorganisms, and drinkable water during SIPCWE. This all-in-one multifunctional textile provides a convenient and sustainable strategy for freshwater production.

Figure 23. (A) Scheme of Ag3PO4-rGO nanocomposite-coated textiles for SIPCWE, photocatalysis and disinfection. Reproduced with permission: Copyright 2020, American Chemical Society.117 (B) Surface plasmon resonance under NIR irradiation leads to a photothermal effect over rGO-TiO2. Reproduced with permission: Copyright 2014, American Chemical Society.183 (C) Schematic illustration of the synchronous photothermal and photochemical solar conversion of a defective D-HNb3O8/PAM aerogel. (D) Schematic illustration of spectrally selective utilization of full solar energy over the D-HNb3O8. (E) Photocatalytic performance of the different samples for RhB degradation. Reproduced with permission: Copyright 2018, Wiley-VCH.185

Gan et al.¹⁸³ prepared rGO-TiO₂ composite as PCM and the photothermal conversion effect of the nanocomposites plays an important role in the enhancement of their photocatalytic performance. Under near-infrared light irradiation, the surface temperature of rGO increases, and photoexcited electrons can obtain additional energy to move rapidly on the hot rGO, thus realizing high-efficiency charge transfer (Figure 23B). The additional ability of photogenerated electrons leads to a significant decrease in the probability of electron-hole complexation, which allows more electrons and holes to participate in the formation of active components and further improves photocatalytic performance. Among them, the degradation effect of photothermal effect on pollutants is as high as ~38%. Of course, this multifunctional structure will also cause pollutant molecules to be adsorbed on the evaporator surface when water evaporates, which will affect the WER.

Inspired by the spectral selective sunlight utilization of plants, Ho et al.¹⁸⁵ assembled a spectrally tailored aerogel for solar energy utilization. It consists of oxygen-vacancy (Ov) defect-rich HNb₃O₈ (D-HNb₃O₈) nanosheets and a polyacrylamide (PAM) framework for all-in-one photochemical and photothermal full solar energy conversion (Figure 23C). Compared with conventional single photochemical and photothermal conversion systems, D-HNb₃O₈/PAM aerogels can selectively utilize the entire solar spectrum. High-energy ultraviolet (UV) photons are converted into electron-hole pairs with high redox potential, while low-energy visible-near-infrared (NIR) photons are converted into heat energy (Figure 23D). The D-HNb₃O₈/PAM aerogel possesses desirable thermal insulation, reactant enrichment, fast mass diffusion, and capillary pumping properties for efficient SIPCWE and photochemical activity. Under 1 sun irradiation, the photocatalytic activity of pure PAM aerogel is negligible because it cannot generate redox carriers. With the introduction of HNb₃O₈, about 60% of RhB was degraded after 120 min. The photoactivity was further enhanced by 1.5% Pd D-HNb₃O₈/PAM, which completely oxidized RhB within 100 min (Figure 23E). This synergistic photochemical and photothermal solar energy conversion, under their respective optimal operating spectra, provides a feasible approach for optimizing and maximizing solar energy for multifunctional applications.

CHALLENGES AND FUTURE PERSPECTIVE

Solar energy, as an abundant and green energy, is expected to solve the shortage problems of fresh water and electric energy. In this review, carbon-based PCMs are reviewed. The photothermal conversion mechanism and the structure of SIPCWE devices are designed and optimized, and their application fields are summarized. In the SIPCWE technology, the first strategy to maximize the use of solar energy is to develop PCMs with high absorbance in the whole solar spectrum to capture as much incident light as possible. The second is to develop various thermal positioning technologies to reduce thermal loss. The heat transfer and heat radiation loss from the top to the water should be minimized without affecting the water supply.

During the SIPCWE process, sufficient water supply is another key to achieving efficient SIPCWE, which requires the capillary action of the porous structure of the evaporator to transport water. Micrometer pores are more suitable for capillary pumping than nanometer pores. In SIPCWE, closed pores also help to reduce the thermal conductivity of the interfacial structure, thus limiting the diffusion of the generated heat to the bulk water and the surrounding environment. In addition to the requirements of appropriate pore size, to achieve a strong water transport effect, it is also necessary for the material to have good water wettability.

Proper insulation design is essential to concentrating heat at the air–water evaporation interface, reducing heat losses (downward conduction losses, ambient convection, and radiation losses), and increasing evaporation rates. In the single-layer evaporation system, in order to reduce the heat loss caused by the downward conduction of the solar evaporator to the lower bulk water, different types of thermal insulation layers can be introduced under the PCM to form a double-layer evaporation structure. Moreover, porous foam is the best choice for the thermal insulation layer, such as PS foam, MF sponge, etc., which has low thermal conductivity and greatly reduces the downward heat transfer and makes the main heat loss to be transferred to the upper surface.

With the rapid development of PCMs, devices, and photothermal applications, SIPCWE technology has been widely used in desalination, sewage purification, electric energy generation, photocatalytic degradation of pollutants, and other fields, but there are still many challenges. In the process of SIPCWE technology, there are mainly the following problems.

• 5.1.

  Due to different experimental conditions (such as temperature, humidity, pressure, etc.) and different experimental devices, there is no unified standard to evaluate the performance of evaporation devices directly from the WER and PCE.

• 5.2.

  The service life and cost of the device are worth considering. To achieve this goal, the material of the evaporation device should be inexpensive and durable, especially when working under the conditions of lake, river, seawater, industrial polluted water, and so forth. It is required to have strong mechanical properties and not easy to be damaged. In addition, it is also required to block pollutant adsorption on the surface of the evaporator during evaporation, which can affect the performance of SIPCWE.

• 5.3.

  In the actual water source, there are many organic volatile substances in the bulk water. In the process of SIPCWE, the organic volatile substances also volatilize, so it is still a huge challenge to use the solar evaporator to obtain a clean water source.

• 5.4.

  There is a gap between the current technology level and practical application. For example, solar evaporators usually evaporate in the daytime, but the WER is very low in dark or rainy weather. If steam can be generated throughout the day, the technology will have a broader development prospect.

• 5.5.

  The energy efficiency of the 2D device is close to the limit, and diffuse reflection and thermal radiation are the main energy losses. The 3D structure of the evaporator can make full use of light and heat energy, so the 3D structure can be used to break through the energy efficiency limit. Although people are constantly looking for better PCMs and heat preservation treatment through the structural design to reduce heat scattering by using incident light as much as possible, the best evaporation efficiency of SIPCWE still requires the simultaneous reduction of radiation, and convection conduction losses of heat as much as possible without affecting light.

While SIPCWE holds promise for solving water scarcity problems, most of them are still in the laboratory stage. Due to the complexity and diversity of working conditions, it is a huge challenge to really put them into actual production. Therefore, how to design and develop adaptive, long-term, and stable PCMs and optimize the structure of solar evaporation devices will become the goal of the next stage. At present, although there are many challenges in many aspects, we believe that this green technology will flourish in various application fields in the future and play an important role in coping with the global crisis, especially in the face of the urgent global water shortage and growing demand for clean energy.

ACKNOWLEDGMENTS

The authors appreciate the support from the National Natural Science Foundation of China (22075122, 52071295), the Natural Science Foundation of Shandong Province (ZR2019MB019), and the Research Foundation for Talented Scholars of Linyi University (Z6122010).

CONFLICTS OF INTEREST STATEMENT

The authors declare no conflicts of interest.