Sustainable polymer materials for triboelectric and hybrid energy harvesting

Environmental pollution due to the use of non-renewable energy and non-degradable materials is a critical issue that must be addressed by employing more sustainable energy sources, materials, and processing methods. Energy harvesting (EH) devices have enormous potential for a novel industrial paradigm for low-power systems, such as sensors or actuators.^1–3 Mechanical energy harvesters are devices that convert small amounts of mechanical energy into electrical energy using the abundant mechanical stimuli dispersed in the surrounding environment. Mechanical energy harvesters that can be developed using polymeric materials typically exploit the piezoelectric and/or triboelectric effects and are known as piezoelectric nanogenerators and triboelectric nanogenerators (PENGs and TENGs, respectively).^4,5 The piezoelectric effect has been investigated for a long time in mechanical EH, providing a high electromechanical coupling and intrinsic properties compared with other transduction phenomena.⁵ The triboelectric phenomenon occurs when two different materials are placed into contact and electric charges are transferred between their surfaces, making this energy harvester technology one of the most efficient in terms of area/volume power ratios.^6,7 The wide availability of potential materials, in particular flexible ones, that can be employed in TENGs compared to PENGs makes this technology more modular.⁸ Of particular interest is the possibility to develop hybrid piezoelectric–triboelectric nanogenerators (PTENGs) that can result in a harvesting device with enhanced energy density.

Polymers are outstanding materials for these types of devices due to their intrinsic properties, such as light weight, easy processability, low cost, and flexibility, allowing processing by additive manufacturing into complex conformations and geometries. Polymers have been proven to be suitable for the development of flexible PENGs,⁹ TENGs,^7,10 and hybrid applications^11,12 and for supplying electrical energy to low-power electronics.^2,11 Typical piezoelectric polymers, such as polyvinylidene fluoride (PVDF), are also valuable triboelectric materials and can be used to develop hybrid harvesters (PTENGs).¹² PVDF and its copolymers are the most commonly used piezoelectric polymers for flexible harvesting devices,^7,13 largely used as piezoelectric materials due to their high coefficients (d₃₃ up to 34 and d₃₁ up to 22 pC/N¹⁴), and can also be used as triboelectric materials (pristine or in the form of composites).^12,15 For EH applications, sustainable bio-based materials of natural origin, such as cellulose,¹⁶ poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV),¹⁷ poly-l-lactic acid (PLLA),¹⁸ and silk fibroin (SF),¹⁹ are in the positive side of the triboelectric series given their strong ability to lose electrons, making them a good sustainable material alternative to ferroelectric polymers and metallic materials largely applied in the literature.¹⁷ Remarkably, these polymers also present piezoelectric properties showing a longitudinal piezoelectric coefficient (d₁₄) of ≈12 pC/N in PLLA,²⁰ ≈1.5–2.0 pC/N in PHBV,²¹ ≈1.5 pC/N in silk,²⁰ and ≈0.2 pC/N in cellulose.²⁰ These sustainable and natural polymers are abundant and exhibit non-toxicity, thermal and chemical stability, and biodegradability.²² Cellulose-based materials, such as hydroxypropyl cellulose (HPC), are the most natural and abundant ones, characterized by flexibility and thermal stability.²³ The first reported TENG based on cellulose appeared in 2016, generating an output voltage and current of 30 V and 90 μA at 10 Hz, respectively.²⁴ Several studies obtained comparable TENG output performance when paired with the most commonly used negative triboelectric polymers, such as fluorinated ethylene propylene (FEP),¹⁷ polytetrafluoroethylene (PTFE),¹⁷ or polyethylene terephthalate (PET).¹⁷ PHBV is also a promising candidate for bio-based TENG due to its biological origin, low cytotoxicity, and degradation under ultraviolet radiation²⁵ and can be tailored to large-scale production with high crystallinity.²⁶ Triboelectric nanogenerators (TENGs) operating in contact-separation mode have been successfully developed using composites of PHBV and PVDF, with the incorporation of various functional fillers. These devices leverage the unique material properties of PHBV, a sustainable polymer known for its mechanical flexibility and triboelectric performance, and PVDF, a polymer widely recognized for its superior piezoelectric characteristics.^27,28 These TENGs showed, respectively, an output voltage, current, and peak power density up to about 1000 V,²⁹ 100 μA,²⁸ and 2 W/m²,²⁷ depending critically on the operating mode and the applied mechanical input. Similarly to PHBV, PLLA also presents tunable crystallinity and biodegradability with promising piezoelectric and triboelectric properties.³⁰ It can be obtained from renewable resources (corn, vegetal starch, or sugar cane) showing piezoelectric properties due to their intrinsic dipolar conformation and stability up to high temperatures of 140 °C.³¹ PLLA and its composites have been incorporated into TENG devices combined with PVDF,³² chitosan,¹⁷ PTFE,³³ and polyimide,³⁴ leading to output signals of few hundred volts, tens of μA, and a few mW/m². The introduction of PLLA-oriented fibers determined an enhancement of TENG performance compared to non-oriented ones, showing an increase in electrical output going from 8 to 45 V and 4 to 9 μA and a cycling stability over 24 000 cycles.³⁵ PVDF and PLLA fibers were used as triboelectric materials in PTENGs that reached near 124 V and 54 μA corresponding to a maximum peak power density of 18 W/m².³² Silk fibroin (SF) is a suitable natural biomaterial that presents biodegradability and biocompatibility with low-cost production.³⁶ The first TENG utilizing SF nanofibers as an active material was successfully developed, employing polyimide as the complementary triboelectric material. This innovative design demonstrated notable energy-harvesting performance, achieving a maximum peak power density of 4.3 mW/m² with reliable and durable performance.³⁷ SF in film or fiber geometry combined with PTFE,¹⁷ PET,³⁸ or PVDF¹¹ showed a larger electrical output and maximum peak power density signals of 395 V, 89 mA/m², and 22 W/m²,³⁹ depending on the roughness or additive manufacturing processing of polymer materials.⁴⁰ Fiber-based PTENGs using electrospun SF and PVDF showed promising properties for healthcare monitoring with an output electrical signal at 2 Hz of 500 V and 12 μA, reaching a maximum peak power density of 3.1 W/m².⁴¹ As reported in the literature, these TENG/PTENG devices can work for thousands of cycles and their degradability starts to happen after a few weeks or months, critically depending on the degradation conditions and polymer composition.⁴² 

