Self‐Contained Monolithic Carbon Sponges for Solar‐Driven Interfacial Water Evaporation Distillation and Electricity Generation

Solar vaporization has received tremendous attention for its potential in desalination, sterilization, distillation, etc. However, a few major roadblocks toward practical application are the high cost, process intensive, fragility of solar absorber materials, and low efficiency. Herein an inexpensive cellular carbon sponge that has a broadband light absorption and inbuilt structural features to perform solitary heat localization for in situ photothermic vaporization is reported. The defining advantages of elastic cellular porous sponge are that it self‐confines water to the perpetually hot spots and accommodates cyclical dynamic fluid flow‐volume variable stress for practical usage. By isolating from bulk water, the solar‐to‐vapor conversion efficiency is increased by 2.5‐fold, surpassing that of conventional bulk heating. Notably, complementary solar steam generation‐induced electricity can be harvested during the solar vaporization so as to capitalize on waste heat. Such solar distillation and waste heat‐to‐electricity generation functions may provide potential opportunities for on‐site electricity and fresh water production for remote areas/emergency needs.     

Solar Absorber Gel: Localized Macro‐Nano Heat Channeling for Efficient Plasmonic Au Nanoflowers Photothermic Vaporization and Triboelectric Generation

Plasmonic nanoparticles with outstanding photothermal conversion efficiency are promising for solar vaporization. However, the high cost and the required intense light excitation of noble metals, hinder their practical application. Herein, an inexpensive 3D plasmonic solar absorber gel that embraces all the desirable optical, thermal, and wetting properties for efficient solar vaporization is reported. The broadband absorption and strong near‐field intertip enhancement of the sparsely dispersed gold nanoflowers contribute to efficient light‐to‐heat conversion, while the macro‐nano thermal insulative silica gel retains and channels the plasmonic heat directly to the water pathways contained within the porous gel. The plasmonic‐based solar absorber gel shows a vaporization efficiency of 85% under solar irradiation of 1 sun intensity (1 kW m^?2). Moreover, the porous gel framework exhibits high mechanical stability and antifouling properties, potentially useful for polluted/turbid water evaporation. Complementary water condensation‐induced triboelectricity can be harvested alongside fresh water condensate, granting simultaneous fresh water production and electricity generation functionalities. The facile sol‐gel synthesis at room temperature makes the solar absorber gel highly adaptable for practical large‐scale photothermal applications.     

Biomimetic MXene Textures with Enhanced Light‐to‐Heat Conversion for Solar Steam Generation and Wearable Thermal Management

2D materials are of particular interest in light‐to‐heat conversion, yet challenges remain in developing a facile method to suppress their light reflection. Herein, inspired by the black scales of Bitis rhinoceros, a generalized approach via sequential thermal actuations to construct biomimetic 2D‐material nanocoatings, including Ti₃C₂T_x MXene, reduced graphene oxide (rGO), and molybdenum disulfide (MoS₂) is designed. The hierarchical MXene nanocoatings result in broadband light absorption (up to 93.2%), theoretically validated by optical modeling and simulations, and realize improved light‐to‐heat performance (equilibrium temperature of 65.4 °C under one‐sun illumination). With efficient light‐to‐heat conversion, the bioinspired MXene nanocoatings are next incorporated into solar steam‐generation devices and stretchable solar/electric dual‐heaters. The MXene steam‐generation devices require much lower solar‐thermal material loading (0.32 mg cm^?2) and still guarantee high steam‐generation performance (1.33 kg m^?2 h^?1) compared with other state‐of‐the‐art devices. Additionally, the mechanically deformed MXene structures enable the fabrication of stretchable and wearable heaters dual‐powered by sunlight and electricity, which are reversibly stretched and heated above 100 °C. This simple fabrication process with effective utilization of active materials promises its practical application value for multiple solar–thermal technologies.     

Interfacial Solar‐to‐Heat Conversion for Desalination

Solar desalination is a promising and sustainable solution for water shortages in the future. Interfacial solar‐to‐heat conversion for desalination has attracted increasing attention in the past decades, due to the heat localization induced high thermal efficiency, simple structure, and low cost. In this review, the authors summarize and analyze the critical processes involved in such a solar desalination system, including the thermal conversion and transport, salt dissipation, and vapor manipulation. Mathematical models of heat transfer and salt dissipation are also built for quantitative analysis of systematic performance relative to properties of employed materials and system designs. Recent efforts devoted to improving the overall thermal efficiency, salt rejection, and water yield are then summarized. Based on the analysis and previous results, opportunities for further interfacial solar desalination development are highlighted.     

