Description:FIELD OF THE INVENTION
[0001] The present invention relates to the integration of atmospheric water harvester (AWH) technology with hydrovoltaic (HV) systems to harvest freshwater and electricity simultaneously from the humid atmosphere even in the arid regions. In particular, the present invention is related to the incorporation of a novel nanocomposite material into the above integrated system, where the nanocomposite material is fabricated by attaching sulfonic groups to the graphitic oxide substrate (SGhO).

DESCRIPTION OF THE RELATED ART
[0002] Water and electricity are the major concerns to the scientific community to fulfil the necessity for human mankind. Freshwater makes up less than 3% of Earth's water, and even this small amount is often unusable due to pollution from industries. Therefore, attempts at freshwater production using accessible energy sources have been remarkably highlighted in the past few years. Notably, nanostructure-based materials were triggered into a considerable possibility of highly efficient freshwater production using the sun as a free source, even though it needed the impure source water for freshwater production.
[0003] An advanced technique called atmospheric water harvesting (AWH) brings a straightforward solution to collect water molecules from a humid environment. Inspired by natural phenomena like beetles and plants that collect moisture, AWH offers a cost-effective solution for water-scarce regions. However, current methods often consume a lot of energy. This challenge is addressed by incorporating hygroscopic salts into a 3D bulk metal–organic framework, which can absorb moisture and then evaporate it through continuous water capture-release cycles during light on-off situations, even though it follows a lot of complex synthesis methods to incorporate hygroscopic materials in the substrate. For instance, a thermoelectric cell integrated with a hydrogel containing hygroscopic material has been developed, as well as a hollow nanocapsule with superhydrophilic photothermic behavior.
[0004] Simultaneously, remarkable developments in electronic devices in the present century require low-power electricity for usage. Several renewable methods are very popular, such as thermoelectric, piezo electric, photovoltaic, etc. However, these methods require cost fabrication and external energy support to perform. An advanced green energy-based method called hydrovoltaics has recently been popularized to generate electricity from water itself. This water may be in the form of moisture, rain, or any source. When water flows through any surface-rich moieties substrate, protonation occurs with the negative moieties of substrates, creating an internal electric field called a specific electric double layer (EDL) formation. The variation of EDL occurs during the flow of water, creating an overall potential across the device called streaming potential or hydrovoltaic potential. During the movement of the proton (H⁺) through the moieties, its respective electrons flow through an external circuit called streaming current. This mechanism was observed and confirmed through experimental data. However, the exact mechanism of hydrovoltaic still needs several theoretical steps and understanding. This gives us an excellent idea to harvest moisture from the environment and its corresponding flow to generate power. The absorbed water molecules can evaporate and collect as freshwater when supplying light illumination. Therefore, the substrate should have a high light absorptivity for evaporating the captured water.
[0005] However, most AWH reports involve adding hygroscopic salts into the network, and there are very few reports on dual applications for AWHs and energy harvesting till now. Instead of adding hygroscopic substance to the substrate, if the substrate itself harvests water from a humid environment, it can work simultaneously to collect the water and evaporate it which could boost the potential usage even in the less abundant water zone. An ideal AWH should be an excellent moisture absorber in humid conditions and release it when external energy, “light or heat”, comes into play.
[0006] Carbon/graphene-based composites are very interesting and have excellent photo thermal effects widely used for energy production, but not that much explored in water harvesting. Graphite is a stacking of graphene layers that possess high light absorption but no moisture-absorbing properties. However, adding oxygen functional groups (graphene oxide/ GO) with that can increase the water uptake. Therefore, it is an excellent suggestion to attach water-absorbing substance to the graphene/graphite substrate. Generally, sulfonic groups are highly hydrophilic, dissociative, and have moisture-absorbing properties. Adding sulfuric acid might be key to manipulating sulfonic groups in the graphitic structure. Hence, a source of sulfuric acid can be used to exfoliate the graphene sheets and switch them into sulfonic groups, which is a straightforward method of attaching them to the graphite structure.
[0007] Jiaxin Bai et al. developed a novel device using a polyelectrolyte that spontaneously causes charge separation and directional movement of H⁺ ions under moisture, leading to electric output. This mechanism, known as moisture-enabled hydrovoltaic electricity generation (MHEG), utilizes a hygroscopic matrix of poly (4-styrene sulfonic acid) (PSSA) to collect moisture and migrate H⁺ ions during the water-capturing process. The sulfonic groups dissociate into SO₃⁻ and H⁺ ions, creating a gradient of hydration levels between the top and bottom electrodes of the film. This gradient drives the H⁺ ions deeper into the film, generating an electric field due to the separation of negative and positive ions. This gives us an idea to attach the sulfonic groups to the graphite/ graphene structure during oxidation.
[0008] Herein, we have successfully synthesized a sulfonic group functionalized graphitic oxide as an atmospheric water harvesting nano- structured material and a simultaneous hydrovoltaic power generator using a slightly modified hummers method followed by a one-step reflux method. The synthesized material shows excellent moisture capture-release stability and portability and gives constant values of capturing water and power generation at a time without adding hygroscopic salts.
[0009] Further, we have achieved the novel integration of atmospheric water harvesting (AWH) with hydrovoltaic (HV) technology, by designing a device, with the incorporation of the synthesized sulfonic group functionalized graphitic oxide (SGhO) substrate. The device exhibits an innovative dual performance to generate ultra-freshwater and electricity simultaneously without adding any hygroscopic material.