The uses of combined triboelectric and piezoelectric polymeric materials should increase the mechanical into electrical energy conversion, which is crucial in the context of the Internet of things (IoT) and Industry 4.0, allowing the implementation and connection of sensors in real time and everywhere for a wide range of applications. The sustainability of these large numbers of devices, especially those designed for short-term use or limited cycles, must be carefully considered. The possible sustainable and biodegradable polymers for TENG devices must be evaluated by comparing their overall properties with conventional high-performance polymers, such as PVDF. This assessment is essential to ensure that these eco-friendly alternatives can meet the functional demands while reducing the environmental impact.

PHBV (molecular weight of 46 064 g/mol, 3% mole fraction of HV, and purity of 99%) was obtained from NaturePlast (France). For the PHBV dissolution, chloroform (purity of 99%) was purchased from Fischer (Portugal). Hydroxypropyl cellulose (HPC) with a molecular weight of 100 000 g/mol and a density of 0.5 g/ml at 25 °C was supplied by Sigma-Aldrich. PLLA with a molecular weight between 217 000 and 225 000 g/mol (Purasorb PL18) was supplied by Purac. N,N-Dimethylformamide (DMF) and dichloromethane (DCM) were obtained from Merck and Sigma-Aldrich, respectively. Bombyx mori silkworm cocoons were supplied by APPACDM from Castelo Branco (Portugal). Sodium carbonate (Na₂CO₃), formic acid (FA), calcium chloride (CaCl₂), and sodium chloride (NaCl) were obtained from Sigma-Aldrich. Distilled water was prepared in our laboratory. Commercial PTFE films with a thickness of 100 μm were used as a negative triboelectric reference material. Commercial films based on poled β-PVDF (PVDF-P) and unpoled β-PVDF (PVDF-uP) with a thickness of 120 μm were acquired from Measurement Specialties, USA (d₃₃ = −33 pC/N and d₃₁ = 23 pC/N for PVDF-P, with similar remaining morphological and physical–chemical properties).⁴³ All reagents and solvents were used as received.

