Can the high throughput yield of solar thermal interfacial evaporation systems be beyond theoretical efficiency?

Abstract

Solar steam generation (SSG) offers a sustainable energy source for generating fresh water worldwide as the world grapples with water scarcity. Collective global efforts have spurred significant developments, leading to solar-to-steam conversion efficiencies surpassing theoretical limits over the last decade. However, the condensate yield remains suboptimal, and thermal losses are under ideal conditions, necessitating a critical evaluation of the practical applicability and scalability of this technology. This comprehensive study conducts an in-depth review of recent advancements in real-time solar steam generation, presenting strategies for high throughput yield of condensate through targeted modifications to core components, achieving higher yields with minimum complexity. The strategies that unlock the maximum possible output from SSG technology are first discussed comprehensively with current limitations and possible solutions. Subsequently, we analyze the environmental factors that significantly boost or drop the theoretical efficiency without being noticed in calculations. We further elucidate potential applications and underlying challenges while discussing contemporary methods to overcome these hurdles. This study ultimately aims to explore the possibility of surpassing theoretical efficiency limits in SSG systems, achieving high-throughput yields that would greatly benefit researchers in developing scalable, next-generation evaporators with unprecedented efficiency.

---

Ghazala Maqsood

Ghazala Maqsood received her Master of Science in Physics from Quaid-e-Azam University, Pakistan in 2016. Then, she joined Prof. Xianbao Wang’s lab at Hubei University as a doctoral student in 2023. Her research interests focus on applying emerging photothermal materials in hybrid solar-driven interfacial evaporation technology.

Muhammad Sultan Irshad

Muhammad Sultan Irshad, a Lecturer at the School of New Energy and Electrical Engineering, Hubei University, Wuhan 430062, P.R. China. Dr. Sultan received his PhD in materials science and engineering with outstanding graduate and PhD dissertation awards from Hubei University, China, in 2022. He engaged in postdoctoral research at Shenzhen University from 2022 to 2024. His research interests focus on applying emerging photothermal materials, efficient energy flow utilization, expanded applicability, and synergistic effects to address the water-fuel-energy crisis.

Xianbao Wang

Prof. Dr. Xianbao Wang received his PhD. degree from the Institute of Chemistry Chinese Academy of Sciences in 2002. He engaged in postdoctoral research at the Katholieke Universiteit Leuven (Belgium) from 2002 to 2004. He is currently the dean of the School of New Energy and Electrical Engineering. His current research activities involve in preparation and functionalization of carbon-based nanomaterials, and their applications in polymer nanocomposites, solar photo-thermal conversion, Li-S cells, fuel cells, supercapacitors, and environmental monitoring.

---

1. Introduction

Freshwater, vital for life, is under growing strain due to population growth, climate change, urbanization, and industrialization. Despite water covering 70% of Earth's surface, 97% is saline and unsuitable for consumption. Conventional desalination methods, like reverse osmosis and thermal distillation, are energy-intensive and depend heavily on diminishing fossil fuels.^1–3 This highlights the urgent need for sustainable, eco-friendly desalination technologies that address rising global freshwater demand while minimizing environmental impact and energy dependence.^4,5 Solar-driven seawater desalination harnesses abundant solar energy to address the water crisis sustainably, meeting freshwater needs while reducing environmental impact and reliance on finite resources.⁶ Research on photothermal materials (PTMs) has advanced significantly, enabling efficient solar energy conversion into heat for water desalination. PTMs facilitate the thermal conversion of aqueous inputs into steam, which condenses into potable water.

However, challenges persist, including water transport logistics, thermal energy loss, and precise collection of salt-free water. Efforts to enhance evaporation rates have surpassed theoretical limits, demonstrating multifunctional applications. For instance, Ho's group developed a hybrid prototype for parallel freshwater production and triboelectricity generation. Using a gold nanoflower solar absorber gel within a sealed design, sunlight-induced vapor condensation on inclined walls, and electrification causes the condensate to produce electrical signals during downward flow and bounce in any direction through the device. As a result, the condensed water was gathered at the vessel's bottom.⁷ In another study, a dual-function reactor combined photothermal-enhanced catalysis with desalination, producing hydrogen on one side and steam on the other. A condenser coil with circulating cold water efficiently captured steam, accelerating condensation and water collection. These advancements highlight the potential of PTMs for integrated clean energy and water solutions.⁸ Despite efforts to achieve 100% solar-to-thermal conversion, thermal losses remain a major challenge, with experimental condensate yields often falling short of expectations, limiting the technology's potential. X. Lan et al. addressed this by designing a condensation device using aluminum metal, which has higher thermal conductivity than conventional glass condensers. This allowed heat to dissipate rapidly from the metal to the bulk water. When tested on a lake for nine days, the device produced 8.09 09 kg m⁻² per day of clean water, achieving a vapor condensation efficiency of 75.7%.⁹ Despite record-high evaporation rates, further improvements in condenser design are needed to enhance condensation and water collection efficiency for practical applications.

While theoretical studies suggest that these systems can achieve near-perfect efficiencies, real-world experimental outcomes consistently fall short of these ambitious predictions. This raises a fundamental question: can we truly surpass theoretical limits in practical applications, or are intrinsic barriers constraining these systems? Exploring this question requires a deep analysis of the fundamental components targeting innovative strategies, recent progress, potential outcomes, and inherent challenges of SSG technology to bridge the gap between theoretical potential and experimental performance. Herein, this review aims to identify key strategies for achieving high condensate efficiency in SSG systems, thereby closing the theoretical-experimental efficiency gap, with implications for various hybrid applications (Fig. 1). First of all, a comprehensive study of the seminal advancements in SSG systems is presented, elucidating the recent progress in overcoming operational limitations to achieve sustained water production through multiple strategies that can enhance condensate yield. To analyze the gap between theoretical predictions and experimental performance of the SSG systems, this research then zeroes in to an in-depth analysis of evaporation rate and efficiency identifying the core problems that raise this discrepancy. Then this study further explores the factors that can potentially lead towards higher condensate yields and also discusses various environmental and operational constraints on SSG systems. Finally, the potential hybrid applications with the current challenges are discussed for future insight. The study aims to offer critical insight for designing a truly efficient SSG system and achieving a high water collection rate rather than merely focusing on a high evaporation rate. Although extensive research has been conducted, reports of surpassing the theoretical limits in solar evaporation systems continue to provoke discussion due to inherent constraints in solar-to-vapor conversion efficiency and evaporation rates. Even under optimal conditions, energy losses persist, preventing the system from achieving 100% efficiency. This review focuses on evaluating the theoretical benchmarks for evaporation rates and energy efficiency in solar-driven processes. By applying energy distribution models, the factors contributing to reported efficiency and evaporation rate anomalies are critically examined. Consequently, this review aims to address misconceptions regarding efficiency claims that exceed theoretical boundaries, offering a framework for researchers to contextualize such advancements within specific experimental conditions. This work ultimately provides a robust theoretical basis to advance the design of efficient solar water purification systems.

Fig. 1 Schematic illustration of solar thermal interfacial evaporation systems for optimized designs and various strategies to design hybrid systems that can practically achieve theoretical efficiency limits.

2. Strategies for optimized solar steam generation

SSG has gained considerable interest in recent years due to its diverse applications, such as wastewater treatment,¹⁰ biofouling,¹¹ desalination,¹² energy generation,¹³ and salt harvesting.¹⁴ The condensate efficiency is deeply affected by every component of the SSG system, however, most literature focuses on the design of highly absorbent advanced nanomaterials or porous structures,^15–19 whereas the coupled strategies such as controlled energy flow,²⁰ and atmospheric water harvesting²¹ have also proven their potential efficacy in achieving higher condensate yield.

2.1 Materials innovation for efficient steam generation

Researchers have developed photothermal materials (PTMs) with broad-spectrum absorption to enhance solar-driven interfacial evaporation for water purification. Various materials—including carbonic, semiconductive, polymeric, metallic, and organic small molecules (OSMs)—have been explored for their photothermal properties, leading to improvements in evaporation rates and system efficiency.

Carbonic materials such as graphene, graphite, and carbon nanotubes are renowned for their excellent photothermal conversion, high absorbance, and low reflectance. They perform well in various environmental conditions, including acidic, alkaline, and salty environments. Their structure can be easily modified to enhance light absorption, but surface reflection remains a challenge, which can be mitigated by nanostructuring these materials to increase light path length and reduce emissivity.^22–26 Semiconductive materials like TiO₂, MoS₂, Cu₂S, FeS₂, and FeTiO₃ (ref. 27) have gained attention for their unique thermalization processes and highly tunable energy band structures, making them promising candidates for PTMs.²⁸ When exposed to light, their energy exceeds the bandgap, generating electron–hole pairs and excitons. These excited charge carriers relax, releasing energy as heat before recombining, a process known as thermalization.²⁹ However, the broad bandgap of semiconductors results in low light-to-heat conversion efficiency due to electron–hole recombination, which releases photons instead of heat. To improve efficiency, strategies such as heteroatom doping are being explored to narrow the bandgap and enhance light-to-heat conversion.^30,31 Polymeric materials are widely used as PTMs due to their thermal stability, low thermal conductivity, high surface area, and chemical resistance. However, their limited energy bandgap restricts their solar spectrum absorption, leading to reduced efficiency. Conjugated polymers, with π-conjugated sp-2 hybridized carbon backbones, possess split energy levels and modified bandgaps, enabling enhanced light absorption and improved solar-to-thermal conversion efficiency. Polypyrrole (PPy),³² polyaniline,³³ polydopamine,³⁴ and poly(1,3,5-hexahydro-1,3,5-triazines)²² are examples of conjugated polymers that demonstrate strong light absorption under ambient conditions, making them promising candidates for efficient PTMs. Metallic nanoparticles are widely used in SSG due to their excellent photothermal conversion efficiency, driven by the high concentration of delocalized electrons. When illuminated, surface plasmons on metal surfaces resonate with light, efficiently emitting phonons. The excited electrons, or hot electrons, release energy as heat through electron-phonon scattering, and as phonons relax, they release further energy. This makes metal nanoparticles with surface plasmon resonances ideal for light-to-heat conversion. However, their narrow absorption spectrum limits their effectiveness in broadband solar light absorption.²³ Moreover, OSMs are promising PTMs due to their ability to convert near-infrared light into phonons via non-radiative decay. Organic conjugate molecules (OCMs) with strong electron donor–acceptor coplanar structures, enhance absorption through electron delocalization and π–π stacking.²⁴ Their small energy gap further improves nonradiative decay, boosting heat generation. They offer flexibility, diverse structures, and tunable properties, making them suitable for SSG applications. However, their limited solar absorption and low efficiency remain challenges. Croconium dyes,^{25,26,35–38} known for exceptional chemical, thermal, and photostability, have shown high photothermal performance surpassing their counterparts.^39–47 Notably, CR-TPE-T, a stable Croconium derivative, absorbs a wide solar spectrum (300–1600 nm) and achieves 87.2% solar-to-vapor conversion efficiency under one-sun irradiance.⁴⁸ A comparison of different types of PTMs is given in Table 1 owing to their evaporation rates, efficiency, and recorded surface temperatures.

