Sol-Gel Cobalt-Free Spinel Oxide as a Selective Solar Absorber: Achieving High Efficiency Without Critical Raw Materials
Abstract
This study develops a cobalt-free CuMnOₓ multilayer solar absorber to replace CuMnCoO4, which offers ~0.95 solar absorption and ~0.15 emissivity. Driven by cobalt's environmental and supply risks, we optimized dipcoating parameters and layer thicknesses on stainless steel using ellipsometry, simulations, and experimental validation. Spectroscopic ellipsometry revealed that cobalt mainly affects the extinction coefficient (k) in the visible range, enhancing absorption via electronic transitions, while its impact in the silica-spinel layer is negligible. The cobalt-free CuMnOₓ showed slower k decay in the near-infrared (1.5-2.0 μm), explaining its higher absorption in this range. Transfer-matrix modeling (TFCalc) predicted an optimal multilayer design with a theoretical α<0xE2><0x82><0x9B><0xE2><0x82><0x92><0xE2><0x82><0x97> of 0.957, closely matched by the measured performance. The empirical correlation between film thickness and withdrawal speed exhibited a regime shift at 7 mm/s, consistent with LandauLevich and gravitational-viscous deposition regimes. Morphological analyses confirmed nanocrystallinity and superior homogeneity for the cobalt-free coating, leading to enhanced reproducibility and scalability. Overall, the cobalt-free CuMnOₓ/CuMnSiOₓ/SiO₂ multilayer exhibits comparable optical performance to its cobaltcontaining counterpart, validating it as a sustainable and industrially viable alternative for solar thermal absorbers.
Keywords
CuMnO_ ₓ _, Selective solar absorber, Cobalt-free, Sol-gel, Dip-coating, Ellipsometry