Thermal analysis of graphene nanosheets/ paraffin nanocomposites with non-ionic surfactant for thermal energy storage

. Thermal energy storage (TES) using phase change materials (PCMs) has been extensively utilized to improve the efficiency of photovoltaic thermal (PVT) systems. High-conductive nanofillers have been an effective method to improve PCM's energy efficiency and thermal management systems. This research modifies high-capacity paraffin using graphene nanosheets (GNS) in small weight fractions of 0.2% and 0.6%. Tween 60 and gum Arabic are added to improve the GNS nanofiller's suspension ability. A higher GNS nanofiller of 0.6 wt% contributes to lower transmittance with higher solar energy absorption. However, GNS/paraffin with Tween 60 results in better thermal stability than gum Arabic surfactant. The improved thermal properties show promising results for TES systems in PVT applications.


Introduction
Energy usage and demand worldwide are increasing, and supporting energy supply from sustainable energy is becoming vital.High usage of non-renewable energy causes environmental and health concern [1].Solar, wind, hydropower, bioenergy, geothermal, marine, and nuclear power are remarkable alternatives to fossil fuels because of their sustainability.Solar energy has the potential to meet energy demand without harming the environmental [2].Excessive energy harvested from the sun can be utilized by capturing it in thermal energy storage (TES).This energy can be used later until it is consumed.The system of TES can be classified as a sensible heat storage system, a latent heat storage system, or a chemical heat storage system [3].With latent heat storage's advantage of being more than 50 times larger than sensible heat storage, the latent heat storage system using PCMs becomes a magnificent choice in various TES applications.PCMs have high storage density and temperature stability compared to sensible heat.However, some issues with low thermophysical properties arise, hence the low-performance rates.PCMs with solid-liquid phase changes are more widely used than solid-solid and liquid-gas phase changes.PCMs fall into three main categories: organic, inorganic, and eutectic.Many researchers mostly prefer organic paraffin as it suits many applications.It comes with properties of large latent heat, chemical stability, nontoxicity, consistent melting properties, and minimum volume change during phase transition [4].However, paraffin has low thermal conductivity, which affects the melting and solidification process and limits its application.
The dispersion of a highly conductive nanofiller creates nano-enhanced PCMs (NePCMs) that will increase thermal conductivity.Since graphene poses high thermal conductivity, it is attractive to disperse small amounts in paraffin PCMs [5].In a research study by Abdelrazik et al [6]; the thermal properties of paraffin wax (PW) PCMs were enhanced using multiwall carbon nanotubes (MWCNT) and graphene nanofiller.The specific heat of PW/graphene is higher than PW/MWCNT by 6.7%.The thermal performance includes the changes in specific heat, latent heat, and melting temperature, resulting in better thermal behavior for PW/graphene compared to PW/MWCNT and pure PW.Nevertheless, thermal conductivity enhancement is higher for MWCNT than graphene.After repeated cycles of heating/cooling on the composites, it still displays good thermal stability.Reji Kumar et al [7] investigated the thermal performance and chemical stability of paraffin-RT50 with graphene.The reduced light transmittance was enhanced by 32% than pure RT50.Moreover, the FTIR analysis shows no subsidiary peak development, which means it is physically and chemically stable.Another study by Tariq et al [8] incorporated RT44HC and RT64HC paraffin with graphene for the thermal management of electronic components.The nanocomposites were tested at heating loads of 0.86 KW/m2, 1.44 KW/m2, and 2.40 KW/m2.RT44HC/graphene nanocomposites performed a maximum reduction of 25% from the baseline temperature for a low heating load, 0.86 KW/m2.At a high heating load of 2.40 KW/m2, RT64HC/graphene performed a maximum temperature reduction of 16.37%.
In certain NePCMs, nanofiller deposition that results in sedimentation and agglomeration degrades the thermal performance [9].Eventually, surfactant addition gives the nanofiller stability advantage and improves the thermal performance of the nanocomposites [10].However, excessive amount of surfactant will result in sedimentation and does not give significant improvements in thermal performance [11].In some cases, it may influence the heat transfer proses by creating thermal resistance between nanofiller and PCMs [12].Various types of surfactants, such as cationic, ionic, and non-ionic, are being used in NePCMs composites.Ionic NK Noran et al [13] studied the impact of surfactant on graphene/paraffin with anionic SDBS surfactant and cationic CTAB surfactant, observing the chemical stability, thermal stability, light transmittance, and thermal conductivity.The study used RT44HC paraffin, which is able to store high latent heat value.Two-step synthesis is being applied to produce graphene/paraffin composites in 0.1 wt% and 0.3wt%.The composites show no chemical interaction as no new peaks are developed in the Fourier Transform Infrared (FTIR) spectra graph.The composites are thermally stable until 230 ℃.As more graphene is added to RT44HC, light transmittance becomes lower, which brings more advantages to the TES application.Thermal conductivity value improves with graphene, and it goes higher with SDBS and CTAB addition.Sivashankar et al [14] improve organic OM35 PCMs using graphene with anionic sodium deoxycholate surfactant.The NePCMs produced were tested on the experimental test facility of a heat sink.With NePCMs integration, the temperature reduction consequently increases the power output and efficiency of the system.In a volume concentration of 0.5%, the power output and efficiency increase up to 7% and 6%.Current work by Jeeja et al [15] tailored eutectic PCMs of (paraffin and palmitic acid) laden with MWCNTs in different concentrations (0 wt% to 0.7 wt%).SDBS surfactant dispersed initially in PCMs to stabilize the nanofiller.The NePCM's thermal stability shows slower decomposition rates, improving the indexed thermal resistance.It also displayed a reliable photothermal performance and high absorptivity.Overall thermal performance improves in a range of 0-0.5 wt% of MWCNTs in thermal effusivity analysis.These NePCMs improve thermal properties to integrate into thermal energy harvesting systems and event thermal management in electrical devices.
Previous works have shown that nanofillers enhanced the thermal properties of PCMs and improved them more with surfactant addition.Carbon-based nanofillers such as graphene and MWCNT have been used extensively with different PCMs and have a prominent impact on PCMs [16].This current research studies the thermal properties on HC paraffin with small weight fraction of GNS (0.2 wt% and 0.6 wt%); RG-0.2 and RG-0.6.Small weight fraction resulting in a more evenly distributed nanofiller [17].Moreover, higher concentration of graphene at 0.8 wt% cause agglomeration, hence reduced the NePCMs's thermal performance [7].Two different non-ionic surfactants are added to the NePCMs composites to improve stability.The NePCMs composites are RGT-0.2and RGT-0.6 with Tween 60 surfactant and RGA-0.2 and RGA-0.6 with gum Arabic surfactant.The NePCM's thermal properties are examine experimentally and compared with pure HC paraffin.The thermal characterization, light transmittance, and thermal stability are analysed and discussed in this paper.

