Preparation and Characterization of Amide-Containing Polyimide Films with Enhanced Tribopositivity for Triboelectric Nanogenerators to Harvest Energy at Elevated Temperatures

Abstract

As triboelectric nanogenerator (TENG) technology continue to evolve, its application in harsh environments has increasingly captivated the interest of researchers. However, the current research on heat-resistant triboelectric materials remains predominantly focused on the development of tribo-negative materials, with scant attention given to their equally crucial tribo-positive counterparts. In this study, the tribo-positive polyimide (PI) material with enhanced tribo-positivity is developed by integrating amide groups with electron-donating effects into the molecular chain. Furthermore, the TENG devices based on this series of tribo-positive PI materials have demonstrated an open-circuit voltage (V_OC) of 242 V, a short-circuit current (I_SC) of 8.13 μA, and a transferred charge (Q_SC) of 117 nC. Notably, these devices also demonstrate the capability to efficiently generate electricity even under elevated temperature conditions. This work not only proposes a potential molecular design strategy for developing high-performance tribo-positive PI materials applicable in TENGs, but also markedly propels the advancement of robust energy-harvesting devices engineered for operation at elevated temperatures.

1. Introduction

Triboelectric nanogenerators (TENGs) represent a groundbreaking advancement in energy-harvesting technology, ingeniously combining the principles of contact electrification and electrostatic induction to efficiently convert mechanical energy into electrical energy. Distinguished from other energy-harvesting technologies, TENGs excel in capturing low-frequency, irregular energy sources [1,2,3,4,5,6,7]. Meanwhile, TENGs are becoming forefront technologies in the arena of new energy devices, attributed to their scalability, versatile design, and a broad range of material choices. With ongoing research and an expanding array of applications, the suitability of these generators for demanding environments characterized by extreme temperatures and humidity is garnering increasing recognition [8,9,10,11,12]. Nevertheless, this broadening introduces new challenges in terms of material selection and the stability of equipment performance.

Extensive research has been dedicated to the selection and preparation of materials to facilitate the broader application of triboelectric nanogenerators (TENGs), ensuring their reliability and performance in harsh environments [13,14,15,16]. For instance, Qian et al. innovatively employed poly(p-phenylenebenzobisoxazole) (PBOA) aerogels in the negative friction layer of TENGs, achieving effective energy harvesting and conversion at high temperatures. The high porosity and substantial specific surface area of PBOA aerogels enabled TENGs to attain a maximum open-circuit voltage of 40 V, a short-circuit current density of 2.9 mA/m², and a charge density of 72 μC/m². Impressively, these TENGs were capable of charging a capacitor to 10 V in just 250 s and efficiently powering several small light bulbs [17]. Additionally, polyimide (PI) materials have attracted considerable attention in the triboelectric field for excellent properties, including outstanding triboelectric characteristics and superior charge storage capacity. The molecular structure of PI, which contains numerous imide and various functional groups, offers abundant charge-trapping sites and excellent electron-withdrawing abilities. These features facilitate effective charge trapping, rendering PI materials ideal for creating high-efficiency friction layers [18,19,20,21]. Furthermore, the exceptional mechanical properties ensure durability and stability through repeated use, while their superior heat resistance broadens the operational temperature range, significantly enhancing the application potential of PI materials [22,23,24,25,26]. For instance, the work conducted by Tao et al. achieved a remarkable enhancement in the triboelectric properties and thermal-charge stability of PI by incorporating potent electron-absorbing groups into its main chain and doping it with BaTiO₃ nanoparticles. These modifications tightened the LUMO (lowest unoccupied molecular orbital) energy band distributions, thereby improving charge-trapping capabilities. Furthermore, the introduction of BaTiO₃ nanoparticles created additional electron traps and interfacial polarization effects, which further augmented the charge-trapping and storage capacity, culminating in a substantial increase in charge density. The TENG based on this composite PI film maintained a 32% effective output at a high temperature of 200 °C in a hot air tunnel, showcasing considerable application potential [27].