The PHBV thin film was prepared by dissolving a 15% fraction (weight/volume) in chloroform under magnetic stirring at 50 °C for 2 h. Then, a film was prepared by spreading the solution by a doctor blade method over clean glass substrates for solvent evaporation overnight at room temperature. Samples with an average thickness of 40 μm were obtained. HPC with a 10% fraction (weight/volume) was dissolved in water by mechanical stirring for 10 min at room temperature. Thin films were obtained by spreading the solution with a doctor blade on a clean glass substrate and left to dry at room temperature for 24 h. Samples with an average thickness of 60 μm were obtained. A PLLA solution with 10 wt. % in a 3:7 vol/vol DMF/DMC mixture was prepared under magnetic stirring (IKA C-MAG HS7) at room temperature until complete dissolution of the polymer, and a transparent, homogeneous, and bubble-free solution was obtained after no longer than 2 h.⁴⁴ Thin films of PLLA were prepared using a doctor blade over clean glass substrates. The samples were then placed in an oven (JP Selecta, Model 2000208) for 30 min at 70 °C for polymer melting and complete removal of the solvent. Next, the films were removed from the oven and allowed to cool at room temperature. Samples with an average thickness of 40 μm were obtained. The cocoons of Bombyx mori were cleaned and cut into pieces of 10 × 10 mm² area. The detailed process of making the cocoons was performed in Ref. 45. First, it is prepared as a solid fibroin, following the process detailed in Ref. 46. The resulting solid SF was then cleaned and dried for an extended time in an oven. After completely removing the impurities and insoluble residues, an SF/FA solution suitable for sample preparation was prepared by dissolving SF in FA (10:1 v/w FA:SF).^46,47 The SF/FA solution was uniformly dissolved under magnetic stirring. Following this, the solution was transferred into a Petri dish and allowed to remain undisturbed at ambient temperature for 24 h to facilitate solvent evaporation.⁴⁶ The films obtained showed a thickness between 40 and 60 μm.

A home-made screen-printing setup was used for printing the 2 × 2 cm² conductive silver ink (Electronic 131 paste DT1201, Hunan LEED Electronic Ink, Zhuzhou, China) electrodes using a squeegee over the screen (100T mesh) placed at 1 mm distance from the samples.⁴⁸ The electrodes were dried for 60 min at 60 °C in an oven.

The morphological structure was investigated by scanning electron microscopy (SEM) using a Hitachi S-4800 at an accelerating voltage of 5 kV with magnifications of ×1 and ×10k. Polymers were metalized with a gold layer (15–20 nm thick) using Polaron SC502 sputter coater equipment.

The hybrid performance of the polymeric materials was measured using a homemade setup, where the mechanical stimulus was provided with a linear motor and force was measured with a load cell. The electrical output was measured with an oscilloscope (Tektronix MSO5000) and a 40 MΩ voltage probe (Tektronix). The current was measured with a resistance commutator and a current amplifier (1211 DL Instruments).

The output power was determined using the output current as a function of the measured load resistance (R_L) through the following equation: $P=RL×Ipeak2$⁠, for each used load resistance. The energy density per cycle was determined as the maximum peak power, while the average power was calculated from the signal integration over 15 cycles for all triboelectric pairs. The devices were operated at frequencies of 7.5 Hz with a 2.5 Hz step and from 10 to 20 N of applied forces. The active area of the samples was 2 × 2 cm², and the largest distance between the materials within the device operating in contact-separation mode was 5 mm for both triboelectric and hybrid PTENG systems. The piezoelectric effect in PENGs was evaluated by applying the same maximum force load, as used for the measurements of the corresponding PTENG. In PENGs, the two materials are in constant contact with the minimum contact force set at 2 N.

