1 Introduction

In recent years with an increase in low power consumption and nano-electronic wearable devices [1, 2], research into energy harvesters has advanced. Wearable devices built for the medical, sports and military industries have yet to resolve the issue of providing constant energy. Energy harvesting, also known as energy scavenging, is the conversion of waste energy captured from the ambient environment into electric energy, including human movement. This form of energy can replace traditional means of power supplies such as batteries which are limited in capacity [3] and constantly need replacing, resulting in high maintenance costs [4, 5].

Different energy harvesting methods capable of powering wearable devices include solar energy [6, 7], thermal energy [8, 9] and mechanical energy [10,11,12]. Solar energy is confined to a light source limiting its efficiency. The output energy for thermal energy is inefficient in addition to requiring a large temperature difference between the human body and the environment; clearly, this is not feasible.

The generation of power from mechanical energy is most favorable due to the diverse and indefinite ambient energy available that can be converted without limitations. The two most popular energy harvesting mechanisms are piezoelectric and triboelectric generators. Even though the development of piezoelectric generators has proven to be effective, their limitations have directed research into triboelectric generators/nanogenerators (TEGs/TENGS) [13]. Evidently, they produce a high output power with great efficiency in addition to a simple and economic production [14,15,16].

Triboelectricity is the conversion of mechanical energy into electrical energy based on the coupling effect of triboelectrification and electrostatic induction [17, 18]. Electrostatic induction gives rise to electrification when two objects coming into contact become spontaneously charged. With a difference in polarity, there is a charge producing a positive charge on one material and a negative charge on the other.

According to their tendency to gain negative or positive charge upon triboelectrification, materials have been classified into the triboelectric series, which suggest that charge is generated from two dissimilar materials located furthest apart in the series [19], with one producing a positive charge and the other a negative charge. Studies have been applied in the area of polymer materials to establish particular triboelectric series [20] or to explore the mechanisms of their triboelectrification [21, 22]. Triboelectricity can be also met between identical polymer materials [23, 24]. Research has yet to determine whether electrons, ions or small amounts of material are transferred between the two materials that create the charged surface during the contact and separation process [25, 26]. Moreover, environmental conditions and contact or friction variables influence the charge transfer.

1.1 Related Work

TEGs have gained much attention since 2012 when Fan et al. [27] built the first prototype. Due to their immense potential as energy harvesters, current research is directed toward improving their efficiency. An ongoing focal point is on reducing their high impedance [28, 29] in order to increase the charge density. A common solution involves modifying the morphology of the electrification surface layer of the TEG [13, 30,31,32,33,34]. Other investigation methods include hybrid energy harvesters, such as electromagnetic-triboelectric and piezoelectric-triboelectric nanogenerators [35,36,37,38,39,40,41]. The innovation of TEGs has led to tremendous applications ranging from energy-harvesting textiles [42, 43] and sensors [44, 45] to biomedical monitoring [43, 46].

Research has demonstrated that according to the conductivity of the participating structures, contact electrification can occur between insulator-insulator [27], insulator-conductor [47, 48] and conductor-conductor [49] structures in four modes: contact mode [50,51,52], lateral sliding mode [29, 53], single electrode mode [54, 55] and free-standing mode [56, 57]. For the contact mode, the conversion of mechanical energy into electrical energy is achieved through periodic contact and separation of the electrification layers under an external vertical force. By applying the lateral sliding mode method, a voltage is generated through periodic rubbing and separation of the two components of the TEG. The single electrode mode constitutes a material coming into vertical contact with a single electrode. The free-standing mode operates only in the sliding mode, consisting of two fixed electrodes separated by a distance with a material sliding on top. In its initial state, the material is in direct contact with one electrode, then it slides to the second electrode producing electrical energy and its final state it comes into direct contact with the second electrode. As the material reverts to the previous electrode an electrical charge is regenerated implying that electronic potential difference (EPD) is generated in motion.

