We developed a poly(vinylidene fluoride)/carbon nanotube (PVDF-MWCNT) filament as a feed for printing of electrically-conductive and corrosion-resistant functional material by fused filament fabrication (FFF). Using an environment-friendly procedure to fabricate PVDF-MWCNT filament, we achieved the best reported electrical conductivity of printable PVDF-MWCNT filament of 28.5 S cm −1 (90 wt% PVDF and 10 wt% CNT). The PVDF-MWCNT filaments are chemically stable in acid, base, and salt solution, with no significant changes in electrical conductivity and mass of the filaments. Our processing method is robust and allow a uniform mixture of PVDF and CNT with a wide range of CNT percentage up to 99.9%. We demonstrated the printing of PVDF-MWCNT filaments to create 3D shapes; printed using a low-cost commercial consumer-grade FFF 3D printer. We found many adjustments of printer parameters are needed to print filament with CNT content >10 wt%, but easier printing for CNT content ≤10 wt%. Since this was due to printer limitation, we believed that PVDF-MWCNT with higher CNT percentage (to a certain limit) and larger electrical conductivity could be printed with a custom-built printer (for example stronger motor). PVDF-MWCNT filament shows higher electrical conductivity (28.5 S cm −1 ) than compressed composite (8.8 S cm −1 ) of the same 10 wt% of CNT, due to more alignment of CNT in the longitudinal direction of the extruded filament. Printable PVDF-MWCNT-Fe 2 O 3 (with a functional additive of Fe 2 O 3 ) showed higher electrical conductivity in the longitudinal direction at the filament core (42 S cm −1 ) compared to that in the longitudinal direction at the filament shell (0.43 S cm −1 ) for sample with composition of 60 wt% PVDF, 20 wt% CNT, and 20 wt% Fe 2 O 3 , due to extrusion skin effect with segregation of electrically insulating Fe 2 O 3 at the shell surface of PVDF-MWCNT-Fe 2 O 3 .

Chemical vapor deposition and filtration are commonly used to fabricate freestanding multiwalled carbon nanotube sheets containing iron(III) oxide (Fe 2 O 3 ) (MWCNT-Fe 2 O 3 sheets). However, the former is relatively expensive, and the latter suffers from poor scalability. Here, the authors develop an inexpensive, scalable, high-throughput, tunable tape-casting method for fabricating flexible, foldable freestanding MWCNT-Fe 2 O 3 sheets. When tested as electrodes in lithium-ion batteries, the sheets perform better than conventional graphite on copper foil (681 mAh g À1 after 20 cycles at 100.5 mA g À1 /0.1 C-rate vs. 72.5 mAh g À1 after 20 cycles at 37.2 mA g À1 /0.1 C-rate). Sheets prepared at Fe 2 O 3 :MWCNT mass ratios ranging from 1/4 to 2/1 are flexible and easy to be separated from the substrate, but a sheet prepared at a 9/1 ratio is not. The ratio can be tuned to suit various applications. Sheets with high (low) Fe 2 O 3 :MWCNT mass ratios has high (low) charge-transfer resistance and are suitable for applications requiring high energy density (high power density). Mechanical compression of the sheets flattens them and increase their density from 0.449 to 0.771 g cm À3 , which in turn increases their electrical conductivity from 3.56 Â 10 3 to 5.73 Â 10 3 S m À1 .