The surface morphology of sustainable polymers was studied using the SEM images shown in Fig. 1. PHBV is porous, presenting some pores with a diameter of 1–2 μm. The remaining polymers show a compact structure without pores. A higher surface roughness can be observed for the polymers processed in the laboratory, which may influence their triboelectric performance. Instead, the commercial PVDF polymers show a smooth surface.

The dielectric properties of PHBV, HPC, PLLA, and SF are shown in Fig. 2. The frequency dependence of the dielectric constant (ε) and dielectric loss (δ) is reported in Figs. 2(a) and 2(b), respectively, showing the typical dipolar regime observed in polymers.⁴⁹ The HPC (ε ≈ 11.1) and SF (ε ≈ 11.5) showed a high and similar dielectric constant at 1 kHz, followed by PHBV (ε ≈ 4.0) and PLLA (ε ≈ 3.6), as represented in Fig. 2(c). The higher dielectric constant of SF and HPC materials⁵⁰ and PHBV and PLLA agrees with the literature.^35,51 The decrease in dielectric constant found for PHBV and PLLA films also leads to lower dielectric losses, reaching δ ≈ 0.02 and δ ≈ 0.01 at 1 kHz, respectively. In HPC and SF, the dielectric losses for both polymeric matrices are near tan δ ≈ 0.16 at 1 kHz. Therefore, HPC and SF materials perform better in dielectric applications than PHBV and PLLA films. The commercial PVDF-P shows a slightly varying dielectric constant between ε ≈ 14 and 12 with increasing frequency and low losses, for both poled and non-poled samples.⁶ 

The dc electrical conductivity of the polymers is presented in Fig. 2(d). As expected by the dielectric measurements, the HPC and SF matrices show a higher electrical conductivity of σ ≈ 2.3 × 10⁻⁸ and 7.0 × 10⁻⁹ (Ω m)⁻¹, respectively. An electrical conductivity nearly four orders of magnitude lower was observed for the PHBV and PLLA samples, showing σ ≈ 7–8 × 10⁻¹³ (Ω m)⁻¹. For the PVDF-uP sample, an electrical conductivity of σ ≈ 1.2 × 10⁻¹¹ (Ω m)⁻¹ was observed. Therefore, while HPC and SF (and PVDF) behave as good dielectric polymers, PHBV and PLLA are closer to low polarizable insulators.

Harvester devices can be used to supply electrical energy to low-power devices, and the ones more sustainable are typically based on polymeric materials showing piezo- and triboelectric properties.¹¹ By selecting materials that can express both triboelectricity and piezoelectricity, the efficiency of a hybrid energy harvester will benefit from both phenomena.⁵² TENGs and PTENGs are typically operated in contact-separation mode as it combines both effects, providing higher performance and energy density, and enables easier integration into practical and industrial applications.^1,53

The harvesting characteristics of PHBV, HPC, PLLA, and SF were evaluated and compared with a high-dielectric PVDF polymer (a negative electron affinity material⁵⁴) using the output current and the load resistance to determine the output power (⁠$P=RL×Ipeak2$⁠). The screen-printed silver film placed on one side of each material was used as the electrode. PTFE and aluminum (Al) were also used as second triboelectric materials due, respectively, to their negative and positive electron affinity (capability of electrons from the highest energy level in a dielectric material to migrate to another material⁵) highlighted in the triboelectric series.^6,55