Superwetting Monolithic Hollow‐Carbon‐Nanotubes Aerogels with Hierarchically Nanoporous Structure for Efficient Solar Steam Generation

Solar steam generation has been proven to be one of the most efficient approaches for harvesting solar energy for diverse applications such as distillation, desalination, and production of freshwater. Here, the synthesis of monolithic carbon aerogels by facile carbonization of conjugated microporous polymer nanotubes as efficient solar steam generators is reported. The monolithic carbon‐aerogel networks consist of randomly aggregated hollow‐carbon‐nanotubes (HCNTs) with 100–250 nm in diameter and a length of up to several micrometers to form a hierarchically nanoporous network structure. Treatment of the HCNTs aerogels with an ammonium peroxydisulfate/sulfuric acid solution endows their superhydrophilic wettability which is beneficial for rapid transportation of water molecules. In combination with their abundant porosity (92%) with open channel structure, low apparent density (57 mg cm^?3), high specific surface area (826 m² g^?1), low thermal conductivity (0.192 W m^?1 K^?1), and broad light absorption (99%), an exceptionally high conversion efficiency of 86.8% is achieved under 1 sun irradiation, showing great potential as an efficient photothermal material for solar steam generation. The findings may provide a new opportunity for tailored design and creation of new carbon‐aerogels‐based photothermal materials with adjustable structure, tunable porosity, simple fabrication process, and high solar energy conversion efficiency for solar steam generation.     

Plasmonic Wood for High‐Efficiency Solar Steam Generation

Plasmonic metal nanoparticles are a category of plasmonic materials that can efficiently convert light into heat under illumination, which can be applied in the field of solar steam generation. Here, this study designs a novel type of plasmonic material, which is made by uniformly decorating fine metal nanoparticles into the 3D mesoporous matrix of natural wood (plasmonic wood). The plasmonic wood exhibits high light absorption ability (≈99%) over a broad wavelength range from 200 to 2500 nm due to the plasmonic effect of metal nanoparticles and the waveguide effect of microchannels in the wood matrix. The 3D mesoporous wood with numerous low‐tortuosity microchannels and nanochannels can transport water up from the bottom of the device effectively due to the capillary effect. As a result, the 3D aligned porous architecture can achieve a high solar conversion efficiency of 85% under ten‐sun illumination (10 kW m^?2). The plasmonic wood also exhibits superior stability for solar steam generation, without any degradation after being evaluated for 144 h. Its high conversion efficiency and excellent cycling stability demonstrate the potential of newly developed plasmonic wood to solar energy‐based water desalination.     

Photothermal Catalytic Gel Featuring Spectral and Thermal Management for Parallel Freshwater and Hydrogen Production

Desalination processes often require large amounts of energy to create clean water, and vice versa for the generation of energy. This interdependence creates a tension between the two essential resources. Current research focuses on one or the other, which exacerbates water‐energy stress, while few tackle both issues jointly. Herein, a low‐carbon technology, H₂O–H₂ co‐generation system that enables concurrent steady freshwater and clean energy output is reported. The water‐energy coupled technology features a spectrally and thermally managed solar harvesting gel for photoredox and photoheating effects. This photothermal catalytic gel exploits interfacial solar heating for heat confinement, and localized plasmonic heating at the catalyst active sites to remarkably improve water and hydrogen production, thus maximizing energy value per area. To this end, a stand‐alone renewable solar desalination system is successfully demonstrated for parallel production of freshwater and hydrogen under natural sunlight. By doing so, the water–energy nexus is transformed into a synergistic bond that offers opportunities to better meet expected demand rather than acting in competition.     

Thermal–Electric Nanogenerator Based on the Electrokinetic Effect in Porous Carbon Film

Converting low‐grade thermal energy with small temperature gradient into electricity is challenging due to the low efficiency and high cost. Here, a new type of thermal–electric nanogenerator is reported that utilizes electrokinetic effect for effective harvesting thermal energy. The nanogenerator is based on an evaporation‐driven water flow in porous medium with small temperature gradient. With a piece of porous carbon film and deionized water, a maximum open‐circuit voltage of 0.89 V under a temperature difference of 4.2 °C is obtained, having a corresponding pseudo‐Seebeck coefficient of 210 mV K^?1. The large pseudo‐Seebeck coefficient endows the nanogenerator sufficient power output for powering existing electronics directly. Furthermore, a wearable bracelet nanogenerator utilizing body heat is also demonstrated. The unique properties of such conversion process offer great potential for ultra‐low temperature‐gradient thermal energy recovery, wearable electronics, and self‐powered sensor systems.     

