Long-term exposure to low concentrations of volatile organic compounds (VOCs) indoors poses a significant threat to human health. Adsorption technology, due to its safety, high efficiency, and ease of integration into compact purification devices, is widely regarded as a key pathway for improving indoor air quality. Porous adsorbents represented by activated carbon, owing to their excellent microporous structure and extremely high specific surface area, have become the preferred material for capturing gaseous organic pollutants. However, a high equilibrium adsorption capacity does not equate to excellent dynamic purification performance. In ventilated environments, the residence time of airflow through the adsorbent layer is extremely short, and pollutant molecules often find it difficult to fully diffuse to internal adsorption sites instantaneously.

The core challenge currently facing structured thin-film adsorption materials lies in the imbalance between mass transfer resistance and adsorption site utilization. Conventional coated activated carbon films have dense structures and poor pore connectivity. Severe intrafilm mass transfer resistance prevents deep adsorbents from functioning, significantly reducing single-pass purification efficiency. Traditional sacrificial templating or direct-writing pore engineering strategies developed to address this bottleneck generally suffer from drawbacks such as reliance on expensive polymers, high-temperature pyrolysis, or organic solvent elution. Not only are the fabrication processes cumbersome, but high energy consumption and high cost also severely hinder their large-scale practical application. Therefore, it is necessary to develop a novel structured adsorption film that is mild, low-cost, and combines efficient mass transfer with in situ regeneration.

To address the above issues, a joint team composed of Shenzhen University, City University of Hong Kong, and Wuhan Second Ship Design and Research Institute, among others, used ZEPTOOLS ZEM series desktop scanning electron microscope for in-depth research, The team developed a mild pore-forming strategy based on water-soluble salt templates, constructing micron-scale gas transport channels in activated carbon coatings, effectively reducing mass transfer resistance under single-pass airflow and achieving in situ electrothermal regeneration.

Title:Salt-templated hierarchical porous activated-carbon films for efficient and regenerable single-pass indoor VOC purification 
Journal:Building and Environment 
URL:http://dx.doi.org/10.1016/j.buildenv.2026.115192

To address the issue of limited purification efficiency caused by the rapid passage of low-concentration volatile organic compounds in indoor ventilation scenarios, the research team proposed a strategy for water-soluble salt template-assisted construction of hierarchically porous activated carbon adsorption films. In this method, micron-sized sodium chloride, natural sea salt, or simulated mixed salt particles are uniformly blended with activated carbon powder and a polymer binder, coated onto a polyimide-copper composite heating substrate, and after low-temperature drying, the salt template is removed by gentle rinsing with deionized water. The dissolution and extraction of water-soluble inorganic salts in situ forms abundant micron-scale gas transport channels inside the dense activated carbon coating, overcoming the high energy consumption and process complexity of traditional sacrificial template methods that rely on high-temperature decomposition or organic solvent elution. This provides a low-cost and environmentally friendly scalable manufacturing route for developing high-efficiency single-pass adsorption films for ventilation conditions.

To precisely elucidate the micron-scale pore evolution mechanism and the structure-performance relationship of mass transfer channels, the research team used the ZEPTOOLS ZEM series desktop scanning electron microscope to perform multiscale morphological characterization of conventional dense coatings and composite films after salt template treatment. The high-resolution microscopic imaging of the ZEPTOOLS ZEM series desktop scanning electron microscope clearly revealed structural differences between the film surface and brittle fracture cross-sections: the coating surface of the control group without template was extremely dense, with activated carbon particles tightly embedded in the polymer matrix and lacking interconnected transport pores. In contrast, samples after water elution desalting exhibited dissolution macropores ranging from tens to hundreds of microns and micron-scale interconnected openings between particles. Combined with nitrogen adsorption-desorption tests, it was confirmed that this microscopic pore reconstruction increased the specific surface area of the coating from 445.9m²/g to 596.9m²/g and the total pore volume from 0.222cm³/g to 0.263cm³/g, directly demonstrating that the introduction of interconnected transport macropores can significantly release the physical adsorption accessibility of deep micropores and mesopores.

In continuous-flow single-pass purification experiments, the study systematically investigated the effects of the activated carbon-to-salt template ratio and airflow velocity on dynamic VOC removal performance. Tests showed that increasing the template doping amount and eluting to form abundant transport channels can effectively shorten the gas diffusion distance. When the mass ratio was adjusted to 1:1, the resulting film achieved an initial single-pass removal rate of 82.2% for toluene at a relatively high face velocity of 1.0m/s, an improvement of about 40% compared with the 58.9% efficiency of the flat control group without template. More importantly, even with a 40% reduction in activated carbon dosage, the film still exhibited dynamic adsorption lifetime and breakthrough characteristics comparable to or even better than those of high-loading samples. In an extended breakthrough test lasting 122.1h, the effective adsorption capacity reached 33.6mg/g, confirming that structured pore engineering can break the intrafilm mass transfer resistance bottleneck of dense coatings and multiply the dynamic utilization efficiency of adsorbents under short contact times.

The study further evaluated the operational adaptability and in situ cyclic regeneration capability of the composite adsorption film under complex environmental conditions. The film not only maintained an initial single-pass removal rate of about 80% when prepared from different natural and industrial salt sources, and exhibited stable purification performance within a toluene concentration range of 1–10ppm, but also maintained a retention rate above 77% for typical aromatic and chlorinated hydrocarbons such as benzene and tetrachloroethylene. In response to the weakened competitive adsorption effect caused by increased relative humidity, the porous film consistently maintained a purification advantage over the control group across the 0%–80% humidity range. In addition, relying on the integrated polyimide-copper thermally conductive substrate, the film could achieve rapid desorption by applying 80°C electrical heating for 20min under a gentle airflow. After 15 adsorption-regeneration cycles in the dry state, the initial single-pass efficiency retention rate remained as high as about 96%, and no cracking or coating peeling occurred after repeated bending, confirming the durable and practical engineering value of this functionalized porous adsorption film in actual indoor air purification systems.

The ZEPTOOLS ZEM series scanning electron microscope is a highly integrated, portable, and cost-effective scientific research instrument. It features rapid vacuum pumping, high imaging speed, and a variety of signal detector options, making it suitable for morphology observation and compositional analysis, and it can also adapt to various in situ experimental requirements. The instrument has low installation environment requirements, is not floor-sensitive, and is simple to operate; non-specialists can quickly get started. It can be widely applied in many fields such as new material R&D, life sciences, failure analysis, and industrial quality inspection, providing scientific research institutes and enterprise users with a powerful microcharacterization solution that combines high performance and high cost-effectiveness.