Co-Mn Oxide Electrodeposition for High Temperature Solar-to-Thermal Energy Conversion Systems

Abstract

Solar concentration systems use solar collectors consisting of a metal tube with a selective coating, and an evacuated, glass vacuum tube to minimize losses by convection, conduction and thermal emission. The glass tubes eventually loose the vacuum, and the exposure to air results in degradation of the selective coating. Manganese cobalt oxides are alternative air-stable selective coatings, related to the spinel structure they form, which is stable up to high temperatures. The optical properties of the selective coating are expected to depend on the proportions of Co, Mn and O in the material; hence, the electrodeposited film needs to be optimized by tailoring the composition. In this work we report on the combinatorial electrodeposition of Co-Mn oxides employing a Hull cell, in order to prepare a compositional materials library and to relate the composition with the solar absorptance and thermal emittance. The Co-Mn oxide material library was deposited in galvanostatic mode using a two-electrode setup, with a stable anode and stainless steel cathode (AISI 304). The electrodeposited cathodes were treated at 600 oC for a short time in order to complete the oxidation of the material. The solar absorptance and thermal emittance in the 200 nm - 15 µm range were obtained from reflectance measurements. From the best optical values, the current density corresponding to the optimal position on the cathode was selected for further characterization of the deposited material. Co-Mn oxide films were then deposited on 6 cm x 4 cm stainless steel substrates for thermal tests at 252 oC for 600 hours in order to evaluate the performance criteria. The film composition was obtained from energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and X-ray diffraction (XRD).