Table 1 Comparison of different types of photothermal materials based on evaporation, efficiency, water collection, and stability of systems

Photothermal material	Material type	Evaporation rate under one sun illumination (1 kW m^−2) (kg m^−2 h^−1)	Water to vapor conversion efficiency (%)	Maximum surface tempera-ture	Freshwater yields	Ref.
Blank hollow spacer fabric	Carbonic	1.4352	86	59.2	—	49
3D cross-linked honeycomb graphene foam	Carbonic	1.30	87	40	2.6 kg m^−2 h^−1 g^−1	50
Super hydrophilic porous carbon foam	Carbonic	1.48	86	92.7	—	51
Three-level pore structure hydrogels	Carbonic	1.65	75	45.2	—	52
PSCH	Carbonic	2.09	80.4	47.1	13 kg m^−2 h^−11	53
Carbonized corn-grooved straw	Carbonic	1.57	85.9	40.7	—	54
Carbonized pomelo peel	Carbonic	1.37	93.7	46.6	—	55
Molybdenum carbide	Carbonic	2.19	96.15	40.5	13.86 kg m^−2 day^−1	56–59
Span80/isodecyl acrylate/divinylbenzene gel emulsion	Carbonic	1.27	90.3	42.3	—	60,61
Ti3C2 MXene/chitosan hydrogel	Semiconductive	1.36	93.7	51	—	62
MnO2 nanowires/Chitosan hydrogels	Semiconductive	1.78	90.6	40.8	17.02 kg m^−2 day^−1	63
Ti^3+-TiO2/polyethylene foam	Semiconductive	1.2	77.1	39.8	—	64
Black TiO2 polystyrene foam	Semiconductive	1.16	77.14	45.4	—	65
Al2O3-coated Cu–Si nanowire	Semiconductive	1.37	86	65.1	—	66
Ti foam	Semiconductive	1.79	90	—	—	67
Ti2O3	Semiconductive	1.32	92	—	—	68
PVA/PPy hydrogel membrane	Polymeric	3.64	96	28	34 Lm^−2	69
PPy/FeCl3 coated pomelo peel	Polymeric	1.22	76.61	50.8	1.38 kg m^−2 h^−1	70
PPy/FeCl3 coated melamine foam	Polymeric	2	90	65.7	—	71
Pyridine-based conjugated microporous polymer aerogels	Polymeric	1.4	80	42.2	—	72
PVA/Chitosan/PPy	Polymeric	3.6	92	33.4	—	73
PDA/PEI/PPy@PI NFM	Polymeric	1.43	86.9	38.5	500 mg L^−1	74
PPy-coated cotton/polystyrene foam	Metallic	1.2	82.4	33.1	—	75
Silicon nanowire/polyethylene foam	Metallic	1.12	72.8	33.9	—	76
Prussian blue @cellulose bioplastic	Metallic	2.22	84.3	70.3	—	77
Tungsten trioxide (WO3−x)/wood	Metallic	1.28	82.5	42	—	78
Silver diatomite filter paper	Metallic	1.39	92.2	44	—	79
Aluminium NP	Metallic	4.9	77.8	—	—	80
Ag NP @CC	Metallic	1.36	92.82	37.5	—	81
CR-TPE-T loaded PU foam	OSM	1.272	87.2	43	—	48
CTC	OSM	1.67	90.3	—	—	82
DPP-INCN	OSM	—	71.8	25.7	—	83
DDPA-PDN cellulose paper	OSM	1.07	56.2	62	—	84

---

Innovations in PTMs are pivotal for achieving higher condensate efficiency and yield in SSG systems, as they enable precise control over material properties to enhance condensation processes. Moving forward, continued exploration of PTM-water-light interactions will be crucial for optimizing these systems, unlocking the potential for more sustainable and efficient water and energy solutions.

2.2 Surface engineering to maximize diffuse reflection

Photothermal surface engineering focuses on optimizing solar steam generation systems by enhancing heat conversion and surface design to maximize energy capture and increase condensate yields. Researchers have explored various approaches, including: (1) transforming 2D surfaces into 3D structures, such as cone-shaped,⁸⁵ origami,^86,87 spiral,⁸⁸ and cup-shaped designs,⁸⁹ to boost diffuse reflection and improve water heating for steam production; (2) designing surfaces that minimize thermal losses while enabling full-spectrum solar absorption and achieving up to 100% solar-to-thermal conversion efficiency;⁹⁰ (3) implementing macroscale modifications to enhance photothermal conversion; and (4) introducing nanoscale changes, such as creating cavities within material layers to improve light trapping.^91–93 Given that radiation losses significantly contribute to energy dissipation, surface engineering plays a critical role in optimizing high-temperature photothermal materials, with a focus on minimizing radiation loss to enhance energy performance. A 3D wave-like fabric structure with carbon black as PTM serves as an all-in-one evaporator for water and electricity generation through collaborative energy coupling (Fig. 2a).¹³ Another carbonized design biomimetic to sunflower head surpasses the evaporation efficiency limit of all natural materials by 100.4% (Fig. 2b).⁹⁴ Chen et al. created an innovative solar steam generator in the form of a wooden flower as shown in Fig. 2c,⁹⁵ coated with silver polydopamine nanoparticles (Ag@DPA NPs) that absorb an impressive 98.65% of sunlight. The hydrophilic properties of Ag@DPA NPs facilitate water absorption on both the upper and lower surfaces of the 3D porous wooden petals. Additionally, the wood's capillary channels and small holes enable evaporation from both surfaces, achieving 97% photothermal efficiency under one sun irradiation. Y. Hu et al. reported a dynamic interfacial evaporation system with deformable conic arrays in response to varying magnetic fields and it successfully boosted the evaporation rate to 25% as compared to static systems (Fig. 2d).⁹⁶ Carbon nanotubes (CNTs) and polyvinyl alcohol-based evaporating foam layer covered with CNTs provide a double-layer heating system where the temperature gradient is generated consequently leading to a higher evaporation rate with cheap materials (Fig. 2e).⁹⁷ Also, a cellulose paper crafted to 3D origami flower (Fig. 2f),⁹⁸ and a 3D wood-cone evaporator (Fig. 2g)⁹⁹ prove better performance than 2D evaporators.

Fig. 2 (a) Enhanced light-trapping with wave-like structure. Reproduced with permission.¹³ Copyright 2023, Wiley-VCH. (b) Engineered carbonized sunflower head. Reproduced with permission.⁹⁴ Copyright 2019, ACS Publications. (c) Wooden flower for enhanced diffuse reflection and high yield. Reproduced with permission.⁹⁵ Copyright 2020, Elsevier. (d) Graphene-wrapped Fe₃O₄ conic arrays with magnetic reconfiguration enhancing evaporation through multidirectional flow. Reproduced with permission.⁹⁶ Copyright 2022, Springer Nature. (e) An array of solar steam generation evaporators with a schematic of its unit structure. Reproduced with permission.⁹⁷ Copyright 2021, American Chemical Society. (f) Surface engineering of cellulose paper coated with PPy and digital picture of solar steam evaporating flower. Reproduced with permission.⁹⁸ Copyright 2019, Wiley-VCH. (g) Surface engineering of a 3D cone-shaped wood evaporator. Reproduced with permission.⁹⁹ Copyright 2023, Springer Nature.

2.3 Porous structures for enhanced water flux

Optimizing water transportation to PTMs is critical for enhancing the efficiency of solar steam generation in SSG systems. The selection of hydrophilic porous materials with unique structures, large surface areas, and high mechanical and thermal stability is essential for improving water flux and ensuring large-scale implementation. In addition to synthetic porous structures, natural porous PTMs, such as carbonized lotus leaves,¹⁰⁰ bamboo,¹⁰¹ and coffee powder,^98,102 are increasingly favored due to their hydrophilic, biodegradable, and heat-insulating properties.¹⁰³ These materials, with their capillary mechanisms, facilitate effective water transport to the PTM and are often combined with thermal insulation to minimize heat loss and optimize water flow, ultimately balancing water supply with evaporation rate for efficient system performance.

Various innovative porous structures have been developed to enhance water flux and evaporation rates in solar steam generation systems. For instance, a 3D fabric evaporator with vertically aligned hemp-yarn arrays and MXene coating (Fig. 3a)¹⁰⁴ achieves an impressive evaporation rate of 3.95 kg m⁻² h⁻¹, with added salt resistance and anti-fouling properties, supporting multifunctional desalination and energy generation. Another example, an MXene-based cellulose fiber with ZIF-67 (Fig. 3b),¹⁰⁵ delivers a 139.4% solar-to-steam conversion efficiency under one sun. Hydrogels, such as a wood-skeleton hydrogel evaporator (Fig. 3c),¹⁰⁶ offer high mechanical strength, salt resistance, and excellent water flux. Additionally, fibrous MXene aerogels (Fig. 3d)¹⁰⁷ provide tunable pore structures, achieving a high evaporation rate and efficiency even in oil-contaminated seawater. PVA/PPy nanostructured gels (Fig. 3e)¹⁰⁸ also function as efficient solar vapor generators with high water purification rates. Recently, hydrogels and hydrogel-derived materials have proven to enhance the efficiency of SSG systems over plasmonic and carbon-based materials, because hydrogels are inexpensive, and they have abundant hydroxyl groups beneficial for high evaporation rates. Moreover, their low thermal conductivity helps to confine heat over the evaporator surface alongside highly porous structure.^109–112 Additionally, wind-induced effects on porous evaporators can further enhance evaporation rates by cooling surfaces and reducing convection losses, boosting overall efficiency.

Fig. 3 (a) Multifunctional solar evaporator. Reproduced with permission.¹⁰⁴ Copyright 2022, Wiley-VCH. (b) MXene-based porous evaporator. Reproduced with permission.¹⁰⁵ Copyright 2023, Elsevier. (c) Hydrogel with wood skeleton. Reproduced with permission.¹⁰⁶ Copyright 2022, Wiley-VCH. (d) MXene-based fibrous aerogel having tuneable pore structures. Reproduced with permission.¹⁰⁷ Copyright 2023, Springer Science. (e) Schematic of porous hydrogel for efficient evaporation and strategy for water confinement. Reproduced with permission.¹⁰⁸ Copyright 2018, Springer Nature.

2.4 Flat band structures toward higher efficiency

In the quest for sustainable and efficient strategies for obtaining high yields in SSG, flat band structures have emerged as a promising approach to enhance the absorption and conversion of solar radiation into heat, thereby amplifying evaporation rates and overall system efficiency. By leveraging the unique electronic properties of materials with flat band structures, researchers aim to overcome existing challenges in SSG. The conversion of photon energy to phonon energy, a crucial process in solar-to-thermal energy conversion, is a complex phenomenon prone to energy losses. The intrinsic mechanism of this conversion varies depending on the material type, with carbon-based materials exhibiting thermal vibrations between HOMO and LUMO (Fig. 4a),¹¹³ metallic-based materials leveraging nanoscale local surface plasmon resonance (LSPR) (Fig. 4b),¹¹⁴ and semiconductive materials utilizing non-radiative relaxation mechanisms (Fig. 4c).¹¹⁵ However, intrinsic material properties often limit light absorptivity, necessitating the design of specialized structures to enhance absorption. Inspired by nature, such as the structural blackness of butterfly wings,¹¹⁶ biomimetic materials—like inverse V-shaped antireflection structures and CNT forests—improve light absorption. These strategies are categorized into nanoscale microstructures, micrometer-scale light traps, macroscale light recovery structures, and omnidirectional designs as illustrated in Fig. 4d.¹¹⁷

Fig. 4 Schematic of: (a) photon to phonon conversion for carbon-based PTM. Reproduced with permission.¹¹³ Copyright 2021, Elsevier. (b) LSPR mechanism in metallic PTM. Reproduced with permission.¹¹⁴ Copyright 2020, Royal Society of Chemistry. (c) Non-radiative relaxation mechanism in semi-conductive PTM. Reproduced with permission.¹¹⁵ Copyright 2012, Wiley. (d) Structural modifications for enhanced light absorption, antireflection, light recovery, and omnidirectional designs. Reproduced with permission.¹¹⁷ Copyright 2023, Wiley.

To maximize solar energy utilization in SSG systems, minimizing reflectance from PTM surfaces is crucial. Nanoscale multistage PTMs, leveraging flat band structures, trap light within optical cavities for multiple reflections, and enhance absorption.¹¹⁸ Optimizing cavity size and light wavelength enables broadband solar spectrum absorption, while anti-reflective coatings and spectrally selective absorbers reduce reflectance and increase solar concentration.¹¹⁹ This improved light absorption enhances light-to-heat conversion, generating the heat needed for steam production. However, heat conduction to bulk water remains a challenge. Volumetric suspension and interfacial systems improve evaporation efficiency and localize heat at the PTM surface. Structural modifications, such as increased mass fraction and reduced thickness, minimize conduction losses. Double-layered photothermal and insulation materials (e.g., melamine, porous carbon foam, or cotton cloth) further reduce heat transfer, highlighting the importance of thermal management. Chao et al. reported contact-free evaporation to avoid heat losses and designed an environment-friendly wood-derived aerogel evaporator to enhance the potential of the SSG system which has good mechanical strength, and thermal stability (Fig. 5a).¹²⁰ An inverted single-stage solar water purifier, with a top selective absorber and a hydrophobic nanostructured copper honeycomb condenser, eliminates vapor-induced optical losses while enhancing heat transfer and condensation efficiency (Fig. 5b).¹²¹ Selective solar absorbers have also been reported for enhanced condensation and reduced emittance as in Fig. 5c.¹²² Researchers redefined traditional 3D evaporators by employing highly thermally conductive materials to bridge evaporation surfaces and bulk water, replacing conventional insulators. This approach enables efficient heat transfer, significantly increasing evaporation rates (Fig. 5d).¹²³ A 3D photothermal spherical evaporator surpasses theoretical efficiency limits with omnidirectional solar absorption and ambient heat capture, using a lightweight, insulating sphere coated with polyamide nanofibers and polypyrrole nanoparticles for efficient heating and water transport (Fig. 5e).¹²⁴

Fig. 5 (a) Graphical illustration of contact-free evaporation system. Reproduced with permission.¹²⁰ Copyright 2020, American Chemical Society. (b) Invert-structured solar evaporator. Reproduced with permission.¹²¹ Copyright 2021, Elsevier. (c) Schematic and reflectance spectra of W-WO_x multilayer films for selective solar absorption. Reproduced with permission.¹²² Copyright 2018, Royal Society of Chemistry. (d) An evaporator with insulation and thermal conduction supports with heat simulation. Reproduced with permission.¹²³ Copyrights 2021, Elsevier. (e) Heat energy flow over the surface of a spherical evaporator. Reproduced with permission.¹²⁴ Copyright 2021, Wiley-VCH.

