Synthesis and electrochemical properties of nanoporous CrN thin film electrodes for supercapacitor applications

[Display omitted] •Sufficient Ni content leads to an interconnected Ni-network in the CrN–Ni coating.•Nanoporous CrN can be obtained by HCl etching of CrN–Ni.•Nanoporous CrN has an ~ 80% higher specific capacitance than CrN.•Guideline to prepare porous transition metal nitrides with a high specific surface area. Transition metal nitrides (TMN) have received widespread consideration as supercapacitor materials for a variety of energy storage applications. An important requirement to such electrode materials is a very high specific surface area, which is typically obtained through a percolating pore-network. Here, we use arc ion plating to prepare CrN-Ni composite coatings with 0, 30.4, 54.2, and 77.6 at.% Ni. Detailed X-ray diffraction and X-ray photoelectron spectroscopy showed that the coatings consist of CrN phases next to metallic Ni. Post-deposition treatments with HCl solutions lead to selective leaching, causing a porous nanostructure for the Ni-containing CrN coatings. From there, the coating with 54.2 at.% Ni in its as-deposited state provided the highest (electrode-geometric-area)-specific capacitance after the HCl treatment, giving 58.5 mF·cm−2 at 1.0 mA·cm−2 in a 0.5 M H2SO4 aqueous electrolyte. This value is about 80 times higher than for the as-deposited coatings or the Ni-free CrN. Consequently, our strategy to combine TMNs with metallic Ni and its subsequent treatment with HCl is highly effective in enlarging the specific surface area and adsorption sites. It allows to significantly enhance the energy storage performance of TMN thin film supercapacitors.