Performance enhancement of triboelectric nanogenerators and exploration of tactile sensing using an electrospun PAN–MWCNT layer through interface manipulation

Abstract

Triboelectricity, being ubiquitous, holds promise as an energy source for achieving net zero emissions and self-powered wearables. Polyacrylonitrile (PAN) fibers, as a dominant material in the textile industry, are a key candidate for such applications. By infusing multiwall carbon nanotubes (MWCNTs) into PAN fibers, the system's longevity is notably enhanced. This study systematically investigates triboelectrification using various configurations of layered electrospun pristine PAN and MWCNT-infused PAN composite (PMC) nanofibers for high-performance triboelectric nanogenerators (TENGs). Among all the configurations (i.e. mono-, bi-, and trilayer), a specific bilayer stacking exhibits a high power density of 48 mW m⁻², a current density of 300 mA m⁻², and an output voltage of 24 V from a 20 mm × 20 mm surface area. This configuration shows a three-fold increase in the output current (I_sc) because of significant reduction in internal impedance. Infusing 0.05 wt% MWCNTs into PAN nanofibers notably improves charge transport capabilities, as reflected by Kelvin probe force microscopy (KPFM) studies. Finite element analysis (FEA) using COMSOL validates the findings and helps to identify the best layer that produces maximum power. Finally, we demonstrate that the device fabricated through these TENG architectures using the PAN–MWCNT composite can serve as a self-powered wearable sensor exhibiting potential applications in gesture-based activities.

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1. Introduction

With the growth of technological advancements, energy demands are increasing by leaps and bounds. To meet the current surge in the energy demand, various sources have been explored, which potentially support the net zero emission movement.¹ The existing energy sources, primarily comprise fossil fuels, may be depleted in the near future.² As a result, numerous studies have been extensively devoted to exploring renewable and sustainable energy resources to mitigate these challenges. In today's world, human life is largely dependent on various power-portable Internet of Things (IoT)-based electronic devices in fields such as health monitoring, medicare, and infrastructure monitoring, which all demand continuous power supplies.³ Although a large part of this power comes from storage devices (e.g. batteries and supercapacitors), there is a growing interest to generate power from various renewable sources that can cater to portable devices, taking advantage of various types of mechanical motion and waste heat utilization, such as piezoelectric, pyroelectric, magnetoelectric, thermoelectric, and triboelectric techniques.⁴ Most of these technologies are dependent on specific material properties and are thereby restricted to the choice of material, except triboelectric technique.

Triboelectric nanogenerators (TENGs) have been recognized as one of the most promising green energy-harvesting mechanisms significantly progressed in the last decade since it was first demonstrated by Wang et al. in 2012.⁵ TENGs can be used for a wide range of self-powered electronic devices for long-term applications because of their simple structure, cost-effective fabrication technique, high output performance, nontoxicity, high efficiency, and high mechanical durability.⁶ The mechanism of TENGs involves the conversion of mechanical energy into electrical energy and is primarily based on the coupling principle of contact electrification and electrostatic induction between two dissimilar material surfaces.⁷ The triboelectrification process occurs when two uncharged bodies become charged upon being brought into contact with each other and are then separated; the corresponding charging effect is known as the triboelectric effect.⁸ The triboelectrification phenomenon can be achieved with different working modes, such as contact separation mode, single electrode mode, lateral sliding mode, and free-standing triboelectric layer mode.⁹ Among them, the contact separation mode has been widely explored, and found to be more efficient and highly durable.¹⁰

TENG enables diverse material combinations for generating triboelectric charges, making material selection essential for enhancing the performance.¹¹ Hence, material selection becomes very important for exploring new TENG devices from the perspective of enhancing the performance. In order to explore energy harvesting from human motion, which is generally abundant in daily life, it is important to choose the correct material that is widely used in human everyday life. Polyacrylonitrile (PAN), a triboelectric negative polymer because of the presence of the predominantly electrophilic CN in the terminating group,¹² offers various applications in textiles, such as knitted clothing like socks, sweaters, and shoes.¹³ Recently, PAN has also been widely utilized in face masks during periods of environmental pollution and pandemics.¹⁴ Thus, PAN fibers are a promising choice for harnessing energy from human motion.¹⁵ In the recent past, PAN has been explored in TENG devices.^16–20 Ye et al. enhanced the voltage and current density by threefold by doping PAN nanofibers with 3 wt% Ga particles.¹⁸ Kınas et al. improved the TENG performance using PAN nanofibers with conductive fillers like graphene oxides (rGO), carbon nanotube (CNT), and carbon black (CB), achieving high voltage (500 V) but low current (<10 nA).¹⁶ Similar results were also obtained by Ozen et al. using sepiolite as a dopant in PAN.¹⁹ Tang et al. achieved ∼2100 V and ∼30 μA current using PAN nanofibers doped with FCNT, and PVDF/PDMS/TiO₂ as the tribonegative layer.²⁰ Lu et al. demonstrated a TENG device with 0.4 and 0.01 wt% MWCNT in PDMS, achieving up to 18.3 μA output current.^21,22 Kınas et al. used PAN/MWCNT (3 wt%) for improved conductivity and triboelectric performance,¹⁶ while Sun et al. used PVDF/PAN/MWCNT (1 wt% of MWCNT).²³ Later, Rani et al. discussed the acoustic-to-electric conversion and triboelectric properties of nature driven with cigarette filters (CFs)-CNT (0.1 wt%) composition.²⁴

To summarize, in all cases, the authors have reported either very high voltage or appreciable current, along with moderately high-power density, but very few have reported systematically on the effective comparative studies in terms of the input impedance, experimental figure of merit, and efficiency. These parameters are of utmost importance to characterizing TENG devices, as they normalize all of the scaling parameters, such as the contact area, input force, and input impedance of the device.

A major challenge in triboelectricity is the effective transportation of charges, which impacts the current from the TENG devices. High voltage generation is common with dielectric materials interacting with other materials, but they often fail to facilitate charge flow due to high internal impedance. This limits the current, despite the high voltage output. To address this, doping conductive fillers into the dielectric matrix, which is commonly used as an active layer in TENG devices, can significantly improve charge flow. Previous studies have largely overlooked these issues, and did not provide a significant depiction of the limitation of current from their respective TENG units.^16,18,23,24 Hence, in order to achieve a large appreciable current, we doped MWCNT in electrospun PAN nanofibers to improve its charge transport capability by reducing the overall impedance of the system. We envisage that MWCNT facilitates the charge transport by aligning itself in a specific direction during the electrospun process. Thus, here we investigate the generation of tribo charges in electrospun PAN nanofibers by doping with a minimal amount of multiwall carbon nanotubes (MWCNT-0.05 wt%). We showed that a wide range of output voltage and current are generated by stacking PAN and PAN–MWCNT composite (PMC) nanofibers when interacted with the Cu electrode. By interface modulation through layer stacking of these nanofibers, we are able to exhibit the maximum output voltage of 24 V and output current of 120 μA, which can be achieved from 20 × 20 mm² surface area when 8 N force was applied. Furthermore, in this study, we comprehensively report the efficiency and various types of figures of merit (FOM) systematically, which is rare in the triboelectric literature and more specifically for PAN-based nanofibers.

