Self-charging device for energy harvesting and storage

The disclosure relates to a self-charging device for energy harvesting and storage. The self-charging device for energy harvesting and storage includes a first electrode, a second electrode spaced from the first electrode, a solid electrolyte bridging the first electrode and the second electrode, and a water absorbing structure. The water absorbing structure is located on the second electrode, absorbs water from external environment and transmits the absorbed water to the solid electrolyte.

This application claims all benefits accruing under 35 U.S.C. § 119 from China Patent Application No. 201910169048.8, filed on Mar. 6, 2019, in the China National Intellectual Property Administration, the disclosure of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a self-charging device for energy harvesting and storage.

2. Description of Related Art

Harvesting energy from environment is a promising strategy to alleviate the global energy shortage and has great application potential in portable electronics. Up to now, various types of generators have been reported. Thermal energy, mechanical energy, and solar energy can all be harvested through thermoelectric, triboelectric/piezoelectric, and photoelectric effects respectively. Recently, great progresses have been made in harvesting energy from the interactions between water and various kinds of materials. In spite of the progress mentioned above, there are still many barriers hindering the effective utilization of water movement for energy generation. For example, the power density is too low because of the large internal resistance. The requirement of rigid working condition and complex experimental configuration is another challenge. Moreover, the conversion of energy from water movement to electric energy often requires artificial water supply, so the movement of water in the natural environment can not be directly utilized.

What is needed, therefore, is a self-charging device for energy harvesting and storage that overcomes the problems as discussed above.

DETAILED DESCRIPTION

Several definitions that apply throughout this disclosure will now be presented. The term “coupled” is defined as connected, whether directly or indirectly through intervening components, and is not necessarily limited to physical connections. The connection can be such that the objects are permanently connected or releasably connected. The term “outside” refers to a region that is beyond the outermost confines of a physical object. The term “inside” indicates that at least a portion of a region is partially contained within a boundary formed by the object. The term “substantially” is defined to essentially conforming to the particular dimension, shape or other word that substantially modifies, such that the component need not be exact. For example, substantially cylindrical means that the object resembles a cylinder, but can have one or more deviations from a true cylinder. The term “comprising” means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series and the like. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail, various embodiments of the present self-charging device for energy harvesting and storage.

Referring toFIGS. 1-3, a self-charging device10for energy harvesting and storage of one embodiment is provided. The self-charging device10for energy harvesting and storage comprises a substrate12, a first electrode13, a second electrode14, a solid electrolyte15, and a water absorbing structure16. The first electrode13and the second electrode14are located on a surface of the substrate12and spaced apart from each other. The first electrode13and the second electrode14are bridged by the solid electrolyte15. The water absorbing structure16is located on the second electrode14, absorbs water from external environment, and transmits the absorbed water to the solid electrolyte15. The self-charging device10for energy harvesting and storage is an asymmetric structure.

The substrate12can be a flexible substrate or a hard substrate. The hard substrate can be made of a material such as glass, quartz, silicon dioxide (SiO2), silicon nitride (Si3N4), alumina (Al2O3), magnesia (MgO). The flexible substrate12can make the self-charging device10for energy harvesting and storage have flexibility, so that the self-charging device10for energy harvesting and storage can be attached to a curved surface. Specifically, the material of the flexible substrate12can be polyethylene terephthalate (PET), polyimide (PI), polymethyl methacrylate (PMMA), polydimethylsiloxane (PDMS), or polyethylene naphthalate (PEN), etc. A shape, a size and a thickness of the flexible substrate are not limited and can be selected according to applications. In one embodiment, the substrate12is a PET sheet.

The first electrode13and the second electrode14can be made of a conductive material such as carbon nanotube, graphene, or carbon fibre. In one embodiment, the first electrode13and the second electrode14both comprise a carbon nanotube structure110. The carbon nanotube structure110comprises a plurality of carbon nanotubes intersected with or spaced from each other and a plurality of openings116defined between the adjacent carbon nanotubes. Containing the openings116, the first electrode13and the second electrode14have good permeability, which is good for water movement. The first electrode13and the second electrode14show good mechanical properties.

The carbon nanotube structure110can be a pure carbon nanotube structure111or a carbon nanotube composite structure112. The pure carbon nanotube structure111means that the carbon nanotube structure110consists of a plurality of carbon nanotubes and does not include other structural components. The carbon nanotube composite structure112comprises a pure carbon nanotube structure111and a functional layer114coated on the pure carbon nanotube structure111.

