Evaluation of Different Mesoporous Silica Supports for Energy Storage

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Evaluation of Different Mesoporous Silica Supports for Energy Storage in Shape-Stabilized Phase Change Materials with Dual Thermal Response Raul - Augustin Mitran, Daniela Berger, Cornel Munteanu, and Cristian Matei J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 Jun 2015 Downloaded from http://pubs.acs.org on June 12, 2015

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The Journal of Physical Chemistry

Evaluation of Different Mesoporous Silica Supports for Energy Storage in Shape-stabilized Phase Change Materials with Dual Thermal Response Raul – Augustin Mitran,†,‡ Daniela Berger, † Cornel Munteanu,§and Cristian Matei †* †

University "Politehnica" of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-

7 Polizu street, Bucharest, 011061, Romania ‡

SARA Pharm Solutions, 266–268 Calea Rahovei, Bucharest, 050912, Romania

§

“Ilie Murgulescu” Institute of Physical Chemistry, 202 Splaiul Independentei, Bucharest,

060021, Romania

ABSTRACT: The high variability and low heat of fusion of composite shape-stabilized phase change materials is a considerable challenge to their widespread application. Here we present the synthesis of shape-stabilized phase change materials composed of lauric acid and mesoporous silica with high heat of fusion through evaporative solution impregnation. Two hexagonal ordered silica with 2.8 and 6.3 nm pores (MCM-41, SBA-15) and two disordered mesocellular foams with 27 – 34.9 nm spherical pores connected by 10.4 - 14.9 nm “windows” are employed. The thermal properties and stability, heat storage efficiency, crystallization, textural and chemical properties are investigated using DSC analysis, small- and wide-angle XRD, nitrogen

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adsorption-desorption isotherms, optical and electron microscopy, as well as FT-IR spectroscopy. MCF-based materials with up to 83% wt. fatty acid show large latent heat (124 Jg1

), almost 90% efficiency with respect to the acid content and two melting – crystallization

temperature ranges associated with nanoconfined and bulk phases. Up to 53 Jg-1 enthalpy change for the nanoconfined phase can be obtained. The melting point depression, heat storage efficiency and the physical state of lauric acid at the mesopore level are correlated with theoretical considerations of thermodynamic and geometric factors, revealing a non-melting interface layer of one organic molecule thickness. This approach provides a facile methodology to estimate the relevant properties of mesoporous silica phase change materials with useful dual temperature ranges.

KEYWORDS shape-stabilized, phase change material, mesocellular foam, lauric acid, nanoconfinement, interface layer

INTRODUCTION Latent heat storage using phase change materials (PCMs) is a promising method to reversibly store and release thermal energy. Due to a high energy storage density and possibility to tailor the phase change temperature, PCMs can be used in a wide range of applications, from solar energy, cold storage and industrial applications1 to heating and cooling buildings,2 seasonal energy storage or textile applications. 3 Organic substances, such as paraffins, fatty acids, and inorganic hydrate salts are often used as solid-liquid phase change materials.4 Pure PCMs exhibit some disadvantages, such as flammability, leakage because of the volume differences during phase change, low thermal conductivity, corrosion or bad odor caused by sublimation.5 In order


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to overcome these drawbacks, an emerging method is the preparation of shape-stabilized PCMs, composite materials obtained through encapsulation into inert materials6 or impregnation of high porosity matrices.7 Shape-stabilized materials can mitigate the leakage or the thermal conductivity decrease caused by the volume change associated with phase transition.8 Mesoporous silica nanoparticles (MSN) are a favorable class of support materials for shapestabilized PCMs due to their high specific area (>1000 m2g-1), pore volume (>1 cm3g-1), good chemical stability and narrow, nanometric pore size distribution (2-50 nm).9 MSN have attracted considerable interest both in the theoretical study of nanoconfinement of small molecules and the effect of physical properties,10-12 as well as in practical applications in the field of PCMs. For example, polyethylene glycol and poly(ethylene glycol) alkyl ether were used to design MSNbased shape-stabilized PCMs.13-14 Recently, the thermal behavior of composite materials based on either SBA-15 silica or CMK-3 carbon and fatty acids was studied, finding strong experimental evidence for nanoconfinement effects on the thermal PCM properties of CMK-3, but not of SBA-15.15 While the melting point depression of nanoscale/nanoconfined crystalline phases is well understood,16 little attention has been paid to the influence of structural and textural parameters of the carrier materials on the latent heat of fusion of the resulting composites and there is no general framework available to assess the enthalpy values and heat storage potential of PCMs containing mesoporous materials.

