Patent Application
20200244188
This work was financially supported by a grant from the European Research Council through an ERC Starting Grant (Grant no. ERC-2014-STG-639526, NANOGEN).
The present invention relates to triboelectric nanogenerators, to elements for triboelectric nanogenerators and to methods for the manufacture of triboelectric nanogenerators and of elements for triboelectric nanogenerators.
The rapidly growing demand for energy solutions for autonomous, wireless, portable and wearable electronic devices has prompted great interest in environment-friendly energy harvesting devices. In this respect, harvesting energy from ubiquitous mechanical vibrations can provide a viable power solution for these devices such as wireless sensors that may be implanted within the body for healthcare monitoring, or embedded within structures for early fault detection [Ref. 1]. Mechanical-to-electrical conversion can be achieved via electrostatic and electromagnetic generators, as well as through piezoelectric materials [Refs. 1, 2]. A more recent approach has been through the use of triboelectric generators based on contact electrification and electrostatic induction between materials having dissimilar electron affinities [Refs. 3, 4]. If two materials with different electron affinities make contact, the resulting charge transfer causes the material which gains charge to become negatively charged, and the other material which loses charge becomes positively charged; a phenomemon known as contact electrification. When the two materials are separated, if there is no conducting path between the two surfaces, then these surfaces are able to maintain their induced charges as static electricity; this process is called electrostatic induction. A periodic potential difference can thus be generated across the materials as a result of a periodic relative motion between the two. Maximising the surface area over which the charge transfer takes place enhances the effect, and thus several nanostructuring routes have been adopted to achieve this, resulting in many different kinds of triboelectric “nanogenerators” (TENGs) that have been reported over the past few years [Refs. 5-9].
For an efficient TENG device, the appropriate pairing of materials should be considered. Versions of empirical “triboelectric series” have enabled such selection through careful consideration of the positions of different materials with respect to one another in the series [Refs. 3, 10, 11]. Although the sequence of this triboelectric series can be affected by many variables, such as electron affinity, surface structure, and dielectric permittivity, etc., materials on the positive and negative series tend to consist of negatively charged and positively charged molecules, respectively [Refs. 12, 13]. As a result, TENG devices based on pairs of materials located on the extreme opposite ends of the triboelectric series are expected to show superior mechanical energy harvesting capability. Interestingly, the majority of the research to date has almost exclusively focused on tribo- negative materials, in other words, electron-accepting materials, such as polytetrafluoroethylene (PTFE) [Ref. 4]. These have been paired with aluminium or copper as the tribo-positive or electron-donating counterpart, even though these metals are not located on the extreme positive end of the triboelectric series. This is because the vast majority of materials on the positive side of the triboelectric series are biological or natural materials, such as human skin and cotton, and have relatively low mechanical stiffness and/or shape controllability [Ref. 14]. An indicative triboelectric series is illustrated in
Nylon, however, as shown in
There have been a few reports where Nylon has been used as a tribo-positive material [Refs. 15-17], but these studies have focused on even-numbered Nylons, such as Nylon- 6 or Nylon-66, showing only slightly better output performance than other tribo-positive materials such as silk or aluminium [Ref. 18].
Odd-numbered Nylons, such as Nylon-11, exhibit ferroelectric properties because of the dipole orientation resulting from the arrangement of polyamide molecules within adjacent chains, similar to more well-known ferroelectric polymers such as poly(vinylidene fluoride) (PVDF) and its copolymers [Refs. 2, 19-21]. Compared to these fluorinated polymers, Nylon-11 shows a similar degree of ferroelectric properties, such as dipole moment [Ref. 22] and piezoelectric coefficient [Ref. 23], with the added advantage of higher thermal stability [Refs. 24, 25]. The effect of remnant polarization in the performance of TENG devices based on ferroelectric polymers has been previously studied, whereby a high voltage was applied across the polymer to achieve dipolar alignment along the direction of the applied field. According to these studies, positively poled PVDF and its copolymer films showed more than double the power output when incorporated into TENGs as compared to the un-poled film [Refs. 17, 26]. This is due to an increase in surface charge density as a result of the remnant polarization [Ref. 27]. These studies indicate that poled Nylon-11 may be well-suited for TENG applications through inherent surface charge density modification. However, in order to maximize the required polarization property of this material, extreme processing conditions are required, including mechanical stretching and/or electrical poling process under high voltage (about 140 MV/m), which are major processing issues that need to be addressed.
In order to realise better polarization in Nylon-11, the present inventors have realised that the material should preferably possess a pseudo-hexagonal structure with randomly oriented hydrogen bonds, referred to as the “δ′-phase”. This is discussed in more detail below. This structure is considered to be beneficial for aligning the dipole moment [Refs. 28, 29]. This δ′-phase, however, is typically achieved through extremely fast crystallization that is required to avoid the formation of large domain size [Refs. 30-33]. As a result, most of the studies regarding the δ′-phase Nylon-11 have been carried out on films grown via melt-quenching.
Although several alternative techniques, such as spin coating [Ref. 34], vapour deposition [Ref. 35], electrospinning [Ref. 36], thermal annealing [Ref. 37], and adding carbon nanostructures [Ref. 38], have been suggested, most of these reported approaches resulted in a lower crystallinity and/or still require harsh processing conditions, such as high temperature (290° C.) and high voltage (40 kV). Moreover, these methods involve further drawing and electrical poling process to enhance the remnant polarization of the Nylon-11 films [Ref. 23].
