New hybrid graphene/inorganic salt composites for thermochemical energy storage: Synthesis, cyclability investigation and heat exchanger metal corrosion protection performance

https://doi.org/10.1016/j.solmat.2020.110601Get rights and content

Highlights

  • New hybrid composites based on graphene sheets incorporated inorganic salts proposed as promising TCMs.

  • Gr-MgS, Gr-MgC, and Gr-MgSC showed interesting heat storage densities with good cycling stability.

  • PU@Gr coating provided high protection efficiency up to ≈99% and act as a physical barrier against TCMs corrosive behavior.

Abstract

Herein, new hybrid thermochemical materials (TCMs) combining MgSO4, MgCl2, and their mixture incorporated into the graphene matrix have been prepared for low to medium temperature heat storage applications. These new hybrid materials were developed to solve agglomeration, cyclability and corrosion issues during hydration/dehydration cycles. The preliminary results of X-Ray diffraction, Raman spectroscopy, and scanning electron microscopy confirmed that the inorganic salts were successfully impregnated into the matrix without providing undesirable reactions. Calorimetric analysis (DSC-TGA) revealed a significant improvement of cycling stability up to 60 hydration/dehydration cycles, and an excellent storage density of 1194.3 J/g and 890 J/g was reached for MgSO4 and MgCl2 composites respectively. In order to protect the heat exchanger metal from the TCMs behaviour, two coating films of polyurethane and polyurethane-graphene have been applied. The scanning electron microscopy showed the good dispersion of graphene into polyurethane film. The electrochemical measurements under storage reactor conditions were examined using conventional methods such as gravimetric tests, polarization techniques and electrochemical impedance spectroscopy (EIS). An inhibition efficiency up to ≈99% for coated metal was reached and suitable corrosion mechanism for uncoated and coated copper was proposed.

Introduction

The exhausting consumptions and the strong energy demand requires the adoption of alternatives policies involving renewable energies [1]. Solar energy is one of the most abundant renewable energy with the potential to meet residential heating needs. The challenge is to employ locally this energy using decentralization system. Indeed, thermal energy storage (TES) has received attractive attention owing to its high storage capacity using different technologies [[2], [3], [4]]. Thermochemical storage (TCS) promotes great storage potential for an indeterminate time without causing thermal losses and high volume storage density which is a critical feature in the building sector. Furthermore, thermochemical storage materials (TCMs) can store energy via reversible chemical reactions and/or solid-gas sorption process. The hydrated salt as TCMs provides excellent storage density with promising efficiency. In this context, several papers discussed the criteria to consider when choosing a TCM [5,6]. However, cyclability is the most common issue for using hydrated salts that mainly depends on the reversibility of hydration/dehydration reactions whereas strong negative effects like: melting, agglomeration or overhydration phenomena could be recorded [[7], [8], [9]].

The high hydration enthalpy of MgCl2 could be useful for TCS, but the hygroscopic behavior of this hydrated salt at the conditions of RH = 33% and a temperature range from 20 °C to 100 °C have a strong impact on the storage density and involve the irreversibility issues. Also, the thermal decomposition of MgC at 150 °C into MgOHCl and HCl as irreversible products causes severe corrosion and cyclic stability problems [[10], [11], [12]]. Even if the hydrated salt of MgSO4 (high DRH≈ 90% at T(°C) = 30) has a promising reaction enthalpy and good structural stability [5,13], its low hydrance kinetics seems to be a problem to admit it as TCM [13,14]. To address the issues and to get desirable hydration enthalpy and good structural stability with appreciable hydration kinetics, we propose herein the use of the mixture of these hydrates with similar cation (Mg2+) to avoid undesirable interactions, to enhance storage properties and to reduce the corrosion issues. A new composites materials (hydrated salt impregnated in a porous matrix) have been widely investigated, Shere et al. [15] have reported that the impregnation of MgSO4 and MgCl2 mixture (50:50 ratio) in zeolite 13X revealed a storage density of 400 kJ/mol, while the use of other salts combinations have significantly reduced the storage density by blocking the sorption pores of the zeolite matrix. Posern K et al. [16] have confirmed that an optimum ratio of MgSO4/MgCl2 (20:80 impregnated in attapulgite) provides heat sorption of 1590 J/g (T = 30 °C; RH = 85%) with a desorption temperature of 130 °C but these results need to be confirmed using cyclic sorption/desorption investigations.