2. Experimental section

2.1 Materials

Polyacrylonitrile (PAN, M_w = 150 kDa), and multiwall carbon nanotube (MWCNT, average diameter 20 nm, average length 2–5 μm) were procured from Sigma-Aldrich and N,N-dimethylformamide (DMF) was supplied by Alfa-Aesar.

2.2 Synthesis and preparation of the PAN solution, MWCNT suspension and the final PMC mixture

The PAN solution and MWCNT suspension were prepared individually by dissolving them in DMF, and both solutions were later mixed together. The final mixture was maintained at a weight ratio of 1 : 0.0005 of PAN to MWCNT, based on their solid content. Initially, to prepare the pristine PAN solution, 1 g PAN and 5 g DMF were mixed using magnetic stirrer at 80 °C for 8 h, and the MWCNT suspension was prepared by mixing 0.5 mg MWCNT and 3.95 g DMF in an ultrasonication bath for 2 h at a frequency and power of 40 kHz and 150 W, respectively. The PMC solution was prepared by mixing both solution and suspension of PAN and MWCNT, respectively, using a magnetic stirrer for 6 h at 80 °C.

2.3 Nanofiber deposition and TENG fabrication

Both PAN and PMC nanofibers were fabricated using the electrospinning process under controlled environmental conditions, with a maintained temperature of 27 °C and a relative humidity (RH) of 40 ± 5%. The electrospinning process was initiated by pumping the PMC solution at a constant rate of 0.8 mL h⁻¹ using a syringe pump (NewEra) through a 20-gauge needle size connected to a 10 mL disposable syringe. The needle was connected to a high voltage DC supply (Genvolt) with the applied voltage of 10 kV. The distance between the needle and the custom-built drum-type metallic collector (negative potential) was 10 cm, rendering the electric field strength to be 1 kV cm⁻¹. The collector rotated at a speed of 200 rpm, and the collection of nanofibers took place for 8 h to collect a peelable membrane. The pristine PAN nanofibers were also deposited, following the same recipe. The schematic of the electrospinning setup with the image of the pristine PAN and PMC nanofiber is shown in Fig. 1(a). For the sake of brevity, herein and after, PAN and PMC will be used to signify the pristine PAN and PMC fibrous samples, respectively.

Fig. 1 (a) Fabrication and structure of the nanofiber-based TENG device via the electrospinning process. (b) Schematic of different TENG devices used in the experiments.

The pristine PAN and PMC nanofibers were used to fabricate TENG samples with different layered structure configurations, as shown in Fig. 1(b). The electrode of TENG is a copper sheet, whereas the secondary plate/electrode consists of different layered structure nanofibers (sample 1 to sample 6) fixed to a copper electrode using conducting silver paste. The contact area of the TENG device samples were fixed to 4.0 cm², and mounted on a transparent acrylic plate (100 mm × 80 mm × 10 mm). The entire process is scalable, and large-scale production of the PAN and PMC nanofibers is viable. The photograph of the membrane consisting of the PMC nanofiber is shown in Fig. S1.†

2.4 Characterization techniques

The crystal structure of pristine PAN and PMC was examined using X-ray diffraction (Rigaku D/MAX 2500 diffractometer). XRD patterns were recorded from 10° to 80° with a step size of 0.03° in Bragg–Brentano geometry. The surface morphology and fiber diameter of PAN and the PMC nanofiber were obtained by scanning electron microscopy (Flex SEM 1000) and image J software. A 514 nm wavelength laser was used on a Raman microscope (Renishaw) with a 100× objective and an integration time of 10 s to obtain Raman spectra. The power of the laser was maintained at 5 mW. Thermogravimetric analysis (TGA) was conducted on pristine PAN and PMC nanofibers using TA instruments Q 600, USA. The study aimed to observe the weight loss patterns of PAN fibers with temperature variations. The measurements were initiated at room temperature (25 °C) with gradual heating to 850 °C at a heating rate of 10 °C min⁻¹ under a nitrogen purge of 60 mL min⁻¹. A platinum pan was used for the TGA study. Transmission electron microscopy (TEM) was utilized to examine the size, distribution, and surface morphology of PAN, MWCNTs, and the PMC nanofibers. The experimental approach involved depositing a thin layer of electrospun nanofibers onto carbon-coated copper grids. Subsequently, the TEM grid films were dried using a 100 watt bulb light. The Technai G2FEI Instrument was used to study the size, distribution, and structure of MWCNT in the composite PAN–MWCNT nanofibers at the accelerating voltage of 200 kV. The distribution of surface potential on the pristine PAN and PMC nanofiber was inspected through Kelvin probe force microscopy (KPFM) mode in an atomic force microscope (Bruker Dimension Icon, USA) using a Pt–Ir coated silicon AFM tip (SCM-PIT) having a work function of 4.7 V. The frequency and amplitude were 75 kHz and 20 nm. The test sample was grounded, and the AFM tip was biased. The dielectric characteristics were measured using the HIOKI IM3570 IMPEDANCE ANALYSER interfaced with a dielectric measurement test set up in the frequency range of 20 Hz to 1.0 MHz. The open circuit output voltage (V_OC), surface charge (σ) and short circuit current (I_SC) were measured by a mixed domain oscilloscope (Tektronix MDO3024), electrometer (Keithley 6517B) and source meter (Keithley 2636B), respectively. The fabricated TENG was integrated to human fingers and wrist for monitoring their movements. All experiments were performed in compliance with the relevant guidelines. Informed consent was obtained for the experiments involving human participants.

