Hybrid Cellulose‐Based Systems for Triboelectrification in Aerosol Filtration, Ammonia Abatement and Respiration Monitoring

The dissipation of charges by aging or under the effect of humid conditions considerably impedes a broader utilization of electrostatic fields in aerosol filtration. This study introduces a respiration‐driven air filter (RAF) that continuously generates triboelectric charges within a pair of tribolayers, which facilitates a sustained filtration performance. Such system is integrated in a multilayer unit that is inserted in personal protective equipment (RAFM) to efficiently capture, sense, and degrade airborne pollutants with no need for external power sources. The triboelectric nanogenerator‐based RAF continuously replenishes static charges and maintains an electrostatic field through breathing by the effect of contact‐electrification between two cellulose‐based tribolayers: a cellulose/metal organic framework cryogel (electron donor) and a cellulose–based electrospun membrane (electron acceptor). Notably, the triboelectric field of the RAF's tribolayer pair substantially enhances both the filtration efficiency (up to 93.8% for 0.3 µm particulate matter) and sensing/catalytic degradation (ammonia; degradation >20%). When integrated in a circuit module, the RAFM effectively monitors respiration dynamics, acting as a breathing indicator/regulator. Overall, this study adds to the promise of tribogeneration through cellulose‐based materials and its application in exposure‐risk operations.


Introduction
Many pathogenic substances are airborne [1,2] and lead to respiratory complications, such as pneumonia or cardiopulmonary failure.Depending on typical symptoms, such as dyspnea, shortness of breath, or irregular breathing patterns, monitoring of the respiration can serve as preventive measure to reduce respiratory diseases. [3,4]Therefore, developing wearable personal protective equipment (PPE) capable of intercepting aerosolized particles and monitoring respiration is highly desirable.The latter involves gas exchange through oxygen inhalation and carbon dioxide exhalation, an essential physiological process. [5,6]ypical PPE, such as facemasks, comprise a thin layer of polypropylene meltblown fabric as middle filtration medium sandwiched between two supporting polyethylene spunbonded layers (face and air side).The face-side layer is hydrophilic to adsorb respiration moisture, while the air-side layer is hydrophobic to repel fluids. [7][13][14][15] Therefore, developing portable and self-driven PPE with high filtration efficiency and respiration monitoring capabilities is an area of active research.
[18] However, relying solely on mechanical filtration is insufficient for removing PM with small sizes, let alone VOCs.One strategy for improving PM removal and VOC degradation in facemasks is to incorporate functional additives into the fibrous medium.For instance, metalorganic frameworks (MOFs) present a porous structure, and large specific surface area.[21] These characteristics render MOFs highly efficacious in the removal of airborne pollutants as well as catalytic degradation.
A supplementary method for enhancing the filtration efficiency of airborne pollutants involves electret phenomena, e.g., the incorporation of electrostatic fields, [22] for instance, by corona charging.Hence, ultrafine PM can be captured by the effect of electrostatic charges without causing a substantial pressure drop. [23,24]Electrostatic adsorption can contribute up to 80% of the overall filtration efficiency. [25,26]Meanwhile, the electrostatic field can further enhance the effectiveness of catalysts, such as MOFs, in degrading ammonia (a common VOC as the primary releases of spoiled seafood and other sources). [27][30][31] Therefore, having an electrostatic field that can be autonomously generated is warranted.
The later challenge brings to the picture the use of triboelectric nanogenerators (TENGs), which involve a coupling effect of contact electrification and electrostatic induction.[39] Therefore, TENG incorporation into PPE can be useful as power source which in turn enhances filtration efficiency.[42] For instance, self-charging masks based on triboelectric fields were designed to capture PMs and maintain continuous electrostatic charges by breathing [26] and self-powered monitoring. [6]Hence, the integration of electrostatic field generated by TENGs into a commercial PPE offers notable advantages as far as improving air filtration efficiency and facilitating continuous respiration monitoring.
Herein, we present a respiratory-driven TENG-based air filter (RAF) that is incorporated into a commercial facemask (RAFM) for high-efficiency PM filtration, ammonia sensing/catalytic degradation, and respiratory monitoring.RAF is demonstrated to continuously generate charges while maintaining a triboelectric field through triboelectrification between an electron donor tribolayer (a composite produced from a cryogel based on cellulose nanofibrils (CNF) combined with titanium dicarboxylate MOF, MIL-125) and an electron acceptor tribolayer (an electrospun mat produced from a suspension containing CNF, polyvinyl alcohol, PVA and polyethylene oxide, PEO).Under the triboelectric field, the tribolayer pair of RAF demonstrated a filtration efficiency as high as 93.8% for PMs 0.3 μm and sustained at a 90.9% (under 60% relative humidity within 12 h) with simultaneous catalytic degradation (NH 3 , >20%).Furthermore, the RAFM facilitates monitoring and regulation of respiration by using a light indicator integrated with a circuit module.The current study substantially expands the application of TENG utilization in airborne pollution and catalytic degradation and demonstrates a great potential for multifunctional intelligent PPE.