SUMMARY OF THE INVENTION
[0010] The present subject matter describes a method (100) for synthesizing a nanocomposite material (200) sulfonic group functionalized graphitic oxide (SGhO), and its applications in atmospheric water harvesting (AWH) and hydrovoltaic (HV) power generation. The SGhO (200) is prepared from graphite powder by a one-step reflux method and exhibits enhanced moisture-capturing and releasing properties, holding significant potential for these applications without the incorporation of additional hygroscopic salts. Additionally, simple fabrication of a dual-function device (300) incorporated with the synthesized nanocomposite SGhO, for simultaneous generation of ultra-freshwater and electricity is disclosed. This presents a promising solution for addressing water scarcity and power generation challenges, particularly in arid regions and areas with high humidity.
[0011] According to one embodiment of the present subject matter, a method (100) for the synthesis of sulfonic group functionalized graphitic oxide (SGhO) (200) by a one-step reflux method is disclosed. The method comprises, mixing (101) 3 grams of graphite powder with 70 ml of sulfuric acid and stirring for 13 hours. Then, 3 grams of potassium permanganate is slowly added (102) to the mixture while maintaining the temperature of the solution at 0°C to minimize effervescence, and retain the mixture for 30 minutes. The method then involves sonicating (103) the mixture for six hours. Then the method involves adding (104) 100 ml of deionized water (DI) to the resultant solution and stirring for 30 min. Subsequently, a mixture of 200 ml DI and 40 ml of hydrogen peroxide (H₂O₂) is added (105) to the above solution and stirring is continued for 15 minutes. Further steps include, collecting (106) the bottom precipitate part of the solution to obtain partially oxidized graphite. The exfoliated graphene oxide is then discarded. The collected precipitate is then washed (107) with deionized water, hydrochloric acid, and ethanol. The washed precipitate undergoes refluxing (108) with sulfuric acid for 30 min, followed by centrifugation and washing. Finally, the refluxed precipitate is dried (109) at 60-70°C for 5-8 hours to obtain a dry powder of SGhO (200) as end product.
[0012] According to another embodiment, a nanocomposite material (200) sulfonic group functionalized graphitic oxide (SGhO), for simultaneous atmospheric water harvesting and hydrovoltaic power generation is disclosed. The nano composite material (200) comprises of graphitic oxide (201) functionalized with sulfonic groups (202). The graphene layers (203) of the graphitic oxide are stacked to form a thick sheet, and the sulfonic groups are attached to side defect carbon sites to form a 3D structure (SGhO) (200). Further, the disclosed nanocomposite material SGhO is synthesized by the above method (100) and it exhibits rapid moisture-capturing and releasing properties without the incorporation of additional hygroscopic salts.
[0013] In various embodiments, the FTIR analysis of SGhO exhibits peaks at 1022-1030 cm⁻¹, 1152-1160 cm⁻¹, 860 cm⁻¹, and 3372 cm⁻¹. The peak at 1022-1030 cm⁻¹ is indicative of the vibration mode of S=O asymmetric stretching, while the peak at 1152-1160 cm⁻¹ is indicative of the symmetric stretching modes of -SO₃H groups. The peak at 860 cm⁻¹ is attributed to C–C stretching vibrations, and the 3372 cm⁻¹ peak is assigned to OH bonds stretching. In various embodiments, the SGhO shows an intense peak at 25.1° in the 2θ range of 10° to 50° in XRD analysis using CuK radiation. The pattern is suggestive of increased crystallinity and attachment of sulfonic groups to the structure. In various embodiments, the SGhO exhibits two prominent peaks in Raman spectra at 1355 cm⁻¹ and 1587 cm⁻¹, corresponding to the D band and G band, respectively. Further, the material has an ID/IG ratio less than 1 (0.75) clearly indicating the formation of graphene stacking in the sample. In various embodiments, the material exhibits high stability for the moisture capture-release process. In various embodiments, the nanocomposite material SGhO exhibits water absorption of 0.3 g/g/h to 2 g/g in 19 hours under 40-60% relative humidity. In some embodiments, the material evaporates all absorbed water within 0.5 hours in one sun illumination (1 kW/m²) at 40-60% relative humidity.
[0014] In various embodiments of the invention a dual-function device (300) for simultaneous atmospheric water harvesting and hydrovoltaic power generation without additional hygroscopic salts is presented. The device includes a moisture harvesting enclosure (301) placed to capture sunlight or other illumination sources (302). It has a top lid (303) that can be opened and closed, an illumination area (304) for absorbing light, a condenser (305) placed at the moisture outlet pathway and two electrodes (307a, 307b) at either end of the enclosure. The enclosure is filled with the nanocomposite material SGhO (200), synthesized by the disclosed method (100), for absorbing and evaporating moisture upon light exposure. The openable lid (303) creates a moisture inlet (306) for atmospheric moisture collection, which is closed during evaporation.
[0015] In various embodiments, the electrodes are typically made of graphite or carbon fibers and are placed in contact with the nanocomposite material. This registers the voltage difference when moisture is absorbed by the material generating electric current. In other embodiments, the condenser (305) collects evaporated moisture and condenses it into fresh water. In alternative embodiments, the enclosure is tilted at an angle of 8-12° to enhance performance. In further embodiments, the moisture inlet area is significantly larger than the illumination area to optimize moisture collection. In various embodiments, a single device with 2 grams of SGhO nanomaterial can achieve a maximum voltage of 0.25V and produce a current of 160 µA under 40-60% relative humidity, demonstrating its efficiency and capability in varying atmospheric conditions.

BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The invention has other advantages and features, which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:
[0017] FIG. 1: illustrates the steps involved in the synthesis of the nano composite material sulfonic group functionalized graphitic oxide (SGhO).
[0018] FIG. 2: shows a schematic illustration of the synthesized SGhO material.
[0019] FIG. 3(a): illustrates the schematic diagram of a hydrovoltaic device setup during the evaporation process under light illumination. It further shows the material (SGhO) placed inside the tube, which is kept at a tilted angle and connected with two electrodes.
[0020] FIG. 3(b): illustrates the schematic diagram of a hydrovoltaic device setup with the lid in open condition for the SGhO material to absorb moisture from the environment.
[0021] FIG. 4(a): illustrates a SEM image of the prepared SGhO.
[0022] FIG. 4(b - d) illustrates TEM images of the prepared SGhO.
[0023] FIG. 5: gives the FTIR spectra of without sulfonic groups attached graphitic oxide (WGhO) and sulfonic groups attached expanded graphitic oxide (SGhO).
[0024] FIG. 6: gives the Raman spectra of WGhO and SGhO.
[0025] FIG. 7: gives the X – ray diffraction (XRD) pattern of WGhO and SGhO.
[0026] FIG. 8 (a): gives a graphical representation of the water uptake and evaporation behavior of a 1g SGhO sample under alternating light conditions (light ON-OFF) and ambient humidity levels ranging from 40% to 60%.
[0027] FIG. 8(b): gives a graphical representation of the Photo-thermal effect and IR images of SGhO sample under one sun (1 kW/m2) illumination.
[0028] FIG. 8(c): demonstrates the water-absorbing property and repeatability of the SGhO sample under several cycles.
[0029] FIG. 8(d): illustrates the cyclic behavior of SGhO under light ON-OFF conditions observed under a microscope.FIG. 9: illustrates the hydrovoltaic power generation performance of SGhO.
[0030] FIG. 9(a) and 9(b) gives graphical representation of the voltage and current obtained from the SGhO HV device under light ON conditions for evaporation.
[0031] FIG. 10 (a - f): illustrates the Recyclability Test of SGhO HV performance with the anticipated mechanism of power generation and graphical representations of the voltage and current changing behaviour under moisture capturing and releasing time.

DETAILED DESCRIPTION OF THE EMBODIMENTS
[0032] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.
[0033] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.
[0034] The present subject matter describes a method 100 for the synthesis of a nanocomposite material 200 comprising sulfonic group functionalized graphitic oxide (SGhO), where the SGhO 200 is prepared from graphite powder by a one-step reflux method. Further, the present subject matter describes the fabrication of a dual-function device 300 for simultaneous generation of ultra-freshwater and electricity without the incorporation of additional hygroscopic salts. This innovative dual performance is achieved by the incorporation of the sulfonic group functionalized graphitic oxide (SGhO) into the dual-function device.
[0035] According to embodiments of the present subject matter, a method 100 for the synthesis of sulfonic group functionalized graphitic oxide (SGhO) by a one-step reflux method is disclosed as illustrated further with reference to FIG. 1. The method comprises, mixing 3 grams of graphite powder with 70 ml of sulfuric acid and stirring for 13 hours as the initial step 101. In the next step 102, slowly adding 3 grams of potassium permanganate to the mixture while maintaining the temperature of the solution at 0°C to minimize effervescence. Then, sonicating 103 the mixture for six hours after 30 min.. Following sonication, the resultant solution is added 104 with 100 ml of deionized water (DI) and stirred for 30 minutes. Subsequently, a mixture of 200 ml DI and 40 ml of hydrogen peroxide (H₂O₂) is added 105 to the above solution and stirring is continued for 15 minutes. Further, the method comprises collecting 106 the bottom precipitate part of the solution to obtain partially oxidized graphite. The exfoliated graphene oxide is discarded. In the next step 107, the collected precipitate is washed with deionized water, hydrochloric acid, and ethanol. The washed precipitate undergoes refluxing 108 with sulfuric acid for 30 min, followed by centrifugation and washing. Finally, the refluxed precipitate is dried (109) at 60-70°C for 5-8 hours to obtain a dry powder of SGhO (200) as end product.
[0036] According to various embodiments, a nanocomposite material 200 for simultaneous atmospheric water harvesting and hydrovoltaic power generation is disclosed with reference to FIG. 2. The nano composite material 200 includes graphitic oxide 201 functionalized with sulfonic groups 202. The graphene layers (203) of the graphitic oxide are stacked to form a thick sheet, and the sulfonic groups are attached to side defect carbon sites to form a 3D structure (SGhO) 200. The disclosed SGhO exhibits rapid moisture-capturing and releasing properties without the incorporation of additional hygroscopic salts.
[0037] In various embodiments, the nano composite material SGhO 200 exhibits peaks in FTIR analysis at 1022-1030 cm-1, 1152-1160 cm-1, 860 cm-1 and 3372 cm-1. The peak at 1022-1030 cm⁻¹ is indicative of the vibration mode of S=O asymmetric stretching and the peak at 1152-1160 cm⁻¹ is indicative of the symmetric stretching vibration modes of -SO₃H groups. The peak at 860 cm⁻¹ is attributed to the vibration of C–C stretching vibrations and the 3372 cm⁻¹ peak is assigned to the stretching vibration of OH bonds. In various embodiments, the synthesized SGhO 200 exhibits an intense peak at 25.1° in the 2θ range between 10° to 50° in X-ray diffraction (XRD) using CuK radiation. The pattern is suggestive of increased crystallinity and attachment of sulfonic groups to the structure. In various embodiments, the nano composite material SGhO 200 exhibits two prominent peaks in Raman spectra at 1355 cm⁻¹ and 1587 cm⁻¹ corresponding to the D band and G band, respectively. Further, the SGhO material has an ID/IG ratio of less than 1 (0.75), clearly indicating graphene stacking nature in the material. In various embodiments, the material exhibits high stability for the moisture capture-release process.