Moreover, Xing et al. made a notable advancement in TENG technology by integrating silica aerogel into the PI electrospinning process, resulting in the development of multilayer triboelectric yarns. In performance tests, the TENG, which utilized the PI nanocomposite layer as applied material, demonstrated a remarkable transferred charge density of 30 nC/cm² and a peak power output of 0.17 mW. Crucially, this device demonstrated the capability to harvest electrical energy and detect biological motion across a wide temperature range of 25 to 400 °C, thereby substantially extending the operational threshold of electronic devices [28]. Despite these advancements, the majority of research has predominantly focused on the development of tribo-negative materials. In contrast, the exploration of tribo-positive materials, which are equally crucial for enhancing surface charge density and facilitating efficient charge transfer, has been comparatively neglected, thus hindering the application of TENG devices in harsh conditions. On the other hand, Li et al. took a different approach by irradiating conventional commercial Kapton^® films that widely applied negative friction layer with low-energy helium (He) ions. The experiments revealed that He ion irradiation effectively altered the surface functional groups of the Kapton^® film, converting tribo-negative imide groups into tribo-positive amide groups. The alteration in chemical structure not only realized the triboelectric conversion of the Kapton^® film, but also endowed it with superb electron-donating capabilities and excellent charge stability, unveiling a significant advancement and a novel tribo-positive material option for TENG technology [29]. Consequently, the diversity in the molecular structure of PI polymers and the correlation with triboelectric properties have prompted further research into modifying molecular structures to develop tribo-positive PI materials suitable for high-temperature environments.

In the present study, the chemical bonding and functional groups of the PI triboelectric polymers were adjusted at the molecular level to develop a novel tribo-positive amide-containing PI material. Furthermore, calculation methods like density functional theory (DFT) were employed to elucidate the relationship between the molecular structure and triboelectric properties from the perspective of changes in molecular orbital energy levels, with a view to inspire the design of triboelectric materials. Based on the above, an all-PI-based TENG was constructed by integrating fluorinated PI composite film assemblies. This design culminates in an effective electrical energy output in high-temperature environments.

2. Materials and Methods

2.1. Materials

The aromatic dianhydride 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA) with a purity exceeding 99.5% was purchased from Beijing Innochem Science & Technology Co., Ltd. (Beijing, China), then being subjected to vacuum drying at 180 °C for 24 h prior to use. The amide-containing diamines, including 4,4′-diaminobenzoylanilide (DABA, purity: 99.6%), 2,3′-dimethyl-4,4′-diaminobenzoylanilide (MMDABA, purity: 99.6%), 2-methyl-4,4′-diaminobenzoylanilide (MeDABA, purity: 99.6%), and 2-chloro-4,4′-diaminobenzoylanilide (ClDABA, purity: 99.6%) were supplied by Hitachi Chemical Company., Ltd. (Tokyo, Japan). Polyamide 6 (PA6) was sourced from Zig Sheng Industrial Co., Ltd. (Taiwan, China) and used directly. Formic acid was obtained from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Other tribo-negative materials including Polytetrafluoroethylene (PTFE) film (thickness: 30 μm) and Polydimethylsiloxane (PDMS) film (thickness: 50 μm) were purchased from Sigma Aldrich, Shanghai, China, Sigma-Aldrich (Shanghai, China) Trading Co., Ltd. (Shanghai, China), and were used without further modification. Furthermore, PVDF (polyvinylidene difluoride) powder was obtained from Arkema (China) Investment Co., Ltd. (Paris, France). Electronic-grade N,N-dimethylacetamide (DMAc, purity: 99.9%, water content < 50 ppm) was also sourced from Beijing Innochem Science & Technology Co., Ltd. (Beijing, China).

