Graphene nanoplatelets as an anticorrosion additive for solar absorber coatings

https://doi.org/10.1016/j.solmat.2017.11.016Get rights and content

Highlights

  • Successful synthesis of one to few layer graphene nanoplatelets by chemical exfoliation.

  • Grahene nanoplatelets used as a corrosion inhibition agent in sol – gel coating.

  • Increase in thermal emittance values with addition of new agent from 0.28 to 0.15.

  • Solar absorptance of 0.91 and thermal emittance of 0.15 on aluminium substrates.

Abstract

Efficiently transforming solar energy into heat requires an understanding of spectrally selective coatings at the atomic, molecular, corrosive and spectrally selective levels. Here, experiments were conducted and analyzed to rationalize and ultimately understand the complex behaviours of spectrally selective coatings in corrosion environments. By exploring reactions that modify graphene nanoplatelets, the preparation of stable dispersions, the incorporation in sol-gel binder, and the preparation of spectrally selective coatings, we demonstrated that (i) a successful graphene nanoplatelet modification is important for their incorporation into binders as sol-gels, (ii) the modified products do not influence the optical properties of the coatings, (iii) incorporating nanoplatelets drastically improves corrosion resistance, and (iv) thinner coatings can be used to acheieve the same anticorrosion properties as other treatments. We believe this experimental insight provides a pathway for the rational design of stable spectrally selective paint coatings that are urgently needed for the development of a new generation of reliable and affordable absorber coatings for efficient solar energy harvesting.

Introduction

Solar heating and cooling can provide low-carbon emission energy from a resource that is widespread throughout the world. Solar heating and cooling describes a wide range of technologies, from mature domestic hot water heaters to those just entering the demonstration phase, such as solar thermally driven cooling. Solar collectors for hot water and space heating could reach an installed capacity of nearly 3500 GWth, annually satisfying approximately 8.9 EJ of energy demand for hot water and space heating in the building sector by 2050. Solar hot water and space heating will account for 14% of space and water heating energy use by that time, according to the International Energy Agency [1].

In the year 2015, a total capacity of 40.2 GWth, corresponding to 57.4 million square meters of solar collectors, was installed worldwide. With a global share of 71.5%, evacuated tube collectors were the predominant solar thermal collector technology, followed by flat plate collectors, with 22.0%, and unglazed water collectors, with 6.2% [2]. Prospects and expectations in this field are very high. Solar absorber surfaces are produced by several deposition procedures such as physical vapour deposition [3], chemical vapour deposition [4], electrodeposition [5], sol-gel [6], and paint-coating [7]. These surfaces have been pursued to see if they meet the criteria of a solar selective coating. Mirosol® TS [8], produced by AlanodSolar, is (in addition to physical vapour deposition coatings) one of the rare industrial processes that enables a wet coil coating process resulting in the mass production of spectrally selective surfaces for solar thermal applications.

To maintain a high collector efficiency during the expected 25-year operation time, substantial effort was made on producing spectrally selective coatings anticorrosion protection using sol-gel [9] and magnetron sputtering methods [10]. Additionally, anticorrosion properties are improved by antireflection layers. Increasing the absorbance by the antireflection layer of solar spectrally selective coatings often involves sacrificing other performance properties, such as thermal emittance and thermal stability of the coating. Market demands are oriented towards low thermal emittance coatings, which are achieved in practice by decreasing the coating thickness. Such coatings are used in areas where corrosion is a large problem, usually because of humid environments such as those found in Brazil or marine environments in coastal regions worldwide.

It seems that the 21st century is becoming the age of graphene, a recently discovered material made from an atomically thick carbon sheet [11]. Graphene has been studied theoretically for the past 65 years [12], [13], [14], [15], but 10 years ago the demonstration of exfoliated graphene allowed for experimental studies [11], [16]. In the following years, graphene was intensely researched for various industrial applications [17], [18], [19], [20], [21], [22], [23], [24]. Its properties are remarkable and include excellent carrier mobility [25], [26], optical transmittance in the visible region (up to 97%) [27], [28], [29], [30], chemical inertness and thermal stability [31], and mechanical strength [32], [33], [34], [35]. Since using graphene confers so many advantages, there are vast opportunities for its use, such as the incorporation into the coatings and paints. Graphene enables a wide spectrum of ultra–thin functional coatings. Among these are anti-bacterial coatings [36], anti-corrosion coatings [36], [37], [38], [39], [40], anti-fog coatings, non-stick coatings, solar coatings [41], [42], [43].

