Experimental Investigation of Co–Cu, Mn–Co, and Mn–Cu Redox Materials Applied to Solar Thermochemical Energy Storage

Abstract

Thermochemical energy storage (TCES) can be achieved via reversible redox reactions based on metal oxides for solar energy storage applications in solar power plants. As cobalt oxide (Co3O4/CoO) and manganese oxide (Mn2O3/Mn3O4) appear as attractive candidates for TCES, an experimental study has been conducted to evaluate the potential of mixed oxides from the Co–Cu–O, Mn–Co–O, and Mn–Cu–O systems. The addition of Cu to Co3O4 allows accessing promising materials with good cycling stability and high reaction enthalpy for TCES applications. As for the Mn–Co–O and Mn–Cu–O systems, the role of the hausmannite phase formation at low Co or Cu contents on the irreversibility of Mn-based mixed oxides is made evident, thus demonstrating the interest of the addition of a secondary metal oxide to the Mn-based material. For the Mn–Co–O system, low amounts of Mn should be favored, since both the oxygen storage capacity and the reaction enthalpy significantly increase with decreasing Mn contents, and the reversibility is lost above 50 mol % Mn because of the formation of the tetragonal spinel phase (hausmannite), which inhibits further reoxidation. Among the mixed oxides of the Mn–Cu–O system, the compositions with Cu amounts below 30 mol % cannot be cycled because of the hausmannite phase formation during reduction, similarly for the case of the Mn–Co–O system. In contrast, the compositions with Cu amounts in the range 40–80 mol % feature promising redox properties with complete reaction reversibility. The tuning of redox properties by the synthesis of mixed metal oxides shows promise for the development of suitable high-temperature TCES materials.

Mixed Metal Oxide Systems Applied to Thermochemical Storage of Solar Energy: Benefits of Secondary Metal Addition in Co and Mn Oxides and Contribution of Thermodynamics

“…Crystalline phases were analyzed, before and after cycling, with X-ray diffraction (XRD) using a PANanalytical XPert Pro diffractometer, Malvern Panalytical, UK (CuKα radiation, λ = 0.15418 nm). Detailed synthesis method and specific properties analysis can be found in previous works [35,36]. Throughout the study, the amount of secondary metal (M') added to the main metal (M) is referred to as x(M') (content of M' in molar fraction), corresponding to the molar ratio on metal basis, n(M')/((n(M) + n(M')).…”

“…This system can achieve a gravimetric energy storage density of 110 to 160 kJ/kg [37,38] and the theoretical reaction enthalpy is reported to be 202 kJ/kg [37]. While Co 3 O 4 /CoO performs redox cycles with rapid reaction kinetics, Mn 3 O 4 is very slow to re-oxidize and also presents a larger hysteresis than cobalt oxide [35][36][37][38]. Thus, the redox properties of Mn 2 O 3 /Mn 3 O 4 need to be improved further to use this material efficiently for TCES application.…”

“…Nonetheless, cobalt oxide is a potentially carcinogenic material and is expensive [6], which gives incentives for the development of other alternative materials. The manganese oxide system (Mn 2 O 3 /Mn 3 O 4 ) features slow kinetics of the oxidation step, which impacts the cycling reversibility of this material and requires performance improvement, but it is a less expensive raw material than cobalt oxide [35][36][37]. This system can achieve a gravimetric energy storage density of 110 to 160 kJ/kg [37,38] and the theoretical reaction enthalpy is reported to be 202 kJ/kg [37].…”

“…It is possible to achieve better redox performance with a metal oxide by modifying its morphology (for example through tailored synthesis), or through the addition of a secondary transition metal. In previous works, this method has been applied to modify single metal oxides, chiefly Co 3 O 4 and Mn 2 O 3 , and the impacts of this modification on the tuning of reaction kinetics, cycling stability and reaction temperature, were reported [34][35][36]. The present work aims to compare experimental assessment of mixed metal oxide systems based on Co 3 O 4 /CoO and Mn 2 O 3 /Mn 3 O 4 through binary mixing between Co, Mn, Fe and Cu oxides to provide an overview of the most suitable systems and to propose perspectives for future development of oxide-based TCES technology.…”

“…However, the re-oxidation rates were low for all compounds except cobalt oxide [7]. Since Co 3 O 4 /CoO and Mn 2 O 3 /Mn 3 O 4 are the two most promising materials for TCES, the potential of Co and Mn-based mixed oxides was investigated through the addition of a secondary metal, for example with copper [36,41], iron [27,35,41,44], or with one another [36,45]. A summary of the experimental data obtained in the present work is provided in Table 1.…”