Based on the insight developed above, the present inventors have carried out research to reduce, ameliorate, avoid or overcome at least one of the above problems. The present inventors have realised that improved triboelectric generators are possible in which at least one of the triboelectric generator elements comprises a templated array of nanowires of a suitable triboelectric material. This constitutes a general aspect of the present invention. Although the insight developed above related to identification of Nylon-11 as a suitable triboelectric material, the applicability of the developments by the inventors is not necessarily limited to this material or related materials.
Accordingly, in a first preferred aspect, the present invention provides a triboelectric generator having a first generator element and a second generator element, the first and second generator elements being arranged so that relative movement between them generates a potential difference between them due to a triboelectrification effect, wherein:
In a second preferred aspect, the present invention provides a method for the manufacture of a triboelectric generator element comprising a templated array of nanowires of a first material, wherein a solution of the first material is allowed to fill an array of channels extending in a template structure by capillary wetting and the solvent is removed from the solution in the channels to solidify the first material into an array of self-poled nanowires.
In a third preferred aspect, the present invention provides a method for the manufacture of a triboelectric generator according to the first aspect, the method including manufacturing the first generator element including the step:
In a fourth preferred aspect, the present invention provides a method of operating a triboelectric generator according to the first aspect, the method including causing relative movement between the first and second generator elements to generates a potential difference between them due to a triboelectrification effect.
The first, second, third and/or fourth aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.
Preferably, the first material is a tribo-positive material. However, the first material may be a tribo-negative material.
The first material may, for example, be a polymeric material. Preferably, the polymeric material is capable of being solution-processed. In this regard, it is preferred that the polymeric material is capable of solidification from a solvent.
The first material may be a polymer with hydrogen bonding. For example, polymers containing carbonyl, carbonate and/or hydroxyl groups may be suitable. Therefore suitable example materials include Nylon, polyurethane, poly(methyl methacrylate) (PMMA), Poly(methacrylic acid) (PMAA), poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), poly(4-vinylphenol) (PVP).
The first material may comprise one or more polymers selected from the group consisting of:
The first material may, for example, be a Nylon material. Of particular interest here are odd-numbered nylons. In one preferred embodiment, the first material is Nylon-11. In this case, preferably the material includes the pseudo-hexagonal polymer structure or the hexagonal/pseudo-hexagonal crystal structure. The material may include the δ′-phase.
Preferably, the degree of crystallinity of the first material is at least 30%, wherein the degree of crystallinity is determined using DSC according to the equation
wherein ΔHm is the equilibrium heat of fusion enthalpy of the first material and ΔH0m is the equilibrium heat of fusion enthalpy of the perfect crystalline equivalent composition of the first material, and wherein ΔHm is determined from the area under the DSC melting peak.
Preferably, the degree of crystallinity of the first material is at least 35%, or at least 40%.
Preferably, the nanowires of the first material are self-poled.
Preferably, the template structure has a base face and a top face. The channels preferably open at the base face and the top face. In the method, the solution of the first material is allowed to fill the channels by contacting the base face of the template structure with the solution. The solution of the first material may therefore fill the channels by capillary wetting.
Preferably, the removal of the solvent from the solution in the channels is assisted by a gas flow. Preferably, the gas flow is directed substantially parallel to the top face of the template structure. The gas flow speed may be at least 0.5 ms−1. More preferably, the gas flow speed is at least 1 ms−1, at least 1.5 ms−1, at least 2 ms−1, at least 2.5 ms−1, or at least 3 ms−1. It is considered that operating with a gas flow in these ranges provides a suitable control over the removal of the solvent and therefore over the temperature distribution in the solution of the first material. In turn, this provides control over the crystallisation of the first material.
Preferably, crystals of the first material nucleate at a free surface of the solution exposed in the channel.
Additionally or alternatively, crystals of the first material nucleate as lamellae adjacent to the internal wall of the channel. In the resultant nanowires, preferably there are formed concentric lamellae, oriented substantially parallel to the internal wall of the channel.
In this manner, it is found to be possible that the nanowires form with a high degree of crystallinity with a high proportion of a phase favourable for triboelectric properties.
Preferably, the width of the channels is at least 50 nm. This permits the solution to infiltrate the channels by capillary wetting. Preferably, the width of the channels is at most 500 nm. It is considered that having channels wider than this causes difficulties with the control of the crystallisation of the first material from the solution. The width of the channel may be expressed as a diameter, this to be understood as the diameter of a circle of equal area as the cross sectional area of the channel.
The second generator element may comprise a template structure having an array of channels extending in the template structure, the channels being substantially filled with the second triboelectric material to define a templated array of nanowires of the second triboelectric material. Thus, the second generator element may have a structure corresponding to that of the first generator element, but maintaining the requirement that the first and second triboelectric materials are different.
The first material may be a tribo-positive material and the second material may be a tribo-negative material.
The second material preferably comprises a polar polymer. The second material may comprise a polymer with hydrogen bonding. The second material may comprise a fluorinated polymer. The second material may comprise a polymer with carbonyl, carbonate and/or hydroxyl groups.
The second material may comprise one or more polymers selected from the group consisting of:
Further optional features of the invention are set out below.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:
Note that in the energy generator system, force amplitude of the magnetic shaker can be controlled by the applied voltage.