Structure and Filtration Principle of the Cellulose-Based Triboelectric System
Maintaining charges on the surface of a filter is a critical factor for efficient electrostatic capture, reported to contribute up to 80% of the total efficiency. [25,26,44,45]To address the electrostatic charge decay, a major limitation of the service life of the filtration unit, we developed a RAF system capable of continuously replenishing the charges, which was then incorporated into the RAFM device (Figure 1a,b).The TENG-structured RAF integrates acceptor and donor tribolayers.The former (acceptor tribolayer) consists of a PVA-CNF-PEO electrospun membrane that serves as face side hydrophobic material while the latter (donor tribolayer) is the air side layer comprising the hybrid CNF-based cryogel/MIL-125 system.
Figure 1c includes the SEM image of the donor tribolayer (CNF-based cryogel/MIL-125) and highlights its threedimensional porous network architecture.Figure S1 (Supporting Information) illustrates the preparation procedure, physical appearance, and thickness of the electrospun acceptor tribolayer (PVA-CNF-PEO).The acceptor tribolayer includes randomly crosslinked nanofibers, with an average diameter of 847 nm ± 319 nm, and numerous three-dimensional micro/nanoscale pores comprising multilayer stacked nanofiber networks.The utilization of nanofibers in RAFM endows a superior mechanical filtration performance compared to typical polypropylene microfibers (Figure 1d and Figure S2, Supporting Information).Furthermore, the esterification that took place in the synthesis of the acceptor tribolayer made it hydrophobic (water contact angle increases from 45°to 109°, Figure S3, Supporting Information), effectively mitigating charge losses due to the moisture generated with exhalation.
As a result of the respiratory process, the donor and acceptor tribolayers engage in rhythmic cycles of contact and separation, generating a triboelectric field.This field acts to draw negativelycharged PM into the tribolayers, where they come into contact with the positively-charged donor tribolayer situated on the outer layer and subsequently gets entrapped (Figure 1e).Simultaneously, uncharged PM, upon traversing the triboelectric field, undergoes a polarization process and is subsequently repelled by the negatively charged acceptor tribolayer, which constitutes the inner layer.The triboelectric field plays a major role in facilitating the interception of PM by successfully combining polarization, repulsion between like-charges, and attraction between oppositecharges.
Notably, within the donor tribolayer, the MIL-125 captures pollutants through three mechanisms (Figure 1f): (1) adsorption onto the exposed metal sites of the MIL-125, (2) interaction with functional groups, and (3) electrostatic interactions.MIL-125 functions as a photocatalyst by initially absorbing photons with energies surpassing the bandgap, leading to the creation of electron-hole pairs.Subsequently, these photogenerated pairs undergo separation and migrate to the surface of the photocatalyst, where they catalyze oxidation-reduction reactions. [27]owever, the recombination of free electrons and holes considerably impedes the catalytic process.In this scenario, the electrostatic field generated by the tribolayers aids in the separation of  electron-hole pairs and facilitates the movement of charges to the photocatalyst's surface (Figure 1g).The separated electrons and holes then interact with water and oxygen to produce hydroxyl groups and superoxide radicals.Ultimately, both holes and functional groups, such as hydroxyl and superoxide radicals, play a vital role in breaking down the pollutant.
Owing to the minimal utilization of raw materials, the cost per unit of tribolayer pair of RAF is estimated to be <0.1 CAD (Table S1, Supporting Information).Besides its high filtration efficiency and electrostatic charge stability, when compared to commercial facemasks the tribolayer pair is also demonstrated for a favorable respiratory resistance, quality factor, cost-effectiveness, and durability (Table S2, Supporting Information).Note: The assessment of cost and durability was conducted using qualitative scoring methods (Notes S1 and S2 Supporting Information, respectively).