[0038] In further embodiments, the nanocomposite material SGhO shows water-absorbing ability of 0.3 g g-1 h-1 to 2 g g-1 in 19 h under 40-60% relative humidity. In some embodiments, the material evaporates all absorbed water within 0.5 h in one sun illumination (1 kW/m2) under the ambient relative humidity of 40-60%.
[0039] In various embodiments of the invention, a dual-function device 300 for simultaneous atmospheric water harvesting and hydrovoltaic power generation without the incorporation of additional hygroscopic salts is presented with reference to FIG. 3(a) and 3(b). The device 300 includes a moisture harvesting enclosure 301 placed to capture sunlight or light from any other illumination source 302. The top portion of the enclosure is provided with a lid 303 that can be opened and closed as needed. Further, the enclosure is provided with an illumination area 304 for absorbing light therefrom. A condenser 305 is placed at the moisture outlet pathway to capture the evaporated moisture from the device. Two electrodes 307a, 307b are provided at either end of the moisture harvesting enclosure 301 to draw power from the device during the evaporation process.
[0040] In further embodiments, the moisture harvesting enclosure 301 is filled with the nanocomposite material sulfonic group functionalized graphitic oxide (SGhO) 200, for absorbing moisture and evaporating the same on exposure to light illumination. The lid 303 is openable to create a moisture inlet having an area 308 to collect moisture from the atmosphere and is configured to be closed during evaporation. In various embodiments, the two electrodes 307a, 307b, may be made of graphite or carbon fibers. These electrodes are placed in direct contact with the SGhO 200 for registering a voltage difference when the material absorbs moisture. This creates an electric current that is harnessed for power generation. The condenser 305 placed at the moisture outlet pathway 306 is configured to condense the evaporated moisture and collect it as fresh water within the condenser vessel, ready for use. In some embodiments, to enhance the performance of the device, the enclosure is tilted at an angle to the horizontal and may be placed to optimally capture natural light or sunlight. In some embodiments, the tilting may be 8-12 with respect to the horizontal.
[0041] Furthermore, in various embodiments of the device, the moisture inlet area (308) is configured to be 5 to 10 times the illumination area 304. The moisture inlet area is configured to maximize the amount of moisture that can be collected from the atmosphere. In some embodiments, a single device equipped with 2 grams of the SGhO 200 nanomaterial may achieve a maximum voltage of 0.25V and produce a current of 160 µA, even in relative humidity conditions of 40-60%.
[0042] In other embodiments, the temperature of SGhO rises from 32°C to 50°C within 1 minute under one sun illumination, demonstrating superior photo-thermal characteristics. Measurements using a thermal imaging camera and solar simulator confirm this behavior, and the moisture capture-release cycle is validated using a traveling microscope. In various embodiments, the material may capture nearly 1.5 times its weight of water. In some embodiments, the device is capable of generating water of high purity. TDS measurements show a salt content of only 25 ppm, meeting global drinking water standards. The pH value of collected water is 6.32, confirming its safety for consumption.
[0043] The presence of sulfonic groups in the graphitic oxide structure of the nanocomposite material SGhO 200, enables moisture absorption without the need for additional hygroscopic substances. The presence of graphitic oxide enhances the photothermal effect to evaporate whole absorbed water upon radiating within a short interval of time. During the moisture capture-release cycle, the water molecules pass through the nanostructure, which creates a net dragging of H+ ions through the graphitic structure, generating a current and voltage across the material. Several cyclic steps were recorded, indicating the dragging of water molecules during capture and release capable of power generation.
[0044] The advantages of the invention are manifold. The novel method 100 provides a one-pot synthesis. The synthesized nano composite material (SGhO) 200 exhibits enhanced moisture-capturing and releasing properties and thereby holds significant potential for applications in atmospheric water harvesting (AWH) and hydrovoltaic (HV) power generation. The material shows an excellent stable moisture capture-releasing property even after 10 cycles. This highlights its efficiency and capability to function well in varying atmospheric conditions. Further, the fabricated dual-function device incorporated with SGhO, generates ultra-freshwater and electricity simultaneous without the incorporation of additional hygroscopic salts. Further, several purification tests were conducted to indicate that the collected water highly meets the global drinking water standards. Thus, device 300 may be used to generate fresh water without any external energy in critical situations/places like desert regions or high-humidity regions.
