Understanding the modulation mechanism of B-site doping on the thermochemical properties of CaMnO_3-: an experimental and computational study

Perovskite materials are considered promising for thermochemical energy storage. B-site substitutional doping can effectively improve the thermal storage performance of perovskite materials. Although the mechanism of B-site doping is well understood, the effects of dopants need to be quantitatively clarified. In this work, the thermal storage performances of two B-site doped materials, CaMn1-xCrxO3 and CaMn1-xFexO3, are comprehensively evaluated. The thermal storage reaction extent and reaction reversibility are significantly improved upon B-site doping. Notably, Cr-doping leads to a slight decrease in the reaction onset temperature by 10 degrees C and an increase in the thermal storage density to 171.0 kJ kg(-1). In contrast, Fe-doping significantly decreases the reaction onset temperature by 66-290 degrees C and leads to an increase in the thermal storage density to 153.5 kJ kg(-1). CaMn1-xCrxO3 and CaMn1-xFexO3 maintain stable structures and high reactivities (>96% and 94%, respectively) after 540 cycles. The DFT+U study reveals that the bond energy strength between the B-site and oxygen is the key factor influencing the reaction onset temperature and reaction enthalpy of the materials. COHP calculations show that doping Cr and Fe breaks the electronegativity and bond energy balance, producing different active sites, which enhances the material reactivity. Cr-doping contributes to a higher reaction enthalpy through the enhancement of overall bonding energy of the material, while Fe-doping lowers the oxygen migration barrier through the reduction of the overall bonding energy.