EP4688651A1

EP4688651A1 - Graphene oxide-based spectrally selective solar absorber coating and process for preparation thereof

Info

Publication number: EP4688651A1
Application number: EP24778489.5A
Authority: EP
Inventors: Selvakumar NAGARAJAN; Srinivas Govindappa; Harish Chandra Barshilia
Original assignee: Council of Scientific and Industrial Research CSIR
Current assignee: Council of Scientific and Industrial Research CSIR
Priority date: 2023-03-28
Filing date: 2024-03-22
Publication date: 2026-02-11
Also published as: WO2024201500A1

Abstract

The present invention relates to a graphene oxide-based spectrally selective coating on metal and nonmetal substrate. More particularly, the present invention provides a spectrally selective coating consisting of Graphene Oxide (GO) based absorber layer followed by deposition of inorganic protective layer using a wet chemical process. The process parameters such as graphite/sulphuric acid concentration ratio, oxidation time, soaking time, and synthesis temperature are optimized to make the GO powder spectrally selective with addition of oxalic acid, ascorbic acid, and aluminum isopropoxide. The spectrally selective coating of the present invention is RoHS and REACH-compliant and exhibits high UV stability, high corrosion resistant, high humidity resistance, and good chemical inertness.

Description

GRAPHENE OXIDE-BASED SPECTRALLY SELECTIVE SOLAR ABSORBER COATING AND PROCESS FOR PREPARATION THEREOF

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a graphene oxide-based spectrally selective coating on metal and nonmetal substrates. More particularly, the present invention provides a spectrally selective coating consisting of a Graphene Oxide (GO) based absorber layer followed by deposition of inorganic protective layer using a wet chemical process. Furthermore, the present invention provides a spectrally selective coating with high absorptance, low emittance, and good thermal stability using a cost-effective spray technique.

BACKGROUND OF THE INVENTION

Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

A large number of solar absorber coatings have been deposited using physical vapor deposition (PVD) and wet chemical process methods. In PVD methods, the sputtering technique has been used widely to develop coatings for solar thermal applications. However, only a few coatings, such as TiNO_x, Ni-NiO, W-A1N, CrN-Cr_xO_y, a-C: H/Cr, eta plus®, SS-C and ALUXID® have been commercialized successfully for low-temperature solar thermal applications. But, scaling up of sputtering process requires a huge initial investment, which makes the process not suitable for small-scale and medium-scale industrial applications.

In recent years, research on the preparation of spectrally selective coatings by wet chemical processes has gained increased attention because of their cost-effectiveness, good thermal stability, and good optical properties. Few coatings exhibit good thermal stability, but the process is not environment-friendly (i.e., RoHS and REACH-compliant). For industrial applications, the process should be green, cost-effective, and easy to scale up. In addition, the source material should be abundant and should exhibit good optical properties. Wet chemical methods such as electrodeposition, electroless deposition, sol-gel, etc. are easily scalable when compared to PVD methods, and also the processes are cost-effective. However, wet chemical methods use hazardous chemicals and are not environment friendly. For example, black chrome coating developed by the electrodeposition method exhibits high a (> 0.95) and low s (< 0.20); however, the use of hexavalent chromium, which is a carcinogen, is a major drawback for the commercialization of these coatings.

References may be made to a Chinese Patent No. 102635964, wherein a solar absorber coating comprising of a composite absorbing layer was deposited on copper and stainless steel substrates. The absorber layer mainly consists of 5-20% by weight of reduced graphene oxide and 80-95% by weight of ethylene- vinyl acetate (EVA) copolymer. The RGO acts as an absorber layer and the copper/SS substrate act as an infrared reflector. The RGO-EVA composite coating exhibits a high absorptance of 0.952 but the emittance values of these coatings have not been reported. It has been claimed that the coatings are stable up to 70°C in the air.

References may be made to a WO 2020244178A, wherein solar thermal conversion material comprises graphene and a hygroscopic polymer. It has been reported that the material has an absorbance greater than 95%. However, the emittance value has not been reported in the patent document and the material cannot be used for industrial applications because of the presence of hygroscopic polymer.