Preparation of the Tribolayer Pair of RAF
The cross-linked three-dimensional CNF-based cryogel framework (9.8-11.2mg cm −3 ) was hybridized by in-situ surface growth of the MOF system (MIL-125) (Figure 2a).The three-dimensional structure exhibited a regular distribution of pore size, thin walls between pores, and a smooth surface morphology (Figure 2b,c).MIL-125 (BET surface area of 1550 m 2 g −1 , Figure 2e), is an attractive highly porous solid with potential applications in capturing airborne PM and catalytic degradation of VOCs.
The incorporation of MIL-125 in the cryogel did not affect its morphology (Figure 2i).The polyhedral MIL-125 particles uniformly adhered to the surface, resulting in a roughened and thicker pore wall (Figure 2j).Compared to the original cryogel, the uniform distribution of the Ti in the donor tribolayer further confirmed the successful loading the MOF (Figure S5,S6, Supporting Information).Moreover, the absorption peaks observed at 680 and at 1380 and 1600 cm −1 in the donor tribolayer were attributed to the vibrations of -O-Ti-O-and -CO 2 -, respectively (Figure S7, Supporting Information).XRD analysis further confirmed the successful preparation of the donor tribolayer, as evidenced by the appearance of characteristic diffraction peaks at 6.7°, 9.6°, 11.8°, and 16.5°, corresponding to MIL-125 (Figure S8, Supporting Information).Compared to the energy peaks of C 1 s and O 1 s in CNF-based cryogel, the Ti 2p peak in the donor tribolayer was observed with a titanium content of ≈1.12% (Figure S9, Supporting Information).The in-situ loading of MIL-125 led to a substantial increase of the AFM surface potential (from 99 to 267 mV, Figure 2d,k).

Working Principle of the Tribolayer Pair of RAF
The conversion of respiration into electricity by the tribolayer pair primarily involves a combination of contact electrification and electrostatic induction.An electron cloud model has been proposed to describe the process of respiration-electricity conversion (Figure S10, Supporting Information).The donor and acceptor tribolayers are periodically actuated by airflow, contacting and separating the tribolayers, thereby converting the energy of respiration into electrical signals (Figure 3a).In the initial state, transferred charges trapped in the surface states of the two non-contacting tribolayers are not apparent (Figure 3a(i)). [46]During exhalation, the transferred charges in the surface states induce a shift in the Fermi level of the corresponding Cu electrodes, resulting in an electrostatic potential between the two Cu electrodes that produce a current flow (Figure 3a(ii)).Upon completion of the exhalation process, the two tribolayers make full contact and their transferred charges, trapped on the surface, compensate each other, resulting in zero current flow across the external circuit (Figure 3a(iii)).During inhalation, the acceptor tribolayer retracts to its original position, and the two tribolayers separate from each other.The presence of trapped charges on the surface induce electrostatic effects, resulting in a potential difference between the two Cu electrodes and generate an opposing current flow (Figure 3a(iv)).After the separation of the acceptor and the donor tribolayers and restoration of the original state, a complete balance of Fermi level difference between the two Cu electrodes is achieved through electron transfer, resulting in no external current flow (Figure 3a(v)).
The CNF-based cryogel and the hybrid CNF-based cryogel/MIL-125 were separately used as electron donor tribolayer to investigate the effect of MIL-125 on the triboelectric properties.Compared to the tribolayer with no MIL-125, the open-circuit voltage of the hybrid CNF-based cryogel/MIL-125 tribolayer increased from 33.2 to 70.5 V (Figure 3b), while the short-circuit current and transferred charge also increased from 6.7 to 11.3 μA (Figure 3c) and from 20.3 to 35.3 nC (Figure 3d), respectively, under a contact area of 3.5 cm 2 , frequency of 2 Hz, force of 10 N, and distance of 2 mm during compressionrelease cycles.This increase is primarily attributed to the strong electron-donating ability of Ti in MIL-125, which enhances the surface charge density of the hybrid tribolayer.Additionally, the in-situ loading of MIL-125 introduced nanostructures on the surface of the CNF-based cryogel, thereby increasing the contact area with the acceptor tribolayer.
An experiment was conducted to investigate the impact of operating frequency and contact force on the output power of the tribolayer pair.With increasing the operating force, the voltage, current, and transferred charge increased from 70.5 V, 35.3 nC, and 11.3 μA to 169.8 V, 65.5 nC, and 34.5 μA, respectively, which can be attributed to a larger contact area (Figure 3e).The voltage and transferred charge remained relatively constant with increasing operating frequency, which can be attributed to the saturation of triboelectric charges on the donor tribolayer surface (Figure 3f).The current, which depends on surface charge density and charge transfer period, increased with increasing frequency, from 7.5 to 21.2 μA.The tribolayer pair was evaluated for its output power in ambient mechanical energy conversion by measuring the resistance as an external load.The voltage exhibited a positive correlation with the load resistance, and at a load resistance of 1 × 10 7 Ω, the instantaneous maximum power output reached 1.41 W m −2 (Figure 3g).