TCMs composites require the harmonization between charging/discharging conditions of matrix and impregnated salt which is often difficult to achieve [17]. The high regeneration temperature of matrices such as zeolite (T(°C) > 180) [18,19], and graphite (T(°C) > 200) [20] present strong issues reducing the global storage efficiency. Moreover, one of the most important factors for a potential TCS system is the heat transfer that depends essentially on the thermal conductivity of the TCMs. The use of porous matrices with high thermal conductivity facilitates the heat exchange during the discharging stage and consequently enhances the TCS system efficiency. Indeed, numerous composites based on different matrices with good thermal conductivity such as expanded graphite [21], graphene oxide aerogel [22], 3D graphene derived from cellulose [23], graphene oxide [24] have been reported.

For a sustainable thermochemical storage system, one of the main drawbacks that need to be improved is the corrosive behavior of hydrated salts in contact with the structural materials such as metals. Indeed, the conditions of moisture and temperature could produce the worst environment increasing the corrosive potential of TCMs in contact with heat exchanger materials, especially copper metal. Few studies were found concerning the evaluation of corrosion behavior of hydrated salts as TCMs under the reactor conditions. Solé et al. [25] reported the gravimetric test of numerous hydrated salts including MgSO4.7H2O and MgCl2.6H2O using a controlled chamber under RH = 99%, a temperature of 60 °C and atmospheric pressure. The authors reported a significant corrosion rate value up to 49.8 mg cm-2 y-1 and 60.2 mg cm-2 y-1 for MgSO4 and MgCl2 respectively. The surface analysis indicated the surface damage of copper metal and a recommendation for a service lower than one year was concluded. Similarly, Fernandez et al. [26] studied the corrosion behavior of copper under severe discharging conditions of MgSO4.7H2O (high temperature of 130 °C and vacuum pressure of 20 mbar). The authors have confirmed a low corrosion rate and good recommendation of copper metal using MgSO4.7H2O as TCM. The electrochemical behavior of copper in the presence of hydrated salts containing SO42− have been studied by Zhao et al. [27]. The authors showed that copper suffers an initial rapid anodic dissolution confirming the increase in current density and the long active region measured by potentiodynamic polarization measurements. The active dissolution tends to be retarded by the formation of oxide passive film confirmed by the current density stabilization at 0.1 mA/cm2. From these studies, it could be concluded that the corrosive medium composition plays a worthy parameter to consider. The differences of the corrosive anions properties have a major effect on corrosion propagation and metal durability. Soltis et al. [28] and Shaheen et al. [29] showed that chlorides migrate fastly due to their high mobility compared to other anions like CO3-, SO42− which explain rapid destruction of the passive film in the media-rich in chloride. Yang et al. [30] have reported that the addition of SO42− ions to chloride media reduce localized corrosive behavior, and a small chloride amount in corrosion products was recorded thanks to the easy adsorption of SO42−. Metal inhibition in chloride media using sulfates has been reported in several investigations [31,32]. Hence, the interest of this work is to evaluate the overall behavior of MgSO4/MgCl2 mixture (MgSC) not only to achieve good storage performance but also to improve HX metal corrosion resistance. To minimize corrosion of copper metal, many compounds have been also studied as corrosion inhibitors [33,34]. Nevertheless, the use of inhibitor is not suitable in the case of TES systems due to their ability to react with hydrated salts and produce undesirable complexes compounds [35]. Polymeric coatings method can be the best way to protect metals against corrosion [36,37]. Various coatings have been tested in the literature owing to their efficiency to isolate metals from electrolytes according to their hydrophilic chemical groups, good adhesion, high cross-linking density, and their great chemical stability [[38], [39], [40], [41]].

