# Hybrid tribo-piezoelectric microgenerator for mechanical energy harvesting

Walid Askri, Julien Le Scornec, Raynald Séveno, Benoit Guiffard

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Walid Askri, Julien Le Scornec, Raynald Séveno, Benoit Guiffard. Hybrid tribo-piezoelectric microgenerator for mechanical energy harvesting. Physica B: Condensed Matter, 2025, 713, pp.417382. 10.1016/j.physb.2025.417382. hal-05415319

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Submitted on 7 Jan 2026

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# Hybrid tribo-piezoelectric microgenerator for mechanical energy harvesting

Walid Askri* , Julien Le Scornec , Raynald Séveno , Benoit Guiffard

Nantes University, CNRS, IETR, UMR 6164, 2 rue de la Houssinière, Cedex 3, 44322, Nantes, France

ARTICLE INFO

Keywords:

Microgenerator

Triboelectric

Piezoelectric

Hybrid generator

Mechanical energy harvesting

A B S T R A C T

In the present study, a hybrid triboelectric/piezoelectric generator was proposed to combine simultaneously the piezoelectric and triboelectric effects to improve the device's performances. When subjected to the shaker motion, the hybrid generator delivers an open circuit voltage of 318 V, a short-circuit current of 4.95 µA and a maximum power of 349 µW dissipated in a load resistance of 40 MΩ under the application of a force of 8.5 N at a frequency of 2 Hz after rectification by two diode bridges. A capacitor of 80 µF capacitance was charged with the piezoelectric, triboelectric and hybrid systems with vibrating shaker. Thanks to the triboelectric-piezoelectric hybridization, the charging time is reduced by 74 % compared to the piezoelectric system alone and 39 % compared to the triboelectric system alone. Finally, a temperature sensor was powered by a 1 mF capacitor charged to 4.5 V by the hybrid tribo/piezoelectric generator.

1. Introduction

In recent years, the advent of the Internet of Things (IoT) [1,2] in all areas of society (home automation, Health, Energy, ...) has led to a proliferation of low-power communicating sensors (range from µW to mW) intended to collect and transmit data with the lowest possible costs [3]. In this context, interest in renewable energy sources [4] continues to grow in order to find an alternative to batteries limited in life. Thus, the harvesting of ambient energy [5] using electroactive materials has become an essential way to reduce the energy footprint of wireless communicating sensors [6,7] and move towards their self-powering [8]. Among these energy sources, the vibrational energy could be mentioned [9–11]. Harvesting mechanical vibrational energy is a promising alternative to ensure the autonomy of electronic devices with low energy consumption. For specific applications of mechanical-electric conversion [12,13] such as the harvesting of low frequency (<100 Hz) ambient micro-energy (range from µW to mW) [14], the potential of flexible piezoelectric [15] and triboelectric [16] materials has been widely demonstrated. In the case of the triboelectric effect, the dynamic Contact-Separation mode [17] is generally preferred for comparing materials performances. According to Hasan and al [18], this mode is the mostly used in practical applications and most efficient in terms of power generation. It consists in periodically contacting the surfaces of two materials of different electronegativity leading to a transfer of

charges between them. An important criterion for the efficiency of triboelectric conversion is therefore the choice of a material with a high electronegative affinity with respect to a metal such as aluminum, also acting as an electrode. The choice of triboelectric materials [19] depends not only on chemical composition but also on other factors such as specific contact area, material shape, roughness and elastic moduli which influence the charge density, and environmental factors such as humidity and temperature [20,21].

In the literature, the polyurethane foam charged with carbon black (PUC) [22] is not commonly used as a charge donor in triboelectricity despite it presents higher voltage than metals like aluminum foil when it is connected with PTFE, so for our study, the electropositive material PUC of the triboelectric pair is a conductive, also acting as an electrode, while the electronegative material is PTFE (Teflon®) [23] which is placed at the bottom of the triboelectric series.

The novelty of this work is to is to not only to ameliorate energy harvesting performance using two connected generators, but also comparing the three generators energy generated at the same impedance to show that tribo-piezoelectric hybridization really does improve energy efficiency. In order to improve the harvesting energy performances, both piezoelectric and triboelectric effects have been combined in two types of connected piezoelectric-triboelectric hybrid generators [24,25], with specific design depending on the input mechanical excitation.

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: walid.askri@etu.univ-nantes.fr (W. Askri).</footnote>
The behavior of each generator TEG (triboelectric generator), PEG (piezoelectric generator) and hybrid (tribo/piezoelectric combination) before and after rectification using diode bridges was studied. Moreover, the three generators energy generated were compared at the same load impedance to show that tribo-piezoelectric hybridization really does improve energy efficiency.

