Thermodynamic and Structural Effects of Fe Doping in Magnesium Manganese Oxides for Thermochemical Energy Storage

Thermochemical energy storage potentially provides a cost-effective means of directly storing thermal energy that can be converted to electricity to satisfy demand, and MgxMn1-xO4 has been identified as a stable, high-energy density storage material. Here, we investigate the effects of doping small quantities of Fe into the MgMnOx system as a means to increase the reduction extent and storage energy via an increase in entropic contributions and the higher reduction energy of Fe as compared to that of Mn. We find that small additions of Fe (Mg0.5Mn0.4975Fe0.0025)3O4 (Fe = 0.5%) show increased reduction extent, but larger quantities of Fe (Mg0.5Mn0.485Fe0.015)3O4 and (Mg0.5Mn0.475Fe0.025)3O4 (Fe = 3 and 5%) show decreased reduction capability. This finding suggests that small quantities of Fe as a substituent change the thermodynamics of the material increasing the reduction extent through entropic effects, but at larger quantities of Fe, the higher reduction energy of Fe lowers the overall reduction capability. Additionally, the reduction occurs through the formation of intermediate phases which co-occur with the oxidized spinel and reduced halite phases; the presence of Fe significantly narrows the window where the intermediate phase is present, narrowing the T range where most of the reduction occurs. This narrowing thus potentially stabilizes the outlet temperature of a thermochemical energy storage system as compared to undoped (MgxMn1-x)3O4.