Improved Thermochemical Energy Storage Behavior of Manganese Oxide by Molybdenum Doping

To improve the thermochemical energy storage (TCS) behavior of Mn₂O₃, several Mn-Mo oxides with varying amounts of MoO₃ (0-30 wt%) were prepared by a precipitation method. The physico-chemical properties of the solids were studied by N₂ adsorption-desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), and H₂-temperature-programmed reduction (TPR), while their TCS behavior was determined by thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC). Apart from Mn₂O₃ and MoO₃ phases, XRD revealed a mixed MnMoO₄ phase for MoO₃ loadings equal or higher than 1.5 wt%. All samples showed a well-formed coral-like surface morphology, particularly those solids with low MoO₃ contents. This coral morphology was progressively decorated with compact and Mo-enriched MnMoO₄ particles as the MoO₃ content increased. TPR revealed that the redox behavior of Mn₂O₃ was significantly altered upon addition of Mo. The TCS behavior of Mn₂O₃ (mostly oxidation kinetics and redox cyclability) was enhanced by addition of low amounts of Mo (0.6 and 1.5% MoO₃) without significantly increasing the reduction temperature of the solids. The coral morphology (which facilitated oxygen diffusion) and a smoother transition from the reduced to oxidized phase were suggested to be responsible for this improved TCS behavior. The samples containing 0.6 and 1.5 wt% of MoO₃ showed outstanding cyclability after 45 consecutive reduction-oxidation cycles at high temperatures (600-1000 °C). These materials could potentially reach absorption efficiencies higher than 90% at concentration capacity values typical of concentrated solar power plants.