Enhanced output and wearable performances of triboelectric nanogenerator based on ePTFE microporous membranes for motion monitoring

Highlights: • ePTFE membranes for TENGs were prepared via low-temperature biaxial stretching. • Microporous structure with high porosity and small pore size was fabricated. • Simple route to balance water proof, moisture permeability and triboelectric output. • TENG device was designed and knee motions were monitored based on an algorithm. • Promising applications in automatically monitoring the body motions. Triboelectric nanogenerators (TENGs) have attracted significant attentions in wearable electronics as self-powered systems. Endowing TENGs with excellent waterproof and breathable functions is critical for their wearable applications. Herein, the expanded polytetrafluoroethylene (ePTFE) calendared sheets were stretched into microporous membranes and used as negative material of TENG device. The excessively expanded micropores of the membrane were considered to sacrifice the surface area, which was adverse to the triboelectric output. Thereby, a series of lower stretching temperatures were chosen for weakening molecular mobility to obtain a membrane (U-300-40) with smaller pore size but maintain a large porosity. As a result, the open-circuit voltage, short-circuit current and transferred charge reached approximately 120 V, 9.5 μA and 44 nC, respectively. Additionally, the water contact angle of 138° and the moisture permeability of 5200 g/m²·24 h presented the potential wearable applications. In order to further improve the wearable performances, the U-300-40 continued to be transversely stretched. The pore size of the membrane (B-200) was only 0.1 µm, which was almost one order of magnitude smaller than that of the uniaxially-stretched membrane owning a similar porosity. Benefiting from the optimized structure, the moisture permeability was further enhanced to 7000 g/m²·24 h, but the output was less impacted. Finally, the fabricated TENG was demonstrated to be a reliable electronic good at monitoring and distinguishing some knee motions. Accordingly, the optimized ePTFE membrane opens an effective route to balance its output and wearable performances, which exhibits a promising application in automatically monitoring the body motions.