Dipolar Energy as an Electrical Power Source: Dipole Rotation in Solids Enables a New Source for the Triboelectric Generator

Herein, the concept of triboelectric generators using dipolar polarization energy as an electrical power source is introduced. Dipolar polarization energy is stored in dielectric films by rubbing, and is conveyed to external circuits for use during the depolarization process. Pyromellitic dianhydride‐4,4’‐oxydianiline polyimide films are used as a possible candidate for triboelectric generators, where polarization is formed by rubbing owing to the orientational alignment of polar molecular groups. The current–voltage measurements are used to evaluate the equivalent‐circuit parameters of the generator. The current source Is is 0.07 nA at 30 °C, which is enhanced to 0.13 nA at 110 °C. It is indicated in these results that the depolarization process proceeds in a short time at higher temperatures, causing an increase in the current. Consequently, the maximum power increases from 18 nW (30 °C) to 33 nW (110 °C). The enhancement of the output power at higher temperatures well supports the idea of using dipolar polarization energy as a power source of triboelectric generators.


Introduction
[4] However, use of triboelectric charges as electrical power sources has recently gained attention.[7] Various types of triboelectric generators have been reported, which include the vertical [8,9] and lateral types, [10,11] among others.These devices are conventional and mainly utilize excessive electronic static charges as a source of power generation; that is, positive and negative charges remain on the two rubbed surfaces, and electrostatic energy is stored in the space between the two surfaces to be used in external circuits.To enhance the power output, the tendency of excessive charges remaining on surfaces is one of the key factors, which is listed in the table of the triboelectric series. [12][19] However, this process is no longer complete.In addition to excessive electronic charges, dipolar polarization charges are available as sources of electrical energy in electric circuits. [20]n contrast to the electrostatic energy accumulation in vertical and lateral triboelectric generators, dipolar energy is stored in the material itself for use in external circuits.Consequently, dipolar energy-based triboelectric generators do not require the relative displacement of electrodes, whereas vertical and lateral types do.There are two microscopic origins for the storage of dielectric polarization energy in materials, the displacement of charges and orientational alignment of permanent dipoles.The power transmission from the polarization energy is maximized under the power-matching condition given by C s R ¼ τ (C s : electrostatic capacitance, R: input resistance of the external circuit, τ: dielectric's relaxation time), leading to 50% usage of the polarization energy in the external circuits.
In this study, we report triboelectric generators based on the dipolar polarization energy stored by aligning dipoles in pyromellitic dianhydride (PMDA)-4,4'-oxydianiline (ODA) polyimide (PI) films.Rubbing the film surface causes DC current to flow from the PI to the rubbing cloth at the interface.By heating, the shortcircuit current increases from I s ¼ 0.07 nA at 30 °C to 0.13 nA at 110 °C.The current-voltage (I-V ) measurement demonstrates that the maximum power increased from 18 nW (30 °C) to 33 nW (110 °C), indicating that the available power output is enhanced at higher temperatures.The results are discussed based on the dipolar energy model. [20]The model indicates that the short relaxation time of depolarization process caused a larger power output.Namely, the polarization energy in the materials is transferred more frequently per unit time, enhancing the output power of the triboelectric generator at higher temperatures.The results well support the idea of dipolar polarization energy as a power source of triboelectric generators.DOI: 10.1002/pssa.202300138Herein, the concept of triboelectric generators using dipolar polarization energy as an electrical power source is introduced.Dipolar polarization energy is stored in dielectric films by rubbing, and is conveyed to external circuits for use during the depolarization process.Pyromellitic dianhydride-4,4'-oxydianiline polyimide films are used as a possible candidate for triboelectric generators, where polarization is formed by rubbing owing to the orientational alignment of polar molecular groups.The current-voltage measurements are used to evaluate the equivalent-circuit parameters of the generator.The current source I s is 0.07 nA at 30 °C, which is enhanced to 0.13 nA at 110 °C.It is indicated in these results that the depolarization process proceeds in a short time at higher temperatures, causing an increase in the current.Consequently, the maximum power increases from 18 nW (30 °C) to 33 nW (110 °C).The enhancement of the output power at higher temperatures well supports the idea of using dipolar polarization energy as a power source of triboelectric generators.