The pure carbon nanotube structure111includes a plurality of carbon nanotubes uniformly distributed therein. The plurality of carbon nanotubes extend along directions substantially parallel to the surface of the pure carbon nanotube structure111. The carbon nanotubes in the carbon nanotube structure111can be combined with each other by van der Waals attractive force therebetween. The carbon nanotubes can be disorderly or orderly arranged in the pure carbon nanotube structure111. The term ‘disorderly’ describes the carbon nanotubes being arranged along many different directions, such that the number of carbon nanotubes arranged along each different direction can be almost the same (e.g. uniformly disordered), and/or entangled with each other. The term ‘orderly’ describes the carbon nanotubes being arranged in a consistently systematic manner, e.g., the carbon nanotubes are arranged approximately along a same direction, or the pure carbon nanotube structure111have two or more sections within each of which the carbon nanotubes are arranged approximately along a same direction (different sections can have different directions). The carbon nanotubes in the pure carbon nanotube structure111can be single-walled, double-walled, or multi-walled carbon nanotubes.

The plurality of carbon nanotubes are tightly connected by Van der Waals forces, so that the pure carbon nanotube structure111and the carbon nanotube composite structure112are a free-standing structure. The term “free-standing” indicates that the carbon nanotube structure110can sustain a weight of itself when it is hoisted a portion thereof without any significant damage to its structural integrity. Thus, the carbon nanotube structure110can be suspended by two supports space apart.

The functional layer114is coated on surfaces of the plurality of carbon nanotubes. In one embodiment, the functional layer114is coated on the surface of each carbon nanotube. The functional layer114is combined with the carbon nanotube structure111by van der Waals attractive force therebetween only. The plurality of carbon nanotubes can be orderly arranged to form an ordered carbon nanotube structure, and apertures are defined in the ordered carbon nanotube structure. The apertures extend throughout the pure carbon nanotube structure111from the thickness direction. The plurality of carbon nanotubes can be single-walled carbon nanotubes, double-walled carbon nanotubes, or multi-walled carbon nanotubes. The plurality of carbon nanotubes are parallel to a surface of the pure carbon nanotube structure111. The surface is the largest surface of the carbon nanotube structure111formed by arranging the plurality of carbon nanotubes substantially parallel in the surface. A length and a diameter of the carbon nanotubes can be selected according to applications. The diameters of the single-walled carbon nanotubes range from about 0.5 nanometers to about 10 nanometers. The diameters of the double-walled carbon nanotubes can range from about 1.0 nanometer to about 15 nanometers. The diameters of the multi-walled carbon nanotubes can range from about 1.5 nanometers to about 500 nanometers. The length of the carbon nanotubes can be greater than 50 micrometers. In one embodiment, the length of the carbon nanotubes can range from about 200 micrometers to about 900 micrometers.

The apertures can be a plurality of holes defined by several adjacent carbon nanotubes intersected with each other or a plurality of gaps defined by adjacent two substantially parallel arranged carbon nanotubes and extending along an axial direction of the carbon nanotubes. The plurality of holes and the plurality of gaps can co-exist in the pure carbon nanotube structure111. Hereafter, a size of each of the plurality of apertures is the diameter of the hole or a width of the gap. The sizes of the apertures can be different. The sizes of the apertures can range from about 2 nanometers to about 500 micrometers, or about 20 nanometers to about 60 micrometers, or about 80 nanometers to about 5 micrometers, or about 200 nanometers to about 1.5 micrometers. The sizes refer to the diameters of the holes or the distances between the gaps in the width direction.