[0045] EXAMPLES
[0046] EXAMPLE 1: Preparation of without sulfonic groups attached graphitic oxide (WGhO) and sulfonic groups attached expanded graphitic oxide (SGhO): Both WGhO and SGhO were prepared from graphite powder by slightly modified Hummer’s method, as already illustrated with reference to FIG. 1. As the first step, 3 g of graphite powder and 70 ml of sulfuric acid were mixed and stirred for 13 h. 3 g of potassium permanganate (KMnO4) was slowly added to the solution, and simultaneously, the solution was maintained at a low temperature (0°C) to reduce effervescence. After 30 min, the solution is allowed to sonicate for six hours. 100 ml of deionized water (DI) was added to the resultant solution and stirred for 30 min. The mixture of 200 ml DI and 40 ml of hydrogen peroxide (H₂O₂) was added to the solution and stirred for 15 min. After adding water and H₂O₂, the bottom precipitate part of the solution was taken (to collect partially oxidized graphite and discard the exfoliated graphene oxide) and washed with DI, hydrochloric acid (HCl), and ethanol. Finally, the collected powder was divided into two halves. One half was named WGhO, and the other was allowed to reflux with sulfuric acid for 30 min, collected by centrifugation, washing, and named SGhO. The two final products (WGhO and SGhO) were allowed to dry at 60–70°C for several hours (5 – 8 h) to get a dry powder. Notably, WGhO doesn’t absorb moisture in a humid environment, whereas the SGhO sample absorbs a high amount of water from the atmosphere within several minutes.

[0047] EXAMPLE 2: CHARACTERIZATIONS AND MATERIAL STUDIES
[0048] Microscopy was done as part of characterizing the SGhO as illustrated in FIG. 4(a) to 4(d). Field Emission Scanning Electron Microscopy was undertaken to analyse the nanostructure and surface morphology of the material using Carl Zeiss, Germany. FIG. 4(a) shows scanning electron microscopic (SEM) image of the prepared SGhO. The morphology shows that the graphene layers are stacked and form a thick sheet. This is because of the precipitate collected from the bottom during the synthesis. Hence, these stacked layers of graphene sheets might pave the way to functionalize the sulfonic groups into the defect site, forming a 3D structure to absorb more moisture than the dispersed graphene sheets. To confirm this postulate, several washing treatments were performed after preparing the SGhO and the FTIR study was taken. The data confirms the functionalization of sulfur and carbon. This confirmation is elaborately discussed later. Therefore, this can increase the hygroscopic property of the material, which leads to absorbing moisture from the surface. Here, the presence of the sulfonic groups on the surface can absorb the moisture and keep the water molecules on its surface.
[0049] TEM images: FIG. 4(b) shows the sample’s TEM image, signifying the graphene’s sheet-like structure. Moreover, the graphene sheets are orderly arranged to form a thick book-like structure; this is shown in FIG. 4(c). Most importantly, in FIG. 4(d), the sheets are orderly arranged under high magnification indicating the d space is 0.37 nm. However, in some places, the gaps are uneven, indicating the probability of functionalization of sulfonic groups. The expected schematic structure of synthesized material is shown in FIG. 2
[0050] XRD analysis of SGhO: The crystallinity of both WGhO and SGhO samples was identified with XRD analysis in the 2θ range between 10° to 50°. It is displayed in FIG. 7. From the obtained XRD patterns, WGhO shows a broad 2θ peak at 26.1° (002), indicating the graphite peak which has less crystallinity, whereas at 43° suggests the formation of graphene oxide. In contrast, the SGhO sample clearly indicates the highly intense peak shift from 26.1° to 25.1°, indicating the interlayer distance between the van-der Waals carbons was slightly increased after the reflux treatment. Notably, the intensity of the 25.1° peak was enhanced significantly after reflux treatment, which may indicate the increase in crystallinity. This might be a probability of attached sulfonic groups with graphite structure during reflux reaction. The elaborate explanation will be discussed later. Moreover, the presence of GO peaks in WGhO at 43° was slightly reduced after the reflux in SGhO, indicating the reduction in oxygen-containing functional groups. From the evidence of the XRD of WGhO and SGhO, we assume the stacking was enhanced in SGhO, depicting a much smaller scope of intercalation due to the negligible variation between graphene sheets. Reports show that the shifting of angle in XRD directly depends on how much sulfuric acid groups penetrate into it. Therefore, in our study, we have varied the time of reflux treatment to 15 min to 30 min and noticed that the XRD 2θ peak shifted from 26.1° to 25.6° and 25.1°, respectively. This indicates that the expansion of d space between sheets would directly be proportional to the rate of reflux treatment. However, the d space does not show any significant change between the graphene sheets, indicating less possibility of intercalation once again. Therefore, the only expectation is anchoring the sulfonic group on the edge of the graphene sheets in a graphitic structure.
[0051] XRD analysis of GO and HGO: To confirm the effect of the reflux method for water harvesting property, we synthesized pure GO using Hummer’s method and divided the washed products into two halves. One half was used to reflux with sulfuric acid (HGO), and the other was kept in the oven for drying (GO). The obtained XRD peak of HGO and GO shows that GO is showing the 2θ value at 11°, 43° indicating graphene oxide, and 26° indicating the graphite peak. The presence of graphite peak is due to the unreacted graphite during the reaction. Simultaneously, the HGO showing the peak at 26° indicated the graphite peak, whereas the peak at 11° of GO was reduced after reflux treatment. This implies that the graphene sheets were stacked during reflux, and its oxygen-containing functional groups may be reduced. This was further proved with transmission electron microscopy analysis (TEM) images of SGhO, indicating the d space is 0.37 nm.
