Recovery of Ni and Co from Nickel-Based Superalloy Scraps Leaching Solution for the Synthesis of NiCo2O4

Abstract

We aimed to recover Ni and Co from the leaching sulfate solution of nickel-based superalloy scraps (NSL) and to synthesize NiCo₂O₄ through oxalate co-precipitation-calcination technique. The structural and compositional analyses of the resultant products were characterized utilizing x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Furthermore, the effects of nickel-cobalt molar ratio and calcination temperature on the oxygen evolution reaction (OER) performance were investigated. The results indicated that under optimal conditions of 0.7 M C₂O₄²⁻ addition, 25°C, and 4 h reaction time, the precipitation efficiencies for Co²⁺ and Ni²⁺ were 99.19% and 95.79%, respectively. The precipitate derived from adjusting the Ni/Co molar ratio in NSL manifested as a solid solution of nickel-cobalt oxalate (Ni_xCo_3-x(C₂O₄)₃). As the molar ratio decreased, the precipitates gradually transformed from a nickel oxalate structure with Ni sites replaced by Co to a cobalt oxalate structure with Ni replacing Co sites. The pure phase structure NiCo₂O₄, synthesized with a Ni/Co molar ratio of 1/2 and calcined at 450°C for 2 h, exhibited an overpotential of 430 mV at a current density of 10 mA·cm⁻² in 0.1 M KOH, a Tafel slope of 93.69 mV·dec⁻¹, and a C_dl of 242.75 mF·cm⁻². Upon exceeding 450°C during calcination, the gradual expulsion of heteroatoms from the matrix resulted in structural destruction. Consequently, the electrocatalytic activity of the sample decreased because of the gradual removal of surface oxygen vacancy concentration and Co³⁺/Co³⁺ + Co²⁺. The methodology employed in this study not only efficiently recovers Ni²⁺ and Co²⁺ from the NSL but also prepares electrocatalysts with superior performance for the oxygen evolution reaction.