2.5 Phase change materials for thermal packs

Various amorphous and crystalline materials are utilized in desalination applications, with some exhibiting unique phase stability at ambient temperature. These materials' distinct physical, electrical, and optical properties are leveraged in SSG systems. Phase Change Materials (PCMs) with high latent heat of fusion can phase transition from solid to liquid or liquid to solid. An ingenious incorporation of PCMs could potentially store the solar irradiation and waste heat in the SSG system during the day and then potentially release it in the absence of sunlight.^125,126 The basic working principle of PCMs is illustrated in Fig. 6a. PCMs are highly stable, pollution-free, and have small volume changes.¹²⁷ Therefore, the combination of interface evaporation and waste heat recovery is a prospective strategy to optimize SSG system performance.¹²⁶ Organic PCMs have proved to be most promising to overcome intermittent solar irradiation by storing heat energy during day time and releasing it as stored energy during night-time for continuous solar thermal desalination. An MXene/polydopamine/iron-oxide with microcapsule PCM (n-docosane) with natural wood as a substrate provided a high evaporation rate under multiple cycles. The use of latent heat supported evaporation in semi-cloudy weather as well (Fig. 6b).¹²⁸ A 3D hydrogel/fiber evaporator with solid-solid phase change materials (SPCMs) to enhance photothermal desalination. The hydrogel-infused fiber matrix improves solar absorption (T = 0.1%, R = 10%), with T being the transmission, and R being the reflection of light through the material and mechanical strength. SPCMs absorb energy during daylight and release latent heat at night, maintaining evaporation at a rate of 3.80 kg m⁻² h⁻¹ in light and 0.31 kg m⁻² h⁻¹ in darkness (Fig. 6c).¹²⁹ Another such solar evaporator has been recently reported to deal with the solar fluctuation challenge while using polyethylene glycol as a PCM (Fig. 6d).¹³⁰ Among various PCMs, paraffin is known as the most efficient owing to its stability over a wide range of temperatures, cost-effectiveness, easy accessibility, and high heat of fusion. It also freezes easily without undergoing super-cooling.¹³¹ A study by Irshad et al. reports stable desalination during intermittent sunlight with 93% photothermal conversion efficiency, and an evaporation rate of 2.13 kg m⁻² h⁻¹ under one sun illumination and minimum evaporation enthalpy 0.85 kg m⁻² h⁻¹ (Fig. 6e).¹³²

Fig. 6 (a) Working principle of PCMs. (b) Latent heat recovery based evaporator endowing magnetic and 2D materials with PCM for long-term sustainable evaporation. Reproduced with permission.¹²⁸ Copyright 2022, American Chemical Society. (c) 3D evaporation with solid PCMs for intermittent sunlight conditions. Reproduced with permission.¹²⁹ Copyright 2022, Wiley-VCH. (d) Solar evaporator with polyethylene glycol as a PCM for solar driven steam generation during natural solar fluctuations. Reproduced with permission.¹³⁰ Copyright 2024, Wiley-VCH. (e) Solar-to-vapor conversion at air–water interface for SrCoO₃@PPy embedded with PCM with recorded surface temperatures, evaporation rate, efficiency, and mineral ion concentration based on experimental results under solar irradiation. Reproduced with permission.¹³² Copyright 2023, Wiley.

However, the PCMs have certain limitations that include poor thermal stability, low thermal conductivity, corrosiveness, high flammability, and significant variations in volume and pressure during phase transitions. These drawbacks hinder their commercial viability in the market.¹³³ In addition, the possible leakage of PCM from evaporator to bulk water can destroy the system compromising its stability and durability. Therefore, the PCMs cannot be the most effective solution to intermittent sunlight and utilizing latent heat. More efficient solutions are needed for SSG systems.

2.6 Zero-liquid discharge strategies

Advancements in solar steam production have focused on mitigating salt accumulation, a key challenge in the zero-liquid discharge (ZLD) strategy for desalination. As salt ions accumulate on the evaporator surface, surface ion concentration, and water salinity increase. A concentration gradient drives ions back into the water, reducing surface salt levels. The interplay between salt ion movement and diffusion influences crystalline salt nucleation, while osmotic pressure and salt crystals in porous media hinder vapor diffusion. Continuous salt buildup under ZLD conditions limits evaporation efficiency, with evaporation rates from saltwater declining exponentially as salt concentration increases.^154–157 The mass flow rate D of density-driven convective salt ion movement and diffusion of salt ions for solar thermal desalination is described by the following equation:where D_diff and D_conv are the mass flow rate due to diffusion and convection, respectively; μ is the porosity; A is the cross-sectional area; α_d and α_c is the coefficient of diffusion and convection, respectively; l is the height of evaporator; S_evap and S₀ are the concentration of salt ions on the evaporator and bulk water, respectively; and ρ_evap and ρ₀ are the densities of saline solutions over the evaporating surface and bulk water, respectively. Traditional salt removal methods hinder evaporation, highlighting the need for novel solar absorption materials with anti-fouling and salt-blocking properties, achieved by balancing salt rejection and diffusion to maintain optimal evaporation performance. Li et al. designed a conical frustum-type evaporator that ingeniously directs salt crystallization to the edges, preventing deposition on the evaporating surface. This innovative design enables efficient desalination of high-salinity solutions (10–25 wt%) while minimizing heat losses as compared to similar cylindrical evaporators as illustrated in Fig. 7a¹³⁴ Yang et al. (Fig. 7b)140 developed a 3D salt evaporation structure that improves evaporation rates and desalination efficiency for brine with over 10% salinity, using mass transport bridges and multiscale channels to enhance salt removal and self-regeneration (Fig. 7b).¹³⁵ Dang et al. developed such a salt-resistant evaporator with carbonized rattan that naturally contains aligned regular multiscale channels and reported 91% photothermal efficiency, with stability for one week in 20 wt% NaCl brine solution under one-sun irradiation (Fig. 7c).¹³⁶ A salt-resistant low-cost aerogel having long-term potential for working conditions achieved 83.4% photothermal conversion efficiency and an evaporation rate of 1.40 kg m⁻² h⁻¹ under one sun irradiation (Fig. 7d).¹³⁷ A Janus evaporator with a super hydrophobic top layer and super hydrophilic bottom layer (Fig. 7e),¹³⁸ salt-resistant evaporator with reduced enthalpy (Fig. 7f),¹³⁹ salt-preventing dual-mode evaporator with minimal heat losses (Fig. 7g),¹³⁴ and evaporators with highly efficient theoretical yield with salt resistance (Fig. 7h)¹⁴⁰ have also been reported. Fig. 7i (ref. 141) shows the salt capacitance model developed by J. Shi et al. and it produced 6 kg m⁻² fresh water per day. With the help of the circuit model, another crucial design factor called salt capacitance, which is connected to the transient heat transfer and salt charging process is identified. Theoretically, by adjusting this value, a low water channel ratio can be used to attain both a high energy efficiency and an effective antifouling feature.

Fig. 7 (a) Schematic of conical 3D hydrogel evaporator for edge-preferential salt deposition. Reproduced with permission.¹⁸⁶ Copyright 2021, Wiley-VCH. (b) Schematic of a 3D salt evaporation structure and its working principle illustrating salt rejection and increased evaporation.¹³⁵ Reproduced with permission. Copyright 2022, Nature Communications. (c) Schematic of salt-resistant evaporator with well-aligned water channels for diffusion flux. Reproduced with permission.¹³⁶ Copyright 2023, Elsevier. (d) Schematic of salt tolerant aerogel. Reproduced with permission.¹³⁷ Copyright 2021, Elsevier. (e) Schematic of super-hydrophobic/super-hydrophilic Janus evaporator for salt collection at edges and steam generation. Reproduced with permission.¹³⁸ Copyright 2023, American Chemical Society. (f) A hydrogel evaporator with a Janus sponge-like structure for highly efficient salt resistance with digital pictures of salt removal for 3 h. Reproduced with permission.¹³⁹ Copyright 2023, Elsevier. (g) Schematic of the salt-resistant evaporator with dual mode of water transportation with salt resistance performance for 200 hours. Reproduced with permission.¹³⁴ Copyright 2022, American Chemical Society. (h) Salt resistance of a lignocellulosic-based sponge. Reproduced with permission.¹⁴⁰ Copyright 2023, Springer. (i) Schematic of the salt capacitance model including PTM, water path, and capacitance layer of salt.¹⁴¹ Reproduced with permission. Copyright 2021, Cell Press.

2.7 Reduced enthalpy

The water molecules that evaporate at a molecular level, need some threshold energy to break the bonds and convert to steam. It is a great idea to reduce this energy either by forming weak or fewer hydrogen bonds. There have been several studies devoted to the activation of water sites and Yu et al. were the first to investigate this, they proposed that the evaporation enthalpy could be reduced if the content of intermediate water is increased and water clusters are formed in a molecular network. The reported evaporator achieved 3.2 kg m⁻² h⁻¹, much higher than the other reported evaporators of its time.¹⁰⁸ Wei et al. integrated the work of Yu et al.^108,142 with Hu et al.¹⁴³ to provide a thermodynamic analysis of vaporization enthalpy which is linked with the potential energy of water.¹⁴⁴ The latent heat of vaporization ΔH_vap is the difference between the enthalpy of the liquid phase and gas phase of water.

U _1-water is the intermolecular potential energy between different water molecules; N is the number of water molecules; k_B is the Boltzmann constant; T is the temperature.¹⁰⁸ Considering the water at 1 atm pressure and room temperature, the intermolecular potential becomes negligible and water vapor can be considered as ideal gas,¹⁴⁵ thus:

Since pressure is such a physical factor that can substantially impact the behavior of any thermodynamic system, we take into account the effect of additional pressure due to surface tension on the enthalpy of evaporation. Specifically, the pressure increases driven by surface tension denoted as ΔP, is incorporated into our analysis.

σ is the surface tension; θ denotes the contact angle; and r is the pore radius.¹⁴⁶Eqn (1), (2), and (3) collectively be presented as:

With this theoretical analysis, eqn (1) and (4) prove that the water evaporation enthalpy is linked with the intermolecular potential energy of water, and therefore the binding potential energy should be reduced for reduced evaporation enthalpy. This is a viable approach to facilitate the formation of weak hydrogen or non-hydrogen bonds, which can be readily dissociated (partially or entirely) during the water evaporation process. Consequently, we propose the following strategies to enhance water evaporation:

(1) Increasing the concentration of intermediate water.

(2) Constructing abundant water clusters.

(3) Promoting the formation of capillary water that partially occupies pore space, thereby mitigating pore saturation.

These are schematically illustrated in Fig. 8a–c¹⁴⁴

Fig. 8 Schematic illustration of a water activation system, showcasing three distinct states: (a) intermediate water: characterized by weakened hydrogen bonds, facilitating easier evaporation, (b) clustered water: exhibiting collective evaporation of multiple water molecules, enhancing vaporization efficiency, (c) capillary water: featuring increased evaporation surface area, allowing for more rapid moisture release. Reproduced with permission.¹⁴⁴ Copyright 2023, Wiley-VCH.

