AIP Conference Proceedings 2233, 020013 (2020); https://doi.org/10.1063/5.0001366 2233, 020013

© 2020 Author(s).

Enhancing the thermal properties of organicphase change material (palmitic acid) bydoping MXene nanoflakesCite as: AIP Conference Proceedings 2233, 020013 (2020); https://doi.org/10.1063/5.0001366Published Online: 05 May 2020

Yathin Krishna, R. Saidur, Navid Aslfattahi, M. Faizal, and K. C. Ng

Enhancing the Thermal properties of Organic Phase Change Material (palmitic acid) by doping MXene Nanoflakes

Yathin Krishna1, R. Saidur2, 3, Navid Aslfattahi 4, M. Faizal1, a), and K.C. Ng5

1Taylor’s University, School of Engineering, Jalan Taylors, 47500 Subang Jaya, Selangor, Malaysia. 2Research Centre for Nano-Materials and Energy Technology (RCNMET), School of Science and Technology, Sunway

University, No. 5, Jalan Universiti, Bandar Sunway, Petaling Jaya 47500, Selangor Darul Ehsan, Malaysia. 3Department of Engineering, Lancaster University, LA1 4YW, United Kingdom.

4Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, Malaysia. 5Department of Mechanical Engineering, Materials and Manufacturing Engineering, The University of Nottingham

Malaysia Campus, Jalan Broga, 43500 Semenyih, Selangor Darul Ehsan, Malaysia.

a) Corresponding author: MohdFaizal.Fauzan@taylors.edu.my

Abstract. Thermal energy storage (TES) is gaining more attention in the solar energy application for production of power round the clock. Phase change materials (PCMs) are a promising solution for TES due to their high energy storage density. However, the PCM materials suffer from low thermal conductivity which results in the low conversion efficiency of solar energy. In this study a novel nanocomposite of palmitic acid/Ti3C2 MXene is synthesised using two-step process. Melting point and enthalpy measurement were conducted using differential scanning calorimeter (DSC). Thermal stability and degradation temperature are studied by the results obtained by thermogravimetric analysis (TGA) up to 300 ⁰C. The functional group and possibility of chemical rearrangement of doping MXene nanoflakes are identified using FT-IR analysis. The nanocomposite showed enhancement in enthalpy by 4.34% and thermal conductivity by 14.45% indicating that the composite is suitable for TES application. FT-IR spectra of the composite revealed that there is no chemical reaction occurring between palmitic acid (PA) and MXene making it more stable composite. Based on the DSC and TGA results enthalpy and thermal conductivity of the composite has improved by doping MXene nanoflakes into Palmitic PCM making it suitable candidate for solar thermal and solar photovoltaic thermal application.

INTRODUCTION

PCM is one of the suitable alternatives available for solar TES [1, 2]. It is gaining a considerable amount of attention for its capability to continue energy production during non-sun hours in solar thermal power plants [3-5]. Therefore, by facilitating PCMs in energy storage systems of renewable energy power systems, the intermittent nature of energy can be overcome [6-10]. Because of the high energy density of latent heat based energy storage, compared to sensible heat energy storage, PCMs are having enormous attentions in solar hot and cold energy storage applications [11-14]. There are many inorganic and organic PCMs which are available in various eutectic compositions and temperature range which have been studied for solar thermal, green building applications and energy storage for fulfilling peak and off-peak energy requirement. Organic PCM (O-PCM) like fatty acids have high latent heat of fusion, prolonged or no supercooling properties, have low vapour pressure, consistent melting temperature, and excellent thermal stability [15]. However, O-PCMs particularly fatty acids have low thermal conductivity, which limits their usage in TES applications. Doping of metal and metal oxide nanoparticles to the O-PCMs is a reliable method for enhancing the thermal properties of O-PCMs [16-19]. PA is a saturated long-chain fatty-acid containing 16 carbon backbone. They are found naturally in dairy products, palm oil, and meat [20]. R.K. Sharma et al. examined the phase change behaviour of palmitic acid by doping TiO2 nanoparticles and found that thermal conductivity of PA

13th International Engineering Research Conference (13th EURECA 2019)AIP Conf. Proc. 2233, 020013-1–020013-7; https://doi.org/10.1063/5.0001366

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increased with increament in weight fraction of TiO2 [21]. Jifen wang et al. studied the improvement in thermal conductivity of PA by doping multi-wall carbon nanotube (MWCNT) and found that the increase in thermal conductivity of the composite mainly relies on the pre-treatment of MWCNTs. Hence, pre-treated MWCNTS containing hydroxyl group resulted in 51.6% increment in thermal conductivity at room temperature by doping 1.0 wt.% MWCNT [22].

