Electrodeposition of cobalt-manganese oxide selective coatings for solar-thermal applications

Abstract

Solar absorber coatings are the heart of solar-thermal power systems and their desired properties intimately depend on the operational temperature. Specifically for medium to high temperature applications, the systems require a thermally-stable selective coating with optimal optical properties. Black metal mixed oxides with the spinel crystal structure are interesting prospects as a new family of selective absorber materials. In this work we report on the indirect electrodeposition of a spinel black oxide of the type MnxCo3-xO4, using a nitrate solution; the electrochemical reduction of nitrate results in the formation of surface hydroxide leading to the co-precipitation of Co and Mn hydroxides, and the resulting mainly amorphous film is subsequently converted to the spinel material by sintering. The optimal precursor chemistry and electrodeposition conditions were obtained using a combinatorial method employing the Hull cell. After optimization and thermal annealing, spinel MnxCo3-xO4 films (x ≈ 0.7) with solar absorptance of α = 89.5% and thermal emittance of ε = 14.9% (at 100 °C) were obtained, illustrating their promising optical properties. The thermal stability of the coatings at medium and high temperatures (250 °C and 560 °C), as well as under harsh accelerated weathering conditions, was found to be excellent. The scalability of the electrodeposition method was confirmed by deposition onto a stainless steel tube.