Thermal properties and shape stabilization of epoxidized methoxy polyethylene glycol composite PCMs tailored by polydopamine-functionalized graphene oxide

Highlights

• EmPEG/PDA-rGO cPCMs present the good stabilized property via the interfacial interaction.

• Good thermal reliability and buffering effect is demonstrated.

• Combined thermal storage and light-to-thermal conversion capability is realized.

Abstract

A series of composite phase change materials (cPCMs) are fabricated through the one-pot method with the epoxidized methoxy polyethylene glycol (EmPEG) as the thermal energy unit and polydopamine-reduced graphene oxide (PDA-rGO) nanosheets as the interfacial enhancement. Structure and thermal properties such as phase change enthalpy, thermal repeatability, and light-to-thermal conversion capacity of EmPEG/PDA-rGO cPCMs with PDA-rGO content varying from 0.5 to 4 wt% have been detailed discussed. A superior stabilization performance of EmPEG/PDA-rGO cPCMs to pristine EmPEG is demonstrated based on the interfacial action from the incorporated PDA-rGO nanosheets. EmPEG/PDA-rGO-3 presents good structure-stabilized behavior after 100 heating-cooling cycles, and no liquid leakage appears up to 105 °C. In the meantime, EmPEG/PDA-rGO-3 illustrates a stable thermal reliability and thermal charging/discharging rate, and the light-to-thermal conversion efficiency reaches 96% under the 100 mW/cm² irradiation. Thus, the structure-stabilized EmPEG/PDA-rGO cPCMs tuned by the interfacial interaction demonstrates the promising application in the field of the thermal energy storage and solar energy conversion.

Introduction

Thermal energy storage (TES) materials have attracted extensive interest in recent years [[1], [2], [3], [4], [5], [6]]. As one kind of important TES materials, phase change materials (PCMs), i.e., organic and inorganic compounds, have been widely explored due to their large phase change enthalpy and the adaptable temperature. And, PCMs with high energy efficiency and the facile preparation is highly desirable for the large-scale thermal management and TES application.

Recently, the structure- and shape-stabilized PCMs are deeply studied, and different techniques such as blending, microencapsulation, and polymer chemistry have been utilized [7]. Lots of supporting materials such as polymer [8], active carbon [9,10], expanded graphite [9,11,12], graphene oxide (GO) [[13], [14], [15], [16]], graphene nanomaterials [[17], [18], [19]], graphene aerogel [20,21] and carbon aerogel [22] have been used, and the shape-stabilized composite PCMs are fabricated through a simple mixing process or vacuum-infiltration method. In terms of the doped concentration of the supporting matrix and the compatibility with PCMs, the desired thermal performance is different, accompanied with the varied thermal conductivity, the stabilization, and the processability [[23], [24], [25], [26], [27], [28]]. In the meantime, the surface structure of supporting matrix and its interfacial action are of importance to approach the stabilized PCMs owing to the varied polarity of organic PCMs [[29], [30], [31], [32]] such as paraffin [33], n-alkyl alcohol [34] and polyethylene glycol (PEG), etc.

Polyethylene glycol (PEG) is biocompatible and eco-friendly polymer PCMs [[35], [36], [37]], and it has been applied in the field of textiles [38], temperature-responsive delivery and the functional coating [39]. During the melting-freezing process of PEG PCMs, however, the surface softening phenomenon and the liquid penetration problem seriously limit the large-scale application. Different methods such as physical blending [40], suction/adsorption [[41], [42], [43]], and chemical modification [44], etc., are exploited to develop the structure-stabilized PEG-based PCMs. Wang et al. reported the preparation of the shape-stabilized PEG/GO cPCMs [15], and a stabilization behavior above the melting temperature of PEG is indicated at loading 10 wt% GO. They thought the formed H-bonding interaction between PEG and GO plays a stabilization role and restricts the flowing of the molten PEG molecules. Xiong et al. also demonstrated the good shape-stable performance of PEG/GO composite based on the formed hydrogen bonding and the capillary force between the GO layers, and no leakage behavior appears at 80 ^oC for 12 h [45]. Besides, Yang et al. prepared a series of hybrid graphene aerogel/PEG cPCMs via vacuum impregnation method [20], and a remarkably enhanced thermal conductivity was indicated. However, the liquid leakage also appeared at 90 ^oC, indicating that the stabilization of PEG cannot get an enhancement under the support of a three-dimensional GO network. Thus, a good shape stabilization performance of PCMs not only relies on the structural state of the supporting matrix, but also on their interfacial interaction and compatibility.

For the infiltrated PCMs supported by the carbon-based matrix or nanoporous materials, the challenge is to maintain the balance between the energy storage capability and the contents of support. As the anchored sites or the interfacial interaction get an increment, the dispersion and interfacial compatibility of PCMs could be obviously enhanced through the π–π stacking and van der Waals interaction. In principles, the chemical modification of GO by dopamine (DA) and mPEG by epoxy groups (EO) is conducted, which is to offer much more reactive groups. As the GO sheets were chemically functionalized by DA derivatives, an increased action between DA and EO groups is formed. Then, the EmPEG components would be locked into the enlarged GO sheets, and the anchored PEG chains onto the GO surface also improve the interfacial compatibility with free ones. Other than the pristine GO, the chemical modified one effectively restricts the liquid flow of PEG components via their interaction offered by the reactive EO and –NH₂ groups. Therefore, excellent thermal stability and reliability of the EmPEG PCMs are approached, and the thermal performance can be enhanced obviously.

Hereby, in this paper, a series of PEG-based (EmPEG/PDA-rGO) cPCMs, composed of epoxidized methoxy polyethylene glycol (EmPEG) and polydopamine reduced GO (PDA-rGO), were fabricated through a facile solution-blending method. The structure, shape stabilization, thermal performance, and energy storage capability of EmPEG/PDA-rGO cPCMs are in-depth characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), optical microscopy, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The shape-stabilized mechanism and the thermal energy storing/releasing efficiency are studied from the aspects of the molecular interaction and the synergetic behavior.