Self-charging electrostatic face masks leveraging triboelectrification for prolonged air filtration

The study addresses the rapid decay of electrostatic charges in conventional electret mask filters, which undermines long-term filtration performance, especially in humid breathing environments. The goal is to develop an efficient, durable, and low-cost air filter that maintains high filtration efficiency over prolonged use without reliance on external power supplies. The authors propose a self-charging air filter (SAF) that leverages triboelectrification between PVDF nanofibers and nylon layers to continuously replenish surface charges during breathing. The purpose includes: (1) optimizing PVDF electrospun filter media for mechanical filtration (balancing efficiency and pressure drop); (2) establishing a quantitative relation between surface electrostatic potential and filtration efficiency; and (3) demonstrating a breathing-excited triboelectric self-charging mechanism that prolongs effective mask lifespan under standardized airflow conditions. This is significant for enhancing respiratory protection, reducing waste from frequent mask replacement, and informing standardized manufacturing of high-performance filters.

The paper reviews strategies to improve mask filtration beyond mechanical mechanisms. Corona electret charging is widely used and can contribute up to 80% of total efficiency but suffers from charge decay, accelerated by humidity due to increased fiber conductivity. Prior works include: (i) continuous high-voltage (20 kV) charging of polydopamine-coated PET maintaining ~99.48% efficiency for 0.3-µm over 30 days; (ii) ionic liquid polymer-coated melamine formaldehyde sponge achieving 99.59% PM2.5 removal at 3 V with stable performance over 21 days; (iii) triboelectric nanogenerator (TENG)-assisted filters, e.g., freestanding sliding TENG delivering 1.8 kV to sustain 94% efficiency for 0.3 µm over 48 h; (iv) a self-powered mask based on triboelectric effect that retained 86.9% efficiency for 0.3 µm after 240 min and remained stable after 30 days of storage; and (v) hybrid piezoelectric/triboelectric films embedded in masks. The key gaps identified are the reliance on external power/batteries, suboptimal efficiency and high respiratory resistance in some triboelectric designs, lack of continuous-wearing performance evaluation, inconsistent airflow test standards across studies, and absence of a quantitative relation between electrostatic charge and filtration efficiency. The present work addresses these by using a breathing-excited self-charging sandwich structure (PVDF/nylon), standardized testing (NIOSH 42 CFR 84-equivalent flow), and deriving efficiency–potential relationships.