2.2. Characterization Methods

To investigate the physical and chemical characteristics of PI films, several analytical techniques were employed. Fourier transform infrared (FTIR) spectra were captured using a Shimadzu Iraffinity-1S FT-IR spectrometer (Kyoto, Japan), encompassing a wavenumber range from 4500 cm⁻¹ to 400 cm⁻¹. The Rigaku D/max-2500 X-ray diffractometer (Tokyo, Japan) facilitated the wide-angle X-ray diffraction (XRD) analysis. Differential scanning calorimetry (DSC) was performed using a TA-Q100 system (New Castle, DE, USA) under a nitrogen atmosphere, with a heating rate of 10 °C/min. The thermal decomposition temperature of PI films was determined via a PerkinElmer STA-8000 thermogravimetric analyzer (Perkin-Elmer, Waltham, MA, USA) in a nitrogen environment at a heating rate of 20 °C/min, spanning from 30 to 750 °C. For thermo-mechanical analysis (TMA), a TMA402F3 system (NETZSCH, Selb, Germany) was conducted in a nitrogen atmosphere with a heating rate of 5 °C/min, spanning temperatures from 50 to 450 °C, to ascertain the coefficients of linear thermal expansion (CTE) of the films within the 50 to 200 °C range. Ultraviolet-visible (UV-Vis) spectra of the PI films were captured at room temperature using a Hitachi U-3210 spectrophotometer (Tokyo, Japan). The color parameters of the PI films were assessed employing the CIE Lab equation, where L* denotes lightness on a scale ranging from 0 (black) to 100 (white). The X-rite color i7 spectrophotometer (Grand Rapids, MI, USA) was utilized to quantify the a* values, with positive values indicating red hues and negative values suggesting green hues.

The electrical properties of the TENGs, including open-circuit voltage (V_OC), short-circuit current (I_SC), and transferred charge (Q_SC), were measured employing a 6517B electrometer (Keithley Instruments, Inc., Cleveland, OH, USA). Kelvin Probe Force Microscopy (KPFM) measurements were performed using the KPFM mode in an atomic force microscopy (AFM) system (Billerica, MA, USA) with a Pt-coated Si probe. The measurements were carried out at an oscillation amplitude of 3 V and a resonance frequency of 1.48 Hz. The geometry of the PI models underwent optimization using Gauss density functional theory (DFT), and the calculations for frontier molecular orbitals provided insights into the electrostatic potentials, the highest occupied molecular orbital (HOMO) energies, the lowest unoccupied molecular orbital (LUMO) energies, and the energy gap (εHOMO−εLUMO). The simulations and calculations were performed with the Gaussian 09 software, employing the B3LYP exchange-correlation function and a 6−31G(d) basis set.

2.3. Preparation of PI Films

The synthesis of aromatic PI commenced with the selection of the dianhydride monomer BPADA and various diamine monomers containing the electron-donating amide group. Employing the thermal imidation method, a series of novel PI polymers with distinct structures, including PI-a (BPADA-DABA), PI-b (BPADA-MeDABA), PI-c (BPADA-ClDABA), and PI-d (BPADA-MMDABA), were successfully synthesized. The specific synthetic route is illustrated in Figure 1a.

The detailed synthesis procedure could be illustrated by the synthesis of PI-a (BPADA-DABA). Initially, DMAC (149.6 g) and diamine monomer DABA (11.3631 g, 0.05 mol) were added to a 500 mL three-necked flask and stirred under a nitrogen atmosphere until a clear solution formed. Subsequently, the pre-measured dianhydride monomer BPADA (26.0243 g, 0.05 mol) was introduced into the reaction mixture, with additional DMAc being used to rinse any remaining reactants from the flask’s interior.

Table 1. Formula for the PI synthesis.

PI	BPADA (g, mol)	DABA (g, mol)	MeDABA (g, mol)	ClDABA (g, mol)	MMDABA (g, mol)	DMAC (g)
PI-a	26.0243, 0.05	11.3631, 0.05	NA a	NA a	NA a	149.6
PI-b	26.0243, 0.05	NA a	12.0631, 0.05	NA a	NA a	152.3
PI-c	26.0243, 0.05	NA a	NA a	14.8130, 0.05	NA a	163.4
PI-d	26.0243, 0.05	NA a	NA a	NA a	12.7630, 0.05	154.8

^a Not applicable.

provides the complete formulation details. After stirring for 24 h at room temperature, the mixture thickened into a high-viscosity poly(amic acid) (PAA) solution. This solution was then evenly cast onto a clean glass plate and subjected to a systematic heat treatment under a nitrogen atmosphere. The temperature increments included 80 °C/h, 120 °C/h, 150 °C/h, 180 °C/h, 200 °C/h, 250 °C/h, and 300 °C/h. Following the heat treatment, the glass plates with the attached films were cooled and submerged in deionized water, resulting in the production of uniform, flat PI films approximately 30 μm thick.