References may be made to Chinese Patent No. 108286833A, wherein the absorber coating consists of an infrared reflecting layer, a barrier layer (nickel oxide), and an absorber layer (graphene). In addition, chrome green has been used as an absorber layer. The magnetron sputtering technique was used to deposit the infrared reflecting and antireflection layers. The absorber solution contains graphene and phenolic resin.

References may be made to Chinese Patent No. 109111808A, wherein a heat-absorbing coating was developed by following parts by weight: 20-30 parts of silicone acrylic emulsion, 15-30 parts of epoxy resin, 7-18 parts of fluorocarbon resin, 6-9 parts of gelatin, 3-7 parts of graphene oxide, 4-8 parts of charcoal, 2-5 parts of ferromagnetic mineral black,

1-3 parts of yttrium stablized zirconium oxide powder, 6-12 parts of modified talc powder,

2-4 parts of leveling agent, 1-3 parts of dispersing agent, 5-8 parts of curing agent, and 30- 50 parts of solvent. The coating exhibits absorptance greater than 93% and also showed good resistance to corrosion, bending, salt fog, and impact.

References may be made to Chinese Patent No. 111397231A, wherein a graphene-based selective absorption film was prepared. The graphene based absorption film comprised of an antireflection layer, an absorber layer, an infrared reflecting layer and a substrate. The absorber layer contains at least one of reduced graphene oxide and graphene material. However, they have used low surface tension solutions at least one of ethanol, acetone, isopropanol and ethylene glycol.

References may be made to Chinese Patent No 106744831 A, wherein a graphene oxidebased photothermal conversion film was prepared. Polyethyleneimine, polyethylene amine, or polyacrylamide was used as the crosslinking agent.

References may be made to Chinese Patent No. 110124532A, wherein a Kynoar photothermal film was developed for seawater desalination applications which contains graphene oxide-PVDF polymer.

References may be made to a European Patent No 3116960, wherein a light- absorbing film comprising two layers was prepared on metal, glass, and ceramic substrates using a wet chemical method. The first layer consists of at least one absorbing material and one binder material and the second layer comprises an infrared-reflecting material. The absorber material was selected from carbon nanotubes, graphene, and fullerenes. Similarly, the binder material was silicon-based and is selected from polyborosiloxane, polysilazane, methyl trimethoxysilane polycarbosilane, silazane, and polysiloxanes and their combinations. The coating exhibits absorptance greater than 0.90 with emittance in the range of 0.1-0.5 in the NIR region. The thermal stability of these coatings has not been disclosed in the patent publication.

References may be made to article by Liao et al. [Adv. Sci 7 (2020) 1903125], wherein GO/TEOS coating was spin-coated on aluminum substrates and reduced thermally at 300°C. After reduction, the GO/TEOS changes into RGO/SiO2 which exhibits a high absorptance of 0.92 and low emittance of 0.04 at room temperature. It has been claimed that the coatings were thermally stable in air at 800°C for 96 h. However, the spin coating process cannot be used for industrial-scale applications. References may be made to “article by A. S. A. Shalaby et al. [Bulgarian Chemical Communications 48 (2016) 38-42 and references therein], wherein RGO-silicate nanocomposites were prepared by a sol-gel process. However, the authors have not studied the optical properties and have reported only the thermal stability of the RGO-silicate nanocomposites.

References may be made to article by J. S. Choi et al. [RSC Advances 5 (2015) 38742 and references therein], wherein RGO-sodium silicate (Na2SiOs) nanocomposites were prepared by a sol-gel process. However, the authors have not studied the optical properties and have measured only the conductivity of the RGO-Na2SiO3 nanocomposites.

References may be made to “article by M. M. Rahman et al. [J. Mat. Sci. Technol. 32 (2016) 1179 and references therein], wherein graphene oxide doped copper-cobalt oxide coatings were prepared by a sol-gel process. However, maintaining the composition of spinel-based coatings for industrial- scale applications is tedious and time-consuming.

References may be made to article by Mehrauli et al. [Appl. Energy 224 (2018) 103], wherein a hybrid plasmonic nanofluid containing silver decorated reduced graphene oxide was prepared. These fluids exhibit both higher absorption and thermal conductance.