Recently, research in 2D grew at a faster phase after the discovery of graphene and their extraordinary thermal and mechanical properties [23, 24]. In 2011, 2D materials comprising of carbonitrides,transition metal carbides, and nitrides were discovered [25]. Since these materials are synthesised using 3D MAX phase compounds, (having general formula Mn+1AXn where M, A, and X represents early d-block transition metals, main group sp elements (IIIA or IVA) and either or both carbon and nitrogen atoms respectively) by selectively etching sp elemental layer, these materials are called MXenes. After the discovery of MXenes, to date, more than seventy MAX phases have been discovered [26]. But the most recognised ones include Ti2C, (V0.5, Cr0.5)3C2, Ti3C2, Ta4C3, (Ti0.5, Nb0.5)2C,Ti3CN, [27], V2C,Nb2C [28] , and Nb4C3 [29]. Since the discovery of MXenes, many researchers reported for their exceptional properties. For instance, the conductivity of MXenes can be compared with multilayer graphene [27]. There are many advantages of using MXene as a dopant for organic base material, such as MXenes can be prepared easily [30], they have strong bonding capacity [25], superior thermal conductivity [31], strong loading capacity[25], and good electrical conductivity[32]. It has been recently found out that Ti3C2Tx exhibits perfect energy conversion property up to 100% [33]. These properties have gained the attention of researchers to explore the realm of MXenes in various fields. Xiaoqiao Fan et. al investigated polyethylene glycol and MXene PEG/Ti3C2Tx composite and found that the composite had a very good electromagnetic waves absorption capacity in UV-Vis-NIR region. Due to localized surface plasmon resonance (LSPR) effect of Ti3C2Tx MXene nanosheets two enhanced absorption bands were observed which enhanced photo-to-thermal storage efficiency up to 94.5% [34]. However, to the best of authors knowledge, there is no experimental or theoretical investigation have been done by doping MXene nanoparticles to palmitic acid to study the thermophysical property enhancement of base palmitic acid.

The objective of this investigation is to prepare a PCM with enhanced thermal properties which will address the TES challenges in solar thermal power plants. Ti3C2 MXene is doped to palmitic acid PCM for modifying or enhancing the thermophysical properties of the palmitic fatty acid. The current study investigates the effect on high-temperature stability and thermal conductivity of the nanocomposite and discusses the reason for the enhancement of the studied thermophysical properties.

MATERIALS AND METHODS

In the synthesis of MXene (Ti3C2,) the following materials were used without any further purification: MAX Phase material (Ti3AlC2) from Y-Carbon Ltd., lithium fluoride (325 mesh powder, 98.5% purity, Alfa Aesar), hydrochloric acid (37% wt, Fisher chemicals), and sodium hydroxide (97% purity, pellets, Sigma Aldrich). Firstly, 15 ml of DI water was added to a 50 ml volume beaker, followed by adding 15 ml of HCL to obtain 30 ml of HCL (6M). Then 3 g of Lithium Fluoride was added to the HCL solution with stirring at 300 rpm for 30 minutes until dissolved. This etching process was continued with adding 3 g of MAX phase material (Ti3AlC2) to the solution slowly (within 15 minutes) to avoid overheating (exothermic reaction), and the resultant solution is left to stir at 40 °C for 48 h. After the etching process, a dilute solution of NaOH was added slowly until the pH of the solution reached six and was filtered and rinsed several times with deionised water. The washing process was conducted using an ultrahigh centrifuge (Sorvall LYNX 6000, Thermo Scientific) for four times (each time of 10 minutes) at 3500 RPM. The achieved multi-layered MXene (m-Ti3C2) was sonicated for one hour using ultrasonic probe sonicator (FS-1200N) to obtain delaminated flakes of the MXene (d-Ti3C2). The synthesised delaminated flakes of MXene nanomaterial was dried in a vacuum oven (VO 500, MEMMERT Germany) overnight.