The harvesting properties of TENGs fabricated with each sustainable material (PHBV, HPC, PLLA, and SF) were calculated by selecting as reference materials well-known electron acceptor and donor materials in the triboelectric series,^5,56 such as PVDF and aluminum (Al), respectively (Fig. 3). Furthermore, we investigated the influence of the poling process on commercial PVDF (unpoled PVDF-uP and poled PVDF-P) on system performances. Figures 3(a) and 3(b) show the output voltages of TENGs and PTENGs, respectively, operating in contact-separation mode, where the PVDF [as an alternative to polytetrafluoroethylene (PTFE) commonly used as a negative triboelectric material that tends to gain electrons⁴] is paired with positive triboelectric materials based on the optimized polymer films or Al films taken as a reference positive tribomaterial. Higher performances are achieved when PVDF-P is combined with PHBV and HPC, when compared to Al films. The output voltage of PVDF-P with HPC, PHBV, and Al reaches near 105, 60, and 50 V, respectively. Noteworthily, our flexible and sustainable materials demonstrate better performance than the positive Al films. Instead, PLLA and SF lead to a decrease in the voltage output compared to the Al films, which were found to be ∼30 and 15 V, respectively. The PVDF-uP polymer showed a similar behavior, with a larger output voltage recorded for HPC (≈60 V) and PHBV (≈40 V) films compared to Al (≈25 V). In addition, the PVDF-uP film showed slightly higher performance with PLLA (≈40 V) and SF (≈25 V) as second triboelectric materials when compared to PVDF-P. We attribute the improved performance observed when pairing the PVDF-P with HPC, PHBV, and Al with respect to the PVDF-uP to the presence of an additional response that is enabled by the high piezoelectric response of the poled films (PTENG systems) as provided by its high piezoelectric coefficient (d₃₃ ≈ −34 pC/N). In addition to PVDF, all other polymers are also piezoelectric, but this effect is not observed in the results (PLLA presents a larger coefficient) due to lower coefficients and shear piezoelectricity.²⁰ It has been observed that despite the higher dielectric constant and electrical conductivity of SF, or the higher electrical conductivity compared to PHBV, these properties do not appear to significantly influence the electrical output of hybrid PTENGs. However, PHBV shows a more structured surface morphology that is responsible for the higher performance of the corresponding PTENG.

The instant power density as a function of load resistance was also determined for PVDF-uP [Fig. 3(c)] and PVDF-P [Fig. 3(d)] as the first triboelectric materials. It is observed that the power density is higher when the PVDF-P is paired with HPC and PHBV compared to Al, at a similar load resistance of 100 MΩ. The PVDF-P showed a maximum power density with HPC (45.0 μW/cm²), followed by PHBV (31.6 μW/cm²), while Al leads to a power density near 28.9 μW/cm². With PLLA and SF, we observe a decrease in power density to 7.2 and 2.4 μW/cm², respectively. Therefore, high-performing PTENGs fully based on polymeric matrices can be achieved by combining the poled PVDF with HPC or PHBV polymeric films. The PVDF-uP film provides enhanced TENG performance with all polymer matrices when compared to Al, as shown in Fig. 3(a), demonstrating that the researched materials are better positive triboelectric materials than Al films. In such a device configuration, where no piezoelectric film is inserted, the PTENG response is dominated by the triboelectric effect only (it operates as a mere TENG) and shows a higher power density with HPC (23.4 μW/cm²), followed by PLLA (18.7 μW/cm²), PHBV (11.3 μW/cm²), and SF (5.2 μW/cm²). Although SF exhibits a larger dielectric constant and conductivity, its triboelectric performance is lower compared to other polymers. The maximum power density occurs at a load resistance near 100 MΩ, except for Al, for which the maximum power density of 3.4 μW/cm² occurs at 50 MΩ [inset in Fig. 3(b)]. In general, PVDF-P presents better performance compared to unpoled PVDF due to the increase in piezoelectric polarizability and the corresponding varying surface charge, optimizing the energy conversion.⁵⁷ 

Overall, when combined with both poled and unpoled PVDF films, the sustainable HPC and PHBV films exhibit superior output performance in terms of voltage and instant power. Both polymers demonstrate remarkable output performance as electron donor materials and represent more positive triboelectric materials than Al along the triboelectric series, where flexible and environmentally sustainable materials are scarce. Their ability to outperform traditional materials, such as Al, in specific configurations represents a significant advancement in sustainable energy harvesting technologies.

To widen the triboelectric series with a novel and sustainable polymers and to provide a better understanding of PHBV as a triboelectric material, its performance was evaluated combining PHBV with other recognized materials that act as good electron donors as for the case of Al or as good electron acceptors as for PTFE (Fig. 4). We also combine PHBV with the remaining sustainable polymers as shown in Fig. 4 to compare their output performance.