Thus, the results of our hybrid generator mounted on a shaker using contact-separating mode (most used in practical applications) were compared with wind tunnel results (real operating conditions simulating natural wind [26]). To obtain the best results in the wind tunnel, structural and aerodynamic tests were carried out to obtain maximum energy harvesting, such as the position of the piezoelectric generator (PEG) and the shape of the bluff body [27,28]. The measurements show that the triboelectric generator (TEG) provides a fairly high output voltage (≈300 V) on shaker, but an output current limited to less than 3 µA, which limits the power dissipated in a load resistor. The parallel hybrid tribo-piezoelectric combining the two transduction systems yields an enhancement of the output power from 234 µW to 349 µW and 2 µW–226 µW for the generator mounted on a shaker and the air flow driven generator, respectively. These improvements are due to the combination of two effects: the summation of the two PEG and TEG currents and the high output voltage value of the hybrid generator similar to that TEG voltage after rectification step.

## 2. Methodology

### 2.1. Fabrication of hybrid systems

As shown in Fig. 1(a), for the hybrid system mounted on shaker, a sheet of commercial PTFE (Goodfellow FP301301/3) with a dimension of length x width x thickness = 90 mm × 70 mm × 100 µm was attached

on a 85 mm × 65 mm × 50 µm aluminum electrode. This set was glued to the moving part of a shaker. The PUC foam 85 mm × 65 mm × 3 mm (Radio Spare 8153512) was glued on the fixed tray in polylactic acid (PLA) installed on the shaker. The piezoelectric generator (PEG) used is a commercial flexible polyvinylidene fluoride film (PVDF) with Aluminum (Al) electrodes (RadioSpare 1-1002908-0) and dimensions of 30 mm × 12 mm × 28 µm, and a capacitance of 1.28 nF. It was firmly attached by its ends (clamped-clamped configuration) to the two PLA plates of the shaker and subjected to bending due to the vertical motion of the top plate. Fig. 1(b) shows the output voltage of the PEG and TEG through a load resistance of 100 MΩ over a contact-separation cycle.

A SEM image of the PUC is presented in Fig. 1(c). The density, porosity and size of cells and carbon nanoparticles influence the conductivity of carbon black-filled polyurethane foam.

Fig. 2 gives a schematic representation of the PEG-TEG hybrid generator and electrical connections for output voltage measurement without or with rectifying circuits. Due to a slight shift of the phases of the PEG and TEG output voltages observed in Fig. 1(b), each generator has been connected to its own rectifying circuit.

As shown in Fig. 3, the hybrid generator tested in wind tunnel (Supplementary information S1) is composed of two TEGs working in contact-separation mode and two PEGs, while retaining the same materials as the hybrid generator mounted on the shaker. The two PEGs combined in parallel can be considered as a single PEG (with capacitance Cp = 2.56 nF) since they are glued on each side and near the clamped end of a stainless steel cantilever beam (180 mm × 45 mm × 200 µm) and therefore subjected to the same mechanical stress amplitude. A bluff body is fixed at the other hand of the cantilever to generate vortex and galloping-induced vibrations [29,30]. The materials used for the TEGs are carbon black-filled polyurethane foam 70 mm × 40 mm × 3 mm as the electropositive material and PTFE as the electronegative

<figure><img src="image_4.png" /><figcaption>Fig. 1. Photograph of the hybrid system and the mechanism of operation. Experimental device (a). PEG and TEG voltages over a cycle (b). SEM image of the PUC (c).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Electrical scheme: without rectification (a), with rectification (b).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Schematic of the hybrid generator for wind tunnel testing.</figcaption></figure>

material with dimensions 72 mm × 42 mm × 100 µm. The PTFE material is a little larger to avoid any short-circuit between aluminium electrode bonded to PTFE and PUC foam.

## 2.2. Experimental setup

To ensure the periodic movement of the TEG and PEG simultaneously mounted on a shaker (Supplementary information S2), the two microgenerators displacements were controlled by a low frequency signal generator through the shaker activation. The contact separation between PUC and PTFE become progressive due to the foam flexibility. The applied external force leads to a gradual increase/decrease of the friction surface.

For all the measurements presented in this part, the chosen frequency is 2 Hz and the applied force is 8.5 N, measured using a force transducer (PCB LW55861). The maximum air gap which separate PUC and PTFE is 10 mm. The choice of 2 Hz is made in order to avoid damaging the shaker and PEG. The output voltages were visualized with an oscilloscope (Keysight DSO 1004A). After loading the PTFE to its

saturation level, Fig. 4(a) shows that in the initial state the surface of the PTFE is negatively charged unlike the positively charged PUC.

