Hierarchical nanostructure engineering of 3D nickel cobaltite ultrafine nanoparticles assemblies for synergistic electrocatalytic water and urea oxidation

Self-assembled complex metal oxides have attracted lots of research interest due to their interesting surface properties for many catalytic applications. The present work focuses on the synthesis of nickel cobaltite using mixed metal oxalates via the micellar route. Microscopic studies of NiCo2O4 confirm the formation of self-assembled ultrafine nanoparticles of size ∼6 nm which were hierarchically arranged to form the microflower structure. Similarly, sodium hydroxide is used as a precipitating agent, and NiCo2O4 with discrete nanoparticles of the size ∼15 nm is formed. The obtained NiCo2O4 microflowers and nanoparticles were used for electrochemical water and urea oxidation in alkaline media. The water oxidation of 3D microflowers shows an overpotential of 300 mV and −206 mV for O2 and H2 evolution whereas nanoparticles show 620 mV and −429 mV overpotential in 1 M KOH, respectively. Further, electrochemical urea oxidation for NiCo2O4 microflowers requires 1.27 V to achieve 10 mA/cm2 current density whereas nanoparticles are inactive towards urea oxidation in 3 M KOH. The overall study suggests that oxalate route synthesis results in hierarchically arranged nanoparticle growth to 3D microflower structure and NiCo2O4 microflower serves as a promising candidate for water and urea electrolysis in alkaline media compared to NiCo2O4 nanoparticle.