Solar Selective Absorber for Emerging Sustainable Applications

Abstract

Solar selective absorbers (SSAs) possess high sunlight absorption (300–2500 nm) and low infrared thermal radiative losses (2.5–25 μm), which are undoubtedly the best choice for photothermal conversion process, and SSAs have been widely used in concentrating solar power, solar water heating, and solar drying. Recently, to promote the realization of “double carbon” goal, SSAs have received widespread attentions, many emerging sustainable applications have been developed. In this review, the recent developments of SSAs and their latest sustainable applications in solar‐driven seawater desalination, multistage solar desalination, atmospheric water harvesting (AWH), wastewater treatment, solar thermophotovoltaics (STPVs), hybrid photovoltaic‐thermal (HPVT), solar‐thermoelectric generators (STEG), personal thermal management (PTM), photothermal catalysis, photothermal deicing, and photothermal sterilization, etc. are systematically summarized. Also, the challenges as well as future opportunities of SSAs are discussion. It is expected that this review will effectively complement the published comments on SSAs and provide more inspirations on sustainable applications of SSAs in the future.

Introduction

Solar energy harvesting can produce huge economic benefits because it is one of the cleanest, inexhaustible energy sources.^[1] To date, solar energy utilization have four ways: photothermal conversion, photoelectric conversion, photocatalysis, and photobiology.^[2,3] Among them, the photoelectric conversion (photovoltaic cells, PV) and photothermal conversion (solar water heating, SWH; concentrating solar power, CSP; solar drying) have proven techniques, which are highly commercialized.^[4–6] However, the manufacturing of PV cells is generally characterized by high costs, high energy consumption and serious environmental pollution. In addition, PV cells conversion efficiency is relatively low because they only work during the day, which greatly hinders the large-scale application.^[7] CSP systems is a common type of high temperature (>550 °C) solar thermal system that has been widely built to meet human electricity needs. Photothermal conversion materials are the key component of very important for photothermal conversion system. Generally, black body materials with high absorption are used for solar energy absorption efficiently, but the high emissivity of them will reduce the photothermal conversion efficiency. Solar selective absorbers (SSAs) with high absorption and low emission are better choice for photothermal conversion, which is the key component of solar thermal conversion systems.^[4,8–11] The two important optical parameters of SSAs, absorptance ($\overline{\alpha }$) and emittance ($\overline{\epsilon }$), are defined as follows^[12]1$\overline{\text{α}}=\frac{{\int }_0^{\infty }\mathrm{d}\lambda \alpha (\lambda )E_{\lambda ,\text{Solar}}(\lambda )}{q_{\text{Solar}}}$2$\overline{\epsilon }=\frac{{\int }_0^{\infty }\mathrm{d}\lambda \epsilon (\lambda )E_{\lambda ,B}(\lambda ,T_A)}{\sigma T_A^4}$where the $\overline{\alpha }$ and $\overline{\epsilon }$ are the total absorptivity and emissivity, respectively. The α(λ) is the absorptivity, ε(λ) is the emissivity as a function of wavelength λ. The E_λ,Solar(λ) is the spectral solar power, and the E_λ,B(λ, T_A) is the blackbody radiation power, q_Solar is the total solar irradiance (AM 1.5G, 1 kW m⁻²); σ is the Stefan−Boltzmann constant, T_A is the object temperature.^[12]

It is known that black bodies have high spectral absorption, but at the same time they possess high emissivity, especially at high temperature. In contrast, SSAs have high α in the solar spectrum (300–2500 nm) and low ε in the infrared range (2.5–25 μm) (Figure 1a,b). This solar selective absorption characterization facilitates maximizing solar energy absorption and minimizing heat radiative loss, thus improving the overall photothermal conversion efficiency of SSAs. Figure 1c shows the heat radiative loss of a blackbody absorber and a commercial SSA (α = 95%, ε = 5%). It is worth noting that the heat radiative loss of black body and SSA both increases with temperature, but the difference is that the increase is significant for the black body and is small for the SSA. For instance, at temperature of 100 °C, the blackbody produces at least ≈650 W m⁻² heat loss; while SSA only have ≈40 W m⁻² heat loss. And their photothermal conversion efficiency are 35% and 91.8%, respectively.^[13]

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Since Tabor et al.^[14] put forward the concept of solar selective absorption coatings (SSACs) in 1964, various SSACs are exploited, such as intrinsic absorber coatings,^[15] textured surface coatings,^[16] multilayer interference stacks,^[17] semiconductor–metal tandem coatings,^[18] and metal-cermet composite coatings.^[19–22] The α of intrinsic absorber relies on materials intrinsic characteristics, such as interband transition, transition metals, depressed plasma frequencies, and so on; while other coatings depend on the structure to improve their inherent high absorption and low emission.^[23] There are at least three limitations in these artificial SSACs: complex manufacturing process, high fabrication cost, and poor thermal stability. First, SSACs are generally manufactured via complex optical manipulation processes, including photonic crystals (PhC), cavity-mode excitation, interference effects, and plasmon resonances to construct complex sub-wavelength metamaterials and metasurfaces.^[24] Second, considering the cost, manufacturing difficulty and environmental friendliness, traditional preparation methods may be not ideal. For instance, manufacturing multilayer interference stacks require costly vacuum-based deposition device and high-purity target.^[18] Third, at present, the thermal stability and spectral selectivity of SSAC are unable to realize together, especially at high temperature. Because the coatings will degrade under long-term high temperature operation, which will inevitably affect their spectral selectivity.^[25]

To promote the realization of “double carbon” goal, many SSAs with excellent performances have been exploited and commercially applied in fields of CSP, SWH, and solar drying. SSAs have received extensive attentions; meanwhile, many comments have been made on emerging sustainable applications. In this review, we summarized the latest progresses of SSAs applications, including solar-driven seawater desalination, multistage solar desalination, atmospheric water harvesting (AWH), wastewater treatment, solar thermophotovoltaics (STPVs), hybrid photovoltaic-thermal (HPVT), solar-thermoelectric generators (STEG), personal thermal management (PTM), photothermal catalysis, photothermal deicing, and photothermal sterilization, etc. We expect this review will effectively complement the published comments on SSAs and provide more inspirations on sustainable applications of SSA in the future.

Recent Advances in SSAs

CSP is a clean and renewable energy power generation mode, which has become a hot spot in new energy fields in recent years due to its incomparable advantages such as high energy conversion efficiency, good energy storage function, good matching with existing power grids, continuous stability, and peak shaving power generation.^[26] It is well-known that cermet-based SSAs exhibit outstanding spectral selectivity. However, the commercial rollout of SSAs is still limited by the high-temperature instability of its metal/ceramic interface. Due to the instability of metal/ceramic interfaces at high temperature, perfect spectral selectivity of SSAs is usually achieved at the expense of thermal stability. This greatly hindered the commercial and large-scale application of SSAs. Generally speaking, the thermal-induced optical degradation of cermet-based SSAs is mainly ascribed to the element migration, coalescence and/or oxidation of metal nanoparticles (NPs), interdiffusion between adjacent layers, and diffusion of substrate atoms into the films.^[25,27] Therefore, it is urgent to develop SSACs with outstanding spectral selectivity, scalable simple structure, and thermal stability to meet the requirements of next-generation CSPs (operating temperature >550 °C). On the other hand, SSAs applied at middle/low and high temperature are manufactured by sophisticated techniques such as high-vacuum deposition and lithographic processes, both of them are costly in large-scale productions. This section summarizes recent progress of SSAs. There are several novel approaches to manufacture SSAs with perfect optical properties and superior thermal stability.

Alloyed Metal NP-Based SSAs

Thermal stability of metal NPs is the most important factor for cermet-based SSACs. Alloying method can effectively enhance the thermal stability of metal NPs. Wang et al. developed WTi–Al₂O₃ cermet-based SSACs by introducing Ti to form WTi alloy NPs. The as-prepared cermet demonstrates ≈93% high solar absorptance and 10.3% low infrared emissivity at 500 °C, and show no signs of deterioration after 840 h of vacuum annealing at 600 °C (Figure 2a).^[28] The results show that the dispersion of refractory alloy and high melting point compound into ceramic substrate is beneficial to improve the heat resistance of W–Al₂O₃ cermet. Yang et al. prepared a high-temperature stable SSA based on TiW–SiO₂ cermet by cosputtering method.^[29] By adding Ti element, the as-obtained SSAC shows remarkable stability in spectrum, structure, and chemistry after annealing at 700 °C in air (Figure 2b).

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High Entropy Alloy-Based SSAs

Alloy composed of five or more main elements is called high entropy alloys (HEAs), which is emerging as a new promising SSA material. It was reported that HEA containing transition metal elements (such as Ti, Ta, Hf, Mo, Cr, and Zr) have excellent thermodynamic and optical properties, such as AlCrTaTiZrN,^[30] (AlCrTaTiZrRu)N_0.7,^[31] Al_0.4Hf_0.6NbTaTiZrN,^[32] AlCrWTaNbTiN,^[33] etc. These have been widely studied because of their excellent thermal stability. He et al. reported a HEA nitride MoTaTiCrN-based double-layer SSAC.^[34] The SSAC exhibits a high solar absorptivity of 92.3% and a low thermal emissivity of 6.5%, due to the combination of antireflective layer (Si₃N₄) and absorption layer (MoTaTiCrN). After short-term annealing at 400–700 °C, the reflection spectrum of the SSAC does not change significantly. After long-term annealing at 550 °C, the SSAC can still maintain its excellent spectral selectivity (Figure 3a). By optimizing elemental composition, He et al. also developed a single layer SSAC based on AlCrTaTiZrN with simple structure, low cost and scalable preparation.^[35] The as-prepared coating can endure 800 °C annealing treatment for 2 h and superior optical performance after annealing at 650 °C for 300 h (Figure 3b). These recent advances in HEAs have promoted the significant development of HEAs in the field of high-temperature photothermal conversion materials and offered immeasurable potential for photothermal conversion systems.