Triboelectric nanogenerators (TENGs) have emerged as potential candidates for mechanical energy harvesting, relying on motion-generated surface charge transfer between materials with different electron affinities. In order to achieve suitable levels of energy harvesting performance, and considering the triboelectric series illustrated schematically in
The fabrication of Nylon-11 in the polarized δ′-phase typically requires extremely rapid crystallization, such as melt-quenching, as well as “poling” via mechanical stretching and/or large electric fields for dipolar alignment.
In the preferred embodiment of the present invention described below, there is disclosed a one-step fabrication process for forming a templated array of nanowires suitable for use as a TENG element. The preferred material provides enhanced surface charge density of highly crystalline “self-stretched” and “self-poled” δ′-phase Nylon-11 nanowires using a novel, facile gas-flow assisted nano-template (GANT) infiltration method. As will be explained in more detail below, when incorporated in a TENG, these Nylon-11 nanowires showed a ten-fold improvement in output power density as compared to a conventional aluminium based TENG device, under similar mechanical excitation.
The embodiment uses a one-step, near room-temperature method. In this disclosure, this is termed a gas-flow assisted nano-template (GANT) infiltration method. This promotes the fabrication of highly crystalline “self-poled” δ′-phase Nylon-11 nanowires, the nanowires being formed within nanoporous anodized aluminium oxide (AAO) templates. The reference to “one-step” indicates that there is no need for subsequent processing such as stretching and/or poling. The gas-flow assisted method allows for a controlled crystallization rate that manifests as a rapid solvent evaporation and a suitable temperature gradient within the nanopores of the template. This is predicted by finite- element simulations, resulting in the δ′-phase crystal structure. Furthermore, self- stretching and preferential crystal orientation originating from template-induced nano- confinement effects lead to self-polarization of the nanowires, increasing average crystallinity up to 40%. The self-poled δ′-phase Nylon-11 nanowires show enhanced surface charge distribution when compared to melt-quenched films as observed by Kelvin probe force microscopy (KPFM). Correspondingly, a TENG device based on GANT Nylon-11 nanowires showed a ten-fold increase in output power density compared to an aluminium based TENG device, when subjected to identical mechanical excitations.
Calculated wt % Nylon-11 (Sigma-Aldrich, [—NH(CH2)10CO—]n) solutions were prepared in the solvent formic acid (Sigma-Aldrich, Reagent grade≥95%) at 70° C. When observing the nanowires using the GANT method, 25 mm diameter AAO templates (Anopore, Whatman) with a diameter of 200 nm and thickness of 60 μm were placed on an 800 μL Nylon-11 solution droplet. To control the crystallization rate of the solution, assisted gas (air) flow was introduced upon the AAO template using a portable mini fan placed immediately next to the floating template. The rate of assisted gas was controlled by fan rotation speed and measured by an anemometer. The whole drying experiment was proceeded under room temperature. Detailed descriptions of the nanowire preparation set-up and treatment were reported previously [Ref. 25]. To obtain the Nylon-11 nanowire arrayed mats, we immersed the nanowire-filled AAO template in a 40 vol% phosphoric acid solution for 4 hr.
Nylon-11 films were produced by quenching the molten film (210° C.) into an ice bath. In the case of stretched films, quenched films were drawn to a draw ratio of 3 at room temperature.
The Nylon-11 nanowires were visualized using field-emission scanning electron microscopy (FE-SEM, FEI Nova Nano SEM)). The crystal phase and crystalline quality of the Nylon-11 nanowires inside the AAO template and Nylon-11 films were measured using an X-ray diffraction (XRD) machine (Bruker D8) using Cu Kα radiation (λ=1.5418 Åu) with a background silicon substrate. Numerical modeling of gas flow, heat transfer around nano-pores, and the triboelectric effect were achieved by COMSOL Multiphysics.
Differential scanning calorimeter (DSC) was carried out using TA Instruments Q2000 DSC at a scanning rate of 5° C/minute. Around 2 mg of samples was used and sealed into T zero aluminum DSC pans. Fourier transform infrared (FTIR) was performed using a Bruker Tensor 27 IR spectrometer in the reflection mode. Kelvin probe force microscopy (KPFM) measurements were carried out using Bruker Multimode 8 with Antimony (n) doped Si (tip radius<35 nm, resonance frequency 150 kHz). AC voltages were applied from a lock-in amplifier. Film thickness was measured by stylus surface profilometer (Veeco Dektak 6M). To study the electrical output performance of the TENG device, a self-made energy harvesting measurement setup [Ref. 41] was utilized, recording the output open-circuit voltage (Voc) and short-circuit current (Isc) by a multimeter (Keithley 2002) and a picoammeter (Keithley 6487), respectively. In the present work, an impacting frequency of 5 Hz with an amplitude of about 1 mm was used. 24 μm thick Aluminum film and 60 μm thick melt-quenched Nylon-11 film with the same diameter (about 2 cm) were prepared to compare the TENG device performance with the nanowire one. To make the TENG device, one side of the Nylon-11 NW filled AAO template and the melt-quenched film was coated by sputter coating about 100 nm thick Au (using k550 Emitech) layer.