3. Results and discussion

3.1 Microstructural characterization of pristine PAN & PMC

The electrospun nanofibers were thoroughly analysed using a field emission scanning electron microscope (FESEM, TESCAN MAGNA LMU). To enhance imaging contrast, the nanofiber was sputter-coated with a 10 nm gold layer, with the coating thickness accounted for in the final calculation of the nanofiber diameter. Image analysis was performed using ImageJ software to accurately measure the fibre diameters. The measured diameters were then used to create histograms of relative frequency in Origin software, applying the lognormal distribution equation to fit the histogram data. Fig. 2(a) presents the results for pristine PAN, while Fig. 2(b) shows the data for PMC. Both pristine PAN and PMC nanofibers show homogenous and uniform structures with smooth surface texture of the fibers or the absence of bumpy features (due to the presence of MWCNTs). This feature indicates the possible orientation of MWCNTs in the longitudinal direction of PAN fibers, stemming from the strong uniaxial elongation during electrospinning. The mean diameters of PAN and PMC fibers are 0.49 μm ± 0.09 μm and 0.60 μm ± 0.08 μm, respectively. The presence of MWCNT was confirmed by obtaining the cross-sectional morphology of the PMC nanofibers, as shown in Fig. S2 (see ESI).† The crystallographic phase information of the electrospun pristine nanofiber PAN, multiwall carbon nanotube powder, and PMC nanofibers was investigated by X-ray diffraction (XRD), as shown in Fig. 2(c). Pristine PAN nanofibers show a strong diffraction peak at 2θ = 16.7°, corresponding to the (010) peak and confirming the hexagonal unit structure [JCPDS (–C₃H₃N)_n 00-048-2119], as shown in Fig. 2(c). On the other hand, the diffraction peak at 2θ = 26° for the PMC (along with 16.7° for PAN) indicates the presence of MWCNT (JCPDS-C-00-058-1638).²⁵ To investigate the surface chemistry and related functional groups of PAN and PMC nanofibers, the samples were characterized using FTIR spectroscopy, and the corresponding FTIR spectra are shown in Fig. 2(d). A strong FTIR peak centered at 2242 cm⁻¹ corresponds to the nitrile group (C≡N).²⁶ Various other peaks located at 2930 cm⁻¹, 1620 cm⁻¹, and 1246 cm⁻¹ correspond to the stretching vibrations of CH, RCOO– and C–O groups, respectively.²⁷ The low-intensity peaks corresponding to the bending vibrations of the CH₂ and CH groups are located at 1450 cm⁻¹ and 1350 cm⁻¹, respectively. The peak at 1075 cm⁻¹ is attributed to the stretching vibration of the C–CN group. The peak at 800 cm⁻¹ is due to the C=C–H bond formation in the PMC nanofibers. In the FTIR spectra, a different peak is observed at 1660 cm⁻¹ that corresponds to the stretching vibrations of the C=C bond.²⁸ The Raman spectra of PAN–MWCNT show distinct modes at 1348 cm⁻¹ and 1573 cm⁻¹, which correspond to the D and G bands, respectively.²⁹ The ratio of I_D/I_G = 0.9, which further confirms the presence of high structural defects in the composite. On the other hand, the Raman spectra of the MWCNT powder show three bands corresponding to the D, G and G′ bands at 1344 cm⁻¹, 1574 cm⁻¹, and 2696 cm⁻¹, respectively. The D band is activated by disorder in carbon system, whereas the G band is assigned to the in-plane vibration of the C–C bond, and the G′ band is attributed to the overtone of the D band.³⁰ The intensity ratio of the D and G bands (0.1) confirms the dominance of the in-plane stretching vibrations (G band) in MWCNT, as shown in Fig. 2(e). Several other characteristic bands of PAN include the CH₂ bending at 1448 cm⁻¹, CH bending at 1312 cm⁻¹, and CC skeletal stretching at 1102 cm⁻¹, as shown in Fig. 2(f).³¹ Thermogravimetry provides a measurable assessment of the moisture content and volatile compounds within the nanocomposite fibers, along with weight loss and thermal degradation analysis. It aids in identifying the degradation mechanism.

Fig. 2 (a and b) SEM micrograph, (c) XRD peaks, (d) FTIR, (e) Raman spectra, (f) zoomed view of the Raman spectra, and (g) TGA characteristics of the pristine PAN and PMC nanofibers.

The initial weight reduction, evident around 100 °C for both materials, is attributed to moisture evaporation from the nanofibers, as depicted in Fig. 2(g). Subsequently, the second weight loss is initiated at 215 °C, corresponding to polymer chain backbone dehydrogenation.³² A subsequent weight decrease, occurring between 280 °C and 480 °C, is linked to the liberation of volatile elements like NH₃, HCN, CH₄, and H₂O.³³Fig. 3(a) presents a TEM image of the PAN nanofibers collected on a copper grid during the electrospinning process. The average diameter size of MWCNT in its powder form was determined to be 30–50 nm, as shown in Fig. 3(b). Fig. 3(c) shows the TEM images of the composite PAN–MWCNT nanofibers containing 0.05 wt% of MWCNT, which were embedded within the polymer matrix and aligned along the composite nanofiber axis. The tubular structure exhibits bending or curvature, occasionally even adopting a helical form due to the presence of non-fully carbon hexagonal rings within the carbon walls of the nanotubes. Some pentagonal or heptagonal rings replace hexagonal ones, causing deformation in the tubular structure.³⁴ The presence of MWCNTs has been indicated with an average diameter size of 30–50 nm at different magnifications, as shown in Fig. 3(d–f).

Fig. 3 TEM image of (a) pristine PAN nanofibers, (b) multiwall carbon nanotube powder, (c) composite PAN-MWCNT, (d–i) presence of MWCNT in PMC nanofibers at different magnifications, (j and l) surface topography of PAN and PMC nanofibers, (k and m) surface potential of PAN and PMC nanofibers, (n) surface potential difference of PAN and PAC nanofibers, and (o) work function of PAN and PMC nanofibers.

The contact potential was obtained between the AFM tip and sample using the KPFM technique, which works in intermittent contact operating mode of AFM. The voltage generated due to the contact potential difference can be derived with eqn (1):

where V_cpd, Φ_tip and Φ_sample are the voltage due to the contact potential difference, work functions of the AFM tip and sample, respectively, and e is the electronic charge. The surface topographies with average roughness of PAN and PMC were 578 nm and 351 nm, respectively, and characterized via atomic force microscopy, as shown in Fig. 3(g, i). By and large, the incorporation of MWCNT into the core of PAN minimizes the undulation in the outer surface of the nanowire, and thereby reduces roughness. One of the reasons is that MWCNT possibly relaxes the surface charge by providing a conductive path, which allows the nanowire to relax instead of being intertwined. Furthermore, the values of the average surface potential measured by KPFM technique were found to be 5.2 V and 0.48 V for PAN and the PMC nanofiber, respectively, as shown in Fig. 3(h, j and k).

The shift in the surface potential of 4.72 V likely occurs due to the incorporation of MWCNT in the PAN nanofibers. As MWCNT serves as an electropositive material, it facilitates the flow of electrons from MWCNT to PAN, and recombines with opposite charge carriers (holes). Thus, this results in a reduction of the surface potential and work function in the PMC nanofiber, as shown in Fig. 3(l). From the KPFM data, it is evident that PAN exhibits a higher surface potential compared to PMC (5.2 V for PAN and 0.48 V for PMC). However, the voltage vs. time graph (Fig. S11†) indicates that the charge carrier lifetime in PAN is higher (rise time: 2.8 ms, fall time: 2.3 ms) than in PMC (rise time: 1.6 ms, fall time: 2.3 ms), as illustrated in Fig. S11.† These findings support the conclusion that MWCNT enhances charge transfer, as demonstrated by KPFM, while also not compromising much the charge carrier lifetime and thereby reducing the recombination time. Therefore, we can conclude that PMC is superior to PAN in terms of the charge transfer efficiency.

