Abstract
A shape-stabilized composite phase change material (SCPCM) made of n-nonadecane infused by capillary forces in a compressed reduced graphene oxide-activated carbon matrix (EFB(rGOAC)-M) was prepared from oil palm empty fruit bunch. The composite exhibited improved thermal properties and was used to fabricate an SCPCM by impregnation, in which the pores of the EFB(rGOAC)-M served as the support, while n-nonadecane was the central envelope. The EFB(rGOAC)-M exhibited a specific surface area of 680 m2 g-1 and an average pore size of 22 Å. The successful infiltration of n-nonadecane into the pores of EFB(rGOAC)-M was confirmed via nitrogen gas adsorption-desorption isotherms and scanning electron micrographs. According to the differential scanning calorimeter analysis, the composite SCPCM-5 exhibited melting and freezing temperatures of 37.25 °C and 25.58 °C, respectively, and an associated latent heat value of 82.72 J g-1 and -62.22 J g-1, respectively. There was no seepage during the phase change process (from solid to liquid, as the n-nonadecane was uniformly dispersed in the pores of the carbon matrix (EFB(rGOAC)-M) and held by the capillary and the surface tension forces of the carbon matrix. This innovative, inexpensive and environmentally friendly shape-stabilized phase change material could be applied for thermal energy storage applications.
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Preparation of Shape-Stabilized Phase Change Material by the Valorization of Oil Palm Waste: Reduced Graphene Oxide-activated Carbon Derived Carbon Matrix for Thermal Energy Storage
Salisu Nasir,a,b,c,* Mohd Zobir Hussein,a,* Zulkarnain Zainal,c and Nor Azah Yusof c
A shape-stabilized composite phase change material (SCPCM) made of n-nonadecane infused by capillary forces in a compressed reduced graphene oxide-activated carbon matrix (EFB(rGOAC)-M) was prepared from oil palm empty fruit bunch. The composite exhibited improved thermal properties and was used to fabricate an SCPCM by impregnation, in which the pores of the EFB(rGOAC)-M served as the support, while n-nonadecane was the central envelope. The EFB(rGOAC)-M exhibited a specific surface area of 680 m2 g-1 and an average pore size of 22 Å. The successful infiltration of n-nonadecane into the pores of EFB(rGOAC)-M was confirmed via nitrogen gas adsorption-desorption isotherms and scanning electron micrographs. According to the differential scanning calorimeter analysis, the composite SCPCM-5 exhibited melting and freezing temperatures of 37.25 °C and 25.58 °C, respectively, and an associated latent heat value of 82.72 J g-1 and -62.22 J g-1, respectively. There was no seepage during the phase change process (from solid to liquid, as the n-nonadecane was uniformly dispersed in the pores of the carbon matrix (EFB(rGOAC)-M) and held by the capillary and the surface tension forces of the carbon matrix. This innovative, inexpensive and environmentally friendly shape-stabilized phase change material could be applied for thermal energy storage applications.
Keywords: Porous carbon matrix; n-Nonadecane; Encapsulation; Shape-stabilized composite; Phase change material; Latent heat; Thermal energy storage
Contact information: a: Materials Synthesis and Characterization Laboratory (MSCL), Institute of Advanced Technology (ITMA), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia; b: Department of Chemistry, Faculty of Science, Federal University Dutse, 7156 Dutse, Jigawa State, Nigeria; c: Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia; *Corresponding authors: salisunasirbbr@gmail.com; mzobir@upm.edu.my
GRAPHICAL ABSTRACT
INTRODUCTION
The most significant challenges facing humanity involve energy, environmental, economic, or security (terrorism) related problems. It is projected that global energy consumption will rise by 48% between 2012 and 2040 (U.S. Energy Information Administration 2016). Currently, about 30% of the total global energy demand comes from the building industry (Cui et al. 2014; Dong et al. 2016; Yang et al. 2017). This sector has garnered global attention from researchers, technologists, and policymakers to reduce the demand or provide an alternative for energy storage and conversion, especially building energy conservation. Novel and sustainable strategies are required to resolve the challenges of future energy demand (due to rapidly growing global population) and environmental effects (caused by fossil fuel exploitation) (Abbasi and Abbasi 2011; Zhao et al. 2019). These advancements in green/renewable technologies should decrease greenhouse gases and the release of noxious materials (Nasir et al. 2019). This is in line with the proclamation made in the 72nd United Nations (UN) general assembly of 2017 that the planet Earth is endangered, but it could be relatively safe if the mean global temperature rise could stay below 2 °C, which could be achieved if all countries reduce their greenhouse gas (CO2, CH4, etc.) emissions generated from fossil-fuel combustion. To systematically end the use of fossil fuels, there are numerous alternatives that harness and store renewable energy supplied by the sun, geothermal, water, wind or biomass sources. However, the major challenge is that the resources considered appropriate to accomplish this mission have to be cost-effective, industrially viable, renewable, and consistently scalable to surpass the performance of existing technologies (White 2015).
