The present invention relates to an electrical device, and in particular to an electrical device configured to generate charge carriers on exposure to moisture; and store the electrical charge carriers.

1. Background

Moisture electric generator (MEG) is a very promising energy harvesting technology owing to its advantage of high efficiency, simple device structure, high power density compared with other energy harvesting technologies. A common method to regenerate the moisture electric generator is using dry gas to remove adsorbed water molecular and then put the device in humidity environment. However, this will greatly limit practical applications of moisture electric generator in our daily life.

In graphene oxide-based Moisture Electric Generators (MEG), GO layer with abundant hydrophilic oxygen-containing groups (e.g., -OH, -COOH, epoxy) harvests moisture through hydrogen bonding. 06 — H6+ bond weakens as water content increases, releasing mobile H+ ions through hydrolysis of functional groups. Functional groups are specific groupings of atoms within molecules that have their own characteristic properties.

2. Summary of the Invention

In embodiments of the invention, a Moisture Electric Generator (MEG) device may be regenerated. The MEG may be regenerated after electrical discharge.

Moisture electric generator devices are energy-harvesting devices based on molecular diffusion of the water in functional materials and the interaction of water with that functional material. MEG devices rely on energy transformation from the chemical potential to electrical output. When exposed to moisture, functional materials spontaneously absorb water molecules leading to the release of positively /negatively charged ions (often protons or hydronium groups) from the hydration interaction of the materials. The charged ions are generated and dissociated from the functional material on exposure to moisture. After dissociation the charged ions create a potential difference across the device. A key functionality of the functional material is the presence of chemical groups that are capable of absorbing water and then dissociating to provide charged ions.

After dissociation, diffusion of these charged ions from high to low concentrations will induce directional movement of charged carriers in external circuits.

In an example MEG device, the functional material may include graphene oxide. The graphene oxide includes abundant hydrophilic oxygen-containing groups (e.g., -OH, - COOH). These groups harvest moisture through hydrogen bonding. 06 — H6+ bond weakens as water content increases, releasing mobile H+ ions through hydrolysis of functional groups. The charge carriers may be protons or hydronium ions.

The functional material may be a semi-conductor. The functional material may be an insulator. The preferred functional material is graphene oxide.

In Moisture Electric Generators (MEG), a potential difference is induced by an ion gradient within the device. During discharge, the potential difference will become smaller and smaller due to the diffusion of ions. In embodiments of the invention, the ion gradient can be rebuilt and a potential difference can be created across the device by applying an external electric field to the device. This process is called regeneration. This has the overall effect of recharging the MEG device. The regeneration process may establish a potential difference across the device which is greater than, equal to or less than a previous potential difference across the device. This electrically induced regeneration may take the form of generating or regenerating moisture sensitive functionality in the functional material so that it may regain its sensitivity to moisture for operation of the MEG device. Furthermore when the electrode material used in the device has the functionality and capacity to store charged ions during operation of the MEG device electrical regeneration may also involve the regeneration of this functionality and capacity through the release of charged ions from the electrode.

Embodiments provide a Moisture Electric Generator (MEG) device that can store electrical charge carriers.

Embodiments provide a regenerable moisture electric generating device. Embodiments provide a dual functional electronic device configured to generate electrical charge carriers when exposed to moisture and to store the generated electrical charge carriers.

Embodiments provide an electronic device having dual functionality (also referred to as dual property). One functionality is the ability to generate charge carriers. The second functionality is to store charge carriers.

Embodiments provide an electrical device with two modes of operation. One mode of operation is the generation of electricity in contact with moisture. The second mode of operation is the storage of potential energy on the application of electric charge to the spent device. Both modes may occur simultaneously. The electrical device may be a hybrid with two modes of operation.

This is a dual mode device with two modes of operation. The dual mode electronic device may be referred to as a dual functional electronic device.

Embodiments provide an electrical device configured to generate electrical charge carriers on exposure to moisture, the electric charge carriers creating a potential difference across the device; and, after discharge of the electrical charge carriers, to regenerate the electrical device on application of an electric field across the device.

In a first aspect the invention provides an electrical device configured to:

- generate electrical charge carriers on exposure to moisture; and,

- store the electrical charge carriers.

An advantage of the invention is that the electronic device can operate as a moisture electric generating device whereby on exposure to moisture the device releases electrical charge carriers. Additionally, the device can also store at least some of the charge carriers. This enables the electrical device to act as a capacitor. The great benefit is that at least some of the electrical charge can be stored in the device. The electrical charge may be protons. The electrical charge carriers may be H+ ions.

The device may include a graphene oxide layer. The graphene oxide layer may include abundant hydrophilic oxygen-containing groups (e.g., -OH, -COOH). These groups harvest moisture through hydrogen bonding. 06 — H6+ bond weakens as water content increases, releasing mobile H+ ions through hydrolysis of functional groups. The charge carriers may be protons or hydronium ions. In embodiments the electrical device is charged when an external electric field is applied across the device.

An advantage of these embodiments is that the charge carriers can be moved when required by applying an external electric field across the device. The electric field may be a voltage. The electric field may be a potential difference. Movement of the charge carriers has the effect of recharging the electrical device. This may also be referred to as regenerating the device. These devices can be regenerated by an external power source, similar to conventional rechargeable batteries. The devices may be regenerated to have a potential difference the same as or greater than the potential difference before recharge.