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All Ceramic-Based SSAs

Despite the breakthrough in thermal stability of HEAs, the intrinsic disadvantages of metal NPs, such as agglomeration, growth, oxidation, and atom diffusion usually lead to irreversible fading of optical properties for SSAs. In contrast, ceramics possess excellent thermal stability than metals NPs at high temperatures, which have been widely used in SSACs as infrared reflectors, diffusion barriers, absorptive layers and surface coatings. Based on the outstanding stability of ceramics. It is promising to exploit a scalable all ceramic based SSA to obtain excellent spectral selectivity and high temperature thermal stability simultaneously. Li et al. developed an all-ceramic SSAC based on TiN/TiNO/ZrO₂/SiO₂ via magnetron sputtering method, which exhibits solar spectrally selectivity (α = 92.2%, ε_1000K = 17.0%).^[36] This produces an unparalleled photothermal conversion efficiency of 82.6% at 100 suns. It is more noteworthy that the SSAC shows preeminent thermal stability after annealing at 1000 K for 150 h, which signifies an operating temperature increasing range of 227 K compared to conventional multilayer absorbers (Figure 4a). In addition, Li et al. also reported an all-ceramic plasmonic metamaterial based on TiN/TiN NPs/SiO₂, in which TiN NPs serving as absorbing layer, and SiO₂ layer serving as anti-reflective layer.^[37] This as-prepared SSAC achieved a high solar absorptivity of 95%, a low thermal emissivity of 3% and superior stability in the range of 100–727 °C (Figure 4b). Wang et al. designed a all-ceramic SSAC based on ZrB₂ with a quasioptical microcavity (QOM) structure.^[38] The SSAC achieved a high solar absorptivity of 96.5% and a low thermal emissivity of 16%. Notably, the SSAC can endure 500 °C annealing treatment for 200 h in air and 800 °C annealing treatment for 200 h in vacuum. The SSAC can retain a superior spectral selectivity and a high photothermal conversion efficiency of 67% under 1000 suns after vacuum annealing. These are mainly attributed to the introduction of the ultrahigh temperature stable ZrB2 phase and the QOM structure (Figure 4c).

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Advanced Preparation Method of SSAs

Moreover, to ensure the optical and thermodynamic properties of SSACs, the feasibility and preparation costs are also crucial. The abovementioned all-ceramic SSAC based on TiN/TiN NPs/SiO₂ can be fabricated by scalable and simple solution-based spin coating method (Figure 5a).^[37] The competitive performance of solution-processed absorbers compared to vacuum deposition absorbers at a significantly lower cost makes them a economical and efficient, versatile solution that provides high efficiency for various photothermal conversion applications. Mandal et al. reported a “dip-and-dry” technique depend on displacement reaction to produce SSACs based on plasmonic metal NPs.^[39] As shown in Figure 5b, Cu plasmonic NP-coated Zn foils was obtained by impregnating Zn foil in a solution that had Cu²⁺ after replacement. The SSAC exhibits a high absorption of 0.97 and has good thermal stability at 200 °C. Moreover, the “dip-and-dry” technique is inexpensive, simple and no pollution, and can replace traditional vacuum deposition process and electrochemical method for SSACs preparation.

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Intrinsic SSAs

Intrinsic materials with high solar absorption and low infrared emission are of great significance for photothermal conversion. However, there has been a lack of intrinsic materials with selective solar absorption in nature. The intrinsic low emissivity of SSAC is usually achieved by constructing complex subwavelength metastructures and metasurfaces. Huang's group reported an 2D Ti₃C₂T_x MXene material that appears black in visible but white in infrared.^[40] The Ti₃C₂T_x MXene film was prepared by simple and scalable vacuum filtration technology with 90% solar absorptivity and 10% low infrared emissivity, surpassing the intrinsic materials with the highest selective solar absorption reported so far (Figure 6). Moreover, the as-prepared Ti₃C₂T_x MXene is free-standing, and is suitable for various substrates, including porous and rough surfaces, which show a wider range of applications than conventional SSACs.

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Sustainable Applications of SSAs

Solar Powered Fresh Water Harvesting

Water is essential for lives, but only about 2.5% water resource is available on Earth, and a portion of that water is still inaccessible.^[41–43] The current scarcity of freshwater resources is a serious challenge shared globally due to a mismatch between the increased global demand for freshwater and the reduction of available freshwater caused by climate warming and water pollution. The harvesting of freshwater from alternative water resources such as seawater, rainwater, atmospheric water and sewage is of big challenge.^[11,44–46] Traditional freshwater harvesting methods is strict to hydrogeological and climatic conditions, which is at the expense of nonrenewable energy. As a result, desalination, multistage distillation, atmospheric water harvesting and wastewater treatment using SSA are promising technologies for producing fresh water in a green and sustainable way that can effectively mitigate and solve the global water crisis.

Solar-Driven Seawater Desalination

Photothermal evaporation technology based on local heating and interface evaporation has been widely used in the field of seawater desalination because of its advantages of nonpollution, safety and reliability, adaptability and economic efficiency.^[43,47–61] However, there are still some challenges that limit the large-scale application of solar-driven desalination technology, such as low solar vapor conversion efficiency, poor salt resistance of solar absorber and low water production due to low condensation efficiency.

Based on this, this section mainly summarizes several strategies for application of SSAs to improve the efficiency of solar desalination.^[13,62–65]

Contact Type

Interfacial solar steam generation (ISSG) is an efficient distillation strategy that uses a self-floating solar absorber to generate vapor at the water-air interface.^[43] By employing this strategy, the heat is locally confined in the solar absorber. Solar absorber is physically separated from the bulk water by the use of a porous thermal barrier with low thermal conductivity. Then water is pumped into the solar absorber through the porous structure driven by capillary force. So, heat can be concentrated on a limited interface between the solar absorber and the pumped water, rather than heating the bulk water, thus significantly reduces heat conduction loss. Solar absorber, a major component of ISSG system, is expected to have the ability to effectively capture the photons in the solar spectrum range and then convert them into heat. At present, photothermal materials with nearly perfect solar absorption can be divided into four types: metal NPs, black polymers, narrowband semiconductors, and carbon-based materials.^{[54–56,59,60]} These photothermal conversion materials have high absorption in the solar spectrum wavelength range, but also have high emission in the infrared spectrum, which inevitably causes significant heat radiative losses.^[12,13] Another thermal management approach is to use SSA to minimize radiative heat losses, thus further improving the overall efficiency of the desalination system.^[65]

In 2016, Chen et al. first introduced a commercial SSA (BlueTec etaplus) to ISSG systems, which can produce water vapor (>100 °C) at below 1 sun radiation in ambient environment by suppressing convective, conductive, and radiative heat losses (Figure 7a).^[66] Following the same thermal management strategy, Chang et al. used a commercial SSA (TiNO_X, α = 95%, ε_100°C = 5%) to develop a 3D porous interfacial evaporator.^[67] Due to the expansion of the effective evaporation area of the SSA and the shift of the vapor outlet from the top of absorber to the side wall, the solar evaporator produced water vapor (>100 °C) and 48% solar-vapor conversion efficiency under 1 sun. (Figure 7b). Wang et al. deposited Ni on anodic aluminum oxide (AAO) by pulsed electro deposition (PED) method and the prepared Ni/AAO SSA exhibited a high absorption of >80% and an emissivity of ≈20%.^[68] Therefore, an overall efficiency achieved 73% under one sunlight. Compared with the graphite absorber, Wangs’ water evaporation rate increased by 10%, which was proved to be attributed to the as-prepared Ni/AAO SSA. In addition, Ding et al. developed a novel SSA based on nickel nanoparticles (SSA–Ni) as an interfacial solar water evaporation platform.^[69] Ni NPs is first encapsulated in carbon and silicon dioxide shells, and then sequentially assembled on infrared reflectors to form a graded refractive index structure. Thus, high solar absorptivity of 0.93 and low thermal emissivity of 0.096 were achieved. Therefore, the floating evaporation system using SSA–Ni can achieve a high evaporation rate of 1.52 kg m⁻² h⁻¹ under 1 sun (Figure 7c).

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The contact solar steam generation system using SSAs can maximize solar absorption and minimize radiative heat loss. The porous thermal barrier can decrease the conductive heat loss from the absorber to the bulk water and environment. Moreover, the convective heat loss is suppressed by a convection cover. Convection cover is a kind of high photothermal conversion material with low thermal conductivity, such as foam wrap, glass, air gap, silica aerogel, etc., which can significantly improve the solar and steam conversion efficiency. However, these direct contact desalination methods are usually restricted by salt precipitation.

