Covalent organic frameworks, or COFs, are crystalline porous polymers built from organic molecular building blocks linked into periodic two- or three-dimensional networks. Their appeal lies in their atomic precision: by choosing the aldehyde and amine monomers, chemists can dictate pore size, topology, and electronic structure with a degree of control that inorganic materials rarely allow. COFs have already proven themselves in gas separation, catalysis, and energy storage, and their low density, high surface area, and tunable conjugation make them natural candidates for photothermal applications. The catch, as the research team led by Yachao Xu, Zhong Zhou, and Bojing Sun, working under the direction of corresponding authors Youxing Liu, Hongbo Li, and Shaojun Guo, points out, is that reported COFs have intrinsically lacked the capacity to harvest infrared light. Because roughly half of the solar energy reaching Earth arrives at wavelengths beyond the visible range, that blind spot translated directly into inferior photothermal conversion efficiency and, consequently, sluggish water evaporation rates.

The solution reported in the new paper is deceptively simple in concept: decorate the COF backbone with chlorine atoms. The researchers synthesized a family of Cl-functionalized COFs by condensing 2,4,6-trichloro-1,3,5-trialdehyde benzene, abbreviated TTB, with p-phenylenediamine, abbreviated pPA, yielding the flagship material TTB-pPA COF. The chlorine substituents proved to be far more than passive appendages. According to the team’s theoretical calculations, they reshape the electronic structure of the framework in ways that extend absorption across an extraordinary 200 to 1,500 nanometer window, spanning the ultraviolet, the entire visible spectrum, and a deep swath of the near-infrared. Equally important, the Cl-functionalized framework exhibits strong non-radiative relaxation capacity, meaning that excited electrons shed their absorbed energy as lattice vibrations, that is, as heat, rather than re-emitting it as light that escapes back into the environment.

The photothermal consequences are dramatic. Under illumination at one sun, the standard intensity of noonday sunlight, the TTB-pPA COF reached a surface temperature of 125.3 degrees Celsius within just 60 seconds. A structurally identical COF lacking the chlorine atoms climbed to only 60.7 degrees Celsius under the same conditions, a difference of nearly 65 degrees that isolates the halogen functionality as the decisive variable. That kind of instantaneous, high-amplitude heating is exactly what an interfacial evaporator needs, because vapor generation is governed by the local temperature at the water-air boundary rather than by the bulk temperature of the water body. The faster and hotter the photothermal layer becomes, the more vigorously water molecules are driven off the surface and into the vapor phase.

To translate material performance into device performance, the team constructed a solar-driven water evaporation device using the TTB-pPA COF as the photothermal conversion layer. In laboratory testing under one-sun illumination, the device achieved a water evaporation rate of 7.62 kilograms per square meter per hour, a figure the authors report as significantly outperforming previously reported water evaporation devices. For context, the theoretical evaporation limit under one sun for a conventional dark absorber sits well below this value, and rates approaching or exceeding this level typically require optical concentration, elaborate thermal insulation, or multi-stage architectures. The chlorine-engineered COF reaches its performance through a combination of broadband absorption, rapid non-radiative heat release, and the porous framework’s ability to draw water to the heated surface through capillary action.

Laboratory metrics, however, have a habit of fading under real skies, where cloud cover, wind, changing solar angles, and salt fouling conspire against even the best evaporators. The team therefore developed a custom water evaporation device and subjected it to long-term outdoor testing under natural light conditions. The device operated continuously for 60 days, demonstrating a level of durability that few photothermal materials can claim, particularly organic polymers, which are often vulnerable to photochemical degradation and hydrolysis over extended deployment. Over that period, the system produced purified water with an impurity content below 0.1 percent at a rate of 30 to 45 liters per square meter per day. The authors calculate that a single square meter of the device can meet the drinking water needs of 12 to 18 people, a strikingly concrete benchmark for a technology aimed at households and small communities rather than industrial desalination plants.

