Jie Yang, Guo-Qiang Qi, Li-Sheng Tang, Rui-Ying Bao, Lu Bai, Zheng-Ying Liu, Wei Yang*, BangHu Xie, Ming-Bo Yang College of Polymer Science and Engineering, Sichuan University, State Key Laboratory of Polymer Materials Engineering, Chengdu 610065, Sichuan, China

Abstract The development of solar energy conversion and storage materials is critical to narrow the mismatch between supply and demand of energy and alleviate the environmental impact related to energy consumption in the coming years. Here, a novel photodriven composite phase change materials (PCMs) based on bioinspired modification of boron nitride (BN) with superior solar-thermal energy conversion and storage performance is reported. The obtained composite PCMs show excellent performance in UV-vis sunlight harvesting, photothermal conversion, thermal energy storage, electrical insulation, shape-stabilization and high thermal conductivity. The preparation procedure is eco-friendly, easy handling, and suitable for the practical application of energy materials based on polyethylene glycol (PEG)/polydopamine (PDA)@BN composites with enhanced performance for energy conversion and storage.

1. Introduction

Solar energy is the renewable and carbon-neutral energy source of sufficient amount to replace fossil fuels. Of critical importance is that high efficient energy storage devices and systems should be developed to improve the performance and reliability of energy systems and narrow the mismatch between supply and demand of energy. Phase change materials (PCMs), as advanced energy saving materials, are widely used to store thermal energy by taking advantage of the massive amount of latent heat, high storage density and isothermal nature during phase change.1-7 Consequently, studies on the development of sustainable solar energy conversion and storage technologies associated with PCMs have attracted a great deal of interest recently. Photodriven PCMs (P-PCMs) can be prepared by the introduction of materials with effective photon capturing ability, including dyes,8-11 carbon materials such as carbon nano-tube (CNT),2, 12-14 expand graphite (EG),15 graphene oxide (GO),16 graphene,17, 18 carbon aerogels,19, 20 graphene aerogel,21 graphite foam,22 alkylated silica aerogels,23 and so on.

It has been universally accepted that the practical applications of PCMs inevitably face one major issue, that is, their low thermal conductivity. To solve this problem, two-dimensional (2D) planar materials and three dimensional (3D) structural materials, boron nitride (BN),24, 25 3D boron nitride nanosheets (3DBNNS) network,26 carbon nano-tube (CNT),27, 28 EG,29, 30 GO,7, 31 graphene nanoplatelets (GNP),7, 32 graphite nanoplatelets,33, 34 graphene aerogel,4, 5, 21, 35 graphite foam,36, 37 have been employed as thermal conductive fillers to improve the thermal conductivity and heat transfer of PCMs. However, the high cost and electrical conductivity of carbon materials limit their applications in some special fields, especially in some electronic devices where electrical insulation is required. Compared with carbon materials, BN has a crystal structure similar to graphene and the intrinsic thermal conductivity of BN has been shown to be of a high value, shedding light on their great potential to be fillers for composites with high thermal conductivity. Although BN has been widely used as thermally conductive fillers for thermoplastic or thermosetting plastics24, 38-42 and rubbers,43-45 there is only limited attempt to prepare composite PCMs with high thermal conductivity.

Dopamine (DOA) is believed to be able to interact strongly with a number of materials,46 such as polymers,47-49 metals,50 carbon nanotubes,51, 52 graphene,53-56 BN24, 57, 58 and barium titanate (BaTiO3).59, 60 Because of the many advantages of the polydopamine (PDA), it has been utilized for green reducing agent of GO,55, 61 protective layer for chemical reduction of GO,62 biomedical applications,58, 63 films to functionalize solid-liquid interfaces,64 and in energy and environmental fields.65 However, studies on the application of PDA in solar-thermal energy conversion and storage are very limited. PDA, as the major pigment of eumelanin, typically shows an absorption property similar to that of the naturally occurring eumelanin. Because of its strong absorbance across the ultraviolet (UV) and visible spectrum, especially in the former regime in which it shows a photoprotection effect,65 PDA could be of particular interest as attractive candidates for energy applications because of their unique structure and properties for sunlight absorption. Additionally, PDA coating can improve the dispersibility of BN and enhance the interaction with the matrix as well,24 by which the energy utilization efficiency and shape-stabilization of polyethylene glycol (PEG)/PDA@BN composites could be enhanced during heat charging and discharging processes. Herein, we describe the successful green aqueous functionalization of BN with amine functional groups of dopamine by taking advantage of the strong π–π interactions as well as van der Waals interactions, and its application as the light captor in PEG based PCMs. Thus, inspired by mussel, a kind of novel environmentally friendly and photodriven composite PCMs with high thermal conductivity and excellent electrical insulation for solar-thermal energy conversion and storage, is developed.

2. Experimental

2.1. Materials

DOA hydrochloride with a purity of over 98% and PEG (Mn = 10,000) were purchased from Aladdin Reagent (Shanghai, China). The BN powder (PW50, purity >99%) was purchased from Zibo Jonye Ceramic Co., Ltd. Ammonia solution (25%) and absolute ethanol (AR) were purchased from Chengdu Jinshan chemical reagent Co., Ltd. and Shanghai Fine Chemical Reagent Co., Ltd., respectively. For all experiments, distilled water was also used.

