Electret mechanisms and kinetics of electrospun nanofiber membranes and lifetime in filtration applications in comparison with corona-charged membranes

Due to recent serious air pollution especially in Asian countries, electret air filters with ultra-high efficiency have attracted much attention. However, the crucial issue of restraining charge dissipation to maintain stable electrostatic capture efficiency is still challenging. Herein, electret mechanisms, charge degradation kinetics and lifetime in filtration applications of electrospun membranes and corona-charged membranes exposed to isopropanol vapor and high humidity were studied. The surface potential degradation was measured in an in-situ discharging and monitoring setup. Electrospun membranes have longer charge retention time compared to corona-charged membranes. A dual-level trap model based on shallow and deep traps was proposed to quantitatively describe charge aging characteristics. It revealed that besides surface charges, volume charges in traps with deep energy levels were also formed during electrospinning. Furthermore, the charge storage mechanism of electrospun membranes was investigated by thermally stimulated depolarization current technique, which demonstrated that volume charges were trapped in the membrane and dipole charges were generated during electrospinning. Direct charge mapping on individual electrospun nanofiber was also achieved by scanning Kelvin probe microscopy. With the benefit of volume charges in deep traps, the filtration efficiency of electrospun membranes only decreased by 3.67% after 40 h at 95% humidity. With abundant volume charges and reinforced charge stability, electrospun membranes consisting of polymers with inherent dielectric characteristics exhibit robust filtration performance and become a promising material for air pollution control.