Previous research into the lateral sliding mode has been undertaken for homogeneous [53], heterogeneous [55] and identical materials [58]. Wang et al. [53] built a sliding mode TEG, with dimensions of 71\(\times\)50 mm (length \(\times\) width), using PA6/6 and PTFE (both polymer materials) with surface etched nanowires on glass substrates. The TEG produced a Voc of 1300 V and a short circuit current density (Isc) of 4.1 mA/m2 with a peak power density of 5.3 W/m2. This was used as a direct power supply to light up a series of LED’s. Yang et al. [55] developed a single electrode-based sliding mode TEG consisting of a PTFE layer with surface-etched nanoparticles and a grounded aluminium (Al) electrode (polymer and metal materials). The TEG produced a Voc of 1100 V, an Isc density of 6 mA/m2, and a maximum power density of 350 mW/m2 with a load of 100 MΩ. Wang et al. [53] and Yang et al. [55] demonstrated that the EPD is generated when the top component of the TEG slides outwards; claiming that the larger the displacement between the two components, the greater the EPD.

It is only until recently that research into the triboelectric effect of identical materials has been demonstrated. Hu et al. [58] built a TEG using an inorganic material Silica. The TEG, measuring 100 × 50 mm, consisted of a flat SiO2 bottom surface and a top surface covered with SiO2 nanoparticles. The Voc was not reported; however, the maximum output power of the TEG reached 12.98 µW and 13.92 nW in contact and sliding model respectively. Apodaca et al. [59] models the contact electrification of identical organic materials as the transfer of charges from individual atoms of them. Henry et al. [25] proposed that the transfer of charges is related to the production of heat when the two materials are rubbed asymmetrically, yet Lowell et al. [23] supports the explanation of asymmetrical rubbing without the production of heat for the charge transfer. Hu et al. [58] confirms Lowell’s theory, by reporting the output power for identical inorganic material built with surface modification altering the surface roughness.

1.2 Research Scope

This paper focuses on two organic polymer materials found at opposite ends of the triboelectric series. Polyamide-6 (PA6) and Polytetrafluoroethylene (PTFE) membranes were used for the hereby insulator-insulator i.e. homogeneous TEGs. Three sets of TEGs were tested, two made of identical materials (i.e., PA6-PA6, PTFE-PTFE) and the third utilizing the two composite materials (i.e., PA6-PTFE). Samples for each TEG of different dimensions were built and repeatedly tested in sliding mode. For this study, no surface modification was performed on the membranes.

The following sections describe the properties of the organic polymer materials selected for this research and the experimental procedure including the fabrication process of the TEGs. This study identifies the maximum Voc (\({V}_{oc(max)}\)) and through data analysis the Voc is investigated for the three sets of TEGs. IBM SPSS Statistics was utilized for data cleaning in which the mean Voc (\({V}_{oc(\underbar{x})}\)) was identified and the data distribution was plotted. To conclude, the influence the membrane coupling, material selection and contact area have on the Voc is discussed.

2 Method

2.1 Materials

PA6 (C6H11NO) and PTFE (C2F4) tribo materials found at opposite ends of the triboelectric series were used for the present work. PTFE has a high chemical resistance. PA6 is most commonly used in tire cords while PTFE has a broader application due to its processability in a wider temperature range of -270 ºC and + 260 ºC.

PTFE is a fluorocarbon-based polymer with a density of 2.16–2.22 g/cm3 and a melting temperature (Tm) of 327–342 ºC. PA6 is a thermoplastic polymer with a density of 1.14 g/cm3 and a Tm of 222 ºC. The PTFE membranes used in the experiments were obtained from Zeus Industrial Products Inc. The PA6 membranes were prepared in the laboratory by dissolving PA6 (TECHNYL 1011 R), obtained from Rhodia Ltd., in formic acid at a temperature of 70 °C. The solution was then spin-coated onto a silicon wafer and immersed in an anti-solvent bath to obtain the PA6 membrane.

2.2 Experimental Procedure

All sets of TEGs built for this research consist of an upper and lower component. Each component constitutes of a substrate, made from acrylic substrate (Plexiglass) 2 mm thick; an electrode, using Al tape 0.2 mm thick; and an electrification layer, PA6 and PTFE membranes 0.1 mm thick (Fig. 1). For each set, the membranes were cut into four sample sizes: sample 1, 30 × 30; sample 2, 40 × 30; sample 3, 50 × 30; and sample 4, 60 × 30 mm. That means that the samples width was kept constant while only the length was gradually increased in such a way that the contact areas of samples 2, 3 and 4 were respectively 33.3%, 66.7% and 100% larger than that of sample 1. The substrates were cut 5 mm longer and wider in each direction to the dimensions of the electrification membranes. The rigid acrylic substrates served as the support basement to place the electrodes and the tested membranes. The sticky side of the Al tape was adhered on the substrates after it was cut in equal to the samples sizes. The membranes were overlaid on the Al tape using a strip of double-sided adhesive tape across each short side. Lead wires connected the electrodes to the oscilloscope Agilent Technologies DSO3102A for measurements (Fig. 2).