The preparation of other modified amide-containing PI films, including PI-b (BPADA-MeDABA), PI-c (BPADA-ClDABA), and PI-d (BPADA-MMDABA), followed a similar procedure as previously described. However, in these preparations, DABA was replaced with MeDABA for PI-b, with ClDABA for PI-c, and with MMDABA for PI-d.

2.4. Fabrication of TENG Devices

The construction of all TENGs adhered to the traditional vertical contact-separation mode, as depicted in Figure 1b. The assembly process involved the utilization of copper conductive tape, approximately 40 μm thick, for the formation of electrodes. Notably, the tribo-positive layers were innovatively fabricated using PI film (thick: ~30 µm), which had an active contact area of approximately 16 cm² (40 mm × 40 mm). Additionally, the preparation of PA6 materials entailed dissolving 10 g of PA6 particles in a 40 g formic acid solution, with continuous magnetic stirring at 40 °C for 5 h to ensure complete dissolution. The resultant PA6 solution was then cast onto a clean glass plate. Following film formation, the film was submerged in deionized water to facilitate phase transition. The film was subsequently peeled off, thoroughly washed to eliminate solvent residues, and dried in an oven at 80 °C for 6 h, yielding the PA6 porous film. For the tribo-negative layer, an FPI composite film with enhanced triboelectric properties was selected, prepared from fluorinated polyimide composite organic semiconductor 1,4,5,8-Naphthalenetetracarboxylic dianhydride (NTCDA) [30]. Furthermore, the integration of the polyethylene terephthalate (PET) film improved the structural integrity of the entire TENG device and greatly increased its durability.

3. Results

3.1. Structural Characterization

The FTIR spectra of the PI films are presented in Figure 2a, exhibiting a sequence of characteristic peaks, indicative of the PI structure within the 450–4000 cm⁻¹ wavenumber range. Notably, two prominent absorption peaks at 1780 cm⁻¹ and 1720 cm⁻¹ correspond to the symmetric and asymmetric stretch vibrations of the imide ring’s carbonyl group (C=O), respectively. Additionally, a peak near 1370 cm⁻¹ aligns with the C–N bond stretch vibration on the imide ring, confirming the successful formation of the imide ring in the polymer. Furthermore, the interval at 2970 cm⁻¹ contains distinctive peaks corresponding to the –CH₃ group’s stretch vibration within the dianhydride monomer BPADA and the diamine monomer, further substantiating the successful synthesis of the target PI polymer. The aggregated state structures of PI with varying configurations were meticulously characterized, with the findings illustrated in Figure 2b. The analysis indicates that the combined effects of ether bonds and methyl substituents result in a more relaxed configuration of the PI, leading to the manifestation of typical amorphous structural characteristics.

3.2. Thermal Properties

For the application of TENGs at high temperatures, it is imperative to select materials with exceptional thermal resistance. PIs, celebrated for their superior heat resistance, are highly advantageous for the development of TENGs intended to function under extreme conditions. The effects of amide functional groups on the high-temperature resistance of PI were investigated using TGA, DSC, and TMA tests, with the pertinent thermal data encapsulated in

Table 2. Thermal properties of PI films.

Samples	T5% a (°C)	Tmax a (°C)	Rw750 a (%)	Tg, DSC a (°C)	Tg, TMA a (°C)	CTE a (×10−6/K)
PI-a	508	538	54.5	267	298	57.3
PI-b	491	526	50.1	272	280	63.7
PI-c	490	545	50.3	237	232	65.6
PI-d	485	513	54.1	257	249	61.2

^a T_5%: Temperatures at 5% weight loss; T_max: Temperatures at the rapidest thermal decomposition rate; R_w750: Residual weight ratio at 750 °C in nitrogen; T_{g, DSC}: Glass transition temperatures according to the DSC measurements; T_{g, TMA}: Glass transition temperatures according to the TMA measurements (peaks of tanδ plots); CTE: linear coefficient of thermal expansion in the range of 50–200 °C.