2.8 Advancements in condenser designs

Efficient condensers are essential for maximizing fresh water production in SSG systems, which use solar energy to produce clean water and help address the global water scarcity crisis. The design and optimization of these condensers are crucial for achieving high condensate yields. Advancements in condenser designs for SSG systems have led to various configurations, materials, and geometries aimed at improving condensation efficiency, reducing costs, and enhancing durability. These innovations include transparent, metallic, and multi-stage condensers, each offering distinct benefits to optimize water collection and heat recovery. Gravity-assisted sloped condensers (Fig. 9a),¹⁴⁷ spherical condensers (Fig. 9b),¹⁴⁸ and dome-shaped condensers (Fig. 9c)¹⁴⁹ have been widely used in SSG systems. However, the use of metal condensers can be highly effective over glass condensers as an internal metal condenser reported a nearly 100% condensation rate with quick water collection (Fig. 9d).¹⁵⁰ Condensers play a key role in releasing latent heat during the vapor-to-liquid phase change, which is usually lost to the environment. By capturing and recycling this heat, evaporation rates can be enhanced. To achieve this, researchers have developed multistage condensers, which have shown a notable freshwater collection rate of 3 kg m⁻² h⁻¹, highlighting the potential of latent heat recovery (Fig. 9e).¹⁵¹ A well-designed system should maximize the number of evaporation–condensation cycles in multi-stage condensers before heat loss to the environment occurs. Additionally, condenser designs with fans can enhance vapor flow, doubling evaporation rates compared to fan-less designs. However, this comes at the cost of increased operational and capital expenses due to the added energy requirements.

Fig. 9 (a) Reduced graphene oxide foam with pyramidal condenser, an outdoor setup is shown. Reproduced with permission.¹⁴⁷ Copyright 2019, Wiley-VCH. (b) Digital images of spherical condenser with and without solar irradiation for solar thermal evaporation. Reproduced with permission.¹⁴⁸ Copyright 2018, Wiley-VCH. (c) Outdoor experiment with 3D hollow compressible evaporator in a dome-shaped condenser. Reproduced with permission.¹⁴⁹ Copyright 2021, Wiley-VCH. (d) A built-in single stage metal condenser with nearly 100% condensation rate. Reproduced with permission.¹⁵⁰ Copyright 2023, American Chemical Society. (e) Multi-stage design with transparent top insulator and metal (aluminium) heat sink. Reproduced with permission.¹⁵¹ Copyright 2018, Springer Nature.

Moreover, in SSG systems, glass condensers are often employed due to their transparency and durability, however, they are significantly more expensive compared to polymer-based alternatives. For cost-effective and scalable SSG systems, the use of cheaper polymeric materials as condensing units is a practical alternative. Polymers offer advantages such as lower cost, lighter weight, and easier manufacturability, making them more suitable for large-scale applications. However, tradeoffs include their lower thermal stability, potential degradation under prolonged UV exposure, and lower optical clarity, which could impact condensation efficiency. Despite these limitations, advancements in polymer engineering, such as the development of UV-stable and thermally robust polymers, can help bridge the gap, offering a viable solution for cost-effective SSG systems without compromising performance significantly.

3. An analysis of the theoretical and experimental efficiency

The photothermal solar-to-vapor conversion efficiency is a crucial metric for evaluating the performance of SSG systems. However, conventional efficiency calculations may overlook significant factors, such as energy absorption due to temperature gradients, environmental influences, and energy losses through conduction, convection, and radiation. To accurately assess the efficiency of these systems, a comprehensive approach must consider these factors and differentiate between various evaporator surfaces and configurations. This article delves into the complexities of photothermal efficiency calculations, exploring the limitations of traditional methods and providing an understanding of solar steam generation efficiency for beginners who are just stepping into this research. The solar-to-vapor conversion efficiency^152,153 to date is calculated using the following equation:where η_sv is the solar-to-vapor conversion efficiency, ṁ is the evaporation rate per unit area, C_opt is the optical concentration, q_i is the irradiated solar intensity (1 kW m⁻² for one sun irradiation), h_LV is the net enthalpy including latent heat of vaporization λ, specific heat capacity of water, C (4200 Jkg⁻¹ K⁻¹) and temperature difference ΔT from water to vapors.ṁh_LV is indeed the heat used to evaporate water and C_optq_i is the solar irradiation energy received by the evaporator. The energy losses to the environment through conduction, convection, and radiation are significant and are calculated following natural science laws. The heat transfer rate due to conduction, Q̇_conduction to the bulk water present beneath the substrate can be calculated¹⁵⁴ using Fourier's law of conduction:where A represents the cross-sectional area, k is the thermal conductivity of underlying bulk water, and ΔT is the gradient of temperature with a time interval of one hour between two thermocouples a distance Δl apart. The negative sign indicates that heat is lost in the opposite direction to the temperature gradient. It is worth noting that the thermal conductivity coefficient k is taken as 0.6 Wm⁻¹ K⁻¹ while calculating the conduction heat losses, it is used to represent the bulk water beneath the evaporator, which is assumed to be at room temperature. This value is closely associated with the kinetic energy of water molecules and is widely adopted in the context of SSG systems. Most SSG systems indeed feature an evaporator in direct contact with the bulk water, however, various evaporators have been reported that are not in direct contact with bulk water and therefore the k value cannot be the same for such systems and it needs to be modified according to a conductive medium such as air for different evaporators. It should also be noted that the surface temperatures of the evaporators are different so the temperature difference should be accurately calculated for conduction heat loss calculations.

In addition, the convective air currents heat transfer rate from the photothermal system to the ambient environment, and these losses can be calculated using the equation:

Q̇_convection = hA (T_s – T_f) (9)

Here h is the convective heat transfer coefficient, A is the area of the object, T_s is the surface temperature and T_f is the surrounding fluid temperature. The convective heat transfer coefficient is commonly taken as 10 Wm⁻² K⁻¹ for SSG systems to represent natural convection between the evaporator surface and the surrounding air under typical ambient conditions. This value is based on the assumption of minimal airflow and standard room temperature, which are conditions often observed in experimental setups. Since most SSG systems operate in these controlled environments, this coefficient is widely used as a standard reference in heat loss calculations. However, the ambient temperature is not the same during different times of the year and for the experiments performed at different places. Moreover, the surface temperature of the photothermal material strongly interacts with ambient temperature modifying the convection heat. It is also known that the convective heat transfer coefficient is dependent on fluid velocity, turbulence, and surface geometry.¹⁵⁵

Moreover, the radiative losses are another significant factor affecting the efficiency of the system, and it can be calculated using Stefan–Boltzmann Law:

Q̇_radiation = εAσ (T_s − T_∞)⁴ (10)

where ε is the emissivity of the PTM, σ is the Stefan–Boltzmann constant (5.669 × 10⁻⁸ Wm⁻² K⁻⁴), T_∞ is the adjacent environment temperature.

The photothermal conversion efficiency is dependent on the photothermal material, and it should be calculated as the energy generated by the photothermal material (which is used to evaporate water and some of it is lost to the environment) to the total solar irradiation received by the photothermal material. Therefore, it can be rewritten as:

Q̇ _net is the total energy generated by the photothermal material which is used to evaporate water under light and dark conditions besides heat lost to the ambient.

Q̇_net = (Q̇_light + Q̇_dark) − (Q̇_conduction + Q̇_convection + Q̇_radiation). (12)

The vapor-to-water conversion efficiency η_vw can be calculated using.

where m_e is the evaporation rate under ambient conditions and m_c is the mass flux rate of condensed vapors when their pressure is increased from ambient in a separately mounted condenser. The decreased mass flux depends on the condensing capacity of condensers and m_c iswhere D_v is the diffused vapor mass in air which can be determined by the physical properties of steam and water, V_sat−e and V_sat−c are the saturation concentration of vapor at the evaporation surface and condensation surface, respectively and d is the distance between the evaporator surface and diffusion surface.

The conventional efficiency calculation method overlooks crucial factors, such as energy absorption due to temperature gradients between the PTM and ambient environment. Accurate energy efficiency calculations require consideration of a single energy source before and after system operation. However, in SSG, only solar irradiation is considered, neglecting evaporation via ambient moisture absorption (sorption) and dark conditions (e.g., using PCMs). Research demonstrates that environmental factors cannot be simply accounted for by subtracting dark evaporation rates. Consequently, the widely used efficiency formula is flawed, with an undefined upper limit exceeding 100%. Furthermore, the formula fails to account for evaporator surface dimensions (1D, 2D, or 3D) and size/shape dependencies, hindering comparisons across research studies and practical large-scale applications.

Significant research has been dedicated to enhancing SSG performance, particularly aiming to exceed the theoretical benchmarks for solar evaporation rate (υ) and solar-to-vapor conversion efficiency (η_sv). However, numerous studies have reported ultrahigh values of υ and η_sv surpassing these limits. Is it truly possible to achieve performance beyond the theoretical thresholds?

In SSG systems, claims of exceeding theoretical efficiency limits warrant critical evaluation to ensure accuracy and reliability. Such claims may result from unaccounted energy inputs, or very general assumptions in efficiency calculations such as the heat coefficients and ambient conditions. Similarly, inaccuracies in determining solar input intensity, material absorption, or condensation losses may lead to overestimations. Besides, glass condensers, which are commonly used in SSG systems for their transparency and durability, add to the system's cost, making large-scale deployment less practical. Polymeric condensers, as a cost-effective alternative, offer advantages like affordability and lighter weight but introduce tradeoffs such as lower thermal stability and potential degradation under prolonged UV exposure. Addressing these challenges requires rigorous analysis of experimental setups, measurement protocols, and material performance. By identifying and accounting for inconsistencies, researchers can enhance the credibility of reported findings while advancing the development of practical and scalable SSG systems.

In addition, the efficiency is fundamentally constrained by the maximum solar energy that can be absorbed and converted into vapor, by the first and second laws of thermodynamics. Efficiency cannot exceed 100%, and it is inherently limited by the temperature difference between the solar-heated evaporator (high-temperature heat source) and the surrounding environment (low-temperature heat sink). This imposes an upper limit η, accounting for inevitable energy losses. Several common errors contribute to overestimations of η, including: (1) Mis-estimating the evaporation area, where nano-scale structures significantly increase the effective surface area compared to the macroscopic one used in calculations. (2) Treating the vaporization enthalpy as a constant, even though it varies with factors such as temperature, chemical interactions, and material nanostructure. (3) Ignoring optical and thermal losses. (4) Neglecting external energy contributions (e.g., environmental radiation or wind energy) in input energy calculations. (5) Applying the commonly used efficiency equation incorrectly in multi-stage configurations, which requires redefining efficiency metrics like solar-to-water conversion efficiency to account for complex processes.

For υ, surpassing the theoretical values is achievable by reducing vaporization enthalpy, leveraging environmental energy, or recycling latent heat. However, a higher evaporation rate does not inherently lead to increased freshwater production. Since the ultimate goal is to maximize condensate yield rather than vapor generation, emphasis should be placed on optimizing υ and enhancing condensation efficiency (vapor-to-water conversion efficiency). High-temperature vapor is more effective for condensation, making it crucial to design systems that integrate high-efficiency evaporation with effective condensation mechanisms. Consequently, achieving superior SSG desalination technology demands a holistic approach that balances rapid evaporation with advanced condensation systems.

4. Factors influencing vapor-to-water conversion efficiency

Maximizing condensation efficiency in SSG systems relies on optimizing the condenser's performance. Efficient condensation is critical for converting vapors into liquid water, and surface temperature control, timely defogging, and scalability play a vital role in achieving this. However, conventional cooling methods, such as flowing water or using fans, have limitations, including reduced solar light transmission and energy requirements.^156,157 Thus, exploring further strategies seems essential for enhancing condensation kinetics and overall system efficiency.

4.1 Surface temperature optimization

The condensation process begins when water vapors strike the condenser surface, cooling down to form water droplets. Surface cooling can significantly accelerate water collection efficiency. Tiwari et al. introduced flowing freshwater across the condenser surface, doubling the water collection rate.¹⁵⁶ However, this method hinders solar light absorption, decreasing evaporation rates over time. Although it increases efficiency by 10%, it's not recommended due to reduced evaporation rates. Moreover, the condensers are typically made of glass or acrylic, therefore the condenser surface suffers from 5–20% optical loss and water droplet accumulation further reduces the efficiency by around 35%.^157,158 Therefore, it is suggested to increase the temperature gradient between the condensing surface and water vapors which can enhance water collection efficiency.