The pure carbon nanotube structure111comprises at least one carbon nanotube film, at least one carbon nanotube wire, or the combination thereof. In one embodiment, the pure carbon nanotube structure111comprises a single carbon nanotube film or two or more carbon nanotube films stacked together. Thus, the thickness of the carbon nanotube structure111can be controlled by a number of the stacked carbon nanotube films. The carbon nanotube film includes a plurality of uniformly distributed carbon nanotubes. The plurality of uniformly distributed carbon nanotubes are arranged approximately along the same direction. In one embodiment, the pure carbon nanotube structure111is formed by folding a single carbon nanotube wire. The carbon nanotube wire can be untwisted or twisted. In one embodiment, the pure carbon nanotube structure111can be a layer structure. The layer structure comprises a plurality of carbon nanotube wires, and the plurality of carbon nanotube wires are parallel to and spaced apart with each other. In another embodiment, the pure carbon nanotube structure111can be a carbon nanotube network structure. The carbon nanotube network structure comprises a plurality of carbon nanotube wires, and the plurality of carbon nanotube wires are intersected or weaved together. A distance between two adjacent carbon nanotube wires can range from about 1 nanometer to about 0.5 micrometers. Gaps between two adjacent carbon nanotube wires are defined as the apertures. The sizes of the apertures can be controlled by controlling the distances between two adjacent carbon nanotube wires. The lengths of the gaps between two adjacent carbon nanotube wires can be equal to the lengths of the carbon nanotube wires. It is understood that any carbon nanotube structure as described above can be used with all embodiments.

In one embodiment, the pure nanotube structure111includes at least one drawn carbon nanotube film. The drawn carbon nanotube film can be drawn from a carbon nanotube array that is able to have a film drawn therefrom. The drawn carbon nanotube film includes a plurality of successive and oriented carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The drawn carbon nanotube film is a free-standing structure.FIG. 5, each of the drawn carbon nanotube films includes a plurality of successively oriented carbon nanotube segments joined end-to-end and side-by-side by van der Waals attractive force therebetween. Each of the carbon nanotube segments includes a plurality of carbon nanotubes parallel to each other, and joined by van der Waals attractive force therebetween. As can be seen inFIG. 3, some variations can occur in the drawn carbon nanotube film. The carbon nanotubes in the drawn carbon nanotube film are oriented along a preferred orientation. The drawn carbon nanotube film can be treated with an organic solvent to increase a mechanical strength and a toughness and to reduce a coefficient of friction of the drawn carbon nanotube film. Diameters of carbon nanotube segments can range from about 10 nanometers to 200 nanometers. In one embodiment, the diameters of nanotube segments can range from about 10 nanometers to 100 nanometers. The drawn carbon nanotube film defines apertures between adjacent carbon nanotubes. The apertures extend throughout the drawn carbon nanotube film along the thickness direction thereof. The apertures can be micro pores or gaps. In one embodiment, the pure carbon nanotube structure111includes one drawn carbon nanotube film. Gaps are defined between the adjacent carbon nanotube segments in the carbon nanotube film. Sizes of the gaps can range from about 1 nanometer to 0.5 micrometers.

The pure carbon nanotube structure111can also include at least two of the drawn carbon nanotube films stacked together. In other embodiments, the pure carbon nanotube structure111can include two or more of the carbon nanotube films which coplanar arranged. Additionally, when the carbon nanotubes in the carbon nanotube film are aligned along the preferred orientations (e.g., the drawn carbon nanotube film), an angle can exist between the preferred orientations of adjacent carbon nanotubes films, whether the carbon nanotube films are stacked together or arranged side-by-side. Adjacent carbon nanotube films can be joined by the van der Waals attractive force therebetween. An angle between the aligned directions of the carbon nanotubes in two adjacent carbon nanotube films can range from about 0 degrees to about 90 degrees. When the angle between the aligned directions of the carbon nanotubes in adjacent stacked drawn carbon nanotube films is larger than 0 degrees, a plurality of micropores are defined by the pure carbon nanotube structure111. In one embodiment, the pure carbon nanotube structure111has the aligned directions of the carbon nanotubes between adjacent stacked drawn carbon nanotube films at 90 degrees. Diameters of the micropores can range from about 1 nanometer to about 0.5 micrometers. The thickness of the pure carbon nanotube structure111can range from about 0.01 micrometers to about 100 micrometers. Stacking the carbon nanotube films will also add to the structural integrity of the pure carbon nanotube structure111.