It is evident from the prior- art literature that most of the spectrally selective coatings developed by wet chemical methods used silicate as a binder/solvent. In addition, few of these coatings are developed using volatile organic compounds and for other coatings, the process is not environment-friendly. To scale up the spectrally selective coatings for domestic and industrial solar water heating applications, the deposition process must be simpler and cost-effective. In addition, the deposition technique should be environment friendly, the raw materials should be cost-effective, the chemicals used should have compliance with RoHS and REACH regulations and the absorber coating must exhibit good thermal stability, high solar absorptance, and low thermal emittance. None of the prior art referred previously fulfills these requirements.

As stated above, the RGO coatings reported in the prior arts have used polymeric materials and silicate as binder/solvents and are not environment-friendly. Preparation of RGO from graphene oxide requires additional chemicals (such as hydrazine hydrate, sodium borate, and ascorbic acid) and needs lots of water for washing and filtration, which is a major problem for industrial applications. Considering the issues associated with RGO-based spectrally selective coating, there is a need to develop cost-effective, spectrally selective, environment-friendly process for solar absorber coatings for domestic and industrial hot water applications.

Thus, keeping in view the drawbacks of the hitherto reported prior arts, there was a need to solve the problem of cost-effective, spectrally selective, environment-friendly process for solar absorber coatings. The present invention overcomes the solar absorber coatings problems with the use of Graphene oxide-based spectrally selective composition. The graphene oxide-based spectrally selective coating composition relates to absorptance in the range of 0.91-0.93 and emittance in the range of 0.21-0.30. The graphene oxide-based spectrally selective coating composition is stable in the air up to a temperature of 175 °C for duration of 600 hrs.

The present invention also addressed the limitation of graphene oxide by introducing oxalic acid, and ascorbic acid in the composition, and optimizing the process parameters (such as graphene oxide concentration, oxidation time, thickness, etc), thus making the graphene oxide absorber layer as spectrally selective.

OBJECTIVES OF THE INVENTION

The main objective of the present invention is to prepare a spectrally selective absorber coating consisting of absorber layer and protection layer using a cost effective and an environment-friendly process.

Another objective of the present invention is to prepare a graphene oxide based solar absorber coating having good thermal stability by using a wet chemical process.

Yet another objective of the present invention is to prepare a molybdenum oxide -based inorganic protective layer by spray pyrolysis technique.

Yet another objective of the present invention is to provide solar absorber coating with inorganic protective layer having high solar absorptance (0.91 to 0.93) and low thermal emittance (0.21 to 0.30) with thermal stability of 175°C in air for a duration of 600 hrs making coating suitable for domestic water heating applications.

Still another object of the present invention is to provide a solar absorber coating with very high chemical inertness, humidity resistant, condensation resistant, RoHS compliant, REACH compliant, UV stability, corrosion resistant, and thermal shock resistance by a wet chemical method.

SUMMARY OF THE INVENTION

In accordance with the above, present invention provides an environment-friendly spectrally selective coating of thickness 4-7 pm is consisting of two layers; graphene oxide based absorber layer containing carbon (50.3 at%) and oxygen (49.7 at%) and an inorganic top protection layer, containing MoOs (2 wt%), talc (2 wt%), ascorbic acid (1 wt%), and water (95%). The spectrally selective coating is obtained by deposition of solutions of said absorber layer and protection layer on metallic and non-metallic substrate at a temperature of 150°C by spray technique.

In an embodiment of present invention, spectrally selective solar absorber coating consisting of Graphene oxide-based absorber layer followed by deposition of protective layer on the top of absorber layer.

In an embodiment, graphene oxide-based spectrally selective composition for solar absorber layer consisting of:

1. Graphene oxide powder: 0.5-2 wt%

2. Oxalic acid: 0.5-1 wt%

3. Ascorbic acid: 8-10 wt %

4. Aluminum isopropoxide: 0.1-1 wt%

5. Water: 90-86 wt%

Still another aspect of the present disclosure, wherein inorganic protective layer for depositing on solar absorber layer selected from the:

1. Molybdenum oxide: 0.12-2 wt%

2. Talc: 0.05 - 2 wt%

3. Ascorbic acid: 10 -14 wt %

4. Water: 82 - 89 wt%

In an embodiment of present invention, solution for absorber layer having viscosity of 58- 60 cP and pH in the range of 1-3 is obtained by dissolving graphene oxide, oxalic acid, aluminum isopropoxide and ascorbic acid in distilled water. In an embodiment of present invention, ascorbic acid is used as a binder, oxalic acid as a chelating agent, aluminum isopropoxide as an enhancer while preparing solution for absorber layer.