Filtration Performance of the RAF's Tribolayer Pair
The filtration performance of the tribolayer pair was evaluated (Figure 4a).A particle-laden air flow (PM0.3,PM2.5, and PM10) was generated by combining an air pump with smoke from burning sandalwood incense, and subsequently injected into an acrylic box (inner dimensions: 40 cm × 40 cm × 50 cm) to traverse the test filter system (diameter: 2.5 cm) (Figure 4b and Video S1, Supporting Information).The air flow rate was maintained constant at 12 cm −1 s, unless otherwise specified.
Notably, the donor and acceptor tribolayer pair were initially contacted and separated for 20 min to increase the surface potential.This is because enhancing surface charge has been proven to be an effective way of increasing filtration efficiency through electrostatic effects, without sacrificing the air permeability of the filter media. [15]The number of PMs was detected using a particulate counter, and the removal efficiency was calculated as follows: where c 0 and c 1 represent the number concentration of PMs upstream and downstream of the filter medium, respectively.The overall efficiency of the filter increased as the PMs size rose from 0.3 to 10 μm (Figure 4c).Hereafter, the filtration efficiency is specified for 0.3-μm PMs unless otherwise indicated (this is because this PM size has the highest penetration or lowest filtration rate).Specifically, the removal efficiency showed a slight increase over that of a commercial surgical mask: the values reached 87.8%, 88.5%, and 93.8% for the commercial mask, tribolayer pair without triboelectric field and tribolayer pair with triboelectric field, respectively.For the CNF-based cryogel and donor tribolayer filter, the efficiency considerably dropped to only 49.7% and 54.7%, respectively.The positive impact of MIL-125 on filtration efficiency stemmed from its specific surface area and small pore size.The filtration efficiency of the tribolayer pair without triboelectric field exceeded that of the donor tribolayer by 33.8% since adding the acceptor tribolayer increased the thickness of filter media, compelling the PMs to travel a longer distance.The tribolayer pair with triboelectric field exhibited 5.3% higher filtration efficiency than the tribolayer pair without triboelectric field.
The increase in removal efficiency comes at the expense of breathing resistance (pressure drop) due to the large surface area, which also results in an increasing air viscous force between the filter media and PMs (Figure 4d).The quality factor (QF) was used as a metric for assessing the filtration efficiency, determined using the following equation [23] : where Δp represents the pressure drop across the test filter.Despite the high efficiency achieved by commercial surgical mask, the pressure drop was also considerable (137 Pa), resulting in a low QF, 15.4 kPa −1 .Although the CNF-based cryogel system exhibited a relatively low pressure drop (62 Pa), it also showed a poor efficiency (low QF, 11.2 kPa −1 ).The tribolayer pair with triboelectric field exhibited an optimal QF (28.7 kPa −1 ), attributed to the well-balanced removal efficiency and pressure drop (97 Pa), as compared to other filter media (Figure 4e).The tribolayer pair demonstrated exceptional filtration efficiency and breathability compared to the values reported in other studies (Table S3, Supporting Information), even when compared to those utilizing external power sources for charge induction or in tests at low air velocities.
The filtration effectiveness of the tribolayer pair (Figure 4f) was assessed using a simulator that replicated the inhaling and exhalation movements, and the entire process of simulating the respiratory function is presented in Video S2 (Supporting Information).Under conditions of air pollution, the particle (PM0.3)concentration was assumed at 2 586 700 particles L −1 .This value was reduced to 186 424 particles L −1 by the tribolayer pair with triboelectric field, resulting in a removal efficiency of 92.8% (Figure 4g).Unfortunately, the removal efficiency inevitably diminished over time, particularly under high-humidity conditions.Under 90%RH, the removal efficiency decreased from 91.2% to 83.4% within 12 h.In contrast, the filtration efficiency remained at 90.9% level at RH <60% over the same testing period (Figure S11, Supporting Information), benefiting from the dry atmosphere that reduces charge decay.The results highlight the ever-present challenge of the continuous exhalation of moist air.In such cases, continuous charge replenishment should be considered to maintain the electrostatic effect.