PA with a melting point of 60-62 ⁰C, the molecular weight of 256.43g/mole and having a purity 99% is procured from Sigma Aldrich company. The method of preparation of palmitic acid-based MXene composite is discussed as follows: Two-step method, was involved in the synthesis of the composite. 0.1 wt.% of MXene Ti3C2 nanoparticles is doped to 49.95g of palmitic acid for thermophysical property enhancement. Initially, 50gm of palmitic acid is measure in microbalance (Shimadzu, TX323L, UNIBLOC) and added to 100ml Borosilicate beaker; the beaker is placed over hotplate (RCT BASIC, IKA) at 100 ⁰C for melting. After complete melting, 0.05gms of MXene is added to the palmitic acid and stirred using magnetic stirrer at 450 rpm at 100 ⁰C for half an hour. Later, uniform and stable dispersion of nanoparticles are ensured by sonicating with probe sonicator for one hour with set on time for 5 sec and set off time

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for 3 seconds maintaining the power supplied to the sonicator at 60%. It is made sure that the temperature of the probe sonicator is maintained higher than the melting point of PA to ensure the composite is in liquid state.

Thermal stability and degradation temperature of the composite is performed using thermogravimetric analysis (TGA) (TGA 4000 PerkinElmer) with the initial temperature of 30 ⁰C up to 300 ⁰C. The heating rate of TGA is maintained at 10 ⁰C/min under nitrogen purging at 20 ml/min. Alumina crucibles are used for placing the sample in TGA furnace. Melting point and enthalpy of the composite is measured using differential scanning calorimeter (DSC-1000/C LINSEIS GERMANY). To obtain standard and reliable measurement, consistent method of characterisation and testing was followed for pure and doped samples. The heating rate for both the samples was fixed to 10 ⁰C/min from 30 to 200 ⁰ with the flow of nitrogen gas at 20ml/min. HDSC value is fixed at 25 μV to get higher resolution, and the mass value was fixed to 13mg for pure and MXene doped samples. In order to obtain the compositional and/or functional group changes, Fourier Transform Infrared Spectroscopy (Spectrum Two FT-IR Spectrometer L160000A, PerkinElmer) has been used between wavenumber between 450 and 4000 cm-1. Thermal conductivity measurement of pure palmitic acid and MXene doped palmitic acid is thermal properties analyser (TEMPOS, Meter Group) using transient line heat source method. SH-3 (3-cm dual needle) sensor is used for obtaining thermal conductivity, thermal resistivity, thermal diffusivity and volumetric specific heat capacity measurement. All the measurement was taken when the solid sample is placed in a water bath (WNB22, MEMMERT) maintained at constant temperature of 25 ⁰C. Ten measurements were recorded for each sample and a time gap of 15 minutes is kept between each measurement to ensure the stabilisation of the temperature.

RESULTS AND DISCUSSION

The important thermophysical properties for any PCM for its application in thermal storage are melting point, enthalpy and thermal conductivity. Experimental results of melting point, enthalpy for pure palmitic acid and MXene doped PA is obtained from DSC, which are presented in FIGURE 1. From the figure, there is no radical change in the melting point of pure palmitic acid and MXene based palmitic acid. However, enhancement in the enthalpy of fusion of palmitic acid by 4.36% has been observed by doping 0.1 wt.% of MXene nanoparticles. This may be due to the supporting role of MXene nanoflakes; they could have possibly had interacted with the thermodynamic behaviour of palmitic acid by enhancing the phase change enthalpies. Nevertheless, the phase change enthalpies for pure palmitic acid and MXene doped palmitic acid is still higher than 153 J/g; this ensures proper synthesis of composite in practical application [34].

