Electrodeposition and characterization of nanostructured black nickel selective absorber coatings for solar–thermal energy conversion

Selective coatings consisting of a bright nickel interlayer and black nickel overlayer for solar-to-thermal energy conversion have been electrodeposited onto stainless steel substrates from a nickel(II) chloride bath. During electrodeposition of black nickel, multiple reduction processes combined with oxidative processes result in a complex film composition, containing α-Ni(OH) 2 , NiOOH, Ni 2 O 3 , NiO, water and metallic Ni, as indicated by a detailed X-ray photoelectron spectroscopy study. Reflectance measurements confirm that the selective coatings have a high absorbance in the wavelength range of the solar spectrum, and that the emissivity in the near to mid infrared is low. Electron microscopy showed that the films consist of nanostructured flakes, oriented perpendicular to the surface. The best coatings were obtained by using a two-pulse galvanostatic deposition method.