Pathways and challenges for efficient solar-thermal desalination

Seawater Desalination by Interfacial Solar Vapor Generation Method Using Plasmonic Heating Nanocomposites

With the ever-growing demand in fresh water supply, great efforts have been devoted to developing sustainable systems which could generate fresh water continuously. Solar vapor generation is one of the promising strategies which comprise an unlimited energy source and efficient solar-to-heat generators for overcoming fresh water scarcity. However, current solar vapor generation systems suffer either from inefficient utilization of solar energy or an expensive fabrication process. In this paper, we introduced a nano-plasmonic approach, i.e., a floatable nanocompoiste where copper sulfide nanorods (Cu_2-xS NRs) are embedded in a polyvinyl alcohol (PVA) matrix, for solar-to-vapor generation. A high solar vapor generation efficiency of ~87% and water evaporation rate of 1.270 kg m⁻² h⁻¹ were achieved under simulated solar irradiation of 1 sun. With the illumination of natural daylight, seawater was purified using Cu_2-xS NRs-PVA gel, with high purity, as distilled drinking water. The plasmonic nanocomposites demonstrated here are easy to fabricate and highly efficient for solar vapor generation, illustrating a potential solution for future seawater desalination.

Three-dimensional self-floating foam composite impregnated with porous carbon and polyaniline for solar steam generation

A promising approach to resolving insufficient freshwater resources is utilizing solar energy for steam generation. Although various types of photothermal conversion materials have been developed, there are still some obstacles, such as complicated system structure fabrication and low energy utilization, that severely hinder their practical application. Herein, we designed and produced a self-floating porous carbon/polyaniline foam (PCPF) evaporator via impregnating melamine foam with porous carbon generated following the bottom-up pyrolytic method and polyaniline, followed by thermal treatment, for efficient solar steam generation. The PCPF obtained with a porous carbon (PC) to polyaniline (PAN) mass ratio of 3:5 (PCPF-3) exhibited a rich pore structure, good hydrophilicity, low thermal conductivity (0.0413 W m^-1 K^-1), and excellent light absorption (96.1%). Our results show that, without additional thermal insulators, the evaporation rate of PCPF-3 reached 1.496 kg m^-2 h^-1, and the photothermal conversion efficiency reached 87.3% under one sun irradiation. Furthermore, it also exhibited good durability and desalination performance. This type of environmentally friendly, low-cost, and stable photothermal conversion material could be used in water treatment and seawater desalination.

Volatile-Organic-Compound-Intercepting Solar Distillation Enabled by a Photothermal/Photocatalytic Nanofibrous Membrane with Dual-Scale Pores

Solar distillation is emerging as a robust and energy-effective tool for water purification and freshwater production. However, many water sources contain harmful volatile organic compounds (VOCs), which can evaporate through the photothermal evaporators and be collected together with distilled water, or even be enriched in the distilled water. In view of the penetration of volatile organic compounds, herein, we rationally demonstrate a dual-scale porous, photothermal/photocatalytic, flexible membrane for intercepting volatile organic compounds during solar distillation, which is based on a mesoporous oxygen-vacancy-rich TiO_2-x nanofibrous membrane (m-TiO_2-x NFM). The dual-scale porous structure was constructed by micrometer-sized interconnected tortuous pores formed by the accumulation of m-TiO_2-x nanofibers and nanometer-sized pores in the m-TiO_2-x individual nanofibers. Consequently, the membrane can sustainably in situ intercept VOCs by providing more photocatalytic reactive sites for collision (mainly by mesopores) and longer tortuous channels for prolonging VOC retention (mainly by micrometer-sized pores); thus, it results in less than 5% of phenol residual in distilled water. As a proof of concept, when the m-TiO_2-x NFM is employed to purify practical river water in an evaporation prototype under real solar irradiation, complex volatile natural organic contaminants can be effectively intercepted and the produced distilled water meets the drinking water standards of China. This development will promote the application prospects of solar distillation.

Flexible and Mildew-Resistant Wood-Derived Aerogel for Stable and Efficient Solar Desalination

Interfacial solar-driven evaporation is an effective and sustainable approach to convert solar energy to heat for desalination. However, maintaining a rapid evaporation rate and long-term stability is a critical challenge that needs to be addressed urgently to facilitate practical applications of this technology. Here, taking advantage of the inherent porosity, wettability, and low thermal conductivity of wood, we develop a flexible and mildew-resistant aerogel derived from natural wood as the substrate of evaporators for stable and efficient solar desalination. The wood-derived aerogel not only possesses good hydrophilicity, low thermal conductivity, and light weight, which are favorable to realize heat localization and efficient water evaporation, but also avoids fragility and mildew problems that seriously restricted long-term stability of wood-based evaporators. With high absorbance (>95%) of Au-rGO coating, the evaporator showed a high solar to vapor efficiency of 90.1% and an evaporation rate of 1.394 kg m^-2 h^-1 under 1 sun. During 120 h desalination, the evaporator transports brine for evaporation in the day and achieves dissolution of salt residues in the night, exhibiting a repetitive self-cleaning behavior, which enables the recovery of desalination stability. The multiple functionalities of the wood-derived aerogel make the evaporator promising as an attractive device for stable and continuous solar desalination.

