Improved Thermochemical Energy Storage Behavior of Manganese Oxide by Molybdenum Doping.

To improve the thermochemical energy storage (TCS) behavior of MnO, several Mn-Mo oxides with varying amounts of MoO (0-30 wt%) were prepared by a precipitation method. The physico-chemical properties of the solids were studied by N adsorption-desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), and H-temperature-programmed reduction (TPR), while their TCS behavior was determined by thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC). Apart from MnO and MoO phases, XRD revealed a mixed MnMoO phase for MoO loadings equal or higher than 1.5 wt%. All samples showed a well-formed coral-like surface morphology, particularly those solids with low MoO contents. This coral morphology was progressively decorated with compact and Mo-enriched MnMoO particles as the MoO content increased. TPR revealed that the redox behavior of MnO was significantly altered upon addition of Mo. The TCS behavior of MnO (mostly oxidation kinetics and redox cyclability) was enhanced by addition of low amounts of Mo (0.6 and 1.5% MoO) 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 MoO 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.