For composites based on

mesoporous matrices, there is a large variation in the reported literature values of recovered heat of fusion with respect to the expected enthalpy of the pure organic component, usually in the 3080% range,13-14,17 which has so far not been correlated with the textural properties of the mesoporous component, nor with the synthesis procedure of the composites.


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To the best of our knowledge, the nanoconfinement of organic phase change materials into the matrix mesopores was not used to obtain shape-stabilized materials having large heat of fusion over two or more temperature ranges. Such materials would provide an alternative method to PCMs with multiple useful phase change temperatures, currently obtained by employing mixtures of phase change agents. These mixtures can be used in applications requiring a larger temperature operating range, as well as to improve the PCM heat transfer rate. 18 By employing only one active phase-change material to obtain multiple phase change temperatures, risks associated with enthalpy and melting temperature change in multicomponent PCMs and storage system complexity could be reduced. In this study, we obtained dual thermal response materials, exhibiting two melting or crystallization temperatures. This effect arises from the existence of both the nanoconfined phase of organic material inside a mesoporous material and the phase adsorbed on the external silica surface, filling the interparticle voids. The shape-stabilized phase change composite materials were produced by using lauric acid as the active heat storage agent and various mesoporous silica matrices (MCM-41, SBA-15 and mesocellular foams). MSN with pore size between 3 – 35 nm and volume between 1.0-3.3 cm3g-1 were employed and the effect of the silica textural properties on the melting point and heat storage efficiency were investigated. Mesocellular foam silica (MCF) was found to be the most suitable for the design of dual temperature PCMs. The prepared materials were characterized in terms of thermal properties and stability, crystalline structure and textural properties. The results of fatty acid nanoconfinement were compared to a straightforward theoretical model based on thermodynamic and pore geometry considerations, yielding insights into the melting point depression, heat storage efficiency and the physical state of lauric acid confined into mesopores. This approach offers a novel methodology to quantify the


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heat loss accompanying PCMs based on mesoporous matrix as a function of the textural properties. EXPERIMENTAL SECTION Materials: MCM-41 silica, tetraethylorthosilicate (TEOS), 1,3,5-trimethylbenzene (TMB), HCl 37% aqueous solution, poly(ethylene glycol)-block-poly(propylene glycol)-blockpoly(ethylene glycol), EO20PO70EO20 (Pluronic® P123, average molecular weight 5800), and hexane were purchased from Sigma-Aldrich. Lauric acid (LA, purity ≥99%) was obtained from Merck. All substances were used as received. Synthesis of mesoporous silica: Mesocellular foam silica (MCF) was prepared according to a slightly modified literature method.19 Thus, a solution of Pluronic P123 in 1.78 M HCl was prepared at 40 °C under stirring. Next, TMB and TEOS were added and the resulting gel was aged at 40 °C under stirring for 20 h. The syntheses were performed using the molar ratio TEOS : Pluronic P123 : HCl : TMB : H2O = 1 : 0.016 : 6.21 : 0.864x : 184, where x denotes the weight ratio of Pluronic P123 to TMB (0.5 or 1). The obtained mesocellular silica materials are denoted “MCFx” (i.e. MCF0.5 and MCF1). The reaction mixture was transferred into a Teflon-lined autoclave and heated at 105 °C for 24 h under auto-generated pressure. The resulted white solid was thoroughly washed with water, ethanol and diluted HCl solution, and then calcined in air at 550 °C for 6 h. The synthesis and characterization of the SBA-15 material was described elsewhere.20 Briefly, the same procedure as for MCF silica was used, without the addition of TMB and changing HCl to H3PO4. The reagents were added in the molar ratio TEOS : Pluronic P123 : H3PO4 : H2O = 1.0 : 0.016 : 0.10 : 160.


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Preparation of lauric acid/silica composite PCMs: The composite materials were prepared by evaporative impregnation, adding mesoporous silica to solutions of LA in hexane (60 – 750 gL-1), at 30 °C. The solvent was removed under vacuum at room temperature and the solids were carefully recovered and used without further treatment. Composite PCMs with different LA weight fractions were prepared by increasing the initial LA amount in the impregnation solution. The samples are denoted “LAy-MSN”, where y is the LA: MSN weight ratio and MSN represents the mesoporous silica matrix (MCM for MCM-41, SBA for SBA-15, MCF0.5 or MCF1). For example, the sample containing 80% wt. LA and MCF0.5 silica is denoted “LA4-MCF0.5”. The LA content of the PCM samples was checked using TG analysis performed in air. The results are in good agreement with the expected LA content employed in the PCM synthesis (i.e. for LA1-MCF0.5, 51.1% mass loss in the 140800 °C range versus 50.4% expected from the synthesis). Characterization: Powder X-ray diffraction (XRD) data were collected using a Bruker D8 Discover diffractometer with CuKα radiation. Fourier transformed infrared spectroscopy (FTIR) was performed on a Bruker Tensor 27 spectrometer. Scanning electron microscopy (SEM) was carried out using a Tescan Vega 3 LM electron microscope and transmission electron microscopy (TEM) was performed on a FEI Tecnai G2-F30 high resolution electron transmission microscope equipped with a field emission electron gun and a maximum accelerating voltage 300 kV. Nitrogen adsorption-desorption isotherms were recorded on a Quantachrome Autosorb iQ2 porosimeter at 77 K for all pristine silica supports and selected composites with high heat storage capacity. The specific surface area was evaluated with the Brunauer–Emmett–Teller (BET) theory and the average pore diameter for MCM-41 and SBA-15 were calculated from the desorption branch using the Barrett Joyner Halenda (BJH) model. The cell and window average