Referring toFIG. 4, in another embodiment, the pure carbon nanotube structure111can include at least one flocculated carbon nanotube film formed by a flocculating method. The flocculated carbon nanotube film can include a plurality of long, curved, disordered carbon nanotubes entangled with each other. The length of the carbon nanotube film can be greater than 10 centimeters. The carbon nanotubes can be randomly arranged and curved in the flocculated carbon nanotube film. The carbon nanotubes can be substantially uniformly distributed in the flocculated carbon nanotube film. The adjacent carbon nanotubes are acted upon by the van der Waals attractive force therebetween, thereby forming an entangled structure with micropores defined therein. Due to the carbon nanotubes in the flocculated carbon nanotube film being entangled with each other, the flocculated carbon nanotube film has excellent durability, and can be fashioned into desired shapes with a low risk to the integrity of flocculated carbon nanotube film. The flocculated carbon nanotube film can be a free-standing structure due to the carbon nanotubes being entangled and adhered together by van der Waals attractive force therebetween. The thickness of the flocculated carbon nanotube film can range from about 1 micrometer to about 1 millimeter. Many of the embodiments of the carbon nanotube structure are flexible and do not require the use of a structural support to maintain their structural integrity. The flocculated carbon nanotube film can be a pure carbon nanotube film only including carbon nanotubes.

The carbon nanotube wire can be untwisted or twisted. Treating the drawn carbon nanotube film with a volatile organic solvent can form the untwisted carbon nanotube wire. Specifically, the organic solvent is applied to soak the entire surface of the drawn carbon nanotube film. During the soaking, adjacent parallel carbon nanotubes in the drawn carbon nanotube film will bundle together, due to surface tensions of the organic solvent as it volatilizes, and thus, the drawn carbon nanotube film will shrunk into an untwisted carbon nanotube wire. Referring toFIG. 6, the untwisted carbon nanotube wire includes a plurality of carbon nanotubes substantially oriented along the same direction (i.e., a direction along a length of the untwisted carbon nanotube wire). The carbon nanotubes are substantially parallel to the axis of the untwisted carbon nanotube wire. More specifically, the untwisted carbon nanotube wire includes a plurality of successive carbon nanotube segments joined end to end by van der Waals attractive force therebetween. Each of the carbon nanotube segments includes a plurality of carbon nanotubes substantially parallel to each other, and joined by van der Waals attractive force therebetween. The carbon nanotube segments can vary in width, thickness, uniformity, and shape. The length of the untwisted carbon nanotube wire can be arbitrarily set as required. A diameter of the untwisted carbon nanotube wire can range from about 0.5 nanometers to about 100 micrometers.

The twisted carbon nanotube wire can be formed by twisting a drawn carbon nanotube film by mechanical forces to turn the two ends of the drawn carbon nanotube film in opposite directions. Referring toFIG. 7, the twisted carbon nanotube wire includes a plurality of carbon nanotubes helically oriented around an axial direction of the twisted carbon nanotube wire. More specifically, the twisted carbon nanotube wire includes a plurality of successive carbon nanotube segments joined end to end by van der Waals attractive force therebetween. Each of the carbon nanotube segments includes a plurality of carbon nanotubes parallel to each other, and joined by van der Waals attractive force therebetween. The length of the carbon nanotube wire can be set as required. A diameter of the twisted carbon nanotube wire can be from about 0.5 nanometers to about 100 micrometers. Further, the twisted carbon nanotube wire can be treated with a volatile organic solvent after being twisted to bundle the adjacent paralleled carbon nanotubes together. A specific surface area of the twisted carbon nanotube wire will decrease, while a density and strength of the twisted carbon nanotube wire will increase.

The carbon nanotube composite structure112can be made by coating a functional layer114on surfaces of the pure carbon nanotube structure111. In some embodiment, each of the plurality of carbon nanotubes is fully covered by the functional layer114. In some embodiment, part of the surface of the pure carbon nanotube structure111is covered by the functional layer114. In one embodiment, the pure carbon nanotube structure111can include two stacked drawn carbon nanotube films, and extension directions of the carbon nanotubes between the adjacent drawn carbon nanotube films are vertical.

The plurality of openings116are defined by the plurality of apertures of the pure carbon nanotube structure111. The plurality of openings116of the carbon nanotube composite structure112and the plurality of apertures of the pure carbon nanotube composite structure111may have a same shape but different in size. The sizes of the plurality of openings116of the carbon nanotube composite structure112are smaller than those of the plurality of apertures because the functional layer114is deposited in the plurality of apertures.

The functional layer114shows good electric conductivity and good wettability with water. The functional layer114can be made of the conductive polymers such as polyaniline (PANI), polythiophene (PT), or polypyrrole (PPy), etc. The materials are not limited to the listed materials mentioned above, as long as it has conductivity and good wettability with water. A thickness of the functional layer114is not limited. In one embodiment, the thickness of the functional layer114ranges from about 5 nanometers to about 150 nanometers. In another embodiment, the thickness of the functional layer114ranges from about 8 nanometers to about 45 nanometers. If the thickness of the functional layer114is greater than 150 nanometers, the plurality of apertures may be fully filled by the functional layer114and the plurality of openings116cannot be obtained.