Interestingly, the storage of thermal energy occurs in a variety of forms, such as the latent heat of fusion, sensible heat stored in a liquid or solid medium, and chemical energy formed as a result of the reversible chemical reaction. Currently, latent heat thermal energy storage is preferred because it is completely based on phase change materials (PCMs). PCMs have the special ability to store and release energy via thawing and solidification. These processes are generally influenced or controlled by the surrounding temperature. PCM-related technology was introduced to improve internal building comfort (temperature) and reduce energy consumption. This technology has attracted attention from various fields due to the high energy storage density and capacity of the PCMs to store energy at a nearly constant temperature.
Since the advent of this technology, various organic and inorganic materials have been exploited for phase change-related applications, such as fatty acids/esters, polyalcohol, paraffin wax (n-alkanes), metals, salts, salt hydrates, and alloys (Zhang et al. 2009; Khadiran et al. 2015, 2016). However, preference is given to n-alkanes with a general formula of CnH2n+2, especially n-hexadecane, n-octadecane, and n-nonadecane, because these substances exhibit self-nucleating properties, slight or no supercooling, and high heat of fusion. Their melting and freezing points fall within the human tolerable temperature zone, and they are chemically and thermally stable (Zhang et al. 2009; Nasir et al. 2019).
Fig. 1. Schematic representation of the preparation of shape-stabilized phase change material (SCPCM-5) by infiltration of n-nonadecane into the pores of reduced graphene oxide-activated carbon matrix, derived from the oil palm waste precursor.
These properties, especially the higher energy storage density and capacity of the PCMs to store energy at an almost constant temperature, could be sufficient for this emerging technology. Despite all these qualities, paraffin wax has shown evidence of low thermal conductivity, poor heat exchange, and large volume changes (which could cause leakage), which limits the rates of energy storage and release during the thawing and solidification processes. Consequently, for paraffin wax (n-alkanes) to be utilized in PCMs, a thermal conductivity enhancer (TCE) or shape-stabilized phase change materials (SPCMs) should be incorporated (as portrayed in Fig. 1).
Activated carbon (AC) is an appropriate material to provide PCM with both superior thermal conductivity and form-stability (Chen et al. 2012; Waisi et al. 2019). The porous structure of AC is the central player in tailoring the behaviour and feature of shape-stabilized PCM (Hussein et al. 2015). To minimize enthalpy loss for shape stabilized PCM application, AC pores with micro- and nano-meter sizes are widely utilized. Nevertheless, it is not always straightforward to obtain the pore structure of AC with similar pore size distribution, geometrical shape, and network-inner-connected; this is attributed to several factors such as carbon source or precursor used and the general method of activation (González-García et al. 2013). For this reason, the study of the thermal energy storage capacity of PCM in the pores of AC is challenging. In this research, AC, reduced graphene oxide (rGO), and their composite were produced using the lignocellulosic-derived materials from oil palm empty fruit bunch. As an abundant waste in Southeast Asia, it is an inexpensive and sustainable carbon precursor for carbon-based materials fabrication. The main goals of this study were to optimize the procedure, modify the low-cost rGO-AC carbon matrix with enhanced or suitable pore structures, and use it as an inorganic framework for the production of shape-stable PCM for thermal energy storage (TES).
Any substance that can absorb, accumulate, and discharge heat in the form of thermal energy is classified as PCM. Thermal energy is stockpiled in the PCM and eventually recovered during the freezing process (Khadiran et al. 2016). This concept is understood by studying the impact of rising and falling of the surrounding temperature where the PCM is situated. For instance, when the heat coming from the surroundings exceeds a thawing temperature of PCM, the bonds holding the PCM will ultimately rupture, and the PCM will take up the heat in the endothermic process. When the surrounding temperature falls below the thawing temperature of PCM, the PCM will consequently discharge out the heat in the exothermic process, and as a result, convert to a freezing or solid state (Khadiran et al. 2016). These interesting properties of PCMs have led to their application as advanced energy storage materials in building with low thermal mass to preserve and enhance its in-house comfort. Many other important applications are reported for heat regulation of electronics, solar energy, and waste heat recovery (Feng et al. 2011).