The mechanistic story behind these numbers is as interesting as the performance itself. When photons strike the Cl-functionalized framework, electrons are promoted into excited states across an unusually wide range of excitation energies. In a typical fluorescent or reflective material, much of that energy would be lost through radiative decay or through charge transport that carries energy away from the absorption site. In the TTB-pPA COF, the chlorine substituents and the resulting electronic asymmetry channel the excitation energy into vibrational modes of the molecular lattice instead. First-principles calculations of the photothermal conversion mechanism, supported by molecular dynamics simulations of heat dissipation, indicate that this non-radiative decay pathway is both fast and efficient, allowing the framework to act as a molecular-scale solar thermal converter. The result is a material that behaves less like a semiconductor absorbing light and more like a blackbody engineered at the level of individual chemical bonds.

The broader significance of the work lies in its design principle rather than in any single number. The authors emphasize that the key to next-generation solar-powered seawater desalination lies in simultaneously engineering two properties that have usually been pursued separately: a broad light absorption range and strong non-radiative electron decay capability. Halogen functionalization, they show, offers a synthetically accessible route to both. Chlorine is cheap, abundant, and compatible with the condensation chemistry used to build COFs, which means the strategy could in principle be extended across many framework chemistries and device formats. If the approach generalizes, it could reshape how photothermal materials for water treatment are designed, shifting the field away from complex plasmonic or ceramic absorbers toward rationally functionalized organic frameworks that are lightweight, scalable, and inexpensive.

Challenges remain before chlorine-functionalized COFs reach the field at scale. Large-area synthesis of crystalline frameworks with consistent quality, mechanical integration into floating evaporator architectures, and cost accounting at production volumes all require further engineering. But the demonstration of a 7.62 kilogram per square meter per hour evaporation rate, a 60-day outdoor operational lifetime, and drinking-quality output sufficient for more than a dozen people per square meter marks a genuine advance in the solar steam literature. As climate change intensifies droughts and groundwater depletion accelerates worldwide, materials that turn ordinary sunlight into safe drinking water with no moving parts and no external energy input are moving from laboratory curiosity to practical necessity. With this work, covalent organic frameworks, long celebrated as precision platforms for molecular chemistry, have now claimed a place at the front line of the global water challenge.

Subject of Research: Chlorine-functionalized covalent organic frameworks with ultra-broadband light absorption for efficient solar-driven water evaporation and desalination

Article Title: Covalent organic frameworks with ultra-broadband absorption for efficient solar-driven water evaporation

Article References: Xu, Y., Zhou, Z., Sun, B., Li, Q., Wang, Y., Zhao, R., Lin, Z., Sun, Z., Liu, Y., Li, H., & Guo, S. (2026). Covalent organic frameworks with ultra-broadband absorption for efficient solar-driven water evaporation. Nature Water, 4(9), 1157-1165. https://doi.org/10.1038/s44221-026-00687-w

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00687-w

Keywords: covalent organic frameworks, solar-driven water evaporation, photothermal conversion, seawater desalination, infrared absorption, non-radiative relaxation, chlorine functionalization, Nature Water, clean water, photothermal materials, interfacial evaporation, water purification

Freshwater scarcity remains a defining challenge of the 21st century, aggravated by climate change, population growth, and industrial expansion. Although 70% of the Earth’s surface is covered by water, more than 97% of it exists in saline form, rendering it unsuitable for direct human consumption and agriculture. Conventional desalination techniques, such as reverse osmosis and multi-stage flash distillation, are energy-intensive and necessitate substantial infrastructural investments. Consequently, solar interfacial evaporation technologies, which utilize photothermal materials to convert sunlight into heat to evaporate water, have gained momentum owing to their low carbon footprint and operational simplicity. However, perfecting the interplay between light absorption, heat retention, and water transport remains a formidable scientific hurdle.

The breakthrough reported involved synthesizing a hybrid material composed of biochar particles embedded within a polyzwitterionic hydrogel matrix. Biochar, which is derived from pyrolyzed biomass residues such as agricultural straw, offers exceptional photothermal properties due to its black coloration and porous carbonaceous structure. The polyzwitterionic hydrogel, known for its high water-retention capacity and ion transport characteristics, provides a supportive scaffold that facilitates efficient water delivery to the evaporation interface. Together, these components generate a harmonized system that maximizes solar energy utilization while minimizing heat dissipation into the bulk liquid.