4.2 Defogging the condensing surface

The widely used transparent condenser surface experiences optical losses due to incomplete light transmission and heavy scattering by water droplets. The droplets in the central region are closer to the light source and therefore these do not condense easily and create fog which resists condensation thereby reducing the reported theoretical efficiency of a system.^159,160 For these reasons, the water droplets accumulation on the surface of the condenser needs serious consideration. The position of water droplets under wettability conditions can be adjusted from top to side surfaces with the use of small heating wires on the condenser surface so the water droplets would move to side walls from top and light transmission will be least affected during continuous vapor to water conversion. The passive heating wires are illuminated through sunlight and can be set outdoors, these can increase the water collection efficiency to 1.25 times.¹⁶¹ The use of small heating wires also increases efficiency by 1.25 times, highlighting the need for innovative defogging techniques to maintain optimal condenser performance.¹⁶² In designing, smart SSG systems, researchers should not only consider the cooling of steam into the water but also pay attention to designing commercially viable condensers with defogging ability to provide reliable SSG systems that can attain 100% efficiency.

4.3 Expanding the surface area

The condenser's surface area plays a crucial role in vapor condensation and water collection efficiency. Increasing the surface area allows more water vapors to condense into droplets, resulting in higher water production in less time. Condensers for SSG systems have not been discussed in detail but research on solar stills, which use similar glass condensers, has shown that expanding the surface area can significantly enhance productivity. Studies have demonstrated that increasing the condenser area boosts condensation efficiency. For instance, Bhardwaj et al. reported an increase in condensation efficiency with a 2.2 m² increase in condenser area.¹⁶³ Experiments have also shown that using multi-slope glass condensers can increase the area, with one condenser receiving light and others acting as cooling surfaces.¹⁶⁴ R. Bhardwaj et al. achieved significant increases in water production by expanding the condenser surface area. They reported a 65% increase in laboratory settings and over 50% increase under sunlight with a 7.5 times increase in condenser surface area. Furthermore, they found that high heat input with a 6.5 times larger condensation area can yield five times more water production, and this can be further increased to eight times with external condenser cooling.¹⁶³

5. Environmental and operational constraints on higher condensate yields

Water collection efficiency in photothermal evaporation systems depends on the evaporation rate (m_e) and condensed vapor mass flux rate (m_c). While evaporation rate optimization is extensive, m_c is often neglected, impacting practical applications. It is also influenced by atmospheric conditions, geographical location, and system design, making careful consideration of these factors crucial for accurate measurement and reliable results.

5.1 Humidity

Humidity has a significant impact on photothermal evaporation systems, affecting their overall performance and water collection efficiency. When humidity levels are high, the evaporation rate slows down as excess moisture in the air hinders water vapor from escaping. Conversely, high humidity also leads to an increase in the condensed vapor mass flux rate, as more water vapor condenses on surfaces.¹⁶⁵ Zhong et al. analyzed relative humidity for a bilayer structure and reported that the increased relative humidity in the environment depressed the vapor diffusion rate from the second layer into the surroundings resulting in a decreased evaporation rate.¹⁶⁶ Pan et al. reported the influence of optimization of condensation architecture on one evaporation performance and different heating wires were used on the condenser for defogging purposes. The water collection rate is improved by active and passive heating wires, however, the passive heating wires are economical.¹⁶¹ Cheng et al. reported a thermally insulated evaporator with a passive condenser and the rate of freshwater collection was increased with the nanoscale antifogging agent.¹⁶⁵ Apart from humidity, evaporation causes some vapors to create fog on the surface of the condenser which affects the evaporation rate as little sunlight can pass through it.¹⁶⁷

5.2 Wind speed

The Earth receives a vast amount of solar energy from the sun, with approximately 1366 watts per square meter (Wm⁻²) of solar irradiance striking the planet's surface. However, this energy is not evenly distributed. The equatorial regions receive the most direct sunlight, while the polar regions receive indirect sunlight due to the Earth's tilt. Additionally, the atmosphere and clouds absorb and scatter a significant portion of the solar radiation, further influencing the distribution. Wind plays a crucial role in redistributing this solar energy across the globe. As the sun heats the surface, it warms the air closest to the ground, causing it to rise and create circulation patterns. The wind then transports this warm air to other regions, distributing the solar energy and influencing local climates. Wind is responsible for transferring approximately 40% of the solar energy from the equatorial regions towards the poles, making it a vital component in shaping our planet's climate and weather patterns. Wind speed affects evaporation efficiency, with rates increasing between 0.05–0.5 ms⁻¹ and remaining constant thereafter, while high wind speeds can further enhance efficiency with suitable porous materials and condenser designs.¹⁶⁸

5.3 Temperature

Temperature is the average kinetic energy that breaks bonds between water molecules and triggers evaporation. The phenomenon occurs at ambient temperature also but the rate of evaporation linearly increases with the increase in temperature. At higher temperatures, more molecules move faster therefore, more molecules get enough energy to form vapors. Therefore, at high temperatures, the evaporation rate is faster, and conversely, at low temperatures, the evaporation rate is slower. For a photothermal solar steam generation system, the temperature difference between bulk water or photothermal surface and the ambient temperature affects the rate of evaporation. If these temperatures are raised, then thermal losses will linearly decrease due to a small temperature gradient between the SSG system and the environment or bulk water. However, the temperature of bulk water has comparatively a larger effect on evaporation efficiency because the heat transfer conversion coefficient of water is larger than air. The effect of temperature is quantified by the equation

η_evap = −615 + 1.839 T_w + 0.4796 T_a (15)

here, η_evap is the evaporation efficiency of water, T_w is the temperature of bulk water and T_a is the temperature of ambient air initially.¹⁶⁹

5.4 Miscellaneous factors

The refractive index (n) and condenser design significantly impact solar steam generation, with higher n values and optimized condenser geometries enhancing light absorption, heat production, and evaporation rates, while also influencing condensation rates and overall system efficiency.^{161,170–172} SSG technology also faces challenges such as intermittent solar energy, PCM leakage, and geographical limitations, but ongoing research aims to address these issues and optimize system design, efficiency, and reliability to harness the full potential of solar energy for sustainable water desalination^173–175 In addition, the water collection efficiency of an SSG system is predominantly influenced by internal and external pressure dynamics. Besides, the glass condensers that are widely used in SSG systems are fragile as compared to the advanced reported condensing materials such as transparent polycarbonate or poly (methyl methacrylate). These materials can overcome the challenges of glass condensers with high optical transmittance, thermal stability, and pressure resistance to ensure reliable long-term performance.^{63,163,164,176–180} Furthermore, solar geometry including latitude, solar declination angle, solar zenith angle, and surface azimuth angle, is another factor that significantly impacts the performance of solar steam generation systems, and optimizing these factors is crucial to -maximize freshwater production efficiency. Researchers have indeed made significant advancements in enhancing evaporation rates through innovative techniques, including 3D evaporators, environmental energy utilization, multistage distillation, hydrogels, and optimized water supply rates, achieving ultra-high evaporation efficiencies and fluxes beyond theoretical limits but the careful consideration of effects of physical factors is crucial for practical applications of SSG systems.

6. Applications of solar thermal interfacial evaporation system

Apart from the production of freshwater from SSG, considerable efforts have been made for hybrid integrated systems to enhance the utilization of solar energy. For instance, 3D-Felt fabric evaporators equipped with concave array structures modified with hydrophilic MXene and a chitosan-dopamine coating have been developed, enabling simultaneous saltwater desalination and electricity generation by recovering thermal energy. This system offers several advantages, including pre-heating saltwater, minimal light absorption, reduced humidity, accelerated evaporation, and effective heat management. By enhancing the efficient utilization of solar energy, this approach outperforms traditional evaporation systems in heat capture (Fig. 10a).¹⁸¹ Besides, few SSG systems can simultaneously perform disinfection activity that could be very useful for sterilization and medical applications. Li et al. investigated this activity using a 3D-porous nano-hydrogel network embedded with MoS₂ nanoflowers (Fig. 10b).¹⁸² This esteemed technology seems to address the food crisis as well because wheat cultivation such as shown in Fig. 10c (ref. 183) and other crop aggregation multifunctional evaporators have been reported. Moreover, the potential for hydrogen generation through photocatalytic and photothermal processes has attracted significant research attention. Ho's group demonstrated an innovative approach by integrating optical and thermal management in a photothermal catalytic gel (PTCG) designed for the simultaneous production of fresh water and hydrogen gas. The PTCG features broadband absorption, aligned porous micro-channels for efficient mass transport, high thermal conductivity, and excellent photocatalytic performance. This system enables parallel water collection and hydrogen production through enhanced solar thermal desalination and photocatalysis (Fig. 10d).¹⁸⁴ The salt collection had been enormously reported for solar evaporators such as Fu et al. reported a nanofluidic photothermal textile umbrella by anchoring MoS₂ nanosheets on cotton fabrics in an asymmetric manner such as shown in Fig. 10e.¹⁸⁵ SSG systems can provide various useful biproducts beside seawater if the fundamental challenges are overcome and the efficiency practically cross theoretical limitations.

Fig. 10 (a) Solar driven water evaporation with electricity generation. Reproduced with permission.¹⁸¹ Copyright 2022, Wiley-VCH. (b) Schematic of hydrogel-based evaporator for simultaneous desalination and disinfection. Reproduced with permission.¹⁸² Copyright 2019, American Chemical Society. (c) Marangoni-driven biomimetic evaporator with wheat cultivation as side application. Reproduced with permission.¹⁸³ Copyright 2023, Elsevier. (d) Hydrogen production besides desalination. Reproduced with permission.¹⁸⁴ Copyright 2020, Wiley-VCH. (e) Salt collection through an umbrella shaped evaporator. Reproduced with permission.¹⁸⁵ Copyright 2021, American Chemical Society.

7. Challenges

Solar-thermal evaporation faces several challenges that limit its efficiency and scalability. Key issues include achieving high photothermal conversion efficiency while minimizing energy losses through thermal conduction and radiation. Conventional materials often have suboptimal solar absorption rates, and even advanced materials struggle to balance light absorption with thermal management.

Additionally, fouling from salts and contaminants can clog evaporative surfaces, reducing long-term functionality and efficiency. Environmental factors, such as ambient temperature and humidity, further complicate control over evaporation rates, making it difficult to ensure consistent performance. Cost-effectiveness and material durability under prolonged exposure to sunlight and water are also critical hurdles in moving towards scalable, sustainable solutions for solar-thermal evaporation, as illustrated in Fig. 11.

Fig. 11 The main challenges are highlighted to meet the gap of high throughput yield of solar thermal interfacial evaporation systems beyond theoretical efficiency.

7.1 System-related challenges

7.1.1. System robustness and longevity. While considerable focus has been placed on innovating at the material level, enhancements at the component level have not received equivalent attention. The interactions among the key elements of the solar evaporator system—such as the bulk water supplier, thermal insulation layers, the environmental context, and the condenser—require substantial optimization. Despite more thorough investigations into aspects like thermal insulation and water supply, there remains significant room for improvement in these areas. For the development of advanced SSG systems, a deep understanding of thermal diffusion kinetics and water transport dynamics is crucial. Moreover, the applicability of SSG in treating water sources contaminated with volatile organic compounds (VOCs) is limited. The presence of VOCs poses a challenge, as these compounds may evaporate alongside water, leading to secondary contamination of the condensed water. This highlights the need for comprehensive system-level approaches that not only consider material properties but also optimize component interactions to address both efficiency and environmental safety concerns.

7.1.2 Condensate yield. Despite significant research on maximizing solar energy absorption and water evaporation rates, the critical output of solar thermal desalination - fresh water collection - remains overlooked. Condensation rate and condenser design are crucial factors, yet thermal losses hinder efficient water collection.^175–182 To address this, optimized condenser structures must be developed, integrating thermal insulation, light absorption, and efficient coolant use to enhance vapor production and freshwater yield.

7.1.3 Mineral deposition and accumulation. Mineral deposition on the surface of the evaporator with continuous desalination is a major hurdle, severely limiting solar steam generation and water production. The combined effects of advection and diffusion accelerate salt deposition until saturation, leading to rapid performance decline.¹⁶³ However, we believe that enhancing substrate porosity, minimizing water channel thickness, and cleverly managing evaporated water can help suppress mineral accumulation, ensuring the long-term stability and efficiency of PTMs in SSG systems.

