Influence of a metallic nickel interlayer on the performance of solar absorber coatings based on black nickel electrodeposited onto copper

Black nickel is an interesting, economical, and abundant solar absorber material for low temperature solar-to-thermal energy conversion systems, and thin films can be prepared by electrodeposition. Critical parameters include the energy conversion efficiency and thermal stability, which depend on the design of the selective coating and the substrate material. In this work, we show that a metallic nickel interlayer is required in order to achieve thermal stability of black nickel coatings on copper, although the solar-to-thermal conversion efficiency for these multilayer coatings is slightly lower than for coatings of black nickel directly electrodeposited onto copper. The black nickel films were electrodeposited from a nickel chloride bath at pH 6.5 in order to avoid thermal stability issues related to the presence of sulfur. XPS and Raman spectroscopy illustrate that the composition of the black nickel film is complex, containing Ni(OH)2, NiO, NiOOH and metallic Ni. We show that the electrodeposition parameters strongly affect the performance of the selective coating.