Novel metal coated nanocapsules of ethyl esters fatty acid eutectic mixture as phase change material with enhanced thermal conductivity for energy storage applications

doi:10.1016/j.tca.2020.178581

Thermochimica Acta

Volume 687, May 2020, 178581

https://doi.org/10.1016/j.tca.2020.178581 Get rights and content

Highlights

•
Nanocapsules of ES-EP eutectic mixture were synthesized by in-situ emulsion polymerization.
•
HRTEM images of nanocapsules exhibit a uniform spherical morphology with core-shell structure.
•
Thermal cycling results of metal coated nanocapsules were found to have acceptable thermal properties even after 1000 cycles.
•
Copper and cobalt coated nanocapsules showed good thermal conductivity of 0.693 ± 0.020 and 0.660 ± 0.019 Wm⁻¹ K⁻¹.

Abstract

In the present study, metal coated nanocapsules of ethyl stearate-ethyl palmitate eutectic phase change material (PCM) with high thermal conductivity were synthesized by in-situ emulsion polymerization followed by trisodium citrate reduction method. HRTEM analysis reveals the formation of core-shell structure with uniform spherical morphology and EDX results showed the presence of metal coating over silica shell. Phase change property of the eutectic mixture and nanocapsules were measured using DSC. Metal coated nanocapsules exhibited better thermal stability, conductivity and reliability determined by TGA, laser flash and thermal cycling analysis. Nanocapsules can persist constant phase change properties even after 1000 melting/ freezing thermal cycles. Most significantly, the thermal conductivity of metal coated nanocapsules increases from 0.257 ± 0.007 (eutectic mixture) to 0.660 ± 0.019, 0.693 ± 0.020 and 0.713 ± 0.021 Wm⁻¹ K⁻¹ for cobalt, copper and silver coated nanocapsules respectively. Similar to silver coated nanocapsules, copper and cobalt coated nanocapsules can also prove to be a promising material for thermal energy storage applications.

Graphical abstract

Introduction

In recent years, the increasing energy consumption and storage of fossil fuels have been a big challenge for the economic development of modern society. Hence resorting to new alternative energy resources for efficient energy storage has become extremely vital. Thermal energy storage (TES) using phase change materials (PCMs) can increase the efficiency of system and help in solving the problem. PCMs can absorb, store and release large amount of latent heat during phase change process within a small range or almost at constant temperature. When PCM is used in building materials, the heat indoors during day time is absorbed by PCM at a temperature above its melting point and then the stored heat is released during night as it reaches temperature lower than its melting point. On considering the chemical compatibility with building materials, most of the research prefers organic PCMs. Among these groups, fatty acid esters and/or their eutectics have been extensively studied as a promising type of solid-liquid PCMs because of their superior properties such as high heat storage capacity, appropriate melting temperature range, little or no super-cooling, non-toxicity, better chemical and thermal stability and non-corrosiveness [1,2].

Only few eutectic mixtures of esters have been used in thermal energy storage applications [[3], [4], [5], [6]], nevertheless it is not easy to be used directly in building construction materials because of leakage during solid-liquid phase transition. Hence encapsulation has been developed to overcome these draw backs [[7], [8], [9]]. The micro/nano capsules pack the PCM core individually with a hard shell that can therefore even handle liquids as solid material. Most attention in synthesizing micro/nano capsules was on using organic shell materials [10,11]. However, there are some limitations with organic shells such as flammability, poor thermal and chemical stability and low thermal conductivity [12]. But the usage of inorganic shell materials such as silica, calcium carbonate and titanium dioxide has considerable attention recently. These inorganic shell materials have presented much higher mechanical strength, rigidity and good thermal conductivity than the organic shells [[13], [14], [15]].