Solar‐to‐Hydrogen Energy Conversion Based on Water Splitting

Artificial photosynthesis provides a blueprint to harvest solar energy to sustain the future energy demands. Solar‐driven water splitting, converting solar energy into hydrogen energy, is the prototype of photosynthesis. Various systems have been designed and evaluated to understand the reaction pathways and/or to meet the requirements of potential applications. In solar‐to‐hydrogen conversion, electrocatalytic hydrogen and oxygen evolution reactions are key research areas that are meaningful both theoretically and practically. To utilize hydrogen energy, fuel cell technology has been extensively investigated because of its high efficiency in releasing chemical energy. In this review, general concepts of the photosynthesis in green plants are discussed, different strategies for the light‐driven water splitting proposed in laboratories are introduced, the progress of electrocatalytic hydrogen and oxygen evolution reactions are reviewed, and finally, the reactions in hydrogen fuel cells are briefly discussed. Overall, the mass and energy circulation in the solar‐hydrogen‐electricity circle are delineated. The authors conclude that attention from scientists and engineers of relevant research areas is still highly needed to eliminate the wide disparity between the aspirations and realities of artificial photosynthesis.     

Empowering Semi‐Transparent Solar Cells with Thermal‐Mirror Functionality

Device architectures for semi‐transparent perovskite solar cells are proposed that are not only highly efficient but also very effective in thermal‐mirror operation. With the optimal top transparent electrode design based on thin metal layer capped with a high‐index dielectric layer for selective transmittance in visible and high reflectance in near‐infrared (NIR) region, the proposed see‐through devices exhibit average power conversion efficiency as large as 13.3% and outstanding NIR rejection of 85.5%, demonstrating their great potential for ideal “energy‐generating and heat‐rejecting” solar windows that can make a smart use of solar energy.     

Recent Advances and Emerging Trends in Photo‐Electrochemical Solar Energy Conversion

Photo‐electrochemical (PEC) solar energy conversion offers the promise of low‐cost renewable fuel generation from abundant sunlight and water. In this Review, recent developments in photo‐electrochemical water splitting are discussed with respect to this promise. State‐of‐the‐art photo‐electrochemical device performance is put in context with the current understanding of the necessary requirements for cost‐effective solar hydrogen generation (in terms of solar‐to‐hydrogen conversion efficiency and system durability, in particular). Several important studies of photo‐electrochemical hydrogen generation at p‐type photocathodes are highlighted, mostly with protection layers (for enhanced durability), but also a few recent examples where protective layers are not needed. Recent work with the widely studied n‐type BiVO₄ photoanode is detailed, which highlights the needs and necessities for the next big photoanode material yet to be discovered. The emerging new research direction of photo‐electrocatalytic upgrading of biomass substrates toward value‐added chemicals is then discussed, before closing with a commentary on how research on PEC materials remains a worthwhile endeavor.     

Pareto Optimal Spectrally Selective Emitters for Thermophotovoltaics via Weak Absorber Critical Coupling

Tailoring the emission spectra of a thermophotovoltaic (TPV) emitter away from that of a blackbody has the potential to minimize transmission and thermalization loss in a photovoltaic receiver. Selective TPV emitters can lead to solar energy conversion with efficiency greater than the Shockley–Queisser limit and can facilitate the generation of useful energy from waste heat. A new design is introduced to radically tune thermal emission that leverages the interplay between two resonant phenomena in simple planar nanostructures—absorption in weakly absorbing nanofilms and reflection in multilayer dielectric stacks. A virtual screening approach is employed to identify promising structures for a selective thermal emitter from a search space of millions, several of which approach the ideal values of a step‐function selective thermal emitter. One of these structures is experimentally fabricated and evaluated, which includes a weakly absorbing alloy with tailored optical properties fabricated by atomic layer deposition (ALD). The versatility of the design and fabrication approach result in an emitter with excellent spectral density (0.8 W cm^?2 sr^?1) and spectral efficiency (46.8%) at 1373 K. Future experimental challenges to a more accurate realization of the optimal structures calculated are also considered.     

