Advancing Indoor Air Purification by Mass Transfer Enhancement: Bridging the Gap Between High-Performance Materials and Technologies

Enze Tian , Qiwei Chen , Yilun Gao , Zhuo Chen , Yan Wang , Jinhan Mo

Engineering ›› 2026, Vol. 60 ›› Issue (5) : 246 -259.

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Engineering ›› 2026, Vol. 60 ›› Issue (5) :246 -259. DOI: 10.1016/j.eng.2025.07.003
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Advancing Indoor Air Purification by Mass Transfer Enhancement: Bridging the Gap Between High-Performance Materials and Technologies
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Abstract

Indoor air purification, as a typical gas-solid interface process, involves the transfer of airborne pollutants to purification material surfaces through mass transfer, enabling their removal. While research on indoor air purification materials has expanded remarkably, studies on enhancing mass transfer have been relatively limited. In this work, we proposed a new concept of “integration of mass transfer and material regulation,” aiming to provide a design methodology for indoor air purification. Taking solid-phase particulate matter (PM) filtration and gas-phase pollutant adsorption as examples, we summarized the novel approach that shifts from passive material usage to active control of mass transfer through multiscale (milli-micro-nano) and multifield (mass-flow-force). For PM removal, the external electric force can enhance the migration velocity of PM towards the fiber approximately fivefold, resulting in 1−3 orders of magnitude higher comprehensive quality factor than commercial filters, considering filtration efficiency, pressure drop, and energy consumption. For gas removal, the hierarchical structure can increase the gas-solid contact area by 58%, resulting in a 37% improvement in single-pass removal efficiency and a 152% enhancement in dynamic adsorption capacity. We bridge the gap between high-performance materials and technologies by providing a design methodology for controlling surface forces and structures to improve mass transfer.

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Mass transfer / Air cleaning / Particulate matter / Formaldehyde / Indoor air quality

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Enze Tian, Qiwei Chen, Yilun Gao, Zhuo Chen, Yan Wang, Jinhan Mo. Advancing Indoor Air Purification by Mass Transfer Enhancement: Bridging the Gap Between High-Performance Materials and Technologies. Engineering, 2026, 60(5): 246-259 DOI:10.1016/j.eng.2025.07.003

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1. Introduction

Indoor air pollution, including both airborne particulate matter (PM) in the solid phase and chemical pollutants in the gas phase, poses paramount global environmental risks for human health [1] and high-end manufacturing industries [2]. Extensive research has established that prolonged or accidental exposure to indoor air pollutants can cause respiratory ailments [3], tumorous diseases [4], cognitive impairments [5], and even death [6]. In 2020, indoor air pollution caused approximately 3.2 million deaths globally, comparable to the 3.5 million deaths caused by outdoor pollution [7]. Moreover, cleanroom airborne contamination control is crucial to product quality and individual safety in high-tech industries such as semiconductors and medicine [8,9].

Healthy buildings and cleanrooms must safeguard people and products from external atmospheric pollution while mitigating internal contamination. Indoor air purification plays a crucial role in achieving this goal. Two air purifiers equipped with high-efficiency particulate air (HEPA) filters can potentially reduce airborne PM exposure by 65% for three individuals in a 54 m2 room [10]. Combining ventilation with air purification in a gym could reduce the PM concentration to 5%−10% of baseline levels, significantly reducing the risk of airborne disease transmission [11]. Clean rooms for semiconductor manufacture require HEPA filters for product yield. The cascaded filtration in two 100 class clean rooms reduced PM concentrations with diameters larger than 0.3 μm to < 1000 count·m−3 [12]. It is worth noting that indoor air purification holds profound strategic implications for human habitation in deep-sea, underground, and outer space environments, as it directly impacts the duration of human stays in such settings.

However, indoor air purification not only plays a crucial role in safeguarding but also in reducing energy costs and greenhouse gas emissions in various inhabited spaces [7]. Due to the low concentration of indoor air pollutants compared to industrial pollutants, conventional purification technologies require a strategic reduction in the gap between the pollutants and the purification medium to ensure efficient and rapid pollutant removal. However, tightly compacted purification medium blocks fluid dynamics, resulting in high wind-pressure drop over the medium and high fan energy consumption. Increased wind-pressure drop or decreased efficiency during operation can also shorten the filters’ service life. Public buildings in China consume 20%−30% of their total power for ventilation and filtration, resulting in an annual power consumption of over 200 billion kW·h. Cleanrooms, with air exchange rates 4−80 times higher than public buildings [13], have even higher energy consumption for ventilation and filtration, leading to significant expenses in industrial production.

In brief, traditional high-efficiency purification methods often struggle to achieve high efficiency and low air resistance due to the necessity for greater medium density, leading to high air resistance. Achieving high efficiency while maintaining low air resistance is considered the “holy grail” challenge in purification research [14]. This challenge stems primarily from the lack of a comprehensive approach that integrates mass transfer enhancement with material regulation for indoor air purification. Despite recent advances in air purification materials such as nanofibers [15], metal-organic-framework (MOF) [16], and two-dimensional materials [17], the challenge lies in the mismatch between material performance and purification efficiency when ignoring mass transfer enhancement.