Still another embodiment of present invention, wherein suspension for protection layer having viscosity of 6-7 cP is obtained by dissolving molybdenum oxide, talc powder and ascorbic acid in distilled water.

In an embodiment of present invention, molybdenum oxide and talc are used as inorganic materials while preparing solution for protection layer.

In an embodiment of present invention, wherein substrate used for coating is metallic and non-metallic materials such as aluminum, copper, galvanized iron, glass, stainless steel and mild steel.

Still another aspect of the present disclosure, wherein a process for preparation of spectrally selective solar absorber coating on metallic and non-metallic substrate, comprising the steps of: a. pre-cleaning the substrate with soap solution/ethanol for 10 min and rinse with distilled water. b. preparing graphene oxide powder from graphite flakes using the modified Hummers’ method, c. preparing graphene oxide solution by adding 400 mg of graphene oxide in 50 ml of distilled water under probe sonication for 90 min, d. adding 0.1 wt% oxalic acid, 0.1 wt% aluminum isopropoxide and 9.8 wt% ascorbic acid to the graphene oxide solution as obtained above and stirring for 30 min, e. depositing the graphene oxide solution as obtained in step (d) on the substrate at 150°C by spray pyrolysis technique to obtain a graphene oxide based absorber layer with a thickness of 3-5 pm, f. preparing MoCh/talc/ascorbic acid solution by adding 30 mg of molybdenum oxide, 10 mg of talc powders and 9.8 wt% ascorbic acid in 20 ml of distilled water under magnetic stirring for 90 minutes, g. depositing the inorganic protective layer by using the MoCh/talc/ascorbic acid solution as obtained in step (f) on top of the GO absorber layer at 150°C by spray pyrolysis technique to obtain a coating with a thickness of 1-2 pm, h. drying of graphene oxide/ MoOa-based spectrally selective coating as obtained in step (g) at 150° C for 5 min.

In an embodiment of present invention, wherein spectrally selective coating is RoHS and REACH-compliant and exhibits high UV stability, high corrosion resistant, high humidity resistance, good chemical inertness and useful in low and mid-temperature applications for harnessing solar energy.

Detailed description of the invention

The foregoing detailed description of the disclosure is elaborated to provide a clear understanding to the person who is skilled in the art. Additional features, embodiments, and advantages of the invention will be described hereinafter which form the subject of the claims of the disclosure, however, the set forth disclosure provided in the specification will best be understood in conjunction with the appended claims and figures as provided heretofore. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent processes do not depart from the spirit and scope of the disclosure as set forth in the appended claims. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and make part of this disclosure.

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.

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. In line with the above objectives, the present invention relates to the spectrally selective coating consisting of Graphene Oxide (GO) based absorber layer followed by deposition of inorganic protective layer using a wet chemical process.

The coating of the present invention is suitable for deposition on metallic and non-metallic substrates like aluminum, glass, copper, galvanized iron, stainless steel, and mild steel. Before deposition, the substrates (dimensions 50 mm x 70 mm x 1 mm) were chemically cleaned in soap solution/ethanol and rinse with distilled water. Graphene oxide (GO) was prepared from graphite flakes (98% purity, 60 mesh, Loba Chemie Pvt. Ltd.) using the modified Hummers’ method. Initially, 9 g of graphite was added to 300 ml of H2SO4 (98% concentrated), and the solution was stirred vigorously for 30 min at room temperature. Subsequently, the solution beaker was immersed in an ice bath (temperature below 0°C), and potassium permanganate (KMnO4, 98.5% concentrated) was added slowly. The solution was stirred continuously for 30 min while maintaining the temperature below 5°C. After complete oxidation, the solution was heated to 45°C on a hot plate and stirred for 6 hrs. Next, distilled water was added slowly into the solution to avoid overheating; the solution temperature was maintained at 95 °C using a hot plate. Finally, 400 ml of distilled water together with 15 ml of H2O2 (30%) was added to terminate the oxidation reaction.

