Enhanced solar absorption by covalency competition in Cr-free (FexCo0.25Mn0.25Cu0.25)3O4 coatings for concentrated solar power

Typically, advanced solar absorber coatings containing chromium (Cr) achieve high solar absorption efficiencies. However, large-scale applications raise concerns regarding the safety and cost due to the potential toxicity of Cr. This study develops a Cr-free (Fe x Co 0.25 Mn 0.25 Cu 0.25 ) 3 O 4 spinel coating based on tuning Fe content, which exhibits enhanced solar absorption and thermal stability. As Fe content increases, a single spinel structure is stabilized, accompanied by lattice expansion. Meanwhile, the covalent competition among cations at the tetrahedral sites in the structure is enhanced, facilitating the migration of Co and Mn ions from tetrahedral to octahedral sites. This process alters the valence distribution and lattice symmetry, leading to a reduction in oxygen vacancies (O V ) and an increase in the bandgap. Consequently, the solar absorptance was enhanced from 90.6 % to 93.2 % by decreasing Fe content in the single spinel structure, while infrared emissivity exhibits a significant increase in emissivity at 2.5–7.8 μm. After 60 thermal cycles at 750 °C, the (Fe 0.25 Co 0.25 Mn 0.25 Cu 0.25 ) 3 O 4 coating exhibits improved solar absorptance (94.9 %) and reduced thermal emissivity (90.2 %), ultimately enhancing the thermal efficiency to 89.3 %. This work provides a new option for Cr-free solar absorption coatings with tailored performance in high-temperature concentrated solar power (CSP) systems.