Contactless Type (Absorber/Emitter)

Solar-driven interface desalination can significantly improve solar-to-steam conversion efficiency. But the corresponding local heat aggregation and rapid generated steam can greatly cause salt accumulation on the evaporator surface, which blocks the steam evaporation channel. This phenomenon is more severe in high concentration seawater than that of low concentration. In contact solar-driven desalination system, the salt accumulation is inevitable, which has negative effect on the absorption, evaporation efficiency and service life. Salt accumulation is a serious impediment to produce large-scale, long-term stable and efficient solar-driven desalination devices for application in underdeveloped areas.^[70–72] As a result, a variety of salt rejection strategies have been proposed, but the conventional approach is still limited to the mode that the water and SSA absorber are contacting.^[73–76]

Recently, a novel and simple noncontact salt rejection strategy was developed by employing SSA and black body emitters.^[13] The SSA is separated from the bulk water by using a support column. SSA can selectively and efficiently absorb solar radiation, convert it into heat, transfer it to the black body emitter, and reradiate infrared thermal photons to indirectly heat water. The indirect heating depth can reach to 100 μm.^[77] Due to the physical isolation between absorber and water, the salt accumulation can be greatly alleviated, thus the steam can continuously escape. Meanwhile, the steam temperature can reach beyond 100 °C, and high-temperature steam is generated under non-pressurized and low solar flux to achieve high solar steam conversion efficiency.

In 2018, Chen et al. proposed a solar-powered evaporator without contact with water.^[77] Solar radiation is absorbed by SSA, and then infrared photons are reradiated through the black body emitter. The infrared photons are directly absorbed by water within a penetration depth of less than 100 μm. (Figure 8a) Scaling can be completely avoided because of the physical separation of SSA and the bulk water. Besides, the temperature of steam can be above boiling point. Superheated steam up to 133 °C can be generated under natural light irradiation and a non-pressurized condition. Similarly, Gu et al. reported a solar thermal photo vapor generator (STPV).^[78] STPV can stably evaporate water at a rate of 1.04–1.19 kg m⁻² h⁻¹ in seawater with high salt concentration after 8 h under 2 suns irradiation. Due to the porous structure and the low infrared reflection of wet salt layer, allowing water vapor to escape unhindered under conditions of salt accumulation (Figure 8c). In comparison, the evaporation rate of conventional solar vapor generator decreases by at least 50% in 1 h and about 70% in 8 h. In addition, there is a “solar umbrella” design. Water in the evaporation pond is shielded from direct sunlight by a photothermal device, which enhance evaporation by more than 100% and increase the efficiency of photothermal conversion up to 43%. (Figure 8b).^[79]

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Bottom Condensation

Although the evaporation rate of the interface solar steam generation system has been significantly improved, the water collection is still limited by the low efficiency of water condensation process. Most single-stage solar purifiers are single or double slope structured, with a transparent cover placed above the solar evaporator to penetrate sunlight and catch vapor condensation.^[80] However, there are some inherent limitations associated with this type of structure. First, the vapor mist that condensed on the lid usually cause light transmittance reduction of the system (up to 35%).^[81] Second, due to the requirements for optical transparency, the top cover is usually made of polymer or glass. These materials possess low thermal conductivity (k < 5 W m⁻¹ K⁻¹), which is ineffective for heat transfer related to condensation. Thirdly, the condensed water on the lid also acts as a thermal barrier, which further hinders condensation. As a result, for most single-stage solar purifiers, the total solar water collection efficiency is only 35% (0.5 kg m⁻² h⁻¹).^[82]

As shown in Figure 9,^[83] Zhu's team designed an inverted single-stage solar water purifier (ISWP). The ISWP contains SSA at the top and copper water condenser with a hydrophobic honeycomb structure at the bottom. This inverted structure not only avoids light loss due to steam condensation, but also achieves enhanced heat transfer and condensation. Ultimately, the system can achieve a condensation efficiency of >70% and a water collection rate of >1 kg m⁻² h⁻¹ at 1 sun, which is excellent in a single-stage solar purification system. In addition, through the fine design and gradual optimization of the whole process of “light absorption-photothermal conversion-steam transfer-condensation heat transfer,” it provides ideas for the understanding of the whole process of evaporation condensation and the design of the device. It is expected that it can also be extended to multistage condensation devices to further improve the condensation rate.

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Multistage Solar Desalination

Contact and noncontact solar steam generators obviously improve the solar steam conversion efficiency. Due to the absorption of solar energy, water evaporation and steam condensation occur on the same side; the escape of steam interfere with light absorption; the conductivity of convection cover is low; these make steam collection and condensation challenging. Single-stage seawater desalination devices based on solar interface evaporation have high evaporation efficiency, but if steam enthalpy is not utilized, the upper limit of solar steam conversion efficiency is only 100%. Therefore, recovering steam enthalpy is the key to further improve the energy conversion efficiency.

Single-stage design based on backside evaporation is upgraded to multistage by modular designing. By recycling the latent heat of steam, the conversion efficiency of solar steam can be increased beyond 100%. As shown in Figure 10a, the multistage solar desalination device is composed of a tightly stacked layered structure. The top includes an SSA, and the evaporator (hydrophilic layer) is immediately below. Driven by capillary pressure, liquid is supplied to the evaporator from the side of the device. In each stage, the evaporator is separated from the condenser (hydrophobic layer) through air gap or hydrophobic membrane.^[84] During working process, solar radiation is converted into heat at the top of the unit, and then heats the evaporator of the first stage to generate steam. The steam condenses into fresh water at the bottom condenser of the first stage, and then releases the latent heat of condensation. The latent heat of steam is transferred to the evaporator of the second stage through the heat conducting sheet and heated to continue to produce steam, so as to realize the utilization of latent heat. In the subsequent stages, the latent heat released by the previous stage will be used as a heat source to drive the evaporation condensation process to obtain additional fresh water.

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For example, in 2018, Xue et al. developed a two-stage desalination device to realize multistage utilization of latent heat.^[85] Under 1 sun irradiation, the two-stage desalination device can achieve a freshwater yield of 1.02 kg m⁻² h⁻¹ and a high solar energy utilization efficiency of 72%. This confirms the importance of recycling latent heat compared to the primary system (0.727 kg m⁻² h⁻¹, 51%) (Figure 10b). Wang et al. reported three-stage and five-stage photovoltaic membrane desalination (PV–MD) devices with commercial SSA (ETA@Al, Alanod Solar), with water production rates of 2.78 and 3.25 kg m⁻² h⁻¹, respectively.^[86] This is about 5 times freshwater productivity of the most advanced traditional solar distiller (Figure 10c). Chiavazzo et al. further investigated the effect of number of stages on the performance of multistage solar desalination device and constructed a compact passive ten-stage multistage desalination device (Figure 10d).^[87] The hydrophilic layer in the device can significantly improve the heating effect of the SSA on the water layer. The diffusion side of the hydrophilic layer is only 1 mm, which makes the mass transfer resistance of steam much lower. Under 1 sun irradiation, the water production rate of the device can reach 3.0 kg m⁻²h⁻¹. Although hydrophobic membrane can completely separate seawater from condenser to avoid pollution, the multistage device can still operate normally after removing the hydrophobic membrane, so it is not a necessary structure. Under the effect of gravity and diffusion, the high concentration of salt water flows back to the bulk water, the device can work efficiently and steadily for 5 days under 8 h of solar irradiation per day. A thermal localized multistage solar still (TMSS) combining solar interfacial evaporation and multistage latent heat recovery was proposed by Xu et al.^[88] The device adopts a ten-stage structure and a design of 5 mm evaporation–condensation distance, these reduce the area required for desalination and increase the flexibility of unit, making it suitable for portable small desalination devices (Figure 10e). It achieving a record solar utilization efficiency of 385% and a yield of 5.78 L m⁻² h⁻¹ under 1 sun irradiation, which is approximately two times higher than the previous efficiency record. Huang et al. Proposed a passive six-stage solar desalination device without membrane.^[89] Each stage of the device is composed of concentric copper cylinders arranged vertically. The inner side of the copper cylinder is the condensation surface and the outer side is the evaporation surface. The feed side of the evaporation layer also uses the capillary force of the water absorbing material to supply water upward, so as to realize the multistage utilization of latent heat in turn (Figure 10f). Due to the vertical distribution structure, the device can avoid pollution even if there is no hydrophobic membrane. In addition, the device can achieve thermal concentration without focusing by controlling the ratio of evaporation area and light absorption area, so that the evaporation layer can reach a very high evaporation temperature. Under 1 sun irradiation, the device can achieve a maximum water production rate of about 1.84 kg m⁻² h⁻¹ and a water production rate of 2.2 kg m⁻² h⁻¹ after 3 times thermal concentration.

Neither a single solar interface evaporation nor a single multi-stage distillation can greatly improve the system efficiency, only a combination of the two can make a breakthrough. The TMSS combines solar interface evaporation and condensing heat recovery, and the latent heat recovery overcomes the performance limitation of single stage evaporation, not only improves water production, but also has excellent salt resistance effect. However, the energy transfer process of the structure is complicated, and the advantages of this structure can be realized only through basic heat and mass transfer analysis and overall efficiency optimization.