. A comprehensive mass change analysis of the PI films was conducted via the TGA technique over a temperature range from 50 °C to 750 °C, with the findings depicted in Figure 2c. As the temperature incrementally rises, a slow decrement in film mass was noted, with a slight weight loss of approximately 5% (T_5%) being recorded within the specific temperature range of 485 °C to 508 °C and the process being accelerated significantly within the narrow temperature range of 513–545 °C. Upon reaching 750 °C, the residual weight ratio (Rw₇₅₀) for the films ranged from 50.1% to 54.5%. Undoubtedly, the variation attests the remarkable thermal stability of the fabricated PI films and their capacity to retain structural integrity under elevated temperatures.

The thermal behavior of the PI films, specifically glass transition temperature (T_g), was meticulously assessed through DSC tests, with the results depicted in Figure 2d. The incorporation of flexible ether bonds generally lowers the T_g of the resulting PI, while the presence of the amide group and side methyl impedes the main chain’s internal rotation, bolstering molecular chain rigidity and thus elevating the T_g values. Consequently, all PI materials maintain a T_g above 237 °C, demonstrating commendable high-temperature resistance. Moreover, the coefficient of thermal expansion (CTE) for this series of PI films was determined using TMA tests, with results depicted in Figure 2e. The findings indicate that all PI samples expand as the temperature rises, especially near respective T_g, where expansion becomes more significant. Due to the influence of more flexible chain segments, the CTE values for PI-a, PI-b, PI-c, and PI-d in the temperature range of 50–200 °C were found to be 57.3 × 10⁻⁶/K, 63.7 × 10⁻⁶/K, 65.6 × 10⁻⁶/K, and 61.2 × 10⁻⁶/K, respectively, demonstrating sufficient thermal stability for high-temperature applications.

3.3. Optical Properties

The specific impact of incorporating amide groups on the optical properties of the materials was examined through UV-Vis spectral analysis and CIE Lab color parameter measurements. Notably, research indicates that PI materials with optimal performance tend to exhibit higher transparency. Consequently, the molecular design and production methods for PI triboelectric materials align with those employed in developing colorless and highly transparent PI films. Thus, it is essential to examine the structure–property relationships of the newly developed PI films. The optical properties of the PI films, including the cutoff wavelength (λ_cut), the transmittances at the wavelengths of 400 nm (T₄₀₀) and 450 nm (T₄₅₀), the CIE color parameters of L*, a* and b*, and the haze, are detailed in

Table 3. Optical properties of the PI films.

Samples	λcut a (nm)	T400 b (%)	T450 b (%)	L* c	a* c	b* c	Haze (%)
PI-a	366	15.7	64.1	92.25	−2.37	12.83	1.00
PI-b	371	5.5	60.9	93.19	−4.25	16.90	0.52
PI-c	371	11.1	58.5	92.33	−1.54	12.85	0.68
PI-d	369	22.2	76.9	93.67	−2.52	8.63	1.16

^a λ_cut: Cutoff wavelength; ^b T₄₀₀, T₄₅₀: Transmittance at the wavelength of 400 nm and 450 nm, respectively; ^c L*, a*, b*: color parameters.

. As shown in Figure 2f, the λ_cut of the PI films all surpassed 366 nm, with the T₄₅₀ values of 64.1% for PI-a, 60.9% for PI-b, and 58.5% for PI-c, respectively. Notably, the T₄₅₀ of the PI-d film attained 76.9%, demonstrating a significant enhancement in optical transmittance compared to the commercial Kapton^® films. Moreover, to evaluate the optical properties in detail, the CIE Lab color parameters of the modified PI films were further measured. The results showed that the b* values of the PI films varied from 8.63 to 16.90 and the haze values from 0.52% to 1.16%, thus verifying the significant improvement in the optical properties. The incorporation of amide groups and associated side groups likely results in the stacking of the PI molecular chains in a more loosely packed arrangement. The arrangement increases the spatial separation between electron donors and acceptors, consequently reducing the charge transfer effect and contributing to the improved optical transparency of the material.