The energy storage of PCM mainly depends upon their thermal conductivity. The heat transfer rate increases with an increase in thermal conductivity; this, in turn, enhances the solidification and melting process by reducing the time required for solidification/melting. Volumetric heat capacity, thermal conductivity, and thermal diffusivity data are presented in

TABLE 1. It has been observed that there is 14.45% and 22.63% enhancement in thermal conductivity and volumetric heat capacity respectively. However, there is a reduction in thermal diffusivity by 10.59%. This may be due to increased molecular vibration in the matrix by doping 0.1 wt.% MXene nanoflakes. This may increase if the temperature of the sample increases leading to furthermore increment in thermal conductivity [22]. The increment in the volumetric heat capacity of MXene doped palmitic acid signifies that there is enhancement in specific heat capacity of the composite because the volumetric heat capacity is directly proportional to the specific heat capacity of the material.

TABLE 1. Thermal Properties enhancement of Palmitic acid by doping MXene Thermal

Conductivity (K) W/mK

Volumetric heat capacity (Cv) MJ/m³•K

Thermal Diffusivity(D) mm²/s

Pure PA 0.1817 1.2004 0.1514 PA+0.1 wt.% MXene 0.2124 1.5515 0.1369 % Enhancement 14.45 22.63 -10.59

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FIGURE 1. DSC curve for Pure PA (Top) and MXene doped PA (bottom).

FT-IR curves of PA and 0.1 wt.% doped MXene nanoflakes, between the wave numbers of 450 and 4000 cm-1 are

shown in FIGURE 2. The peak at 2913 cm-1 and 2848 cm-1 signifies the symmetrical stretching vibration of -CH3 and -CH2 group of palmitic acid. C=O stretching vibration peak is proved by the peak formed at 1695 cm-1. The peak at 1464 cm-1

represents the deformation-vibration of -CH2 and -CH3 group of palmitic acid. The peak at 939 cm-1 and

1299 cm-1 characterises the out-plane and in-plane vibration of the -OH group of PA respectively. At 723 cm-1 and 685

cm-1 swinging vibrations of the -OH group of palmitic acid is represented. The FT-IR spectrum of MXene doped palmitic acid neither show any new peaks, nor they show peak shifts in the pure PA. This indicates that there is only physical interaction is occuring between PA and MXene nanoparticles. Also, chemical rearrangements of the functional groups are not detected in the FT-IR spectra, indicating no chemical reaction occurring between palmitic acid and MXene.

Thermal stability of the palmitic acid and palmitic acid doped MXene nanoflakes was investigated by TGA, and it is presented in

FIGURE 3. From the figure, it can be seen that the pure sample decomposes at 270 ⁰C. By doping MXene nanoparticles the degradation temperature enhances to 290 ⁰C. This indicates that the presence of MXene has enhanced the thermal stability of palmitic acid. This may be due to the thermal retardation caused by doping MXene nanoparticles [21]. This indicates that the nano particle doped composite can be used in thermal storage and also as heat transfer fluid in low and medium temperature solar thermal applications.

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FIGURE 2. FT-IR Spectrum of pure palmitic acid (Top] and palmitic acid doped with MXene (Bottom).

FIGURE 3. TGA analysis of pure and MXene doped PA.

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CONCLUSION

The experimental investigation on the synthesis and characterisation of PA /MXene (Ti3C2) was conducted in this paper. 0.1 wt.% of MXene was doped into the melted palmitic acid to determine its effect on its thermophysical properties of base palmitic acid. The composite is subjected to test for changes in melting point, enthalpy, thermal conductivity, and thermal stability. Enhancement in enthalpy by 4.36% is disclosed by DSC readings. However, there was no significant decrease in the melting point of the composite is found by doping 0.1 wt.% MXene to palmitic acid. However, there was 14.45% enhancement in the thermal conductivity (k) of the composite due to the increased molecular vibration by doping MXene nanoparticles. The FT-IR analysis of the composite revealed that the composite is more stable even after doping of MXene nanoflakes showing no sign of chemical decomposition or structural modification of the composite. It was also found that 22.63% increase in volumetric specific heat capacity of the composite making it suitable for TES applications.

ACKNOWLEDGEMENT

The authors would like to show gratitude for the financial aid provided by the Sunway University through the project no# STR-RCTR-RCNMET-001-2019. This current work was also supported by Taylor’s University through its TAYLOR’S RESEARCH SCHOLARSHIP Programme”.

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