Medium-temperature thermochemical energy storage with transition metal ammoniates – A systematic material comparison

Abstract

Materials with high volumetric energy storage capacities are targeted for high-performance thermochemical energy storage systems. 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. In the present study, 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. 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.