3.4. Triboelectric Properties

The impact of amide group incorporation on output performance was evaluated through the engineering of a novel all-PI-based TENG, with the modified PI film serving as the positive friction layer and the FPI composite film as the negative friction layer. The performance of the specifically structured modified PI film within the TENG framework was thoroughly analyzed, with PA6 material also utilized in TENG devices for comparative purposes. Each TENG device underwent rigorous testing to measure its output under standardized conditions: a maximum contact pressure of 30 N, a working frequency of 2 Hz, and a spacer distance of 10 mm. The electrical outputs from these trials are detailed in

Table 4. Triboelectric properties of the TENG based on PI films.

Samples	VOC (V) a	ISC (μA) b	QSC (nc) c
PA6	147	3.14	46
PI-a	169	7.37	77
PI-b	181	5.62	80
PI-c	179	6.13	88
PI-d	242	8.13	117

^a V_OC: open-circuit voltage; ^b I_SC: short-circuit current; ^c Q_SC: transferred charge.

. It is evident that TENG devices with distinct material combinations exhibit significant performance differences. As depicted in Figure 3a, the open-circuit voltage (V_OC) of the TENG with a PA6 tribo-positive material registered at 147 V. In contrast, the V_OC increased to approximately 169–181 V with PI-a to PI-c, and further elevated to about 242 V with PI-d as the positive friction layer. This trend persisted, as corroborated by the short-circuit current (I_SC) data in Figure 3b, where the TENG with PA6 generated merely 3.14 μA compared to the 5.62–8.13 μA output when implemented with the modified PI. These results unequivocally validate the superior triboelectric characteristics of molecularly modified PI films as a positive friction layer in TENG devices.

In the transferred charge (Q_SC) test, the TENGs featuring the modified PI film demonstrated consistent performance in both voltage and current, as illustrated in Figure 3c. Notably, a TENG utilizing a PA6 material achieved a modest transferred charge of approximately 46 nC. In stark contrast, a TENG with a PI-d film as the positive friction layer attained a peak charge transfer of nearly 117 nC. This pronounced discrepancy underscores the pivotal role of modified PI materials in enhancing charge transfer efficiency. To further validate the superior performance of tribo-positive modified PI, the exceptional PI-d film was combined with conventional materials, including PA6 and commercial Kapton^®. Various TENG devices were then constructed, and V_OC as compared under identical conditions, as shown in Figure 3d. The TENG made from PI-d/Kapton^® material generated a positive voltage of 45 V, whereas the TENG built with PI-d/PA6 material yielded a negative voltage of 40 V. The outcome not only signifies the successful transition of PI from a traditionally tribo-negative materials to a tribo-positive materials, but also highlights its superior performance compared to the conventional PA6 material. These findings unequivocally affirm the exceptional triboelectric properties of the modified PI material, further cementing its advantages in triboelectric applications.

To meticulously assess the impact of load resistance on TENG’s output, the systematic variations in load resistance were applied while monitoring changes in output voltage and power density. Figure 3e demonstrates that with a 2 MΩ external load, the peak output voltage reaches 8 V. With an increase in load resistance, the peak output voltage progressively rises, culminating in a significant surge to 258 V when the load resistance attains 500 MΩ. Conversely, as depicted in Figure 3f, the instantaneous power density initially increased with the increasing load resistance, eventually peaking before declining. At a load resistance of 100 MΩ, the TENG achieved a maximum instantaneous power density of 298 mW/m². These findings underscore that optimizing the load resistance can significantly enhance the TENG’s output power and expands its potential for a range of practical applications.