After the Graphene oxide (GO) solution settled down, the supernatant was removed, and the sedimented graphite oxide was filtered using a Whatman filter paper (42 grade, 0.2 pm). The filtered graphite oxide was washed several times with dilute HC1 (35%) to remove the metal ions. A dark brown gel of graphite oxide was obtained which was vacuum dried at 40°C for 12 hrs.

The absorptance and emittance of the spectrally selective coatings were measured using a solar spectrum reflectometer and emissometer, respectively (Devices and Services, USA). The measurement accuracies of the absorptance and emittance were ±0.002 and ±0.01, respectively. The coating of the present invention was deposited on various substrates as given in the examples accompanying this specification. The novelty of the present invention is in providing an efficient spectrally selective solar absorber coating having a solar selectivity in the order of 3 to 4. Novelty of present invention also lies in employing a novel combination of materials and optimizing the operating conditions to provide spectrally selective coating suitable for domestic hot water applications.

The above-mentioned novelty of the present invention has been achieved by adopting the following non-obvious inventive steps:

1. Depositing absorber layer from graphene oxide-based composition, wherein distilled water has been used as a solvent.

2. Synthesis of graphene oxide in an appropriate C/O concentration so that the composite solar absorber coating exhibits high absorptance, low emittance good thermal stability, and good adhesion at medium temperatures (i.e., up to 200°C).

3. A protective layer containing molybdenum oxide and talc was deposited on top of the GO absorber layer.

The candidate materials selected for the coatings of the present invention i.e., graphene oxide exhibits good optical properties and high thermal stability. In the present invention, graphene oxide (GO) was prepared by the modified Hummers’ method, using ROHS and REACH-compliant chemicals. The absorber material graphene oxide (GO) exhibits high solar absorptance and good thermal stability. This has been reflected in the optical properties of the coated substrate as given in the present invention. The candidate materials such as graphite flakes molybdenum oxide, ascorbic acid, oxalic acid, talc, and aluminum isopropoxide are RoHS and REACH compliant. Thus, the present invention qualifies the novelty and inventiveness of invention.

EXAMPLES

The following examples, which include preferred embodiments, will serve to illustrate the practice of this invention, it being understood that the particulars shown are by way of example and for purpose of illustrative discussion of preferred embodiments of the invention.

Example-1 Graphene oxide powder was prepared using the modified Hummers’ method. Graphite flakes (9 g) were used as source material and it was added in 300 ml of sulphuric acid. Subsequently, the addition of KMnCM, H2O2, and HC1 in the above-mentioned solution with probe sonication for 30 min resulted in graphene oxide. Four hundred milligrams of GO (0.5-2 wt%) was added to 50 ml of distilled water to prepare the GO solution. The solution was probe sonicated for 30 min and later mixed with oxalic acid (0.1 wt%), ascorbic acid (9.8 wt%), and aluminum isopropoxide (0.1 wt%). After stirring for 30 min, the GO-based dispersion was applied on the substrates by spray pyrolysis. The thickness of the coating was 3-5 pm. Before deposition, the substrates (metallic) were chemically cleaned using an soap solution/ethanol for 10 min and rinse with distilled water. The graphene oxide solution was applied to the aluminum substrate by spray pyrolysis technique. Similarly, an inorganic protective layer was applied on top of the GO-based absorber coating. Molybdenum oxide (0.12 wt%), talc (0.05 wt%), and water (90 wt%) were mixed suitably to produce the molybdenum oxide solution. Subsequently, 9.8 wt% ascorbic acid was added to the solution and stirred for 30 min. The absorptance and the emittance values of Al substrate, GO-absorber coating, and GO/MoOs coating are given in Table 1. The optimized spectrally selective coating exhibits absorptance in the range of 0.91 - 0.93 and emittance in the range of 0.21 - 0.30.

Table-1: Absorptance and emittance of different layers of the solar selective coating system

Example-2

The solar absorber coating of the present invention as mentioned in Example 1 was also deposited on other metal/non-metal substrates like copper, aluminum, glass, stainless steel, mild steel, and galvanized iron. The substrates were chosen for low and mid-temperature applications because these substrates are also used for domestic hot water applications. The absorptance and the emittance values of the GO-based solar absorber coating on these substrates are given in Table 2.

