Piezoelectric polymers are promising energy materials for wearable and implantable applications, for replacing bulk batteries in small and flexible electronics. Therefore, many research studies are focused on understanding the behaviour of polymers at a molecular level and designing new polymer-based generators using polyvinylidene fluoride (PVDF). In this work, we investigated the influence of voltage polarity and ambient relative humidity in electrospinning of PVDF for energy harvesting applications. A multi-technique approach combining microscopy and spectroscopy was conducted to study the content of β-phase and piezoelectric properties of PVDF fibers. We shed new light on β-phase crystallization in electrospun PVDF and showed the enhanced piezoelectric response of PVDF fiber-based generator produced with the negative voltage polarity at a relative humidity of 60%. Above all, we proved that not only crystallinity but also surface chemistry is crucial for improving piezoelectric performance in PVDF fibers. Controlling relative humidity and voltage polarity, increased d33 piezoelectric coefficient for PVDF fibers more than 3 times and allowed to generate power density of 0.6 µW·cm-2 from PVDF membranes. This study showed that electrospinning technique can be used as a single-step process for obtaining a vast spectrum of PVDF fibers exhibiting different physicochemical properties with β-phase crystallinity reaching up to 73%. The humidity and voltage polarity are critical factors in respect of chemistry of material on piezoelectricity of PVDF fibers, which establishes a novel route to engineer materials for energy harvesting and sensing applications.

Wettability of electrospun fibers is one of the key parameters in the biomedical and filtration industry. Within this comprehensive study of contact angles on three-dimensional (3D) meshes made of electrospun fibers and films, from seven types of polymers, we clearly indicated the importance of roughness analysis. Surface chemistry was analyzed with X-ray photoelectron microscopy (XPS) and it showed no significant difference between fibers and films, confirming that the hydrophobic properties of the surfaces can be enhanced by just roughness without any chemical treatment. The surface geometry was determining factor in wetting contact angle analysis on electrospun meshes. We noted that it was very important how the geometry of electrospun surfaces was validated. The commonly used fiber diameter was not necessarily a convincing parameter unless it was correlated with the surface roughness or fraction of fibers or pores. Importantly, this study provides the guidelines to verify the surface free energy decrease with the fiber fraction for the meshes, to validate the changes in wetting contact angles. Eventually, the analysis suggested that meshes could maintain the entrapped air between fibers, decreasing surface free energies for polymers, which increased the contact angle for liquids with surface tension above the critical Wenzel level to maintain the Cassie-Baxter regime for hydrophobic surfaces.

Triboelectric generators are excellent candidates for smart textiles applications due to their ability to convert mechanical energy into electrical energy. Such devices can be manufactured into yarns by coating a conductive core with a triboelectric material, but current triboelectric yarns lack the durability and washing resistance required for textile-based applications. In this work, we develop a unique triboelectric yarn comprising a conducting carbon nanotube (CNT) yarn electrode coated with poly(vinylidene fluoride) (PVDF) fibers deposited by a customized electrospinning process. We show that the electrospun PVDF fibers adhere extremely well to the CNT core, producing a uniform and stable triboelectric coating. The PVDF–CNT coaxial yarn exhibits remarkable triboelectric energy harvesting during fatigue testing with a 33% power output improvement and a peak power density of 20.7 μW cm^–2 after 200 000 fatigue cycles. This is potentially due to an increase in the active surface area of the PVDF fiber coating upon repeated contact. Furthermore, our triboelectric yarn meets standard textile industry benchmarks for both abrasion and washing by retaining functionality over 1200 rubbing cycles and 10 washing cycles. We demonstrate the energy harvesting and motion sensing capabilities of our triboelectric yarn in prototype textile-based applications, thereby highlighting its applicability to smart textiles.