Electrospun PVDF filter fabrication: PVDF (M = 1,000,000) was dissolved in DMF/acetone (4:5 wt/wt), stirred at 60 °C to form a uniform solution, and electrospun using an NS-1 machine. A 5 mL syringe with 20-gauge needle dispensed at 1.5 mL/h toward a grounded drum (10 cm gap). Voltage, PVDF usage, and solution concentration were tuned to control fiber diameter and basis weight. Films were dried in situ at 60 °C overnight. Parameters are in Supplementary Table 4. Corona electret treatment: PVDF films were suspended, exposed to a negative high-voltage needle (≈2 cm spacing) to inject charge. Surface potential was set via voltage/time (Supplementary Table 5). Surface potential was measured in situ with an electrostatic tester at ~25 mm probe distance; films were reset using an anti-static gun before each charge. SAF assembly: The self-charging air filter (SAF) is a sandwich structure with an electrospun PVDF nanofiber layer between two triboelectric layers (nylon fabric primary; also tested copper mesh and conductive fabric). Copper meshes were attached externally as current collectors for electrical characterization. The SAF was integrated into a commercial mask shell. Before durability tests, the PVDF layer was corona-charged to about −3.3 kV. Filtration testing platform: Aerosols were generated by burning incense and entrained by compressed air. PM2.5 concentration was held at 500 µg/m³. Two A4-CG laser sensors measured particle number concentrations upstream and downstream to compute efficiency η = (C0 − C1)/C0 × 100%. Unless otherwise specified, the airflow velocity was 0.35 m/s (≈80 L/min, NIOSH 42 CFR 84), with grade efficiencies reported for particle size bins centered at 0.3, 0.5, 1, 2.5, 5, and 10 µm. Differential pressure (ΔP) was measured with a Testo 510 gauge; air velocity with a Testo 405-V1 anemometer. Quality factor QF = −ln(1 − η)/ΔP was calculated. Mechanical optimization: Films with different fiber diameters (141 nm, 407 nm, 694 nm, 915 nm, 2.86 µm) and basis weights (2–10 g/m² at 694 nm) were tested to determine efficiency, ΔP, and QF. Velocity effects were examined from 0.15 to 0.55 m/s. Electrostatic effect and efficiency–potential relation: Films were negatively charged via corona to various surface potentials (e.g., −0.5 to −3.3 kV). SEM was used to visualize particulate capture dynamics on uncharged vs charged fibers over time. Efficiency across particle sizes and QF at 0.3 µm were measured at each potential; pressure drop changes were recorded. Humidity-dependent charge decay was studied at 20% and 50% RH over 5 days. Triboelectric characterization: The SAF’s triboelectric output under simulated breathing (peak flows 80, 260, 620 L/min for slow, moderate, fast) was measured by oscilloscope, demonstrating contact–separation induced alternating voltage. A symmetric three-layer structure (nylon–PVDF–nylon) was compared with a two-layer structure; alternative tribo-materials were screened against nylon. Piezo vs tribo mechanism was distinguished using a PVDF cantilever piezo device (no signal) versus a contact–separation tribo device (distinct signal). COMSOL simulation: Electrostatic potential distributions were modeled (Electrostatics module) for PVDF/nylon and PP/PE pairs with assigned surface charge densities (±34.5 µC/m² for PVDF/nylon; ±22 µC/m² for PP/PE). Geometry: 1 mm × 25 mm interfaces with 1 mm separation; infinite element boundary; extremely fine mesh. Human wear tests: Four participants (2 females, 2 males, 22–32 years) wore the self-charging mask 10 h/day for 3 days (total 60 h, with 30 h wearing). Electrical outputs before and after wear and filtration performance (efficiency and ΔP) were measured. Ethics: IRB approval (City University of Hong Kong, protocol 2-2021-49-E) with informed consent. Characterization: SEM (ZEISS EVO MA10), XRD (PANalytical X’pert³, Cu Kα), thickness gauge (AICE), oscilloscope (Rohde & Schwarz RTE1024).

• Optimized mechanical filtration: A PVDF nanofiber film with 694 nm fiber diameter and 6 g/m² basis weight achieved 92.7% efficiency for 0.3-µm particles with 86 Pa pressure drop, yielding a quality factor (QF) of 30.5 kPa⁻¹. Finer fibers (141 nm) increased efficiency to 98.0% but with high ΔP = 198 Pa (QF = 19.8 kPa⁻¹), while larger fibers (2.86 µm) had low ΔP = 62 Pa but poor efficiency (77.2%, QF = 23.8 kPa⁻¹). Increasing basis weight (2→10 g/m² at 694 nm) increased efficiency (82.3%→94.7%) but raised ΔP (86→230 Pa), reducing QF beyond 6 g/m². - Air velocity effects: Increasing velocity from 0.15 to 0.55 m/s reduced 0.3-µm efficiency from 95.0% to 87.3% and increased ΔP from 38 to 115 Pa, reducing QF. - Efficiency–potential relation: Charging the 694 nm film to negative potentials increased grade efficiencies without increasing ΔP. At −3.3 kV, efficiency gains vs uncharged were +7.39% (0.3 µm), +6.86% (1 µm), +6.95% (2.5 µm), +6.25% (5 µm). QF at 0.3 µm rose up to 64.1 kPa⁻¹. SEM showed more and faster coalescence of captured particles on charged fibers. - Humidity-driven charge decay: At 50% RH, surface potential decayed from −3.05 kV to −0.23 kV in 5 days (≈92.5% decay). At 20% RH, −0.7 kV remained after 5 days. - Triboelectric self-charging: The SAF (nylon–PVDF–nylon) generated breathing-excited voltages from >1 V (slow, 80 L/min) up to ~10 V (fast, 620 L/min). The symmetric three-layer structure outperformed two-layer configurations; material selection was critical, with PVDF/nylon far superior to PP/PP (PP/PP yielded 0.1 V). - Charge retention during wear: With initial corona charging (−3.3 kV) plus continuous triboelectric replenishment, the SAF maintained strong electrical output after 60 h (including 30 h wearing). Comparative potential decay after 10 h wearing: PVDF/nylon retained −0.75 kV from −3.3 kV, while PP/PP decayed from −2.9 kV to −0.27 kV. - Long-term performance: After 60 h (30 h worn), the SAF retained 95.8% efficiency for 0.3 µm with minimal ΔP increase (85.1→86.4 Pa), meeting N95 criteria and outperforming commercial surgical masks in efficiency and durability. SEM showed no notable fiber morphology degradation after testing. - Cost-effectiveness: Minimal raw materials enabled an estimated per-unit cost of 0.47 HKD for the SAF, outperforming commercial masks in a radar-chart comparison including cost and durability.