2.2. Surface Modification of BN

4 g BN powder was dispersed in the mixed solution of 600 mL of distilled water and 200 mL of ethanol with the aid of ultrasonic bath for 1h, and then 1.6 g DOA hydrochloride was added followed by intense stirring for 10 min. 10 mL of ammonia solution (25%) was dripped into the mixture to trigger the selfpolymerization of DOA at room temperature for 30min. 54 After that, the modified BN powder, denoted as PDA@BN, was filtered and washed by distilled water and ethanol for several times before being dried in a vacuum oven at 50 °C for more than 24 h.

2.3. Preparation of Composite Phase Change Materials

PEG/PDA@BN composite PCMs were prepared by a solution blending method. A certain amount of PDA@BN powder was dispersed in absolute ethanol with the aid of ultrasonic bath treatment for 30 min. Then PEG was added, and the mixture was vigorously stirred at 90 °C for 3 h to evaporate solvent and form PEG based composites containing different contents of BN. Finally, the samples were dried in vacuum oven to constant weight at 50 °C. For comparison, pure PEG and PEG/unmodified BN samples were also prepared in the same way. The obtained PEG/PDA@BN and PEG/BN composite PCM samples with different BN mass content were labeled as 10PDA@BN, 15PDA@BN, 30PDA@BN and 10BN, 15BN, 30BN, respectively.

2.4. Characterization

The morphologies of original and modified fillers were visually characterized via A JOEL JSM-5900LV field-emission scanning electron microscopy (SEM) instrument (Japan) and a Tecnai G2 F20S-TWIN transmission electron microscopy (TEM) instrument at an accelerating voltage of 20 kV and 200 kV, respectively. X-ray photoelectron spectroscopy (XPS) experiments were carried out on an XSAM800 (Kratos Company, UK) with Al Kɑ radiation (hv = 1486.6 eV). The Raman spectra were obtained on a DXRxi Raman Microscope (Thermal Scientific, USA) with an excitation wavelength at 532 nm. Thermogravimetric analysis (TGA, TG209F1, Netzsch, Germany) was carried out at a heating rate of 10 °C min-1 from 30 to 700 °C in an air stream with a flow rate of 20 mL min-1. Dynamic rheological measurements were performed on a stress-controlled rheometer (AR 2000, TA Instruments) equipped with parallelplate geometry (diameter of 25 mm). The frequency sweep was conducted in a frequency rang e of 0.01–100 Hz with a strain of 0.1%. Disk-shaped samples with the diameter of 25 mm and the thickness of 1.5 mm were used. The test temperature was 80 °C and the gap between two plates was 1.2 mm. Fourier transform infrared (FT-IR) spectroscopy patterns were recorded over the wavenumber range of 4000 – 400 cm-1 using a Nicolet 6700 FTIR spectrometer (Nicolet Instrument Company, USA). The Rigaku UltimaIV diffractometer (Rigaku, Japan) with Cu K α radiation (λ=0.15406 nm) was used to obtain the X-ray diffraction (XRD) pattern of the samples. Samples were scanned in the range of diffraction angle 2θ=5–40° at a scanning speed of 3 ° min-1 at room temperature.

The optical properties of the composite PCMs were studied using an Ultraviolet-Visible Near-Infrared (UV-vis-NIR) spectrophotometer (UV3600,Shimadzu, Japan). The photo-to-thermal energy conversion test was carried out using a CEL-HXUV300 xenon lamp (CEAULIGHT, China) and CEL-NP2000 optical power meter (CEAULIGHT, China). 

The sample temperature was recorded using a paperless recorder with thermocouples (OMEGA). Disclike specimens with a diameter of 25 mm and a thickness of 1.5 mm were used to test the AC conductivity using a Concept 50 Broadband Dielectric-impedance Spectrometer (Navocontrol technologies, Germany), and the thermal conductivity (TC) was measured using a Hot Disk Thermal Constant Analyzer (TPS 2500, Hot Disk AB Company, Sweden) by a transient plane heat source method. Furthermore, to measure the thermal responses of PCMs, samples were placed on a hot plate. A photo shot was taken by an infrared thermal imager (Fluke Ti27) to record the temperature distribution. The melting and freezing temperatures and the latent heat of the PCMs sealed in an aluminum pan were obtained by differential scanning calorimeter (DSC) measurements using a TA Q20 instrument (USA) with a heating/cooling rate of 10 °C min-1 in purified nitrogen atmosphere. Dimensional stability measurements were performed on a thermomechanical analyzer (TMA, TA Q200, USA). Temperature ramp was recorded at a rate of 5 °C min-1 over the temperature range of 20-120 °C in purified nitrogen atmosphere and a constant normal force was set to be 0.01 N.

3. Results and Discussion

3.1. Surface Modification of BN

Generally, the surface modification of BN is troublesome owing to its chemical inertness. However, PDA with a similar molecular structure of 3,4-dihydroxyL-phenylalanine which is the major origin of the wet adhesion property of mussels, has been a novel coating material since 2007.46 It is believed that substrates with conjugate structure such as graphene, 56 carbon nanotubes,66 and BN58 show strong π−π interaction with PDA. Therefore, the biomolecule DOA inspired by the mussel adhesion mechanism was introduced to develop a simple and green route for aqueous functionalization of BN. When DOA is introduced to the aqueous solution of BN (without any toxic solvents involved) at room temperature, strong adhesion can be expected owing to strong π−π interactions as well as van der Waals interactions between BN and the DOA molecule.57 Detailed structure was schematically shown in Scheme 1a, in which BN has a six-member ring graphite-like structure consisting of alternating B and N atoms.