Prior art MEG devices are typically required to be dried using dry gas to remove absorbed moisture and then regenerated by applying moisture again to the device to generate more charge carriers. In embodiments, the charge carriers can be maintained in a mobile state and pushed across the device to regenerate the device and so the device includes a charge distribution across the device which produces a potential difference across the device. Embodiments of the invention provide a more practical process for regenerating the device without requiring drying and remoisturising of the device. This practical regeneration process is expected to be useful for practical applications.

In embodiments the electrical device is configured such that after electrical discharge of the potential difference across a short circuit, the electrical charge carriers are stored in the device and the device is regenerated when an external electric field is applied across the device.

Such embodiments provide a device in which charge carriers may be diffused across the device after discharge to regenerate the device by creating a charge distribution across the device to generate a potential difference across the device. It could be concluded that there are two charging processes: First is the natural charging process in which the interaction of moisture with the device produces charge carriers in the device; and the second is the regeneration process in which charge carriers can be diffused, or pushed, across the device to generate a potential difference across the device. It is surprising to find that this strategy is efficient, and the MEG operates as a capacitor that stores energy.

In embodiments the electrical device is configured so that the generation of electrical charge carriers produces a potential difference across the device on exposure to moisture. In embodiments the electrical device configured to create a moisture absorption gradient across the device.

In embodiments the electrical device configured to create a charge carrier gradient across the device, the charge carrier gradient producing the potential difference across the device.

Embodiments are configured to produce a charge gradient across the device on exposure to moisture. Such embodiments provide a potential difference across the device on exposure of the device to moisture. Some embodiments achieve a charge gradient by providing asymmetric moisture absorption across the device. This may be achieved by exposing one surface of the device to moisture, and preventing exposure of a second surface of the device to moisture or reducing the exposure of the second surface to moisture.

In embodiments the electrical device comprises a functional layer, a first electrode and a second electrode, the functional layer being disposed between and being electrically connected to the first electrode and the second electrode.

Embodiments comprise a functional material, preferably in the form of a layer. The material may be one functional material or may be a composite of two or more functional materials, or a composite of one or more functional materials with one or more other materials. The composites may comprise functional material mixed with a binder. The composite may be a homogeneous mixture of one or more carbon active materials and one or more binders, preferably an organic binder such as a polymer. Examples of binders include: polyvinyl alcohol (PVA); polyvinyl butyra (PVB); Poly (methyl methacrylate) (PMMA);

Polyvinylpyrrolidone (PVP); PVDF; PS SNA. PVA has abundant hydroxyl group which allows moisture to be absorbed from the environment. PVA also has good viscosity, which makes it a good candidate for printing. As discussed above in relation to binders, PVA provides stable attachment of the functional layer to an electrode.

The functional material may be any material that contains groups capable of generating charge carriers on contact with H2O and may be any material that can act as a transport medium for these charge carriers under an applied electrical load.

The functional layer may be referred to as a charge generating layer.

The functional layer releases mobile electrical charge carriers on exposure to moisture. The first electrode and the second electrode may be asymmetric. The first electrode and the second electrode may comprise different electrode materials. The first electrode and the second electrode have different moisture permeability properties. The functional layer may comprise a graphene-based material. The functional layer may comprise graphene oxide. The functional layer may comprise GO-PVA. The electrical charge carriers may be protons. The electrical charge carriers may be hydronium ions. The electrical charge carriers may be hydrogen ions. The hydrogen ions being released by the functional layer through the hydrolysis of functional groups in the functional layer.

The first electrode may comprise at least one of carbon nanotubes, mxene, graphene; or carbon black. The second electrode may comprise silver nanowires.

In embodiments, the first electrode acts as a charge reservoir to store electrical charge carriers. The first electrode may have functionality to allow it to reversibly act as a charge reservoir. Such functionality may be chemical groups that reversibly interact with and retain charge carriers such as protons.

In embodiments, the stored electrical charge carriers stored at the charge reservoir are released and mobilised on applying an external electric field across the device.

The advantage of such embodiments is that the charge carriers are stored within the device and may be moved on applying an electric field to the sample to regenerate the sample without requiring additional new charge carriers to be created. The charge carriers may be stored in a mobile state in order that they can be moved and redistributed within the sample on application of an external electric field. The mobile charge carriers may be accumulated at the interface between the functional layer and the electrode. For example, the charge carriers may be accumulated at the interface between the GO layer and the electrode.

After discharge, the mobile ions may be redistributed in the GO layer, or migrate back towards the countered immobile ions group for recombination.

In embodiments, the overall resistance of the device is low and the charge carriers can easily move in the device and a high current can be achieved. The device has high ionic conductivity.

In embodiments, the first electrode is configured to provide a bonding between the first electrode and the electrical charge carrier, the bonding being overcome when an electric field is applied across the device. Candidate materials for the first electrode include carbon based nanomaterials including graphene, carbon nanotubes (CNT) and MXene. CNT has good conductivity, and plenty of functional groups that can act as charge carrier reservoir so when the electric field is applied the charge can be pushed back to the other electrode so the ion gradient can be rebuilt.

In embodiments the first electrode provides a high hydrogen binding energy.

In embodiments the first electrode is one of: carbon nanotubes, mxene, graphene; or carbon black.

The advantage of these carbon based materials compared with many metals is that metals do not have functional groups that can bond with protons so they cannot act as charge reservoirs. Compared with metal electrodes, the CNT has some functional groups and larger surface area, which may reserve H+ and is beneficial for a higher capacity.

