Effects of Ni(OH)₂ Structures on the Electrochemical Conversion of KA Oil

Efficient conversion of KA oil (cyclohexanol/cyclohexanone mixture) to adipic acid (AA) holds great significance. Herein, a series of Ni(OH)2 samples with well-defined morphologies (i.e., nanorod and nanosheet) and crystal structures (i.e., α and β phases) were synthesized and evaluated for electrochemical conversion of KA oil. The catalytic performance of the samples was evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometric tests. The products of KA oil oxidation were analyzed using high-performance liquid chromatography (HPLC). Among the samples, the α-Ni(OH)2 nanorod exhibited the best electrochemical activity, with a maximum current density of 17.6 mA mg–1 at 1.57 V. Such superior activity is related to the large surface area and formation of active NiOOH at lower potentials. HPLC results indicate that the β-Ni(OH)2 nanosheet exhibited the highest adipic acid selectivity of 26% at 1.63 V. Notably, the α-Ni(OH)2 nanosheet shows both poor activity and product selectivity. Open-circuit potential (OCP) and double-layer capacitance (Cdl) measurements demonstrated that the adsorption capacity of KA oil over the samples follows the sequence of β-Ni(OH)2 nanosheet > α-Ni(OH)2 nanorod > α-Ni(OH)2 nanosheet. Operando Raman and electrochemical impedance spectroscopy (EIS) results suggest that the oxidation of KA oil by the α-Ni(OH)2 nanorod mainly follows the indirect oxidation mechanism, while the oxidation of KA oil by the α-Ni(OH)2 nanosheet follows the direct oxidation mechanism. For the β-Ni(OH)2 nanosheet, both direct and indirect oxidations of KA oil take place.