Microscopic mechanisms of Mn-doped CaCO3 heat carrier with enhanced optical absorption and accelerated decomposition kinetics for directly storing solar energy

Abstract

The ability of calcium-based composites to simultaneously improve optical absorption and accelerate decomposition kinetics represents a major endeavor direction in the direct solar-driven thermochemical energy storage system. Doping strategy has been macroscopically proved to be an effective way in boosting optical absorption properties and calcination kinetics, but the underlying microscopic mechanisms are still unclear. This paper aims to reveal the mechanisms of optical absorption enhancement and accelerated decomposition kinetics for Mn-doped CaCO3 through both experiments and density function theory (DFT). In the experiment, the optical absorption of Mn-doped CaCO3 was much higher than that of pure CaCO3. The sample CaCO3–7Mn displayed satisfied optical absorption and fabulous cycling stability. The optical absorption reached 74% and energy storage density only decreased 2.6% after 20 cycles. Meanwhile, the decomposition reaction rate equations of pure CaCO3 and Mn-doped CaCO3 were obtained via non-isothermal kinetic analysis. The results indicated that doping Mn could accelerate decomposition kinetics of CaCO3 by reducing the apparent activation energy. In the DFT calculation, the complex refractive index of Mn-doped CaCO3 was lower in real part (refractive index) and higher in imaginary part (extinction coefficient) within the wavelength range from 300 to 2500 nm compared with pure CaCO3, which was closely related to the electron cloud distribution in the calcite crystal structure. Moreover, Mn-doped CaCO3 has lower transition state energy barrier and dissociation energy than pure CaCO3. The calculation results revealed the mechanisms of doping Mn in CaCO3 to enhance the light absorption and accelerate the decomposition kinetics from the microscopic perspective at atomic level. This work contributes to understanding the physical mechanisms of doping elements on boosting the performance of calcium-based composites at the molecular level.