The smart shape-stabilized phase change composite materials (SCPCM) developed in this study are feasible options to reduce the energy consumption of buildings. This is possible because when material changes phase within a particular temperature range, heat is stored and released. This phenomenon generally increases the building inertia, decreases the temperature flux, and makes the indoor climate very steady. As a result, the inner building temperature will fall within the human comfort zone (thermally comfortable) that does not necessarily require any cooling or heating system, which in turn reduces energy consumption. When electricity consumption is minimized, the demand for fossil fuels decreases, leading to reduced greenhouse gas emissions, which is one of the global efforts towards sustainable development.
The higher energy density exhibited by the PCM is generally ascribed to the latent heat of fusion. Even when there is no considerable change in the measured temperature, it is pragmatic that huge amount of energy is absorbed and discharged when materials change phase (Whiffen and Riffat 2013). The variation in the intermolecular forces between the phases is the main driver that influences the change in energy.
Despite multitude studies on PCMs, rGO, and AC, there has been limited research on the use of the rGO-AC matrix (derived from lignocellulosic feedstock) in PCMs. Most reports used paraffin waxes encapsulated in the porous cavity of the activated carbon derived from other sources (e.g., peat soil). It is informative to observe that the mean pore size of the material plays an influential role in SCPCM performance. For example, a slow molecular motion of the PCMs is usually caused if the pore size is very small, which affects the latent heat storage capacity. Similarly, it is also imperative to retain the PCM during the latent heat storage process by an adequate capillary force action. If the average pore size is too large, the capillary force will not be enough to retain the liquid PCM during the phase change. Consequently, leakage and lower thermal conductivity of pure PCMs have discouraged their widespread applications. Several techniques have been introduced to address these drawbacks. One of the principal methods is incorporating a form-stable material with PCMs to form a composite.
In the present study, a mixture of the lignocellulosic-based rGO and activated carbon with tunable pore size distributions and suitable thermal capacity was designed and prepared for stabilizing the shape of the PCM. This material has been proven useful for the outstanding performance of the phase change system.
EXPERIMENTAL
Materials
Zinc chloride (ZnCl2) (SystermChemAR, Shah Alam, Malaysia), ethyl alcohol (99.7%) (RandM Chemicals, Semenyih, Malaysia), n-nonadecane (99%) (Sigma-Aldrich, St. Louis, MO, USA), and deionized water were utilized. The empty fruit bunch (EFB) was obtained from the Seri Ulu Langat Palm Oil Mill, Dengkil, Selangor, and was utilized as the starting raw material for the reduced graphene oxide and activated carbon. It was washed thoroughly with deionized water to remove dust particles, oven-dried at 100 °C, and pulverized by a grinder before it was impregnated with activating agents.
Methods
Preparation of reduced graphene oxide using lignocellulosic materials derived from oil palm by-product
As a facile and scalable production process, first, graphene oxide was prepared as described by Marcano et al. (2010), with a slight modification as previously reported (Nasir et al. 2017, 2018). Briefly, the EFB-derived graphite-like powder (3 g) was mixed with concentrated H2SO4/H3PO4 (360:40 mL), and 18 g of KMnO4 was gradually added to the mixture, raising the temperature of the reaction mixture to 35 to 40 °C. The mixture was kept under controlled stirring for 12 h at 50 °C. The reaction was allowed to cool to room temperature, and 400 mL of icy-deionized-water was poured concurrently with 3 mL of 30% H2O2. It was then washed and filtered with 200 mL of water, 200 mL of 30% HCl, and 200 mL of ethanol. This process was repeated twice, and the GO was obtained by grouping the particles together with 200 mL of ether and finally dried overnight at room temperature. The as-fabricated GO was later deoxygenated to graphene-like material (in the form of the reduced graphene oxide) through thermal treatment using a low-temperature annealing reduction method. This process was done at 300 °C in a furnace under the controlled steady flow rate of high purity N2 at 150 cm3/ min.
Preparation of the empty fruit bunch-derived activated carbon
The ZnCl2 activation step of the waste EFB was performed as follows: typically, 10 g of the pulverized EFB was impregnated with different concentrations (0, 11, 22, and 33% w/w) of ZnCl2 by stirring in an aqueous solution of the said chemical. This was followed by an evaporation step at 80 °C. The impregnation ratio was 1:0, 1:1.1, 1:2.2, and 1:3.3, respectively, which by definition is the mass ratio of activating agent to dried samples. The dried EFB/ZnCl2 mixture was then carbonized at 900 °C for 3 h at a heating rate of 10 °C/min under nitrogen atmosphere. Carbonization was performed to purge low-melting-point and low-boiling-point organic compounds leading to carbon with a surface area and good pore size distribution. Following the carbonization, the resulting activated carbon samples were further pulverized, using mortar and pestle. The sample was repeatedly washed (refluxes) with 1 L of 3 M HCl solution and later with deionized water in order to remove the remnant of the ZnCl2 and attain a neutral pH value. The sample was activated at 500 °C, and the final product was designated as activated empty fruit bunch (EFBAC-0, EFBAC-11, EFBAC-22, and EFBAC-33), where 0, 11, 22, and 33 were corresponding to the ZnCl2 treatment concentrations, respectively.