This pair was chosen because of PTFE's ability to capture electrons and the ability of the PUC to divest them [31]. The distance between the two layers gradually reduces (Fig. 4(b)) until full contact (Fig. 4(c)). Upon contact, a current migrates from the aluminum electrode to the PUC foam since it plays a dual role: electropositive material complementary to PTFE to form the triboelectric pair and electrode thanks to its conductive properties. This is due to the change in polarization and change in electric field which are directly related to speed (frequency 2Hz) at which the two charged surfaces are moving with respect to their electrodes [32]. On the other hand, the PEG gradually bends thus producing a current that increases until it reaches a value corresponding to the maximum deformation.

Then, as the PTFE and PUC foam gradually separate, an opposite current migrates from the PUC foam to the aluminum electrode until completely separated. Similarly, the piezoelectric current decreases until the PEG returns to the initial position (Fig. 4(d)).

For the hybrid generator mounted in wind tunnel, the PEG position and different bluff body shapes were tested to harvest the maximum of energy.

The position of the PEG is an important parameter for generating the maximum possible power, this is why the PEG's performance should be tested. Fig. 5 shows an explanatory scheme of the different positions tested. Position x = 0 mm corresponds to the clamped end of the stainless-steel cantilever beam, whereas the first position tested is x = 20 mm.

In addition to the geometric study carried out to optimize the position of the PEG, the aerodynamic aspect was studied by varying the shape of the bluff body that presents the first point of contact between the wind and the system under study (Supplementary information S3).

<figure><img src="image_7.png" /><figcaption>Fig. 4. Charge transfer in a single cycle of triboelectric effect.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 5. PEG position tests: Hybrid generator on wind tunnel (a). PEG mounted on stainless steel (b).</figcaption></figure>

# 3. Results and discussion

## 3.1. Energy harvesting with shaker

The hybrid microgenerator is mounted on a shaker allowing the movement of the moving part in the vertical direction. The voltage and current delivered independently by the TEG and PEG, as well as by the hybrid generator are measured according to a variable load resistance. The voltage amplitudes of the PEG, TEG and hybrid signals rectified by two diode bridges are also compared.

Supplementary information S4 (a) shows the piezoelectric and triboelectric voltages visualized simultaneously before rectification. The peaks of the TEG are wider than the narrow peaks generally known in triboelectricity. This is due to the effect of PUC foam which makes the contact longer compared to a PTFE/Metal contact for example. The two positive peaks of PEG and TEG which correspond to the moment of full contact are in phase, on the other hand the separation peaks (the two negative peaks) are out of phase because at the moment of separation, the TEG has mechanically stopped working while the PEG has not yet returned to its initial position.

The hybrid signal without rectification shown in Supplementary information S4 (b) is of the same shape and amplitude quite close to that of the piezoelectric signal: This is due to the parallel configuration of two power sources and it is therefore the one with the smallest internal impedance that dominates [33], related to the capacitance of TEG ($C_T$ = 0.68 nF) and PEG ($C_p$ = 1.28 nF). For this reason, two full-wave diode bridges were independently connected to both the piezoelectric and triboelectric outputs. The rectified output signals were then connected in parallel. Supplementary information S4 (c) shows the individual output signals after rectification by the piezoelectric and triboelectric generators. Supplementary information S4 (d) shows the voltage obtained after rectification by the hybrid (PEG and TEG) generator, which is clearly the superposition of the different peaks that form this signal.

Without rectification and as shown in Fig. 6(a) and (c), which give the evolution of the voltage, current amplitudes and power as a function of the load resistance, the PEG and TEG deliver an open circuit voltage Voc of 74 V and 330 V, a short-circuit current $I_{sc}$ of 2.5 µA and 2.8 µA, respectively.

For each generator, the power reaches a maximum at the optimum load resistance ($R_{opt}$ = $1/C_ω$). For the piezoelectric generator, the maximum power of 27 µW is reached with an optimal resistance of 62 MΩ against 240 µW at 117 MΩ for the triboelectric generator.

In summary (Table 1), without rectification, the hybrid current $I_{sc}$ = 4.8 µA shown in Fig. 6(e) is close to the sum of piezoelectric and triboelectric currents (4.8 µA ≈ 5.3 µA). This sum results from the

parallel configuration used for the hybrid system. But in terms of power, the obtained value ($P_{max}$ = 64 µW) is lower than those obtained for TEG alone ($P_{max}$ = 240 µW) and this can be due to the phase shift between the two piezoelectric and triboelectric signals [33], yielding a Voc of 98 V, while Voc of TEG is 330 V.

After rectification of the PEG and TEG signals, the electrical outputs obtained are voltages amplitude of Voc = 72 V and Voc = 325 V, currents $I_{sc}$ = 2.2 µA and $I_{sc}$ = 2.7 µA, and maximum powers of 25 µW and 234 µW, respectively (Fig. 6(b)–(d)).