Remnant polarization, without the need of subsequent stretching and/or high-voltage poling, is achieved via nanoconfinement during the template wetting process [Refs. 39- 41]. This method has been shown to enable self-poling of a ferroelectric polymer through grapho-epitaxial alignment in the lamellae [Refs. 20, 25, 41, 42]. Recent work from our group has shown that the template-induced nanoconfinement led to self-poling in poly(vinylidene difluoride-trifluoroethylene) (P(VDF-TrFE)) nanowires [Ref. 20] as well as Nylon-11 nanowires [Ref. 25], resulting in a highly efficient piezoelectric nanogenerators [Ref. 41]. However, controlling the exact crystal structure through nanoconfinement methods remains challenging, in part owing to the crystal growth initiation site. For a typical template-wetting process where a polymer melt or solution is introduced into the top surface of the template via spin coating, nucleation is initiated from the surface of the bulk solution and propagated into the nano-sized pores [Ref. 43]. Such bulk-initiated nucleation and growth mechanism imposes restrictions on the control of polymer crystallinity, due to the difficulties in controlling the rate of crystallization. We mitigate this problem by using a reverse template infiltration technique with assisted gas flow.
Capillary forces allow the target solution to infiltrate from the bottom, directly exposing the solution to air. Furthermore, assisted gas flow enables precise control of the crystallization rate of the Nylon-11 polymer solution, resulting in the desired δ′-phase crystal structure. We note that this phase was not prominent in the XRD diffraction pattern reported in our previous work [Ref. 25] where a more typical template-wetting method was employed. We find that the strong presence of the δ′-phase in Nylon-11 nanowires leads to enhanced surface polarisation, and thus improved control of this crystalline phase through our GANT method is useful in enhancing the triboelectric energy harvesting performance.
The GANT fabrication procedure is schematically illustrated in
Stretched and entangled Nylon-11 nanowire strands (white thread-like regions) were observed in cross-sectional SEM images of a cleaved template—see
Detailed crystal structural characterization was subsequently carried out by X-ray diffraction (XRD, Bruker D8). At room temperature, a melt-quenched film with pseudo- hexagonal δ′-phase typically showed broader reflection peak at 2θ of about 21.6° corresponding to (hk0) plane, which is merged planes of (100), (010), and (110) [Ref. 44]. However, without assisted gas-flow, Nylon-11 nanowires displayed a relatively strong peak at 2θ=22.6° with a very weak δ′-phase peak (2θ=21.6° . This is shown in
In order to compare the crystallography of the nanowires, melt quenched Nylon-11 films with δ′-phase, were grown.
It can therefore be surmised that the actual amount of crystalline region in the nanowire per unit area is larger than that of the melt-quenched film. These results suggest that even through the conventional melt-quenching method, the presence of large amorphous regions cannot be prevented in the films due to the thickness of the film and the low thermal conductivity of Nylon [Ref. 33]. However, this problem is mitigated via the GANT method where the δ′-phase in Nylon-11 nanowires is realised to a larger extent.
The crystal structure of Nylon-11 have been extensively studied due to their extensive degree of polymorphism [Refs. S1-S6]. Nylon-11 displays several crystal structures depending on the processing condition as summarised below.
At room temperature, in Nylon-11 there have been demonstrated at least four crystalline forms (the triclinic a and a′, the monoclinic β, and the pseudo-hexagonal y phase) and one smectic-like pseudo-hexagonal phase [Ref. 44].
In terms of electrical property, although all Nylon-11 has a polar crystal structure due to its molecular configuration, the electric polarisation can be maximised from a specific type of crystalline phase. In the case of the most stable and well-organized α-phase, Nylon-11 does not display remnant polarisation due to the strong hydrogen bond, which is originated from the highly-packed crystal structure. In contrast, the highest electric polarisation could be observed from the meta-stable pseudo-hexagonal phase (γ, γ′, δ, δ′) [Refs. 28, 29, 33]. Disordered, short-range hydrogen bond and breakage of gauche bonding originated from rapid crystallisation are likely to enable dipole reversal [Ref. 33].
Despite disordered configuration in the perpendicular direction to the chain and randomly oriented hydrogen bonds, pseudo-hexagonal phase shows more ordered crystalline structure along the chain direction, which was referred to be a smectic-like phase, with aligned amide groups [Ref. S11].
The crystallization mechanism initiated by the GANT method was verified using finite element analysis using COMSOL Multiphysics.
Using COMSOL Multiphysics simulation, the simulation is demonstrated based on these three different effects: the turbulent flow of the assisted gas, heat transfer in all components, and the vaporisation of the solvent in the Nylon solution. For the turbulent flow, we assumed that the gas flow rate and pressure field are independent of the property of gas, such as moisture content level and temperature. Heat transfer in the model is considered to have two different aspects: conduction and convection. The heat transfer between the template wall and the solution is governed by conduction. In the gas flow, the heat transfer is originated from the convection and the effect of turbulent flow. The cooling effect during the solvent evaporation needs to be considered as well based on the heat of vaporisation Hvap. To calculate the amount of vaporised solvent to the air, the material transport equation is used with the turbulent flow as a diffusion coefficient.
To further investigate the rapid crystallization process from the polymer solution, both the solvent evaporation and temperature of the solution have been considered. In terms of solvent evaporation, our simulations revealed that through the assisted gas-flow, the artificially generated dry and cool turbulent air flow effectively encourages the evaporation of the solvent.
The velocity of the turbulence, evaluated 10 nm above the surface of the solution, was 36 μm/s for an assisted-gas velocity of 3.1 m/s. This is a high flow rate considering the size of nanopores are about 200 nm in diameter. As a result, nucleation can be initiated at the surface of the exposed solution by rapid solvent evaporation.