3D Nanostructures for the Next Generation of High‐Performance Nanodevices for Electrochemical Energy Conversion and Storage

Among the different nanostructures that have been demonstrated as promising materials for various applications, 3D nanostructures have attracted significant attention as building blocks for constructing high‐performance nanodevices. Particularly over the last decade, considerable research efforts have been devoted to designing, fabricating, and evaluating 3D nanostructures as electrodes for electrochemical energy conversion and storage devices. Although remarkable progress has been achieved, the performance of electrochemical energy devices based on 3D nanostructures in terms of energy conversion efficiency, energy storage capability, and device reliability still needs to be significantly improved to meet the requirements for practical applications. Rather than simply outlining and comparing different 3D nanostructures, this article systematically summarizes the general advantages as well as the existing and future challenges of 3D nanostructures for electrochemical energy conversion and storage, focusing on photoelectrochemical water splitting, photoelectrocatalytic solar‐to‐fuels conversion from nitrogen and carbon dioxide, rechargeable metal‐ion batteries, and supercapacitors. A comprehensive understanding of these advantages and challenges shall provide valuable guidelines and enlightenments to facilitate the further development of 3D nanostructured materials, and contribute to the achieving more efficient energy conversion and storage technologies toward a sustainable energy future.     

MoS2‐Based All‐Purpose Fibrous Electrode and Self‐Powering Energy Fiber for Efficient Energy Harvesting and Storage

Here an all‐purpose fibrous electrode based on MoS₂ is demonstrated, which can be employed for versatile energy harvesting and storage applications. In this coaxial electrode, ultrathin MoS₂ nanofilms are grown on TiO₂ nanoparticles coated carbon fiber. The high electrochemical activity of MoS₂ and good conductivity of carbon fiber synergistically lead to the remarkable performances of this novel composite electrode in fibrous dye‐sensitized solar cells (showing a record‐breaking conversion efficiency of 9.5%) and high‐capacity fibrous supercapacitors. Furthermore, a self‐powering energy fiber is fabricated by combining a fibrous dye‐sensitized solar cell and a fibrous supercapacitor into a single device, showing very fast charging capability (charging in 7 s under AM1.5G solar illumination) and an overall photochemical‐electricity energy conversion efficiency as high as 1.8%. In addition, this wire‐shaped electrode can also be used for fibrous Li‐ion batteries and electrocatalytic hydrogen evolution reactions. These applications indicate that the MoS₂‐based all‐purpose fibrous electrode has great potential for the construction of high‐performance flexible and wearable energy devices.     

A Portable and Efficient Solar‐Rechargeable Battery with Ultrafast Photo‐Charge/Discharge Rate

The solar‐rechargeable electric energy storage systems (SEESSs), which can simultaneously harvest and store solar energy, are considered a promising next‐generation renewable energy supply system. However, the difficulty in meeting the demands of higher overall photoelectric conversion and storage efficiency (PCSE) with both high power density and large energy density in the current SEESSs severely limit their practical application. Herein, a new class is demonstrated of portable and highly efficient SEESS that uniquely integrates a perovskite solar module (PSM) and an aluminum‐ion battery (AIB) directly on a bifunctional aluminum electrode without any external circuit. Such nanostructural design in the SEESS not only exhibits fast photo‐charge/discharge rate (less than one minute) with high power density (above 5000 W kg^?1), but also delivers a high energy density (above 43 Wh kg^?1). By rationally matching the maximum power point voltage of PSM with AIB charging voltage, an excellent solar‐charging efficiency of 15.2% and a high PCSE of 12.04% are achieved, which is among the best in all reported portable SEESSs. Moreover, enhanced PCSE is observed as the light intensity decreases, which makes such SEESS immune from the geographical location and climate limitations for diverse practical applications.     

Over 17% Efficiency Stand‐Alone Solar Water Splitting Enabled by Perovskite‐Silicon Tandem Absorbers

Realizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p⁺nn⁺ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.     