The method of making the carbon nanotube composite structure112comprises the following steps: providing a pure carbon nanotube structure111, and conductive polymer monomer solution A, wherein the conductive polymer monomer solution A is placed in a container; immersing the pure carbon nanotube structure111into the conductive polymer monomer solution A in the container; dropping the conductive polymer monomer solution B into the container, and compounding conducting polymer monomers with the pure carbon nanotube structure111to form a carbon nanotube-conducting polymer composite. The conductive polymer monomer can be the material of aniline, pyrrole, thiophene, acetylene, or p-benzene and p-styrene.

In one embodiment, the placing the carbon nanotube structure110on the surface further comprises solvent treating the substrate12with the carbon nanotube structure110thereon. Because there is air between the carbon nanotube structure110and the surface of the substrate12, the solvent treating can exhaust the air and allow the carbon nanotube structure110to be closely and firmly adhered on the surface of the substrate12. The solvent treating can be applying a solvent to entire surface of the carbon nanotube structure110or immersing the entire substrate12with the carbon nanotube structure110in a solvent. The solvent can be water or volatile organic solvent such as ethanol, methanol, acetone, dichloroethane, chloroform, or mixtures thereof. In one embodiment, the organic solvent is ethanol.

The first electrode13and the second electrode14are located on a surface of the substrate12, and spaced from each other. The first electrode13and the second electrode14can be directly attached to the surface of the substrate12due to the adhesion of the carbon nanotube structure110, and can also be attached to the surface of the substrate12by a binder. The first electrode13and the second electrode14can be bridged by solid electrolyte15.

The solid electrolyte15has no conductivity in a dry state, but become conductive after absorbing water. The conductivity of the solid electrolyte15increases as the absorbing more and more water. When electrolyte diffusion/fluid passes through a channel or porous structure, a potential is induced across the channels. An electric potential is formed across the solid electrolyte15when the self-charging device10for energy harvesting and storage exposed to a moisture gradient. This is ascribed to asymmetric reorientation and dislocation of ions under water diffusion in the solid electrolyte15. The material of the solid electrolyte15can be solid polymer which is soluble in water. The solid polymer can be polyvinyl alcohol (PVA) gel or composite gel of polyvinyl alcohol with some ionic compounds such as HCl/PVA, H2SO4/PVA, H3PO4/PVA, etc. In one embodiment, the material of the solid electrolyte15is HCl/PVA gel. The ionic compounds can ionize cations and anions in water. Cations and anions are separated under the electrolyte diffusion because of their asymmetric interactions with the solid electrolyte15.

The water absorbing structure16is an ionic compound, which can produce anions and cations in solvents, such as calcium chloride, calcium sulfate, magnesium sulfate, sodium sulfate, or potassium carbonate, etc. In one embodiment, the material of the water absorbing structure16is anhydrous calcium chloride (CaCl2). In another embodiment, the material of the water absorbing structure16is calcium chloride dihydrate (CaCl2.2H2O). The water absorbing structure16not only absorbs water, but also acts as an electrolyte. The CaCl2on the surface of the second electrode14can absorb water more quickly so that a local CaCl2solution can be formed in a shorter time. Therefore, the CaCl2solution will be formed in the second electrode14in a short time and diffuses towards the solid electrolyte15. The different interactions with solid electrolyte15can lead to a net separation of cations and anions under the automatically-formed diffusion of CaCl2solution, which leads to a potential across the bridge. The upstream of diffusion has a negative potential, indicating that cations are more likely to be carried along by the diffusion flux. That is, negative potential is formed at the second electrode14, and positive potential is formed at the first electrode13. Referring toFIG. 8, current-voltage curve (CV curve) of the self-charging device10for energy harvesting and storage after exposed to the humid environment after different time at a scan rate of 25 mV s−1is provided. The area of the CV curve has been enhanced by 47 times after 140 min, indicating that the mobility enhancement of ions in the HCl/PVA bridge. This will assist the electrolyte diffusion in HCl/PVA bridge.