In the computational model, we also considered the effect of the solution cooling within the pore channel: both turbulent flow induced evaporative cooling and thermal conductive cooling via the nano-template walls. The overall cooling mechanism is demonstrated in
Self-poled δ′-phase nanowires through GANT method Differential scanning calorimetry (DSC) was carried out to determine the thermal and structural properties of Nylon-11 nanowires as shown in
Using the average data of the four distinct DSC peaks of Nylon-11, the crystallinity was calculated by means of the equation
where ΔHm and ΔHm are the equilibrium heat of fusion enthalpies of the semi-crystalline Nylon-11 samples and the perfect crystalline Nylon-11 (ΔH0m is about 189 J/g) [Ref. 48], respectively. ΔHm is achieved from the area under the DSC melting peak. The results of the calculations are given in Table 2 below.
Typically, the degree of crystallinity of melt-quenched films is usually around 30% since the rate of crystallisation is very fast compared to other polymers [Refs. 33, 48]. The crystallinity of δ′-phase nanowires is similar to the crystallinity of stretched film due to the self-stretching effect.
Returning now to a consideration of
The thermal properties of the nanowires can be explained on the basis of several factors: nanowire size effect, nano-template effect and crystal ordering effect. Both nanowires within the template, as well as those freed from the tempate displayed double melting behaviour. The additional low temperature melting peak can be explained by nanoscale size effect. According to the Gibbs-Thomson equation [Refs. 46,47], the melting point depression ΔT_m is given by
ΔTm=Tm−Tm(d)=4σstTm/(DΔHfρs)
where Tm is the normal (bulk) melting point, Tm(d) is the melting point of crystals of size d, σsl is the surface tension of the solid-liquid interface, d is particle size, ΔHf is the bulk enthalpy of fusion (per g of material), and ρs is the density of the solid [Ref. 46]. This shows that the melting point of the nanowires was depressed due to the nano-confined structure and surface tension of the Nylon-11/template interface.
From nanowires in the template, the dominant melting peak was observed at a lower temperature (185° C.), as compared to the template-freed nanowires which displayed dominant melting peaks at a higher temperature (190° C.). This is because, in case of nanowires in the AAO template, both the very thin nanowire diameter as well as the surface tension between Nylon-11 and the nano-template interface affect the melting behaviour [Ref. 46]. In the cooling cycle, the nano-template effect was confirmed by the relatively broader crystallization temperature of the nanowire in the template. Thermal behaviour of template-freed nanowires, however, makes us infer an additional effect that influences the property of the nanowires. This is because cooling behaviour of template- freed nanowires and stretched film are substantially similar and have the same Tc (162° C.). In addition, the δ′-phase nanowires displayed much higher degree of crystallinity (39%) than those of the melt-quenched film (29%) [Refs. 33, 48]. The crystallinity of the δ′-phase nanowire is rather similar with those of stretched film (40%), even though those nano-confined structure showed much lower latent heat of fusion ΔHm [Ref. 47]. All of these phenomena suggests that, during the GANT process, δ′-phase nanowires were likely to have experienced crystalline ordering. Similar to the molecular alignment from the film-drawing process, grapho-epitaxial alignment originating from the template-pore walls resulted in highly ordered structure, giving rise to similarities in the thermal behaviour and high crystallinity observed in the stretched film. Our results indicate that the GANT method successfully enabled highly ordered δ′-phase nanowires to be formed without additional drawing process.
The molecular bond structure of δ′-phase nanowires was measured using room temperature Fourier transform infrared (FT-IR) spectroscopy and the results shown in
In order to investigate the direction of polarization and quantify the surface charge variations between Nylon-11 nanowires and melt-quenched films arising from the self- poled nature of the former, we employed Kelvin probe force microscopy (KPFM), where the surface charge potential difference between the material and an atomic force microscope (AFM) tip can be measured. The results are shown in
A TENG device was fabricated using self-poled nanowires embedded within AAO template. The bottom side of the AAO template was coated by an Au electrode (about 100 nm thickness), and an Au-coated Teflon film was prepared as a counterpart substrate. Aluminium films and melt-quenched Nylon-11 film were also prepared to compare the device performance with the nanowire-based TENG.
We conducted analytical simulations using a finite element method (FEM) with COMSOL Multiphysics software to further confirm the effects of self-polarization on triboelectric potential.
To confirm the working mechanism of the self-poled nanowire based TENG, we carried out a theoretical analysis based on Gauss theorem [Refs. S13, 11]. In the dielectric-to- dielectric contact mode TENGs, the electric potential difference (ΔV) between two electrodes can be given by:
where E is electric field strength, t1 and t2 are the thickness of the two surfaces, d is the distance between two different layers, Q is the value of transferred charges, S is the area of the electrode, σ is the triboelectric charge density, ϵ0 is the vacuum permittivity, and ϵr1 and ϵr2 are the relative permittivity (dielectric constant) of dielectric materials, respectively. In the conductor-to-dielectric structure, t1/ϵr1 can be ignored because the metal layer acts as triboelectric layer and electrode.
Under open-circuit conditions, the value of transferred charges (Q) become zero since no charge is transferred between the two top and bottom electrodes. Thus, if we assume electric potential of the bottom electrode to be zero, the equation for the open-circuit voltage (Voc) can be calculated by
Using experimentally determined Vocof each TENGs, we can obtain a theoretical triboelectric charge density (σ):
When d is maximum (0.5 mm) and σ of aluminium, the melt-quenched Nylon film, and the nanowire-filled template are 0.78, 1.06, and 1.95 μC m−2, respectively.