The extensive utility of modified PI as an exceptional tribo-positive material has been exemplified through the construction of TENGs, integrating PI films with various widely used tribo-negative materials, including PTFE, PDMS, and PVDF. A comprehensive evaluation of triboelectric properties was subsequently conducted, with the experimental findings presented in Figure 4. Comparative analysis revealed that TENGs with PI as the positive friction layer significantly outperformed those with PA6 in both V_OC and I_SC. These results underscore the superior performance of PI and confirm its remarkable adaptability and compatibility when paired with a diverse array of commonly used tribo-negative materials.

During the comprehensive evaluation of TENG performance, the I_SC response characteristics of modified PI films were systematically studied under variational contact force, working frequency, and spacer distance. Initially, the I_SC changes in the TENG device within the contact pressure range of 10 N to 40 N were recorded, with the working frequency set at 2 Hz and the spacer distance at 10 mm, as illustrated in Figure 5a,b. It was observed that the I_SC stabilized from 7.32 μA to 10.27 μA as the contact force increased from 10 N to 30 N, and peaking of 12.16 μA occurred at the contact pressure of 40 N, which indicates that increased contact pressure promotes the formation of additional microscopic contact points between the two materials, effectively enhancing the triboelectric effect by increasing the frictional contact area. Concurrently, the increase in contact pressure also induces the deformation of the material at the contact interface, further increasing the contact area. However, the increase in contact area will no longer be significant for the generation and accumulation of frictional charges as the plastic deformation of the material is close to or reaches its limit when the contact pressure reaches a certain threshold, leading to a saturation point in the improvement of performance. Additionally, the I_SC of the TENG was evaluated at a contact force of 30 N and a spacer distance of 10 mm, with varying working frequencies, as shown in Figure 5c,d. The data indicated that the I_SC significantly increased with the working frequency. The value of I_SC sharply rose from 7.43 μA to 11.42 μA when the frequency doubled from 1 Hz to 2 Hz. At a frequency of 4 Hz, the I_SC further escalated to 20.99 μA. The increase in working frequency is associated with an enhanced rate of charge transfer, which in turn increases the current output. As the frequency rises, the rate of material separation correspondingly increases, culminating in a more rapid charge transfer process. Consequently, a greater amount of charge traverses the circuit per unit time, resulting in a higher peak short-circuit current.

The influence of the separation distance between the two triboelectric materials on the TENG’s output performance was also examined. Under a constant contact force and working frequency, the I_SC was measured across a range from 5 mm to 20 mm, as depicted in Figure 5e,f. The I_SC exhibited a marked increase as the spacer distance grew from 5 mm to 15 mm, and it began to plateau beyond this distance. These findings indicate that at a steady operating frequency, an increase in spacer distance correlates with a higher velocity during the contact and separation of the triboelectric layers, thereby promoting electron flow in the external circuit and boosting the TENG’s electrical output performance.

3.5. The Mechanism of Performance Enhancement

Polymers have emerged as essential materials for engineering high-performance TENGs due to their diverse functional group. The electron-donating or electron-withdrawing capabilities of these groups in polymers are predominantly influenced by inductive and conjugative effects. Amide groups, typically recognized as electron donors, play a pivotal role in the formation of tribo-positive materials. Within the PI structural system, the conjugation of an amide group with a benzene ring markedly amplifies its electron-donation potential, which in turn enhances the material’s triboelectric characteristics. In this study, the triboelectric series of various polymer materials is presented, as delineated in Figure 6a.

The contact potential differences (CPD) were visualized to nullify as a voltage (V_CPD) applied between the tip and sample using Kelvin probe force microscopy (KPFM), which is crucial for evaluating charge transfer and retention [31]. As illustrated in Figure 6b,c, the PI-d film’s CPD value was measured at 1.38 V, considerably surpassing the PA6 material’s −0.07 V. This pronounced disparity underscores the PI-d film’s enhanced positive triboelectric traits and its superior capacity to donate electrons during material interactions. Consequently, PI’s efficacy in charge transfer, when utilized as a tribo-positive material, facilitates a higher electrical energy output in TENG devices. The findings unequivocally demonstrate the exemplary performance of this series of PI films as positive triboelectric layers. To further elucidate the performance advantages of PI-based TENGs, the density functional theory (DFT) method was employed to calculate the orbital energy levels of PI molecules. The results, as presented in Figure 6d, highlight that the trend in the orbital energy levels upon the introduction of amide-based electron-donor groups is diametrically opposed to that observed with –CF₃ electron-withdrawing group [30,32]. Specifically, the HOMO energy level in the new PI series is significantly elevated, which is an advantage for the electron-donating capability. Furthermore, modifications in molecular structure yield energy gap (△ε, |εHOMO—εLUMO) values of 3.53 eV, 3.44 eV, 4.08 eV, and 3.85 eV, respectively, are considerably higher than the 2.80 eV band gap observed with Kapton^®, allowing more localization for charge transfer to be maintained. Both experimental and theoretical investigations demonstrate that the molecular orbital of polymers play an important role in determining the triboelectric performance, thereby offering new possibilities for performance optimization in the design of tribo-positive materials for TENG applications.