To provide a design methodology for indoor air purification engineering, we introduce the concept of “integration of mass transfer and material regulation” in this work. Section 2 demonstrates the principle of increasing air purification efficiency while saving energy. 3 Upgrade indoor PM filtration performance with electrostatic force, 4 Improving indoor gaseous pollutant removal by multiscale mass transfer enhancement offer principle-based indoor air purification technologies with low energy consumption, high efficiency, and long service life, using particulate filtration and chemical pollutant adsorption as examples. Section 5 summarizes the main viewpoints and promotes next-generation indoor air purification technology design.

2. Increasing efficiency and reducing energy consumption of air purification

The clean air delivery rate (CADR) is an important parameter that characterizes an air cleaner’s indoor air purification ability. It quantifies the rate at which an air cleaner can generate clean air over a specific unit of time, which Eq. (1) determines [18]:

$\text{CADR}=G\cdot \left(1-{\text{e}}^{-{\text{NTU}}_{\text{m}}}\right)$

where G is the airflow rate, m3·s−1, which contributes most importantly to CADR [19]; NTUm is the number of the mass transfer unit and is further determined by three dimensionless parameters, as shown in Eq. (2) [18]:

${\text{NTU}}_{\text{m}}={A}^{\ast }\cdot {\text{St}}_{\text{m}}\cdot \eta $

where A* is the dimensionless characteristic surface area for mass transfer (the ratio of the purification area of the targeted pollutants to the cross-sectional area of the air purification module); Stm is the Stanton number for mass transfer, which characterizes the interfacial mass transfer and is between 0 and 1; η is a dimensionless coefficient related to material properties, which describes the material characteristic and is also between 0 and 1. The three parameters have equal weights in determining NTUm and further CADR. It means when the breakthrough of material performance is hard to realize, the performance of an air cleaner can be effectively improved by optimizing the structure of the cleaner (e.g., enhancing the contact area of airborne pollutants to the materials) or by strengthening the interface mass transfer of airborne pollutant to the materials (Fig. 1).

For the structural design of air cleaners, the contact area of airborne pollutants to the materials, that is, the mass transfer area, should be increased. Taking gas-phase air cleaning as an example, inspired by the heat transfer fins, the sub-milli-sized fin structure could be considered in fabricating adsorption components [20]. Besides, Refs. [21,22] reported that the net-like honeycomb structure can also be applied as it has a high specific surface area.

Eq. (3) can be used to guide the enhancement of interfacial mass transfer:

$\varphi ={h}_{\text{m}}\cdot \left({C}_{\infty }-{C}_{\text{interface}}\right)$

where φ is the mass transfer flux per unit area, mg·m−2·s−1; hm is the convective mass transfer coefficient, m·s−1; C∞ is the pollutant concentration in the inflow, mg·m−3; Cinterface is the pollutant concentration on the interface of the airborne pollutant to the materials. Eq. (3) provides insight into the parameters determining φ, including hm and the difference between C∞ and Cinterface.

Among them, hm can be realized by enhancing the turbulence of the flow, which can be realized by building some surface microstructures [23]. For particulate filtration, hm depends on the external field forces to which the particles are subjected, such as gravity, centrifugal force, electrostatic force, thermophoretic force, and so forth. The greater the external field force exerted toward the fibers, the faster the PM’s transfer velocity and the greater the hm. For gas purification, the interfacial mass transfer includes two steps: external mass transfer from the airflow to the mass transfer boundary layer and internal mass transfer from the boundary layer into the inner material. However, for most adsorption mass transfer processes, the internal mass transfer is the primary resistance limiting the whole process. Maintaining a large concentration gradient is essential for a sufficient external mass transfer driving force. It requires keeping the Cinterface at a low level compared to C∞ over time, which necessitates the rapid internal diffusion of adsorbed gas from the adsorbent surface. The goal is to ensure that the adsorbate is uniformly distributed throughout the material instead of accumulating on the surface. Therefore, enhancing the diffusion mass transfer inside the material is paramount. Since adsorbents are generally porous materials, optimizing the porosity and the pore size distribution is usually the most effective method [24,25]. Furthermore, an interconnected pore structure is crucial for facilitating the diffusion transfer of gas molecules within the material [26,27].

3. Upgrade indoor PM filtration performance with electrostatic force

Indoor PM pollution has gained significant attention due to respiratory epidemics such as corona virus disease 2019. Efficient PM (including bioaerosol) removal technologies are essential for ensuring clean and healthy built environments [28]. Filtration is frequently used among these technologies because of its low cost, easy operation, and high efficiency. Synthetic polymers such as polyethylene and polypropylene are often used in commercial air filters because of their affordability and adaptability. Electrospinning methods have been developed to generate nanofibers from diverse synthetic polymers, improving filtering efficacy and reducing the wind-pressure drop. Natural polymers, including cellulose and chitosan, have demonstrated commendable filtration efficiency and recyclability for their superior biocompatibility and biodegradability. Besides, nanomaterials, including MOF, carbon nanotubes, and graphene oxide, have been deposited onto fiber substrates for superior mechanical properties, hydrophobicity, and surface area. For applications in extreme conditions, inorganic and metallic fibers, including SiO2, Al2O3, Si3N4, and SiC, have been developed for PM filtration at elevated temperatures [29].