Fig. 1
figure 1

The layering of the components as used in the hereby TEG

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Fig. 2
figure 2

Detailed image of the experimental set-up

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Fig. 3
figure 3

Sketch of the experimental set-up

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In its original position, the two components were vertically aligned with the bottom component fixed and the top component unrestrained to slide in the lateral plane. As previewed in Fig. 3, the rotation of a disk drove a connected arm to push the top component forward and backward, thus the motion’s direction was linear. Consequently, the top membrane could slide from its original position and to fully separate from the bottom membrane before returning to its original position. All of the different size samples 1 to 4 were positioned so that they would slide longitudinally, meaning across their longest dimension. The total contact area of the top and bottom membranes were in full contact under a vertical center force of 9.8 N; this was transferred through a cylinder 3 cm in height and 1.5 cm in diameter with a contact area of 1.7 cm2. At a speed of 0.02 m/s, the TEGs completed periodic cycles.

3 Results and Discussion

As a result of a small area of friction, the surface of one component of the TEG gains positive charges whilst the other gains negative charges producing a potential difference. It was observed that the EPD was generated when the top component was returning to its original position after fully separating from the bottom component before completing a full cycle.

The process was repeated, and an indicative presentation of the Voc behaviors as recorded by the oscilloscope, for each sample of each set is presented below. For the PA6-PA6 TEG (Fig. 4), it was found that the Voc peak values increased from sample 1 to sample 4, such to say with the increase of the participating surface area.

Fig. 4
figure 4

PA6-PA6 TEG \({\varvec{V}}_{\varvec{o}\varvec{c}}\) indicative values for: (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4

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A similar behavior was noticed for the PTFE-PTFE TEG (Fig. 5), however for this set the corresponding Voc peaks were higher than for PA6-PA6 TEG.

Fig. 5
figure 5

PTFE-PTFE TEG \({\varvec{V}}_{\varvec{o}\varvec{c}}\) indicative values for: (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4

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Finally, the corresponding Voc peaks for PA6-PTFE TEG (Fig. 6) were between those of the PA6-PA6 TEG and the PTFE-PTFE TEG. In all three TEGs, the tension of Voc to increase with the increase of the surface area was maintained. This suggests that the surface area significantly influences the Voc of the TEGs.

Fig. 6
figure 6

PTFE-PA6 TEG \({\varvec{V}}_{\varvec{o}\varvec{c}}\) indicative values for: (a) sample 1, (b) sample 2, (c) sample 3 and (d) sample 4

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The peak values of Voc (\({V}_{oc\left(max\right)}\)) were collected for each of the four samples of each set, and their mean values were calculated and presented in Table 1.

Table 1 Mean \({{V}}_{{o}{c}({m}{a}{x})}\) values (V) per sample per TEG
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From the results presented in Table 1, it is clear that there is a direct relationship, between the surface area and \({V}_{oc\left(max\right)}\); and also there is a distinction between the identical and composite TEGs. Sample 1 of the identical TEG PA6-PA6 produced a mean \({V}_{oc\left(max\right)}\) of 317 V, the composite TEG PA6-PTFE generated 428 V; whilst the PTFE-PTFE TEG generated 598 V. Accordingly, sample 2 of the identical TEG PA6-PA6 produced a mean \({V}_{oc\left(max\right)}\) of 464 V, the composite TEG PA6-PTFE generated 605 V; whilst the PTFE-PTFE TEG generated 805 V; sample 3 of the identical TEG PA6-PA6 produced a mean \({V}_{oc\left(max\right)}\) of 713 V, the composite TEG PA6-PTFE generated 797 V; whilst the PTFE-PTFE TEG generated 900 V; and sample 4 of the identical TEG PA6-PA6 produced a mean \({V}_{oc\left(max\right)}\) of 826 V, the composite TEG PA6-PTFE generated 931 V; whilst the PTFE-PTFE TEG generated 1095 V. The form of change of the \({V}_{oc}\) in relation to the surface area for each TEG, was very close to a linear form, with R2 > 0,98 as can be seen in Fig. 7.