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diameters for MCF materials were computed using the modified Broekhoff–de Boer method with Frenkel–Halsey–Hill form of the isotherm (BdB-FHH), employing a cylindrical model for windows (adsorption branch) and a spherical model for cells (desorption branch).21 Bright-field and polarized light microscopy was performed using an Optika B353-POL polarizing microscope equipped with a digital camera and image capture software. Thermogravimetric analyses (TG) were recorded on a Setaram Labsys Evo thermogravimetric under N2 flow. Differential scanning calorimetry (DSC) measurements were collected using a Setaram 131 Evo calorimeter under N2 flow, at a scan rate of 2 °C min-1. Heating-cooling cycles were performed, discarding the first heating run in order to remove the thermal history of the samples. Leakage assessment: The leakage behavior of the silica-LA PCMs was evaluated by placing a set amount of sample on a clean glass slide at 80 °C for 2 h. The macroscopic aspect of the samples (leakage, color change) was visually assessed and optical microscopy was performed before and after the heat treatment. RESULTS AND DISCUSSION Structural characterization Small-angle X-ray diffraction (Figure 1A) was employed to highlight the existence of an ordered mesopore array in the case of pristine MCM-41 and SBA-15 materials, as well as the existence of the characteristic scattering pattern associated with the structure of MCF materials (Figure 1A, inset).22 The MCF-type silica consists of hollow spheres (“cells”) interconnected by hexagonal pores (“windows”), usually called “ink-bottle” pore type and having the small-angle diffraction pattern arising from the X-ray scattering produced by monodispersed spherical cells.22 The inclusion of LA into the mesoporous materials has not yielded changes in the diffraction peaks position, indicating that the mesopore structure is not affected by the impregnation


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procedure. For SBA-15 containing PCM, a reduction of the (100) peak intensity is noticed, which can be explained by the decrease of electronic density difference between mesopores and framework caused by the presence of confined lauric acid molecules.23

Figure 1. A) Small-angle and B) wide-angle XRD of silica matrices, commercially available lauric acid (A-super form) and subjected to a melt-crystallization cycle (C form) and their representative PCMs. Inset shows the patterns of MCF-type materials with the background subtracted. The wide-angle XRD (Figure 1B) provides details on the crystallinity of LA after encapsulation into the mesoporous silica materials. The commercially available LA consists of the A-super form24, which is transformed into the more stable C form25 by melting and subsequent crystallization. For the PCM materials with low LA content (LA0.33-MCM, LA1-SBA and LA1-MCF0.5) no crystalline phase of lauric acid could be noticed by wide-angle XRD. The lack of a LA crystalline phase can be explained by the complete adsorption of the organic molecules into the pores and on the outer particle surface of the mesoporous silica. As demonstrated in several


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cases, adsorbed molecules in small pores form a disordered layer at the interface.26-29 This layer behaves as a liquid and does not crystallize even at low temperature. Upon increasing the LA content, the PCM samples exhibit XRD patterns associated with the presence of crystalline organic phase, irrespective of the average pore diameter of the silica matrix. This could be explained by: i) the complete filling of the available mesopore volume, leading to the formation of a crystalline phase outside the mesopores, if the LA content is higher than the silica pore volume; ii) for silica materials with larger pore diameter (SBA-15, MCF) LA crystallization inside the mesopores might also occur as a result of a higher amount of encapsulated organic material. Both mechanisms can occur, as evidenced by the DSC data. Nitrogen sorption measurements provide comprehensive information regarding the textural parameters of the pristine silica and PCMs (specific surface area, mesopore volume and pore size distribution), which will be used to construct the theoretical model of LA nanoconfinement. The nitrogen adsorption-desorption isotherms for MCF silica and LA1-MCF0.5 PCM (Figure 2) have the characteristic type IV shape with a pronounced hysteresis in the 0.5-0.95 relative pressure range. The reduction in adsorbed volume for the LA containing PCMs as compared with the pristine supports indicates that the lauric acid is present inside the mesopores. This is also supported by the pore size distribution for the samples (Figure 2, inset), showing a slight reduction in window size and practically unchanged cell size for the PCMs. This fact could be explained as a preferential adsorption of LA molecules on the smaller window pores surface. The BET specific surface area and total pore volume of the LA composites have lower values than the corresponding silica matrices (Table 1).