7.1.4 Microscopic thermal transfer. The interaction between water and the material must be further investigated at the molecular level despite its exceptional photothermal conversion efficiency. The development of computational models is essential for clarification of the intricate mechanism of thermal transfer because it is challenging for current experimental methods to entirely understand the mechanism at the molecular level. Additionally, the methods of preparation for the used PTMs can be quite detrimental to the environment and cost a lot of money. The following efforts should concentrate on the production of affordable, robust materials with long-term stability in order to support the commercialization of this revolutionary innovation.

7.2 Environmental challenges

The solar steam generation system is a complex assembly comprising various components such as photothermal materials, substrates, and a condensing structure, all of which are integral to its function. The effectiveness of this system is critically dependent on the rates of water evaporation and collection, which are, in turn, significantly influenced by a multitude of environmental variables in practical, real-world settings. Key among these environmental variables is relative humidity, which not only varies with climatic conditions but also is dynamically altered by the steam generated by the system itself. This interaction complicates the maintenance of consistent humidity levels around the photothermal surface and the surrounding air, thereby impacting system performance. Furthermore, because experimental conditions cannot fully replicate these dynamic real-world interactions, comparing the efficiency of different solar steam generation systems using various materials and designs becomes problematic. Additionally, wind speed introduces another layer of complexity. It varies throughout the day, between seasons, and according to geographical location, with coastal areas generally experiencing stronger winds than more sheltered inland or desert regions. Such variability can affect the heat transfer processes crucial for efficient steam generation. Compounding these challenges are the empirical fluctuations in temperature and pressure, which are natural and ubiquitous across different parts of the globe. These fluctuations further strain system performance, hindering its potential to address global water and energy crises. To overcome these challenges, a more robust and adaptive design is crucial for optimal solar steam generation.

7.3 Measurement and standardization challenges

Despite significant advancements in solar evaporation efficiency, reaching beyond 100%, the lack of standardized measurement methodologies hinders practical progress. A comprehensive formula incorporating key factors such as ambient conditions, thermal conductivity, stability, altitude, latitude, refractive index, transmittance with broadband light absorption, thermal losses, evaporation and condensation rates, and solar irradiance is urgently needed to bridge the gap between experimental success and commercialization of SSG systems. Currently, the inability to differentiate quantized thermal energy to and from ambient and water complicates accurate efficiency measurements. Establishing widely recognized standards for measurements and calculations would greatly benefit the solar thermal desalination community. Therefore, we strongly recommend developing a standardized efficiency evaluation method based on thermodynamic analysis and integrated environmental factors.

8. Conclusion

SSG systems have proven their potential in addressing global water scarcity and enabling resource recovery. Their ability to harness solar energy for multifunctional applications has brought them to the forefront of sustainable technologies. While theoretical advancements have pushed the efficiency of PTMs to 100%, experimental condensate yields still fall significantly short of these predictions. This discrepancy arises from fundamental thermodynamic constraints and unaccounted factors, such as energy losses and errors in efficiency estimation, emphasizing the need for a more nuanced understanding of the practical limits of SSG systems, as we reported in details of section 3 of this article.

Over the years, researchers have explored numerous strategies to enhance condensate yields and improve condensation efficiency such as surface engineering, flat band structures, optimized tunable porous channels to improve heat and mass transfer, and reduced enthalpy designs with ZLD for modified condensers that hold promise for improving the performance of SSG systems and addressing freshwater scarcity. However, inaccuracies in measuring evaporation area or applying efficiency equations in multi-stage configurations can lead to overestimated solar-to-vapor conversion efficiency (η) beyond its theoretical maximum. Similarly, while higher solar evaporation rates (υ) may be achievable through latent heat recycling or leveraging environmental energy, these gains do not directly translate to increased freshwater yields unless accompanied by efficient condensation mechanisms.

Despite these advancements, significant challenges remain in achieving consistently high condensate yields and bridging the gap between theoretical and practical efficiencies. Many strategies fall short due to issues like material degradation, insufficient durability, and limited scalability for real-world applications. Furthermore, the inability to precisely quantify and mitigate physical losses—such as heat dissipation, co-precipitation of impurities, and suboptimal vapor dynamics—hampers progress. Future efforts must focus on developing advanced materials with superior thermal and chemical stability, refining heat recovery mechanisms, and crafting designs that integrate latent heat reuse seamlessly. Additionally, accurate modeling and experimental validation of efficiency parameters are critical to address the underlying physical and operational inefficiencies. A robust scientific framework that addresses fundamental thermodynamic limits and operational inefficiencies will unlock the full potential of SSG technology, making it a viable solution for sustainable water recovery and resource management.

Data availability

The data supporting the findings of this study are available from the corresponding authors upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Department of Science and Technology of Hubei Province, China (No. 2024BAB096).

