Effects of additive integration on pore morphology and airflow resistance in cellulose nanofibril air filters

Cellulose nanofibril (CNF, derived from renewable plant cell walls,) films fabricated via doctor blade-freeze-drying offer a potential sustainable platform for air filtration. However, how functional additive integration pathways alter pore morphology and airflow behaviour remains insufficiently understood. This study investigated how structural variation and two additive integration pathways influenced pore morphology, permeability, and filtration performance in doctor blade-freeze-dried CNF filters. Graphene oxide (GO) and green-synthesised ZIF-L (LE-ZIF-L) were selected as representative 2D sheet and microcrystalline additives. First, film thickness and CNF concentration were varied to establish baseline morphology-airflow and filtration relationships. Then, bulk incorporation and surface-localised deposition of selected additives were examined. Increasing film thickness and CNF concentration improved filtration efficiency by up to 9-fold but reduced permeability from (3.8 ± 0.4) × 10^-11 to (8.1 ± 2.5) × 10^-13 m² while increasing normalised pressure drop. Bulk additive incorporation increased network densification and structural heterogeneity, yielding high normalised pressure drops for GO (24.9 ± 2.5 Pa.m².g^-1) and LE-ZIF-L (10.4 ± 2.1 Pa.m².g^-1), accompanied by reduced permeability. Surface-localised deposition of additives resulted in lower pressure drops than their bulk incorporation at the investigated loadings, but still increased airflow resistance relative to baseline CNF films, even at low mass loadings, likely due to restriction of surface pore entrances. Although 25-pass LE-ZIF-L deposition achieved the highest PM_2.5 filtration efficiency (43.3%), the accompanying pressure loss prevented a substantial improvement in the filtration quality factor. Overall, the results demonstrated that structural modification and additive distribution altered pore morphology and airflow resistance, highlighting their importance in the design of doctor blade-freeze-dried CNF filters.