3.2 Triboelectric characterization

Fig. 4(a) depicts the schematic block diagram of the in-house developed horizontal contact-separation mode triboelectric setup, consisting of various electrical and mechanical components, such as a servo motor, power supply, load cell, controller, oscilloscope, and source measuring unit (SMU). The experiment was performed using the in-house built TENG test set up shown in Fig. 4(c and d).³⁵ To minimize interference, the TENG setup was placed inside a metallic rack (coated with black paint) that was fully enclosed on three sides, as illustrated in Fig. S3 (see ESI).† The TENG device, measuring 20 mm × 20 mm, was properly assembled with the layers securely attached. Conductive silver paste was applied along the four edges of the layers, and the structure was then pressed using a standard electric press. Subsequently, one of the layers, depending on the stacking arrangement, was glued to a copper sheet, as depicted in Fig. 4(e). The other end of the sample, serving as the primary plate, was connected to a base plate made of acrylic in the setup. The working mechanism of the TENG is shown in Fig. 4(b) and Video S1 (see ESI).† On one side, a copper plate was used as a primary triboelectric layer (positive material) and on the other side, a bilayer configuration of the PAN/PMC nanofiber was employed as the secondary triboelectric layers (negative material). Initially, when no force was applied to the TENG device and both plates (primary and secondary) were far apart, no charge was generated on either side of the system, as shown in Fig. 4(b) (described as stage 1). When the force was applied to the TENG device perpendicular to its surface, both plates containing different materials possessing distinct electron affinity came into contact. This allowed it to generate and accumulate the charge carriers, and the polarization of charge carriers can proceed (as shown in Fig. 4(b), stage 2). Furthermore, the charge carriers flow from one material to the other side. When the applied force is removed from the secondary plate, the distance between the two plates starts increasing. This results in the depolarization and generation of the image charge to their counter electrodes, as shown in Fig. 4(b), stage 3. Consequently, the electrons start moving from one material (plate) to the other due to the built-in potential difference. The charge generation and flow of electrons stop when both plates return to their initial positions, causing a diminishing of the built-in potential, as shown Fig. 4(b) in stage 4. Accordingly, no flow of electric current was observed in the output circuitry because of charge neutralization in both plates and the corresponding electrodes. At stage 5 (as shown in Fig. 4(b)), as the force is applied to the TENG device again, the plates come closer to each other. This results in the flow of electrons in the opposite direction in the external circuitry until both plate surfaces touch each other, which completes the triboelectricity generation cycle. The complete cycle of applying and releasing the force on the TENG device generates an alternating current that flows in the external circuit.³⁶

Fig. 4 (a) Block diagram of the in-house built contact separation triboelectric nanogenerator setup. (b) Mechanism of charge transfer in the triboelectric nanogenerator. (c) Pictorial view of the in-house built triboelectric nanogenerator instrument. (d) Zoomed view of the TENG plates and (e) PMC nanofiber pasted on the metal sheet.

To understand the open circuit output voltage (V_OC) and short circuit output current (I_SC), it is important to investigate the surface charge density and dielectric behaviour of pristine PAN and PMC. The charge density (Q_SC) under the short circuit condition can be calculated using the following eqn (2):³⁷

where σ, x, d, and ε_r are the surface charge density, airgap distance, dielectric thickness, and dielectric constant of the material used as secondary layer, respectively. As described in the above eqn (1), the Q_SC is directly proportional to σ and inversely proportional to d/ε_r. In the case of pristine PAN (sample 1) and PMC (sample 2), the thickness of the dielectric is merely ∼110 μm. As they are single layer (see Table 2), these showed less surface charge density (σ), i.e., 4.5 μC m⁻² and 5.1 μC m⁻², respectively. As the thickness (∼360 μm) was increased in the secondary layer by fabricating a two-layer dielectric combination (in sample 3 and sample 4), the surface charge density was improved to 5.8 μC m⁻² (in sample 3) and reached a maximum value of 6.1 μC m⁻² (in sample 4), as shown in Table 2. Further increment in the dielectric thickness of the secondary layer by fabricating a Tri layer (∼410 μm) configuration (in sample 5 and sample 6, as shown in Table 2) led to the reduction in the surface charge density due to poor interfacial charge polarization among the layers.³⁸

The different layered, i.e., monolayer, bilayer, and tri layer configuration of TENG is shown in Fig. 5(a). The dielectric constant (ε′), dielectric loss (ε′′), and conductance (κ′) for all TENG samples are presented in Fig. 4(b, c, and d) and summarized in Table 2. The standard deviation error bars, indicating data variability and reproducibility, are depicted in Fig. S4 (refer to ESI).† The samples 1 and 2 show the dielectric constants of 1.3 and 1.4, respectively, and remain constant over a frequency range from 20 Hz to 1 MHz, as shown in Fig. 5(b). Whereas, sample 3 and sample 4 show a dielectric constant value of 1.7 and 2.1, respectively. In sample 4, the higher dielectric constant values are attributed to PAN, where the dielectric constant remains stable at high frequencies. This stability occurs because only electronic polarization, involving the rapid displacement of electron clouds, can keep up with the fast oscillations of the electric field. The molecular dipoles associated with nitrile groups (–C≡N) have a relaxation time that limits their ability to respond to electric fields above a certain frequency.^39,40 Beyond this point, their contribution to the dielectric constant effectively drops to zero. Slower polarization mechanisms, such as dipolar polarization from nitrile groups, ionic polarization, and interfacial polarization, are unable to keep up with the high frequency, and thus no longer contribute to the dielectric constant. As a result, the dielectric response stabilizes, and the constant value at high frequencies is determined solely by the frequency-independent electronic polarization. The increase in ε′ value may be due to the increase in their dielectric strength. Furthermore, the porosity of the electrospun nanofibers might affect their dielectric behaviour calculation, as shown in Table S1 (see ESI).† Sample 1 (pristine PAN) exhibited porosity of 98.6%, which further increased slightly to 98.9% after adding the conductive filler (MWCNT). On the other hand, in the case of the trilayer configurations, i.e., sample 5 and sample 6, the dielectric constant value decreases to 1.3 and 1.25, respectively, due to the increase in the thickness in overall layer, as well as resulting poor interfacial polarization at the interfaces. Furthermore, the dielectric loss in all of the samples remains very low and constant in the frequency range of 20 Hz to 1.0 MHz. However, variations of losses in all of the samples were observed in the low frequency regime, as shown in Fig. 5(c). High dielectric loss in the materials used in TENGs means more energy is lost as heat, rather than being converted into electrical energy. This reduces the overall energy conversion efficiency of the TENG. When a dielectric material has high dielectric loss, it can absorb and dissipate a significant portion of the input mechanical energy. This absorption reduces the amount of charge separation that can occur, leading to lower output voltage and current. Consequently, the electrical output of the TENG is diminished.

Fig. 5 (a) Thickness-dependent charge induction mechanism and frequency-dependent (b) dielectric constant, (c) dielectric loss, and (d) conductance of different TENG samples.