A COMSOL Multiphysics simulation demonstrated the triboelectric potential difference of three different structure TENGs with Teflon as the top part and aluminium (
The ideal electric potential of self-poled Nylon-11 nanowire based TENGs can be calculated based on the experimental result of remnant polarisation [Ref. S15]. Under short-circuit conditions (V=0), the transferred charges (Qsc) are given by
The value of Qsc can therefore be varied as a function of d. When the generator is fully released (d=0.5 mm), the maximum transferred charges per unit area was theoretically calculated to be 0.65, 0.94, and 1.75 μC m−2 for aluminium, melt-quenched Nylon film, and the nanowire-filled template, respectively. The transferred charges (Qsc) are also calculated using the integration of short-circuit current (Isc). The short circuit current is shown in
Thus, in the work presented here, when compared with the aluminium and Nylon film based TENGs, a higher triboelectric potential is obtained from the self-poled nanowire based TENG, because the self-poled nanowires increase the charge density (σ) on the electrified surfaces.
The electrical power output of the TENG was measured across different resistors. Peak output power density of 1.03 W·m−2, 0.19 W·m−2, and 0.099 W·m−2 were observed from the Nylon-11 nanowire, Nylon-11 (melt-quenched) film, and aluminium based device respectively under impedance-matched conditions at a load resistance of about 20 MΩ (
The electrical output was found to increase with increasing impact frequencies and amplitudes (see
To further confirm the effect of self-polarization on TENG devices, we also measured the output performance of the template-free nanowire mat where the nanowires are lying on the substrate with randomly oriented polarization directions.
To confirm the energy generated by the δ′-phase nanowire, TENG is also feasible for energy storage. As illustrated in
To summarize, we have demonstrated for the first time, a novel and facile gas-flow assisted nano-template (GANT) infiltration method for the fabrication of highly oriented, self-poled δ′-phase Nylon-11 nanowires, as a rarely synthesized tribo-positive material. Assisted gas-flow controlled the crystallization speed, resulting in the δ′-phase crystal structure. Preferential crystal orientation originated from the nano-confinement effect in the template, resulting in self-poling of the Nylon-11 nanowires with an increased average crystallinity of up to about 40%. When self-poled Nylon-11 nanowires were combined with counterpart tribo-negative surfaces, such as Teflon, the resulting output power were observed to be about 6 times and about 10 times higher than those of melt- quenched Nylon-11 film and aluminium based TENGs, respectively. The output power generated by the Nylon-11 nanowire-based TENG device was high enough to drive commercial electronic components such as LEDs and capacitors without external power sources. It is therefore now possible to provide enhanced surface charge density for TENG elements for use in high-performance TENG devices.
The present invention is not necessarily limited to nylon materials or to Nylon-11 specifically. The inventors have conducted additional investigations into other materials that are suitable for use in embodiments of the present invention, whether as tribo- positive materials or tribo-negative materials.
Cellulose, a major constituent of our natural environment and a structured biodegradable biopolymer, exhibits inherent shear piezoelectricity. It has therefore been studied by the inventors' research group with a view to its potential applications in energy harvesters, biomedical sensors, electro-active displays and actuators. Here are disclosed self- assembled cellulose nanofibers (SA-CNFs), also referred to here as cellulose nanowires, fabricated using a template-wetting process, whereby parent cellulose nanocrystals (CNCs) introduced into a nanoporous template bunch together to form rod-like cellulose clusters, which then assemble into SA-CNFs. Subsequent thermal annealing is found to enhance crystallinity, hardness and shear piezoelectric response, as observed using quantitative nanomechanical mapping (QNM) and non-destructive piezo-response force microscopy (ND-PFM). The inventors have found a distinct chiral-nematic hierarchical structure in their template-grown SA-CNFs as revealed by scanning electron microscopy
(SEM) and high resolution transmission electron microscopy (TEM).
It is considered that oriented cellulose crystallites are responsible for the observed piezoelectricity of wood due to stress-induced orientation of dipoles, possibly stemming from the OH groups in cellulose molecules. Recently, thick 45 μm films of cellulose nanofibrils were reported for their use in piezoelectric sensors with corresponding vertical sensitivities of 4.7-6.4 pC/N in ambient conditions [Reference C31].
SA-CNFs were fabricated from an aqueous dispersion of CNCs using a simple template- wetting method (drop-cast) on an AAO template, followed by a low-temperature annealing process. SA-CNFs were formed from CNCs which above a critical aqueous concentration, exhibit left-handed chiral nematic (cholesteric) liquid crystallinity as observed by transmission electron microscopy (TEM). The SA-CNFs displayed helicoidal arrangement of rod-like cellulose clusters, where the helicoidal axis follows the longitudinal axis of the pores of the anodised alumina (AAO) templates used. SA-CNFs showed higher crystallinity resulting in enhanced mechanical properties attributed to annealing, as determined using QNM on individual SA-CNFs. PFM scans further show evidence of preferential arrangement with single SA-CNFs. (In recent years, QNM and PFM have emerged as advanced scanning probe tools used to assess the mechanical and electromechanical properties of materials at the nanoscale respectively.)
As explained below, it is possible, by confinement in a suitable template, to promote self- assembly of cellulose nanocrystals into nanofibers. This is unexpected, since the cellulose nanocrystals would be expected to repel one another rather than self-assemble due to their surface charge. Such self-assembly is not seen in bulk conditions. It is considered that the nanoconfined environment of the template forces the self-assembly, which then leads to a degree of polar alignment along the nanofiber. This polar alignment manifests as an enhanced triboelectric effect due to this self-assembly.