The rectified hybrid signal takes the shape of the rectified triboelectric signal with a close voltage amplitude Voc = 318 V. The short-circuit current of the hybrid generator and the maximum power are equal to 4.9 µA and 349 µW respectively (Fig. 6(f)).

To conclude, without rectification, the combination of PEG and TEG in parallel configuration leads to a sum of currents but the voltage amplitude is similar to the PEG voltage amplitude due to the domination of the PEG generator with lowest impedance.

After rectification, in addition to current summation, the output voltage of the hybrid generator is close to that of the TEG due to the parallel connection.

Jung and al [33] attributed this voltage increase of the hybrid generator after rectification to the attenuation of the effect of the internal resistance mismatch of the two microgenerators PEG and TEG.

Different values obtained with rectification of the signals are summarized in Table 1. The hybrid tribo-piezoelectric combination has improved performance in terms of energy harvesting compared to PEG alone or TEG alone. Thus, a gain of 115 µW corresponding to a 33% increase was achieved compared to TEG alone.

To better understand the effect of the tribo-piezoelectric hybrid combination, an energy comparison was made at 2 Hz controlled by the signal generator and at a 1 kΩ resistive load imposed by the current amplifier (Keithley 428). So, from the currents variation as a function of time, the power curves as a function of time can be deduced (Fig. 7(a), (b), (c)). After integration over a period the energy generated by PEG, TEG and hybrid generator is obtained. The results show that the energies produced per cycle (E(generator/cycle)) by the hybrid, TEG and PEG generators are 2 µJ/cycle, 1.24 µJ/cycle and 0.53 µJ/cycle, respectively.

This energy comparison confirms the beneficial effect of the tribo-piezoelectric hybrid combination and is illustrated by the charging times for a 80 µF capacitor.

An experimental and computational comparison of capacitor charging times by the 3 generators PEG, TEG and hybrid was carried out. The energy required to charge a capacitor at 10.6 V is given by the following expression:

$$E (capa) = \frac{1}{2} CU^2 \quad (1)$$

where C is the capacitance (80 µF) and U the voltage reached (10.6 V). The energy required to charge the capacitor at 10.6 V is equal to 4494 µJ, so for each generator, the required charging time was calculated using the following equation:

$$Charging\ time = \frac{1}{f} \cdot \frac{E(capa)}{E(generator/cycle)} \quad (2)$$

where f is the frequency. The hybrid generator is the fastest to reach 10.6 V (maximum voltage limited by the acquisition board (NI myDAQ 195509F-01L) in 18mn55s experimentally, compared with 18mn43s calculated. The TEG is ranked 2nd with an experimental charging time of 30mn41s against 30mn20s calculated. Finally, the PEG charged the capacitor in 71mn12s experimentally versus 70mn40s calculated. The time dependence of the capacitor voltage and charging times are given in Fig. 7(d).

Comparing the electrical performances of tribo-piezoelectric hybrid systems in the literature for energy harvesting at low frequencies (<20 Hz) seems difficult because the parameters used for comparison are
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Electrical characterization of the different microgenerators. Voltage, current and power of: PEG without rectification (a), PEG with rectification (b). TEG without rectification (c), TEG with rectification (d), hybrid without rectification (e), hybrid with rectification (f).</figcaption></figure>

<table><caption>**Table 1**

Voltage, current and power for different systems without and with rectification on shaker.</caption><thead><tr><th rowspan="2">System</th><th colspan="3">Without rectification</th><th colspan="3">With rectification</th></tr><tr><th>Voc(V)</th><th>Isc(µA)</th><th>Pmax(µW)</th><th>Voc(V)</th><th>Isc(µA)</th><th>Pmax(µW)</th></tr></thead><tbody><tr><td>PEG</td><td>74</td><td>2.5</td><td>27</td><td>72</td><td>2.2</td><td>25</td></tr><tr><td>TEG</td><td>330</td><td>2.8</td><td>240</td><td>325</td><td>2.7</td><td>234</td></tr><tr><td>Hybrid</td><td>98</td><td>4.8</td><td>64</td><td>318</td><td>4.9</td><td>349</td></tr></tbody></table>

numerous (triboelectric excitation mode, geometrical dimensions, frequency, force ...). Nevertheless, Table 2 summarizes the main characteristics of tribo-piezoelectric hybrid microgenerators in uniaxial contact-separation mode. For most of the works devoted to tribo-piezoelectric hybrid combination, surface area and frequency appear to be the most important parameters.

