The pressure effect on the carbonation behavior of CaO as model compound is studied under mild hydrothermal conditions, as relevant to sustainable geological CO_² sequestration and for potential utilization in thermochemical energy storage. Reaction yields are determined experimentally by means of in-situ powder X-ray diffraction using CaO powder samples in a controlled reaction with CO₂ under gas pressures between 1.0 and 5.0 MPa and at temperatures between 298 and 373 K. The results show a two-step conversion of CaO to CaCO₃, involving Ca(OH)₂ as a reactive intermediate, with differing influences of the microstructures on the individual reaction sub-steps. A kinetic evaluation of the experimental data delivers a high rate-enhancing effect of temperature on the hydration reaction, whereas the CaCO3 formation is strongly dependent on the available CO2 gas pressure. With this systematic investigation the optimal pressure and temperature conditions for this reaction system can be determined delivering a contribution to a sustainable climate and energy management. The pressure effect on the carbonation behavior of CaO as model compound is studied under mild hydrothermal conditions, as relevant to sustainable geological CO₂ sequestration and for potential utilization in thermochemical energy storage. Reaction yields are determined experimentally by means of in-situ powder X-ray diffraction using CaO powder samples in a controlled reaction with CO₂ under gas pressures between 1.0 and 5.0 MPa and at temperatures between 298 and 373 K. The results show a two-step conversion of CaO to CaCO₃, involving Ca(OH)2 as a reactive intermediate, with differing influences of the microstructures on the individual reaction sub-steps. A kinetic evaluation of the experimental data delivers a high rate-enhancing effect of temperature on the hydration reaction, whereas the CaCO₃ formation is strongly dependent on the available CO₂ gas pressure. With this systematic investigation the optimal pressure and temperature conditions for this reaction system can be determined delivering a contribution to a sustainable climate and energy management.

“Pressure Dependence of the Low Temperature Carbonation Kinetics of Calcium Oxide for Potential Thermochemical Energy Storage Purposes and Sustainable CO₂ Fixation”, G. Gravogl, F. Birkelbach, D. Müller, C.L. Lengauer, P. Weinberger, R. Miletich, Adv. Sustainable Syst., (2021) 2100022 (1-11).

https://doi.org/10.1002/adsu.202100022

Materials with high volumetric energy storage capacities are targeted for high-performance thermochemica 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.

“Medium-temperature thermochemical energy storage with transition metal ammoniates – a systematic comparison”, D. Müller, C. Knoll, G. Gravogl, C. Jordan, E. Eitenberger, G. Friedbacher, W. Artner, J.M. Welch, A. Werner, M. Harasek, R. Miletich, P. Weinberger, Applied Energy, 285 (2021) 116470, 1-11.
https://doi.org/10.1016/j.apenergy.2021.116470

Systematic variation of the dehydration temperature and time enables the preparation of highly reactive magnesium oxide for thermochemical energy storage purposes. The reactivity of the MgO, resulting from varying dehydration conditions has been studied by a comparative approach, including reactive surface area, particle morphology and reactivity towards rehydration. For the rehydration an in-situ powder X-Ray diffraction setup is used, allowing for continuous monitoring of Mg(OH)2 formation. The outcome of this investigation was subsequently applied to MgO from natural magnesites to assess the impact of impurities in the material on rehydration reactivity.

“Tuning the performance of MgO for thermochemical energy storage by dehydration – From fundamentals to phase impurities”, D. Müller, C. Knoll, G. Gravogl, W. Artner, J. Welch, E. Eitenberger, G. Friedbacher, M. Schreiner, M. Harasek, K. Hradil, A. Werner, R. Miletich, P. Weinberger, Applied Energy 253, 2019, 113562
https://doi.org/10.1016/j.apenergy.2019.113562

Metal carbonates are attractive materials for combining carbon capture and thermochemical energy storage. Carbonate materials feature high decomposition and formation temperatures and may be considered in applications in combination with concentrating solar power. In the present study in-situ P-XRD carbonation (1–8 bar CO₂) and reactor-based experiments (1–55 bar CO₂) are combined focusing on the effect of elevated CO₂ pressures on carbonation of metal oxides. Carbonation of MnO and PbO at CO₂ pressures between 8 and 50 bar in the presence of moisture resulted in reaction with CO₂, forming the corresponding carbonates at notably lower temperatures than under dry CO₂ atmosphere of 1 bar. This enables the application of metal oxide/metal carbonate reaction couples for energy storage at temperatures between 25 and 500 °C. Based on the reversible carbonation/decarbonation of PbO under varying CO₂ pressures, an isothermal storage cycle between PbO/PbCO₃ · 2 PbO, triggered by changing the CO₂ pressure between 2 and 8 bar, was developed.

„Pressure effects on the carbonation of MeO (Me=Co, Mn, Pb, Zn) for thermochemical energy storage“, G. Gravogl, C. Knoll, W. Artner, J. Welch, E. Eitenberger, G. Friedbacher, M. Harasek, K. Hradil, A. Werner, P. Weinberger, D. Müller, R. Miletich, Applied Energy 252, 2019, 113451
https://doi.org/10.1016/j.apenergy.2019.113451