Crystal phase-dependent optical properties of CoMn-based spinel oxides for solar thermal conversion

Abstract

Spinel materials are promising solar absorbing candidates in concentrating solar power (CSP) due to their excellent high-temperature stability and adjustable light-absorbing properties. However, it is still unclear how the crystal structure of spinels affects their solar absorption properties and further solar-to-heat conversion efficiency. In this work, we found that both the absorption coefficient and skin depth of cubic phase Co₂MnO₄ are higher than those of tetragonal phase CoMn₂O₄. The difference in light absorption properties between Co₂MnO₄ and CoMn₂O₄ is attributed to the different excitation modes and d–d transitions of the materials. The cubic Co₂MnO₄ allows a direct electronic transition process and multiple allowed d–d transitions, which is responsible for the improved absorption properties of Co₂MnO₄. Therefore, the cubic Co₂MnO₄ spinel exhibits higher macroscopic absorption than its tetragonal counterpart in the UV-vis-IR diffuse reflectance spectra, especially at around 1400 nm. As a result, Co₂MnO₄ displays an internal photothermal conversion efficiency of 94.0%, which is higher than that of CoMn₂O₄ (69.4%). This work reveals the intrinsic correlation of the crystal phase of CoMnO-based spinel materials with its optical properties and solar thermal conversion efficiency.