The results of this work are obtained using low cost commercial materials (PUC: 0.01€/cm² for RadioSpare 8153512 and PTFE: 0.046€/cm² for Goodfellow FP301301/3), unlike other works which have used rather expensive materials such as gold (345.91€/cm² for Goodfellow
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Energy and capacitor charge. Energy generated at 1kΩ resistive load by PEG (a), TEG (b) and hybrid (c). Charge of 80 µF capacitor by the 3 generators (d).</figcaption></figure>

<table><caption>**Table 2**
Comparison of different hybrid tribo-piezoelectric generators in contact-separation mode.</caption><thead><tr><th>Work</th><th>Hybrid materials</th><th>Frequency (Hz)</th><th>Power density (µW/cm²)</th></tr></thead><tbody><tr><td>Singh and al [34]</td><td>Au/PTFE (ZnO/PVDF)</td><td>1.1</td><td>24.5</td></tr><tr><td>Chen and al [35]</td><td>PDMS/(PVDF/TrFE)</td><td>3</td><td>0.46</td></tr><tr><td>Šahu and al [36]</td><td>Al/(PDMS-BTO)</td><td>1</td><td>40</td></tr><tr><td>Katragadda et al. [37]</td><td>ITO_PET/(BKBFT PDMS_MWCNT)</td><td>1</td><td>0.072</td></tr><tr><td>Sutka et al. [38]</td><td>β-Phase PVDF and ITO (electrode)</td><td>1</td><td>2400</td></tr><tr><td>This work</td><td>PUC/PTFE (PVDF)</td><td>2</td><td>5.92</td></tr></tbody></table>

AU00-FL-000148). In fact, polarized ferroelectric materials play an important role to increase energy harvesting. Šutka et al. [38] reported a power density of 24 W/m² thanks to the polarized β-Phase PVDF. The maximum power obtained was equal to 349 µW which is lower than those obtained by Sahu and al [36] (360 µW). Despite the use of PUC/PTFE and non-doped PVDF, our hybrid system exhibits a power equal to 349 µW, higher than those of Singh and al [34], Wang and al [39], Chen and al [35], Lee and al [40] and comparable to that of achieved by Sahu et al. [36].

### 3.2. Energy harvesting with wind tunnel

In recent years, triboelectricity has proven its ability to generate electricity and power many low-consumption electrical applications from natural environmental excitations, such as the vibratory mechanical energy that can be harvested from air flow. But remembering that

triboelectric current is limited, the TEG and PEG were combined to improve the current and therefore the total harvested power. In this section, the influence of structural variations on the hybrid generator was studied, in particular the position of the PEG and the shape of the bluff body (elliptical, Y shape with different angles) which is the first point of contact between the wind generated in wind tunnel and the generator. The influence of the wind speed on the harvested power is assessed. Finally, a comparison of the hybrid generator energy harvesting results in wind tunnel and on shaker was studied.

#### 3.2.1. PEG position variation

Fig. 8(a) shows PEG voltage and current amplitudes for positions x = 20 mm to x = 50 mm with a step size of 5 mm. The voltage curves show that the open-circuit voltage decreases away from the clamped end of the cantilever beam. The same applies to current, which shows the same behavior. The results show that the best position for electrical performance is the one closest to the clamped end of the cantilever beam. Fig. 8(b) shows the power curves for different distances and clearly confirms that the best position in terms of electrical performance is x = 20 mm. This position generates an open-circuit voltage of 15 V, a short-circuit current of 2.6 µA and a maximum power of 2.32 µW.

The PEG's performance in wind tunnel is limited owing to the limitation of the bending angle (8°) caused by the presence of boundaries which generate the triboelectric effect.

#### 3.2.2. Bluff body tests

The parameters describing the criteria for choosing a bluff body are numerous. Since the objective is to achieve the highest electrical response, experiments were carried out to choose the best shape and geometry for the bluff body was based on the PEG output voltage under
<figure><img src="image_11.png" /><figcaption>Fig. 8. Electrical performances of the PEG at different positions. Voltage and current amplitudes (a). Power (b).</figcaption></figure>

wind excitation in the wind tunnel. Fig. 9 shows the evolution of the piezoelectric voltage amplitude for the different shapes tested (Elliptical, Y45°, Y60°, Y90° and Y120°) as a function of wind speed (2 m/s to 10 m/s). The curves show that the amplitude voltages delivered by the PEG using the Y120° and Y90° shaped bluff body do not exceed 6 V over the whole speed range tested. The Y60° shape performs well at low speeds up to 8 m/s, when the amplitude voltage begins to decrease. The output voltage amplitudes of the two remaining shapes, elliptical and Y45°, are larger, with increasing voltage amplitudes from the lowest speed of 2 m/s to the highest speed of 10 m/s, with a slight advantage for the elliptical shape (see Fig. 10).