Nanoconfined Fluids: What Can We Expect from Them?

Nanoconfined fluids (NCFs), which are confined in nanospaces, exhibit distinctive nanoscale effects, including surface effects, small-size effects, quantum effects, and others. The continuous medium hypothesis in fluid mechanics is not valid in this context because of the comparable characteristic length of spaces and molecular mean free path, and accordingly, the classical continuum theories developed for the bulk fluids usually cannot describe the mass and energy transport of NCFs. In this Perspective, we summarize the nanoscale effects on the thermodynamics, mass transport, flow dynamics, heat transfer, phase change, and energy transport of NCFs and highlight the related representative works. The applications of NCFs in the fields of membrane separation, oil and gas production, energy harvesting and storage, and biological engineering are especially indicated. Currently, the theoretical description framework of NCFs is still missing, and it is expected that this framework can be established by adopting the classical continuum theories with the consideration of nanoscale effects.

Latest development in salt removal from solar-driven interfacial saline water evaporators: Advanced strategies and challenges

Solar-driven interfacial water evaporation, which gets rid of the limitation of saline waters, enables to supply potable water in the worldwide, especially in remote areas where only solar energy and water are available. This technique has also exhibited great potential applications in fields such as seawater desalination, steam sterilization, and fuel production. However, the evaporation efficiency decreases during continuous operation in saline water due to the blockage of the solar absorber resulting from crystalline salt deposition. Therefore, it is still a great challenge to design a stable and efficient solar-driven interfacial saline water evaporator. Herein, a variety of structural designs and engineering strategies for salt removal of evaporators in the latest years were reviewed. We classified these strategies as remaining unsaturated evaporation of saline water, preventing salt ions from contacting the solar absorber, dissolving and/or migrating back of crystalline salts, and keeping salt crystallization away from evaporation area. Finally, the current challenges and future research opportunities were discussed. The purpose of this review was: (1) to provide ideas to solve the problem of the reduced efficiency causing by salt deposition during saline water evaporation and (2) to promote the application of solar-driven interfacial saline water evaporation technology by providing the latest achievements in structural designs for salt removal.

Photothermal Transduction Efficiencies of Plasmonic Group 4 Metal Nitride Nanocrystals

The photothermal transduction efficiencies of group 4 metal nitrides, TiN, ZrN, and HfN, at λ = 850 nm are reported, and the performance of these materials is compared to an Au nanorod benchmark. Transition metal nitride nanocrystals with an average diameter of ∼15 nm were prepared using a solid-state metathesis reaction. HfN exhibited the highest photothermal transduction efficiency of 65%, followed by ZrN (58%) and TiN (49%), which were all higher than those of the commercially purchased Au nanorods (43%). Computational studies performed using a finite element method showed HfN and Au to have the lowest and highest scattering cross section, respectively, which could be a contributing factor to the efficiency trends observed. Furthermore, the changes in temperature as a function of illumination intensity and solution concentration, as well as the cycling stability of the metal nitride solutions, were studied in detail.

Facile preparation of magnetic porous carbon monolith from waste corrugated cardboard box for solar steam generation and adsorption

Porous carbon monoliths (PCMs) were prepared from waste corrugated cardboard box (WCCB) via slurrying in FeCl₃ solution followed by molding and thermal treatment. The thermal process was analyzed by a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer. The evolution of physicochemical characteristics of PCMs was studied. The photothermal conversion and solar steam generation performances of the optimal sample (PCM_Fe/600) were evaluated. The adsorption properties of PCM_Fe/600 for methylene blue (MB) were investigated. Results showed that Fe³⁺ promoted the breaking of cellulose chains in WCCB, leading to the occurrence of pyrolysis of WCCB at lower temperatures and the reduction of activation energy by 76.63 kJ mol^-1. Char yield raised because volatile radicals were captured by FeCl₃-derived amorphous Fe(III) species, then involved in char formation. Amorphous Fe(III) continuously converted into Fe₃O₄ crystallites with carbonization temperature increasing from 400 to 700 °C, then α-Fe was formed at 800 °C via the carbothermal reduction of Fe₃O₄. FeCl₃ was favorable to the formation of a developed microporous structure. Surface area significantly increased with carbonization temperature increasing from 400 to 600 °C due to the removal of volatiles. The etching of carbon by Fe₃O₄ above 700 °C also led to the increase of surface area. PCM_Fe/600 exhibited higher optical absorption than other samples due to its high graphite degree and porosity. It also had excellent photothermal performance; thus, solar steam yield was 1.46 times that of the pure water with the assistance of PCM_Fe/600. PCM_Fe/600 in floating state was effective in adsorption of MB from water. Besides, the adsorption behavior fitted Langmuir model with a monolayer adsorption capacity reached up to 70.9 mg g^-1.