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Figure 2. N2 adsorption-desorption isotherms for A) MCF0.5, B) MCF1 and C) LA1-MCF0.5. Inset shows the BdB-FHH pore size distribution for the same materials, with full symbols for cell diameter and open for window size. Table 1. Textural parameters (specific surface area, pore diameter, mesopore volume and total pore volume) for the mesoporous supports and representative PCMs

SBET

dBJH

dwindow

dcell

[nm]

[nm]

Vpore (cm3 g-1)

Pore filling

d≤50 nm

total

[%]

Material

LA [%wt]

[m2g-1]

[nm]

MCM-41

-

912

2.76

-

-

1.05

1.37

-

SBA-15

-

696

6.28

-

-

1.10

1.19

-

MCF0.5

-

876

-

10.4

27.0

2.47

2.49

-

MCF1

-

892

-

14.9

34.9

3.25

3.33

-

LA1-MCF0.5

50

127

-

9.3

28.0

0.71

0.72

57.5

LA2-MCF0.5

66.7

92

-

9.8

27.9

0.49

0.49

60.8

LA4-MCF0.5

80.0

57

-

9.3

27.9

0.33

0.33

68.8

LA4-MCF1

80.0

69

-

12.7

28.1

0.45

0.45

70.8

LA5-MCF1

83.3

37

-

12.7

27.9

0.24

0.24

46.1


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FTIR spectroscopy (see Supporting Information, Figure S1) highlights the presence of the silica matrix and lauric acid in all composite samples. No significant peak shifts from the spectra of the starting materials can be noticed, signifying weak, physical interactions between the silica and LA. The morphology of the mesoporous silica supports (Figure S5) varies from rods for SBA-15 to spheres in the cases of MCF0.5 and MCF1. TEM investigation (Figure 3) revealed “inkbottle” mesopores of the MCF0.5 and MCF1 materials, with averages sizes in good agreement with the nitrogen sorption data.

Figure 3. TEM micrographs of A) MCF0.5 and B) MCF1 Thermal stability The thermal stability of the PCMs is a critical parameter for durable materials for practical heat-storage applications. The thermal stability of the PCM was evaluated by TG analysis (Figure S2). All samples exhibit good thermal stability, as no mass loss could be noticed up to 150 °C.


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The shape stability on heating was investigated by optical microscopy (Figure S3). MCFbased PCMs with LA contents up to 80% weight have retained both their macroscopic shape and white color upon heating at 80 °C. In the case of LA5-MCF1 (83.3% LA), no shape change was noticed; however the material changed on heating from white to semi-transparent. For higher LA content (85.7% for LA6-MCF0.5) a visible loss of material shape, caused by the organic acid melting was noticed (Figure S3), similarly to pure LA. The optical microscopy images showed the crystallization of a separate LA phase for the LA6-MCF0.5 material, which cannot be noticed for the other shape-stabilized PCM samples (Figure S4). The optimum LA weight fraction for preparing shape-stabilized PCMs was found to be 80% for MCF0.5 and 83.3% for MCF1. Thermal properties of silica-lauric acid composite PCMs The DSC analyses of LA0.33-MCM and LA1-SBA (Figure 4A) show no appreciable melting and crystallization events between 5-70 °C. For the LA1-MCM sample, single melting and crystallization events can be noticed, both depressed with 2-2.5 °C with respect to bulk LA and with no thermal hysteresis. The difference between the LA1-MCM and LA1-SBA samples can be rationalized based on the difference of average pore diameters between the silica matrices. The smaller MCM-41 pores (2.76 nm) are only 1.5 times larger than the length of a lauric acid molecule (1.78 nm), likely leading to a more disordered organic amorphous phase inside the mesopores than for SBA-15, which favors the crystallization of lauric acid on the MCM-41 particle surface. The presence of crystalline organic acid for LA1-MCM, but not in the case of LA1-SBA is also confirmed by XRD data. In the case of LA1-MCF0.5, a small melting effect can be noticed (Tonset=31.4 °C; ∆Hfusion= 18.2 Jg-1). The crystallization temperature shows a significant hysteresis (Tonset=14.4 °C). The


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melting point decrease with respect to bulk LA (Tonset=42.7 °C) and the melt-crystallization hysteresis indicates that the LA is confined in the MCF0.5 pores. PCM samples having a high LA content (higher than 66% wt.) exhibit two melting events upon heating (Figure 4B), which can be associated with the LA melting inside the mesopores and on the particle surface, respectively. As expected, the nanoconfined phase shows a decreased melting temperature and significant melt-crystallization hysteresis (Table 2), while the melting and crystallization of the surface-adsorbed LA phase occurs at the bulk melting point.