### 3.2.3. Energy harvesting performances with wind driven hybrid tribo-piezoelectric generator

In this section, the performance of the air flow-based hybrid generator at a frequency of 2 Hz (2 m/s wind speed) is studied. Supplementary information S5 shows the time dependence of the output voltage of the various PEG, TEG and hybrid generator without and with rectification. Supplementary information S5(a) and Supplementary information S5(b) show the PEG curves without and with rectification, respectively. Supplementary information S5(c) and Supplementary information S5(d) show the curves for TEG 1 (same form for TEG 2 with different voltage amplitudes Supplementary information S5(e), Supplementary information S5(f)) without and with rectification. The peaks of the TEG 1 curve are narrower than those of the TEG curve under the shaker-induced mechanical excitation. These two aspects are due to the different mechanical movements between the shaker and the wind tunnel. On the other hand, the hybrid signal without rectification Supplementary information S5(g), takes on the same shape and almost the same amplitude as that of the PEG without rectification: the same behavior as the hybrid generator under the shaker was observed. This is due to the dominance of the voltage with the lowest impedance in parallel configuration. With rectification, only the hybrid signal (Supplementary information S5(h)) changes completely in shape and voltage amplitude compared to the one without rectification: the same behavior as for the hybrid generator under the shaker was observed.

Supplementary information S5 and Fig. 11 show the different voltage, current amplitudes and power curves for the PEG, TEG 1, TEG 2 and hybrid generators without and with rectification. Without rectification, the PEG, TEG 1 and TEG 2 generators have open-circuit voltages of 15 V, 228 V and 219 V, respectively and short-circuit currents of 2.60 µA, 2.04 µA and 1.94 µA, respectively. At maximum power, PEG, TEG 1 and TEG 2 generate 2.3 µW, 152 µW and 136 µW, respectively. For the hybrid generator, the dominance of the PEG voltage limits the open-circuit voltage and maximum power to 24 V and 9.5 µW respectively, while the short-circuit current (6.24 µA) is close to the sum of the currents of the three generators PEG, TEG 1 and TEG 2. The problem of low hybrid power is solved by using diode bridges for each TEG and PEG. With rectification, the following performances for PEG, TEG 1 and TEG 2 were respectively obtained: 14 V, 225 V and 216 V for open-circuit voltages; 2.55 µA, 1.97 µA and 1.88 µA for short-circuit currents and 2 µW, 142 µW and 132 µW for maximum powers. For hybrid generators, the short-circuit current still increases slightly (6.35 µA) compared with the current before rectification, but the radical change is seen in the voltage, which rises from 24 V before rectification to 144 V after rectification and the power also rises from 9.5 µW to 226 µW. The results are summarized in Table 3.

So, at 2 Hz (2 m/s wind speed), the surface power density of hybrid generator excited by the shaker and positioned in the wind tunnel after rectification could be compared. It may be noticed that the shaker is a laboratory instrument that do not correspond to real outdoor mechanical excitations, unlike the wind tunnel, which is a use case for the hybrid generator harvesting energy from environmental air flow.

Finally, the surface power density of the hybrid generator tested in wind tunnel (3.57 µW/cm²) represents 60 % of the surface power density of the hybrid generator installed on shaker (5.91 µW/cm²).

<figure><img src="image_12.png" /><figcaption>Fig. 9. Amplitude Voltage evolution of PEG for different bluff body shapes as a function of wind speed.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>Fig. 10. Electrical characterization of the hybrid microgenerator without rectification. Voltage and current amplitudes and power of PEG (a), TEG 1(b), TEG 2(c) Hybrid (d).</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 11. Electrical characterization of the hybrid microgenerator with rectification. Voltage and current amplitudes and power of PEG (a), TEG 1(b), TEG 2(c) Hybrid (d).</figcaption></figure>
<table><caption>**Table 3**
Voltage, current and power for different systems without and with rectification in wind tunnel.</caption><thead><tr><th rowspan="2">System</th><th colspan="3">Without rectification</th><th colspan="3">With rectification</th></tr><tr><th>Voc(V)</th><th>Isc(µA)</th><th>Pmax(µW)</th><th>Voc(V)</th><th>Isc(µA)</th><th>Pmax(µW)</th></tr></thead><tbody><tr><td>PEG</td><td>15</td><td>2.60</td><td>2.3</td><td>14</td><td>2.55</td><td>2</td></tr><tr><td>TEG 1</td><td>228</td><td>2.04</td><td>152</td><td>225</td><td>1.97</td><td>142</td></tr><tr><td>TEG 2</td><td>219</td><td>1.94</td><td>136</td><td>216</td><td>1.88</td><td>132</td></tr><tr><td>Hybrid</td><td>24</td><td>6.24</td><td>9.5</td><td>144</td><td>6.35</td><td>226</td></tr></tbody></table>