The water absorbing structure16can be located on any area of the second electrode14as long as the water absorbed by the water absorbing structure16can be transmitted to the solid electrolyte15. The water absorbing structure16can be contacted with the solid electrolyte15. The water absorbing structure16can also be not in direct contact with the solid electrolyte15. Since the carbon nanotube structure110comprises a plurality of openings116which are conducive to water transfer, water absorbed by the water absorbing structure16can be transmitted to the solid electrolyte15by the carbon nanotube structure110. In one embodiment, the water absorbing structure16is located on the second electrode14and in direct contact with the solid electrolyte15.

In one embodiment, the water absorbing structure16is located on part surface of the second electrode14. The method of locating on the water absorbing structure16on the part surface of the second electrode14comprises the following steps: preparing the CaCl2solution by dissolving 25 g anhydrous CaCl2in 100 mL deionized (DI) water; adding about 10 microlitres (μL) CaCl2solution to one side of the second electrode14near the solid electrolyte15to form a prefabricated structure A; and removing the deionized water from the CaCl2solution by drying the prefabricated structure A in a vacuum oven at 30° C.

In another embodiment, the water absorbing structure16is located on entire surface of the second electrode14. The method of locating the water absorbing structure16on the entire surface of the second electrode14comprises the following steps: preparing the CaCl2solution by dissolving 25 g anhydrous CaCl2in 100 mL deionized (DI) water; forming the carbon nanotube composite structure (CaCl2/CNT composites) by immersing the carbon nanotube structure110into the CaCl2solution and ultrasound treating to form a prefabricated structure B; and removing the deionized water from the CaCl2solution by drying the prefabricated structure B in a vacuum oven at 30° C. Referring toFIG. 9, contact angles of pure carbon nanotube structure (CNT) and the CaCl2/CNT composites are provided. The contact angle between the pure CNT and deionized water (DI water) is about 121.6°, and the contact angle between the CaCl2/CNT composites DI water is about 49.1°, which indicated that the hydrophilia of CNT was greatly improved after being composited with CaCl2.

The self-charging device10for energy harvesting and storage can further comprise a first terminal pad17located on the surface of first electrode13away from the substrate12and a second terminal pad18located on the surface of the second electrode18away from the substrate12. The material of the first terminal pad17and the second lead-put terminal18is conductive material such as metal or conductive polymer. In one embodiment, the material is conductive silver paste. The first terminal pad17and the second terminal pad18are connected to an electronic device through wires to supply power to the electronic device.

The mass of the self-charging device10for energy harvesting and storage is small, and can be in a range from about 30 milligram (mg) to 100 mg. In one embodiment, the mass of the self-charging device10for energy harvesting and storage ranges from about 40 mg to 60 mg. In another embodiment, the mass of the self-charging device10for energy harvesting and storage without the substrate12is 50 mg.

In order to research the effect of the solid electrolyte15and water absorbing structure16on the property of the self-charging device10for energy harvesting and storage, the following experiments were carried out.

A device10′ of a comparative example 1 was provided. The device10′ is similar to the self-charging device10for energy harvesting and storage except that the device10′ is a symmetric structure without the water absorbing structure16.

The device10′ is put into a humidity cabinet whose relative humidity (RH) is 80% for a period of time. As shownFIG. 10, without CaCl2, the magnitude of the open circuit voltage (VOC) is no more than 14 mV and became more and more stable around zero point when exposed to humid condition.

Then, 10 L of DI water and CaCl2solution were dropped to one side of the HCl/PVA gel bridge of the device10′ respectively. As shown inFIG. 11, a variation curve of the VOCvs. time is provided. There is an output under the flow of DI water, which likely results from the separation of Cl−and H+. When CaCl2solution was used, the output reached a higher value, showing the synergistical contribution of Ca2+and H+to the electric output.

In order to research the output performance of the self-charging device10for energy harvesting and storage, the following experiments were carried out. The device10was short-circuited and kept in vacuum oven at 30° C. for a period of time to evaporate extra water before measurement.