The conductivity of all samples increases with respect to the increase in frequency, according to eqn (3):

where, κ^'(ω), κ_o, ω, and are the real part of the complex conductivity, DC conductivity, angular frequency, and dielectric loss, respectively. Sample 4 shows the highest conductance compared to any other samples at low frequencies, as shown in the enlarged view of Fig. 5(d).⁴¹

3.2.1 Optimization of the MWCNT content in PAN. To optimize the conductive filler (MWCNT) doping wt% in the PAN matrix, we have doped 0.02, 0.05, and 0.08 wt% of MWCNT into PAN. Furthermore, the V_OC and I_SC were obtained for all three samples and tabulated in Table 1, and the waveforms are shown in Fig. S5 (see ESI).† The V_OC and I_SC increased significantly as the doping percentage of MWCNT increased from 0.02 to 0.05 wt% due to enhancement in the dielectric property. Further increase in the doping percentage (0.08 wt%) caused a decrement in V_OC and I_SC due to the percolation limit. Beyond the percolation limit, the conductive and dielectric properties of the composite nanofiber deteriorate.

Table 1 Output performance parameters for 0.02, 0.05, and 0.08 wt% of MWCNTs doped into PAN

Sample name	Open circuit voltage (VOC) (V)	Short circuit current (ISC) μA
0.02 wt%	6.0	22.0
0.05 wt%	23.5	45.0
0.08 wt%	12.0	20.0

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Table 2 Surface charge on different TENG samples of the secondary layer

Sample	Sample configuration			Surface charge density (μC m^−2)	Dielectric constant (1 kHz)
	Layer 1	Layer 2	Layer 3
1	Pristine PAN	—	—	4.5	1.3 ± 0.1
2	PMC	—	—	5.1	1.4 ± 0.09
3	Pristine PAN	PMC	—	5.8	1.7 ± 0.14
4	PMC	Pristine PAN	—	6.1	2.1 ± 0.16
5	Pristine PAN	PMC	Pristine PAN	4.2	1.3 ± 0.14
6	PMC	Pristine PAN	PMC	4.1	1.25 ± 0.12

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The open circuit output voltage (V_OC), short circuit output current (I_SC), along with the voltage and current at different output load resistances for all six TENG samples (as mentioned in Table 3) are shown in Fig. 6(a, d, g, j, m, p), (b, e, h, k, n, q), and (c, f, i, l, o, r), respectively. We calculated the standard deviation error bars for each sample (S1, S2, S3, S4, S5, and S6), and observed relatively small error margins (5–7%) in the voltage and current data. The corresponding graph is presented in Fig. S6 (see ESI).† The peak-to-peak values of V_OC and I_SC for different TENG samples were obtained at infinite load (R_L = ∞) and no load (R_L = 0) resistance conditions at an applied force of 8 N operated at a frequency of 2.0 Hz. The initial distance between the primary and secondary plates, before starting the experiments, was kept constant at 6 mm. Table 3 depicts the V_OC and I_SC of all the TENG samples studied here. The peak values of V_OC and I_SC were observed to be 19.0 V and 38.0 μA, respectively, for sample 1. For sample 2, the values were found to be 20.7 V and 51 μA, respectively. The observed enhancement in electronic transport in the case of sample 2 is due to the addition of MWCNT into pristine PAN.⁴² Thus, the enhancement in charge transport mainly occurs owing to the improvement in the surface charge density and electronegativity of the secondary layer in sample 2.²⁰ This happens due to the addition of MWCNT into PAN, as the former creates a conductive matrix, which facilitates charge transport into the system, thereby improving the short circuit current.

Table 3 Triboelectric output parameter

Sample	Peak to peak voltage (VOC) (V)	Peak to peak short circuit current (ISC) (μA)	Internal impedance (kΩ)
1	19.0 ± 1.08	38.0 ± 4.1	1018
2	20.7 ± 1.3	51.0 ± 5.0	1000
3	23.0 ± 1.0	31.0 ± 7.1	648
4	24.0 ± 1.5	120.0 ± 12.0	130
5	17.5 ± 1.1	32.0 ± 6.9	1020
6	14.0 ± 1.8	53.0 ± 6.9	664

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Fig. 6 Open circuit voltage (a, d, g, j, m, and p), short circuit current (b, e, h, k, n, and q), and V_OC and I_SC variation with respect to load resistance (c, f, i, l, o, and r) for different TENG samples (1, 2, 3, 4, 5, and 6).

Sample 3 shows the peak values of V_OC and I_SC at 23 V and 31 μA, respectively. For sample 4, the maximum values of V_OC and I_SC were observed to be 24 V and 120 μA, respectively. The peak-to-peak V_OC and I_SC of sample 5 and sample 6 were found to be 17.5 V and 14.0 V, and 32 μA and 53 μA, respectively.

We found that sample 2 shows a 1.5-fold enhancement of current compared to sample 1. The reason is as follows. The I_SC comprises mainly two types of current components, i.e., drift current (I_drift) and diffusion current (I_diff), as per eqn (4):

where κ, E, q, D_n, and n are conductivity, electric field, electric charge, diffusion constant, and electron doping density, respectively.

Due to the interaction between the top layer and copper electrode, the tribo charge carriers are generated on the top layer. This induces the opposite charges on the corresponding electrodes, thus producing an electric field between the layers. As a result, the drift current is influenced, as per eqn (4). Furthermore, it indicates that the drift current will be more pronounced depending on the conductivity of the layers. A careful observation delineates that the layer having higher conductivity will produce more drift current. Hence, as discussed between sample 1 and 2, sample 2 has higher conductivity as compared to sample 1. This is because of the greater number of charge carriers arising due to the presence of MWCNT in the matrix. On the other hand, sample 4 shows an order of magnitude higher current as compared to sample 3. It is important to note that the layered architectures of samples 3 and 4 are just opposite to each other. In the former, PMC (layer 2, see Table 2) serves as the top layer; whereas, in the latter, it acts as a bottom layer, which is connected to the back electrode unlike the former.

The higher I_SC in sample 4 is due to the intermediate conductive PMC layer, which forms a space charge matrix (holes and electrons) at the PAN/MWCNT interface, triggering interface polarization and improving the capacitance.³⁸ The enhancement in capacitance is also due to the improvement in the dielectric constant of the PMC layer. From the relation mentioned in eqn (5), it is discernible that the current generated in a system depends on the capacitance, as can be found from eqn (6). More precisely, the generation of current would be larger in a movable system, where variable capacitance contributes to current generation. In other words, from eqn (6), it is seen that the larger capacitance of a movable device may result in higher current in a time-varying system.

q = cv (5)

where q, c, v, and i are the charge, capacitance, voltage, and current, respectively. In sample 5, the charge carriers are generated after friction and contact in the PAN layer, which is transferred to the conductive PMC and insulating PAN layer. Hence, many charge carriers recombine or diffuse in these two intermediate layers due to there being less carrier lifetime, resulting in a lower current. Sample 6 carries more current in comparison to sample 5 due to the presence of more conductive PMC layer, and less diffusion of charge carriers in the intermediate layers. The rectified V_OC and I_SC for different TENG samples (1, 2, 3, 4, 5 and 6) are shown in Fig. S7 and S8 (see ESI),† respectively.

