Flame-retardant and solid-solid phase change composites based on dopamine-decorated BP nanosheets/Polyurethane for efficient solar-to-thermal energy storage

Highlights

• Novel photothermal filler was synthesised via self-polymerisation of dopamine on black phosphorus nanosheets.

• The synthesised photothermal filler effectively improved the solar-thermal conversion efficiency (88.5%) of composites.

• The synthesised phase change composites possessed high phase change enthalpy ranged in 124.4–129.8 J/g.

• The synthesised photothermal filler significantly enhanced the flame retardancy of phase change composites.

Abstract

For utilisation of solar energy, the development of form-stable phase change material (PCM) composites with excellent flame retardancy and superior solar–thermal conversion performance is critical. Here, by incorporating dopamine-decorated black phosphorus nanosheets (PDA@BP) into polyethylene glycol-based polyurethane (PEG-PU), novel form-stable PCM composites (PBPCMs) having superior solar–thermal conversion performance and excellent flame retardancy were fabricated. PDA@BP, covalently bonded to PEG-PU, acted as an efficient photothermal filler to capture solar energy and convert it to thermal energy, while PEG-PU absorbed and stored the thermal energy via phase transition. A light irradiation experiment and thermal conductivity test demonstrated that introducing PDA@BP effectively improved the solar–thermal conversion efficiency (up to 88.5%) and thermal conductivity (69.4% increase) of PCM composites. Moreover, by introducing PDA@BP into PBPCMs, the total heat release and heat release rate decreased considerably, while the limiting oxygen index value and char yield increased, demonstrating the considerably enhanced flame retardancy of PCM composites. PBPCMs with superior solar–thermal conversion performance and excellent flame retardancy show considerable potential in the solar–thermal conversion and storage field.

Introduction

Solar energy, an inexhaustible, renewable and clean energy resource, is regarded as an ideal substitute for fossil fuels [[1], [2], [3]]. Among all the methods for harnessing solar energy, photothermal conversion has attracted considerable because of its operational simplicity and high energy conversion efficiency [4,5]. However, as solar energy is intermittent, it is necessary to develop energy storage devices that enable it to be stored and made available 24 h a day. For thermal energy storage (TES) application, phase change materials (PCMs) are regarded the most promising candidate because of their capacity for storing and releasing large amounts of thermal energy during the phase change process within a narrow temperature range [[6], [7], [8]]. Currently, many solid–liquid PCMs have been developed for TES because of their high latent heat capacity, nonreactivity and low or even no phase segregations [[9], [10], [11]]. However, the limitations of solid–liquid PCMs, such as volume shrinkage and leakage, limit their application. To address the issue of liquid leakage during phase transition, form-stable PCMs (FSPCMs) have been extensively investigated via porous absorption, microencapsulation, chemical cross-linking and polymer blending [12,13]. One of the most promising FSPCMs were polyethylene glycol-based polyurethane (PEG-PU) in which PEG covalently bonds to the polyurethane structure to form a solid–solid PCM [14].

However, applying PEG-PU in solar energy utilisation systems is considerably limited by its poor solar–thermal conversion efficiency and low thermal conductivity [15,16]. Furthermore, the high flammability of organic PEG-PU greatly limits further application, thus requiring high flame retardancy. Combustion gas and smoke, including carbon monoxide, have been responsible for many deaths [17]. Moreover, the incorporation of inorganic photothermal fillers (such as graphene nanoplatelets, reduced graphene oxide, MXene, titanium oxide and carbon nanotubes) into PEG-PU for thermal energy storage (TES) applications is an effective strategy to enhance the solar–thermal conversion efficiency and thermal conductivity of PCM composites [[18], [19], [20], [21], [22]]. However, the introduction of photothermal fillers has not significantly improved the flame retardancy of PCM composites; hence, it is important to develop form-stable PCM composites that have excellent flame retardancy, superior solar–thermal conversion performance and high thermal conductivity, which can then be applied to solar energy storage systems.

Black phosphorus (BP) nanosheets, as novel 2D nanomaterials, have recently attracted considerable attention owing to their desirable properties, including excellent photothermal effect, large surface area and high thermal conductivity [[23], [24], [25], [26], [27]]. In addition to their unique optical, electronic and mechanical properties, BP nanosheets can generate a protective carbonaceous layer on the combustion surface and effectively enhance the flame retardancy of organic materials [28,29]. This phenomenon is attributed to the expanded flame-retardant mechanism of non-toxic phosphorus-containing compounds (phosphonates and phosphates) generated from BP nanosheets during combustion [30]. Therefore, using BP nanosheets in PEG-PU-based solar energy storage systems may considerably improve the solar–thermal conversion performance and flame retardancy of PCM composites. However, the agglomeration of BP nanosheets inevitably occurs in PCM composites because of their poor compatibility and intrinsic poor dispersibility. In PCM composites, polydopamine (PDA), which is accompanied by near-infra-red (NIR) absorption capacity and high solar–thermal conversion efficiency, can effectively increase the dispersibility and compatibility of BP nanosheets [[31], [32], [33]]. In our previous research, form-stable PCM composites based on cellulose nanofiber (CNF), n-octacosane and BP nanosheets were prepared by impregnating n-octacosane into CNF/BP aerogel [34]. The incorporation of BP nanosheets effectively increased the thermal conductivity and solar-thermal conversion efficiency of PCM composites. However, the slight leakage of n-alkane in the melting state from the porous aerogels still limit their large-scale application.

Herein, dopamine-decorated 2D-layered BP nanosheets (PDA@BP) having excellent photothermal effects and improved stability were prepared using the in situ self-polymerisation of dopamine on BP nanosheets. Then, form-stable PDA@BP/PCM composites (PBPCMs) were fabricated by incorporating PDA@BP into PEG-PU. PDA@BP, covalently bonded with PEG-PU, acted as an efficient photothermal filler to capture solar energy and convert it to thermal energy, while PEG-PU absorbed and stored thermal energy via phase transition. The morphology, solar-thermal conversion performance, thermal storage property, thermal conductivity, thermal stability, and flammability property of the PBPCMs were systemically investigated. As expected, the introduction of PDA@BP effectively improved the solar–thermal conversion efficiency of PCM composites (up to 88.5%). Moreover, the flame retardancy of the synthesised PBPCMs was considerably enhanced by introducing phosphorus-containing PDA@BP.