Temperature effect on crystallinity and chemical states of nickel hydroxide as alternative superior catalyst for urea electrooxidation

Developing a highly efficient and stable catalyst, composited by inexpensive, earth-abundant and non-toxic materials, is the critical issue for electrochemical urea oxidation. Among the all other physical parameters, the reaction temperature plays the key role in catalyst performance. Herein, we have studied the effect of temperatures on the synthesis of nickel hydroxide-carbon nanotubes composites via a facile hydrothermal method and the catalytic performance towards urea electro-oxidation in detail. Our studies indicate that the higher temperature enhances the crystallinity of β-Ni(OH) ₂ species. Furthermore, the Ni(III) species in Ni(OH) ₂ -CNTs composites show an optimal point upon the changes of the reaction temperature, and the highest Ni(III) content appears at 80 °C of β-Ni(OH) ₂ -CNTs. Meanwhile, electrochemical studies show that the electrochemically active surface areas (ESA) of the optimized β-Ni(OH) ₂ -CNTs (80 °C, 95.6 m ² g ⁻¹ ) catalyst is 4.51- and 2.76-fold higher than that of β-Ni(OH) ₂ -CNTs synthesized at 20 and 140 °C, respectively. The higher ESA of β-Ni(OH) ₂ -CNTs (80 °C) also accompanies a superior electrochemical urea oxidation with a peak current density of 98.5 mA cm ⁻² , significantly higher than all the other studied catalysts. Additionally, the optimal β-Ni(OH) ₂ -CNTs (80 °C) also demonstrates the highest initial and limiting current densities after uninterrupted long-term operation. These excellent performance of β-Ni(OH) ₂ -CNTs (80 °C) catalyst indicates the positive effect of Ni(III) content which provides more catalytically active species along with the unique lamellar structures of CNTs as the support to facilitate electron/mass transfer and gas emission.