CuSO4/[Cu(NH3)4]SO4-Composite Thermochemical Energy Storage Materials

Abstract

The thermochemical energy-storage material couple CuSO₄/[Cu(NH₃)₄]SO₄ combines full reversibility, application in a medium temperature interval (<350 °C), and fast liberation of stored heat. During reaction with ammonia, a large change in the sulfate solid-state structure occurs, resulting in a 2.6-fold expansion of the bulk material due to NH₃ uptake. In order to limit this volume work, as well as enhance the thermal conductivity of the solid material, several composites of anhydrous CuSO₄ with inorganic inert support materials were prepared and characterized with regard to their energy storage density, reversibility of the storage reaction, thermal conductivity, and particle morphology. The best thermochemical energy storage properties were obtained for a 10:1 CuSO₄-sepiolite composite, combining an attractive energy storage density with slightly improved thermal conductivity and decreased bulk volume work compared to the pure salt.

Keywords: CuSO4/[Cu(NH3)4]SO4; composite material; thermal conductivity; thermochemical energy storage; thermochemistry.

Figure 1

2.6-fold expansion of the bulk material during reaction of CuSO₄ (left, white solid) with four equivalents of NH₃, forming the [Cu(NH₃)₄)]SO₄ complex (right, blue solid).

Figure 2

Enthalpy of reaction (energy content) of pure CuSO₄ (entry 1, [23]) and those corresponding to the samples of different composite materials. Entries 2 to 4 correspond to CuSO₄ co-precipitated from the suspension of fine-grained matrix material in water. Entries 5 to 6 are the composite carrier materials prepared from impregnation with dissolved CuSO₄.

Figure 3

Initial study on reversibility by performing a complete thermal charging–discharging cycle for all various composites. For comparison, the equivalent NH₃ charging–discharging of pure [Cu(NH₃)₄]SO₄ is given on top. An X-ray powder diffraction (P-XRD) comparison for all composites before and after reaction with NH₃ are given in the supplementary materials, Figures S2–S6. Comparing the support materials after loading and ammoniation with the P-XRD pattern for pure CuSO₄ and [Cu(NH₃)₄]SO₄ shows good agreement. Nevertheless, a detailed structural analysis of eventual changes of the support material or interactions with the storage material based on the powder patterns is impossible, due to non-existing comparability with previously reported structures/patterns. This would also exceed the scope of this study.

Figure 4

Consecutive charging–discharging cycles number 2 and 3 for [Cu(NH₃)₄]SO₄ in a sepiolite matrix under an NH₃-atmosphere.

Figure 5

Consecutive charging–discharging cycles 2 and 3 for [Cu(NH₃)₄]SO₄ on vermiculite under an NH₃-atmosphere.

Figure 6

Comparison of the thermal conductivity for all CuSO₄ and [Cu(NH₃)₄]SO₄ composites: (a) thermal conductivity at 30 °C, (b) thermal conductivity at 55 °C. The results for the pure salts are represented in black, for charcoal (in the legend stated as (1) in orange, for zeolite 13X (2) in blue, for sepiolite (3) in violet and for vermiculite (4) in green.

Figure 7

SEM-images of the pristine support materials (left column), CuSO₄ loaded to the support materials (middle column) and the loaded support materials after reaction with NH₃ (right column). On top SEM-images of CuSO₄ and [Cu(NH₃)₄]SO₄ are given for comparison. All images were taken ad a 10,000-fold magnification, the white bar corresponds to a length of 10 µm.

Figure 8

Volumetric expansion of CuSO₄ during reaction with NH₃, resulting [Cu(NH₃)₄]SO₄ in the case of (a) pure CuSO₄, (b) CuSO₄ on sepiolite 10:1 wt. % and (c) CuNaX zeolite. For the zeolite sample the color change during reaction with NH₃ towards the darker color of [Cu(NH₃)₄]SO₄ is hardly visible.