Fig. 7
figure 7

Mean \({\varvec{V}}_{\varvec{o}\varvec{c}\left(\varvec{m}\varvec{a}\varvec{x}\right)}\) values per sample per TEG

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By plotting the \({V}_{oc}\) generated for each test of each TEG sample, in Fig. 8 we can see a stable behavior of each sample. Only partially, two \({V}_{oc}\) values of sample 4 for PTFE-PTFE TEG deviated significantly; however taking into account the rest values of the sample tests, we may consider that those two low values were due to some measuring error.

Fig. 8
figure 8

\({\varvec{V}}_{\varvec{o}\varvec{c}}\) produced for PA6-PA6, PTFE-PTFE and PA6-PTFE TEG: (a) Sample 1, (b) Sample 2, (c) Sample 3 and (d) Sample 4

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Essentially, we can see that the contact area influences significantly the generated energy of the TEG. In both the three different made TEGs, no matter if they were homogenous or composite, we have seen an almost linear change of the \({V}_{oc}\) in relation to surface area (Fig. 7).

Additionally, we can also notice the importance of the material choice or combination. PTFE is a tribo material that has a high tendency to acquire a negative charge, ranked low in the polymer triboelectric series, and PA6 is ranked high in the triboelectric series and acquires a high tendency to gain a positive charge, so theoretically one might expect their positions to be an excellent combination according to the triboelectric series theory and provide the highest \({V}_{oc\left(max\right)}\). Contrary to this theoretic expectation following the triboelectric series table, hereby the intensity of the triboelectric phenomenon coming from the actuation of the homogenous PTFE-PTFE TEG, was higher than that of the composite PA6-PTFE TEG. More specifically, the PTFE-PTFE TEG provided the highest \({V}_{oc\left(max\right)}\) values throughout this experiment, the second was the composite PA6-PTFE TEG and the third the identical PA6-PA6 TEG.

4 Conclusions

Through repeated experiments, the electrical energy generated from mechanical energy was examined for various sized identical and composite homogeneous TEGs. The selected tribo materials, PA6 and PTFE, are located on opposite ends of the triboelectric series where PA6 has a tendency to acquire a positive charge and PTFE a negative charge. These materials were selected to compare the Voc generated from the identical TEGs (i.e., PA6-PA6 and PTFE-PTFE) with the composite TEGs (PA6-PTFE). For each set of TEGs, four samples were fabricated with dimensions: sample 1, 30 × 30; sample 2, 40 × 30; sample 3, 50 × 30; and sample 4, 60 × 30 mm. Therefore, an observation could be made on the following parameters: tribo material selection, the type of coupling (identical or composite) and the size of the contact area.

Firstly, it was identified that for any sized identical or composite TEG the Voc generated was not constant and the range of the data set was wide. The mean \({V}_{oc\left(max\right)}\) was reported for each TEG and a comparison of the TEGs was made. It was shown that the use of PTFE membrane in a TEG provided better results than the use of PA6 membrane. Moreover, the best results came from the actuation of an identical TEG and not a composite TEG. Indeed, the use of PTFE on both the contact areas of the TEG provided the highest mean \({V}_{oc\left(max\right)}\) values (from 598 V for sample 1 up to 1095 V for sample 4). This verifies that the triboelectric phenomenon can happen between identical materials, and it is not limited to follow the triboelectric series theory which refers to the use of materials with different charge affinities. Finally, for all the examined TEGs, both identical or composite, it was shown that the increase of the contact surface area increased the outcoming \({V}_{oc\left(max\right)}\) of each sample in a very close to linear form.

Although much research has gone into identical TEGs with surface modification or composite TEGs, this research demonstrates that identical TEGs can be as efficient as composite TEGs without the need for surface modification. However, the tribo material selected is of importance; for example, the PTFE-PTFE TEG had the largest \({V}_{oc}\) antithesis to the PA6-PA6 TEG which had the lowest. Thus for an energy harvesting system, PTFE-PTFE would be better suited given that the energy generated over a number of cycles is greater and consistent.

This research might be further extended by fabricating triboelectric fibres to produce triboelectric textile surfaces which can be used for attire, where the woven or knitted fibres will be rubbed over a large surface area, to harvest the energy produced from human movement and power wearable devices.