References

1. C. Forrestal, P. Xu and Z. Ren, Energy Environ. Sci., 2012, 5, 7161–7167.
2. A. Steinfeld, Sol. Energy, 2005, 78, 603–615.
3. M. Rodell, J. S. Famiglietti, D. N. Wiese, J. T. Reager, H. K. Beaudoing, F. W. Landerer and M.-H. Lo, Nature, 2018, 557, 651–659.
4. A. Ghermandi and R. Messalem, Desalin. Water Treat., 2009, 7, 285–296.
5. A. M. Delgado-Torres and L. García-Rodríguez, Renew. Sustain. Energy Rev., 2012, 16, 44–53.
6. V. D. Dao, Sci. Total Environ., 2021, 759, 143490.
7. L. Zhu, M. Gao, C. K. N. Peh and G. W. Ho, Nano Energy, 2019, 57, 507–518.
8. M. Gao, P. K. N. Connor and G. W. Ho, Energy Environ. Sci., 2016, 9, 3151–3160.
9. X. Lan, P. Wan, Y. Guo, X. Xu, L. Fan, W. Liu and J. Wang, J. Mater. Chem. A, 2022, 10, 11784–11792.
10. A. K. Pandey, R. R. Kumar, B. Kalidasan, I. A. Laghari, M. Samykano, R. Kothari, A. M. Abusorrah, K. Sharma and V. V Tyagi, J. Environ. Manage., 2021, 297, 113300.
11. T. Ding, Y. Zhou, W. L. Ong and G. W. Ho, Mater. Today, 2021, 42, 178–191.
12. W. He, G. Huang and C. N. Markides, Desalination, 2023, 552, 116424.
13. C. Ge, D. Xu, H. Du, X. Zhang, Z. Song, H. Zhao, Z. Chen, B. Song, Z. Shen, C. Gao, G. Yan, W. Xu and J. Fang, Small Methods, 2023, 7, 2300227.
14. K. Chen, L. Li and J. Zhang, ACS Appl. Mater. Interfaces, 2021, 13, 59518–59526.
15. Y. Zhang, S. K. Ravi and S. C. Tan, Nano Energy, 2019, 65, 104006.
16. P. Wang, Environ. Sci.:Nano, 2018, 5, 1078–1089.
17. Y. Zeng, K. Wang, J. Yao and H. Wang, Chem. Eng. Sci., 2014, 116, 704–709.
18. P. Zhang, Q. Liao, T. Zhang, H. Cheng, Y. Huang, C. Yang, C. Li, L. Jiang and L. Qu, Nano Energy, 2018, 46, 415–422.
19. Q. Zhang, X. Xiao, G. Wang, X. Ming, X. Liu, H. Wang, H. Yang, W. Xu and X. Wang, J. Mater. Chem. A, 2018, 6, 17212–17219.
20. B. Xu, M. Ganesan, R. K. Devi, X. Ruan, W. Chen, C. C. Lin, H. Chang, E. Lizundia, A. K. An and S. K. Ravi, Adv. Mater., 2025, 37, 2406666.
21. C. Onggowarsito, A. Feng, S. Mao, L. N. Nguyen, J. Xu and Q. Fu, ChemSusChem, 2022, 15, e202201543.
22. Q. Chen, Z. Pei, Y. Xu, Z. Li, Y. Yang, Y. Wei and Y. Ji, Chem. Sci., 2018, 9, 623–628.
23. Z. Liu, L. Liu, H. Lu, P. Zhan, W. Du, M. Wan and Z. Wang, Sci. Rep., 2017, 7, 43803.
24. H. M. Kim, H. K. Lee, J. M. Lee, S. Kim and J. P. Kim, Prog. Org. Coat., 2022, 170, 106978.
25. L. X. Guo, M. H. Liu, S. M. Sayed, B. P. Lin, P. Keller, X. Q. Zhang, Y. Sun and H. Yang, Chem. Sci., 2016, 7, 4400–4406.
26. L. Liu, M. H. Liu, L. L. Deng, B. P. Lin and H. Yang, J. Am. Chem. Soc., 2017, 139, 11333–11336.
27. I. Ibrahim, D. H. Seo, A. M. McDonagh, H. K. Shon and L. Tijing, Desalination, 2021, 500, 114853.
28. Y. Zhao, H. Pan, Y. Lou, X. Qiu, J. Zhu and C. Burda, J. Am. Chem. Soc., 2009, 131, 4253–4261.
29. M. Umlauff, J. Hoffmann, H. Kalt, W. Langbein, J. M. Hvam, M. Scholl, J. Söllner, M. Heuken, B. Jobst and D. Hommel, Phys. Rev. B, 1998, 57, 1390.
30. N. Yu, Y. Hu, X. Wang, G. Liu, Z. Wang, Z. Liu, Q. Tian, M. Zhu, X. Shi and Z. Chen, Nanoscale, 2017, 9, 9148–9159.
31. W. Choi, A. Termin and M. R. Hoffmann, J. Phys. Chem., 2002, 98, 13669–13679.
32. L. Zhang, B. Tang, J. Wu, R. Li and P. Wang, Adv. Mater., 2015, 27, 4889–4894.
33. J. Yang, J. Choi, D. Bang, E. Kim, E. K. Lim, H. Park, J. S. Suh, K. Lee, K. H. Yoo and E. K. Kim, Angew. Chem. Int. Ed., 2011, 50, 441–444.
34. Q. Jiang, H. G. Derami, D. Ghim, S. Cao, Y. S. Jun and S. Singamaneni, J. Mater. Chem. A, 2017, 5, 18397–18402.
35. G. T. Spence, S. S. Lo, C. Ke, H. Destecroix, A. P. Davis, G. V Hartland and B. D. Smith, Chem. - Eur. J., 2014, 20, 12628–12635.
36. S. Guha, G. K. Shaw, T. M. Mitcham, R. R. Bouchard and B. D. Smith, Chem. Commun., 2016, 52, 120–123.
37. L. Tang, F. Zhang, F. Yu, W. Sun, M. Song, X. Chen, X. Zhang and X. Sun, Biomaterials, 2017, 129, 28–36.
38. X. Song and J. W. Foley, Dyes Pigm., 2008, 78, 60–64.
39. H. S. Jung, P. Verwilst, A. Sharma, J. Shin, J. L. Sessler and J. S. Kim, Chem. Soc. Rev., 2018, 47, 2280–2297.
40. C. Ji, W. Cheng, Q. Yuan, K. Müllen and M. Yin, Acc. Chem. Res., 2019, 52, 2266–2277.
41. C. Liu, S. Zhang, J. Li, J. Wei, K. Müllen and M. Yin, Angew. Chem., Int. Ed., 2019, 58, 1638–1642.
42. R. Jiang, S. Cheng, L. Shao, Q. Ruan and J. Wang, J. Phys. Chem. C, 2013, 117, 8909–8915.
43. M. Camerin, G. Jori, L. Della Ciana, S. Fabbroni, S. Bonacchi, M. Montalti and L. Prodi, Photochem. Photobiol. Sci., 2009, 8, 1422–1431.
44. B. D. Rihter, M. E. Kenney, W. E. Ford and M. A. J. Rodgers, J. Am. Chem. Soc., 1993, 115, 8146–8152.
45. M. Camerin, S. Rello-Varona, A. Villanueva, M. A. J. Rodgers and G. Jori, Lasers Surg. Med., 2009, 41, 665–673.
46. S. Liu, X. Zhou, H. Zhang, H. Ou, J. W. Y. Lam, Y. Liu, L. Shi, D. Ding and B. Z. Tang, J. Am. Chem. Soc., 2019, 141, 5359–5368.
47. Z. Zhao, C. Chen, W. Wu, F. Wang, L. Du, X. Zhang, Y. Xiong, X. He, Y. Cai and R. T. K. Kwok, Nat. Commun., 2019, 10, 768.
48. G. Chen, J. Sun, Q. Peng, Q. Sun, G. Wang, Y. Cai, X. Gu, Z. Shuai and B. Z. Tang, Adv. Mater., 2020, 32, 1908537.
49. F. Wang, D. Wei, Y. Li, T. Chen, P. Mu, H. Sun, Z. Zhu, W. Liang and A. Li, J. Mater. Chem. A, 2019, 7, 18311–18317.
50. Y. Yang, R. Zhao, T. Zhang, K. Zhao, P. Xiao, Y. Ma, P. M. Ajayan, G. Shi and Y. Chen, ACS Nano, 2018, 12, 829–835.
51. C. Wang, J. Wang, Z. Li, K. Xu, T. Lei and W. Wang, J. Mater. Chem. A, 2020, 8, 9528–9535.
52. Y. Chen, H. Qiu, X. Li, Q. Tong, M. Jensen, Q. Li and N. Wang, Appl. Surf. Sci., 2022, 582, 152483.
53. L. Hao, N. Liu, H. Bai, P. He, R. Niu and J. Gong, J. Colloid Interface Sci., 2022, 608, 840–852.
54. X. Hou, H. Sun, F. Dong, H. Wang and Z. Bian, Chemosphere, 2023, 315, 137732.
55. C. Zhang, P. Xiao, F. Ni, L. Yan, Q. Liu, D. Zhang, J. Gu, W. Wang and T. Chen, ACS Sustain. Chem. Eng., 2020, 8, 5328–5337.
56. F. Yu, J. Wang, L. Yan, Z. Guo, Z. Chen, M. S. Irshad, J. Qian, T. Mei and X. Wang, Sol. Energy Mater. Sol. Cells, 2022, 243, 111738.
57. M. Aizudin, M. K. Sudha, R. Goei, S. Kuang Lua, R. P. Pottammel, A. I. Y. Tok and E. H. Ang, Chem.–Eur. J., 2023, 29, e202203184.
58. F. Yu, Z. Chen, Z. Guo, M. S. Irshad, L. Yu, J. Qian, T. Mei and X. Wang, ACS Sustain. Chem. Eng., 2020, 8, 7139–7149.
59. P. Srimuk, J. Halim, J. Lee, Q. Tao, J. Rosen and V. Presser, ACS Sustain. Chem. Eng., 2018, 6, 3739–3747.
60. J. Liu, X. Chen, H. Yang, J. Tang, R. Miao, K. Liu and Y. Fang, Mater. Chem. Front., 2021, 5, 1953–1961.
61. J. Liu, H. Yang, K. Liu, R. Miao and Y. Fang, ChemSusChem, 2020, 13, 749–755.
62. I. Mahar, F. H. Memon, J. W. Lee, K. H. Kim, R. Ahmed, F. Soomro, F. Rehman, A. A. Memon, K. H. Thebo and K. H. Choi, Membranes, 2021, 11, 869.
63. M. S. Irshad, X. Wang, N. Arshad, M. Q. Javed, T. Shamim, Z. Guo, H. R. Li, J. Wang and T. Mei, Environ. Sci.:Nano, 2022, 9, 1685–1698.
64. J. Huang, Y. He, L. Wang, Y. Huang and B. Jiang, Energy Convers. Manag., 2017, 132, 452–459.
65. M. Ye, J. Jia, Z. Wu, C. Qian, R. Chen, P. G. O'Brien, W. Sun, Y. Dong and G. A. Ozin, Adv. Energy Mater., 2017, 7, 1601811.
66. X. Song, H. Song, N. Xu, H. Yang, L. Zhou, L. Yu, J. Zhu, J. Xu and K. Chen, J. Mater. Chem. A, 2018, 6, 22976–22986.
67. K. Yin, S. Yang, J. Wu, Y. Li, D. Chu, J. He and J. A. Duan, J. Mater. Chem. A, 2019, 7, 8361–8367.
68. J. Wang, Y. Li, L. Deng, N. Wei, Y. Weng, S. Dong, D. Qi, J. Qiu, X. Chen and T. Wu, Adv. Mater., 2017, 29, 1603730.
69. Y. Shi, O. Ilic, H. A. Atwater and J. R. Greer, Nat. Commun., 2021, 12, 2797.
70. H. Liu, T. Zou, X. Cui, M. Ding, B. Zhang, X. Yang, Y. Han, S. Nie, Y. Zhang and E. Wang, Chem. Eng. J., 2023, 468, 143619.
71. J. Chen, B. Li, G. Hu, R. Aleisa, S. Lei, F. Yang, D. Liu, F. Lyu, M. Wang and X. Ge, Nano Lett., 2020, 20, 6051–6058.
72. Z. Zhu, P. Mu, Y. Fan, W. Bai, Z. Zhang, H. Sun, W. Liang and A. Li, Eur. Polym. J., 2020, 126, 109560.
73. Y. Guo, X. Zhou, F. Zhao, J. Bae, B. Rosenberger and G. Yu, ACS Nano, 2019, 13, 7913–7919.
74. Y. Xu, H. Xu, Z. Zhu, H. Hou, J. Zuo, F. Cui, D. Liu and W. Wang, J. Mater. Chem. A, 2019, 7, 22296–22306.
75. S. Hakkim, M. Devassy and I. J. Creat, Res Thoughts, 2020, 8, 2320–2882.
76. A. Mittal, R. Brajpuriya and R. Gupta, Mater. Today Sustain., 2023, 21, 100319.
77. L. Sun, L. Li, X. An and X. Qian, Molecules, 2021, 26, 4449.
78. Z. Li, M. Zheng, N. Wei, Y. Lin, W. Chu, R. Xu, H. Wang, J. Tian and H. Cui, Sol. Energy Mater. Sol. Cells, 2020, 205, 110254.
79. J. Fang, Q. Liu, W. Zhang, J. Gu, Y. Su, H. Su, C. Guo and D. Zhang, J. Mater. Chem. A, 2017, 5, 17817–17821.
80. L. Zhou, Y. Tan, J. Wang, W. Xu, Y. Yuan, W. Cai, S. Zhu and J. Zhu, Nat. Photonics, 2016, 10, 393–398.
81. P. Qiao, J. Wu, H. Li, Y. Xu, L. Ren, K. Lin and W. Zhou, ACS Appl. Mater. Interfaces, 2019, 11, 7066–7073.
82. B. Wang, S. Zhang, J. Meng, L. Min, J. Luo, Z. Zhu, H. Bao, R. Zang, S. Deng and F. Zhang, Adv. Mater., 2021, 33, 2103999.
83. S. P. Prakoso, S. S. Sun, R. Saleh, Y. T. Tao and C. L. Wang, ACS Appl. Mater. Interfaces, 2021, 13, 38365–38374.
84. Y. Cui, J. Liu, Z. Li, M. Ji, M. Zhao, M. Shen, X. Han, T. Jia, C. Li and Y. Wang, Adv. Funct. Mater., 2021, 31, 2106247.
85. Y. Wang, C. Wang, X. Song, M. Huang, S. K. Megarajan, S. F. Shaukat and H. Jiang, J. Mater. Chem. A, 2018, 6, 9874–9881.
86. P. Han, W. Martens, E. R. Waclawik, S. Sarina and H. Zhu, Part. Part. Syst. Charact., 2018, 35, 1700489.
87. S. Sarina, E. R. Waclawik and H. Zhu, Green Chem., 2013, 15, 1814–1833.
88. Y. Wang, X. Wu, T. Gao, Y. Lu, X. Yang, G. Y. Chen, G. Owens and H. Xu, Nano Energy, 2021, 5, 2100053.
89. Y. Shi, R. Li, Y. Jin, S. Zhuo, L. Shi, J. Chang, S. Hong, K. C. Ng and P. Wang, Joule, 2018, 2, 1171–1186.
90. H. Liang, Q. Liao, N. Chen, Y. Liang, G. Lv, P. Zhang, B. Lu and L. Qu, Angew. Chem., 2019, 131, 19217–19222.
91. J. Cai and L. Qi, Mater. Horiz., 2015, 2, 37–53.
92. M. K. Hedayati and M. Elbahri, Materials, 2016, 9, 497.
93. J. Li, X. Gao, B. Liu, Q. Feng, X. B. Li, M. Y. Huang, Z. Liu, J. Zhang, C. H. Tung and L. Z. Wu, J. Am. Chem. Soc., 2016, 138, 3954–3957.