Table-2: Absorptance and emittance of solar selective coating deposited on various substrates.

# Formation of copper oxide at 150°C; *Semi-transparent

Example-3

The thermal stability of the GO-based spectrally selective coating of the present invention, as prepared in Example 1, was tested by heating in the air in a resistive furnace at temperatures in the range of 150-250°C for a duration of 7 hrs. The temperature of the sample was increased from room temperature to the desired temperature at a slow heating rate of 5°C/min, and the temperature was maintained for 7 hrs. The temperature controller has an accuracy of ±1°C. The values for absorptance and emittance of the spectrally selective GO coatings deposited on Al substrates after heat treatment are listed in Table 3. The optical properties of the coating are almost unchanged even after annealing in air at 200°C.

Table-3: Effect of annealing on optical properties of the GO/MoOa solar selective coating deposited on Al substrate under cyclic heating conditions.

Example-4

The GO/MoOs -based spectrally selective coatings, as prepared in Example 1 of the present invention, was heat-treated in the air for long periods. The heating rate of the furnace and the accuracy of the temperature controller were 5°C/min and ±1°C, respectively. The absorptance and the emittance values of the heat-treated coatings on Al substrates are listed in Table 4. No significant change in the absorptance was observed even after annealing in air at 175° C for 600 hrs. However, the emittance was reduced drastically from 0.31 to 0.18. This indicates that the coatings of the present invention are thermally stable in air at 175°C for longer durations as per International Energy Standards.

Table-4: Effect of annealing (in the air at 175°C) on optical properties of the GO/MoOs solar selective coating deposited on Al substrate under cyclic heating conditions.

Example-5

The coating deposited on aluminum substrates was subjected to thermal shock resistance tests at 150°C in air. For this, the sample was loaded directly into a furnace preheated to 150°C for 10 min and was taken out immediately. The sample was kept in an open-air atmosphere for two min. Subsequently, the sample was placed back inside the furnace, and the above-mentioned procedure was repeated 150 times. The absorptance and the emittance values are shown in Table 5. The results clearly show that there is no significant degradation in the optical properties of the coatings up to 150 cycles.

Table-5:Absorptance and emittance data of GO/MoOa solar selective coating deposited on the aluminum substrate after thermal shock test (150 °C).

Example-6 The humidity resistance of the GO-based spectrally selective coating of the present invention, as prepared in Example 1, was tested by exposing the sample at 40°C at 95% RH for a period of 600 hrs in a climate chamber. The coatings were tested as per the International Energy Agency Task 27 standard. The values for absorptance and emittance of the spectrally selective GO coatings deposited on Al substrates after the humidity test are listed in Table 6.

Table-6:Absorptance and emittance data of GO/MoOa solar selective coating deposited on the aluminum substrate after humidity test (40 °C and 95% RH).

Example-7

The coating as prepared in Example 1 of the present invention was exposed accelerated weathering test for 300 hrs as per ASTM G 154 cycle 1. The test parameters are: Lamp type: UVA-340; Irradiance - 0.89 W/m²; UV cycle - 8 hours (Temperature = 60°C);

Condensation cycle - 4 hours (Temperature = 50°C). The absorptance and emittance values are listed in Table 7.

Table-7: Absorptance and emittance data of solar selective coating deposited on aluminum substrate after accelerated weathering test. Example-8

The corrosion resistance of the GO-based spectrally selective coating of the present invention, as prepared in Example 1 , was tested by exposing the sample to sulfur dioxide gas at 20°C with 95% RH for a period of 150 hrs in a gas chamber. The concentration of the SO2 gas was 500 ± 20 ppb. The coating was tested as per the ISO 10062: 2006 standards. The values for absorptance and emittance of the spectrally selective GO coating deposited on Al substrates after the test are listed in Table 8.

Table-8: Absorptance and emittance data of GO/MoOa solar selective coating deposited on the aluminum substrate after corrosion resistance test (20 °C; 95% RH and SO2: 500 ± 20 ppb).

