Ionic liquid crystals/nano-nickel oxide-decorated carbon nanotubes composite for electrocatalytic treatment of urea-contaminated water

Abstract

Electrochemical treatment is an efficient approach for urea-decontamination from aqueous media. Reliable electro-catalysts are therefore crucial for water decontamination applications. We introduce in this work a newly developed composite by modifying a nickel substrate with nickel oxide nanoparticles (NiO), multiwalled carbon nanotubes (CNTs) and ionic liquid crystals (ILCs). The composite modifier (Ni/NiO + CNTs) exhibit an outstanding specific catalytic current for 0.33 M urea electro-oxidation in 1 M KOH of 1303 A·g−1·cm−2. The optimum regime of the catalyst administration to the substrate surface is a layer of ILCs followed by a mixture of CNTs and NiO with a ratio of 1:1 by mass (Ni/ILCs/NiO + CNTs (1:1)). The inclusions of ILCs assist the ionic exchange of the electrode surface and increase substantially the performance of the catalyst. The change in the carbon-based component of the catalyst affects the electro-catalytic current; reduced graphene oxide (RGO) exhibit relatively lower value compared to CNTs. The Tafel slope, the exchange current density, charge transfer coefficient, diffusion coefficient, and heterogeneous rate constant in 0.33 M urea/1.0 M KOH for the optimized electrode (Ni/ILCs/NiO + CNTs (1:1)) are: 33.0 mV·dec−1, 2.89 × 10−5 A·cm−2, 0.91, 1.70 × 10−3 cm2·s−1, 1.51 × 105 mol−1·L·s−1, respectively. The calculated activation energy of the electrochemical conversion of urea is 5.28 kJ·mol−1. The electrode shows high stability when used for extended time to constant applied potential of about +0.5 V for urea electro-oxidation. The conversion efficiency increased in the order Ni/ILCs/NiO + CNTs > Ni/NiO + CNTs > Ni/NiO compared to the Ni surface.