Prof. Zhong Zhaoxiang & Assoc. Prof. Kang Yutang from Nanjing Tech University: Graphene Oxide Induced Thermal-Oxidation Polyacrylonitrile Nanofibrous Membrane for High-Temperature PM2.5 Filtration

PM2.5 emissions mainly originate from high-temperature processes including fossil fuel combustion, transportation, biomass burning, and waste incineration. Statistics show approximately 58% of particulate emissions occur at 50-300°C. Direct PM2.5 removal from high-temperature emission sources is crucial for developing energy-efficient air purification technologies, requiring separation materials with excellent heat resistance and flame retardancy.

Electrospun nanofibrous membranes have gained extensive research attention in air purification due to their high particulate filtration efficiency and low air resistance. However, common low-cost polymer materials (e.g., polyacrylonitrile, polyvinylidene fluoride, polyurethane, polystyrene) exhibit poor heat resistance and flame retardancy, limiting their high-temperature applications. While specialty engineering plastics (e.g., polyether ether ketone, aramid, polyphenylsulfone, polyimide) demonstrate better performance, their high material costs, poor solubility, and inferior spinnability hinder widespread adoption.

Recently, Prof. Zhong Zhaoxiang and Assoc. Prof. Kang Yutang's team published their research "Graphene oxide induced thermal-oxidation polyacrylonitrile nanofibrous membrane with superior heat resistance and flame retardancy for high-temperature air filtration" in Journal of Membrane Science. The team utilized graphene oxide (GO) to regulate the thermal-oxidation process of polyacrylonitrile (PAN) nanofibrous membranes, lowering the reaction temperature and heat release rate, effectively preventing PAN nanofiber melting and coalescence, and maximally maintaining the structural integrity of oxidized PAN (OPAN) membranes. This work provides guidance for preparing low-cost, easily processable nanofibrous membranes with heat-resistant and flame-retardant properties.

The thermal oxidation process of polyacrylonitrile (PAN) polymer, also known as pre-oxidation, is an essential intermediate step for converting PAN fibers into carbon fibers. During thermal oxidation (240-350°C in air atmosphere), PAN's molecular structure transforms into a nitrogen-containing ladder structure, which endows the oxidized PAN (OPAN) nanofibrous membranes with excellent heat resistance and flame retardancy. However, PAN's thermal oxidation is an intense exothermic process, and its melting temperature range overlaps with the oxidation temperature range. This causes PAN nanofibers to melt and coalesce during thermal oxidation, severely damaging the pore structure of OPAN nanofibrous membranes (as shown in Figures 1b and 1c).The incorporation of graphene oxide (GO) effectively inhibits the melting and coalescence of PAN nanofibers (Figures 1d and 1e). The resulting GO-OPAN nanofibers only fuse at their contact points (Figures 1f and 1g). 

Figure 1: (a) Preparation process of GO-OPAN membranes, (b-c) PAN membranes before/after oxidation, (d-e) GO-PAN membranes before/after oxidation, (f-g) TEM images of GO-PAN and GO-OPAN membranes

Raman spectroscopy (Figure 2a) shows that after combining with PAN, GO's G-band shifts from 1567 cm⁻¹ to 1602 cm⁻¹ (a total shift of 35 cm⁻¹), indicating strong interactions between GO and PAN.Thermogravimetric curves of PAN and GO-PAN nanofibrous membranes (Figure 2b) demonstrate the effect of different GO loadings on PAN's weight loss. PAN nanofibrous membranes show almost no weight loss below 300°C, with significant weight loss beginning at 350°C. As GO loading increases, the weight loss temperature of GO-PAN membranes gradually decreases. At 0.36% GO content, significant weight loss occurs at 280°C. The distinct weight loss curves with different GO loadings indicate that GO alters PAN's pyrolysis pathway. Furthermore, in nitrogen atmosphere, GO-PAN membranes also show significant weight loss below 300°C, with greater loss than PAN at 500°C, suggesting that oxygen-containing functional groups on GO's surface participate in PAN's thermal oxidation and decomposition processes.As shown in Figure 2c, GO addition broadens the exothermic peak of PAN's cyclization reaction while lowering its temperature. This phenomenon suggests that electronegative groups (e.g., -COOH) on GO's surface nucleophilically attack PAN's -C≡N groups, inducing cyclization at lower temperatures and slowing heat release (Figure 2d). This inhibits melting and coalescence of PAN nanofibers during thermal oxidation, maximally preserving membrane pore integrity (Figures 2e-f).

Figure 2: (a) Raman analysis, (b) TGA, (c) DTA, (d) GO's induction mechanism for PAN thermal oxidation, (e) melting/coalescence of PAN nanofibers, (f) GO's inhibition effect

The prepared GO-OPAN membranes exhibit outstanding heat resistance and flame retardancy. Their microstructure remains intact at 350°C (Figures 3a-c), with only 0.2% weight loss at 350°C (Figure 3d) and no change in molecular structure (Figure 3c). As shown in Figures 3h and 3j, GO-PAN membranes completely burn within 3 seconds upon flame contact, whereas GO-OPAN membranes show no burning, flaming, smoking, or dripping. Their limiting oxygen index (LOI) reaches 49.7% (Figure 3i), far exceeding most polymer materials and specialty engineering plastics.

Figure 3: (a-c) Microstructure of GO-OPAN membranes at different temperatures, (d) heat resistance, (e) FTIR characterization at different temperatures, (f-g) comparison of GO-PAN and GO-OPAN before/after combustion, (h-j) combustion process, (i) LOI

The GO-OPAN membranes demonstrate excellent high-temperature particulate filtration performance. At 350°C, they achieve 98.16% PM2.5 filtration efficiency with a pressure drop of 158.7 Pa and a quality factor of 0.025 Pa⁻¹. This work successfully transforms inexpensive, easily spinnable PAN nanofibrous membranes into heat-resistant, flame-retardant OPAN membranes, demonstrating their potential for high-temperature particulate filtration applications.

Figure 4: (a) PM filtration efficiency, (b) pressure drop, (c) quality factors at different temperatures, (d) SEM of GO-OPAN after 2-hour PM2.5 filtration at 350°C, (e) effect of airflow rate on filtration efficiency, (f) pressure changes, (g) long-term PM filtration performance of GO-OPAN at 350°C

Paper Link: https://www.sciencedirect.com/science/article/pii/S0376738825002571