Ammonia Sensing by the Tribolayer Pair of RAF
The tribolayer pair was used to assess the sensing potential of ammonia, a hazardous gas that is the primary constituent of the releases of spoiled seafood and other sources.Triboelectric field acts to draw NH 3 , a polar molecule, into the tribolayers, where they come into contact and get entrapped with the airside, positively charged donor tribolayer.Simultaneously, NH 3 molecules are subsequently repelled by the face-side (negatively charged) acceptor tribolayer.For the purpose of the present investigation, the system was tested with the aim of monitoring ammonia concentration level as an indicator of seafood freshness (Video S3, Supporting Information).The adsorption mechanism of gas molecules by MIL-125 primarily involves binding to exposed metal sites, interaction with functional groups, and electrostatic interactions (Figure 5a).Meanwhile, the 3D porous microchannels of the cryogel offered an opportunity for MIL-125 to act as active ammonia adsorption sites: MIL-125 binds NH 3 molecules, which are then degraded by a process represented by the following reactions (Figure 5b) [47][48][49][50] : The triboelectric field in our tests showed a NH 3 concentration reduction to 76.4% within 180 min (in its absence, it only decreased to 85.6%, Figure 5c).Subsequently, the output performance of the tribolayer pair was assessed in the presence of NH 3 to investigate whether the catalytic degradation of NH 3 impacts the triboelectric behavior.At an NH 3 concentration of 100 ppm, the open-circuit voltage exhibited a decline from 70.5 to 15.8 V (Figure 5d), accompanied by substantial reduction in both shortcircuit current and transferred charge, which decreased from 11.3 to 3.2 μA (Figure 5e) and from 35.3 to 10.1 nC (Figure S12, Supporting Information), respectively.This is primarily due to the adsorption of NH 3 molecules onto the active sites of MIL-125, hindering the binding of the acceptor tribolayer to these sites and consequently resulting in a reduction in the microcapacitor network, leading to a decrease in charge storage capacity. [51]Furthermore, the generation of electrons through NH 3 degradation led to a reduction in the charge density of positively charged triboelectric materials.To investigate the sensitivity of the tribolayer pair to NH 3 , voltage responses were measured at varying NH 3 concentrations.The observed decrease in voltage, from 62.2 to 24.8 V, with NH 3 concentration going from 20 to 80 ppm is attributed to the high levels of NH 3 that reduce the microcapacitor network (Figure 5f).
The response and recovery times of the tribolayer pair at a NH 3 concentration of 100 ppm were 14 and 23 s, respectively, which is critical for real-time gas sensing applications (Figure 5g).The gas sensitivity of the tribolayer pair was defined as the slope of the voltage response versus NH 3 concentration, where V0 and V represent the voltage in air and in the presence of NH 3 , respectively.The optimal response of (|V0 − V|/V0) (Y) to NH 3 concentration (X) was best described by the following equation: Y = 0.1614X − 0.1681 (Figure 5h).The tribolayer pair exhibited a response time of 60 s to NH 3 at a concentration of 100 ppm, indicating the stable adsorption and degradation capacity of NH 3 on the donor tribolayer (Figure S13, Supporting Information).
The tribolayer pair exhibited excellent sensing capability to NH 3 at concentrations of 20, 60, and 100 ppm during the 3 h testing, demonstrating the long-term sensitivity of the donor tribolayer (Figure 5i).Although NH 3 is the primary gas emitted from expired seafood, the interference caused by other common gases released simultaneously plays a crucial role in determining NH 3 selectivity.Interestingly, the response of the tribolayer pair to other gases (CH 2 O, CH 3 OH, NO 2 , and H 2 S) released at equivalent concentrations was negligible (Figure 5j).The donor tribolayer used in this study exhibited superior response and recovery times upon exposure to ammonia as compared to other NH 3 sensing systems (Table S4, Supporting Information).To embrace the Internet of Things paradigm and facilitate the instant detection of food spoilage, a wireless sensing system that interfaces with mobile devices was developed, as illustrated in Figure 5k.The extent of prawn spoilage was directly correlated with variations in ammonia concentration, a parameter that was continuously tracked via the voltage signal generated by the tribolayer pair (Figure 5l).The trend of the voltage signal received by the mobile terminal was similar to that shown in Figure 5f, demonstrating the potential of the tribolayer pair for wireless real-time detection of NH 3 concentration.