It is in general considered that any polymeric material that is capable of sustaining a surface charge will work suitable as a triboelectric material in the context of the present invention. Suitable polymers include ferroelectric polymers are a good example, though they may need to be poled if they are not self-poling. Other polar materials also work in this way. Advantages related to templated polymeric nanowires are (1) better crystallinity, (2) self-poling (relevant particularly for ferroelectrics) and (3) texturing/alignment of polar molecules.
Extraction and stabilization of CNCs as aqueous suspension. Cellulose nanocrystals (CNCs) were extracted according to the procedure by Beck-Candanedo et al. [Ref. C60].
The source material for the suspension was bleached, softwood Kraft pulp (TEMBEC). TEMBEC board was cut into strips and dried overnight at 50° C. The strips were mixed with sulfuric acid and stirred at 45° C. for 45min, at the ratio of 1:17.5, 40 g TEMBEC with 700 mL sulfuric acid (64%). Then, the sample was diluted 10 times in cold double distilled water (DDW), and the mixture was left standing for 1 hour. The acidic upper phase was decanted and discarded, and three wash cycles were performed on the bottom phase according to the following sequence per cycle: the material was centrifuged (20° C., 6K rpm, 10 min), the supernatant was discarded, and the pellet was rinsed with DDW. The pellet from the final cycle was collected with the addition of DDW, and dialyzed against DDW until the pH of the suspensions stabilized. Finally, the suspension was sonicated (Q500 Qsonica; 6 mm probe) on ice to avoid overheating, until the suspension appeared uniform (15 kJ/g). The sonicated suspensions were filtered (Whatman 541) and toluene (100 μL/L) was added to the suspensions to avoid bacterial growth.
Preparation of cellulose nanofibers. SA-CNFs were prepared by template-based drop- cast wetting method from an aqueous dispersion of CNCs. In the process, charged CNC dispersion (1.25%) is pooled on top of the anodised aluminium oxide (AAO) porous template (Anapore, Whatman) with nominal pore diameters of about 250 nm and of thickness 60 μm. The suspension pool of the CNC dispersion is then allowed to infiltrate the pores by gravity. The template was then left under ambient conditions, allowing the evaporation of water and self-assembly of CNCs within the pore channels. Post-heat treatment of the infiltrated template at approximately 80° C. was carried out for 30 minutes to remove bound water and to facilitate the increase in the degree of bonding between CNCs. SA-CNFs are released from the template by dissolving the AAO template in 3.2 molar potassium hydroxide aqueous solution, followed by repeated washing with deionised water, and centrifugation to neutralise and isolate the SA-CNFs for further characterisation.
Following the guidance set out above for nylon nanowires, it is also possible to form cellulose nanowires by infiltration of the aqueous dispersion of CNCs upwards through the template.
The phase and crystalline quality of the SA-CNFs was characterized by X-ray diffraction (XRD, Bruker D8 diffractometer, equipped with Lynx Eye position-sensitive detector and using Cu Kα radiation (λ=1.5418θ)). Peak shifts due to sample misalignment were adjusted while performing the XRD scans and background correction was solved by using a zero background silicon substrate for collecting the scans. Morphological analyses were systematically studied by scanning electron microscopy (SEM, FEI Nova ). Differential scanning calorimetry (DSC) measurements of the samples were carried out using a Q2000 TA Instruments differential scanning calorimeter. High resolution transmission electron microscopy (HR-TEM) of CNC and SA-CNFs were acquired using a FEI Tecnai T12 G2 Spirit Cryo-TEM and FET Tecnai T20 STEM equipped with Gatan Imaging Filter, respectively. For cryo-TEM of CNCs, 0.1 wt. % sample were placed on a carbon grid and vitrified (rapid freezing) in liquid ethane using a Vitrobot Mark IV (FEI Company) and cryogenically transferred in liquid nitrogen to the cryo-TEM holder, which was then inserted into the microscope (FEI Tecnai T12 G2 Spirit). The temperature of the sample was maintained at approximately −175 ° C. to prevent crystallization of ice. The goal of this method is to directly observe the nanoparticles as they exist in aqueous suspension. For standard room temperature TEM of SA-CNFs, released nanofibers were drop cast on copper grids and imaged alternatively between 100-120 kV at 3 spot-size to avoid destruction of the SA-CNFs by electron beam.
AFM measurements were carried out using a Bruker multimode 8 (with Nanoscope V controller). Several scanning modes were used: 1) tapping mode using an MESP- RC V2 (Bruker) tip for topographic measurements; 2) QNM measurements were carried out with a DDESP-V2 tip, where deflection sensitivity was calibrated using a sapphire standard, and elastic modulus was then calibrated on a polystyrene film standard of known elastic modulus (2.7 GPa); 3) PFM measurements were performed by adapting the QNM mode to yield PFM data, in a non-destructive intermittent contact mode (ND-PFM). Calibration of the ND-PFM signal was carried out by scanning a periodically poled lithium niobate reference with a reported d33=7.5 pmV−1. An MESP-RC V2 tip was used for ND-PFM scanning atop the dispersed NFs, which were lying on a conducting indium tin oxide (ITO) substrate. An alternating voltage of amplitude 4.0 V at a frequency 125 kHz was applied between the sample and the tip.