From Table 2, it is discernible that samples 2 and 4 show reasonably higher voltage and current, as compared to the other samples investigated in this work. To understand the charge generation and transfer within different triboelectric layers (i.e., pristine PAN and PMC), the V_OC and I_SC waveforms of samples 2 and 4 are shown in Fig. 7(a) and (b) by overlapping them together. In sample 2, the generation of charge carriers occurs due to the interaction between copper (primary electrode) and PMC (secondary material), which is normally observed in any two combinations of triboelectric material when they contact each other.⁴³ However, the generated charges then transfer between the two electrodes at a comparatively faster pace than the other samples due to the presence of MWCNT in the matrix, which facilitates charge transport and thereby produces sufficiently high output current. On the other hand, in sample 4, as the bilayer configuration was used, the charge generation happens primarily due to the interaction between copper (primary electrode) and the PAN layers (layer 2, i.e., top layer) like sample 2, as mentioned before. However, layer-1 (PMC) in sample 4 facilitates the occurrence of efficient charge transport due to the presence of the conductive filler, i.e., MWCNT (in the matrix), which is directly connected to the back electrode (copper). This produces a very large current, i.e., 120 μA, as compared to sample 2 and the other samples, as shown in Fig. 6(a) and (b), and as depicted in Table 2. The width of the current generating cycle is also higher in sample 4. This is possibly due to the larger carrier lifetime and diffusion length. Similarly, such comparisons are also made across various configurations used in this study, such as between monolayers (i.e., sample 1 and sample 2), between monolayer and bilayer (i.e., sample 1 and sample 3), between bilayers and tri layers (i.e., sample 4 and sample 6), monolayer and tri layer (i.e., sample 1 and sample 6), and between tri layers (i.e., sample 5 and sample 6). Fig. S9 (see ESI)† represents the V_OC and I_SC of all such possible combinations of sample configurations investigated here. Asymmetry in the negative and positive voltage peaks in a triboelectric nanogenerator was observed due to several factors, such as surface roughness, non-uniform contact, charge trapping and recombination on the surface. As mentioned before, the output current and voltage with respect to different load resistance (10 Ω to 10 GΩ) were obtained to calculate power, as shown in Fig. 6(c, f, i, l, o, and r). According to the maximum power transfer theorem, the maximum power can be derived from an electrical circuit when the output load resistance is equal to the device internal impedance. Thus, the maximum power was delivered to the output load, where the current and voltage curve intersect each other.

Fig. 7 (a and b) Enlarged views of the open circuit voltage and short circuit current of samples 2 and 4, and (c) power density at different load resistances for all the TENG samples. Open circuit voltage (V_OC) at (d) different separating distances, (e) different frequencies, and (f) different forces. (g) Durability test of TENG sample 4 for 1600 cycles, voltage generation in the (h) first cycle and (i) 1600th cycle (last cycle), and (j) durability test for 3200 cycles.

The internal impedance for different TENG samples is found to vary within one order of magnitude depending on the sample configuration, as mentioned in Table 2. Sample 4 showed a minimum internal impedance of 130 kΩ, whereas sample 5 showed the maximum internal impedance, i.e., 1020 kΩ.

In the AC circuit, the impedance can be derived as

where Z, R_L, and X_C are the impedance, output resistive load, and capacitive reactance, respectively.

For TENG sample 4, the capacitance is the highest in comparison to all TENG samples, as shown in Fig. 6(b). Hence, both X_C and the impedance are the lowest. In other words, the impedance is inversely proportional to the conductivity of the sample. From Fig. 5(d), it is evident that sample 4 has five times more conductivity than the other samples. Therefore, sample 4 reflected the lowest impedance.

The power density increases as the load resistance increases from 1 Ω to 10 MΩ, and further increasing the load resistance causes a reduction in power density due to impedance mismatch. The power densities of the TENG samples were calculated (P = V²/R) using the voltage at different load resistors, as shown in Fig. 7(c). The maximum power density for sample 4 was achieved at around 48 mW m⁻². The effects of the variation in separating distance ‘d’ and applied frequency ‘f’ on the output V_OC are shown in Fig. 7(d) and (e), respectively. The separating distance ‘d’ was optimized, and it was found that a 6 mm distance provides the maximum V_OC. When the gap between the triboelectric surface changes, it affects the capacitance and the electric field distribution between the surfaces. The capacitance decreases with the increase in the separating distance up to 6 mm. Further increasing the distance causes a decrease in voltage due to there being less carrier lifetime and diffusion length. The magnitude of V_OC increases as the frequency increases from 1.2 Hz to 2 Hz, and further increasing the frequency (2.2 to 2.5 Hz) results in saturation in V_OC. The voltage can be influenced by the frequency due to the charge accumulation rate and capacitance reactance. In the charge accumulation rate at a higher frequency (>2.5 Hz), the rate of contact and separation cycles increases, which can enhance the rate of charge transfer. However, if the frequency is too high, the time available for complete charge transfer during each cycle might be insufficient, potentially leading to lower effective voltage. The output voltage (V_OC) was obtained under the impact of various applied forces ranging from 2 N to 12 N. The V_OC increases linearly with the increase in applied force from 2 N to 8 N. Further increasing the applied force did not cause any change in V_OC. To assess the data variability, voltage measurements were repeated five times for each parameter, including distance, frequency, and force, with standard deviation error bars showing small margins (5–6%) for each operation. The corresponding graph is shown in Fig. S10 (see ESI).†

After cyclic pressing and friction tests, the interface integrity between MWCNT and PAN nanofibers largely depends on the initial interfacial adhesion, mechanical properties of both components, any functionalization of MWCNTs, the morphology of PAN nanofibers, and the testing environment. To confirm the interface integrity, a durability test was conducted on the TENG sample for the 1600th and 3200th cycles, as shown in Fig. 7(g) and (j), respectively. No degradation in output parameters was observed between the first cycle and the 1600th cycle, as depicted in Fig. 7(h) and (i), respectively. After the 3200th cycle, the surface morphology of the dielectric layers in the TENG samples was analysed using FESEM images taken before and after the durability test.

