Structure and electrochemical activity of nickel aluminium fluoride nanosheets during urea electro-oxidation in an alkaline solution

Abstract

An electrocatalyst of potassium nickel aluminium hexafluoride (KNiAlF₆) nanosheets has been prepared using solid-phase synthesis at 900 °C. X-ray diffraction, scanning electron microscopy, and conductivity studies confirmed the formation of KNiAlF₆ nanosheets having a cubic defect pyrochlore structure with an average thickness of 60–70 nm and conductivity of 1.297 × 10³ S m⁻¹. The electrochemical catalytic activity of the KNiAlF₆ nanosheets was investigated for urea oxidation in alkaline solution. The results show that the KNiAlF₆ nanosheets exhibit a mass activity of ∼395 mA cm⁻² mg⁻¹ at 1.65 V vs. HRE, a reaction activation energy of 4.02 kJ mol⁻¹, Tafel slope of 22 mV dec⁻¹ and an oxidation onset potential of ∼1.35 V vs. HRE which is a significant enhancement for urea oxidation when compared with both bulk Ni(OH)₂ and nickel hydroxide-based catalysts published in the literature. Chronoamperometry and impedance analysis of the KNiAlF₆ nanosheets reveal lower charge transfer resistance and long-term stability during the prolonged urea electro-oxidation process, particularly at 60 °C. After an extended urea electrolysis process, the structure and morphology of the KNiAlF₆ nanosheets were significantly changed due to partial transformation to Ni(OH)₂ but the electrochemical activity was sustained. The enhanced electrochemical surface area and the replacement of nickel in the lattice by aluminium make KNiAlF₆ nanosheets highly active electrocatalysts for urea oxidation in alkaline solution.