Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est Thermogravimetric analysis of Cu, Mn, Co, and Pb oxides for thermochemical energy storage Mahyar Silakhori ⁎ , Mehdi Jafarian, Maziar Arjomandi, Graham J. Nathan School of Mechanical Engineering, Centre for Energy Technology, The University of Adelaide, Adelaide, SA 5005, Australia ARTICLE INFO Keywords: RedOx cycles Chemical looping Thermochemical heat Pressure swing Solar energy Energy storage ABSTRACT The reversible reduction and oxidation (RedOx) reactions of CuO/Cu 2 O, Co 3 O 4 /CoO, Mn 2 O 3 /Mn 3 O 4 , and Pb 3 O 4 /PbO have been assessed experimentally with thermogravimetric Analysis (TGA). The temperature was maintained constant during charging and discharging of the thermochemical energy storage via pressure swing for a range of oxygen partial pressures spanning from 0.05 to 0.8 bar. The rate of oxidation reactions were assessed for a range of partial pressures, while changes to the structure of the materials was assessed with X-Ray diffraction spectra (XRD) before and after 10 successive reduction and oxidation cycles. The results show that the Co 3 O 4 /CoO, Mn 2 O 3 /Mn 3 O 4 , and CuO/Cu 2 O pairs have a potential for chemical storage at temperatures ranges from 900 °C to 1000 °C, while no thermochemical reaction was observed for Pb 3 O 4 up to a temperature of 550 °C. 1. Introduction Thermal energy storage systems have received significant attention because they offer potential for low cost energy storage for Concentrating Solar Thermal (CST) energy plant [1], in comparison to other renewable energy technologies [2–4]. Among the different types of thermal energy storage systems, thermochemical energy storage of- fers potential to achieve both a high energy density and operating temperatures in the range 700 °C–1100 °C [5], which is compatible with the more efficient power cycles such as super-critical CO 2 and gas turbine combined cycles. However, high temperature thermochemical energy storage systems are still at an early stage of development. One of the barriers to temperature swing systems is the enthalpy loss of heating and cooling, while another is the potential for sintering of materials and thermal hysteresis during reduction and oxidation [6,7]. Hence, more work is needed to assess the potential of suitable materials at different thermodynamic condition. Reduction and Oxidation (RedOx) reactions of metal oxides can be described as follows: → + > − ( ) Δ Reduction reaction: MeO O MeO H 0 , δ1 δ δ 2 2 δ2 2 1 (1) + → < − ( ) Δ Oxidation reaction: MeO O MeO H 0. δ2 δ δ 2 2 δ1 2 1 (2) MeO δ1 and MeO δ2 represent two states of oxidation of the metal Me. Since the reduction reaction (Eq. (1)) is endothermic, it offers potential to be used in the charging cycle of a thermochemical thermal energy storage (TES) system. Similarly the exothermic oxidation reaction (Eq. (2)) can be used to release the stored heat. Based on Le Chatelier’s principle, two alternative states of oxidations of MeO δ1 and MeO δ2 can be achieved by either changing the partial pressure of oxygen (P- swing), the temperature of the reaction (T-swing) or a combination of them [8]. However, the relative advantages and disadvantages of these two alternative approaches on the rate of the reaction has not pre- viously received much attention. The pairs of Co 3 O 4 /CoO and Mn 2 O 3 /Mn 3 O 4 and CuO/Cu 2 O have been selected here as potential candidate materials for thermochemical storage due to their high reaction enthalpy (Co 3 O 4 /CoO = 5.15 GJ/m 3 , CuO/Cu 2 O = 5.11 GJ/m 3 and Mn 2 O 3 /Mn 3 O 4 = 0.918 GJ/m 3 ), high operating temperature of approximately 1000 °C and reaction reversi- bility [9–11]. These materials has been extensively assessed for ther- mochemical energy storage with temperature swing [9–11]. However, gaps remain in the understanding of their performance under some relevant conditions such as RedOx reactions at isothermal condition with changing the oxygen partial pressure. The exact temperatures for the RedOx reactions (Eqs. (1) and (2)) and the conversion of multivalent metal oxides in temperature-swing system depend on the partial pressure of gaseous components involved in the reactions or experimental condition [12]. Table 1 presents the operating temperature of the selected metal oxides in various en- vironments such as N 2 ,O 2 , Ar and Air. Neises et al. [13] reported a reduction temperature of 800 °C for Co 3 O 4 /CoO oxides in air at ambient https://doi.org/10.1016/j.est.2019.03.008 Received 9 October 2018; Received in revised form 4 March 2019; Accepted 8 March 2019 ⁎ Corresponding author. E-mail address: mahyar.silakhori@adelaide.edu.au (M. Silakhori). Journal of Energy Storage 23 (2019) 138–147 2352-152X/ © 2019 Elsevier Ltd. All rights reserved. T