Figure 4. A) and B) DSC analyses of pure LA and PCM samples, and DSC analysis variation on storage of C) LA4-MCF0.5 and D) LA4-MCF1. Inset shows the cooling scan of LA1-MCF0.5 at 0.5 °C min-1.


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The melting and crystallization points for the nanoconfined LA phase increase with the pore size of silica support, as quantified by the Gibbs–Thomson equation (Equation 1). The crystallization of LA inside MCF1 pores occurs in two stages, which can be explained by the “ink-bottle” structure of the mesocellular support. For MCF0.5-based materials, the two types of crystallization effects might be overlapped, resulting in only a single DSC endothermic event. A DSC analysis of LA1-MCF0.5 performed at 0.5 °C min-1 (Figure 4A, inset) shows two resolved endothermic peaks with Tonset of 26.2 and 14.1 °C, respectively. The heat storage efficiency was computed as the ratio of PCM total heat of fusion to the theoretical heat of fusion for the same bulk LA quantity. The experimental efficiency is proportional with the LA content and it usually increases with the pore diameter of the silica matrix (Table 2). These results confirm that a fraction of the fatty acid does not melt or crystallize, as expected in the case of a non-freezing layer associated with nanoconfined solids.811

The highest heat of fusion values are obtained for the MCF1 materials (110 and 123 J g-1 for

LA4-MCF1 and LA5-MCF1, respectively). For the later shape-stabilized PCM, almost 90% of the theoretical heat of fusion is recovered. With respect to the enthalpy change associated with the melting of the nanoconfined phase for MCF1-based materials, the largest value of 42 Jg-1 was obtained for the LA4-MCF1 material. Unexpected, a lower value was obtained for LA5-MCF1 (28.4 Jg-1), which can be attributed to a partial blocking of the mesopores by the fast crystallization of LA during PCM synthesis, leading to lower pore filling. The PCM stability was evaluated by DSC after storage in ambient conditions for 2 months, by performing 3 additional DSC heating-cooling cycles (Figure 4C and Figure 4D). No differences were noticed after storage, with the exception of the first heating run of the LA4-MCF1 sample. It is interesting to note that after storage the MCF1 sample exhibit the same different behavior


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between the first and subsequent analyses, indicating that the matrix pores are large enough to allow a rearrangement of organic molecules into the mesopores. Table 2. Thermal data for lauric acid and PCMs melting a)

crystallization a)

∆Ht

∆Hb

Tb

∆Hnc

Tnc

∆Ht

∆Hb

Tb

∆Hnc

Tnc

Efficie ncy c) [%]

Sample

xLA

[Jg-1]

[Jg-1]

[°C]

[Jg-1]

[°C]

[Jg-1]

[Jg-1]

[°C]

[Jg-1]

[°C]

LA

1

166.0

166.0

42.7

-

-

163.4

163.4

41.0

-

-

100

LA0.33-MCM

0.25

-

-

-

-

-

-

-

-

-

-

0

LA1-MCM

0.50

34.9

34.9

40.1

-

-

30.2

30.2

39.1

-

-

42.0

LA1-SBA

0.50

-

-

-

-

-

-

-

-

-

-

0

LA2-SBA

0.67

73.7

63.3

41.7

10.4

11.5

62.6

62.6

41.1

- b)

- b)

66.6

LA1-MCF0.5

0.50

18.2

-

-

18.2

31.4

1.9 b)

-

-

1.9 b)

14.4

21.8

LA2-MCF0.5

0.66

68.1

11.0

42.0

53.2

32.2

61.1

9.8

40.7

51.2

14.4

61.8

LA4-MCF0.5

0.80

103.9

74.8

42.1

25.5

32.5

90.9

75.2

41.3

24.4

14.3

78.4

LA6-MCF0.5

0.86

123.6

104.8

41.9

17.1

31.7

120.2

105.8

41.0

17.1

14.0

86.8

LA4-MCF1

0.80

110.5

66.1

42.4

42.0

33.7

110.3

65.8

41.3

40.9

21.1; 83.2 16.2 20.9; LA5-MCF1

0.83

123.7

90.9

42.3

28.4

34.0

118.8

92.8

41.2

25.1

89.5 16.1

a) indices t, b, and nc represent total, bulk and nanoconfined values for heat of fusion and Tonset; b) crystallization occurs at lower temperatures than recorded experimentally on the cooling run (i.e. < 8 °C) c) the experimental heat storage efficiency is computed as 100* ∆Ht, PCM /( xLA* ∆Ht, LA), where xLA represents the LA weight fraction in PCM, and ∆Ht, PCM, ∆Ht, LA refer to the total heat of fusion of PCM and LA, respectively.