Materials Selection
High-capacity paraffin (HC paraffin) from Rubitherm Technologies GmbH is selected for this study.The phase change temperature ranges from 41-44℃, with a low thermal conductivity of 0.2 W/mK and density of 0.8 kg/m3 with a maximum operating temperature of 70℃.It has the advantage of a high thermal energy storage capacity of 250kJ/kg and is suitable for use in PVT systems in the Malaysian climate [18].The thermal properties of HC paraffin are given in Table 1.GNS nano-filers from XFNANO with an average thickness of 3-10nm with a thickness of 5-10 micron are used in this study.It comes in grey-black powder form with more than 99.5% carbon content.The surfactant reduces the agglomeration of GNS nano-filler NePCMs, improving the thermal properties [19].Non-ionic surfactants used in this study were Tween 60 by Chemiz and Gum Arabic.The materials used in this experiment are analytical reagent grade without further purification.

Preparation of NePCMs
The preparation of HC paraffin containing different weight percentage of GNS is carried out using two-step synthesis method [20].Fig. 1 shows the stages in preparing GNS/paraffin NePCMs.In stage 1, HC paraffin is heated to 70℃ until it turns into liquid phase.Next, in stage 2, GNS is added to the liquified HC paraffin in a weight ratio of 0.2% and 0.6%.Probe sonication is used for the proper distribution of the sample.The samples are prepared in three mixtures of NePCMs: i) without surfactant, ii) with Tween 60 surfactant, and iii) with gum Arabic surfactant.The surfactant is added in a ratio of 1:2 of surfactant/GNS for better dispersion of nano-filler.Finally, in stage 3, the NePCMs are prepared in solid form for further analysis.The prepared NePCMs are RG-0.2,RG-0.6, RGT-0.2,RGT-0.6,RGA-0.2 and RGA-0.6.

Characterization
The prepared NePCMs composites are characterized to determine the thermal properties using various equipment.Fourier Transform Infrared (FTIR) equipment was used to analyze the composition from the spectra graph.The spectrum transmittance ranges 400-4500 cm-1 using spectral grade KBr pellets.The light transmittance of the composites is analyzed using an Ultra-violet visible spectrometer (UV-Vis) with the capability to measure the range of 200-3300 nm.Thermal gravimetric analysis (TGA) equipment was used to measure the weight loss percentage of the composites using a ceramic crucible.It was analyzed at 30 to 600 °C, where the composites decomposed, oxidized, or lost volatiles (such as moisture).Differential scanning calorimetry (DSC) was used to measure the thermal energy storage capacity of the composites as well as the melting temperature.