Dipolar Energy Model
Here, the equivalent circuit of the dipolar energy model is briefly described. [20]Figure 1a presents the initial conditions induced by rubbing, which orients the polar molecular groups in the films, and a nonzero initial polarization P ¼ P 0 is established.Electrode charges Q 1 ¼ þP and Q 2 ¼ ÀP are induced on the top and bottom electrodes, respectively.In this situation, the polarization energy is stored as a result of the orientational alignment of the permanent dipoles; that is, nonzero polarization energy is stored in the film owing to the entropy change caused by the alignment of the dipoles, [21] and this energy is consumed in the external circuits.
As depolarization proceeds in dielectric materials, the stored polarization energy transfers to an external load.A transient current flows during this energy-transfer process.A mathematical analysis of this transient current provides the equivalent-circuit model the triboelectric generator, as shown in Figure 1b.The left side of terminal A-A' is a triboelectric generator, and the right side is the external load, that is, the conductance G.The current source I s and internal conductance G i of the triboelectric generators are expressed by using the initial polarization P 0 , electrode area A, dielectric's relaxation time τ, and electrostatic capacitance C s as follows Equation ( 1) and ( 2) connect the concepts of the electric-circuit analysis and dielectric material properties.Accordingly, the maximum electrical power P m available at the external conductance G is as follows The external conductance providing P m is indicated by the power-matching condition G ¼ G i .Note, it is possible to induce polarization via mechanical rubbing.Rubbing process has been utilized to provide alignment layers for liquid crystal (LC) cells. [22]By rubbing the polymer surface with cloth, the polymer molecules are orientationally aligned, and the resulting anisotropic intermolecular interaction between the polymer and LC molecules causes alignment of the LC director. [23]This rubbing process is known to provide polar orientational ordering of polymers. [24]Namely, the polar molecular groups are capable of orientationally ordering in one direction at the PI surface.Accordingly, nonzero electrical polarization can be induced in polymers with permanent dipole moments.

Experimental Section
3.1.Sample PI films with polar groups, that is, PI from PMDA and ODA, were studied as candidates for triboelectric generators based on the rotation of permanent dipoles.Various types of electrically insulating materials have been candidates for vertical and lateral triboelectric generators.In contrast, for generators based on the dipolar polarization mechanism, dielectric properties are key.As a possible candidate that generates electrical power based on dipole rotation, PMDA-ODA PI is used considering the following: i) the rotation of the permanent dipole during the course of the depolarization process is a source of power generation.Consequently, materials possessing nonzero permanent dipole moments are candidates.ii) By mechanical rubbing, nonzero electrical polarization is formed to store dipolar polarization energy in materials.Consequently, the dipole moment should be capable of rotating through mechanical rubbing.
The repeating segment of the PMDA-ODA PI is considered to be bonded PMDA and ODA units, where semiempirical quantum chemical calculations gave a permanent dipole moment of 1.3 D. This permanent dipole is primarily located in the ODA unit.Mechanical rubbing of the PMDA-ODA PI induces polar orientational order. [24]Accordingly, the permanent dipoles associated with the ODA molecular units can be rotated by mechanical rubbing, making PMDA-ODA PI a candidate for dipolar energy-based triboelectric generators.
PI/indium-tin-oxide (ITO) devices were used as samples in this experiment.As a precursor of PI, polyamic acid (PAA) from PMDA and ODA (M v ¼24 000) was used.PAA was dissolved in N,N-dimethylacetamide (DMAc) at 4.1 wt%, and spin-coated onto ITO/glass substrates (3 Â 3.5 cm).Subsequently, the PAA film was heated at 270 °C for 2 h in a dry nitrogen atmosphere and changed to a PI film (thickness: 910 nm).The resulting PI/ITO devices (Figure 2) were used as triboelectric generators.Note that the center area (0.7 Â 2 cm) of the device was rubbed with cotton.