FIG. 12shows a typical self-charging process of the self-charging device10for energy harvesting and storage. The open circuit voltage (VOC) began to rise after the device10was transferred to the humidity cabinet whose RH is 80%. After a slow pace at the beginning, the charging rate speeds up that the VOCquickly rose to 0.300 V after 1 hour. Then the charging rate slows down and reaches a maximum value of 0.348 V. When a 100 load is added to the circuit, the voltage decreased to 0.040 volt (V), which corresponds to a current density of 0.40 milliampere per square centimeter (mA cm−2) and output power of 16 microwatt per square centimeter (μW cm−2). The discharging process is shown inFIG. 2a. The short circuit current (ISC) density of the self-charging device10for energy harvesting and storage is 0.655 mA cm−2(FIG. 2b) after being charged to 0.287 V, corresponding to a maximum power of 47 μW cm−2. This current density is 2-3 orders higher than that of fluid or evaporation generator.

The relation between humidity and VOCwas further studied by tuning the humidity in the humidity cabinet. Referring toFIG. 13, it is found that increasing humidity can boost the self-charging process. The charging rates are 4.1 microvolt per second (μV/s), 25.9 μV/s, 37.1 μV/s and 62.0 μV/s for the relative humidity of 15%, 55%, 75%, and 87% respectively. When the device10was taken out of the cabinet, the VOConly decreased from 0.232 V to 0.160 V for 6880 s. This means that the induced electricity can be well stored in the device10under a dramatic humid change. Even if the device10is removed from the humid environment, the water absorbed by the water absorbing structure16can remain for a long time. This is very different from other fluid generator. The output of the other fluid generator vanishes within seconds once the moisture disappears from the environment, and the energy generated by the other fluid generator cannot be timely and effectively stored.

The self-charging curves under different humidity are shown inFIG. 14. The VOCis 7.5 mV, 20.5 mV, 147.4 mV, and 189.1 mV after 7500 s under the relative humidity of 15%, 38%, 65%, and 80%. When the relative humidity is 40%, the VOCrose from 2.6 mV initially to 26.0 mV and 161.2 mV after 4400 s and 18000 s, indicating that the device10can also be charged to higher voltage in low humidity but requires a longer time. These results demonstrate that the device10can work in a common ambient condition with relative humidity ranging from 40% to 80%, which surely promotes the device10application potential.

Referring toFIG. 15, a self-charging device20for energy harvesting and storage of another embodiment is provided. The self-charging device20for energy harvesting and storage comprises a substrate12, a first electrode13, a second electrode24, and a solid electrolyte15. The first electrode13and the second electrode24are located on a surface of the substrate12and spaced apart from each other. The first electrode13and the second electrode24are bridged by the solid electrolyte15. The second electrode24comprises a carbon nanotube structure110and a water absorbing structure16located on the entire surface of the carbon nanotube structure110. The self-charging device20for energy harvesting and storage is an asymmetric structure. The self-charging device20for energy harvesting and storage further comprises a first terminal pad17located on the surface of first electrode13away from the substrate12and a second terminal pad18located on the surface of the second electrode18away from the substrate12.

The self-charging device20for energy harvesting and storage is similar to the self-charging device10for energy harvesting and storage above except that, in the self-charging device20for energy harvesting and storage, the second electrode24comprises a carbon nanotube structure110and a water absorbing structure16located on the entire surface of the carbon nanotube structure110. As shown inFIG. 16, each of the carbon nanotubes is entirely coated by the CaCl2.

Referring toFIG. 17, a self-charging device30for energy harvesting and storage of another embodiment is provided. The self-charging device30for energy harvesting and storage comprises a substrate32, a first electrode13, a second electrode14spaced from the first electrode13, a solid electrolyte15, and a water absorbing structure16. The first electrode13and the second electrode14are spaced apart from each other. The substrate32comprises a first surface322and a second surface324opposite to the first surface322. The first electrode13is located on the first surface322, and the second electrode14is located on the second surface324. The substrate32further comprises a plurality of through holes320. The plurality of through holes320are spaced apart with each other. The plurality of through holes320extend throughout the substrate32from the first surface322to the second surface324. The solid electrolyte15is located in the through holes320and connects the first electrode13and the second electrode14. The water absorbing structure16is located on the entire surface of the second electrode14. The self-charging device30for energy harvesting and storage further comprises a first terminal pad17located on the surface of first electrode13away from the substrate32and a second terminal pad18located on the surface of the second electrode14away from the substrate32.