These innovative materials allow the creation of diverse air filtration products, such as face masks, smart windows, indoor air purifiers, bag filters, and gasoline filters, enhancing air quality and safeguarding human health. However, current filter materials often struggle to balance filtration efficiency, wind-pressure drop, and PM holding capacity. Many studies [30,31] have demonstrated that electrostatic assistance enhances filtration efficiency and reduces the wind-pressure drop. The following section reviews current advances in the mechanism, device build-up, material preparation, and electrostatically assisted air filtration performance for indoor PM removal. The goal is to improve the fundamental understanding of high-performance PM filtration from the perspective of mass transfer enhancement by electrostatic force.

3.1. Motivation for coupling PM filtration with electrostatic force

In a typical indoor PM filtration process, fibers capture PM in the air flow while air molecules penetrate them. Air molecules are expected to pass through the fibers with minimal resistance to maintain an extremely low wind-pressure drop, while all airborne PM is expected to be removed for 100% filtration efficiency. The pressure drop of the fibrous materials depends on the filter’s intrinsic properties and the airflow characteristics. The random distribution of fibers within the filtration material poses challenges in accurately calculating the pressure drop. Despite this, the low-pressure drop is typical of fibrous materials with large pores, high porosity, and thin thickness. However, these preferable parameters for low-pressure-drop fibrous material can also lead to low filtration efficiency, as indicated by Eqs. (4), (5) [32]:

$\text{Δ}p=64\mu {v}_{\text{air}}\frac{{\alpha }^{1.5}·\left(1+56{\alpha }^{3}\right)L}{{d}_{\text{f}}^{\text{2}}}$
${\eta }_{\text{f}}=1-\text{exp}\left(\frac{4\alpha L}{\text{π}\left(\alpha -1\right){d}_{\text{f}}}{\eta }_{\text{s}}\right)$

where Δp refers to the pressure drop of a porous filter (Pa); μ is air viscosity (Pa·s); vair indicates the face air velocity over the filter (m·s−1); α represents the fiber volume percentage in the filter; L represents the filter thickness (m); df refers to the fiber diameter (m); ηf refers to the filtration efficiency of a fibrous filter made up of cylinder fibers of the same size, placed perpendicular to the airflow and packed evenly; and ηs refers to the filtration efficiency of a single cylinder fiber.

To achieve minimal pressure drop while enhancing the filtration efficiency of the fibrous filter, it is crucial to improve ηs, as indicated by Eqs. (4), (5). The contact probability between a particle and a fiber positively correlates with the PM migration velocity towards the fiber and then determines ηs. The PM migration velocity is influenced by various external fields, including electric, magnetic, temperature, and centrifugal force fields, which drive the mass transfer process of PM onto the fiber surface. For PM2.5 (particulate matter with aerodynamic diameters less than 2.5 μm), which is of significant concern for indoor air quality, electrostatic force plays a crucial role in enhancing the particle migration velocity while minimizing energy dissipation (Fig. 2) [33].

Numerous researchers have utilized electrostatic forces to enhance the mass transfer of PM onto fibers (Fig. 3 [[34], [35], [36], [37], [38]]). Typical technologies include electret filters [39], electrospun nanofiber filters [15], tribo-charge enhanced filters [34], electric field induced-charge enhanced filters [35], and ion-assisted filters [36]. These approaches for charging either PM or fibers offer simple construction and convenient replacement. However, when only PM or fibers are charged, the electrostatic force on the PM is referred to as dielectrophoretic force (FD) and image force (FI), which are comparatively weaker than the Coulomb force generated when both PM and fibers are charged [40].

To further enhance the mass transfer of PM towards fibers, some researchers simultaneously charge PM and fibers with opposite charges, remarkably improving filtration efficiency. Microscopic charge-coupled device (CCD) cameras have been employed to observe the enhanced PM migration velocity boosted by an electric field [41]. As a result of the application of an electrostatic field, the migration velocity of charged particles significantly increases when they move toward the polarized fiber surface. When located at a distance of about 6 μm from the fiber surface, the migration velocity experiences an approximately fivefold increase compared to the scenario without an electric field (Fig. 4 [41]).

3.2. System of the two-stage electrostatically assisted air (EAA) filtration process

Researchers have proposed various approaches for simultaneously charging PM and fibers [37,38]. In these approaches, particles acquire charge through ion generators, corona discharge, and plasma discharge. Fibers can be classified into two types: conductive fibers and dielectric fibers. Conductive fibers acquire charge by being connected to a power supply, such as a battery [42] or a nanogenerator [43]. Dielectric fibers acquire a one-off charge through electret or electrospinning techniques. Alternatively, continuous polarization in an electric field can obtain a long-lasting induced charge. Charge decay is a concerning issue in one-off charging and ion generators [44], and plasma discharge and continuously charging conductive fibers have drawbacks of high energy consumption and cost [31]. Consequently, simultaneously charging PM by corona discharge and charging dielectric fibers by continuous polarization, namely two-stage EAA filtration [45], has emerged as a promising electrostatic-force enhanced filtration technology with potential applications.