Typically, high-performing devices are obtained by combining materials that present a colossal difference in their electron affinity, which accounts for a higher probability of triboelectric charge transfer.⁵⁸ PTFE lies in the most negative side of the triboelectric series, so it is expected to present a high output voltage when coupled with a more electropositive material, such as PHBV. TENGs based on PHBV and PTFE reached an output voltage of 35.3 V, which decreased for Al, HPC, and PLLA to 12.9, 8.7, and 7.1 V, respectively. Therefore, PHBV is a more electropositive material compared to PLLA, HPC, and Al. Interestingly, the output performance of PHBV paired with PVDF-uP or PTFE is comparable (≈35–40 V), PTFE being known as one of the most electronegative tribomaterials.⁵ In fact, fluorinated polymers demonstrate strong electron-attracting characteristics.⁵ 

The instant power density as a function of load resistance is represented in Fig. 4(b). PHBV showed a higher power density of 6.1, 2.1, 0.8, 0.7, and 0.3 μW/cm² for PTFE, PVDF, Al, HPC, and PLLA, respectively.

We also investigated the improved electron affinity of the HPC films, by fabricating TENGs integrating the HPC films as one tribomaterial and the established PLLA, PTFE, and Al as the second triboelectric material. The optimized TENGs are operated in contact separation mode with a maximum force of 20 N and a frequency of 7.5 Hz [Fig. 5(a)]. HPC combined with PLLA shows an output voltage near 7.7 V, while it presents output voltages near 23 and 46 V when combined with Al and PTFE, respectively. The lower electron affinity of HPC compared to PHBV polymer, Fig. 4, is responsible for the increased performance observed when combined with Al and PTFE. Similar to PHBV performance, the HPC–PVDF-uP output voltage is larger than when paired with PTFE, under similar mechanical stimuli.

The instant power density as a function of load resistance is presented in Fig. 5(b). The TENG based on PTFE presents the highest power density (15.6 μW/cm²) at a load resistance of 100 MΩ. For the remaining materials, Al presents a maximum instant (or peak) power density of near 3.8 μW/cm² for the processed samples and PLLA with 0.5 μW/cm² at a load resistance of 50 MΩ.

Table I summarizes the maximum energy density per cycle determined at the maximum peak power with an average of integrating over 15 cycles and the instant power density determined for all triboelectric pairs. High-performing devices can be obtained using hybrid PTENGs incorporating PVDF-P/HPC and PVDF-P/PHBV pairs of materials as optimal sustainable positive triboelectric polymers that can be used to convert mechanical to electrical energy in flexible PTENGs.⁵⁹ The PVDF-P presents the maximum output power near 322 and 228 nJ/cm² when paired with HPC and PHBV, respectively, being larger when compared to the Al pair (224 nJ/cm²). The PTENG performance is enhanced, showing that fully polymer devices can be used with excellent performance to harvest mechanical into electrical energy. For PLLA or SF, the energy density values obtained decrease to 62.6 and 21.5 nJ/cm², respectively.

As mentioned, the TENG performance of PVDF-uP was higher with the HPC matrix, showing a value near 200.3 nJ/cm², which was followed by 164.1 nJ/cm² when in combination with PLLA. PHBV and SF showed energy density values of 123.7 and 81.0 nJ/cm², respectively. Despite the absence of piezoelectric behavior on the PVDF-uP matrix, an increase of nearly eight times for HPC and nearly seven times for PLLA polymers compared to Al was observed. The TENG also showed enhanced performance, nearly five and three times higher for PHBV and SF, respectively.

We should also make a comment on why HPC performs better than PHBV. Looking at the conductivity and dielectric studies, it is possible to conclude that the higher conductivity and good dielectric properties of HPC compared to PHBV might be associated with its higher polarizability, and similarly, the conductivity measures also point out an improved capacitive charging at the interface between the electrode and the HPC concerning the PHBV (despite the higher surface structuring shown by PHBV).