3.6. Application Exploration

A durability test was conducted over an extended period to confirm the reliability of TENG devices fabricated with PI films. The linear motor cycle parameters were configured to standardize the frequency, force, and separation distance for each contact and separation event, ensuring repeatable test conditions. As depicted in Figure 7, the amplitude of the transferred charge exhibited no notable decline throughout the testing period during the continuous 1000 s cycles of contact and separation. The findings strongly suggest that the PI film-based TENG device demonstrates remarkable stability and long-term reliability.

The operation of TENG is contingent upon the principles of electrostatic induction and the contact-separation mechanisms that occur between two distinct materials, thereby generating an alternating current. To fulfill the power needs for powering electronic devices, it is necessary to convert the alternating current output into the direct current output using a full-bridge rectifier circuit. Figure 8a illustrates the rectifier bridge circuit. The converted energy is then stored in capacitors or batteries, providing power to various external electronics. Under consistent experimental conditions, the TENG constructed from PI film was utilized to charge capacitors with varying capacities (1 μF, 3.3 μF, 4.7 μF, and 10 μF). The outcomes, displayed in Figure 8b, demonstrate that capacitors reached voltage levels of 10.2 V, 7.4 V, 5.8 V, and 3.4 V in 60 s, respectively. It was observed that the charging rate of the capacitors decreases with increasing capacitors. The phenomenon can be attributed to the limited quantity of charge produced by the TENG within a set timeframe, whereas larger capacitors require more charge to reach the same voltage level.

To rigorously assess the robustness of the full PI-based TENG under high-temperature conditions, a test system comprising a heater, a linear motor, and an electrostatic meter was constructed. As depicted in Figure 8c,d, the impact of temperature fluctuations on the TENG’s open-circuit voltage performance was evaluated. A decline in V_OC was observed as the testing environment’s temperature increased, primarily attributable to the intensified thermionic emission effect, which facilitates charge escape from the PI material’s surface. Nevertheless, the all-PI-based TENGs sustained a V_OC of 62 V even at extreme temperatures reaching 200 °C, as illustrated in Figure 8e. Furthermore, as evidenced by Figure 8f, a string of 30 series-connected LEDs consistently emitted light at temperatures up to 200 °C during real-world application tests. These observations conclusively demonstrate the PI-TENG’s capability to deliver a reliable electrical output, even under severe high-temperature conditions.

4. Conclusions

In this work, a novel tribo-positive PI material was synthesized by incorporating amide groups into the PI polymer backbone. This was achieved via molecular structural modifications, effectively altering the triboelectric characteristics of PI materials and the corresponding range of applications. The modified PI exhibited significantly superior tribo-positivity compared to traditional tribo-positive PA6 materials, attaining an open-circuit voltage of up to 242 V, a short-circuit current at 8.13 μA, and a transferred charge reaching 117 nC. Supporting evidence is provided by KPFM potential measurements, which reveal that the surface of PI exhibits more positive potentials compared to PA6. Additionally, DFT calculations suggest that the PI molecule possesses higher HOMO values and a larger band gap, corroborating the experimental findings. It is especially worth noting that these characteristics facilitate the PI-based TENG functionality at high temperatures, allowing for electricity generation and the illumination of a series of small LED bulbs at temperatures up to 200 °C.