Metals show excellent thermal conductivity and mechanical properties. The utilization of novel metal coating on silica shell materials were challenging, only few works were reported [[16], [17], [18]] previously and were limited with single PCM. Therefore in the present study, novel metal coated nanocapsules composed of Ethyl Stearate (ES) and Ethyl Palmitate (EP) eutectic mixture as core and SiO₂ as shell material were synthesized by in-situ emulsion polymerization technique. To strengthen the silica shell layer, a bilayered SiO₂-Ag/Cu/Co shells are formed using trisodium citrate reduction method. Moreover, the morphologies, size, chemical structure and thermal properties of the as-synthesized novel metal coated nanocapsules were thoroughly investigated. These metal coated nanocapsules with enhanced thermal conductivity will benefit their application in thermal energy storage devices mainly in building materials.

Section snippets

Reagents and materials

Ethyl Stearate (ES) and Ethyl Palmitate (EP) used as PCM were supplied by Sigma-Aldrich, USA. Triton-X 100 as emulsifier, Ammonium Chloride (NH₄Cl) as nucleating agent, Urea (CH₄N₂O) and Trisodium citrate (Na₃C₆H₅O₇) were used as reducing agent, Acetic acid (CH₃COOH) and Sodium Hydroxide (NaOH) pellets used as pH regulators were obtained from SRL, India. Tetra ethyl ortho silicate (TEOS), Silver Nitrate (AgNO₃), Cupric Nitrate Trihydrate (Cu(NO₃)₂.3H₂O), Cobalt Nitrate Hexahydrate (Co(NO₃)₂.6H₂

Morphological study with elemental analysis and particle size of nanocapsules

The morphological structure and particle size of the as-synthesized samples were observed by HRTEM with EDX and PSD analysis as shown in Fig. 3. i, ii and iii respectively. From the HRTEM images it is clear that nanocapsules exhibit a perfect spherical morphology with average particle size in the range of 510−580 nm. It is very significant to observe that all the nanocapsules show a well defined core-shell structure and the silica shell appears solid and very compact. The HRTEM images of

Conclusion

In the present work, nanocapsules of ES-EP eutectic mixture were synthesized by in-situ emulsion polymerization method. Novel metal coating over the nanocapsules were done using trisodium citrate reduction method.

a
The FTIR spectra confirmed that the ES-EP eutectic mixture was successfully encapsulated within the silica shell material and has no chemical interaction between the ES-EP core and silica shell material.
b
HRTEM images confirmed that the nanocapsules exhibit a uniform spherical morphology

Authorship contributions

Conception and design of study: S. Dhivya, S. Imran Hussain, S. Kalaiselvam.

Acquisition of data: S. Dhivya, S. Imran Hussain.

Analysis and/or interpretation of data: S. Dhivya, S. Kalaiselvam.

Drafting the manuscript: S. Dhivya, S. Imran Hussain;

Revising the manuscript critically for important intellectual content: S. Dhivya, S. Kalaiselvam.

Approval of the version of the manuscript to be published (the names of all authors must be listed): S. Dhivya, S. Imran Hussain, S. Kalaiselvam.

Declaration of Competing Interest

None.

Acknowledgments

The authors gratefully acknowledge Department of Science and Technology (DST), New Delhi, India, for providing financial support to carry out this research work under DST – Women Scientist Scheme A (WOS-A) [DST File No. SR/WOS-A/CS-5/2017]; DST - TDT [DST File No. DST/TDT/LCCT-03/2017]. One of the authors, Ms. S. Dhivya is thankful to DST, New Delhi, for the award of DST-Women Scientist fellowship and expresses her sincere thanks to Ms. J. Sandhya for improving the language of the manuscript.

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Novel metal coated nanocapsules of ethyl esters fatty acid eutectic mixture as phase change material with enhanced thermal conductivity for energy storage applications

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Reagents and materials

Morphological study with elemental analysis and particle size of nanocapsules

Conclusion

Authorship contributions

Declaration of Competing Interest

Acknowledgments

Sol. Energy

Thermochim. Acta

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Appl. Therm. Eng.

J. Colloid Interface Sci.

Energy Conserv. Manage.

Energy Build.

Appl. Energy

Polymer

Energy

Thermochim. Acta

Appl. Therm. Eng.

Energy

Sol. Energy Mater. Sol. Cells