[0052] Analysis of Raman spectrum of the SGhO: Additional confirmation was done with Raman spectra analysis, and the obtained peaks are shown in FIG. 6. It shows the typical Raman spectra for SGhO and WGhO samples. The Raman spectra exhibit two prominent peaks of wavenumbers at 1355 cm⁻¹ and 1587 cm⁻¹, which are assigned to the D band (related to sp³ carbon atoms disorder induced phonon mode of vibrations) and G band (associated with the first order scattering of E2g mode for sp² carbon lattice of the graphitic domain), respectively. Usually, studying the ratio of ID and IG of carbon-based material is beneficial to evaluate the structural changes during the chemical process. In our study, the ID/IG ratio of SGhO is less than 1 (0.75), whereas WGhO is higher than 1 (1.01), clearly indicating the formation of graphene stacking nature in the SGhO sample. In the Raman peak of WGhO, the intensity of the D band is high, indicating the disorder is high; this may be because of the abundance of oxygen functional groups. However, SGhO clearly shows that the G band enhanced enormously, possibly due to the stacking behaviour of graphene sheets being orderly with less oxygen abundance. Raman spectra of SGhO clearly depict the same as pure graphite. Therefore, the stacking is increased after reflux treatment, which is also supported by XRD studies.
[0053] Raman spectra of GO and HGO: Furthermore, to understand the evidence of the effect of reflux treatment, we have also done the Raman analysis for GO and HGO, and the data was interpreted. It clearly showed two peaks for the D and G bands; further the intensity of the G band is much less in the GO sample, whereas the intensity was increased in the HGO sample. This is well-matched with previous data. This indicates that the material’s graphitization increased after the reflux treatment.
[0054] FTIR analysis of SGhO: Fourier transform infrared spectroscopy study was utilized to analyze the presence of functional groups in the sample. As synthesized, the two samples were allowed to check and ensure the effectiveness of the reflux reaction. The obtained FTIR spectra of WGhO and SGhO are shown in FIG. 5. It is clear that the peak at 1023 and 1153 cm⁻¹ in SGhO indicates the vibration mode of S = O asymmetric and symmetric stretching vibration of -SO₃H groups, and these peaks are absent in the WGhO sample. The observed peak matches well with the existing reports. To understand the repeatability of reflux reaction, we have done four trials of WGhO and SGhO synthesis and carried out the FTIR studies. In all obtained FTIR spectra, the presence of S = O asymmetric and symmetric stretching vibration modes of -SO₃H groups are present in the same wavenumber range of 1022–1030 cm⁻¹ and 1152–1160 cm⁻¹, respectively. Moreover, the wavenumber at 860 cm⁻¹ is attributed to the vibration of C–C stretching vibrations, 1565 cm⁻¹ indicates the asymmetrical vibrational modes of COO⁻, 1695 cm⁻¹ groups assigned to C = O, and the broad peak at 3372 cm⁻¹ assigned to the stretching vibration of OH bonds. Furthermore, washing the SGhO with ethanol, DI, acetone, and hydrochloric acid several times, we obtained the same S = O and sulfonic groups peaks. This indicates the possibility of ionic interaction of sulfonic groups with graphite substrate. This conveys the stability of the material for the moisture capture-release process.
[0055] TGA analysis: Thermal gravimetric analysis (TGA) was taken by a thermos-gravimetric analyser (T.A. instruments, U.S.A. with weighing Precision: ±0.1 %, sensitivity: ±2%) from the temperature range of 50°C to 800°C with a rate of 10°C/min under the nitrogen temperature to analyse the thermal stability of the sample. Before the TGA analysis, the sample was kept dry at 50°C for an hour under a vacuum. The thermal stability of the SGhO nano-structured material was analyzed using TGA measurements in the temperature range from 50°C to 700°C. The obtained TGA curve of the SGhO material depicts the weight reduction in the range of 70°C and 150°C completely due to water dehydration. Next, a rapid weight loss in the range of 200°C and 500°C is mainly associated with the reduction of sulfonic groups. Also, the amount of weight was reduced by nearly 35%, indicating the high amount of sulfonic groups present in the sample. From all material characterisation results of SGhO, signifying the huge abundance of sulfonic groups in the graphite structure could be a reason for water absorption.
[0056] EXAMPLE 3: ANALYSIS OF MOISTURE ABSORBING-RELEASING PERFORMANCE
[0057] The presence of sulfonic groups in SGhO is from the source “sulfuric acid,” a crucial factor for the water harvest and graphite for the release cycle process. To signify the SGhO rather than bare sulfuric acid, a mixture of sulfuric acid and pure graphite was made and kept in humid conditions to compare the effect of the reflux process. Notably, considerable moisture absorption occurs because of the presence of H₂SO₄; however, water harvesting via evaporation is not possible due to irreversible reactions. After several hours, we observed that graphite powder and water containing H₂SO₄ were phases separated.
[0058] Furthermore, direct reflux treatment was done with commercial graphite and sulfuric acid, which did not perform any water evaporation after the sample had absorbed the moisture in a humid environment. Hence, this is also a helpful evidence to conclude that the fabricated SGhO can absorb high atmospheric water content and release all absorbed water under the light on condition by phonon vibration. Generally, the moisture-absorbing property of sulfuric acid in humid conditions is very high, but the release of absorbed water is significantly less because of the ionisation reaction. When H₂O gets absorbed by H₂SO₄, it converts into HSO₄⁻ + H₃O⁺. This reaction is completely irreversible due to protonation. Here, sulfonic groups attached to graphite sheets give a reversible process that can standby for cyclic water collection.
[0059] Several steps were taken to analyse the SGhO’s physical properties, such as light absorbance, thermal effect, and cyclic water ability. 1 g SGhO was kept in a humid environment with an RH of 45–60% for one hour. After one hour of observation, the sample has collected 0.3 g of water from the environment. To measure the water collection for an extended period, the same amount of sample was kept for 19 h in the RH of 40–60%. FIG. 8(a) indicates the 1 cm² surface area sample can absorb up to 2 g/g of moisture in 19 h, and the falling peak shows the weight reduction when exposing light on the surface.