Example-9

The coating as prepared in Example 1 of the present invention was exposed to freezing conditions in a freezer for long durations (4000 hrs) to determine the stability of the coating by placing them at low temperatures (< -2°C). The optical properties of the sample before and after exposure to freezing conditions are listed in Table 9.

Table-9: Absorptance and emittance data of solar selective coating deposited on aluminum substrate after freezing tests. ADVANTAGES OF THE INVENTION

1. Cost-effective, ecofriendly process and abundantly available cheap materials (graphite, oxalic acid, ascorbic acid, and aluminum isopropoxide) for the preparation of spectrally selective coating;

2. No hazardous chemicals and volatile organic compounds were used to prepare the GO- based dispersion;

3. Thermally stable coating in the air up to 175 °C for 600 hrs, having high solar absorptance (0.90 on Al substrates) and easy to scale up for industrial applications;

4. Optical properties of the coatings do not degrade even after heating in the air for longer durations;

5. Coating of the present invention is UV resistant, humidity resistant, chemically inert, RoHS compliant and REACH compliant.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obviously, many modifications and variations are possible in the light of the above teaching.

The embodiments were chosen and described to best explain the principles of the present invention and its practical application, to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated.

Claims

We Claim:

1. A graphene oxide-based spectrally selective solar absorber coating on metal and non- metal having a thickness of 4-7 pm is essentially consisting of two layers; i. graphene oxide based absorber layer containing carbon (50.3 at%) and oxygen (49.7 at%); and ii. an inorganic top protection layer containing Molybdenum oxide (2 wt%), talc (2 wt%), ascorbic acid (1 wt%), and water (95%).

2. A process for preparing spectrally selective solar absorber coating as claimed in claim 1 , comprising the steps of: a. pre-cleaning the metallic and non-metallic substrate with soap solution or ethanol for 10-20 min and rinse with distilled water; b. preparing graphene oxide powder from graphite flakes using the modified Hummers’ method; c. mixing 400-500 mg of graphene oxide powder as obtained in step (b) in 50-70 ml of distilled water under sonication for 90-100 min to make graphene oxide solution; d. adding 0.1-0.3 wt% oxalic acid, 0.1-0.3 wt% aluminum isopropoxide and 9-10 wt% ascorbic acid to the graphene oxide solution as obtained in step (c) and stirred for 30-40 min; e. depositing the graphene oxide solution as obtained in step (d) on the metallic and non-metallic substrate at 150°C by spray pyrolysis technique to obtain graphene oxide based absorber layer with a thickness of 3-5pm; f. preparing inorganic protective layer solution by adding 30-50 mg of inorganic materials and 9-10 wt% ascorbic acid in 20-30 ml of distilled water under magnetic stirring for 90-100 minutes; g. depositing the inorganic protective layer solution as obtained in step (f) on top of the graphene oxide absorber layer at 150°C-170 °C by spray pyrolysis technique to obtain a coating with a thickness of 1-2 pm, h. drying of graphene oxide/MoCh-based spectrally selective coating as obtained in step (g) at 150° C for 5-10 min .

3. The process as claimed in claim 2, wherein metallic and non-metallic substrate selected form the group consisting of aluminum, copper, galvanized iron, glass, stainless steel and mild steel.

4. The process as claimed in claim 2, wherein inorganic materials used in step [f] selected from group consisting of molybdenum oxide and talc for preparing inorganic protection layer solution.

5. The process as claimed in claim 2, wherein solution for the absorber layer having viscosity of 58-60 cP and pH in the range of 1-3 is obtained by dissolving graphene oxide, oxalic acid, aluminum isopropoxide and ascorbic acid in distilled water.

6. The process as claimed in claim 2, wherein the ascorbic acid is act as a binder, oxalic acid as a chelating agent, aluminum isopropoxide as an enhancer while preparing solution for absorber layer.

7. The process as claimed in claim 2, wherein solution for the inorganic top protection layer having viscosity of 6-7 cP is obtained by dissolving molybdenum oxide, talc powder and ascorbic acid in distilled water.

8. A graphene oxide-based spectrally selective solar absorber coating as claimed in claim 1, exhibits absorptance in the range of 0.91 - 0.93 and emittance in the range of 0.21- 0.30.