Enhanced Performance of Nickel–Cobalt Oxides as Selective Coatings for Flat-Plate Solar Thermal Collector Applications

Solar thermal collectors represent a practical option to capture energy from the sun, providing low-cost domestic and industrial heating and decreasing the dependency on fossil fuels. Spinel-type metal oxides show interesting physicochemical properties and so can be used as active materials for converting solar energy to electrical, chemical, and heat energy. We report the synthesis and characterization of nickel–cobalt mixed metal oxides used as an active phase in selective paints for solar absorber coatings applied to a domestic flat collector. The nickel–cobalt mixed oxides crystallized in the cubic phase related to the spinel structure, exhibiting good thermal stability and reproducibility. These mixed oxides presented oxidation states (2+ and 3+) for both nickel and cobalt. The coatings fabricated from the selective paints based on nickel–cobalt mixed oxides showed a solar absorptance value of 94%, while for the commercial paint Solkote®, the value was 93%. A representative coating based on the NiCo2O4 composition was evaluated for the first time in a domestic-type flat solar collector for water heating under real operating conditions, achieving an outstanding performance that competes with that of commercial collectors. The potential application of nickel–cobalt mixed oxides in solar collectors opens up new opportunities for future innovations and developments in functional absorber coatings.