Application Demonstration of a Smart RAFM
The RAFM was combined with two circuit modules featuring distinct types of relays to establish a human-machine interface, demonstrating the potential of the RAF's tribolayer pair in the intelligent wearable devices and smart medical applications.The process of data acquisition for the smart facemask with the RAF in the human-machine interface is illustrated in Figure 6a.When the intelligent RAFM collected respiratory energy to generate electrical signals, these signals were initially fed into a signal conversion and transmission module.After amplification and conversion, they were wirelessly transmitted and received by a signal receiving and converting module.The converted signals were then used to control the relay for switching on or off an electrical appliance.
The voltage signals captured by the intelligent RAFM were recorded for three respiratory patterns-normal, rapid, and deep.Rapid breathing (50-70 breaths min −1 ) resulted in a greater number of voltage peaks compared to normal breathing (20-40 breaths min −1 ).Furthermore, during deep breathing, voltage peaks were observed to be higher and wider than those produced during rapid breathing.These findings clearly demonstrate the impact of different patterns of breathing on magnitude and frequency of the generated voltage (Figure 6b).More details are shown in Videos S4,S5 (Supporting Information).
The breathing indicator (light) remained inactive during normal respiratory patterns since the respiratory signal did not exceed the threshold of the conversion and transmission module (Figure 6c).However, in case of deep respiratory patterns, the breathing signal surpassed the threshold.The electric signal was then amplified, converted, and transmitted via a transmission module, before being received and converted by a control module.Finally, the converted signal activate the light indicator (Figure 6d).More details are shown in Video S6 (Supporting Information).

Conclusion
The decay of charge in an electret membrane undermines the filtration efficiency against harmful airborne PMs and gases.This study introduces an efficient, durable, and cost-effective hybrid cellulose-based TENG filter that continuously replenish electrostatic charges.The system leverages the triboelectric field between the donor and acceptor tribolayers to efficiently cap-ture, sense, and degrade airborne pollutants with no need for external power sources.The tribolayer pair with triboelectric field substantially enhances both the filtration efficiency (up to 93.8% for 0.3 μm PM) and sensing/catalytic degradation (NH 3 ; degradation >20%).Under 60%RH, the removal efficiency remains at a 90.9% level within 12 h.Furthermore, the tribolayer pair is found most useful in respiratory monitoring and to regulate breathing indicators when integrated with a circuit module.
The present study considerably broadens the scope of the triboelectric field by considering cellulose-based materials in multifunctional wearable PPE.