For a charged PM and a dielectric cylindrical fiber in an external electric field for polarization, the radial Coulomb force FE (N) between them can be described by Eq. (6) [32]:

${F}_{\text{E}}={q}_{\text{p}}{E}_{\infty }\left[1+\left(\frac{{\epsilon }_{\text{f}}-1}{{\epsilon }_{\text{f}}+1}\right)\frac{{d}_{\text{f}}^{2}}{\text{4}{r}^{2}}\right]$

where qp (C) is the charge on the particle; E∞ (V·m−1) refers to the external electric field intensity perpendicular to the fiber’s center line and passes through the center of the particle; εf is the relative dielectric constant of the fiber; r (m) is the distance between the particle and the fiber.

According to Eq. (6), the migration process of charged PM to fibers can be categorized into three regions based on r, namely the accelerating region, the capturing region, and the adhesion region (Fig. 5(a)) [46]. In the accelerating region (r >> df), the fiber-related parameters (including εf, df, and r) exhibit minimal influence on FE, while qp and E∞ primarily determine FE. PM accelerates in the air flow due to the near-consistent FE until reaching a force equilibrium. In the capture region (r∼df), the induced electric field around the dielectric fibers significantly influences FE. The shorter r/df, the larger εf, the stronger the FE. As a result, the PM experiences an increasing FE as they migrate towards the fibers until being captured on the fiber surfaces. In the adhesion region, where the PM is trapped by the fiber (r ≈ dp/2 + df/2), the interaction between PM and fibers involves FE and van der Waals forces. Depending on the surface energy and roughness of the fiber, the van der Waals forces can be significantly stronger than FE, deciding whether the captured PM would be released.

A fundamental method to construct an EAA filtration device involves creating two sets of charging electrodes (Fig. 5(b) [45]). The particle charging electrodes consist of a (nearly) flat grounding electrode and an electrode with a narrow curvature radius connected to a high-voltage power supply. The two types of electrodes create an uneven electric field that enables corona discharge for particle charging. On the other hand, the fiber charging electrodes consist of two perforated plates or nets that are parallel to each other, with one connected to high voltage and the other to the ground. This electrode pair ensures a uniform electric field over the filter material between them, effectively charging the filter through polarization [45].

In cases where a more compact device is desired, the two high-voltage electrodes can be merged into one [50]. However, there would be a significant coupling between the discharge and polarization zones, which may hinder optimal overall filtration performance. When the device’s size is not concerned, an additional grounding electrode can be installed between the two high-voltage electrodes [46]. To minimize interference between the discharge and polarization zones, the distances between the additional grounding electrode and the two high-voltage electrodes should not be shorter than the distances between the two high-voltage electrodes and their corresponding grounding electrodes.

Furthermore, the PM charging and filter polarizing structural units can be improved. For example, W-shaped polarization electrodes were designed to expand the filtration area, reducing air filtration velocity and air resistance and enhancing filtration efficiency and dust-holding capacity (Fig. 5(c)) [47]. A two-stage cascaded corona charger (Fig. 5(d) [48]) and a particle-free air protection box (Fig. 5(e) [49]) were designed to reduce PM contamination and ozone generation on the high-voltage tip electrodes [48,49].

3.3. Electrostatically responsive fiber

As indicated in Eq. (6), FE on the PM (acting as the mass transfer force) is influenced by the relative dielectric constant of the fibers (εf) when the PM is close to the fibers. Experiments demonstrated a positive correlation between εf and the filtration efficiency enhanced by PM charging and fiber polarization [50]. Besides, the EAA filtration is influenced by factors such as fiber distribution and pore distribution. Notably, even when the pressure drops of filters are comparable (11.6 to 16.9 Pa at 1.1 m·s−1), the electrostatically assisted filtration efficiency for PM0.3-0.5 can vary significantly from 50.6% to 95.4% [45]. Therefore, the design of the fibrous material is essential to the system-level performance of the EAA filtration devices.

We proposed a concept of “electrostatically responsive filters” involving coating a small quantity of heterogeneous material with a high relative dielectric constant onto the fibers with low air resistance (Fig. 6(a)) [41]. The fibrous material can maintain a large physical porosity after coating to sustain its low wind-pressure drop. High-dielectric-constant caking results in much more induced charges on their surface in a polarizing electric field (Fig. 6(b) [41]). This phenomenon facilitates the migration and capture of charged PM, enhancing the PM filtration efficiency. In this way, inexpensive coarse filters can be transformed into highly efficient electrostatically responsive filters with low wind-pressure drop, making the best use of the material.