Structurally, cellulose chains are linear and usually aggregation occurs via both intra- and intermolecular hydrogen bonds. With a strong affinity to itself and toward materials containing hydroxyl groups, CNCs can easily self-assemble in water. Rod-like CNCs with only a few nanometer of lateral dimensions show right-handed chiral twisting along the rods. The formation of hydrogen bonds at the cellulose/water interface is also observed to be highly dependent on the orientation of the CNCs in the chains, and it has been argued that significant contribution from Van der Waals forces contribute to the strong cohesive energy within the CNC network. In this work, to fabricate SA-CNFs, charged CNCs within an aqueous dispersion were drop-cast onto AAO templates facilitating self-assembly of CNCs within the nanoporous channels.
Following an annealing process to remove the adsorbed water molecules, SEM imaging of an intentionally fractured AAO template revealed well-formed SA-CNFs, as shown in
TEM images of individual SA-CNFs revealed the presence of helicoidal structure (
Similar rod-like structures have been recently reported from a mixture of charged gold nanoparticles (AuNPs) and CNCs [Ref. C35]. The formation of chiral rod-like structures was obtained by neutralization of the inter-CNC electrostatic forces. In our case, we suggest that the confinement of the CNCs within the nanopore channels forces the self- assembly process to rod-like clusters, and finally to SA-CNFs, which help retain the morphological integrity even after freeing from the template. Although the exact self- assembly mechanism is not yet clear, it might be related to the attraction due to the charged AAO template walls.
The X-ray diffractometry (XRD) spectra of different SA-CNF samples before and after annealing are shown in
A wide range of values is often quoted for the Young's modulus of cellulose in the literature which are highly dependent on the degree of fibre alignment. It is expected that the SA-CNFs reported here show lower moduli than bulk cellulose-based materials, as the latter have chains which cross between crystalline regions, while the present SA- CNFs are formed from self-assembled rod-like CNC clusters with no covalent cross- linking between them. Since the bonding between CNCs are primarily intermolecular, this will lead to an intrinsically lower modulus than for covalent bonding. Thus, it is expected that the SA-CNFs will have a modulus more comparable to that of CNC films.
QNM imaging of the SA-CNFs was carried out. Elastic moduli of 1.6±0.6 GPa and 0.18±0.05 GPa were respectively obtained for annealed and non-annealed SA-CNFs that were freed from the template and dispersed onto an indium tin oxide (ITO) coated glass substrate. We note that the modulus was extracted by fitting the data to an infinite plane indentation, which might have a fundamentally different mechanical response than cylinder indentation. Nonetheless, the qualitative distinctions between the materials before/after annealing are still valid. Furthermore, based on previous preliminary simulations of this problem regarding the exact geometry of indentation, the fitted values are not expected to deviate significantly from the true value of the modulus. It is therefore clear that the SA-CNF modulus (hence the stiffness) had increased due to the annealing process, which might be due to stronger bonding between constituent CNCs as a result of the release of intra-molecular water. These values are lower than those quoted in the literature for CNC films and suggest there may be weaker bonding between the CNCs in the SA-CNFs than in a CNC film.
Piezo-response force microscopy studies were carried out on individual SA-CNFs. Overall the results indicated that there is a preferential orientation of CNCs within the SA- CNFs and agrees with the observation of the oriented rod-like cellulose clusters from TEM studies (
Thus, the template-wetting technique for cellulose gives rise to a remarkable two-stage hierarchical self-assembly process. In this process, constituent CNCs first form rod-like clusters of larger dimensions which finally assemble into SA-CNFs. The presence of helicoidal structure in a SA- CNF is clearly visualized using TEM, which is a result of locking of the chiral nematic phase of the constituent CNCs. Post-deposition annealing of the infiltrated template enhances the crystallinity and hence the stiffness of the prepared SA-CNFs, as observed by QNM. PFM measurements on individual SA-CNFs are found to be affected by their chirality.
In the arrangement shown in
A second triboelectric generator element was formed using PVDF-TrFE. An embodiment of the invention was formed by drop casting a 5wt % solution of PVDF-TrFE in MEK onto an AAO template. For comparison, a second triboelectric generator element was formed by spin coating a PVDF-TrFE film onto ITO coated PET of thickness 1.571 μm. In each case the second triboelectric generator element was assembled with a first triboelectric generator element as shown in
A second triboelectric generator element was formed using cellulose. An embodiment of the invention was formed by drop casting a cellulose dispersion as discussed above onto an AAO template. For comparison, a second triboelectric generator element was formed by spin coating a cellulose film onto a highly p-dope Si wafer. In each case the second triboelectric generator element was assembled with a first triboelectric generator element as shown in
A second triboelectric generator element is formed using PLLA. An embodiment of the invention is formed by floating an AAO template on a PLLA solution at 100° C. Residual film at the surface can be removed suitably. For comparison, a second triboelectric generator element is formed by hot pressing. In each case the second triboelectric generator element is assembled with a first triboelectric generator element as shown in
While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.
All references referred to above are hereby incorporated by reference.
13 S. Shin, Y. E. Bae, H. K. Moon, J. Kim, S. Choi and Y. Kim, ACS Nano, 2017, 11, 6131-6138.
Sci. Adv., 2017, 3, e1700015.
Environ. Sci., 2013, 6, 3576.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/068810 | Jul 2017 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/072742 | 9/11/2017 | WO | 00 |