While the TENG assembly displayed slight architectural deformation, individual nanofibers retained their morphology without any visible change. This is possibly due to the high glass transition temperature of PAN, as illustrated in Fig. S12 (see ESI).†

The electrical output performances of all six TENG samples were assessed by calculating their standard figure of merit (FOM). The FOM calculation aims to evaluate and compare the efficiency and effectiveness of the TENG devices in the generation of electrical energy output across the various samples. The structural, performance, materials, and normalized materials figure of merit for TENG sample 4 have been calculated using the expression , σ², and , where E_m is the maximum input energy, σ is the surface charge, ε_o is the permittivity of vacuum, and A and x_max are the area and maximum gap between the TENG layers, respectively.⁴⁴ These parameters were calculated to be 0.95, 35.34 × 10⁻¹² C² m⁻⁴, 37.2 × 10⁻¹² C² m⁻⁴, and 4.20 J m⁻³. The energy conversion efficiency was calculated as the ratio of energy output to the energy input. The maximum input energy generated by the TENG device is 4.0 μJ, which is derived by ½mv², where m is the mass and v is the velocity of the primary plate. The energy conversion efficiency was calculated to be 3.7%. The same has been obtained for all samples (1, 2, 3, 4, 5 and 6), as shown in Table 4.

Table 4 Performance parameters of the TENG

Sample	Efficiency (η) (%)	Maximum input energy (μJ)	FOMS	FOMP (×10^−12 C^2 m^−4)	FOMnormalized material (J m^−3)	FOMmaterial (×10^−12 C^2 m^−4)
1	1.2	4	1.78	36.04	2.28	20.2
2	1.0	4	1.35	35.11	2.93	26.0
3	0.9	4	1.05	35.32	3.80	33.6
4	3.7	4	0.95	35.34	4.20	37.2
5	0.7	4	2.0	35.28	1.99	17.6
6	0.9	4	2.10	35.30	1.89	16.8

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The performances of the layered TENG-based devices have been compared with respect to the other available state-of-the-art layered TENGs devices, and are shown in Table 5. Most existing solutions are tailored for vertical and rotating movements or sliding motions. However, our proposed device is specifically designed to operate in a horizontal contact mode, making it ideal for addressing such applications. The proposed TENG-based device demonstrates significantly improved efficiency and other output performance parameters compared to other reported topologies, as shown in Table 6. Various studies showed high impedance and low current, especially at larger device areas. However, our proposed TENG device offers enhanced output performance in terms of less contact area, applied pressure, and frequency, all while maintaining low impedance.

Table 5 Performance parameters of selected examples of TENGs with different layers

S. no.	TENG structure	Contact area (cm^2)	V OC (V)	I SC (μA)	Device impedance (MΩ)	Frequency (Hz)	Force (N)	Power density W m^−2	FOMs	Ref.
1	Graphene/PDMS/PET	—	47.1	7	—	—	—	—	—	45
2	PTFE/Graphene	1	3.3	0.3	—	3	—	—	—	46
3	MWCNT/PANI	—	0.2	12.5	0.15	—	—	4.2 × 10^−5	—	47
4	PVDF-TrFE/CNT	—	24	8	—	—	—	—	—	48
5	CNT/PDMS		500	20	—	—	—	0.15	—	49
6	CNT/PDMS		5	240	—	—	25	—	—	50
7	PAN nanofibers		80	25	—		—	—	—	51
9	MWCNT-PAN	16	442	—	10		—	13.2	—	16
10	MXene/CNT/PEDOT	—	184.1	4.42	—	4	—	—	—	52
11	MXene/CNF	—	25	0.95	—	2	—	—	—	53
13	Sample pristine PAN/PAN–MWCNT (sample 4)	4	24	120	0.13	2	8	48 × 10^− ^3	0.95	This study

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Table 6 Performance parameters of selected examples of the TENG with different wt% values of MWCNTs in PAN

S. no	Materials	wt% of MWCNTs	V OC (V)	I SC (μA)	Area (cm^2)	Power density (W m^−2)	Ref.
1	PAN/MWCNT	3	442	—	16.0	14	16
2	PDMS/MWCNT	0.4	720	18.3	6.0	11.62	21
3	PDMS/MWCNT	0.01	132	—	36.0	—	22
4	PAN/PVDF/MWCNT	1	120	30	6.25	2.2	23
5	CF/CNT	0.1	60	1.80	6.25	0.11	11,24
6	PDMS/MWCNT	10	102	-	16.0	-	54
7	PDMS/CNT	3	5	1.5	4.0	0.003	55
8	PDMS/CNT	4	77.8	—	6.0	3.29	56
9	PAN/MWCNT	0.05	24	120	4.0	0.048	This study

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In addition to the above points, it is important to address the charge accumulation of all the samples tested in this study. We have performed the detailed calculations, and presented them in the ESI in Table S4.† Sample 4 has shown the maximum accumulated charge, i.e., 2.67 μC, in one complete alternating cycle of current. Charge saturation and leakage significantly impact the efficiency of triboelectric nanogenerators (TENGs) by limiting charge transfer and retention, as evident in sample 5 and sample 6. Charge saturation occurs when the triboelectric material reaches its maximum charge density, the output voltage and current also reach their highest level, and the device reaches the maximum efficiency and prevents further efficiency gains despite increased mechanical input force (>8 N), as shown in Fig. 7(f). In contrast, charge leakage results from the dissipation of accumulated charges due to material imperfections, environmental factors like humidity, or inadequate insulation, leading to energy loss and degraded long-term performance. Together, these phenomena restrict the device's ability to maintain high output power, requiring strategies like material optimization, surface engineering, improved insulation, and protective encapsulation to enhance charge storage, minimize leakage, and maximize efficiency,^57,58 which demands further investigation in a separate forum.

4. Simulation of TENG devices using finite element analysis

A comprehensive quantitative study was conducted to analyse the real-time output parameters, i.e., surface potential and electric field distribution, across the layered structure of a TENG device using a theoretical simulation model based on the finite element method (FEM) implemented in COMSOL Multiphysics. The model comprises bi-layer distinct polymer nanofibrous structures, with each layered sample area at 20 mm × 20 mm and each pristine PAN and PMC layer having thicknesses of 100 μm and 150 μm, respectively. The primary objective of the simulation was to elucidate the electric potential and electric field distribution across the layers and electrodes of the TENG sample during the contact separation mode. The device model was created based on the simulation parameters listed in Table S3 (see ESI).†

When two triboelectric layers with unequal electron affinities come into contact, contact electrification occurs, leading to an accumulation of equal and opposite charges on the surfaces of both materials. These charges are subsequently transferred through electrodes via electrostatic induction. The electric potential and field were calculated based on Maxwell's Equation below:

∇D = ρ_v (10)

E = −∇V (11)

where σ_s, Q, S, D, ρ_v, E, V, σ, and ε are the surface charge density, surface charge, surface area, electric flux density, volumetric charge density, electric field, voltage, surface charge, and permittivity of the material, respectively. The detailed calculations to obtain σ_s, D, and E have been illustrated in the ESI Section.† The maximum electric potential of 30 V occurred at 6 mm distance, aligning closely with both theoretical predictions and experimental observations, as shown in Fig. 8(i)(a–d). The electric field between the triboelectric layers was quantified at 31.0 × 10⁴ kV m⁻¹ at the same 6 mm distance, as shown in Fig. 8 (ii)(a–d).