The preparation methods for electrostatically responsive filters can be categorized into physical and chemical. In physical methods, a high-dielectric-constant material is coated onto the fiber through physical adhesion (Figs. 6(c) and (d) [51]). Tian et al. [45] employed a rapid and extensive roll-to-roll gel squeezing technique to incorporate manganese dioxide, activated carbon (AC), zinc oxide, copper oxide, and barium titanate onto polyurethane (PU) foams. The MnO2-coated polyurethane foams demonstrated 82.5% removal efficiency of undesired ozone produced during PM charging [51]. The physical method usually exhibits a higher loading capacity, shorter preparation time, and lower cost for large-scale production. However, the coatings can be easily released due to the absence of chemical bonding between the high-dielectric-constant material and the fiber substrate. Consequently, the physical method may not be applicable in specific scenarios that demand exceptionally high filtration efficiency, such as cleanrooms.

In the chemical method, a high-dielectric-constant material grows on the fiber, forming a stronger chemical bond. Tian et al. [46] grow thin polydopamine (PDA) coatings on the polyethylene terephthalate (PET) coarse filter through in situ dopamine polymerization, achieving a high filtration efficiency of 99.48% for PM0.3-0.5 and a low wind-pressure drop of 9.5 Pa at 0.4 m·s−1 filtration velocity. Dong et al. [52] further doped the PDA-coated PET with MOF crystals via suction filtration. Gao et al. [53] doped the PDA-coated PET with manganese oxide (MnOx) via dip-coating of potassium hypermanganate precursors (Figs. 6(e) and (f) [53]), demonstrating a 96.8% removal efficiency for 100 ppb level ozone. While chemical approaches provide solutions for coating release, they tend to be expensive and time-consuming, thus posing challenges for achieving large-scale manufacture.

3.4. Improved PM filtration performance

Compared to commercial filter materials with similar filtration efficiency for PM0.3-0.5, EAA filters illustrated a significant reduction in air resistance, ranging from 1 to 3 orders of magnitude (Fig. 7(a) [41,[45], [46], [47], [48],50,51,[53], [54], [55], [56], [57]]). However, it is inappropriate to compare EAA filters with conventional filters in quality factor (QF), which incorporates the ratio between the benefit (filtration efficiency) and the payout (wind-pressure drop) for evaluating the overall performance of filtration materials (Eq. (7)). As EAA filtration consumes additional energy for creating electric fields apart from the fan energy consumption, Tian et al. [54] proposed a comprehensive quality factor (CQF) by assuming that the extra energy consumption for filtration efficiency improvement is equivalent to an extra equivalent pressure drop (Eq. (8)):

$\text{QF}=\frac{-\mathrm{ln}\left(1-{\eta }_{f}\right)}{\text{Δ}p}$
$\text{CQF}=\frac{-\mathrm{ln}\left(1-{\eta }_{f}\right)}{\text{Δ}p+\frac{{\eta }_{\text{fan}}{P}_{\text{E}}}{{v}_{\text{air}}}}$

where ηfan is the estimated efficiency of the fan in an HVAC system, 0.71; PE (W·m−2) refers to the energy consumption for creating electric fields in an air filtration device.

The research on EAA filtration can be categorized into three types for different method to improve the performance: external mass transfer enhancement, materials enhancement, and structure enhancement. Substantial improvements in CQF existed with the advancement of electrostatically responsive new materials and device structural designs (Fig. 7(b)). A study from Germany rated the EAA filter research as “very good” in a relevant review for indoor environments [58]. Due to the limited research on device structures, further investigations are necessary to enhance the overall performance and our understanding of EAA filtration techniques.

Several existing surface modification techniques can be applied to the future development of electrostatically responsive fibers, such as electrospinning [59] and self-polarized assembly technique [60]. Nevertheless, some may not be suitable or require improvement when utilizing coarse fibrous materials as substrates, primarily due to their huge pore size. Examples of such techniques include scrape, spin, and slot coating. Moreover, considering the low cost and limited resistance to high temperatures, air filter substrates are not appropriate for expensive surface modification techniques that involve high-temperature procedures, such as atomic layer deposition and chemical vapor deposition. Therefore, further exploration is required to build a low-cost and scalable surface modification method for electrostatically responsive fibrous materials. Moreover, assessing the porosity, pore size distribution, surface morphology, and surface voltage potential is challenging with conventional methods. Therefore, it is essential to clearly understand the relationship between parameters and the performance of electrostatically responsive filters through thorough characterization and principled investigations.

4. Improving indoor gaseous pollutant removal by multiscale mass transfer enhancement

Extensive research has been carried out on gas adsorption, including gas separation, CO2 capture, and removal of indoor gaseous pollutants [[61], [62], [63]]. Catalysis can complement adsorption as an effective method for enhancing the sustainability of gas purification or separation processes [64]. However, conventional adsorptive and catalytic materials produced via granulation frequently encounter issues related to elevated air resistance, resulting in increased energy consumption. Monoliths, characterized as multilayered net-like structures with interconnecting channels, have been increasingly utilized as adsorption components to address these limitations [65]. The hierarchical porous structures of monoliths and the consequent large gas-solid interfacial contacting areas facilitate improved mass transfer kinetics and significantly reduce pressure drop compared with granular adsorption/catalytic components [66]. This section focuses on the impact of such monolithic structures on the mass transfer process in adsorption and catalysis, thereby improving the removal of gaseous pollutants. The relevant materials and manufacturing process are summarized.