Preparation of reduced graphene oxide-activated carbon matrix EFB(rGOAC)-M
Graphene oxide and reduced graphene oxide have applications in composite materials due to their 2D structure and excellent properties (Hazra and Basu 2016). In this study, a template or matrix of these materials was formed as follows. An equal ratio (3 g each) of the rGOEFB and EFBAC-22 were mixed, homogenized, and ground together. The composite mixture was refluxed in 1 L deionized water overnight and subsequently ultrasonicated for 6 h. After drying in an oven, the composite was reactivated in the furnace at 300 °C under a controlled flow rate of high purity nitrogen gas before encapsulation with n-nonadecane. The graphitic composite materials retained several of the properties of the rGOEFB and the physicochemical properties of the activated carbon (EFBAC-22) sample.
Preparation of shape-stabilized n-nonadecane/ EFB(rGOAC)-M composites (SCPCMs)
The shape-stabilized phase change material (SCPCM) was synthesized by a simple impregnation process. n-Nonadecane with the melting point of 32 °C was selected as the phase change material, while reduced graphene oxide-activated carbon matrix [EFB(rGOAC)-M] derived from the EFB feedstock was used as supporting material. Initially, the n-nonaadecane (C19H40) was heated to slightly above its melting temperature (32 °C). The resulting melted C19H40 was dissolved in 30 mL of absolute ethanol. The reduced graphene oxide-activated carbon matrix EFB(rGOAC)-M was added into the n-nonadecane solution, and the mixture was stirred at 500 rpm for 4 h. Finally, the mixture was oven-dried at 80 °C for two days to evaporate all residual ethanol. The content of the n- nonadecane in the composite PCM was varied from 10, 30, 50, 70, and 90 wt%, (Table 1). The prepared materials were stored in sample bottles.
Table 1. n-Nonadecane and EFB(rGOAC)-M Compositions Utilized for the Production of the Shape-stabilized n-Nonadecane/EFB(rGOAC)-M Composites
The efficacy of the encapsulation of the n-nonadecane into the pores of EFB(rGOAC)-M was evaluated by Eq. 1, taking into account the enthalpy of the pure n-nonadecane,
(1)
where ΔHm is the enthalpy of thawing of the SCPCM composite (J g-1), and ΔHpcm is the enthalpy of thawing for the pure n-nonadecane (J g-1).
One of the drawbacks associated with the organic PCMs, more especially paraffin, for thermal energy storage application is related to their leakage when the PCM melts. For this reason, seepage was evaluated in the prepared composite materials. The SCPCM (precisely SCPCM-5) was kept in the oven at 90 °C for 72 h. To determine the capability of the EFB(rGOAC)-M carbon matrix to clasp the n-nonadecane during the thawing phase, one gram (1 g) of the SCPCM-5 was measured on a filter paper and subjected to 45 °C for 12 h. Afterward, the sample was put in the oven at 90 °C for 72 h, as described earlier. The mass and the latent heat of the composite material after this testing period were evaluated.
Materials characterization
The BET (Brunauer-Emmet-Teller) surface area, total pore volume, and average pore size of the rGOEFB, EFBAC-22% ZC, EFB(rGOAC)-M, and all of the SCPCMs nanocomposites were measured by the nitrogen gas adsorption-desorption method at 77 K (liquid nitrogen temperature) on a Micromeratics Tristar II plus (Norcross, GA, USA). The analysis was also used as a yardstick to validate the encapsulation or impregnation of the n-nonadecane-based PCM into the pores of the as-prepared EFB(rGOAC)-M. Before the experiment; the samples were outgassed at 290 °C for 12 h under vacuum. The BET equation and Barrett-Joyner-Halenda (BJH) methods were applied to evaluate the specific surface area and the pore size distributions of the materials. Fourier transform infrared (FTIR) spectra of the as-prepared materials were generated using a Nicolet 6700 models (Thermo Scientific, Waltham, MA, USA). The pellets were obtained by mixing 1 mg of the sample and 200 mg of potassium bromide (KBr) and compressing the mixture in a manual hydraulic press, and absorbance was recorded from 400 to 4000 cm-1. The graphitic traits of the materials were examined using Raman spectroscopy, with a WiTec Raman spectrometer (WiTec, Ulm, Germany) using a laser excitation wavelength of 532 nm. The intensity ratio between the D-line (~1350 cm-1) and the G-line (~1597 cm-1) of the Raman spectra was used as a yardstick for the graphitization condition of the samples. The surface morphology of the materials was observed using a Nova Nanosem 230 field emission scanning electron microscope (FESEM; Hillsboro, OR, USA). Prior to the analysis, the dried samples were disseminated on a conductive carbon adhesive tape surface that was attached to a FESEM stub and then gold-coated using sputter coater equipment.