Atmospheric Water Harvesting

Solar-driven seawater desalination provides a low-cost and portable freshwater harvesting to meet the individual daily water needs in underdeveloped areas.^[90,91] However, solar-powered desalination is immensely depend on natural water source distribution, which is usually abundant in sea, river, and lakes.^[92,93] In contrast, atmospheric water is more ubiquitous and not geographically restricted. The Earth's atmosphere stores water in the form of water droplets or water vapor, which accounts for 10% of freshwater resources. In addition, the natural hydrological cycle can achieve sustainable water supply.^[11,94] Therefore, atmospheric water harvesting (AWH) has become a promising strategy for freshwater production to overcome the challenges of long-distance transportation or drinking water delivery for rural areas. The working mechanism of AWH is liquefying moisture to produce collectible liquid water.^[95,96] The traditional moisture collecting is cooling the ambient air below the dew point and collecting the condensate.^[97,98] But a large amount of energy is used in the condensation process to power the cooler and overcome the latent heat, which greatly increased the cost.^[99]

An AWH device typically employed porous materials such as metal organic frameworks (MOFs) to absorb moisture in humid air at night, and release moisture during the day by solar heating.^[96,100] Wang et al. reported a dual-stage AWH device, using a new type of adsorption material (a commercial zeolite composed of microporous aluminum iron phosphate with a large internal surface area) to replace expensive MOFs.^[101] This new material is easy to obtain, widely distributed, and stable in nature. It can absorb the trace moisture contained in almost dry air and exhibits good adsorption performance. As shown in Figure 11a, the device top is a selective absorber to collect solar energy, and then heat the zeolite to release the water absorbed in the night. The process of steam condensing on the collector plate also releases heat, which was collected by copper sheet plate that directly placed above the second zeolite layer. This condensation heat is used to release the steam of the next layer. The water droplets collected from each layer can be collected into a tank. Thus, the water collection efficiency of this dual-stage device rapidly raised to about 0.77 L m⁻² day⁻¹, which is twice than that of single-stage device. Although similar dual-stage systems have been used as seawater desalination, the authors believe that no one has used this method to collect atmospheric water before.

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In addition, passive atmospheric water harvesting (PAWH) collects freshwater by condensing atmospheric moisture below the dew point using natural temperature drops, without additional energy input and moisture adsorption materials. At night, atmospheric moisture will radiate heat to the cold outer space (≈3 K) through atmospheric window, resulting in a significant drop in temperature and condensation of moisture into dew. This process is called dew phenomenon and inspired the PAWH.^[102,103] It is ideal for producing fresh water in areas that are not suitable for solar steam generation, such as inland and deserts. In addition, PAWH is particularly effective at night when there is no sunlight. Xu et al. demonstrated an all-day freshwater generation system that combines dew collection capabilities and interfacial solar desalination.^[104] A dual function film (DFF) with both passive radiation cooling and solar heating capabilities is a key part of the system. The DFF consists of multi-walled carbon nanotubes deposited by one-step spraying on a flexible radiative cooling substrate. The prepared DFF exhibited high solar absorption of ≈0.95 in the solar spectral region and high emission of ≈0.9 in the far infrared wavelength range (8–13 μm). During the day, the DFF can efficiently absorb solar energy and then drive desalination process to produce fresh water; while in the night, dew deposited on the surface of the substrate can be collected by decreasing the surface temperature of the substrate to below the dew point. By integrating DFF into a three-stage desalination system, its potable freshwater rate can be advanced to 1.13 L m⁻² h¹ under 1 sun during desalinating; meanwhile, the dew collecting rate is also enhanced to 0.1 L m⁻²h¹ at night. This simple structure, low-cost, efficient, easily operation and environment-friendly all-day freshwater generation system is suitable for many arid areas, underdeveloped areas, and islands (Figure 11b).

Wastewater Treatment

Modern industries, such as desalination plants, produce high-concentrated waste brine as by-products everyday.^[105–107] This waste water is discharged directly into nearby open water sources, such as lakes, rivers, coastal seawater. The rest is discharged to evaporate ponds and deep underground wells for treatment.^[108,109] However, these traditional methods have adverse effects on terrestrial vegetation systems and aquatic ecosystems.^[110,111] Zero liquid discharge (ZLD) has drawn renewed attention as an effective method of wastewater treatment. Its purpose is to remove all waste liquids and produce solid salt as the only product.

The ZLD facility typically consists of a concentration process and a crystallization process. The highly concentrated brine is concentrated to near saturation by a concentration system and all residual water is removed by a crystallization system to obtain solid salt.^[102] At present, brine crystallization is generally realized through evaporation ponds or brine crystallizers. However, the traditional brine crystallizers have high energy consumption and high cost, The large-scale application of evaporation pond is restricted because of its low solar energy utilization efficiency, high land cost and small capacity.^[112,113] As a result, the crystallization system has received little attention due to the lack of new strategies, limiting ZLD brine treatment. In solar-driven desalination process, salt precipitation on the photothermal material inevitably affects light absorption, but the salt concentration of brine only has a slight effect on the evaporation rate, and the latent heat of pure water is higher than high-concentrated brine.^[114] Therefore, in order to achieve the ZLD goal, a feasible method is use traditional technology (such as MD and RO) to produce fresh water from ordinary seawater and concentrate low-concentration brine to near saturation, then crystallize solid salt from nearly saturated brine by the ISSG.

The noncontact strategy mentioned above can be used for ZLD high-salinity industrial wastewater treatment. Gu et al. proved that an interfacial solar thermal photo-vapor generator (iSTPV) can evaporate water steadily under the condition of solute accumulation.^[115] The iSTPV was able to evaporate continuously for 32 h at a high rate of 1.94 kg m⁻² h⁻¹ despite the continuous solute accumulation. This ability can not only recover valuable metals from wastewater having heavy metal ions while producing pure water, inspiring researchers to found more advanced solar-driven water treatment equipment (Figure 12a). Prasher et al. demonstrated a passive and noncontact device, which is particularly suitable for the treatment of various high-salinity waste water.^[79] Under 1 sun irradiation, the device enhances evaporation by >100%. By optimizing the structure of thermal management design, it is possible to increase the evaporation capacity of the photothermal umbrella by 160% compared to a conventional evaporation pond. Wang et al. reported a 3D solar crystallizer device that uses a dead-end solar-driven water removal model.^[116] As shown in Figure 12b, the inner surfaces and bottom of the device are SSAC with an absorption rate of up to 0.99 for absorbing solar energy, and the outer wall is a porous hydrophilic material as a water evaporation surface and a salt crystallization surface. An aluminum plate with high thermal conductivity is conducive to effectively transfer the heat generated at the bottom of the device to its wall for water evaporation. As the concentrated brine continues to evaporate and concentrate, scaling occurs on the outer walls, forming a dense layer of salt that subsequently falls off automatically. The device can operate continuously and steadily for 288 h under 1 sun irradiation with an average evaporation rate of 2.42 kg m⁻² h⁻¹.

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We summarize the contribution of SSA in solar-driven freshwater collection (Table 1). We found that the evaporation rate and evaporation efficiency of the single-stage seawater desalination methods are not very ideal. By optimizing the device structure design and regulating the thermal management strategy, the water output of the multistage distillation has been significantly improved, and the highest solar energy utilization efficiency has reached a record 385%. In addition, atmospheric water collection and wastewater treatment provide ideas for collecting fresh water from alternative water resources, and their performance is excellent.

Table 1 Summary of SSA for solar powered fresh water harvesting

Application	Material and optical property	Performance	Ref.
Solar-driven seawater desalination	Cermet-based SSA with 93% $\overline{\alpha }$ and 7% $\overline{\epsilon }$	100 °C steam under 1 sun	[66]
TiNO x with 95% $\overline{\alpha }$ and 5% $\overline{\epsilon }$	48% solar-to-vapor conversion efficiency under 1 sun	[67]
Nickel on AAO (with >80% $\overline{\alpha }$ and ≈20% $\overline{\epsilon }$)	73% solar-to-vapor conversion efficiency under 1 sun	[68]
Ni NPs based SSA with 93% $\overline{\alpha }$ and 9.6% $\overline{\epsilon }$	Evaporation rate of 1.52 kg m−2 h−1 under 1 sun	[69]
SSA with 92% $\overline{\alpha }$ and 8% $\overline{\epsilon }$	Up to 133 °C steam	[77]
TiNO X on Al with 95% $\overline{\alpha }$ and 4% $\overline{\epsilon }$	Evaporation of >100% and a 43% solar-thermal efficiency under 1 sun	[78]
TiNO X on Al with 92% $\overline{\alpha }$ and 4% $\overline{\epsilon }$	Evaporates rate of 1.04 − 1.19 kg m−2 h−1 under 2 suns	[79]
Multistage solar desalination	SSA (Bluetec, Germen) with 95% $\overline{\alpha }$ and 5% $\overline{\epsilon }$	Evaporation rate of 0.98 kg m−2 h−1 under 1 sun, water productivity of 1.02 kg m−2 h−1, solar efficiency up to 72%	[84]
SSA (ETA@Al, Alanod Solar) with 94% $\overline{\alpha }$ and 12.3% $\overline{\epsilon }$	Water production rate >1.64 kg m−2 h−1 under 1 sun	[85]
TiNOx with 95% $\overline{\alpha }$ and 4% $\overline{\epsilon }$	3 L m−2 h−1 output from seawater under 1 sun	[86]
Commercial SSA (B-SX/T-L/Z-Z-1.88, Linuo-Paradigma)	Solar-to-vapor conversion efficiency of 385% with r evaporation rate of 5.78 L m−2 h−1	[87]
CrAlO-based SSA with 93.5% $\overline{\alpha }$ and 15% $\overline{\epsilon }$	Water yield up to 2.2 kg m m−2 h−1 under 1 sun illumination and a 3× thermal concentration.	[88]
Atmospheric water harvesting	SSA (Alanod eta plus) with 93% $\overline{\alpha }$ and 4.2% $\overline{\epsilon }$	Water harvesting productivity of 0.77 L m−2 day−1, an 18% increase over the single-stage device.	[101]
Dual functional film (DFF) with 95% $\overline{\alpha }$ and 90% $\overline{\epsilon }$ in the 8–13 μm wavelength	Freshwater collection efficiency of 71.1% under 1 sun and a dew water collection rate of 0.1 L m−2 day−1 at night.	[104]
Wastewater treatment	SSA (Almeco) with 92% $\overline{\alpha }$ and 4% $\overline{\epsilon }$, Black paint emitter (4SD Hi-Temp Paint)	Evaporate at a high speed of 1.94 kg m−2 h−1during a persistent solute accumulation process for 32 h	[115]
Commercially SSA (Alanod) with 99% $\overline{\alpha }$	Water evaporation rate of 2.42 kg m−2 h−1under 1 sun when treating real concentrated seawater brine (21.6 wt%), the solar crystallizer shown a water evaporation rate of 48.0 kg m−2 day1	[116]