Experimental Section
PVA-CNF-PEO Membrane (Acceptor Tribolayer): An electrospun nonwoven or membrane was produced using a suspension comprising PVA (MW = 17 000 ± 50, Sigma-Aldrich), 3% TEMPO-oxidized CNF (CNF, 2 wt.%,Sigma-Aldrich, based on the dry weight of PVA), and 8% PEO (1500 g mol −1 , Sigma-Aldrich, based on the total dry weight of PVA and CNF).The aqueous suspension was injected into a 10 mL syringe to generate the fluid jet under ambient humidity (45% relative humidity (RH)) and at a temperature of 25°C.This process was conducted with an applied voltage of 30 kV while maintaining a spinning speed of 2 mL h −1 for 6 h.
CNF-Based Cryogel: Initially, 0.3 g of PVA was dissolved in a 100 mL suspension of TEMPO-oxidized CNF at a concentration of 0.5% (by weight).The solution underwent magnetic stirring at a temperature of 90°C for duration of 2 h, followed by subsequent cooling to room temperature.Subsequently, a mixture comprising 1.2 g of citric acid (CCA, 192.124 g mol −1 , Sigma-Aldrich) and 1.2 mL of phosphoric acid (H 3 PO 4 , 80 wt.%, Sigma-Aldrich) was introduced into the suspension under continuous stirring for a duration of 1 h.The suspension was thereafter placed into a plastic beaker and promptly subjected to quick freezing by immersion in liquid nitrogen for duration of 5 min.After achieving complete freezing, the samples were subjected to a freeze-drying procedure to generate the cryogel.To further strengthen the cryogel, hornification of the cellulose was achieved by heating the samples at 60 °C for 2 h in an oven.After this, the samples were meticulously washed with deionized water and subsequently dried at ambient temperature.
MIL-125 Synthesis: The process of synthesizing MIL-125 (Ti 8 O 8 (OH) 4 [O 2 C-C 6 H 4 -CO 2 ] 6 ) involved the following steps: a combination of 1.5 mmol (170 mg) of terephthalic acid or 1,4-benzenedicarboxylic acid (with a purity of 98% from Aldrich) and 1 mmol (0.2 mL) of titanium isopropoxide Ti(OiPr) 4 (with a purity of 98% from Acros Organics) was prepared in a solution consisting of 3 mL of dimethylformamide (extra dry from Acros Organics) and 0.4 mL of dry methanol (with a purity of 99.9% from Aldrich).This mixture was gently stirred for 5 min at a temperature of 20 °C.Afterward, it was transferred into a 23 mL Teflon liner and inserted into a metallic digestion bomb, where it was subjected to a temperature of 150 °C for a period of 15 h to facilitate the synthesis of MIL-125.After reverting to room temperature, the solid was recovered through filtration, followed by two washes with acetone and subsequent drying under ambient air conditions.The residual solvent was eliminated via drying at a temperature of 220°C for duration of 16 h.

CNF-Based Cryogel/MIL-125 Assembly (Donor Tribolayer):
The CNFbased cryogel was combined with the MIL-125 system, yielding a hybrid material with a yellowish hue.This hybrid material underwent two rounds of washing with acetone and was subsequently dried under ambient air conditions.
Assembly of the RAF and RAFM: The CNF-based cryogel/MIL-125 and PVA-CNF-PEO membrane were cut into 2-cm circles, designating them as the positive and negative tribolayers, respectively.Two ring-shaped copper foil electrodes, each with a thickness of 0.1 mm, were attached to the outer edges of these tribolayers.To ensure a uniform distance between the two tribolayers, a circular support layer composed of polylactic acid, measuring 1 mm in thickness, was utilized.The tribolayers were each electrically connected to wires to enable the transmission of electrical current to an external circuit.This entire setup, referred to as the RAF system, was integrated into a commercially available mask (RAFM) to create a self-charging PPE unit.
Characterization: To understand the properties and composition of the CNF-based cryogel, the powder form of MIL-125, and the hybrid CNFbased cryogel/MIL-125 system, a scanning electron microscope (Jeol Neoscope JCM-5000) was employed to examine their surface morphology and elemental makeup.The surface potential of the samples was measured using an AFM system (Hitachi, AFM5100N, Japan), [43] the crystal structures of the samples were determined through XRD analysis conducted with a Smartlab-3KW + UltimaIV3KW instrument.The XRD analysis involved scanning the samples at a speed of 5°min −1 within a range of 5°-30°.The chemical characteristics were evaluated using ATR-FTIR spectroscopy (TENSOR II, Brook Technology, Germany).Chemical elemental analysis was employed using K-Alpha XPS (Thermo Fisher Scientific, ES-CALAB 250XI+, USA) with the test voltage and current set to 12 kV and 6 mA, respectively.Contact-angle measurements were conducted using a Ramé-Hart model 500-F1 contact-angle measurement system (Ramé-Hart Instrument Co.).The electrical output performance of the TENG-based air filter was evaluated using a Keithley 6514 system electrometer, while a digital linear motor (LinMot H01-23386/160) was employed to execute the contact and separation cycle at various forces and frequencies.
Filtration Performance: The filtration performance was evaluated using a dedicated testing platform.The PMs were produced by sandalwood incense combustion and transmitted via compressed air.The compressed air flow rate was regulated to manage the PM concentration.The PM counter was used to determine the PM concentration in 1 L of air.A differential pressure gauge was utilized to measure the pressure drop across the tested sample.After positioning the tested sample, a thermal anemometer was deployed to accurately measure and record the air flow rate.
NH 3 Sensing Testing Platform: The CNF-based cryogel/MIL-125 tribolayer was cut into circle (5 cm) and served as NH 3 sensing unit.The NH 3 concentration was regulated using a rotameter and monitored with a specialized ammonia detector.The NH 3 employed for real-time testing was derived from the gas produced by spoiled prawns.
To simplify terminology, the hybrid CNF-based cryogel/MIL-125 was referred as the "donor tribolayer" and the PVA-CNF-PEO membrane was referred as the "acceptor tribolayer."