I-V Measurement With and Without Rubbing
Figure 3a presents the arrangement of the triboelectric generators and electrical connections for the I-V measurement.The PI surface was rubbed with a cotton rubbing cloth (width and length: 2 Â 7.8 cm) in one direction, whereas the ITO was used as the electrode connected to the DC voltage source.The cotton rubbing cloth was attached to an aluminum (Al) bar (diameter of 25 mm, Figure 3b) by using conducting adhesive tape, and the Al bar was connected to an ammeter.These arrangements were electrically insulated from their surroundings by using poly(tetrafluoroethylene) blocks.In this setup, the cotton rubbing cloth acted as the top electrode of the model system shown in Figure 1a, and the initial polarization P 0 points in the direction from the top to the bottom electrode.During depolarization, a positive current (I s > 0) flows through the ammeter under short-circuit condition (V ¼ 0).If the initial polarization direction is opposite, negative current flows during depolarization.Note that the relative motion of the cotton cloth to the PI surface causes rubbing between the cloth and PI, whereas the ITO electrode and Al bar are not displaced.Accordingly, the power-generation process in the rubbing motion illustrated in Figure 3a differs from that of conventional triboelectric generators that uses the vertical and lateral displacement of the electrodes.
The equivalent-circuit model for the I-V measurements is shown in Figure 3c.In the I-V measurement, the DC voltage V was increased from 0 to þ10 V, decreased to À200 V, and returned to 0 V.The sample temperature was maintained at a constant value by using a heater block.The torque on the Al bar rotating at 10 rpm is increased by ΔN ¼ 2 mNm with rubbing.Under these conditions, DC current flows through the ammeter.The samples were maintained in a dry nitrogen atmosphere.The I-V measurement was also carried out with the Al bar stopped.This I-V curve was considered as that without rubbing.

Results and Discussion
Figure 4 presents the I-V characteristics of the PI/ITO device at 30 °C.The solid and broken curves indicate measurements with and without rubbing, respectively.With rubbing, under the short-circuit condition V ¼ 0, nonzero positive current (I > 0) flows stably through the circuit, whereas the current is zero without rubbing.The short-circuit current with rubbing is I s = 0.07 nA at V ¼ 0. The polarity of the current is positive, indicating that rubbing induced initial polarization pointing in the direction from the cotton rubbing cloth to the ITO.The I-V curves were nearly independent of the external voltage and fairly fit the linear function of the external voltage.The internal conductance, calculated from the slope of the linear fit to the I-V curve, was 0.07 pS.This result indicates that the leakage current crossing the PI film was small, allowing electrical power to be transferred without loss in the film.The maximum power was calculated as P m = 18 nW by using Equation (3).These parameters are listed in Table 1. Figure 5 presents the I-V curves measured at 110 °C.The short-circuit current increases to I s ¼ 0.13 nA, and internal conductance is G i ¼0.13 pS.The power is calculated as 33 nW.It is useful to estimate the output power of a triboelectric generator based on dipolar energy in comparison with the sliding-triboelectric nanogenerator. [10]We tentatively assume that a sliding-triboelectric nanogenerator with a peak output power of 0.4 W m À2 is operating under a power-matching condition, where the lateral speed is 1 m s À1 and the intermittent output is repeating at 1 Hz with a duration of 50 ms.Triboelectric generators based on dipolar energy produce a DC power of 2.4 Â 10 À4 W m À2 , that is, 33 nW divided by the rubbing area, with rubbing at 0.013 m s À1 .Preliminary experiments on the PI/ITO device demonstrated that the output current increased in proportion to the rubbing speed (data not shown), suggesting a DC power of 18 mW m À2 at a rubbing speed of 1 m s À1 .Assuming that the DC power based on dipolar energy is compressed in 50 ms, and similar to that of sliding-triboelectric nanogenerators, the apparent peak power of the PI/ITO device is estimated to be 0.4 W m À2 .
The capability of the equivalent-circuit model shown in Figure 3c to fit the experimentally obtained I-V curves is noteworthy.The equivalent-circuit model suggests that the I-V curve is linear if the equivalent-circuit elements are constant.This equivalent-circuit model was derived by assuming that the dielectric properties of the materials were constant [20] ; that is, the dielectric constant, permanent dipole moment, and relaxation time were independent of the external applied voltage.In the I-V measurement, the average electric field formed by the external voltage was 2.2 MV cm À1 at most, and conventional dielectric materials, such as PMDA-ODA PI, provide constant values.Accordingly, we believe that the concept of the equivalent-circuit model is useful for discussing the experimentally obtained I-V characteristics.If these dielectric properties depend on the applied voltage and other factors, the I-V curves will deviate from the linear I-V curves expected from the model.
The results indicated that the output power of the triboelectric generator increased with increasing temperatures.Based on the dipolar energy model, the current source I s and internal conductance G i were expressed by using the dielectric properties of the films (Equation (1) and ( 2)). [20]At higher temperatures, the relaxation time of the depolarization τ is expressed as follows [25] τ where H is the activation energy, k is the Boltzmann constant, T is the absolute temperature, and τ 0 is the pre-exponential factor.Equation (4) indicates that the relaxation time τ is significantly short at high temperatures.Accordingly, Equation (1) and ( 2) indicate that the current source I s and internal conductance G i increase at high temperatures, demonstrating good agreement with the I-V measurement results shown in Table 1.Note that various experimental methods are available for determination of the relaxation time τ; these include the thermally stimulated current [25][26][27] and impedance measurement, [28][29][30] among others.These measurements will be helpful for investigating power generators based on dipolar polarization energy in terms of relaxation time.