The triboelectric performance of HPC is lower when combined with PLLA (5.6 nJ/cm²) and larger when combined with PTFE (149.3 nJ/cm²), demonstrating, respectively, the closest and furthest electron affinity between them.

With these electroactive polymers, it is demonstrated that fully flexible TENG or PTENG devices show competitive harvester performance, as shown in Table I. It is worth noting that HPC can replace Al as a novel and sustainable polymer for triboelectric applications, demonstrating a larger capability for donor electrons within the triboelectric phenomenon.⁵ The most used polymers in TENG systems are summarized in Table II. Al and PTFE are well established as positive and negative triboelectric materials compared with the remaining materials. The ideal and structured order of materials in the triboelectric series is complex due to the intricacy of evaluating or comparing different works, where the specificity of individual studies on materials morphology, geometry, electrodes, or electronic circuits strongly changes. However, SF, HPC, PHBV, or PLLA polymers are on the positive side of the triboelectric table (electron donor materials), where there are less flexible materials with higher output performance in the literature.^4,10,56,60 In this way, our work contributes to expanding the triboelectric series with sustainable polymers for triboelectric harvesting devices.

In this work, we provide evidence that sustainable polymers, such as cellulose (HPC), biodegradable PHBV, PLLA, and natural SF, show enhanced harvesting performance due to their low electron affinity compared to most common positive triboelectric materials, such as Al or PTFE. The highest output voltage and instant power density are found when PHBV and HPC are combined with PTFE as a negative triboelectric layer into the TENG, presenting output voltage and power density about ≈30 V and 6.1 μW/cm² and ≈50 V and 15.6 μW/cm², respectively. It is also found that HPC and PHBV are more positive triboelectric materials than PLLA and SF polymers; thus, they are better materials to be integrated into mechanical TENG energy harvesters. All these sustainable polymers can be placed on the more positive electron affinity side of the triboelectric series, a novelty in the current panorama of triboelectric materials.

In addition, it was shown that when coupled with a negative triboelectric and piezoelectric material, such as poled PVDF, hybrid PTENGs show an improved electrical output compared to TENGs, harvesting energy from both effects. Moreover, in PTENG device configurations, HPC and PHBV present the largest output voltage providing about 100 and 50 V, respectively, decreasing about one-third for unpoled PVDF where only the triboelectric effect drives the electrical output. This evidence further shows that PTENG devices can provide a higher energy density compared with the corresponding TENG based on similar materials.

This work demonstrates a broad contribution to the field of energy harvesting materials focusing on sustainable and easily processable positive triboelectric materials that are shown to provide improved performances compared to more common metals, allowing the development of conformable devices, suitable for being processed by solvent based additive manufacturing. Further tailoring these polymers with physicochemical modifications to enhance the surface charge density or their polarity will further increase the energy harvested with these flexible materials.

The authors acknowledge the Fundação para a Ciência e Tecnologia for financial support under strategic funding (Grant No. UID/FIS/04650/2021) and acknowledge European Union for support through NextGenerationEU (Grant No. PRR - C644936001-00000045). T.R.-M. and P.C. acknowledge support from FCT (Grant Nos. SFRH/BD/140242/2018 and 2023.07491.CEECIND). This study forms part of the Advanced Materials program supported by MCIN with funding from European Union NextGenerationEU (Grant No. PRTR-C17.I1) as well as by IKUR Strategy under the collaboration agreement between Ikerbasque Foundation and Fundación BCMaterials on behalf of the Department of Education of the Basque Government. Finally, the authors acknowledge funding by the Basque Government Industry Department under the ELKARTEK program.

The authors have no conflicts to disclose.

T. Rodrigues-Marinho: Investigation (equal); Writing – original draft (equal). R. Brito-Pereira: Investigation (equal); Methodology (equal). G. Pace: Supervision (equal); Writing – review & editing (equal). C. R. Tubio: Methodology (equal); Software (equal). S. Lanceros-Méndez: Conceptualization (equal); Writing – review & editing (equal). P. Costa: Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article.