Template-free hydrothermal synthesis of nickel sulfide nanocrystals on MWCNTs: efficient and stable bifunctional electroactive material for oxygen electrocatalysis

Abstract

Developing efficient and low-cost bifunctional electrocatalysts for oxygen electrocatalysis is extremely desirable in energy conversion and storage systems. In this work, a facile growth of nickel sulfide on the multi-walled carbon nanotubes (MWCNTs) is achieved hydrothermally via a template-free approach. Further, the material is annealed to obtain a highly active nanostructure with much more exposed active sites. The annealed product (Ni–S@MWCNTs-500) and the control samples are characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis, Raman spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy techniques. The prepared catalyst, Ni–S@MWCNTs-500 in 1.0 M KOH displays an improved oxygen evolution reaction (as compared to the benchmark catalyst, RuO₂) exhibiting an overpotential of 320 mV at 10 mA cm⁻² current density and a Tafel slope value of 84 mV dec⁻¹. In addition, Ni–S@MWCNTs-500 demonstrates a highly selective two-electron reduction of oxygen with an onset potential of 0.92 V. It reveals a high mass activity (115.59 A g⁻¹) and high electrochemically active surface area (17.0 cm²). The durability of the catalyst is studied by chronopotentiometry and continuous cyclic voltammetry cycles. The oxygen electrocatalysis activity is surprisingly stable without any noticeable change in the current response and overpotential/onset potential. Ni–S@MWCNTs-500 improves the electrical accessibility of the active sites and decreases the chances of nanoparticle aggregation.