[0060] Weight reduction is rapid when exposed to one sun illumination, indicating high light absorption and evaporation of captured water molecules. The critical target of AWH is to capture the moisture and evaporate the captured water. This is dependent on the photo-thermal property of the absorbing material. Moreover, to investigate the light absorption property, we mixed 1 g of dry SGhO sample with 2 ml of water to make a paste. The paste was coated directly on 1 cm² of cleaned polyurethane foam. The foam was kept in an oven under 70°C for 3 h to evaporate the existing water groups from the foam. After that, one sun illumination was provided from the solar simulator to the top surface of the foam to study the photo-thermal properties of the AWH. Notably, the temperature has suddenly raised from 32°C to 50°C within 1 min, confirming the superior photo-thermal characteristics of the material. The temperature variation and the actual image of coated foam are shown in FIG. 8(b). This extraordinary light capturing property is due to the presence of graphite.
[0061] Furthermore, the moisture-absorbing repeatability was tested by keeping a 0.010 g of SGhO sample on a micro slide and exposed in a humid environment. The collected moisture was measured and evaporated into water vapour by light illumination. During light-off conditions, the sample starts to absorb moisture and readily evaporates the whole absorbed water after exposure to solar light every 10 minutes. The following 10 minutes is the moisture-absorbing time, and evaporation occurs again during light on condition. FIG. 8(c) shows the graphical representation of the amount of moisture captured over ten cycles. The bar graph indicates that the sample shows nearly the same water-capturing property which will be useful for continuous water capture-release cycles for long-term purposes.
[0062] The major role of graphitic oxide in SGhO is to evaporate whole captured water from the material. Therefore, it’s a major necessity to confirm the dry and wet changes on the surface of the material. To confirm this, a microscopic analysis of the surface change was performed using a microscope. FIG. 8(d) shows the microscopic view of the surface change of SGhO during absorbtion and release of water over time. The inlet FIGS in 8(d) show the surface change during the moisture absorption. The moisture is captured by the material and forms a closed texture with water content on the sample. During evaporation, the material evaporates the whole water content absorbed by the material and came back to its original stage. The three-time repeatability shows the exact cyclic behaviour of the material.
[0063] The change in the relative length of the sample before and after absorbing moisture is calculated using the ImageJ software and is observed that the material can capture nearly 1.5 times the water. The FIGS in 8c and 8d confirm the repeatability of the SGhO for water harvesting in real applications. This work aims to generate fresh water without any external energy in critical situations/ places like desert regions and high-humidified regions. Therefore, utilizing this material to show the water harvesting process is initiated.
[0064] The material is placed in a watch glass under ambient relative humidity in the range of 50–80% for a day to collect the maximum amount of moisture from the surroundings. The moisture obtained from the sample is kept under a solar simulator/ direct sunlight for evaporation and in a condenser to collect the water. The condensed water is highly transparent without any contaminants. To check the purity of the collected water, the UV–Vis absorption spectroscopic study was used, and the obtained data indicates that the collected water is ultrapure and safe to drink. Further confirmation was done by checking the salt content present in the water by TDS (total dissolved solute) measurement. The TDS meter shows only 25 ppm which highly meets the global drinking water standards. Additional confirmation was done by measuring the pH value of collected water which shows only 6.32 which would be safe to drink.
[0065] EXAMPLE 4: PERFORMANCE OF THE HYDROVOLTAIC POWER GENERATION DEVICE
[0066] The moisture capture-release repeatability due to the presence of sulfonic groups also might be a major suggestion to generate electricity. Using sulfonic groups in the material can harvest electricity during moisture absorption. Elaborately, during moisture capturing, the presence of sulfonic groups can dissociate into SO3⁻ and H⁺ ions in the SGhO material. The kinetic of H⁺ ions can separate from the graphitic substrate and can drag into the material. Thus, this may create a non-uniform distribution of ions throughout the material, which develops a potential across the electrodes. It is noted that the SO3⁻ will not move easily due to the heavy ions, and it has a high attraction to the carbon substrate. From the experimental observation, the material exhibits the power generation property only when absorbing moisture. The graphical image of the hydrovoltaic device is shown in FIG. 3(a) and FIG. 3(b). Two opening pathways are provided for moisture ingress and vapour out. FIG. 3(b) shows the material kept in the tube. The tube is tilted at a small angle, and the adjustable lid is kept open to absorb moisture.
[0067] During the lid open condition, the material started absorbing the environment’s moisture. A formation of moisture was observed on the interface of SGhO. During moisture absorption, the connected voltmeter shows that the voltage in negative. After opening for a few minutes, we closed the opening and exposed one sunlight illumination to the illumination area. During the evaporation time, the voltage was varied and attained positive. This ensures the movement of water directly opposite compared to water capture progress. The generated voltage is shown in FIG. 9(a) and it attained its saturated point. This confirms the generation and variation of voltage due to the alteration of water movement through SGhO. Specifically, we measured the electric current during evaporation, shown in FIG. 9(b). The current attained a maximum of nearly 160 μA and saturated. All these experiments were conducted in a relatively humid environment of 60–65%.