Conclusion
The I-V curves of the triboelectric generators were analyzed based on the dipolar energy model.The PMDA-ODA PI/ITO device was used as a triboelectric generator that utilized the alignment of permanent dipoles in the materials as a source of electrical power.The I-V curve at 30 and 110 °C provided calculated maximum output powers of P m ¼ 18 and 33 nW, respectively.Namely, the output power was enhanced at higher temperatures.The dipolar energy model suggests that the relaxation time of the permanent dipole in materials is short at high temperatures, resulting in a higher power output.These results indicate that PMDA-ODA PI, which has polar molecular groups, is a candidate for transforming mechanical rubbing energy into electrical power through the orientational change of permanent dipoles in materials.

Figure 1 .
Figure 1.a) Dipolar energy model of triboelectric generator.By rubbing, the initial polarization P ¼ P 0 is induced in the film, and electrode charges Q 1 ¼ þP and Q 2 ¼ ÀP are induced on the top and bottom electrodes.During the depolarization process, polarization energy transfers to load G in the external circuit.b) Equivalent-circuit model of the triboelectric generator using dipolar polarization energy as a power source.The current source I s and internal conductance G i are expressed based on the dielectric properties of the film as I s ¼ P 0 A=τ and G i ¼ C s =τ, respectively (A: film area, C s : electrostatic capacitance, τ: relaxation time of the dielectric).

Figure 2 .
Figure 2. a) A sample picture of a polyimide/indium-tin-oxide (PI/ITO) device for triboelectric generators (3 Â 3.5 cm).The dotted rectangle indicates the area (0.7 Â 2 cm) rubbed by cotton.The PI layer covers the substrate surface.The top corner area of the PI is removed for electrical contact with the ITO through conductive paste.b) Device structure; PI thickness (d) is 910 nm.

Figure 3 .
Figure 3. Experimental system for the current-voltage (I-V ) measurement of triboelectric generators.a) PI/ITO device, rotating cotton rubbing cloth, and electrical-circuit arrangement for measuring the I-V curve of triboelectric generators.b) Image of the cotton rubbing cloth attached to the Al bar.The Al bar rotates at 10 rpm and the cloth rubs the PI/ITO device (not shown) facing the cloth.c) The equivalent circuit where the external I-V measurement system is connected to the triboelectric generator.

Table 1 .
Current source I s and internal conductance G i obtained from I-V measurement at 30 and 110 °C.The maximum power P m is calculated by using Equation (3).