Theoretical model of nanoconfined phase melting and thermodynamic aspects The melting point difference between the bulk and nanoscale or nanoconfined crystalline phase are usually explained by one or more of the following: i) A melting point decrease accompanies nanoparticles with high surface area, following the Gibbs–Thomson (Equation 1), where M is the species molar mass, ρ is the particle density, Tbulk


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is the bulk melting temperature, T (r) is the melting temperature of condensed phase at a certain radius r, γnl is the surface tension, θ is the interfacial contact angle between the nucleating phase and support surface and ∆Hfus is the bulk molar heat of fusion.28 ( )

=−

··

· cos

(1)

ii) The nature and strength of interactions between the adsorbate and the pore wall influences the melting point, as strong attractive interactions lead to an increased m.p., while weak or repulsive interactions cause a decrease of its value.30-31 iii) Especially for encapsulated materials, the difference in molar volume between the solid and liquid phase lead to a pressure difference which influences the melting point, according to the Clausius–Clapeyron equation (Equation

2), where T is the melting temperature, p is the

pressure, ∆Vm and ∆Hm are the molar volume and enthalpy change during the solid-liquid phase transition.32

This effect is not expected to significantly influence the PCMs based on

mesoporous silica, as they contain open pores.

= ⋅ !

(2)

!

Taking into account that no strong hydrogen bonding between the carboxyl groups of LA and the silica support are formed, we consider the Gibbs–Thomson equation for quantifying the melting point depression. It is worth noting that for small pores, the particle radius, r, can be significantly lower than the average pore radius due to the presence of the non-melting interphase layer. Several authors have accounted for this effect by using r = dpore/2 – t, where t is the layer thickness in Equation 1. 33-34 The experimental melting points obtained from DSC analysis were fitted with both the modified and classical Gibbs-Thompson formula by using Equation 3, where ∆T=Tbulk – T(r) and parameter A contains the constants (Table S1).


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" =

#

(3)

($%&' ()

A layer thickness value of 1.84 ± 0.31 (adjusted R2 = 0.973) was computed with the modified Equation 1, correlating well with the length occupied by a single lauric acid molecule in the Cform crystal structure (1.78 nm). The modified Gibbs – Thompson equation accounting for the non-melting layer presents a better fit for the experimental data points (Figure S6) than the classical equation with no interfacial layer (for t=0, adjusted R2 = 0.823). It can also be seen that the pore diameter of MCM-41 is lower than 2t, signifying that no crystallization under nanoconfinement can take place for this silica material. Based on the thickness of the non-melting layer, pore diameter values and geometrical considerations, the volume occupied by the non-melting phase inside the mesopores can be computed, assuming cylinder shaped pores, as the ratio of inter-phase layer to total mesopore volume Vlayer/Vpore = 1-[(dpore - 2t)2/dpore2]. The cylinder pore model can be readily applied to SBA-15 and MCM-41-type silica. Due to the “ink-bottle” structure of the mesocellular silica, a more complex approach, consisting of tangent spheres with diameter equal to dcell (Table 1), connected by a cylinder region with diameter equal to dwindow is necessary (Figure S7). The computed non-melting layer volume ranges between 0.8 – 0.9 cm3g-1 for the SBA-15 and MCF supports (Table 3), which can be rationalized on the basis of the similar surface area of all the mesoporous silica. However, in the case of SBA-15 the non-melting layer occupies almost 83% of the total mesopore volume, while for MCF0.5 and MCF1 supports the layer volume represents only 35% and 27%, respectively. The high percentage of non-melting layer volume found for SBA-15 and MCM-41 (100%), the most commonly studied supports, can explain the lack of reported fatty acids or paraffin PCM with high heat of fusion for the nanoconfined phase.