4.1. Mass transfer in gas adsorption and catalysis

Insufficient mass transfer may reduce the concentration gradient of target gas or reactants on the gas-solid interfaces of the materials [67], thus slowing down the adsorption flux and the reaction rates. Enhancing mass transfer kinetics is more economically viable through modifications to the geometric structures and internal porous architecture of adsorption components rather than concentrating on developing high-performance materials with high specific surface area or particular functional groups [68]. In adsorption and catalytic beds, materials typically form into beads with 103−104 μm diameters. The specified size maintains an adequate distance between particles, preventing excessive air resistance [69].

Fig. 8 [70] (up) illustrates that the millimeter-sized beads had a long diffusion path for gas molecules, resulting in diffusion resistance being the primary limitation in the overall adsorption mass transfer process. Generally, a smaller pore size is beneficial for producing a larger specific surface area but is not conducive to mass transfer. An ideal porous adsorbent should have a uniform structure with micropores (< 2 nm) and hierarchical pores (mesopores, 2 nm < pore size < 50 nm; macropores, pore size > 50 nm) [71]. This design maximizes surface area for the distribution of active sites, reduces mass transfer resistance, and enhances access to micropores. However, the pores inside the adsorbent are mainly within the range of 10−1−100 nm [72]. The sizes are less than the average free path of most indoor air pollutants, such as formaldehyde and toluene [73], resulting in slow diffusion rates. The considerable mass transfer resistance inside the material leads to the concentration accumulation at the gas-material interfaces, reducing the gas concentration gradient and, consequently, the mass transfer flux.

Novel adsorbent structures are significant for improving the mass transfer kinetics. For example, the monolith structure with a millimeter-micron-nanometer morphology, a novel honeycomb-like structure consisting of filaments with gaps in 102 μm (Fig. 8, down), can offer several advantages for enhancing adsorption mass transfer. ① The monoliths reduce the gas diffusion path at the macro level compared to the densely packed adsorbent, thereby decreasing gas molecule transport durations. This reduction in inner diffusion resistance mitigates the decline in mass transfer flux. Besides, the honeycomb structures provide sufficient gas-solid interfacial contact, expanding the mass transfer area and enhancing the overall adsorption/catalysis performances [74,75]. ② At the micro and nano scales, optimizing the pore distribution within the porous material, such as increasing the average pore size and providing diffusion channels ranging from hundreds of nanometers to several microns, can accelerate the rapid diffusion and migration of adsorbate molecules inside the porous material. However, achieving this ideal multi-scale structural design requires careful consideration of material selection, preparation procedures, and component structure, which necessitates further investigation. Besides, manufacturing these specifically designed porous monoliths using conventional techniques is challenging due to limitations in fabrication resolution.

Besides, the external field force also promotes gas mass transfer. For instance, non-thermal plasma and the associated ionic wind effect can potentially enhance mass transfer in gas adsorption and catalysis [76,77]. The electric field generated by the non-thermal plasma creates an ionic wind that promotes convective mass transfer, leading to more efficient delivery of pollutants to the catalyst/adsorbent surface [78]. This enhanced transport overcomes diffusion limitations, particularly in scenarios with low flow rates or stagnant boundary layers.

4.2. Preparation of adsorbents/catalysts via direct ink writing (DIW)

Traditionally, monolithic adsorbents and catalysts are manufactured using hydraulic extrusion technology [79]. However, the geometry of the extrusion heavily depends on the pre-fabricated die, requiring additional procedure, equipment, and time. This limitation constrains the geometric flexibility of purification equipment and hinders the material’s versatility. DIW, a three-dimensional (3D) printing technique, has been widely utilized for its diverse suitable materials, cost-effectiveness, and fast production [80].

In the DIW process, it is essential to choose appropriate adsorbents or catalysts to formulate a printable suspension, which is also known as printing ink. The rheological properties, including dispersion, viscosity, and viscoelasticity, should be meticulously determined to guarantee a smooth printing process [81]. Adding appropriate dispersants, binders, and plasticizers effectively enhances the rheological property and the consequent printing product [82]. For DIW-fabricated monoliths, the gas molecule diffusion inside the adsorbent becomes challenging due to the high internal diffusion mass transfer resistance. The reason is that the intrinsic pore size of the porous adsorbent is concentrated at 10−1−100 nm, which is shorter than the mean free path of the gas molecules diffused in it. Consequently, gas molecules are continually captured and desorbed by the pore wall during the diffusion, leading to a winding diffusion path and presenting a high resistance in diffusion (known as Knudsen diffusion) [83]. To enhance the diffusion mass transfer, building more pores within the diameter range of 10−1−100 µm in the material is essential.