PV/PT

The two main ways of solar energy utilization typically include photovoltaic and photothermal technologies. However, Solar cells cannot utilize photons with energy below the band gap of the semiconductor, resulting in low solar energy utilization efficiency.^[117,118] Solar thermal systems convert solar energy into electricity, with a cheap heat storage system.^[119] But the price is still more expensive than that of photovoltaic systems. The photovoltaic thermal (PVT) system that combines photovoltaic technology and solar thermal technology, achieving simultaneous converting incident sunlight into electrical and thermal energy. PVT system has always been regarded as the most attractive invention in solar thermal technology. The traditional PVT system can take away heat through the working fluid (refrigerant) to reduce the temperature of the PV module. However, since the solar absorber is pasted on the back of the PV module, the high temperature will inevitably affect the working efficiency and service life of the PV module, and reduce the overall solar energy utilization efficiency.^[120]

Solar Thermophotovoltaics

STPVs by using a high temperature absorber/emitter to absorb sunlight and convert it into thermal radiation, thereby tuning the photons above the bandgap of PV cell (Figure 13a).^[7] It offers a broader path for the existing methods of power generation. As shown Figure 13b, STPV consists of concentrating optics, SSA, emitter, heat sink and solar cell.^[121] The concentrating optic focuses parallel incident sunlight into a spot which is projected onto the SSA. The SSA is heated to about 1300 k and heat is transferred to the emitter. Then, the emitter re-radiates a specific wavelength spectrum that closely matches the absorption characteristics of the PV cell. The selective heating device is an important component that affects the efficiency of the STPV system. Wang et al. Reported a planar compact integrated solar thermophotovoltaic device.^[121] The device integrates the absorber with multiwall CNT and the emitter with 1D Si/SiO₂ PhC on the same substrate (Figure 13b). The solar energy utilization efficiency of the device can reach 3.2%, which is attributed to the nanostructures on the surfaces of the absorber and emitter, as well as the optimization of the absorption and emission regions to balance the energy of the STPV device. Tian et al. proposed a STPV device with wavelength selectivity.^[122] As shown in Figure 13c, the device contains a 1D multilayer photonic structure, W layer serves as an infrared reflection layer and HfO₂ layer acts as an anti-reflection layer, showing an absorption of 0.92 in the solar spectrum and an emission of 0.1 in the midinfrared region. The proposed design of SSA and emitter based on metamaterials can improve the performance of STPV system, as well as can also be applied to traditional TPV system and solar energy collection. Cui et al. demonstrated a W-CNT PhCs with spectral selectivity.^[123] In a STPV system with GaSb PV cells, the W-CNT PhCs serve as SSA and emitter. The W-CNT PhCs shows negligible degradation in optical properties after 168 h of annealing at 1000 °C, exceeding the previously known high-temperature PhCs that because of its stable composition and structure (Figure 13d). According to spectral characteristics of the W-CNT PhCs, it can be predicted that the efficiency of the STPV system will exceed the Shockley–Queisser efficiency limit under moderate operating temperature and input power.

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Although STPV has shown great potential in the field of power generation, efficient collection of solar energy in the SSA and the modulation of spectrum in the emitter are still challenging, especially at high temperature operation.

Hybrid Photovoltaic-Thermal

Although STPV has shown great advantages in utilizing full solar spectrum, the preparation of selective thermal devices in STPV system is very complicated and the spectral regulation is not easy. Furthermore, its thermal stability should be considered under high temperature operation. These limit the wide application of STPV. To utilize the broadband solar spectrum, HPVT based on spectral splitting design is an emerging technology, solar spectrum can be split into multiple spectral bands by filter, where photons with energy just above the PV cell bandgap are introduced into the PV cell. while photons with energy lower and higher than the PV cell bandgap are absorbed as heat for solar thermal applications. Spectral splitting also reduces the temperature of the solar cell, which can prevent it from overheating and reduce cell efficiency.^[124]

For example, Cao et al. proposed a four-band spectrum splitting strategy that is expected to achieve efficient utilization of the full solar spectrum by suppressing radiative heat loss, improving the overall performance of HPVT system.^[125] As shown in Figure 14, The beam splitter is composed of a SiO₂/TiO_x top interference filter and a bottom SSA with low IR emissivity. The incident solar radiation within the wavelength range of 725–1100 nm is absorbed and converted into electric by Si solar cell in the form of reflection, the remaining is absorbed and converted into heat by the SSA in the form of transmission. Thus, the overall efficiency of HPVT is improved. However, the reflectance of PV band still has a gap with the ideal splitter.

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In short, on the one hand, different from the traditional PVT system, STPV and HPVT systems make efficient use of SSAs. STPV systems can absorb solar energy efficiently by SSA, and then radiate photons matching the band gap wavelength of PV cells; while HPVT systems divide the solar spectrum into multiple bands through beam splitter, the bands matching the PV cells bandgap are directly utilized by the PV module, and the other bands are absorbed by SSAs. On the other hand, the PV modules of these two systems are not in direct contact with the PT system, which prevent the PT system from overheating and affecting the efficiency and lifetime of the PV modules. Both systems are to make full use of the solar spectrum and combine the advantages of PV and PT technology, so as to produce more useful electric energy and thermal energy.

Solar-Thermoelectric Generators

PV cells and CSP systems are two main methods of convert sunlight into electricity. The aforementioned STPV and HPVT devices both contain PV cells, and the overall utilization efficiency of solar energy can be improved by combining PV and PT system. However, both the absorber/emitter device and the spectrum splitter include complex PhCs structures, and the spectrum is not easy to control. CSP systems are only suitable for large power plants due to the requirement of mechanical heat engines and expensive concentrators. There is a power generation system with stronger portability and practicability. STEGs are solid state devices which can directly convert heat into electricity.^[126–128]

As shown in Figure 15b, thermoelectric (TE) module typically consists of p-type and n-type semiconductors with two charge carriers (electrons and holes). They are connected thermally in parallel and electrically in series to produce a large TE voltage.^[129] When a temperature difference is created between the hot and cold sides of the STEG, the STEG can directly convert heat into electricity using the TE module, this is known as the Seebeck effect.^[130] STEGs systems are simple, the operation process is environmentally friendly, and the photoelectric conversion efficiency is high, thus enabling wider applications.

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For example, Chen et al. demonstrated a flat-panel STEG with a high thermal concentration.^[131] As shown in Figure 15a, the use of SSA rather than expensive optical concentrators achieves a large temperature gradient along the TE module that converts solar radiation into heat and concentrates its heat onto the TE element by lateral heat conduction. The results shown that the STEG has a TE conversion efficiency up to 4.6–5.3%, it was 7–8 times higher than the previously reported.^[126] Besides, the STEGs are inexpensive, do not need to track the sun, and can generate electricity from waste heat.

Personal Thermal Management

Heating

The traditional indoor heating strategies generally uses the building heating, ventilation, and air conditioning (HVAC) system to heat large empty indoor space to improve human thermal comfort, but it will consume a lot of power and energy, exacerbate the trend of global warming, and cause serious environmental and social problems.^[132,133] Personal thermal management (PTM) based on thermal localization accurately heat the human body by regulating the temperature of the small space between the human body and the fabric, which is an energy-saving and effective heating strategy.^[134–138]

Human skin is an excellent IR emitter (ε = 0.98), hence the thermal radiation emitted by human body is mainly in the mid-IR wavelength range of 7–14 μm.^[139] Radiation plays an indispensable role in human body heat dissipation, contributing to more than 50% of total heat loss in typical scene (such as an office).^[140] To achieve passive radiative heating (PRH) effect, the radiative heat generated by a human body should be suppressed through wearable textiles with mid-IR emissivity, especially in indoor and outdoor environments without direct sunlight.^{[138,141,142]} However, it is insufficient to heat a human body only by PRH. A fabric with high absorption can maximize the absorption of solar energy and then heat a human body. So SSACs with high absorption and low emission is undoubtedly the second to none choice. An effective strategy is to deposit ultrathin SSAC on various flexible substrates, where PRH and solar heating can be realized, no matter in any weather or environment.

