Zinc Nitrate Hexahydrate Pseudobinary Eutectics for Near-Room-Temperature Thermal Energy Storage

Abstract

Stoichiometric salt hydrates can be inexpensive and provide higher volumetric energy density relative to other near-room-temperature phase change materials (PCMs), but few salt hydrates exhibit congruent melting behavior between 0 and 30 °C. Eutectic salt hydrates offer a strategy to design bespoke PCMs with tailored application-specific eutectic melting temperatures. However, the general solidification behavior and stability of eutectic salt hydrate systems remain unclear, as metastable solidification in eutectic salt hydrates may introduce opportunities for phase segregation. Here, we present a new family of low-cost zinc-nitrate-hexahydrate-based eutectics: Zn(NO₃)₂·6(H₂O)-NaNO₃ (T_eu = 32.7 ± 0.3 °C; ΔH_eu = 151 ± 6 J·g^-1), Zn(NO₃)₂·6(H₂O)-KNO₃ (T_eu = 22.1 ± 0.3 °C; ΔH_eu = 140 ± 6 J·g^-1), Zn(NO₃)₂·6(H₂O)-NH₄NO₃ (T_eu = 11.2 ± 0.3 °C; ΔH_eu = 137 ± 5 J·g^-1). While the tendency to undercool varies greatly between different eutectics in the family, the geologic mineral talc has been identified as an active and stable phase that dramatically reduces undercooling in Zn(NO₃)₂·6(H₂O) and all related eutectics. Zn(NO₃)₂·6(H₂O) and its related eutectics have shown stability for over a hundred thermal cycles in mL scale volumes, suggesting that they are capable of serving as robust and stable media for near-room-temperature thermal energy storage applications in buildings.

Figure 1

Cost-energy density as a function of (a) melting temperature of the PCM and (b) volumetric energy density of the PCM with reference data taken from Hirschey et al. and Ahmed et al.^, Figure adapted with permission from ref (7). Copyright 2021, retained by authors. Solid shapes represent salt hydrates in the liquid phase, and outlined represent the solid phase. Yellow diamonds are Zn(NO₃)₂·6H₂O eutectics (reported here), maroon squares are LiNO₃·3(H₂O) eutectics, pink triangles are paraffin, and blue circles are other salt hydrates. The blue shaded section in (a) indicates the desired temperature range for TES media in buildings.

Figure 2

(a) The observed powder diffraction pattern of talc in blue, with the calculated spectra illustrated in red, and a difference curve in black. A supercell of the resultant crystal structure of talc in the top right corner, showing the layering of tetrahedral-octahedral-tetrahedral sites observed in talc, resulting in a pseudohexagonal in-plane structure. (b) SEM image of talc taken using secondary electrons at 10 keV. The image shows the flaky nature of talc.

Figure 3

DSC curve of ZNH (a) as well as DSC curves for composition arrays of tested pseudobinary ZNH eutectics, ZNH-NaNO₃ (b, purple), ZNH-KNO₃ (c, green), and ZNH-NH₄NO₃ (d, blue), as measured at heating and cooling rates of 10 °C·min^–1 to determine the eutectic composition to within 1 wt % (highlighted in bold and in color). Dashed lines refer to the reported T_fus or T_eu collected from the Q2000 DSC. Eutectic compositions were selected for having a narrow peak width, indicative of minima in the liquidus temperature (associated with the eutectic point). Mixtures with higher concentrations of anhydrous nitrate are above the solubility limit at room temperature. A ternary phase diagram is shown on each plot with a dot indicative of eutectic composition, and a tick mark indicative of solublity limit.

Figure 4

Resultant DSC curves of ZNH and related eutectics taken at 1 °C·min^–1 on Microcalvet DSC.

Figure 5

Thermal conductivity of liquid ZNH and related eutectics, compared to water, measured on both heating and cooling. Deionized water (DIW) is provided for comparison as well as reference values from NIST. The black triangle at 40 °C is a previously reported thermal conductivity for ZNH from Lane.

Figure 6

Various NPs were tested alongside neat ZNH to determine the effect on undercooling. 1 wt % quantities of NP were added to a DSC pan with ZNH.

Figure 7

Isothermal crystallization times at various temperatures for ZNH (red) and eutectics containing NaNO₃ (purple), KNO₃ (green), and NH₄NO₃ (blue) in the presence and absence of the talc nucleation particles. Darker shades of the two colors presented at a temperature represent samples that contain 2 wt % of talc. Curves represent isothermal nucleation times for 25, 50, and 75% probability of nucleation, calculated as described in the text.

Figure 8

Samples were aged for short periods and periodically sampled in hermetically sealed DSC pans initially (dashed line), and were then extracted from vials and measured (solid line). Neat systems (light shades) were compared with talc-inclusive systems (dark shades).

Figure 9

Calculated difference between the cell thermocouple reading and the control thermocouple in an unhoused cell are plotted against temperature for (a) ZNH, (b) ZNH-NaNO₃, (c) ZNH-KNO₃, and (d) ZNH-NH₄NO₃. The top two curves of each plot are talc-inclusive samples, and the bottom two are neat samples.

Figure 10

Three sets of graphs for: (left) normalized integrated area of the melting curves, (middle) melting temperature, and (right) degrees of undercooling for (a) ZNH, (b) ZNH-NaNO₃, (c) ZNH-KNO₃, and (d) ZNH-NH₄NO₃, over many cycles. Red and orange points indicate neat samples A and B respectively for each system, and blue and green indicate talc samples A and B for each system. The vertical line indicates a pause between sets of cycles in all systems but ZNH where it did not have one.