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The volume of free space available for nanoconfined crystalline LA permits the calculation of its corresponding maximum theoretical heat of fusion, depending on support type (Table 3). Table 3. Theoretical values of non-melting layer specific volume and maximum heat of fusion posible for the nanoconfined LA phase Vlayer

Vlayer/Vpore

Max. ∆Hnc a)

[cm3g-1 support]

[%]

[Jg-1 support]

MCM-41

1.05

100

0

SBA-15

0.911

82.8

31.6

MCF0.5

0.857

34.7

269.6

MCF1

0.886

27.3

395.2

Support

a)

∆Hnc = ∆Ht, LA * ρLA * (Vpore- Vlayer) The experimental values for the heat storage efficiency (Figure 5) show good agreement with

the theoretical values obtained from the layer volume model (Equation S4). The notable exception is LA1-SBA, as no experimental melting and crystallization was found, leading to a large discrepancy between experimental and predicted efficiency values. This fact can be explained by the lack of crystallization of the nanoconfined LA phase in the investigated temperature domain. The ratio of maximum possible nanoconfined LA phase heat of fusion to total PCM enthalpy (Equation S5) is higher than the experimental values (Figure 5). These results are in agreement with the sorption isotherm data, which shows only partial mesopore filling with the organic phase change material.


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Figure 5. Experimental (full symbols) and calculated (line) heat efficiency values, percentage of experimental (open symbols) and calculated maximum (dashed line) nanoconfined phase heat of fusion values for the A) SBA-15, B) MCF0.5 and C) MCF1 PCMs. CONCLUSION We successfully obtained shape-stabilized phase change composite materials containing lauric acid and various mesoporous silica materials through an evaporative solution impregnation method. Mesocellular foam silica with high pore volume is a promising matrix in designing shape stabilized PCMs with high fatty acid content (80-83% wt.) and large total phase transition enthalpy values, up to 124 Jg-1. Moreover, the adsorption of the organic species inside the silica mesopores results in two different melt-crystallization regimes for the nanoconfined and bulk phases of lauric acid, and therefore in dual melting and crystallization points. The encapsulation of organic phase change substances into mesocellular foam silica offers a new possibility of obtaining shape-stabilized PCMs with good thermal performance over an adjustable temperature range. The melting point depression for the nanoconfined phase was quantified with a modified Gibbs–Thomson equation, which includes a non-melting interfacial layer between the silica pore


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wall and nanoconfined crystalline lauric acid phase. The 1.84 nm thickness of this layer was found to roughly correspond to the length of the lauric acid molecule. Using the experimental textural parameters of the silica matrices and the obtained thermal data for the PCM samples, a straightforward theoretical model was constructed based on geometric considerations of mesopore size and shape in order to determine the specific volume of the nonmelting interfacial layer. The methodology presented herein offers an attractive and easy way to estimate relevant latent heat of fusion values for PCMs containing mesoporous silica as function of organic component content. The experimental and theoretical heat storage efficiencies were found to be in good agreement. The shape-stabilized PCMs show partial pore filling and reduced nanoconfined phase enthalpy with respect to the theoretical maximum. Nonetheless, heat of fusion values up to 53 Jg-1 for the nanoconfined phase were obtained and even higher values could be reached after careful selection of the organic phase and optimization of composites preparation. MCM-41 and SBA-15 mesoporous silica with hexagonal ordered pore array were demonstrated to have reduced potential for obtaining dual temperature PCMs because of the high fraction of mesopore volume occupied by the non-melting layer. On the other hand, our results proved that high pore volume mesocellular silica foams are attractive supports for obtaining dual temperature shape-stabilized PCMs with high heat of fusion.

ASSOCIATED CONTENT Supporting Information. FTIR spectra of silica supports and composite PCMs, TG analyses of LA and PCMs, optical microscopy of samples before and after heating at 80°C for 2 hr, SEM analyses of silica supports, experimental melting point depressions and fitting functions based on


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Gibbs–Thomson equation, equations used on the non-melting layer etc. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132395. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132395. REFERENCES


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(1) Oró, E.; de Gracia, A.; Castell, A.; Farid, M. M.; Cabeza, L. F., Review on phase change materials (PCMs) for cold thermal energy storage applications. Appl. Energ. 2012, 99, 513-533. (2) Baetens, R.; Jelle, B. P.; Gustavsen, A., Phase change materials for building applications: A state-of-the-art review. Energ. Buildings 2010, 42, 1361-1368. (3) Sarier, N.; Onder, E., Organic phase change materials and their textile applications: An overview. Thermochim. Acta 2012, 540, 7-60. (4) Sharma, A.; Tyagi, V. V.; Chen, C. R.; Buddhi, D., Review on thermal energy storage with phase change materials and applications. Sust. Energ. Rev. 2009, 13, 318-345. (5) Sarı, A., Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications. Energ. Convers. Manage. 2003, 44, 2277-2287. (6) Jin, Z.; Wang, Y.; Liu, J.; Yang, Z., Synthesis and properties of paraffin capsules as phase change materials. Polymer 2008, 49, 2903-2910. (7) Li, B.; Liu, T.; Hu, L.; Wang, Y.; Nie, S., Facile preparation and adjustable thermal property of stearic acid–graphene oxide composite as shape-stabilized phase change material. Chem. Eng. J. 2013, 215–216, 819-826. (8) Fan, L.; Khodadadi, J. M., Thermal conductivity enhancement of phase change materials for thermal energy storage: A review. Renewable and Sustainable Energy Reviews 2011, 15, 24-46. (9) Colilla, M.; Gonzalez, B.; Vallet-Regi, M., Mesoporous silica nanoparticles for the design of smart delivery nanodevices. Biomater. Sci. 2013, 1, 114-134.