In this paper, we focus on spectrally selective coatings with improved anticorrosion properties. Scientists agree that, there is an eminent problem in maintaining the high efficiency of solar absorber coatings during their 25-year lifetime. The performance criterion (PC) is a useful criterion that describes the influence of changes in the solar absorbance (ΔaS) and thermal emittance (ΔeT) with respect to the solar fraction: PC = -ΔaS + 0.5ΔeT ≤ 0.05 [44]. Metal substrates require protective coatings to increase the life span of the substrate, as oxidation causes increased emittance. The US Department of Defense estimates that corrosion-related expenses are approximately $20 billion per year [45]. Because of its structure, graphene forms a natural diffusion barrier between the oxidants/reactants and the protected surface [46]. Researchers rapidly developed a graphene–based, oxidative barrier coating for metal substrates [45], [47], [48], [49], [50], [51] but we believe no one used graphene for solar applications. Considering the large quantities of absorber surfaces being used around the world, and its expected growth until 2050, graphene-based solar selective coatings produced by coil coating could become the most viable and popular method for solar energy harvesting. More durable and longer performing materials would strongly reduce the overall lifetime costs by reducing maintenance and lengthening service intervals.

The mass production of graphene from graphite results in graphene platelets (3–5 layers thick) [52]. These graphene nanoplatelets quickly agglomerate [53], [54]. Consequently, they normally need to be functionalized to create easily applied dispersions and coatings that are more homogenous. Furthermore, because of the planar structure of the graphene nanoplatelets, unimpeded solar absorptivity is expected [55].

To the best of our knowledge, there are no reports in the literature detailing how graphene nanoplatelets influence the anti-corrosion properties of spectrally selective coating. With the industrial production of graphene nanoplatelets by companies such as Talga Resources ltd., additives with large-scale productions can be used in products such as spectrally selective coatings.

The objective of this paper is to show how to properly prepare and incorporate functionalized graphene nanoplatelets into spectrally selective paint coatings to improve their anticorrosion properties for solar thermal application, where the coating thickness is of extreme importance. The preparation and properties of the graphene nanoplatelets additives and coatings using these additives are reported, discussed and supported by several analyses in detail.

Section snippets

Materials

Graphite flakes, sodium chloride (NaCl), iodopropane, butanol, dimethoxyethane (DME), tetraethoxysilane (TEOS), (3-iodopropyl)trimethoxysilane (ITMS) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 37%), 2-propanol and tetrahydrofuran (THF) were obtained from Merck. Perfluorooctyl iodide was purchased from Apollo scientific Ltd. Sodium (Na) sticks were obtained from Fluka and potassium (K) was obtained from ABCR GmbH. Ethanol (absolute) was acquired from Carlo Erba Reagents.

Scanning electron microscopy (SEM) analysis

Using scanning electron microscopy (SEM), we evaluated the success rate of our exfoliation. In Fig. 3a, the graphite flake before the exfoliation can be seen. A stack of graphene layers is evident in the SEM image. Fig. 3b shows the exfoliated graphene nanoplatelets after functionalization with iodopropane (Sample C). As expected, the multilayer graphene is present. The sheets are also twisted and wrinkled. There are different sizes and shapes of graphene nanoplatelets present.

Characterization of the graphene nanoplatelets by Raman spectroscopy

Raman

Discussion

Graphene, thanks to its mechanical, electronic, thermal and optical properties, can fulfil multiple functions in spectrally selective coatings. The effectiveness of the solar thermal absorbers depends on various properties of the coating, among which the values of solar absorbance and thermal emittance are of critical importance. In the interest of outstanding spectral selectivity of the coating, the paints require great pigment loading, which increases the solar absorptivity and low thermal

Conclusions

We report the synthesis and properties of functionalized graphene nanoplatelets used as an anticorrosion additive for spectrally selective coatings. The structure and morphology of the modified graphene platelets was uncovered by scanning electron microscopy, Raman spectroscopy, transmission electron microscopy, atomic force microscopy and thermogravimetric analysis. It was demonstrated that modified multilayer graphene nanoplatelets display outstanding dispersibility in many solvents suitable

Acknowledgements

The authors acknowledge financial support from the Slovenian Research Agency (research core funding No. P2-0393 and High-Performance Nanostructured Coatings–breakthrough in concentrated solar power; J2-7371). E. Šest is grateful to the National Institute of Chemistry for funding his Master's degree. The authors would like to thank Dr. Dimitrios Peros for providing a sol-gel binder and the aluminium substrates samples.

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