The new class of transition metal salt ammoniates as new promising thermochemical energy storage materials

Abstract

In the attempt to reduce the carbon footprint new solutions for our energy supply system are crucial. This holds for the more efficient use or recovery of industrial waste heat, especially. A promising approach is based on thermochemical energy storage materials with high volumetric energy storage capacities. The reaction of transition metal salts with ammonia, forming reversibly the corresponding ammonia-coordination compounds, is still an under-investigated area for energy storage purposes, although, from a theoretical perspective this should be a good fit for application in medium-temperature storage solutions between 25 °C and 350 °C. The potential of reversible ammoniation of a series of transition metal chlorides and sulphates with gaseous ammonia for suitability as thermochemical energy storage system was investigated. Among the investigated metal chlorides and sulphates, candidates combining high energy storage densities and cycle stabilities were found. For metal chlorides, during the charging / discharging cycles in the presence of ammonia slow degradation and evaporation of the materials was observed. This issue was circumvented by reducing the operating temperature and cycling between different degrees of ammoniation, e.g. in the case of NiCl₂ by cycling between [Ni(NH₃)₂]Cl₂ and [Ni(NH₃)₆]Cl₂. In contrast, sulphates are perfectly stable under all investigated conditions. The combination of CuSO₄ and NH₃ provided the most promising result directing towards applicability, as the high energy storage density of 6.38 GJ m⁻³ is combined with full reversibility of the storage reaction and no material degradation over cycling. During reaction with ammonia, a large change in the sulphate 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. The results of this comparative systematic material evaluation encourage for a future consideration of the so far underrepresented transition metal ammoniates as versatile thermochemical energy storage materials.