Spectroscopic analysis revealed that incorporating biochar transforms the hydrogel from a translucent to an opaque material, with the hybrid achieving over 95% light absorption efficiency across a wide spectral range. This broad-spectrum absorption is critical in harnessing the full intensity of the solar spectrum, including visible and near-infrared wavelengths. The enhanced photothermal conversion directly translates to higher local surface temperatures at the evaporative interface, thereby accelerating the phase change of water molecules from liquid to vapor.

Microscopic investigations provided further insights into the structural modifications induced by biochar addition. Scanning electron microscopy images demonstrated that the presence of biochar particles induces the formation of a denser and more interconnected pore network within the hydrogel matrix. Such a microstructure significantly improves capillary-driven water transport channels, ensuring a continuous replenishment of water at the evaporative surface. This seamless water supply chain is imperative to sustain high evaporation rates without drying out the active layer or wasting thermal energy.

Beyond the macroscopic enhancements in photothermal absorption and water transport, the study delves into subtle molecular interactions that contribute to evaporation efficiency. The surface chemistry of biochar introduces functional groups that engage with the hydrogen-bonding network of water molecules inside the hydrogel. This interaction increases the proportion of “intermediate water”—a phase where water molecules are neither tightly bound nor entirely free. Intermediate water requires substantially less evaporation enthalpy compared to bulk water, enabling the system to lower the total energy input for vaporizing a gram of water to 877.79 joules. This molecular-level modulation of water behavior represents a paradigm shift in designing solar evaporators.

The synergy between enhanced light absorption and modified water molecular states culminates in a solar evaporation performance that outperforms many existing engineered materials. Notably, the hybrid hydrogel retains its efficacy under saline conditions, making it an excellent candidate for seawater desalination. Typically, salt accumulation and crystallization impair the function of many solar evaporators, but the robust water transport pathways and stable pore architecture in this hybrid material mitigate salt fouling issues effectively.

Sustainability is a pivotal consideration in this study. The team emphasizes that biochar production utilizes waste biomass, such as sorghum straw, aligning with circular economy principles and reducing environmental impacts. This contrasts starkly with many photothermal materials that rely on rare or expensive metals. The accessibility and renewability of biochar give this technology significant advantages in scalability and affordability, especially for deployment in resource-constrained or remote regions facing acute water stress.

The authors underline that their research extends beyond material innovation; it offers a comprehensive strategy that addresses multiple bottlenecks in solar desalination concurrently. By harmonizing photothermal conversion, microstructural optimization, and water state regulation, the hybrid hydrogel embodies a holistic approach to maximizing solar water evaporation efficiency. This integrative design philosophy could become a blueprint for next-generation solar evaporators, facilitating widespread clean water production with minimal energy consumption.

Considering the escalating global demand for freshwater, technologies that enable efficient, low-carbon desalination are vital. The biochar-enhanced hydrogel represents a significant advance toward this goal by combining affordability, performance, and environmental stewardship. Future research will likely explore scale-up methodologies, operational durability, and integration into existing water treatment systems, fostering pathways toward commercial adoption.

In a broader context, this breakthrough exemplifies how interdisciplinary approaches, merging materials science, chemistry, and environmental engineering, can yield practical solutions to pressing global challenges. It also highlights the latent potential of biomass-derived materials in advancing sustainability goals across sectors. Leveraging waste streams to produce high-performance photothermal materials offers a model for circular innovation that could ripple across industries.

This study, published in the journal Biochar, sets the stage for transformative developments in solar desalination technology. It invites the scientific community to reconsider conventional paradigms around water evaporation energetics and material design, opening new avenues for research and application. As freshwater scarcity continues to intensify, innovations like these will be instrumental in ensuring equitable access to this most precious resource.

Subject of Research: Experimental study on solar-driven water evaporation efficiency enhancement in biochar-polymer hybrid materials.

Article Title: Heat loss and water transport capacity regulation in hybrid evaporators

News Publication Date: April 27, 2026

Web References:
http://dx.doi.org/10.1007/s42773-026-00604-0

References:
Wang, S., Yang, J., Wang, A., et al. Heat loss and water transport capacity regulation in hybrid evaporators. Biochar 8, 97 (2026).

Image Credits:
Sihui Wang, Jiaqi Yang, Aijie Wang & Wenzong Liu

Keywords

Applied sciences and engineering, Life sciences, Hydrogels, Polymer chemistry, Evaporation