The melting and freezing temperatures as well as the enthalpy or the latent heat (generally referred as the thermal energy storage properties) of the pure n-nonadecane and the nanocomposite were investigated by a differential scanning calorimeter (DSC Q20 V24.10 Build 122, TA Instrument, New Castle, DE, USA) equipped with a refrigerated cooling system. Briefly, 6 mg of the sample was placed in an aluminium pan. The experiment was started from -20 to 100 °C for the heating cycle phase and vice versa for the cooling cycle stage in a persistent flow of pure nitrogen gas with the flow rate of 50 mL/min. The thermal stability of the SCPCMs was determined by using a TGA Q500 V20.13 Build 39 (TA Instruments, Newcastle, DE, USA). The sample was analyzed under a nitrogen atmosphere from room temperature to 1000 °C with a heating rate of 10 °C/min and flow rate of 20 mL/min.
RESULTS AND DISCUSSION
The materials prepared in the present study were subjected to thorough characterization, and their thermal energy storage potentials were analyzed. The results are discussed in detail below.
Structure and Chemical Characterization of Empty Fruit Bunch-based Reduced Graphene Oxide-Activated Carbon Matrix
The chemical analysis of the raw EFB and its resulting rGO (acquired after chemical oxidation and thermal reduction processes), and activated carbon (acquired after ZnCl2 treatment and carbonization-activation process) was done using FTIR spectroscopy, and the results are presented in Figs. 2 and 3. The detailed information on the interactions between the components of the composite was revealed using the same analysis (Fig. 4).
Fig. 2. FTIR spectra of (a) raw empty fruit bunch sample, (b) graphene oxide derived from empty fruit bunch precursor (c) reduced graphene oxide derived from empty fruit bunch precursor
Fig. 3. FTIR spectra of the raw empty fruit bunch sample and its corresponding activated carbon prepared using different concentrations of zinc chloride
Based on the spectra in Figs. 2 and 3, the raw EFB sample exhibited several absorption peaks in the fingerprint region between 1649 and 665 cm−1 (Fig. 2) and 1642 and 678 cm−1 (Fig. 3) as compared to its corresponding GO, rGO, and activated carbon, respectively. The occurrence of those multiple absorption peaks is an indication of extra chemical functional groups (mostly primary aliphatic alcohol, esters, and aliphatic hydrocarbons) in the raw EFB samples. After the modification and conversion of the raw EFB feedstock into GO, rGO, and activated carbon, the surface functional groups had been converted to an aliphatic carboxylic acid, aliphatic hydrocarbons, and secondary aliphatic alcohols.
In Fig. 4, the characteristic absorption peak at 2921 and 2851cm-1 are the C-H stretching bands of n-nonadecane. The peaks at 1468 cm-1 are ascribed to the CH2 bending vibrations, whereas the peak at 723 cm-1 connotes to the trembling and out-of-plane bending vibration of CH2 in n-nonadecane. It is apparent from the FTIR spectra of the composite materials (Fig. S.1) that the impregnation of n-nonadecane was partially achieved in 10 and 30 wt%. However, at 50 wt% and above of n-nonadecane concentrations, the impregnation and the contributions of the n-nonadecane to the FTIR spectra of the composite materials were properly observed. The best impregnation or encapsulation was realized at 90 wt% of the n-nonadecane concentration (Fig. S.1).
Interestingly, no shift in the absorption peaks of n-nonadecane was observed in the FTIR spectra of all composite materials. This result is an indication that no chemical interaction occurred between the n-nonadecane and the EFB(rGOAC)-matrix. Thus, the n-nonadecane had been properly adsorbed and disseminated into the pores of reduced graphene oxide-activated carbon matrix, EFB(rGOAC)-M through capillary and surface tension forces. In effect, the problem of seepage of the molten PCM from the composite material was avoided.