In a further aspect the invention provides a moisture-electric generation device comprising a functional material having a potential difference associated with an ion gradient across the functional material, wherein the ion gradient has been generated by the application of an external electric field across the device.

The moisture-electric generation device may comprise bound charge carriers prior to application of the external electric field.

The moisture-electric generation device may comprise a functional material and charge carriers bound within the device capable of being transported through the device and functional material upon the application of an external electric field to produce a potential difference across the device associated with an ion gradient across the functional material.

The functional material may be moisture treated to reduce the ion gradient of the material prior to application of the external electric field. The moisture treatment is via moistureelectric generation.

The ion gradient of the device after application of an external electrical field may be the same or greater than the ion gradient produced through the moisture-electric generation treatment of the device.

In a further aspect the invention provides a regenerated moisture-electric generation device comprising a functional material having a potential difference associated with an ion gradient across the functional material that is depleted after moisture-electric generation, wherein the depleted ion gradient has been regenerated by the application of an external electric field across the device.

In a further aspect the invention provides a capacitor comprising electrodes and a capacitor element comprising a functional material having had a potential difference associated with an ion gradient across the functional material that was generated and then depleted by moistureelectric generation and the depleted potential difference and ion gradient being capable of regeneration by the application of an external electric field across the device.

In a further aspect the invention provides a method of generating electricity from a moistureelectric generation device comprising a functional material the method comprising the steps of: a) exposing a surface of the functional material to moisture in order to generate charge carriers within the functional material and a potential difference with associated ion gradient across the device, b) applying a load to the device to deplete the potential difference and associated ion gradient of the device via the generation of electricity, c) applying an electric field across the depleted electronic device to regenerate the potential difference and associated ion gradient across the device, and d) repeating steps b) and c) with optional further repetition of step a).

In one aspect the invention provides an electrical device comprising a functional material, the functional material dissociates electrical charge carriers on exposure to moisture to generate a potential difference across the device; wherein the electrical device stores at least some of the electrical charge carriers.

In one aspect the invention provides an electrical device comprising a functional material comprising graphene-oxide, the functional material dissociates electrical charge carriers on exposure to moisture to generate a potential difference across the device; wherein the electrical device stores at least some of the electrical charge carriers.

In one aspect the invention provides an electrical device comprising a functional material comprising graphene-oxide, the functional material dissociates electrical charge carriers on exposure to moisture to generate a potential difference across the device; after discharge of the potential difference the electrical device stores at least some of the electrical charge carriers, wherein a potential difference is regenerated across the device on applying an electric field across the discharged device.

In one aspect the invention provides a moisture-electric generation device comprising a functional material with electrically generated or regenerated moisture sensitive functionality.

In one aspect the invention provides a moisture-electric generation device comprising an electrode with electrically generated or regenerated proton storage capacity.

The device may further comprise an electrode with electrically generated or regenerated proton storage capacity.

The functional material may comprise graphene-oxide.

The electrode may comprise carbon nanotubes and the electrically generated or regenerated proton storage capacity is due to the forced exit of protons.

3. Brief Description of the Figures:

In order that the invention be more clearly understood and put into practical effect, reference will now be made to preferred embodiments of an assembly in accordance with the present invention. The ensuing description is given by way of non- limitative example only and is with reference to the accompanying drawings, wherein:

Figure 1 is a first side view of a MEG device.

Figure 2 is a front facing view of the MEG device of Figure 1.

Figure 3 is a flow diagram showing the method steps for charging, discharging and regenerating a MEG device.

Figure 4 shows circuit diagrams for charging and discharging a MEG device.

Figure 5 Charge and discharge mechanism of GO/CNT based moisture electric generator.

Figure 6 Charge-discharge curve of MEG sample 1.

Figure 7 Charge and discharge capacity of MEG sample 1.D0-D6 represent discharge cycle 0-6, and C1-C6 are charging cycle 1-6. Figure 8 Charge curve and capacity comparison of MEG sample 2.

Figure 9 discharge curve and capacity comparison of MEG sample 2.

Figure 10 Charge curve and capacity comparison of MEG sample 3.

Figure 11 discharge curve and capacity comparison of MEG sample 3

Figure 12 Charge-discharge curve of MEG sample 4.

Figure 13 Charge-discharge curve of MEG sample 5.

4. Detailed Description

The electrical device described could be in any form, an example of which is described in earlier international patent application PCT/AU2022/050036 filed on 25 January 2022 the contents of which are incorporated by reference.

Sample fabrication

The CNT-PVA-GO-NF MEG device is fabricated as follows. First, 6mL of carbon nanotube (CNT) paste is mixed with 24 mL of diluted (DI) water to obtain 20 wt% of CNT solution, and 30mL of the as-prepared solution is coated on a flexible polyethylene terephthalate (PET) substrate (10 x 10 cm ² ). A homogeneous CNT film is obtained to work as the bottom conductive layer after natural evaporation for 12 h. Second, 1.5 g of graphene oxide (GO) powder is added to 48.5 g of DI water, and the solution is probe- sonicated for 1.5 h to get 3 wt% of GO suspension. The abovementioned CNT film is uniformly printed with 200 pL of 2 wt% polyvinyl alcohol (PVA), and the as-prepared GO suspension is deposited on the CNT- PVA film. The composite is placed in a well-ventilated environment for 12 h and the GO suspension gradually becomes a smooth film that acts as the top working layer of the MEG device. Third, the CNT-PVA-GO film is placed in a sealed container with 32% HC1 vapor for 12 h to tune the carbon-oxygen bonding, followed by drying in an oven at 50 °C for 12 h to remove extra HC1 and moisture. Finally, the whole composite is cut into different testing sizes (e.g. 2 x 1 cm ² for a standard sample), and 0.3 mm thick nickel foam (NF) of equal size is pressed on top of the GO film using a hydraulic machine with 2-ton pressure. The NF serves as the top conductive layer and the porous structure allows moisture to penetrate the working area. Each sample is vacuum- sealed separately at room temperature and taken out to be tested under various conditions. The sample was put into plastic bags and vacuumed to remove moisture inside. The charge and discharge experiments were done by using Keithley 2450 source meter.