### 3.2.4. Performance of the hybrid generator at different wind speeds

To better understand the behavior of the hybrid system in wind tunnel, a study was carried out varying the wind speed from 2 m/s up to 10 m/s with a 2 m/s step based on results in section 3.2.1 and 3.2.2. The relationship between the hybrid generator vibrating frequency and wind speed is shown in Supplementary information S7. The voltage amplitudes, current and power curves for the five tested speeds are shown in Fig. 12(a) and (b). Open-circuit voltage, short-circuit current and maximum hybrid system power after rectification increase with speed. Between the lowest speed of 2 m/s and the highest speed of 10 m/s, the increases of open-circuit voltage, short-circuit current and maximum power are respectively from 144 V to 165 V, from 6.35 µA to 6.92 µA, and from 226 µW to 289 µW.

As mentioned for the comparison of our results with previous works in Table 2, the comparison in Table 4 is based on two important parameters: active surface and frequency, except that in this table only wind-excited hybrid tribo-piezoelectric systems are compared. This work presents the best area power density to frequency ratio. The surface power density per frequency obtained was 1.79 µW/cm²/Hz which is 3 times larger than in the study of Wang and al [39] (0.59 µW/cm²/Hz) because of the high voltage delivered by the triboelectric pair (PUC/PTFE), despite the use of macro-fiber composite (MFC) in Ref. [29] with high piezoelectric response but also quite expensive material. However some works in litterature reach high tribo-piezoelectric energy harvesting airflow. Sutka et al. [41] reported a power density of 77.4 mW/cm² thanks to the use of recycled poly-styrene (rPS), very low active surface area and high frequency of 15 Hz at an impedance of 100 MΩ.

## 3.3. Temperature sensor supply

### 3.3.1. Sensor energy consumption

The chosen sensor is the eZ430-RF2500, a complete wireless development tool from Texas Instruments (TI). The sensor energy consumption is measured using the circuit shown in Supplementary information S7. The aim of this setup is to measure the current $i_s$ consumed by the sensor. To do this, an oscilloscope (myDAQ card connected to the PC: the myDAQ card is capable of recovering voltages on a reduced time scale (ms) unlike a conventional oscilloscope) displays the voltage $U_R$ across R of 15 Ω ($U = R \times I$). To measure $U_R$, the sensor is supplied by a DC generator with a voltage $U_s$ of 3V to take temperature measurements every 12 s [43]. Fig. 13(a) shows the five phases of sensor power consumption on power-up and Fig. 13(b) shows the consumption phases of the sensor during temperature acquisition.

<table><caption>**Table 4**
Hybrid tribo-piezoelectric generator wind energy harvester.</caption><thead><tr><th>Work</th><th>Tribo/ Piezoelectric generator</th><th>Active surface (cm²)</th><th>Frequency (Hz)</th><th>Maximum of power (µW)</th><th>Power density (µW/ cm²)</th></tr></thead><tbody><tr><td>Wang et al. [39]</td><td>Al/FEP (MFC)</td><td>27</td><td>15</td><td>237.7</td><td>8.8</td></tr><tr><td>Ge et al. [42]</td><td>Al/FEP (PVDF)</td><td>29.5</td><td>5</td><td>2.4</td><td>0.08</td></tr><tr><td>This work</td><td>PUC/PTFE (PVDF)</td><td>63.2</td><td>2</td><td>226</td><td>3.57</td></tr></tbody></table>

Tables 5 and 6Tables 5 and 6 summarizes phase durations and average current consumption at power-up.

The average current consumed by the sensor during temperature acquisition is approximately 56.4 µA.

### 3.3.2. Energy storage

The schemes in Supplementary information S8 and Supplementary information S9 shows the energy storage circuits under shaker and in wind tunnel respectively at a frequency of 2 Hz and a force of 8.5 N.

Under shaker, charging time varies according to capacitor capacity. For a 1 mF capacitor, it takes 1h36mn to reach a voltage of 4.5 V (the maximum voltage that the eZ430-RF2500 sensor can accept), compared with 7h31mn to charge the 4.7 mF capacitor (Fig. 14(a)). In wind tunnel and at a speed of 2 m/s, the 1 mF capacitor reaches 4.5 V in 1h14mn, compared with 10h22mn for a 4.7 mF capacitor (see Fig. 15).

The energy stored by the capacitor is 10.13 mJ for the 1 mF capacitor versus 47.59 mJ for the 4.7 mF capacitor.

