Manganese–iron binary oxide for solar thermochemical energy storage

Electricity can be generated from heat stored in molten salt for less than $0.03/kWhel, which is far less than an aggressive $0.10/kWhel estimate for future battery storage1. Although numerous metal oxide systems have been... [ view full abstract ]

Electricity can be generated from heat stored in molten salt for less than $0.03/kWh_el, which is far less than an aggressive $0.10/kWh_el estimate for future battery storage¹. Although numerous metal oxide systems have been investigated for thermochemical energy storage (TCES) redox processing to improve on molten salt storage, none of these systems has fulfilled all the requirements of an ideal storage material (low cost, robustness, safety, and high energy density). For instance, pure manganese oxide is an inexpensive material that reacts at an acceptable temperature and has a moderate reaction enthalpy. However, its re-oxidation reaction is slow and its cyclability (robustness) is poor.  On the other hand, iron oxide has a much higher reaction enthalpy and faster reaction kinetics, but suffers from a relatively high reduction temperature and deactivation upon sintering.   Recent work has focused on a binary 1:3 Fe₂O₃:Mn₂O₃ system as a possible TCES system².  However, it seems likely that a higher Fe content will be desirable for faster kinetics and improved energy storage.

So, in this work, a binary mixture of 2:1 Fe₂O₃:Mn₂O₃ which forms iron manganese oxide spinel (MnFe₂O₄) on calcination is investigated as a potentially suitable TCES material. The XRD patterns in Figure 1 prove the presence of cubic spinel phase for the reduced state and show a bixbyite phase for the oxidized state of the material. The reduction reaction of this system has been studied using Thermogravimetric Analysis (TGA). Master plot analysis (Figure 2), and multivariate non-linear regression (Figure 3) are used to identify the reaction mechanism and its associated kinetic parameters for inert gas reduction. The reduction reaction follows the first order mechanism and the reaction rate law is da/dt=3.077e+11exp(-296/RT)(1-a). Results reveal several key advantages of the proposed binary oxide system over pure manganese and iron oxides.  It ensures high enthalpy of reaction and improved re-oxidation reaction behavior while preserving an appropriate reduction reaction temperature.  Comparative results for reduction in air have also been studied for comparison.

¹ Branz, H.M. et al., Energy & Environmental Science, 8, 3083 (2015)  

² Wokon, M et al., Solar Energy, 153, 471 (2017)