Liu et al. [42] showed an improvement in corrosion resistance of coated Q235 steel/waterborne epoxy against salt spray and confirmed that the addition of graphene (Gr) to epoxy film prevents the diffusion phenomenon through the coating pores. Similarly, Zhou et al. [43] have studied the corrosion behavior in 3.5 %wt NaCl of coated steel metal with reinforced epoxy polymer using several structures of Gr fillers. In this experiment, the authors reported good anti-corrosive metal resistance and highlighted the impermeability of the used coatings against the aggressive species. Additionally, the Gr amount into the coating composite could have a strong impact on the anti-corrosive efficiency of the coating. Alhumade et al. [44] confirmed that an optimum amount of 1 wt% Gr is required to prevent metal corrosion and to obtain a good dispersion of Gr into polyetherimide film. Indeed, the Gr dispersion into the coating film is a key factor related to the synthesis method of the composite coating film [45]. Furthermore, the superhydrophobic and impermeability properties of the coating film using graphene could be beneficial to minimize water uptake, avoid electrolyte diffusion and protect the metal substrate [46,47].

The article's number concerning the synthesis of TCMs, their properties, and their storage efficiency has increased, but few studies have been reported considering other parameters such as the cyclability of TCMs composites and the corrosive behavior of hydrated salts under TCS storage conditions. In this paper, new hybrid TCMs based on graphene matrix incorporated inorganic hydrated salts are developed and an assessment of their performance as TCMs during the dehydration phase is examined using several hydration/dehydration cycles. Furthermore, the corrosive behavior of uncoated and coated copper metal using graphene reinforced polyurethane PU@Gr coating in hydrated salts of MgS, MgC, and MgSC is investigated under storage conditions using surface analyses, immersion corrosion test, and electrochemical measurements.

Section snippets

Synthesis of graphene and TCMs composites

Graphite oxide (GO) was synthesized using Hummers' method [48], 1 g of graphite powder and 0.5 g of NaNO3 were mixed, then 23 ml of H2SO4 was added under constant stirring. After 1 h, 3 g of KMnO4 was added gradually to the solution at a temperature below 20 °C, an ice bath was used to prevent overheating or explosion (Fig. 1). After stirring for 12 h at 35 °C, the resulted solution was diluted by adding 500 ml of water under vigorous stirring (very exothermic reaction). To ensure the complete

Graphene and TCMs composites materials characterization

As shown in Fig. 4, the diffractogram of graphite shows an intense diffraction peak at 2θ = 26.32°, corresponding to an interlayer spacing (d002) of 0.3385 nm [52,53]. The intensity of this peak was almost completely disappeared after the exfoliation process while a new peak (d001) at 2θ = 10.18° was found with a new calculated d001 interlayers distance of 0.8675 nm. This result confirmed that graphite was completely altered and transformed into GO compound [53,54]. The reduction of GO using

Conclusions

In this paper, new hybrid materials composites consisting of MgS, MgC, and MgSC incorporated into the graphene sheets matrix was successfully synthesized and characterized. This work aimed to investigate the ability of these materials for thermochemical heat storage applications over a temperature (T(°C) ≤ 150). Cyclic wetness impregnation protocol was proposed to improve the cycling stability and ensure good incorporation of hydrated salts into graphene matrix, while the corrosion behavior of

CRediT authorship contribution statement

Hanane Ait Ousaleh: Conceptualization, Funding acquisition, Formal analysis, Writing - original draft. Said Sair: Conceptualization, Funding acquisition, Formal analysis, Writing - original draft. Said Mansouri: Funding acquisition. Younes Abboud: Funding acquisition, Formal analysis. Abdessamad Faik: Conceptualization. Abdeslam El Bouari: Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We express our great thanks to the center of material and energy thematic research (CeRTheME) for the technical supports of this research.

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