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(10) Johari, G. P., Origin of the enthalpy features of water in 1.8 nm pores of MCM-41 and the large Cp increase at 210 K. J. Chem. Phys. 2009, 130, 124518. (11) González Solveyra, E.; de la Llave, E.; Scherlis, D. A.; Molinero, V., Melting and Crystallization of Ice in Partially Filled Nanopores. J. Phys. Chem. B 2011, 115, 14196-14204. (12) Lan, X. Z.; Pei, H. R.; Yan, X.; Liu, W. B., Phase behavior of dodecane—tetradecane binary system confined in SBA-15. J. Therm. Anal. Calorim. 2012, 110, 1437-1442. (13) Zhang, L.; Shi, H.; Li, W.; Han, X.; Zhang, X., Structure and thermal performance of poly(ethylene glycol) alkyl ether (Brij)/porous silica (MCM-41) composites as shape-stabilized phase change materials. Thermochim. Acta 2013, 570, 1-7. (14) Feng, L.; Zhao, W.; Zheng, J.; Frisco, S.; Song, P.; Li, X., The shape-stabilized phase change materials composed of polyethylene glycol and various mesoporous matrices (AC, SBA15 and MCM-41). Sol. Energ. Mat. Sol. C. 2011, 95, 3550-3556. (15) Kadoono, T.; Ogura, M., Heat storage properties of organic phase-change materials confined in the nanospace of mesoporous SBA-15 and CMK-3. Phys. Chem. Chem. Phys. 2014, 16, 5495-5498. (16) Riikonen, J.; Salonen, J.; Lehto, V.-P., Utilising thermoporometry to obtain new insights into nanostructured materials. J. Therm. Anal. Calorim. 2011, 105, 811-821. (17) Wang, W.; Wang, C.; Li, W.; Fan, X.; Wu, Z.; Zheng, J.; Li, X., Novel phase change behavior of n-eicosane in nanoporous silica: emulsion template preparation and structure characterization using small angle X-ray scattering. Phys. Chem. Chem. Phys. 2013, 15, 1439014395.


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TOC/graphical abstract. Dual temperature phase change materials with high latent enthalpy are obtained using lauric acid as melting phase and mesoporous silica as support material. A nonmelting interface organic layer model explains the thermal behavior and energy storage efficiency, representing a novel tool for engineering phase change composite materials.


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A) Small-angle and B) wide-angle XRD of silica matrices, commercially available lauric acid (A-super form) and subjected to a melt-crystallization cycle (C form) and their representative PCMs. Inset shows the patterns of MCF-type materials with the background subtracted. 69x30mm (300 x 300 DPI)


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N2 adsorption-desorption isotherms for A) MCF0.5, B) MCF1 and C) LA1-MCF0.5. Inset shows the BdB-FHH pore size distribution for the same materials, with full symbols for cell diameter and open for window size. 79x61mm (300 x 300 DPI)



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TEM micrographs of A) MCF0.5 and B) MCF1 79x39mm (300 x 300 DPI)


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A) and B) DSC analyses of pure LA and PCM samples, and DSC analysis variation on storage of C) LA4MCF0.5 and D) LA4-MCF1. Inset shows the cooling scan of LA1-MCF0.5 at 0.5 °C min-1. 119x89mm (300 x 300 DPI)



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Experimental (full symbols) and calculated (line) heat efficiency values, percentage of experimental (open symbols) and calculated maximum (dashed line) nanoconfined phase heat of fusion values for the A) SBA-15 B) MCF0.5 and C) MCF1 PCMs. 79x61mm (300 x 300 DPI)


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Dual temperature phase change materials with high latent enthalpy are obtained using lauric acid as melting phase and mesoporous silica as support material. A non-melting interface organic layer model explains the thermal behavior and energy storage efficiency, representing a novel tool for engineering phase change composite materials. 49x45mm (600 x 600 DPI)


Evaluation of Different Mesoporous Silica Supports for Energy Storage

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