At present, the primary methods to achieve pore regulation in printed adsorbent materials are the gas volatilization method (Fig. 9(a)) and the sacrificial template method (Fig. 9(b)). The gas volatilization method is realized by adding a material to the ink that can decompose and generate gas. After printing, the gas can be generated by controlling the temperature of the printing platform [22] so that the pores would form inside the adsorbent. In the sacrificing template method, a template material is pre-mixed into the ink and removed after printing to form pores. As shown in Fig. 9(c), Chen et al. [22] added ammonium carbonate to the adsorbent ink and heated it to generate NH3 and CO2 after printing. The generated gas could form micron-sized pores inside the printed adsorbent (Figs. 9(d) and (e) [22]), and the inward gas diffusion rate of the adsorption film could accelerate. In another approach, Huang et al. [84] utilized micro-nano-sized polymethyl methacrylate (PMMA) beads as the sacrificial template. The beads were first suspended in the polymethylsiloxane/isopropanol solutions to formulate ink and then removed in a high-temperature inert atmosphere (99.99% N2) following the printing (Fig. 9(f)). As shown in Figs. 9(g) and (h) [84], the porosity and pore size of the material could be tailored by changing the size and amount of the PMMA beads [27,84]. By combining the gas volatilization and sacrifice template methods, DIW can construct adsorption components with multi-scale morphology from millimeter to micron to nanometer.

Additional post-processing methods, including surface loading and grafting, are extensively used to enhance gas-solid interactions via specialized physical and chemical interactions [85]. For example, Lawson et al. [86] printed and infused the MOF monolith with amine-dense tetraethylenepentamine (TEPA) or polyethyleneimine (PEI), achieving high CO2 adsorption capacities of 1.8 and 2.8 mmol·g−1, respectively. Medina et al. [87] developed a hyper-crosslinked adsorption monolith by adhering hyper-porous carbon powder to the printed polymer monolith. Pellejero et al. [88] used atomic layer deposition to apply ZnO nanosheets on the printed honeycomb filters, and zeolitic imidazolate framework-8 (ZIF-8) can be obtained via in situ hydrothermal synthesis on ZnO, allowing for efficient VOC removal.

4.3. Improving adsorption performance by structural enhancement

Recent progress in indoor gas adsorption materials includes activated carbon, ceramic adsorbents, MOFs, and polymer adsorbents [89]. These materials are utilized in different structures, including packed adsorption beds, fiber adsorption components, and monolithic adsorption components. The packed adsorption bed is the most widely used component due to its simple structure and convenient manufacturing [90]. It can achieve a lifetime of more than 60 h with high-performance materials loaded [91]. However, densely packed adsorption beds face relatively high resistance, leading to poor mass transfer kinetics and low utilization rates of adsorbed materials [92], resulting in rapid decay in most cases [[93], [94], [95]]. The lifespan of fiber adsorbers is generally shorter because the thickness of typical fibers is limited [96], and excessively thick fibers introduce high wind resistance [97], making them more challenging for application. In many studies, the surface velocity is maintained very low, typically ranging from 10−3 to 10−2 m·s−1, resulting in an initial pass efficiency of 100% [74,90,94,95,98]. However, the breakthrough performances declined rapidly, many falling within 10 h and some even within 1 h [74,93,94,98].

To overcome this, Chen et al. [99] developed a laminated adsorption component by assembling temperature-controllable films coated with adsorbent (adsorption films) in parallel (Fig. 10(a) [99]). This structure resembles a monolith construction, where gas flows through channels, and the monolith adsorbs the target gas. Once the adsorbent saturates, adsorption films can undergo in-situ electro-thermal regeneration. Compared to the traditional regeneration method, which preheats the airflow first and subsequently heats the material, in-situ thermal regeneration has been proven more energy-efficient owing to reduced heat dissipation and heightened regeneration efficiency [99]. The film-structured adsorption component can achieve 82% and 81% single-pass removal efficiency for formaldehyde and toluene at 0.8 m·s−1 face air velocity [22].

The adsorption capacity of gaseous pollutants is influenced by both the equilibrium adsorption capacity and the dynamic adsorption capacity. The equilibrium adsorption capacity is primarily determined by the specific surface area (SSA) of the adsorbent, which decreases as pore size increases. Micropores (pore size < 2 nm) provide a significantly larger SSA compared to mesopores (2 nm < pore size < 50 nm) and macropores (pore size > 50 nm). However, the small size of micropores can limit the diffusion rate of gaseous pollutants, such as formaldehyde and toluene, due to their average free path being larger than the pore diameter [73]. In contrast, the dynamic adsorption capacity, which reflects the adsorption amount [101] and the capturing rate, is more critical in real-world applications. To optimize dynamic adsorption capacity, an ideal adsorbent should possess a hierarchical pore structure that balances high SSA (for equilibrium adsorption) and efficient mass transfer (for dynamic adsorption) [22,102]. Increasing the mesoporous ratio under similar porosity conditions can enhance diffusion rates [103], but an excessively high mesoporous ratio may reduce the SSA and compromise the equilibrium adsorption capacity. Therefore, achieving an optimal pore size distribution is essential to maximize the dynamic adsorption capacity, requiring a careful balance between diffusion efficiency and equilibrium adsorption performance.