Effect of Change of Forest Carbon Storage on Net Carbon Dioxide Balance of Wood Use for Energy in Japan

This study analyzed the net carbon dioxide (CO₂) emission reductions between 2005 and 2050 by using wood for energy under various scenarios of forest management and energy conversion technology in Japan, considering both CO₂ emission reductions from replacement of fossil fuels and changes in carbon storage in forests. According to our model, wood production for energy results in a significant reduction of carbon storage levels in forests (by 46% to 77% in 2050 from the 2005 level). Thus, the net CO₂ emission reduction when wood is used for energy becomes drastically smaller. Conventional tree production for energy increases net CO₂ emissions relative to preserving forests, but fast‐growing tree production may reduce net CO₂ emissions more than preserving forests does. When wood from fast‐growing trees is used to generate electricity with gas turbines, displacing natural gas, the net CO₂ emission reduction from the combination of fast‐growing trees and electricity generation with gas turbines is about 58% of the CO₂ emission reduction from electricity generation from gas turbines alone in 2050, and an energy conversion efficiency of around 20% or more is required to obtain net reductions over the entire period until 2050. When wood is used to produce bioethanol, displacing gasoline, net reductions are realized after 2030, provided that heat energy is recovered from residues from ethanol production. These results show the importance of considering the change in carbon storage when estimating the net CO₂ emission reduction effect of the wood use for energy.     

High‐Performance Solar Steam Device with Layered Channels: Artificial Tree with a Reversed Design

Solar steam generation, combining the most abundant resources of solar energy and unpurified water, has been regarded as one of the most promising techniques for water purification. Here, an artificial tree with a reverse‐tree design is demonstrated as a cost‐effective, scalable yet highly efficient steam‐generation device. The reverse‐tree design implies that the wood is placed on the water with the tree‐growth direction parallel to the water surface; accordingly, water is transported in a direction perpendicular to what occurs in natural tree. The artificial tree is fabricated by cutting the natural tree along the longitudinal direction followed by surface carbonization (called as C‐L‐Wood). The nature‐made 3D interconnected micro‐/nanochannels enable efficient water transpiration, while the layered channels block the heat effectively. A much lower thermal conductivity (0.11 W m^?1 K^?1) thus can be achieved, only 1/3 of that of the horizontally cut wood. Meanwhile, the carbonized surface can absorb almost all the incident light. The simultaneous optimizations of water transpiration, thermal management, and light absorption results in a high efficiency of 89% at 10 kW m^?2, among the highest values in literature. Such wood‐based high‐performance, cost‐effective, scalable steam‐generation device can provide an attractive solution to the pressing global clean water shortage problem.     

High Frequency Thermal Energy Harvesting Using Magnetic Shape Memory Films

A new method for thermal energy harvesting at small temperature difference and high cycling frequency is presented that exploits the unique magnetic properties and actuation capability of magnetic shape memory alloy (MSMA) films. Polycrystalline films of the Ni_50.4Co_3.7Mn_32.8In_13.1 alloy are tailored, showing a large abrupt change of magnetization and low thermal hysteresis well above room temperature. Based on this material, a free‐standing film device is designed that exhibits thermomagnetically induced actuation between a heat source and sink with short heat transfer times. The cycling frequency of the device is tuned by mechanical frequency up‐conversion to over 200 Hz. An integrated pick‐up coil converts the thermally induced change of magnetization as well as the kinetic energy to electricity. For a temperature change of 10 K, the maximum peak power density is in the order of 5 mW cm^‐3.     

A Long Lifetime Aqueous Organic Solar Flow Battery

Monolithically integrated solar flow batteries (SFBs) hold promise as compact stand‐alone energy systems for off‐grid solar electrification. Although considerable research is devoted to studying and improving the round‐trip efficiency of SFBs, little attention is paid to the device lifetime. Herein, a neutral pH aqueous electrolyte SFB with robust organic redox couples and inexpensive silicon‐based photoelectrodes is demonstrated. Enabled by the excellent stability of both electrolytes and protected photoelectrodes, this SFB device exhibits not only unprecedented stable continuous cycling performance over 200 h but also a capacity utilization rate higher than 80%. Moreover, through comprehensive study on the working mechanisms of SFBs, a new theory based on instantaneous solar‐to‐output electricity efficiency toward more optimized device design is developed and a significantly improved solar‐to‐output electricity efficiency of 5.4% from single‐junction silicon photoelectrodes is realized. The design principles presented in this work for extending device lifetime and boosting round trip energy efficiency will make SFBs more competitive for off‐grid applications.