The work demonstrates that combining optimized mechanical filtration via electrospun PVDF nanofibers with triboelectric self-charging effectively addresses the central challenge of charge decay in electret masks. The established quantitative relation between surface potential and filtration efficiency clarifies how electrostatic adsorption supplements mechanical mechanisms without incurring additional pressure drop, enabling higher quality factors. Nylon–PVDF pairing, selected for large electron affinity differences and configured in a symmetric sandwich, provided reliable charge transfer under natural breathing, maintaining electrostatic adsorption over extended wear despite humid exhalation. The result is a durable, high-efficiency mask that sustains N95-level performance over 60 h with negligible increase in respiratory resistance, offering practical benefits for public health and reduced environmental burden from frequent mask replacement. The standardized testing conditions (80 L/min, NIOSH-inspired) and COMSOL-supported electric field comparisons further validate and contextualize performance gains relative to conventional PP-based surgical masks.

This study presents an efficient, durable, and low-cost self-charging air filter that leverages triboelectrification between an electrospun PVDF nanofiber layer and nylon fabric to continuously replenish surface charges during breathing. Mechanically optimized PVDF media (694 nm, 6 g/m²) provided a strong baseline (92.7% at 0.3 µm, 86 Pa), while electrostatic charging substantially increased efficiency and quality factor without added pressure drop. The breathing-excited SAF sustained 95.8% efficiency for 0.3 µm after 60 h (30 h worn) with minimal ΔP increase, outperforming commercial surgical masks in durability and efficiency. The study also establishes a quantitative efficiency–potential relationship, informing standardized, high-efficiency filter design. Future work could explore scalability and manufacturability, broader triboelectric material combinations and structures, long-term field tests across diverse environmental conditions and user behaviors, and integration with decontamination or reuse protocols without compromising triboelectric performance.

• Environmental scope: Charge decay and performance were characterized primarily at 20% and 50% RH; broader humidity and temperature ranges, and real-world fluctuating conditions, were not fully explored. - Aerosol surrogate: Incense smoke was used as the particle source; while size-resolved, it may not fully represent complex bioaerosols or diverse environmental PM. - Wear study scale: Human wear tests involved four participants over 3 days; larger cohorts, longer durations, and varied activity levels would strengthen generalizability. - Initial charging: Durability results combined corona pre-charging with triboelectric replenishment; the long-term performance of purely tribo-charged filters without initial electret treatment was not isolated. - Standardization breadth: Core tests were conducted at 80 L/min (NIOSH-like); cross-standard comparisons (e.g., ASTM, GB) and additional flow regimes could further contextualize performance. - Reusability and maintenance: Effects of cleaning, decontamination, or repeated donning/doffing on triboelectric performance were not evaluated.