Chen et al. [100] employed laser etching to create submillimeter-scale vertical penetration channel structures on the adsorption film (Figs. 10(b)−(e)). Formaldehyde and xylene were chosen as the target adsorbates. The two pollutants were limited in the China National Standard GB/T 18883 and are most widely studied for indoor VOC adsorptive removal [104,105]. Additionally, the adsorption properties of xylene can reflect the purification capabilities of adsorption materials for benzene, toluene, and other aromatic compounds [100]. Compared to adsorbents without channel structures, the adsorption capabilities for formaldehyde and xylene were enhanced by 100% and 80%, respectively. Physical adsorption was demonstrated by X-ray photoelectron spectroscopy and X-ray diffraction spectrum, showing no new functional groups or chemical bonds forming after formaldehyde and toluene adsorption [22]. Therefore, the adsorption films can be extended to other gaseous pollutants, making it a widely available method for air purification. Besides, the adsorption film can be modularized into a laminated structure and scaled to a normalized adsorption module. The efficiency of the adsorption module remains consistent regardless of size, while the airflow volume and pollutant removal rate increase in direct proportion to the windward area [102].

Furthermore, a sub-milli ribbed structure was constructed on the adsorption films using the DIW method (Figs. 10(f)−(h)) [20]. The ribbed structure effectively increases the gas-solid contact area by 58% and accelerates mass transfer by enhancing the airflow turbulence. Experimental results indicate a 37% increase in the one-pass efficiency of formaldehyde, leading to a remarkable enhancement in dynamic adsorption capacity by 152%. Through interfacial structural modification and internal pore regulation, the sub-milli-net hierarchical-porous adsorption plate (SHARP) can achieve outstanding mass transfer kinetics in conjunction with the inherent nanopores of the material. The gradient porous structures in SHARP are promising not only in the field of indoor air purifying but also in broader energy and environmental studies due to their adequate heat and mass transfer dynamics [106]. Similarly, Tammaro et al. [107] combined 3D printing with foaming technology, which made a 20% increase in surface area from 255 to 314 m−1, showing a 96.0% removal efficiency for 500 ppm SO2 at 0.13 m3·h−1 flow rate. Besides, the integration of internal cooling can further improve 28.42% benzene removal performance, primarily due to the reduction in liquid desiccant solution temperature and the corresponding decrease in Henry’s constant, which enhances the driving force for benzene transfer from air to the desiccant solution [108]. Humidity regulation is also essential for improving mass transfer efficiency by reducing the competitive adsorption of water molecules on the catalyst surface, which otherwise obstructs the adsorption and activation of volatile organic compounds such as toluene [109].

5. Conclusions and perspectives

Traditionally, air pollution purification has focused on the qualities of the materials themselves, often neglecting the importance of mass transfer, which ultimately results in high air resistance. This study summarized the novel approach that shifts from passive material usage to active control of mass transfer, setting out to better understand bridging the gap between high-performance materials and high-performance technologies. The main viewpoints can be summarized as follows:

(1) Significance of mass transfer enhancement. This study highlights that optimizing mass transfer can significantly improve air purification technologies’ efficiency and energy performance. For PM removal, we suggest using electrostatic assistance in filtration, which can enhance the migration velocity of PM towards the fibers, resulting in 1−3 orders of magnitude higher CQF compared to commercial filters. We suggest constructing hierarchical structures for gas-phase pollutant removal, which can increase the gas-solid contact area by 58%, leading to a 37% improvement in single-pass removal efficiency and a 152% enhancement in dynamic adsorption capacity.

(2) Approaches to mass transfer enhancement. This study highlights optimizing mass transfer through multiscale (milli-micro-nano) and multifield (mass-flow-force) approaches. The physical fields may include electric, magnetic, thermal, microwave, and ultrasonic fields. These physical fields can improve mass transfer efficiency by influencing the adsorption kinetics, reducing the diffusion boundary layer thickness, and modifying the interactions between adsorbents and adsorbates.

(3) Other influencing factors in real-world applications. Organic pollutants such as VOCs can affect the adsorption performance by occupying the pores of purification materials, thereby reducing the effective surface area available for adsorption. Additionally, these pollutants can alter the dielectric properties of materials, impacting the electrostatic field’s enhancement of mass transfer. Besides, limited indoor space often necessitates the tight compression of purification components, increasing air resistance but enhancing contact between pollutants and materials. Balancing space constraints, resistance, and mass transfer performance is a critical challenge. Furthermore, the ventilation rate (face velocity) significantly influences pollutant-material contact time, thus strengthening or weakening mass transfer. Optimal air flow rates should be determined in different ventilation circumstances to ensure a sufficient clean air supply and efficient mass transfer.

(4) Future research and application guidelines. Future research should focus on developing new interaction mechanisms, innovative fabrication methods, and optimized ventilation strategies to further advance the field. For future applications, substantial implementation expenses, the need for specialized apparatus, and a lack of skilled staff may impede adoption. The long-term performance and stability of EAA filtration and SHARP systems should be further evaluated. Research and development should prioritize cost-effective solutions to address these challenges, and collaborations between industry and academia should be enhanced. The insights and methodologies presented in this study have the potential to revolutionize indoor air purification and contribute to broader applications in various sectors, including chemical, pharmaceutical, and environmental industries.

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