Chemical Characterization
The FTIR analysis is carried out to confirm the material present after the nanofiller is doped into HC paraffin.The composition of NePCMs is prepared through two-step methods, and the nanomaterials are expected not to react chemically.FTIR spectra analysis for HC paraffin, GNS, RG, RGT, and RGA are measured in a 4000-400 cm-1 wave number range and presented in Fig 2 .Three significant peaks are shown in HC paraffin as it relates to the CnH2n+2 formula.The intense peak at 2955 cm-1, 2913 cm-1, and 2848 cm-1 presents the stretching vibration of the -CH2 and -CH3 group, 1471 cm-1 represents the deformation vibration of -CH2 and -CH3 group, and 716 cm-1 of the lowest peak value stand for rocking motion of -CH2 group [21].However, GNS does not have a carboxyl group; hence, it does not develop any peak in the transmittance spectra [22].The peaks develop as GNS adds 0.2 w% and 0.6 wt%, similar to HC paraffin.It does not show any chemical reaction took place, and only physical interaction occurs [18].Surfactant addition in RGT and RGA composites does not form any new peak as it does not react chemically.Regardless of all the GNS, Tween 60 and gum Arabic in different wt% show a similar trend to the HC paraffin, and only physical interaction takes place.

Light Transmittance
Light transmittance is observed using UV-Vis equipment from 2100 nm to 250 nm.The results obtained are referred to the recent solar irradiance observation of extra-terrestrial spectrum data in a range of 280 nm to 1700 nm [23].The UV-Vis spectrum data of HC paraffin, RG, RGT, and RGA composites are shown in Fig. 3. HC paraffin shows a high transmittance value of 90.45%, as most of organic PCMs, especially paraffin, is white transparent.However, the carbon black color of GNS shows a low transmittance value of 21.91%.
The light transmittance of nanocomposites relies on the concentration of the nanofiller, as a higher concentration is expected to improve the reduced light transmittance and absorb more energy.The light from Fig. 3. (a) of RG-0.2, RGT-0.2, and RGA-0.2 are 38.19%,40.14%, and 38.81%, respectively.Lower light transmittance is observed in Fig. 3. (b) of RG-0.6, RGT-0.6, and RGA-0.6 with transmittance percentages of 23.71%, 25.88%, and 23.87%.The GNS nanofiller results from altering HC paraffin color to greyish, resulting in lower transmittance and higher absorption capability [24].The transmittance percentage event increases with Tween 60 and gum Arabic surfactant addition as more clear white substances accumulate.Overall transmittance percentage are observed as higher nanofiller loading, transmittance percentage reduces.The transmittance reduction is also relative to the type of surfactant being used.

Thermal Stability
Thermogravimetric analysis is measured over time in which the weight % degrades with increments in temperature.The derivatives curves of RG, RGT, and RGA at 0.2 wt% and 0.6 wt% are shown in Fig 4 .These nanocomposites characterize high stability until they reach 155℃ with no trace of weight loss fraction.The RGT nanocomposites give better improvements that show higher degradation temperatures than RGA nanocomposites.The derivatives curves do show that Tween 60 is better than gum Arabic, which acts as a thermal barrier, improving the thermal degradation temperature.The degradation of more than 10% weight loss is being observed as a limit of good nanocomposite conditions [25].The onset temperature at which 10% weight loss is tabulated in Table 1.The HC paraffin degradation occurs at 199.68℃ and increases as the nanocomposites with surfactant are added.The degradation temperature is the highest improvement for RGT nanocomposites at 0.2 wt%.Surfactant addition to 0.6 wt% does lower the degradation temperature for RGT and RGA nanocomposites.The degradation temperature for RGA-0.6 goes lower than RG-0.6 and RGT-0.6;hence, it improves from the HC paraffin PCMs.

Conclusion
This research analyzes the thermal behavior of HC paraffin, RG, RGT, and RGA composites in low-weight percentages.The nanocomposites were prepared using two-step synthesis with 0.2 wt% and 0.6 wt% of GNS.Tween 60 and gum Arabic of non-ionic surfactant are added to the GNS nanofiller in a ratio of 1:2.The nanocomposites prepared were analyzed by adopting chemical characterization, transmittance ability, and thermal stability.The chemical characterization RG, RGT, and RGA does not show any new peak evolved as compared to HC paraffin.It does not develop new functional groups with a mixture of GNS, Tween 60, and gum Arabic.High absorption occurs with the addition of 0.6 wt% of GNS, resulting in a low transmittance of 23.71%.It gives significant improvement from HC paraffin that transmits 90.45% due to the nature of the white transparent color of paraffin.The degradation temperature does improve for all composites, especially with the Tween 60 addition.It is thermally stable, with no traces of weight loss until 155 ℃.At 10% weight loss, RGT-0.2 degradation temperature is the highest at 221.31 ℃.This research work provides insight into RGT and RGA nanocomposites that can be applied in photovoltaic thermal systems and other thermal management systems.The HC paraffin with GNS and surfactant was synthesized successfully with improved heat storage of solar absorptivity.

Table 1 :
Degradation Temperature of RG, RGT and RGA