MEG Device Structure

Figure 1 shows an example moisture electric generating (MEG) device. The view in Figure 1 is a side view of the MEG device. Referring to Figure 1, the MEG device includes a functional material 110 positioned between two electrodes 150 and 130. Functional material 110 is in the form of a layer, a functional layer 110. When exposed to moisture, functional materials spontaneously absorb water molecules leading to the release of positively/negatively charged ions (often protons or hydronium groups) from the hydration of the materials. The charged ions are dissociated from the functional layer on exposure to moisture. For a graphene oxide functional layer, H+ ions are dissociated on exposure to moisture.

The functional material 110 may be a semi-conductor. The functional material 110 may be an insulator.

The functional layer is disposed between a first electrode 130 and second electrode 150. First electrode 130 may be attached to a first surface 120 of the functional layer 110. Second electrode 150 may be attached to a second surface 160 of the functional layer 110. In the example of Figure 1, first surface 120 and second surface 160 are opposite faces of the functional layer. For the purposes of the description electrode 130 may also be referred to the bottom electrode and electrode 150 may also be referred to as the top electrode. The orientation of MEG device is not limiting and these labels are used for the purposes of description only.

The functional layer 110 has a length dimension (E) and a depth dimension (D) (not shown in the side view of Figure 1 but shown in the front view of Figure 2) and a thickness dimension (E). The length dimension (L) and the depth dimension (D) are much greater than the dimension of the thickness (E). For example, the thickness of the functional layer may be around 0.5mm, the length may be is around 1 cm and the depth may be is around 1 cm. The surface area of surfaces 120 and 160 (for example the dimension L x D) are large compared with the thickness of the functional layer and the cross -sectional surface area of the functional layer (for example the dimension E x D). The MEG is arranged in a vertical configuration with the functional layer disposed between the first electrode 130 and the second electrode 150. The first electrode 130 may be attached to a surface of the functional layer. The second electrode 150 may be attached to a surface of the functional layer.

In the vertical configuration the length and depth dimensions of the functional material and the electrodes are much greater than the thickness. The surface area of the length/width plane is much greater than the surface area of the cross-sectional surface area of the thickness/length or thickness/depth planes. This arrangement allows the surface area of the electrodes to be large and also the surface area of the interface between the electrodes and the functional layer to be large. This is a layered structure.

The first electrode 130 may be mounted onto a substrate. For example the substrate may be a flexible polyethylene terephthalate (PET) substrate

Preferably the first electrode 130 may have a similar material composition to the functional layer. For example, the first electrode 130 and the functional layer 110 may be carbon based.

The MEG is arranged to provide asymmetric exposure to moisture between the first surface 120 of the functional layer 110 and the second surface 160 of the functional layer 110 when the device is exposed to a moisture, for example in an environment having humidity. This arrangement can provide a moisture gradient across the functional layer when the device is exposed to moisture.

In the example of Figures 1 and 2, the MEG device is configured to promote absorption of moisture into the second surface 160 of the functional layer 110 and resist absorption of moisture into the first surface 120 of the functional layer 110. This configuration helps to create a moisture absorption differential between the surfaces of the functional layer when moisture is applied to the MEG device. This promotes an abundance of moisture absorbed into the second surface of the functional layer 160 and lack of moisture absorbed into the first surface of the functional layer 120.

In the embodiment of Figure 1 electrode 150 covers only a portion of second surface 160 of the functional layer. The electrode does not fully cover surface second surface 160. The remainder of the second surface of the functional layer may be uncovered. This allows direct contact of moisture onto the second surface of the functional layer. Larger second electrodes can result in larger current carrying capacity through the functional layer through the electrode joint. However, if the electrode is too large then water can be prevented from escaping from the functional layer and also moisture may be prevented from contacting surface through the electrode.

In further embodiments second electrode 150 may cover the whole of the second surface 160 of the functional layer. Such electrodes should be moisture absorbent and allow moisture to penetrate through the electrode and onto the functional layer. Such electrodes may be porous.

An example of a porous second electrode 150 is porous nickel foam (NF). The foam has a three-dimensional porous structure, which is beneficial for the moisture penetration. In addition, the high work function and good stability of NF will improve the overall performance of MEG device. Other examples of a porous second electrodes include porous silver nanowires.

In the embodiment of Figure 1 electrode 130 extends across the first surface of the functional layer. Electrode 130 covers the first surface of the functional layer.

By covering the first surface, electrode 130 reduces the penetration of moisture into the functional layer 110 through electrode 130. Preferably electrode 130 prevents penetration of moisture into the functional layer 110. Preferred electrodes have moisture insulating properties to resist penetration of moisture into the functional layer.

Preferably, first electrode 130 has moisture insulating properties, to resist penetration of moisture into the functional layer 110. The penetration of moisture through the first electrode 130 and into the functional layer is reduced by using an electrode with moisture insulating properties. Preferably the first electrode 130 prevents the penetration of moisture into the functional layer 110 through electrode 130. In the example of Figure 1 electrode 130 extends across the entire bottom surface of the functional layer.

The first electrode 130 may be any material. The electrode may be porous. The electrode may be hydrophobic. The electrode may be any material capable of acting as a storage for charge carriers. Preferably the electrode is a nanotube based material, preferably a carbon based material, capable of regenerating mobile charge carriers on the application of an electrical load to the device.

Examples of suitable material for the first electrode include carbon based materials, for example carbon nanotubes or graphene. Other suitable materials for the bottom electrode include FTO, ITO, MXene, Au, Pt and carbon black. The first electrode may include defects. The first electrode may include surface defects.

Additional resistance to moisture penetration can be provided by mounting the first electrode 130 onto a separate substrate. First electrode 130 may be mounted on a PET support. This can improve the moisture resistive properties of the bottom layer by requiring that any moisture penetrating the bottom layer 120 of the battery cell must first penetrate through the substrate and then penetrate through the first electrode 130 in order to penetrate into the functional layer.

In the example of Figure 1, electrode 130 covers the surface of the functional layer 110. As shown in Figure 1 the first electrode 130 extends across the full bottom surface of the functional layer 110. This configuration covers the entire surface from direct contact with moisture. As described above this helps reduce the penetration of moisture across the entire bottom surface of the cell.

The functional material may include carbon. The functional material may be a carbon-based material. The functional material may be GO/PVA.

For the bottom electrode, carbon-based materials with good electrical property and many functional groups are desired, such as carbon nano-tubes. This kind of carbon-based materials can form good interface with the functional layer (for example GO/PVA) due to similar material composition. The functional groups in the carbon-based bottom electrodes attract the charge carriers in the functional layer or work as charge carrier reservoir for electricity generation and recharging behaviour. In addition, CNT as a typical carbon-based material also exhibits excellent stability. In our study, better performance can be realized by using similar carbon-based materials as bottom electrode and functional layer.

Different from metal oxides, the bottom electrode is carbon-based material. Carbon based electrodes have similar material composition (carbon) to a carbon based functional material. And its functional groups in CNT can also facilitate the contact between CNT and, for example GO functional material, which can improve the interface.

The asymmetric moisture absorbing configuration provides that when the MEG device is exposed to moisture, the second surface of the functional layer is exposed to moisture while the first surface is resisted from exposure to moisture due to the water-proof layer with bottom electrode. The formed ion concentration gradient between the top and bottom electrodes leads to the generation of a vertical electric field, i.e. across the thickness (E) direction of the electrical device. And the power output can be enhanced in a device with a bigger top electrode. The electrical performance of the MEG device is also improved by selecting electrode materials with suitable mechanical properties. Improved electrical performance may be achieved by using electrodes comprising a material with similar mechanical properties to the functional layer, for example a material having a similar thermal expansion coefficient to the functional layer. By having a similar thermal expansion coefficient, the functional layer 110 and electrode 130 tend to expand and contract proportionally. This maintains adhesion between the composite layer 110 and the electrode 130 during use. A small amount of PVA between the GO and electrodes also improves the mechanical properties. This helps prolong usage of the MEG battery cell by preventing electrical contact failure and increased resistivity of the interface between the electrode and the composite layer over time.

In the example of Figure 1 the first electrode is carbon nanotube (CNT) film. The functional layer is GO/PVA. The interface between the first electrode and the GO/PVA film is enhanced by pre-pressing using a hydraulic machine.

A further benefit of the first electrode 130 extending across the entire surface of the functional layer 110 is that the contact area of the functional layer 110 and the electrode 130 is increased compared with an electrode which partially extending across the functional layer. This greater contact area results in an increased surface area of the electrical joint. The greater contact area can reduce the electrical resistivity of the joint.

Preferably, the bottom electrode 130 includes one or more of the following properties: moisture repellent, waterproof, moisture proof, soft, provides good adhesion on the interface with the composite layer, flexible, lightweight, similar thermal expansion coefficient to the composite layer, good adhesion on the GO/carbon nanotube interface, highly conductive, flexible.

The considerations for the top electrode 150 are different from those of bottom electrode 130. To produce a moisture gradient across the functional layer of the MEG battery cell, absorption of moisture into top surface of 160 of the functional layer is promoted.

In an embodiment shown in Figure 1, electrode 150 is configured to cover only a portion of top surface 160 of the functional layer. The remainder of the top surface is left uncovered and exposed to allow direct contact of moisture onto the top surface when the MEG battery cell is exposed to moisture. Electrode 150 may be porous. Electrode 150 may be porous to moisture. Larger top electrodes, having larger contact area with the functional layer, can result in larger current carrying capacity through the functional layer to electrode joint. However, if the electrode is too large then water can be prevented from escaping from the functional layer and also moisture may be prevented from contacting the surface through the electrode.

In the example of Figure 1, the second electrode is Nickel foam.

In the MEG electrical devices, the structure may be described as a vertical structure. The functional material is disposed between first and the second electrode. The structure is arranged as a sandwich arrangement with the functional material positioned between the first and the second electrodes. The first and second electrodes may contact he functional material.

In an illustrative embodiment, a moisture -electric generating cell is provided where the work function of one of the first or second electrodes is higher than the work function of the functional layer and the work function of the other of the first or second electrodes is lower than the work function of the functional layer, to create a work function gradient between the first electrode, functional layer and the second electrode.

Electrical performance of a moisture electric generating cell and selection of electrode material is also affected by the work function of the materials. GO has a work function of around 4.7 to 4.9eV.

Suitable electrodes configuration can induce a Schottky barrier at the electrodes/GO interface that can match well with the direction of diffusion of protons in GO, thus enhancing the voltage output. In particular, GO has a work function around 4.7 to 4.9eV, so a top electrode with smaller work function would prevent the recombination of electrons and protons. An example of a suitable material is zinc. Zinc which has a work function of 4.3 which is much smaller than that of GO. In an example embodiment, the top electrode is zinc foil, having a thickness of around 0.5 mm.

Preferably the bottom electrode has a work function higher than the GO/PVA functional layer. This creates a work function gradient across the MEG battery device from the first electrode to the GO/PVA functional layer to the second electrode.

The work function gradient may increase from the first electrode to the second electrode or increase from the second electrode to the first electrode. So the work function of one electrode is higher than the work function of the GO/PVA functional layer and the work function of the other electrode is lower than the work function of the GO/PVA functional layer. When the work function of one of the first or second electrodes is higher than the work function of the composite layer, and the work function of the other of the first or second electrodes is lower than the work function of the composite layer, a work function gradient is created between the first electrode, composite layer and the second electrode.

The functional group density (the number of functional groups per molecule) of the MEG electrical device can be tuned to change the electrical properties of the MEG electrical device. Functional group density can be tuned by acid treatment.

Discharge and Regenerate:

Figure 3 is a flow diagram illustrating method steps taken to charge, discharge and regenerate a MEG device.

At 310 a surface of the functional material is exposed to moisture. Depending on the configuration of the MEG device, the surface of the functional material may be exposed directly to moisture, for example the surface of the MEG device is uncovered and the MEG device is placed into an environment including moisture. For example, in the MEG device shown in Figure 1, areas of the top surface 160 are uncovered. When the MEG device of Figure 1 is exposed to a moisture environment, the uncovered areas of top surface 160 are exposed to moisture.

On exposure to moisture, charge carriers are generated within the functional material. Charge carriers are generated at or around the top surface 160 of the functional layer. This generation of charge carriers at step 310 represents an initial charging cycle for the MEG device. The initial charging cycle is represented in the circuit diagram of Figure 4 (a).

On exposure to moisture, in samples in which the bottom surface is covered with a second electrode, the bottom surface is not exposed to moisture.

Generation of charge carriers within the functional material creates an ion gradient across the device. Generation of the charge carriers within the functional material creates a potential difference across the device,

At 320 a load is applied across the device. When a load is applied to the device, the potential difference is depleted across the device. The potential difference is depleted via the generation of electricity. On applying a load to the device the potential difference and associated ion gradient of the device are depleted via the generation of electricity. When applying a load across the device the device is discharged. The discharge cycle is represented by the circuit diagram shown in Figure 4(b) in which the MEG is discharged across a load R. Charge carriers diffuse across the device. In some examples the device is discharged until a relatively stable current is achieved. In some examples, the sample may be discharged for a period of around 5 hours. At this point the current provided by the MEG may be too low to be applicable for typical MEG powered applications, for example IOT devices.

During and after discharge charge carriers may be bound within the device. These bound charge carriers provide a charge reservoir within the device. Charge carriers in the charge reservoir may mobilised on application of an electric field across the device. The bound charge carriers may be transported across the device on application of an electric field across the device.

The charge carriers may be bound by being trapped by defects within the electrical device. The charge carriers may be trapped in the first electrode. The charge carriers may be bound by being physically absorbed into the first electrode. For example, the charge carriers may bound by defects or vacancies or surface defects in the electrical device. These defects or vacancies or surface defects may be in the first electrode.

Other charge carriers may be recombined, for example and become neutral hydrogen.

At 330 an electric field is applied across the device to regenerate a potential difference across the device. The bound charge carriers may be released and transported through the device upon the application of an electric field to produce a potential difference across the device associated with an ion gradient across the device. The charge carriers may be transported across the functional material. A potential difference and associated ion gradient is regenerated across the device.

The bound charge carriers are released by the electric field providing them with sufficient energy to overcome binding energy binding them.

In some examples, the ion gradient of the device after application of an external electrical field is the same or greater than the ion gradient produced through the moisture-electric generation treatment of the device. The potential difference of the device after application of an external electric field is the same or greater than the poten6ial difference produced through the moistureelectric generation treatment of the device. Steps 310 and 320 may be repeated by applying a load across the device after a potential difference is regenerated to deplete the potential difference. The device may again be regenerated by applying an electric field across the device.

Step 310 may also be repeated by further exposing a surface of the functional material to moisture.

The regenerated moisture-electric generation device may include a functional material having a potential difference associated with an ion gradient across the functional material that is depleted after moisture-electric generation, where the depleted ion gradient has been regenerated by the application of an external electric field across the device.

The MEG device can be described as a capacitor having electrodes and a capacitor element comprising a functional material having had a potential difference associated with an ion gradient across the functional material that was generated and then depleted by moistureelectric generation and the depleted potential difference and ion gradient being capable of regeneration by the application of an external electric field across the device.

In one example device, referring to Figures 1 and 2, the functional layer 110 is a composite including a functional material and a binder. The functional material is graphene oxide (GO), the binder is a polymer, a preferred polymer is PVA. The functional layer is disposed between the first electrode 130 and second electrode 150. The first electrode 130 is carbon nanotube (CNT). The first electrode is attached to the first surface 120 of the functional layer 110, so there is an interface between the first electrode and the functional layer. The first electrode covers the first surface 120 of the functional layer 110. The first electrode is mounted on a PET substrate. The second electrode 150 is nickel foam. The second electrode is porous to moisture. The second electrode 150 is attached to a second surface 160 of the functional layer 110 and so there is an interface between the second electrode and the functional layer. In this example device the second electrode does not cover the whole surface area of the second surface of the functional layer, and so some of the second surface of the functional layer is exposed. The first surface 120 and second surface 160 of the functional layers are opposite surfaces of the functional layer as shown in Figures 1 and 2. The arrangement of this example device is a vertical structure where the interfaces of the electrodes with the functional layer are the large surface area faces of the functional layer in the length / depth plane, rather than the small cross sectional surface area faces of the functional layer in the thickness / length plane or the depth / thickness plane. The cycle for charging, storing and recharging the device is now described with respect to Figure 5. The gradient distribution of H+ drives diffusion and generates potential difference. At Figure 5(a) no moisture is applied to the device. At 5(b) moisture is applied to the MEG device. The first surface of GO attached to the carbon nanotube electrode is not exposed to moisture since the PET is impervious to moisture. The second surface of GO is exposed to moisture and moisture contacts the second surface. At Figure 5(b) moisture contacts the second (top) surface of the GO. Protons are released and accumulated in GO layer owing to the functional groups. At Figure 5(c) the protons diffuse. The protons (H+) diffuse to the CNT layer due to concentration gradient. CNT layer is positively charged, and internal voltage is built.

At 5(d) the device is discharged. An external load is attached across the electrodes. Electrons flow to CNT through external circuit, which neutralize the built-in voltage. Some protons are recombined with electrons to form neutralized hydrogen atoms, and some are trapped in CNT defects or vacancies. Electrons flow from GO to CNT layer owing to the potential gap, which eventually neutralize the built-in voltage, thereby reducing the current. After some time of discharge, the current reaches a relatively low and stable level.

At Figure 5(e) the device is regenerated. A voltage is applied to the electrodes. The proton and electrons are pushed back to GO owing to the applied voltage. A voltage is applied across the device using an external voltage source. The external voltage source may be a sourcemeter or other stable voltage source. A positive voltage is applied to the CNT electrode and a negative voltage is applied to the second electrode to create a potential difference across the sample. The positive potential at the first electrode pushes at least some of the bound H+ charge carriers across device. A higher applied voltage may push more H+ charge carriers across the device.

After removing the voltage at 5(f), again, H+ diffuses to the CNT layer due to concentration gradient, and the internal voltage is re-built. A further discharge across a load is shown in 5(g) and, again, electrons flow to CNT through external circuit due to the re-built internal potential. Further recharge and discharge cycles may be applied.

Results

Sample 1 (1X2 cm2):

Short-circuit current was measured at 60% humidity. Referring to Figures 6 and 7, an electric field (IV) is applied periodically for 5 cycles (every 10 min), and the sample generates instantaneous high current. For this sample, The discharge capacity for Cycle 1 and t2 is higher than the initial value, indicating charging with an applied field has positive impact on discharging. However, there is a dropping trend of the following cycles, in accordance with the decreasing trend of short-circuit current of new samples.

It’s worth noting that a higher electric field (2V) is applied for Cycle 6, and the discharge capacity rises again, meaning a higher charging voltage helps with energy storage and promotes discharging.

Positive values show discharge capacity, and negative values are charge capacity.

Sample 2:

Different to sample 1, the current was measured with 1 kQ loading at 60% humidity. The sample was discharged for 4h to achieve a relatively stable current with an end value of 42.67 uA. An electric field (IV) is applied periodically for 4 cycles (every 10 min). The sample generates instantaneous high current after charging, and the current could all be maintained higher than the initial 42.67 uA in 4 cycles. The results from Sample 2 are shown in Figure 8 and 9.

To eliminate the effect of humidity accumulation during the 10 min charging, a blank control test of Cycle 5 was performed. The measured current was much lower than the previous 4 cycle, confirming the high discharge current is not only from the electron accumulation during the charging waiting period, but mainly from charging via applied electric field. The discharge capacity for all the cycles is higher than the initial value, even for the last cycle that only rest in the 60% humidity without applying electric field.

It could be concluded that there are two charging ways included in the process: First is the natural charging process, i.e., to rest in humid atmosphere without current output. This process helps water molecules to accumulate and could be applied in devices requiring pulse current output. Second is the electric charging process. It is surprising to find that this strategy is efficient and the as-made MEG could work as a capacitor that stores energy.

Sample 3( 6X6 cm2) Shown in Figures 10 and 11 cycles were tested to study the charge and discharge process (DO stands for the initial discharge round immediately after continuous current test for 19 hours; D1-D8 represent discharge cycle 1-8; C1-C8 are charging cycle 1-8. All the discharging cycles are set to be 10 min. The current was measured with 1 kQ loading at 80% humidity. The sample was discharged continuously for 19h to achieve a relatively stable current with an end value of 31.86 uA.

The sample generates instantaneous high current after each charging cycles, and the current could all be maintained higher than the initial 31.86 uA. Compared to DO, the increased discharge capacity (D1-D8) benefits from:

The natural charging process (Cycle 7 and Cycle 8, i.e., to rest in humid atmosphere without current output), which helps water molecules to accumulate.

The electric charging process by applied voltage, which is more efficient and is affected by the value of applied voltage (0.5 V for Cycle 5, 1.5 V for Cycle 6, and 1 V for the others) and charge duration (20 min for Cycle 3 and Cycle 4, and 10 min for the others). Compared to DI and D2, charging for a longer period (C3 and C4) results in more increased capacity as shown in D3 and D4, indicating charging duration has a positive correlation to the discharge performance; Lower/higher charging voltage (C5 and C6) leads to a less/more increased capacity as shown in D5 and D6, demonstrating discharge capacity is positively associated with charging voltage.

Sample 4 (6x6 cm2, acid treated GO)

Loading: 15 Q. Humidity: -80% RH.

The sample was first discharged continuously (nearly completely) for 50 h to achieve a stable current before charging and discharging process. Charging process: applied voltage of IV; lasting for 10 min. Discharging process: testing for 4 h immediately after the above-mentioned charging process. The charge and discharge curve of Sample 4 is shown in Figure 12.

The sample was discharged continuously for 50 h to achieve a relatively stable current with an end value of 120 uA. The sample generates instantaneous high current after charging, and the current could be maintained higher than the initial end value of 120 uA for 2.7 h. Sample 5: (1.6*1.7=2.72cm2, Au as bottom electrode to replace CNT)

Loading: 15 Q. Humidity: -80% RH. The sample was first discharged continuously for 5 h to achieve a stable current before charging and discharging process. Charging process: applied voltage of IV; lasting for 10 min. Discharging process: testing for 4 h immediately after the above-mentioned charging process. The charge-discharge curve of Sample 5 is shown in Figure 13.

The sample generates instantaneous high current after charging, and the current could be maintained higher than the initial end value of 38.82 uA for 30.7 min. Compared to previously tested regular CNT-GO samples, Au, as the positive electrode, does not work efficiently as CNT in two aspects: The peak current is much lower (~0.1 mA compared to >0.5 mA);

The discharge interval higher than end value is not as long as expected (usually a lower end current, corresponding to a more complete initial discharge, should lead to a longer interval. This is because Au does not have a high hydrogen binding energy and superior hydrogen storage ability as CNT, and it is restricted as the positive electrode.

For the first time, we demonstrated that moisture electric generator can be regenerated by an external power source, similar to conventional rechargeable batteries. A very small voltage (~1V) and short time (~10min) can regenerate MEG to a high capacity. The MEG device can store charge carriers. The carriers may be bound, for example at the first electrode. One of the possible mechanisms is carbon nanotube first electrode can act as hydrogen reservoir, so during natural charging process (harvesting energy from the moisture), the protons are generated and diffuse to the surface of carbon nanotubes to generate potential difference. The charge carriers may be bound at defects or surface defects or vacancies. To push proton back to the GO side, a voltage is applied across the device using an external voltage source. The external voltage source may be a sourcemeter or other stable voltage source. A positive voltage is applied to the first electrode (for example carbon nanotube) and a negative voltage is applied to the second electrode to create a potential difference across the sample. The positive potential at the first electrode pushes at least some of the bound H+ charge carriers across device. Higher potential difference may push more H+ charge carriers across the device. On the other hand, MEG itself can act as a capacitor which can storage some charge. Our comparing experiments indicate that if Au was used as bottom electrode, the charging performance is much lower as Au cannot hold proton. A candidate material for the first electrode is carbon nanotubes (CNT), compared with metal electrodes the CNT with a high surface area can easier reserve charge carriers for a high capacity.

The examples provided are gradient structures. Asymmetric and heterogeneous MEG structures may also be recharged.

The MEG device capable of storing charge carriers is regarded as a capacitor. The porous CNT structure with large surface- volume ratio might be able to trap electrons or protons and prevent the recombination to some extent. Higher humidity could boost natural recharging, which adds on the electrical recharging and leads to a high columbic efficiency (CE) above 100%.

In example MEG devices which include a carbon-based functional layer (for example GO) and a carbon based electrode (for example carbon nano-tube), the MEG device is a combination of two carbon materials, which are suitable for capacitor design. The capacitance of this structure, is able to “bind” some charge carriers and ready to send them reversely upon applied electrical field.

When the MEG exhibits capacitance behaviour, it can be recharged easily. For a typical capacitor, porous functional materials with a high surface area are desired for a high capacity, and the charge carrier mobility needs to be reduced. For the GO, capacitor performance can be realized through functionalization, and functional groups are needed such as those that may be present in GO or oxidised CNT. In addition, moisture is also required, which will help the dissociation of functional groups and generation of protons. For the bottom electrode, it can be waterproof or just deposited onto a waterproof substrate to avoid the moisture penetration. For the top electrode, porous structure can benefit the moisture intake for electricity generation. In addition, better performance can be expected with asymmetrical electrodes. For example, higher work function difference will improve the performance.

The MEG device may not be moisture saturated or water saturated. Initially it is dry when exposed to moisture.

CNT could be specially treated to create more vacancies to trap protons/electrons and diminish recombination. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, namely, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

It is to be understood that the aforegoing description refers merely to preferred embodiments of invention, and that variations and modifications will be possible thereto without departing from the spirit and scope of the invention, the ambit of which is to be determined from the following claims.