[0068] To show the repetition of moisture-induced power generation, we have done experiments with the moisture capturing-releasing process by opening and closing the tube lid and measuring the voltage change in behaviour. The device is under open conditions for moisture absorption as shown in FIG. 10(a). The dissociation mechanism is schematically shown in FIG. 10(b). The mechanism shows during the humid condition, the moisture is being absorbed by the sulfonic groups. During the moisture-capturing process, the dissociated H⁺ ions move into the bottom side which generates the potential difference along the two sides and the drifting of these ions generates current. The SO3⁻ in the graphic substrate does not move which generates a negative effect on the top. FIG. 10(c) shows that while the lid is closed and light is applied, the movement of H⁺ is reversed from bottom to top which shows the reversibility nature of the material. This mechanism is schematically shown in FIG. 10(d). This opposite movement of ions creates an opposite/reverse in potential and current. Therefore, during the moisture capture release process, the voltage and current were measured and shown in FIG. 10(e) and 10(f). This signifies the voltage and current are being reversed during moisture collecting and evaporation time.
[0069] Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed herein. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, operation and details of the system and method of the present invention disclosed herein without departing from the scope of the invention as described here and as delineated in the claims appended herewith.
, Claims:1. A method (100) for the synthesis of sulfonic group functionalized graphitic oxide (SGhO) for both atmospheric water harvesting and hydrovoltaic power generation by a one-step reflux method, the method comprising:
mixing (101) 3 g of graphite powder with 70 ml of sulfuric acid and stirring the mixture for 13 hours;
slowly adding (102) 3 g of potassium permanganate (KMnO₄) to the mixture while maintaining the solution at a temperature of 0°C to reduce effervescence;
sonicating (103) for six hours after 30 min;
adding (104) 100 ml of deionized water (DI) to the resultant solution and stirring for 30 minutes;
adding (105) a mixture of 200 ml DI and 40 ml of hydrogen peroxide (H₂O₂) to the above solution and stirring for 15 minutes;
collecting (106) the bottom precipitate part of the solution to obtain partially oxidized graphite and discarding the exfoliated graphene oxide;
washing (107) the collected precipitate with DI, hydrochloric acid (HCl), and ethanol;
refluxing (108) the washed precipitate with sulfuric acid, followed by centrifugation and washing;
drying (109) the refluxed precipitate to obtain a dry powder of sulfonic group functionalized graphitic oxide (SGhO).
2. The method (100) as claimed in claim 1, wherein the refluxing (108) with sulfuric acid is carried out for 30 minutes.
3. The method (100) as claimed in claim 1, wherein the drying (109) of the refluxed precipitate is carried out at 60-70°C for 5-8 hours.
4. A nanocomposite material (200) for simultaneous atmospheric water harvesting and hydrovoltaic power generation, the nanocomposite material (SGhO) synthesized without the incorporation of additional hygroscopic salts, comprising:
graphitic oxide (201) functionalized with sulfonic groups (202), wherein the graphene layers (203) are stacked to form a thick sheet, and the sulfonic groups are attached to side defect carbon sites to form a 3D structure (SGhO) (200).
5. The material (200) as claimed in claim 4, wherein the SGhO is characterized by FTIR with a peak at 1022-1030 cm-1 , 1152-1160 cm-1 , 860 cm-1 and 3372 cm-1.
6. The material (200) as claimed in claim 4, wherein the SGhO gives two prominent peaks at 1355 cm-1 and 1587 cm-1 in the Raman spectra and the ID/IG ratio is less than 1 (0.75).
7 The material (200) as claimed in claim 4, wherein the SGhO gives an XRD pattern with intense peak at 25.1°.
8. The nanocomposite material (200) as claimed in claim 4, wherein the material shows water-absorbing ability of 0.3 g g-1 h-1 to 2 g g-1 in 19 h under 40-60% relative humidity, or
wherein the material evaporates all absorbed water within 0.5 h in one sun illumination (1 kW/m2) under the ambient relative humidity of 40-60%.
9. A dual-function device (300) for simultaneous atmospheric water harvesting and power generation without the incorporation of additional hygroscopic salts, comprising:
a moisture harvesting enclosure (301) placed to capture sunlight or other illumination source (302), having a top portion with a lid (303), an illumination area (304) for absorbing light therefrom, a condenser (305), placed at the moisture outlet pathway (306), and two electrodes (307a, 307b), at either end of the moisture harvesting enclosure (301),
wherein the moisture harvesting enclosure (301) is filled with the nanocomposite material (200), having sulfonic group functionalized graphitic oxide (SGhO), for absorbing moisture and evaporating the same on exposure to light illumination,
wherein the lid (303) is openable to create a moisture inlet (306) having an area (308) to collect moisture from the atmosphere and configured to be closed during evaporation,
wherein the electrodes are in contact with the nanocomposite material filled within the moisture harvesting enclosure (301), and configured to register a voltage difference generated therebetween on moisture absorption and draw current; and the condenser (305), is configured to collect the moisture as fresh water.
10. The device (300) as claimed in claim 9, wherein the electrodes (307a, 307b) used are graphite and carbon fibres.
11. The device (300) as claimed in claim 9, wherein the enclosure is placed at a tilting angle of 8-12 with reference to the horizontal for enhanced hydrovoltaic performance.
12. The device (300) as claimed in claim 9, wherein the moisture inlet area is 5-10 times the illumination area.
13. A device (300) as claimed in claim 9, wherein a single device, with 2 g of as-fabricated (SGhO) nanomaterial (200) exhibits a maximum voltage of 0.25V and produces a current of 160 µA under a relative humidity of 40–60%.