The self-charging device30for energy harvesting and storage is similar to the self-charging device10for energy harvesting and storage above except that the substrate32further comprises a plurality of through holes320spaced apart from each other. The first electrode13is located on the first surface322, and the second electrode14is located on the second surface324. Since the substrate32further comprises a plurality of through holes320spaced apart from each other, and each of the plurality of through holes320comprises the solid electrolyte15, the self-charging device30can be divided into several small self-charging devices and used independently. The small self-charging devices can include one through hole320, two through holes320, three through holes320, or other number of through holes320less than the number of the through holes320of the self-charging device30. In one embodiment, the substrate32comprises six through holes320. The self-charging device30for energy harvesting and storage can be divided into six small self-charging devices, and each small self-charging device have one through hole320. In another embodiment, the substrate32comprises six through holes320. The self-charging device30for energy harvesting and storage can be divided into four small self-charging devices, and two of the small self-charging device have one through hole320, and two of the small self-charging device have two through holes320. Alternatively, the self-charging device30for energy harvesting and storage can be used as a single self-charging device. Referring toFIG. 18together, a method for dividing the self-charging device30for energy harvesting and storage into several small self-charging devices comprises the following steps: providing the self-charging device30for energy harvesting and storage; square cutting the area of the substrate32between adjacent through holes320along a cutting line321.

Referring toFIG. 19, a self-charging device40for energy harvesting and storage of another embodiment is provided. The self-charging device40for energy harvesting and storage comprises a first electrode43, a second electrode44, and a solid electrolyte15. The solid electrolyte15comprises a third surface150and a fourth surface152opposite to the third surface150. The first electrode43is located on the third surface and the second electrode44is located on the fourth surface152. The first electrode43is a carbon nanotube composite structure (PANI/CNT) comprising a carbon nanotube structure110and polyaniline (PANI). The second electrode44is a carbon nanotube composite structure (CaCl2/PANI/CNT) comprising a carbon nanotube structure, CaCl2, and PANI. The self-charging device40for energy harvesting and storage is a sandwich-like device.

The PANI/CNT composite was synthesized using a in situ method. In one embodiment, the carbon nanotube structure was immersed in an aniline/HCl solution, then equal amount of ammonium peroxidisulfate (ASP) solution was added in to the aniline/HCl solution with the carbon nanotube structure and kept at 0° C. for 24 h. The PANI/CNT composite was sonicated in the CaCl2solution and dried in an oven to prepare the CaCl2/PANI/CNT. The sandwich-like device was fabricated by assembling PANI/CNT and CaCl2/PANI/CNT together with HCl/PVA gel.

The self-charging device40for energy harvesting and storage is a sandwich-like device. The first electrode43, the solid electrolyte15, and the second electrode344are stacked in succession. The sandwich type device can increase the effective area and reduce diffusion distance. The moisture induced self-charging process is shown inFIG. 20andFIG. 21. The charging rate became slower after 2 hours and finally reached a charging voltage of 0.243V. However, the short-circuited current (ISC) density was enhanced to 4.00 mA cm−2and the outpower density reached 0.243 mW cm−2(12.15 mW cm−3), which is 5.2 times of the planar self-charging device10for energy harvesting and storage with pure carbon nanotube structure as electrodes. This demonstrates that the improvement of output performance can be realized by proper material election and rational structure design.

The self-charging device for energy harvesting and storage as disclosed has the following advantages. Firstly, the self-charging device for energy harvesting and storage can be fabricated by a simple process. The self-charging device for energy harvesting and storage can be used without complicated external conditions, and not only can realize self-charging but also can store the energy. Secondly, the carbon nanotube structure is used as electrodes. The porous structure of the carbon nanotube structure facilitates the diffusion of water and ions. Thirdly, the device has excellent self-charging and storing energy capacity. The self-charging device for energy harvesting and storage can be charged to 0.348V in a humid environment of 80% relative humidity and the energy can be effectively stored. Output performance of the self-charging device for energy harvesting and storage can be significantly improved by providing a sandwich type device using PNAI/CNT electrodes, which possesses the maximum power density of 0.243 mW cm−2. Fourthly, the electrolyte diffusion was driven by absorbing water from moisture automatically. Separation of cations and anions is realized under the diffusion because of their different mobilities in the HCl/PVA gel. Furthermore, the self-charging device for energy harvesting and storage is lightweight, flexible, and has great potential in portable and smart electronics.

It is to be understood that the above-described embodiments are intended to illustrate rather than limit the disclosure. Any elements described in accordance with any embodiments is understood that they can be used in addition or substituted in other embodiments. Embodiments can also be used together. Variations may be made to the embodiments without departing from the spirit of the disclosure. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure.