Moisture-driven energy harvester based on two-dimensional vanadium pentoxide nanosheets

Abstract

A moisture-driven electrical generator includes a vanadium-pentoxide membrane. In some embodiments, it includes the use of a nanofluidic membrane of reconstructed vanadium pentoxide nanosheets to harvest energy from ambient moisture for use in a moisture-driven electrical generator. The vanadium pentoxide membrane is fabricated by using vacuum filtration to restack vanadium pentoxide nanosheets.

Description

[0001] MOISTURE-DRIVEN ENERGY HARVESTER BASED ON TWO-DIMENSIONAL VANADIUM PENTOXIDE NANOSHEETS

[0002] CROSS REFERENCE TO RELATED APPLICATIONS

[0003] This application claims the benefit of the filing date of U.S. Application No. 63 / 602,854, filed on November 27, 2023, the contents of which are hereby incorporated by reference in their entirety.

[0004] STATEMENT OF GOVERNMENT RIGHTS

[0005] This invention was made with government support under grant W911NF-22- 1-0239 awarded by the US Army and under grant 1935555 awarded by the National Science Foundation. The government has certain rights in the invention.

[0006] FIELD OF INVENTION

[0007] The invention relates to extraction of electrical energy from water molecules in the natural environment.

[0008] BACKGROUND

[0009] It has been found that certain inorganic materials with extensive surface areas, abundant voids, high hydrophilicity, and charged surfaces are useful for use as moisture-driven electrical generators. Examples of useful materials include graphene, porous carbon films, and metal oxides.

[0010] However, it is worth noting that most moisture-driven electrical generators based on these inorganic materials tend to be cost-prohibitive and offer limited power output sustainability. In typical cases, a significant drop occurs after only about fifty seconds of continuous operation. This hinders their practical applicability.

[0011] A moisture-driven electrical generator’s operation depends a great deal on the reversible transition between hydration and dehydration states. This transition leads to the release of preformed ionizable groups. When exposed to moisture, a concentration gradient occurs. This gradient causes the hydrated ions to migrate from regions of high concentration to those with lower concentrations. This, in turn, develops electrical energy.

[0012] To take advantage of this phenomenon, a variety of moisture-driven electrical generators have been developed using two-dimensional materials, such as graphene oxide and a variety of Mxenes. These systems generate power in response to changes in humidity. However, the tightly packed stacking structure, substantial size, and limited active sites of such two-dimensional sheets tend to impede adsorption of moisture and the transport of hydrated ions.

[0013] SUMMARY

[0014] The invention concerns the use of a nanofluidic membrane of reconstructed vanadium pentoxide nanosheets to harvest energy from ambient moisture for use in a moisture-driven electrical generator. The vanadium pentoxide membrane is fabricated by using vacuum filtration to restack vanadium pentoxide nanosheets. Owing to their outstanding proton conductivity, vanadium-pentoxide-based devices display high energy density. Moreover, the nanofluidic membrane of vanadium pentoxide has the unique property of being able to heal structural damages when in the presence of a water droplet. This promotes its longevity.

[0015] In one aspect, the invention features a moisture-driven electrical generator that includes a cation-selective membrane that includes vanadium-pentoxide.

[0016] Among the embodiments are those in which the generator further includes first and second electrodes, the first electrode being a perforated electrode. In such embodiments, the membrane is disposed between the electrodes. As a result, the perforations expose a face of the membrane.

[0017] In other embodiments, the generator further includes an insulator having an opening. In such embodiments, the cation-selective membrane is disposed in the opening.

[0018] In still other embodiments, the membrane includes nanochannels (32) extending therethrough.

[0019] Also among the embodiments are those in which the membrane includes a stack of nanosheets, each of which includes a crystal having vanadium pentoxide repeating units.

[0020] Further embodiments of the generator include those having an ion-infused gel in contact with the cation-selective membrane. Among these are embodiments in which the gel is a hydrogel and embodiments in which the gel is an aerogel. Still other embodiments include those that have first and second compartments, each of which includes a gel that has been infused with an ion. In such embodiments, the cation-selective membrane separates the first and second compartments and also permits ions to pass therethrough in response to a difference in concentrations of the ion in the first and second compartments.

[0021] Also among the embodiments are those that include an anion-selective membrane. In such embodiments, an ion-infused gel separates the anion-selective membrane and the cationselective membrane.

[0022] Still other embodiments include those having first, second, and third compartments, each of which includes a gel that has been infused with a cation and an anion. In these embodiments, the cation-selective membrane separates the first and second compartments and the anionselective membrane separates the second and third compartments. The cation-selective membrane permits the cation to pass from the first compartment to the second compartment in response to a difference in concentrations of the cation in the first and second compartments. Meanwhile, the anion- selective membrane permits the anion to pass from the third compartment into the second compartment in response to a difference in the anion’s concentration in the second and third compartments.

[0023] Among the embodiments are those in which the generator also includes a nickel hydroxide membrane. In these embodiments, an ion-infused gel separates the nickel hydroxide membrane and the cation-selective membrane.

[0024] Other embodiments include an ion-infused gel and electrodes in contact with the ion- infused gel. In such embodiments, the cation-selective membrane is disposed between the electrodes and embedded in the ion-infused gel.

[0025] In another aspect, the invention features a method of manufacturing a moisture-driven electrical generator. Such a method includes manufacturing a cation-selective membrane that includes a stack of vanadium-pentoxide layers and providing electrodes in electrical communication with the cation- selective membrane. Among the practices of the invention are those in which manufacturing the cationselective membrane includes exfoliating bulk vanadium pentoxide to form a dispersion of vanadium pentoxide nanosheets and vacuum filtering the dispersion to stack the vanadium pentoxide nanosheets. Among these are embodiments in which the exfoliant is hydrogen peroxide.

[0026] Still other practices include providing an insulating sheet having a cutout sized to accommodate the cation-selective membrane, inserting the cation-selecting membrane in the cutout, and placing the electrodes on either side of the insulating sheet. In such practices, one of the electrodes includes perforations to permit fluid to reach the cation-selecting membrane.

[0027] Still other practices include infusing a gel with ions to produce an ion-infused gel and embedding cation-selective membrane in the gel. In such practices, the cation-selective membrane permits ions to cross from ion-infused gel on one side of the cation-selective membrane to ion-infused gel on another side of the cation-selective membrane.

[0028] These and other features of the invention will be apparent from the following detailed description and the accompanying figures, in which:

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 shows an exploded view of a generator;

[0031] FIG. 2 shows two nanolayers from the ion-selective membrane shown in FIG. 1;

[0032] FIG. 3 shows steps in manufacture of the ion-selective membrane shown in FIG. 1 ;

[0033] FIG. 4 shows the generator of FIG. 1 in assembled form;

[0034] FIG. 5 shows a single-cell generator that relies on an ion-selective membrane as shown in FIG. 1;

[0035] FIG. 6 is a multi-cell generator that relies on ion-selective membranes of alternating polarity; and

[0036] FIG. 7 is an isometric view of a multi-cell generator similar to that shown in FIG. 6. DETAILED DESCRIPTION

[0037] FIG. 1 shows an exploded view of a moisture-driven generator 10 comprises an ion- selective membrane 12, a perforated electrode 14, a solid electrode 16, and an insulator 18 that separates the perforated electrode 14 from the solid electrode 16. The insulator 18 includes an opening 20 that faces perforations 22 in the perforated electrode 14. The ion-selective membrane 12 is sized and shaped to fit inside the opening 20. In a preferred embodiment, the electrodes 14, 16 comprise copper in metallic form and the insulator 18 comprises polyethylene terephthalate.

[0038] The ion-selective membrane 12 is either cation selective or anion selective. In the former case, the ion-selective membrane 12 permits only cations to cross; in the latter case, the ion- selective membrane 12 permits only anions to cross. As used herein, cation-selective membranes and anion-selective membranes will be referred to as having opposite polarities. For ease of exposition, those ions that are not permitted to cross the ion-selective membrane 12 will be referred to as “counter-ions” and those that are permitted to cross the ion-selective membrane 12 shall be referred to as “co-ions.”

[0039] The ion-selective membrane 12 includes an upper face 24 and a lower face 26. The upper face 24 comprises portions that are exposed through the perforations 22 and portions that make electrical contact with the perforated electrode 14. The lower face 26 of the ion-selective membrane 12 makes electrical contact with the solid electrode 16.

[0040] Referring to FIG. 2, the ion-selective membrane 12 comprises a stack of ultra-thin nanosheets 28, 30, two of which are shown. In the illustrated embodiment, each nanosheet 28, 30 is a crystalline arrangement whose repeating unit 34 is vanadium pentoxide. When stacked, the nanosheets 28, 30 form nanochannels 32.

[0041] In a preferred embodiment, the nanosheets 28, 30 result from having exfoliated a precursor 36 with an exfoliant and re-stacking the resulting nanosheets 28, 30, as shown in FIG. 3. In one embodiment, the precursor 36 comprises vanadium pentoxide and the exfoliant is hydrogen peroxide. A solution of thirty-percent hydrogen peroxide is sufficient for exfoliation when starting with powdered vanadium-pentoxide precursor 36. Exfoliation results in a dispersion of nanosheets 28, 30. This dispersion is then filtered over a polytetrafluorethylene support membrane to yield a flexible and free-standing ion-selective membrane 12. The re- stacking step is carried out by vacuum filtration. Observation of the membrane’s ion-transport properties and X-ray diffraction provide a way to confirm the formation of the nanochannels 32 in the ion-selective membrane 12.

[0042] In a preferred embodiment, exposure to hydrogen peroxide is carried out under ice-cold conditions. In one practice, powdered vanadium-pentoxide precursor 36 is dissolved in deionized water, placed in an ice bath, and exposed to hydrogen peroxide. This results in vigorous bubbling and formation of precipitate. The precipitate slowly gels. This gel is then diluted with water and bath sonicated for more than half an hour. This gives a homogeneous dispersion of vanadiumpentoxide nanosheets 28, 30. The resulting dispersion is then vacuum filtered over a polytetrafluorethylene filter with a one-hundred nanometer average pore size and allowed to dry in the ambient atmosphere to obtain the freestanding ion-selective membrane 12.

[0043] The vacuum-assisted filtration process create spaces between vanadium pentoxide nanosheets 28, 30. Through the use of atomic force microscopy, it was found that the lateral size distribution of the vanadium pentoxide nanosheets 28, 30 ranges from one-hundred nanometers to one-hundred-twenty nanometers, with an average thickness of between three and six nanometers. The ^-potential analysis of the vanadium pentoxide dispersion yielded a negative voltage of forty millivolts. This suggests a prevalence of negative surface charge on the membrane’s upper surface 26 and also provides a basis for determining the electrostatic behavior of the nanosheets 28, 30.

[0044] The foregoing vacuum filtration process results assembles the nanosheets 28, 30 into a well-organized lamellar structure to form a freestanding and highly adaptable ion-selective membrane 12. An X-ray diffraction peak at 1.2 nanometers, which corresponds to the interlayer spacing in the crystal’s 001 -plane, suggests highly ordered stacking of the nanosheets 28, 30 within the ion-selective membrane 12.

[0045] The intercalated water molecules expand the layered structure in the restacked vanadium pentoxide nanosheets 28, 30. This results in network of hydrated vanadium-pentoxide nanochannels 32. These nanochannels 32, which are negatively charged, are useful for promoting ion transport through the ion-selective membrane 12. As is apparent from FIG. 2, the vanadium pentoxide nanosheets 28, 30 develops an orthorhombic crystal structure. This results in relatively weak bonding between the nanosheets 28, 30. The crystal structure includes zigzagging double-chains that connect to each other via an oxygen atom, referred to as a “bridge oxygen.” This contributes to the formation of square -based pyramids around each vanadium atom. The oxgyen atoms promote conductivity across the ion- selective membrane 12 and in particular, to the ability to transport ions through the ion-selective membrane 12.

[0046] When exposed to humid air, an ion-selective membrane 12 along the lines of the foregoing generates an open-circuit voltage and a short-circuit current. In one example, a one- square-centimeter ion-selective membrane 12 having an overall thickness of sixty-four micrometers exposed to air with eighty-five percent relative humidity generates an open-circuit voltage of slightly less than half a volt and a short-circuit current of three-hundred nanoamperes.

[0047] In particular, when water molecules contact the upper face 24, significant charge develops. The resulting electric field becomes strong enough to split water molecules into hydroxide ions and hydronium ions. The resulting hydronium ions then cross the ion-selective membrane 12. This mobility ultimately results in electric current.

[0048] Because they are so thin, the nanosheets 28, 30 are vulnerable to mechanical breakage. The resulting fractures impair the membrane’s ability to operate efficiently. A particularly useful property of the ion-selective membrane 12 made from vanadium pentoxide is its ability to be healed simply by exposure to water.

[0049] FIG. 4 shows the components in FIG. 1 assembled to form a sandwich in which the exposed portions of the membrane’s upper face 24 are exposed to water molecules through the perforations 22. The water molecules interact with the exposed upper face 24 to induce a negative charge thereon.

[0050] On exposure to moisture, the membrane’s upper face 24 develops a negative charge, thus creating a charge differential between the upper face 24 and the lower face 26. As a result, an electric field forms across the ion-selective membrane 12 between the upper face 24 and the lower face 26. This electric field promotes migration of ions across the ion-selective membrane 12 using the nanochannels 32.

[0051] The foregoing ion-selective membrane 12 permits crossing of cations. The counter-ions are thus anions and the co-ions are cations. However, a similar procedure enables construction of an ion-selective membrane 12 that has the opposite polarity, i.e., one in which the counter-ions are cations and the co-ions are anions.

[0052] A process for constructing an ion-selective membrane 12 that is selective for anions begins by exfoliation of a layered double -hydroxide, i.e., a two-dimensional layered nanomaterials belonging to the anionic clay family. The layered double-hydroxide can be synthesized by coprecipitation, hydrothermal synthesis, microwave treatment, or ion exchange. However, a particularly fast and simple method is to carry out microwave-assisted synthesis of the nanosheets 28, 30.

[0053] A particularly useful precursor 36 is nickel hydroxide. This results in nickel-hydroxide nanosheets.

[0054] The nickel hydroxide (or Ni(OH)2) nanosheets 28, 30 for the fabrication of the anionselective membrane 12 will be synthesized by stirring NiCh ftO in water at room temperature, adding ammonia solution, and exposing the result to microwaves. The resulting product is then isolated by centrifuge, washed with distilled water and alcohol, and dried. This is followed by dispersal in water and filtration as described above.

[0055] A typical method includes dissolving 2 mmol of NiC12-6H2O in 200 mL of water with vigorous stirring for ten minutes at room temperature followed by adding 2 mL of twenty-eight percent ammonia solution to form a transparent solution after vigorous stirring. The resulting solution is then transferred into a 500 mL three-neck flask and treated under microwave irradiation in a microwave reactor at 100°C for thirty minutes at six hundred watts. The resulting product is then isolated by centrifuge, washed with distilled water and ethanol three times, and then dried in warm air overnight for further characterization. The nanosheets 28, 30 are then dispersed in water and filtered over a polytetrafluorethylene membrane to get the freestanding anion selective membrane. Detailed characterization of the structural morphology of the Ni(OH)2 sheets will be achieved by atomic force microscopy, transmission electron microscopy, and field emission scanning electron microscopy. The surface of the exfoliated Ni(0H)2 sheets is then characterized by (^-potential analyzer, X-ray diffraction, infrared, and ultraviolet- visible spectroscopy.

[0056] Connecting the upper face 24 of the ion-selective membrane 12 to the perforated electrode 14 and the lower face 26 of the ion-selective membrane 12 to the solid electrode 16 results in an interface for electron transfer. This arrangement promotes conversion of mechanical energy derived from ion movement into electrical energy. Connecting individual generators 10 as described herein provides a basis for controlling voltage and current output as desired.

[0057] The potential difference and output current values are believed to arise from continuous diffusion of water molecules from the perforated electrode 14 to the solid electrode 16. Vanadium pentoxide nanosheets 28, 30, being oxides, tend to have hydroxyl groups on their exposed surfaces and edges. Notably, the extent of the vanadium pentoxide nanochannels (approximately 1.3 nanometers) is significantly smaller than the Debye length of deionized water. The presence of surface-charge-govemed ionic conductivity affirms that the electrical double layers of opposing walls of vanadium pentoxide overlap, thus influencing the transport characteristics of the nanochannels 32. In these conditions, the concentration of ionic species within the nanochannels 32 is determined by the surface charges of the vanadium pentoxide sheets.

[0058] When water molecules move through the nanochannels 32, these hydroxyl groups dissociate. This results in hydronium ions and hydroxide ions. The negative charges on vanadium pentoxide, which arise from the water-assisted dissociation of surface hydroxyl groups, attract counterions and repel co-ions, establishing an electrical double-layer at the vanadium pentoxide / water interface. The negative surface charges of the vanadium pentoxide nanochannels 32 selectively allow the passage of hydronium ions. The continuous diffusion of charged species, propelled from the top to the bottom electrode, maintains a constant potential difference between the two electrodes 14, 16.

[0059] FIG. 5 shows a single cell 38 of an osmotic generator 40. The cell 38 includes an ion- selective membrane 12 embedded in an ion-infused gel 42 with first and second electrodes 44, 46, disposed on either side of the ion-selective membrane 12 and in contact with the ion-infused gel 42. In some embodiments, the electrodes 44, 46 take the form of a sheet coated with an ink that provides ions. Examples of materials for use in such a sheet include polymer and paper. Examples of a suitable ink include silver and silver-chloride ink.

[0060] The resulting flux of co-ions across the ion-selective membrane 12 causes a salinity gradient between a high-concentration compartment 48 surrounding the first electrode 44 and a low-concentration compartment 50 surrounding the second electrode 46. This results in a potential energy difference across the ion-selective membrane 12. When the first and second electrodes 44, 46 are connected via a load 52, this potential energy difference drives a current between the electrodes 44, 46. In a preferred embodiment, the concentrations in the high- concentration compartment 48 and that in the low-concentration compartment 50 differ by a factor of a thousand.

[0061] FIG. 6 shows an osmotic generator 40 having plural cells 38. In the multi-cell osmotic generator 40 of FIG. 6, the membranes 12 alternate between being cation selective and anion selective. In one embodiment, the membranes 12 alternate between those comprising vanadium pentoxide and those comprising nickel hydroxide. Connecting a load 52 between the first and second electrodes 44, 46 pennits current to flow through the load 52.

[0062] FIG. 7 shows an isometric view of an osmotic generator 40 having two complementary membranes 12, i.e., a cation-selective membrane and an anion-selective membrane. The central compartment between the two membranes 12 is a high-concentration compartment 48 from which cations and anions flow towards lower concentrations thereof in opposite directions, thereby causing an electric field to come into existence in much the same way that it would in a PN junction.

[0063] In the illustrated embodiment, the ion-infused gel 42 is a hydrogel infused with potassium ions and chloride ions. Other embodiments feature the use of sodium ions instead of potassium ions. The use of sodium or potassium ions is advantageous because they are small enough to pass easily through the nanochannels. An ion-infused gel 42 in the form of a hydrogel is particularly useful because its high water-content promotes absorption and retention of ions, thus providing an ideal environment for ion storage. Additionally, hydrogel is amenable to being tailored to exhibit selective ion binding, thereby enabling targeted capture and release of specific ions. Moreover, hydrogel offers biocompatibility, low cost of synthesis, easy availability, high water retention, and a matrix that promotes efficient diffusion of ions, thereby promoting osmosis and ensure a steady ionic flow.

[0064] A suitable procedure for preparing a hydrogel for use in the generator shown in FIG. 5 includes crosslinking gelatin in a solution having the desired ionic species and concentration. The solution is then placed into a cast and gelled for about twenty minutes at 4°C. The resulting ion-infused gel 42 is then inspected by a field emission scanning electron microscope to confirm that adequate porosity has been achieved. It is then inspected by a universal testing machine to confirm that it has the desired mechanical properties, such as strength and flexibility. The ionic conductivity of the ion-infused gel 42 is then measured using an electrochemical measurement device.

[0065] In other embodiments, the ion-infused gel 42 is an aerogel. This results in a dry cell that has the ability to generate power in response to exposure to an electrolyte-laden liquid such as those excreted during various biological processes.

[0066] A particular advantage of using an aerogel as the ion-infused gel 42, in addition to light weight, is that the ions remain fixed to the aerogel matrix when no electrolyte is present. As a result, ions only flow when the generator 40 is actively being used, i.e., only when a liquid carrying electrolytes is present. This avoids degradation as a result of continuous ion diffusion across the ion-selective membrane 12.

[0067] A suitable method of ionic aerogel is to begin by freeze-drying an ion-infused hydrogel, for example by freezing ion-infused hydrogels of appropriate ionic concentration at -80°C for six hours followed by freeze-drying to form an aerogel.

[0068] The overall structure of a generator that relies on an aerogel is similar to that already discussed in connection with generators that rely on a hydrogel. Such a generator is particularly useful for powering devices that are to be activated only in the presence of an electrolyte-laden liquid, such as perspiration, gastric juices, saliva, and urine.

[0069] An ion-selective membrane 12 comprising vanadium pentoxide is particularly advantageous because of its high proton conductivity, its flexibility, and the ease with which it can be fabricated. In addition, fractures in vanadium pentoxide nanolayers as described herein are easily repaired by exposing the fractures to liquid water. A ion-selective membrane 12 that comprises nickel hydroxide is advantageous because of its high anion selectivity and its stability in water.

[0070] Other possibilities for use as an ion-selective membrane 12 include those with both high surface charge and ultrafine nanochannels to promote transport of co-ions. Examples of such materials include graphene oxide, Mxenes, and various metal oxides.

[0071] Examples of suitable loads 52 include electronic devices that would otherwise use conventional batteries. Examples of such devices include medical devices and diagnostic sensors such as blood glucose meters, digital thermometers, portable ECG monitors, handheld pulse oximeters, and ingestible devices.

[0072] Osmotic generators 40 as described herein provide a way to replace non-recyclable lithium-ion batteries in many applications. As such, the osmotic power generators described herein contribute to environmental sustainability by reducing electronic waste and minimizing pollution associated with traditional battery disposal. In addition, the materials required for manufacturing osmotic power generators are abundant, far more so than lithium. In addition, gelbased osmotic power generators offer enhanced safety and reliability. For instance, an osmotic power generator as described herein is not at risk of bursting into flame in the same way as a lithium-ion battery.

[0073] Additional details are set forth in the accompanying appendices, which are incorporated herein by reference.

Claims

CLAIMS1. An apparatus comprising a generator (10), said generator being a moisture-driven electrical generator that comprises a cation-selective membrane (12), wherein said cationselective membrane comprises vanadium-pentoxide.

2. The apparatus of claim 1, wherein said generator further comprises first and second electrodes (14, 16), wherein said membrane is disposed between said first and second electrodes and wherein said first electrode is perforated to expose a first face (24) of said membrane.

3. The apparatus of claim 1, wherein said generator further comprises an insulator (18) having an opening and wherein said cation-selective membrane is disposed in said opening.

4. The apparatus of claim 1, wherein said membrane comprises nanochanncls (32) extending therethrough.

5. The apparatus of claim 1, wherein said membrane comprises a stack of nanosheets (30), each nanosheet comprising a crystal having vanadium pentoxide repeating units (34).

6. The apparatus of claim 1, wherein said generator comprises an ion-infused gel (42) in contact with said cation-selective membrane.

7. The apparatus of claim 1, wherein said generator comprises an ion-infused hydrogel (42) in contact with said cation-selective membrane.

8. The apparatus of claim 1, wherein said generator comprises an ion-infused aerogel (42) in contact with said cation-selective membrane.

9. The apparatus of claim 1, wherein said generator further comprises a first compartment (48) and a second compartment (50), each of which comprises a gel that has been infused with an ion, wherein said cation- selective membrane separates said first and second compartments, and wherein said cation- selective membrane permits said ion to pass therethrough in response to a difference in concentrations of said ion in said first and second compartments.

10. The apparatus of claim 1, wherein said generator further comprises an anion-selective membrane and wherein said anion- selective membrane and said cation-selective membrane are separated from each other by an ion-infused gel.

11. The apparatus of claim 1, wherein said generator further comprises a first compartment, a second compartment, and a third compailment, each of which comprises a gel that has been infused with a cation and an anion, wherein said cation- selective membrane separates said first and second compartments, wherein said anion- selective membrane separates said second and third compartments, wherein said cation-selective membrane permits said cation to pass from said first compartment to said second compartment in response to a difference in concentrations of said cation in said first and second compartments, and wherein said anion- selective membrane permits said anion to pass from said third compartment into said second compartment in response to a difference in concentration of said anion in said second and third compartments.

12. The apparatus of claim 1, wherein said generator further comprises a nickel hydroxide membrane and wherein said nickel hydroxide membrane and said cation-selective membrane are separated from each other by an ion-infused gel.

13. The apparatus of claim 1, further comprising an ion-infused gel and electrodes in contact with said ion-infused gel, wherein said cation- selective membrane is disposed between said electrodes and embedded in said ion-infused gel.

14. A method comprising manufacturing a generator, said generator being a moisture-driven electrical generator, wherein said method comprises manufacturing a cation- selective membrane that comprises a stack of vanadium-pentoxide layers and providing electrodes in electrical communication with said cation-selective membrane.

15. The method of claim 13, wherein manufacturing said cation-selective membrane comprises exfoliating bulk vanadium pentoxide to form a dispersion of vanadium pentoxide nanosheets and vacuum filtering said dispersion to stack said vanadium pentoxide nanosheets.

16. The method of claim 13, wherein manufacturing said cation-selective membrane comprises exposing bulk vanadium pentoxide to hydrogen peroxide to form a dispersion of vanadium pentoxide nanosheets and vacuum filtering said dispersion to stack said vanadium pentoxide nanosheets.

17. The method of claim 13, further comprising providing an insulating sheet having a cutout sized to accommodate said cation-selective membrane, inserting said cation-selecting membrane in said cutout, and placing said electrodes on either side of said insulating sheet, wherein one of said electrodes comprises perforations to permit fluid to reach said cation-selecting membrane.

18. The method of claim 13, further comprising infusing a gel with ions to produce an ion- infused gel and embedding cation- selective membrane in said gel, wherein said cationselective membrane permits ions to cross from ion-infused gel on one side of said cationselective membrane to ion-infused gel on another side of said cation-selective membrane.

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