Multilayer SSACs based on cermets are widely used, but the coating is relatively thick because of the complex multilayer structure, and it is generally deposited on rigid substrates such as glass, stainless steel, Si wafer, Al sheet, and so on. Therefore, ensuring excellent photothermal properties while reducing SSA thickness is still a challenge for thermal management fabrics. It is difficult for PTM fabrics to reduce SSAC thickness while ensuring superior optical performance. Wang's group used self-doped single target magnetron sputtering technology to prepare a series of ultrathin SSACs that can be deposited on flexible substrates. For example, a SSAC based on WO_x with a nanogradient that is only about 100 nm thick has a high solar absorption of 0.93 and excellent thermal stability at 300 °C.^[143] A SSAC based on MoO_x with a nanogradient, when SSAC is coated on fabric and polymer, under 1 sun irradiation, the surface temperature reaches 90 °C and 78 °C, respectively, which realizes efficient photothermal conversion due to the excellent absorption of SSAC.^[144] A SSAC of amorphous carbon doped by Ag NPs with a thickness of only 130 nm, it can achieve 87% photothermal conversion efficiency due to a significant enhancement of light absorption by the plasmonic effect of Ag NPs.^[145] These SSACs have superior spectral selectivity and can realize solar heating and passive radiative heating. Because of their ultra-thin characteristics, they can be generally deposited on flexible substrates, and have bendability, strong adhesion and excellent mechanical strength. Therefore, they are very promising in flexible wearable applications.

In addition to passive radiative heating and solar heating capabilities, it is necessary to pay attention to their actual needs such as wear resistance and aesthetics. Luo et al. reported an ultra-thin (thickness about 16 μm) colored textile with nanophotonic structure. The textile exhibits 50% solar absorption and 10% low infrared emissivity, allowing for both outdoor solar heating and indoor passive radiative heating. (Figure 16a).^[146] By depositing ultrathin germanium (Ge) and gold (Au) layers on nanoporous polyethylene (nPE) textiles and changing the deposition time of Ge, the textile can show various bright colors (purple, magenta, orange, and blue). In addition, its wear resistance, wettability, and air permeability are comparable to traditional fabrics such as black sweatshirts and polyester film blankets.

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Besides the abovementioned multilayer SSACs, there is also an intrinsic material with selective absorption-MXene, which can be used in PTM. Wang's team demonstrated an energy-saving all-day personal precision heating system, which includes three heating modes: solar heating, passive radiative heating, and Joule heating (Figure 16b).^[147] The surface of nanoPE fabric was decorated by Ti₃C₂T_x MXene with low infrared emissivity. The MXene/nanoPE textile with a thickness of only 12 μm has a low IR emissivity of 0.176 in the atmospheric window wavelength, showing excellent indoor passive radiative heating performance. The heating temperature is 4.9 °C higher than that of 576 μm thick traditional cotton textile. Due to MXene/nanoPE textile has excellent solar energy absorption and electrical conductivity, the outdoor solar heating temperature and electric heating temperature of MXene/nanoPE textile reach 73.5 and 55 °C (5 V), respectively. By freely switching or arbitrarily combining the three heating modes, the MXene/nanoPE textile can precisely warm the human body in a variety of environment, including sunny/cloudy, day/night and indoor/outdoor. Moreover, the textile also shows excellent wearable properties, including breathability, wind resistance, electromagnetic interference shielding, quick-drying, mechanical strength, flame retardancy, antibacterial, etc., demonstrating its huge application prospects in the field of precision PTM. In addition, as shown in Figure 16c, the ultrathin Ti₃C₂T_x MXene films (as low as 1 μm) have excellent high-temperature indoor/outdoor thermal camouflage performance due to the low mid-infrared emissivity of the film (0.19). At the same time, it has long-term high temperature or fire stability, and electromagnetic interference shielding capability.^[148]

Dual-Mode Thermal Management

At present, most textiles only have a single function of solar heating or radiative cooling.^{[141–147,149,150]} It is necessary to combine the two seemingly opposite thermal functions in order to fully protect people from outdoor overheating/undercooling environments and drastic temperature changes. Cui et al proposed a dual-mode textile that can use the same textile to achieve passive radiative heating and cooling without any energy input.^[151] In this design, double-layer heat emitters with different emissivity on each side are embedded in a nanoPE layer, and each side also has an asymmetric thickness. Since nanoPE is infrared transparent, which can freely radiate energy to the surrounding environment. As shown in Figure 17a, in the cooling mode, the environment is exposed to the high emissivity layer, moreover, there is a small space between the emitter and the skin, guaranteeing efficient heat transmission from heated human skin to the transmitter, the temperature of the transmitter will rise as a result. With a high amount of outgoing emissivity and a relatively short distance to the skin, this combination will result in a high coefficient of heat transfer, and therefore the fabric will be cooling. The cooling effect will be as strong as just wearing nanoPE to cool the textile if both the fabric and the skin have a thermal emissivity of 0.98 and there is no thermal resistance between the fabric and the emitter. On the reverse side, the side with low emissivity faces inward, and as the distance from the emitter to the skin increases, the thermal conductivity decreases; therefore, the textile is in heating mode. Therefore, two different heat transfer coefficients can result from asymmetric emissivity and nano-PE thickness, and heat is produced when the low emissivity layer faces outward, and when the high-emissivity layer faces outwards, cooling can be achieved by moving the high emissivity layer outward.

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Luo et al. described a textile that provides simultaneous thermal management and energy generation for outdoor personal thermal management.^[152] As shown in Figure 17b, when heated, the Janus textile is dark gray, with an absorption rate greater than 0.8, and the temperature can be increased by 8.1 °C compared with ordinary black cotton; in the cooling mode, it is white, showing a high reflectivity of 0.91 and a high emissivity of 0.87. Compared with ordinary white cotton cloth, the temperature can be lowered by 6 °C. Through free turning, local heating and cooling can be carried out, providing all-day temperature control outside, it is able to provide excellent outdoor temperature adjustment capabilities. Dual-function Janus textile is capable of maintaining a temperature gradient between its inside surface and the skin in both modes thanks to its superb local temperature regulation ability. Wearable TE devices may therefore benefit from its improved performance in the generation of TE power. By connecting Janus textiles to a commercial thermoelectric generator (TEG) with a thickness of 4.3 mm, the maximum output power density of TEG can be reached in the range 69 mW m⁻² during the day and 24 mW m⁻² at night. The fabrication of Janus textiles will allow for the maintenance of a comfortable microclimate for users outside as well as serve as a platform for ubiquitous power generation.

Photothermal Catalysis

Solar energy can be efficiently stored in chemicals by directly converting sunlight into heat energy.^[153–155] The black body material can absorb almost all wavelength sunlight,^[156] but its high emissivity leads to great heat radiation loss. The temperature is frequently lower than 100 °C when exposed to natural sunshine. which makes it difficult to start most catalytic reactions (usually above 200 °C).^[157] So far, it is still difficult to achieve photothermal catalysis with a weak solar source (1 kW m⁻²) and no additional energy input.^[158,159] The SSAs, which can convert weak sunlight into high temperatures, enabling catalytic reactions to be driven without secondary energy inputs, for example, water splitting to produce hydrogen, CO₂ methanation, photocatalytic organic synthesis reaction, selective catalysis of volatile organic pollutants, etc. The following are some specific applications of solar-driven thermal catalysis, which rely on vacuum glass tube-based photothermal conversion device. In the device, SSAC is coated on the inner wall of the glass tube to maximize solar energy absorption and minimize IR radiation, the vacuum layer can reduce heat conduction loss, and the metal film between the inner wall of the glass tube and the SSAC can reflect IR radiation. These ensure the full utilization of weak sunlight, and the heat is concentrated to promote the photothermal catalytic reactions in the vacuum glass tube.

As the concentration of CO₂ in the atmosphere continuously increase, the greenhouse effects become more obvious, and extreme weather occurs frequently. Methane (CH₄) is the primary component of natural gas and is commonly employed as the energy that is clean and has a low carbon footprint. Ambient sunlight-driven CO₂ methanation is of great significance for reducing CO₂ emissions.^{[158,160–162]} A photothermal system using SSA was constructed by Li et al. to generate high temperatures (up to 288 °C) under weak solar radiative (1 kW m⁻²), which is 3 times higher than that of traditional photothermal catalytic systems.^[163] Furthermore, ultra-thin amorphous Y₂O₃ nanosheets containing limited single nickel atoms (Ni/Y₂O₃) with improved CO₂ methanation activity were created. To attain the high temperature, the exterior of a quartz tube is coated with SSA, as illustrated in Figure 18a. On the interior of a flow-type quartz tube, Ni/Y₂O₃ nanosheets were coated. Therefore, with the aid of SSA, SA Ni/Y₂O₃ achieves 80% CO₂ conversion efficiency and 7.5 L m⁻² h⁻¹ CH₄ under solar irradiative (from 0.52 to 0.7 kW m⁻²), demonstrating that the system may be utilized as a platform for directly converting carbon dioxide into useful compounds using solar energy.

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Chlorinated volatile organic compounds (CVOCs) are produced by a variety of industrial operations, as well as municipal and medical waste incineration.^[164,165] Because of their acute toxicity, high bioaccumulation potential, and high environmental stability, aryl chlorides are regarded as one of the most dangerous organic pollutants.^[166] Photocatalytic CVOCs breakdown, among other technologies, is a viable solution to solving this environmental issue caused by solar energy. Based on a scalable CuMnCeO_x gel and a novel photothermal conversion device, Li et al developed a new solar-driven catalytic system for CVOCs combustion.^[167] Figure 18b shows the combination of a new photothermal device and CuMnCeO_x gel, which was incorporated into the device's interior. At 250 °C, the CuMnCeO_x gel achieved 99% elimination of CVOCs, which is 25 times higher than that of bulk CuMnCeO_x. Furthermore, the CuMnCeO_x gel may be heated to 300 °C using the novel photothermal conversion device under 1 sun and show a stable CVOCs combustion rate of 6.8 mmol g⁻¹ h⁻¹, which was 7.8 times greater than the current status of photocatalytic CVOCs breakdown. As a result, the high-activity natural sunlight-driven thermal CVOCs combustion system with negligible secondary emissions has the potential for large-scale industrial applications.

Hydrogen (H₂) has long been considered the most potential future fuel. Solar-powered methanol hydrogen production is a promising technique for conserving nonrenewable energy while creating hydrogen.^[168,169] Bai et al. demonstrated that Pt NPs doped in CeO₂ nanosheets (Pts–CeO₂) show outstanding methanol dehydrogenation activity and catalytic stability of 500 h, which is 11 times greater than Pt NPs/CeO₂.^[170] Under 1 sun irradiation, Pts–CeO₂ can be heated to 299 °C by using a photothermal conversion device (Figure 18c), owning to efficient full sunlight absorption and low heat irradiation loss. As a consequence, with a 481.1 mmol g⁻¹ h⁻¹ of H₂ production rate and a high solar-to-hydrogen (STH) efficiency of 32.9%, the Pts–CeO₂ achieves an exceptionally high methanol dehydrogenation performance.

Steam reforming is another dominant approach for hydrogen production due to its relatively lower cost and higher production rate. But it consumes a lot of energy. Concentrated solar energy could be employed for the endothermic reaction in the steam reformer to produce H₂. However, solar absorber has huge heat radiative loss at high temperatures. To avoid the large radiative heat loss and obtain higher surface temperature, Bai et al. proposed a 2D photonic crystal (PhC) SSAs, which is made of titanium nitride (TiN) thin film.^[171] On the top of TiN thin film, nanocavity arrays structured Al₂O₃ film is deposited (Figure 18d). The as-fabricated SSAs have excellent high temperature stability (800 °C for 2 h). Compared to blackbody absorber, the SSAs show higher surface temperature, higher C₃H₈ conversion rate, and higher H₂ production rate, which indicate 2D PhC SSAs is a good candidate in the steam reforming application for H₂ production.

Nitrogen oxides (NO_x) is one of the most toxic gases that causes major environmental issues.^[172,173] NO_x SCR (photocatalytic selective catalytic reduction of nitrogen oxides) is a green and efficient way to reduce NO_x emissions. A commercial AlN_x SSA selected by Bai et al. can heat the catalysts up to 270 °C under 1 sun irradiation due to its full sunlight absorption and low radiation characteristics, thus enabling NO_x SCR to begin (Figure 18e).^[174] Furthermore, to produce W doped Fe₂O₃ nanosheets on a large scale, a polyvinyl alcohol aided graphene oxides templated technique was devised. This method may be employed as a NO_x SCR catalyst with moderate SO₂ and H₂O resistance and positive N₂ selectivity. As a consequence, without a second energy input, the AlN_x SSAs aided W doped Fe₂O₃ nanosheets displayed 92% and 90% NO conversion rates under 1 sun irradiation and outside sunlight irradiation, respectively. With this method, natural sunlight-driven photothermal NOx SCR was achieved without the need for additional energy.

Photothermal Anti-Icing

Icing is an inevitable phenomenon in nature. Ice accumulation can seriously affects the operation of high-voltage power lines, aircraft and wind turbines, which can lead to serious economic losses and even casualties.^[175–179] Traditional mechanical and chemical deicing procedures are either energy-intensive or harmful to the environment.^[180] Recently, many passive anti-icing strategies were developed. Superhydrophobic surfaces, slick surfaces, antifreeze compounds, and polyelectrolyte brush coatings are only a few examples.^[181–184] However, the efficiency of these passive anti-icing methods is too low, as none of these solar anti-icing or deicing devices take the spectrum properties of thermal radiation into account. In fact, heat leakage from the absorber to the environment is nonnegligible, severely reducing the total efficiency of solar absorbers. Dash et al. described a “photothermal trap” that effectively deices ice by converting solar energy to heat at the ice-substrate interaction.^[185] The photothermal trap in Figure 19a is based on the complementary features of three layers: a SSA layer for solar radiation, a thermal spreader layer for lateral dispersal of heat, and insulation layer to minimize transverse heat loss. As a result, solar illumination can lead to a temperature rise of 37 °C, forming a thin lubricating melt layer, making it an extremely energy-efficient deicing method.

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Although SSAs can efficiently absorb almost the entire solar spectrum, the lack of a superhydrophobic surface prevents the timely removal of meltwater, which can reduce the photothermal efficiency of the absorber and lead to reicing. For anti-icing and deicing applications, Ma et al. established a superhydrophobic selective solar absorber (SHSSA) on an aluminum substrate that combines superhydrophobicity with spectrally selective solar-thermal conversion.^[186] This SHSSA surface, in particular, is made up of hierarchical micro/nanostructures that allow for stable superhydrophobicity (contact angle [CA] ∼162°; sliding angle [SA] ≈5°). A hierarchical surface is coated with ceramic plasmonic titanium nitride (TiN) NPs for solar-thermal conversion. Due to light trapping and plasmonic resonance effect allowed by the hierarchical architecture, the SHSSA has considerably enhanced spectrum selectivity: high solar absorption (≈90%) and low IR absorption/emission over the mid-IR region (>2.5 μm; ≈42% at 25 °C). The SHSSA produces a higher temperature of 61 °C under 1 sun illumination than a nonselective blackbody-like surface (52 °C) due to its low thermal radiation loss (Figure 19b). As a consequence, the SHSSA exhibits exceptional icephobicity at a record-low temperature of 60 °C when illuminated by 1 sun. With the synergistic effect of enhanced solar thermal conversion and superhydrophobicity, the SHSSA possesses excellent deicing, defrosting performance, and high durability.

Photothermal Sterilization

Healthcare-associated infections (HCAI) place a heavy burden on the health care system and patients.^[187] In medical sterilization methods known as autoclaving, saturated steam (>121 °C and >205 kPa) is normally utilized. Although the standard sterilization is effective, it is challenging to generate high temperature and high-pressure steam without a reliable power or fuel supply. Although photothermal sterilization is a promising strategy, solar-powered steam generation at such high temperatures and pressures often necessitates the use of costly optical concentrators.^[188,189]

Based on this, Wang's group developed a stationary solar thermal equipment that can provide the required saturated steam and achieve a stable output of saturated steam with temperature up to 128 °C in a static solar collector.^[190] Because of its high solar absorption rate, low infrared emissivity, and high thermal conductivity, the tube-fin absorber is built of copper with a SSAC. The back of the absorber is insulated by a 50 mm-thick fiberglass board. The absorber is protected by an aluminum casing, and a piece of borosilicate glass covers the top hole. Ten super-transparent aerogel tiles are placed on top of the absorber surface to replace the air gaps in traditional solar collectors. This kind of aerogel collector can still achieve high temperature and high-pressure steam output under nonideal outdoor lighting conditions (Figure 20a). This research provides an effective solution for the shortage of medical steam disinfection resources in underdeveloped areas. It also provides a new design idea for a new generation of low-cost and high-efficiency solar thermal utilization technology. For high-efficiency superheated steam production under ambient solar irradiation, Chang et al. constructed an interfacial evaporator within a solar vacuum tube.^[191] The commercial double-layer solar vacuum tube is employed as a solar collector in this experiment, and its high solar absorption rate and low thermal emissivity are utilized to reduce heat loss (Figure 20b). The solar thermal energy gathered heats the porous evaporator, resulting in efficient interface evaporation and the production of water vapor. Without any need for pressurization, the produced vapor is further heated using a copper-mesh-based heat exchanger inside the solar vacuum tube, resulting in superheated steam. Under 1 sun irradiation, the steam generator demonstrated adjustable steam temperature of 102 to 165 °C and solar-to-steam conversion efficiency of 26% to 49%. It allows stable production of steam over 121 °C under ambient variable solar illumination with an averaged solar flux of ≈600 W m⁻² with little heat loss from the solar vacuum tube and the interfacial evaporation design.

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SSA can be used as an antimicrobial agent in its own right, in addition to the application of solar-powered steam sterilization. Ag–TiO₂ has been widely used in self-cleaning and photocatalysis, and Ag–TiO₂ can also use as SSA for solar thermal applications. Prasad et al. have successfully developed a nano-cermet Ag–TiO₂ SSAs with antibacterial activity.^[192] The nano-cermet coating is manufactured on the economically available SS 202 substrate. And a mesoporous MgF₂ layer is then coated on it, which acts as a tandem absorption layer and exhibits good optical properties (α = 0.93; ε_{200 °C} = 0.19). In addition, the coating has strong antibacterial properties due to the photocatalytic reduction of silver ions under the sun, which is capable of eradicating Gram-positive Bacillus subtilis and Gram-negative Escherichia coli. This provides applications opportunities for SSAs in water disinfection, hot water, industrial heating, and swimming pools.

Solar-Driven Soft Robots

Researchers have designed biomimetic soft robots with a compliant construction that can mimic biological systems and conduct intricate robotic tasks. The driving force is heat created by different mechanisms (e.g., light, magnetic induction, or Joule heating). Wang et al. proposed a low-cost, high performance, and easy-implement technique to drive untethered soft robots.^[93] The pressure chamber (which contains the low-boiling point liquid and the solar absorber sheet) and the soft robot prototypes made of compliant silicone rubber are the two main components of the gadget. A glass syringe was used as the pressure chamber, with one end sealed with a heat sink and the other ends attached to a soft robot. A commercially available selective sun absorption screen is used to enhance the evaporation of the low boiling point liquid under light irradiation. In the visual spectrum, these coatings absorb more than 95% of sunlight (Figure 21). The method uses collimated light or sunshine to excite soft robots over a great distance, allowing it to be used in settings where electric or pneumatic sources are unavailable.

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We summarize the contribution of SSA in sustainable applications (Table 2). In addition to solar-powered freshwater harvesting, SSA also make a significant contribution in these applications. In PV/PT, W-CNT PhCs have the superior high-temperature thermal stability and have great potential in integrated CSP and PT systems. Although the IR emissivity of Ti₃C₂T_x MXene is higher than that of other SSACs, it has great advantages in PTM because of its low cost, simple preparation and high conductivity. In the photothermal catalytic, TiON_x can heat the photothermal conversion device to a higher temperature, but AlN_X make the conversion efficiency of the catalytic reaction higher, which may be related to the type of catalyst and reaction, we will not be discussed further. TiN NPs layer provides excellent de-icing performance at −60 °C. In the photothermal sterilization, TiON_x can assist devices to generate higher vapor for medical sterilization. In conclusion, SSA has a very broad prospects in many solar applications, and it is worthwhile for researchers to further explore.

Table 2 Summary of SSA for emerging sustainable application

Application	Material and optical property	Performance	Ref.
PV/PT	Multiwalled CNTs as SSA and a 1D Si/SiO2 PhC as selective emitter	Solar energy utilization efficiency of 3.2%.	[121]
1D HfO2/Al2O3-W NPs/W/Al2O3/W multilayered photonic structure as selective thermal devices	Absorption = 0.92, emission = 0.1	[122]
W-CNT PhCs serve as SSA and emitter	Endure 168 h annealing (1273 K), exceeded the Shockley–Queisser efficiency limit	[123]
W-Ni-SiO2 cermet SSA with 95.6% $\overline{\alpha }$ and 15.0% $\overline{\epsilon }$	400 °C thermal stability in vacuum	[125]
SSA with 95% $\overline{\alpha }$ and 5% $\overline{\epsilon }$	Peak efficiency η (η = η ot η te) of 4.6% under 1 sun, 7–8 times higher than the previously reported STEG	[126]
Personal thermal management	Nanogradient WO x -based SSAC with 93% $\overline{\alpha }$ and 5.6% $\overline{\epsilon }$	Flexible polyimide sheet available; endure annealing for 300 h (300 °C in vacuum, 275 °C in air)	[143]
Nanogradient MoO x -based SSAC with 93% $\overline{\alpha }$ and 5.6% $\overline{\epsilon }$	Surface temperatures of 78 °C (polymers) and 90 °C (fabrics) under 1 sun	[144]
Ag NPs encapsulated in amorphous carbon based SSAC with 92.3% $\overline{\alpha }$ and 6% $\overline{\epsilon }$	Flexible substrates available; solar-thermal conversion efficiency of 87%; surface temperature of 94 °C under 1 Sun (cotton textiles)	[145]
Ti3C2T x MXene with 91.3% $\overline{\alpha }$ and 17.6% $\overline{\epsilon }$	All-day energy-saving personal precision thermal management	[147]
Photothermal catalysis	AIN x ceramic/AI foil layer	Temperature up to 288 °C under 1 sun; CO2 conversion efficiency of 80%; CH4 production rate of 7.5 L m−2 h−1	[163]
TiON x with 98% $\overline{\alpha }$ and 2% $\overline{\epsilon }$	Temperature up to 299 °C under 1 sun; H2 production rate of 481.1 mmol g−1 h−1; solar-to-H2 (STH) efficiency of 32.9%	[170]
AlN x ceramic	Temperature up to 270 °C; NO conversion rate of 92% under 1 sun	[174]
Photothermal anti-icing	SSA with 95% $\overline{\alpha }$ and 3% $\overline{\epsilon }$	Surface temperature of 37 °C under 1 sun	[185]
TiN NPs layer with 90% $\overline{\alpha }$	Anti-icing of a sessile droplet at −60 °C	[186]
Photothermal sterilization	Tube-fin SSA with 91.7% $\overline{\alpha }$	Steam temperature up to 128 °C	[190]
TiNO x with 93% $\overline{\alpha }$ and 7% $\overline{\epsilon }$	Tunable steam temperature (102 to 165 °C); solar-to-steam conversion efficiency (26% to 49%) under 1 sun	[191]
Solar-driven soft robots	SSAC (eta plus AL) with 95% $\overline{\alpha }$	Lift and grasp objects used in daily life	[193]

Conclusion

In recent years, solar selective absorbers have attracted a lot of research attention for sustainable applications, with the majority of them being employed in concentrating solar power, solar water heating and solar drying. In this review, we summarized some impressive and promising applications in recent years, which include solar-driven seawater desalination, multistage solar desalination, atmospheric water harvesting, wastewater treatment, solar thermophotovoltaics, HPVT, STEGs, personal thermal management, photothermal catalysis, photothermal deicing, photothermal sterilization, etc. (Figure 22). For specific uses, they use solar thermal energy instead of regular energy. And the key is managing the heat from solar radiation efficiently and effectively.

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Compared with blackbody materials, solar selective absorbers with high absorption and low infrared emission can maximize solar absorption and minimize heat radiative losses, which are undoubtedly the best choice for photothermal conversion process. In addition, the synergy of composite/hybrid solar selective absorbers with photothermal devices is important for functionalization and performance enhancement. For example, a solar selective absorber combining spectrally selective and superhydrophobicity can be used for photothermal deicing and personal thermal management textile. Unlike steam power generation, solar thermoelectric generator using temperature difference power generation is more portability and practicability. Both catalysis and sterilization need higher temperature, which is usually realized by the design of vacuum tube. The solar selective absorber coating coated on the inner wall of a solar vacuum tube can inhibit heat losses and assist the tube to obtain better performance. In the multistage solar desalination device, solar interface evaporation and multistage condensation latent heat recovery are skillfully combined, so that the overall water production efficiency is significantly improved. The absorber/emitter device can be used for seawater desalination and wastewater treatment, due to the device is separated from water, salt and impurities can be prevented from blocking the absorber. The absorber/emitter device can also be used in full-wavelength solar thermo photovoltaics system, which can not only generate electricity but also store thermal energy. In conclusion, the potential of solar selective absorbers in sustainable utilization of clean energy has just begun to be explored. More creative applications will emerge in the near future.

To date, there are still many challenges as well as future opportunities of solar selective absorbers. For example, current SSACs faces an unfavorable trade-off between spectral selectivity and thermal stability, in high temperature CSP system, the photoelectric conversion efficiency is reduced due to high temperature and large heat radiative loss. Although the high temperature resistance has made a lot of progress such as alloyed NPs, high entropy alloys and all-ceramic coatings, great progress has been made, the oxidation of metal NPs in the alloys is inevitable. As a result, the long-term thermal stability will be greatly affected. Another challenge is the very complex structure of the all-ceramic coating. In addition, the vast majority of the currently reported all-ceramic coatings are prepared vacuum deposition techniques, which is very costly. Definitely, there are some simple and low-cost chemical methods, such as solution method and “dip-and-dry” technique. But they are not easy to control, and the prepared coatings exhibit poor performances. The inherent low mid-infrared emissivity of Ti₃C₂T_X MXene is of great significance in the field of photothermal conversion, but MXene is unstable when exposed to water–oxygen environment. It still has many works to do with the intrinsic materials that possess spectral selective absorption and stability. In addition, the corrosion resistance of the coating is rarely studied, but it has great influence in practical application. For example, in long-term seawater desalination, under the long-term corrosion of air, water and salt, its optical performance and stability will inevitably decline. In brief, we still need to develop high-temperature resistant selective absorbers with excellent corrosion resistance and low cost, which can be prepared on a large scale, to cope with more emerging applications.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (nos. 51905326 and 52075309), the Youth Innovation Team of Shaanxi Universities, the Shaanxi Provincial Science and Technology Department (grant no.2021-6), the Science and Technology Department, Shaanxi Province (2021GY-248).

Conflict of Interest

The authors declare no conflict of interest.

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