Fig. 8 Finite element simulation of the double-layer TENG device (Sample 4) reveals the following observations: (i) electric potential at different positions—0 mm, 2 mm, 4 mm, and 6 mm (a–d), and (ii) electric field distribution at corresponding positions—0 mm, 2 mm, 4 mm, and 6 mm (a–d).

Furthermore, the electric potential and electric field across the TENG, obtained at various separating distances of 0 mm, 2 mm, and 4 mm for all the TENG samples, have been compiled in Table S4 (see ESI),† and their distribution profiles are shown in Fig. S13–S18 (see ESI).† There was no tribo-induced charge in our device before initial contact. The surface charge density was assigned to TENG before the simulation began. In this work, the separation between both triboelectric plates was varied between 0, 2, 4, and 6 mm. For each value of separating distance (d), the FEM simulation was carried out, and the corresponding potential and electric field were demonstrated. There is some potential observed for the 0 mm separating distance, i.e., both plates are in full contact and triboelectrification begins. Subsequently, the surface potential and electric field were varied at various distances, as depicted in Fig. 8.

5. Application of TENG devices

To investigate the capacitor charging behaviour, different capacitors (1 μF, 2.2 μF, and 4.7 μF) were used for electric energy storage for 200 s. They were all charged up to 5.5 V, 4.2, and 3.1 V using the capacitor, as shown in Fig. 9(a and b). The obtained power was utilized to illuminate multiple LEDs with the help of an electrical circuit, as shown in Fig. 9(c) (see ESI Video S2†).

Fig. 9 (a) Full wave bridge rectifier, (b) capacitor charging through the TENG output, and (c) LED glowing. Application of the TENG to generate output voltage: (d–i) human fingers (thumb, index, middle, ring, little) and wrist. Voltage generated from using a keyboard, finger tapping and hammering (j, k, and l).

In addition to energy harvesting, TENG devices were used for Human–Machine Interaction (HMI). Our development involves a real-time monitoring system for finger movements, achieved by integrating a TENG with fingers. This integration not only enables seamless interaction, but also establishes a self-powered sensor mechanism. The different voltage responses, as shown in Fig. 9(d–i), were obtained from the thumb, index, middle, ring, pinky and wrist movements, and are synchronous with the movement. The fundamental operation of our proposed TENG device hinges on a key principle. As the device is set into motion, it progressively harnesses the kinetic energy generated by finger movements, converting them into discernible electrical signals. Each finger of the human hand naturally exerts different bending forces and shows different motions upon similar movement, both under static and dynamic conditions. For instance, for a right-handed person, the same force exerted by the body to the right-hand fingers will show different bending angles and frequency for different fingers. Thus, the index finger will show a greater bending angle and higher bending amplitude and frequency, followed by the middle, ring, pinky, and thumb. Hence, similar observations have been made while performing the electrical measurements, as shown in Fig. 8. We also explored the generation of voltage pulses due to the bending of the human wrist, as shown in Fig. 9(i). This observation indicates that the designed architecture of sample 4 can be used as a good tactile sensor for future applications. To effectively explore human–machine interfaces, especially during computer-related activities, it is essential to monitor human behaviour by capturing data related to mouse clicks and keyboard typing.

In Fig. 9(j, k, m, and n), (see ESI Videos S3 and S4†), a photograph depicts the actions of clicking the mouse and typing on the keyboard, and generates the voltage output from the TENG device, showing distinct voltage and current wave patterns corresponding to each movement. Continuous usage of the mouse produced a V_OC of 6.1 V. Subsequently, during keyboard use, a V_OC of 5.0 V is generated. Consequently, through accurate identification of single operations, real-time monitoring of human behaviour during computer work can be achieved. During the hand hammering on the TENG samples, a V_OC of 12.0 V is generated, as shown in Fig. 9(l and o) (see ESI Video S5†).

6. Conclusions

A multi-layered TENG device was developed using electrospun pristine PAN and PMC nanofibers, specifically designed for energy harvesting and advanced tactile sensing applications in human motion detection. Among various configurations, the architecture of sample 4 exhibited the highest performance, achieving a maximum power density of 48 mW m⁻², a short-circuit current (I_SC) of 120 μA, an open-circuit voltage (V) of 24 V, and a surface charge density (σ) of 6.1 μC m⁻². The accumulated charge during a complete AC cycle was measured at 2.7 μC. These outstanding results are attributed to the incorporation of MWCNTs within the PAN fibers, which enhance charge transport by modifying the surface potential of the fibers, as confirmed by TEM, Raman spectroscopy, and KPFM analysis. FEM simulations using COMSOL verified that the specific device architecture (sample 4) optimizes charge generation and transport mechanisms.

The device demonstrated the ability to generate distinct electrical signals in response to human finger movements and gestures, indicating its potential for integration into wearable human motion sensors. It can be utilized to detect intricate tactile interactions, such as typing, drawing, or hand gestures, making it suitable for applications in human–machine interfaces, virtual reality systems, and robotic controls. Additionally, its high sensitivity to subtle movements suggests potential use in health monitoring systems for detecting tremors, muscle activity, or rehabilitation progress. The robust energy generation and tactile sensing capabilities of this TENG architecture position it as a promising candidate for next-generation wearable devices that combine energy harvesting with precise motion tracking and gesture recognition.

Data availability

The data supporting this article have been included as part of the ESI.†

Author contribution

Shailendra Kumar: conceptualization, data curation, formal analysis, investigation, methodology, software, validation, visualization, writing – original draft, writing – review & editing. Rajesh Kumar Jha: conceptualization, data curation, formal analysis, investigation, methodology, software, validation, visualization, writing – original draft, writing – review & editing. Bhavesh Thakur: data curation, methodology. Tulip Biswas: data curation, methodology. Jay Krishna Anand: data curation, investigation, methodology. Chhotrai Soren: data curation, software. Durgesh Banswar: software. Shalini: data curation. Sonika Singh: data curation. Sumit Sinha-Ray: conceptualization, data curation, formal analysis, investigation, methodology, resources, supervision, validation, visualization, writing –review & editing. Ankur Goswami: conceptualization, data curation, formal analysis, funding acquisition, investigation, methodology, project administration, resources, software, supervision, validation, visualization, writing – original draft, writing – review & editing.

Conflicts of interest

The authors have no conflicts to disclose.

Acknowledgements

The authors would like to thank Prof. N. Ravishankar, Professor at the Material Research Centre, Indian Institute of Science Bangalore for his insightful comments on TEM analysis. National Research Facility and Central Research Facility at Indian Institute of Technology Delhi are gratefully acknowledged for the XRD and FESEM study. This work was supported by Start-up Research Grant [No. SRG-(2019/001984)] SERB-DST, CRG (CRG/2023/006778), FIST (SR/FST/ET-I/2020/628), SEED Grant (PLN12/04 MS), FIRP, and MFIRP grant of IIT Delhi. This experiment was carried out at the Advanced Electronics Materials and Systems (AEMS) laboratory, Department of Materials Science and Engineering, IIT Delhi.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta07120d

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