Structure Architecting for Salt-Rejecting Solar Interfacial Desalination to Achieve High-Performance Evaporation With In Situ Energy Generation

The past few years have witnessed a rapid development of solar-driven interfacial evaporation, a promising technology for low-cost water desalination. As of today, solar-to-steam conversion efficiencies close to 100% or even beyond the limit are becoming increasingly achievable in virtue of unique photothermal materials and structures. Herein, the cutting-edge approaches are summarized, and their mechanisms for photothermal structure architecting are uncovered in order to achieve ultrahigh conversion efficiency. Design principles to enhance evaporation performance and currently available salt-rejection strategies for long-term desalination are systematically investigated. The guidelines to utilize every component in solar desalination systems for simultaneous in situ energy generation are also revealed. Finally, opportunities and challenges for future works in this field are also discussed and concluded.

MOF water harvesters

The advancement of additional methods for freshwater generation is imperative to effectively address the global water shortage crisis. In this regard, extraction of the ubiquitous atmospheric moisture is a powerful strategy allowing for decentralized access to potable water. The energy requirements as well as the temporal and spatial restrictions of this approach can be substantially reduced if an appropriate sorbent is integrated in the atmospheric water generator. Recently, metal-organic frameworks (MOFs) have been successfully employed as sorbents to harvest water from air, making atmospheric water generation viable even in desert environments. Herein, the latest progress in the development of MOFs capable of extracting water from air and the design of atmospheric water harvesters deploying such MOFs are reviewed. Furthermore, future directions for this emerging field, encompassing both material and device improvements, are outlined.

Scaling Resistance in Nanophotonics-Enabled Solar Membrane Distillation

This study compares the scaling behavior of membrane distillation (MD) with that of nanophotonics-enabled solar membrane distillation (NESMD). Previous research has shown that NESMD, due to its localized surface heating driven by photothermal membrane coatings, is an energy-efficient system for off-grid desalination; however, concerns remained regarding the scaling behavior of self-heating surfaces. In this work, bench-scale experiments were performed, using model brackish water, to compare the scaling propensity of NESMD with MD. The results showed NESMD to be highly resistant to scaling; a three times higher salt concentration factor (c/c₀) was achieved in NESMD compared to MD without any decline in flux. Analyses of the scaling layer on NESMD membranes revealed that salt deposition was 1/4 of that observed for MD. Scaling resistance in NESMD is attributed to its lower operating temperature, which increases the solubility of common scalants and decreases salt precipitation rates. Precipitation kinetics measurements revealed an order of magnitude faster precipitation under heated conditions (62 °C, k = 8.7 × 10^-2 s^-1) compared to ambient temperature (22 °C, k = 7.1 × 10^-3 s^-1). These results demonstrate a distinct advantage of NESMD over MD for the treatment of high scaling potential water, where scaling is a barrier to high water recovery.

Electrically Driven Interfacial Evaporation for High-Efficiency Steam Generation and Sterilization

Electrically driven steam generation is a critical process for many heating-related applications such as sterilization and food processing. Current systems, which rely on heating up the bulk water to generate steam, face the dilemma in achieving a large evaporation flux and fast thermal response. Herein, we report a self-floating electrically driven interfacial evaporator for fast high-efficiency steam generation independent of the amount of loaded bulk water in the system. Through localized heating of the wicked water at the air-water interface, the evaporator has achieved an electrical-to-steam energy conversion efficiency of ∼90% at a heating power density of 10 kW/m² and a fast thermal response of 20 s. The interfacial evaporation design not only achieves a high evaporation efficiency within a broad range of heating power densities by using different wicking materials, but also enables attaining a high evaporation temperature under low heating power densities by tuning the ratio of the vapor outlet area and the evaporation surface area. By integrating an interfacial evaporator within a sanitizer, the resultant system has demonstrated a faster steam temperature rise and superior steam sterilization performance than the commercial bulk heating-based approach.