94. P. Sun, W. Zhang, I. Zada, Y. Zhang, J. Gu, Q. Liu, H. Su, D. Pantelic, B. Jelenkovic and D. Zhang, ACS Appl. Mater. Interfaces, 2019, 12, 2171–2179.
95. S. Chen, Z. Sun, W. Xiang, C. Shen, Z. Wang, X. Jia, J. Sun and C. J. Liu, Nano Energy, 2020, 76, 104998.
96. Y. Hu, H. Ma, M. Wu, T. Lin, H. Yao, F. Liu, H. Cheng and L. Qu, Nat. Commun., 2022, 13, 4335.
97. P. Cao, L. Zhao, J. Zhang, L. Zhang, P. Yuan, Y. Zhang and Q. Li, ACS Appl. Mater. Interfaces, 2021, 13, 19109–19116.
98. W. Li, Z. Li, K. Bertelsmann and D. E. Fan, Adv. Mater., 2019, 31, 1900720.
99. M. Xie, P. Zhang, Y. Cao, Y. Yan, Z. Wang and C. Jin, Clean Water, 2023, 6, 12.
100. Y. Bian, Y. Shen, K. Tang, Q. Du, L. Hao, D. Liu, J. Hao, D. Zhou, X. Wang and H. Zhang, Glob. Challenges, 2019, 3, 1900040.
101. A. M. Saleque, S. Ma, A. K. Thakur, R. Saidur, T. K. Han, M. I. Hossain, W. Qarony, Y. Ma, R. Sathyamurthy and Y. H. Tsang, Desalination, 2023, 554, 116488.
102. C. Shi, X. Zhang, A. Nilghaz, Z. Wu, T. Wang, B. Zhu, G. Tang, B. Su and J. Tian, Chem. Eng. J., 2023, 455, 140361.
103. A. G. Slater and A. I. Cooper, Science, 2015, 348, aaa8075.
104. Z. Lei, S. Zhu, X. Sun, S. Yu, X. Liu, K. Liang, X. Zhang, L. Qu, L. Wang and X. Zhang, Adv. Funct. Mater., 2022, 32, 2205790.
105. K. Zhou, L. Yin, K. Gong and Q. Wu, Chem. Eng. J., 2023, 464, 142616.
106. L. Li, N. He, S. Yang, Q. Zhang, H. Zhang, B. Wang, T. Dong, H. Wang, B. Jiang and D. Tang, EcoMat, 2023, 5, e12282.
107. F. Wu, S. Qiang, X. D. Zhu, W. Jiao, L. Liu, J. Yu, Y. T. Liu and B. Ding, Nano–Micro Lett., 2023, 15, 1–15.
108. F. Zhao, X. Zhou, Y. Shi, X. Qian, M. Alexander, X. Zhao, S. Mendez, R. Yang, L. Qu and G. Yu, Nat. Nanotechnol., 2018, 13, 489–495.
109. X. Li, J. Li, J. Lu, N. Xu, C. Chen, X. Min, B. Zhu, H. Li, L. Zhou and S. Zhu, Joule, 2018, 2, 1331–1338.
110. Y. Guo, J. Bae, Z. Fang, P. Li, F. Zhao and G. Yu, Chem. Rev., 2020, 120, 7642–7707.
111. J. Sun, S. Zhao, X. Wang, W. Kong, W. Li, S. Wang, S. Liu and S. Nie, Mater. Today, 2024, 80, 619–647.
112. A. Hu, Y. Zhao, Q. Hu, C. Chen, X. Lu, S. Cui and B. Liu, Nat. Commun., 2024, 15, 9484.
113. W. He, L. Zhou, M. Wang, Y. Cao, X. Chen and X. Hou, Sci. Bull., 2021, 66, 1472–1483.
114. X. Liu, D. D. Mishra, X. Wang, H. Peng and C. Hu, J. Mater. Chem. A, 2020, 8, 17907–17937.
115. H. Tong, S. Ouyang, Y. Bi, N. Umezawa and M. Oshikiri, Adv. Mater., 2012, 24, 229–251.
116. P. Vukusic, J. R. Sambles and C. R. Lawrence, Proc. Roy. Soc. Lond. B Biol. Sci., 2004, 271, S237–S239.
117. M. S. Irshad, N. Arshad, G. Maqsood, M. S. Asghar, P. Wu, N. Mushtaq, M. A. K. Y. Shah, L. Lin, X. Li, I. Ahmed, T. Mei, M. Sabir, H. Wang, P. V. Pham, H. Li, H. X. Nang, V. Dao, J. Guo and X. Wang, Small, 2024, 2407280.
118. X. Wu, G. Y. Chen, G. Owens, D. Chu and H. Xu, Mater. Today Energy, 2019, 12, 277–296.
119. F. Cao, K. McEnaney, G. Chen and Z. Ren, Energy Environ. Sci., 2014, 7, 1615–1627.
120. W. Chao, X. Sun, Y. Li, G. Cao, R. Wang, C. Wang and S.-H. Ho, ACS Appl. Mater. Interfaces, 2020, 12, 22387–22397.
121. F. Wang, N. Xu, W. Zhao, L. Zhou, P. Zhu, X. Wang, B. Zhu and J. Zhu, Joule, 2021, 5, 1602–1612.
122. W. Wang, H. Wen, S. Ling, Z. Li, J. Su and C. Wang, J. Mater. Chem. A, 2018, 6, 15690–15700.
123. Y. Wang, X. Wu, P. Wu, J. Zhao, X. Yang, G. Owens and H. Xu, Sci. Bull., 2021, 66, 2479–2488.
124. H. Liu, H. Ye, M. Gao, Q. Li, Z. Liu, A. Xie, L. Zhu, G. W. Ho and S. Chen, Adv. Sci., 2021, 8, 2101232.
125. Z. Wei, N. Arshad, C. Hui, M. S. Irshad, N. Mushtaq, S. Hussain, M. Shah, S. Z. H. Naqvi, M. Rizwan, N. Shahzad, H. Li, Y. Lu and X. Wang, Nanomaterials, 2022, 12, 3206.
126. X. Li, W. Xie and J. Zhu, Adv. Sci., 2022, 9, 2104181.
127. Z. Guo, W. Zhou, N. Arshad, Z. Zhang, D. Yan, M. S. Irshad, L. Yu and X. Wang, Carbon, 2022, 186, 19–27.
128. Z. Zheng, W. Li, H. Liu and X. Wang, ACS Appl. Mater. Interfaces, 2022, 14, 50966–50981.
129. Y. Liang, J. Guo, J. Li, J. Mao, A. Xie, L. Zhu and S. Chen, Adv. Sustainable Syst., 2022, 6, 2200236.
130. S. Lei, Z. Li, H. Xu, J. Liu, W. Cai, L. Song and Y. Hu, Sol. RRL, 2024, 8, 1–11.
131. Z. Guo, Z. Chen, Z. Shi, J. Qian, J. Li, T. Mei, J. Wang, X. Wang and P. Shen, Sol. Energy Mater. Sol. Cells, 2020, 204, 110227.
132. M. S. Irshad, N. Arshad, J. Zhang, C. Song, N. Mushtaq, M. Alomar, T. Shamim, V. D. Dao, H. Wang, X. Wang and H. Zhang, Adv. Energy Sustainability Res., 2023, 4, 2200158.
133. V. V Tyagi, K. Chopra, R. K. Sharma, A. K. Pandey, S. K. Tyagi, M. S. Ahmad, A. Sarı and R. Kothari, Sol. Energy Mater. Sol. Cells, 2022, 234, 111392.
134. P. Yuan, C. Men, L. Zhao, P. Cao, Z. Yang, Y. Niu, Y. Zhang, Y. Yu and Q. Li, ACS Appl. Mater. Interfaces, 2022, 14, 15549–15557.
135. K. Yang, T. Pan, S. Dang, Q. Gan and Y. Han, Nat. Commun., 2022, 13, 1–8.
136. C. Dang, X. Zhang, L. Huang, G. Xu, L. Gu, X. Cao and M. Zhu, Desalination, 2023, 550, 116408.
137. D. You, W. Yang, Y. Zhao, H. Yu, Y. Ma, D. Lin, Q. Pan and S. Song, Sol. Energy, 2021, 227, 303–311.
138. C. Shi, Z. Wu, Y. Li, X. Zhang, Y. Xu, A. Chen, C. Yan, Y. Shi, T. Wang and B. Su, ACS Appl. Mater. Interfaces, 2023, 15, 23971–23979.
139. A. Chu, M. Yang, J. Chen, J. Zhao, J. Fang, Z. Yang and H. Li, Green Energy Environ., 2024, 9, 1698–1710.
140. P. Qiao, M. Yang, J. Zhao, Z. Wang, G. Li, J. Fang, Y. Qian and H. Li, Cellulose, 2024, 31, 1–12.
141. J. Shi, X. Luo, Z. Liu, J. Fan, Z. Luo, C. Zhao, X. Gu and H. Bao, Cell Rep. Phys. Sci., 2021, 2, 100330.
142. X. Zhou, F. Zhao, Y. Guo, B. Rosenberger and G. Yu, Sci. Adv., 2019, 5, eaaw5484.
143. C. Jia, Y. Li, Z. Yang, G. Chen, Y. Yao, F. Jiang, Y. Kuang, G. Pastel, H. Xie and B. Yang, Joule, 2017, 1, 588–599.
144. D. Wei, C. Wang, J. Zhang, H. Zhao, Y. Asakura, M. Eguchi, X. Xu and Y. Yamauchi, Adv. Mater., 2023, 35, 2212100.
145. J. Wang and T. Hou, J. Chem. Theory Comput., 2011, 7, 2151–2165.
146. S. Gao, X. Dong, J. Huang, J. Dong, F. Di Maggio, S. Wang, F. Guo, T. Zhu, Z. Chen and Y. Lai, Glob. Challenges, 2019, 3, 1800117.
147. H. S. Mohaisen, J. A. Esfahani and M. B. Ayani, Renewable Energy, 2021, 168, 170–180.
148. Z. C. Xiong, Y. J. Zhu, D. D. Qin, F. F. Chen and R. L. Yang, Small, 2018, 14, 1–12.
149. T. Gao, X. Wu, Y. Wang, G. Owens and H. Xu, Sol. RRL, 2021, 5, 2100053.
150. T. A. Wani, V. Gupta, P. Garg and A. Bera, ACS Sustain. Chem. Eng., 2023, 26, 9595–9600.
151. E. Chiavazzo, M. Morciano, F. Viglino, M. Fasano and P. Asinari, Nat. Sustain., 2018, 1, 763–772.
152. N. Arshad, I. Ahmed, M. S. Irshad, H. R. Li, X. Wang, S. Ahmad, M. Sharaf, M. Firdausi, M. Zaindin and M. Atif, Nanomaterials, 2020, 10, 2510.
153. J. Xu, Z. Wang, C. Chang, B. Fu, P. Tao, C. Song, W. Shang and T. Deng, Desalination, 2020, 484, 114423.
154. J. H. Schon, Therm. Prop., 2011, 8, 337–372.
155. J. Sarkar, Renew. Sustain. Energy Rev., 2011, 15, 3271–3277.
156. G. Tiwari and V. S. V. B. Rao, Desalination, 1984, 49, 231–241.
157. S. A. Lawrence, S. P. Das Gupta and G. Tiwari, Energy Convers. Manag., 1990, 30, 277–285.
158. V. Velmurugan, M. Gopalakrishnan, R. Raghu and K. Srithar, Energy Convers. Manag., 2008, 49, 2602–2608.
159. M. D. Mulroe, B. R. Srijanto, S. F. Ahmadi, C. P. Collier and J. B. Boreyko, ACS Nano, 2017, 11, 8499–8510.
160. G. Ni, S. H. Zandavi, S. M. Javid, S. V Boriskina, T. A. Cooper and G. Chen, Energy Environ. Sci., 2018, 11, 1510–1519.
161. C. Pan, Y. Yang, M. Xie, Q. Deng, X. Cheng, X. Wang, S. Zhao, Y. Wei and W. Que, Membranes, 2022, 12, 899.
162. Y. Xu, J. Yin, J. Wang and X. Wang, Rev. Adv. Mater. Sci., 2019, 58, 226–247.
163. R. Bhardwaj, M. V. ten Kortenaar and R. F. Mudde, Appl. Energy, 2015, 154, 480–490.
164. S. K. Patel and K. V. Modi, J. Clean. Prod., 2020, 268, 122260.
165. S. Cheng, Z. Sun, Y. Wu, P. Gao, J. He, Z. Yin, L. Liu and G. Li, J. Mater. Chem. A, 2021, 9, 22428–22439.
166. J. Zhong, C. Huang, D. Wu and Z. Lin, Int. J. Heat Mass Transfer, 2019, 128, 860–864.
167. Y. Chen, Y. Wang, J. Xu, M. R. Ibn Raihan, B. Guo, G. Yang, M. Li, H. Bao and H. Xu, Sol. RRL, 2022, 6, 2200202.
168. A. F. Muftah, M. A. Alghoul, A. Fudholi, M. M. Abdul-Majeed and K. Sopian, Renew. Sustain. Energy Rev., 2014, 32, 430–447.
169. J. Zhong, C. Huang and D. Wu, Appl. Therm. Eng., 2018, 144, 331–341.
170. M. Keshavarz Hedayati and M. Elbahri, Materials, 2016, 9, 497.
171. H. Fattahi Juybari, H. B. Parmar, A. D. Alshubbar, K. L. Young and D. M. Warsinger, Desalination, 2023, 545, 116129.
172. W. Han, J. Gao, J. Yu, R. Wang and Z. Xu, Desalination, 2022, 544, 116132.
173. R. Perez, K. R. Rábago, M. Trahan, L. Rawlings, B. Norris, T. Hoff, M. Putnam and M. Perez, Energy Policy, 2016, 96, 27–35.
174. M. Fripp, Environ. Sci. Technol., 2012, 46, 6371–6378.
175. G. He, A. P. Avrin, J. H. Nelson, J. Johnston, A. Mileva, J. Tian and D. M. Kammen, Environ. Sci. Technol., 2016, 50, 5467–5473.
176. X. Liu, D. D. Mishra, X. Wang, H. Peng and C. Hu, J. Mater. Chem. A, 2020, 8, 17907–17937.
177. Y. Kuang, C. Chen, S. He, E. M. Hitz, Y. Wang, W. Gan, R. Mi and L. Hu, Adv. Mater., 2019, 31, 1900498.
178. M. S. Irshad, N. Arshad and X. Wang, Glob. Challenges, 2021, 5, 2000055.
179. Y. Xia, Y. Kang, Z. Wang, S. Yuan, Y. Li, L. Gao, H. Wang and X. Zhang, J. Mater. Chem. A, 2021, 9, 6612–6633.
180. H. Fu, M. Dai, H. Song, X. Hou, F. Riaz, S. Li, K. Yang, I. Ali, C. Peng and M. Sultan, Energies, 2021, 14, 7050.
181. Z. Lei, B. Hu, P. Zhu, X. Wang and B. Xu, Nano Energy, 2024, 122, 109307.
182. Y. Li, M. Zhao, Y. Xu, L. Chen, T. Jiang, W. Jiang, S. Yang and Y. Wang, J. Mater. Chem. A, 2019, 7, 26769–26775.
183. Y. Chen, J. Yang, L. Zhu, S. Wang, X. Jia, Y. Li, D. Shao, L. Feng and H. Song, Desalination, 2023, 548, 116287.
184. M. Gao, C. K. Peh, L. Zhu, G. Yilmaz and G. W. Ho, Adv. Energy Mater., 2020, 10, 2000925.
185. H. Peng, D. Wang and S. Fu, ACS Appl. Mater. Interfaces, 2021, 13, 47549–47559.
186. L. Li, N. He, B. Jiang, K. Yu, Q. Zhang, H. Zhang, D. Tang and Y. Song, Adv. Funct. Mater., 2021, 31, 2104380.

---

Footnote

† These authors contributed equally to this work.

---