To study the sensor power supply, we choose to discharge the 1 mF

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Electrical characterization of the hybrid microgenerator with rectification for different speeds. Voltage and current amplitudes (a) power (b).</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 13. Sensor power consumption. (a) on power-up. (b) during temperature acquisition.</figcaption></figure>

<table><caption>**Table 5**
Duration and current consumption of the five phases on power-up.</caption><thead><tr><th>Phase</th><th></th><th>Duration (ms)</th><th>Average Current (mA)</th></tr></thead><tbody><tr><td>1</td><td>Sensor off</td><td>-</td><td>0</td></tr><tr><td>2</td><td>Connection established</td><td>57,8</td><td>15,12</td></tr><tr><td>3</td><td>Asleep</td><td>5,5</td><td>0,05</td></tr><tr><td>4</td><td>Temperature acquisition</td><td>24,8</td><td>3,32</td></tr><tr><td>5</td><td>Asleep</td><td>12000</td><td>0,05</td></tr></tbody></table>

<table><caption>**Table 6**
Duration of temperature acquisition steps and their average current consumption.</caption><thead><tr><th>Phase</th><th></th><th>Duration (ms)</th><th>Average Current (mA)</th></tr></thead><tbody><tr><td>1</td><td>Asleep</td><td>12000</td><td>0,05</td></tr><tr><td>2</td><td>Microcontroller wake-up</td><td>1,62</td><td>0,5</td></tr><tr><td>3</td><td>Temperature measurement</td><td>3,6</td><td>1,9</td></tr><tr><td>4</td><td>RF transmission</td><td>28,8</td><td>13,9</td></tr><tr><td>5</td><td>Standby</td><td>17,9</td><td>1,6</td></tr></tbody></table>

capacitor. The 1 mF capacitor was chosen for two reasons.

* The use of a 1 mF capacitor is sufficient to power the eZ430-RF2500 temperature sensor, due to its low power consumption.

* The capacitor has a faster recharge time (1h36mn under shaker) than other higher-capacity capacitors, making ambient temperature acquisition more convenient.

<figure><img src="image_17.png" /></figure>

### 3.3.3. Sensor supply

With an interval of 4 min between two temperature acquisitions, shows the discharge time of the 1 mF capacitor.

The 1 mF capacitor is capable of powering the sensor for 30mn50s for a total of 5 stable acquisitions before measuring false temperatures and then shutting down. The temperature is stable (24.5 °C) for voltages above 1.7V. Below this voltage, the temperature is no longer correct (≈31 °C). The voltage level becomes insufficient for a valid temperature measurement.

## 4. Conclusion

In this work, a hybrid tribo-piezoelectric microgenerators were developed and tested on two mechanical excitation systems: shaker and wind tunnel. Using the shaker inducing contact-separation mode, the triboelectric pair (carbon black-filled polyurethane foam/Teflon) delivers an open-circuit voltage of 325 V and a short-circuit current of 2.7 µA. A commercial PVDF piezoelectric microgenerator was combined with the triboelectric microgenerator to enhance maximum harvested power. The hybrid connection of the two generators (TEG and PEG) mounted on shaker delivers, with rectification using the two diode bridges and under the application of a 8.5 N force and a frequency of 2 Hz, an open circuit voltage of 318 V, a short circuit current of 4.95 µA and a maximum power of 349 µW for an optimum resistance of 40 MΩ, compared with a maximum power obtained without rectification equal to 64 µW. A 80 µF capacitor was charged with the piezoelectric only, triboelectric only, and hybrid generators. The results show a 39 % and 74 % gain in capacitor charging time with hybrid combination, compared to triboelectric and piezoelectric microgenerators,

<figure><img src="image_18.png" /><figcaption>Fig. 14. Charging of 1 mF, 2 mF, 3 mF and 4.7 mF capacitors by hybrid generator. (a) Under shaker. (b) In wind tunnel.</figcaption></figure>

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. 15.** Evolution of 1 mF capacitor discharge and temperature for acquisitions every 4 min.</figcaption></figure>

respectively. In the wind tunnel, at a frequency of 2 Hz (2 m/s wind speed), the hybrid generator generates a maximum power of 226 µW. The maximum surface power density of the hybrid generator in wind tunnel (3.57 µW/cm²) represents 60 % of the maximum surface power density of the hybrid generator tested on a shaker (5.91 µW/cm²). A temperature sensor was powered by a 1 mF capacitor charged to 4.5 V by the hybrid tribo/piezoelectric generator.

### CRediT authorship contribution statement

**Walid Askri:** Writing – original draft. **Julien Le Scornec:** Writing – review & editing. **Raynald Séveno:** Writing – review & editing, Methodology. **Benoit Guiffard:** Writing – review & editing, Supervision.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

We thank P. DERVAL, technician at IETR, Nantes University for the assembly of the experimental device.

### Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.physb.2025.417382.

### Data availability

Data will be made available on request.

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