Figure 1 .
Figure 1.a) Schematic illustration of the RAFM comprising an integrated TENG-based RAF system.b) Photograph of the RAFM assembled with a tribolayer pair.c) Photograph (i) and SEM image (ii) of the donor tribolayer (CNF-based cryogel/MIL-125).d) Photograph (i) and SEM image (ii) of the acceptor tribolayer (PVA-CNF-PEO).e) Schematic illustrating of particle matter (PM) capture under a triboelectric field.f) Effect of the MOF (MIL-125) in the capturing mechanism.g) Promotion of MIL-125 catalytic efficiency by the triboelectric field.

Figure 2 .
Figure 2. a) Schematic of the fabrication process of CNF-based cryogel/MIL-125.b) Photograph, c) SEM image, and d) surface potential distribution of CNF-based cryogel.e) SEM images, f) Ti, C, and O mapping images, g) molecular structure, and h) infrared spectra of MIL-125.i) Photograph, j) SEM image, and k) surface potential distribution of CNF-based cryogel /MIL-125.

Figure 3 .
Figure 3. a) Triboelectric charge generation of RAF's tribolayer pair during respiration.b) Output voltage, c) current, and d) transferred charges of a cellulose-based cryogel tribolayer free of MIL-125 (CNF-based cryogel) and loaded with MIL-125 (CNF-based cryogel/MIL-125).Output voltage, current, and transferred charges of the tribolayer pair e) at different forces ranging from 10 to 50 N and f) at excitation frequencies ranging from 1 to 5 Hz.g) The power density of the tribolayer pair under different load resistances.

Figure 4 .
Figure 4. a) Schematic illustration of the experimental setup utilized to evaluate the PM filtration performance.b) Optical photograph of the experimental setup.c) PM diameter-dependent filtration efficiency.d) Filter media-dependent pressure drop.e) PM diameter-dependent corresponding quality factor.f) Photograph of a system designed to mimic the dynamics of respiration.g) Values of PM removal efficiency from the simulator.

Figure 5 .
Figure 5. a) Schematic illustration of the mechanism of ammonia adsorption by the CNF-based tribolayer pair.b) Schematic of the NH 3 sensing mechanism by the tribolayer pair.c) The concentration changes in NH 3 degradation process with and without a triboelectric field.d) Output voltage and e) current of a tribolayer pair in air and NH 3 .f) Output voltage plots of the tribolayer pair at different NH 3 concentrations.g) Real-time response/recovery process of the tribolayer pair at 100 ppm NH 3 .h) Response fitting curves of the tribolayer pair at different NH 3 concentrations.i) Tribolayer pair long-term stability.j) Selectivity of the tribolayer pair to different interfering gases.k) Optical photograph showing the respiration simulator and l) corresponding wireless sensing of voltage signals upon prawn decomposition.

Figure 6 .
Figure 6.a) Control circuit diagram of a RAFM.b) Real-time voltage signals to record normal, rapid, and deep breathing states.c) Switching off and d) on the emergency indicator light driven by the RAFM.