Storing Solar Energy in Sodium Acetate-Based Hand Warmers Using Light-Absorbing Particles Bhim P. Kafle,* Sijan Devkota, Bibek Uprety, Kyun Y. Park,* Niroj Basnet, Aavash Shakya, and Santosh K. Tiwari* Cite This: https://dx.doi.org/10.1021/acsaem.0c01933 Read Online ACCESS Metrics & More Article Recommendations ABSTRACT: We have fabricated an eco-friendly, affordable, and recyclable heat-releasing chemical bag (hence forth termed as hand warmers), which melts when exposed to solar radiation and can be suitable for warming the hands and feet of mountain dwellers, hikers, and motorbike/vehicle riders during winter. Our hand warmer uses sodium acetate trihydrate (CH 3 COONa·3H 2 O, SAT)-based composites consisting of a suitable photo- thermal-responsive additive to shorten the melting time of the hand warmer for practical applications. Under simulated solar radiation, activated charcoal powder, graphite powder, graphene flakes, and copper nanoparticles (Cu-NPs) were tested as photothermal- responsive additives, which reduced the melting time of the hand warmer by a factor of 0.68, 0.65, 0.63, and 0.57 respectively, when compared to the sample without any additives. Moreover, the stability of the hand warmer was maintained for two-three months (at which point the samples were deliberately discharged) by incorporating 9 mL of extra water (8% wt) in the SAT composites, which demonstrates the long-term heat storage capacity of the hand warmer. KEYWORDS: hand warmer, sodium acetate trihydrate, phase change materials, solar energy storage device, photoresponsive additives 1. INTRODUCTION New materials are changing our lives every day with cutting- edge technologies. 1,2 In this regard, phase change materials (PCMs) are playing a vital role in the development of new technologies for our comfort. 3-5 PCMs efficiently absorb, store, and release large amounts of latent heat over a defined temperature range when the material changes its physical state. PCMs are classified as organic, inorganic, and eutectic and have been extensively used for thermal energy storage applications. 3,6-8 Among the different types of PCMs, hydrated salts of inorganic PCMs are cheap, sustainable, and efficient for the solar energy storage, recycling of industrial and domestic waste, and small energy storage device (such as heat pads and for hand warmers) fabrication. 3-5,9-15 One such inorganic PCM material that has been widely used in both the large 3-5,9-15 and small 9,13-22 thermal heat storage systems is sodium acetate trihydrate (SAT), with a melting point of 58 °C and heat of fusion of 264-289 J/g. Small heat storage devices such as hand warmers usually comprise a solution of SAT with a triggering element, usually a disc- shaped and corrugated thin metallic plate, which is bent alternatively inward and backward to activate the supercooled SAT to crystallize and release heat. 13 Heat is stored in such systems when external energy such as boiling water and microwave or electric heating is applied to melt the crystallized solution to the liquid state. 16,18,20-22 However, these charging techniques (melting the crystallized chemical to the liquid state) are impractical for low-income mountain dwellers and inconvenient during outdoor activities such as hiking and biking. On the other hand, for larger energy storage applications, SAT systems are usually coupled with a solar water heating or electrical heating system (connected with heat exchangers) for the efficient charging of SAT. 16-18,20,21 Still, coupling with heat exchangers or solar panels for charging the small systems is also impractical. Alternatively, the direct use of solar energy to melt the pristine SAT systems could be a viable solution. However, as discussed above, it takes a longer time to fully melt the SAT because of its poor optical absorbance in the visible and near- IR region of the solar spectrum (from 0.4 to 2.5 μm) and also because of the poor thermal conductivity of SAT. These issues are very important in terms of frequent (daily) usage of solar hand warmers in the winter season. In this regard, introduction of a small amount of nano/micromaterials with good optical absorption and thermal properties (hereafter, we will name Received: August 11, 2020 Accepted: November 2, 2020 Article www.acsaem.org © XXXX American Chemical Society A https://dx.doi.org/10.1021/acsaem.0c01933 ACS Appl. Energy Mater. XXXX, XXX, XXX-XXX Downloaded via SAN FRANCISCO STATE UNIV on November 20, 2020 at 02:59:07 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.